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An Insight Into the Production, Characterization, and Mechanisms of Action of Low-Cost Adsorbents for Removal of Organics From Aqueous Solution a

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a

S. Kushwaha , H. Soni , V. Ageetha & P. Padmaja

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Department of Chemistry, Faculty of Science , The Maharaja Sayajirao University of Baroda , Vadodara , India Accepted author version posted online: 22 Mar 2012.Published online: 08 Feb 2013.

To cite this article: S. Kushwaha , H. Soni , V. Ageetha & P. Padmaja (2013) An Insight Into the Production, Characterization, and Mechanisms of Action of Low-Cost Adsorbents for Removal of Organics From Aqueous Solution, Critical Reviews in Environmental Science and Technology, 43:5, 443-549, DOI: 10.1080/10643389.2011.604263 To link to this article: http://dx.doi.org/10.1080/10643389.2011.604263

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Critical Reviews in Environmental Science and Technology, 43:443–549, 2013 Copyright © Taylor & Francis Group, LLC ISSN: 1064-3389 print / 1547-6537 online DOI: 10.1080/10643389.2011.604263

An Insight Into the Production, Characterization, and Mechanisms of Action of Low-Cost Adsorbents for Removal of Organics From Aqueous Solution S. KUSHWAHA, H. SONI, V. AGEETHA, and P. PADMAJA Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India

A number of industries currently produce varying organic pollutant-laden waste streams, which is a serious environmental prblem owing to their potential human toxicity. Amongst all the treatments proposed, adsorption is one of the more popular methods for the removal of pollutants from wastewater. In particular, considerable work has been carried out on the use of natural materials, nonconventional adsorbents, and their modifications. These natural materials and nonconventional materials in many instances are relatively cheap, abundant in supply, or thrown away as waste and have significant potential for modification that ultimately leads to enhancement of their adsorption capabilities. The authors review and evaluate literature dedicated to the preparation of adsorbents such as activated carbon and clays and their modifications/activation by recycling different types of waste materials and also to their application in organic pollutant containing aqueous-phase treatments. It should be noted that the reported adsorption capacities of the adsorbents vary, depending on the characteristics of the individual adsorbent, the extent of chemical modifications, the concentration of solutes, and the ionic strength and pH of the medium. Thus characterization of the adsorbents for their functional groups and their nature using IR, XRD, and pHZpc has been discussed. The authors further discuss how several key factors such as pH, competitive adsorption, and temperature influence the adsorption taking instances from literature. The authors also provide a summary of recent information obtained using Address correspondence to P. Padmaja, Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, India. E-mail: [email protected] 443

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S. Kushwaha et al.

batch studies and deals with the various adsorption mechanisms involved. KEY WORDS: mechanism

agrowaste, organics, pretreatment, adsorption


1. INTRODUCTION Environmental pollution, in general, and water pollution, in particular, is a subject of growing concern to the scientific community worldwide. There has been increasing concern about the release of heavy metals, dyes, pesticides, phenols and other organic and inorganic compounds to the environment as a result of various industrial agricultural activities, due to the potential toxic, carcinogenic, and mutagenic effects of these substances. Organic compounds such as phenols, dyes, and nitroaromatics are considered to be hazardous wastes, which are released into the aquatic environment by industries such as coke ovens in steel plants, petroleum refineries, phenolic resin, fertilizer, pharmaceutical, chemical, and dye industries and have been reported in hazardous wastes sites. The literature shows a greater amount of studies on the adsorption of organic compounds as compared with the inorganic ones,[1] probably due to the complexity associated with the mechanisms regarding the adsorption of organic compounds and hazardous nature of a greater variety of organic contaminants. The major classes of organic pollutants are shown in Figure 1. There are several reported methods for the removal of pollutants from effluents and surface waters using a wide variety of treatment technologies Organics

Dyes

Pesticides

Ionic

Non-ionic

Cationic Chlorinated HC's Basic Organo phosphoric Acidic Dinitro anillides Miscellaneous

Direct Acid Basic Disperse Mordant Ingrain Vat Reactive Sulphur

Phenols Chlorophenols Nitrophenols

FIGURE 1. Major classes of organic pollutants.

Others (Nitro) Nitrosamines Nitrobenzenes (PAHs) Naphthalene Acenaphtalene


Removal of Organics From Aqueous Solution

445

such as precipitation, coagulation–flocculation, sedimentation, flotation, filtration, membrane processes, electrochemical techniques, biological process, chemical reactions, adsorption, and ion exchange. Several other advanced methods available for removal include photocatalytic degradation, combined photo-Fenton and biological oxidation, advanced oxidation processes, aerobic degradation, nanofiltration membranes, ozonation, and adsorption.[2–23] The various aspects of the microbiological decomposition of synthetic dyes were reviewed by Stolz.[24] The benefits and drawbacks of the use of microbial consortiums for the decomposition and decolorization of various dyes have been reviewed by Banat et al.[25] Because of the high cost and disposal problems, many of these conventional methods for treating organic pollutant containing wastewater have not been widely applied on a large scale.[26–29] Each method has its own merits and limitations in applications (Table 1). Further, most of the pesticides, dyes and other organics present in water are not removed or transformed during the physical/chemical treatment processes (i.e., coagulation/flocculation, sedimentation, and filtration).[30] In accordance with the abundant literature data, adsorption process, a surface phenomenon, is one of the most widely used methods for the removal of pollutants from wastewater. Adsorption process is simple in operation, inexpensive (compared to other separation processes), and without sludge formation. Thus this process provides an attractive alternative for the treatment of contaminated waters, especially if the sorbent is inexpensive and does not require an additional pretreatment step before its application.[31–38] Adsorption on activated carbon is the most widespread technology used to purify water contaminated by pesticides and other hazardous chemicals.[39–43] A notable surge in the use of activated carbon, as an effective and widely used adsorbent for removal of organics from aqueous solutions has been observed.[44] The natural materials, waste materials from industry and agriculture and biosorbents, can be employed as inexpensive adsorbents. Some review articles discussing low-cost alternative adsorbents are already available.[45–61,67–72,78–94] However, such studies are restricted to either a particular type of waste, preparation procedures, or adsorbate specific applications. For instance, the available technologies for the abatement of phenol from water and gaseous streams are briefly reviewed, and the recent advancements are summarized by Busca et al.[62] The fate of organophosphorus pesticides in the aquatic environment via processes such as adsorption, hydrolysis, oxidation, and photochemical degradation has been reviewed by Pehkonen and Zhang.[63] Bhatnagar et al. in their review provided a summary of recent information obtained using batch studies and the various adsorption mechanisms involved in the use of coconut based biosorbents.[64] The effects of parameters such as the chitosan characteristics, the process variables, the chemistry of the adsorbate, and the solution conditions used in batch studies on the biosorption

446

Emerging removal processes

Established recovery processes

Conventional treatment processes

Rapid and efficient process No sludge production little or no consumption of chemicals efficiency for recalcitrant dyes Particularly useful for treating nonbiodegradable pesticides and dyes useful for treating multiple combinations of pesticides and dyes in a single step. Process is accelerated by solar, UV, or ultrasonic radiation Economically attractive regeneration is not necessary high selectivity Low operating cost good efficiency and selectivity no toxic effect on microorganisms

Oxidation Advanced oxidation process

Selective biosorbents Biomass

No loss of sorbent on regeneration effective low power consumption and low operational cost

Removes all dye and pesticide types produce a high-quality treated effluent no phase change and no chemical conditioning required Low energy consumption compared to advanced oxidation process

The most effective adsorbent great capacity produces a high-quality treated effluent

Economically attractive capable of treating wide range of pesticides and dyes that cannot be treated by membrane and chemical processes

Simple, economically feasible

Advantages

Ion-exchange

Adsorption on activated carbons Membrane separations

Coagulation Flocculation Biodegradation

Technology

TABLE 1. Merits and demerits of water treatment technologies in applications

Slow process performance depends on some external factors (e.g., pH, salts)

Requires extra energy sources such as solar, UV, or ultrasonic radiation Requires chemical modification nondestructive process

Formation of chlorine or hypochlorite due to the potential of chlorine oxidation

Fouling takes place due to the presence of large spectrum of pollutants present in water resulting in short lifetime The concentrate resulting after treatment has to be disposed off or treated Not effective for disperse dyes and nonionic pesticides improper for treatment of chlorinated pesticides requires different types of exchangers for different constituents The presence of microorganisms can reduce the exchange capacity Disposal of regeneration solution High energy cost chemicals required Economically unfeasible, formation of by-products, technical constraints

High pressures, expensive, incapable of treating large volumes

High sludge production handling and disposal problems Not useful for treatment of pesticides Slow process due to slow digestion rates necessary to create an optimal favorable environment, maintenance and nutrition requirements Requires large area for implantation of treatment and biomass separation units Hampered by the presence of biorefractory organics (humic acids and surfactants) Limited suitability in treating less biodegradable pesticides and dyes due to the recalcitrant nature of its organic carbon Ineffective against disperse and vat dyes the regeneration is expensive and results in loss of the adsorbent nondestructive process

Disadvantages




447

capacity and kinetics were presented and discussed by Crini and Bhatnagar in their respective reviews.[65,66] The sorption behaviors of layered double hydroxides (LDHs) with various oxyanions, and the kinetic models adopted to explain the adsorption rate of oxyanions from aqueous solution onto LDHs, have been comprehensively reviewed.[73] The use of coconut-based biosorbents for water pollution control, highlighting and discussing key advancement on the preparation of novel adsorbents utilizing coconut wastes, its major challenges, and future perspectives has been discussed.[64] The review also summarizes the equilibrium and kinetic models reported for adsorption, which are important to determine the adsorption capacity and to design treatment processes. Adsorption of metal contaminants and organics on carbon nanotubes (CNTs) were extensively reviewed, but the sorption of biological contaminants on CNTs needs to be understood in greater detail.[76] Pearce et al.[77] made a comprehensive review of a method using bacterial cells to remove dyes from textile wastewater. The role of saw dust materials in the removal of pollutants from aqueous solutions has been reviewed recently.[35,78] Wang et al.[79] discussed the novel applications of red mud as coagulant, adsorbent, and catalyst for environmentally benign processes in their comprehensive review. Methods of dye wastewater treatment have been reviewed by Pokhrel and Viraraghavan,[80] Robinson et al.,[81] Slokar and Le Marechal,[82] Delee et al.,[83] Banat et al.,[25] Cooper,[84] Crini,[51] and Gupta and Suhas.[72] Fungal and bacterial decolorization methods have been reviewed by Aksu,[47] Wesenberg et al.,[85] Pearce et al.,[77] McMullan et al.,[86] Fu and Viraraghavan,[87] and Stolz.[24] Although many review articles have been published discussing the importance of low-cost adsorbents in water pollution control, the specific mechanism by which the adsorption of several organic compounds takes place is still ambiguous and controversial. The adsorption process results from interactions between the adsorbent surface and the adsorbate. These interactions can be electrostatic or nonelectrostatic. When the adsorbate is an electrolyte the interactions are electrostatic which depend on the (a) charge density of the adsorbent surface, (b) chemical characteristics of the adsorbate, and (c) ionic strength of the solution. Nonelectrostatic interactions include (a) van der Waals forces, (b) hydrophobic interactions, and (c) hydrogen bonding. According to Moreno-Castilla,[1] the properties of the adsorbate that mainly influence the adsorption process are (a) molecular size, (b) solubility, (c) pKa (ability for dissociation), (d) logarithm of octanol–water partition coefficient (log Kow) and dipole moment (D), and (e) nature of the substituents (in the case of aromatic adsorbates). The molecular size determines the accessibility of the adsorbate to the pores of the adsorbent, the solubility determines the degree of hydrophobic interactions between the adsorbate and the carbon surface and pKa controls the dissociation of the adsorbate (if it is an electrolyte). When the adsorbate is aromatic, the substituents of the aromatic


448

S. Kushwaha et al.

ring have the ability to withdraw or release electrons, which therefore affects the nonelectrostatic interactions between the adsorbate and the adsorbent surface. When the adsorbent is in contact with an aqueous solution, an electric charge is generated. This charge results from either the dissociation of the surface functional groups of the carbon or the adsorption of ions from the solution, and strongly depends on the solution pH, ionic strength, and the surface characteristics of the adsorbent.[95,96] The key issue for ion adsorption from an aqueous medium is the understanding of the mechanisms by which ionic species become attached to the carbon surface. Many studies have been made in order to understand thoroughly the adsorption mechanisms involved in these processes.[74,75,98–104,304] The influence of adsorbent, adsorbate, and solution properties on adsorption uptake was analyzed by many researchers. In the case of adsorption of aromatic derivatives the properties of functional groups on the adsorbate influence the sorption mechanism to a certain extent.[1,95,107–121] Radovic et al.,[97] in their review, discussed that (a) adsorption of aromatic compounds is partly physical and partly chemical and (b) p–p interactions might prevail under favorable conditions the strength of which can be modified by ring substitution on both the adsorbate and the adsorbent. The most important characteristics of adsorbent in the adsorption of organic compounds are pore size distribution, surface chemistry (functionality), and mineral matter content.[98–106] The oxygen groups on the adsorbent and dissolved oxygen exhibit a significant influence on the adsorption capacity.[105,106] The adsorption capacity depends on the accessibility of the organic molecules to the microporosity, which depends on their size. Thus, under appropriate experimental conditions, small molecules such as phenol can access micropores, natural organic matter can access mesopores, and bacteria can only access macropores. Thus, the investigations on adsorbate properties, medium as well as modification and characterization of carbonaceous materials with regard to increase their effectiveness in the adsorption processes are of fundamental importance. However, no previous review is available where readers can get an overview of the adsorption capacities of various adsorbents used for the removal of different aquatic organic pollutants reported in literature. An overview of some low-cost adsorbents for the removal of organics is also presented in this article along with their adsorption capacities. In the present review we (a) present an extensive list of low-cost adsorbents, recycled waste materials, and activated carbons prepared by utilizing different types of waste materials with their adsorption capacities, for various organic pollutants as available in the literature (see Table 2); (b) describe their characteristics, advantages, and limitations; and (c) discuss the applicable adsorption isotherm models, kinetic models, and various mechanisms involved. The effects of various parameters such as adsorbent characteristics,

449

100–1,800 mg/L, pH 7.0; 25 C, -, 50 mg/L, pH (1-6), -, 60 min

reactiveyellow42 reactivered45

Phenanthrene Pyrene Reactive Red 195 Methylene blue Direct blue 71

Methylene blue dye

Crystal violet Basic Blue 41 Nitrobenzene Cyanosine model basic dye Phenol

Neutral Red

Basic dyes

Reactive azo dyes

Modified pine bark

Pinus sylvestris L. Phoenix tree leaf powder Wheat shells

Carica papaya seeds

Grape fruit peel Novel biosorbent (Canola hull) Cost crop biological wastes Coconut husks Luffa cylindrica Pinus pinaster bark packed bed

peanut husk

dried Seagrape (Caulerpa lentillifera) chemically treated Citrus sinensis waste biomass

5.184 mg/g

◦

—

113.64 mg/g

36.36 mg/g 106.38 mg/g, 51.47 mg/g — 22.69 mg/g 9.39 mg/g 7.38 mg/g 200 mg/L, -, 50◦ C, 30 and 50 mgl-1, pH 4-10, 293 K, -, 149 mg/g 50–250 mgL−1, -, 293, 303 and 46.30 mg g−1 at 313 K, -, 313 K 10–400 mg L−1, pH 7.04–7.5, 1250 mgg−1 298K, 2 h 10-600 mg/L, -, -, -, 254 mg/g 67.6 mg/g 50 mg/L, pH 8, 20◦ C, -, 96% -, -, 25 ◦ C, 72 h 4 x10-5 M, pH3.6, 30 oC, -, — 20–100 mgL−1, -, 308K, 5 h 29.4 mg/g 0.01, 0.1 and 1 g/L, pH(4.5–9.5), -, 0.3768 mg/g -, 60 mg/L, pH 2-7, 295K, 4 h, 37.5 mg/g

60 120 min

30 g/L, -, -, 90 min

Free Citrus sinensis biomass

Cashew nut shell

Maximum adsorption capacity

Domestic waste and agromaterials 294.12 -, -, 25◦ C, 25 mg/L/1 g/L, pH 6, -, >60 min 254.16 mg/g -, -, -, 5,303 K,24 h

Basic Green4

◦

217.39 and 285.71 mg/g 303 mg/g (nitrogen atm) 312.5 mg/g (pyrolysis)

270.3 mg/g 178.6 mg/g

45.5 mg/g

200 mg/L, -, 298 and 318K, 4.5 h

—

Neutral Red

-, -, 30◦ C, -,

0.123 mg/g 0.189 mg/g 0.079 mg/g 0.116 mg/g 1.66, 0.560 mol/g 200 mg/g, 204 mg/g, 113 mg/g 40.49 mg/g

25–100 mg/L, -, 298 K, -

Synthetic water based ink solution Dichloromethane

Activated carbon- coconut shells

-, -, (30, 40 ◦ C), -, 120 mg/dm3, -, 30◦ C, 24h

-, -, -, 75-135 min

Rotenone

Indigocarmine Acetaminophen

Deoiled mustard Charcoal Activated carbon -wood

Commercial Coconut GAC NH3 modified activated carbon (NH4 )2 S2 O8 modified activated carbon Impregnated activated carbonsTypha orientalis HCl treated activated carbon from Durian peel under Nitrogen atmosphere vaccum pyrolysis Activated carbon-Hevea brasiliensis ZnCl2 activated coir pith carbon

Bromopropylate (isopropyl 4,40-dibromobenzilate)

Activated carbons- corn cob

[151]

[295]

[290]

[288]

Physisorption/Chemisorption [237] Langmuir, Freundlich/ [218] Pseudo2nd order (Continued on next page)

—

—

Freundlich/Pseudo 2nd order

Langmuir pseudo second order

[285]

Langmuir/Pseudo 2nd order

[181] [135]

[172]

[219] [146]

[142]

Langmuir, Freundlich, Redlich Peterson Freundlich, Langmuir/Pseudo 2nd order Langmuir

Langmuir, Freundlich —



452 Adsorbates

Acid yellow 117

Methyl tertiary butyl ether Pesticide waste Acid blue-25

Pharmaceuticals and an endocrine disrupting Compound Phenolic compounds

Trichloroethylene adsorption

Carbon- from Brazilian-pine fruit Reactive orange 16 shell

334 mg/g

—

—

0.45–0.71, 0.62–0.84, 0.23–0.42, 0.29–0.40, mmol/g 1000 ppm MB, -, -, 72 h 321.75 and 133.33 mg/g MB and Phenol respectively -, pH 2.0–10.0, 298–323 k, 0.25–8 h 34 mgg−1, 456 mgg−1

—

0.01–150 mg/L, -, 22 ± 2 ◦ C, 24h 20 μg/L, -, -, -

-, -, 310K, -

[279]

[238]

Sips/Fractionary-order

[231]

[276] [277]

[61]

[275]

[274]

[273]

[272]

[271]

[269]

[270]

[269]

[268]

[267]

Reference

Langmuir

Redlich-Peterson

Langmuir

Freundlich

Freundlich

1 μg/mg

-, pH (7.5–7.9), 23 ◦ C, -,

Langmuir, Freundlich

—

Langmuir–Freundlich

-, pH 7.0, -, -,

2 gL−1, -, -, -

Organic compounds

—

47.8 mg/g

—

—

Langmuir–Freundlich, Freundlich/Pseudo 2nd order Freundlich

Langmuir

Freundlich

Isotherm/kinetics

491 and 448 mg/g, — phenol and salicylic acid respectively — Dubinin–Astakhov

100 mg L , -, 25 C, -,

◦

Organic water pollutants

−1

-, -, RT, 10 min

—

Aromatic adsorbate

Organic materials

—

—

-, pH 2.2, 25 ◦ C, 24 h

99.4% —

—


100 mg/L, -, 30 C, -,

◦

Initial metal concentration, pH, temperature, equilibrium time

Organic micropollutants

Granular activated carbon (GAC) Methylene blue and from black stone cherries Phenol

Activated carbon-carbonaceous material Granular zeolites Activated carbon Activated carbons from tyre rubber waste

Activated carbon-commercial grade and pre loaded with Laurentian humic acid Activated carbon—coal-based

Activated carbon F400-form Calgon Carbon Corp Activated carbon-commercial Norit ROW0,8 supra Adsorbents made from sewage sludges Adsorbents obtained from sewage sludge Active carbon-commercial activated carbons CVC1123

Activated carbon purchased Organic chlorinated from Norit Italia compound Carbon beads loaded with metal Phenol oxides Mesoporous carbons Organic solutes

Adsorbents

TABLE 2. Low cost adsorbents, adsorbates, their operating conditions with maximum adsorption capacities (Continued)


453

Basic dye

2,4-D

Phenol and 3-nitrophenol

Phenol

Anions, Heavy Metals, Organics and Dyes Phenol Fluroxypyr Catechol Resorcinol 1. phenol, 2. hydroquinone, 3. m-cresol, 4. p-cresol and 5. p-nitrophenol Lindane- insecticide

Activated carbon- date stones

Conventional and novel carbons

Activated coconut shell

Activated Carbon from Jatropha Husk palm seed coat activated carbon Activated carbon ACC Activated carbon cloth (ACC)

Granular activated carbon (GAC)

Activated carbon cloth (ACC)

50–300 mg/L, pH (4–10), 25–50 C, -, 50-400 mg/L, pH 2-11, 30 ◦ C, 9 h 30 min —

p-nitrophenol o

1.84, 1.86, 2.26, 1.92, 2.10 mmol/g for 1 to 5 respectively

1 × 10−4 M, -, 24 h, -,

—

— —

72 mg/g

10–60 mg/L, pH 4–9, -, -, 5 to 90 mg/L, -, 288–308 K, -, 100 mg/L, pH 7.4, 30◦ C, 48 h

10 mg/L, -, 25 ◦ C, -,

—

205.842 mg/g

—

238.10 mg/g

221.23 mg/g at 50 C

8.69 mg/g

10 mgL−1, pH 2–11, 30◦ C, 3h

100–500 mg/L, -, 30◦ C, 48 h

500 mg/g,

-, pH 5.0–6.0, 298 K

Methylene blue

Activated carbon- Euphorbia rigida (H2 SO4 activation) Activated carbon- olive stones-Carbon AC Apricot stone activated carbon ◦

114.45 mg/g

Malachite green

Carbon- Arundo donax root

— 89.5 mg/g

— 25–500 mgL−1, pH6.0, 60 ◦ C), 120 min 10–100 mg/L, pH (3–10), (293, 303, 313 K), 3h, 200 mg/L, pH 3–10, 40 ◦ C, -,

Effect of NaOH activation Phenol

Activated carbon- plum kernels Carbonised beet pulp

99.04% removal At pH = 7.0 and 50 min

Malachite green

Activated carbon- epicarp of Ricinus communis

Initial conc. (25–200 mg L−1, at (27 ± 2◦ C). pH (2–10), 0.2–1.0 g/ 50 ml of adsorbent

[252]

Dubinin–Radushkevich [253] (Continued on next page)

Langmuir, Freundlich, Tempkin/1st order kinetics

[249] [250] [251]

Freundlich/1st order Freundlich Pseudo 2nd order

[247] [248]

[246]

[245]

[244]

Pseudo 1 order, pseudo 2 order, Natarajan and Khalaf, Elovich Langmuir, Freundlich/pseudo 2nd order —

st

Langmuir/pseudo1st order nd

[243]

Langmuir isotherm/pseudo 2nd order — Langmuir, Freundlich

[242]

Langmuir/pseudo 2nd order

[110]

[240] [241]

[239]

Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Tempkin/Pseudo 2nd order Langmuir isotherm Freundlich/pseudo 2nd order


454

Phenol Organic pollutants

Malachite green

Methyl Orange

Lignite Activated coke

Activated carbon- Lignite

Ultrafine Coal Powder

(100, 150 and 200 mg/L), -, (25, 40 and 50◦ C), 40 min 100 mg/L, -, (303, 313, 323 K), 3 h

100 mg/L, -, -, -, -, 40◦ C, 6h

(100, 150, 200 mg/L), -, (25, 40, 50 ◦ C), -, -, 303◦ C, -

Malachite green

Methyl Orange

300–500 mg/L, pH 7.0 and 3.0, 65 ◦ C, -

-, -, 368 K, -

4.5 × 10−5M, -, 25 ◦ C, 48h

—

anionic and cationic dyes

Bentazon and Propanil-pesticides Dibenzofuran

Activated carbon – lignite (T3K618): 1000 m2/g modified ultrafine coal powder

Activated Carbon-Norit RB1, Chemviron, Monolith activated carbon- purchased from Calgon, France

—

-, pH (3, 7, 11), 30 ± 0.1◦ C, 10 h

Benzene and Toluene

Phenolic compounds

—

-, pH (8, 10, 12), -, -

Pentachlorophenol

Granular activated carbon (GAC)- bituminous coal Activated carbon- Commercial coal based Granular activated activated carbon fibers (ACFs)- ACC15 Activated carbon-cloth

nd

Langmuir, Freundlich/Pseudo 1 order, Pseudo 2nd order 100% — 91.6% of COD; 90% of Redlich Peterson color 200 mg/g Langmiur/Pseudo 2nd order kinetics — 1st order and pseudo 2nd order kinetics

[209]

[208]

[210] [211]

[209]

[208]

Langmuir/Pseudo 2nd order st

[266]

[265]

[263]

[262]

[261]

[260]

[258] [259]

[256] [253] [257]

[254] [255]

Reference

order Langmuir/Pseudo 2

—

708.8 and 568.5 mg/g pH 7.0 and 3.0 respectively 149, 200 mg/g

Freundlich

Langmuir

Langmuir, Freundlich/1st order Pseudo 2nd order Langmuir Prausnitz–Radke isotherm

Langmuir

18.52 mg/g

st

Langmuir, Freundlich/1 order

Freundlich

Dubinin–Astakhov Pseudo 1st order, Pseudo 2nd order

Isotherm/kinetics

0.307 g/g

—

—

— —

Aromatic organic acids Avermectins- insecticide

Activated carbon cloth Activated carbon

200, 1000 mgL−1, -, 23 ± 1 ◦ C, -, — 1.96 × 10−4 M, pH 3.7, 25 ◦ C, 125 min. 1.70–1.75 × 10−4 M, -, -, 2, 4, 6 g/l, -, 303.15 K, -

Phenolics Chlorinated organics Benzoic acid

— 1.171, 1.908, 1.886, 2.254, 2.430, 2.257, 2.272 mmol/g — — —


Activated carbon- bituminous Granulr activated carbon (GAC) Activated carbon cloth

— 1.5 mmol/L, -, 25 ◦ C, 24 h


n-butane Phenol, 2-CP, 4- CP, DCP, TCP, 4- NP, DNP

Adsorbates

Pitch based activated carbon Activated carbon fibers

Adsorbents



455

Methylene blue

Direct Rede80 (DRe80) Mordant Bluee9 (MBe9) Dimethomorph Pyrimethanil Isoproturon Naproxen Reactive blue 19 Dispersed blue Acid blue 74 Acid red 27 Reactive black 5 Reactive blue 19 Dispersed blue Acid blue 74 Acid red 27 Reactive black 5 Reactive Red 2

Pyrolysed petrified cement

Phanerochaete chrysosporium

Consortium of white rot fungi

Surfactant modified macro fungus Chlorellavulgaris

Laccase immobilized on porous glass beads

Trametes versicolor Free Laccase

Supranol Red 3BW Lanaset Red 2GA Levafix Navy Blue EBNA Naphthalene, Acenaphthene, Fluorene, Phenanthrene Pyrene

Phenol

Lignite

Scenedesmus obliquus Scenedesmus quadricauda

Organic pollutants

Activated coke

0.2–0.25 mg/L, 0.002–2.5 mg/L 0.008–1.7 mg/L 0.001–1.0 mg/L 0.0002–0.1 mg/L, -, -, -

—

600 μg/L 600 μg/L 10 μg/L respectively, -, -, 4 days 55 μg/L, -, -, 5 h 0.036 mM, -, -, 0.072 mM, -, -, 0.036 mM, -, -, 0.036 mM, -, -, 0.036 mM, -, -, 0.036 mM, -, -, 0.072 mM, -, -, 0.036 mM, -, -, 0.036 mM, -, -, 0.036 mM, -, -, 100 mg/L; -, 25◦ C, -

Microorganisms 10–200 mg/L, -, -, -

—

(100, 1000 mg/L), -, -, -

(3.22 × 103–1.60 × 104 mg/L), -, 40 ◦ C, 6 h

—

256.4 mg/g 345 mg/g 188.7 mg/g —

92% 94% 24% 10% 58% 95% 90% 87% 68% 80% 18% 77% 78% 74% 40% 12% 141.53 mg/g

10 mg/g

90%

Freundlich

[170]

[169]

[168]

(Continued on next page)

—


[167]

[166] [167]

— —

—

[165]

[164]

[125]

[210]

[211]

— —

—

Redlich–Peterson isotherm/pseudo-second order Freundlich/1st order rate equation Langmuir, Freundlich/Pseodu 1st order


456

Direct Blue 199

Adsorbates

Kaolinite-type clay, SDS

Untreated clay Humic acid-immobilized amine modified polyacrylamide/bentonite composite Modified organo bentonites Raw sepiolite Modified sepiolite

o-nitrophenol, m-nitrophenol, p-nitrophenol

-, -, 298K, -

50 to 1000 mg/L, pH 5.5, -, 24 h -, pH 6.5, -, -

Phenol Crystal violet (CV)

Clay -, pH 2.0, -, 15 min -, pH 6.0–8.0, -, 4 h

—

100 mg/L, pH 7.0, 25 ± 2 ◦ C, -

39% 833 mg/g

-, -, -, 5 h 100 mg/L, pH 7.0, 303 K, 750 mg/L, pH 7.5 25 ◦ C, FLU, PHE, FLA, PYR and BAP at 1, 1, 0.25, 0.15 and 0.10 μgL−1 and 0.05, 0.05, 0.5 and 0.05 μgmL−1, of Cd, Cu, Ni & Zn ions respectively, -, 121 ◦ C, -, 50–500 mg/L, -, 35 ◦ C, —

— 77 mg/g 319 mg/g 1.50 mol/g, 8.86 mol/g

1133.1 mg/g 656.5, 648.4, 510.4 μmol/g

113

—

300–600 mg/g.

99%

44%

29.96 mg/g


133 mg/L, -, -, 42 h

400 mg/L, -, 45 C, -

◦


Acid red 88 Malachite Green (MG) Methylene Blue (MB) Crystal Violet (CV)

—

Cationic Pollutants

Citric acid-treated bacterial biosorbents

Activated sludge

Acid dyes

Fungal biomass

Remazol Brilliant Blue R Laccase on mesostructured silica Naphthalene A. filiculoides Basic Orange Staphylococcus epidermidis Triphenylmethane dyes Green alga Selenastrum Effects of metals on capricornutum Biosorption of mixed polycyclic aromatic hydrocarbons

Laccase

Aspergillus Niger

Adsorbents

Langmuir

— Langmuir/Pseudo 2nd order

Langmuir/Pseudo 2nd order Freundlich/Pseudo 1st order

[194]

[192] [193]

[190] [191]

[189]

[302]

[301]

Langmuir, Freundlich/Pseudo 1st order, Pseudo 2nd order Redlich-Peterson/Pseudo 1st order, Pseudo 2nd order, Intra-particle —

— —

[297] [298] [299] [300]

[296]

[171]

Reference

— Langmuir

Langmuir, Freundlich/Pseudo 2nd order —

Isotherm/kinetics



457

Organo-clays Organophillic bentonites Kaolinite, Montmorillonite Natural Rectorite Modified Rectorite Bentonite Sepiolite Raw montmorillonite Treated montmorillonite Acid-activated beidellite Acid-activated bentonite Organo-bentonite from Pacitan

organobentonites (ODTMA-B, HDTMA-B) poly(epicholorohydrin dimethylamine) modified bentonite Bentonites HDTMA Bentonite, BTEA bentonite Organo-modified Basaltic clay, Bentonite HDTMA bentonite Sepiolite

Clinoptilolite

Hematite, Kaolinite, Montmorillonites Regenerated bleaching earth

48.309 mg/g 59.53 mg/g 0.5 mmol/m2 ≤5 ppm 46 mg/g 3.72 mg/g

600 ppm; 48 h; 250C 0.3–1.5 mM, -, 20◦ C, 24 h 0.5–5 ppm, -, 20◦ C, 24 h 10–500 mg/L, pH 7, 333K, 1000 mg/L, -, 50◦ C, 60 min

Benzimidazole

Simazine γ -picoline Acid green 25

100% 300 mg/g 70.42, 22.02, 3.3 mg/g —

Methylene Blue

— —

50 mg/L, -, 25◦ C, 24 h 0.01 M, pH 6.7-7.4, -, 0.003 M, pH 2.74, -, — 60 mg/L, pH (6.0, 2.0, 3.0), 250C, 300 mg/L, pH 3-9, 260C, -

—

— 12500 mg/Kg

43.29, 39.682, 21.55, 18.52 mg/g 148.6, 88.4, 106.7 mg/g

50.76, 69.93, 344.83 mg/g 0.0112, 0.05513 mg/g

200–2000 μg/L, -, -, -

100–1200 μmol/L, -, 25◦ C, 4 h —

-, pH 5, 25 ± 2 ◦ C, -

-, pH 3, 313 K, -

80 mg/L, pH 7.0, 2 C, -

◦

5–20 mg/L, pH 7.0, 25◦ C, 120 min

-, -, 273K, -

Acid Orange 10 Diquat, Paraquat, Methyl green Fluridone Chlorophenol Triclosan Phenol

2,4-D Acetochlor benzene, trichloroethene, 1,2-dichlorobenzene Phenol

1,10-phenanthroline 2,2-bipyridyl Basic violet 4, Basic violet 3, Basic red 9 Amido Black 10B Safranine T benzoic acid hydroquinone Direct Fast Scarlet, Eosin Y, Reactive Violet K-3R

1.11 mol/g, 8.33 mol/g 1.12 mol/g, 6.36 mol/g 6.85 × 10−20 mol/g —

—

—

[147]

[229]

[205] [206] [227] [228]

[203] [204]

[202]

[200] [201]

[199]

[198]

[197]

[196]

[195]

— [148] Langmuir/Pseudo 2nd order [149] nd Langmuir/Pseudo 2 order [150] (Continued on next page)

Langmuir

— Freundlich/Pseudo 1st order

Langmuir

Pseudo 2nd order —

Langmuir

Freundlich Langmuir


Langmuir, (D–R)

Langmuir, Freundlich/Pseudo 2nd order Langmuir


458

2,4-dinitrophenol (DNP)

2-methyl-4,6-dinitrophenol (DNOC) 1,2,4-trichlorobenzene — 1,1,1-trichloroethane 2,4-D -, pH 3, 6, 11 respectively, -, Clopyralid Picloram Siliceous materials Congo red -, -, 298 K, -

MgAl LDH’s

Calcined LDH’s

5–200 mg/dm3 (pyridine); 2 g/dm3 43.9, 51.6, 45.6 mg/g (phenol) -, -, 5.0, 8.4, 7.2 mg/g 5 to 50 mg/L, -, 282 K, 4 h -, pH 5, 300C, -

Pyridine

Phenol

Nitrobenzene Methylene blue

Porous hydroxyapatite Mesoporous apatite of phosphate rock Nanocrystalline hydroxyapatite Abu-Tartour phosphate rock

8.993 mg/g 101.13 mg/g

29.5 mg/g 190–240 mg/g

Phosphate rock

4.8 mg/L, pH 5, 25◦ C, -, -, 35–60◦ C, 4.5 h

Azo dye Methylene blue

74.07, 175.44, 192.31 mg/g

813 μmol/g 2564 μmol/g 1176 μmol/g

—

4.89, 2.54 mmol/g

2.9 mg/g 3.7 mg/g 7.6 mg/g

333.3 mg/g —

536 μmol/g 852 μmol/g


Organovermiculite based adsorbent CEC-50%, 100%, 200%; Algerian dolomite powders Peat

Calcined hydrotalcite

Surfactant modified LDH’s

Everzol Black, Everzol Red, Everzol Yellow

Clinoptilolite

LDH’s 1–40 mM, (pH 5, 7, 11), RT, 0.5–24 h

Methyl orange 4-nitrophenol Basic yellow 28

Calcined Lapindo volcanic mud Iron organo–inorgano pillared montmorillonite clay

50 μmol/L (pentachlorophenol) 285–4000 μmol/L (safranine), -, 22◦ C, 24 h -, -, -, 20 min 80 μmol/L (4-nitrophenol), 34.6 μmol/L (Basic Yellow 28), pH 5, 7, 9, 24 h 25 mg/L, -, -, 2 h


Pentachlorophenol, Safranine

Adsorbates

Alginate encapsulated pillared clays

Adsorbents

Freundlich Langmuir/Elovich

— Langmuir/Endothermic/ Spontaneous Freundlich/Inter particular diffusion, pseudo 1st order

[156] [230]

[155]

[225] [226]

[224]


[223]

[220]

[162]

[153] [154]

[152]

Reference

[163]

nd

Langmuir

—

—

—

Langmuir/Pseudo 2nd order Freundlich/Pseudo 1st order

Freundlich Langmuir/Pseudo 2 order

Isotherm/kinetics



459

Powdered waste sludge

Deoiled mustard Charcoal Modified basic oxygen furnace slag (BOF slag)

Reactive Blue 19 Reactive Black 5 Reactive Red 120 Remazol red RR Chrisofonia direct yellow 12 Sumifix Blue Turquoise blue G

Indigocarmine

n-hexane n-heptane Atrazine

Zeolites

Acid-activated zeolite-rich tuffs

Nitrosamines

Methylene Blue; Bisphenol A Phenol 2-Chloro phenol 4-Chloro phenol 2,4-Dichloro phenol 3, 5-Dichloro phenol

-, -, -, 6h

500 mg/L, pH 2, -, -

Industrial byproducts -, (pH 8.8, 2.3), (30, 40 ◦ C), -

1.25–125 mg/L, -, -, 10 days

—

4.4 × 10−4 M, -, 20◦ C, 1h

-, pH 4.0-10.5, 25◦ C; 22 h

20 mg/dm3, pH7, 25◦ C, -

10 g/L–2500 mg/L, pH 8.5, -, 24 h 150 mg/L, -, -, 24 h

—

87 77 22 78

mg/g mg/g mg/g mg/g

1.66, 0.560 mol/g 60, 76, 55 mg/g

169 mg/g 177 mg/g

— 0.287 mmol/g 102 × 103 mg/kg

0.437 mmol/g 0.323 mmol/g 0.323 mmol/g

144.9 mg/g 5.44 mg/g, 6.81 mg/g, 7.27 mg/g —

10.33 mg/g 2.39 /mg

400 mg/L, pH 6.4, 60◦ C, 2 h -, pH 7, 300C, 8 h

Phenol Methylene Blue

Methylene Blue Congo red

434.78 mg/g

-, pH 4–10; 50◦ C, -

Disperse Blue SBL

Zeolites

Natural zeolite

Magnesium silicate Australian Kaolins: Q38, K15Gr and Ceram Zeolite

Poorly crystalline Hydroxyapatite Hydroxyapatite nanopowders Pyrolyzed petrified sediment

[233]

[232]

[219]

[161]

[160]

[159]

[158]

[196]

[127] [132]

[222] [125]

[221]


Langmuir/Second order

Langmuir, Redlich Peterson/Pseudo 1st order

Langmuir, Freundlich

—

Langmuir, Freundlich/Pseudo 1st order Dual site Langmuir

Langmuir

Freundlich/Pseudo 2nd order Langmuir and Freundlich Pseudo 1st order Langmuir/Pseudo 2nd order Langmuir Pseudo 2ndorder Pseudo 2nd order

Langmuir/Exothermic


460

Dimethyl terephthalate distillation residue

Baggase fly ash

Fly ash

Alkaline treatment of biomass fly ash Fuel oil fly ash

De-Oiled Soya Bottom Ash

Shale oil ash

Adsorbents

Chlorophenol, Chloroaniline Methylene blue Reactive Red 23 Reactive Blue 171 Acid Black 1 Acid Blue193 Color from waste water DDD DDE Lindane Malathion Malachite green Congo red Orange-G Methyl violet Auramine-O Brilliant green Phenol 2-picoline Safranine-T Brilliant Cresyl Blue Nile Blue Brilliant Green

Reactive Black 5

Drim yellow-K4G Drim blue-KBL Drim red K4BL Methyl orange

Adsorbates

500 mg/L, -, -, -

-, - (60, 70 ◦ C), 24h 29.07 mg/g 7.69 mg/g 6.67 mg/g 2.51 mg/g 2.08 mg/g 170.3 mg/g 11.89 mg/g 18.80 mg/g 26.25 mg/g 31.18 mg/g 116.2 mg/g 23.83 mg/g 59.88 mg/g 17 mg/g 13 mg/g 22 mg/g 107 mg/g

2.102, 1.860, 10.331, 10.937 mg/g

70, 36, 47 mg/g

1000 mg/L, -, 25◦ C, 24 h -, pH 5–8.5, 293 K, 60 min

107.53 mg/g

13.46 × 10−4 13.35 × 10−4

—


50–700 mg/L, -, -, -

-, pH 3.0, -, -

50–380 mg/L, -, 25◦ C, -


[145]

[173] [174] [175] [176] [177] [178] [179] [180]

[172]

[217]

[216]

[215]

[235]

[234]

Reference



Langmuir, Freundlich, Redlich Peterson

Langmuir, Freundlich/ Pseudo 2nd order

Langmuir

Langmuir, Freundlich/Intraparticle diffusion Langmuir/ Pseudo 2nd order

Langmuir

Isotherm/kinetics



461

Granular bioplastic- propagules of Phanerochaete chrysosporium Chitosan–zinc oxide nanoparticle N ,O-carboxymethylchitosan/montmorillonite nanocomposite Chitosan-Montmorillonite Chitosan bead loofah composite

Tannic acid Surfactant, organic acids and dyes

Direct Blue 78 (DB78) Acid Black 26 (AB26) Congo red

Oseltamivir (Tamiflu)pharmaceuticals

-, pH 4, -, (LAS, tannic acid, humic acid, RR222, AO51, and MB) 100–1000, 100–1000, 20–200, 150–1500, 60–600, and 50–500 g/L, respectively, -, 30◦ C, -

200 mg/L, pH 7.5, 30 C, 480 min

◦

(25, 50, 75, 100 mg/L), pH2, -, -

Biopolymers —

240 g kg−1 1546 g/kg for Tannic acid

74.24 mg/g

34.48, 52.63 mg/g

—

Freundlich Langmuir, Freudlich/ Pseudo 1st order

Temkin Langmuir/Pseudo 2nd order Langmuir/Pseudo 2nd order


[214] [294]

[213]

[212]

[303]

462

S. Kushwaha et al.

the activation conditions, the process variables, the chemistry of the adsorbate, and the experimental conditions used in batch systems, on adsorption are presented and discussed. In the case of adsorption from solutions the complex nature of carbon surface and the role of surface functional groups in adsorption mechanisms were especially considered.


2. TYPES OF ADSORBENTS Table 2 presents a summary of the adsorbents used along with their adsorption capacities, experimental conditions for various organic pollutants as available in literature.

2.1 Plant Wastes Plant wastes have been exhaustively used as adsorbents as they are inexpensive and very less or no economic value. Adsorbents from seed, seed coat, stem, and stalk of different agricultural products; peels of different fruits such as orange,[593] grape fruit,[123] tea,[122] and coffee waste[341] shells and/or stones of fruits such as nuts,[64,136,139,187,247,283,410] peanuts,[287,489,491] olive wastes,[335,345,412,477] almonds,[186] apricot stones and date stones,[245,336,476] and cherries;[344] and wastes resulting from the production of cereals such as rice,[346,347,349,350,351,352,433] maize and corn,[342,343] and sugar cane bagasse,[172–181,280,353,354,449] coir pith,[295,478,491] and wheat waste[36,292] have been used. Among lignocellulosic materials, saw dust is produced annually in large scale by timber industries that is either incinerated or dumped in the environment. However, studies dealing with the removal of dyes[130,172] and other organic pollutants[35,128,365] from water by saw dust from show that this material is a very promising adsorbent. However, untreated plant wastes as adsorbents have several limitations such as adsorption capacity, high chemical oxygen demand (COD) and biological chemical demand (BOD) as well as total organic carbon (TOC) due to release of soluble organic compounds contained in the plant materials.

2.2 Activated Carbons Activated carbon has been widely used for removal of organics from aqueous solutions. Wood,[361–365,376,429,492] nutshells,[365] and fruit stones,[410,476,477] peat,[92,226,574] charcoal,[219,528] lignite,[210] bituminous coal,[487] and petroleum coke[151] are materials with high carbon content and low inorganic components, and, consequently, they have been used for the production of activated carbon.


463

2.3 Layered Double Hydroxides


Anionic clays, also called LDH or hydrotalcites (HT), are natural or synthetic materials containing magnesium(II) hydroxide layers, with some isomorphic substitution of Mg(II) by Al(III). Anionic clay structure results in their ability to be used as suitable materials for adsorption of anionic contaminants from water. The sorption behaviors of LDHs with various oxyanions have been explored.[8,462]

2.4 Bio Polymers Recently, natural polymers and their composites have been studied for the development of cheaper and more effective adsorbents. Among these, polysaccharides such as chitin and starch,[66,513] and their derivatives such as chitosan,[43,52,55,65,66,212] deserve particular attention. These biopolymers are considered as attractive alternatives as adsorbents because of their structure, physicochemical characteristics, chemical stability, high reactivity, and excellent selectivity toward aromatic compounds and metals, which are attributed to the presence of chemical reactive groups (hydroxyl, acetamido, or amino functions) in polymer chains.[294]

2.5 Clays Among natural materials clays occupy a prominent position being low cost, available in abundance, and having good sorption properties. There are various types of clays such as ball clay, bentonite, common clay, sepiolite, fire clay, Fuller’s earth (attapulgite and montmorillonite varieties), and kaolin.[132,195,227,331,501,516]

2.6 Siliceous Materials Apart from clay, the siliceous materials such as silica,[297,321,431,446] glass beads,[167] and zeolites[159,160,276,443,501,503,565,566,587] have also been proposed for removal of various organic pollutants. The use of natural siliceous adsorbents for waste water is increasing because of their high abundance, easy availability, and low cost. Many researchers have suggested that hydroxyapatites[155,455] show a good affinity for organic pollutants. Their availability, structure, ionic exchange property, adsorption affinity, and characteristic to establish weak bonds with organic molecules of different sizes have led to the wide use of this material as adsorbent.

464

S. Kushwaha et al.


2.7 Industrial Wastes Widespread industrial activities generate huge amount of solid waste materials as by-products. Some of this material is being put to use while others find no proper utilization and are dumped as waste. The industrial waste material is available almost free of cost and causes major disposal problem and hence would be advantageous to use as adsorbent. Thus, a number of industrial wastes have been investigated with or without treatment as adsorbents for the removal of pollutants from wastewaters. Fly ash (the major solid waste by-product of thermal power plants based on coal burning),[172–180,215–217,369] red mud (a solid waste product of aluminum industry produced during bauxite processing),[79] fertilizer industry waste,[9] paper industry waste,[51,80,94] and waste sludges[54,272,374,377–379,424–427] have been used. The steel industry also produces a number of wastes in large quantities such as blast furnace slag,[232] dust,[365,492] and sludge,[189,233,271] which have been investigated as adsorbents.

2.8 Micro-organisms In recent years, a number of studies have focused on some microorganisms, which are able to biodegrade and biosorb dyes in waste waters. A wide variety of microorganisms capable of decolorizing a wide range of dyes and organics include some bacteria[77,302] and fungi.[85,170] It is evident from Table 2 that it is very difficult to compare adsorption performance of various adsorbents as the adsorption capacities were evaluated at different experimental conditions (different pH’s temperatures, adsorbate concentration ranges, and adsorbents). Some adsorption capacities were reported in batch experiments and others in continuous column modes. These cannot be readily compared with each other. In batch experiments, the adsorption capacities were computed by the Langmuir or Freundlich isotherm. This makes comparison all the more difficult.

3. METHODS OF PRETREATMENT AND PREPARATION Various methods are used to manufacture activated carbons and modify adsorbents such as fly ash, saw dust, and industrial waste. Almost any carbonaceous material may be used as a precursor for the preparation of activated carbon materials. However, in practice, wood, nutshells and fruit stones, peat, charcoal, soft coal, lignite, bituminous coal, and petroleum coke are materials with a high carbon content and low inorganic components (mineral matter), and, consequently, they are adequate for the production of activated carbon. The selection of the raw material is based mainly on the following criteria[312]:


465


• Low inorganic matter content, • Ease of activation (e.g., calcined coke is a difficult material while wood char is readily activated), • Availability and low cost, and • Low degradation upon storing. The porous structure of activated carbon or any adsorbent is a function of the precursor used in the preparation, the activation method followed and the extent of activation.[312] The porous structure in the case of activated carbon is not fully understood. It consists generally of small graphite crystallites with highly disordered, irregular, rough, and heterogeneous surfaces. In general, activated carbon is sometimes described as having a crumpled layered surface, in which flat sheets are broken and curved back upon themselves. The activated carbon porous structures may comprise carbon nanotubes.[304] Four main zones in the heterogeneous internal surface of the activated carbons can be distinguished: the carbon basal planes, edges and crystal defects the chemical groups, and the inorganic ash content. The heterogeneity of adsorbent surfaces arises from two sources known as geometrical and chemical ones. Geometrical heterogeneity is the result of differences in size and shape of pores, and cracks, pits, and steps. Chemical heterogeneity is associated with different functional groups,[102] mainly oxygen groups that are located most frequently at the edges of the turbostratic crystallites, as well as with various surface impurities. Both chemical and geometrical heterogeneities contribute to the unique sorption properties of activated carbons.[305] The chemical groups are mainly placed on the edges of the graphitic basal planes. Surface functional groups can be classified as acidic (carboxyl, carbonyl, phenolic, hydroxyl, lactone, anhydride) and basic (chromene- and pyrone-like structures).[306,307] The acidic or basic complexes formed on adsorbent surface determine the charge, the hydrophobicity, and the electron density of the graphene layers causing π -π dispersive interactions, hydrogen bonding, water adsorption, and donor–acceptor interactions between the adsorbate and adsorbent.[56,101,110,308–311] There are two principal methods of modifying and activating adsorbents namely physical and chemical activation (Figure 2). Physical activation comprises carbonization of the raw material in an inert atmosphere followed by partial gasification of the resulting char with steam, CO2 , or their mixture.[313,431,480,482–485] Evolution of microporosity is relatively similar for those two activating agents, with a maximum at about 40–50% mass burnoff. Further gasification results in a decrease in micropore volume for the case of CO2 . Steam produces a continuous increase with burnoff, which indicates enlargement of micropores and their size shift toward mesopores.

466

S. Kushwaha et al. Biomass Feedstock (Carbon Source)

Pretreatment Methods (Pyrolysis, Carbonisation)


Mechanical Pretreatment Shearing

Thermal Pretreatment 1. Temperature variations 2. Steam explosions 3. Liquid hot water

Acid Pretreatment Strong acids HCL, HNO3, H2SO4, Tartaric, Citric and Thioglycollic acid

Organic Alkaline Pretreatment Compunds Strong Alkali NaOH, Ca(OH)2, EDA, HCHO, CH3OH, Na2CO3 Epichlorhydrin

Oxidative Pretreatment Oxidizing agent 1. Hydrogen peroxide 2. Peracetic acid

Activation

Chemical 1. Acid 2. Alkali 3. Alkali metal salts

Thermal High temperature

Physical 1. Steam 2. Carbon di oxide 3. Nitrogen

FIGURE 2. Methods of activation.

3.1 Pyrolysis Biochar, a carbon-enriched and porous material is produced from a variety of biomass. The production of biochar by pyrolysis is a carbonization process in which the content of carbon increases with temperature accompanied by a simultaneous decrease in oxygen and hydrogen contents. The properties of biochar are different from activated carbon, though both of them are carbon-rich material. Generally, biochar is a not fully carbonized product because its production by pyrolysis is often operated under low temperatures (< 500◦ C).[377,378] The properties of biochar are different from activated carbon, though both of them are carbon-rich material. The production of activated carbon from organic materials includes a two-step process: carbonization and activation. After physical and chemical activation, the surface areas and internal pore structures of activated carbon are reported to be greatly enhanced and improved compared to biochar that only experiences the carbonization treatment.[272] Thus, biochar may act as a precursor to manufacture activated carbon. Biochar can not only strongly sorb many cationic chemicals such as ammonium ions and a variety of metal ions,[271,378] but also efficiently remove anionic nutrients such as phosphate from aqueous solutions.[377]

3.2 Chemical and Physical Activation In chemical activation, the raw material is impregnated by a compound such as H3 PO4 or ZnCl2 , and the impregnated product is pyrolyzed and



467

then washed to remove the activating agent.[428–430,486] Chemical activation is usually carried out if the raw material is wood or peat.[487,489–490] There is dehydration of the cellulosic material during pyrolysis, which results into charring and aromatization of the carbon skeleton, and the creation of the porous structure. All chemicals are dehydrating agents and inhibit tar formation. The yield of the process is relatively large (30 mass%) as compared to that of physical activation. Other possible advantages of chemical activation are (a) simplicity, no need of the previous carbonization of raw material, (b) lower temperatures of activation, and (c) good development of the porous structure.[314,337] Depending on the nature of the parent material and by adjustment of the reaction conditions and temperature, different pore sizes that cover the micro-, meso-, and macropore ranges can be obtained. Table 3 shows some raw materials, the type of AC usually prepared from them, and the changes produced on the adsorbent surface by different methods of pretreatment. Chemical activation of sewage sludge is reported to produce materials with good adsorbent properties. Comparing the adsorptive capacities of the only pyrolyzed sewage sludge and those that had been previously impregnated with sulfuric acid, it has been claimed that with activation a large number of pores has been created and also enlargement resulting in a remarkable increase in surface area. Research has been focused on making use of sludges to prepare the carbon-bearing adsorbents by different activating ways due to their high carbon content,[214,424–427] and application of the adsorbents to the removal of organics in the final stages of water cleaning.[428–430]

3.3 Thermal Treatment Thermal treatment of montmorillonites can generate new adsorption sites from the movement of octahedral cations produced from dehydration and dehydroxylation processes during calcination of clay minerals.[320–323] For instance, enhanced adsorption of hydrophobic herbicide metolachlor was linked to the increase of the montmorillonite’s specific mesopore surface area produced by calcination, which was promoted by the exposure of the aluminum ions at the surface edges.[324] Different milling processes such as processing in an oscillating mill were also used to obtain clay structure transformations.[325,439] High-energy ball milling has been reported to produce submicrometer rounded particles, as well as nanometric particles of montmorillonite.[326] Ionized argon interaction generates mechanical deformations with modification of surface characteristics and particle size distribution, but without particle agglomeration or compaction.[326] High-energy ultrasound generates exfoliation in the c-axis with some sheet rupture in other directions and minimal crystallinity transformation,[327] which has been demonstrated to be important in order to obtain a controlled release of herbicides.[328] Liu

468

calcination

High energy ball mill

bentonites and montmorillonites

Montmorrilonite Clay

Kaolinite

Phosphoric acid activation

Sorghum grain

Thermal treatment: 500 and 750◦ C

Ionized argon interaction

Phosphoric acid activation

Physical

Chemical

Activation method

Pecan shell Lignin

Biomass derived precursors

Precursor

TABLE 3. Precursors and the method of activation Carbons with network of narrow micropores. Enlarge the dimensions of the micropores.

[331]

[328]

[326]

[319]

[317]

[316]

For carbon precursors with close reactivities (biomass-derived precursors), the porosity development is more strongly related to the activation procedure than to the chemistry of the precursor which relates to the chemical composition of the resulting activated material.

Acidic surface groups form due to (1) hydrolysis of starting materials at acidic condition resulting in formation of carboxylic groups of different strengths, and some of these groups begin to decompose at low temperatures (170 ◦ C) (2) the reaction between activation agent and starting material or the hydrolysis products of starting material resulting in the formation of phosphorus-containing groups, which are stable at high temperatures, and (3) the reaction between starting material and air resulting in weak and strong acidic groups. i) Simultaneous carbonization and activation after impregnation resulting in carbons with moderate surface area and high mesoporosity. ii) Carbonization of the precursor followed by activation of the char with phosphoric acid resulting in carbons with large surface area and high microporosity. New adsorption sites originate from the movement of octahedral cations produced from dehydration and dehydroxylation processes during calcination of clay minerals, increase of the specific mesopore surface area with the calcination temperature, exposure of the aluminum ions at the surface edges occurs which increases the adsorption. Sub-micrometer rounded particles, as well as agglomeration and compaction of larger ones, a progressive reduction of the d (001) lattice spacing of the montmorillonite from 1.50 to 1.30 nm as a function of time, induces a structural destabilization in Ca-montmorillonite both in the interlayer and in the intralayer with progressive losses of interlayer water. Interlayer destabilization with water loss Mechanical deformations with modification of surface characteristics and particle size distribution without particle agglomeration or compaction. Structural breakdown produced by dehydroxylation of kaolinite and the subsequent formation of metakaolinite which induces the change of a substantial fraction of the surface Al from octahedral to tetrahedral coordination. Further heating at 900 and 980◦ C brings about the formation of a phase (g-Al2 O3 ) in which Al occupies octahedral positions.

Reference

Inference


469

Thermal treatment at 750 and 950◦ C

Diatomite

Waste Tyre

VS were impregnated for 2 h with 60 wt.% H3 PO4 solution at room temperature, 50 and 85◦ C. The three impregnated products were carbonised at 400◦ C. The product impregnated at 50 ◦ C was heated either first at 150–250 ◦ C and then at 400 ◦ C or simply at 350–550 ◦ C in N2 atmosphere pre-treated by H2 SO4 prior to CO2 activation at 1223 K

Thermal treatment Thermal treatment at 550◦ C

Diatomite Diatomite

Activated carbon by vine shoots (VS)

Thermal treatment

Montmorrilonite

Mechanical treatment

[333]

[375]

[332]

[231] Have higher yield, higher surface area (1118 m2/g) and micropore volume (0.490 cc/g) compared with those without treatment. The removal of catalytic metallic species such as Ca and K after acid treatment reduced the reactivity of the chars, thus allowing thorough diffusion of CO2 into the inner carbon matrix and leading to higher porosity.the adsorbent surface can be positively or negatively-charged. δ+ + δ+ Positive surface C = S OH + H → C = S + H2 O Negative surface C = Oδ− H+ → C = Oδ− + H+ The adsorbent surface can be positively or negatively-charged (Continued on next page)

induces the appearance of a significant amount of surface Al in tetrahedral coordination, as well as a 20% decrease of the Si/Al atomic ratio in the kaolinite surface. Al(VI) turned into Al(IV) in octahedral sheets with the change in isoelectric point. Changes its specific area and pore volume distribution. Specifically increased the specific and external surface area of the diatomite of about 12.76% and 20.47%, respectively. This is probably attributed to the removal of admixtures or adsorbed volatile compounds, through volatilization due to the high temperature of 550◦ C. The active groups on the diatomite surface (–OH groups) are removed and the surface acquires stronger hydrophobic properties The modification processes of the diatomite changed the surface area, porosity, diffusion properties and accessibility to internal sites. High temperatures cause destruction of vicinal micropores, which leads to a decrease in micropore and a corresponding increase in mesopore content. The total pore volume of diatomite decreases at temperatures as high as 950 ◦ C gives a more “basic character” to its aqueous solutions, probably due to the fact that acidic sites in diatomite are weak and decrease.Theaverage pore radius increases, specific and external areas decrease and micropore volume tends to zero. Better developments of surface area and microporosity are obtained when the impregnation of VS with the H3PO4 solution is effected at 50 ◦ C and for the products heated isothermally at 200 and 450 ◦ C. The mesopore volume is also usually higher for the products impregnated and heated at intermediate temperatures while changes produced in the surface functional groups and structures by effect of the activation with H3 PO4 are stronger at the highest carbonisation temperature.


470

ZnCl2 and H2 SO4

Alkali

KOH

KOH

Steam

H3 PO4 and ZnCl2

Fly ash

Cassava Peel

Olive pits (OP)

Olive waste Cake

Date Pit

Activation method

Sewage sludge

Precursor

TABLE 3. Precursors and the method of activation (Continued) Accelerates the activation process of dewatering, condensation, wetting and expansion, and making the carbon-bearing chemical compound within the sludges condense to non-volatile carbon. It’s activation effect is better than the single activator, and the transition pore structure of the activated products are more flourishing, correspondingly the abilities of the carbon-bearing adsorbents increase greatly. Dissolves SiO2 from fly ash and leave increased amount of unburned carbon as a residue. excessively high temperature and treatment time appeared to alter the fly ash structure and characteristics and may have been the reasons for a resultant decrease in the dye adsorption capacity (optimum 100◦ C and 12 h). Activation time showed no significant effect on the pore structure. Potassium acts as the catalyst for oxidation reaction; therefore more carbon atom in surface is oxidized leading to widening of the pore structures. Maximum surface area and pore volume were obtained using: impregnation ratio of 5:2 and carbonization temperature of 750 ◦ C. PAC adsorption capacity was the highest at 75% (w/w) KOH. The characteristics of the starting materials, the activating agent concentration, and the carbon particle size influenced adsorption capacity of the prepared carbons.Adsorptive properties were highest in the OP powdered carbon obtained at 75% KOH concentration. The best AC was obtained activating for 68 min at 1095 K. Doehlert’s matrix was used as a tool to optimize the activation conditions Iodine number increased with increasing activation temperature and higher with phosphoric acid. Justification was given from literature that. it appears to function both as an acid catalyst to promote bond cleavage reactions and formation of cross-links via processes such as cyclization and condensation and to combine with organic species to form phosphate and polyphosphate bridges that connect and crosslink biopolymer fragments, various surface acidic functional groups (oxygen- and/or phosphorus-containing groups) are developed through the surface oxidation as well as attachment of different oxygen/phosphorous groups to the surface, while developing required porosity. Dehydration of cellulose by phosphoric acid occurs at higher temperatures the phosphorous oxides act as Lewis acids and can form C–O–P bonds. At temperatures higher than 900 ◦ C phosphorus-bearing species leave the carboneous surface.

Inference


[336]

[335]

[45]

[334]

[215]

[271]

Reference

471

Macadamia nutshell

Walnut shell

KOH and ZnCl2

KOH

KOH

H3 PO4

[338] Porosity and cellular structure of raw material influenced acid concentrations and/or temperatures. Best AC was obtained using 50 and 60% (w/w) H3 PO4 . Excess acid reduced raw material activation. Chemical modifications occur by penetration, particle swelling, partial dissolution of the biomass, bond cleavage and reformation of new polymeric structures resistant to thermal decomposition. In addition, raw date pits are proposed to be composed of a low-porosity and compact cellular structure that needs higher acid concentrations and/or temperatures to attain the optimum effect normally reached at lower temperature in case of other feedstocks of botanical origin. Macrocroporous texture with more homogeneous pore size distribution [45] than the ones prepared from olive pits. 2 −1 −1 [339] High-surface area (2259.4 m g ) and pore volume (1.10 cm g ) where obtained using the following activation conditions: 0.5% (w/w) KOH, 3 h at 800 ◦ C. Removal of cross-linking and stabilizing of carbon atoms in the crystallites. Potassium metal liberated at the reaction temperatures may intercalate and force apart the separate lamellae of the crystallite. Removal of these potassium salts (by washing) and carbon atoms (by activation reaction) from the internal volume of the carbon creates the micropores in the structure high temperature would cause several atomic layers of carbon being widened and hence, forming large pores. Some possible reactions were proposed: 2KOH → K2 O + H2 O Dehydration C + H2 O → H2 + CO Water − gas reaction CO + H2 O → H2 + CO2 (Water − gas shift reaction) K2 O + CO2 → K2 CO3 (Carbonate Formation) When the activation temperature exceeds 700◦ C, a considerable amount of metallic potassium is formed due to the following possible reactions: K2 O + H2 → 2K + H2 O (Reduction by hydrogen) K2 O + C → 2K + CO (Reduction by Carbon) [340] Chemical activation of MNS (with both chemicals) the impregnation method with a shorter carbonization time produces activated carbons with a well developed pore structure but there is a loss in bulk density. (Continued on next page)


472

Steam, CO2 high activation temperature 1073 and 1173 K

Corn cob

Steam, CO2

KOH

H3 PO4 , NaOH

Cherry stone

Olive Seed waste residue

Rice Husk

ZnCl2

H3 PO4

Activation method

Coffee bean husk

Precursor

TABLE 3. Precursors and the method of activation (Continued) Characteristics could be easily controlled by varying H3 PO4 impregnation ratio. High impregnation ratios yielded essentially mesoporous carbons with high surface areas and pore volumes.Low impregnation ratios lead to microporous ACs with almost no mesopores. At intermediate impregnation ratios, ACs with wide PSDs (from micropores to mesopores) are obtained. Microporosity was obtained using both agents. Surface areas of 1315m2g−1 could be reached. BET surface area, pore volume, and average pore diameter of the resulting activated carbon generally increase with the extent of burn-off. The higher activation temperature can overcome the drawbacks of a longer period of activation required to attain larger surface area High-surface area (about 1400 m2g−1) and microporosity was obtained using the following conditions: 175% (w/w) ZnCl2 , 773K for pyrolysis, 0.5–1.0 h for soaking time. Steam activation more effective than CO2 activation. Surface area: 1200 m2g−1. Activation in carbon dioxide requires higher temperatures (9OOOC) and gives a carbon of slightly lower activity. Carbon from the hull, or hard outer portion of the fruit stone, provides essentially all of the adsorption capacity; the inner kernel does not form a microporous material. The hull structure is dominated by 0.4-micron pores which facilitate access to internal microporosity. This structure requires that the carbon be ground to less than 75 micron particles to achieve reasonable adsorption rates. The activated carbons from olive-seed waste residue showed considerably higher surface areas and could be characterized as “super-activated carbons”. The pores are composed of micropores at the early stages of activation and of both micropores and mesopores at the late stages. Surface area and the pore volume were found to increase with the degree of burn-off, i.e. the activation time and temperature. The reaction of phosphoric acid and organic constituents such as lignin was exothermic. Physical changes on rice husks were only observed when the husks were treated with 20% H3PO4. The husks remained intact when treated with 10% NaOH even at 100◦ C. However, the NaOH was able to remove most of the silica, leaving behind a porous structure.

Inference


[346, 347, 348]

[345]

[344]

[343]

[342]

[341]

Reference

473

i) steam, carbon dioxide or phosphoric acid ii) phosphoric acid

CO2 (H2 SO4 Pre-Treatment)

Cedar Wood

Steam

Tartaric acid H2 SO4

i) Pecan Shell ii) almond shell

Sugar cane Baggasse

NaOH, KOH

NaOH, NaHCO3 , Epichlorhydrin

[349] Recovery of rice husk after base washing was 84.7%, which was quite high as compared with other agricultural by-products. The purpose of washing was to remove water-soluble surface matter or in the case of base washing, base soluble materials on the rice husk surface that might have interfered with it’s adsorptive properties. The base wash, appeared at least partially successful in exposing the groups, which can enhance the uptake of metal. The cost of sodium carbonate is about three times less as compared to sodium hydroxide. Outer activation is stronger, for the boiling point of sodium is higher [350] than potassium. So the potassium can enter into the interiorof carbon structure and make the activation freely. Porous carbons prepared by KOH have a well developed microporosity and lower pore size, and larger pore were obtained by NaOH-activation.Surface chemistry of activated carbons were further changed by HNO3 and H2 O2 treatments. Esterification of carboxyl groups was caused. [351, 352] [353] Sulphuric acid undergoes intercalation reactions that results in physical and chemical changes modifying the thermal degradation process of the major components of bagasse (lignin, cellulose, and hemicellulose). presented high surface areas (614–1433 m2g−1) and well developed microporous texture. Low temperature chemical carbonization and gasification was effective. The steam activated carbons have high adsorptive capacity, low ash [354] content and surface areas greater than 400 m2/g that could be expected to be as effective as a commercial activated carbon. [355] Acid activation appeared to result in carbons with high surface area and under the same activation conditions. Almond shells produced a higher surface area than pecan shells. Carbon dioxide is more expensive than steam in addition to yielding a lower surface area under the same activation conditions of time and temperaturealmond shell carbons were softer with higher attrition values. [356] Activated Carbon resulted with improved the porous texture and the adsorption capacity due to dehydration of the raw material with H2 SO4 . The dehydration of the samples increases the yield of the carbonization process. The yield of the activation process is higher for samples prepared at 700 and 800◦ C, probably due to the presence of sulfur surface complexes. Such complexes are removed at 900◦ C, which results in a lower yield. The previous dehydration of the raw material also gives rise to an improvement in the porous texture and adsorptioncapacity of the activated carbon. (Continued on next page)


474

KOH

Fir wood and pistachio shells

Oak, corn hulls and corn stover

Vaccum pyrolysis followed by steam activation, atmospheric pyrolysis

Teak Saw-dust

CO2

Steam

CO2 (H2 O2 Pre-Treatment)

Activation method

Cedar wood and its shavings

Precursor

TABLE 3. Precursors and the method of activation (Continued) Development of porous structures probably due to the elimination of surface complexes produced during the activation step.The activation treatment favors the development of medium and narrow-size micropores whereas the carbonization process leads to the development of wide micropores of size close to that corresponding to mesopores. Surface area of the char obtained from vacuum pyrolysis is lower than those from the atmospheric one. Micropore surface area and micropore volume of vacuum pyrolysis char are slightly higher than atmospheric char. At vacuum pyrolysis, the decomposition of sawdust is not complete since the temperature of 600◦ C is not high enough to achieve complete decomposition. The atmospheric pyrolysis char has higher surface area due to the contribution of mesopores structures. In atmospheric pyrolysis, the residence time of organic vapor formed during pyrolyis process in the reactor is considerably longer than vacuum pyrolysis. At this condition, the secondary reactions of these organic vapor and thermal degradation of the pore wall occurs causing the widening of micropores and collapsing the structure of carbon. Activation with steam as gasifying agent involves the C–H2O reaction, resulting in the removal of carbon atoms and causing the main weight loss of resulting char. Also, at high temperature, destruction of high porosity by external ablation of carbon particles is more pronounced than development and widening of micropore. AC with a surface area of 1150 m2g−1 and pore volume of 0.43 cm3g−1 was obtained. Activated carbons with high yield, high porosity having surface areas up to 1096 m2g−1 were obtained. Produces mainly micropores. Chemical activation process is that it normally proceeds at lower temperature and takes shorter time than those required in physical activation. The higher yield from chemical activation is because the chemical agents used are substances with dehydrogenation properties that inhibit formation of tar and reduce the production of other volatile products. Rate of external surface adsorption was higher using steam, but the rate of intraparticle diffusion was much lower.s. Steam-activated carbons have two types of pores, micropores (0–2 nm) and mesopores (3.5–4.5 nm). Both the surface area and the nature of porosity are significantly affected by the conditions of activation, the extent of which depends on the nature of the precursors. The higher the activation temperature, the greater are the surface areas and micropore volumes of the resultant activated carbons. For oak, the longer the activation duration, the greater the adsorption capacity of the resultant activated carbons, and vice versa for corn hulls and corn stover.

Inference


[360]

[359]

[358]

[357]

Reference

475

H3 PO4

CO2

Compression of wood followed by Steam treatment

H3 PO4 , ZnCl2 , CO2

CO2 (metal oxide impregnation)

Heat treatment with sulphuric acid (chemical activation) followed by steam activation KOH

Heat teated in two stages were 250–270 ◦ C for 2–5 h and 360–430 ◦ C for 2 h, respectively followed by carbonization and activation

Chestnut, Cedar, Walnut (shavings)

Quercus agrifolia wood waste

Cedar wood

Tropical tree wood

Pinewood sawdust

PET

Poly Vinyl Chloride

[367]

[107]

[366]

[365]

[364]

[363]

[362]

[361]


The best textural properties were obtained by chemical activation using 36% (w/w) H3 PO4 . Effect of chemical activation with H3 PO4 is better when lower acid concentration is used. Fractal dimension is quite similar in all the ACs. Porosity development seemed to be strongly influenced by the kinetic reaction stage and the reactant gas concentration.Activated carbons with well-developed heterogeneous location of a reacting carbon atom (more weakly bonded porous system and high surface areas were obtained. The developed micropores were predominantly supermicropores. Transition from diffusion to a chemical-controlled process seems to be the ruling mechanism that affects porosity development. Variation in the reactant gas concentration gradient, established between external and internal parts of carbon particles appears to be the ultimate factor that determines both microporosity and mesoporosity evolution. The formation of micropores by steam reaction was not affected by macropore volume. AC of double density could be produced by compression treatment. Homogeneous porosity and high surface area was obtained by choosing appropriate wood.physically activated cubes were more fragile and had significant deformations (convex planes) not noticed in chemically activated cubes.the surface area obtained by physical activation was less influenced by the type of wood. surface areas obtained were significantly smaller than that obtained by chemical activation (2000–3000 m3/g). Suitable support for metal oxide catalyst. Before CO2 activation metal oxide impregnation leads to adequate porous texture. Chemical activation was more effective than physical activation. Micro- and mesoporous activated carbons with surface areas up to 1030 m2g−1 were obtained concluding that combined treatment might replace physical activation. Nitrogen functionalized (basic) adsorbents could be obtained by cocarbonization of PET waste and N-compounds. Activated Carbon Fibres obtained were mainly composed of micropores with surface areas between 1000 and 2000 m2g−1.


476

Activation method

ZnCl2 activation followed by pyrolysis

ZnCl2

Digested sewage sludge with additive coconut husk

Stabilization at 25◦ C followed by stepwise sequential heat treatment followed by activation with Steam Steam, CO2

Steam (addition of ferrocene)

Pyrolysed at 900◦ C followed by activation with CO2 CO2

K2 CO3

Sewage Sludge

Tire

Acrylic Fabric waste

Heavy oil Fly Ash waste Mixtures of oil refining pitch and carbon-fired fly ash Pitch waste

Waste polyurethane

Precursor

TABLE 3. Precursors and the method of activation (Continued)

Pitch-based spherical AC with high mesopore texture was obtained. Catalytic activation reaction took place at the vicinity of iron particlesand meso and macropores were produced. Excess content of iron was a disadvantage Activation at 900 ◦ C for 5 min significantly increased specific surface area (to 2400 m2g−1) and total pore volume (1.15 cm3g−1). ACF performance was superior to those of conventional carbons. Surface areas were comparable with those of commercial carbons. Tire char air reactivity is dependent on pyrolysis temperature. Tire chars present higher reactivities with steam than with CO2 . Active carbons, produced from tire chars, possess surface areas comparable with those of commercially available active carbons (areas around 1100 m2g−1). AC presented remarkable micropore and mesopore surface area and distinct physical and chemical properties from commercial carbons. The activated carbon was acidic in nature with the quantities of oxygen-containing functional groups higher than the commercial activated carbons. The anaerobically undigested sludge had higher carbon content and lower ash content than the digested one, therefore yielding a better AC. low concentration of activating agent ZnCl2 tended to increase the microporosity. Heating temperature had impact on surface area and PSD

High specific surface areas (2800 m2g−1) where obtained mainly due to the activating agent action. Optimal conditions were: carbonization temperature of 1073 K, impregnation ratio of 1.0.K2 CO3 promoted charring during carbonization, and the carbonization behavior changed below 700 K and above 1000 K, the carbon in polyurethane char was consumed by K2 CO3 reduction, and the specific surface area increased.This activated carbon had a very sharp micropore size distribution, compared with commercial activated carbon. Low surface area carbons with mesoporous texture suggested the possible use in adsorption of large molecules. AC with high surface area, high content of oxygen, and surface sites with prevalent basic characteristics were obtained.

Inference


[374]

[377]

[373]

[372]

[371]

[370]

[369]

[368]

Reference



477

and Zhang reviewed the adsorption properties of the raw clays, activated clays by acid-treatment or calcinations, organic-modified clays with small molecules or polymers for the adsorption and removal of organic dyes from aqueous solutions.[330] Thermal and mechanical treatments on raw kaolinite and montmorillonite are claimed to modify the aluminum coordination that changes from octahedral to tetrahedral,[329,331] accounting for the isoelectric point (IEP) increase from pH around 3–7, which should alter the electrostatic attraction of organic molecules and consequently the adsorption capacity of the clay. Thermal treatment of diatomite to a temperature of 550◦ C is also reported to cause changes in its specific area, external surface area, and its pore volume distribution, which is attributed to the removal of admixtures or adsorbed volatile compounds. On the other hand thermal treatment at 750 and 950◦ C is reported to cause destruction of vicinal micropores walls, which leads to a decrease in micropore and a corresponding increase in mesopore content.[332] Thermal treatment of diatomite is also claimed to give a more basic character to its aqueous solutions, probably due to the fact that acidic sites in diatomite are weak and decrease when thermal treatment at temperatures between 650 and 1000◦ C is applied.[416]

3.4 Acid Treatment Acid treatment of clay is reported to leach out part of the octahedral Al3+ ions. Also a part of nonsilicon cations, such as Na, Al, Fe, and Mg, were leached out, which resulted in the increase in basal spacing, specific surface area, and pore volume.[414,415,417] A scheme to explain the acid treatment process and adsorption of simazine in acid activated clay minerals is presented in Figure 3.[148] If the natural inorganic interlayer cations are replaced by certain organic cations, the resulting organoclay minerals are claimed have the capability to adsorb nonionic organic compounds as well as anions. Commonly, monovalent organic cations are used for this purpose.[418]

FIGURE 3. Schematic acid activation of beidillite and simazine adsorption.


478

S. Kushwaha et al.

The surface chemistry of HNO3 and thermal treated F-400 samples obtained from Boehm titration, along with the pHPZC values of the carbon surface were studied by Wibowo and coworkers.[261] They found that the number of oxygenated acidic surface groups increases upon oxidation or treatment with nitric acid, and decrease upon thermal treatment. Also HNO3 being a strong oxidizing acid it would oxidize carbon atoms and cause the carbon surfaces to lose its electrons and become positively charges. Simultaneously, oxygen anions existing in solution would be adsorbed to form surface oxides. In the thermal treatment, the surface oxide decompose to carbon monoxide and carbon dioxide, highly reactive sites remain on the carbon surface that have free-radical character to some relatively small extent. They can react with oxygen present in air, giving new surface oxide, which can be assigned to some carbonyls, pyrone, and chromene-type structures.[261,399,400] These groups have a basic character, conferring basic properties to sample. As expected, for HNO3 treatment, the pHPZC and basicity values decrease with the increase in the amount of oxygenated acidic surface groups, while the opposite is observed for the thermal treatment.

3.5 Alkaline Treatment A particular type of activated carbon is produced from petroleum pitch using potassium hydroxide activation. This method leads to a very unique and sophisticated sorbent, Maxsorb, with maximum theoretical surface area (around 2000 m2/g calculated from density functional theory) and a very ˚ [376] Interestingly it has been high volume of pores, all smaller than 30 A. observed that regardless of the chemistry of the carbon precursor there are close similarities in the porosity of adsorbents prepared from the same activation procedures. Chemical activation has shown to lead to carbons with network of narrow micropores while physical activation is known to enlarge the dimensions of the micropores, which in fact define the chemical composition of the resulting activated material.[314] To increase the amount of unburnt carbon in the fly ash that is known to be effective for dye removal, an alkaline solution is commonly used by dissolving SiO2 from fly ash and leaving unburned carbon as a residue. The reaction between NaOH and SiO2 is as follows[433]: SiO2 + 2NaOH→Na2 SiO3 + H2 O. On alkaline treatment, Pengthamkeerati et al. observed that SiO2 dissolution deforms the net-such as structure of the original fly ash and changes the rough surface into a smooth thin surface after alkaline treatment. SiO2 dissolution deforms the net-like structure of the original fly ash and changes the rough surface into a smooth, thin surface.[215]


479

Yu et al. modified vermiculite using hexadecyl trimethylammonium bromide (HDTMAB), which led to the replacement of interlayer cations in vermiculite by HDTMA changing hydrophilic vermiculite to hydrophobic.[224]


3.6 (NH4 )2 S2 O8 Oxidation Upon (NH4 )2 S2 O8 oxidation there was found to be dramatic increase of total acidic sites.[135] This behavior is attributed to generation of new oxygen functionalities during (NH4 )2 S2 O8 oxidation.[405] On the other hand, ammonia treatment led to a significant enhancement of basic surface groups, which are measured to be almost twofold higher than on virgin carbon. The treatment with (NH4 )2 S2 O8 , mostly generates lactonic groups, although concomitant increases in carboxylic and phenolic groups are recorded. These results contrast with that published by Santiago et al.[408] reported that (NH4 )2 S2 O8 treatment mostly generates phenolic groups. The authors[135] were not clear about the contradiction but attributed it to the higher surface density of carboxylic groups present on plain AC, used in this study, which suggests their possible condensation with phenolic groups to form lactones.[407,408] The modification with ammonia was found to drastically reduce the concentration of carboxylic groups, while phenolic and lactonic groups were not affected. The carbon matrix does not consist of carbon atoms alone, but is also formed by other heteroatom such as hydrogen, oxygen, nitrogen, halogen, sulfur, and phosphorus.[406] These heteroatoms bonded to the edges of the carbon layers, which govern the surface chemistry of activated carbon.[409,410]

4. ADSORPTION MECHANISMS AND EFFICIENCY Usually the high adsorption activity of activated carbon and other adsorbents is attributed to the surface functional groups and porosity. Several methods can be used to characterize the adsorbents and also adsorbent-adsorbate interactions. XPS, Boehm titrations, X-ray powder diffraction (XRD), BET, and scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) can be used to identify and characterize the adsorbent as well as adsorbent-adsorbate interactions. XRD is an effective method used to study the degree of intercalation. The specific surface area of the adsorbent, pore volume and pore size can be measured by the BET method (N2 adsorption–desorption). The surface images of samples are obtained using SEM. DTA–TGA is employed to analyze the population of surface functional groups. FTIR analysis is applied to determine the surface functional groups and to study adsorbate-adsorbent

480

S. Kushwaha et al.

interactions. FTIR spectroscopy and XRD were used as main experimental techniques to assess the interactions among the adsorbent and the adsorbates. The common functional groups identified in various adsorbents by FTIR and their basal plane angles by XRD are summarized in Table 4. The adsorbent and adsorbate properties influencing the adsorption process are discussed subsequently.


4.1 Nature of Functional Groups Oxygen chemisorbs on the surface of AC to form carbon–oxygen functional groups such as carboxyls, phenols, lactones, aldehydes, ketones, quinines, hydroquinones, anhydrides, or ethereal structures. These groups may also interact among themselves. Some of the groups (e.g., carbonyl, carboxyl, phenolic hydroxyl, lactones), are acidic.[400,401] The existence of pyrone, chromene, and quinone structures as well as oxygen-free Lewis basic site on the graphene layers was postulated to account for the basic nature of the carbon surface. The Lewis basicity of delocalized electrons is influenced by the aromatic system on the carbon surface.[400,402,403] The surface chemistry of carbons has been indicated to have a significant effect on the uptake of small molecules of organic compounds. Many attempts have been made to characterize surface-oxygen groups using experimental techniques such as Boehm titrations, infrared and X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For example, in terms of XPS, deconvoluting the spectrum identifies a defined number of peaks. The specified binding energy corresponds to each Gaussian peak and indicates the type of surface group(s), in which a given element (i.e., C or O) is present. Additional information obtained from XP spectra is the element percentage in a given functional group. The distributions of the carbon and oxygen structures derived from the XP C1s and XP O1s spectra, respectively, are published in tabulated form.[406]

4.2 Physical Structure of Adsorbent The surface chemistry of activated carbon is related to the presence of heteroatoms (oxygen, hydrogen, and nitrogen) other than carbon atom within the carbon matrix.[315,409] The nature of surface groups in activated carbon can be modified through physical, chemical, and electrochemical treatments.[207,411–413] The treatment using oxidizing chemicals will selectively remove some of the functional groups.[399]

4.3 Nature of Adsorbate It is reported that the sorption of planar, aromatic, and π -electron donor or acceptor compounds in general is greater than that of their opposites

481

Nanocomposites N,O-carboxymethylchitosan/montmorillonite (N,O-CMC–MMT)

50 HDTMA 100 HDTMA 200 HDTMA EPIDMA/bentonite

Vermiculite Vermiculite with different organic loadings of HDTMA

Organo Vermiculite

FTIR

3424 cm−1

3442 cm−1 3430, 3428, 3424 cm−1

3425 cm−1

1421, 1474 cm−1

721, 688, 458 cm−1 721, 692, 459 cm−1 721, 694, 457 cm−1 3100–2800 cm−1

3423cm−1 2850 cm−1 2919 cm−1 1467 cm−1 719, 684, 458 cm−1

3567 cm−1

Frequencies

References

O–H stretching vibrations of Si–OH and Si–O–Si groups of the [224, 434–437] silicate layers –OH vibration of the tetrahedral sheets Characteristic stretching vibration of the –CH2 Characteristic stretching vibration of the –CH3 The bending vibration of –CH3 Deformation and bending modes of the Si–O bonds It was noted that with the increase of organic loading from 100 to 438 200% CEC, the absorption bands of –CH2 and –CH3 had few changes. This may indicate that the exchange between HDTMA+ and the exchangeable cations approached anequilibrium. The absorption at 1637 cm−1 was primarily due to water directly coordinated to the exchangeable cations of the clay. It can be seen from the spectra that this band weakened with the increase of HDTMA+. Therefore, this phenomenon is a good indication of the replacement of the interlayer cations with the HDTMA+. Moreover, the property of the silicate layer has been changed from hydrophilic to hydrophobic. 439 Asymmetric and symmetric stretching vibrations of –CH2 and –CH3 groups of EPIDMA/bentonite. Bending vibrations of –CH2 and –CH3 groups supporting the intercalation of EPIDMA molecules. Intensity of broad bands of H–O–H stretching was reduced revealing less content of adsorbed water in EPIDMA/bentonite than that in bentonite. corresponding to –OH stretching vibration of H2 O of MMT 440 Shifting to lower wave number Suggests the vibration bands in N ,O-CMC (O–H and N–H stretching, 3423 cm−1) overlap with the bands of MMT (–OH stretching of H2O). Enhanced band suggests the vibration bands in CTS (O–H and N–H stretching) overlap with the bands of MMT (–OH stretching of H2O). (Continued on next page)

Inferences

TABLE 4. Functional groups identification in various adsorbents by FT-IR and XRD


482

Plant Material (GFP)

Zeolite rich tuffs

FTIR

2909.64 cm−1

3000–3600 cm−1

2600–3600 cm−1

455 cm−1 1040 cm−1

609 cm−1

1050 cm−1

1530 cm−1

1630 cm−1

2923, 2882 cm−1 1424, 1380 cm−1

Frequencies The intercalated CTS (C–H stretching and bending of methyl and methylene groups) are observed in the spectra of the nanocomposites, and the intensity of the both adsorption bands increased with increasing the molar ratio of CTS to MMT. Intensity of band increased indicating the first NH–CO group stretching vibration of CTS overlap with –OH bending vibration of H2 O of the MMT. Showed deformation vibration of the protonated amine group of CTS also becomes stronger with increasing the molar ratios of CTS to MMT. IR spectra indicate that the molar ratio of CTS to MMT could influence chemical environment of the nano-composites, and then may have an influence on absorption properties of the nanocomposites. Asymmetrical stretching vibrations of the tetrahedral of pure phillipsites shifted towards 1095 cm−1 at severe acid activation, it was deduced that acid activation also induced breakdown and dealumination of phillipsite. Loss of crystallinity in the acid-activated samples as a function of the band width near 1050 cm−1, a larger band width revealing a lower crystallinity Characteristic band of clinoptilolite decreased with the severity of acid activation. Diagnostic of amorphous silica, increased with acid activation. Band shifted to 1090 cm−1 reflected clinoptilolite breakdown and dealumination. Two major band stretches 3000–3600 cm−1 and light stretch at 2909.64 cm−1. The O–H stretching vibrations occur within a broad range of frequencies indicating the presence of “free” hydroxyl groups and bonded O–H bands of carboxylic acids. A broad and strong band stretch (free or hydrogen bonded O–H groups (alcohols, phenols and carboxylic acids) as in pectin, cellulose and lignin on the surface of the adsorbent. (Light stretch) stretching of symmetric or asymmetric C–H vibration of aliphatic acids

Inferences

TABLE 4. Functional groups identification in various adsorbents by FT-IR and XRD (Continued)


[123, 447–450]

[161, 441–446]

References

483

Hydroxyapatite (HAP)

Pine Bark

660–520 cm−1

1050 cm−1 1100, 1085, & 1050 cm−1 1040–1113 cm−1

630 cm−1

1545 cm−1

1469, 1429 and 875 cm−1 875 cm−1 930 cm−1

1650–1500 cm−1 1450 cm−1 1150–1000 cm−1

1736 cm−1

3600–3200 cm−1

1050 cm−1

1641 cm−1 to 1415 cm−1 1384 cm−1 1233 cm−1

1730 cm−1

[230, 453–457]

[451]

Splitting of bands may be assigned to either hap or to stretching vibrations of Si–O in quartz mineral. Distinct phosphate band of t4 bending mode whereas CO3 Ap gives a single band while HAP has three bands. (Continued on next page)

A strong evidence for the presence of CO3 Ap in the apatite matrix A barely discernible shoulder refers to carbonate radical in the apatite mineral which ensures the type of apatite present. Characteristic of CO2 −3 ions substituting for OH− ions (A-type substitution). Characteristic OH vibrational mode for hydroxyapatite (absence of this band HAP suggests a very low hydroxyl content in the apatite matrix). A single intense band of phosphate in CO3 Ap. In HAP it appears as three distinct bands.

Stretching vibration of -C=O bond due to nonionic carboxyl groups (–COOH, –COOCH3 ) and may be assigned to carboxylic acids or their esters. The O–H stretching vibrations occur within a broad range of frequencies indicating the presence of “free” hydroxyl groups and bonded O–H bands of carboxylic acids). Asymmetric and symmetric stretching vibrations of ionic carboxylic groups (–COO−). Symmetric stretching of –COO− of pectin. Aliphatic acids group vibration to deformation vibration of –C-O and stretching formation of –OH of carboxylic acids and phenols Assigned to the stretching vibration of C–OH of alcoholic groups and carboxylic acids. Broad band for –OH stretching vibrations of polymeric compounds especially polysaccharides (cellulose). C=O stretching for esters was unconspicuous, consistent with its comparable low suberin content. C=C bending vibration of aromatic skeletal mode of lignin. Phenolic OH groups. C–O–C and OH vibration of polysaccharides. Pine bark sorbents was mainly composite of polymeric OH groups, phenolic OH and carboxylate groups, C=C of lignin, and OH groups of polysaccharides. Characteristic of B-type precipitated carbonate flourapatite.


484

Hydrotalcite and its adsorption products2,4-D, clopyralid, picloram

FTIR

1700 cm−1

1244 cm−1 3472 & 3374 cm−1

1617 & 1369 cm−1 1051, 1147 cm−1

1707 cm−1

1315, 1089 cm−1 1735 cm−1 1231 cm−1 1626 & 1369 cm−1

1371 cm−1 1476, 1422 cm−1

1640 cm−1 3000 cm−1

3480 cm−1

601 & 667 cm−1 569 cm−1 466 cm−1 3404 & 3534 cm−1 2113 cm−1 2932 cm−1

Frequencies Characteristic for gypsum. Stretching vibration of Si–O–Mg in talc. Si–O–Si bending vibrations. Characteristic of OH− vibrations. Shoulder identifies the presence of a Si–P bond C–H stretching vibration of minor organic material is manifested by the shoulder. A wide band of hydrotalcite attributed to the O–H vibrations of free and H-bonded hydroxide groups. Low intensity band due to the water-bending mode O–H. Hydrogen bonding between H2 O and the anions present in the interlayer. Stretching-vibration band of carbonate anion. C=C vibrations of the aromatic ring (2,4-D also corresponds to same vibrations). Antisymmetric and symmetric vibration C–O–C Indicating the band corresponding to –COOH group O–H deformation coupled with C–O stretching vibrations. Bands are due to the CMO vibration of the carboxylate anion, confirming the presence of 2,4-D in the anionic form in the interlayer of the hydrotalcite Corresponds to CMO vibration of carboxyl group, substituted in HT500-Clopyralid adsorption product. Bands of carboxylate anion. Stretching and deformation vibration of the C–H of the pyridine ring. C–N aromatic vibration. Corresponds to the N–H deformation vibration of Picloram which is also present in the adsorption product. C–O vibration of carboxyl group of Picloram and the presence of this band in the adsorption product suggesting adsorbed Picloram in a molecular form in HT500-Picloram adsorption product.

Inferences

TABLE 4. Functional groups identification in various adsorbents by FT-IR and XRD (Continued)


[223, 458–461]

References

485

Activated carbon from plant source

Palygorskite

LDH modified by surfactant

1155–1033 cm−1

700–500 cm−1 2950–2850 cm−1 1240–1210 cm−1 800–500 cm−1 1300 cm−1 1385 cm−1 2950–2850 cm−1 2950–2850 cm−1 3000–2800 cm−1

1700–1610 cm−1 1383 cm−1

1378 cm−1 1735 cm−1 3600–3300 cm−1

1239 & 1105 cm−1 1700 cm−1

1600 cm−1 1536 & 1450 cm−1 =N–H vibration of Picloram. Deformation vibration C=N and C=C stretching vibration of pyridine. -C–H vibration of pyridine. Relative intensity at 1700 cm−1decreases with respect to that at 1600 cm−1 in HT500-Picloram with respect to the pure pesticide, indicating the presence of anionic form of Picloram. Characteristic for carboxylate (HT500-Picloram). –COOH groups of 2,4-D disappears after adsorption on bentonite Broad OH stretching of (associated water molecule, SDS-LDH, [462–467] RL-LDH) in brucite-like layer. Bending assigned to O–H bond in water molecules. The bending vibration is recognized with the nitrate anion. Disappears in SDS-LDH and RLLDH of SDS and RL due to ion exchange Metal–oxygen bending vibration. -CH2 stretching vibration. -O-SO3 − asymmetric and symmetric stretching mode. Metal-oxygen bonding vibration. -C–O–C bending vibration, -C–H bonds, Carbon chains of aliphatic hydrocarbons. Assigned to glycolipid O–H bond and brucitelike layer vibration. [468–471] Symmetric and asymmetric stretching of –CH2 of organic molecules. Unmodified palygorskite does not give any vibration in this region. Both asymmetric and symmetric vibration shift towards lower frequencies with increasing surfactant loadings. Organopalygorskites synthesised from DMDOA shows shift in frequencies towards lower side (2923 cm−1 and 2852 cm−1) for asymmetric and symmetric stretching This can be attributed to the structure and chain length of the surfactant molecules. [239, 266, 472, Most prominent peaks in the spectrum originate from OH 473] vibrations, CH2 and CH3 asymmetric and symmetric stretching vibrations. Corresponds to lignin and hence it is possible that cellulose, hemicellulose as well as lignin, having many OH groups in their structure, make up most of the absorbing layer. (Continued on next page)


486

Activated Carbons of olive kernels, corn cobs, soya stalks and rapeseed stalks (the abbreviations of the samples in the study are OK, CC, SS and RS, respectively).

C. Sinensis

FTIR

cm−1 & 2850 cm−1 & 1558 cm−1 cm−1 cm−1 cm−1

∼1000 cm−1 3400 cm−1 2950 & 2850 cm−1 2350 cm−1 1650 cm−1

∼1600 cm−1

∼1700 cm−1

3500 cm−1 3400 cm−1 pHPZC . The IEP denotes the pH value of zero potential. pHIEP is usually close to pHPZC , but it is lower than pHPZC for activated carbons. Depending on the pH of the solution the adsorbate can also be charged as a result of dissociation or protonation.[1] Thus the pKa or pKb values of the ionizable molecule are also important along with the solution pH for determining the extent of ionization. It is commonly assumed, that for pH < pKa adsorption of nonionized organics does not depend on the surface charge of adsorbent. However, for pH > pKa compounds such as phenols are dissociated, and adsorption of their ionic form depends on the surface charge. The pH of the medium thus affects the adsorption process by controlling electrostatic interactions between the adsorbent and the adsorbate. The fact that pH and pKa values affect the degree of ionization of the phenolic compounds has been clearly observed.[124,511] Phenol, 2chlorophenol, 4-chlorophenol, and 2,4-dichlorophenol were calculated to have quite different ratio of their anionic forms at the same pH value owing



495

FIGURE 4. Dependence of surface potential upon pH for hypothetical activated carbons.

to their different pKa values. This was attributed to the decrease in pKa value with increase in number of substituted chlorine atoms and also with the nearness of the chlorine atom position to the OH group in chlorophenols having the same number of substituted chlorine atoms When pH = pKa , the anionic forms and neutral forms would be 50%, respectively. As the pH increases, the anionic form was proposed to increase while neutral form decreases. The adsorption capacity of phenol was observed to decrease rapidly with pH > 8, which was attributed to the large decrease in neutral form of phenol limiting the adsorption of phenol in the organic matter of aged-refuse. In addition, pH is also reported to affect the surface property of the biomass and hence influence the equilibrium adsorption process. At lower pH, the overall surface charge of the biomass becomes positive leading to donor–acceptor interactions between the aromatic ring of the phenolic compound, especially chlorinated phenol activated by the –Cl, and the surface of the biomass.[510] Similar to phenols anionic dyes show various acid–base properties in solution, which could be described by acidity constants (Ka ). Sun et al. explained pH effect on adsorption of fly ash by electrostatic interaction between fly ash and dye molecules.[217] In the pH range of 7.5–8.5, the surface of fly ash was found to be positively charged (pHpzc = 8.4) and reactive dyes were negatively charged (pKa = 5–7), revealing strong electrostatic attraction. Al-Degs et al.[512] also made a similar observation and justified that at acidic pH, the sulfonate groups present in the reactive dyes were protonated (SO3 H, i.e., neutral). Furthermore, the protonation of nitrogen


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S. Kushwaha et al.

atoms, especially those not involved in aromatic systems, was also probable.[512] All this was suggested to result in reduced attraction between reactive dye molecules that were neutral or positive charged and fly ash. There was low dye removal in highly basic medium due to repulsive interaction between negatively charged fly ash and deprotonated reactive dye molecules as well as competition between OH− (at high pH) and dye ions for positively charged adsorption sites. It was also hypothesized that the spatial structures of dye molecular may influence the adsorption process.[217] Similarly Salehi et al.[212] and Kumar[513] also explained the adsorption of anionic dyes onto chitosan to be due to the strong electrostatic interaction between the –NH3 + of Chitosan and dye anions in acidic pH conditions. When considering the charge of the two different organic species, pyridine and phenol have a pKa of 5.20 and of 9.95, respectively. Therefore, at the beginning of the reaction (pH = 6), both molecules are in the neutral form. The surface charge of kaolinite was found to change according to the condition of the aqueous dispersion by Vimonses and coworkers.[515] pHzpc of kaolinite was reported to be approximately 4–4.7 and the kaolinite edge was considered to have a pHzpc of 6–7.[516] The surface charge on apatite is observed to result from the balance between P OH, P O and Ca2+ groups by Buoyarmane and coworkers.[155] Therefore they expected pyridine (or phenol) interactions with apatite surface were to be two-fold: hydrogen bonding with P OH/P O- groups and Lewis acid-base interactions involving Ca2+ ions as also reported by Tanaka et al.[517] The latter is expected to be particularly efficient for pyridine bearing a lone free pair of electrons and the former for phenol due to its aromatic acidic hydroxyl group. Both phenomena are hypothesized to limit further protonation of the surface phosphate groups by solvating water. It was observed that the amount of sorbed pyridine increases with pH over the whole range. This was explained by the argument that under low pH conditions where pyridinium ions are present, they cannot significantly interact with Ca2+ but may develop attractive electrostatic interactions with P O groups. With increasing pH, the relative amount of pyridine over pyridinium increases that can more easily bind to the calcium ions and also P OH . . . NC6 H5 hydrogen bonds may arise. In contrast, phenol sorption was observed to be more sensitive to the variation of the charge of the apatite surface due to its interactions with different groups of the mineral surface, with minimum sorption at pH 7–8 probably corresponding to the apatite pH point of zero charge (pHPZC ), which has been reported by other researchers.[514,518] Below pHPZC , the overall apatite surface is positively charged and the oxygen atom of phenol may interact via Lewis acid–base interactions with Ca2+. Above pHPZC , the overall apatite surface charge is negative and hydroxyl groups of phenol may interact via hydrogen bonding with P O moieties. The possible modes of interaction of pyridinium, pyridine and phenol with the apatite surface are depicted in Figure 5. Brønsted acid–base reaction between pyridine and P OH are also reported to occur

497



FIGURE 5. Possible modes of interaction of (a) pyridinium, (b,c) pyridine, and (d,e) phenol with apatite surface.

but because this occurs at the apatite surface, Bouyarmane and coworkers feel that the resulting ionic pair P O /C6 H5 N H+ remains associated, as in the case of direct pyridinium-phosphate interactions. Hence they proposed that the upright position of pyridine is the most favorable for Lewis acid–base interactions between the nitrogen atom and the calcium ion whereas the

HO HO

HO

O

O

Salicylic Acid

Benzoic Acid

O

HO

NH2 O

N

Nicotinic acid

HO

4-Amino benzoic acid

FIGURE 6. Configuration of phenol.


498

S. Kushwaha et al.

configuration of phenol shown in Figure 6 should provide a better conformation for hydrogen bond formation. In another study despite being an anionic dye, Vimonses and coworkers observed that the amount of congo red (CR) adsorbed onto all kaolins was significant even in alkaline conditions (over 85% dye removal took place at a pH range 3–11).[132] They had justified the observation by the fact that in the aqueous system, sulfonate groups of the dye (D SO3 Na) are dissociated and the dye becomes anionic. At a pH lower than 5–7, a high electrostatic attraction exists between negative charges of anionic dyes and positive charges of both kaolinite and its edges, thereby increasing the dye adsorption capacity. As the pH increased to more than 7, the edge charges turned negative and did not favor the adsorption of dye anions at pH ∼8 attributed to ionic repulsion between the clay surface and the anionic dye molecules. Also, an abundance of OH− ions in basic solution was assumed to create a competitive environment with anionic ions of CR for the adsorption sites causing a decrease in adsorption. Also CR is a zwitterionic molecule (H3 N+–R–SO3 −) at low pH. The negatively charged site of CR can be adsorbed at the edges of kaolinite crystals, whereas the ammonium groups will interact with the permanent negative surface sites and further the two primary amines (–NH2 ) attached to the two naphthalene rings located at the two ends of CR molecule can be protonated into –NH3 + at the initial pH of 6, thus getting attracted to the negatively charged surface of the adsorbent as also reported by Fu and Viraraghavan.[519] The behavior of adsorption of organic acids has been explained by Ayranci and Duman based on pHZpc , structure of organic acids and their different percentage of ionization.[258] In general the rate and extent of adsorption was observed to be highest from solutions in water or in 0.4 M H2 SO4 , the lowest from solutions in 0.1 M NaOH and intermediate from solutions at pH 7.0 for all the organic acids studied and shown in Figure 6. They hypothesized that organic acids are found as mixtures of two forms in water due to partial ionization and adsorbate solutions in water are slightly acidic due to partial ionization of organic acids. At pH values of solutions lesser than pHZPC ( = 7.4) the carbon surface is positively charged. Salicylic acid (SA) being neutral and having two functional groups (OH and COOH), has the highest rate and extent of adsorption through charge dipole and dispersion interactions followed by benzoic acid (BA) having one less functional group (only COOH) than SA. Nicotinic acid (NA) and para-aminobenzoic acid (PABA) showed the least rate and extent of adsorption since they both have a positive charge on their N centers and the carbon surface has also a net positive charge. So, the relatively high rate and extent of adsorption observed in water solutions was attributed to both electrostatic attractions of positively charged surface and anionic adsorbate species and also from the dispersion interactions between the carbon surface and neutral adsorbate molecules. At pH values greater than pHZPC carbon surface is negatively



499

charged and the adsorbates are also negatively charged resulting in least adsorption in basic solutions. In neutral solutions the carbon surface is almost neutral. All four adsorbates in this solution are in singly charged anionic state (> 99%), the charge being on the carboxylate group. So, they concluded that order is determined mainly by the remaining structure (other than COO− group) of the molecule. SA, having an OH substituent (in ortho position to carboxylate) that possesses two lone pairs of electrons on O atom, showed the highest rate and extent of adsorption via dispersion and hydrogen bonding interactions. An intramolecular hydrogen bonding is also hypothesized in SA. In alkaline solutions the carbon surface is negatively charged since pH values of these solutions are much greater than pHZPC . SA in this solution is anionic, which is slightly reduced by the intramolecular hydrogen bonding between the negatively charged O atom of carboxylate group and partial positively charged H atom of OH group in ortho position to carboxylate group. So, SA experiences the least electrostatic repulsion from the carbon surface among the four adsorbates and thus shows the highest rate and extent of adsorption in this solution. BA is completely in an anionic state with a full negative charge on it in this solution and thus experiences more electrostatic repulsion than SA. So it shows smaller rate and extent of adsorption than SA. NA and PABA have a functional group having a lone pair of electrons in para and meta positions to the carboxylate group, respectively, in addition to a full negative charge on carboxylate group. So these two adsorbates experience the most electrostatic repulsion from the surface resulting in the least rate and extent of adsorption in this solution.[258] They had also similarly explained the adsorption of benzoic acid on activated carbon cloth based on electrostatic and dispersion interactions.[257]

5.2 Ionic Strength The organic contaminants are always present in effluents with significant amounts of soluble salts. The electrolyte concentration or ionic strength of the contaminated water can greatly influence the adsorption capacity of an adsorbent by altering its surface potential. Apart from dyes, the wastewaters from textile-manufacturing or dye-producing industries also contain various types of suspended and dissolved compounds such as acids, alkalis, salts, surfactants, and metal ions. Anions such as chloride, sulfate, carbonate, and nitrate are the most common ions present in textile effluents. So, it is necessary to evaluate the adsorption performance of an adsorbent under various electrolyte concentrations. Behera et al. observed that triclosan sorption onto activated carbon was almost constant up to ionic strength of 1 × 10−1M followed by an increase in sorption from 37.54 to 44.88 mg/g when ionic strength was increased from 1 × 10−1 to 5 × 10−1M at pH 3. This is attributed to the fact that


500

S. Kushwaha et al.

sorbent particles and triclosan molecules are both surrounded by an electric double layer due to electrostatic interactions.[227] This justification was based on the reference of Rashid and coworkers to the Gouy–Chapman theory of the diffuse double layer, which states that the thickness of the double layer is compressed by an increase in the ionic strength of the solution as.[521] At higher ionic strength, the sorption of triclosan will be high owing to the partial neutralization of the positive charge on the sorbent surface and a consequent compression of the electrical double layer by the chloride ion. The chloride ion can also enhance the sorption of triclosan ion by pairing their charges, and hence reducing the repulsion between the triclosan molecules sorbed on the surface. This initiates the adsorbent to adsorb more positive triclosan ions. A similar phenomenon was observed on the sorption of an herbicide onto activated carbon by Belmouden and coworkers.[522] The increased sorption of triclosan onto clay minerals with increase in ionic strength could also be due to the decrease in solubility of triclosan in salt solution, which is characterized as a salting out effect.[520] Irrespective of the increase in ionic strength, triclosan sorption onto all the sorbents was almost constant at pH 9 in contrast to pH 3. This could be due to the fact that at pH 9 the anionic form of triclosan (88%) is dominant in solution and the overall surface charge on all the sorbents is negative. This led to very high electrostatic repulsion between the deprotonated triclosan and the negatively charged sorbent surfaces, and was most likely prevailed over the effect of ionic strength at this pH.[227] The adsorption capacity of APTC for RhB was slightly affected by the presence of NaCl. There is a carboxyl group (–COOH) in the RhB molecule, which imparts a negative charge to the chromophore and a positive charge is also contributed by the amino group. When the ionic strength increased, the electrical double layer surrounding the APTC surface was compressed, resulting in a decrease in RhB adsorption onto APTC while at the same time NaCl could screen the electrostatic interaction of the opposite-charged groups in the zwitterionic RhB molecules, and the adsorbed amount will increase with the increase of NaCl concentration. As a result of these two opposite influences, adsorption capacity of APTC for RhB is little affected by ionic strength.[184] Eren et al. also observed that increasing the ionic strength of the solution causes an increase in the adsorption of CV+ onto the MCS surface due to coated sepiolite will be negatively charged inducing the attractive forces between the CV+ cation and the MCS surface.[193] Also a number of intermolecular forces such as van der Waals forces, ion-dipole forces, and dipole-dipole forces occur between dye molecules in the solution causing aggregation of the dye molecules, which increased upon the addition of salt to the dye solution.[193,524] Substances that remain in molecular form are not affected by the presence of other ionic substances as observed by Andini et al.[216] The data of



501

their study relative to chlorophenol (CP) show that when HCl or KOH are added to the initial solution, CP adsorption is depressed, as both H+ and OH− compete favorably with this organic molecule. Furthermore, when KCl is added to the initial solution, the presence of both cations and anions has a synergistic effect and CP adsorption takes place to a very low extent. They hypothesized that the reason for this behavior may be the fact that, at equilibrium, CP is present in molecular form (pKa = 8.94) and its adsorption is unfavored with regard to ionic species. The addition of both HCl and KOH favors the adsorption of Chloroaniline (CA), and the same happens when KCl is present in the solution. At equilibrium, as for CP, CA is present in molecular form, but its behavior may be different from that of CP because the presence of an electronic doublet on nitrogen makes CA a Lewis base with nucleophilic character. This may favor its adsorption on the partially neutralized adsorbent sites. The adsorption of MB is not affected by the addition of any of the chemicals HCl, KOH, and KCl. This may be due to the fact that the MB molecule has both hydrophilic and hydrophobic character; then, its adsorption is favored on any type of adsorbent site.

5.3 Oxic and Anoxic Conditions Molecular oxygen in the aqueous phase is reported to promote chemical transformation, such as oligomerization of the organic compounds adsorbed onto the carbon surface,[523] thus improving the adsorptive capacity. Studies of the adsorptive properties of granular activated carbon (GAC) have shown that molecular oxygen plays an important role in the adsorption of phenolic compounds.[526–529] In these systems, GAC surfaces are thought to act as catalysts for the oligomerization of phenolic compounds by oxidative coupling reaction.[530] Higher extraction efficiencies of the carbons used in the oxic isotherms (presence of molecular oxygen) were obtained for the carbons that exhibited lower increases in capacities when compared to anoxic isotherms absence of molecular oxygen.[256] ACFs also showed significant increase in adsorptive capacity under oxic conditions, which was attributed to the oligomerization of adsorbates (o-cresol and 2-ethylphenol) on the surface of adsorbents.[256] Adsorption of phenolic compounds from aqueous solutions by activated carbons is one of the most investigated of all liquid-phase applications of carbon adsorbents. Recently, an up-to-date comprehensive survey of various approaches to this subject was given by Radovic et al.[97] Surface functional groups on the adsorbent also are known to play a role in the oligomerization of phenolic compounds on activated carbon surface when molecular oxygen is present.[256,525,528,529] Vidic et al. in their studies on the impact of surface properties of activated carbons on oxidative coupling of phenolic compounds postulated


502

S. Kushwaha et al.

that oxygen containing basic surface functional groups are primarily responsible for the catalytic properties of activated carbon toward oxidative coupling of phenolic compounds.[531] Vidic et al. also studied the impact of oxygen containing groups on activated carbon adsorption of phenols, and they found that acidic surface functional groups hindered to a certain extent the ability of activated carbon to promote oxidative coupling reactions on its surface.[531] Although metal and metal oxides that are present on the surface of activated carbon were found to be not a key factor in promoting oligomerization of adsorbates in the presence of oxygen,[256,531] in another study on five GACs (three bituminous base, one lignite base, and one wood base), it was postulated that manganese could be one of the causes of the oligomerization reaction.[256] ACFs also showed significant increase in adsorptive capacity under oxic conditions that was attributed to the oligomerization of adsorbates (o-cresol and 2-ethylphenol) on the surface of adsorbents.[262]

5.4 Natural Organic Matter Natural organic matter (NOM) is found in varying concentrations in all natural water sources and mainly comprises compounds of humic acid, fulvic acid, and other humic substances. With NOM in the system, the adsorption capacity for target compounds is usually reduced.[21,521,522] Investigations undertaken to examine the prominent role of NOM in assessing an adsorption system also attributed the decrease in adsorption capacity, to the changes of the surface polarity distribution, displacement effect, pore blockage, pore constriction, and direct competition for the adsorption sites (Figure 7).[40,432,533,562,563,569] For instance, the NOM molecules are excluded from the ZSM-5 pores as the NOM molecular size is expected to be much larger than the pore size

FIGURE 7. Pore constriction



503

of ZSM-5.[276,564] Gonzalez-Olmos et al. came to the same conclusion in their work on the degradation of MTBE with hydrogen peroxide, catalyzed by the iron-containing zeolite (Fe-ZSM-5) in the presence of humic acid.[565] GAC performance for MTBE removal from water was adversely affected by the presence of NOM in water.[276,563,565–568] Shih et al. suggested that NOM can reduce GAC adsorption capacity for trace organics by pore blockage or by the competition between NOM and the target organics for adsorption sites, thus reducing the total available adsorption sites.[543] The direct competition effects on micropollutants are mainly attributable to the small portion of the background NOM, which is similar in size and structure to the target compound.[568]

5.5 Octanol–Water Partitioning Coefficient (Kow ) Solute hydrophobicity is usually represented by the octanol–water partitioning coefficient (log Kow ). The qe increased with the Kow increase (i.e., more hydrophobic organic substances tend to result in a higher equilibrium adsorption capacity.[536–538] When the adsorbent surface is hydrophilic, lower solute removal has been observed than when the adsorbent surface is hydrophobic.[539,540] These authors determined surface hydrophobicity by water vapor adsorption and by the enthalpy of (water) displacement with calorimetry. Adsorbent surface hydrophobicity is related to the presence of oxygen-containing functional groups. However, while these functional groups promote water adsorption, they can also facilitate H-bond donor and π –acceptor interactions between solutes and adsorbent surface.[270] Increasing the amount of oxygen-containing functional groups on the activated carbon surface decreases the adsorption of organic solutes, indicating preferential adsorption of water molecules over organic solutes at these sites.[112,270,541] On the other hand in nonpolar solvents (e.g., cyclohexane), the presence of oxygen containing functional groups on the carbon surface enhanced the removal of MTBE,[542] phenol and aniline.[108] But Boyd et al. in a study of benzene and TCE sorption onto HDTMA smectite, observed that benzene sorbed more strongly than TCE, a result that would not be expected based on the aqueous solubility of the solutes.[34] Bartelt-Hunt et al. attributed the observation to the fact that compounds such as benzene are partly polarizable due to electron delocalization, which may cause an electrostatic attraction between benzene and the clay mineral surface that does not occur for TCE. Compounds such as benzene are partly polarizable due to electron delocalization, which may cause an electrostatic attraction between benzene and the clay mineral surface that does not occur for TCE. Another possible explanation for the lower sorptive capacity of HDTMA bentonite for TCE than for benzene may be that TCE is unable to access the interlamellar spaces of the HDTMA bentonite, and therefore does not have access to the entire partition medium. Benzene, being a planar


504

S. Kushwaha et al.

molecule, would be able to access the interlamellar spaces, resulting in a higher sorptive capacity for benzene than TCE. They also proposed that TCE may have a lower micelle–water partition coefficient than benzene when the micelle phase is composed of HDTMA cations. In general, micelle–water partition coefficients are thought to be linearly related to octanol-water partition coefficients (Kow ). Based on this assumption, TCE, having a larger Kow than benzene, should be more soluble in HDTMA. The partly polar nature of benzene results in benzene being more soluble than TCE in a cationic surfactant phase such as HDTMA.[544]

6. DESCRIPTION OF ADSORPTION 6.1 Isotherms Proper analysis and design of adsorption processes requires relevant adsorption equilibria as one of the vital information.[309,310] In general, sorption processes were found to proceed through varied mechanisms such as external mass transfer of solute, intraparticle diffusion, and adsorption at sites. Unless extensive data are available, it is impossible to predict the rate-determining step involved in the process. However, sorption isotherm equations can be used to explain the process at equilibrium conditions. Several adsorption isotherms originally used for gas phase adsorption are readily adopted to correlate adsorption equilibria in solution. The maximum adsorption capacities achieved for different classes of adsorbates such as dyes, phenols, pesticides and other organic adsorbates are shown in Figures 8–11. For pesticides the maximum adsorption capacity was achieved with Filtrasorb as adsorbent and carbofuran as adsorbate (∼525 mg/g). On the other hand maximum adsorption capacity was achieved with activated carbon for phenols (∼500 mg/g). Crosslinked chitosan beads were found to have good adsorption capacity for dyes (∼2500 mg/g for reactive blue 2).

6.2 Kinetics Adsorption equilibria studies are important to determine the efficacy of adsorption. In spite of this, it is also necessary to identify the adsorption mechanism type in a given system. Kinetic models have been used to test the experimental data to understand the mechanism of adsorption and its potential rate-controlling steps. In practice, kinetic studies are carried out in batch systems using various initial sorbate concentrations, sorbent doses, particle sizes, agitation speeds, pH values, and temperatures to select the optimum conditions. Then, linear regression is used to determine the best-fitting kinetic rate equation. As an additional step, linear least-squares method can



505

FIGURE 8. Maximum adsorption capacity of dyes (Color figure available online).

also be applied to the linearly transformed kinetic rate equations for confirming the experimental data and kinetic rate equations using coefficients of determination. Several adsorption kinetic models have been established to understand the adsorption kinetics and rate-limiting step as summarized in Table 5.

FIGURE 9. Maximum adsorption capacity of phenols (Color figure available online).

506

qe qm

=

K L Ce 1+K L C e

Temkin

qe qm

=

RT Q

Ho’s model q =

Ln (K T C e )

2kqe2 1+2kqe t

Freundlich qe = K f C e1/n

C Type-isotherm

L Type-isotherm

Langmuir

Model

Assumptions/comments

Two-parameter models The Langmuir theory is valid for monolayer adsorption onto a surface containing a finite number of identical sites. Langmuir model for isotherm modeling were unsuccessful in the low concentration. Qm and KL represent maximum adsorption capacity and energy of adsorption; 0 < RL (separation factor) < 1 implies favorable adsorption; RL values indicate the type of isotherm to be irreversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1) or unfavorable (RL > 1). Represents boundary layer diffusion of solute molecules; Uniform energies of adsorption onto the surface and no transmigration of the adsorbate in the plane of the surface. Based on the film resistance and homogeneous solid phase. Two site Langmuir - Presence of two different types of sites (hydrophobic and hydrophillic) on adsorbent favors the applicability of two site model. Corresponds to monofunctional adsorbate which is strongly attracted to a sorbent, mostly by electrostatic or ion–ion interactions, reaching the saturation value in the plateau of the isotherm. Corresponds to a constant partition of solute between the solvent and the sorbent. The C type isotherm is characteristic when the solute is in low concentration with respect to the adsorption sites of the sorbent, and the isotherm shape could change if the concentration of the solute is increased and can later on give L-type isotherm reaching the plateau. High values for Kf shows high adsorption capacity; Heterogeneous adsorbent and multilayer adsorption and the adsorption capacity is related to the concentration of adsorbate at equilibrium. The constant n is the empirical parameter related to the intensity of adsorption, which varies with the heterogeneity of the material. To describe heterogeneous systems 1/n values indicate the type of isotherm to be irreversible (1/n = 0), favorable (0 < 1/n < 1) and unfavorable (1/n > 1) shows the adsorption capacity of an adsorbent, The adsorption process occurs on the heterogeneous surfaces. If the n is below one, then the adsorption is chemical process; otherwise, the adsorption is physical process. Describe chemical and physical adsorption on heterogeneous surface. Correlates with surface area and surface charge. Parameters from Temkin model describes The heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy; KT equilibrium constant corresponds to maximum binding energy and B1 is related to heat of adsorption.

TABLE 5. Common kinetics and isotherm models


[575]

[574]

[573]

[452, 572]

[452, 572]

[570,571]

Reference

507

[584]

[583]

[582]

[581]

[580]

[579]

[576.577]


Reflects the combined feature of the Langmuir and Freundlich isotherm equations. For example, if nK equals 1, the equation reduces to the Langmuir isotherm, whereas the equation can be simplified to a Freundlich-type isotherm when value of the term bKCf is much greater than unity.

L

Khan isotherm Q max bK C f Q= (1 + bK C f )aK

(K L )((n−1)/n) C e (qm )1/n

It represents the Langmuir model for n = 1 and Henry’s model for n = 0. That is This model approaches the Freundlich model at high concentrations and is in accordance with the low concentration limit of the Langmuir equation.

Three parameter models Empirical model combining the parameters of the Langmuir and Freundlich equations. There are two limiting behaviors: the Langmuir form for β = 1 and Henry’s law form for β = 0. the mechanism of adsorption is a hybrid and does not follow ideal monolayer adsorption. can be applied either in homogeneous or heterogeneous systems. The isotherm has a linear dependence on concentration in the numerator and an exponential function in the denominator. Diffusion coefficient can be calculated; Maximum adsorption amounts depends on the dynamic diameter of the adsorbate molecule. Can give the adsorption capacity which helps in calculating the heat of adsorption using Clausius–Clapeyron equation. generally better validated when n > 1 the adsorption reaction stoichiometry would be n solute molecules per free adsorbent site. At low sorbate concentrations, the Sips isotherm effectively reduces to the Freundlich isotherm and thus does not obey Henry’s law. At high sorbate concentrations, it predicts a monolayer sorption capacity characteristic of the Langmuir isotherm. In the liquid phase, it is generally used as an adaptation of the Langmuir model. applicable to heterogeneous adsorbent surface Applicable to adsorption on heterogeneous surfaces. This equation describes well many systems with sub-monolayer coverage and reduces to Langmuir equation when t = 1. The Toth model is derived from potential theory

Radke–Prausnitz 1/n Ce = (K q1m)1/n + qe

Toth ( Cqee )n = ( q1m )n × (C e )n + ( qm1K L )n

Sips or Langmuir-Freundlich qe (kL C e )n = 1+(k n qm L Ce )

1+aR P C e

Redlich–Peterson qe = kR P C e β

Flory Huggins Account for the degree of surface coverage characteristics of the sorbate on the sorbent. Log Cθ0 = LogK F H + nF H Log(1 − θ) If the isotherm, instead of leveling off to some saturated value at high concentrations is able to rise BET qe = BCe qmax Ce −C∗ [1+(B−1(C indefinitely indicating that the initial adsorbed layer can act as a substrate for further adsorption. ∗ ))] /C e s) s ( Equation is unpopular in the interpretation of liquid phase adsorption data for complex solids.


508

Langmuir kinetic equation dθ t = kads (1 − θt )n Ct dt −kdes θtn

− π62 exp(−Bt) Ln(1 − F) = −Kfd t

Elovich model qt = 1 Ln (αβ) + β1 Lnt β Boyd modelor Liquid film diffusion model qt =1 qe

Intraparticle qt = K i t 0.5

Pseudo 1st order dq = K i (qe − qt ) dt Pseudo 2nd order q2 K t qt = 1 +e q2e K t

Model

Assumptions/comments

Applicable when flow of the reaction from the bulk liquid to the surface of the adsorbent determines the rate constant This Langmuir model is applied when the adsorption kinetics is governed by the rate of surface reactions and it gives kinetic parameters for both adsorption and desorption simultaneous by using the data obtained from the adsorption experiment.

KINETICS Generally do not fit well in whole range of contact time, only follows in initial stage of sorption. The adsorbate uptake, q, increases with increasing the initial concentration. The adsorption rate of adsorbate depends on the concentration of adsorbate at the absorbent surface. Mechanism being the rate controlling step, involve valency forces through sharing or exchange of electrons between adsorbate and adsorbent. Kinetic performance is proportional to the adsorption rate. Based on the sorption equilibrium capacity. The adsorption process and the overall rate of the adsorption process appears to be controlled by the chemical reaction. Chemisorption significantly contribute to the adsorption process Commonly used to identify the steps involved in adsorption. If it passes through origin it infers the applicability of intraparticle diffusion whereas presence of intercept shows the surface adsorption/ boundary layer effects. Adsorption is said to be intra-particle diffusion controlled if the reaction sites are internally located in the porous adsorbents and the external resistance to diffusive transport process is much less than the internal resistance.The term kdif calculated from the slope is indicative of an enhancement in the rate of adsorption. The value of C from intercept gives an idea about the boundary layer thickness. Adsorption is said to be intra-particle diffusion controlled if the reaction sites are internally located in the porous adsorbents and the external resistance to diffusive transport process is much less than the internal resistance. Considers that the rate-controlling step is the diffusion of the adsorbate molecules. The Elovich equation describes chemisorption. Determines whether the main resistance to mass transfer is in the thin film (boundary layer) surrounding the adsorbent particle, or in the resistance to diffusion inside the pores.

TABLE 5. Common kinetics and isotherm models (Continued)


[591]

[587]

[134, 515]

[588, 589]

[585]

[586]

Reference



509

FIGURE 10. Maximum adsorption capacity of pesticides (Color figure available online).

6.3 Competitive Adsorption Wastewaters usually do not contain a single pollutant. Thus, a better understanding of the multicomponent adsorption from aqueous solutions is needed to improve treatment designs. Competitive adsorption on the adsorbent surface takes place when several aromatic compounds are present in

FIGURE 11. Maximum adsorption capacity of others. (Color figure available online).


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the water[201,264,545–548,559–561] in such way that compounds with similar properties (i.e., size, polarity, interaction energy) compete for a limited number of adsorption sites. Though studies of multicomponent systems[550,551] are very important for improving the efficiency of water treatment, they are less common, possibly as a consequence of experimental limitations and the difficulty of interpreting isotherms. For competitive adsorption, several mathematical models have been proposed, including the Langmuir competitive adsorption model, models developed by Jain and Snoeyink, and by Mathews and Weber.[549,551,552] However, the model with the most thermodynamically accepted foundation is the IAST, originally proposed by Myers and Prausnitz for gas mixtures[553] and later developed by Radke and Prausnitz for dilute liquid solutions.[554] IAST has been used to describe multicomponent equilibria of several adsorbates taking competitive adsorption into account.[555] The basic assumptions and equations of IAST have been discussed in detail.[394] However, IAST only considers the direct site competition mechanism and has been shown to under-predict removal of target compounds at trace levels in the presence of DOM at much higher levels.[556,557] The discrepancy has been attributed to the IAST assumption that all adsorption sites are equally available to all adsorbates. High molecular weight molecules are subject to size exclusion in small GAC pores, a violation of the IAST assumption. Li et al. found that a high molecular weight model compound blocked pores, which resulted in slower adsorption kinetics of a target compound than without the competing substance, but the pore blockage had little effect on adsorption capacity.[558] Adsorption capacity reduction was mostly attributed to direct site competition from competing materials in the same molecular weight range as the target compound. Schideman et al. also proposed a third mechanism where accumulated DOM increases external mass transfer resistance in film diffusion.[559] To illustrate the competitive adsorption Sheindrof-Rebhun-Sheintuch (SRS) equation, a multicomponent Freundlich type equation, is used which is based on the assumption of exponential distribution of adsorption energies available for each solute.[560]

7. DESORPTION AND REGENERATION Desorption studies help to elucidate the mechanism of adsorption and regeneration of biosorbent making the treatment process more economical as the adsorbent can be used for a number of cycles. Desorption of the adsorbates from the adsorbents has been done by either changing the pH, use of electrolytes or organic solvents. The medium in which quantitative desorption takes place many a time helps in understanding the interaction between adsorbent and adsorbate.



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R. oryzae biomass was found to be a good adsorbent for the removal of organochloro pesticide lindane from water by Ghosh et al. The adsorption was found to be independent of pH. They hypothesized that being predominantly independent of pH, binding of lindane with ROB is likely to take place mainly through interaction other than ionic type and physical forces and hydrophobic interactions possibly play major role in this type of interaction. Their hypothesis was confirmed with the help of desorption studies where they observed that organic solvents (e.g., acetone and hexane) desorbed lindane from the loaded ROB to the extent of 85–100%.[592] Desorption studies were conducted by Eren et al. to gain insight into the mechanistic aspects of cationic dye (crystal violet, CV+) adsorption onto manganese oxide coated sepiolite (MCS). They found that the use of aqueous KCl and ethanol solutions for CV+ desorption is ineffective. Very low desorption of CV+ with these solutions led them to suggest that some complex formation takes place between the active sites of MCS and the cationic group of CV+. Even mixtures of aqueous ethanol solutions with KCl did not greatly improve the CV+ desorption. This led to their presumption that CV+ was bound onto the MCS through an electrostatic interaction binding force. Ethanol did not help to break this binding interaction.[193] Namasivayam et al. studied desorption of CR, procion orange, and rhodamine B form waste orange peel. CR and procion orange being anions showed increased adsorption in acid medium and increased desorption in alkaline medium indicating the these dyes that are held by an ion-exchange mechanism. In the case of rhodamine B an increase in pH from 3 to 11 did not show marked change in percentage desorption (17–27%), which was similar to adsorption. This led to their presumption that ion exchange might not have a significant role in adsorption of rhodamine B and it might be chemisorption. According to Namasivayam et al. if the adsorbed dyes on the solid surface can be desorbed by water, then the attachment of dyes on the adsorbent is by weak bonds. If sulfuric acid (1N) or alkali (pH 12) can desorb the dye and the adsorptions is by ion exchange. If organic acids such as acetic acids can desorb the dye, then the dye is held by the adsorbents through chemisorptions.[593] Desorption isotherms have also been used to elucidate the mechanism and the reversibility of the process. Li et al. studied the interaction between 2,4-D and inorganic–organic bentonites. Desorption isotherms plotted in Figure 12, along with the respective adsorption isotherms, revealed the different strength of adsorption between 2,4-D and various kinds of bentonites. Both Fe/CTMA60-bentonite (60% cetyltrimethyl ammonium ion) and CTMA60-bentonite showed reversible 2,4-D adsorption, whereas Febentonite showed an obvious hysteresis of desorption. Li et al. explained this observation by presuming that in nearly neutral media, 2,4-D species exist mainly as anion. The –COO− groups may be adsorbed by the positive iron cations in Fe-bentonite. According to them, the large hysteresis of desorption on


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FIGURE 12. Desorption and regeneration.

Fe-bentonite implied strong interactions between 2,4-D and the inorganic bentonite. In comparison, the reversible adsorption of 2,4-D on CTMA60bentonite indicated weaker interactions, which were dominated by the van der Waals interaction between herbicide molecules and the interlayer space They hypothesized that these interactions may also occur between 2,4-D molecules and the interlayer space of inorganic–organic bentonites, which made adsorption on Fe/CTMA60-bentonite reversible.[199]

8. CONCLUSIONS Chemical contamination of water from a wide range of toxic compounds, in particular organic compounds, is a serious environmental problem owing to their potential human toxicity. Adsorption techniques are widely used to remove organic pollutants from waters, especially those that are not easily biodegradable. In recent years, increasing costs and environmental considerations associated with the use of commercial adsorbents have led to a significant amount of research work aimed at developing new low cost adsorbents derived from renewable resources that involve both natural and nonconventional adsorbents. Their removal is based on various factors that include adsorbate–adsorbent interactions, size of adsorbate, role of functional groups on both adsorbate, and adsorbent and experimental conditions used for the adsorption process (e.g., pH, ionic strength, temperature,



513

existence of competing organic or inorganic compounds in solution, initial adsorbate/adsorbent concentration, contact time and speed of rotation, particle size of adsorbent). Numerous chemicals have been used for modifications that include acids, bases, oxidizing agent, and organic compounds. In this review, we have tried to highlight and discuss the significance of the previously mentioned factors. Chemical modification in general improved the adsorption capacity of adsorbents probably due to higher number of active binding sites after modification, better ion-exchange properties and formation of new functional groups that favors adsorbate uptake. Distinctive adsorption equilibria and kinetic models are of extensive use in explaining the adsorption process, denoting the need to highlight and summarize their essential issues, which has been done in this review. Knowledge of multicomponent adsorbate equilibrium (organics) and rate removal systems is required. The review has dwelled on the problem, showing the development of investigations on the subject, presenting some of the latest most important results and providing a source of up-to-date literature on it. The adsorption properties of the adsorbents prepared have usually been studied with regard to only a few adsorbates. Most studies on adsorption have been directed toward the uptake of single adsorbate in preference to multicomponent systems. There are virtually no studies available on the uptake of multicomponent systems involving different groups of organics such as phenols, dyes, and pesticides though studies are being done of late from adsorbates of the same group, however, these studies are also few. The development in the field of adsorption process using low-cost adsorbents essentially requires further investigation of testing these materials with real industrial effluents. Many of the adsorbents reported in literature have not been studied for the recovery and reuse of adsorbed substances, which is a necessity so that there is no sludge generation.

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