phosphorus removal from wastewater using oven ...

PHOSPHORUS REMOVAL FROM WASTEWATER USING OVEN DRIED ALUM SLUDGE By SARMAD A. RASHID DR. WADOOD T. MOHAMMED Chemical Engineering Department, College of Engineering, University of Baghdad, Baghdad, Iraq

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Contents

Page

Contents Nomenclature

2 4

1. Introduction

6

2. Literature Survey

9

2.1 Relevance of phosphorus in small-scale Applications 2.2 Phosphorus Removal Technologies 2.2.1 Chemical precipitation 2.2.2 Adsorption 2.2.3 Biological phosphorus removal 2.2.4 Recirculating Media Filter 2.2.5 Membrane Processes 2.2.6 Ecological Removal Technologies 2.3 Previous Works 2.3.1 Batch Adsorption Studies 2.3.2 Fixed Bed Column Studies 2.4 Applications for Small-Scale Phosphorus Treatment 2.5 Adsorption Phenomena 2.6 Principle of Adsorption 2.7 Fundamental of Adsorption 2.8 Types of Adsorbents 2.8.1 Alum Sludge 2.8.2 Activated Carbon 2.9 Adsorption Equilibria 2.9.1 Adsorption Kinetics 2.9.2 Adsorption Isotherm 2.10 Modes of Operation 2.10.1 Counter Current Moving Bed 2.10.2 Expanded-Bed up Flow 2.10.3 Fixed Bed down Flow 2.11 Contacting Systems 2.11.1 Batch Type Contacting System 2.11.2 Continuous-Flow System 2.12 Rate Limiting Factors 2.12.1 Effect of Flow Rate 2.12.2 Effect of Bed Height 2.12.3 Effect of Influent Concentration 2.12.4 Effect of Adsorbent Particle Size

9 10 11 12 12 13 13 14 15 15 17 18 20 22 23 24 25 25 26 26 27 32 32 32 33 35 35 36 37 37 38 38 39

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Contents

Page

3. Experimental Design

40

3.1 Introduction 3.2 Materials 3.2.1 Adsorbent 3.2.1.1 Alum Sludge 3.2.1.2 Granulated Activated Carbon (GAC) 3.2.2 Adsorbate 3.3 Wastewater 3.4 Experimental Arrangements 3.4.1 Adsorption Column 3.4.2 Auxiliary Apparatus 3.5 Experimental Work 3.5.1 Batch Experiments 3.5.2 Fixed Bed Column Experiments 3.6 Tests for Characterizing Oven Dried Alum Sludge 3.6.1 IR Spectroscopy 3.6.2 X-Ray Diffraction 3.6.3 Specific Surface Area

40 40 40 40 42 43 43 44 45 46 47 47 47 49 49 49 49

4. Results and Discussion

50

4.1 Introduction 4.2 Batch Experiment 4.2.1 Effect of Mass of Oven Dried Alum Sludge on the Adsorption Process 4.2.2 Effect of pH on the Adsorption Process 4.2.3 Equilibrium Isotherm Experiments 4.3 Fixed Bed Experiment 4.3.1 Effect of Initial Concentration 4.3.2 Effect of Particle Size 4.3.3 Effect of Flow Rate and Bed Depth 4.4 Oven Dried Alum Sludge Compared to Granular Activated Carbon

50 51

5. Conclusions and Recommendations

68

5.1 Conclusions 5.2 Recommendations for further study

68 69

References

70

3


51 55 59 63 63 64 64 67

Nomenclature Symbols a

Langmuir constant (L/mg)

A

A constant in equation (2.11)

b

Langmuir constant (mg/g)

C

Concentration of solute in solution at any time (mg/L)

C1

Concentration at given time (mg/L)

Ce

Concentration of solute in solution at equilibrium (mg/L)

Co

Initial concentration of adsorbate (mg/L)

Cs

Saturation concentration of solute in solution (mg/L)

k

Equilibrium rate constant of adsorption (1/h)

k´

The second-order reaction rate constant for adsorption (g/mg.h)

m

Mass of solute adsorbent (g)

n

Freundlich constant indicative of adsorption intensity

H

Bed depth ( m)

pH

Acidity

Q

Flow rate (L/hr)

qe

Amount of metal ion adsorbed at equilibrium (mg/g)

qmax

The maximum adsorption density in Langmuir equation

qt

Amount of metal ion adsorbed at any time (h)

R

Separation factor

R2

Correlation coefficients

t

Time

x

Mass of solute adsorbed (mg)

Xm

Amount of solute adsorbed in forming a complete monolayer (M/M)

4


Subscript o

Initial

e

Equilibrium

s

Saturation

x

Phosphate ions

Abbreviations P

Phosphorus

ODS

Oven dried alum sludge

MTZ

Mass transfer zone

GAC

Granulated activated carbon

BET

The Brunauer, Emmett and Teller

QVF

Quality vessels fabrication

ASTM

American society for testing and materials

USEPA

United states environmental protection agency

UV

Ultra violet

ID

Inner diameter

n.r

Not reported

b.p.v

Break through pore volume

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1. Introduction Wastewater or contaminated water is a big environmental problem all over the world. Not only does wastewater pollute but also those materials that enter into wastewater produce bacteria. For these reasons, attention has been devoted to industrial wastewater contamination since the technologist is interested in minimizing this quantity for the down stream processing costs. In industrial plants, contaminants may be a result of side reactions, rendering the water stream an effluent status. These impurities are at lowlevel concentration but still need to be further reduced to levels acceptable by various destinations in the plant. Surface waters contain certain level of phosphorus (P) in various compounds, which is an important constituent of living organisms. In natural conditions the phosphorus concentration in water is balanced, i.e., accessible mass of this constituent is close to the requirements of the ecological system. When the input of phosphorus to waters is higher than it can be assimilated by a population of living organisms the problem of excess phosphorus content occurs. Regulatory control on phosphorus disposal is evident all over the world in recent years (Environment Canada, 2000; USEPA, 2000; Department of Justice, 2004). Strict regulatory requirements decreased the permissible level of phosphorus concentration in wastewater at the point of disposal (i.e. 1 mg/L). This has made it very important to find appropriate technological solution for treatment of wastewater prior to disposal. Phosphorus removal is considered as a major challenge in wastewater treatment, particularly for small-scale wastewater treatment systems. Processes available for P-treatment are generally classified into three general categories: chemical, physical or biological-based treatment 6


systems. Among physical-chemical methods, phosphorus removal is achieved using ion exchange (Zhao and Sengupta, 1998; Liberti et al., 2001), dissolved air flotation (Penetra et al., 1999; Jokela et al., 2001), and membrane filtration (Yu et al., 2000; Dietze et al., 2002). Filtration has been used either alone or in conjunction with a coagulation process as a means to remove phosphorus from wastewater (Xie et al., 1994; Jonsson et al., 1997). High rate sedimentation has also been attempted in some studies (Rogalla et al., 1992; Zeghal et al., 1998). Among the various

physical-chemical

methods,

coagulation

with

chemical

precipitation and adsorption are the most common techniques being used for removing phosphorus. Enhanced biological methods for removing phosphorus are also used with success (Wareham et al., 1995; Louzeiro et al., 2002). For small scale applications (e.g., aquaculture) biological methods may not be appropriate for phosphorus removal because of the low carbon concentrations, which increases cost and time involved in biological methods (Park et al., 1997). Alternatively, physical-chemical methods can offer advantages for small industries because of lower initial costs involvement. These methods are also easier to use and do not require high level of expertise to maintain. Physical-chemical methods can also accommodate recycling sludge to reduce further costs involved in handling sludge. However, finding an effective and feasible material is a significant challenge in physical chemical approach. This problem has not been addressed so far as a complete solution. The key problem is to find a suitable material, which is easily available and effective to remove phosphorus from small-scale wastewater applications. Biosolid management is considered very important, as there are considerable amounts of biosolids generated due to anthropogenic reasons. Alum sludge, a biosolid generated in the coagulation process in a water treatment plant is one such type. Divalent and trivalent cation based 7


materials are known to be effective for phosphorus removal, Therefore, aluminum based residuals (i.e., alum sludge) are a viable option for being an effective phosphorus removal material. Alum is typically effective in phosphorus removal in chemical precipitation process (Aguilar et al., 2002). Therefore, use of alum sludge can be effective for phosphorus removal. Air dried alum sludge has also been attempted in limited manner by some researchers with success (Kim et al., 2003a). However, the use of waste material (alum sludge) not only can provide low cost appropriate technological alternative for small-scale applications, but also reduce hazard and cost related to the disposal of large amount of alum sludge.

Aim of this work was: 1. To investigate the effectiveness of oven dried alum sludge for adsorption of orthophosphate from deionized water. 2. To study the effect of key operating parameters such as influent concentration (Co), particle size, flow rate (Q) and bed depth (H) on the dynamic behavior of adsorption process in fixed bed using oven dried alum sludge. 3. To compare the adsorption of orthophosphate on oven dried alum sludge to other conventional adsorbent (i.e. activated carbon).

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2. Literature Survey

2.1 Relevance of Phosphorus in Small-Scale Applications Water treatment for phosphate removal is a very old and established process, although most of the processes are dedicated to drinking and sewage water treatments. Little attention has been given to the industrial wastewater contamination since these processes are quite specific in nature. Most of the processes however were established and dedicated to municipal wastewater treatment where the emphasis is more on ecological considerations. Industrial wastewater treatment is done with dual motive of environmental as well as economic consideration. (Fahad, 2004). Discharges

from municipal

waste

pollution

control

plants

are typically noted among the major contributors of phosphorus to the receiving water (Nutt, 1991). The contributors of phosphorus emission are both point source and non-point source polluters (Figure 2.1). Phosphorus removal from point source polluters has been a concern for many years. A large number of these point source polluters are small-scale in nature. Phosphorus can be present in both solid and liquid phase in water (Maruf et al., 2006).

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Phosphorus source to surface water Non-point source

Point source Industrial Industrial

Wastewater Wastewater treatment plant treatment plant

Dairy

Agriculture Agriculture

Municipal

Aquaculture

Storm water

Residential

Swine

Poultry

Figure 2.1, Example of various sources of point source phosphorus in surface water (Maruf et al., 2006)

2.2 Phosphorus Removal Technologies Phosphorus removal technologies have been developed using chemical, biological processes or a combination of both processes. Physical processes are also used as complementary technology to both the processes above. Physical processes are used to separate solids from liquid. Soluble phosphorus found in environment is mostly present as orthophosphate (Mann, 1996), which has been considered as the target for most of the treatment technologies. Chemical processes on the other hand, are used to bring soluble phosphorus to bulk phase. Once the soluble phosphorus is in the solid phase, physical processes are used for removal. Reviews on phosphonates discussed similar chemical behavior of adsorption and chemical precipitation (Nowack, 2003). Constructed wetlands, reverse osmosis or evaporation have also been used for phosphorus removal. However, due to cost and operational limitations they are not very suitable for small-scale industries. 10


2.2.1 Chemical Precipitation Chemical precipitation is a reaction that causes insoluble precipitates to settle and is strongly dependent on pH, phosphorus and coagulant concentration. Precipitation of phosphorus from municipal wastewater with the addition of divalent and trivalent metal salts has been widely reported in North America and Europe (Nutt, 1991). Common metal coagulants that have been examined are iron salts (ferric chloride, ferric sulfate, ferrous chloride, ferrous sulfate), aluminum salts (aluminum sulfate, aluminum chloride, sodium aluminate, polyaluminum chloride) or calcium-based compounds (lime and gypsum) (Narasiah et al., 1994). The removal efficiencies of individual materials are different and based on their individual reaction kinetics. The reactions between the metal salts and phosphorus are complex due to the presence of different species of phosphorus in the wastewater. The generally accepted theory is that a primary reaction occurs between the metal ion and orthophosphate to precipitate the insoluble metal phosphate. Chemical reactions create different types of complexes. The simplest forms of precipitation reactions are: Al+3 + PO4-3 → Al PO4 ↓

..… 2.1

Fe+3 + PO4-3 → Fe PO4 ↓

….. 2.2

Ca+2 + PO4-3 → Ca3 (PO4)2 ↓

….. 2.3

Stable precipitates of FePO4 and AlPO4 are created in the pH range of 57 (Jiang and Graham, 1998). The calcium precipitate at pH above 7 is predominantly Ca10(PO4)6(OH)2. Generalized precipitates of Al/Fe hydroxo-phosphates are Al(OH)3-x(PO4)x and Fe(OH)3-x(PO4)x. The value 11


of x in these precipitates depends on the extent of hydrolysis and pH of the precipitation reactions. These precipitates produce large amounts of sludge. However, safe and environment friendly disposal of this sludge is a big concern for the wastewater treatment utilities.

2.2.2 Adsorption Chemical adsorption can also be associated with chemical reactions that occur in many precipitation processes. The adsorption of phosphorus occurs when orthophosphate attaches to the surface of the adsorbent. Phosphorus adsorption may be physical or chemical in nature. Alum is well known to exhibit strong adsorption characteristics (Eckenfelder, 1999). In addition, waste materials those are high in divalent and trivalent metal concentration have also found to be useful phosphorus adsorbents. The materials that have been examined are blast furnace slag (Drizo et al., 2002), opoka (Johansson, 1999a; Johansson and Gustafsson, 2000), clay minerals (Ioannou and Dimirkou, 1997), activated red mud (Pradhan et al., 1998), polyacrylamides (Sherman et al., 2000), podzolised forest soil (Johansson, 1999a), zeolite (Sakadevan and Bavor, 1998), manganese nodules (Parida and Mohanty, 1998), calcium sulfate (Theis and Fromm, 1977) and other waste residual.

2.2.3 Biological Phosphorus Removal Biological treatment is one of the most widely used treatment alternative for phosphorus removal. The uptake of phosphorus in excess of metabolic requirements is well documented survival mechanism for many bacteria (Barnard, 1975). Although phosphorus uptake in the activated sludge process is very complex, nitrate removal and anaerobic stage is known to be a precondition for effective biological phosphorus

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uptake (Mann, 1996). The presence of volatile fatty acid in wastewater is needed for biological phosphorus removal (Randall et al., 1992). Alternative anaerobic and aerobic reactors for micro organisms provided the effectiveness of phosphorus removal. Biological treatment reduces the waste generated during the treatment process. However, biological wastewater treatment systems are notorious for their effectiveness and stability (Park et al., 1997).

2.2.4 Recirculating Media Filter Recirculating biofilters are normally used to treat septic tank effluents. It provides advanced secondary treatment of settled wastewater. In general, sand and/or gravel are used to treat wastewater. Chemical adsorption onto the media surface can play a significant role on removal of phosphorus (USEPA, 2004). However, a limited number of adsorption sites often limit phosphorus removal after a certain period of time. Sand and gravel as commonly used filter media are not known to be efficient adsorbents of phosphorus. Studies conducted on sand, crushed glass, peat and geotextile biofilters showed 6.2%-12.5% phosphorus removal (Hu and Gagnon 2005).

2.2.5 Membrane Processes Membrane treatment processes provide a physical barrier for removing sub-micron particles (Lozier et al., 1997). Membrane is a fast growing technology in wastewater treatment (Hillis, 2000). Small wastewater systems require simple solutions which are easy to operate and maintain. Membrane technologies can provide simple, yet elegant, treatment solutions that are easy to operate. Though membrane technology is not common in small-scale wastewater treatment, it has

13


potential for development as phosphorus removal alternative for small wastewater systems. For phosphorus removal, the use of membrane was reported limitedly (Yu et al., 2000; Gnirss and Dittrich, 2000; Dietze et al., 2002). In particular, microfiltration was effective in removing phosphorus from secondary municipal effluent. Membrane technology in conjunction with other chemical or biological process was observed to be effective in removing phosphorus from small-scale wastewater systems (Ratanatamskul et al., 1995).

2.2.6 Ecological Removal Technologies The use of ecological technologies for removing phosphorus from wastewater has drawn considerable interest (Rectenwald and Drenner, 2000). The concept has been used in the form of wetland (Mann and Bavor, 1993; Nnadi and Addasi, 1999; Serodes and Normand, 1999; Comeau et al., 2001), solar aquatics (Teal and Peterson, 1993; Industry Canada, 1997) and hydroponics (Ayaz and Saygin, 1996; Soto et al., 1999; Vaillant et al., 2003). Among these concepts, wetlands are the most widely used ecological technology that has been used for removing phosphorus from wastewater. However, the required land area, costs and lack of operational history limited the acceptance of solar aquatic and other innovative treatment systems (Stephens 1998). Ecological technologies are growing because of their robustness for treating variable wastewater streams and low reliance on high energy or chemical input.

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2.3 Previous Works 2.3.1 Batch Adsorption Studies Batch adsorption experiments under static hydraulic conditions have been reported in the literature (Table 2-1). Air dried alum sludge, limestone, blast furnace slag, activated red mud; manganese nodules and some clay minerals have shown a high adsorptive capacity for phosphorus (Ioannou and Dimirkou, 1997; Sakadevan and Bavor, 1998; Pradhan et al., 1998; Parida and Mohanty, 1998; Johansson, 1999a; Kim et al., 2003a). Higher removals were associated with either higher adsorbent concentration or lower initial phosphorus concentration. Optimal pH levels for the highest phosphorus removal varied widely for different adsorbents. For instance, optimal P removal using blast furnace slag was associated with a high pH of 10 (Sakadevan and Bavor, 1998). In comparison, manganese nodules and activated red mud showed high removal at a pH of 5-6 (Pradhan et al., 1998; Parida and Mohanty, 1998). Under optimal pH condition, blast furnace slag achieved higher phosphorus removal than manganese nodule and activated red mud. Opoka and different soil and sand samples did not show significant phosphorus removal. Particle size was not mentioned as a key experimental factor in many of the studies, although particle size influences adsorption behavior (Sakadevan and Bavor, 1998; Pradhan et al., 1998).

15


Table 2-1, Comparative batch adsorption results on several adsorbents using synthetic water (Maruf et al., 2006) Adsorbent

Max. %

(g/L)

removal

100-4000

33

Blast furnace Slag

5-10000

Blast furnace Slag

Adsorbent

P (mg/L)

pH

Reference

Alum sludge

>80

3.5-11

Kim et al., 2003a

20-100

50-90

5-10

Yamada et al., 1986; Sakadevan and Bavor, 1998; Johansson and Gustafsson, 2000

500-10000

100

50

n.r.

Sakadevan and Bavor, 1998

Opoka

5-25

0.02

20

7-8.9

Johansson, 1999a; Johansson and Gustafsson,2000

Activated red Mud

24-190

2

97

5.2

Pradhan et al., 1998

Limestone

5-25

20

85

8.9

Johansson, 1999a

Podzolised forest soil

5-25

20

50

4.9-5.4

Johansson, 1999a

Soil samples

500-10000

100

>50

n.r.


Zeolite

500-10000

100

>50

n.r.


Manganese Nodules

1.3

2-10

90

5.23

Parida and Mohanty, 1998

Hematite

1.3-13.8

1

n.r.

3.8-9

Ioannou and Dimirkou, 1997

Kaolinite

1.3-13.8

1

n.r.

3.8-9


Kaolinite hematite System

1.3-13.8

1

100

3.8-9


Sand

2.5-160

10

2.8

83%

6.5-7.1

Huang and Chiswell, 2000

0.04

n.r.

0-700b.p.v.

3-12

Kim et al., 2003a

3-20

35

0-4

75% at 25 Day

7

Johansson, 1997

LECA* + Opoka

3-20

35

0-4

63% at 25 Day

7

Johansson, 1997

Blast Furnace Slag

10

25

0.25-4

95% at 56 week

9-11.2

Johansson, 1999b

Alum sludge (air dried)

Alum sludge (air dried)

* Light expanded clay aggregate, n.r. – not reported, b.p.v. – breakthrough pore volume

2.4 Applications for Small-Scale Phosphorus Treatment Physical chemical processes were shown previously to be effective technology for phosphorus removal. The use of waste materials can provide very cost effective means of phosphorus removal. Table 2-3 summarizes the key features for selecting any physical chemical treatment technologies. Adsorption as a process uses materials more efficiently than chemical precipitation. Therefore, adsorption as a treatment option for small-scale.

18


Table 2-3, Schematic framework for issues in major physical chemical phosphorus removal processes (Maruf et al., 2006) Process

Coagulation

Adsorption

Max %

Operational issues

removal

100

Applications

-Low initial cost -Coagulant required on-site -Sludge disposal concerns -Residual chemical concerns

-Wastewater having high particulate

-Easy to operate -Low maintenance costs -Material used more efficiently

-Treatment of secondary effluent -Places, where low cost adsorbents are

100

phosphorus.

-Places, where low cost coagulants are available

available

than coagulants

-Space efficient 

Membrane

90

-High initial cost -Simple to operate -Backwashing and fouling concerns

-Wastewater having large particulate phosphorus

-Treatment of secondary effluent -Places need reuse of water

-Can be successful with other treatment process

-Success depends on loading Membrane

90

rate and detention time. -Use of effective adsorbents in constructed wetland can increase efficiency -Wildlife considerations are needed

-Hydraulic loading rate should Hydroponic system

90.6

be consistent with plant uptake -Reduces sludge disposal

-Availability of existing wetland -Availability of land area is important for natural wetland

-Availability of land area -Suitable where water is scarce and reuse is essential

concerns

-Stable and resilient system that Solar aquatics

84.8

can adapt to effluent changes -Venerable to changes in hydraulic loading -Reduces sludge disposal

-Can be applied where greenhouses are available

-Suitable where sunlight is available for longer hours

concerns

19


Applications can reduce the hazard and sophistication of handling waste. The use of adsorbents after recirculating biofilter can also provide some cost effective advantage for on-site wastewater systems. Small municipalities can upgrade their existing treatment facilities with addition of suitable physical chemical treatment alternatives. However, selection of low cost locally available material is needed for the successful application of these technologies. Membrane technologies require further research for development as a cost effective phosphorus treatment alternative. Ecological wastewater treatment technologies were also shown to be effective in recent years, especially for rural or small scale wastewater. These technologies can provide sustainability as part of technological options. However, most ecological treatment technologies require large land area and often dependent on hydraulic detention time (Maruf et al., 2006).

2.5 Adsorption Phenomena Adsorption is a mass transfer operation in which substances present in a liquid or gas phase are adsorbed or accumulated on a solid, and thus removed from the liquid or gas, adsorption process are used in drinking water treatment (Crittenden et al., 2005). Adsorption may be defined as the selective removal of a component of a fluid mixture by contacting the fluid with a solid adsorbent. Applications include the purification of drinking water, removal of harmful pollutants from wastewater effluents. It also has application in air pollution control and many processes in chemical engineering (Mckay, 2001).

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It is well known that liquid-phase adsorption is one of the most efficient methods for the removal of colours, odors, and organic pollutants from process of waste effluents (Juang and Tseng, 2001). Adsorption plays an important role in industry of recovery of valuable substances. It’s most important uses is in the removal of organic contaminants from polluted sources. The removal of organic pollutants, particularly the synthetic variety, can be of particular significance to the environment. Most organic substances are ultimately biodegradable and can lead to oxygen depletion in receiving waters. Organic compounds can cause colour, taste and odour and large numbers of synthetic organic substances could have carcinogenic effects if the water is to be used downstream as a water supply or for packaged water (Federal Register, 1987). The relative advantages of adsorption over other conventional advanced treatment methods are: 1. It can remove both organic as well as inorganic constituents even at very low concentrations. 2. It is relatively easy and safe to operate. 3. Both batch and continuous equipment can be used. 4. No sludge formation. 5. The adsorbent can be regenerated and used again (Mohanty et al., 2005). Adsorption of substance involves its accumulation at the interface between two phases, such as liquid and solid or a gas and a solid (Bhatia, 2001). All adsorption processes involve a decrease in the free energy of the system. The decrease in the heat contain of the system is called the heat of adsorption (Cassidy, 1951).

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2.6 Principle of Adsorption The constituent that undergoes adsorption is referred to as the adsorbate and the solid into which the constituent is adsorbed in referred to as the adsorbent .During the adsorption process, dissolved species are transported into the porous solid adsorbent granule by diffusion and are then adsorbed onto the extensive inner surface of the adsorbent. Dissolved species are concentrated on the solid surface by chemical reaction (chemisorption) or physical attraction (physical adsorption) to the surface. Physical adsorption is a rapid process caused by non-specific binding mechanisms such as van der Waals forces. Physical adsorption is reversible, that is, the adsorbate desorbs in response to a decrease in solution concentration. Physical adsorption is the most common mechanism by which adsorbates are removed in water treatment (Richardson, 1989). The Physical adsorption process is exothermic with a heat of adsorption that is typically 4 to 40 KJ / mole (Graham, 1959; Waadalla, 2006). Chemisorption is more specific because a chemical reaction occurs that entails the transfer of electrons adsorbent and adsorbate and a chemical bond with the surface can occur. The heat of adsorption for chemisorption is typically above 200 KJ/mole (Treybel, 1981).Chemisorption is usually not reversible, and adsorption if it occurs, is accompanied by a chemical change in the adsorbate (Sawyer and McCarty, 1978). The commonly referred to as “irreversible adsorption” is chemisorption because the adsorbate is chemically bonded to the surface. While physical adsorption and chemisorption can be distinguished easily at their extremes, some cases fall between the two, as a highly unequal sharing of electrons may not be distinguishable from the high degree of distortion of and electron cloud that occurs with physical adsorption (Kipling, 1965; Satterfield, 1980; Adamson, 1982). 22


Table 2-4 shows a comparison of the physical and chemical adsorption (Crittinden, 2005).

Table 2-4, Comparison of adsorption mechanisms between physical adsorption and chemisorption (Crittinden, 2005)

Parameter

Physical Adsorption

Chemisorption

Use of water treatment

Most common type of adsorption mechanism

Rare in water treatment

Process speed

Limited by mass Transfer

Variable

Type of bonding

Non specific binding Mechanisms such as Vander Waals forces Vapour condensation

Specific exchange of electrons chemical bond at surface

Type of reaction

Reversible, exothermic

Typical non Reversible Exothermic

Heat of adsorption

4-40 KJ/mole

>200 KJ/mole

2.7 Fundamental of Adsorption For the adsorption of a solute onto the porous surface of an adsorbent, the following steps are required (Seader and Henly, 1999): • External (interphase) mass transfer of the solute from the bulk fluid by convection, through a thin film or boundary layer surrounding the adsorbent, to the outer solid surface of the adsorbent (film diffusion). • Internal (interphase) mass transfer of the solute by pore diffusion from the outer surface of the adsorbent to the inner surface of the internal porous structure. • Surface diffusion along the porous surface. • Adsorption of the solute onto the porous surface. This involves the attachment of solute (adsorbate) to the adsorbent at an available adsorption site.

23


Because adsorption occurs in series of steps, the slowest step in the series is defined as the rate limiting step. In general if physical adsorption is the principal method of adsorption one of the diffusion transport steps will be the rate limiting because the rate of physical adsorption is rapid. Where chemical adsorption is the principle of the adsorption, the adsorption step has often been observed to be the rate limiting (Tchobanoglous et al., 2002). Adsorption process proceeds through various mechanisms such as external mass transfer of solute onto adsorbent followed by interaparticle diffusion. Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules are adsorbed onto the surface is equal to the rate at which they are desorbed and then the capacity of the adsorbent has been reached (Kumar et al., 2005).

2.8 Types of Adsorbents Adsorbents may be classified according to porosity .Non – porous adsorbents are materials such as glass and steel beads, and clay. Their relatively small external adsorptive surface areas limit their commercial application. Porous adsorbents must have a high internal surface area which accessible to the components being removed from the fluid (Shahlaa, 2008). The high internal surface area of an adsorbent creates the high capacity needed for a successful separation or purification process. For practical applications, the range of internal surface area is normally restricted to about 300-1600 m2/g. Such porous adsorbents solids may be carbonaceous or inorganic in nature, synthetic or naturally occurring, such as activated carbon. Silica gel, alumina, bentonite, synthetic polymers, etc. (Crittenden and Thomas, 1998).

24


2.8.1 Alum Sludge Alum sludge is a waste material generated during the coagulation / sedimentation process in a drinking water treatment plant. Interestingly, it has been shown that adsorption is the main mechanism for P-removal during alum coagulation (Galarneau and Gehr, 1997). Alum sludge that is generated from drinking water treatment contains precipitated alum hydroxides and the contaminants that are specific to the raw water chemistry.

2.8.2 Activated Carbon Due to their high porosity, activated carbon is the most widely used materials for adsorption of chemical (Cooney, 1999). The adsorption process of a solute on an activated carbon takes place at the liquid- solid boundary; it is thus clearly a “heterogeneous” reaction, and the interfaces of the two phases represent a special environment under dimensional or topological constraints (Gaspard et al., 2006). Activated carbon, both in granular and powdered form, has unsupassed internal adsorptive surface area ranging between 800 and 1600 m2/g which is contained predominantly within micro pores in the 1 to 3 nm diameter.

25


2.9 Adsorption Equilibria 2.9.1 Adsorption Kinetics In a solid-liquid system, adsorption results in the removal of an adsorbate from solution and simultaneously the concentration of adsorbate at the surface of adsorbent increases. This process continues until the concentration of the adsorbate remaining in solution achieves a dynamic equilibrium with the concentration of adsorbate on the adsorbent surface. The time required for this stage to be completed is called "equilibrium time" for adsorption. When the equilibrium stage is reached, the adsorption process is assumed to be complete and no further removal of adsorbate takes place. The first rate equation developed by Lagergren (1898) has been used to study the kinetics of heavy metal adsorption (Jassim, 2008). The Lagergren model has the following form:

= (

−

)

….. 2.4

Where: k = equilibrium rate constant of adsorption (1/h) qe = amount of metal ion adsorbed at equilibrium (mg/g) qt = amount of metal ion adsorbed at any given time (h)

Integrating equation (2.4) for the boundary conditions t=0 to t=t and qt=0 to qt=qt, given (Ho and McKay 1999):

=

−

exp (− )

Which is the integrated rate low for a pseudo-first order reaction.

26


….. 2.5

Ho et al., (1996) have presented a pseudo-second order reaction rate model for the kinetics of adsorption of heavy metals on peat:

=

΄

+

….. 2.6

Where: k´ is the second-order reaction rate constant for adsorption (g/mg.h).

2.9.2 Adsorption Isotherm Models showing the relationship between the solid phase concentration of an adsorbate (or the amount of adsorbate adsorbed per unit weight of adsorbent) and the solution phase concentration of an adsorbate at equilibrium condition at a particular temperature is called as an "adsorption isotherm". Adsorption data can be described in many forms of adsorption isotherms. The most common forms of adsorption isotherms used in chemical-environmental engineering are the Langmuir, the Freundlich, and the Brunauer, Emmett and Teller (BET) (Weber 1972; Metcalf and Eddy 1979; Eckenfelder 1981; Benefield et al., 1982; Reynolds and Richards 1996). The Langmuir and BET models were derived from first principles in chemical engineering process to describe the adsorption of gases onto solids. The Freundlich isotherm is an empirical model. The Langmuir model describing adsorption can be described as:

Where:

=

….. 2.7

x = mass of solute adsorbed to the solid (mg) 27


m = mass of adsorbent used (g) Ce = concentration of solute in solution at equilibrium (mg/L) a = Langmuir constant; the amount of solute adsorbed per unit weight of an adsorbent in forming a complete monolayer (L/mg) b = Langmuir constant (mg/g) Assumptions made in developing the Langmuir model are as follows (Weber 1972): (a) The maximum adsorption corresponds to a saturated monolayer of solute molecules on the adsorbent surface. (b) The energy of adsorbent is constant. (c) There is no transmigration of adsorbate in the plane of the surface. (d) The adsorption is reversible. The Langmuir isotherm model applies strictly to homogeneous surfaces. Weber and DiGiano (1996) found that the Langmuir model can be used in describing adsorption of an adsorbate on locally homogeneous sites on generally heterogeneous adsorbents. Hall et al., (1966) found that the Langmuir constant (b) can be expressed in terms of a dimensionless constant, separation factor or equilibrium parameter (R). The separation factor (R) can be described by the following relationship:

=

….. 2.8

Where: Co is the initial concentration of adsorbate (mg/L).

28


The separation factor (R) provides important information about the nature of adsorption process as indicated in table 2-5. Figure 2.2 shows the shapes of various isotherms for the above four sets of conditions defined by Weber and Chakravorti (1974). A convex-shapes isotherm in which the equilibrium solid phase concentration of an adsorbate increases sharply from a low value to a high one shows a favorable adsorption process and is called "Type I isotherm". A concave-shaped isotherm is an indication of an unfavorable adsorption process and is called "Type III isotherm". If the equilibrium solid phase concentration of an adsorbate increases linearly with equilibrium concentration of an adsorbate in the liquid phase, the isotherm is termed linear or "Type II isotherm". The Freundlich isotherm is an empirical model and was developed for heterogeneous surfaces. The Freundlich adsorption model is of the form (Weber 1972):

=

….. 2.9

Where: k = Freundlich equilibrium constant indicative of adsorptive capacity n = Freundlich constant indicative of adsorption intensity. Combination of Langmuir-Freundlich Isotherm Model, i.e the Sips model for single component adsorption (Sips, 1984) is:

29


=

….. 2.10

The BET isotherm model is of the form:

=

(

)

(

….. 2.11

)

Where: A = a constant describing energy interaction between the solute and the adsorbent surface. Xm = amount of solute adsorbed in forming a complete monolayer (M/M). C = concentration of solute in solution at equilibrium. Cs = saturation concentration of solute in solution. The BET isotherm model has been developed on the following assumptions (Weber 1972): (a) The model describes multilayer adsorption at the adsorbent surface. (b) The Langmuir model applies to each layer. (c) A given layer need not complete formation prior to initiation of subsequent layers.

30


Table 2-5, Use of the separation factor as an indication of the nature of

Value of R

Nature of adsorption process

R>1

Unfavorable

R=1

Linear

0