Preparation, characterization and application of

Fuel Processing Technology 149 (2016) 75–85

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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc

Research article

Preparation, characterization and application of polystyrene based activated carbons for Ni(II) removal from aqueous solution L. Gonsalvesh a,⁎, S.P. Marinov a, G. Gryglewicz b, R.Carleer c, J.Yperman c a b c

Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Department of Polymer and Carbonaceous Materials, Faculty of Chemistry, Wrocław University of Technology, Gdańska 7/9, 50-344 Wrocław, Poland Research Group of Applied and Analytical Chemistry, CMK, Hasselt University, Agoralaan — gebouw D, B-3590 Diepenbeek, Belgium

a r t i c l e

i n f o

Article history: Received 9 September 2015 Received in revised form 21 March 2016 Accepted 23 March 2016 Available online xxxx Keywords: Polystyrene waste Activated carbon Nickel Adsorption Kinetics

a b s t r a c t The production of activated carbon from polystyrene waste is tested in order to limit its negative environmental impact through conversion to value added products. For this purpose modification of the precursor, slow pyrolysis and subsequent activations, i.e. high temperature steam activation and low temperature air oxidation, are applied. The physical/chemical properties as well as adsorption capacities of obtained activated carbons (ACs) towards Ni(II) removal in aqueous solutions are explored. Steam activated carbon S-ACMPS performs superior in Ni(II) removal at applied circumstances. Ni(II) adsorption by this AC has been investigated using different process parameters and occurs through cation exchange mechanism optimal at pH range of initial solution of 4–8. Several reaction based kinetic models, i.e. pseudo-first, pseudo-second and Elovich models, and intra-particle diffusion model, are applied on experimental data. The adsorption kinetics of Ni(II) is best approximated by a pseudo second-order model. The equilibrium adsorption data best fits the Langmuir adsorption isotherm. Calculated maximum adsorption capacity for S-ACMPS is 40.8 mg g−1. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Activated carbons (ACs) are highly effective adsorbents with wide range of applications that are generally produced through pyrolysis and subsequent physical or chemical activation of different materials with high carbon and low inorganic content. However, due to the high production cost, ACs tend to be more expensive than other adsorbents and their widespread application is somewhat limited. This instigated a growing interest into production of low cost activated carbons through the usage of low-cost raw materials that are economically attractive and at the same time show similar or even better performance than the conventional ones. Therefore, cheaper and common precursors as lignocellulosic biomasses have been widely tested for ACs preparation [1]. Recently, a large number of studies are dealing with the preparation of ACs from various polymeric wastes as well [2–4]. These materials are successfully used for the production of high yield of ACs characterized by low ash content, high adsorption capacity and considerable mechanical strength. Thermoplastic polymers, i.e. polypropylene, polyethylene, polyvinylchloride, polystyrene, polyamide, etc., are the major constituents of municipal solid waste. More than 25 million tons of plastic waste ⁎ Corresponding author at: Central Scientific Research Laboratory, Assen Zlatarov University, Yakimov Str. 1, Burgas, Bulgaria. E-mail address: [email protected] (L. Gonsalvesh).

http://dx.doi.org/10.1016/j.fuproc.2016.03.024 0378-3820/© 2016 Elsevier B.V. All rights reserved.

is annually generated in the region of European countries [5]. This creates significant ecological concern since the degradation of plastic waste on a landfill is an extremely slow process, ongoing for centuries. Consequently, the use of these waste materials for higher-value products preparation such as fuels, carbon nanotubes, and porous carbons is very attractive to decrease the negative impact on the environment and the costs of waste disposal or treatment. Polystyrene (PS) is a petroleum-based plastic which is available as a solid or foamed. Several papers have been published in recent years on ACs preparation from “pure” PS wastes or their blends with an additional carbon source [6–9] as well as from polystyrene-based macroreticular ion-exchange resin spheres (copolymer of polystyrene and divinylbenzene) [10–11]. Although these studies focus on physical and chemical characterization of obtained ACs, information concerning ACs adsorption efficiency towards heavy metals removal is rather scarce. Water polluted by heavy metals can be problematic due to their stability, mobility and toxicity. A number of technologies have been used to remove heavy metals, i.e. chemical precipitation, ion exchange, membrane separation, flotation, electrocoagulation, etc. from wastewaters. However, most of them suffer from disadvantages such as incomplete removal, expensive equipment/reagent usage, production of toxic sludge requiring disposal, and long treatment time. [12]. An alternative and attractive choice for heavy metal removal from aqueous solutions appears to be their adsorption since it is considered as a simple,

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L. Gonsalvesh et al. / Fuel Processing Technology 149 (2016) 75–85

relatively low-cost and effective method. The main limitation for the industrial application of this process is the high cost of the commercial available adsorbents. Therefore, the production of low cost adsorbents with specific characteristics for heavy metals removal is promising as can been seen from the number of publications in the last decade. Among the group of heavy metals, Ni(II) have widespread application and is released in waste waters of various industries like metallurgical, chemical and refractories [13]. In view of its toxic effects (highly mutagenic and carcinogenic) it is of extreme importance to treat Ni(II) polluted industrial effluents before discharging them in water bodies. Several papers examining Ni(II) adsorption by biomass based ACs have shown the effectiveness of these adsorbents towards Ni(II) removal from aqueous solutions [14–17]. However, there are missing references about Ni(II) removal from aqueous solution by ACs prepared from solid polystyrene waste. The aim of the present study is to produce ACs from solid PS waste suitable for heavy metals removal through appropriate procedure. To achieve this goal, pyrolysis of PS followed by different activation treatments, i.e. steam activation and air oxidation at low temperature, are tested. Additionally, the evaluation of physical and chemical properties as well as adsorption efficiencies and kinetic behavior of produced ACs with respect to Ni(II) removal from aqueous solutions is considered. 2. Materials and methods 2.1. Preparation of activated carbons Precursor material for ACs production is solid PS waste in the form of post-consumer solid plastics of disposable cutlery, cups, plates and containers for dairy products. A three-stage process, comprising i) modification of the raw PS (step 1), ii) carbonization or so called slow pyrolysis (step 2) and iii) physical activation (step 3), was carried out for ACs preparation. During the first step, initial PS material was cut into small pieces and heated up to the temperature of PS melting point (~240 °C) under continuous stirring and dropwise addition of concentrated H2SO4. After cooling, the obtained modified PS (denoted as MPS) was carbonized in a covered silica crucible under slow heating rate of 10 °C min−1 in an inert atmosphere of nitrogen until the desired pyrolysis temperature was reached. In our study a temperature of 600 °C, kept isothermal for 30 min, was applied for pyrolysis. Thus obtained char was denoted as C-MPS. Two procedures for physical activation of C-MPS were tested, i.e. low temperature air oxidation and high temperature steam activation. The steam activation of C-MPS (particle size in the range of b 1.5 mm to N0.8 mm) was carried out in a laboratory tubular stainless steel reactor. Process parameters included heating rate of 5 °C min−1 in an inert atmosphere up to activation temperatures of 750, 800, 850, 900 and 950 °C, hold isothermal for 30, 60, 90 and 120 min (activation time). When the activation temperature was reached, the inert atmosphere was switched to steam with a constant steam flow of 2 mL min−1. The low temperature air oxidation of C-MPS (particle size b0.2 mm) was performed in a quartz boat placed in a horizontal tubular furnace under flow of air. Several activation temperatures, i.e. 300, 350 and 400 °C, kept isothermal for 120 min were applied. 2.2. Characterization of MPS, C-MPS and ACs samples 2.2.1. Proximate and elemental analysis Proximate analysis was performed by TGA analysis using a DuPont Instruments 951 Thermogravimetric Analyzer according to Warne method [18]. The ultimate analysis, i.e. C, H, N, and S was performed with a Thermo Electron Flash EA1113 elemental analyzer. C, H, N and S can be determined simultaneously. The oxygen content was calculated by difference. The instrument was calibrated using 2,5-bis (5-tert-butylbenzoxazol-2-yl) thiophene provided from Thermo Electron. All samples were analyzed at least in duplicate.

2.2.2. Iodine number The surface activity of activated carbons towards iodine was determined applying the ASTM standard method [19]. 2.2.3. Surface area and pore size distribution A textural characterization was carried out by measuring nitrogen adsorption isotherms at 77 K on an automatic apparatus Autosorb sorption analyzer (Quantachrome) and then analyzed to obtain information about: (i) the surface area by the BET method (SBET) [20]; (ii) the total pore volume calculated from N2 adsorption at a relative pressure of 0.98; (iii) the micropore volume (VDR) by using the DubininRadushkevich equation up to p/po ≤ 0.15 as well as the average micropore diameter Lo [21]; and (iv) pore size distribution, i.e. micropore and mesopore volumes, obtained by applying the Quenched Solid Density Functional Theory (QSDFT) on N2 adsorption data [22]. 2.2.4. Surface oxygen-containing groups and pHpzc determination The content of oxygen-containing functional groups with acidic character on the carbon surface was determined applying the Boehm method by neutralization with bases of increasing strength, i.e. NaHCO3, Na2CO3, NaOH and sodium ethoxide, while the total number of basic sites was determined with 0.05 N HCl as described in [23]. pHpzc (pH at point of zero charge) of ACs was determined by mixing 250 mg of AC with 10 mL freshly boiled and cooled Milli-Q water (CO2 free water) following the experimental procedure described by Moreno-Castilla et al. [24]. The mixture was stirred with magnetic stirrer for 48 h at ambient temperature. The pH of the solution measured after 48 h can be considered as the pHpzc of ACs. 2.2.5. FTIR analysis Fourier Transform Infrared (FTIR) spectroscopy was used to analyze the surface functional groups of MPS and prepared ACs. The KBr pellet technique was applied. The IR spectra were recorded on Tenzor-27, Bruker Instrument by co-adding 250 scans in the range 400– 4000 cm−1 at resolution 2 cm−1. 2.3. Adsorption of Ni(II) 2.3.1. Adsorption experiments The adsorption experiments were performed in a thermal shaker at a temperature of 25 °C for a period of 24 h using 100 mL Erlenmeyer flasks containing a certain amount of investigated ACs (0.010–0.050 g) and 50 mL of Ni(II) solutions with concentration varying in the range of 1 to 150 mg L−1. The experimental stock solution containing 1000 mg L− 1 Ni(II) was prepared by dissolving analytical grade NiCl2·6H2O in Milli-Q water and stored under refrigeration. Aliquots of the stock solution were appropriately diluted with Milli-Q water to get experimental solutions of known concentration. The initial pH of the solutions was adjusted by adding 0.1 N HCl or 0.1 N NaOH and was not further altered during the adsorption experimental run. The exact Ni(II) concentrations of initial and equilibrium solutions were determined by Perkin Elmer Optima 3000 DV ICP-AES apparatus. The nickel adsorption capacities were calculated using the following equation: qe ¼

ðC 0 −C e ÞV m

ð1Þ

where qe is the adsorption capacity at equilibrium (mg g−1), Co and Ce are the Ni(II) initial and equilibrium concentrations (mg L−1), respectively, V is the volume of Ni(II) solutions (L) and m is the weight of AC (g). 2.3.2. Adsorption isotherm and kinetic models For adsorption process evaluation, different equations and models are developed. The adsorption isotherm model developed by Langmuir


[25] and expressed in Table 1 is one of the most widely used. It is based on (1) the assumption that only a single layer of molecules of adsorbate will adsorb to the adsorbent, (2) the immobility of the adsorbate after being adsorbed, and (3) equal enthalpy of adsorption for all molecules of adsorbate [26]. In the Langmuir equation, KL (L mg− 1) and qm (mg g−1) are coefficients related to the adsorption energy and the maximum adsorption capacity representing the mass of adsorbate adsorbed in a complete monolayer per mass of adsorbent, respectively. Another widely used isotherm model is the Freundlich model (Table 1). It is based on heterogeneous surface and adsorption heat. It applies to multilayer adsorption, with non-uniform distribution of adsorption heat and affinities over the heterogeneous surface [27]. The adsorbed amount is the summation of adsorption on all sites, with the first occupation of stronger binding sites, until adsorption energy exponentially decreased upon completion of adsorption process. The Freundlich parameters KF and 1/n are indicators of adsorption capacity and adsorption intensity, respectively. In order to analyze the adsorption kinetics, different models, i.e. Lagergren pseudo-first order, pseudo-second order, Elovich equation and intra-particle diffusion, were applied to experimental data as well. The pseudo-first order rate equation of Lagergren (Eq. (4), Table 1) and the pseudo-second order kinetic model (Eq. (5), Table 1) are the most widely applied for liquid adsorption studies [28–33]. In those equations qe and qt (mg g−1) are the adsorption capacities at equilibrium and at time t (min), respectively, while k1 and k2 are the rate constants of the pseudo-first order adsorption and pseudo-second order adsorption, respectively. Beside the correlation coefficient, a real measure of the validity of pseudo-first (Eq. (4)) and pseudo-second order (Eq. (5)) kinetic models is the comparison between the experimentally determined qe values and those obtained from the plots of log(qe-qt) vs. t and t/qt vs. t, respectively. Elovich's equation (Eq. (6)) is another rate equation based on the adsorption capacity which is used successfully to describe second order kinetics [31,33–36]. It assumes that the actual solid surfaces are energetically heterogeneous and that neither desorption nor interactions between the adsorbed species could significantly affect the kinetics of adsorption at low surface coverage. In this equation α and β represent the initial sorption rate and the desorption constant. When diffusion of adsorbate into adsorbent's pores is the slowest step and determines the overall rate of adsorption and the kinetics of adsorption process, the intra-particle diffusion model will be a valid model describing the experimental data. The intra-particle diffusion model proposed by Weber and Morris [28,30,32–33,35–37] is described in Table 1, where the C is the boundary layer thickness and kid is the intra-particle diffusion rate constant. Greater the value of C, the greater is the effect of the boundary layer on the adsorption process. The plot of qt vs t0.5 should yield a straight line with a slope equal to kid and an intercept equal to C.

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3. Results and discussion In general, thermoplastics, similarly to coking coals, are not appropriate raw material for ACs production unless they are pretreated, i.e. low temperature pre-oxidized [38]. The pre-oxidation treatment is required since it stabilizes the thermoplastic by creating cross-links within its structure and thus prevents the formation of fluid or pseudo-fluid phase and creates solid skeleton during subsequent carbonization. In our study, thermal pre-oxidation treatment of PS with concentrated sulfuric acid was applied since the last is an oxidative and dehydrating agent that promotes the formation of cross-linked carbon structures. This treatment usually involves formation of oxygen containing structures and oxygen consumption coinciding with polymerization and polycondensation reactions [39–40]. Furthermore this treatment should promote higher yields of char from PS appropriate for ACs preparation. Indeed, high yield of C-MPS char of about 50 wt.% is achieved due to applied modification and pyrolysis conditions. For comparison the char yield due to carbonization of not preliminary modified PS with sulfuric acid is about 7 wt.%. A widely spread perception is that the best ACs for positively charged species removal are carbons with acidic functional groups [38, 41]. Thus adsorbents with prevailing acidic surface are expected to be a better choice for the removal of positively charged Ni(II) ions. The development of acidic surface groups on AC surface can be carried out by low temperature activation in the presence of air as oxidant [42–43]. On the other side, ACs with well developed surface area and high presence of microporous can also be considered as favorable adsorbents for Ni(II) removal from aqueous solutions due to the small size of Ni(II) species (Ni metallic radius is 0.125 nm). Furthermore it is known that physical activation with steam at higher temperature produces activated carbons with high surface area and high microporous volume [44]. Therefore, both types of physical activation, i.e. low-temperature air oxidation and high temperature steam activation, were tested for production of ACs capable of Ni(II) removal. The first stage of our study is directed to the evaluation of the impact of the activation parameters, i.e. activation temperature and activation time, on the surface area of ACs described by iodine number and on the ACs yields. Fig. 1A and B show iodine adsorption capacities and yields of ACs obtained through steam activation versus activation temperature and activation time, respectively. It can be revealed (Fig. 1A) that at a constant activation time of 30 min iodine adsorption capacities increase continuously from 650 to 970 mg g− 1 with increasing activation temperature in the range of 750–900 °C and then decrease to 950 mg g− 1 at activation temperature of 950 °C. ACs yields, calculated on the base of C-MPS sample in wt.%, are also influenced by an increase in activation temperature. Yield decreases from 70 to 25 wt.% with the increasing activation temperature.

Table 1 Adsorption models. Model Isotherms models Langmuir

Non-linear form

Linear form

m KL C e qe ¼ q1þK L Ce

Ce qe

¼ q1 Ce þ KL1q m

Eq. No

Plots

Parameters

(2)

Ce qe

KL ; L mg−1 qm ; mg g−1

m

vs Ce

Freundlich

qe = KFC1/n e

; logqe ¼ n1 ; logCe þ ; logK F

(3)

log qe vs log Ce

K F ; L1=n mg1−1=n g−1 1=n

Kinetic models Pseudo-first order

qt = qe[1 + exp(−k1t)]

k1 ; logðqe −qt Þ ¼ ; logqe − 2:303 t

(4)

log(qe − qt) vs t

(5)

t qt

qe;cal ; mg g−1 k1 ; ; min−1 qe;cal ; mg g−1 k2 ; g mg−1 ; min−1

Pseudo-second order

t qt

1 qt ¼ β1 ; ln ðαβÞ þ β1 ; ln ðt þ αβ Þ

qt = β ln(αβ) + β ln t

(6)

qt vs ln t

qt = kidt0.5 + C

(7)

qt vs t0.5

2

e

Elovich Intra-particle diffusion (Weber and Morris model)

¼ k 1q2 þ qt

k2 qe t qt ¼ 1þk 2q t

2 e

e

vs t

α; g mg−1 ; min−1 β; g mg−1 kid, mg g−1 min−1/2

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Fig. 2. Influence of activation temperature at a constant activation time of 120 min during low-temperature air oxidation.

3.1. Characterization of ACs Characteristic data of samples under study are gathered in Table 2. From proximate analysis data the relatively high ash content, especially in the case of S-ACMPS carbon, is remarkable. The increase in the ash content of ACs (relative to the final weight of AC) compared to C-MPS and MPS is due to the net carbon loss occurring as a result of the activation process. As mentioned, this effect is superior in the case of steam activation. As expected for AC samples, the content of Cfix is also considerably high. With regard to ultimate characteristics, a significant increase in C/H value of S-ACMPS is observed. This is due to polymerization and condensation processes through dehydrogenation of cyclic and

Fig. 1. Influence of process parameters during high temperature steam activation: A) different activation temperatures at constant activation time of 30 min; and B) different activation times at activation temperature of 850 °C.

Nevertheless, a good compromise between sufficient AC yield and a well developed surface may be considered to be reached at 850 °C at which the AC yield is 48 wt.% and iodine adsorption capacity is 920 mg g− 1. Therefore the influence of activation time on AC yields and surface area is monitored at an activation temperature of 850 °C (Fig. 1B). The iodine adsorption capacity increases from 920 to 1000 mg g− 1 with increasing activation time in the range of 30–120 min, while AC yields decrease from 48 wt.% to 33 wt.%. Apparently the influence of activation time at a constant activation temperature is less pronounced compared to the influence of activation temperature at a constant activation time. Thus, produced AC by steam activation at 850 °C for 30 min with iodine adsorption capacity of 920 mg g− 1 and a yield of 48 wt.% is considered as appropriate and is subjected for an in depth study. This AC is denoted as S-ACMPS. In Fig. 2 the effect of activation temperature on the yields and iodine adsorption capacities of ACs obtained through low-temperature air oxidation for 120 min is visualized. It is obvious that an increase in activation temperature in the range of 300–400 °C does not significantly alter the iodine adsorption capacity. The AC yield decreases slightly with a temperature increase to 350 °C but rather significantly when temperature increases to 400 °C. However, the AC activated at 350 °C for 120 min through low-temperature air oxidation approach and characterized with iodine adsorption capacity of 326 mg g− 1 and yield of 92 wt.% is chosen for a deeper study. This AC is denoted as A-ACMPS.

Table 2 Characteristics of the samples under consideration. Parameter

MPS

C-MPS

A-ACMPS

S-ACMPS

Proximate analysis (wt.%) Wad 0.8 3.0 Ashdb db 85.9 VM db 11.1 Cfix

0.9 6.1 6.6 87.4

1.8 6.7 11.4 81.9

1.4 12.9 5.0 82.2

Ultimate analysis (wt.%, daf) 89.1 Cdaf H 7.6 N 0.1 S 1.6 1.6 Odiff C/H 1.0 … pHPZC

84.0 2.7 0.2 2.4 10.7 2.6 …

81.4 2.5 0.2 2.9 13.0 2.7 5.6

85.8 1.2 0.2 0.8 12.0 6.0 10.6

Acidic groups (mequiv·g−1) Carboxyls … Lactones … Phenols … Carbonyls … Total acidic … Total basic …

… … … … … …

1.01 0.06 0.43 1.64 3.14 0.97

0.50 1.00 0.40 1.68 2.78 3.01

Porous texture parameters … SBET, m2 g−1 … VT, cm3 g−1 3 −1 … VDR, cm g … Vmes, cm3 g−1 … VDR/VT … Lo

267 0.166 0.107 0.059 0.64 1.42

567 0.343 0.208 0.135 0.61 0.74

842 0.682 0.314 0.368 0.46 0.95

MPS - modified PS; C-MPS - carbonized MPS; S-ACMPS - activated by steam C-MPS; A-ACMPS - activated by air C-MPS. ad Air dried. db Dry basis. daf Dry, ash free basis.


aromatic molecules, which results in the formation of larger fused and condensed polycyclic aromatic planar structures. Sulphur located in samples under consideration is related to sulphuric acid treatment. Its final amount is reduced during steam activation, acting as desulphurization process. For both ACs considerably high oxygen contents are registered. Since surface functional groups can significantly affect the adsorption process of heavy metals it is of relevance to investigate the surface chemistry of ACs as well. Therefore, FTIR spectroscopy was applied. Combined IR spectra of S-ACMPS and A-ACMPS are illustrated in Fig. 3. For comparison the FTIR spectrum of MPS sample is shown as well. Typical bands of MPS correspond to the stretching of PS [45]: i) stretching vibrations of Csp2-H from aromatic groups at 3081 cm−1 and 3026 cm−1; ii) asymmetric and symmetric stretching of methylene CH2 groups at 2924 cm− 1 and 2850 cm−1; iii) strong bands around 1600 cm−1, 1493 cm−1 and 1452 cm−1, due to C_C stretching of an aromatic ring; and iv) peaks round 900 cm−1, 757 cm−1 and 698 cm−1 indicate C\\H out of plane bending of a mono-substituted aromatic ring. The structural changes that appear due to subsequent carbonization and activation can be traced in the IR spectra of the ACs. It is clear that oxygen containing groups appear on ACs surface: i) the strong stretching at 3440 cm−1 arises from the OH– stretching vibration of alcohols, phenols or carboxylic acids; and ii) strong bands at 1700 cm−1 is due to stretching vibration corresponding to carbonyl and carboxyl groups [46]. This is in accordance with ultimate analysis. However, the last mentioned bands are more dominant for A-ACMPS sample indicating the stronger oxidative action of low-temperature air oxidation. Absorption in the range of 1050–1300 cm−1 is also observed especially in the spectrum of S-ACMPS sample. It could also be attributed to the presence of oxygen containing structures, i.e. phenoxy or C\\O in ethers (aliphatic and alicyclic compounds) [40]. Additional information for surface chemistry can be obtained from determination of surface oxygen containing groups through Boehm titration and pHpzc values of ACs. The pH of aqueous slurry of ACs (or pHpzc) constitutes a useful indicator of the nature of the functionalities present on the carbon surface. In solution, acidic groups of the carbon surface lose their protons and the AC surface becomes negatively charged. On the other hand, basic groups of the carbon surface bind protons and AC becomes positively charged. The results for surface oxygen containing groups and pHpzc values of S-ACMPS and A-ACMPS (Table 2) indicate their overall basic and acidic surface nature, respectively. The expected acidity of A-ACMPS carbon, as confirmed also from FTIR study (strong bands at 3440 cm− 1 and 1700 cm−1), can be assigned to an enhanced amount of acidic surface groups, i.e. carboxyl, carbonyl,

Fig. 3. FTIR spectra of MPS, S-ACMPS and A-ACMPS samples.

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phenols, lactones, with a prevalence of carboxyl and carbonyl groups. The basic character of S-ACMPS sample can be attributed not only to its higher amount of basic groups, i.e. pyrones, ether, furanic and/or chromene-like structures, but probably also to its mineral matter. Actually, very little attention has been paid to the inorganic matter usually found in carbon materials, and to its contribution on the acidic/basic character of the carbon. From the point of view of the basicity of carbons, the effect of inorganic impurities has been usually inferred from indirect experiments; this is through reducing the ash content by acid washing procedures and it has been reported that the pH of basic carbons is significantly reduced after a substantial drop in the ash content [38]. The N2 adsorption/desorption isotherms of the studied samples are visualized on Fig. 4A. According to IUPAC classification N2 isotherms of A-ACMPS and S-ACMPS samples represent a type I\\IV hybrid shape. Type I isotherm is usually considered to be indicative of adsorption in micropores or monolayer adsorption due to the strong adsorbentadsorbate interactions (chemisorption) [38]. In the case of nonpolar gases, i.e N2, chemisorption is unlikely and therefore type I reflects usually adsorption on microporous solids. However, when a type I isotherm does not level off below the relative pressure of 0.1, the sample is likely to exhibit an appreciable amount of mesopores. This is the case for both studied samples. At high relative pressure a hysteresis is observed in the isotherms, they resembling therefore IV shape as well. The appearance of H4 type hysteresis loop in N2 adsorption-desorption isotherm of A-ACMPS can be mainly attributed to differences in adsorption mechanism by capillary condensation and desorption phenomena by capillary evaporation in narrow slit-like mesopores. H3 type hysteresis loop observed in N2 adsorption-desorption isotherm of S-ACMPS is typical for materials comprised of aggregates (loose assemblages) of plate-like particles forming slit-like pores. Thus, it appears that mesoporous texture of both ACs is somewhat different. The examination of porous texture parameters determined by N2 sorption at 77 K (Table 2) reveals that S-ACMPS is characterized by a larger surface area (SBET) and total pore volume (VT) in contrast to A-ACMPS, due to the stronger porous texture development action of water vapor activation. SBET of SACMPS is about 842 m2 g− 1 while SBET of A-ACMPS amounts to 567 m2 g−1. Pore size distribution visualized in Fig. 4B shows that the size of micropores of both samples also varies although slightly. This difference in micropore sizes of S-ACMPS and A-ACMPS is also reflected in the calculated average micropore diameter, Lo, which is higher in the case of S-ACMPS sample (see Table 2). 3.2. Adsorption of Ni(II) The adsorption of metal cations by ACs from aqueous solutions generally depends on the physicochemical characteristics of the carbon surface, i.e. surface area, pore-size distribution, electrokinetic properties, and the chemical structure of the carbon surface, as well as on the nature of the metal ions in the solution. Most of the as-received ACs are hydrophobic and thus more suitable for the adsorption of neutral or nonpolar organic compounds and show little affinity for polar and ionic pollutants. However, AC surfaces are almost invariably associated with a certain amount of heteroatoms, i.e. oxygen, hydrogen, nitrogen, halogens, and sulphur. These heteroatoms originate either from the source raw material and become a part of the chemical structure during carbonization and activation, or are associated with additional/subsequent treatments. Although all of these surface groups influence the adsorption of inorganics from aqueous phase, the carbon-oxygen surface groups are by far the most important chemical structures that are present on all activated carbons and influence the adsorption characteristics under all practical situations [47]. The amount of these carbon-oxygen surface groups can be enhanced significantly by surface oxidation of carbons, the more important oxidizing agents being, nitric acid, ammonium persulfate, hydrogen peroxide, and sodium hypochlorite in aqueous solutions, and oxygen or air at temperatures of 300 to 400 °C. These

80


Fig. 4. Porous texture characteristics of prepared ACs: A) N2 adsorption/desorption isotherms; B) pore size distribution.

oxidative treatments result in the formation of different types of carbon-oxygen surface groups [38,41,47]: i) acidic surface groups which are polar, render the carbon surface hydrophilic and enhance the properties of the carbons towards removal of polar and ionic pollutants, i.e. cations; ii) basic surface groups that have been postulated as pyrone and chromene structures demonstrating somewhat anion exchange properties; and iii) nonacidic, neutral surface groups that have been postulated as quinones. 3.2.1. Influence of initial Ni(II) solution pH The removal of heavy metals from aqueous solution by ACs is a complex process that requires a thorough understanding of the speciation of heavy metals in solution under various conditions and the role of ACs properties in the removal of these species. In this sense, pH of solution is a crucial factor controlling adsorption from solution since it affects surface properties of ACs as well as the state and stability of different species of heavy metals in the solution. In aqueous solution Ni(II) exists as four main species, i.e. Ni2 +, Ni(OH)+, Ni(OH)2(s), Ni(OH)− 3 , which distribution depends on pH (Fig. 5). In our study for the pH range of initial solution, i.e. pH 2–8, the dominant Ni(II) form is Ni2+. In general, Ni2+ can be adsorbed through direct surface complexation or cation exchange mechanisms depending on AC surface properties and its charge in aqueous solution [14,48–50]. As mentioned earlier an important factor to be considered during adsorption of different contaminants by ACs is the pHpzc, or in other words the pH at which the external surface

Fig. 5. Speciation diagram for Ni(II) as a function of pH adopted from Krishnan et al. [14].

charge is zero. This characteristic reveals the surface charge of the activated carbon at different pH of aqueous solutions. As a general rule, at pH of solution below pHpzc, the carbon surface is positively charged due to protonation of AC surface groups, and the number of positive charges gradually decreases with pH increase. It has been observed that π-electron system of the basal planes of AC is also sufficiently basic to bind protons from aqueous solutions as well [41]. The pHpzc values of S-ACMPS and A-ACMPS carbons are 10.6 and 5.6, respectively. This means that, at the studied pH range from 2 to 8 of initial solutions the surface S-ACMPS is predominantly positively charged and Ni2+ adsorption can occur through a cation exchange mechanism. With regards to A-ACMPS, at a pH of initial solution below 5.6 this AC is positively charged and adsorption of Ni2+ will appear through cation exchange mechanism, while at a pH of initial solution above 5.6, it will be loaded with negative charge which will induce an electrostatic attraction of Ni2+ followed by a direct surface complexation. The effect of initial solution pH in the range of 2 to 8 on the adsorption of Ni(II) by studied ACs is shown in Fig. 6. Adsorption at pH above 8.0 was not carried out to avoid the interference from metal precipitation in solution. The following Ni2+ adsorption behavior by S-ACMPS

Fig. 6. Influence of initial solution pH on Ni(II) adsorption. Experimental conditions: m = 0.030 g, V = 50 mL, Co = 20 mg L−1, T = 25 °C, 24 h.


can be distinguished: i) significant increase of adsorption up to pH 4; and ii) gradual and slight increase of adsorption in the pH range 4–8. Since S-ACMPS is positively charged at the studied pH range, the cation exchange mechanism between counter ions H+ and Ni2+ followed by complexation of Ni2 + seems to be the actual occurring adsorption mechanism. As the adsorption of Ni2+ does not occur at strongly acidic pH 2 and occurs weakly at pH 3, the binding of protons is more favorable than the complexation of Ni(II) and S-ACMPS acts rather as a weakly acidic cation exchanger [51]. As the pHpzc is clearly basic, basic functional groups have small pKb value and their corresponding acidic forms high pKa values, or thus very weak acids. The increase in dissociation with pH increase, because of a neutralization process, is actually the explanation of the increase of Ni(II) adsorption in the solution pH range of 3–8. Moreover, with pH increase the competition between H+ and Ni2+ turns in favor of Ni2+. The range of initial solution pH favorable for Ni(II) adsorption is between 4 and 8, as is clearly demonstrated in Fig. 6. In order to get insight in the mechanism of Ni(II) adsorption by S-ACMPS the pH of the system Ni(II) solution/S-ACMPS carbon during and after adsorption of Ni(II) is also traced. From the obtained result in Fig. 7 it can be seen that addition of S-ACMPS to the initial solution of Ni2+ at pH 5.0 leads to an increase in the pH of solution from 5.0 to 8.9 in the first 20 min due to protonation of AC surface and consumption of H+ from the initial solution. Thus, at pH around 8.9, besides Ni2 +, minor amounts of Ni(OH)+ and Ni(OH)2(s) species are possibly also present. However, as process proceeds, the pH of the system decreases down to 8.0 (± 0.1) due to H+ exchange reflecting Ni(II) adsorption, i.e as Ni2+ (dominant form) and as Ni(OH)+, within the ion exchange capacity of that S-ACMPS sorbent. Ni(II) adsorption behavior of A-ACMPS is also shown in Fig. 6. According to pHpzc value of 5.6 for this AC, it is clear that Ni(II) adsorption occurs through different mechanisms in the studied initial solution pH range. Adsorption of Ni(II) in the pH range of 2–5 is much weaker for A-ACMPS compared to S-ACMPS. In this pH range, A-ACMPS is rather positively charged and a repulsion of the positive charged Ni-ion occurs. In the range of initial solution pH of 6–8, A-ACMPS is loaded with a negative charge and adsorption occurs through electrostatic attraction followed directly by complexation of Ni(II) which increases with increasing initial solution pH. Nevertheless, the adsorption of Ni2+ by A-ACMPS is not that high as in the case of S-ACMPS. Its maximal Ni(II) removal efficiency is about 11 mg g−1, being much lower compared to S-ACMPS demonstrating a Ni(II) adsorption capacity of about 32 mg g−1. This can be linked to the different nature of A-ACMPS organic functional groups in terms of their stronger acidity and weaker

complexation ability as well as to A-ACMPS smaller surface area, different porous texture and eventual accessibility towards the surface organic groups. However, when dealing with the adsorption of a heavy metal, the surface chemical parameters are expected to exert a higher influence than textural ones [52–54]. Both ACs differ in mineral matter content, thus this could also be an option for the difference in performance. Our unpublished results on adsorption of Ni(II) by manure based ACs having about 40% ash content as received revealed that adsorption of Ni(II) was drastically decreased upon rather mild de-ashing with diluted HCl acid. 3.2.2. Influence of carbon dosage and ion concentration Fig. 8A, B and C reveal that the adsorption of Ni(II) by S-ACMPS strongly depends on adsorbent dosage and initial ion concentration. In Fig. 8A can be seen that the adsorption capacity of S-ACMPS towards Ni(II) gradually decreases with carbon dosage increase. This can be explained by the fact that more adsorption sides are available for less Niions present, leading to a smaller amount of Ni(II) adsorbed per gram of adsorbent. In Fig. 8B the total amount of Ni(II) adsorbed as a function of the amount of used AC is also presented. It can be seen that for C0 = 150 mg L− 1, a steady increase of adsorbed Ni(II) by S-ACMPS is observed. In the case of the C0 = 20 mg L− 1 this trend is not found, it goes to a maximum and the total adsorbed Ni(II) is also much lower than in the case of the higher initial Ni(II) solution concentration. This can be dedicated to the equilibrium describing the exchange mechanism between H+ and Ni2 +. High Ni(II) concentration is in favor of Ni-ions adsorption, in the case of low Ni(II) concentration Ni-ions are not able to replace H+ bound on the AC. From Fig. 8C, describing the influence of initial ion concentration, it is revealed that the amount of adsorbed Ni(II) per unit weight of S-ACMPS increases by increasing the initial concentration. This is related to the fact that at low Ni(II) concentrations, the adsorption sites are not completely occupied. The increase of initial Ni(II) concentration results in higher probability for exchange of Ni(II) with H+ respectively and in higher occupation of active sites and thus in higher amount of adsorbed Ni(II). When the initial Ni(II) concentration is further increased, the adsorbed amount of Ni(II) reaches a plateau, i.e. maximum occupation of the active sites on the surface of the adsorbent is reached and further adsorption of Ni(II) is strongly hindered. 3.2.3. Equilibrium study The isotherms, i.e experimental and theoretical (Langmuir and Freundlich), for Ni(II) adsorption by S-ACMPS are shown in Fig. 9. According to Giles classification the isotherm shape can be attributed to H-type or so called “high affinity” type indicating strong interaction between Ni(II) ions and AC surface at low concentration. Langmuir and Freundlich constants of Ni(II) adsorption isotherms together with model correlation coefficients are also calculated and presented in Table 3. The Langmuir equation for the applied adsorption processes by S-ACMPS gives a somewhat better fit than Freundlich. Calculated Langmuir monolayer adsorption capacity qm is 40.82 mg g−1. An information for Ni(II) adsorption process favorability can be obtained from the dimensionless separation factor, i.e. RL, which is defined in the literature as [27,55–56]:

RL ¼

Fig. 7. pH change during adsorption of Ni(II) by S-ACMPS. Experimental conditions: initial Ni(II) solution pH = 5, m = 0.030 g, V = 50 mL, Co = 20 mg L−1, T = 25 °C, 24 h.

81

2 1 þ K L Co

ð8Þ

where C0 is the highest initial concentration of Ni(II). The values of RL determine feasibility and favorability of adsorption processes. The magnitude of RL indicates the type of Langmuir isotherm: irreversible (RL = 0), favorable (0 b RL b 1), linear (RL = 1), unfavorable (RL N 1). The calculated value of RL, shown in Table 3, indicates that Ni(II) adsorption on S-ACMPS is strongly favorable.

82


Fig. 9. Equilibrium Ni(II) adsorption isotherms for S-ACMPS sample, i.e. experimental and theoretical. Experimental conditions: initial Ni(II) solution pH = 5.5, m = 0.030 g, V = 50 mL, Co = 10–150 mg L−1, T = 25 °C, 24 h.

surface of the adsorbent; ii) external diffusion or transport of adsorbate from the bulk of solution across the stationary layer of water, called the hydrodynamic boundary layer, liquid film or external film, that surrounds the adsorbent particles; iii) intra-particle (internal) diffusion involving transfer of adsorbate to sites within AC particles; and iv) interaction with AC surface atoms leading to chemisorption or physisorption. Any of these steps might be the slowest determining the overall rate of adsorption. However, many experimental AC sorption systems, i.e. our experimental set-up, are designed to eliminate or limit the effect of bulk solution transport by rapid mixing. Thus, the AC adsorption is controlled by a chemical process and (or) by a diffusion process inside the AC. As a general rule, if the equilibrium is attained within three hours the adsorption process is controlled by a chemical process and above twenty-four hour it is diffusion controlled [58]. Either or both processes might be rate controlling in the three to twenty-four hour period. The adsorption of Ni(II) on S-ACMPS was studied as a function of contact time at the optimal initial solution pH and for two different initial concentrations. Corresponding results are visualized in Fig. 10. In both cases, i.e. at C0 = 20 mg L−1 and C0 = 40 mg L−1, Ni(II) adsorption rate by S-ACMPS is higher at the start of the experiments. The adsorption rapidly increases in the first 30 min, where about 75% and 68% of the equilibrium sorption capacities are obtained for C0 = 20 mg L−1 and C0 = 40 mg L−1, respectively. The very high initial adsorption rate is related to the fact that available AC surface area or active sites are larger compared to the density of Ni(II) ions. Then the adsorption slowly approaches to equilibrium (attained at the 8th hour) due to the decrease in bare surface/active site fraction and subsequent competition of Ni(II) ions for adsorption sites (in other words, saturation of the AC). Thus, the interaction is slowed down and the adsorption rate in this

Table 3 Langmuir and Freundlich parameters for adsorption of Ni(II) by S-ACMPS. Isotherm parameters Fig. 8. Influence of carbon dosage (A and B) and initial Ni(II) concentration (C) on Ni(II) adsorption by S-ACMPS. Experimental conditions: A, B) initial Ni(II) solution pH = 5.5, V = 50 mL, T = 25 °C, 24 h; C) initial Ni(II) solution pH = 5.5, m = 0.030 g, V = 50 mL, T = 25 °C, 24 h.

3.2.4. Kinetic study The mechanism of adsorption is generally considered to involve several steps [33,57]: i) bulk solution transport (advection) which is expressed as the transport of the adsorbate from the bulk to the external

Langmuir model KL L mg−1 qm mg g−1 R2 RL

0.84 40.82 0.9995 0.01

Freundlich model KF L1/n mg1−1/n g−1 n R2

29.83 15.29 0.9946


Fig. 10. Effect of contact time on adsorption of Ni(II) by S-ACMPS. Experimental conditions: initial Ni(II) solution pH = 5.5, m = 0.030 g, V = 50 mL, T = 25 °C.

range of contact times seems to become predominantly dependent on Ni(II) ions diffusion in AC interface. Apparently, the kinetics of Ni(II) adsorption by S-ACMPS might depend on chemical and diffusion processes. Therefore, several reaction based kinetic models, i.e. pseudo-first, pseudo-second and Elovich models, and diffusion based kinetic model, i.e. intra-particle diffusion model, are applied to experimental adsorption data. Estimated kinetic parameters as well as correlation coefficients of the kinetic models are presented in Table 4. It can be seen that the correlation coefficients for pseudo-first order kinetic model described by Lagergren equation are lower compared to correlation coefficients of pseudo-second order kinetic model. Additionally, the experimental qe(exp) values do not reflect the calculated ones. Obviously, Lagergren equations is not suitable for describing kinetic of Ni(II) adsorption by S-ACMPS. In our study, pseudo-second order kinetic model gives the best fits. This is suggested not only from the high correlation coefficients which are close to 1 in all cases but also from the good agreements (match) between the experimental qe(exp) and calculated ones. This means that the overall rate of Ni(II) adsorption is best approximated by a pseudo second-order model and might eventually be controlled by a chemical process through sharing electrons between adsorbent and adsorbate. However, this issue needs to be additionally confirmed and supported, for example by determining the energy of Ni(II) adsorption through Arrhenius plot. As expected k2 is found to depend on Ni(II)

Table 4 Parameters of kinetic modeling of adsorption of Ni(II) by S-ACMPS. Parameters −1

Pseudo-first order

Pseudo-second order

Elovich

Intra-particle diffusion

qe (exp) (mg g ) qe (cal) (mg g−1) k1 (x102) (min−1) R2 qe (cal) (mg g−1) k2 (x102) (g mg−1 min−1) h⁎ R2 qe (cal) (mg g−1) α (g mg−1 min−2) β (g mg−1) R2 C1 Kid1 (mg g−1 min-1/2) R21 Kid2 (mg g−1 min-1/2) R22

20 mg L−1

40 mg L−1

27.50 6.99 0.62 0.9505 27.47 0.27 203.78 0.9997 27.45 43.32 2.55 0.9672 16.43 0.75 0.9778 0.22 0.9782

30.42 7.94 0.6 0.9843 30.40 0.23 212.49 0.9994 31.25 5.13 3.45 0.9072 14.25 1.22 0.9652 0.22 0.8801

⁎ h - initial adsorption rate of pseudo-second order kinetic defined as: h=k2q2e .

83

initial concentration in the bulk phase and decreases with the increase in Ni(II) initial concentration. Indeed, the higher the initial adsorbate concentration the longer the time for reaching equilibrium, respectively k2 value decreases. Nevertheless, the initial adsorption rate, h, increases with increasing Ni(II) initial concentration. Elovich equation is also widely used to deduce adsorption kinetics. In our study, application of this equation to Ni(II) adsorption by SACMPS gives again lower correlation coefficients compared to pseudosecond order model. However, there is a good agreement between experimental qe(exp) and calculated ones indicating that adsorption might be explained by this model as well. This is expected since it is quantitatively proved that both the pseudo-second order and Elovich equations exhibit identical behavior when considering the values of the fractional surface coverage lower than about 0.7 [33]. The Elovich equation describes well chemical adsorption on highly heterogeneous adsorbents, but definite mechanism of interaction between adsorbate and adsorbent is uncertain [59]. Nevertheless, the good fit of this equation with the experimental data gives kind of an indication that Ni(II) ions are held rather strongly on ACs surfaces by chemisorptive bonds [59]. This is one more evidence that overall rate of Ni(II) adsorption might be controlled by chemical process. The intra-particle diffusion model was also tested in order to determine whether adsorption process is diffusion controlled. According to Weber-Morris, if the rate limiting step is governed only by intraparticle diffusion then the plot of qt versus t0.5 will give a single straight line with intercept C equal to zero. However, if adsorption is controlled by more than one mechanism, then plot of qt versus t0.5 will be multilinear. In case of adsorbents whose pore size range is extensive, including micro-, meso- and macropores, up to three linear sections have been observed [58]. In this case the first step (sharper section) could be attributed to boundary layer effect, the second step describes the adsorption stage where intra-particle diffusion is rate limiting, the third portion could be attributed to the final equilibrium stage. Fig. 11 represents the Weber-Morris equation modeling of Ni(II) adsorption by SACMPS at contact times ranging up to the 8th hour. As can be seen the plots are not linear over the whole time range, implying that more than one process affects the adsorption. The occurrence of an external boundary layer diffusion process can be deduced due to the fact that plots do not pass through the origin. Additionally, the thickness of the boundary layer decreases (although rather slightly) with increasing Ni(II) initial concentration. It can be assumed that intra-particle diffusion occurs in two stages: i) pore diffusion (or so termed mesopore diffusion), which is the molecular diffusion of solutes in fluid-filled pores; and ii) surface or solid diffusion (termed micropore diffusion) representing the diffusion of solutes along the adsorbent surface after adsorption takes place. Micropore diffusion only occurs if the surface attractive forces are not strong enough to prevent surface mobility of molecules [57]. It is most likely to be significant in porous adsorbents as ACs with a high surface area and narrow pores. Obviously this is the case in our study. As can be seen from Fig. 11 the slope of the linear part corresponding to diffusion in the small micropores is lower than that corresponding to diffusion in mesopores. This is an indication that intra-particle diffusion into the smaller micropores is the rate controlling step in this region of contact times. The influence of initial Ni(II) concentration on diffusion process could also be distinguished. The increase of initial Ni(II) concentration does not change the rate of Ni(II) diffusion into smaller micropores but increase the rate of Ni(II) diffusion into meso- and wider micropores. 4. Conclusions Our study demonstrated the production of ACs with sufficient yields and properties from solid PS waste through appropriate experimental strategy constituting precursor modification, pyrolysis and two different physical activation treatments. The produced ACs, using high temperature steam activation and low temperature air oxidation demonstrated

84


Fig. 11. Weber-Morris diffusion model for Ni(II) adsorption by S-ACMPS at Co = 20 mg L−1 (squares) and Co = 40 mg L−1 (triangles).

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