INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 2, No 4, Copyright 2010 All rights reserved Integrated Publishing Association

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1 INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 2, No 4, 2012 Copyright 2010 All rights reserved Integrated Publishing Association Research article ISSN Heavy metal adsorption by Ligand loaded granular activated carbon: Thermodynamics and kinetics Department of Chemical Engineering, Sinhgad College of Engineering, Vadgaon(Bk), Pune , Maharashtra, India doi: /ijes ABSTRACT This paper describes the adsorption of Cu metal ions from aqueous solutions by IQSA, [7- iodo-8-hydroxyquinoline-5-sulphonic acid] an adsorbed chelating species on F-400 granular activated carbon [GAC]. The optimum shaking speed, adsorbent mass, contact time and ph were determined and adsorption isotherms were obtained for Cu metal ions ranging from 25mg/l to 120mg/l. The adsorption process follows pseudo second order reaction kinetics as well as validated to Freundlich, Langmuir and D-R adsorption isotherms. The paper discusses the thermodynamic parameters of the adsorption process like Gibb s free energy, entropy and enthalpy. Our results demonstrate that the adsorption process was spontaneous and slightly exothermic under natural conditions. The maximum removal efficiency was found to be 95% for Cu [II] at ph 5.5. Keywords: Heavy metals, Adsorption Isotherms, Granular Activated Carbon and Ligands. Notations: q e = equilibrium ligand concentration on the GAC (moles/g) C 0, C e = initial and equilibrium concentration of ligand (moles/l) V= volume of solution (liters) d' = weight of GAC (g) q e = concentration of Cu ion on ligand loaded GAC (mg/l of ligand) C 0, C e = initial and equilibrium concentrations of Cu ion in solution (mg/l) d= weight of ligand on GAC (millimole) q =concentration of Cu on the GAC ligand system (mg/millimole of ligand) C t = concentration of Cu ion in solution present at any time t (mg/l) b, K= Langmuir constant with respect to sorption capacity and energy of adsorption. K, n = Freundlich constants; indicating the adsorption capacity and adsorbent intensity. ε = Polanyi potential = adsorption capacity of the sorbent (mg/g) = constant related to the adsorption energy (mol 2 /kj 2 ) R = gas constant (kj/k.mol) Received on March 2012 Published on May

2 T = temperature (K). q t = metal uptake per unit weight of GAC (mg g 1 ) at time t k 1 k 2 = rate constants of the first-order and pseudo-second-order kinetics equations. (min 1 ), (g mg 1 min 1 ) 1. Introduction Industrial complexes have now become focii of environmental pollution. The emphasis now is thus on removal of toxic polluting chemicals. The control of these toxic priority pollutants is to be based on the application of the best available technology economically achievable. Metals of particular concern in treatment of industrial wastewaters include Cu, Zn, Cd, Pb, Ni, Ag, Hg, Cr and Fe which are toxic to living organisms at fairly low concentrations and tend to either biologically magnify or accumulate in plant and animal systems. Thus, the disposal of heavy metals bearing wastes into water bodies or on land is of great concern due to their adverse effects. The metal processing, plating and metal finishing industries, e -wastes etc are sources of such metal wastes. In general, to be removed from wastewater, the metals must be precipitated or otherwise attached to an insoluble form through ion-exchange or adsorption. The focus on removal of copper was initiated as it is the most widely used non-ferrous metals in the industry. It is required as a trace element in the creation of RBC s and some enzymes but in high dosages it is highly toxic to humans and animals. Absorption of excess of Cu by man results in Wilson s disease, in which excess Cu is deposited in the brain, skin, liver, pancreas and myocardium. Consequently its removal from wastewater assumes importance. The treatment technologies more frequently cited for removal of heavy metals are carbon adsorption, wet oxidation, solvent extraction, precipitation, ultra filtration, reverse osmosis, ion-exchange, etc. (Kurniawan, 2006) GAC adsorption can be selective, cheap and relatively inert and the high surface area along with the ability to chemically regenerate and reuse makes it very useful. Huang using 8- hyrdroxyquinoline as an effective chelating agent improved Cu ion adsorption. (Huang, 1978) Natarajan et al. separated Cu ions from aqueous medium either alone or in admixtures with other divalent ions using GAC containing adsorbed 8-hydroxyquinoline-5-sulphonic acid (Deshmukh, 1989). Similarly, Liu et al. used activated carbon loaded with 8- hydroxyquinoline for recovery of vanadium. (Liu, 2000) Activated carbon can, therefore be used to enhance the adsorption capacity for metal ions in aqueous solutions by ligands adsorbed on the surface. For the present study activated carbon adsorption with adsorbed chelating species has been chosen as a viable method for removing heavy metals from wastewater. 2. Materials and method 2.1 Adsorbents and reagents F-400 Granular Activated Carbon gifted by Calgon Corporation Inc., Pittsburg, USA was used for the present study. The ligand chosen was IQSA, a derivative of oxine (E-Merck, India). The chelating properties of oxine are well documented in literature (Moeller, 1950). It was found to be more selective towards copper ions thus having an edge over EDTA as a complexing agent which would complex almost all metal ions in solution. Oxine behaves as a 2127

3 bidentate (N, O) univalent ligand to form chelates with several transition metals ions. The sulphonic acid derivatives have a hydrophilic character conferring to its metal complexes an unusually high water solubility (Bailer, 1956).Further the ligand has a crystalline nature and is easily soluble in hot water thus suitable for chemical analysis. The iodo derivative was specifically chosen for the present work to study the effect of a bulky substituent on the adsorption process. All the chemical reagents used in these studies were of analytical grade. Stock solutions (1000 mg/l) were prepared from BDH grade hydrated copper sulphate. These were then diluted with distilled water to obtain solutions used in the adsorption experiments. For analysis of Copper ions and the ligands a Systronics digital spectrophotometer type 166 was readily available in the lab. 2.2 Preparation and modification of adsorbent For the present work Calgon F-400 grade having a mesh size of 12x18 was easily amenable for experimentation. As per the ASTM method DD4749 (American Society for Testing and Materials, 1994), the particles that passed through 18 mesh size was finally chosen. The sample was washed several times in distilled water and the leachate was scanned in the UV- Vis Spectrophotometer showing no undesirable impurities. The sample was air dried and then placed in a dessicator. Weighing of the sample intermittently confirmed constant composition in about 10 days time. For modification of its surface IQSA, a derivative of oxine was used as a chelating agent. 2.3 Characterization of the adsorbent The GAC sample was subjected to both SEM and FTIR to have an idea about the basic structure of GAC and also identify the surface groups responsible for adsorption. The SEM studies were carried out on a Cambridge stereo scan S 250 MK 111 model at RSIC centre, Nagpur, while the FTIR was recorded on a Nicolet Magna IR 550 spectrometer series instrument at the same centre. The SEM photograph and FTIR spectra are shown in figures 1 and 2 respectively. The properties of F-400 GAC as an adsorbent are well documented in literature, shown in table 1. [a] 2128

4 [b] Figure 1: SEM photographs of F-400 and F-200 GAC [a] F-400 IQSA, [b] F-200 GAC Table 1: Characteristic Properties of GAC Carbon Type* Surface Area[N- BET] Particle Density Apparent Density True Density Pore Volume Porosity sq.m./gm g/cc g/cc g/cc cc/g F F F F *Origin for all carbon types is Bituminous Coal [a] 2129

5 2.4 Adsorption isotherm studies [b] Figure 2: FTIR spectra of [a] F-200 and [b] F-400 GAC Isotherms were determined for the F-400 grade carbon by the usual method of contacting various dosages with M IQSA for a period of 8 hours in a closed thermostat and agitated flask. The final equilibrium concentrations were determined by UV Spectrophotometry after the carbon was filtered off. The procedure involved samples weighing in the range of 0.1 g-1.5 g of GAC in 200 ml each of M solution of IQSA. The stoppered glass bottles were fixed in the Tempo make mechanical shaker placed in a trough containing water which acts as a thermostat maintaining a temp of 25±1 C for a period of 8 hours to reach equilibrium. The concentration of the filtrate was analyzed on spectrophotometer to measure the amount of ligand adsorbed by F-400 GAC. The residual ligand concentration in solution gives the value of C e, the equilibrium concentration in solution, using this q e was calculated as Where q e = the equilibrium ligand concentration on the GAC in moles/g C 0, C e = Initial and equilibrium concentration of ligand in moles/l V = Volume of solution in liters d = weight of GAC in g A typical adsorption isotherm at the given temperature was obtained by plotting qe Vs Ce for a given GAC ligand system. For ligand IQSA the adsorption isotherm was determined with F-400 and F-200 GAC. The adsorption isotherm with F-200 GAC was determined for comparison only. The data used are shown in figure 3 and figure 4. (1) 2130

6 Figure 3: Adsorption isotherm system F-400 IQSA Figure 4: Adsorption isotherm system F-200 IQSA The figures clearly indicate that these isotherms are all of the favorable type referred to in literature. (Chereminisoff, 1978) With increasing values of C e the values of q e reaches the saturation level of the adsorbate on the adsorbent i.e., formation of a monolayer of the adsorbate on the surface of the adsorbent. This value actually represents the maximum amount of ligand that the GAC could hold under a given set of experimental conditions. In order to verify to what extent the isotherms adhere to D-R, Langmuir and Freundlich adsorption isotherm representative plots of ln q e vs e 2, 1/q e vs 1/C e and ln q e vs ln C e are accordingly shown in figures 5, 6 and 7. These plots indicate satisfactory adherence to isotherms within the range of concentrations employed in this work. Figure 5: D-R Adsorption isotherm system F-400 IQSA 2131

7 Figure 6: Langmuir Adsorption isotherm System: F-400 IQSA Figure 7: Freundlich adsorption isotherm System: F-400 IQSA Since in the present work it was necessary to determine the adsorption of copper in the presence of the ligands, the ligands were first fixed onto the GAC to hinder direct adsorption of copper and thus facilitate adsorption through the ligand. For this purpose 0.5g of the GAC was shaken with 100 ml of M ligand solution for eight hours, the solution was filtered off and washed with distilled water to remove all adhering ligand solution. The residual ligand solution was checked for its absorbance to ensure that the same amount of ligand was adsorbed in subsequent systems. To the washed ligand loaded GAC was added 100 ml of mg/l of copper ion solution and the contents were shaken for around 8 hrs. in order to obtain the isotherm curve, the initial concentration C 0 and equilibrium concentration C e was determined spectrophotometrically at 520 ηm by the Na-DDC method. [Vogel, 1979] The amount of Copper on the adsorbed chelating species was determined using a similar relation (2) Where q e = concentration of Cu ion on ligand loaded GAC in mg/l of ligand C 0, C e = initial and equilibrium concentrations of Cu ion in solution in mg/l V= volume of solution in liters, d= millimole of ligand on GAC 2132

8 A plot of q e against C e is shown in figure 8. A plot of C e / q e vs Ce, ln q e vs ln C e and a plot of ln q e vs e 2 were also determined and are shown in figures 9, 10 and 11. Figure 8: Adsorption isotherm System: F-400 IQSA Figure 9: Langmuir Adsorption isotherm System: F-400 IQSA Cu Figure 10: Freundlich adsorption isotherm System: F-400 IQSA Cu Figure 11: D-R Adsorption isotherm system F-400 IQSA -Cu 2133

9 2.5 Kinetic Studies A study of the rates of diffusion could throw light on the type of diffusion operating in the adsorption process of ligand by GAC. For the experimental set up, a one liter round bottom flask containing 500 ml of ligand solution was placed in an indigenously built in-house thermostat. The thermostat was constructed using a heavy duty refrigerating compressor with its coils dipped in a water bucket. The bucket was insulated with a wooden box containing saw dust. The temperature control was done using contact thermometer connected through an electrical relay using cold water pump circulation. Remi stirrer (Model RQ 122) was used for continuous stirring and 5ml samples of ligand solution were withdrawn after every 15 minutes for the first hour and then every 30 minutes for the remaining 3 hours. The speed was controlled at 1000 rpm to eliminate film diffusion using an electronic stroboscope (M/s Toshniwal, Mumbai). The absorbances were measured at 350 ηm on the spectrophotometer. Using Beer s law relation the concentration of the ligand in solution C t at various time intervals was determined (Chatwal, 2007). A plot of C t vs t is shown in figure 12. Figure 12: Kinetic studies for system F-400 IQSA At definite time intervals the value of qt the amount of ligand adsorbed on GAC at these time intervals was calculated as (Yenkie, 1985) (3) where q and C t represent values at the given time intervals and not equilibrium values. However it was possible to find out the equilibrium concentration of the ligand on the GAC by using this value of Ct, and in conjunction with the adsorption isotherm curve found for the ligand-gac combination the value of qe was read at the value equal to C e = C t. This represented the value of q*. A plot of both q* and q vs time thus indicated the approach to equilibrium in the process. The difference between the value of q* and q at any time is the driving force operative in the process leading to adsorption on the GAC (refer figure 13). 2134

10 Figure 13: Adsorbent phase approach to equilibrium for system F-400-IQSA Study of the rate of adsorption of Cu ions on GAC containing adsorbed ligand followed a similar procedure 0.5g of GAC F-400 was shaken with 100ml of 0.002M IQSA for 8 hrs. The ligand loaded GAC was washed thoroughly and placed in a 1 lit RB flask containing 500ml of Cu ions solution. Experimental samples 2 ml each was withdrawn as before for a period of 4 hrs and the value of q obtained as described earlier (refer figure 14) where q represented the concentration of Cu on the GAC ligand system in mg/millimole of ligand C 0 & C t = concentration of Cu ion in solution present initially and at any time t in mg/l V= Volume of Cu solution d= millimoles of ligand on 0.5 g of GAC. From the value of q, q* is obtained as in the previous case for adsorption of copper on ligand loaded GAC and the adsorbent phase approach to equilibrium is obtained as shown in figure 15. (4) Figure 14: Kinetic studies for system F-400 and IQSA Cu 2135

11 Figure 15: Adsorbent phase approach to equilibrium for system F-400-IQSA-Cu 3. Results and discussion 3.1 Selection of the adsorbent and their characterization: The SEM images of the F-400 and F-200 grade carbon shown in plates 1 &2 show layered, loosely packed structure with lots of cavities, cracks, pores. It is apparent that the F-400 has large number of pores as compared to F-200. (Refer figures 1a and 1b). The FTIR study (figure 2) showed presence of various functional groups on surface of GAC. It showed predominant presence of free OH stretch vibration of Phenol at 3500cm -1 and an asymmetric C=O stretch vibration around cm -1. Both these observations significantly have an effect on the uptake of ligand IQSA used in the present study and also on removal of Cu ions from solution by adsorbed chelating species. As the GAC is a bituminous coal, a large number of free COOH groups are expected on the surface thereby favoring adsorption of ligands. (Mattson, 1969) 3.2 Validity of Isotherm The various plots for validating Freundlich, Langmuir and D-R equations are shown in figures 5, 6, 7, 9, 10 and 11. On plotting log q e vs log C e and C e /q e vs C e satisfactory linear plots were obtained over the range of concentrations used for the ligands. 3.3 Effect of mass of adsorbent To estimate optimum amount of ligand on GAC for subsequent kinetic studies with the Cu ions; 0.5g of GAC F-400 was shaken with 100ml 0.002M IQSA solution. The adsorption gradually increases with increase in adsorbent mass to attain a maximum value of 120 mg/lt for Cu. Further increase did not improve the percentage removal efficiency. From this it can be concluded that increase in number of adsorption sites due to increase in adsorbent mass did not have any effect once equilibrium was reached. (Refer figure 16). 2136

12 Figure 16: Effect of mass of adsorbent system F-400 and IQSA -Cu 3.4 Effect of contact time It was observed that a period of 4 hrs was optimum as initially there is rapid increase in uptake of Cu ions in the first hour but once equilibrium is achieved the increase in contact time had no effect on removal efficiency which may be due to saturation of the adsorbent bed with Cu ions followed by adsorption- desorption process that occurs after saturation as shown in figure Effect of shaking speed Figure 17: Effect of contact time system F-400 IQSA-Cu The change in concentration with time at difference speeds of the stirrer was determined for the system F-400- IQSA-Cu to get a comparative performance and to determine optimum speed for experimentation. The maximum removal efficiency was 95% for Cu in F-400- IQSA-Cu system at 850 rpm. The efficiency remained the same up to 1200 rpm, however declined slightly at higher speeds which may be due to decrease in boundary layer thickness around the particles and also because of the possibility of breakage of newly formed bonds due to additional energy provided by the increase in speed. (Refer figure 18). 2137

13 3.6 Effect of ph Figure 18: Effect of shaking speed system F-400 IQSA-Cu The ph of the solution has significant effect on the adsorption process. The removal efficiency was 95% for Cu at ph 5.5. At higher ph values Cu precipitated as hydroxide. It is possible that in acidic range lower than 5.5 the surface of the adsorbent is protonated which does not favor the uptake of adsorbate species. At low ph the surface of the adsorbent would be closely associated with hydronium ions which hinder the access of metal ions by repulsive forces, to the surface functional groups, consequently decreasing the percentage of metal removal, however in alkaline range the surface of the adsorbent gets deprotonated and hence there should be an uptake in adsorption under restricted limits due to hydrolysis. Thus, maximum uptake is attributed to both adsorption and chemical precipitation as shown in figure 19. Figure 19: Effect of ph system F-400 IQSA-Cu 3.7 Determination of adsorption isotherms for Cu on GAC ligand Initially a dynamic equilibrium is achieved when the adsorbate and the adsorbent interact. The concentration of metal ion was varied between 25 to 120 mg/lit at an optimum ph 5.5 with a fixed adsorbent dose of 0.5g. On increasing the adsorbent dose further the removal efficiency decreases as the adsorption sites already saturated are unavailable for increased adsorbent dose. Further there is possibility of decreased covalent interactions and simultaneous increase in electrostatic interaction thereby decreases adsorption with increase in initial metal ion concentration. The adsorption isotherm obtained is shown in figure 9. The isotherm data was fitted to Langmuir isotherm model (Altin, 1998) using equation 2138

14 (5) Where q e = mg of metal ion adsorbed per unit mass of the adsorbent C e = equilibrium concentration of metal ion adsorbed b and K= Langmuir constant with respect to sorption capacity and energy of adsorption. The Freundlich isotherm model (Freundlich, 1906) is an empirical equation which does not indicate a finite uptake capacity of adsorbent and can thus only be applied in case of low and intermediate concentration ranges. The model equation is given by Where K and n are the Freundlich constants; indicating the adsorption capacity and adsorbent intensity respectively. The isotherm data was also fitted to Dubinin Radushkevich D-R isotherm model [Bering, 1972] which assumes the heterogeneous surface, is expressed as follows: Where ε (the Polanyi potential) =RTln (1+1/C e ), q e is the amount of metal ions adsorbed per unit weight of ligand, is the adsorption capacity of the sorbent mg/g, C e is the equilibrium concentration of metal ions in solution mg/l, is a constant related to the adsorption energy mol 2 /kj 2, R is gas constant (kj/k.mol) and T is the temperature (K). 3.8 Adsorption thermodynamics The changes in Gibbs free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) for the adsorption process were obtained using the following equations [Mohan, 2002] where R is the ideal gas constant (kj mol 1 K 1 ) and T is the temperature (K). The enthalpy change (ΔH) and the entropy change (ΔS) are calculated from a plot of ln b (from the Langmuir Isotherm) versus 1/T (Figure 20). The results of these thermodynamic calculations are shown in Table 2. The negative value for the Gibbs free energy for copper metal shows that the adsorption process is spontaneous and that the degree of spontaneity of the reaction increases with increasing temperature. The overall adsorption process seems to be slightly exothermic (ΔH= ). This result also supports the suggestion that the adsorption capacity increases with increasing temperature. Table 2 also shows that the ΔS values were positive (i.e., that entropy increases as a result of adsorption). This occurs as a result of redistribution of energy between the adsorbate and the adsorbent. Before adsorption occurs, the heavy metal ions near the surface of the adsorbent will be more ordered than in the subsequent adsorbed state and the ratio of free heavy metal ions to ions interacting with the adsorbent will be higher than in the adsorbed state. As a result, the distribution of rotational and translational energy among a small number of molecules will increase with increasing (6) (7) (8) (9) 2139

15 adsorption by producing a positive value of ΔS and randomness will increase at the solid solution interface during the process of adsorption. Table 2: Adsorption Thermodynamics Parameters. T in ⁰ C T in K b ln b ΔG ΔH ΔS Figure 20: Adsorption Thermodynamics system F-400 IQSA-Cu 3.9 Adsorption Kinetics For the kinetic studies 0.5 g of F-400 GAC was adsorbed on 100ml of M IQSA by shaking for 8 hours on a mechanical shaker. 500 ml of Cu solution at a concentration equivalent to a value from the descending portion of the adsorption isotherm of Cu F-400 IQSA system was chosen. The run lasted for 4 hrs by which the fall in concentration of the Cu ion in solution stabilized. The data were then regressed against the Lagergren equation, which represents a first-order kinetic equation, (Namasivayam, 1995), (Parab,2005) and against a pseudo-second-order kinetic equation (Ho, 1995): (10) (11) where q t is the metal uptake per unit weight of GAC (mg g 1 ) at time t, q e is the metal uptake per unit weight of GAC (mg g 1 ) at equilibrium, and k 1 (min 1 ) and k 2 (g mg 1 min 1 ) are the rate constants of the first-order and pseudo-second-order kinetics equations, respectively. The slopes and intercepts of these curves were used to determine the values of k 1 and k 2, as well as the equilibrium capacity (q e ). The first-order kinetics model was considered initially which gave R 2 value (0.951) (Figure 21); however, the linearized pseudo-second-order kinetics model (Figure 22), provided R 2 values (0.956). As a result, the sorption system appears to follow pseudo-second-order reaction kinetics. 2140

16 Figure 21: First Order Kinetic Model system F-400 IQSA-Cu Figure 22: Pseudo second order kinetic model system F-400 IQSA-Cu 4. Conclusion The present investigation reveals that ligand loaded GAC could function effectively in scavenging copper metal ions from aqueous solution. The percentage removal observed was around 95% at ph 5.5. Operation parameters like shaking speed, contact time, effect of temperature, and amount of adsorbent and initial metal ion concentration significantly affect removal efficiency. The adsorption isotherms of Cu on ligand adsorbed carbon behave in a favorable manner. This is reflected in Langmuir, Freundlich and D-R isotherms which were used to model the isotherm sorption studies. The kinetics was found to be best suited to a pseudo second order Lagergren kinetic equation. Based on the results of this analysis, external mass transfer appears to control the rate of adsorption. The adsorption process was also thermodynamically spontaneous under natural conditions. 5. References 1. Kurniawan, T.A., Chan, G.Y.S., Lo, W.H., Babel, S., (2006), Physico-chemical treatment techniques for wastewater laden with heavy metals, Chemical Engineering Journal, 118, pp Huang C.P, (1978), Carbon Adsorption Handbook, edited by P.N. Chermisinoff & F Elllerbusch, (Ann Arbor Science Publishers, Michigan), pp

17 3. Deshmukh K, Pande H, Natarajan S and Nageshwar G., (1989), Indian Journal of Environmental Protection, 9, 428, CA 113:102772x. 4. Liu Oi, Li Quarunin & Zhang Yongcai, Fenxie Huaxue, (2000), , CA 132:241520u 5. Moeller T & Jackson, (1954), Analytical Chemistry, 22, 1950, pp John C. Bailer Jr & Daryle H Busch, (1956), The chemistry of the coordination compounds, Reinhold Publishing Corporation, New York, Chapman and Hall, London. 7. American Society for Testing and Materials, Method D4749, (1994), Standard Test Method for performing the Sieve Analysis of Coal and Designating Coal Size, USA. 8. Chereminisoff, P. and Ellerbusch, F., (1978), Chemical interactions between Inorganics and Activated Carbon, pp , Carbon Adsorption Handbook. Ann Arbor Science Publishers, Ann Arbor, MI. 9. Vogel, A., (1979), Quantitative Inorganic Analysis, ELBS. Longman Group Limited, England, pp Chatwal G., Anand S., (2007), Instrumental methods of chemical analysis, Himalaya Publishing Company, New Delhi, pp Yenkie, M.K.N., (1985), Ph.D. Thesis, Nagpur University, Nagpur. 12. James S.Mattson, Havy B.Mark, Walter J. Weber, (1969), Analytical Chemistry, 41(2), pp O.Altin, H.O. Ozbelge, T. Dogu., (1998), Use of general-purpose adsorption isotherms for Heavy metal-clay mineral interactions, Journal of Colloid Interfacial Science, 198, pp H. Freundlich, Over the adsorption in solution, (1906), Journal of Physical Chemistry, 57, pp B. P. Bering, M.M. Dubinin, V.V. Serpinsky., (1972), On thermodynamics of adsorption in micropores, Journal of Colloidal Interfacial Science, 38, pp D. Mohan, K.P. Singh., (2002), Single- and multi-component adsorption of cadmium and zinc using activated carbon derived from bagasse: an agricultural waste, Water Research, 36, pp C. Namasivayam, R.T.Yamuna., (1995), Adsorption of chromium (VI) by a low-cost sorbent: biogas residual slurry, Chemosphere, 30 (3), pp