International Research Journal of Environment Sciences________________________________ ISSN 2319–1414Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 64 Adsorption Isotherm, Thermodynamic and Kinetic Study of Arsenic (III) on Iron Oxide Coated Granular Activated Charcoal Saikia Ananta *, Banerjee Saumen and Veer Vijay Defence Research Laboratory, DRDO, Post Bag No. 2, Tezpur -784001, INDIA Available online at: www.isca.in, www.isca.me Received 17th December 2014, revised 11th January 2015, accepted 21st January 2015 AbstractIn this paper, adsorption of arsenic (As) (III) as arsenite on iron oxide coated granular activated charcoal (FeOGAC) has been studied. Percentage adsorption of As (III) on FeOGAC was determined with respect to pH, contact time and adsorbent dose. The study revealed that the removal of As (III) was best achieved at pH range from 7.5 to 9.5. The initial As (III) concentration (100 µg/L) came down to less than 10 µg/L at contact time 90 min with adsorbent dose of 7.5g/50mL. The adsorption was studied for Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms. The Langmuir isotherm model was found to be fitted well. During the study the thermodynamic parameters like enthalpy change (H°), free energy change (G°), entropy change (S°) and activation energy (E) were determined for the better understanding of the adsorption process. The adsorption was found to be kinetically pseudo-second order controlled process. Keywords:Arsenic, iron oxide, granular activated charcoal, adsorption, kinetic. Introduction Water is one of the most essential requirements for all life on earth and it is considered as very important resource for human civilization. Safe source for pure and affordable water is certainly one of the basic humanitarian goals, and is identified as a major global challenge for the 21st century. Water contamination is a most important critical issue facing the world due to an increase in population growth and has become a serious threat to environmental and human health. Therefore, decontamination of polluted water is become one of the most important environmental matter in the recent decade. The availability of arsenic in groundwater specially in drinking water, is causing a major environmental and health issue1,2 in several developing regions3-5. The minimum lethal dose for adult humans for various compounds has been reported to be 100 to 200 mg. Various effects like carcinogenic, cardiovascular, respiratory, dermal, gastrointestinal, mutagenic etc. may be occurred due to short-term exposure to arsenic and also it may disturb the immune system of our body. Cancer of the skin, kidney, bladder, lungs, nasal passages, liver and prostate may be resulted due to long-term exposure to arsenic. All these health problems occur mainly due to the consumption of drinking waters containing high dose of arsenic.Arsenic pollution has been reported recently in USA, China, Chile, Bangladesh, Taiwan, Mexico, Romania, United Kingdom, Argentina, Poland, Canada, Hungary, New Zealand, Vietnam, Japan and India. In India, NE states along with Jharkhand, Bihar, West Bengal, Madhya Pradesh, Uttar Pradesh, etc. are badly affected by groundwater contaminated with arsenic 8-11. Thus, the World Health Organization (WHO, 2006) the United States Environmental Protection Agency (USEPA, 2009) and also Turkish Standards Institute (TSE-266, 2005) have applied the maximum allowable level of arsenic in drinking water as 10 g/L (10 ppb). Arsenic contamination of water systems is mainly occurs from natural and anthropogenic sources12,13. Beside these, another primary contamination source of arsenic is the industrial wastewater. Arsenic in aqueous systems exists in both organic and inorganic forms; inorganic arsenic is mostly found in natural water systems. In general inorganic arsenic is more poisonous to loving systems than organic arsenic14. Generally, in natural aqueous systems inorganic arsenic has two different oxidation states, i.e. trivalent (+3) and pentavalent (+5)15,16. The mobility of arsenical forms in waters is very dependent on pH, Eh conditions and presence of different chemical species12Pentavalent arsenic [As (V), arsenate] is stable in oxidative condition and exists as a monovalent (HAsO) or divalent (HAsO) anion, while trivalent arsenic [As (III), arsenite] is stable in reductive conditions 17 and exists as an uncharged (HAsO) or anionic species (HAsO). Inorganic As(III) is approximately ten times more toxic than As(V) 18. Recent report 19 reveals that the toxicity order of arsenic is as follows: inorganic As3+�organic As3+�organic As5+� inorganic As5+20,21. In the last two decades huge attention has gained for the development of cost effective technologies to remove arsenic along with other heavy metals from drinking water. During this period several physiochemical techniques such as adsorption, ion-exchange or chelation, lime softening, reverse osmosis (membrane techniques), coagulation and precipitation have been applied to remove arsenic as well as other heavy metals from aqueous systems22-34. But these techniques have several International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 65 drawbacks like incomplete metal removal, high capital and operational cost or the disposal of the residual metal sludge, and sometimes are not suitable for small-scale industries. Among these technologies, adsorption processes are effective techniques and they have long been used in the water and wastewater industries to remove inorganic and organic pollution for its ease of handling, minimal sludge production, cost effectiveness and its regeneration capability1,35. The technique is also popular because of the availability of a wide range of adsorbents such as activated alumina, activated carbon, fly ash, ferric hydroxide, chitosan-coated biosorbent, oxides, clay minerals and zero valent iron26-40. Among all types of conventional and non-conventional adsorbents, granular or powdered activated carbon is the most widely used adsorbent for arsenic removal37 because of their high surface area, micro/mesoporous structure and significant adsorption capacity. Activated carbon modified with iron has been used for the removal of various contaminants from water 41-43. However, some of these processes are not economic or require the control of pH and/or other parameters to achieve the optimum arsenic removal; therefore, a more effective and economical technique would be highly desirable. Arsenic removal from drinking water using iron compounds has been reported by several authors44-46. Elementary iron40-42, granular iron hydroxides and ferrihydrites50-53 have been proposed for the arsenic decontamination of water. Most of the adsorption processes investigated so far was reported in the review of Mohan and Pittman28. In the present study experiments were carried out to evaluate the performance of iron oxide coated granular activated charcoal (FeOGAC) for As (III) removal. The process parameters such as effect of adsorbent dose, pH, initial arsenic concentration and contact time were studied. Four isotherms like Langmuir, Freundlich Temkin and Dubinin-Radushkevich models were studied. Various thermodynamic parameters were also calculated for the process to complete the investigation for efficacy of FeOGAC in adsorption of arsenic from contaminated water. The process was also analysed for kinetic models like pseudo first order, pseudo second order, Elovich and Intra-particle diffusion models at different experimental conditions. Material and Methods Materials: The granular activated carbon (GAC) used in this study was obtained from Clariant Chemicals. The GAC, prior to use in this study was backed in an oven at 200°C for 24 hrs and washed several times with deionised water. After washing the GAC was completely dried in oven at 110°C, cooled in desiccators, and stored at room temperature in a covered glass container until further use. The As, FeCl, HCl and NaOH etc. used were LR grade from Merck and used as such. Stalk solution of 1000 µg/L arsenic solution was prepared by dissolving 0.00132 g of As in 1000 ml of deionised water with a pallet of NaOH. All the instruments used during the experiments were calibrated as per standard procedure. The initial pH of the arsenic solutions was adjusted using NaOH (0.1 M) and/or HCl (0.1M) solutions as and when necessary and analysed by Cyberscan pH 510 (Eutech) instrument. The determination of concentration of arsenic (III) was done using AA 7000 Atomic Absorption Spectrophotometer (Labindia) having detection limit 5µg/L. Adsorbent Preparation: Iron oxide coated GAC (FeOGAC) was prepared by mixing 200 g of GAC with 100 ml of 2M FeCl solution and gradually adding 5M NaOH solution (nearly 180 ml) under slow agitation until the pH of the final mixture was attained at around 11.0. The mixture was then allowed to settle for few hours so air trapped in pores could escape. After that the mixture was completely dried in an oven at 95°C, allowed to cool to room temperature, and washed several times with deionised water to remove excess Fe-oxide. The FeOGAC thus obtained was then dried in an oven at 100°C and stored at room temperature in air tight glass container until use. Adsorption Experiments: The experiments were carried out by batch process to obtain rate and equilibrium data. Two parallel experiments were conducted using control granular activated charcoal (GAC) and FeOGAC (figure-1) separately to compare few parameters. The reaction mixture consisting of 50 mL known concentration of As (III) solution and known quantity of adsorbent was shaken in a temperature controlled orbital shaker with shaken speed of 175 rpm at three different temperatures of 298 K, 303 K and 308 K. The effect of adsorbent dose was studied by varying the adsorbent dose from 2.5 to 10g/50 ml and maintaining pH of the solution at 7.5 with a constant contact time of 90 min. In this study the spiked water arsenic concentration was fixed at 100 µg/L. The study of the effect of initial pH of the solutions on arsenic adsorption by the FeOGAC was done by using 100 µg/L of the adsorbent at varying pH of the solutions. The effect of contact time was studied with varying contact time from 30 to 180 minutes keeping pH of the solutions and dose of the adsorbent constant. The effect of initial arsenic concentration in feed water was studied with various adsorbent doses with varying initial arsenic concentration from 50 to 250 µg/L. The adsorption isotherm was also performed by using 7.5g of FeOGAC with 50 mL spiked water at different initial concentrations of arsenic. The kinetics and thermodynamic parameters for arsenic adsorption on FeOGAC were established by conducting the experiments at different contact times and at three different temperatures respectively. Figure-1 Image of FeOFAC International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 66 Desorption experiments: To determine the reusability of the FeOGAC samples the adsorption/desorption cycles were carried out. The saturated FeOGAC was prepared by treating 30 g of FeOGAC with 200 ml of 250 µg/L arsenic solution under agitation for about 3 hours at shaken speed of 175 rpm. Then filtered the arsenic saturated FeOGAC and dried in oven 100°C. The arsenic saturated FeOGAC (7.5 g) was agitated for about 3 hours at shaken speed of 175 rpm with 100 mL of NaOH solution (0.1, 0.3 and 0.5 M solution were used separately). The aqueous phases were then separated and concentrations of arsenic in that phase were determined. Results and Discussion Effect of adsorbent dose: The effect on arsenic (III) removal at fixed initial arsenic (III) concentration 100 µg/L by adsorbent dose is shown in figure-2. It shows that the adsorption efficiency of arsenic (III) from the solution increases rapidly with increase in adsorbent dose from 2.5-7.5g/50 mL; a marginal increase is observed on further increase in the adsorbent dose for both GAC and FeOGAC. The increase in the removal efficiency of arsenic (III) may be attributed to the fact that with the increase in adsorbent dose, available surface for adsorption increases for the solute to be adsorbed54,55. 24681030405060708090100 GAC FeOGAC% Removal of arsenicAdsorbent dose (g/50 ml) Figure-2 Plots of percent removals of arsenic versus adsorbent dose Effect of pH: The effect of pH on the adsorption of arsenic (III) onto FeOGAC and GAC were studied in the pH range of 5.5 to 11.5. Different pH solutions were prepared by adding dilute NaOH or HNO solutions drop wise to achieve pH values. The figure-3 reveals that the efficiency of arsenic (III) removal increases with increasing pH from 5.5 to 7.5 after that the marginal increase was observed. In the study highest arsenic removal was achieved at pH range from 7.5 to 9.5. The results suggested that the removal efficiency of FeOGAC and GAC were found to decrease both at lower and higher than pH 9.5. This may be due to the fact that highly and extreme low protonated surface of FeOGAC and GAC are not favourable for arsenic (III) adsorption 56. The variation of adsorption with respect to the initial solution pH can be understood from the zero point charge (pHzpc) of the adsorbent. pHPZC is the point of pH, at which the solid surface of the adsorbent has the net zero charge. The pHzpc of the FeOGAC is determined by plotting initial pH of the working solution versus the final pH of the solution. The pHzpc is the pH corresponds to the point at which the curve crosses the straight line of plot Initial pH=Final pH (figure-4). The pHzpc was found to be at pH 8.2. At pH �pHzpc the surface of FeOGAC become negatively charged and the As(III) is positively charged. In this situation the electrostatic attraction between the metal ions and the adsorbent surface increases resulting in increased adsorption of As(III) on to the FeOGAC surface is observed. But in the case, at pHpHzpc the surface of FeOGAC become positively charged leading to a decreased adsorption of the As(III) due to the higher concentration of the H ions in the solution that will compete with the positively charged As(III) ions for the active sites. 56789101112405060708090100 % Removal of arsenicpH GAC FeOGAC Figure-3 Plots of percentage removal of arsenic versus pH 56789101112101112 Final pHInitial pH FeOGAC pHzpc=8.2 Initial pH=Final pH Figure-4 Determination of point zero charge (pHpzc) of FeOGAC International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 67 Effect of contact time: Effect of duration of agitation time was studied with various adsorbent doses by varying the agitation time using feed water containing 100 µg/L of arsenic. From the figure-5 it is clear that adsorption efficiency increases with increasing the contact time, reaching nearly maximum removal at contact time of 90 minutes; thereafter removal becomes nearly constant for both the cases of FeOGAC and GAC. The reason may be due to the fact that, initially all adsorbent sites were empty and the arsenic concentration was high. Afterward, the arsenic (III) adsorption capacity of the adsorbent was decreased significantly due to the decrease in available adsorption sites as well as arsenic concentration 57. It was also observed that the removal efficiency increases with increasing the adsorbent dose. The fact has already been discussed during the discussion of effect of adsorbent dose. Figure-5 Effect of duration of agitation time on the adsorption of As (III) onto FeOGAC using various adsorbent doses Effect of initial arsenic concentration: The influence of initial arsenic concentration in feed water was studied using various adsorbent doses by varying initial arsenic (III) concentration. Figure-6 reveals that adsorption efficiency is higher at lower initial arsenic concentration (50 µg/L) and a slow decrease in arsenic (III) adsorption by FeOGAC as well as GAC were observed with increasing initial arsenic (III) concentration. The reason for the decrease in As (III) removal efficiency at higher initial concentration may be due to saturation of the active sites of the adsorbent by the arsenic (III) and hence, further increasing the arsenic (III) concentration not leading to the increase in absorption significantly 58. From the result it is revealed that for 50 ml feed water containing maximum 100 µg/L arsenic the minimum effective adsorbent dose to reduce the arsenic content to less than 10 µg/L (WHO limit) is 7.5 g. Desorption Study: The experimental results (table-1) from the desorption study revealed that trend of desorption percentage at different concentration of NaOH is in the 0.1M 0.3M 0.5M. The maximum desorption of arsenic was found to be 71.4 % with 0.5 M NaOH solution. From the study it is revealed that the used FeOGAC can be regenerated satisfactorily and reused for further arsenic (III) removal due to which the process is seems to be cost effective and suitable for the application in the rural areas. Figure-6 Effect of initial arsenic concentration on the percentage removal of As (III) using various adsorbent doses Table-1 Desorption Study of FeOGAC Adsorbet adsorbed by adsorbent at saturated point (µg/g) Eluent used Conc. of eluent (M) Adsorbet eluted from adsorbent (µg/g) Regeneracy of adsorbent (%) 1.4 NaOH 0.1 0.42 30 1.4 NaOH 0.3 0.59 42 1.4 NaOH 0.5 1.0 71.4 Adsorption Isotherms Study: Langmuir isotherm: The main assumptions of the Langmuir adsorption isotherm are; i. The adsorption is mainly due to saturated monolayer adsorption of adsorbate molecules on the adsorbent surface, ii. the energy of adsorption is constant and iii. in the plane of adsorbent surface, no transmigration of adsorbate molecules. The Langmuir adsorption isotherm59 can be expressed by the equation given below: eeme bC bC + = 1 Or Where; e (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent, (µg/L) = the unabsorbed adsorbrent concentration in solution at equilibrium condition,(µg/g) = the maximum amount of adsorbate adsorbed per unit mass of adsorbent to form a complete monolayer on the surface (L/µg)= Langmuir constant. International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 68 The Langmuir adsorption model was adopted for the determination of maximum arsenic uptake () at different arsenic (III) concentration (50 µg/L- 250 µg/L). The linear plot of versus (figure-7) with greater correlation coefficient value indicates the monolayer adsorption on FeOGAC. The values of and were calculated from the slope and intercept respectively are presented in table-2. From the results it was observed that the maximum arsenic uptake () and the value of Langmuir constant (), related to the affinity of the binding sites were increased with the increase of temperature. Freundlich isotherm: Both the monolayer (chemisorption) and multilayer adsorption (physisorption) can be studied by the Freundlich adsorption isotherm. The isotherm considered the adsorption of the adsorbate onto the heterogeneous surface of an adsorbent60. The Freundlich isotherm61 can be expressed by a linear equation as: Or efeC n logloglogWhere (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent, [µg/g (L/µg)1/n] = a constant, sorption capacity of the adsorbent, = favourability of the adsorption process, constant related to energy of intensity of adsorption. Figure-7 Langmuir isotherms of FeOGAC for arsenic adsorption at different temperatures Table-2 Adsorption isotherms 298°K 303°K 308°K Adsorption isotherm Adsorption parameters Values Langmuir m (µg/g) 1.0771 1.1773 1.2000 (L/µg) 0.5781 0.7696 0.8454 R 2 0.9980 0.9968 0.9967 Freundlich n 4.2141 4.1982 5.5835 K f [µg/g (L/µg) 1/n ] 0.4272 0.4687 0.6058 R 2 0.9350 0.9413 0.9774 Temkin B 0.3534 0.3753 0.2816 A (L/g) 18.7893 22.0525 269.841 b (kJ/mol) 7.0107 6.7123 8.9458 R 2 0.9434 0.977 0.9823 Dubinin-Radushkevich (µg/g) 0.9162 0.975 0.9772 ( mol/kJ) 3x10-7 3x10-7 4x10-8 (kJ/mol) 1.291 1.291 3.5355 R 2 0.843 0.868 0.837 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 69 The value of and (table-2) were obtained from the linear plot (figure-8) of log versus log The values of lying between 1 and 10 indicating chemisorptions62. Isotherms with �1 are known as L-type isotherms indicating a high affinity between the adsorbate and the adsorbent and is denotes the chemisorption63. The Freundlich isotherm constant, , which is associated with the adsorption capacity of the adsorbent, increased with temperature, reflecting the endothermic adsorption process. The relatively high value of correlation coefficient () value indicates the satisfactory fitness of the data with the Freundlich isotherm model. Temkin Isotherm: The Temkin isotherm model is based on the assumptions that the adsorption energy decreases linearly with the surface coverage, which is due to the adsorbent-adsorbate interactions. The Temkin isotherm model64 is expressed by the linear equation as; log(AC b RTOr loglogWhere: b RT, (J/mol) = Temkin constant, related to heat of sorption, (L/g) = Temkin isotherm constant also, called equilibrium binding constant, = gas constant (8.314 J/mol.k), (k) = absolute temperature. The linear plot (figure-9) of versus loge at different temperatures for Temkin adsorption isotherm, which consider the chemisorptions of an adsorbate onto the adsorbent65, fitted satisfactorily with correlation coefficients () �0.94 (table-2). This supports the findings that the adsorption of arsenic onto FeOGAC is a chemisorption process66. Figure-8 Freundlich isotherms of FeOGAC for arsenic adsorption at different temperatures Figure-9 Temkin Isotherms of FeOGAC for arsenic adsorption at different temperatures International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 70 Dubinin Radushkevich (D-R) Isotherm: Dubinin–Radushkevich (D-R) isotherm model predicts that the adsorption process follows a pore filling mechanism and is expressed by a semi-empirical equation. The assumptions of the model are that the adsorption has a multilayer character, involves van der Waals forces and is applicable for physical adsorption processes67. The D-R isotherm model is presented linearly by the equation as given below; loglogWhere; log(RT(µg/g)= theoretical saturation capacity, (mol/kJ) = constant related to adsorption energy, = gas constant (8.314 J/mol.K), (k) = absolute temperature, (kJmol-1) = the mean energy of sorption , The magnitude offurnishes the information on the nature of adsorption process i.e. physical/ chemical., From the plot (figure-10) log versus at different temperatures, it is revealed that the values are not complying with the previously determined Langmuir isotherm q values. The correlation coefficients for the D-R isotherm are the lowest compared to those values of the other three isotherm models (table-2) suggest that the arsenic (III) adsorption onto FeOGAC is not a physical process66. Dimensionless equilibrium parameter (R): Dimensionless equilibrium parameter or Langmuir isotherm constant parameter ), the essential characteristics of the Langmuir isotherm provides the valuable information on the adsorption efficiency of the isotherms process. The can be related with Langmuir isotherm constant by the following equation68; bC Where is the Langmuir isotherm constant and is the initial arsenic concentration (µg/L). The shape of the isotherms can be predicted by the value. The Langmuir isotherm is favourable if the value is 0 R1, if = 0 then it is irreversible, if = 1 it is linear and if &#x-6.2;ᜆ1 the isotherm is unfavourable. values at different temperatures studied were calculated (table-3) and the values are ranges from 0.0047 to 0.0334, which implies that the Langmuir isotherm is favourable. 030000006000000900000012000000-0.5-0.4-0.3-0.2-0.10.00.10.2 25 Deg. cel. 30 Deg. cel. 35 Deg. cel.log e ValueStandard Error 25 deg. cel. Intercept-0.037970.04995 Slope-3.10903E-77.72466E-8 30 deg. cel. Intercept-0.011040.04867 Slope-2.56864E-75.76135E-8 35 deg. cel. Intercept-0.010440.05413 Slope-4.27829E-81.08948E-8 Figure-10 Dubinin–Radushkevich (D-R) isotherms of FeOGAC for arsenic adsorption at different temperatures Table-3 The and values of arsenic adsorption onto FeOGAC Temp R L values 2 values for Adsorption kinetics (°K) 50 µg/L 100 µg/L 150 µg/L 200 µg/L 250 µg/L Pseudo first order Pseudo second order 298 0.0334 0.0170 0.0114 0.0086 0.0069 9.9459 0.7027 303 0.0253 0.0128 0.0086 0.0065 0.0052 308 0.0231 0.0117 0.0078 0.0059 0.0047 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 71 2 (Chi square) analysis: The difference between the experimental and various models data can be measures with the help of Chi-square test analysis. Mathematically it can be expressed as: ( ) expcalcalWhere, e,exp is experimental equilibrium capacity data and e,calis the equilibrium capacity from a model. If data from the model are similar to experimental data, value will be small and if they differ, will be large69. The values were calculated (table-3) for Pseudo first and Pseudo second order kinetic models. The lower value (i.e. 0.7027) with respect to the pseudo-second order model revealed that arsenic (III) adsorption onto FeOGAC obeyed the pseudo-second order kinetics. The pseudo-first order model provided higher values (i.e 9.9459) suggesting poor pseudo-first order fit to the data for arsenic (III) adsorption onto the FeOGAC. Thermodynamic investigations: The thermodynamic parameters like enthalpy change (H°), free energy change G°), entropy change (S°) and activation energy () are the parameters, which determine the spontaneity of a process. A process will be spontaneous if there is a decrease in G° value with increasing temperature 70. The temperatures used in the thermodynamic study were 298, 303 and 308 K. The thermodynamic parameters were calculated based on the following equations: RT G RT H R Sb ° D -= ° D - ° D log ° D - ° D = D Where: b is the equilibrium constant, R is the universal gas constant (8.314 J/mol K), and T is the temperature (). The calculated values of the thermodynamic parameters are listed in the table-4. The positive enthalpy change, H° ( + 12.629 kJ/mol), implies the endothermic nature of the adsorption process, due to which the adsorption efficiency increases with the increase of temperature. The positive entropy change, S° (+ 0.0404 J/mol K), reveals that the randomness increases at the solid/liquid interface during the adsorption onto the FeOGAC. The positive and weak values of free energy change, G° which is decreasing with increasing temperature, indicates that the process is favourable but non spontaneous. The sticking probability, Sof an adsorbate on adsorbent can be expressed by a modified Arrhenius-type equation related to the surface coverage ( q ). The S is a function of the adsorbate/adsorbent system and it is the measure of the ability of an adsorbate to remain on the adsorbent indefinitely, 71 and it is expressed as; exp1( RT Or RT loglog( Where, q is surface coverage, Ea is activation energy. 1( Where, Co and C are the initial and equilibrium arsenic concentrations respectively. From the plot of log( q - versus T with intercept of logand slope of R , the value of S* and E were calculated (table-4). The value of S* was found to be 1.17×10-20 which is very close to zero indicates that adsorption mechanism follows chemisorptions 72 whereas the determined activation energy (E) for the process was 45.934kJ/mol. Adsorption kinetics: The adsorption mechanism of arsenic (III) onto FeOGAC was investigated by studying the pseudo first order, pseudo second order, Elovich and Intra-particle diffusion kinetic models at various experimental conditions. Pseudo first order kinetic model: The linear form of pseudo-first-order kinetic model73 is expressed as: 303 . 2 loglog( Where;K (1/min)= rate constant, qe (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent at equilibrium, qt (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent at time t, t (min) = time. Table-4 Thermodynamic parameter Thermodynamic parameters Temp. (K) o (KJ/mol) o (kJ/mol) o (kJ /mol K) a (kJ/mol) S* 298 0.56 12.629 0.0405 45.934 1.17×10 - 20 303 0.3575 308 0.155 International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 72 The Lagergren plots of log( versus for various initial arsenic concentration viz. 50, 100, 150, 200 and 250 µgL at 303 K are given in figure-11. The values of at five different initial arsenic concentrations were calculated from slopes of the respective linear plots and also the correlation coefficient () was computed and the values are given in table-5. Pseudo second order model: The most popular pseudo second order kinetic model 74, linear form of equation is expressed as; Where: K2 (g/µg min.) = rate constant, qe (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent at equilibrium, qt (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent at time t t (min) = time. From the plot of versus for arsenic adsorption on FeOGAC at different initial arsenic concentration viz. 50, 100, 150, 200 and 250 µgL-1 at 303 K, the experimental value of and 2 of the pseudo second order equation were obtained. The plot (figure-12) 20406080100120-3.0-2.5-2.0-1.5-1.0-0.50.00.5 50 µg/L 100 µg/L 150 µg/L 200 µg/L 250 µg/Llog-q (Min.) ValueStandard Error 50 µg/g Intercept0.275390.4786 Slope-0.030880.00739 100 µg/g Intercept0.438750.58682 Slope-0.024210.00905 150 µg/g Intercept0.508450.26101 Slope-0.019760.00318 200 µg/g Intercept0.815710.44334 Slope-0.022680.0054 250 µg/g Intercept0.541290.2755 Slope-0.015380.00335 Figure-11 Lagergren plots of log( versus for various initial arsenic concentrations Table-5 Kinetic parameters for the adsorption of arsenic (III) on FeOGAC o (µg/ L) e (µg/g Pseudo first order kinetic model Pseudo second order kinetic model Elovich kinetic model Intra-parti cle diffusion kinetic model 1 (1/min) q e (µg/g) [g/(µg min)] e (µg/g) [µg/(g min)] (g/µg) id[µg/(g min1/2)] (µg/g) 50 0.333 0.0712 1.885 0.9459 0.0830 0.4021 0.9767 1.3362 0.2115 0.8361 0.0193 0.1061 0.7292 100 0.666 0.0557 2.746 0.8773 0.0240 0.879 0.9659 0.3141 0.4976 0.8944 0.0465 0.1108 0.8187 150 0.970 0.0456 3.224 0.9508 0.0116 1.3729 0.9617 0.1502 0.7877 0.9327 0.0742 0.0747 0.8665 200 1.253 0.0523 6.542 0.8983 0.0063 1.9342 0.9377 0.0860 1.1019 0.9397 0.104 -0.0044 0.8774 250 1.4 0.0355 3.478 0.9132 0.0053 2.1838 0.9632 0.0752 1.2238 0.9693 0.1167 -0.032 0.9233 gives a straight line with higher correlation coefficient, values, which was higher than that of the observed pseudo-first- International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 73 order model indicating the better fitness of the pseudo second order model. The values of , and are shown in table-5. The value of e increases with the increase in initial arsenic concentration; this is due to the higher availability of arsenic to adsorb at higher initial arsenic concentration. The values of rate constant, decreases with increase with initial arsenic concentration which indicates the saturation of the FeOGAC with arsenic at higher initial arsenic concentration. Elovich model: The equation of Elovich model is expressed 75as below; exp(dqWhere; (µg/(g min) = Elovich coefficients represents initial adsorption rate, (g/µg) = the adsorption coefficient, Assuming t �� 1, = 0, = 0 and = at t = t loglog( b ab b + = The figure-13 shows a plot of versus log , a linearization form of Elovich model at all concentrations studied. From the slope and intercept of the plot the constant and the initial adsorption rate were calculated. The results with the correlation coefficients are shown in Table-5. The comparatively low correlation coefficients for the Elovich kinetic model suggest that this adsorption system is not an acceptable for this system. Intraparticle diffusion model: The intraparticle diffusion model is based on the assumptions that the adsorbate species are transported through intraparticle diffusion from the bulk of the solution into the solid phase and is often considered as the rate controlling step in many biosorption processes. The intraparticle diffusion model is expressed as76,77, id2/1Where: (µg/g) = the amount of adsorbed adsorbate per unit mass of adsorbent at time t, Kid(µg/(g min1/2)= rate constant of Inter Particle Diffusion Model, (min) = time, I = constant associated with the boundary layer thickness. According to this model, if we plot versus 1/2 it will give a straight line, and then the biosorption process is controlled by intraparticle diffusion. The larger values of intercept the greater is the boundary layer effect. Figure-14 show a plot of versus 1/2 at all concentrations studied. From the plot it was observed that the external surface adsorption was completed before 90 minute, and then the intraparticle diffusion control stage was attained and continues from 90 minute to 150 minute. Finally, final equilibrium adsorption started after 150 minute. The arsenic (III) was slowly transported via intraparticle diffusion into the particles and was finally retained in the micropores. The rate parameters along with the correlation coefficients are presented in table-5. Figure-12 Plots of versus for various initial arsenic concentrations International Research Journal of Environment Sciences______________________________________________ ISSN 2319–1414 Vol. 4(1), 64-77, January (2015) Int. Res. J. Environment Sci. International Science Congress Association 74 Figure-13 Plots of versus log for various initial arsenic concentrations Figure-14 Plots of versus 1/2 for various initial arsenic concentrationsAcknowledgement The authors are thankful to DRDO HQs, New Delhi for providing necessary financial support to carry out the research work. References 1.Ranjan D., Talat M., Hasan S.H., Rice polish: an alternative to conventional adsorbents for treating arsenic bearing water by upflow column method. 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