Research Journal of Recent Sciences _________________________________________________ ISSN 2277-2502 Vol. 3(7), 18-27, July (2014) Res.J.Recent Sci. International Science Congress Association 18 Red Mud as Adsorbent to Remove Lead (II) from Aqueous SolutionsKumar Sujata, Singh D., Mishra A.K., Upadhyay M. and Kumar SarojDepartment of Chemistry, Kirodimal Institute of Technology Raigarh, CG, INDIA Department of Chemistry, Dr. C.V. Raman University Bilaspur, CG, INDIA Department of Chemistry, K. Govt. Arts and Sc. College Raigarh, CG, INDIA Available online at: www.isca.in , www.isca.me Received 14th February 2014, revised 18th June 2014, accepted 1st July 2014Abstract Batch experiments under different experimental conditions have been performed to evaluate the adsorption characteristics of red mud. Freundlich and Langmuir adsorption isotherm models have been used to discuss the data obtained. Kinetics of adsorption have been discussed using Lagergren first-order equation, pseudo-second-order equation and intra-particle diffusion models. Thermodynamic parameters such as change in free energy G, change in enthalpy H and change in entropy S have been evaluated and discussed to know the spontaneity and feasibility of the process. Keywords: Red mud, adsorption, Pb(II) ion, langmuir isotherm, lagergren first-order equation, pseudo-second-order equation. IntroductionLead is used in many industries such as battery, plumbing, painting, petrochemical, smelting etc. The effluents from these industries contain lead which pollute soil and river and cause serious problem to environment and human health. Adsorption method is effective and economical among various methods to remove heavy metals from aqueous system. Though activated carbon is a very good adsorbent but it is expensive. A large number of substances have been used as adsorbents along with red mud1-5. Red mud is a by-product of aluminium industry. It mainly consists of oxides of aluminium, iron, silicon and calcium and has been suggested as a cheap adsorbent to remove heavy metals from aqueous system6-10. The aim of this study is to examine the removal characteristics of red mud to remove lead(II) from aqueous solutions. The adsorption of lead(II) ion on red mud has been evaluated as a function of initial lead ion concentration, contact time, temperature, pH and particle size. The equilibrium data has been discussed by Langmuir and Freundlich adsorption isotherm and kinetics have been discussed using Lagergren first order, pseudo second order and intraparticle diffusion model. Besides, some thermodynamic parameters such as change in Gibbs free energy G, change in enthalpy H and change in entropy S have been calculated and discussed. Material and Methods Red mud was obtained from BALCO, Korba(C.G). For characterization and morphology of red mud SEM and FTIR were obtained from SAIF-IIT Bombay. Stock solutions of Pb(II) was prepared from A.R. quality Pb(NO. 1.0 g of red mud was added in 25 ml aqueous solution of Pb(II) of given concentration in different glass bottles. It was then agitated in a shaking machine. After pre-determined time interval, the solutions were centrifused, filtered and analyzed for concentration by spectrophotometer. Initial Pb(II) concentrations used were 100, 150, 200 and 250 mgL-1. Different contact time intervals 20, 40, 60, 80, 100, 120 and 140 min. Various pH values were 2.0, 4.0, 6.5 and 8.0. Different temperatures for adsorption were 303K, 313K and323K and particle size (45µ, 75 µ and 150µ). For equilibrium study, initial Pb(II) concentration used were 25, 50, 75, 100, 125, 150, 175, 200, 225 and 250 mgL-1 . The following mass balance equation11 was used to calculate the amount of Pb(II)ion adsorbed : q = V (C – C) /m where C and Ce are Pb(II) ion concentration in mgL-1 before and after adsorption respectively, V is the volume of adsorbate in litre, and m is the weight of the adsorbent in grams. The percentage of removal of Pb(II) ion was calculated from the following equation11: Removal % = 100 ( C – C )/ Ci Results and Discussion Characterization of red mud: Different red mud contain the same basic chemical elements but in different proportions. Different compounds present are Fe, Al, SiO, CaO, NaO and TiO. Figure-1(a) is the SEM spectrum of red mud before adsorption and 1(b) after adsorption. It is evident from figure 1(b) that adsorption of lead has taken place between 1.7 to 2.7 keV. The FTIR spectra of red mud before and after adsorption is shown in figure-2. It shows a broad band around 3500 cm-1, Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 19 which is attributed to surface -OH group of silanol groups ( -Si-OH) and adsorbed water molecules on the surface12. A peak around 1400 cm-1 –1600 cm-1 is attributed to presence of carbonate. A strong peak at 995.22 cm-1 is due to stretching vibration of Si(Al)-O group13. Figure 2(b) shows new peaks at 2922.94 cm-1, 2262.68 cm-1 and 837.69 cm-1. These additional bands and variation in vibrational frequencies indicates the presence of lead on red mud surface. Figure-1 (a) Before adsorption (45µ)Figure-1 (b) After adsorption (45µ) Figure-2 (a) FTIR before adsorption 3620.75 3523.04 3283.06 3103.62 1642.07 1456.77 1406.70 995.22 803.24 686.73 564.32 457.51 500 1000 1500 2000 2500 3000 3500Wavenumber cm-1 30 40 50 60 70 80 90 100Transmittance [%] Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 20 Figure-2 (b) FTIR after adsorption Effect of initial Pb(II) ion concentration: Percentage removal of Pb(II) ion versus initial concentration has been shown in figure-3. It shows that with increase in initial Pb(II) ion concentration, the percentage removal of Pb(II) ion decreases from 85.20% at 100 mgL-1 to 70.08% at 250 mgL-1. The reason may due to the fact that adsorbents possess a limited number of active sites and these sites become saturated at certain concentration. However, q(adsorbed amount at equilibrium in mgg-1) increases with initial concentration of lead(II) ion which is evident from figure-4. It increases from 2.13 mgg-1at 100 mgL-1 to 4.38 mgg-1at 250 mgL-1. The necessary driving force to overcome the mass transfer resistance of Pb(II) ion between the aqueous and the solid phase is possibly provided by the initial concentration of metal ion. The increase in Pb(II) ion concentration also increases the interaction between Pb(II) ions in the aqueous phase and the red mud surface resulting in higher adsorption of Pb(II) for the given mass of red mud14. Figure-3 Effect of initial conc. on Pb(II) adsorption Figure-4 Effect of initial conc. on Pb(II) adsorption Effect of contact time: Removal of Pb(II) ion by red mud has been shown in figure-5. Adsorption increases with time and equilibrium is reached in 120 min. Adsorption rate is fast initially which may be due to more number of active sites on adsorbent surface. As adsorption progresses, number of active sites decreases and the rate of adsorption slows down15-16. Effect of pH: Figure-6 shows the effect of pH on adsorption of Pb(II) ion on red mud. The amount of Pb(II) adsorbed on red mud increased from 1.88 mgg-1 (75.2 %) to 2.41 mgg-1 (96.4 %) by increasing pH of solution from 2.0 to 8.0. Speciation studies17 have shown that at low pH lead remains in the form of Pb++ and at higher pH in the form of Pb(OH). It is probable that in acidic medium positively charged surface of adsorbent does not favour the association of cationic adsorbate species. In alkaline medium negatively charged surface offers the suitable sites for the adsorption of Pb++ and Pb(OH). 3621.09 3522.42 3281.83 3101.22 2922.94 2262.68 1643.22 1404.51 995.00 837.69 803.79 684.44 563.85 453.73 500 1000 1500 2000 2500 3000 3500Wavenumber cm-1 40 50 60 70 80 90 100 Transmittance [%] 0.0010.0020.0030.0040.0050.0060.0070.0080.0090.000100200300% Removal Ci mgL-1 0.51.52.53.54.50100200300qe , mgg-1 Ci mgL-1 Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 21 Effect of temperature: Figure-7 shows the effect of temperature on adsorption. It is evident that adsorption of Pb(II) ion on red mud increases from 2.13 mgg-1 (85.20 %) to 2.39mgg-1 (95.6 %) by increasing temperature from 303K to 323K indicating the process to be endothermic The rate constant of adsorption are 2.09 x 10-2, 3.74 x 10- and 4.11 x 10-2 per min at 303K, 313K and 323K respectively which indicate that the rate of adsorption also increases with temperature. Figure-5 Effect of contact time on adsorption of Pb(II) ion on red mudFigure-6 Effect of pH on adsorption of Pb(II) ion on red mud. Figure-7 Effect of temperature on adsorption of Pb(II) ion on red mud -1050100150200Amount adsorbed mgg-1 Time, min. 100mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Poly. (100mgL-1) Poly. (150 mgL-1) Poly. (200 mgL-1) Poly. (250 mgL-1) 0.51.52.5050100150200Amount adsorbed, mgg-1 Time , min. pH 2 pH 4 pH 6.5 pH 8 Poly. (pH 2) Poly. (pH 4) Poly. (pH 6.5) Poly. (pH 8 ) 0.51.52.5050100150200Amount adsorbed, mgg-1 Time , min. 303K 313K 323K Poly. (303K) Poly. (313K) Poly. (323K) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 22 Effect of particle size: The effect of particle size of red mud on adsorption of Pb(II) ion has been shown in figure-8. The amount of Pb(II) ion adsorbed on red mud increases from 1.91 mgg-1 (76.4%) to 2.13 mgg-1 (85.20 %) by decreasing particle size of red mud from 150 µ to 45 µ. This increase in amount of Pb(II) adsorbed on red mud is due to increase in surface area of red mud particles with decreasing particle size. Adsorption Isotherm: According to the Langmuir adsorption isotherm model, adsorption occurs on a homogeneous surface by monolayer adsorption without interaction between the adsorbed molecules18. The linear form of Langmuir isotherm18is given as: /q = 1/.b + C where q is the amount of lead adsorbed per gram of the adsorbent at equilibrium, C (mgL-1) is equilibrium concentration of Pb(II) and and b are Langmuir constants related to adsorption capacity and adsorption energy respectively. The plot of C/q versus C has been shown in figure-9. It is linear which shows that Langmuir isotherm is applicable. The value of and b have been calculated from slope and intercept of the straight lines obtained and are given in table-1. The result shows that the values of and b increase on increasing the temperature. Figure-8 Effect of particle size on adsorption of Pb(II) ion on red mud Figure-9 Langmuir adsorption isotherm for the adsorption of Pb(II) ion on red mud Table-1 Adsorption isotherm constants for adsorption of Pb(II) on red mud Langmuir Isotherm Results Freundlich Isotherm Results Temp.(K) Correlation coefficient, R 2 b Correlation coefficient, R 2 K f n 303 0.994 5.95 0.038 0.983 0.469 1.86 313 0.993 5.95 0.072 0.973 0.771 2.09 323 0.992 6.45 0.103 0.943 0.971 2.06 0.51.52.5020406080100120140160180Amount sorbed, mgg-1 Time , min. 45 µ 75 µ 150 µ Poly. (45 µ) Poly. (75 µ) Poly. (150 µ) y = 0.168x + 4.471R² = 0.994y = 0.168x + 2.324R² = 0.993y = 0.155x + 1.512R² = 0.9921012141618020406080Ce/qe(gL-1) Ce (mgL-1) 303 K 313 K Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 23 Freundlich adsorption isotherm model can be applied to heterogeneous surfaces involving multilayer adsorption19The linearized Freundlich equation is represented as: logq = log K + 1/n log Ce where qe is the amount of Pb(II) ion adsorbed (mgg-1), C is the equilibrium concentration of Pb(II) ion in solution (mgL-1) and and n are constants for the adsorption capacity and intensity of adsorption respectively. Plots of logq versus logC has been shown in figure-10 and values of K , n and R (correlation coefficient) value have been obtained and given in table-1. Comparing R value shows that both isotherms are applicable. However, experimental data fits better in Langmuir equation. A dimensionless separation factor (R) has been calculated using following equation19: L = 1/1+b.C where C is the initial concentration in mgL-1 and b is Langmuir constant (L/mg) related to adsorption energy. It gives important information about the nature of adsorption. If 0R1, it indicates the adsorption process to be favourable and if R&#x-3.3;女1 the process is unfavourable. It can also be explained that when b&#x-3.3;女0, adsorption system is favourable16. The calculated values are given in table- 2. The values 0R1 and b&#x-3.3;å ¦0 suggest that the process is favourable. Adsorption kinetics: The Lagergren first order20, pseudo-second-order21 and Intraparticle diffusion kinetic models22 have been used to discuss the adsorption kinetics. The Lagergren first order kinetic model: The Lagergren first order rate equation is represented as : log (q – q) = log q – k.t/2.303 where q and q are the amounts of Pb(II) adsorbed (mgg-1) at equilibrium and at time t , respectively. K is the Lagergren rate constant (min-1). Plots of log (q – q) versust has been shown in figure- 11. Values of q and K at different initial concentrations have been calculated from the slope and intercept respectively. These values have been given in table-3. Table-2 Dimensionless separation factor (R) ----- R L Ci (mgL - 1 ) 303 K 313 K 323 K 25 0.515 0.356 0.282 50 0.347 0.217 0.164 75 0.262 0.156 0.116 100 0.210 0.122 0.089 125 0.175 0.100 0.073 150 0.151 0.084 0.061 175 0.132 0.073 0.053 200 0.117 0.065 0.047 225 0.106 0.058 0.042 250 0.096 0.052 0.038 The pseudo-second-order kinetic model: The adsorption data have been applied to pseudo-second-order kinetic model also . The equation is represented as t/q = 1/K.q + t/qt where K is the rate constant of second order adsorption (g/mg/min.). Plots of t/q versus t has been shown in (figure-12). Values of K and q have been calculated from the slope and intercept of the graph respectively. These values have been given in table- 3. . Figure-10 Freundlich adsorption isotherm for adsorption of Pb(II) ion on red mud y = 0.539x -0.329R² = 0.983y = 0.478x -0.113R² = 0.973y = 0.485x -0.013R² = 0.9430.00.10.20.30.40.50.60.70.80.90.00.51.01.52.0logqe logCe 303 K 313 K 323 K Linear (303 K) Linear (303 K) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 24 . Figure-11 Lagergren first-order kinetic plot for adsorption of Pb(II) ion on red mud . Figure-12 Peudo-second-order kinetic plot for adsorption of Pb(II) ion on red mud Table-3 Kinetic parameters for adsorption of Pb(II) ion on red mud Lagergren first order Pseudo- second- order Intraparticle diffusion Conc. mgLmin-1expmgg-1calmgg-1g/mg/min calmgg-1mg/g.min1/2I R 100 2.30x10 - 2 2.13 1.127 0.993 2.96x10 - 2 2.353 0.998 0.105 1.01 0.961 150 2.99x10 - 2 3.02 1.845 0.954 2.46x10 - 2 3.311 0.998 0.129 1.652 0.988 200 2.76x10 - 2 3.86 1.927 0.988 2.10x10 - 2 4.219 0.999 0.174 2.064 0.896 250 2.76x10 - 2 4.38 1.717 0.885 2.10x10 - 2 4.739 0.997 0.194 2.406 0.766 The Intraparticle diffusion model: TheWeber and Morris intraparticle diffusion model is expressed as: = K . t1/2 + I where I is the intercept which reflects the boundary layer effect and K is the intra-particle diffusion rate constant. Plot of qversus t1/2 has been shown in figure-13. From the slope and intercept the value of K and I have been calculated and are given in table- 3. If the plot of q versus t1/2 is linear and passes through the origin then Intraparticle diffusion is considered to be the sole rate-limiting step11. As the linear plots did not pass through the origin, it is evident that intraparticle diffusion is not the only rate limiting step. y = -0.010x + 0.052R² = 0.993y = -0.013x + 0.266R² = 0.954y = -0.012x + 0.285R² = 0.988y = -0.012x + 0.235R² = 0.885-1.4-1.2-1.0-0.8-0.6-0.4-0.20.00.20.4020406080100120log(qe -qt) Time, min 100 mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Linear (100 mgL-1) y = 0.425x + 6.101R² = 0.998y = 0.302x + 3.703R² = 0.998y = 0.237x + 2.679R² = 0.999y = 0.211x + 2.124R² = 0.997102030405060050100150t/qt Time, min. 100 mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Linear (100 mgL-1) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 25 Figure-13 Intraparticle diffusion model for adsorption of Pb(II) ion on red mud It is evident from table- 3 that pseudo-second-order kinetic model shows high correlation coefficient (R � 0.99) at all the studied concentration in comparision to the other kinetic models. Moreover, qe(cal) values agree better with the experimental data in the case of pseudo-second-order kinetic model. In general the rate constant K decreases with increase in concentration. The reason for this may be the possibility of lower competition for surface active sites of adsorbent at lower concentration. As the concentration of the metal ion increases, the competition for the surface active sites increases which decreases the rate23. Thermodynamic treatment of the adsorption process: The thermodynamic parameters such as free energy, enthalpy and entropy changes have been calculated using the following equations 24. c = C/C G = - RT ln Kc log K = S/2.303 R - H/2.303 RT where C is the equilibrium concentration in solution in mgL-1 and C is the equilibrium concentration on the adsorbent in mgL and K is the equilibrium constant. The Gibbs free energy, G was calculated from the above equation. The values of H and S have been calculated from the slope and intercept of the plot between logK versus 1/T shown in figure-14. All these values are listed in table- 4. Figure-14 Plot of logKvs 1/T Figure-15 Plot of log(1-) vs 1/T The values of activation energy (E) and sticking probability (S*) have been calculated from the experimental data. They were calculated using modified Arrhenius type equation related to surface coverage() as follows25 = ( 1- C/C) S* = (1- )e -Ea/RT The sticking probability, S*, is a function of the adsorbate/adsorbent system under consideration, depending on temperature and should satisfy the condition 0S*1 .The values of E and S* has been calculated from slope and intercept of the plot of ln(1-) versus 1/T shown in figure-15 respectively and have been given in table-4. Table-4 Thermodynamic parameters for adsorption of Pb(II) ion on red mud Temp. K G , kJ/mol H , kJ/mol S , J/mol , kJ/mol S*, J K mol-1 303 -4.411 54.11 193.19 49.705 3.93X10-10 313 -6.501 323 -8.268 It is evident from table-4 that as G values are negative , the process is spontaneous. Endothermic nature of adsorption is y = 0.105x + 1.010R² = 0.961y = 0.129x + 1.652R² = 0.988y = 0.174x + 2.064R² = 0.896y = 0.194x + 2.406R² = 0.7660.001.002.003.004.005.00051015qt t1/2 100 mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Linear (100 mgL-1) y = -2826.x + 10.09R² = 0.997log Kc 1/T y = 2596.x -9.405R² = 0.998-1.5-1-0.50.00300.00310.00320.00330.0034log(1 -) 1/T Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 26 indicated by positive H value . The positive value of S shows the affinity of the adsorbent for the Pb(II) ions. The value of E has been found to be 46.048 kJ mol-1 for the adsorption . The endothermic nature of the adsorption process is supported by positive value of E .This is in accordance with the positive values of H. Since S*1, it indicates that the probability to stick on surface of red mud is very high26. Mechanism: Speciation17 of Pb(II) with varying pH has been shown in figure-16. Figure-16Speciation of Pb(II) with varying pH It is evident that at lower pH, lead is in the form of Pb+2 and at higher pH it is in the form of Pb(OH) . It is probable that in acidic medium positively charged surface of adsorbent does not favour the association of cationic adsorbate species. In alkaline medium negatively charged surface offers the suitable sites for the adsorption of Pb+2 and Pb(OH) species27,28. OHM – OH ---------- MOMO + Pb+2 ---------- MOPb MO + Pb(OH) ---------- MOPb(OH) where M represents the adsorbent surface. Conclusion It is evident that initial Pb(II) ion concentration, contact time, pH and temperature have marked effect on adsorption. The equilibrium data are best explained by Langmuir adsorption isotherm. Kinetics of adsorption follows second order rate equation. Thermodynamic parameters also favour the adsorption. It is expected that red mud may be used as an efficient adsorbent under suitable conditions. Acknowledgement We are thankful to SAIF, IIT Bombay, for SEM and FTIR analysis of red mud. References 1.Bhatnagar A. and Minocha A.K., Conventional and nonconventional adsorbents for removal of pollutants from water – A review, Indian J.Chem.Tech.,13, 203-217 (2006)2.Karthika C. and Sekar M., Removal of Hg(II) ions from aqueous solution by acid acrylic resins : A study through adsorption isotherms analysis, I.Res.J.Environment.Sci., 1(1), 34-41(2012) 3.Singh Dhanesh and Singh A., Chitosan for the removal of chromium from waste water., I.Res.J.Environment.Sci., 1(3), 55-57(2012) 4.Samuel P., Ingmar P., Boubia C. and Daniel L., Trivalent chromium removal from aqueous solutions using raw natural mixed clay from BURKINA FASO., I.Res.J.Environment Sci., 2(2), 30-37 (2013) 5.Kini S.M., Saidutta M.B., Murty V.R.C. and Kadoli S.V., Adsorption of basic dye from aqueous solution using ACl treated saw dust (Lagerstroemia microcorpa): Kinetic , Modeling of Equilibrium, Thermodynamic, I. Res. J. Environment.Sci., 2(8), 6-16 (2013) 6.Haq B.I.U., Elias N.B. and Khanam Z., Adsorption studies of Cr(VI) and Fe(II) aqua solution using rubber tree leaves, I.Res.J.Environment.Sci., 2(12), 52-56 (2013) 7.Nadaroglu H. and Kalkan E., Removal of cobalt (II) ions from aqueous solutions by using alternative adsorbent industrial red mud waste material.l, Int.J.Phy.Sciences., , 1386-1394 (2012)8.Han S.W., Kim D.K., Hwang I.G. and Bae J.H., Development of Pellet-type Adsorbents for Removal of Heavy Metal Ions from Aqueous Solutions using Red Mud, J.Ind.Eng.Chem., 8(2), 120-125 (2002)9.Kim J.S., Han S.W., Hwang I.G., Bae J.H. and Tokunaga S., Astudy on removal of Pb++ ion using pellet-type red mud adsorbents, Env.Eng.Res., 7(1), 33-37(2002)10.Mobasherpour I., Salahi E. and Asjodi A., Research on the batch and fixed bed column performance of red mud adsorbents for lead removal, Canadian Chemical Transactions, 2(1), 83-96 (2014) 11.Das B., Mondal N.K., Roy P. and Chatterji S., Equilibrium, Kinetic and Thermodynamic Study on chromium(VI) removal from aqueous solutions using Pistia Stratiotes Biomass, Chem Sci Trans., 2(1),85-104 (2013)12.John C., Interpretation of Infrared Spectra, A Practical Approach,Encyclopedia of Analytical Chemistry, R.A.Heyers(Ed.), John Wiley & Sons Ltd. Chichester, 10815 – 10837 (2000) 13.Ekrem Kalkan, et.al., Bacteria – Modified Red Mud for Adsorption of Cadmium Ions from Aqueous Solutions, Pol.J.Environ. Stud., 22(2), 417–429 (2013)14.Tsai W.T. and Chen H.R., Removal of malachite green from aqueous solution using low-cost chlorella-based biomass, J Hazard Mater., 175(1-3), 844-849 (2010) Research Journal of Recent Sciences ______________________________________________________________ ISSN 2277-2502Vol. 3(7), 18-27, July (2014) Res. J. Recent Sci. International Science Congress Association 27 15.Sarin V. and Pant K.K., Removal of chromium from industrial waste by using eucalyptus bark, Bioresource Technol., 97(1), 15-20 (2006) 16.Wongjunda J. and Saueprasearsit P.,Biosorption of Chromium(VI) using rice husk ash and modified husk ash Environ Res. J., 4(3), 244-250 (2010)17.Brummer G.W., Importance of Chemical Speciation in Environmental Process (Springer Verlag, Berlin) (1986) 18.Bansal M. Singh D, Garg V K, Rose P., Use of agricultural waste for the removal of nickel ions from aqueous solutions: Equilibrium and kinetic studies, World Acad.Sci.Eng.Technol., 51, 431-437(2009)19.Anirudhan T.S. and Radhakrishnan P.G., Thermodynamics and kinetics of adsorption of Cu(II) from aqueous solutions onto a new cation exchanger derived from tamarind fruit shell, J.Chem.Thermodynamics., 40(4), 702-709 (2008) 20.Lagergren S., About the theory of so-called adsorption of soluble substsnces,der Sogenanntenadsorption geloster stoffe Kungliga Svenska psalka de Miens Handlingar., 24, 1-39(1898) 21.Ho Y.S. and Mckay G., The kinetics of sorption of divalent metal ions onto sphagnum moss peat., Water Res. 34(3),735-742 (2000) 22.Weber W.J. and Morris J.C., Kinetics of adsorption on carbon from solution, J. Saint. Eng. Div. Am. Soc. Eng., 89, 31-60 (1963) 23.Kumar P.S., Ramakrishnan K., Kirupha S.D and Sivanesan S. Thermodynamic and Kinetic studies of cadmium adsorption from aqueous solution onto rice husk, Braz.J.Chem.Eng., 27, 347 (2010) 24.Arivoli S., Hema M., Karuppaiah M. and Saravanan S., Adsorption of chromium ion by acid activated low cost carbon-Kinetic, Mechanistic,Thermodynamic and Equilibrium studies, E-Journal of Chemistry., 5(4), 820-831 (2008) 25.Senthilkumar P., Ramalingam S., Sathyaselvabala V., Kirupha D.S. and Sivanesan S., Desalination, 266(1-3), 63-71 (2011) 26.Nevine K.A., Removal of direct blue-106 dye from aqueous solution using new activated carbons developed from pomegranate peel: Adsorption equilibrium and kinetics, J. Haz. Mat.., 165(1-3), 52-62 (2009) 27.Singh Dhanesh and Rawat N.S., Bituminous coal for the Removal of Cd rich water, Ind. J. Chem. Technol., , 266-270 (1994) 28.Singh Dhanesh and Rawat N.S., Sorption of Pb(II) by bituminous coal, Ind. J. Chem. Technol., , 49-50 (1995)