Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 44 A Study on Removal of Cadmium(II) from Aqueous solutions by Adsorption on Red MudKumar Sujata, Singh D., Upadhyay M., Mishra A.K and Kumar Saroj Department of Chemistry, K. Govt. Arts and Sc. College Raigarh, CG, INDIA Department of Chemistry, Kirodimal Institute of Technology, Raigarh, CG, INDIA Department of Chemistry, Dr. C.V. Raman University, Bilaspur, CG, INDIA Available online at: www.isca.in, www.isca.me Received 10th February 2014, revised 15th March 2014, accepted 14th April 2014Abstract The present study aims to evaluate the removal characteristics of red mud as adsorbent to remove Cd(II) ion from aqueous solutions by batch experiments under various experimental conditions. Freundlich and Langmuir adsorption isotherm models have been used to discuss the data obtained. Lagergren first-order equation, pseudo-second-order equation and intra-particle diffusion models have been used to discuss the kinetics. To have an idea about spontaneity and feasibility of the adsorption process, thermodynamic parameters such as change in free energy G, change in enthalpy H and change in entropy S have been evaluated and discussed. Keywords: Red mud, adsorption, Cd(II) ion, Langmuir isotherm, Lagergren first-order equation, pseudo-second- order equation. Introduction Cadmium is used in many industries including paints, batteries, alloys, electric contacts etc. The wastes of these industries contain cadmium 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. 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 oxides of aluminium, iron, silicon and calcium and has been suggested as a cheap adsorbent to remove heavy metals from aqueous system6-10. To evaluate adsorption characteristics of red mud this study has been carried out. Different conditions of experiments are: Initial Cd(II) ion concentration, contact time, pH, temperature and particle size. Adsorption kinetics, different isotherms and thermodynamic parameters have been 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 Cd(II) was prepared from A.R. quality Cd(NO. 1.0 g of red mud was added in 25 ml aqueous solution of Cd(II) of given concentration in different glass bottles and was shaken in shaking machine.At different time intervals, the solutions were centrifused, filtered and analyzed for concentration by spectrophotometer. Different parameters were : Initial Cd(II) concentration (100,150, 200 and 250 mgL-1), contact time (20,40,60,80,100,120 and 140 min.), pH (2.0, 4.0, 6.5 and 8.0), temperature (303K, 313K and 323 K) and particle size (45µ, 75µ and 150µ) Initial Cd(II) concentration used were 25, 50, 75, 100, 125, 150, 175, 200, 225 and 250 mgL-1 for the equilibrium study. The following mass balance equation11 was used to calculate the amount of Cd(II)ion adsorbed : = V (C – C) /m where C and Ce are Cd(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 Cd(II) ion was calculated from the following equation11 : Removal % = 100 ( C – C )/ Ci Results and Discussion Characterisation 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 cadmium has taken place between 3 to 4 keV. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 45 The FTIR spectra of red mud before and after adsorption is shown in figure-2. It shows a broad band around 3500 cm-1 , 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 group12. Figure 2(b) shows a new peak at 861.61 cm-1 which is associated with Cd-O(H) stetching vibrations13 showing the adsorption. Figure-1(a) Before adsorption (45µ) Figure-1(b) After adsorption (45µ) 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 [%]Figure-2(a) FTIR before adsorption Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 46 3781.62 3621.05 3522.58 3281.82 3101.06 2262.62 1643.51 1411.01 994.93 861.61 804.32 708.14 564.27 453.74 500 1000 1500 2000 2500 3000 3500Wavenumber cm-1 50 60 70 80 90 100Transmittance [%]Figure-2(b) FTIR after adsorption Effect of initial Cd(II) ion concentration: Graph between percentage removal of Cd(II) ion versus different initial concentrations has been shown in figure-3. It is evident that with increase in initial Cd(II) ion concentration, the percentage removal of Cd(II) ion decreases from 77.60% at 100 mgL-1 to 65.92% at 250 mgL-1. It may due to the fact that adsorbents possess a limited number of active sites and these sites become saturated at certain concentration. Figure-4 shows the plot between adsorbed amount at equilibrium, q (mgg-1) and initial concentration of Cd(II) ion. It is evident that q increases with increase in concentration. It increases from 1.94 mgg-1(77.6%) at 100 mgL-1 to 4.12 mgg(65.92%) at 250 mgL-1. The necessary driving force to overcome the mass transfer resistance of Cd(II) ion between the aqueous and the solid phase is possibly provided by the initial concentration of metal ion. The increase in Cd(II) ion concentration also increases the interaction between Cd(II) ions in the aqueous phase and the red mud surface resulting in higher adsorption of Cd(II) for the given mass of red mud14. Figure-3 Effect of initial conc. on Cd(II) adsorption Figure-4 Effect of initial conc. on Cd(II) adsorption Effect of contact time: Figure-5 shows that removal of Cd(II) ion by red mud increases with time. The time of saturation (120 min.) is independent of concentration. 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: The effect of pH on adsorption of Cd(II) ion on red mud has been shown in figure-6. The amount of Cd(II) adsorbed on red mud increased from 0.74 mgg-1( 29.6 %) to 2.28 mgg-1 (91.2 %) by increasing pH of solution from 2.0 to 8.0. Speciation studies17 have shown that at low pH cadmium remains in the form of Cd++ and at higher pH in the form of Cd(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 Cd++ and Cd(OH) . Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 47 Effect of temperature: Temperature has a marked effect on adsorption. From figure-7 it is evident that adsorption of Cd(II) ion on red mud increases from 1.94 mgg-1 (77.60 %) to 2.32 mgg-1 ( 92.8 %) by increasing temperature from 303K to 323K indicating the process to be endothermic The rate constant of adsorption are 1.5 x 10-2, 1.9 x 10- and 2.6 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 Cd(II) ion on red mud Figure-6 Effect of pH on adsorption of Cd(II) ion on red mud Figure-7 Effect of temperature on adsorption of Cd(II) ion on red mud Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 48 Effect of particle size: Figure-8 shows the effect of particle size of red mud on adsorption of Cd(II) ion. The amount of Cd(II) ion adsorbed on red mud increases from 1.73 mgg-1(69.2%) to 1.94 mgg-1 (77.60 %) by decreasing particle size of red mud from 150 µ to 45 µ. This increase in amount of Cd(II) adsorbed on red mud is due to increase in surface area of red mud particles with decreasing particle size. Adsorption Isotherm: The linear form of Langmuir isotherm18is given as: /q = 1/.b + C where C (mgL-1) is equilibrium concentration of Cd(II) and and b are Langmuir constants related to adsorption capacity and adsorption energy respectively. The linear plot of C/qversus C shown in figure-9 suggests that Langmuir isotherms is applicable. From slope and intercept of the straight lines obtained, values of and b have been calculated 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 Cd(II) ion on red mud Figure-9 Langmuir adsorption isotherm for the adsorption of Cd(II) ion on red mud Table-1 Adsorption isotherm constants for adsorption of Cd(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 0.99 5.99 0.023 0.995 0.313 1.71 313 0.988 6.41 0.037 0.993 0.457 1.76 323 0.998 6.45 0.079 0.975 0.793 1.95 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 49 The Freundlich equation19 has also been used for the adsorption of cadmium (II) on red mud which is represented as:logq = log K + 1/n log Cwhere qe is the amount of Cd(II) ion adsorbed (mgg-1), C is the equilibrium concentration of Cd(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;女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 Cd(II) adsorbed (mgg-1) at equilibrium and at time t , respectively. K is the Lagergren rate constant (min-1). Plots of log (q – q) versus t 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. Figure-10 Freundlich adsorption isotherm for adsorption of Cd(II) ion on red mud Table-2 Dimensionless separation factor (R) ----- L Ci (mgL - 1 ) 303 K 313 K 323 K 25 0.635 0.519 0.337 50 0.465 0.351 0.202 75 0.367 0.265 0.145 100 0.303 0.213 0.113 125 0.258 0.178 0.092 150 0.225 0.153 0.078 175 0.199 0.134 0.068 200 0.179 0.119 0.060 225 0.162 0.107 0.053 250 0.148 0.098 0.048 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 50 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. 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. Figure-11 Lagergren first-order kinetic plot for adsorption of Cd(II) ion on red mud . Figure-12 Peudo-second-order kinetic plot for adsorption of Cd(II) ion on red mud Table-3 Kinetic parameters for adsorption of Cd(II) ion on red mud Lagergren first order Pseudo- second- order Intraparticle diffusion Conc. mgL-1min-1expmgg-1calmgg-1 2 Kg/mg/min calmgg-1 2 Kmg/g.min1/2I R 2 100 2.07x10 - 2 1.94 1.41 0.998 1.46x10 - 2 2.342 0.993 0.136 0.444 0.99 150 2.07x10 - 2 2.77 1.01 0.994 3.67x10 - 2 2.941 0.998 0.097 1.718 0.969 200 2.53x10 - 2 3.47 1.25 0.982 3.51x10 - 2 3.676 0.999 0.115 2.263 0.909 250 2.30x10 - 2 4.12 1.25 0.970 3.39x10 - 2 4.329 0.999 0.121 2.841 0.913 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 51 Figure-13 Intraparticle diffusion model for adsorption of Cd(II) ion on red mud It is evident from table- 3 that the kinetic data are better explained by pseudo-second-order kinetic model as it 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 rate. Other studies also support it23. 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 K, 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 Cd(II) ion on red mud Temp. K G , kJ/mol H , kJ/mol S , J/mol , kJ/mol S*, J K mol-1 303 -3.130 53.34 185.99 46.048 2.70X1009 313 -4.639 323 -6.866 It is evident from table-4 that as G values are negative, the process is spontaneous. Endothermic nature of adsorption is indicated by positive H value. The positive value of S shows the affinity of the adsorbent for the Cd(II) ions. The value of E has been found to be 46.048 kJ mol-1 for the adsorption. The Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(4), 44-53, April (2014) Res. J. Chem. Sci. International Science Congress Association 52 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: Speciation23 of Cd(II) with varying pH has been shown in figure-16. Figure-16 Speciation of Cd(II) with varying pH It is evident that at lower pH, cadmium is in the form of Cd+2and at higher pH it is in the form of Cd(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 Cd+2 and Cd(OH) species27,28. OHM – OH ---------- MOMO + Cd+2 ---------- MOCd MO + Cd(OH) ---------- MOCd(OH) where M represents the adsorbent sites on surface.Conclusion It is evident that initial Cd(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 The authors are thankful to DR.Surekha Thakkar, Vice-Chancellor, DR.C.V.Raman University, Bilaspur(C.G.), Sri Shailesh Pandey, Registrar, DR. C.V.Raman University, Bilaspur(C.G.) and DR. A.K.Shrivastava, Principal, K.Govt. Arts & Sc. College Raigarh(C.G.) for providing research facilities and co-operation to carry out the work. One of the authors is thankful to Prof.R.S.Tomar,Director,K.I.T. 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