Research Journal of Chemical Sciences ______________________________________________ISSN 2231-606XVol. 4(5), 20-28, May (2014) Res.J. Chem. Sci. International Science Congress Association 20 Removal of Zinc(II) from Aqueous Solution Using Fly Ash Saroj Kumar1*, Mishra A.K., Upadhyay M., D Singh, M Mishra and Sujata KumarDepartment of Chemistry, K.Govt. Arts and Sc. College, Raigarh, CG, INDIA Department of Chemistry, Dr. C.V.Raman University, Bilaspur, CG, INDIA Department of Chemistry, Kirodimal Institute of Technology, Raigarh, CG, INDIA Available online at: www.isca.in, www.isca.me Received 19thFebruary2014, revised 22nd March 2014, accepted 5thApril 2014Abstract The removal characteristics of fly ash has been evaluated to remove Zn(II) from aqueous solution under different conditions. Batch experiments have been carried out for this purpose. Kinetics of adsorption have been discussed using Lagergren first order equation, pseudo second order equation and intraparticle diffusion models. Langmuir and Freundlich adsorption isotherms have been used to discuss the data. Different thermodynamic parameters such as change in Gibbs free energy G, change in enthalpy H and change in entropy S have been calculated to discuss the spontaneity of the process. Various experimental conditions are : initial Zn(II) ion concentration, temperature, pH and particle size. Keywords: Fly ash, adsorption, zinc (II) ion, Langmuir isotherm, pseudo-second-order equation, Intraparticle diffusion model, Freundlich isotherm. IntroductionZinc and its alloys are mostly used in galvanization, diecasting, plastic, paints and cosmetic industries.The effluents of these industries pollute soil and water. It causes liver damage and other health problems. Out of various methods to remove heavy metals from aqueous system, adsorption method is effective and economical. Studies have shown that there are number of substances which can be used as adsorbent1-6. In the present study, the removal characteristics of fly ash, an industrial waste of thermal power plant, have been evaluated as a low cost adsorbent. Effects of four different parameters – initial Zn(II) ion concentration, temperature, pH and particle size on adsorption have been investigated. Kinetics of adsorption, different isotherms and thermodynamics of adsorption have been discussed. Material and MethodsThe fly ash used in this study was of JSPL, a thermal power plant at Raigarh (C.G).To characterise it, XRF, FTIR and SEM image were obtained. A.R quality Zn(NO has been used to prepare stock solution of Zn(II) ion. 1.0 g of fly ash of desired size was added to 25 mL of the aqueous solution of Zn(II) of known concentration at fixed pH. Shaking machine was used to shake it at desired temperature. After pre-determined time interval,it was centrifused and filtered. The remaining concentration of Zn(II) ion was determined by spectrophotometer. Different initial Zn(II) ion concentration for rate study was 100,150,200 and 250 mgL-1. At 303K,313K and 323K studies were performed. Various pH values were 2.0, 4.0, 6.5 and 8.0. Particle sizes were 45µ, 75µ and 150µ. For equilibrium studies 25, 50, 75, 100, 125, 150, 175, 200 and 250 mgL-1 of Zn(II) were used. Following equation has been used to determine the adsorbed amount of Zn(II) in mgg-1 . = V (C – C) /m Where represents Zn(II) ion concentration in mg/L , represents the concentration of Zn(II) ion after adsorption in mg/L , is the volume of Zn(II) ion in solution in and is the mass of fly ash in. Removal (%) of Zn(II) ion was determined using equation mentioned below: Removal % = 100 ( C – C )/ Ci Results and DiscussionCharacterisation of fly ash: XRF studies of fly ash sample shows the composition as given in table-1 Table-1 Chemical composition of fly ash Constituent wt (%) SiO 2 43.170 Al 2 O 3 13.248 Fe 2 O 3 41.198 CaO 1.090 MgO 0.727 TiO 2 1.262 It may be seen that SiO and Al constitute about 56.42% of fly ash and Fe and CaO constitute about 42.29 %. As CaO content is less than 10% and SiO, Al and Fe is greater Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 21 than 70% so the fly ash used in this study may be classified as class F. Figure-1 presents the SEM image of fly ash which shows that particles of small size (45µ) are mainly spherical whereas the big size (150µ) particles are porous in nature and irregular in shape. Figure- 1(c) is the SEM image of fly ash(45µ) after adsorption. The FTIR spectra of fly ash before and after adsorption is shown in figure-2. The main broad band at 1084.88 cm-1 in the fly ash before adsorption, corresponds to asymmetric stretching vibrations of Si-O-Si and Al-O-Si becomes sharper and shifts toward lower frequency 1061.97 cm-1 as a result of the formation of new reaction products. (a) (b) (c) Figure-1 (a) Before adsorption (b)Before adsorption (150µ) (c) After adsorption Figure-2 (a) FTIR Before adsorption Figure-2 (b) FTIR After adsorption 3917.08 3855.17 3793.08 3707.42 3625.07 3439.72 3103.71 2923.40 2854.39 2262.71 2225.49 1815.30 1728.14 1591.85 1434.21 1381.50 1061.97 775.15 736.16 565.66 466.62 500 1000 1500 2000 2500 3000 3500Wavenumber cm-1 93 94 95 96 97 98 99 100Transmittance [%] Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 22 Effect of initial Zn(II) ion concentration: The relationship between C and % removal has been represented in figure-3 . It is evident that the % removal decreases from 76.4% at 100 mgL-1 to 64.61% at 250 mgL-1. It means as C increases the percentage removal decreases.. The reason might be the possibility of saturation of active sites present in the adsorbent at certain concentration10. It may also be seen that when the initial concentration of Zn(II) is 100mgL-1 , the adsorbed amount at equilibria, qe , is 1.91 mgg-1 whereas at initial concentration of 250 mgL-1 it is 4.04 mgg-1. This means that actual amount adsorbed increases with concentration.The reason might be that higher concentration of sorbate provides required driving force so as to exceed the mass transfer resistance of Zn(II) ion of the two phases i.e. the liquid and solid phase. Besides, due to high concentration of Zn(II), interaction of Zn(II) with fly ash surface increases resulting more adsorption10. Figure-3 Plot of Cvs % removal Figure-4 Plot of qvs CEffect of contact time: Amount adsorbed(mgg-1vs time(min.) has been shown in figure-5. It is evident that amount adsorbed increases till saturation. It has been found that initially rate of adsorption is fast and slows down till saturation. The reason might be that before adsorption, fly ash surface possess a large number of active sites. So, the initial rate of adsorption is high. But as adsorption goes on, the number of active sites get reduced and consequently the rate of adsorption also slows down11-12. Effect of pH: Adsorption of Zn(II) ion on fly ash is much influenced by pH of the medium. Amount adsorbed vs pH has been shown in figure-6. It is evident that amount adsorbed increases with increase in pH . It increases from 1.62 mgg(64.8%) to 2.36mgg-1(94.4%) by increasing pH of solution from 2.0 to 8.0. Studies have shown13 that at low pH fly ash particles are positively charged and at high pH negative charge is dominant. When pH is increased, there is more electrostatic attraction between Zn2+ ion which is positively charged and negatively charged fly ash surface. In morealkaline medium both adsorption and precipitation take place. Effect of temperature: Temperature influences much the process of adsorption. As temperature increases, adsorption increases. It increases from 1.91mgg-1(76.4%) at 303K to 2.26mgg-1(90.4%)at 323K. The rate constant of adsorption are 2.8x10-2 and 3.4x10-2 per min at 303K and 323K respectively which shows that the adsorption rate increases with temperature. It indicates that nature of adsorption is endothermic. Effect of size of fly ash particle: Figure-8 shows the effect of particle size on adsorption. The amount adsorbed is 1.71mgg-1(68.4%) for 150 µ and 1.91mgg-1 (76.4%)for particle size of 45µ. It is evident that as particle size of fly ash decreases, the amount of Zn(II) ion adsorbed increases. The reason might be that as size of particle decreases, its surface area increases. As a result the number of active sites increases, thereby, increasing the adsorption. Figure-5 Effect of contact time on adsorption of Zn(II) ion on fly ash 60657075800100200300% Removal , mgL-1 0100200300qe , mgg-1 Ci, mgL-1 -1050100150200Amount adsorbed , mgg-1 Time , min 100mgL-1 150mgL-1 200mgL-1 250mgL-1 Poly. (100mgL-1) Poly. (150mgL-1) Poly. (200mgL-1) Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 23 Figure-6 Effect of pH on adsorption of Zn(II) ion on fly ashFigure-7 Effect of temperature on adsorption of Zn(II) ion on fly ash Figure-8 Effect of particle size on adsorption of Zn(II) ion on fly ash Adsorption Isotherm: The linear form of the Langmuir and Freundlich isotherm14 have been used to analyze the experimental data. The Langmuir isotherm is given by the following equation /q = 1/.b + C where C (mgL-1) is equilibrium concentration of Zn(II) and and b are Langmuir constants. is related to adsorption capacity and b is related to adsorption energy. The plot of /qversus C shown in figure-9 is linear which suggests that Langmuir isotherms is applicable. Slope and intercept of the straight line obtained has been used to calculate and b respectively. These values have been given in table-2. It can be seen that as temperature increases values of and b also increases. Adsorption data have been discussed using Freundlich equation which is given as: logq = log K + 1/n log Ce -0.50.51.52.5050100150200Amount adsorbed, mgg-1 Time , min. pH 2 pH 4 pH 6.5 pH 8 Poly. (pH 2) Poly. (pH 4) Poly. (pH 6.5) -0.50.51.52.5050100150200Amount adsorbed, mgg-1 Time , min. 303K 313K 323K Poly. (303K) Poly. (313K) Poly. (323K) 0.51.52.5020406080100120140160180Amount sorbed, mgg-1 Time , min. 45 µ 75 µ 150 µ Poly. (45 µ) Poly. (75 µ) Poly. (150 µ) Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 24 where qe represents the amount of zinc ion adsorbed (mgg-1), the equilibrium concentration of zinc ion in solution(mgL-1) is represented by C. Kis the adsorption capacity and n is the intensity of adsorption. 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-2. Comparing R value obtained from Langmuir plots and Freundlich plots clearly shows that the experimental data fits better in Langmuir equation. A dimensionlessseparation factor (R) gives important information about the nature of adsorption and is defined as15 L = 1/1+b.Cwhere C is the initial concentration in mgL-1 and b is Langmuir constant (L/mg) related to adsorption energy. For favourable adsorption 0R1 and for unfavourable adsorption &#x-15.;䌒1. Besides, when b&#x-15.;䌒0, adsorption system is favourable16. The calculated values are given in Table-3. The values 0R1 and b&#x-3.3;女0 suggest that the process is favourable. Figure-9 Langmuir adsorption isotherm for the adsorption of Zn(II) ion on fly ash Figure-10 Freundlich adsorption isotherm for adsorption of Zn(II) ion on fly ash Table-2 Langmuir and Freundlich isotherm constants for adsorption of Zn(II) on fly ash Langmuir Isotherm ResultsFreundlich Isotherm Results Temp.(K) R 2 b R 2 K f n 303 0.995 6.33 0.020 0.988 0.262 1.597 313 0.997 6.80 0.031 0.974 0.406 1.672 323 0.982 6.99 0.044 0.990 0.527 1.721 y = 0.158x + 8.057R² = 0.995y = 0.147x + 4.745R² = 0.997y = 0.142x + 3.337R² = 0.98210152025020406080100Ce/qe(gL-1) Ce(mgL-1) 303 K 313 K 323 K Linear (303 K) y = 0.626x -0.582R² = 0.988y = 0.598x -0.392R² = 0.974y = 0.581x -0.278R² = 0.9900.00.10.20.30.40.50.60.70.80.00.51.01.52.02.5logqe logCe 303 313 323 Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 25 Table-3 Dimensionless separation factor (R) ----- R L Ci (mgL - 1 ) 303 K 313 K 323 K 25 0.671 0.563 0.478 50 0.505 0.392 0.314 75 0.405 0.301 0.234 100 0.338 0.244 0.187 125 0.290 0.205 0.155 150 0.254 0.177 0.133 175 0.226 0.156 0.116 200 0.203 0.139 0.103 225 0.185 0.125 0.093 250 0.169 0.114 0.084 Kinetics of adsorption: Theadsorption kinetics have been discussed by usingLagergren first order17, pseudo-second-order18 and Intraparticle diffusion kinetic models19. The Lagergren kinetic model for first order: First order rate equation of Lagergren is given as: log (q – q) = log q – k.t/2.303 where q and q are the amounts of Zn(II) sorbed (mgg-1) at equilibrium and at time t , respectively. K is the first order rate constant (min-1). Plots of log(q – q) versus t has been shown in figure- 11. From the slope and intercept q and K have been calculated respectively and have been given in table-4. 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 (g mg-1min-1). Figure-12 shows plot of t/q versus t. Values of K and q have been calculated from the straight lines obtained. These values have been given in table-4. Figure-11 Lagergren first-order kinetic plot for adsorption of Zn(II) ion on fly ash Figure-12 Peudo-second-order kinetic plot for adsorption of Zn(II) ion on fly ash y = -0.016x + 0.189R² = 0.998y = -0.014x + 0.235R² = 0.992y = -0.012x + 0.144R² = 0.957y = -0.013x + 0.220R² = 0.995-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.0020406080100120log(qe -qt) Time , min. 100 mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Linear (100 mgL-1) y = 0.458x + 7.393R² = 0.999y = 0.326x + 4.330R² = 0.999y = 0.262x + 3.161R² = 0.998y = 0.226x + 2.489R² = 0.99810203040506070020406080100120140t/qt Time, min. Zn on F.A 100 mgL - 1 150 mgL- 200 mgL- 250 mgL- Linear (100 mgL-1) Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 26 The Intraparticle diffusion model : The data obtained have been analyzed by Weber and Morris intraparticle diffusion model which is represented as: = K . t1/2 + I where I is the intercept . It reflects the boundary layer effect. is the diffusion rate constant. Figure-13 shows the graph betweenq and t1/2. Straight lines are obtained from which values of K and I can be calculated. Slope gives the value of and intercept gives the value of I. These values have been given in table-4. Intraparticle diffusion is considered as rate-limiting step if the straight lines obtained pass through the origin. It is evident from the figure that the linear plots did not pass through the origin, so, intraparticle diffusion is not the only rate limiting step and some boundary layer effect is indicated. It is evident from table- 4 that the kinetic data follows pseudo-second-order kinetic model as R� 0.99 which is higher in comparison to the other kinetic models. Moreover, e(cal)obtained from pseudo-second-order kinetic model is in better agreement with the q(exp) Thermodynamics of adsorption: Thermodynamic parameters give some insight into the process of adsorption. Whether the adsorption process is feasible or not can be predicted by the parameters such as free energy change G, enthalpy change H and entropy change S. These parameters have been calculated using the following equations20. c = C/CG = - RT ln Klog K = S/2.303 R - H/2.303 RT In the above equation C represents the concentration of Zn(II) in solution in mgL-1 at equilibrium. C represents the concentration of Zn(II) on the fly ash in mgL-1 at equilibrium. is the equilibrium constant. The value of K was used to calculate the value of G . Plot between logK and 1/T gives a straight line shown in figure-14. The slope and intercept of this straight line has been used to calculate H and S. These values have been given in table-5. Table-4 Kinetic parameters for adsorption of Zn(II) ion on Fly ash Conc. mgL-1First order Lagergren Pseudo- second- order Intraparticle diffusion min-1expmgg-1calmgg-1g/mg/min calmgg-1mg/g.min1/2I R 100 3.68x10-2 1.91 1.55 0.998 2.84x10-2 2.183 0.999 0.110 0.787 0.891 150 3.22x10-2 2.76 1.72 0.992 2.45x10-2 3.067 0.999 0.139 1.329 0.89 200 2.76x10-2 3.47 1.39 0.957 2.17x10-2 3.817 0.998 0.171 1.722 0.821 250 2.99x10-2 4.04 1.66 0.995 2.05x10-2 4.425 0.998 0.190 2.126 0.809 Figure-13 Intraparticle diffusion model for adsorption of Zn(II) ion on fly ash y = 0.110x + 0.787R² = 0.891y = 0.139x + 1.329R² = 0.890y = 0.171x + 1.722R² = 0.821y = 0.190x + 2.126R² = 0.8090.000.501.001.502.002.503.003.504.004.50051015qt t1/2 100 mgL-1 150 mgL-1 200 mgL-1 250 mgL-1 Linear (100 mgL-1) Linear (100 mgL-1) Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. International Science Congress Association 27 Figure-14 logKvs 1/T Figure-15 log(1 – ) vs 1/T Sticking probability (S*) and activation energy (E) also give insight into the process and mechanism of adsorption. These parameters have been calculated by using modified Arrhenius equation. This equation, related to surface coverage () is represented as21: = ( 1- C/C) S* = (1- )e -Ea/RTThe sticking probability, S*, is a function of the adsorbate/adsorbent system and should satisfy the condition 0S*1. Plot of ln(1- ) versus 1/T gives straight line (figure-15). The slope and intercept of this straight line have been used to calculate E and S* respectively and have been given in table-5. Table-5 Thermodynamic parameters for adsorption of Zn(II) ion on fly ash Temp. K G , kJ/mol H , kJ/mol S , J/mol , kJ/mol S*, J K mol-1 303 -2.959 43.48 153.46 36.61 1.138X1007 313 -4.639 323 -6.023 Table-5 shows that as G values are negative, the process is spontaneous. Positive H value indicates that the nature of adsorption is endothermic. Positive S shows the affinity of the adsorbent for the Zn(II) ions. The value of E has been found to be 36.61 kJ mol-1. Positive value of E also indicates the endothermic nature of the process. Since S*1, it indicates that the probability of Zn(II) ion to stick on surface of fly ash is very high22. Mechanism: Speciation23 of Zn(II) with varying pH has been shown in figure-16. Figure-16 Speciation of Zn(II) with varying pH It is evident that at lower pH, zinc is in the form of Zn+2 and at pH 8 it is in the form of Zn(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 Zn+2 species24,25. Zn(OH) may possibly be adsorbed on adsorbent as shown below : OHM –OH ---------- MOMO + Zn+2 ---------- MOZnMO + MOZn ---------- ( MO)Zn OH OH O + Zn M O Zn OH OH Conclusion It is evident that initial Zn(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 due to chemical composition, structure, more adsorption sites, cheap, availability in plenty etc. fly ash may prove to be an efficient adsorbent. Acknowledgement The authors are thankful to SAIF, IIT Bombay, for XRF, SEM and FTIR analysis of fly ash. y = -2271.x + 8.015R² = 0.9960.00.20.40.60.81.01.20.0030.00310.00320.00330.0034log Kc 1/T y = 1912.x -6.944R² = 0.998-1.2-1-0.8-0.6-0.4-0.20.00300.00310.00320.00330.0034log(1 -) 1/T Research Journal of Chemical Sciences ___________________________________________________________ISSN 2231-606XVol. 4(5), 1-6, May (2014) Res. J. Chem. Sci. 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