Research Journal of Chemical Sciences ______________________________________________ ISSN 2231-606X Vol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 36 Equilibrium and Kinetic Studies on removal of Cd(II) from Aqueous Solutions by Sulphuric Acid Activated Sesamum Indicum Carbon Tharanitharan Venkatesan1*, Dhanabal Thangaraj, Nagashanmugam Bommannan, Kannan Kulanthai and Srinivasan Krishnamoorthy5 1,4Department of Chemistry, Government College of Engineering, Salem-636 011, Tamil Nadu, INDIA Department of Chemistry, Muthayammal College of Engineering, Rasipuram-637 408, INDIA Research and Development Centre, JSW Steel (P) Ltd, Salem – 636 453, INDIA Department of Chemistry, Gnanamani College of Technology, Pachal, Namakkal, Tamil Nadu, INDIA Available online at: www.isca.in, www.isca.me Received 25th January 2014, revised 1st February 2014, accepted 16th March 2014Abstract Activated carbon prepared from oil cake of Sesamum Indicum by sulphuric acid treatment was used as adsorbent for the removal of Cd(II) ions from aqueous solutions. Various parameters as a function of contact time, initial pH, initial adsorbent dosage and metal ion concentrations were studied. The equilibrium adsorption data were fitted to Langmuir and Freundlich adsorption isotherm models. The equilibrium adsorption isotherms confirmed that activated carbon has high affinity and sorption capacity for Cd(II) with monolayer sorption capacities of 35.32 mg/g. The kinetic study indicated that the pseudo- second order rate equation better described the adsorption process. The adsorbents were also tested for the removal of cadmium (II) from synthetic electroplating wastewater. The results indicated that the prepared activated carbon is an efficient (99.9%), alternative low-cost adsorbent for the removal of cadmium(II) from aqueous solutions. Keywords: Cd(II) removal, activated carbon, isotherms, kinetics, wastewater. Introduction Water resources are being contaminated by heavy metal ions released from various industries. Large volumes of waste generated from a variety of industries are one of the main reasons for the contamination of water and other environmental resources with heavy metals. Heavy metals are harmful pollutants and due to their non-biodegradability and persistence, which is able to accumulate in living organisms causing various diseases and disorders. The efforts on reducing cadmium concentration in the industrial wastewaters are focussed by the toxic effects of cadmium on the aquatic world and the risk of contamination of water resources designated for human consumption. Phosphate fertilizers and sewage sludge, cigarette smoking and industrial uses of cadmium have been known as a major cause of widespread distribution of the metal at trace levels into the general environment and human foodstuffs. Cadmium can be accumulated in human body, causing erythrocyte destruction, nausea, salivation, diarrhoea and muscular cramps, renal degradation, chronic pulmonary problems and skeletal deformity. According to U.S. Environmental Protection Agency (EPA) standards, the permissible limit of cadmium discharge in industrial effluents into water bodies is limited to 0.1 mg/l. Therefore, removal of heavy metal ions from water and wastewater is significant in terms of human health. Various physical and chemical methods have been used for the removal of heavy metal ions from wastewaters. The most widely used methods which comprise ion exchange, chemical precipitation, reverse osmosis, evaporation, and membrane filtration. Most of these methods suffer from some disadvantages such as deficient removal of metal ions, high capital and operational cost and the further disposal of toxic sludge. Among various techniques, the adsorption processes used exclusively in water treatment and many studies has been carried out to find economical and viable adsorbent4-6. Many studies have been reported on the increase of low cost activated carbons from cheaper and easily available materials. Activated carbons are excellent and versatile adsorbents with their high surface area, micro porous character and chemical nature of their surface have made them potential adsorbents for the removal of heavy metal ions from industrial wastewater.There are many studies in the literature regarding preparation of activated carbons from various biomaterials such as coconut shells fruit stones, pyrolyzed coffee residues10, pine bark11, neem leaf12, and olive stones13, peanut husks14, almond tree leaves15, black gram husk16, maize cob husk17, apricot stone18and their application for the removal of various heavy metal ions from water and wastewater. In the present work, the removal of cadmium (II) ion from water and wastewater by using activated carbon produced from Sesamum Indicum oil cake by sulphuric acid treatment was investigated. The adsorption capacity of adsorbent was investigated using batch experiments. The influence of pH, contact time, metal ions and adsorbent concentrations were investigated. The experimental data obtained were evaluated and fitted using adsorbent equilibrium isotherms, and kinetic models. Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 37 Material and Methods Preparation of activated carbon: 50 g of washed and dried Sesamum Indicum oil cake was mixed with 200 g (1:4 ratio, wt. basis) of concentrated sulphuric acid. The mixing was done by adding small quantities of oil cake to acid taken in 1000 mL beaker with vigorous stirring. Charring of the cake occurs immediately accompanied by evolution of fumes. When the reaction subsided, the mixture was left in air oven at 140-160 C for a period of 24 h. The product was then washed with approximately 4.0 - 4.5 L of distilled water to remove free sulphuric acid and dried at 110 C. The material was sieved (80–120 ASTM size) and used for adsorption experiments. The activated carbon after sulphuric acid treatment was indicated as STGOC. The characteristics of STGOC are given in table-1. Table -1 Characteristics of adsorbent Parameter STGOC Pore volume (cm 3 g 1 ) BET surface area (m2 1) Bulk density (g cm-3) pH Matter soluble in water (%) Matter soluble in acid (%) Average pore size (A) Decolorizing power (mg g-1) Ion-exchange capacity (m equiv/g) Iron content (%) Phenol number Proximate analysis Moisture (%) Ash (%) Fixed carbon (%) Volatile matter (%) Elemental analysis Carbon (%) Sulphur (%) Nitrogen (%) Oxygen (%)0.02 36.78 0.79 3.7 0.84 4.90 89.56 33.1 0.72 0.28 57.00 2.20 4.75 51.75 41.30 53.42 3.26 2.72 15.59 Preparation stock solution: All chemicals and reagents used for experiments and analysis were of analytical grade. Stock solution of 1000 mg/L of Cd(II) was prepared from 3 CdSO.8HO ( S.D. Fine, Mumbai, India) in double distilled water. The solution was diluted as required to obtain the working solution. The initial pH of the working solution was adjusted using 0.1 N HNO or 0.1 N NaOH solutions. Fresh dilutions were used for each study. Adsorption studies: Batch mode adsorption experiments were executed by mixing known weight of adsorbent and 100 mL of Cd(II) ion solution of known concentration adjusted to a known pH. The mixture was taken in a polythene bottle of 300 mL capacity and shaken in a mechanical shaker (200 rpm) for a predetermined period at 30 ± 1C. Then the equilibrated solutions were centrifuged and the concentration of Cd(II) ions in the supernatant solution was measured by Atomic Absorption Spectrophotometer. Adsorption isotherm and kinetic studies were carried out with different initial concentrations of Cd(II) ions by maintaining the adsorbent dosage at constant level. Adsorption capacities were calculated from the difference in the metal ion concentration in the aqueous phase before and after the experiment according to the following equation: (1) where q, adsorption capacity per unit mass of adsorbent (mg/g); , initial concentration of Cd(II) in the aqueous solution (mg/L); C is the final equilibrium concentration of test solution (mg/L); m, mass of adsorbent (g); and v, volume of sample (L). Results and DiscussionEffect of contact time: Contact time is a significant factor for the successful use of adsorbents for practical applications20 Effect of contact time on the adsorption of Cd(II) by STGOC was studied in the range of 30 to 240 min and the results are shown in figure-1. It could be seen that the removal of Cd(II) increases with increase in contact time and attains equilibrium at 120 min. Basically the removal rate of adsorbate is rapid, but it gradually decreases with time until it reaches equilibrium. The rate in percent of metal removal is higher in the beginning due to the larger surface area of the adsorbent being available for the adsorption of the metals. The removal efficiency was found to be 99.9 % for an initial concentration of 10 mg/L of Cd(II). Therefore, optimum contact time was selected as 120 min for further experiments. Effect of pH: The pH of the aqueous solution is a central controlling parameter in the heavy metal ions adsorption process21. Moreover, due to the different functional groups on the adsorbent surface, this became active sites for the metal binding at a specific pH. The effect of pH on percentage removal of Cd(II) for pH ranging between 1 to 10 is shown in figure-2. It could be seen that 99.9 % removal of Cd(II) was achieved by the adsorbent over the pH range of 5.0 – 8.0. It is evident from figure-2, the adsorption of efficiency of Cd(II) increased with increasing the pH of the medium until reaching to the optimum pH range. At lower pH values (2-4), H ions compete with Cd2+ ions for exchange sites in the adsorbent. Cd2+ uptake was decreased because the surface area of the adsorbent was more protonated. Competitive adsorption occurred between H protons and free Cd2+ ions and therefore decrease in Cd(II) adsorption. When the pH value increased (5-8), adsorbent surfaces were more negatively charged and functional groups of the adsorbent more deprotonated which results higher attraction of Cd(II) ions. The Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 38 decrease in Cd(II) removal beyond pH 8.0 and more basic pH conditions, may suggest the possibility of Cd(OH)2 precipitates reside in the adsorption sites and prevent further removal of Cd(II)21. Further experiments were carried out at pH 5.0. Effect of adsorbent dose: Adsorbent dosage is an important factor because it determines the capacity of an adsorbent for a given concentration of the adsorbate22. The influence of STGOC dosage on the removal of Cd(II) ion is shown in figure-3. Cd(II) removal increases with increasing the adsorbent dosage. The removal efficiency was found to be 99.9 % at an adsorbent dose of 200 mg for an initial concentration of 10 mg/L. The results showed that the adsorption increases with the increase in the dose of adsorbent. This is because of presence of more binding sites on the surface at higher concentration of the adsorbent for binding of metal ions. Effect of initial Cd(II)concentration: The removal of Cd(II) ions was carried out at different initial Cd(II) ion concentrations ranging from 10 to 60 mg/L at pH 5.0. The results are presented in figure-4. Cd(II) removal percentage increases when the initial Cd(II) ion concentration decreases. At low Cd(II) concentration the surface active sites to the total metal ions in the solution is high and hence all the Cd(II) ions may interact with the binding sites of the adsorbent and may be removed from the solution. However, the amount of Cd(II) adsorbed per unit weight of adsorbent (q) is higher at high concentration. Adsorption isotherms: Adsorption isotherm is considered by certain constant values, which states the surface properties and attraction of the adsorbent. It can also be used to compare the adsorptive ability of the adsorbent for different pollutants23 The Langmuir and Freundlich models are the most frequently employed models. In this work, both models were used to describe the relationship between the amount of Cd(II) ions adsorbed and its equilibrium concentration in solution at room temperature for 24 h. Langmuir isotherm: The main postulation of the Langmuir method is that adsorption occurs regularly on the active part of the surface, and when a molecule is adsorbed on an active site, the other molecules could not be interacted with this active24. The linear form of Langmuir equation may be written as Ce 1 C = + (2) q qb qwhere q is the amount of solute adsorbed per unit weight of adsorbent (mg/g) and C is the equilibrium concentration of solute in the bulk solution (mg/L) while q is the monolayer adsorption capacity (mg/g) and b is the constant related to the free energy of adsorption (L/mg). A linear plot of C/q versus exhibits that the adsorption obeys the Langmuir isotherm and values of Langmuir constants (q and b) calculated from the slope and the intercept (figure-5) are presented in table-2. Table -2 Langmuir and Freundlich constants for Cd(II) removal Metal ion Langmuir Model Freundlich Model o (mg/g) b (L/mg) F (mg/g) 1/n (L/mg) Cd(II) 35.32 0.1852 0.8872 3.63 0.5907 0.8282 Figure-1 Effect of contact time [Cd(II) concentration : 10mg/L, adsorbent dose: 200mg/100mL, pH : 5.0 ± 0.1 ] Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 39 Figure-2 Effect of pH [Cd(II) concentration : 10mg/L, equilibrated time : 2 hrs, adsorbent dose : 200 mg/100mL] Figure-3 Effect of adsorbent dose [Cd(II) concentration : 10mg/L, equilibrated time:2 hrs, pH : 5.0±0.1 ] Figure-4 Effect of initial Cd(II) concentration [Equilibrated time : 2 hrs, pH : 5, adsorbent dosage : 200 mg/100 mL ] Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 40 The vital characteristics of the Langmuir isotherm can also be articulated in terms of a dimensionless constant of separation factor or equilibrium parameter, R, which is defined as25 1 = (3) 1 + b Cwhere b is the Langmuir constant and C is the initial concentration of Cd(II) ion. Separation factor demonstrate the nature of adsorption process and its value indicates the sorption process could be favourable, linear and unfavourable when 0 1, R =1, R &#x-3.3;夀 1, respectively. The RL values at different concentrations were found be in the range of 0 to 1 indicated a highly favourable adsorption of Cd(II) ions onto adsorbent. Freundlich isotherm: It is an experimental expression that takes into account the heterogeneity of the surface and multilayer adsorption to the binding sites located on the surface of the sorbent26. The logarithmic form of Freundlich model is expressed as follows log x/m = log K + 1/n log Ce (4) where C is the equilibrium concentration (mg/L) and x/m is the amount of metal ion adsorbed per unit weight of adsorbent (mg/g). The K is Freundlich constant related to the adsorption capacity (mg/g) and n shows the adsorption intensity (L/mg). The linear plot of log q versus log C (figure-6) exhibits that the adsorption obeys the Freundlich isotherm and value of Freundlich constants (K and 1/n) calculated from the intercept and slope of the plot are presented in table-2. The adsorption intensity 1/n value was found to be between zero and one which indicate the favourable adsorption of Cd(II) ions onto surface of adsorbent. The correlation coefficient (R) value of Langmuir model is found to be higher than Freundlich model. These results indicated that the Freundlich model is not proficient to describe effectively the relationship between the amounts of cadmium(II) ions adsorbed and their equilibrium concentration in the solution. Hence, it could be concluded that the Langmuir isotherm model was found to be a best fit with the equilibrium data since R values were closer to unity. Kinetic studies: Adsorption kinetics is important as it gives valuable insights into the reaction pathways and the mechanism of the reactions. Several kinetic models are used to explain the mechanism of the adsorption processes. Two well known kinetic models were used for this study name pseudo-first-order and pseudo-second-order. A simple pseudo-first order equation was given by Lagergren equation27 log (q-q) = log q - k t /2.303 (5) where q and q are the amounts of Cd(II) adsorbed (mg/g) at equilibrium time and any time t, respectively, while k is the rate constant of adsorption (min 1). Plot of log (qe - q) versus t gives a straight line for first order adsorption kinetics (figure-7) which allows calculation of the rate constant k and its values are given in table-3. The pseudo-second order equation based on equilibrium adsorption is expressed as28 t/qt = 1/k + t/q (6) where k is the pseudo-second order rate constant (mg g 1min 1), q and q represent the amount of Cd(II) adsorbed (mg/g) at equilibrium and at any time. The plot of (t/q) versus t produces straight line with slope of 1/q and intercept of 1/k. It indicated the applicability of pseudo-second-order model (figure-8). The overall rate constants k and other constants of pseudo-second-order kinetics are given in table-3. The correlation coefficients value (R) was also calculated and presented in table-3. In order to evaluate the applicability of kinetic models in fitting to data, the percent relative deviation (P) was calculated using the experimental data as given by the following equation29 100 q(exp) – q (theo) P = ------ { } (7) N q (exp) where qe(exp) is the experimental value of qat any value of C, e(theo) the corresponding theoretical value of q and N is the number of observations. It is identified that lower the value of percentage deviation (P), better is the fit. It is generally accepted that when P value is less than 5, the fit is considered to be excellent29. The results were analyzed using equations 5 and 6. The experimental data fitted well in both equations. The values of qe(theo) calculated from these models are compared with experimental values qe(exp) and shown in table-3. It is found that values of qe(theo) calculated from the pseudo-first-order kinetic model differed appreciably from the experimental values qe(exp). The percent deviation (P) is also very high. On the other hand, values of qe(theo)are found to be very close to qe(exp) when pseudo-second-order rate equation was applied. The percent deviation (P) is well with in the range and values of correlation coefficients (R) are very high for pseudo-second-order when compared with pseudo-first-order kinetics. These results indicated that the adsorption of Cd(II) ions onto STGOC was governed mainly by pseudo-second-order kinetics. Thus, it could be inferred that the binding of Cd(II) onto modified STGOC appeared to occur by chemical interactions relating valence forces due to sharing or exchange of electrons between Cd(II) and STGOC30. Thermodynamic studies: The free energy of adsorption () can be connected with the equilibrium constant K (L/mol), corresponding to the reciprocal of the Langmuir constant, b, by the following equation31-33 = -RT ln b (8) Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 41 where R is the universal gas constant (8.314 J/ mol K) and T is the absolute temperature (K). Gibbs free energy change () was calculated to be -25.04 kJ/mol for Cd(II). Negative value of designated the feasibility and spontaneous nature of the adsorption. Desorption studies: It was performed to recover Cd(II) and regenerate the adsorbent using 0.01 – 0.25 N HNO. Results showed that 97% of Cd(II) could be desorbed from STGOC under optimum concentration of 0.15 N. After the desorption of Cd(II), these sorbents were washed thoroughly with distilled water. The adsorption capacities of these sorbents were again tested and five cycles of successive sorption-desorption operations were carried out. Results indicated that the adsorption capacity of HNO regenerated STGOC was decreasing in the range of 99.9 – 55.8 %. Results indicated that 0.15 N HNO is suitable for regeneration of adsorbent. Application to wastewater treatment: Experiments were carried out to determine the effectiveness of STGOC with respect to treatment of electroplating wastewater (synthetic)34. Characteristics of electroplating wastewater before and after treatment are presented in table-4. Experiments were carried out with 100 mL of cadmium(II) wastewater solution at pH 5.0 in the presence of varying amounts of STGOC ranging from 100-1700 mg/100 mL and the results are presented in figure-9. Optimum dosage was found to be 1300 mg/100 mL for the maximum removal (99.8%) of Cd(II) from the wastewater. In addition to the removal of Cd(II) ions, the STGOC was able to effectively decrease the concentration of other metal ions in the wastewater. Therefore, it could clearly be recognized that the STGOC can be considered as an effective and alternative adsorbent for the treatment of wastewater containing Cd(II) ions. Figure-5 Langmuir adsorption isotherm [Temperature : 30±1C, metal ion concentration : (10-60mg/L), equilibration time : 24h, pH : 5.0 ± 0.1, adsorbent dose: 200mg/100mL] Figure-6 Freundlich adsorption isotherm [Temperature: 30±1C, metal ion concentration: (10-60mg/L), equilibration time:24h, pH : 5.0 ± 0.1, adsorbent dose : 200mg/100mL] Table -3 Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 42 Pseudo – first order and pseudo- second order kinetic constants for the adsorption of cadmium(II) onto STGOC Conc. (mg/L) Expt.q(mg/g) Pseudo-first-order kinetics Pseudo-second -order kinetics (1/min) e(theo)(mg/g) 2 P K(mg g-1 min-1) e(theo)(mg/g) 2 P 3 2.98 0.0263 1.52 0.901 48.99 0.0506 3.05 0.998 2.29 5 4.95 0.0232 2.00 0.956 59.59 0.0353 5.03 0.999 1.61 7 6.89 0.0162 3.76 0.811 45.42 0.0268 7.07 0.996 2.61 10 9.82 0.094 5.23 0.887 46.74 0.0152 9.92 0.992 1.01 Figure-7 Pseudo-first order kinetic plot for adsorption of cadmium(II) onto STGOC [Tempereture: 30±1C, pH : 5.0 ± 0.1, adsorbent dose : 200mg/100mL] Figure-8 Pseudo-second order kinetic plot for adsorption of cadmium(II) onto STGOC [Temperature: 30±1C, pH : 5.0±0.1, adsorbent dose : 200mg/100mL] Research Journal of Chemical Sciences ___________________________________________________________ ISSN 2231-606XVol. 4(3), 36-44, March (2014) Res. J. Chem. Sci. International Science Congress Association 43 Figure-9 Effect of adsorbent on removal of metal ions from electroplating wastewater [Temperature: 30±1C, equilibrated time: 2 hrs, pH: 5.0±0.1]Table -4 Characteristics of electroplating wastewater (synthetic) Parameter Before treatment (mg/L) After treatment (mg/L) Removal (%) Copper(II) 15 14.66 97.4 Nickel(II) 30 29.55 98.5 Cadmium(II) 5 4.99 99.8 Zinc(II) 20 19.86 99.3 Conclusion The presented study signified that the activated carbon obtained from oil cake of Sesamum Indicum (STGOC) was employed as an adsorbent for the removal of Cd(II) from aqueous solution. The operating parameters such as, contact time, pH, adsorbent dosage and initial Cd(II) concentration were effective on the adsorption efficiency of Cd(II) ions. Experimental results are good agreement with Langmuir isotherm model and have shown a better fitting to the experimental data. The kinetics of Cd(II) adsorption onto STGOC was found to follow more reliably pseudo second order kinetics. Negative Gibbs free energy value ) indicated the feasibility and spontaneous nature of the process. Desorption of Cd(II) was effectively be achieved with 0.15 N HNO from the adsorbent. Experiments with wastewater clearly indicated that the STGOC is an effective adsorbent for the removal of higher concentrations of Cd(II) and other metal ions from wastewater. Based on results, it could be concluded that STGOC can be used as a potential adsorbent to treatment of wastewater containing Cd(II) ions since it is efficient, economical and locally available. References1.Madhava Rao M., Ramana a D K. , Seshaiah a K. , Wang b M.C. , and Chang Chien S.W. , Removal of some metal ions by activated carbon prepared from Phaseolus aureus hulls, J. Hazard. 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