Equilibrium and Kinetic Batch Studies of Cadmium and Lead sorption using Low Cost Biosorbents

Abstract: Agricultural wastes; Albizia coriaria, Coffea canephora, Cyperus papyrus, Erythrina abyssinica and Musa spp were evaluated for uptake of aqueous Cd2+ and Pb2+ ions in single- and binary-component solutions. Untreated, base-treated and peroxide-treated biomasses were employed in batch studies. The optimal conditions for Cd2+ and Pb2+ ions biosorption were  pH 3.5 – 5 for contact time 3.0 – 3.5 hours and biosorbent dosage 10 – 12.5 g/L. Base-treated biosorbents showed a 10 – 17 %  sorption enhancement for Cd2+ ions and a 1.6 – 2.3 % uptake reduction for Pb2+ ions. The sorption capacities for Cd2+ and Pb2+ ions for base-treated biosorbents were between 1.738 and 1.760 mg g-1 compared to 1.415 – 1.539 mg g-1 for untreated materials. The maximum biosorption of peroxide treated materials in single component media was between 0.819 and 1.595 mg/g for Pb2+ ions and between 0.044 and 1.343 mg/g for Cd2+ ions while in binary component media it was between 0.472 and 1.303 mg/g for Pb2+ ions and between 0.008 and 0.195 mg/g for Cd2+ ions. The pseudo-second order kinetic model suitably fitted the Cd2+ and Pb2+ ions biosorption data with regression coefficients (R2); 0.892 – 1.000 for peroxide-treated materials and 0.9784 – 0.9999 for base-treated biosorbents which implied that the biosorption was mainly a chemisorption process. The base treated biosorbents had better sorption performance for Cd2+ ions than peroxide treated materials and untreated biomass whereas the order for Pb2+ ions biosorption was untreated > base treated > peroxide treated materials. All materials showed superior selectivity for Pb2+ ions biosorption in comparison to that of Cd2+ ions in single- and binary-component media. A. coriaria performed best of the base treated biosorbents while C. canephora performed best amongst peroxide treated materials for Cd2+ and Pb2+ ions biosorption in single-and binary-component media. In the competitive biosorption, Cd2+ ions showed higher synergistic effects than Pb2+ ions although Pb2+ ions were preferentially sequestered even when the Cd2+:Pb2+ ions ratio was increased through 3:2, 2:1, 3:1 and 5:1. Of the three isotherm models evaluated against the experimental data, the Langmuir model generally fitted the sorption data for both metals. Regression coefficients for the Langmuir model were; 0.983 ? R2 ? 1.000 for single-component and 0.939 ? R2 ? 1.000 for binary-component solutions which revealed that the biosorption was potentially monolayer. The biosorption equilibrium coefficient                          values and change in Gibbs’ free energy  values showed that Pb2+ ions biosorption was more thermodynamically favoured than that of Cd2+ ions in single-component and binary-component media. The materials studied displayed potential for use as biosorbents for remediation of aqueous Cd2+ and Pb2+ ions.