Sorption of Chromium(Vi) and Copper(Ii) from Aqueous Solution by Chemically-Modified Rice Hull

The sorption of Cr(VI) and Cu(II) from single and binary metal solutions by rice hull modified using different chemical reagents were investigated. Modification of rice hull using citric acid and quaternary ammonium salt was investigated and optimized to yield a material (citric acid-quaternized rice hull – CQRH) which was capable of removing both Cr(VI) and Cu(II) simultaneously and efficiently. Batch and column studies were carried out in order to understand the sorption characteristic of Cr(VI) and Cu(II) by CQRH. The parameters studied were pH, contact time, agitation rate, initial concentration of metal solutions, presence of other cations and chelators, competitive cation and anion, particle size, bed depth, flow rate and sorption-desorption cycles using synthetic solution and electroplating wastewater. In batch studies, sorption was pH dependent and equilibrium was attained in less than 30 minutes. Kinetic study showed that sorption followed the pseudo-second order kinetics. The sorption of Cr(VI) from single and binary metal ion solutions was found to be an exothermic and endothermic process, respectively while sorption of Cu(II) was an endothermic process. For single metal ion solution, the sorption process conformed to the Langmuir isotherm with maximum sorption capacities of 0.46 and 0.12 mmol/g at 28±2oC for Cr(VI) and Cu(II), respectively. For binary metal ion solution, sorption capacity of Cr(VI) was 0.53 mmol/g while that of Cu(II) was similar to that in single metal solution. In the presence of chelators, metal uptake decreased and the uptake of Cr(VI) and Cu(II) were not affected by agitation rate and particle size. The results of column studies showed that breakthrough depended on bed depth and initial concentration of metal ion. However, sorption was flow rate independent. Predicted breakthrough curves obtained from two parameter mathematical model agreed well with experimental values for Cr(VI) from binary metal ion solution and Cu(II) from single and binary metal ion solutions. For Cr(VI) from single metal ion solution, predicted breakthrough curves obtained from two parameter mathematical model did not agree well with experimental values. In sorption-desorption study, Cr(VI) and Cu(II) could be removed effectively by eluting with 0.05 M NaOH and H2SO4, respectively. Sorption-desorption process for Cr(VI)-CQRH system can be repeated for at least 5 cycles without deterioration capability of CQRH while in the case of Cu(II)-CQRH system, it could only be used once. In the case of wastewater, the poor capability of CQRH to remove Cr(VI) from electroplating wastewater under continuous flow conditions could be due to the wastewater containing other anionic species such as SO42- and PO43-. Besides that sorption of Cr(VI) from electroplating wastewater needed longer time to reach equilibrium compared to synthetic solution which may affect the uptake of Cr(VI) in column study.

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