Kinetics, equilibrium and mechanism of Cd2+ removal from aqueous solution by mungbean husk

Mungbean husk (mbh), an agrowaste material, was investigated as a new sorbent for the removal of Cd2+ from aqueous solution. The maximum removal of Cd2+ was found to be 35.41 mg g(-1) at pH 5.0, 500 mg l(-1) initial Cd2+ concentration and 5 g l(-1) sorbent dosage. Sorption kinetics and equilibria followed pseudo-second order and Langmuir isotherm equations. The mechanism of Cd2+ adsorption on mbh was investigated by DRIFT spectroscopy, SEM-EDX analysis, and by monitoring the release of alkali and alkaline earth metal cations (Ca2+, Mg2+, K+ and Na+) during the uptake of Cd2+. DRIFT spectra of mbh showed the presence of amino, carboxyl, carbonyl and hydroxyl as the major functional groups involved in the sorption of Cd2+. The sum of Ca2+, Mg2+, Na+ and K+ released from mbh with the quantitatively equivalent uptake of Cd2+ indicated that the main mechanism of Cd2+ adsorption was ion exchange. EDX analysis data supported the hypothesis of the involvement of ion exchange, as alkali and alkaline earth metal cations were noted to disappear in EDX spectrum of mbh after the uptake of Cd2+. No significant loss in Cd2+ sorption capacity of regenerated mbh was noted during reuse for five successive sorption-desorption cycles. (C) 2009 Elsevier B.V. All rights reserved.