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 Cd(2+) from aqueous solution. The maximum removal of Cd(2+) was found to be 35.41 mg g(-1) at pH 5.0, 500 mg l(-1) initial Cd(2+) concentration and 5 g l(-1) sorbent dosage. Sorption kinetics and equilibria followed pseudo-second order and Langmuir isotherm equations. The mechanism of Cd(2+) adsorption on mbh was investigated by DRIFT spectroscopy, SEM-EDX analysis, and by monitoring the release of alkali and alkaline earth metal cations (Ca(2+), Mg(2+), K(+) and Na(+)) during the uptake of Cd(2+). DRIFT spectra of mbh showed the presence of amino, carboxyl, carbonyl and hydroxyl as the major functional groups involved in the sorption of Cd(2+). The sum of Ca(2+), Mg(2+), Na(+) and K(+) released from mbh with the quantitatively equivalent uptake of Cd(2+) indicated that the main mechanism of Cd(2+) 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 Cd(2+). No significant loss in Cd(2+) sorption capacity of regenerated mbh was noted during reuse for five successive sorption-desorption cycles.