Selective removal of heavy metals from contaminated kaolin by chelators

Abstract

The competitive desorption/dissolution of kaolin-adsorbed heavy metal mixtures and mixtures of adsorbed Cd with Mg and/or Ca by four chelators (NTA, EDTA, EGTA, and DCyTA) was investigated. Metals were adsorbed on kaolin at pH 7 and the effects of chelator type and concentration on the extent of metal dissolution was studied at a solution pH of 10. EGTA addition. EGTA was the most effective chelator in selectively removing Cd from kaolin in the presence of adsorbed alkaline-earth metals. Approximately 90% of the adsorbed Ca and Mg were retained on the kaolin until almost all (> 80%) of the cadmium was dissolved by EGTA chelator. NTA was the least effective chelator in selectively dissolving Cd from kaolin contaminated with both Cd and Ca (≈ 45% of the adsorbed Cd could not be removed). All four chelators exhibited some desorption/dissolution selectivity for Cd, Cu, and Pb adsorbed on kaolin. When the concentration of chelator in solution was insufficient to dissolve all adsorbed metals, the observed metal ordering for chelation and dissolution was Cd > Cu > Pb (for EGTA), Cd > Pb > Cu (for EDTA and DCyTA), and Cu > Cd > Pb (for NTA).