Desorption-complexation-dissolution characteristics of adsorbed cadmium from kaolin by chelators

In order to provide a sound experimental background for the remediation of metal-contaminated soil by chelators, the desorption/complexation/dissolution characteristics of Cd from kaolin as a representative soil component by four chelators (NTA, EDTA, EGTA and DCyTA) have been investigated as a function of solution pH. For all chelating agents under examination, the ratio of Cd (desorbed from kaolin) to chelator was found to be 1:1. The chelation/dissolution of Cd was strongly dependent on the solution pH for NTA and EDTA. In the NTA system, 100% Cd dissolution occurred only at a pH = 8 and pH < 3.2; under weakly acidic conditions only 45% of the Cd on kaolin was dissolved due to readsorption of CdNTA(-) complex on kaolin. At a pH greater than or equal to 10, Cd dissolution decreased, due to Cd hydroxide precipitation. Only 85% of the total Cd on kaolin desorbed under weakly acidic conditions in the EDTA system, indicating metal complex readsorption similar to that found in the Cd-NTA system. Zeta potential measurements showed that the surface charge of Cd-loaded kaolin became more negative after addition of EDTA and NTA with a shift in the pH at the point of zero charge to a lower value. As compared to the EDTA and NTA systems, DCyTA and EGTA complexed strongly with Cd (100% Cd dissolution) over a wide pH range (2.5-12.0). The zeta potential of kaolin did not change and no Cd readsorption was found after addition of EGTA and DCyTA. The capacity of the four chelators for removing Cd from kaolin was found to be in the order DCyTA > EGTA > EDTA > NTA.