Cadmium adsorption on hydroxyaluminosilicate-montmorillonite complex as influenced by oxalate and citrate

The hydroxyaluminosilicate (HAS)-montmorillonite (Mt) and hydroxyaluminum (HyA)-Mt complexes are important constituents of soil colloidal complexes in acidic environments. Oxalate and citrate are dominant organic acids in the root exudates of many plants. These acids, among other factors, strongly influence the adsorption behavior of HAS-Mt and HyA-Mt complexes. In this study, we investigated the adsorption phenomena of Cd on the HAS-Mt and HyA-Mt complexes as influenced by oxalate and citrate. Without addition of oxalate and citrate, the adsorption of Cd on Mt was easy. However, in the presence of oxalate and citrate, the adsorption was strongly inhibited. In contrast, the HAS-Mt and HyA-Mt complexes in the absence of oxalate and citrate hardly adsorbed Cd. In the presence of oxalate, the HAS-Mt and HyA-Mt complexes were able to adsorb Cd. Thus, optimal concentration of oxalate for Cd adsorption on both complexes could be determined. Similarly, the HyA-Mt complex showed an optimal concentration of citrate for Cd adsorption. However, the HAS-Mt complex did not adsorb Cd in the presence of citrate. We concluded that the optimal concentrations of oxalate and citrate for Cd adsorption depended on the form of Cd ions and their proportion in the solution. Based on the amount of Cd adsorbed and the distribution of Cd species in the solution, the adsorption of Cd on the HAS-Mt and HyA-Mt complexes took place in various chemical forms.