The role of anatase impurity in the leaching of Cd(II) from contaminated kaolinite

Abstract

This study was performed to enhance the understanding of the Cd(II) leaching mechanism from contaminated kaolinite with a focus on the role of anatase impurities. Experimental data obtained from titration and batch Cd(II) adsorption experiments were analyzed using a surface complexation model coupled with ion exchange reaction. Anatase showed significantly higher Cd(II) adsorption capability than those of kaolinite and goethite (α-FeOOH), with inner-sphere complex formation as the dominant adsorption mechanism. Under acidic and near-neutral conditions, kaolinite absorbs Cd(II) mainly via outer-sphere complex formation, while both outer-sphere and inner-sphere complex formation are important at higher pH. Binding constants obtained from modeling analyses were then used to predict the leaching extent of Cd(II) from contaminated kaolinite with different contents of anatase impurity at pH 4.0–7.0. The experimental data and model predictions consistently showed that the leaching of Cd(II) decreased with increasing anatase content and this trend was more significant under acidic conditions. These results showed that the content of anatase impurity is a factor controlling the leaching extent of Cd(II) from kaolinite when exposed to acid rain.