Chemical speciation and distribution of adsorbed Cd(ii) on goethite: influence of pH and sulfate

Abstract

It has been widely believed that the adsorption mechanism of cadmium on iron (oxyhydr)oxide is solely the inner-sphere complexation based on extended X-ray absorption fine structure spectroscopy (EXAFS). However, these results were obtained at specific experiment settings (high ionic strength and high cadmium surface loading). In natural cadmium-polluted soil and water environments, such as acidic mineral drainage (AMD) areas, the pH/ionic strength is relatively low. Therefore, under practical conditions, the bonding pathway of cadmium could be different and may comprise other potential mechanisms. This work focuses on the proportion of different cadmium sorption pathways on goethite by employing a chemical extraction procedure. Cadmium is mostly adsorbed on goethite via electrostatic adsorption and inner-sphere complexation in a pH range of 5.5–7.5 under 10 mM ionic strength conditions. Through quantitative calculation, it is confirmed that the amount of Mg(NO₃)₂-extracted cadmium can refer to the content of electrostatically adsorbed cadmium. Two types of surface hydroxyls contributed differently to inner-sphere complexed Cd(II) at different pH values, and the emergence of surface-induced precipitates at higher pH could also be included in the retention mechanism. The behavior of electrostatically adsorbed cadmium and inner-sphere complexed Cd(II) was found to be different at different pH values when sulfate was introduced. Based on zeta potential and total reflectance Fourier transform infrared spectral analyses, sulfate would modify the surface charge of goethite, but, moreover, sulfate might form pseudo ion-group species with cadmium, which could influence the sorption pathway of cadmium.