Surface precipitation of Mn²⁺ on clay minerals enhances Cd²⁺ sorption under anoxic conditions

The influence of Mn²⁺ on the sorption of metal(loid)s onto clay minerals is still unclear despite its relevance in suboxic and anoxic environments which often exhibit elevated dissolved Mn²⁺ concentrations. In this study, the effects of Mn²⁺ on Cd²⁺ sorption to two types of clay minerals, a well-crystalline natural kaolinite (KGa-1b) and a synthetic montmorillonite (Syn-1), were investigated. Batch experiments on Mn²⁺ and Cd²⁺ sorption to Ca-saturated KGa-1b and Syn-1 were conducted under anoxic conditions. At low Mn²⁺ and Cd²⁺ concentrations (1 and 5 µM), both metals exhibited similar affinity for sorption to the clays, suggesting that elevated Mn²⁺ concentrations might effectively decrease Cd²⁺ sorption as predicted using a three-plane surface complexation model. However, competitive Mn-Cd experiments at higher concentrations (≥50 µM) revealed that for both clay minerals, the presence of Mn²⁺ increased Cd²⁺ sorption to the solid phases. Although solutions were undersaturated with respect to known Mn(ii) solid phases, analysis using X-ray absorption spectroscopy (XAS) evidenced the formation of Mn(ii)-containing solid phases which can specifically adsorb or incorporate Cd²⁺. This process, which was mediated by the presence of clay minerals, overcompensated the decrease in Cd²⁺ adsorption to clay surfaces due to competition with Mn²⁺. We conclude that, contrary to predictions based on a competitive surface complexation model, elevated Mn²⁺ concentrations can contribute to decrease dissolved Cd²⁺ concentrations in anoxic clay-containing environments, such as contaminated sediments or flooded paddy soils.