Surface precipitation of Mn on clay minerals enhances Cd sorption under anoxic conditions.

The influence of Mn 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 Mn concentrations. In this study, the effects of Mn on Cd 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 Mn and Cd sorption to Ca-saturated KGa-1b and Syn-1 were conducted under anoxic conditions. At low Mn and Cd concentrations (1 and 5 µM), both metals exhibited similar affinity for sorption to the clays, suggesting that elevated Mn concentrations might effectively decrease Cd 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 Mn increased Cd 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 Cd. This process, which was mediated by the presence of clay minerals, overcompensated the decrease in Cd adsorption to clay surfaces due to competition with Mn. We conclude that, contrary to predictions based on a competitive surface complexation model, elevated Mn concentrations can contribute to decrease dissolved Cd concentrations in anoxic clay-containing environments, such as contaminated sediments or flooded paddy soils.