Surface precipitation of Mn2+ on clay minerals enhances Cd2+ sorption under anoxic conditions. Issue 8 (6th July 2020)

• Record Type: Journal Article
• Authors: Van Groeningen, Natacha; Glück, Blanche; Christl, Iso; Kretzschmar, Ruben
• Is Part Of: Environmental science. Volume 22:Issue 8(2020)
• Page Start: 1654
• Page End: 1665
• Publication Date: 2020-07-06
• Subjects: Environmental monitoring -- Periodicals; Biological monitoring -- Periodicals; Environmental chemistry -- Periodicals; 363.7363
• DOI: 10.1039/d0em00155d
• Languages: English
• ISSNs: 2050-7887

Abstract

Abstract : Under anoxic conditions, clay minerals sorb dissolved Mn(ii ) predominantly by forming surface precipitates, which increase Cd sorption. Abstract : The influence of Mn 2+ 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 2+ concentrations. In this study, the effects of Mn 2+ on Cd 2+ 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 2+ and Cd 2+ sorption to Ca-saturated KGa-1b and Syn-1 were conducted under anoxic conditions. At low Mn 2+ and Cd 2+ concentrations (1 and 5 µM), both metals exhibited similar affinity for sorption to the clays, suggesting that elevated Mn 2+ concentrations might effectively decrease Cd 2+ 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 2+ increased Cd 2+ 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 2+ . This process, which was mediated by the presence of clay minerals, overcompensated the decrease in Cd 2+ adsorption to clay surfaces due to competition with Mn 2+ . We conclude that, contrary to predictions based on a competitive surface complexation model, elevated Mn 2+ concentrations can contribute to decrease dissolved Cd 2+ concentrations in anoxic clay-containing environments, such as contaminated sediments or flooded paddy soils.