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Issue 8, 2020
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Surface precipitation of Mn2+ on clay minerals enhances Cd2+ sorption under anoxic conditions

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Abstract

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.

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

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Supplementary files

Article information


Submitted
07 Apr 2020
Accepted
23 Jun 2020
First published
06 Jul 2020

This article is Open Access

Environ. Sci.: Processes Impacts, 2020,22, 1654-1665
Article type
Paper

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

N. Van Groeningen, B. Glück, I. Christl and R. Kretzschmar, Environ. Sci.: Processes Impacts, 2020, 22, 1654 DOI: 10.1039/D0EM00155D

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