Issue 8, 2020

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

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

Supplementary files

Article information

Article type
Paper
Submitted
07 Apr 2020
Accepted
23 Jun 2020
First published
06 Jul 2020
This article is Open Access
Creative Commons BY-NC license

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

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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