Surface precipitation of Mn2+ on clay minerals enhances Cd2+ sorption under anoxic conditions.,Environmental Science: Processes & Impacts

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Surface precipitation of Mn2+ on clay minerals enhances Cd2+ sorption under anoxic conditions.
Environmental Science: Processes & Impacts ( IF 4.3 ) Pub Date : 2020-07-06 , DOI: 10.1039/d0em00155d
Natacha Van Groeningen ₁ , Blanche Glück , Iso Christl , Ruben Kretzschmar

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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.

中文翻译：

在缺氧条件下，Mn2 +在粘土矿物上的表面沉淀会增强Cd2 +的吸附。

尽管Mn ²⁺在通常表现出较高的溶解Mn ²⁺浓度的低氧和缺氧环境中具有相关性，但对金属（胶体）在粘土矿物上的吸附的影响仍然不清楚。在这项研究中，研究了Mn ²⁺对Cd ²⁺吸附到两种类型的粘土矿物上的影响，这两种晶体是高结晶度的天然高岭石（KGa-1b）和合成的蒙脱石（Syn-1）。在缺氧条件下进行了对Mn ²⁺和Cd ²⁺吸附到Ca饱和的KGa-1b和Syn-1的分批实验。Mn ²⁺和Cd ²⁺低时浓度（分别为1和5 µM），两种金属对粘土的吸附均表现出相似的亲和力，这表明，如使用三平面表面络合模型所预测的，升高的Mn ²⁺浓度可能有效降低Cd ^2+的吸附。但是，在较高浓度（≥50µM）下的竞争性Mn–Cd实验表明，对于两种粘土矿物，Mn ²⁺的存在都会增加Cd ²⁺对固相的吸附。尽管溶液相对于已知的Mn（II）固相而言不饱和，但使用X射线吸收光谱法（XAS）进行的分析表明，形成了可以特异性吸附或掺入Cd ^2+的含Mn（II）固相。。该过程是由粘土矿物的存在介导的，该过程过度补偿了由于与Mn ^2+的竞争而使Cd ²⁺对粘土表面的吸附减少。我们得出结论，与基于竞争性表面络合模型的预测相反，升高的Mn ²⁺浓度可有助于降低含氧粘土环境中的溶解Cd ²⁺浓度，例如受污染的沉积物或淹没的稻田土壤。

更新日期：2020-08-19

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