Department of Geological Sciences, Stanford University, Stanford, California 94305-2115, United States.
Department of Environmental Geosciences, Centre for Microbiology and Environmental Systems Science, University of Vienna, 1090 Vienna, Austria.
Environ Sci Technol. 2020 Mar 17;54(6):3237-3244. doi: 10.1021/acs.est.9b06502. Epub 2020 Feb 28.
Groundwater contamination by As from natural and anthropogenic sources is a worldwide concern. Redox heterogeneities over space and time are common and can influence the molecular-level speciation of As, and thus, As release/retention but are largely unexplored. Here, we present results from a dual-domain column experiment, with natural organic-rich, fine-grained, and sulfidic sediments embedded as lenses (referred to as "reducing lenses") within natural aquifer sand. We show that redox interfaces in sulfur-rich, alkaline aquifers may release concerning levels of As, even when sediment As concentration is low (<2 mg/kg), due to the formation of mobile thioarsenates at aqueous sulfide/Fe molar ratios <1. In our experiments, this behavior occurred in the aquifer sand between reducing lenses and was attributed to the spreading of sulfidic conditions and subsequent Fe reductive dissolution. In contrast, inside reducing lenses (and some locations in the aquifer) the aqueous sulfide/Fe molar ratios exceeded 1 and aqueous sulfide/As molar ratios exceeded 100, which partitioned As(III)-S to the solid phase (associated with organics or as realgar (AsS)). These results highlight the importance of thioarsenates in natural sediments and indicate that redox interfaces and sediment heterogeneities could locally degrade groundwater quality, even in aquifers with unconcerning solid-phase As concentrations.
地下水受到天然和人为来源砷的污染是一个全球性的问题。时空上的氧化还原非均质性很常见，会影响砷的分子水平形态，从而影响砷的释放/保留，但这在很大程度上尚未得到探索。在这里，我们展示了一项双重域柱实验的结果，其中天然富含有机质、细颗粒和含硫沉积物作为透镜（称为“还原透镜”）嵌入在天然含水层砂中。我们表明，富硫、碱性含水层中的氧化还原界面即使在沉积物砷浓度较低（<2mg/kg）时，也可能释放出令人担忧的砷水平，这是由于在水合硫化物/Fe 摩尔比 <1 的情况下形成了可移动的硫代砷酸盐。在我们的实验中，这种行为发生在还原透镜之间的含水层砂中，归因于硫化条件的扩散和随后的 Fe 还原溶解。相比之下，在还原透镜内部（和含水层中的一些位置），水合硫化物/Fe 摩尔比超过 1，水合硫化物/As 摩尔比超过 100，将 As(III)-S 分配到固相（与有机物或雄黄（AsS）有关）。这些结果强调了硫代砷酸盐在天然沉积物中的重要性，并表明氧化还原界面和沉积物非均质性可能会局部降低地下水质量，即使在固相中砷浓度不令人担忧的含水层中也是如此。
