Environmental Microbiology Laboratory (EML), EPFL-ENAC-IIE-EML , Ecole Polytechnique Fédérale de Lausanne (EPFL) , Station 6 , CH-1015 Lausanne , Switzerland.
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC, CNRS-UPMC-IRD-MNHN UMR 7590) , Centre National de la Recherche Scientifique (CNRS)-Université Pierre et Marie Curie (UPMC Paris 6) , Campus Jussieu, 4 place Jussieu , 75005 Paris , France.
Environ Sci Technol. 2018 Mar 20;52(6):3431-3439. doi: 10.1021/acs.est.7b05177. Epub 2018 Mar 1.
Arsenic contamination in groundwater is pervasive throughout deltaic regions of Southeast Asia and threatens the health of millions. The speciation of As in sediments overlying contaminated aquifers is poorly constrained. Here, we investigate the chemical and mineralogical compositions of sediment cores collected from the Mekong Delta in Vietnam, elucidate the speciation of iron and arsenic, and relate them to the sediment depositional environment. Gradual dissolution of ferric (oxyhydr)oxides with depth is observed down to 7 m, corresponding to the establishment of reducing conditions. Within the reduced sediment, layers originating from marine, coastal or alluvial depositional environments are identified and their age is consistent with a late Holocene transgression in the Mekong Delta. In the organic matter- and sulfur-rich layers, arsenic is present in association with organic matter through thiol-bonding and in the form of arsenian pyrite. The highest arsenic concentration (34-69 ppm) is found in the peat layer at 16 m and suggests the accumulation of arsenic due to the formation of thiol-bound trivalent arsenic (40-55%) and arsenian pyrite (15-30%) in a paleo-mangrove depositional environment (∼8079 yr BP). Where sulfur is limited, siderite is identified, and oxygen- and thiol-bound trivalent arsenic are the predominant forms. It is also worth noting that pentavalent arsenic coordinated to oxygen is ubiquitous in the sediment profile, even in reduced sediment layers. But the identity of the oxygen-bound arsenic species remains unknown. This work shows direct evidence of thiol-bound trivalent arsenic in the Mekong Delta sediments and provides insight to refine the current model of the origin, deposition, and release of arsenic in the alluvial aquifers of the Mekong Delta.
地下水砷污染普遍存在于东南亚三角洲地区，威胁着数百万人的健康。受污染含水层上覆沉积物中砷的形态仍不清楚。本研究调查了越南湄公河三角洲采集的沉积物岩芯的化学和矿物组成，阐明了铁和砷的形态，并将其与沉积环境联系起来。随着深度的增加，观察到铁（氧）氢氧化物逐渐溶解，直到 7 米深处，这对应于还原条件的建立。在还原沉积物中，确定了源自海洋、沿海或冲积环境的层位，并且它们的年龄与湄公河三角洲全新世晚期的海侵一致。在富含有机物和硫的层位中，砷与有机物通过巯基结合，以砷黝铜矿的形式存在。在 16 米处的泥炭层中发现了最高的砷浓度（34-69ppm），这表明由于形成了与硫结合的三价砷（40-55%）和砷黝铜矿（15-30%），导致了砷的积累，这种环境为古红树林沉积环境（约 8079 年前）。在硫有限的情况下，鉴定出菱铁矿，并且氧和巯基结合的三价砷是主要形态。值得注意的是，即使在还原沉积物层中，砷也普遍以氧结合的五价砷形式存在。但是，氧结合的砷物种的身份仍然未知。这项工作提供了直接证据，表明在湄公河三角洲沉积物中存在与硫结合的三价砷，并深入了解了当前湄公河三角洲冲积含水层中砷的来源、沉积和释放模型。
