Molecular Characterization and Reaction Mechanisms of Arsenic Speciation in Stream Hyporheic Zone Sediments

砷在环境中的化学转化控制着它的流动性、生物有效性和毒性。这些过程包括无机或生物氧化和还原、生物介导的甲基化、吸附到矿物表面和从矿物表面解吸，以及矿物沉淀、共沉淀和溶解。砷是一种浓度较高的众所周知的毒素，在长期接触中是一种已知的致癌物质。由于砷通常存在于美国西部干旱和半干旱地区的溪流和地下水中，了解地下水系沉积物的滞留机制对于评估砷在饮用水水源中的迁移和迁移非常重要。我们正在使用同步辐射X射线吸收光谱和辅助表征方法来确定两条溪流浅层带沉积物中砷的主要形态形态，一条溪流来自地热源，另一条溪流来自地热源，另一条溪流来自硫化物尾矿的开采和风化，其砷和金属浓度升高。我们将在分子水平上确定沉积物中的物种组成，记录氧化状态和配基环境随沉积物深度的变化，并评估生物中介的重要性。我们将使用X射线吸收近边结构(XANES)和扩展X射线吸收精细结构(EXAFS)分析相结合的方法，以及适当的砷参比化合物，来定量确定砷的形态。补充表征方法将包括实验室和同步加速器粉末X射线衍射法、扫描电子显微镜和微探针以及选择性萃取法。硫同位素比值的二次离子质谱仪(SIMS)将用于鉴定生物沉淀的硫化物。这些分子信息将通过现场测量的地表和深水带水化学的热力学计算来评估，以确定稳定和亚稳定的物种以及砷转化反应的化学亲和力。现场样品中确定的分子形态将与简化的实验室系统中测量的砷氧化或还原反应速度有关。我们将开发一种加窗流动池，用于在简化的系统中实时、原位地测量砷的氧化还原反应。其结果将是在分子尺度上对生物和非生物形态反应的定量理解，这些反应控制着河流低渗流区中的砷分配。这一水平的理解对于发展砷的氧化和还原反应的通用速率表达式非常重要，这些反应可以在不同的地球化学环境中转移。

0073984O'DayChemical transformations of arsenic in the environment control its mobility, bioavailability, and toxicity. These processes include inorganic or biological oxidation and reduction, biologically mediated methylation, adsorption to and desorption from mineral surfaces, and mineral precipitation, co-precipitation, and dissolution. Arsenic is a well-known toxin at elevated concentrations and a known carcinogen in chronic exposure. Because arsenic is commonly found in streams and groundwaters of the arid and simi-arid western U.S., understanding of retention mechanisms by stream subsurface sediments is important for evaluating arsenic mobility and migration into drinking water sources.We are using synchrotron X-ray absorption spectroscopy and supporting characterization methods to determine the primary modes of arsenic speciation in hyporheic zone sediments from two streams, one receiving As(III) from geothermal sources, and one with elevated arsenic and metal concentrations from mining and weathering of sulfide tailings. We will identify speciation in sediments at the molecular level, document changes in oxidation state and ligand environment as a function of sediment depth, and assess the importance of biological mediation. We will use a combination of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses, and appropriate arsenic reference compounds, to quantitatively determine arsenic speciation. Complementary characterization methods will include laboratory and synchrotron powder X-ray diffraction, scanning electron microscopy and microprobe, and selective extraction methods. Secondary ion mass spectrometry (SIMS) of sulfur isotope ratios will be used to identify biologically precipitated sulfur compounds. This molecular information will be evaluated with thermodynamic calculations of surface and hyporheic zone water chemistry measured in the field to determine stable and metastable species and the chemical affinity of arsenic transformation reactions. Molecular speciation identified in field samples will be linked to reaction rates of arsenic oxidation or reduction measured in simplified laboratory systems. We will develop a windowed flow-through cell for real-time, in situ X-ray aborption measurements of arsenic redox reaction in simplified systems. The outcome will be a quantitative understanding at the molecular scale of biotic and abiotic speciation reactions that control arsenic partitioning in stream hyporheic zones. This level of understanding is important for development of general rate expressions for arsenic oxidation and reduction reactions that are transferable among different geochemical environments.