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了它的移动性、生物利用度和毒性。 这些过程包括无机或生物氧化和还原、生物介导的甲基化、矿物表面的吸附和解吸以及矿物沉淀、共沉淀和溶解。 砷是一种众所周知的高浓度毒素，也是一种已知的慢性接触致癌物。 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，了解河流地下沉积物的保留机制对于评估砷的流动性和向饮用水源的迁移是很重要的。我们正在使用同步加速器X射线吸收光谱和支持的表征方法来确定两条河流的潜流区沉积物中砷形态的主要模式，一条河流接收来自地热源的As（III），另一个是由于硫化物尾矿的开采和风化，砷和金属浓度升高。 我们将在分子水平上识别沉积物中的物种形成，记录氧化态和配体环境随沉积物深度的变化，并评估生物调解的重要性。 我们将结合使用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，以及适当的砷参考化合物，以定量确定砷的形态。 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、扫描电子显微镜和微探针以及选择性提取方法。 硫同位素比率的二次离子质谱法（西姆斯）将用于鉴定生物沉淀的硫化合物。 这一分子信息将进行评估，在现场测量的表面和潜流区水化学热力学计算，以确定稳定和亚稳物种和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实验室系统中测得的砷氧化或还原反应速率相关联。 我们将开发一种窗口流通池，用于实时，原位X射线吸收测量简化系统中的砷氧化还原反应。 其结果将是一个定量的了解，在分子尺度上的生物和非生物形态的反应，控制流hyporheic区的砷分区。 这一水平的理解是很重要的砷氧化和还原反应，是不同的地球化学环境之间的转移的一般速率表达式的发展。

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.