Quantifying thioarsenate formation constants to advance understanding of arsenic biogeochemical cycling in anoxic waters

砷（As）被认为是环境中最常见的天然致癌物，大多数受慢性砷中毒影响的人都因饮用砷浓度升高的饮用水而暴露于这种元素。虽然南亚和东南亚有超过1亿人的饮用水中含有高水平的砷，导致一些人称之为人类历史上最大的自然灾害，但美国境内有许多地方的当地居民也暴露于饮用水中天然存在的高水平砷。因此，了解将砷从地质材料动员到天然饮用水源的地球化学过程对于预测砷浓度升高的地方或随着时间的推移而发展，以及设计修复策略以确保当前和未来人口的安全饮用水资源至关重要。该项目的一个重要的更广泛的影响是，它将提供一种手段来准确预测缺氧天然沃茨中砷的形态，这是目前不可能的。这种方法将允许其他研究人员使用熟悉的工具，如地球化学平衡和反应路径模型，以更好地预测这种涉及砷和溶解硫化物之间反应的过程在低温天然沃茨中可能采取的方向。拟议的研究将涉及一名研究生和多达三名本科生的“动手”地球化学研究经验，并发展学术界和联邦机构（美国地质勘探局，美国环保署）之间的密切合作。外联活动将包括：（1）通过路易斯安那州少数民族参与计划联盟指导至少一名本科生参与该项目;（2）通过杜兰大学的STEM项目女孩向5至7年级女孩介绍环境地球化学的职业机会;和（3）吸引来自路易斯安那州数学，科学，自然沃茨中的砷迁移可以通过许多过程发生，包括间接微生物介导的铁氧化物还原溶解和相关砷释放到溶液中，氧化砷的直接酶促（微生物）还原为以亚砷酸盐含氧阴离子形式的更移动的还原砷，含砷硫化物矿物如黄铁矿的氧化，以及通过更丰富的阴离子竞争从矿物表面释放。直到最近，通过微生物硫酸盐还原产生溶解的硫化物被认为导致通过砷硫化物矿物或其他砷清除硫化物矿物（例如，黄铁矿、毒砂）。然而，砷可以在缺氧沃茨中与硫结合联合收割机，形成溶解的砷硫化合物（硫代砷酸盐和硫代亚砷酸盐），在某些情况下，这些化合物在溶液中表现出高度移动的和持久性（硫代砷酸盐）。尽管人们越来越认识到硫代砷物种是砷地球化学的一个重要方面，硫化物条件影响砷在自然沃茨中循环的途径是复杂的，知之甚少。这种认识不足是显而易见的，在缺乏平衡热力学数据的硫代砷物种，这妨碍了可靠的预测建模的丰度和分布在自然沃茨。本研究的目的是测量描述四种同源硫代砷酸盐物种（即，一硫代砷酸盐、二硫代砷酸盐、三硫代砷酸盐和四硫代砷酸盐），并开发可用于预测天然水中这些砷硫化合物的形成和丰度的地球化学模型。

Arsenic (As) is recognized as the most common, naturally occurring carcinogen in the environment and most people who are affected by chronic arsenic poisoning are exposed to this element from consumption of drinking water with elevated arsenic concentrations. Although in excess of 100 million people in South and Southeast Asia are known to be exposed to high levels of arsenic in their drinking water, leading to what some have referred to as the largest natural disaster in human history, there are a number of locations within the United States where local populations are also exposed to high levels of naturally occurring arsenic in their drinking water. Consequently, understanding the biogeochemical processes that mobilize arsenic from geologic materials to natural drinking water sources is critical for both predicting where elevated arsenic concentrations may be found or develop over time, and for designing remediation strategies to ensure safe drinking water resources for current and future populations. An important broader impact of the project is that it will provide a means to accurately predict the speciation of arsenic in anoxic natural waters, which is presently not possible. This approach will allow other researchers to employ familiar tools like geochemical equilibrium and reaction path models to better predict the direction that such processes involving reactions between arsenic and dissolved sulfide are likely to take in low-temperature, natural waters. The proposed research will involve a graduate student and up to three undergraduates in 'hands-on' biogeochemical research experiences, and develop close collaborations between academia and federal agencies (USGS, US EPA). Outreach will include: (1) mentoring of at least one undergraduate on the project through the Louisiana Alliance for Minority Participation program; (2) presentations on career opportunities in environmental biogeochemistry to 5th through 7th grade girls through Tulane University's Girls in STEM program; and (3) engaging high school students from the Louisiana School of Math, Science, and Arts in 'hand-on' research experiences related to the project.Arsenic mobilization in natural waters can occur by a number processes that include indirect microbially mediated reductive dissolution of iron oxides and release of associated arsenic to solution, direct enzymatic (microbial) reduction of oxidized arsenic to more mobile reduced arsenic in the form of the arsenite oxyanion, oxidation of arsenic-bearing sulfide minerals like pyrite, and release from mineral surfaces by competition by more abundant anions. Until recently, production of dissolved sulfide by microbial sulfate reduction was thought to lead to arsenic removal from waters by precipitation of arsenic sulfide minerals or other arsenic-scavenging sulfide minerals (e.g., pyrite, arsenopyrite). However, arsenic can combine with sulfur in anoxic waters forming dissolved arsenic-sulfur compounds (thioarsenates and thioarsenites), which appear in some cases to be highly mobile and persistent (thioarsenates) in solution. Despite the growing recognition that thioarsenic species are an important aspect of arsenic geochemistry, the paths by which sulfidic conditions affects arsenic cycling in natural waters are complex and poorly understood. This poor understanding is apparent in the lack of equilibrium thermodynamic data for many of the thioarsenic species, which prevents reliable predictive modeling of their abundances and distributions in natural waters. The goals of this study are to measure the equilibrium constants that describe the formation of the four, homologous thioarsenate species (i.e., monothioarsenate, dithioarsenate, trithioarsenate, and tetrathioarsenate), and develop a geochemical model that can be used to predict the formation and abundances of these arsenic-sulfur compounds in natural water.