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自然致癌物，大多数慢性砷中毒患者因饮用砷浓度较高的饮用水而接触到这种元素。尽管已知南亚和东南亚有超过 1 亿人的饮用水中含有高浓度的砷，从而导致了一些人所说的人类历史上最大规模的自然灾害，但在美国的许多地方，当地居民的饮用水中也含有高浓度的天然砷。因此，了解将砷从地质材料转移到天然饮用水源的生物地球化学过程对于预测砷浓度随时间推移可能发现或发展的位置升高以及设计修复策略以确保当前和未来人口的安全饮用水源至关重要。该项目的一个重要的更广泛的影响是，它将提供一种准确预测缺氧天然水域中砷形态的方法，而目前这是不可能的。这种方法将使其他研究人员能够利用地球化学平衡和反应路径模型等熟悉的工具来更好地预测涉及砷和溶解的硫化物之间的反应的此类过程在低温天然水中可能发生的方向。拟议的研究将涉及一名研究生和最多三名本科生的“实践”生物地球化学研究经验，并在学术界和联邦机构（美国地质勘探局、美国环保署）之间建立密切合作。外展活动将包括：(1) 通过路易斯安那州少数族裔参与联盟计划对至少一名本科生进行该项目的指导； (2) 通过杜兰大学 STEM 项目的女生向 5 至 7 年级女生介绍环境生物地球化学方面的职业机会； (3) 让路易斯安那数学、科学和艺术学院的高中生参与与该项目相关的“实践”研究经验。天然水中的砷可通过多种过程发生，包括间接微生物介导的氧化铁还原溶解和相关砷向溶液中的释放、氧化砷直接酶促（微生物）还原为亚砷酸盐氧阴离子形式的更具流动性的还原砷，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的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可以通过沉淀砷硫化物矿物或其他清除砷的硫化物矿物（例如黄铁矿、毒砂）来从水中去除砷。然而，砷可以在缺氧水中与硫结合，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具有高度流动性和持久性（硫代砷酸盐）。尽管人们越来越认识到硫砷物质是砷地球化学的一个重要方面，但硫化物条件影响自然水域砷循环的途径是复杂的且知之甚少。这种认识不足的表现是缺乏许多硫砷物质的平衡热力学数据，这阻碍了对其在天然水中的丰度和分布进行可靠的预测建模。本研究的目标是测量描述四种同源硫代砷酸盐（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）形成的平衡常数，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.