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

砷被认为是环境中最常见的自然致癌物，大多数受慢性砷中毒影响的人是通过饮用砷浓度升高的饮用水而接触到这种元素的。虽然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，导致了一些人所说的人类历史上最大的自然灾害，但在美国境内的许多地方，当地居民也暴露于饮用水中自然存在的高浓度砷。因此，了解将砷从地质物质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升高的地方或随着时间的推移而发展，以及设计补救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，它将提供一种方法来准确预测在缺氧的自然水中砷的形态，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，以更好地预测涉及砷和溶解的硫化物之间反应的过程在低温自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“动手”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）之间发展密切合作。外展将包括：(1)通过路易斯安那州少数民族参与联盟项目指导至少一名本科生；(2)通过杜兰大学女生STEM项目向5 - 7年级女生介绍环境生物地球化学领域的就业机会；(3)让路易斯安那州数学、科学和艺术学院的高中生参与与该项目相关的“动手”研究经验。自然水体中的砷动员可通过多种过程发生，包括间接微生物介导的氧化铁的还原性溶解和伴生砷释放到溶液中，直接酶（微生物）将氧化砷还原为亚砷酸盐氧化阴离子形式的更具流动性的还原性砷，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的溶解硫化物被认为是通过沉淀硫化砷矿物或其他清除砷的硫化矿物（如黄铁矿、毒砂）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）。尽管越来越多的人认识到硫砷是砷地球化学的一个重要方面，但硫化条件影响自然水体中砷循环的途径是复杂的，而且人们对其知之甚少。这种不充分的认识在许多硫砷物种的平衡热力学数据的缺乏中是显而易见的，这妨碍了对其在自然水域中的丰度和分布进行可靠的预测建模。本研究的目的是测量描述四种同源硫代砷酸盐（即单硫代砷酸盐、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，并建立一个地球化学模型，用于预测这些砷硫化合物在天然水中的形成和丰度。

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.