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质，大多数慢性砷中毒患者在饮用砷浓度较高的饮用水时都会接触到砷。尽管南亚和东南亚已知有超过1亿人暴露在饮用水中的高水平砷中，导致了一些人所说的人类历史上最大的自然灾害，但在美国的一些地方，当地居民也暴露在饮用水中天然存在的高水平砷中。因此，了解将砷从地质材料转移到天然饮用水水源的生物地球化学过程，对于预测随着时间的推移可能发现或发展高砷浓度的地方，以及制定补救战略以确保当前和未来人口的安全饮用水资源都是至关重要的。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中砷的形态的手段，而这在目前是不可能的。这种方法将允许其他研究人员使用熟悉的工具，如地化平衡和反应路径模型，更好地预测这种涉及砷和溶解硫化物之间的反应的过程可能在低温自然水域中发生的方向。这项拟议的研究将涉及一名研究生和最多三名本科生在生物地球化学研究方面的实践经验，并发展学术界和联邦机构(美国地质调查局、美国环保局)之间的密切合作。外展活动将包括：(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)通过杜兰大学的女生STEM项目向5至7年级的女孩介绍环境生物地球化学领域的职业机会；以及(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际研究经验。天然水中砷的动员可以通过一系列过程进行，包括间接微生物介导的铁氧化物的还原溶解和相关砷的释放到溶液中，直接酶(微生物)还原氧化的砷以亚砷酸根的形式还原为更具流动性的砷，氧化含砷的硫化物矿物如黄铁矿，以及通过更丰富的阴离子竞争从矿物表面释放砷。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物矿物(例如黄铁矿、毒砂)来从水中去除砷。然而，在缺氧的水中，砷可以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表现出高度的流动性和持久性(硫代砷酸盐)。尽管人们越来越认识到硫代砷形态是砷地球化学的一个重要方面，但硫化物条件影响自然水体中砷循环的途径复杂且知之甚少。许多硫代砷物种缺乏平衡热力学数据，阻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，这一认识不足是显而易见的。这项研究的目的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，并建立一个可用于预测这些砷硫化合物在天然水中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.