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

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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.