Relevance and environmental fate of methylthiolated arsenates in geothermal waters

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Geothermal waters are one of the most important sources in the geochemical arsenic cycle. Investigations typically cover arsenite and arsenate, which can travel considerable distances from a hot spring discharge with negative impact on downstream sediments, rivers, or groundwaters used as drinking water. Lately, it has been shown that thioarsenates (AsVSnO4-n3-; n = 1 - 4) which form from arsenite and reduced sulfur can dominate geothermal arsenic speciation. Their long-distance transport, however, is likely limited by their reactivity towards oxygen.Here, we postulate that methylation is a much more common process at geothermal springs than currently assumed and that methylthiolated arsenates can contribute significantly to total arsenic, especially at slightly acidic pH, high sulfide and dissolved organic carbon concentrations. We further postulate that compared to inorganic thioarsenates, methylthiolated arsenates are less susceptible to abiotic and microbially mediated species transformation and that compared to arsenite and arsenate they show less and slower sorption to iron mineral phases and natural organic matter, enhancing their mobility in the environment and long-distance impact away from geothermal sources.To test our hypotheses, we will carry out field studies at two geothermal areas in China (Rehai, Yunnan and Daggyai, Tibet) investigating arsenic speciation at the geothermal vents and following their fate and transformation along natural drainage channels as well as in on-site incubation studies. We will identify other abiotic or biotic factors contributing to arsenic species transformation. In the laboratory, we will synthesize methylthiolated arsenates and study their formation and stability varying S/As ratios, temperatures, pH, and presence and amount of oxidizing agents. We will further study sorption extent and kinetics on commonly occurring iron mineral phases (ferrihydrite, goethite, mackinawite, pyrite) and a model natural organic matter compound in batch experiments. Adding complexity, we will identify the mineralogical and chemical compositions of sediments in the hot spring vents and their outflow channels and determine their sorption potential for methylthiolated arsenates.The study will be carried out in close cooperation between Prof. Dr. Britta Planer-Friedrich (Germany), an expert in thioarsenic chemistry and analytics, and Prof. Dr. Qinghai Guo (China), an expert in geothermal water geochemistry. Cooperation includes co-supervised PhD and MSc students, joint field trips in China, and joint lab experiments in Germany. A kickoff meeting in Germany and a closing workshop in China will frame the project. The overall output of the present proposal will be to derive a new model for geothermal arsenic species formation, transport, transformation, and fate, illustrate the importance, and predict occurrence of methylthiolated arsenates also in other environments.