Relevance and environmental fate of methylthiolated arsenates in geothermal waters

地热水是地球化学砷循环中最重要的来源之一。调查通常涉及亚砷酸盐和亚砷酸盐，它们可能会从温泉流出相当长的距离，对下游沉积物、河流或用作饮用水的地下水造成负面影响。最近的研究表明，硫代砷酸盐(AsVSnO4-n3-；n=1-4)由亚砷酸盐和还原硫形成，可主导地热砷的形态形成。然而，它们的长途运输可能受到它们对氧气的反应性的限制。在这里，我们假设地热泉水中的甲基化过程比目前假设的要常见得多，甲基硫代砷酸盐可以显著地贡献总砷，特别是在弱酸性的pH、高硫化物和溶解的有机碳浓度下。我们进一步假设，与无机硫代砷酸盐相比，甲基硫代砷酸盐更不容易受到非生物和微生物介导的物种转化的影响，与亚砷酸盐和砷酸盐相比，它们对铁矿相和天然有机物的吸附更少和更慢，增强了它们在环境中的流动性和远离地热源的远程影响。为了验证我们的假设，我们将在中国的两个地热区(云南热海和西藏达格亚)开展实地研究，调查地热喷口砷的形态，并跟踪其在自然排水渠道和现场培养研究中的去向和转化。我们将确定导致砷物种转化的其他非生物或生物因素。在实验室中，我们将合成甲硫化砷盐，并研究其形成和稳定性随S/砷比、温度、pH以及氧化剂的存在和用量的变化。我们将在间歇实验中进一步研究常见的铁矿相(亚铁、针铁矿、麦基纳威石、黄铁矿)和一种模型天然有机物化合物的吸附程度和动力学。为了增加复杂性，我们将确定温泉喷口及其流出通道中沉积物的矿物学和化学成分，并确定它们对甲硫化砷的吸附潜力。这项研究将由硫代砷化学和分析专家Britta Planer-Friedrich教授(德国)和地热水地球化学专家郭青海博士(中国)密切合作进行。合作包括共同指导博士生和硕士研究生，在中国联合实地考察，在德国联合实验室实验。在德国举行的动员会和在中国举行的闭幕研讨会将确定该项目的框架。本提案的总体成果将是得出地热砷形态形成、迁移、转化和归宿的新模型，说明其重要性，并预测甲硫化砷酸盐在其他环境中的发生情况。

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.