Relevance and environmental fate of methylthiolated arsenates in geothermal waters

地热水是地球化学砷循环的重要来源之一。调查通常包括亚砷酸盐和砷酸盐，它们可以从温泉排放物传播相当远的距离，对下游沉积物、河流或作为饮用水的地下水产生负面影响。最近研究表明，由亚砷酸盐和还原硫形成的硫代砷酸盐（AsVSnO4-n3-; n = 1 - 4）在地热砷的形成中起主导作用。然而，它们的长距离运输可能受到它们对氧气的反应性的限制。在这里，我们假设甲基化在地热泉中是一个比目前假设的更常见的过程，甲基硫化砷酸盐可以显著地增加总砷，特别是在微酸性pH、高硫化物和溶解有机碳浓度的情况下。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，它们对铁矿物相和天然有机物的吸附更少、更慢，从而增强了它们在环境中的流动性和远离地热资源的远距离影响。为了验证我们的假设，我们将在中国的两个地热区（云南热海和西藏大盖）进行实地研究，调查地热喷口的砷形态，并跟踪它们的命运和沿着自然排水通道的转变，以及现场孵化研究。我们将确定其他促进砷物种转化的非生物或生物因素。在实验室中，我们将合成甲基硫代砷酸盐，并研究它们的形成和稳定性，包括S/As比、温度、pH值、氧化剂的存在和数量。我们将进一步研究常见的铁矿物相（水合铁、针铁矿、镁铁、黄铁矿）和一种模式天然有机物的吸附程度和动力学。更复杂的是，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，并确定它们对甲基硫代砷酸盐的吸附潜力。这项研究将在硫砷化学和分析专家Britta Planer-Friedrich教授（德国）和地热水地球化学专家青海Guo教授（中国）的密切合作下进行。合作包括共同指导博士和硕士学生，在中国的联合实地考察，以及在德国的联合实验室实验。在德国举行的启动会议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推导出地热砷物种形成、运输、转化和命运的新模型，说明其重要性，并预测甲基硫代砷酸盐在其他环境中的发生。

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.