Mercury isotope ratios as a tool to trace and quantify Hg species transformations in contaminated soils and aquifers (MIRACOSA)

汞是一种有毒的污染物元素，对环境有很大的影响。不同的汞物种(如汞(II)Cl2、汞(0)、汞)表现出广泛的物理化学性质。因此，汞的形态转化对汞的环境迁移性和生物有效性具有重要影响。然而，认识到这种变化及其对环境中汞的意义一直是具有挑战性的。然而，在拟议的工作中，我们探索使用汞同位素分馏作为解决这一问题的新工具。在汞物种转化(如还原/氧化、吸附、沉淀、挥发)过程中，汞的稳定同位素分布发生了可测量的变化，可以利用汞同位素比值来追踪环境系统中汞的动态变化。不同的过程产生了汞同位素特有的质量相关和质量无关的分馏特征。这使得能够对汞物种转化的生物地球化学控制获得新的见解，并有可能量化自然样品中不同过程的相对重要性。我们选择了德国西南部的两个现场(都是由于在木材浸渍中使用汞(II)Cl2而被汞污染)作为单一汞点源的自然实验室，以研究土壤和地下水系统中汞的形态转化和相关的汞同位素分馏。我们收集了一组初步数据，确定了不同汞物种的存在，并揭示了土壤和地下水样本中汞同位素的显著变化。我们假设，土壤和地下水中的汞同位素特征将提供过去和现在汞形态转化的完整图景，并允许在固溶体界面和不同固相汞池之间追踪过程。除了收集大量现场样本外，还将进行实验室实验，调查相关个别过程中的汞同位素分馏，以获得解释和定量描述现场数据所需的分馏因素和机制。此外，将开发一个包含不同汞物种之间汞同位素分馏的反应迁移模型，以提供对地下水系统中汞羽流随时间变化的预测性了解。液态、固态和气态样品的汞浓度、汞形态(使用热解-热解吸法)和汞同位素(使用冷蒸汽-MC-ICPMS)分析的新组合将显著提高我们预测汞的行为和归宿的能力，不仅在研究的野外地点，而且在其他受污染的和原始的含水层和土壤系统。与已经记载的来源示踪剂的使用相反，汞同位素特征作为过程示踪剂在受污染地点的应用尚未得到描述，目前文献中也没有关于地下水的汞同位素数据。

Mercury (Hg) is a toxic pollutant element of great environmental concern. Different Hg species (e.g., Hg(II)Cl2, Hg(0), HgS) exhibit a wide range of physico-chemical properties. Thus, Hg species transformations exert a major influence on the environmental mobility and bioavailability of Hg. However, recognizing such transformations and their significance for Hg in the environment has been challenging. However, in the proposed work we explore the use of Hg isotope fractionation as a novel tool to address this problem. The stable isotope distribution of Hg is altered to a measurable extent during Hg species transformations (e.g., reduction/oxidation, sorption, precipitation, volatilization) may be used to trace Hg dynamics in environmental systems using Hg isotope ratios. Different processes cause characteristic mass-dependent and mass-independent fractionation signatures of Hg isotopes. This allows obtaining new insights into biogeochemical controls of Hg species transformations and potentially quantifying the relative importance of different processes in natural samples. We have selected two field sites in SW-Germany (both Hg-contaminated due to the use of Hg(II)Cl2 in wood impregnation) serving as natural laboratories with a single Hg point source to investigate Hg species transformations and associated Hg isotope fractionation in soil and groundwater systems. We have collected a set of preliminary data identifying the presence of different Hg species and revealing significant Hg isotope variations in soil and groundwater samples. We hypothesize that Hg isotope signatures in soil and groundwater will provide an integrated picture of past and present Hg species transformations and allow tracing processes at solid-solution interfaces and between different solid phase Hg pools. In addition to collecting a large set of field samples, laboratory experiments investigating Hg isotope fractionation during relevant individual processes will be conducted to obtain fractionation factors and mechanisms needed for the interpretation and quantitative description of the field data. Furthermore, a reactive transport model incorporating Hg isotope fractionation between different Hg species will be developed to provide a predictive understanding of Hg plumes in groundwater systems over time. The novel combination of Hg concentration, Hg speciation (using pyrolytic-thermodesorption), and Hg isotope (using cold vapor-MC-ICP-MS) analysis of liquid, solid, and gaseous samples will significantly improve our capability to predict the behavior and fate of Hg not only for the studied field sites but also for other contaminated as well as pristine aquifer and soil systems in general. In contrast to the already documented use source tracer, the application of Hg isotope signatures as process tracer at contaminated sites has not been described yet and no Hg isotope data for groundwater are currently available in the literature.