Transport and fixation of arsenic in geothermal eviroments - a case study of the fossil geothermal system at Sailauf

世界各地许多活跃的地热系统排放大量的砷，这是造成地下水和土壤中地源砷污染的重要原因。地热系统中砷的迁移和固定过程，特别是表面吸附和络合作用涉及到铁、锰氧化物和氢氧化物，目前还不完全清楚。在高温和高压条件下，水热系统中砷迁移的地球化学模型的实质性改进需要将精心设计的现场分析研究与新的数值地球化学模拟方法相结合。建议以德国斯皮萨特赛劳的化石地热系统为例进行这样一项综合研究。这一系统非常适合作为案例研究，因为它的特点是砷的显著浓缩，以及含有少量/微量砷的不同铁、锰氧化物和氢氧化物相的丰富。这项研究将涉及以下目标：(1)详细了解砷在地热系统中共沉淀的不同矿物相中的分布，(2)利用适用于热液条件的改进的表面络合模型，评估砷在铁、锰氧化物和氢氧化物相上的吸附作用，(3)评估流体-岩石相互作用和流体沸腾对砷分布和浓缩的相对作用，以及(4)建立适用于世界其他地热系统的一致的砷迁移和固定的地质-地球化学模型。

Many active geothermal systems worldwide discharge large amounts of arsenic, which contributes significantly to the geogenic arsenic contamination of ground waters and soils. The processes responsible for transport and fixation of arsenic in geothermal systems, in particular the role of surface adsorption and complexation involving Fe and Mn oxide and hydroxide phases, are not fully understood. Substantial improvement of geochemical models for arsenic mobility in hydrothermal systems at elevated temperatures and pressures requires the integration of well-designed field-based analytical studies with novel approaches to numerical geochemical modeling. It is proposed to perform such an integrated study using the fossil geothermal system at Sailauf, Spessart, Germany, as a key example. This system is ideally suited to serve as a case study, because it is characterized by significant enrichment in arsenic and the abundance of different Fe and Mn oxide and hydroxide phases hosting minor/trace concentrations of arsenic. The study will adress the following objectives: (1) gaining detailed insight into the distribution of arsenic among various mineral phases that have coprecipitated in the geothermal system, (2) assessing the role of arsenic adsorption onto Fe and Mn oxide and hydroxide phases using an improved surface complexation model that is applicable at hydrothermal conditions, (3) evaluating the relative role of fluid-rock interaction and fluid boiling for arsenic distribution and enrichment, and (4) development of a consistent geological-geochemical model for the transport and fixation of arsenic that is applicable to other geothermal systems worldwide.