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

世界上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砷污染的重要原因。地热系统中负责砷的运输和固定的过程，特别是涉及铁和锰氧化物和氢氧化物相的表面吸附和络合作用，尚未完全了解。高温高压下热液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基于现场的分析研究与新颖的数值地球化学模拟方法相结合。建议以德国Spessart的Sailauf化石地热系统为例进行综合研究。这个系统非常适合作为一个案例研究，因为它的特点是砷的显著富集，不同的Fe和Mn氧化物和氢氧化物相的丰度含有少量/痕量的砷。这项研究将探讨以下目标：(1)详细了解了在地热系统中共沉淀的各种矿物相中砷的分布情况；(2)利用一种改进的适用于热液条件下的表面络合模型，评估了砷在铁锰氧化物和氢氧化物相上的吸附作用；(3)评估了流体-岩石相互作用和流体沸腾对砷分布和富集的相对作用。(4)建立了适用于全球其他地热系统的砷运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.