西藏搭格架高温热泉中砷的地球化学异常及其存在形态

西藏搭格架高温热泉是我国大陆少有的大型间歇性喷泉，砷元素作为对人类威胁极大的环境问题普遍存在于热泉之中，搭格架高温热泉中砷元素质量浓度最高已达到了9.75 mg/L，其对地表水和浅层地下水的污染不容忽视。硫代砷是富含硫化物热泉中砷的存在形态之一，鉴于国内相关研究较少，本文对西藏搭格架地热区的热泉样品进行了水化学分析，并利用水文地球化学模拟软件PHREEQC开展了对热泉中砷元素存在形态的地球化学模拟。结果表明：西藏搭格架热泉中砷元素的存在形态有亚砷酸盐、砷酸盐和硫代砷，其中亚砷酸盐与砷酸盐是砷的主要存在形态，且在pH影响下两者之间存在相互转化关系；各种硫代砷按质量浓度由高至低依次为一硫代砷酸盐、三硫代砷酸盐、二硫代砷酸盐、一硫代亚砷酸盐、四硫代砷酸盐；硫代砷形态占总砷浓度比例主要受热泉中硫化物质量浓度、Eh（氧化还原电位）和pH等因素的控制，在硫化物质量浓度总体偏低的情况下，硫化物质量浓度的上升可促进其他形态的砷向硫代砷形态转化，强还原性环境有利于硫代砷形态的存在；此外，在中性环境下，硫代砷占总砷浓度比例随pH上升亦有上升趋势。

Geochemical Anomalies of Arsenic and Its Speciation in Daggyai Geothermal Springs,Tibet

Daggyai geothermal system in Tibet includes the biggest geyser in mainland China. Arsenic, as one of the most harmful substances, commonly exists in geothermal water, and the highest arsenic concentration detected in Daggyai geothermal springs reaches 9.75 mg/L, unneglectable potential arsenic pollution in shallow groundwater and rivers should be noticed. Thioarsenic usually exists as the dominant species of arsenic in sulfide-rich thermal springs;however there are few related studies in China. We focused on Daggyai geothermal system, and analyzed the chemical components of the geothermal spring samples. Thioarsenic species in each sample were calculated by using the hydro-geothermal simulation software PHREEQC. The results are as follows:Arsenic in Daggyai geothermal water consists of arsenate, arsenite, and thioarsenic, among which the arsenate and arsenite are the main species. There are interchanges between arsenate and arsenite when pH changes. The descending order of thioarsenic species in terms of their average ratio is monothioarsenate, trithioarsenate, ditthioarsenate, monothioarsenite and tetrathioarsenate. The proportion of thioarsenic to total arsenic concentration is mainly controlled by sulfide concentration, pH and Eh in geothermal water. High concentration of sulfide can promote the process of arsenic changing into thioarsenic, and strong reducing environment is required for the existence of thioarsenic. Moreover, the percentage of thioarsenic seems to have a positive correlation with pH value.