高温富硫化物热泉中硫代砷化物存在形态的地球化学模拟:以云南腾冲热海水热区为例

Geochemical Simulation of Thioarsenic Speciation in High-Temperature,Sulfide-Rich Hot Springs:A Case Study in the Rehai Hydrothermal Area,Tengchong,Yunnan

天然水环境中地质成因砷的存在是世界范围内对人类威胁极大的环境问题之一.在高温富硫化物地热水中,硫代砷化物是砷的主要存在形态之一.在国内尚无硫代砷化物定量检测方法的背景下,以云南腾冲地热带的热海水热区为典型研究区,基于不同类型硫代砷化物的最新化学热力学数据wateq4f.dat,利用水文地球化学模拟软件PHREEQC开展了不同类型热泉中砷的存在形态的地球化学模拟.结果表明,热海热泉中砷的主要形态是硫代砷酸盐,砷酸盐和亚砷酸盐次之,硫代亚砷酸盐则含量极低;在各类硫代砷酸盐中,按平均百分含量降序依次为:一硫代砷酸盐→三硫代砷酸盐→四硫代砷酸盐→二硫代砷酸盐.pH、Eh和总硫化物含量是热泉中砷的形态分布的控制性因素.在酸性条件下,砷以硫代砷酸盐和亚砷酸盐为主要存在形式;而在中性/偏碱性条件下,砷的形态则以硫代砷酸盐为主,砷酸盐次之.偏还原环境和高硫化物含量是硫代砷化物、特别是三硫代砷酸盐和四硫代砷酸盐稳定存在的有利条件.

The occurrence of geogenic arsenic in natural water environment is one of the significant hazards to human beings in the world. In high-temperature, sulfide-rich geothermal waters, thioarsenicals are likely the major species of arsenic. In view that there has been so far no quantitative test method for thioarsenic species in China, a hydrogeochemical code, PHREEQC, with its wateq4f.dat database being updated by the latest thermodynamic data for thioarsenicals, was used for calculating the arsenic species distribution in various types of hot springs from the Rehai hydrothermal area located in the Tengchong geother- mal belt, Yunnan Province. The results show that the major species of arsenic in the Rehai hot springs is thioarsenate, arsenate and arsenite comes second, and the thioarsenite concentrations are extremely low; the descending order of thioarsenate species in terms of their average percentages is： monothioarsenate, trithioarsenate, tetrathioarsenate, and dithioarsenate, pH, Eh and total sulfide concentration are the controlling factors for the arsenic speciation in hot springs. Under acidic condition, the main species of arsenic are thioarsenate and arsenite, whereas under neutral/weak alkaline conditions, thioarsenate is predominant with arsenate being the second most important species. Comparative reducing environment and high sulfide concentration in ge-othermal water are favorable for the stable existence of thioarsenicals, especially trithioarsenate and tetrathioarsenate.