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

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 geothermal 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 geothermal water are favorable for the stable existence of thioarsenicals, especially trithioarsenate and tetrathioarsenate.