Speciation of arsenic and antimony in geothermal water affected by their competitive hiolation: A case study in several typical Ali hydrothermal areas, Tibet

Arsenic and antimony are typical harmful constituents in geothermal water, in which they usually show different speciationfrom other types of natural waters, as a result of the unique hydrochemical conditions there. With the Langjiu, Quseyongba, Menshi and Moluojiang geothermal areas in Ali, Tibet asthe study areas,, the arsenic and antimony speciation in the sulfide-bearing geothermal waters under the influence of competitive thiolation between their oxyanions was investigated. Owing toinput of arsenic- and antimony-rich magmatic fluids and/or enhanced leaching of reservoir hostrocks at elevated temperature, the arsenic and antimony concentrations in the geothermal waters discharged from the sehydrothermal areas range from 5833 to 20750 μg/L and from 579 to 2129 μg/L, respectively. Arsenite and arsenate are the main species of arsenic in the geothermal waters, but thioarsenates exist as well with their proportions in total arsenic ranging from 0.1 to 55.1%. Different from arsenic, the species of antimony in all the geothermal water samples are antimonite and/or antimonatewith thioantimonates being undetected. Considering that the S/Sb molar ratios of a large part of the geothermal waters are high enough for formation of thioantimonates and that arsenic in all the samples is more enriched than antimony to varying degrees, we concluded that thiolation of antimony oxyanions in the geothermal waters was strongly inhibited by coexisting arsenic oxyanions. Provided that there were no a large excess of sulfide over the sum of arsenic and antimony in geothermal water, competitive thiolation of arsenic oxyanions would be the most critical factor impeding formation of thioantimonates. The present work and the results obtained in this study would be helpful for an in-depth understanding of the environmental geochemical behaviour of arsenic and antimony in geothermal water environments in Tibet.