THIOARSENATES AND ARSINES - THE LESSER KNOWN ARSENIC SPECIES IN GEOTHERMAL SYSTEMS

Geothermal systems commonly contain high concentrations of geogenic arsenic, in Yellowstone National Park e.g. up to 5.4 mg/L. It is typically believed that most hot springs are dominated by arsenite at the point of discharge due to the presence of reducing agents, such as sulfide, and that their further environmental fate depends on oxidation to arsenate by microbial catalysis.

We, however, showed by anion-exchange chromatography inductively coupled plasma mass spectrometry that inorganic (AsS_nO_4-n^3-; n=1-4) and methylated (CH₃AsSO₂^3-_, (CH₃)₂AsSO^3-) thioarsenates occur in sulfidic geothermal waters over a pH range of 2.1 to 9.3. Under alkaline conditions, they dominate arsenic speciation (up to 83% of total arsenic). Previous reports on arsenite dominance in sulfidic waters either missed the arsenic portion of thioarsenates completely due to precipitation of arsenic-sulfide mineral phases or co-determined them with arsenite or arsenate due to their instability towards changes in pH and oxygen. Incubation field experiments and laboratory growth studies on thioarsenates were conducted with the hyperthermophile microorganism Thermocrinis ruber. Thioarsenate transformation was shown to be linked to the sulfur cycle with microbially enhanced sulfide oxidation coupled to reduction of arsenate to arsenite enhancing abiotic desulfidation of thioarsenates. However, we were also able to show that at least monothioarsenate could be used as sole source for aerobic autotrophic growth.

Volatile arsenic release from geothermal sources was observed especially under acidic conditions (pH < 4). Speciation using solid-phase micro-extraction fibers and analysis by gaschromatography coupled to mass spectrometry showed the presence of trimethylarsine and, for the first time in natural systems, methylchloroarsines ((CH₃)₂AsCl, CH₃AsCl₂) and a dimethylarsenomercaptane ((CH₃)₂AsSCH₃). The source of the arsines is yet unclear and could be both primary subsurface steam phase separation and secondary microbial transformation. Photosynthetic cyanobacteria have recently been shown to be capable of arsenic volatilization. An alternative are yet unknown volatilization mechanisms of thermophiles. The fate of volatile arsenic upon release from the geothermal features was tracked by active moss-monitoring.