ARSENIC SPECIATION IN SULFIDIC GEOTHERMAL WATERS
Samples of geothermal waters in Iceland were collected and analyzed using this method. The samples included hot springs and well discharges and their outflow. The arsenic concentration of the sampled water was 7-116 ppb, pH was 8.56-9.60 and the H2S concentration was <0.01-77.6 ppm. Nine arsenic species were identified considered to be among others arsenite (HnAsIIIO3n-3), thio-arsenite (HnAsIIIS3n-3), arsenate (HnAsVO4n-3) mono-thioarsenate (HnAsVSO3n-3), di-thioarsenite (HnAsVS2O2n-3), tri-thioarsenate (HnAsVS3On-3) and (terta-) thioarsenate (HnAsVS4n-3) where n = 0-3. Some peaks could be unambiguously identified whereas others remain somewhat uncertain.
The thermodynamic speciation of arsenic was calculated from the major elemental composition of the waters and compilation of various arsenic species stability constants. The calculated and measured species concentrations are in reasonable agreement. The results of the thermodynamic and measured arsenic speciation reveal that the distributions of the various arsenic species in natural geothermal waters are both dependent on sulfide concentration and pH. Temperature may also play a role. At low sulfide concentrations and in alkaline waters the predominant species are AsIII oxyanions. With increasing sulfide concentration AsIII and AsV thio-species become also important and eventually predominant at high sulfide concentration. Upon oxidation AsV oxyanions may also be present and at high sulfide concentrations also AsV thio complexes.