ARSENIC SPECIATION IN SULFIDIC GEOTHERMAL WATERS

The speciation of arsenic in natural sulfidic geothermal waters was studied using chemical analysis and thermodynamic aqueous speciation calculations. The analytical setup used consisted of ion chromatography system connected with a Hydride Generation Atomic Fluorescence Spectrometer (IC-HG-AFS). Using this method, arsenite, arsenate and thio-arsenite and thio-arsenate compounds could be analyzed down to ~1-2 ppb within ~5 minutes of sampling on sample site.

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 (H_nAs^IIIO₃^n-3), thio-arsenite (H_nAs^IIIS₃^n-3), arsenate (H_nAs^VO₄^n-3) mono-thioarsenate (H_nAs^VSO₃^n-3), di-thioarsenite (H_nAs^VS₂O₂^n-3), tri-thioarsenate (H_nAs^VS₃O^n-3) and (terta-) thioarsenate (H_nAs^VS₄^n-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 As^III oxyanions. With increasing sulfide concentration As^III and As^V thio-species become also important and eventually predominant at high sulfide concentration. Upon oxidation As^V oxyanions may also be present and at high sulfide concentrations also As^V thio complexes.