Sulfur speciation in natural hydrothermal waters, Iceland

The speciation of aqueous dissolved sulfur was determined in hydrothermal waters in Iceland. The waters sampled included hot springs, acid-sulfate pools and mud pots, sub-boiling well discharges and two-phase wells. The water temperatures ranged from 4 to 210°C, the pH_T was between 2.20 and 9.30 at the discharge temperature and the SO₄ and Cl concentrations were 0.020-52.7 and <0.01-10.0mmolkg^-1, respectively. The analyses were carried out on-site within ~10min of sampling using ion chromatography (IC) for sulfate (SO₄^2-), thiosulfate (S₂O₃^2-) and polythionates (S_xO₆^2-) and titration and/or colorimetry for total dissolved sulfide (S^2-). Sulfite (SO₃^2-) could also be determined in a few cases using IC. Alternatively, for few samples in remote locations the sulfur oxyanions were stabilized on a resin on site following elution and analysis by IC in the laboratory. Dissolved sulfate and with few exceptions also S^2- were detected in all samples with concentrations of 0.02-52.7mmolkg^-1 and <1-4100μmolkg^-1, respectively. Thiosulfate was detected in 49 samples of the 73 analyzed with concentrations in the range of <1-394μmolkg^-1 (S-equivalents). Sulfite was detected in few samples with concentrations in the range of <1-3μmolkg^-1. Thiosulfate and SO₃^2- were not detected in <100°C well waters and S₂O₃^2- was observed only at low concentrations (<1-8μmolkg^-1) in ~200°C well waters. In alkaline and neutral pH hot springs, S₂O₃^2- was present in significant concentrations sometimes corresponding to up to 23% of total dissolved sulfur (S_TOT). In steam-heated acid-sulfate waters, S₂O₃^2- was not a significant sulfur species. The results demonstrate that S₂O₃^2- and SO₃^2- do not occur in the deeper parts of <150°C hydrothermal systems and only in trace concentrations in ~200-300°C systems. Upon ascent to the surface and mixing with oxygenated ground and surface waters and/or dissolution of atmospheric O₂, S^2- is degassed and oxidized to SO₃^2- and S₂O₃^2- and eventually to SO₄^2- at pH >8. In near-neutral hydrothermal waters the oxidation of S^2- and the interaction of S^2- and S⁰ resulting in the formation of S_x^2- are considered important. At lower pH values the reactions seemed to proceed relatively rapidly to SO₄^2- and the sulfur chemistry of acid-sulfate pools was dominated by SO₄^2-, which corresponded to >99% of S_TOT. The results suggest that the aqueous speciation of sulfur in natural hydrothermal waters is dynamic and both kinetically and source-controlled and cannot be estimated from thermodynamic speciation calculations.