SULFUR CHEMISTRY IN GEOTHERMAL FLUIDS, ICELAND

Sulfur (S) is among the main components in natural geothermal waters and its compounds are important in many (bio)geochemical processes and transport and deposition of metal(loid)s. The most common oxidation states are sulfide (S^2-) and sulfate (SO₄^2-). Sulfur species with intermediate oxidation states (sulfite SO₃^2-, thiosulfate S₂O₃^2-, polythionates S_xO₆², polysulfides S_x^2-, elemental sulfur S⁰) may also be present, these however not typically analyzed in geothermal water.

We determined sulfur speciation in geothermal water samples collected from hot springs, acid-sulfate pools and mud pots, and well discharges in Iceland. Water temperatures were in the range of 4-210 °C, pH_t 2.20-9.30, and Cl concentrations <0.01–10 mmol/kg. Sulfur species were preferably determined on site within 10 min of sampling using IC for SO₄^2-, S₂O₃^2-, SO₃^2- and S_xO₆^2- and UV-Vis spectrophotometry and/or titrations for S^2-. Alternatively, sulfoxyanion samples were stabilized on ion-exchange resins. The main dissolved sulfur species were SO₄^2- (0.02-52.7 mmol/kg ) and S^2- (<1-4100 μmol/kg ). Thiosulfate was detected in most samples in concentrations in the range of <1- 394 μmol/kg (sulfur equivalents). It was found in significant concentrations in alkaline and neutral pH hot springs, whereas it was not a significant species in steam-heated acid-sulfate waters where SO₄^2-dominated. Traces of SO₃^2- were detected in few surface geothermal water samples. Intermediate sulfur species were not detected in sub-boiling well discharges, and S₂O₃^2-was present only at low concentrations in high-temperature well discharges.

The results indicate that intermediate sulfur species do not occur in the aquifer fluids at depth at <150°C and are only present in trace concentrations at >150°C. The intermediate sulfur species are considered to form at or near the surface by oxidation of S^2- at pH>8 and S^2- oxidation and interaction of S^2- and S⁰ at near-neutral pH. At a lower pH, the reactions seemed to proceed relatively rapidly to SO₄^2-. Sulfur speciation in natural geothermal waters is dynamic and both kinetically and source-controlled, and can not be estimated by thermodynamic calculations.