Paper No. 4
Presentation Time: 1:55 PM


KAASALAINEN, Hanna, Institute of Earth Sciences, University of Iceland, Sturlugata 7, Reykjavik, 101, Iceland and STEFÁNSSON, Andri, Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavik, 101, 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 (S2-) and sulfate (SO42-). Sulfur species with intermediate oxidation states (sulfite SO32-, thiosulfate S2O32-, polythionates SxO62, polysulfides Sx2-, 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, pHt 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 SO42-, S2O32-, SO32- and SxO62- and UV-Vis spectrophotometry and/or titrations for S2-. Alternatively, sulfoxyanion samples were stabilized on ion-exchange resins. The main dissolved sulfur species were SO42- (0.02-52.7 mmol/kg ) and S2- (<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 SO42-dominated. Traces of SO32- were detected in few surface geothermal water samples. Intermediate sulfur species were not detected in sub-boiling well discharges, and S2O32-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 S2- at pH>8 and S2- oxidation and interaction of S2- and S0 at near-neutral pH. At a lower pH, the reactions seemed to proceed relatively rapidly to SO42-. Sulfur speciation in natural geothermal waters is dynamic and both kinetically and source-controlled, and can not be estimated by thermodynamic calculations.