Isotope exchange reactions with radiolabeled sulfur compounds in anoxic seawater

Abstract

The isotope exchange between35S-labeled sulfur compounds of sulfate (SO4 2−), elemental sulfur (S0), polysulfide (Sn 2−), hydrogen sulfide (ΣHS−: H2S + HS− + S2−), iron sulfide (FeS), and pyrite (FeS2) was studied at pH 7.6 and 20 °C in anoxic, sterile seawater. Isotope exchange was observed between S0, S2 2− ΣHS−, and FeS, but not between35S labeled SO4 2− or FeS2 and the other sulfur compounds. Polysulfide mediated the isotope exchange between S0 and bisulfide (HS−). The isotope exchange between S0 and Sn 2−) reached 50% of equilibrium within < 2 min while exchange between S2 2− and HS− approached equilibrium within 0.5-1 h. In all the experiments ΣHS−, revealed a fraction exchange from 0.79 to 1.00. Isotope exchange between S2− and FeS took place only via S2 2− and/or HS−. The isotope exchange between iron sulfide and the other sulfur compounds was not complete within 24 h as shown by a fraction exchange of 0.07–0.83. This lack of equilibrium (fraction exchange < 1) was due to the isotope exchange between dissolved compounds and surfaces of sulfur particles. The isotopic exchange reactions limit the usefulness of radiotracers in process studies of the inorganic sulfur species. Exchange reactions will also affect the stable isotope distribution among the sulfur species. The kinetics of the isotopic exchange reactions, however, depend on both pH and temperature.