Theoretical estimation of equilibrium sulfur isotope fractionations among aqueous sulfite species: Implications for isotope models of microbial sulfate reduction

Abstract:

Sulfite (sensu lato), an intermediate in a variety sulfur redox processes, plays a particularly important role in microbial sulfate reduction. It exists intracellularly as multiple species between sets of enzymatic reactions that transform sulfate to sulfide, with the exact speciation depending on pH, T, and ionic strength. However, the complex speciation of sulfite is ignored in current isotope partitioning models of microbial sulfate reduction and simplified solely to the pyramidal SO₃^2- (sulfite sensu stricto), due to a lack of appropriate constraints. We theoretically estimated the equilibrium sulfur isotope fractionations (³³S/³²S, ³⁴S/³²S, ³⁶S/³²S) among all documented sulfite species in aqueous solution, including sulfite (SO₃^2-), bisulfite isomers and dimers ((HS)O₃^-, (HO)SO₂^-, S₂O₅^2-), and SO_2(aq), through first principles quantum mechanical calculations. The calculations were performed at B3LYP/6-31+G(d,p) level using cluster models with 30-40 water molecules surrounding the solute. Our calculated equilibrium fractionation factors compare well to the available experimental constraints and suggest that the minor and often-ignored tetrahedral (HS)O₃^- isomer of bisulfite strongly influences isotope partitioning behavior in the sulfite system under most environmentally relevant conditions, particularly fractionation magnitudes and unusual temperature dependence. For example, we predict that sulfur isotope fractionation between sulfite and bulk bisulfite in solution should have an apparent inverse temperature dependence due to the influence of (HS)O₃^- and its increased stability at higher temperatures. Our findings highlight the need to appropriately account for speciation/isomerization of sulfur species in sulfur isotope studies. We will also present similar calculation results of other aqueous sulfur compounds (e.g., H₂S/HS^-, SO₄^2-, S₂O₃^2-, S₃O₆^2-, and poorly documented SO₂^2- species), and discuss the implication of our results for microbial sulfate reduction models and other sulfur-redox processes in nature.