Kinetics and mechanism of polythionate oxidation to sulfate at low pH by O2 and Fe3+

Polythionates (SxO62-) are important in redox transformations involving many sulfur compounds. Here we investigate the oxidation kinetics and mechanisms of trithionate and tetrathionate oxidation between pH 0.4 and pH 2 in the presence of Fe3+ and/or oxygen. In these solutions, Fe3+ plus oxygen oxidizes tetrathionate and trithionate at least an order of magnitude faster than oxygen alone. Kinetic measurements, coupled with density functional calculations, suggest that the rate-limiting step for tetrathionate oxidation involves Fe3+ attachment, followed by electron density shifts that result in formation of a sulfite radical and S3O30 derivatives. The overall reaction orders for trithionate and tetrathionate are fractional due to rearrangement reactions and side reactions between reactants and intermediate products. The pseudo-first order rate coefficients for tetrathionate range from 10(-11) s(-1) at 25degreesC to 10(-8) s(-1) at 70degreesC, compared to 2 x 10(-7) s(-1) at 35 degreesC for trithionate. The apparent activation energy (E-A) for tetrathionate oxidation at pH 1.5 is 104.5 +/- 4.13 kJ/mol. A rate law at pH 1.5 and 70degreesC between 0.5 and 5 millimolar [Fe3+] is of the form: r = 10(-6.61+/-0.3)[S4O62-](0.3+/-0.08)[Fe3+](0.15+/-0.09) where rate, r, is in units of mol L-1 s(-1). In pH <1 solutions where trithionate forms by rearrangement of tetrathionate, trithionate oxidation is the rapid pathway for conversion of tetrathionate to sulfate. The kinetics of both trithionate and tetrathionate oxidation are several orders of magnitude slower than the formation of polythionates from thiosulfate in acidic, Fe3+-rich solutions. Copyright (C) 2003 Elsevier Ltd.