Kinetics and mechanism of trithionate and tetrathionate oxidation at low pH by hydroxyl radicals

The oxidation kinetics of trithionate (S3O62-) and tetrathionate (S4O62-) with hydroxyl radicals (OH*) have been investigated in systems analogous to acid mine drainage (AMD) environments. The discovery of hydroxyl radical (OH*) formation on pyrite surfaces (Borda et al., 2003) suggests hydroxyl radicals may affect the oxidation kinetics of intermediate sulfur species such as tetrathionate. Cyclic voltammetry experiments in acidic solutions indicate that the reaction of S4O62- with OH* goes through an unknown intermediate, tentatively assigned as S3O4n-. An outer-sphere electron transfer mechanism for the reaction of S4O62- with OH* to form S3O4n- is proposed based on experimental results. Oxidation rates for trithionate and tetrathionate in the presence of Fenton's reagent (which forms hydroxyl radicals) are too fast to be directly measured using UV-Vis spectrophotometry, electrochemical, or stop-flow spectrophotometry methods. Competitive reaction kinetics within the context of the Haber-Weiss mechanism suggests that the rate constant for the oxidation of trithionate and tetrathionate with OH* is in excess of 10(8) M-1 sec(-1).