The kinetics of aqueous thiolated arsenic oxidation

Thiolated arsenic (As) compounds have been identified in various natural and engineered environments worldwide and are important for the biogeochemical cycling of As, yet quantitative data regarding their stability and transformation rates remains scarce. This study investigates the oxidation kinetics of mono-, di-, and tri-thioarsenate at varying pH, Fe, and (thio-)As concentrations in the aqueous phase. Experiments conducted over four weeks revealed that all thioarsenates were oxidized faster at lower pH, with rates of up to several μmoles/L/d at a pH of 3. Trithioarsenate demonstrated approximately two orders-of-magnitude faster oxidation rates than di- and monothioarsenate and these rates exhibited a higher sensitivity to pH and dissolved As and Fe concentrations. The presence of Fe enhanced the oxidation rates of trithioarsenate but had less impact on di- and monothioarsenate. Kinetic data were subsequently used to parameterize oxidation rate equations and determine reaction orders, and to calibrate a kinetic model that was leveraged to determine rate constants. The fundamental insights and kinetic parameters derived for thio-As oxidation in this study are important for predicting the mobility of thio-As compounds and for assessing the potential environmental impacts of As across ambient aquatic systems.