Reaction kinetics of organic sulfur compounds with ozone in aqueous phase

Abstract

Sulfur functional groups are abundant in natural organic compounds, pesticides and pharmaceuticals. Kinetic data of their aqueous reaction with ozone is only available for a very small set of compounds and sometimes contradictory, making predictions on their fate during ozonation processes difficult. Therefore, second-order rate constants k_O₃ were determined for 37 sulfur-containing compounds and trends in reactivity were identified. For the determination of k_O₃ via competition kinetics, aldicarb was established as pH-independent competitor (k_O₃=1.0 × 10⁶M⁻¹s⁻¹). Sulfides (thioethers) react very fast with ozone (k_O₃≈2 × 10⁶M⁻¹s⁻¹). Exceptions are sulfides, which are π-conjugated to electron-withdrawing groups (e.g., thiocarbamates, k_O₃≈10⁰−10³M⁻¹s⁻¹), and β-lactam antibiotics (k_O₃≈10³M⁻¹s⁻¹), which are deactivated to different degrees. Thiols exhibit pH-dependent reactivities, though the dependency is much smaller than previously reported, ranging from k_O₃+SH≈10⁴M⁻¹s⁻¹ to k_O₃+S⁻≈10⁶M⁻¹s⁻¹, but an unprecedented pH-dependency is observed. A deactivating effect of neighboring groups renders the reactivity of the disulfide cystine and similar compounds pH-dependent (k_O₃(pH3)≈10²M⁻¹s⁻¹, k_O₃(pH7)≈10⁵M⁻¹s⁻¹) while disulfides without acidic functional groups exhibit pH-independent reactivity (k_O₃≈2 × 10⁵M⁻¹s⁻¹). Further oxidized functional groups like sulfoxides or thiosulfinate esters are unreactive (k_O₃≈10⁰−10¹M⁻¹s⁻¹), but sulfinates are very susceptible to ozonation (k_O₃≈2 × 10⁶M⁻¹s⁻¹). A definitive explanation for this difference is still lacking. For non-N-alkylated isothiazolones, an unexpected pH-dependency is observed, similar to cysteines. The reactivity at pH 7 differs by 4 orders of magnitude, whether or not N-alkylated (k_O₃≈10²M⁻¹s⁻¹, k_O₃≈10⁶M⁻¹s⁻¹, respectively). Overall, the data presented provides novel insights into the ozone reactivity of sulfur compounds as a basis for a better assessment of their fate during ozonation.