Iron cycle tuned by thiosulfate in Fenton reactions: Kinetic modelling and mechanisms

The widespread application of Fenton reaction have been limited by the rapid precipitation of Fe(III) and the slow Fe(III) reduction. This work presents a strategy to address these concerns by adding metal-free reducing agent thiosulfate (S2O32−). Compared to the classical Fenton reaction at pH 3.0, S2O32− significantly accelerated the degradation of sulfamethoxazole (SMX) over a wide pH range of 3.0–7.0. The enhancement of SMX degradation was attributed to the complexation and reduction role of S2O32−, preventing the precipitation of iron ions and accelerating the reduction of Fe(III). A first-principles kinetic model was established and revealed that the interaction of S2O32− and Fe(III) was a fast equilibrium reaction, affecting the oxidation kinetics of SMX. Both HO• and Fe(IV) were found to be involved in the SMX degradation. Compared to HO•, Fe(IV) is much more selective, and thus SMX degradation was less affected by water matrices, maintaining high efficiency in real water samples. Transformation products analysis demonstrated that (i) sulfonamide bond (S-N or C-S bonds) cleavage, (ii) hydroxylation and (iii) nitrosubstitution and deamination were the major transformation pathway. This study could improve our understanding the role of S2O32− in Fe(III)/H2O2 system and provide a novel promising Fenton-like reaction for wastewater treatment.