Degradation of sulfamethoxazole by the heterogeneous Fenton-like reaction between gallic acid and ferrihydrite

In soils, the Fenton-like reaction can be initiated when phenolic acids (PCs) existed simultaneously with iron oxides and dissolved O2, which would have great impact on transformation of organic pollutants. This study probed the mechanism of the Fenton-like reaction that occurs in a heterogeneous system containing ferrihydrite (Fh) and gallic acid (GA), and evaluated its performance in sulfamethoxazole (SMX) degradation. In the absence of dissolved O2, only reductive dissolution of Fh by GA occurred. It was further showed that Fh is capable of catalyzing the oxidation of GA by O2, in which the Fenton-like reaction was involved with the formation of reactive oxygen species (ROS) (semiquinone free radicals, superoxide, singlet oxygen, hydroxyl radical and H2O2) together with the adsorbed and aqueous Fe(II). At pH 4.0, this Fenton-like reaction could lead to SMX degradation at a rate of 38.2% and 65.6% when GA concentration were set at 0.1 and 0.2 mM, respectively. Elevating pH inhibited SMX degradation process. Citric acid had no effect on SMX degradation, while ascorbic acid showed a promotive effect. Moreover, HPLC-MS showed the presence of 12 intermediate products, and the proposed pathways for SMX degradation included cleavage, demethylation, oxidation and electrophilic substitution. This work could enhance our understanding on how the abiotic soil Fenton-like reaction controls the fate of SMX in soil environments.