Autocatalytic effect of in situ formed (hydro)quinone intermediates in Fenton and photo-Fenton degradation of non-phenolic aromatic pollutants and chemical kinetic modeling

While the electron-shuttling properties of (hydro)quinones can facilitate the redox cycling of iron species (Fe2+ and Fe3+), the impact of in situ formed (hydro)quinones from degradation of non-phenolic aromatic pollutants in Fenton-type processes has not been reported. This study investigated the catalytic effect of (hydro)quinone intermediates produced from degradation of p-arsanilic acid (p-ASA) and roxarsone (ROX) in Fenton and photo-Fenton processes, and its chemical kinetic modeling. The bimolecular rate constants of the reactions of OH with p-ASA and ROX were determined to be (2.79 ± 0.29) × 109 and (2.03 ± 0.07) × 109 M−1·s−1, respectively. However, ROX degraded faster than p-ASA in Fenton process under the same conditions, which was attributed to the greater catalytic effect of the in situ formed (hydro)quinone intermediates. p-Hydroquinone, p-benzoquinone, and 1,2,4-benzenetriol were identified among the oxidation intermediates of p-ASA, while 2-nitrohydroquinone, 2-nitrobenzoquinone, 1,2,4-benzenetriol, and 2-hydroxy-benzoquinone were found for ROX oxidation. The autocatalytic degradation of p-ASA and ROX in Fenton and photo-Fenton processes could be well described by a chemical kinetic model after accounting for the reactions of the (hydro)quinone intermediates. Both experimental and modeling results consistently showed that the redox cycling of iron promoted by the in situ formed (hydro)quinone intermediates was critical for the oxidation of p-ASA and ROX in Fenton process. Chemical kinetic modeling revealed that (hydro)quinone-related reactions and photo-reduction of Fe3+-complexes were responsible for producing the vast majority of OH that sustained the continuous degradation of p-ASA and ROX in photo-Fenton process with the presence of limited amount of Fe2+. The autocatalytic effect observed for phenylarsonic acid compounds and the chemical kinetic model developed in this study could guide the development of Fenton and solar photo-Fenton treatment for other non-phenolic aromatic pollutants.