Analysis of the steady-state concentrations of reactive species and their role in contaminant degradation by the iron-biochar/persulfate advanced oxidation process: Comparison of probe compound and quenching agent methods

Abstract

Reactive species, including hydroxyl radicals (OH), sulfate radicals (SO₄), singlet oxygen (¹O₂), superoxide radicals (O₂), and Fe(IV), are generated by the iron-biochar activated persulfate (Fe-BC/PS) process. These reactive species can be leveraged for treatment of micropollutants, such as the sulfamethoxazole antibiotic. In this study, the steady-state concentrations and contributions of OH, SO₄, ¹O₂, O₂, and Fe(IV) to sulfamethoxazole degradation were calculated for different operating conditions in the iron-biochar/persulfate (Fe-BC/PS advanced oxidation process. Electron paramagnetic resonance was employed to confirm the production of each reactive species. The nitrobenzene, benzoic acid, furfuryl alcohol, p-chlorobenzoic acid or p-benzoquinone, and phenyl methyl sulfoxide probe compounds were added to experimental solutions in isolation, as mixtures, and at different concentrations to calculate the steady-state concentrations of OH, SO₄, ¹O₂, O₂, and Fe(IV) and determine their contributions to sulfamethoxazole degradation at variable pH conditions. The results not only informed the primary mechanisms of sulfamethoxazole degradation by the Fe-BC/PS system, but also highlighted best practices for the use of probe compounds and quenching agents in persulfate-based advanced oxidation processes. In particular, the initial concentration of the probe compounds should be as low as possible to avoid impacts on target contaminant degradation and misinterpretation of the role of each reactive species. Furthermore, quenching-based approaches to determination of the key reactive species were less consistent than evaluation by probe compounds. The overall outcomes of this work inform sulfamethoxazole treatment by the Fe-BC/PS system and emphasize the need for internal validation of kinetics results using a multi-pronged approach.