Biochar-loaded iron oxide as a novel electrode for the electro-Fenton degradation of sulfaquinoxaline: Performance evaluation, mechanistic insights, and toxicity transformation.

In this work, carboxymethyl cellulose (CMC)-modified biochar (BC)-supported Fe3O4 was prepared for the degradation of sulfaquinoxaline (SQX) in a heterogeneous electro-Fenton process. The degradation rate of 10 mg/L SQX reached 94.2 % after 180 min of Fe3O4(CMC)/BC treatment, compared to 61.2 % with Fe3O4/BC. CMC allowed Fe3O4 particles to be more evenly distributed on the biochar surface, and its electron transfer capacity effectively activated the in situ generated H2O2 on the electrode with a maximum H2O2 yield of 17.9 mg/L. The produced 1O2 and ⋅O2- are the primary contributors to SQX degradation. The aniline of SQX is susceptible to electrophilic attack, whereas quinoxaline is susceptible to free radical attack, with bis-methylation, heterocyclic oxidation, and amino oxidation being the major reactions in the decomposition of SQX. Toxicity assessment by ECOSAR and T.E.S.T. modeling showed that all the intermediates were considerably less biotoxic than the parent compound. Density functional theory calculations showed that the O2 adsorption and H2O2 decomposition processes are spontaneous reactions, and the intermediates absorbed on the Fe atom have an increased energy potential and a tendency to be less active. The results of material cycling tests and metal ion leaching experiments confirmed the good reusability of the prepared cathode. Additionally, Fe3O4(CMC)/BC achieved excellent performance in livestock wastewater (SQX removal of 93.4 % and COD removal of 79.9 %), demonstrating the possibility of practical application. This study offers a theoretical foundation for the use of novel composite cathode materials to degrade persistent pollutants.