pH-driven integrated impacts of the calcium sulfite/Fe(VI) system: In-depth insights into active species evolution and mechanisms

The CaSO 3 /Fe(VI) system was demonstrated to efficiently degrade emerging contaminants under alkaline conditions. However, the integrated impacts of pH-driven mechanisms remain to be fully elucidated. The present study investigated the degradation of sulfonamide antibiotics (SAs) in both buffer solution (BS) and deionized water (DW) by conducting comparative experiments under varying pH conditions, evaluating the pH-driven degradation efficiency and mechanisms, and clarifying the integrated impacts of pH variations in the CaSO 3 /Fe(VI) system. The results indicated that pH significantly influenced the degradation performance and kinetics of the CaSO 3 /Fe(VI) system toward SAs, identifying neutral pH as the optimum condition. The dynamic increasing in pH in DW enhanced the oxidation capacity under acidic conditions. In neutral and alkaline conditions, Fe(V) was the primary active species, while alkaline conditions favored radical generation. Conversely, acidic conditions favored the formation of Fe(IV) as the predominant species. Furthermore, the active sites of SAs were identified, and degradation pathways were proposed, confirming that the process simultaneously achieves contaminant removal and toxicity reduction. In terms of engineering implications, the CaSO 3 /Fe(VI) system offers a highly competitive treatment cost of only 0.45 CNY/m 3 , while also reducing chemical dosage requirements in potential processes such as coagulation and disinfection. Moreover, the end products SO 4 2− and Fe(III) both promote coagulation, demonstrating broad application potential. Such findings underscore the significance of pH in the CaSO 3 /Fe(VI) system, elucidate the integrated impacts of pH-driven mechanisms, and provide data support for engineering applications.