The role of dissolved oxygen in the sulfite/divalent transition metal ion system: degradation performances and mechanisms

Abstract

• S(IV) coupled with Fe(II) showed the fastest BrO 3 − and HA decay than other Me(II). • S(IV)/Fe(II) selectively and effectively reduced BrO 3 − over organics and inorganics. • Dissolved oxygen can regulate the interconversion between AOPs and ARPs. • Sulfite radical and Fe(II) resulted in rapid BrO 3 − reduction at DO 0 < 8 mg/L. • Hydroxyl and sulfate radicals led to fast HA degradation at DO 0 > 8 mg/L. In this study, we employed a sulfite/divalent transition metal ions system (S(IV)/Me(II)) to highly efficiently degrade bromate (BrO 3 − , a model inorganic oxidizing pollutant) and humic acid (HA, a model natural organic matter). Amongst the S(IV)/Me(II) system, S(IV)/Fe(II) system had the highest removal performance. Its rate constants for BrO 3 − reduction were 1.81 ± 0.08 min −1 and for HA oxidation were 1.22 ± 0.03 min −1 at a DO of 1.0 mg/L and 20.0 mg/L, respectively, orders of magnitude faster than previous studies. Of note, not only process performance but degradation mechanisms can be managed by controlling DO concentrations. Sulfite radicals mainly and Fe 2+ led to rapid BrO 3 − reduction at a low DO (DO 0 < 8.0); hydroxyl radicals primarily and sulfate radicals contributed to fast HA degradation at a high DO (DO 0 > 8.0). Further, bromide, sulfate, and carbon dioxide were the primary reaction products, and degradation pathways were proposed accordingly. Moreover, enhancing Fe(II) and S(IV) concentrations and lowering pH and DO levels facilitated BrO 3 − reduction mainly due to increases in reactive species. Comparatively, NO 3 − , Cl − , and SO 4 2− had a minor effect, whereas HCO 3 − and HPO 4 2− exhibited a powerful inhibition on BrO 3 − reduction primarily because of decreases in Fe 2+ levels in the S(IV)/Fe(II) system. Collectively, S(IV)/Fe(II) system could efficiently decontaminate inorganics (i.e., BrO 3 − ) and organics (e.g., HA) from potable waters by regulating DO levels.