Enhanced Photolysis of Sulfite by Far-UVC (222 nm) and Efficient Photoreductive Degradation and Dehalogenation of Halogenated Organic Pollutants

UV/sulfite technologies offer promising advanced reduction processes (ARPs) for the degradation and dehalogenation of halogenated organic compounds (HOCs), but are limited by the low absorption of sulfite at a conventional wavelength of 254 nm (UV254). This study introduces a wavelength-optimized far-UVC KrCl* lamp (UV222)/sulfite as transformative ARPs. Sulfite exhibited 45.6-fold stronger absorption at 222 nm compared with 254 nm, coupled with a higher innate quantum yield (Φinnate) of 0.128 mol·Ein–1 versus 0.115 mol·Ein–1. This wavelength optimization generated 29.2-fold higher steady-state concentration of eaq–, with optimal performance at basic pH (>9) and moderate sulfite dosages (0.4–0.75 mM). HOC degradation and dehalogenation showed clear structure-dependent patterns. The enhancive effect of UV222 over UV254 negatively correlated with HOCs’ susceptibility to UV photolysis due to competitive absorption between sulfite and target compounds. Electron-withdrawing halogens significantly favored reductive degradation over oxidation, enabling UV222/sulfite to outperform UV222/H2O2 by 1.3–37.1 times in degradation and 1.9–48.6 times in dehalogenation efficiency, with prominent enhancement particularly for polyhalogenated HOCs. In real water matrices, nitrate and nitrite inhibited HOCs’ degradation through UV absorption and eaq– scavenging. Compared with UV254/sulfite, UV222/sulfite reduced energy costs by 92.7% and required lower sulfite doses. This study provides insights into the wavelength-matching mechanism for UV/sulfite, offering an economical and efficient solution for the remediation of recalcitrant HOCs.