Hydrogen radical is also a critical reductive defluorination species in the Far-UVC-activated sulfite process

Sulfite photolyzed with far-UVC light is increasingly used for the dehalogenation of halogenated contaminants due to its high yields of hydrated electron (e_aq^-), but hydrogen radicals (•H) were always overlooked in previous studies. This work first revealed the critical roles of •H in rapid reductive degradation and dehalogenation of florfenicol (FLO) under near-neutral conditions. Results demonstrated that sulfite activated by far-UVC process achieved 95.2% degradation of FLO within 15 min and the defluorination and dechlorination efficiency were 92.4% and 100.0% within 30 min, respectively. Increasing pH value from 5.0 to 9.0 improved the FLO degradation and dehalogenation efficiency owing to the increase in quantum yield of sulfite, from 0.077 to 0.178 mol Einstein^-1. The second-order rate constants of e_aq^- and •H with FLO were determined as 1.26 × 10⁹ and 2.82 × 10⁸ M^-1s^-1, respectively. Kinetic modeling coupled with probe experiments quantified the contributions of •H and e_aq^- to FLO degradation as 56.4% and 18.2% at pH 7.5, respectively. Furthermore, •H contributed 47.3% and 41.8% to FLO dechlorination and defluorination at pH 7.5, respectively. While Cl^- and SO₄^2- had minimal effect on FLO degradation, HA and bicarbonate slightly inhibited FLO degradation by quenching reactive species or/and light shielding. Products analysis revealed that •H promotes defluorination through both direct H/F exchange and indirect facilitation via accelerated dechlorination. This work offers an innovative perspective on the potential of •H for dehalogenation of halogenated pollutants under near-neutral conditions in far-UVC/sulfite process.