Far-UVC Activation of Performic Acid Enables Concurrent Oxidative and Reductive Micropollutant Abatement

Abstract Conventional UV-driven advanced oxidation and reduction processes generally emphasize one redox regime and can be inhibited by wastewater constituents. Here, we characterize the photochemistry of performic acid (PFA) under 222 and 254 nm irradiation and evaluate the capacity of PFA solution, prepared from H2O2 and formic acid and thus containing residual H2O2 and formate, to generate concurrent oxidative and reductive pathways. Relative to 254 nm, 222 nm irradiation increased the fluence-normalized photodecay rate constants of neutral PFA (PFA0) and its conjugate base (PFA–) by 1.21- and 3.81-fold, respectively. The increase arose from 16.1- and 10.1-fold larger molar absorption coefficients at 222 nm, despite lower fitted innate quantum yields (0.367 and 0.804 molecule photon–1 at 222 nm versus 0.587 and 0.819 molecule photon–1 at 254 nm). The UV222/PFA solution system generated the reductant •CO2– together with oxidative HO• and oxygen-centered organic radicals. Pathway analysis attributed 34% of carbamazepine removal to •CO2– and 19% of naproxen removal to R-O• at 222 nm. Chloride and bromide enhanced fluence-normalized degradation through reactive-halogen chemistry; the largest increases were observed for carbamazepine (47.5-fold at 254 nm and 21.9-fold at 222 nm). In real municipal wastewater, UV222/PFA solution decreased total organic carbon by 33.5% within 60 min, compared with 21.6% under UV254/PFA solution, at an initial PFA concentration of 80 μM. These results suggest far-UVC activation of PFA solution as a dual-redox treatment platform, with the observed advantages maintained in the tested wastewater effluents.