Enhanced chlorine photolysis and 1H-benzotriazole Degradation in Water under the Far-UVC (UV222) irradiation

Far-UVC at 222   nm (UV 222 ) is a promising alternative of conventional 254   nm (UV 254 ) lamps for microorganism inactivation and recalcitrant pollutant removal. This study evaluates UV 222 combined with chlorine (HOCl/OCl⁻) for removing 1H-benzotriazole (BTZ), a widely used UV-absorber and anticorrosion agent ubiquitously detected in wastewater, surface water, and swimming pools. UV 222 achieved 1.4−21.7 times faster direct photolysis of BTZ compared to UV 254 , attributed to higher quantum yield (3.0−32.4 enhancement). UV 222 /chlorine demonstrated strong synergies (factors of 3.7–16.3 across pH 5.0–10.0), consistently outperforming UV 254 /chlorine by 1.09–2.02-fold. Kinetic modeling revealed that UV 222 generates reactive radicals more efficiently, with innate quantum yields for HOCl and OCl⁻ photolysis being 1.7-fold and 4.0-fold higher than UV 254 , respectively, resulting in elevated steady-state concentrations of • OH, Cl • , and ClO • . Reactive chlorine species contributed 47.5%–72.9% to BTZ degradation. Notably, under alkaline conditions where OCl⁻ dominates, UV 222 /chlorine showed substantially greater advantage over UV 254 /chlorine, as UV 222 appears to mitigate the marked reduction in OCl⁻ efficiency. In complex water matrices, nitrate (≥0.5   mM) acted as a photosensitizer, enhancing BTZ removal, while humic acid caused moderate inhibition (16.8%−44.0%). In tap water and secondary effluent, UV 222 /chlorine achieved 1.39–3.86 times higher degradation than in deionized water, with nitrate-driven radical generation outweighing natural organic matter inhibition. Importantly, UV 222 /chlorine produced 23% lower chlorinated disinfection byproducts (DBPs) and 17% lower toxicity-index than UV 254 /chlorine, demonstrating its potential as an alternative of UV 254 -driven oxidation process.