Tuning reactive species formation in dithionite-based advanced reduction processes for the degradation of carbamazepine and perfluorooctanoic acid.

Advanced reduction processes (ARPs) offer a promising alternative for degrading contaminants that are resistant to oxidation. However, dithionite (DTN)-activated ARPs have rarely been studied, and the underlying mechanisms of radical generation are not well elucidated. In this study, we systematically investigated how irradiation wavelength regulates reactive species formation in dithionite-based ARPs and consequently controls pollutant-specific degradation pathways. By comparing UV310, UV254, and VUV irradiation conditions, the transition between SO2•--dominated and eaq--dominated reduction pathways were elucidated, providing a mechanistic framework for wavelength-controlled contaminant removal. Radical quenching and ESR analyses revealed that SO2•⁻ dominated under UV254 irradiation, whereas hydrated electrons (eaq-) dominated under VUV irradiation, leading to different degradation mechanisms. In the UV254/DTN system, carbamazepine (CBZ) was rapidly degraded (> 90% in 20 min), while perfluorooctanoic acid (PFOA) removal remained below 10%. In contrast, the VUV/DTN system achieved complete PFOA degradation within 120 min with 56% defluorination, while CBZ was almost completely degraded within only 5 min. The monochloroacetic acid (MCAA) probe results showed that the large difference in PFOA degradation was due to different eaq- yields. The steady-state concentration of eaq- was 0.342 pM in the VUV/DTN system but only 0.03 pM in the UV254/DTN system. Water matrix constituents, particularly carbonate and natural organic matter, preferentially inhibited eaq--driven pathways. These results establish a wavelength-controlled framework linking reactive species formation (SO2•⁻ versus eaq-) to pollutant-selective degradation in DTN-based advanced reduction processes, providing a mechanistic basis for the process-oriented design of reduction technologies.