# Light-Induced Advanced Oxidation Processes as PFAS Remediation Methods: A Review
## 5 Methods Based on a Sacrificial Radical Source  
A different approach involves the presence of a sacrificial agent in the solution, enabling pathways based on the generation of radical species. Among these latter, it is worth mentioning the use of aqueous electrons to perform reductive defluorination, as explored by [PERSON] et al. [131]. These authors employed potassium iodide to generate hydrated electrons from a water-iodide complex under 254 nm wavelength irradiation from a 15 W lamp. A combination of photolysis and reduction by electrons then yielded a 96% defluorination for 0.025 mM PFOA in a 12 h experiment. Another report along the same lines is that by [PERSON] et al. [137], though in this case aqueous electrons, along with hydroxyl and other radicals, were generated by the more exotic approach of irradiating water with \(\gamma\)-rays. In addition to mineralization of around 86% and nearly 100% defluorination for 20 mg/L PFOA in a 22 h experiment, the report highlights the synergistic effect of hydroxyl radicals and aqueous electrons, which are both necessary to achieve efficient removal. Indeed, hydroxyl radical-based processes such as Fenton, photo-Fenton, and hydrogen peroxide activation are ineffective due to the inactivity of OH\({}^{\circ}\), as also recently clarified by [PERSON] et al. [136], who found no significant difference in PFOA degradation under UV irradiation and UV/H\({}_{2}\)O\({}_{2}\). In contrast, several processes based on sulfate radicals (SO\({}_{4}\)\({}^{\bullet-}\)), obtained by persulfate activation, have been reported to be effective, though defluorination efficiencies remain low. [PERSON] et al. used a 200 W Xe-Hg UV-visible lamp to activate persulfate (50 mM) for treating 1.35 mM PFOA in a 4 h experiment, observing an 11-fold increase in decomposition with respect to direct photolysis [138]. Similarly, [PERSON] and [PERSON] used UV-activated persulfate on PFOA, reporting that while using 185 nm irradiation, decomposition was proceeded by both direct photolysis and sulfate radicals, under 254 nm light the latter was the prevalent mechanism [139]. A step forward was made by introducing ferrous iron as a persulfate activator in the system, enabling a low activation energy pathway for the decomposition of persulfate to sulfate radicals [140]. Using this approach, [PERSON] et al. recently reported 64% degradation of PFOA (initial concentration: 1.64 \(\upmu\)M) in a 4 h anoxic experiment [141]. However, the role of light was not clarified and defluorination not discussed, leaving room for further investigations. A further effort in this direction was made by [PERSON] et al. [11], who combined a Ga\({}_{2}\)O\({}_{3}\) photocatalyst with peroxmonosulfate as a sulfate radical sacrificial source. Under illumination from either a 32 W 185 nm light or a 32 W 254 nm light, complete degradation of a PFOA solution in the 50 ng/L to 50 mg/L range was claimed, although defluorination was not quantified. A more detailed examination of persulfate-based methods (out of the scope of this work) is being currently studied, and can be found in a recent article by [PERSON] et al. [142]. Overall, methods based on sacrificial radical sources look promising in terms of yields, but may have similar drawbacks to direct photolysis because radical generation is often performed by activation with the same artificial UVC light sources, with all its related problems. Furthermore, the sacrificial nature of the reagents used to generate the radicals implies extra costs with respect to direct photolysis or photocatalysis. In this respect, further work is needed not only to clarify the reaction mechanisms and improve efficiency but also to evaluate the economic viability and industrial scalability of this approach with respect to current methods or other methods under study.