Better Together Than Apart: Degradation of Emerging Contaminants in Concentrated Streams Using UV/Sulfite/Iodide and UV/Hydrogen Peroxide Treatment Trains

Abstract

This study evaluates the performance of homogeneous UV-based advanced reduction processes (ARPs) and advanced oxidation processes (AOPs), applied individually and in sequential treatment trains as a largely unexplored approach for the degradation of aqueous contaminants. UVC/SO₃^2– and UVC/SO₃^2–/I^– were the reference ARPs, while UVC/H₂O₂ was used as the reference AOP. A mixture of three model contaminants was used in all cases, consisting of perfluorooctanoic acid (PFOA, as a model perfluoroalkyl substance─PFAS), ibuprofen, and benzoic acid. The UVC/SO₃^2– treatment achieved 70% defluorination of 100 μM PFOA after 24 h of irradiation, while nearly complete defluorination was obtained with UVC/SO₃^2–/I^– in the same period. Benzoic acid exhibited the fastest degradation rates with both ARPs (k_obs = 3.9 × 10^–1 min^–1, 100 μM concentration), whereas ibuprofen was considerably slower, being comparable to its direct photolysis (k_obs = 2.9 × 10^–2 min^–1, 100 μM concentration). In contrast, the UVC/H₂O₂ process efficiently mineralized benzoic acid and ibuprofen (>99% within 2 h) but showed negligible degradation of PFOA (<1% in 2 h). The treatment trains (AOP → ARP and ARP → AOP) were decisively more efficient when dealing with the pollutant mixture degradation in the presence of scavengers. With the AOP → ARP approach, for matrices containing 50 mg L^–1 of humic acids, the UVC/H₂O₂ pretreatment led to faster PFOA defluorination rates than ARPs alone (65% and 5% in 2 h, respectively). The inverse configuration, ARP → AOP, allowed the mineralization of PFOA, a result that could not be achieved when either ARPs or the AOP were applied individually. Experiments in simulated retentate wastewater, containing 1 μM of each contaminant and high concentration of anions and organic matter, were consistent with the preliminary results. This work is one of the first to offer valuable information on the degradation of PFAS and non-PFAS pollutants by the aforementioned treatment trains, especially in concentrated wastewater treatment streams.