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Comparative evaluation of UV/Fe(C2O4)33−/PS and Fe2+/PS advanced oxidation processes: degradation efficiency, mechanisms, and environmental risks

Mian Li,Lian-Yu Qiu,Tian-Yang Zhang,Bin Xu,Dan Yang,Ting Ye,Heng Luo

Advanced oxidation processes (AOPs) based on Fe 2+ /persulfate (PS) have been widely applied in water treatment but face critical challenges: inefficient radical utilization due to excess Fe 2+ scavenging, unsustainable reactivity from irreversible Fe 2+ -to-Fe 3+ conversion, and ambiguous dominant reactive species (e.g., SO 4 − / OH vs. high-valent iron). Ferric oxalate complexes address these limitations by enabling light-driven iron photoreduction and the Fe 2+ /Fe 3+ cycle. However, direct comparisons of degradation efficiency, mechanisms, and environmental risks between Fe 2+ /PS and UV/Fe(C 2 O 4 ) 3 3− /PS remain scarce. Using iohexol as a model, this study directly compared the two systems. The UV/Fe(C 2 O 4 ) 3 3− /PS system achieved 98.8 % iohexol degradation in 60 min (vs. 69.9 % for Fe 2+ /PS) and reduced iron sludge by 70–80 %. Both systems relied on SO 4 − and OH, but UV/Fe(C 2 O 4 ) 3 3− /PS enhanced radical yield via photocatalytic iron cycling, whereas Fe 2+ /PS depended on Fe(IV). In Fe 2+ /PS, contributions were Fe(IV) (29.4 %) > SO 4 − (25.9 %) >  OH (20.2 %); in UV/Fe(C 2 O 4 ) 3 3− /PS, SO 4 − (63.6 %) >  OH (23.7 %) > Fe(IV) (23.7 %). Electrical energy per order (EE/O) analysis showed UV/Fe(C 2 O 4 ) 3 3− /PS consumed 1.166 kWh mM −1 , 60.2 % lower than Fe 2+ /PS (2.524 kWh mM −1 ). Acute toxicity tests with Vibrio fischeri exhibited 67.7 % inhibition in Fe 2+ /PS, significantly higher than 36.4 % in UV/Fe(C 2 O 4 ) 3 3− /PS. Ecological Structure-Activity Relationships (ECOSAR) modeling and disinfection by-products (DBPs) risk assessment further confirmed lower risks in UV/Fe(C 2 O 4 ) 3 3− /PS, with 50 % lethal concentration ((LC 50 ) for aquatic organisms 2.3-fold higher and genotoxicity potency 41.7 % lower than Fe 2+ /PS. These results bridge the efficiency-risk gap between the two systems and offer a three-dimensional (efficiency-cost-safety) framework for AOP selection.