Hydroxyl and sulfate radicals formation in UVA/FeIII-NTA/S2O82- system: Mechanism and effectiveness in carbamazepine degradation at initial neutral pH

Ferrous ion (Fe-II)-induced activation of persulfate (S2O82-), reference to the generation of sulfate radical (SO4'), has attracted considerable attention in water treatment for refractory organic contaminants removal. However, two major challenges, i.e., the required acidic pH condition and the extremely slow Fe-III/Fe-II redox cycle, limit the application of Fe-II/S2O82- process. Herein, the present work addresses S2O82- activation in aqueous solutions at neutral pH by a ferric-nitrilotriacetic (Fe-III-NTA) complex under low pressure UVA irradiation. The results showed that UVA irradiation-induced Fe(III-)NTA reduction could effectively enhance Fe-III/Fe-II redox cycle for the S2O82- activation under neutral pH condition. Electron spin resonance spectroscopy (ESR) and radicals quenching tests indicated that the major reactive species included SO4'- and hydroxy radicals (HO center dot) in UVA/Fe-III-NTA/S2O82- process, which contributed more than 80% degradation of a model compound carbamazepine (CBZ). In addition, comprehensive investigations were conducted on the important parameters including NTA:Fe-III molar ratio, initial pH, Fe-III-NTA and S2O82- dosages and their impacts on CBZ degradation kinetics. The critical NTA:Fe-III molar ratio was determined at 1:1, above this ratio the excess NTA would act as a scavenger of SO4 center dot-/HO center dot thereby inhibiting the degradation of CBZ. The secondary reaction rate constant of NTA with SO4 center dot- was determined at 5.89 x 10(7) M-1 s(-1). Moreover, the degradation of CBZ by UVA/Fe-III-NTA/S2O82- process in real water matrix (lake water, groundwater and tap water) with environmentally relevant concentration were conducted, and the efficacies were compared with UVA/Fe-III-NTA/H2O2 (i.e., photo-Fenton-like) process. The degradation mechanism and pathway of CBZ were proposed on the basis of the identified intermediates by using LC-QTOF-MS technique.