Comparison of the oxidation of halogenated phenols in UV/PDS and UV/H2O2 advanced oxidation processes

摘要

UV/peroxydisulfate (PDS) and UV/hydrogen peroxide (H _(2) O _(2) ) can effectively degrade halophenols (HPs, e.g. , 2,4-bromophenol and 2,4,6-trichlorophenol); meanwhile, information about the discrepancies in the related degradation kinetics and mechanisms of these two processes is limited. To gain this knowledge, the degradation of two typical HPs ( i.e. , bromophenols and chlorophenols) in UV/PDS and UV/H _(2) O _(2) processes were investigated and compared. The results showed that the degradation rates of HPs with different substitution positions in the UV/PDS process were in the order of para -substituted HPs ( i.e. , 4-BP and 4-CP) > ortho -substituted HPs ( i.e. , 2-BP and 2-CP) > meta -substituted HPs ( i.e. , 3-BP and 3-CP), while in the UV/H _(2) O _(2) process, these rates were in the order of para -substituted HPs > meta -substituted HPs > ortho -substituted HPs. These discrepancies were ascribed to the different reaction activities of SO _(4) ˙ ~(?) and HO˙ with HPs, which were calculated based on the competition method. Further density functional theory (DFT) calculations suggested that SO _(4) ˙ ~(?) reacts more readily with HPs via electron transfer than HO˙. In the presence of water matrices (such as Cl ~(?) , HCO _(3) ~(?) and natural organic matter (NOM)), the degradation of 2-BP in both UV/PDS and UV/H _(2) O _(2) treatment processes was inhibited due to the scavenging of free radicals by these background substances. The degradation products and pathways further confirmed that SO _(4) ˙ ~(?) is a strong one-electron oxidant that reacts with HPs mainly via electron transfer, while HO˙ reacts with HPs via electron transfer and hydroxyl addition.