Simulation degradation of bromophenolic compounds in chlorine-based advanced oxidation processes: Mechanism, microscopic and apparent kinetics, and toxicity assessment.

Chlorine-based advanced oxidation processes (AOPs) have been extensively studied to remove contaminants through generating HO and reactive chlorine species, including ClO and Cl. In this work, 2,4,6-tribromoanisole (246TBA) and 2,4,6-tribromophenol (246TBP) were selected as model to investigate the reaction mechanisms and micro-kinetics of brominated contaminants with HO, ClO and Cl in chlorine-based AOPs. Also, the apparent degradation kinetics of two compounds were simulated at pH 3.0-9.5 under UV/HO, UV/chlorine and UV/NHCl. Calculated results showed that neutral 246TBA and 246TBP exhibited similar reactivity to HO and ClO, which was different from anionic 2,4,6-tribromophenolate (246TBPT): radical adduct formation (RAF) and H atom abstraction (HAA) were predominant mechanisms for the HO and ClO initiated reactions of 246TBA and 246TBP, while RAF and single electron transfer (SET) for 246TBPT; the reaction rate constants of 246TBA and 246TBP with HO and ClO were lower than 10 M s, and such rate constants dramatically increased to 10 M s once 246TBP was deprotonated to 246TBPT. The apparent degradation kinetics of 246TBA at pH 3.0-9.5 was simulated in the order of UV/NHCl > UV/chlorine > UV/HO, and UV/chlorine and UV/NHCl were more effective for the removal of 246TBP and 246TBPT than UV/HO. UV and/or Cl dominated 246 TBA degradation under three AOPs. The main radicals mediating 246TBP and 246TBPT degradation are respectively HO under UV/HO, ClO under UV/chlorine, and HO and Cl under UV/NHCl. The transformation products of 246TBA, 246TBP and 246TBPT, especially methoxylated and hydroxylated polybrominated diphenyl ethers (MeO-PBDEs and HO-PBDEs), were still toxic pollutants.