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/H2O2, UV/chlorine and UV/NH2Cl. 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 107 M-1 s-1, and such rate constants dramatically increased to 1010 M-1 s-1 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/NH2Cl > UV/chlorine > UV/H2O2, and UV/chlorine and UV/NH2Cl were more effective for the removal of 246TBP and 246TBPT than UV/H2O2. UV and/or Cl• dominated 246 TBA degradation under three AOPs. The main radicals mediating 246TBP and 246TBPT degradation are respectively HO• under UV/H2O2, ClO• under UV/chlorine, and HO• and Cl• under UV/NH2Cl. The transformation products of 246TBA, 246TBP and 246TBPT, especially methoxylated and hydroxylated polybrominated diphenyl ethers (MeO-PBDEs and HO-PBDEs), were still toxic pollutants.

Acetaminophen degradation by hydroxyl and organic radicals in the peracetic acid-based advanced oxidation processes: Theoretical calculation and toxicity assessment.

The research on the mechanisms and kinetics of radical oxidation in peracetic acid-based advanced oxidation processes was relatively limited. In this work, HO• and organic radicals mediated reactions of acetaminophen (ACT) were investigated, and the reactivities of important organic radicals (CH3COO• and CH3COOO•) were calculated. The results showed that initiated reaction rate constants of ACT are in the order: CH3COO• (5.44 × 1010 M-1 s-1) > HO• (7.07 × 109 M-1 s-1) > CH3O• (1.57 × 107 M-1 s-1) > CH3COOO• (3.65 × 105 M-1 s-1) >> •CH3 (5.17 × 102 M-1 s-1) > CH3C•O (1.17 × 102 M-1 s-1) > CH3OO• (11.80 M-1 s-1). HO•, CH3COO• and CH3COOO• play important roles in ACT degradation. CH3COO• is another important radical in the hydroxylation of aromatic compounds in addition to HO•. Reaction rate constants of CH3COO• and aromatic compounds are 1.40 × 106 - 6.25 × 1010 M-1 s-1 with addition as the dominant pathway. CH3COOO• has high reactivity to phenolate and aniline only among the studied aromatic compounds, and it was more selective than CH3COO•. CH3COO•-mediated hydroxylation of aromatic compounds could produce their hydroxylated products with higher toxicity.