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.

Ozonation of diclofenac in the aqueous solution: Mechanism, kinetics and ecotoxicity assessment.

The pharmaceutical and personal care products (PPCPs) in aquatic environment have aroused more interest recently. Many of them are hard to degrade by the typical biological treatments. Diclofenac (DCF), as a significant anti-inflammatory drug, is a typical PPCP and widely existed in water environment. It is reported that DCF has adverse effects on aquatic organisms. This work aims to investigate the mechanism, kinetics and ecotoxicity assessment of DCF transformation initiated by O3 in aqueous solution, and provide a solution to the degradation of DCF. The O3-initiated oxidative degradations of DCF were performed by quantum chemical calculations, including thirteen primary reaction pathways and subsequent reactions of the Criegee intermediates with H2O, NO and O3. Based on the thermodynamic data, the kinetic parameters were calculated by the transition state theory (TST). The total reaction rate constant of DCF initiated by O3 is 2.57 × 103 M-1 s-1 at 298 K and 1 atm. The results show that the reaction rate constants have a good correlation with temperature. The acute and chronic toxicities of DCF and its degradation products were evaluated at three different trophic levels by the ECOSAR program. Most products are converted into less toxic or harmless products. Oxalaldehyde (P3) and N-(2,6-dichlorophenyl)-2-oxoacetamide (P6) are still harmful to the three aquatic organisms, which should be paid more attention in the future.

Theoretical study on the mechanisms, kinetics and ecotoxicity assessment of OH-initiated reactions of guaiacol in atmosphere and wastewater.

The OH-initiated transformation mechanisms, kinetics and ecotoxicity assessment of guaiacol (2-methoxyphenol) in the presence of O2/NOx were investigated both in atmosphere and wastewater. The solvent effect lowers the energy barriers of initial OH-addition reactions more than H-abstraction reactions, leading to much higher addition branching ratio (Γadd) of 0.92 in aqueous solution than that of 0.42 in gas-phase. At 298 K, the overall rate constants of the title reactions in atmosphere and wastewater are 5.56 × 10-12 and 1.41 × 10-11 cm3 molecule-1 s-1 with corresponding half-lives of 34.6 h and 0.82 s, respectively. In atmosphere, all the proposed favorable products including nitroguaiacols, methoxybenzoquinone, 2-hydroxyphenyl formate, 2-methoxybenzene-1, 3-diol and dialdehyde could contribute to secondary organic aerosols (SOAs). In wastewater, NO2 addition reactions lead to higher toxicity of products (nitroguaiacols and 2-methoxybenzene-1, 4-diol) than that of parental guaiacol. However, O2/NO addition pathways may generate less harmful products except for methoxybenzoquinone (P3) which is with higher toxicity than guaiacol. Therefore, more attention should be focused on the products formed from OH-initiated reactions of guaiacol both in atmosphere and wastewater.

Theoretical study on gas-phase reactions of nitrate radicals with methoxyphenols: Mechanism, kinetic and toxicity assessment.

Creosol and 4-ethylguaiacol are two important methoxyphenols, lignin pyrolysis products, which are discharge into the atmosphere in large quantities. In this work, theoretical calculations of the reaction mechanism towards the two compounds with NO3 radicals was performed using DFT method. The rate constants and toxicity assessment were also investigated. The atmospheric lifetime for creosol and 4-ethylguaiacol were 0.82 and 0.19 h, respectively. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl, which has not yet been clarified in previous studies. The toxicity of methoxyphenols and their degradation products is closely related to their hydrophobicity. Although most degradation products are less toxic, they also should be pay more attention, especially for nitro-substituents. A new reaction pathway was proposed for the transformation of methoxyl into hydroxyl. The toxicity is closely related to their hydrophobicity.