Oxidation of the odorous compound 2,4,6-trichloroanisole by UV activated persulfate: Kinetics, products, and pathways

Abstract

The transformation efficiency and products of an odorous compound 2,4,6-trichloroanisole (TCA) at the wavelength of 254 nm in the presence of persulfate were investigated for the first time. The effects of water matrix (i.e., natural organic matter (NOM), pH, carbonate/bicarbonate (HCO3−/CO32−), and chloride ions (Cl−)) were evaluated. The second order rate constant of TCA reacting with sulfate radical (SO4−) was determined to be (3.72 ± 0.10) × 109 M−1 s−1. Increasing dosage of persulfate increased the observed pseudo-first-order rate constant for TCA degradation (kobs), and the contribution of SO4− to TCA degradation was much higher than that of HO at each experimental condition. Degradation rate of TCA decreased with pH increasing from 4.0 to 9.0, which could be explained by the lower radical scavenging effect of dihydrogen phosphate than hydrogen phosphate in acidic condition (pH < 6). NOM significantly decreased kobs due to the effects of radical scavenging and UV absorption with the former one being dominant. kobs decreased from 2.32 × 10−3 s−1 to 0.92 × 10−3 s−1 with the CO32−/HCO3− concentration increased from 0.5 mM to 10 mM in the UV/persulfate process, while kobs slightly decreased from 2.54 × 10−3 s−1 in the absence of Cl− to 2.10 × 10−3 s−1 in the presence of 10 mM Cl−. Most of these kinetic results could be described by a steady-state kinetic model. Furthermore, liquid chromatography/electrospray ionization-triple quadrupole mass spectrometry at powerful precursor ion scan approach was used to selectively detect oxidation products of TCA. It was found that 2,4,6-trichorophenol (TCP) was the major oxidation product (i.e., the initial yield of TCP was above 90%). The second order rate constant between TCP and SO4− was estimated to be (4.16 ± 0.20) × 109 M−1 s−1. In addition, three products (i.e., 2,6-dichloro-1,4-benzoquinone and two aromatic ring-opening products) were detected in the reaction of TCP with SO4−, which also appeared in the oxidation of TCA in the UV/persulfate process. A tentative pathway was proposed, where the initial one-electron oxidation of TCA by SO4− and further reactions (e.g., ipso-hydroxylation and aromatic ring-cleavage) of the formed cation intermediate TCA were involved.