This study systematically explored the effect of humic acid (HA) (as model of natural organic matter) on the kinetics, products and transformation pathway of triclosan (TCS) by laccase-catalyzed oxidation. It was found that TCS could be effectively transformed by laccase-catalysis, with the apparent second-order rate constant being 0.056 U−1 mL min−1. HA inhibited the removal rate of TCS. HA-induced inhibition was negatively correlated with HA concentration in the range of 0–10 mg L−1 and pH-dependent from 3.5 to 9.5. FT-IR and 13C NMR spectra showed a decrease of aromatic hydroxyl (phenolic) groups and an increase of aromatic ether groups, indicating the cross-linking of HA via C-O-C and C-N-C bonds during enzyme-catalyzed oxidation. Ten principle oxidative products, including two quinone-like products (2-chlorohydroquinone, 2-chloro-5-(2,4-dichlodichlorophenoxy)-(1,4)benzoquinone), one chlorinated phenol (2,4-dichlorophenol (2,4-DCP)), three dimers, two trimmers and two tetramers, were detected by gas chromatograghy/mass spectrometry (GC-MS) and high performance liquid chromatography/quadrupole time-of-flight/mass spectrometry (HPLC/Q-TOF/MS). The presence of HA induced significantly lesser generation of self-polymers and enhanced cross-coupling between HA and self-polymers via C-O-C, C-N-C and C-C coupling pathways. A plausible transformation pathway was proposed as follows: TCS was initially oxidized to form reactive phenoxyl radicals, which self-coupled to each other subsequently by C-C and C-O pathway, yielding self-polymers. In addition, the scission of ether bond was also observed. The presence of HA can promote scission of ether bond and further oxidation of phenoxyl radicals, forming hydroxylated or quinone-like TCS. This study shed light on the behavior of TCS in natural environment and engineered processes, as well provided a perspective for the water/wastewater treatment using enzyme-catalyzed oxidation techniques.
Environmental Pollution – Elsevier
Published: Mar 1, 2018
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