The transformation of triclosan by laccase: Effect of humic acid on the reaction kinetics, products and pathway

Rong-Ni Dou; Jing-Hao Wang; Yuan-Cai Chen; Yong-You Hu

doi:10.1016/j.envpol.2017.10.119

The transformation of triclosan by laccase: Effect of humic acid on the reaction kinetics, products and pathway

Environ Pollut. 2018 Mar:234:88-95. doi: 10.1016/j.envpol.2017.10.119. Epub 2017 Nov 21.

Authors

Rong-Ni Dou¹, Jing-Hao Wang¹, Yuan-Cai Chen², Yong-You Hu³

Affiliations

¹ School of Environment and Energy, South China University of Technology, Guangzhou 510006, China.
² School of Environment and Energy, South China University of Technology, Guangzhou 510006, China; Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, South China University of Technology, Guangzhou 510006, China. Electronic address: chenyc@scut.edu.cn.
³ School of Environment and Energy, South China University of Technology, Guangzhou 510006, China; Ministry of Education Key Laboratory of Pollution Control and Ecological Remediation for Industrial Agglomeration Area, South China University of Technology, Guangzhou 510006, China.

PMID: 29172042
DOI: 10.1016/j.envpol.2017.10.119

Abstract

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.

Keywords: (13)C NMR; Cross-coupling; Humic acid (HA); Laccase; Triclosan (TCS).

MeSH terms

Anti-Infective Agents, Local / chemistry*
Carbon-13 Magnetic Resonance Spectroscopy
Catalysis
Chromatography, Gas
Chromatography, High Pressure Liquid
Humic Substances*
Kinetics
Laccase / chemistry*
Mass Spectrometry
Oxidation-Reduction
Spectroscopy, Fourier Transform Infrared
Triclosan / chemistry*

Substances

Anti-Infective Agents, Local
Humic Substances
Triclosan
Laccase