title: Transformation, products, and pathways of chlorophenols via electro-enzymatic catalysis: How to control toxic intermediate products.
abstract: Chlorophenols can be easily oxidized into chlorobenzoquinones (CBQs), which are highly toxic and have been linked to bladder cancer risk. Herein, we report the transformation, products, and pathways of 2,4-dichlorophenol (DCP) by horseradish peroxidase (HRP) and electro-generated hydrogen peroxide (H2O2) and suggest methods to control the formation of toxic intermediate products. After a 10-min electroenzymatic process, 99.7% DCP removal may be achieved under optimal conditions. A total of 16 reaction products, most of which are subsequently verified as DCP polymers and related quinone derivatives, are identified by using ultra-performance liquid chromatography-time-of-flight mass spectrometry (UPLC-TOF-MS). A five-step reaction pathway for DCP transformation, including HRP-driven substrate oxidation, substitution and radical coupling, quick redox equilibrium, nucleophilic reaction and precipitation from aqueous solution, is proposed. Current variations and the presence of CO2 could significantly affect these reaction pathways. In particular, higher currents enhance the hydroxylation process by promoting alkaline conditions and abundant H2O2 formation. As both OH(-) and H2O2 are strong nucleophiles, they easily react with CBQ products to form hydroxylated products, which can significantly reduce solution toxicity. An adequate supply of CO2 can provide favorable pH conditions and facilitate enzymatic steps, such as substrate oxidation and radical coupling, to generate precipitable polymerized products. All of the results suggest that toxic intermediate products can be effectively reduced and controlled during the electro-enzymatic process to remove DCP and other phenolic pollutants from wastewaters.
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China; University of Chinese Academy of Sciences, Beijing, 100049, China.
authors.forename: Penghui
authors.initials: P
authors.lastname: Du
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China. Electronic address: hzhao@ipe.ac.cn.
authors.forename: He
authors.initials: H
authors.lastname: Zhao
authors.affiliation: Environmental Protection Institute of Light Industry, Beijing Academy of Science and Technology, Beijing, 100089, China.
authors.forename: Haitao
authors.initials: H
authors.lastname: Li
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
authors.forename: Di
authors.initials: D
authors.lastname: Zhang
authors.affiliation: School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, United States.
authors.forename: Ching-Hua
authors.initials: CH
authors.lastname: Huang
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
authors.forename: Manfeng
authors.initials: M
authors.lastname: Deng
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
authors.forename: Chenming
authors.initials: C
authors.lastname: Liu
authors.affiliation: Beijing Engineering Research Center of Process Pollution Control, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China.
authors.forename: Hongbin
authors.initials: H
authors.lastname: Cao
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry*; D002733:Chlorophenols / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D004553:Electric Conductivity; D055664:Electrochemical Techniques; D006735:Horseradish Peroxidase / Q000737:chemistry; D013058:Mass Spectrometry; D010084:Oxidation-Reduction; D014874:Water Pollutants, Chemical / Q000737:chemistry*
chemical_list: D002733:Chlorophenols; D014874:Water Pollutants, Chemical; D002245:Carbon Dioxide; D006735:Horseradish Peroxidase; C004762:2,4-dichlorophenol
keywords: 2,4-Dichlorophenol; Electro-enzymatic catalysis; Horseradish peroxidase (HRP); Oxidative coupling; Reaction mechanism