ClO2 pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

Tao Li; Chii Shang; Yingying Xiang; Ran Yin; Yang Pan; Mengge Fan; Xin Yang

doi:10.1016/j.watres.2022.118341

ClO₂ pre-oxidation changes dissolved organic matter at the molecular level and reduces chloro-organic byproducts and toxicity of water treated by the UV/chlorine process

Water Res. 2022 Jun 1:216:118341. doi: 10.1016/j.watres.2022.118341. Epub 2022 Mar 21.

Authors

Tao Li¹, Chii Shang², Yingying Xiang³, Ran Yin¹, Yang Pan⁴, Mengge Fan⁵, Xin Yang⁶

Affiliations

¹ Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
² Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China; Hong Kong Branch of Chinese National Engineering Research Center for Control and Treatment of Heavy Metal Pollution, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China.
³ Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China. Electronic address: cexiangyy@ust.hk.
⁴ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China.
⁵ School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China.
⁶ School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China; Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, China. Electronic address: yangx36@mail.sysu.edu.cn.

PMID: 35367942
DOI: 10.1016/j.watres.2022.118341

Abstract

The formation of undesirable chloro-organic byproducts is of great concern in the UV/chlorine process. In this study, chlorine dioxide (ClO₂) pre-oxidation was applied to control the formation of chloro-organic byproducts and the toxicity in UV/chlorine-treated water. The molecular-level changes in dissolved organic matter (DOM) were tracked by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and ClO₂ pre-oxidation was found to preferentially react with DOM moieties with high aromaticity level and with a carbon number of > 18, producing compounds with a higher degree of oxidation and lower aromaticity. The ClO₂-treated DOM was found to be less susceptible to attack by radicals and free chlorine in the UV/chlorine process compared to the raw DOM. ClO₂ pre-oxidation resulted in a significant decrease in the number of unknown chloro-organic byproducts (i.e., -17%) and the total intensity of organic chlorine detected by FT-ICR-MS (i.e., -31%). The molecular characteristics, such as O/C, aromaticity index, and the average number of chlorine atoms, of these unknown chloro-organic byproducts generated in the scenarios with and without ClO₂ pre-oxidation were also different. Additionally, ClO₂ pre-oxidation reduced the genotoxicity (SOS/umu test) and cytotoxicity (Hep G2 cytotoxicity assay) of UV/chlorine-treated water by 26% and 20%, respectively. The findings in this study highlight the merits of ClO₂ pre-oxidation for controlling chloro-organic byproducts and reducing the toxicity of water treated by the UV/chlorine process in actual practice.

Keywords: Chlorine dioxide; Chloro-organic byproducts; Cytotoxicity; FT-ICR-MS; Genotoxicity; UV/chlorine.

MeSH terms

Chlorides
Chlorine / analysis
Chlorine Compounds*
Disinfection
Dissolved Organic Matter*
Oxides
Water
Water Pollutants, Chemical* / analysis
Water Purification*

Substances

Chlorides
Chlorine Compounds
Dissolved Organic Matter
Oxides
Water Pollutants, Chemical
Water
Chlorine