Unveiling the mystery of chlorine radical in water by a simple methanol-based spin trapping

Pu Wang; Lingli Wang; Ronghui Xiao; Sifan Qiu; Jinhui Cao; Yu Fu; Shandi Bai; Zhaohui Wang

doi:10.1016/j.watres.2025.124100

Unveiling the mystery of chlorine radical in water by a simple methanol-based spin trapping

Water Res. 2025 Jun 24:285:124100. doi: 10.1016/j.watres.2025.124100. Online ahead of print.

Authors

Pu Wang¹, Lingli Wang¹, Ronghui Xiao², Sifan Qiu¹, Jinhui Cao¹, Yu Fu¹, Shandi Bai¹, Zhaohui Wang³

Affiliations

¹ Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Sciences, East China Normal University, Shanghai 200241, China.
² Shanghai Research Institute of Materials, Shanghai 200437, China.
³ Shanghai Key Lab for Urban Ecological Processes and Eco-Restoration, School of Ecological and Environmental Sciences, East China Normal University, Shanghai 200241, China; Technology Innovation Center for Land Spatial Eco-restoration in Metropolitan Area, Ministry of Natural Resources, 3663 N. Zhongshan Road, Shanghai, 200062, China; Shanghai Engineering Research Center of Biotransformation of Organic Solid Waste, Shanghai 200241, China. Electronic address: zhwang@des.ecnu.edu.cn.

PMID: 40580832
DOI: 10.1016/j.watres.2025.124100

Abstract

Both hydroxyl radical (^•OH) and chlorine radical (Cl^•) are recognized as significant reactive species in aquatic and atmospheric sciences. While ^•OH can be readily detected with the conventional spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), Cl^• is often considered difficult to detect by electron paramagnetic resonance (EPR) technique and has therefore been largely underexploited. In this work, we clarify the complicated reactions of Cl^• with DMPO, which pose challenges for the direct detection of Cl^•. To overcome this limitation, we take advantage of the differences in the products formed when methanol reacts with Cl^• (i.e. ^•OCH₃) compared to its reaction with ^•OH (i.e. ^•CH₂OH), thereby developing a novel approach for detecting the elusive Cl^•. The reliability and adaptability of this solvent-independent method are validated across various natural and engineered scenarios. Therefore, this work unlocks opportunities for identifying Cl^• in the saline environments and further elucidating its roles in biogeochemical cycles of elements and environmental remediation.

Keywords: Chloride; Electron paramagnetic resonance; Hydroxyl radical; Saline environment; Secondary radical spin-trapping method.