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

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.