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.