The overlooked risk of activated peroxymonosulfate: Significant free chlorine and volatile disinfection byproduct formation

Peroxymonosulfate-based advanced oxidation processes (PMS-AOPs) have been widely studied for water treatment, but in chloride-containing matrices, they may unintentionally produce free chlorine during PMS activation. This study directly quantified the unintended Cl₂ generation from chloride during PMS activation by Co²⁺, Fe²⁺, and UV, using membrane introduction mass spectrometry. The Co²⁺/PMS system exhibited the most pronounced free chlorine production (k = 0.25 ± 0.02 M^-1·s^-1), nearly two orders of magnitude higher than non-catalytic PMS (k = (3.88 ± 0.33) ×10^-3 M^-1·s^-1). Chlorinated DBPs were formed in the presence of organic precursors in the Co²⁺/PMS/Cl^- system, and their brominated analogues were generated when bromide was present. Radical quenching results were consistent with sulfate radical initiated chloride oxidation, while transient chlorine radical and hydroxyl radical intermediates cannot be ruled out. Crucially, under simulated realistic water treatment conditions, Co²⁺/PMS generated volatile DBPs similar in magnitude to those from conventional chlorination. The study suggests that, under the tested Co²⁺/PMS conditions, even near-background Cl^- (0.1-5 mM) can produce substantial chlorinated volatile DBPs, especially under activation/acidic conditions.