Selective Oxidation of Cyanotoxins (Microcystins) by Peroxyacetyl Radical: Kinetic Studies and Mechanistic...

Microcystins (MCs) are one group of the most toxic cyanobacterial metabolites in surface waters, yet their selective and efficient removal under environmentally relevant conditions remains a major challenge. Here, we identify the peroxyacetyl radical (PAR) as a previously unrecognized oxidant for MC degradation. We first quantified the bimolecular reaction rate constants of PAR with 10 MC congeners, which fell within a narrow range of (3.94-6.69) × 10⁷ M^-1 s^-1. Our results revealed that PAR selectively attacks the conjugated diene in the Adda side chain, yielding dihydroxylation products while leaving the aromatic ring and Mdha largely unreacted. High-resolution MS/MS confirmed the formation of three dominant dihydroxylated products, accounting for ∼80% of total yield. The environmental relevance of PAR was further demonstrated in solar/peracetic acid (PAA) systems, where MCs were degraded mainly via two parallel pathways: HO^• oxidation and PAR oxidation. Quantitative radical budget analysis revealed that PAR contributed 1.5-3.5 times more to MC degradation than HO^• across varied PAA/H₂O₂ ratios and DOM levels. Overall, this study provides the first mechanistic and kinetic evidence that PAR is a selective oxidant capable of efficiently degrading MCs under sunlight and realistic water matrices, highlighting its potential for next-generation cyanotoxin treatment.