pH-Dependent Reactivity, Radical Pathways, and Nitrosamine Byproduct Formation in Peroxynitrite-Mediated Advanced Oxidation Processes

Peroxynitrite (ONOOH/ONOO−) is increasingly recognized as a key intermediate in advanced oxidation processes (AOPs), yet its role in water treatment remains insufficiently defined. This study provides mechanistic insights into peroxynitrite-mediated AOPs through competition kinetics method, demonstrating that both decomposition and pollutant degradation are strongly pH-dependent, with ONOOH dominating stability and radical pathways across pH 5.0−9.0, while its decay rate decreases from 1.2 s−1 to 0.0022 s−1. The interplay of HO• and diverse reactive nitrogen species (RNS, including reactive nitrogen radicals and peroxynitrite) dictates pollutant-specific degradation efficiencies, with RNS showing a unique reliance in degrading bisphenol A—contributing up to 66.7% at pH 8.0. Buffer chemistry further modulates these pathways: bicarbonate accelerates peroxynitrite decay via CO2 and CO3•−-mediated acceleration (resulting in a 361.9% increase at pH 9.0), while borate promotes reactive nitrogen radical formation but suppresses HO• contributions. Importantly, peroxynitrite was shown to facilitate N-nitrosodimethylamine formation in the presence of dimethylamine, with yields maximized under alkaline conditions and attenuated by bicarbonate. These quantitative findings underscore the critical roles of pH and buffer chemistry in optimizing peroxynitrite-based water treatment while mitigating byproduct risks.