Nonradical Oxidation Pathways in Advanced Oxidation: Promise, Pitfalls, and a Roadmap to Real-World Water Treatment Implementation

Abstract

Advanced oxidation processes (AOPs) are integral to modern water treatment and water reuse, where the oxidation of trace organic microcontaminants and taste-and-odor compounds complements biological and physical barriers. A growing research trend intentionally tunes AOPs away from highly reactive radicals (hydroxyl radical, •OH; sulfate radical, SO4•–) toward nonradical pathways, most notably singlet oxygen (1O2), surface-mediated electron transfer on engineered carbons, and high-valent metal–oxo oxidants on tailored oxides. The driving rationale is improved matrix tolerance, given that natural organic matter (NOM), carbonate/bicarbonate, halides, and other background constituents suppress radical pathways in real waters. However, matrix tolerance comes at the cost of reduced reaction breadth, particularly against electron-poor contaminants. Moreover, nonradical systems often rely on advanced catalysts whose cost, stability, operational feasibility, potential metal leaching/nanoparticle release, and synthesis footprints require careful evaluation. Nonradical oxidation can also stop at partial transformation and can promote coupling reactions (oligomers/polymers) or generate halogenated byproducts, raising concerns about toxicity and residuals management. This Perspective critically examines the intrinsic advantages and limitations of nonradical AOPs. We call for aligning materials innovation with feasibility across water treatment applications and identify scenarios where nonradical selectivity delivers real benefits. Finally, we outline a research agenda spanning contaminant/matrix panel testing, continuous-flow validation, feasibility metrics (energy, oxidant demand, catalyst durability/leaching), transformation-product/toxicity profiling, and integrated techno-economic/life-cycle assessments.