Alcohols as reactive species modulators: Rethinking quenching in UV/chlorine process

Alcohols are widely used as quenchers in UV/chlorine process currently to evaluate the roles of radicals, principally relying on the premise that they selectively react with target radicals (e.g., HO^• and Cl^•) without affecting other reactions. However, this study revealed several unrecognized alcohol-dependent phenomena in this system that clearly challenge this principle. Specifically, tert‑butanol inhibited chlorine decay (k_obs = 0.14 min^–1), whereas methanol, ethanol, and isobutanol accelerated it by 1.2–2.3 folds (k_obs = 0.23–0.45 min^–1). Mechanistic analysis indicated that alcohols quenched HO^• and Cl^• while simultaneously generating secondary species that altered chlorine consumption. In the case of tert‑butanol, radical quenching dominated and suppressed indirect chlorine decay, so that tert‑butanol actually inhibited the conversion of chlorine. In contrast, other monohydric alcohols formed HO-R^• that reacted with O₂ to O₂^•−, a stronger driver of chlorine loss. A quantitative structure-activity relationship model (R² = 0.949) identified molecular polarizability (α = 13.8–42.9) and the lowest unoccupied molecular orbital energy (E_LUMO = 2.41–2.60 eV) of alcohols as predictive descriptors of chlorine decay. For representative micropollutants, using methanol as quencher inflated the apparent roles of primary radicals (up to 83.6 %) but underestimated secondary species such as ClO^•, whereas tert‑butanol provided more reliable qualitative indication. To assess radicals reliably, we propose a two-step strategy that avoids misattribution: (1) using tert‑butanol for qualitative indication of radical involvement; (2) applying multi-probe competition kinetics and modeling radicals adduct speciation for radical quantification. This work established quantitative bounds on alcohol-induced perturbations and offers a protocol for reliable radical quantification in advanced oxidation research.