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 iso-butanol 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 O 2 to O 2 ●− , a stronger driver of chlorine loss. A quantitative structure-activity relationship model (R 2  = 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.