Aqueous-Phase Degradation Mechanism of Parabens, Emerging Contaminants, by Peroxynitrite

Advances in science and technology have improved the quality of life but have also contributed to environmental pollution through additives like parabens, commonly used as preservatives in personal care products. This study investigates the degradation of a series of parabens (methylparaben, ethylparaben, propylparaben, and butylparaben) in aqueous-phase solution initiated by peroxynitrous acid (HOONO), an oxidant present in environmental waters. A stopped flow system was employed to follow this oxidation reaction. A reaction mechanism, considering experimental variables, such as pH, temperature, ionic strength, radical scavengers, and reaction products detected, was proposed. This mechanism involves two competitive pathways: a radical attack producing hydroquinone and quinone and a nucleophilic attack by HOONO yielding <i>p</i>-hydroxybenzoic acid, a less ecotoxic compound than parabens. The acidic equilibrium constants of the carbonyl group of parabens and the rate constants for the nucleophilic attack of HOONO on parabens were experimentally determined. Additionally, a structure-reactivity correlation analysis (QSAR) through the Taft equation was applied for the determined equilibrium and nucleophilic attack rate constants. The derived QSAR can be applied to other water-soluble parabens.