Effect of pH on UV/H₂O₂-mediated removal of single, mixed and halogenated parabens from water

Adjusting pH values in aqueous environments can significantly improve the efficiency by which parabens and halo-parabens are removed. In this study, 20 neutral and deprotonated species were selected as models to investigate their pH-dependent removal mechanisms and kinetics by a UV/H₂O₂ process using density functional theory (DFT). Compared to neutral species, deprotonated species exhibit higher reactivity to HO^• due to their high electron cloud density. H atom abstraction (HAA) reactions on the substitution groups are the most favorable pathways for neutral species, while radical adduct formation (RAF) reactions are the most favorable for deprotonated species. Single electron transfer (SET) reactions can be neglected for neutral species, while these reactions become a viable route for deprotonated molecules. The total reaction rate constants range from 1.63 × 10⁹ to 3.74 × 10¹⁰ M^{- 1} s^{- 1} at pH 7.0, confirming the experimental results. Neutral and weakly alkaline conditions are favorable for the degradation of MeP and halo-parabens in the UV/H₂O₂ process. The order of removal efficiency at optimum pH is dihalo-parabens > mono-halo-parabens ≈ F, F-MeP > MeP. Furthermore, the transformation products must undergo oxidative degradation due to their high toxicity. Our findings provide new insights into the removal of parabens and their halogenated derivatives from wastewater.