The overlooked role of benzalkonium chlorides in enhancing chlorination of organic contaminants

Benzalkonium chlorides (BACs), a major class of quaternary ammonium compounds, are widely used disinfectants in hospital settings and commonly enter wastewater treatment systems. Although the role of tertiary amines in promoting disinfectant byproduct (DBP) formation has been well documented, the contribution of BACs to chlorination reactions has been largely overlooked. This study demonstrates that BACs significantly enhance the chlorination of phenol, promoting the formation of toxic chlorophenols and their downstream transformation products. The enhancing effect is attributed to the generation of highly reactive R 3 N⁺-Cl intermediates via BAC oxidation by chlorine. The strong electrophilic nature of R 3 N⁺-Cl facilitates chlorination, leading to the increased formation of 2,4-dichlorophenol and 2,4,6-trichlorophenol, with shorter-chain BACs (C12) exhibiting stronger effects than longer-chain counterparts (C14, C16). Further oxidation of 2,4,6-trichlorophenol leads to the formation of trichloro-hydroxy-cyclopentene-dione, a highly toxic DBP. Density functional theory calculations suggest that while the electronic properties of BAC-derived chloramine intermediates are similar, the reactivity of longer-chain BACs decreases due likely to lower aqueous solubility and diffusion rates. These findings highlight BACs as important contributors to chlorinated DBP formation in chlorination processes and underscore the need to assess their impact in wastewater treatment, particularly in hospital effluents with high BAC concentrations.