Photochemical transformation of benzalkonium chloride in seawater

The fate and risks of quaternary ammonium compounds (QACs) have gained increasing attention due to their widespread use during and after the COVID-19 pandemic. This study investigates the photodegradation of benzalkonium chloride (BAC C12) in seawater, revealing that its degradation rate (0.0928 h^-1) is slower compared to pure water, where the rate is 0.132 h^-1. Chloride (Cl^-) and bromide (Br^-) ions promote degradation by generating halogen radicals (Cl·, Br· and ClBr·^-), nitrate ions (NO₃^-) can generate reactive oxygen species (ROS) and nitrogen-containing free radicals (e.g., ·NO₂) via direct photolysis or indirect photosensitization reactions, while Fe³⁺, Mn²⁺ and Cu²⁺ accelerate the reaction by promoting ROS production to accelerate the reaction. The effect of dissolved organic matter (DOM) on degradation is concentration-dependent: at low concentrations, it inhibits degradation through photocompetition, while at high concentrations, it promotes degradation via photosensitization and electron transfer. Quenching reactions and probe experiments indicate that ·OH is the dominant species (contributing over 50 %), followed by ¹O₂ (more than 10 %) and halogen radicals (over 5 %), all of which play significant roles in degradation. Quantum chemical calculations and mass spectrometry identify key degradation pathways, including alkyl chain cleavage, benzene ring substitution (primarily at C2 and C5), and N-demethylation. ECOSAR simulations indicate that the chlorine and hydroxyl substitution products are more toxic. These findings provide a scientific basis for the environmental risk assessment and mitigation strategies of QACs.