From scavenger to catalyst: the emerging role of chloride in peroxymonosulfate-based advanced oxidation processes

Abstract

Peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) effectively degrade persistent and emerging organic pollutants in water. Although chloride ions were once dismissed as inhibitors due to radical scavenging, research now reveals their dual role: at low levels they inhibit reactions, but at higher concentrations they enhance degradation via reactive chlorine species, singlet oxygen, and high-valent metal-oxo species. These intermediates heighten treatment efficacy across applications like disinfection, ammonia removal, membrane cleaning, and emerging contaminant breakdown. However, chloride can also generate chlorinated by-products (CBPs) and absorbable organic halides (AOXs), raising ecological concerns. While the toxicity of some chlorinated products may initially increase, prolonged treatment typically mitigates these risks. The key lies in optimizing chloride concentration and treatment conditions to ensure both efficiency and environmental safety.