Advances in cold atmospheric plasma for water purification: reactive species generation, diagnostics, and degradation of persistent organic contaminants

Cold atmospheric plasma (CAP) has emerged as a promising non-catalytic and chemical-free advanced oxidation and reduction process (AORP) for treating persistent organic contaminants (POCs) in water. CAP is a non-equilibrium, partially ionized gas containing energetic electrons, ions, radicals, metastable species, neutral molecules, and photons. Upon interaction with liquids, CAP generates complex plasma-liquid interfacial processes that produce a broad range of reactive oxygen and nitrogen species (RONS), including hydroxyl radicals (•OH), atomic oxygen (O•), ozone (O 3 ), hydrogen peroxide (H 2 O 2 ), reactive nitrogen species, hydrogen radicals (•H), and solvated electrons ( e - sol ). These short- and long-lived species promote pollutant transformation through oxidative and reductive pathways; however, the extent of degradation, mineralization, defluorination, and detoxification depends on reactor configuration, interfacial transport, pollutant properties, and water matrix composition. This review presents an integrated perspective on CAP-based treatment of POCs by connecting discharge characteristics, CAP-liquid interfacial interactions, reactive-species generation and diagnostics, pollutant transformation pathways, reactor engineering, and computational optimization. Recent advances in transient species generation and characterization, ultrafast spectroscopy, POCs transformation mechanisms, multiphysics modeling, and machine learning-assisted process optimization are highlighted. By bridging fundamental plasma processes with treatment performance, this review identifies critical knowledge gaps and provides insights for the rational development of efficient and reliable CAP-based water purification technologies.