UV light sources for advanced oxidation processes in water treatment: Photochemical mechanisms and comparative...

The increasing detection of trace organic micropollutants (OMPs) in water is intensifying the need for efficient and sustainable treatment. Ultraviolet-based advanced oxidation processes (UV-AOPs) have emerged as an important class of options for OMP control, but diversification of UV sources and wavelengths complicates selection. This review integrates mechanistic insights and performance metrics to guide UV-AOP design and source selection. Krypton-chloride excilamps emitting at 222 nm represent an emerging far-UVC source with growing research interest, but are constrained by low wall-plug efficiency and short lifetimes. Low-pressure mercury lamps remain a robust choice for large-scale applications, whereas ultraviolet light-emitting diodes offer flexible wavelength combinations and modularity but still suffer from limited wall-plug efficiency. Shorter wavelengths enhance direct photolysis and radical formation in peroxide-based systems, while activation of free chlorine is favored at 265-300 nm and chlorine dioxide responds most strongly in the UVA. In real waters, dissolved organic matter, nitrate, halides, and carbonate alkalinity redistribute photons and reshape reactive species pathways, often attenuating gains observed in clean matrices. Faster degradation does not necessarily translate into lower electrical energy per order. No single type of UV source is universally optimal; implementation should be application driven, matching wavelength-oxidant combinations to matrix conditions while balancing performance and energy efficiency across diverse treatment contexts.