Comparing the degradation of organic micropollutants by 222 nm far-UVC and 254 nm irradiation: A critical review

The degradation of organic micropollutants (OMPs) in water remains a major challenge for conventional ultraviolet (UV) treatment at 254 nm, which suffers from poor photon energy and low quantum yields for many contaminants. Krypton-chloride (KrCl*) excimer lamps emitting at 222 nm (far-UVC) offer a promising alternative due to their higher photon energy and favorable safety profile at occupational exposure limits. This critical review systematically compares the performance of 222 nm and 254 nm irradiation for OMPs degradation, with a focus on direct photolysis and UV-based advanced oxidation processes (UV-AOPs). The higher energy at 222 nm facilitates direct bond cleavage and enhances the generation of reactive radicals from oxidant precursors such as H₂O₂, persulfate, peracetic acid, and free chlorine. Across multiple studies, 222 nm-based AOPs increase radical formation and reduce oxidant demand compared to 254 nm systems. However, the elevated photon energy also promotes unintended transformation products (e.g., nitrated aromatics and nitrogenous disinfection byproducts), raising toxicity concerns that are highly dependent on water matrix composition (e.g., nitrate, dissolved organic matter (DOM)). This review highlights the transformative potential of 222 nm far-UVC technology while identifying critical knowledge gaps, including byproduct management, energy efficiency, and regulatory readiness. We conclude that 222 nm irradiation represents a meaningful advancement for water treatment, provided that its implementation is guided by a risk-based, context-specific framework.