Distinct effects of inorganic and organic carbon on nitrate photolysis and micropollutant degradation under Far-UVC (222 nm) irradiation

Nitrate photolysis under UV222 irradiation has increasingly attracted attention as an advanced oxidation process, while fundamental mechanisms remain incompletely understood in various water matrices. This study revealed that the observed nitrate photodecay under UV222 represented a dynamic balance between innate photolysis and significant reformation, with the innate quantum yield (0.52 mol·Ein −1 ) substantially 7.1–11.0 times higher than the observed quantum yield (0.047–0.073 mol·Ein −1 ). Inorganic carbon (IC) and organic carbon (OC), ubiquitous water matrix components, influenced UV222/nitrate through distinctly different mechanisms: IC suppressed nitrate photodecay 12.6–58.3 % via enhancing ONOO⁻-mediated nitrate reformation without affecting innate photolysis, while OC promoted observed nitrate photodecay 19.6–54.5 % by scavenging reactive intermediates involved in nitrate reformation. UV222/nitrate effectively degraded various micropollutants via direct photolysis (e.g., N-nitrosodimethylamine), • OH oxidation (e.g., carbamazepine), and a combination of pathways including reactive nitrogen species (RNSs) oxidation (e.g., sulfamethoxazole). IC selectively promoted RNSs oxidation while suppressing • OH oxidation pathway, leading to selective effects for various micropollutant degradation and the increased nitration byproducts formation. In contrast, OC uniformly reduced all oxidation pathways through non-selective scavenging of reactive species. This mechanistic understanding explained the complex effects of water matrix components on UV/nitrate performance and provides a fundamental basis for optimizing UV222 driven advanced oxidation processes for micropollutant control in diverse water conditions.