In Situ Photochemical Utilization of Endogenous Biotreatment-Derived Species toward Sustainable Micropollutant Abatement in Water

The pervasive presence of recalcitrant micropollutants in aquatic environments undermines progress toward the sustainable development goals (SDGs) for clean water, health, and ecosystem protection. Conventional wastewater treatment is often ineffective for their removal, while advanced oxidative or reductive processes typically require substantial external chemical inputs. This perspective presents an integrative strategy that synergistically connects biological and physicochemical treatment stages by repurposing residual species generated from biological treatment (e.g., nitrate/nitrite, sulfide/sulfite, and effluent organic matter) as endogenous photochemical precursors. It examines the inherent photoreactivity of common biological-derived species e.g., nitrate/nitrite, sulfide/sulfite, and effluent organic matter (EfOM). Ultraviolet (UV) irradiation of these matrices generates reactive species (e.g., HO•, NO•, and SO3•–) in situ, enabling degradation of micropollutants and transformation of the photosensitizers themselves. This approach minimizes or eliminates the need for external chemical reagents. We systematically review the photochemical fundamentals, including absorption characteristics, quantum yields, and radical generation mechanisms of these biological-derived species, alongside their critical wavelength-dependent behaviors. Finally, we discuss operational parameters, challenges of water matrix components, and future research directions required to implement this synergistic concept. This strategy promises a reduction in chemical use, lower operational costs, and a smaller environmental footprint, effectively establishing a functional linkage between biological treatment and advanced photochemical processes for a more sustainable water treatment paradigm.