From enemy to ally: Dissolved organic matter synergizes contaminants removal under light-driven processes

Dissolved organic matter (DOM), ubiquitous in natural and engineered waters, has long been regarded as an "enemy" in organic micropollutant (OMP) removal due to its photon competition, radical quenching, and catalyst fouling. Recent evidence, however, reveals a paradigm shift: under light-driven conditions, DOM can possibly act as a powerful "ally" that promotes OMP degradation via its intrinsic photochemical activity. This review provides a comprehensive framework for understanding this transition. We summarize the optical properties of DOM, the formation pathways of photochemically produced reactive intermediates (PPRIs), and the environmental factors controlling their activity. Special attention is given to DOM's synergistic mechanisms: DOM can promote OMP removal via redox and energy transfer reactions by PPRIs, photosensitized activation of oxidants, and enhancing its photo-reactivity via adsorption. Then, the methods for investigating the involvement of PPRIs in the degradation of OMPs are summarized, as well as the contribution of DOM to contaminant removal in various light-driven systems. Furthermore, we identify key research needs to advance the utilization of DOM for improving the performance of photochemical systems in OMP treatment. By bridging fundamental photochemistry with engineering practice, this review offers new insights into leveraging DOM to design low-energy, high-efficiency water treatment processes.