Recent Advances in Photocatalysis–Microalgae Coupled Systems for Removing Recalcitrant Organic Pollutants from Water

Recalcitrant organic pollutants are characterized by stable molecular structures, high toxicity, and low biodegradability, making it difficult for a single treatment process to simultaneously achieve rapid transformation, extensive mineralization, and ecological safety. Microalgal treatment can support pollutant biotransformation, nutrient recovery, and carbon fixation, but its efficiency is often constrained by pollutant toxicity and limited bioavailability. Photocatalysis can rapidly break down stable molecular structures through reactive oxygen species, but may generate intermediates that resist further transformation. This review summarizes the working principles, system configurations, and performance-determining factors of photocatalysis–microalgae coupled systems for removing recalcitrant organic pollutants from water. Particular attention is given to photocatalytic activation, microalgal biotransformation, extracellular polymeric substance (EPS)-mediated interfacial regulation, and the effects of pollutant properties, material characteristics, microalgal physiological status, and reactor conditions. Recent advances show that the field is moving beyond pollutant removal efficiency as the sole endpoint and toward an integrated assessment of pollutant abatement, microalgal activity, and photocatalyst recovery. Mechanistic understanding has also progressed from empirical descriptions of synergy to systematic analyses of intermediate fate, interfacial electron transfer, and EPS regulation. Future studies should further elucidate pollutant transformation pathways and interactions at the nanomaterial–microalgae interface, and use these mechanistic insights to guide the coordinated design of photocatalytic materials, microalgal species, and reactor configurations, thereby improving treatment performance and long-term operational stability.