Recent advances in photocatalytic technologies for removing microplastics from aquatic environments

As emerging contaminants, microplastics (MPs) are becoming ubiquitous in aquatic environments. Owing to their small size, resistance to degradation, and high capacity to adsorb coexisting pollutants, MPs pose severe threats to ecosystems and human health. Conventional water treatment processes, such as coagulation, sedimentation, and filtration, exhibit limited removal efficiencies for MPs and fail to achieve complete mineralization. Consequently, there is an urgent need for green, efficient, and advanced technologies. Photocatalysis has emerged as a promising technology with unique advantages. Operating under mild conditions (such as ambient temperature, pressure, and near-neutral pH), it utilizes solar energy to excite semiconductors, generating reactive oxygen species (ROS) that nonselectively attack polymer backbones. This drives oxidative chain scission and complete mineralization into CO2 and H2O, thereby avoiding high energy consumption and secondary pollution. Moreover, the photoreforming pathway enables the selective cleavage of polymer chains into value-added chemicals or clean H2 fuel via the hydrogen evolution reaction, achieving waste-to-resource upcycling. Photocatalysis directly utilizes renewable solar energy without the need for external oxidants and allows catalysts to be designed in recoverable forms, offering a low carbon footprint. This review systematically examines the sources, pollution characteristics, and environmental and health risks of MPs, while elucidating the core reaction mechanisms of photocatalytic MP conversion, including direct photodegradation, photocatalytic oxidation, and photoreforming. Furthermore, the current research progress and modification strategies for five mainstream material families are comprehensively summarized. TiO2-based materials exhibit high UV mineralization efficiency; for instance, they can achieve 98.4% mineralization of 400 nm polystyrene within 12 h. Strategies such as doping, carbon hybridization, and heterojunction construction extend the light absorption of these materials into the visible region. ZnO likewise provides high electron mobility, facile morphology control (yielding structures such as nanorods and nanoflowers), and low-cost green synthesis. For instance, ZnO nanorods in continuous-flow reactors achieved >65% volume reduction of polypropylene MPs after 456 h under visible light. Furthermore, Pt/ZnO, Fe-doped ZnO (Fe–ZnO), and GO/ZnO composites, as well as the coupling of ZnO with persulfate, further enhanced visible-light activity. g-C3N4-based materials benefit from a suitable bandgap and a two-dimensional structure. Nitrogen-vacancy engineering raised polystyrene removal to 34.2% (vs. 8.6% for bulk g-C3N4) and reduced product toxicity, while a WO3/g-C3N4 Z-scheme heterojunction enabled simultaneous polyethylene terephthalate degradation and H2 evolution. Additionally, bismuth-based photocatalysts utilize internal electric fields to facilitate charge separation. For example, hydroxy-rich BiOCl-X yielded a 5.38% weight loss of high density polyethylene (HDPE) in 5 h, which is 24-fold higher than that of pristine BiOCl. Lastly, metal–organic frameworks offer exceptionally high surface areas; specifically, NH2-MIL-88B(Fe)/MoS2 heterojunctions degraded HDPE and coexisting antibiotics, while a BiOI/MIL-101(Fe) composite utilizing photo-Fenton synergy raised the carbonyl index of PE by 0.127 in 6 h. Finally, a radar diagram compares the five families across seven criteria, revealing that no single material excels in all dimensions. This review also evaluates critical environmental factors influencing degradation performance. For instance, a low pH (3) and low temperature (0 °C) enhanced HDPE degradation, resulting in a 71.77% weight loss, while inorganic anions and dissolved organic matter acted as light filters and ROS quenchers. Furthermore, the generated intermediate products and associated ecotoxicity are compiled. Aromatic MPs generate benzene-containing intermediates that induce oxidative stress, whereas PLA degrades to low-toxicity lactic acid oligomers. Nevertheless, core bottlenecks remain, including low conversion efficiencies for C—C-dominated polyolefins, the requirement for harsh, alkali-dependent pretreatment, unclear interference mechanisms, and insufficient long-term ecotoxicity assessments. To address these issues, this work proposes a roadmap involving machine-learning-assisted catalyst screening, microfluidic-reactor-coupled in-situ spectroscopy for mechanistic elucidation, and pilot-scale, continuous-flow photoreactors operating under real wastewater conditions that integrate low-alkali pretreatment and product valorization. Overall, this study provides a valuable theoretical reference and technical support for developing photocatalytic MP remediation technologies.