Advances in micro-nano bubbles technology for the removal of emerging contaminants in wastewater: Mechanisms, synergistic integrations, and future perspectives

Emerging contaminants (ECs), including antibiotics, endocrine-disrupting chemicals (EDCs), per- and polyfluoroalkyl substances (PFAS), microplastics, and pharmaceuticals and personal care products (PPCPs), pose critical threats to aquatic ecosystems and public health due to their environmental persistence and chronic toxicity at trace concentrations. Micro-nano bubbles (MNBs) provide a promising process-intensification strategy because of their high specific surface area, prolonged residence time, charged gas-liquid interfaces, and enhanced gas-transfer characteristics. This review critically evaluates MNB generation, physicochemical properties, EC-removal mechanisms, and integration with advanced treatment processes. Across the reviewed studies, MNBs enhanced contaminant removal mainly through gas-liquid mass-transfer intensification, interfacial enrichment and flotation, reactive oxygen species generation, and biological stimulation. Representative systems achieved > 99% degradation of tetracycline and ibuprofen during MNB-assisted ozonation, 98.73% and 99.21% removal of PFOA and PFOS, respectively, by MNB-enhanced foam fractionation, and > 98% removal of microplastics from laundry wastewater by microbubble flotation. MNB-enhanced electro-peroxone treatment also increased ibuprofen removal from 47.3% under conventional aeration to 98.4%. Coupling MNBs with ozonation, Fenton-like oxidation, photocatalysis, non-thermal plasma, adsorption, and membrane processes generally improves mass transfer and reactive-species utilization, although the magnitude of enhancement remains strongly dependent on water chemistry and reactor configuration. Major challenges include standardized MNB characterization, stable operation in complex matrices, efficient PFAS defluorination, scale-up of bubble generators, and energy consumption. Future research should prioritize molecular-level interfacial mechanisms, functionalized bubble interfaces, multi-contaminant systems, and energy-efficient reactor design.