Micro- and nanobubble-assisted oil–water separation: Mechanisms, performance, and challenges

Oily wastewater from industries such as petroleum refining, petrochemical processing, and metalworking poses significant treatment challenges, particularly when oil is present as fine droplets or stable emulsions. Conventional methods, including gravity separation, coagulation–flocculation, membrane filtration, and gas flotation, often show limited effectiveness under these conditions. Micro- and nanobubble (MNB) has emerged as a promising enhancement to flotation processes due to their high interfacial area, extended residence time, and distinct interfacial properties in aqueous systems. This review critically examines the application of MNB in oil–water separation, focusing on bubble generation, characterization, and mechanisms of bubble–oil interactions. Key operational parameters, including bubble size distribution, water chemistry, and hydrodynamics, are evaluated. Reported oil removal efficiencies typically range from 70% to >95%, with improvements of 10–30% over conventional flotation under optimized conditions. MNB-assisted systems operate within the general flotation framework of bubble–droplet collision, attachment, and buoyant transport; however, their distinct physicochemical properties, including enhanced bubble stability, high specific interfacial area, and surface charge effects, can influence interaction dynamics and separation behavior in ways that are not fully captured by classical flotation models, particularly in emulsified systems under complex water chemistry conditions. Despite promising performance, variability in experimental conditions and lack of standardized reporting hinder direct comparison across studies. Overall, MNB shows strong potential for improving oil–water treatment, particularly for emulsified systems; however, further pilot-scale validation and integrated techno-economic and environmental assessments are needed to support large-scale application.