Microemulsions for produced water treatment: A review of mechanism, performance, and industrial challenges

Produced water (PW) is a complex effluent characterized by high salinity, recalcitrant organic compounds, metals, and dispersed hydrocarbons. These properties challenge conventional separation technologies. Microemulsions (MEs) offer thermodynamic stability, ultralow interfacial tension, and the ability to extract hydrophobic and ionic contaminants simultaneously. However, the literature remains fragmented and dominated by studies using synthetic produced water (SPW), while investigations involving real produced water (RPW) represent approximately 30% of publications. This imbalance limits mechanistic interpretation and restricts the direct extrapolation of laboratory-scale data to industrial conditions. This review consolidates current knowledge on the phase behavior, interfacial mechanisms, and solubilization performance of MEs in both SPW and RPW systems. The analysis highlights the influence of salinity, divalent ions, naturally occurring surfactants, and temperature on shifting Winsor transitions and controlling stability. Reported ME systems remove up to 100% of oils and greases and achieve more than 95% removal of benzene, toluene, ethylbenzene, and xylenes ( BTEX). Some systems also reduce salinity by up to 47% and achieve metal recovery efficiencies above 98%. Despite these advances, relevant gaps persist regarding ME regeneration, stability in high–salinity and high–total dissolved solids (TDS) matrices, and offshore scalability. This review critically synthesizes available knowledge on the phase behavior, interfacial mechanisms, and solubilization performance of MEs in SPW and RPW systems, while highlighting key industrial constraints.