Performance and mechanism study on surfactant-stabilized oil microdroplets extraction from oily wastewater

Petrochemical, coal chemical, and metal smelting industries generate substantial amounts of oily wastewater. The major obstacle in treating these wastewaters is surfactant-stabilized oil microdroplets. Solvent extraction has shown significant application potential in this kind of oil microdroplet removal due to its high-efficiency, economy, and environmental friendliness. However, mechanistic study and other in-depth investigation are still needed to support the more high-performance extractant screening and process design. In this study, in terms of the surfactant-stabilized oil microdroplet extraction from oily wastewater, the mechanism was analyzed by characterizing the compositions and properties of the intermediate layer and extractant phase. It was demon-strated that, unlike conventional liquid-liquid extraction system containing molecular solutes, the extraction of surfactant-stabilized oil microdroplets is accomplished by the mass transfer of extractant from the exterior to the interior of the oil microdroplets and the rise of extractant-containing oil droplets in the aqueous phase with critical influences. This behavior of mass transfer and droplet rise is supported by the mechanism of the emulsion liquid membrane extraction technique and the Stokes' law. In addition, we illustrated the positive or negative effects of several factors (e.g. molecular extractant type, extractant dosage, stirring time, stirring speed and so on) on extraction efficiency. This confirmed the mechanism proposed above. And multistage extraction exper-iments showed that this solvent extraction technology was able to treat oily wastewater to discharge standards. Finally, by combining with the feasibility analysis of extractant regeneration, non-polar extractants were considered as the more suitable extractant than polar extractant because of the realizability of regeneration, lower costs for regeneration and better extraction performance.