Hollow fiber membranes with Janus surfaces for continuous deemulsification and separation of oil-in-water emulsions

Janus membranes with asymmetric surface wettability are regarded as promising materials for oil/water separation. However, their performances and applications are still limited by diminutive flat structure, dead-end filtration mode, batchwise separation process, low disposal capacity and heavy oil preference. Herein, Janus hollow fiber membranes (JHFMs) are reported to be fabricated by a circulated co-deposition of polydopamine/polyelectrolyte onto lumen surfaces of membranes for addressing these issues. A thin, hydrophilic and charged layer is uniformly and thickness-controllably coated to achieve an asymmetric wettability and a deemulsification function simultaneously. The as-prepared JHFMs feature a high efficiency to separate surfactant stabilized oil-in-water emulsions through successive deemulsification and unidirectional oil delivery. Furthermore, the membrane module assembled by multiple pieces of JHFMs exhibits a large-scale and remarkable ability to separate both heavy and light oils from their oil-in-water emulsions in a cross-flow filtration mode. More importantly, the hollow fiber module shows a record-high recovery ratio toward light oil compared with flat membranes. The purities of filtrates and feeds after the separation totally meet the international purification standards. This work provides an exhilarated opportunity of Janus membranes in the practical application of oily wastewater separation, especially for those surfactant-stabilized oil-in-water emulsions.