Hierarchically superhydrophilic poly(vinylidene fluoride) membrane with self-cleaning fabricated by surface mineralization for stable separation of oily wastewater

Abstract

Long-term separation of highly emulsified oily wastewater is still challenging in advanced membrane technologies. Herein, β-FeOOH nanorods were in-situ inlaid on a hierarchically superhydrophilic poly(vinylidene fluoride) (PVDF) ultrafiltration membrane surface via a facile biomimetic mineralization process. The β-FeOOH-decorated hierarchical PVDF membrane surface with the organic-inorganic hybrids showed excellent hydrophilicity and ultralow underwater oil adhesion force. After in-situ anchoring of β-FeOOH nanorods, the polydopamine/polyethylenimine (PDA/PEI) modified hierarchical PVDF membrane showed improved permeation flux behaviors, higher oil removal, demulsification, and better anti-oil-fouling properties during 1440-min continuous separation of stabilized soybean oil-in-water (o/w) and crude o/w emulsions. In contrast, the unmodified hierarchically superhydrophilic PVDF membrane maintained efficient emulsion separation for a much shorter period (120 min). Most importantly, the surface-immobilized β-FeOOH exhibited excellent photocatalytic activity for cleaning the oil foulants (soybean oil and crude oil) through the photo-Fenton reaction in the presence of peroxide, endowing the membrane with self-cleaning capability. As a result, the unwanted foulants were successfully removed from the membrane surface, restoring separation performance of the superhydrophilic PVDF membrane for reuse. Therefore, the self-cleaning membrane can treat more oily wastewater and produce more clean water before replacement. The mineralized superhydrophilic PVDF membrane with hierarchical composite micro-/nano structures exhibits satisfactory stability under simultaneous separation and photocatalysis cleaning, showing great potential for tricky oily wastewater treatment and oil resource recovery.