Dense Janus membrane with designed nanostructure and surface properties for sustainable desalination from hypersaline silica wastewater in membrane distillation.

Silica-induced membrane scaling is a major constraint to membrane distillation (MD) that limits desalination performance for hypersaline wastewater, due to unique scaling formation pathway and unclear membrane anti-scaling mechanism. Herein, we proposed an electro-assisted ultrasonic atomization method to prepare a specifically designed dense Janus membrane (SL-MPD-PVDF JM), featured precisely controllable PA-based nanostructure and sulfonic group dominant surface on hydrophobic substrate. Pristine PVDF substrate suffered severe performance degradation, with initial water flux of 36.5 LMH and normalized flux of 0.46 within 1 h under 9.0 mM Na2SiO3, pH 9.0, and 50.0 g L-1 NaCl. In comparison, SL-MPD-PVDF JM exhibited even higher initial water flux (38.6 LMH) with negligible decline (normalized flux 0.94) and salt invasion (7.53 μS cm-1) during 12 h. Corresponding pore distribution analysis and surface functional group detection demonstrated that nanochannel (mean pore size: 0.42 nm) was the prerequisite for avoiding silica intrusion, while less-carboxyl group containing surface (1.05 nm-2) supported the trade-off breakthrough between permeability and anti-scaling. Calculated negative interfacial energy (-56.55 mJ m-2), measured extremely low adhesion force (-0.26 N), and DFT analysis between silica-membrane surface further confirmed that newly-developed membrane not only suppressed silica species invasion into membrane matrix but also avoided silica aggregates accumulation on membrane surface. Its application potential was also verified via real silica-laden industrial wastewater treatment and long-term cyclic experiment. This study would underline the potential of dense Janus membranes with precisely modulation of nanostructure and surface properties for efficient desalination of hypersaline silica wastewater in MD.