Engineering a protective surface layer to resist membrane scaling and scale-induced wetting in membrane distillation for the treatment of hypersaline wastewater

Membrane distillation (MD) is a promising technology in hypersaline wastewater treatment, while its performances such as permeate flux and quality are still hindered by inorganic scaling/wetting. Enhancing surface hydrophobicity is often regarded as the unique way to fabricate anti-scaling MD membranes. However, the weak capability in resisting the scale intrusion (i.e., scale-induced wetting process) and commonly complicated fabrication processes restrain the practical application of this strategy. Herein, we report a facile method for engineering the membrane with a protective surface layer to simultaneously resist membrane scaling and scale-induced wetting during MD. Results showed that the coating of a PVA layer significantly mitigated membrane scaling and prohibited the occurrence of scale-induced wetting during the MD scaling tests. Specifically, the engineered PVDF/PVA-GA Janus membrane (JM) achieved a higher water recovery rate (80 %) with less flux decline (11.6 %) compared with that of the pristine PVDF membrane (9 % and 86.7 %), implying the great potential of the JMs for the near-ZLD treatment of hypersaline wastewater. Morphological characterization and salt diffusion tests demonstrate that the salt rejection capability and dense structure of the surface layer are primary factors preventing scale crystals from intruding into the membrane matrix (anti-scale-induced-wetting effect). Besides, the PVA-GA surface layer, possessing a high energy barrier for gypsum nucleation and low adhesion to gypsum crystals, effectively mitigated membrane scaling (anti-scaling effect). Using polyelectrolytes as additives to increase the inner charge of the PVA layer further improved the salt rejection of JMs. In addition, the JMs exhibited comprehensively anti-organic fouling/wetting capabilities (e.g., surfactants and oils) and longterm stability (similar to 87 h) during the near-ZLD treatment of hypersaline wastewater.