Engineering construction of robust superhydrophobic two-tier composite membrane with interlocked structure for membrane distillation

The major challenge in membrane distillation for seawater desalination and/or wastewater purification is the development of high-efficiency membrane distillation (MD) membranes with high porosity, hydrophobicity, and adequate mechanical strength for long-term operation. Herein, a superhydrophobic two-tier interlocked composite membrane based on a hierarchically structured isotactic polypropylene (iPP) coating and an electrospun poly(vinylidene fluoride) (PVDF) nanofibrous support was engineering constructed. The obtained coating layer possessed a deformed micro/nanostructured microsphere surface with robust superhydrophobicity and further glorious anti-fouling property, which was owed to the synergistic effect of the low surface free energy material and hierarchical roughness. Specially, the middle transitional interlocking zone between the crystalline iPP microsphere coating and PVDF nanofibers endowed the resultant composite membrane with excellent structural integrity including the remarkable enhancement in mechanical performance compared with PVDF flat sheet or nanofibrous membranes, and also resulted in the iPP/PVDF composite membranes with robust durability against ultrasonication in isopropanol and strong acid/base attacks. Moreover, for the simulated high salinity sunset yellow (SY) wastewater, the optimized superhydrophobic composite membrane exhibited a competitive permeate flux of 53.9 kg/(m²·h) and complete rejection over 50 h operation (ΔT = 40 °C) because of its highly porous nanofibrous support and firmly sustainable liquid repellency.