Solar Membrane Crystallizer with Ultralow Salt Adhesion for Zero Liquid Discharge of Hypersaline Wastewater

Solar-driven zero-liquid discharge (ZLD) is a promising wastewater management strategy for freshwater recovery and salt resource harvesting. However, currently developed interfacial solar crystallizers fail to maintain high evaporation capability when treating hypersaline wastewater due to the salt scaling problem. The accumulated salt on the solar crystallizers hinders the efficiency of solar-driven ZLD. This study reports a solar membrane crystallizer consisting of CuO nanoarrays covered by a hydrophobic and smooth fluoride-co-hexafluoropropylene (PVDF-HFP) coating, capable of preventing salt scaling during desalination. During solar-driven water evaporation, salt crystals migrate and aggregate inward on the crystallizer surface and finally agglomerate together, forming a discontinuous contact with the crystallizer surface. The salt crystals exhibit extremely low adhesion to the crystallizer with adhesive force as low as 1.50 mN mg^-1. With this ultralow adhesion, salt crystals can be easily detached from the surface by tilting the solar crystallizer, leaving a regenerated surface that is used continuously. During a 6-hour outdoor test (from 9:30 to 15:30), the solar crystallizer shows a treatment capacity of 7.52 kg m^-2 for hypersaline wastewater with 25 wt.% salt content under natural sunlight irradiation. This work provides a low-cost and feasible ZLD strategy for sustainable hypersaline wastewater treatment.