Ultrahigh and Stable Water Recovery of Reverse Osmosis-Concentrated Seawater with Membrane Distillation by Synchronously Optimizing Membrane Interfaces and Seawater Ingredients

Membrane distillation holds promise for further recovery of reverse osmosis (RO)-concentrated seawater to realize zero-liquid discharge; however, severe scaling induced by high-concentration salts and fouling triggered by a naturally occurring substance would result in severe wetting of the hydrophobic distillation membrane. To retard membrane wetting during the concentration process, the membrane’s interfacial structure and the seawater’s ingredients were systematically investigated to improve the membrane’s durability. The results demonstrated that the dual-layer membrane with hierarchically rough beads-on-a-string structured fibers could enlarge the water evaporation area on the membrane surface to improve membrane water flux, and superhydrophobicity can reduce the contact area between the salt and membrane surface to prevent membrane scaling. Significantly, the membrane’s durability can be further improved by adjusting the seawater ingredients. This finding revealed that the scaling resulting from Ca²⁺ and Mg²⁺ when they exist alone is more severe than when they exist together. Therefore, eliminating the dominant Ca²⁺, Mg²⁺, and naturally occurring substance in seawater and maintaining it in a neutral state can stably concentrate the seawater with a high water recovery of ∼85%. The regulation of the membrane interface integrated with optimal seawater ingredients is essential for the application of membrane distillation in the treatment of RO-concentrated seawater.