Superhydrophobic sponge with nanoneedle-like surface for efficiently separating immiscible and emulsified oil-water mixtures

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To increase the collision probability between emulsified droplets and sponge surface for more efficient separation of emulsions, a durable superhydrophobic melamine sponge (MS) with nanoneedle-like surface was prepared through a simple and inexpensive route in this work. The fabrication processes included the formation of ZnO nanoneedles on MS surface via a hydrothermal method, followed by coating with polybenzoxazine based bisphenol A and dodecylamine (PB-D). As excepted, the resulting superhydrophobic sponge (PB-D/ZnO/MS) could effectively separate various surfactant-stabilized water-in-oil and oil-in-water emulsions, with a high permeation flux of up to 14,050 L m−2 h−1 and separation efficiency above 98.5 %, driven solely by gravity due to its nanoneedle-like surface. Moreover, PB-D/ZnO/MS also displayed outstanding absorption capacities for various oils and organic solvents (57.4–123.0 g/g) as well as prominent reusability, enduring 30 cycles of repeated absorption-squeezing without any evident decrease in water contact angle and absorption capacity (maintaining 90.9–96.6 % of initial value). In addition, the superhydrophobicity of PB-D/ZnO/MS remained almost unchanged after being immersed in strong acidic, alkaline and salty solutions for 48 h. Furthermore, PB-D/ZnO/MS not only showed outstanding flame retardancy but also remained rapid oil absorption rate under combustion conditions. Thus, the obtained superhydrophobic MS is a promising candidate for practical oil/water separation.