Switchable wettability of grain-stacked filter layers from polyurethane plastic waste for oil/water separation

Hypothesis Polyurethane plastic waste (PUPW), a port-abundant solid waste, is difficult to degrade naturally and poses a severe threat to the environment. Hence, the effective recycling of PUPW remains a challenge. Experiments Herein, a strategy of converting PUPW into stacked oil/water filtration layer grain through a layer-by-layer (LBL) assembly process is investigated. Notably, such PU-based, grain-stacked, and switchable wettability of the oil/water filter layer is first reported. Findings The grain-stacked filter layers are flexible for separating immiscible oil/water mixtures, water-in-oil emulsions (WOE), and oil-in-water emulsions (OWE) under gravity over 10 cycle-usages. They can withstand strong acid/alkali solutions (pH = 1–14) and salt solutions over 12 h. Besides, 100-times scale-up experiments have indicated that the obtained filter layers exhibit an upper to 98.2 % separation efficiency for 10 L real industrial oil/water emulsion in the 24 h continuous operation. The demulsification mechanism for emulsions is that the electrostatic interaction along with adsorption between emulsion droplets and grains leads to the uneven distribution of surfactants on the interface film of the emulsion droplets, increasing the probability of tiny droplets colliding and coalescing into large droplets to achieve oil/water separation. This work proposes an effective and economical method of abundant plastic waste for industrial-scale oil–water separation rather than just on the laboratory-scale.