PDA-PEI copolymerized highly hydrophobic sponge for oil-in-water emulsion separation via oil adsorption and water filtration

Oil adsorption from oily wastewater using three-dimensional porous materials is a popular mode of separation, but the adsorption of oil droplets from oil-in-water (O/W) emulsions is difficult. In this study, hydrophobic melamine sponges, prepared by coating with dopamine (DA) and polyethyleneimine (PEI) via controlled copolymerization, were utilized to separate O/W emulsions on a self-designed “oil adsorption-water filtration” device. Unexpectedly, the sponge obtained via DA/PEI modification was highly hydrophobic. Molecular dynamics simulations revealed that this was due to Schiff base production upon copolymerization of polydopamine (PDA) and PEI on the melamine sponge. The sponge showed excellent adsorption capacities, 67.2–178.6 times its own weight, for various oils. More importantly, a surfactant-free O/W emulsion was separated with 93.5% separation efficiency and 24.9 g/g sponge oil adsorption. The sponge also exhibited perfect separation efficiency for an anionic-surfactant-stabilized O/W emulsion. During filtration, negatively charged oil droplets were captured on the amino-rich sponge by electrostatic attraction. Furthermore, the hydrophobic effect was found to be the driving force in adsorbing and collecting oil on the sponge skeleton while repelling the continuous water phase. Since the as-prepared sponge can simultaneously adsorb oil droplets and filter the water phase in O/W emulsions, it has major potential for application in oily wastewater treatment.