A facile strategy to make expanded graphite granules recyclable in oil-water separation via incorporating them into the framework of monolithic cellulose nanofibril aerogels

Advanced materials for oil-water separation have attracted a great deal of attention recently. This work incorporated expanded graphite (EG) granules into the framework of monolithic cellulose nanofibril (CNF) aerogels using polyvinyl alcohol (PVA) (together with glutaraldehyde (GA) in some cases) as crosslinkers to make them recyclable in oil-water separation. The methyltrimethoxysilane (MTMS) modified EG/CNF/PVA/GA composite aerogels (M-EG/CNF/PVA/GA) exhibit an interconnected 3D hierarchical pore structure with a porosity of 97.11 %, showing a high adsorption capacity for various oils (i.e., trichloromethane, 44.12 g/g; real waste machine oil, 18.38 g/g), being able to separate 100 % of light oil (hexane) and heavy oil (trichloromethane) from an immiscible mixture of oil and water, 74.29 % of soybean oil, and 25.71 % of real waste machine oil from the emulsions of oil-in-water (100 g/L) by static adsorption, and 94.35 % of soybean oil, and 58.46 % of real waste machine oil from the oil-in-water emulsions(100 g/L) by dynamic adsorption. Acting as 3D filters, the polydimethylsiloxane (PDMS) functionalized EG/CNF/PVA composite aerogels (P-EG/CNF/PVA) manifest an outstanding continuous separation performance for the immiscible oil-water mixture based on filtration. For the continuous separation driven by gravity, a constant permeate flux at approximately 5600 L/m 2 h is obtained for repeated separations, the water retention time fluctuates around 20 s, and the separation efficiency reaches 100 %. The permeate fluxes range from 50 to 107 L/hg for the vacuum-driven separation, and the filtrates are free of water uptake. The composite aerogels demonstrate favorable recyclability in adsorption and filtration-based oil-water separation.