Study On Construction And Oil-Water Separation Performance Of Superhydrophobic Composites With Micro-Nano Hierarchical Structure

Since oil-water mixtures and emulsions in rivers and ocean are greatly harmful to ecological environment,it is of great necessity to develop oil-water separation technology.Superwetting materials have shown huge potential in separation of oil-water mixtures due to virtues of the extreme wettability to water or oil.Graphene is recognised as an excellent candidate for the fabrication of superwetting materials the due to its unique features such as the low density and high specific surface area.In this study,three-dimensional(3D)superhydrophobic graphene-based composites with the micro-nano hierarchical structure were prepared first.Then a series of high-performance composites for oil-water separation were developed by dip-coating,vacuum-assisted filtration and ultrasonic atomization spraying.The structure-function relationship between the surface structure and the separation efficiency was systematacially studied.On this basis the separation and demulsification mechanism of these composites for surfactant stabilized water-in-oil emulsions(SSEs)was further studied.First,3D wrinkled spherical ZIF-8@reduced graphene oxide(r GO)composites were developed by embedding ZIF-8 nanocrystals between GO nanosheets,followed by high-temperature reduction self-assembly.The ZIF-8@r GO microspheres(ZGS)featured a unique micro-nano hierarchical structure with well-dispersed ZIF-8 nanocrystals.After optimizing preparation conditions,the superhydrophobic/superoleophilic composites exhibited a water contact angle(WCA)of 158° and fast oil adsorption(within 0.15 s).The adsorption capacity of ZGS for organic solvents could reach to 260 wt.%,showing a good cyclic stability and the thermal stability.Besides,ZIF-7@r GO,ZIF-11@r GO and ZIF-67@r GO microspheres were successfully prepared by mixing ZIFs nanocrystals into GO dispersion instead of in situ growing ZIFs,with uniform micro-nano hierarchical structure.And the universality of high-temperature reduction self-assembly for the fabrication of different nanocrystals doped composites was studied.Second,the superhydrophobic/superoleophilic ZIF-8@r GO@Sponges were successfully prepared by dip-coating commercial sponges in ZGSpolydimethylsiloxane(PDMS)mixtures.The micro-nano hierarchical structure on the surface of ZGS turned the wettability of sponge from superhydrophilic/superoleophilic to superhydrophobic/superoleophilic.Under the optimum preparation conditions(ZGS loading of 0.04 g·cm-3,2 PDMS layers),the superhydrophobic ZIF-8@r GO@Sponge exhibited the WCA of171°,and the adsorption capacity of organic solvent could reach to 2400 wt.%.Moreover,the ZIF-8@r GO@Sponge showed high separation efficiency(>98%)for oil-water mixture,and an excellent performance in cyclic stability and acid,alkali and salt solution resistance stability.Third,the superhydrophobic/enhanced oleophilicity ZIF-8@r GO@PDMS/PTFE(ZGPP)membranes were fabricated by vacuum assisted dip-coating with well-dispersed ZGSs.Due to the micro-nano hierarchical structure of ZGS,the ZGPP membranes displayed superhydrophobicity(WCA=155°).When the loading of ZGS was 2 mg·cm-2 and 2 PDMS layers,the separation flux of ZGPP membranes for SSEs could reach to 2254L·m-2·h-1(0.15 bar,room temperature),and the separation efficiency was99.6%.Besides,ZGPP membranes could complete the demulsification of SSEs within 70～200 minutes,even for the nano-emulsions(average droplet size less than 100 nm).The demulsification process was observed and analyzed by ultraviolet-visible spectrum and optical microscope.Molecular dynamics simulation was used to research the contact demulsification mechanism.The simulation results showed that the micro-nano hierarchical structure on the surface of ZGS enhanced the attraction to surfactant and oil phase in SSEs,which destroyed the phase equilibrium of oil and water,leading to the emulsified droplets coalescence and demulsification.Finally,ultrasonic atomization spraying was developed for the preparation of ZIF-8@r GO@PDMS/PTFE(S-ZGPP)oil-water separation membranes.ZGSs were doped into PDMS-n-heptane solution,followed by spraying assisted layer-by-layer assembly.The superhydrophobic S-ZGPP membranes were prepared with well-dispersed ZGS.Due to the superhydrophobic micro-nano hierarchical structure on the membrane surface,S-ZGPP membrane exhibited a WCA of 152°.By the optimum of preparation conditions,the separation efficiency of S-ZGPP membrane for SSEs could reach to 99.6%,with improved separation flux of 4478 L·m-2·h-1(1 bar,room temperature).