Preparation And Performance Of Cellulose-Based Membrane For Oil-Water Separation

Oily wastewater and frequent oil spills have led to serious ecological pollution,threatening aquatic life and human health.The treatment of oily wastewater consumes a lot of resources and energy.By constructing micro-nano composite structures on the surface of porous substrates and regulating the surface free energy,superhydrophobic（water contact angle>150°）or superhydrophilic（water contact angle～0°）membranes can be obtained,which have great potential in the field of oily wastewater treatment.The use of low-pollution,low-energy,sustainable and degradable materials instead of ceramics,alloys and petrochemical derivatives is the direction of membrane technology development.Therefore,in this paper,we developped superhydrophobic or superhydrophilic cellulose membranes for oil-water separation from both degradable substrates and fluorine-free modified materials,using woody（wood）and herbaceous（loofah）plants as substrates,as follows.The pathways of graft-condensation reactions of methyltriethoxysilane and its three hydrolysis products on the cellulose surface in ethanol solvent were investigated by DFT method,and the simulation results showed that the incompletely hydrolyzed products of methyltriethoxysilane in ethanol solution environment tended to graft on the cellulose surface by dehydration reaction.The product CH₃Si（OEt）（OH）₂ with two hydroxyl groups after hydrolysis of methyltriethoxysilane is more favorable for the growth of silane on the cellulose surface.The-OSi（OEt）（OH）CH₃ on the cellulose surface tends to be further dehydrated and condensed with the free methyltriethoxysilane hydrolysis product in solution,which in turn leads to multilayer growth.The interaction of interfacial water molecules with cellulose and hydrophobically modified cellulose was simulated using molecular dynamics.The hydrophobic modification resulted in a reduction of the interfacial energy between the interfacial water molecules and cellulose,and the interfacial water molecules formed a high-density region close to the cellulose and a low-density region in the siloxane distribution layer.A degradable fluorine-free superhydrophobic walnut membrane was prepared by a one-step sol-gel method using walnut wood as raw material.Its tensile strength was 222%of that of delignified wood aerogel.The maximum water contact angle of the membrane surface was 164.2°.After rubbing on the surface of240 mesh sandpaper（2.5 kPa（G）pressure）for 100 cm or impregnating in acid-base solution for 12 h,the contact angle was still greater than 150°.The cross stacking resulted in efficient oil-water（oilwater density ratio of 1.491）separation with 98.5%separation efficiency and separation flux（dichloromethane）of about 1.3×10³ Lm^-2h^-1.The composite cellulose membrane is obtained by pore control using loofah cellulose fibers as raw material.The filtration and support layers are dried by water activation followed by superimposed hot pressing and spontaneously bonded under hydrogen bonding without the addition of binder.The tensile strength of the composite membrane is 177.8 kN/m,which is 388.7%of that of commercial PVDF membrane.The superhydrophobic cellulose membrane obtained by one-step sol-gel method with tetraethyl orthosilicate as the precursor and dodecyltriethoxysilane as the modifier using this membrane as the substrate has a contact angle of more than 150°,an oil-water（oil-water density ratio of 1.491）separation efficiency of 98.1%,and a flux（dichloromethane）of 2349 Lm^-2h^-1.The effects of heat treatment temperature on the surface wettability,crystallinity and degree of order of cellulose films were nonlinear and the trends of the effects on the wettability,crystallinity and degree of order of cellulose cellulose films obtained using walnut and loofah as templates differed significantly.The contact angle of the walnut cellulose film was highest at 225℃,and the crystallinity increased rapidly at 200℃;the contact angle of the lucerne cellulose film was highest at 200℃,and the crystallinity peaked at 200℃.Water-stable superhydrophilic oil-water separation membranes based on heattreated walnut and loofah cellulose membranes were prepared by a one-step sol-gel method using tetraethyl orthosilicate as the precursor and 3-aminopropyl 3ethoxysilane as the modifier.The separation efficiency of the superhydrophilic walnut cellulose membrane is higher than 97.8%with a flux of about 1398 Lm^-2h^-1,and the separation efficiency of the superhydrophilic lucerne cellulose membrane is higher than 98.2%with a flux of 1879 Lm^-2h^-1.