Fabrication Of Biomass-based Special Wettability Materials And Its Oil/Water Separation Properties

The frequent occurrence of crude oil spill accidents and the discharge of oily wastewater from industry and human daily life have caused harm to water resources such as oceans,surface water and groundwater,and then serious threat to the ecological environment and human health.Oily wastewater has various types,complex components,and poor biodegradability,while traditional oil-water separation technologies have some problems,such as low separation efficiency,poor selectivity,high energy consumption,complex equipment requirements,secondary pollution,and non-biodegradation of discarded separation materials.Thus,it is urgent to research and develop effective oil-water separation methods.Due to the opposite wettability to water and oil,special wetting materials show attractive application prospects in the efficient and selective oil-water separation.However,the practical applications of ceramic membranes,artificial polymer membranes,aerogels,and metal mesh materials are limited by unsustainable raw materials,high cost,low flexibility,and poor recyclability and durability,etc.Biomass is a sustainable green resource in nature,which has excellent properties including extensive sources,biodegradability,low toxicity,and abundant active functional groups.It is an ideal material to replace the matrix of oil/water mixture separation materials.In this paper,renewable biomass resources were selected as the substrate for oil-water separation.After adjusting the wetting properties,biomass-based superwetting materials were used for multi-system oily wastewater separation and the separation effect was studied.The win-win goal of removing oil pollution from water and biomass resource utilization is achieved.The main research contents are as follows:1.Preparation of superhydrophobic biomass-based porous membrane and study on the separation performance of oil-water mixture（1）Considering the microstructure and chemical composition of two intrinsic elements that affect the wetting performance,using waste cellulose as a matrix,a hierarchical layered double hydroxide（LDH）nanosheet layer was in-situ constructed on its surface.The LDH/cellulose porous membrane with superhydrophobic/superoleophilic properties was prepared via physical deposition.The cellulose-based porous membrane shows a lotus leaf-like superhydrophobicity with a water contact angle of 155°and a rolling angle of only 2°.It has good separation effect for light oil/water mixture and heavy oil/water mixture,and the separation efficiency is higher than 94.4%.At the same time,the surfactant-stabilized water-in-oil emulsion also showed excellent separation performance（water content of filtrate less than 25 ppm）and good flux(more than 537 L m^-2 h^-1).The superhydrophobic LDH/cellulose membrane still maintains a higher water angle than 151°in after being immersed in strong acid（p H=1）and alkaline（p H=12）solutions for 8 h,indicating that the membrane has stable properties,good wetting properties,and good corrosion resistance.（2）Based on the superhydrophobic wettability mechanism and the in situ growth mechanism,the construction of the LDH-based hierarchical structure and the surface hydrophobic modification process are integrated in a special oil-water two-phase reaction system:the hydrothermal reaction technology is used to control the growth of LDH crystals on the cellulose surface in the water phase;with the evaporation of the water phase,the cellulose membrane enters the oil phase,and the stearic acid dissolved in the oil self-assembles on the surface of the LDH crystal,forming a low surface energy layer,thereby realizing the one-step fabrication of a superhydrophobic cellulose membrane.The surface structure,chemical composition,wetting properties and corrosion resistance of superhydrophobic cellulose membranes were studied in detail.Taking the water-in-oil emulsion containing surfactant and non-surfactant as the separation target,with the assistance of vacuum pump,the emulsion separation efficiency of superhydrophobic cellulose membrane is as high as 99.6%,and its permeation flux is 4800 L m^-2 h^-1 bar^-1.Moreover,the membrane has good stability and recyclability.2.Preparation of superhydrophobic biomass-based aerogel composites and study on the separation performance of water-in-oil emulsions（1）To achieve water-in-oil emulsion separation with both high flux and efficiency,a compressible,highly hydrophobic cellulose composite（BPWP）aerogels aerogel with high pore tortuosity was prepared using waste paper and banana peels as building blocks via ice crystal method,freeze-drying and pyrolysis processes.The microflakes formed by banana peel with the aid of an ice template can strengthen the hierarchical structure of aerogel,which can effectively promote the polymerization and demulsification of emulsion,achieving fast gravity-driven separation with high separation efficiency of up to 99.6%and permeation flux of up to 8550 L m^-2 h^-1;the aerogel also achieved good oil adsorption performance(35-115 g g^-1).After ten compression-release cycles,there was no collapse or fracture of the aerogel.It is suggested that the mechanical entanglement of fibers and the banana peel-derived sheet-like structure enable the good compressive properties of BPWP aerogels.（2）Under the premise of ensuring the separation efficiency,to further improve the permeation flux,a new type of cellulose-based aerogel/membrane composite（CAMC）with large pore size was prepared via combining facile freeze-drying and hydrophobic modification using cellulose fibers as building blocks.An ultrathin silanization coating layer on cellulose fibers surface is constructed via directly coupling alkoxysilane and endows the composite with excellent superhydrophobicity（161°）and superoleophilicity（0°）.CAMC can separate surfactant-stabilized water-in-oil emulsions with high separation efficiency（99.5%）and high permeation flux(12890 L m^-2 h^-1).Based on the separation process of emulsified droplets in the aerogel layer and the membrane layer,an emulsion separation mechanism of"demulsification,coalescence,and rejection"is proposed,which provides a new idea for the efficient separation of emulsions from large-pore superwetting materials.3.Preparation of wetting-switchable biomass-based separation materials and study on on-demand emulsion separation performance（1）In order to overcome the difficulty that a single wettability cannot satisfy different types of emulsions,waste biomass rapeseed pollen was selected as the oil-water separation medium,and the pollen with a porous hollow sphere structure was prepared by solvent extraction and calcination treatment technology.After further oxygen-barrier calcination and water vapor modification in turn,biomass-based separation materials with switchable wettability between high-hydrophobicity and superhydrophilicity were prepared.The separation performance of the material for oil-in-water and water-in-oil emulsions was studied respectively.The permeation fluxes for separation of toluene-in-water,chloroform-in-water,and chlorobenzene-in-water emulsions could reach 6874,5955 and 6732 L m^-2 h^-1 bar^-1,and the emulsion separation efficiency is higher than 99.12%.Moreover,the separation flux can be as high as 11874L m^-2 h^-1 when separating water-in-chloroform and water-in-chlorobenzene emulsions,and the separation efficiency is higher than 98.85%.After 10 cycles of emulsions separation,the separation efficiency is still higher than 97.5%,suggesting it has good recycling performance.The mechanism of the pollen filter layer adsorbing emulsified droplets to achieve emulsion separation is proposed.（2）In order to further realize the switching of rapid wetting properties,using waste biomass cellulose as raw material,a superhydrophobic Zn O/cellulose membrane layer was constructed using in-situ growth and hydrophobic modification technology,and ultra-long Mn O₂ nanowires were further deposited on the surface of the membrane layer,forming Janus membrane with a superhydrophobic membrane side and a superhydrophilic membrane side.The mechanical entanglement between Mn O₂nanowires and Zn O/cellulose can better realize the compatibility of Zn O/cellulose membrane layer and Mn O₂ nanowire layer.The Janus membrane with opposite wettability realizes the conversion of wetting properties and the on-demand separation of emulsion through membrane inversion,for chloroform-in-water,toluene-in-water,diesel-in-water,and hexadecane-in-water emulsions.The permeation fluxes were 8380,11550,12320,and 13740 L m^-2 h^-1 bar^-1,respectively.The separation efficiency of Janus membranes for these emulsions is as high as 99.8%,and the oil content in the filtrate is less than 18 ppm.After 5 cycles,the Janus membrane still maintains a stable permeation flux,indicating that the Janus membrane has potential application prospects in the field of actual oil-water separation.