| Nowadays,the energy industry mianly relies on petroleum-based oil.Separating water/oil emulsions is of worldwide urgency due to frequent oil spills and increasing oil pollution from industrial wastewater.Scince the oil-water emulsions are stable and highly viscous,separating the oil-water emulsion is difficult.Membrane separation technology has the advantages of simple operation,low-cost and high separation efficiency.However,the membrane surface is easily fouled by oil and the pores can be plugged during the membrane separation.Synthetic polymers are currently the most common membrane materials;however,they are difficult to decompose and will pollute the environment throughout the manufacturing process.In view of above problems,the environmentally-friendly materials,cellulose fiber with high hydrophilic and degradable were used to prepare a series of underwater superoleophobic paper-based mambrane functional materials.Underwater superoleophobicity of the membrane was realized by chemical modification and micro/nano rough structure construction.Meanwhile,the material pore structure can be controlled by fiber morphology control,chemical crosslinking and pore formation,so as to achieve its oil-water separation performance.Futhermore,the relationship between pore structure,flux and separation performance of paper-based membrane materials were discussed.This paper firstly systematically studied the oil-water separation mechanism of paper-based membrane materials,and then prepared high flux paper-based membrane materials by selecting fiber materials.Then,in order to improve the filter fineness of paper-based membrane materials,a double layer of cellulose paper-based composite membrane was constructed.The paper-based membrane materials with high flux were fabricated by using a straight-through structure under high filter fineness.At the same time,the paper-based composite membrane with smart gate was further prepared.The mian work of this dissertation includes:（1）In order to investigate the relationship between paper-based membrane materials pore structure and surface hydrophilicity on the separation performance of emulsified oil with different oil droplet sizes,high wet strength and underwater superoleophobic cellulose paper-based membrane materials with controllable pore structure were prepared by fiber morphological control and chemical modification.The average pore size of the paper can be controlled from 6.64μm to 18.9μm as well as the water flux from25.8 L·m-2·h-1 to 4,920L·m-2·h-1under gravity.The underwater oil contact angle UOCA and wet strength of the paper can reach 167.8°and 36.5 N·m/g,respectively after BTCA modification.Paper-based membrane materials with low pore diameter and high carboxyl content can improve the separation efficiency of paper.For emulsified oil with the average oil droplet is 0.883μm,the separation efficiency can reach 99.3%.The zigzag pore structure of paper sheets,which is favorable to form liquid bridge on the surface as well as collision demulsification in the Z-direction channel,thereby promoting the mechanical interception effects.（2）In order to obtain high flux paper-based membrane materials,organic/inorganic hybrid membranes were constructed by utilizing the glass fiber and cellulose fiber.A stable cross-linking network can be formed between cellulose fibers of micro and nano size by hydrogen bond bonding,so that glass fibers can be inserted and filled to form composite membranes with interpenetrating network structure.Since the surface of glass fiber is smooth,it can effectively reduce the water resistance.Besides,the micro-sized cellulose fibers and nano-sized BC can effectively fix the glass fibers,making the composite membrane has good mechanical strength.The wet strength of the composite membrane can reach 6.55 N·m/g after maleic anhydride（MA）modification.Micro-nano roughness were constructed by plasma treatment,which realized the underwater superoleophobicity of the composite,and the UOCA was more than 150°.The separation efficiency of emulsified oil is more than 98.1%,and the flux of 12,070 L·m-2·h-1 can be obtained under gravity.In addition to mechanical interception,inertial collection occurs when an oil droplet,because of its inertia,cuts across the fluid streamlines and impinges on a fiber placed normal to the flow during the oil-water separation.The oil droplets then collide with each other on the membrane surface,thus eventually leading to the demulsification.（3）Since the separation efficiency of emulsified oil is low,a kind of all-cellulose composite membrane with support/barrier double layers was prepared by casting bacterial cellulose（BC）barrier layer on the filter paper.The pore structure was controlled by slow gelation and acidification.The crosslinking reaction between 1,2,3,4-butanetetracarboxylic acid（BTCA）and fibers was used to improve the paper wet strength and surface hydrophilicity.The UOCA is more than 150°.Combined with the micron pore structure of BC barrier layer,the composite paper shows good emulsified oil separation performance.The denser BC layer,the higher the separation efficiency and the lower the flux.With the increase of BC concentration from 0%to0.1%,the separation efficiency of soybean emulsified oil with average particle size of 13μm increases from 14.7%to 99.3%.Increasing the content of low-polar solvent in BC dispersion can improve the porosity of barrier layer.When the ratio of ethanol/water in BC dispersion is 80%,the flux of the composite paper can be significantly increased to 1320 L·m-2·h-1·bar-1,and the separation can reach 99.2%.（4）In order to solve the contradiction between pore structure flux and filter fineness,BC paper-based ultrafiltration membranes with high flux,high wet strength and antibacterial properties was prepared by using nano silica as poreogen and polyhexamethylguanidine polymer（PHGHE）as crosslinking agent and fungicide.Since BC and nano silica are both negatively charged on the surface,they have good compatibility in the water.Therefore,even if the addition amount of nano silica is high,the prepared membrane is uniform.A straight-throughout channel can be constructed after HF treatment.According to the Hagen-Poiseuille（H-P）equation,flux is inversely proportional to capillary length.The flux of the ultrafiltration membrane was increased to 7147 L·m-2·h-1·bar-1 by constructing Z-throughout channels in the membrane with high dagose of nano silica,and the effective pore size of the membrane was controlled below 90 nm.The wet strength of the ultrafiltration membrane modified by PHGHE is up to 4.47 MPa,and the antibacterial performance is excellent（antibacterial zone=15.0-15.2 mm）.The underwater superoleophobicity and nanosized pore of ultrafiltration membrane can effectively separate nanoparticles and emulsions.（5）In view of the problem that the pore structure of the paper-based membrane material is monotonous,paper-based smart gating materials were prepared for the first time inspired by the mechanism of stomatal opening and closing.Cellulose nanofiber（CNF）containing carboxyl group was modified by N-Isopropyl acrylamide（NIPAM）and mix with carbon nanotube（CNT）to vacuum filter to be smart gating membranes.The amide groups and carboxyl groups on modified CNF make it is thermal sensitive and p H responsive.After mixing with CNT,the photoresponsiveness of the membrane was realized through the photothermal effect of CNT.CNT was not only conducive to the construction of micro-nano roughness,but also created more nano channels in the membrane.The maxium UOCA of the membrane was increased to 162.3°,and the flux was 620±9 L·m-2·h-1·bar-1.At p H=1,20℃,the effective filtration size and pore size of the membrane increased from 28-48 nm to 48-70 nm compared with that at p H=7,20℃.At p H=7,45℃and p H=1,45℃,the effective filtration size and pore size of the membrane further increased to 70-91 nm.Due to the excellent underwater superoleophobicity,the separation efficiency of emulsified oils is greater than 99.6%. |
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