Mechanism And Application Of Inorganic Porous Membranes In Liquid Separation

In recent years,the frequent marine oil-spill accidents during offshore oil exploration and transportation processes as well as the oil pollution caused by the textile industry,food manufacturing industry and petrochemical industry have become one of the most pressing global environmental problems.Therefore,the separation of immiscible liquids represented by the oil/water separation has attracted widely spread attention.Along with the deep understanding and learning of special wetting phenomena in nature,a large number of advanced functional materials with superwettability have been developed to separate the immiscible liquid mixtures.Comparing with the traditional liquid separation materials,these advanced materials exhibit high flux,excellent selectivity and recyclability.However,the separation of immiscible non-aqueous liquids has been less explored.The separation of immiscible non-aqueous liquids with a smaller difference in surface free energy(SE)than that between water and most oils is more difficult,especially for the challenging separation of immiscible liquids with a extremely low difference in SE(~ 2 m J m-2).Current liquid separation strategies are primarily based on covalent modification to control the membranes’ SE,or are based on gating mechanisms by accurately tuning the gating threshold of the transport substance.Inorganic porous materials have been widely used in the area of catalysis,adsorption,separation and biological medicine due to their large pore volume and surface area,orderd pore structure,flexible and adjustable chemical composition and easy to functionalized surface.Especially inorganic porous membrane materials have found new applications in the fields of microfiltration,nanofiltration,reverse osmosis and pervaporization.Combing the advantage of inorganic porous materials with special surface wettability will be beneficial to explore and develop new and high efficient liquid separation materials.Under the guidance of this concept,we explore the application of inorganic porous membranes in liquid separation,and proposed a universal separation strategy based on regulating the polar and dispersive components of SE.Any immiscible liquids,even with a surface energy difference as small as 2 m J m-2,or emulsions stabilized by emulsifiers,and successive separation of multiphase liquids can be achieved.The main results are as follows:1.We develop a universal polarity-based separation protocol which can be used for effectively separation of arbitrary immiscible liquids,as well as for successive multiphase liquid separation.This separation protocol is based on interfacial polar interaction between liquids and membranes,without any external stimulation or covalent modification.Flexible Si O2–Ti O2 composite porous nanofibrous membranes(STPNMs)are prepared by electrospinning technology.The wetting behavior of the superamphiphilic STPNMs can be regulated by infusing a liquid with high polar component of SE(PSE)into the STPNMs and forming a relatively stable liquid-infusion-interface to repel the immiscible low PSE liquid.Even immiscible liquids with a surface energy difference as small as 2 m J m-2,or emulsions stabilized by emulsifiers can be effectively separated.Furthermore,the infused liquid can be directly substituted by another immiscible liquid with a higher PSE,affording successive separation of multiphase liquids.We further study the principle of the polarity-driven wetting behavior from the following aspects: the principle of minimization of a system’s free energy,components of SE of STPNMs,density functional theory and the desorption activation energies of liquid molecules on STPNMs,all of which prove that hierarchical porous structure and abundant polar hydroxyl groups in the inner and outer surfaces of STPNMs are the key factors of the above performance.2.We investigate the principle of designing surface with under liquid dual superlyophobicity(under water oleophobic and under oil hydrophobic)from the perspective of polar and despersive components of SE(PSE and DSE),and explore its application in selective and continuous liquid separation.First,the silicon nanowire arrays prepared via chemical etching method are used as the rough surface,and clean silicon wafers are used as the smooth surface.Various functional groups with different PSE and DSE are modified on these two surfaces respectively.We find that the under liquid wettability of surface depends on the ratio of PSE to DSE(f): the lower f value,the more oil-affinity of the material;the higher f value,the more water-affinity of the material;and when f value is in a suitable range(0.390 ~ 0.993),the under liquid dual superlyophobic surface can be formed cooperating with substrate’s rough structure.Guided by this theory,the composite porous nanofibrous membranes with under liquid dual superlyophobicity can achieve selective and continuous liquid separation,and the separation efficiency of both water and oil are above 99.5 %.3.We prepare amino-functionalization composite porous nanofibrous membranes via vapor deposition method for the removal of both oil and heavy metal ions from wastewaters.The superhydrophilicity and underwater superoleophobicity endow the membranes excellent oil removal efficiency,whether oil slick or emulsified oil.Meanwhile,the modified amino group can selectively absorb various heavy metal pollutants in water,and hierarchical porous structure endows the membranes with excellent adsorption capacity.Composite membranes with suitable modification time(15min)maintain the under oil superhydrophilicity,which is beneficial to clean the oily contamination by washing with water.Therefore,the amino-functionalization composite porous nanofibrous membranes can efficiently remove both oil and heavy metal ions(Pb,Cr,Ni)from simulated industrial wastewaters to meet the national sewage disposal standards.In addition,the used membranes can be regenerated by washing with HCl solution and show good recyclability.