Design And Performance Study Of Hydrophilic Antifouling Separation Membranes

With the fast development of modern society,oil-related industrial activities and daily life generate tremendous amount of wastewater.Environmental pollution is thus becoming increasingly serious,and the shortage crisis of drinkable fresh water is continuously aggravated.High-performance separation membranes have gained more and more attention in the field of water purification with high efficiency and low energy consumption.However,traditional polymeric,ceramic and metal separation membranes usually have weak hydrophility due to their hydrophobic components and suffer from severe membrane fouling,resulting in quick decline of separation efficiency and permeation flux,as well as shortened service life.It has been proved an effective strategy to improve the hydrophilicity and antifouling ability of traditional membranes by introducing hydratable components on membrane surfaces.In this paper,we constructed some highly hydratable polymers on traditional membrane surfaces and fabricate a series of hydrophilic antifouling membranes via designing new surface chemicals and exploring new modification methods.We studied the oil/water separation performance and antifouling performance of the membranes towards highly viscous and polluting crude oil.Meanwhile,we systematically investigated the effects of polymer hydratability,the modification layer thickness and the interaction between polymer and membrane on the wettability,effective pore size,separation performance and long-term stability of the polymer-modified membrane.1.Ionized hydrogel embedded poly(vinylidene fluoride)(PVDF)membrane material.Hydrogel,a typical polymer that can form a large number of hydrogen bonds and have relatively strong hydratability,is an ideal material for modifying separatioin membranes to improve the hydrophilicity and antifouling ability.However,ordinary hydrogel-modified membrane materials are still susceptible to the adhesion and fouling by crude oil.In order to endow the hydrogel with stronger hydratability,and thus to endow the hydrogel-modified membrane with anti-crude oil-fouling ability,we ionized the hydrophilic groups of hydrogel to obtain more hydrogen bonds and more complete hydrogen bond network structure,and fabricated the ionized hydrogel-modified membrane material.In chapter 2,we fabricate the ionized hydrogel sodium polyacrylate-grafted PVDF(PAAS-g-PVDF)membrane material via a alkaline-induced phase inversion process,using polyacrylic acid-grafted-poly(vinylidene fluoride)(PAA-g-PVDF)powder as the raw material.The ionized hydrogel PAAS is embedded and modified in PVDF via covalent bond.Thanks to the synergistic effect of its enhanced strong hydratability and electronegativity,PAAS-g-PVDF exhibits underwater ultralow-oil-adhesive superoleophobicity to crude oil(negatively charged)with oil contact angle above 165°,sliding angle below 2.3°,nearly zero oil adhesion force and excellent antifouling ability in water.Molecular dynamics simulation demonstrates that ionized PAAS-g-PVDF induces stronger hydratability and more stable hydrated layer than unionized PAA-g-PVDF,and the tightly bounded hydrated layer forms a physical and energetic barrier to prevent the adhesion and fouling of oil.2.Ionized hydrogel/PVDF blend membrane.The ionized hydrogel PAAS embedded and modified PVDF membrane material shows excellent antifouling ability for crude oil.However,caused by the strong hydratability,the swelling behavior of the membrane material uaually gives rise to the variation of membrane pore structure,leading to the blocking of membrane pores and loss of membrane flux In order to modify the ionized hydrogel into traditional PVDF to obtain an antifouling membrane and avoid pore blocking,we should endow the membrane with moderate hydratability,that is,to accurately control the proportion of ionized hydrogel in membrane material.In chapter 3,we adopt the blending method that can precisely control the component proportion to quantificationally modify the ionized hydrogel PAAS into PVDF,and fabricate the PAAS-g-PVDF/PVDF blend ultrafiltration(UF)membrane.The quantitative embedding modification of PAAS in PVDF is thus realized.The blend membrane is fabricated via a two-step process including the first non-solvent-induced phase inversion of PVDF/PAA-g-PVDF blends and the subsequent in-situ ionization of PAA to PAAS.The hydrophilicity,underwater superoleophobicity,pore size and water flux of the blend membrane can be well controlled by adjusting the proportion of PAA-g-PVDF.The blend membrane can effectively separate crude oil-in-water emulsion with ultrahigh separation efficiency of 99.97%and high water flux similar to traditional UF membranes,simultaneously with good cyclic and antifouling performance.The membrane also exhibits good stability during a 3-year-long test.3.Thickness-controllable hydrogel modified PVDF membrane.Compared with surface grafting and blending,surface coating is an effective and widely used method for membrane modification.The challenge in modifying filtration membranes with ionized hydrogel is to design an ultrathin hydrogel layer with sufficient hydratability and controllable thickness,thus endowing the membrane with anti-crude-oil-fouling ability and not blocking the micro-and nanosized membrane pores.In chapter 4,we construct an ultrathin ionized hydrogel copper alginate(CuAlg)with precisely controllable thickness at the nanometer scale on a carboxylated PVDF membrane via a layer-by-layer self-assembly method.The CuAlg-modified PVDF UF membrane is thus fabricated.Both the nanosized pores and the high water flux of original carboxylated PVDF membrane are well maintained.The CuAlg-modified PVDF membrane behaves a superhydrophilicity,underwater superoleophobicity and antifouling ability for crude oil.The membrane is capable of efficiently separating crude oil-in-water emulsion with ultrahigh water flux of 1230 Lm-2 h-1bar-1,very high efficiency of 99.82%,and outstanding cyclic and antifouling ability.What's more,the membrane exhibits good salt-tolerance,antibacterial ability and long-term stability.4.Construction of highly stable hydrogel modification layer with gradient strong adhesivity and strong hydratability.Hydrogel modification layer is filled with a large amount of water molecules that occupy most volume of the hydrogel and greatly reduce the interaction sites between the hydrogel polymer chains and the substrate membrane.Therefore,the interaction force between the hydrogel and the membrane is usually weak,and the hydrogel layer is easy to peel off during use.Designing a hydrogel modification layer with both strong adhesivity and strong hydratability so as to achieve both the highly stable hydrogel/membrane interface and the strong hydrated protective layer is a key to construct separation membranes with stable anti-crude oil-fouling ability.In chapter 5,we design a hydrogel modification layer with gradient strong adhesivity and strong hydratability,which consists of gradiently distributed inner protocatechuic acid with strong adhesivity and outer calcium alginate with strong hydratability.A metal mesh membrane modified by the gradient hydrogel exhibits stable separation performance and nearly 100%flux recovery rate during 280-hour continuous separation of crude oil production fluid.The oil content in the filtrate water is as low as 12 ppm and the water flux is as high as 6500 Lm-2h-1.