Research progress on preparation and application technology of ceramic membrane for water treatment
Membrane technology is considered as one of the best water treatment technologies in the 21st century. Membrane material technology and membrane separation technology have been greatly developed in recent ten years and have been widely used in the field of water treatment. The filtration and separation performance of ceramic membrane for water treatment is closely related to its pore size and distribution, porosity and surface morphology. The active separation layer of ceramic membrane is formed by random accumulation of particles, and the porosity is usually 3 ~ 35%, and it is difficult to control the tortuosity factor, so the water treatment efficiency of ceramic membrane is limited. It is the research focus in the field of ceramic membrane for water treatment to study the new technology of preparation, modification and process optimization of ceramic membrane to improve its filtration, separation and anti-pollution efficiency.
1. preparation technology of ceramic membrane for water treatment
1.1 preparation technology of pore-forming agent
pore-forming agent is a simple and economical method to improve the porosity of water treatment ceramics, and pore-forming agents can be divided into inorganic substances and organic substances. Inorganic pore-forming agents include ammonium carbonate, ammonium bicarbonate and ammonium chloride, which are easily decomposed at high temperature, or inorganic carbon such as graphite and pulverized coal. Organic porogens mainly include natural fibers and high molecular polymers, such as sawdust, starch, polystyrene (PS), polymethyl methacrylate (PMMA) and so on. Yang et al. used Al2O3 as membrane matrix, bentonite as sintering aid and corn starch as pore-forming agent to prepare ceramic membrane through extrusion, crosslinking, drying and sintering. It was found that with the increase of starch content, the maximum pore size and average pore size of Al2O3 support increased, and the porosity of ceramic membrane increased from 24% to 38%.
1.2 template preparation technology
template can effectively control the morphology, structure and size of the synthesized materials, and prepare microporous, mesoporous and macroporous materials with orderly pore structure, uniform pore size and large porosity. Template method has rich material selection and flexible adjustment means, and it is very promising to prepare ceramic membrane for water treatment by template method. Xia et al. prepared three-dimensional ordered polyurethane macroporous materials with pore size of 1nm by UV polymerization with organic polystyrene microspheres as template. Sadakane et al. used PMMA as template to prepare metal oxide materials with three-dimensional ordered macropores with porosity ranging from 66% to 81%. Surfactants can form self-assemblies such as micelles, microemulsions, liquid crystals and vesicles in solution, and are often used as organic templates in self-assembly technology. Ordered mesoporous molecular sieve MCM-41 can be prepared by using surfactant cetyltrimethyl ammonium bromide as template, which has a variety of symmetrical pores with pore sizes ranging from 2 to 5 nm. Choi et al. used Tween8 as template to prepare TiO _ 2-Al2O3 ceramic membrane with gradient pore size structure, and the permeability of ceramic membrane was greatly improved.
1.3 fiber lamination preparation technology
ceramic fiber materials can quickly deposit bridges on the surface of the support during the film formation process, which obviously reduces the infiltration of the film layer, and easily obtains higher porosity and specific surface area, which has a significant effect on improving the permeability of the film material. Ke et al. used TiO2 _ 2 fiber as raw material to prepare ceramic fiber membrane with an average pore size of 5nm by spin coating method. The rejection rate of spherical particles was over 95% and the membrane flux was over 9Lm-2h-1.
1.4 Sol-gel preparation technology
Sol-gel technology mainly controls the separation accuracy of ceramic membrane separation layer by adjusting the material size. Sol-gel method can form nano-scale sol, and the obtained ceramic membrane has small pore size and narrow pore size distribution, which is suitable for the preparation of ultrafiltration membrane and nanofiltration membrane with high permeability selectivity. Tsuru et al. prepared TiO2 _ 2 nanofiltration membrane with an average pore size of .7 ~ 2.5 nm by polymerization sol route, and the molecular weight of PEG was 5 ~ Da, and the rejection rate of Mg2+ was 88%.
2. water treatment ceramic membrane modification technology
2.1 chemical vapor deposition modification technology
chemical vapor deposition (CVD) is a very effective method to deposit silicon oxide or metal oxide on the surface of ceramic membrane to improve its pore structure and filtration performance. Lin et al. modified Al2O3 ceramic membrane with an average pore size of 4nm by CVD technology, and prepared SiO2 ceramic membrane with a pore size range of .4 ~ .6 nm. Generally, CVD methods need to be carried out in high temperature and vacuum environment, and the precursors are required to have certain volatility.
2.2 atomic layer deposition modification technology
atomic layer deposition technology (ALD) can deposit substances on the surface of ceramic membrane layer by layer in the form of monatomic films, thus constructing micro-nano structure on the surface of ceramic membrane. Li et al. deposited alumina layer on the surface of ceramic membrane with an average pore size of 5nm by atomic layer, and adjusted the average pore size of the membrane by controlling the number of atomic layer deposition. After modification, the rejection rate of BSA by ceramic membrane increased from 2.9% to 97.1%.
2.3 surface grafting modification technology
surface grafting technology is often used to control the surface properties of membrane materials, and the grafting process will change the pore structure of the membrane and achieve the purpose of reducing the pore size. Generally, the surface of ceramic membrane will absorb water to form a large number of hydroxyl groups, and an organic molecular layer can be modified on the surface of mesoporous membrane by grafting organosilane. By adjusting the chain length and functional groups of grafted molecules, the pore size can be adjusted and special surface properties can be obtained. Singh et al. found that grafting silane coupling agent can further reduce the pore size of porous ceramic membrane. Cohen et al. grafted hydrophilic PVP on the surface of ceramic ultrafiltration membrane, and the pore size of the modified membrane decreased, the interception performance improved and the anti-pollution performance improved, which can be used for oil-water separation.
3. Optimization of preparation and modification process of water treatment ceramic membrane
3.1 Selection of ceramic membrane materials and additives
The preparation of water treatment ceramic membrane mainly focuses on raw materials and sintering process. Adding sintering AIDS to reduce sintering temperature, adopting low-cost easy-to-sinter raw materials to reduce raw material cost, and using advanced sintering process to achieve low-cost control are the research focuses of ceramic membrane. During the preparation of ceramic membranes, some liquid or solid sintering AIDS are often added to the basement membrane materials. Natural silicate clay minerals such as kaolin and potash feldspar can be melted at a low temperature to form a liquid phase, which wets and wraps the matrix particles of membrane material under the action of capillary force between particles, and binds the particles together, supplemented by the good mechanical strength of porous ceramic membrane. Metal oxides such as titanium oxide and zirconium oxide can form multi-oxide solid solution with ceramic membrane matrix, which will reduce the sintering temperature, which is beneficial to the preparation of ceramic membrane.
3.2 Optimization of firing process of ceramic membrane
Porous ceramic membrane must be sintered many times, which has the problems of long sintering process cycle and high energy consumption. In addition to using sintering AIDS or easy-to-sinter materials to reduce sintering temperature, reducing sintering time or shortening preparation period can also achieve the purpose of reducing sintering process cost. In the aspect of reducing sintering time, microwave sintering technology is a non-contact technology. Heat is transmitted by electromagnetic wave, which can directly reach the inside of the material, minimizing the non-uniformity of sintering, and reducing the sintering temperature while shortening the sintering time. Microwave technology is mostly used to prepare nearly dense ceramic composites, and it can also be used to prepare porous ceramic composites because it can improve the structure and properties of materials. In terms of shortening the sintering cycle, some researchers refer to the successful application of low-temperature sintering technology in the packaging field of multilayer ceramic components, and propose to adopt * * * sintering technology to reduce the sintering times, thus reducing the sintering cost.
4. Conclusion
The preparation technology of water treatment ceramic membrane aims at improving the overall performance of ceramic membrane, and the breakthrough of ceramic membrane preparation technology can be achieved by regulating the microstructure of ceramic membrane. At present, the preparation technologies of ceramic membranes, such as porogen preparation technology, template preparation technology, fiber lamination preparation technology, sol-gel technology and solid particle sintering technology, have attracted increasing attention. The research on the preparation technology of ceramic membrane for water treatment will lead and promote the development of ceramic membrane technology and industry, and alleviate the bottleneck pressure of upgrading and upgrading water plants and improving water quality. ;