A Brief Introduction to the Development History of Membrane Science and Technology

The observation of membrane separation phenomena can be traced back to the 17th century when scientists began to notice certain natural phenomena, such as osmosis. In 1748, the French scholar J.A. Nollet observed in experiments that water could naturally diffuse into a pig's bladder filled with an alcoholic solution, marking the earliest research on membranes and membrane processes. Subsequently, R.J.H. Dutrochet defined the phenomenon of osmosis in 1828, and J.H. Van’t Hoff proposed the osmotic formula and the theory of ideal solutions in 1887. These early studies laid the foundation for the development of membrane science. Since then, membrane science and technology have gradually grown and developed from basic theoretical research to the development of artificial membranes and then to a wide range of applications, eventually forming a multidisciplinary high-tech industry that has had a profound impact on modern society and industrial production.

Membrane separation technology refers to the selective separation of mixtures of molecules of different sizes at the molecular level when passing through a semipermeable membrane (separation membrane or filter membrane). According to the pore size of the separation membrane, it can be divided into: microfiltration membranes (MF), ultrafiltration membranes (UF), nanofiltration membranes (NF), reverse osmosis membranes (RO), etc. The separation characteristics of pressure-driven membrane processes are shown in Figure 1. Typically, the appropriate pore size separation membrane can be selected according to the size of the target molecules to achieve efficient separation. Based on the different membrane materials, they can be divided into organic membranes and inorganic membranes. Organic membranes are made of polymer materials, such as cellulose acetate, aromatic polyamides, polyether sulfone, polyfluoropolymers, etc. Inorganic membranes are divided into ceramic membranes, carbon membranes, and metal membranes, which have lower filtration accuracy and smaller selectivity. Membrane separation technology has the advantages of mild operating conditions, no phase change, no chemical reaction, good selectivity, simple operation, low energy consumption, and recyclable use.

Figure 1. Separation characteristics of pressure-driven membrane processes

Ultrafiltration is a membrane permeation method separation technology that can purify, separate, or concentrate solutions. The pore size is between microfiltration and nanofiltration, at 1-50 nm, and it belongs to pressure-driven membrane separation technology. The basic principle is the sieve separation process. The early research and experimental stage of ultrafiltration membranes can be traced back to 1861 when the German scientist Schmidt first published the experimental results of bovine heart cell membrane retention of soluble gum arabic, which was the earliest experiment on ultrafiltration membranes. Four years later, Traube made the world's first artificial ultrafiltration membrane. In 1907, Bechhold systematically studied ultrafiltration membranes and first used the scientific and technological term "ultrafiltration". In 1926, Membrane Filter GmbH developed commercial ultrafiltration membranes. After that, ultrafiltration membranes flourished, and in 1963, Michaels developed asymmetric cellulose acetate ultrafiltration membranes with different pore sizes. In 1966, the United States Amicon developed laboratory polysulfone and polyvinylidene fluoride ultrafiltration membranes. In 1967, Amicon developed the first hollow fiber membrane. In 1969, Abcor established a commercial tubular UF device for electrophoretic paint recovery. By the 1970s, ultrafiltration had developed from an experimental separation method to an important industrial separation unit operation technology. Our country began to study ultrafiltration technology in the 1970s, and after decades of development, it has gradually caught up with the international advanced level. Organic ultrafiltration membrane preparation methods are usually phase inversion method, stretching method, sintering method, nuclear track etching method, and composite membranes, etc. Ultrafiltration has a wider range of uses than microfiltration. In addition to removing particles, sub-particles, and fine particles, it can also remove enzymes, viruses, and other smaller particle substances, but it can hardly retain inorganic ions. Precisely because microfiltration membranes can remove viruses, their application in the biomedical field has attracted more and more attention in recent years.

Nanofiltration (NF) is a new type of membrane separation process driven by pressure between reverse osmosis and ultrafiltration. The retention range of nanofiltration is usually between 200 and 1000 relative molecular weights. Its development is relatively late and can be traced back to the late 1970s when J. E. Cadotte's research team began to study the NS-300 membrane (then called "low-pressure reverse osmosis membrane" or "loose reverse osmosis membrane"), marking the beginning of nanofiltration membrane research. In 1988, Erikassnon and others first clearly proposed the concept of "nanofiltration", and nanofiltration membranes had their own name. Subsequently, the development of nanofiltration technology has been rapid, and the membrane components were commercialized in the mid-1980s. Most nanofiltration membranes are derived from reverse osmosis membranes, including cellulose acetate (CA) membranes, cellulose triacetate (CTA) membranes, aromatic polyamide composite membranes, and sulfonated polyether sulfone membranes, etc. The emergence of nanofiltration technology has filled the technical gap between ultrafiltration and reverse osmosis and has become one of the hot topics in the field of global membrane separation research. Nanofiltration membrane research in our country began in the 1990s, and the earliest report on domestic nanofiltration membranes was in 1995. Common preparation methods for nanofiltration membranes include composite method, L-S phase inversion method, plasma treatment, chemical crosslinking, and polymer grafting, etc. The application range of nanofiltration technology is wide, including medicine, chemical industry, water treatment, metallurgy, petroleum, and many other fields, especially in the preparation of pure water, water treatment, seawater desalination, and brackish water desalination, etc.

Reverse osmosis technology usually operates under pressure higher than the osmotic pressure of the solution, allowing only water from the solution to pass through the membrane, while all large and small molecules of organic and inorganic salts in the solution are completely retained. The ideal reverse osmosis membrane should be considered poreless, and its separation principle is dissolution diffusion (there is also a capillary flow theory). The "membrane pore size" is 1-10 Å, and the pressure used is 0.6 to 10 MPa. The development history of reverse osmosis membranes can be traced back to the 1960s. In 1961, an American company first proposed the manufacturing method of the spiral membrane element; in 1964, the United States developed a spiral reverse osmosis element; in 1965, the University of California developed a tubular reverse osmosis device for brackish water desalination; in 1979, Filmtec Corporation applied for the world's first patent for the preparation of reverse osmosis membranes. Since the 1960s, reverse osmosis technology has increasingly attracted attention, and the development of reverse osmosis membranes has made significant breakthroughs. Membrane materials have evolved from the initial single-cellulose acetate asymmetric membranes to new types of materials and efficient membranes made by surface technology, such as cross-linked aromatic polyamide composite membranes. It was not until the late 1990s that our country began to master the production technology of reverse osmosis membranes. Over the past 30 years, reverse osmosis (RO) technology has been widely used in the preparation of pure water, desalination of brackish water, oily and degreasing wastewater, wastewater from the fiber industry, paper industry wastewater, radioactive wastewater, and other industrial water treatments, as well as in the pharmaceutical industry, semiconductor industry, and wastewater treatment of high-rise buildings.

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