On activated carbon adsorption technology and its application in water treatment

Activated carbon, as a relatively special carbonaceous material, is widely used in various fields such as environmental protection due to its developed pore structure, huge specific surface area, good stability, strong adsorption capacity and excellent regeneration ability. Focus on the application of activated carbon adsorption technology in water treatment.

1. The physical and chemical properties of activated carbon

1.1 Activated carbon (AC)

Activated carbon is a commonly used non-polar adsorbent with stable performance and corrosion resistance, so it is widely used. It is a general term for carbon-based substances with adsorption properties. The carbon-containing organic matter is heated and carbonized to remove all volatiles, and then activated by chemicals (such as ZnCl2, etc.) or water vapor to form a porous carbon structure adsorbent. There are two types of activated carbon, powdered and granular, and granular activated carbon is mostly used in industry.

Due to the different raw materials and preparation methods, the pore size distribution is different, generally divided into: carbon molecular sieve, pore size below 10×10-10m; activated coke, pore size below 20×10-10; activated carbon, pore size below 50×10-10m .

1.2 Activated Carbon Fiber (ACF)

Activated carbon fiber is a new type of adsorption functional material, which uses lignin, cellulose, phenolic fiber, polypropylene fiber, pitch fiber, etc. as raw materials, through carbonization and activation. Compared with activated carbon, it has a unique microporous structure, higher outer surface and specific surface area, a variety of functional groups, and a smaller average pore diameter. It can be obtained in waste water, waste gas treatment, and water purification through physical adsorption and physical chemical adsorption. It is widely used.

The micropore volume of fibrous activated carbon accounts for about 90% of the total pore volume, and most of its micropore diameter is about 1nm, without excessive pores and macropores. The specific surface area is generally 600-1200m2/g, even up to 3000m2/g. Activated carbon fiber has a fast desorption regeneration rate, short time, and its performance is unchanged, which is better than activated carbon. Like activated carbon, activated carbon fiber has no selectivity when adsorbing. It is mainly used to adsorb organic pollutants and is generally used for comprehensive wastewater treatment in oil refineries.

2. The adsorption function and adsorption form of activated carbon

2.1 Activated carbon treatment

Refers to the related process of using activated carbon as adsorbent and catalyst carrier. Mainly used in deep purification of drinking water, urban sewage treatment, and industrial wastewater treatment.

2.2 Adsorption and form of adsorption

The effect of gathering solutes on the solid surface is called adsorption. The surface of activated carbon has an adsorption effect. Adsorption can be regarded as a surface phenomenon, so adsorption is closely related to the surface characteristics of activated carbon. Activated carbon has a huge internal surface and pore distribution. The effect of its outer surface area and surface oxidation state is small, and the outer surface provides many channels communicating with the inner cavities. The main function of surface oxides is to make the hydrophobic carbon skeleton hydrophilic, so that activated carbon has affinity for many polar and non-polar compounds. Activated carbon has surface energy, and its adsorption effect is caused by the imbalance of the forces on the carbon atoms forming the surface of the pore wall, causing surface adsorption. 

3. Application of activated carbon adsorption technology in water treatment

3.1 Overview of the application of activated carbon adsorption technology in water treatment

Practice has proved that activated carbon is an ideal adsorbent for water and wastewater treatment. It has been a decade of research on activated carbon for water and wastewater treatment. In the past two decades, the problem of activated carbon regeneration has been satisfactorily solved, and the manufacturing cost of activated carbon has also been reduced. The activated carbon adsorption technology has gradually been promoted and used at home and abroad. smelly. In the early 1960s, European and American countries began to use activated carbon as an effective means of water purification. my country has used activated carbon in the treatment of carbon disulfide wastewater in the 1960s. Since the early 1970s, the treatment of industrial wastewater by granular activated carbon has developed rapidly in terms of technology, application scope and treatment scale. The treatment of oil refinery wastewater, explosives wastewater, printing and dyeing wastewater, chemical wastewater, electroplating wastewater, etc. has formed a large-scale application in production and has achieved satisfactory results.

3.2 Application of activated carbon in wastewater treatment

Activated carbon has different forms. At present, the water treatment is still mainly in granular and powder form. Powdered carbon is used for intermittent adsorption, that is, add powdered carbon to the water to be treated in a certain proportion, mix it evenly, and separate the carbon from water by precipitation or filtration. This method is also called static adsorption. Granular carbon is used for continuous adsorption. The treated water passes through the carbon adsorption bed to purify the water. This method is exactly the same as the fixed bed in form, and is also called dynamic adsorption. There are many substances that can be adsorbed by activated carbon, including organic or inorganic, ionic or non-ionic. In addition, the surface of activated carbon can also play a catalytic role, so it can be used in many different occasions.

Activated carbon has a strong adsorption capacity for soluble organic matter in water, and it is effective in removing most organic pollutants in water, such as phenol and benzene compounds, petroleum and many other synthetic organic matter. Some organic pollutants in water are difficult to remove by biochemical or oxidation methods, but they are easily adsorbed by activated carbon.

Because the cost of activated carbon adsorption treatment is higher than other general treatment methods. Therefore, when the concentration of organic matter in the water is high, other economical methods should be adopted to reduce the content of organic matter to a certain extent before processing. In wastewater treatment, the activated carbon adsorption process is usually placed after the biochemical blowing, called activated carbon tertiary wastewater treatment, to further reduce the content of organic matter in the wastewater, remove those pollutants that are not easily decomposed by microorganisms, and make the water after the activated carbon treatment It can meet the requirements of discharge standards, or the treated water can be returned to the production process for repeated use, so as to achieve the purpose of closed circulation of production water.

The ability of activated carbon to adsorb organic matter is very large. In tertiary wastewater treatment, the COD adsorbed by each gram of activated carbon can reach dozens of percent of its own mass. The addition of three-stage wastewater treatment in the wastewater treatment plant can make the removal effect of BOD reach 95%. Activated carbon removes organic matter in water in the form of physical adsorption, and the adsorbed properties remain unchanged before and after adsorption. If appropriate desorption methods can be used, valuable substances in water can be recovered. If the powdered activated carbon is put into the gas explosion equipment, the carbon powder and microorganisms form a kind of agglomerate, which can make the treatment effect exceed the general two-stage biological treatment method, and the effluent quality is close to the three-stage treatment.

In addition, it can also make the activated carbon sludge dense and firm, reduce the turbidity of the effluent, and increase the hydraulic load of the secondary treatment. Powdered carbon can be added intermittently, and the treatment effect can be improved without increasing the investment in the tertiary treatment for the existing secondary treatment plant.

3.3 Application of powdered activated carbon in water supply treatment

Powdered activated carbon has been used in water treatment for about 70 years. Since the first use of powdered activated carbon in the United States to remove the odor generated by chlorophenols, activated carbon has become one of the effective methods for removing color, smell, taste and organic matter in water treatment. A large number of foreign studies on the adsorption performance of powdered activated carbon have shown that: powdered activated carbon contains organic substances in trichlorophenol, pesticides, trihalomethanes and precursors, and disinfection by-products such as trichloroacetic acid, dichloroacetic acid and dihaloacetonitrile. All have good adsorption effect, and the removal effect of color, smell and taste has been recognized.

Powdered activated carbon is widely used in Europe, the United States, Japan and other places. In the early 1980s, the United States used about 250,000 tons of powdered activated carbon in water supply treatment every year, and it has a trend of increasing year by year. In the late 1960s, my country began to pay attention to the problem of deodorization and deodorization of polluted water sources. Powder activated carbon has been tried in Shanghai, Harbin, Hefei, and Guangzhou.

The main characteristics of the application of powdered activated carbon are low equipment investment, low price, fast adsorption speed, and strong adaptability to short-term and sudden water pollution. The application of powdered activated carbon adsorption technology in water plants is a very promising technology. However, due to the failure to solve the limitations of the technology in application, it is still difficult to take advantage of the powdered activated carbon technology, resulting in the application of the technology cannot achieve practical results.

The application in the water plant must solve two major types of problems: theoretical basis and application. The problems that should be solved theoretically mainly include the following aspects:

1. According to the water quality of the raw water in the water plant, especially the distribution of the molecular weight of the organic matter, determine the carbon type of powdered activated carbon and the effect of different carbon types on the removal of organic matter;

2. According to the actual water quality of the water plant, determine the appropriate and reasonable dosing point and dosing method to solve the contradiction between powdered activated carbon and coagulant adsorption competition, and improve the efficiency of powdered activated carbon.

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