As a special carbon material, activated carbon is widely used in various fields such as environmental protection due to its developed pore structure, large specific surface area, good stability, strong adsorption capacity and excellent regeneration capacity. This paper will focus on the application of activated carbon adsorption technology in water treatment.
1. Physical and chemical characteristics of activated carbon
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 kind of carbon - based substance with adsorption property. Carbon containing organic materials are heated and carbonized, all volatile substances are removed, and porous carbon structure adsorbent is made after activation by drugs (such as ZnCl2, etc.) or water vapor. Activated carbon has powder and granular two, industrial use of granular activated carbon. Due to different raw materials and production methods, the pore diameter distribution is different, generally divided into: carbon molecular sieve, the pore diameter is 10×10-10m below; Active coke with pore diameter below 20×10-10; Activated carbon with pore diameter below 50×10-10m.
Activated carbon fiber (ACF) is a new type of adsorptive material, which is prepared by carbonization and activation with lignin, cellulose, phenolic fiber, polypropylene fiber and asphalt fiber as raw materials. Compared with activated carbon, it has a unique micropore structure, higher outer surface and specific surface area as well as a variety of functional groups, and a smaller average pore diameter. It has been widely used in the field of wastewater, waste gas treatment and water purification through physical adsorption and physical chemical adsorption. The micropore volume of fibrous activated carbon accounts for about 90% of the total pore volume, and most of the micropore diameter is about 1nm, without excessive pores or large pores. Specific surface area is generally 600 ~ 1200m2/g, or even up to 3000m2/g. Activated carbon fiber desorption regeneration rate is fast, short time, and its performance is unchanged, which is better than activated carbon. Similar to activated carbon, activated carbon fiber is non-selective in adsorption. It is mainly used for adsorption of organic pollutants and is generally used for comprehensive wastewater treatment in oil refineries.
2. Adsorption effect and adsorption form of activated carbon
Activated carbon treatment refers to the process of using activated carbon as adsorbent and catalyst carrier. It is mainly used for deep purification of drinking water, urban sewage treatment and industrial wastewater treatment.
2.2 adsorption and adsorption form
The aggregation of solutes on solid surfaces is called adsorption. Activated carbon has adsorption on the surface. Adsorption can be regarded as a surface phenomenon, so adsorption is closely related to the surface characteristics of activated carbon. Activated carbon has a large internal surface and pore distribution. Its surface area and surface oxidation state are less important, and the outer surface provides many channels to the inner hole. The main function of surface oxides is to make the hydrophobic carbon skeleton hydrophilic and the activated carbon has affinity for many polar and non-polar compounds. Activated carbon has surface energy, and its adsorption is caused by the imbalance of carbon atoms on the surface of the pore wall, which leads to surface adsorption.
The adsorption forms of activated carbon can be divided into physical adsorption and chemical adsorption. Physical adsorption is the adsorption of molecular forces, which is related to the interaction between dipoles and the weak van der Waals forces dominated by hydrogen bonds. It is strong enough to trap molecules in the liquid. Physical adsorption is caused by molecular force, and the adsorption force is small. Physical adsorption requires activation energy and can be performed at low temperatures. This adsorption is reversible. At the same time, the adsorbed molecules move away from the solid surface due to heat. This phenomenon is called desorption. Chemical adsorption combined with valence bond force is an exothermic process. Chemisorption is selective and ACTS only on one or more specific substances. Chemisorption is irreversible, stable and not easy to desorption. The adsorption process of activated carbon is divided into three stages. Firstly, the adsorbed material forms water film diffusion on the surface of activated carbon, which is called membrane diffusion, and then spreads to the internal pores of carbon, which is called pore diffusion, and finally adsorbed on the pore surface of carbon. Therefore, the adsorption rate depends on the diffusion of adsorbed material to the surface of activated carbon. In physical adsorption, the diffusion velocity in pores and the adsorption reaction velocity on the surface of carbon particles are mainly related to the first two terms.
3. Application of activated carbon adsorption technology in water treatment
3.1 overview of the application of activated carbon adsorption technology in water treatment
It has been proved that activated carbon is an ideal adsorbent for water and wastewater treatment. In the past 20 years, as the regeneration of activated carbon has been relatively satisfactory solution, at the same time, the manufacturing cost of activated carbon has also been reduced, activated carbon adsorption technology at home and abroad to gradually promote the use, the most commonly used is three-stage wastewater treatment and water deodorization. In the early 1960s, European and American countries began to use a large number of activated carbon adsorption water purification effective means. Activated carbon has been used in the treatment of carbon disulfide wastewater in China in the 1960s. Since the early 1970s, granular activated carbon treatment of industrial wastewater has developed rapidly in terms of technology, application range and treatment scale. It has been widely used in the treatment of oil refining wastewater, explosive wastewater, printing and dyeing wastewater, chemical wastewater, electroplating wastewater, etc.
3.2 application of activated carbon in water and wastewater treatment
Activated carbon has different forms, at present in water treatment is still in granular and powder two main. Powdered carbon is used for intermittent adsorption, that is, according to a certain proportion, the powdered carbon is added to the treated water, mixed evenly, by precipitation or filtration to separate the carbon, water, this method is also known as static adsorption. Granular carbon is used for continuous adsorption. The treated water is purified through the carbon adsorption bed, which is exactly the same as the fixed bed, also known as dynamic adsorption. Activated carbon can be adsorbed a lot of substances, including organic or inorganic, ionic or non-ionic, in addition, activated carbon surface can also play a catalytic role, so can be used in many different occasions. Activated carbon has a strong adsorption ability to dissolved organic matter in water, and is effective in removing most organic pollutants in water, such as phenols and benzene compounds, petroleum and many other synthetic organic matter. Some organic pollutants in water are difficult to be removed by biochemical or oxidation methods, but they are easy to be adsorbed by activated carbon. The cost of activated carbon adsorption treatment is higher than other general treatment methods. Therefore, when the concentration of organic matter in water is high, other economic methods should be adopted to reduce the content of organic matter to a certain degree before treatment. In wastewater treatment, usually put activated carbon adsorption technology behind biochemical blown, known as activated carbon tertiary wastewater treatment, further reduce the content of organic matter in wastewater, removal of the microorganism is not easy to decompose pollutants, make after dealing with the activated carbon of water can reach the requirement of discharge standard, or make the treated water can back to the production process of reuse, achieve the goal of production water closed cycle. Activated carbon adsorption capacity of organic matter is very large, in the three-level wastewater treatment, COD adsorption per gram activated carbon can reach its own mass of dozens of percent. The removal efficiency of BOD can reach 95% by adding three levels of wastewater treatment in the wastewater treatment plant. Activated carbon in the form of physical adsorption to remove organic matter in water, adsorption before and after the nature of the adsorption did not change, if appropriate desorption method can be used, but also can recover valuable substances in the water. If the powdered activated carbon is put into the blasting equipment, the carbon powder and microorganism form a kind of condensate, which can make the treatment effect exceed the general secondary biological treatment method, and the effluent quality is close to the tertiary treatment. In addition, activated carbon sludge can be made rigorous and firm, reduce the effluent turbidity, improve the hydraulic load of secondary treatment. Powdered charcoal can be added intermittently to improve the treatment performance of existing secondary treatment plants without increasing the investment in tertiary treatment.
3.3 application of powdered activated carbon in feed water treatment
The application of powdered activated carbon in water treatment has a history of about 70 years. Since the first use of powdered activated carbon in the United States to remove the odor of chlorophenol, activated carbon has become one of the effective methods to remove color, smell, taste and organic matter in water treatment. A large number of studies on the adsorption performance of powdered activated carbon in foreign countries have shown that the powdered activated carbon has a good adsorption effect on the organics contained in trichlorophenol, pesticide, trihalomethane and its precursor, as well as disinfection by-products trichloroacetic acid, dichloroacetic acid and dihaloacetonitrile, and the removal effect on 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 annual use of powdered activated carbon in water treatment in the United States was about 250,000 tons, with a trend of increasing year by year. In the late 1960s, China began to pay attention to the problem of odor and odor removal of polluted water sources. Powdered activated carbon has been tested in Shanghai, Harbin, hefei and guangzhou. The main characteristics of powdered activated carbon application are low investment, low price, fast adsorption speed, and strong adaptability to short-term and sudden water pollution. Powdery activated carbon adsorption is a promising technology in waterworks. However, it is still difficult to give full play to the advantages of powdery activated carbon technology because of the limitation of the technology in application. Application in waterworks must solve two kinds of problems: theoretical basis and application. Theoretically, the following problems should be solved:
1. According to the water quality of raw water in the water plant, especially the distribution of molecular weight of organic matter, determine the influence of powdered activated carbon and different kinds of activated carbon on the removal effect of organic matter;
2. According to the actual water quality of the water plant, determine the appropriate and reasonable adding point and adding method, in order to solve the contradiction between powdered activated carbon and coagulant adsorption competition, improve the use efficiency of powdered activated carbon.
3.4 application of granular activated carbon in deep treatment of drinking water
Because activated carbon has excellent adsorption characteristics to trace organic pollutants in water, as early as the early 1920s, foreign countries began to use powdered activated carbon to remove the odor and flavor of water. In 1930, the first use of granular active carbon adsorption pool deodorization water plant built in Philadelphia, USA in the late 1960 s and early '70 s, due to the large number of production and coal particles carbon regeneration equipment, developed countries carried out by using activated carbon adsorption removal of trace organic compounds in water research, carried out on the water depth processing, granular activated carbon purification plant in the United States, Europe, Japan and other countries were completed and put into operation in succession. In the United States, more than 90% of water plants using surface water as their source have adopted activated carbon adsorption process. There are currently hundreds of water plants in operation around the world that use granular carbon adsorption. At present, the most commonly used water treatment in foreign countries is the downflow activated carbon adsorption pool. It is reported that the maximum area of a single pool is 160m2, the thickness of a single layer of carbon is 0.7-2.5m, and the contact time of an empty bed is 6-20min.