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Columnar activated carbon adsorption pretreatment technology refers to the method of removing pollutants in water by using the adsorption property or exchange action of substances. Among them, the most used are hydrophobic substances which have strong adsorption effect on organic pollutants and odors in water. However, after the columnar activated carbon participates in the coagulation and precipitation process, it remains in the sludge, and there is no good recovery and regeneration method at present, resulting in high treatment costs and difficult to popularize and apply. Although the clay mineral adsorbents are abundant in supply, cheap in price and have good adsorption properties, a large amount of clay is put into the coagulant to increase the sludge discharge of the sedimentation tank, which brings certain difficulties to the production and operation.France generally uses granular activated carbon in the drinking water treatment process, when used for odor removal and deodorization, the sand filter is placed before the activated carbon adsorption step, so that the water treatment effect is better, and the activated carbon life is more than three years. When activated carbon is used to remove all organic matter in the water, the sand filter is used as the second stage, and the activated carbon adsorption method or the biological activated carbon method used in combination with aeration is used as the first stage, so that the water treatment effect is increased by 20% to 40%.In order to protect the drinking water in a low-lying area from benzene, toluene, chloroform, dichloroethylene and other organic matter pollution, the United States Environmental Protection Agency has built two activated carbon adsorption devices, using the granular activated carbon produced by Calgore company, each adsorption tower is filled with activated carbon 9t, and the drinking water is treated every day (the water plant mixes surface water with groundwater) 4546t.In 1979, a dye factory in Dalian, China used cooling, settling, activated carbon adsorption, expanded limestone neutralization process to treat 1000-2000mg dinitrochlorobenzene containing 0.5% acid washing wastewater. The content of dinitrochlorobenzene in the effluent reached 5mg/L. The saturated carbon was desorbed with chlorobenzene, and was regenerated in the tower by purging steam regeneration.Activated carbon adsorption method is an effective way to treat water, in recent years, activated carbon water treatment application methods continue to deepen the research, and gradually widely used, but it should be emphasized that activated carbon adsorption method for sewage treatment, often used as a secondary treatment, or even tertiary treatment. Wastewater treatment in the world's industrial developed countries is characterized by a large proportion of secondary treatment, such as the United States and the former West Germany in more than 70%, the United Kingdom accounted for almost 100%, many countries have begun to study the application of sewage treatment, including activated carbon adsorption, the wastewater as a second water source to be used. In addition, it should be pointed out that the combined water treatment process, including activated carbon adsorption method, has better water treatment effect and wider application range.At present, the adsorbents used for water treatment are columnar activated carbon, diatomite, silica, activated alumina, zeolite and ion exchange resin. In recent years, some new adsorption materials have been developed, such as porous synthetic resin, columnar activated carbon fiber and so on.

First, the nature of activated carbon1. AdsorptionThe adsorption property is the primary property of activated carbon. Activated carbon has microcrystals that resemble graphite grains but are arranged randomly. During the activation process, pores of different shapes and sizes are generated between microcrystals. Assuming that the pores of activated carbon are cylindrical pores, the radius of the pores can be divided into two categories according to a certain method:(1) According to IUPAC score:Micropore <1.0nmCenter hole 1-25nmLarge hole >25nm.(2) According to habits:Micropore <150nmMiddle hole 150-20000nmLarge hole >20000nm.Because these pores, especially micropores, provide a huge surface area.The pore volume of micropores is generally only 0.25-0.9mL/g, the number of pores is about 1020 /g, and the surface area of all micropores is about 500-1500m2/g, which is usually measured by BET method, and there are also as high as 3500-5000m2/g. Almost 95% or more of the surface area of activated carbon is in micropores, so in addition to some macromolecules can not enter, micropores are an important factor in determining the adsorption performance of activated carbon. The pore volume of the middle hole is generally about 0.02-1.0mL/g, and the surface area can reach several hundred square meters, which is generally only about 5% of the total activated carbon silkworm species. Its function can adsorb steam, and can provide a channel for the adsorption into the micropore, and can directly adsorb larger molecules.The pore volume of large pores is generally about 0.2-0.5mL/g, and the surface area is only about 0.5-2m2/g. One of its functions is to enable adsorbent molecules to quickly penetrate into the smaller pores inside the activated carbon. Second, when the catalyst is used as a catalytic carrier, a small amount of catalyst is often precipitated in micropores, mostly in large and medium pores.The surface area of the activated carbon should include the internal surface area and the external area, in fact, the adsorption properties are mainly from the huge internal surface area, so it can not be mistaken for: grinding the activated carbon will significantly improve the surface area to improve the adsorption force.Many adsorption is reversible physical adsorption, that is, the adsorbed material is a fluid, at a certain temperature and pressure is adsorbed by activated carbon, at high temperature and low pressure adsorbed material and desorption, activated carbon inner surface return to its original state. This is a widely used physical adsorption, also known academically as van der Waals adsorption.2. ChemistryAdsorption of activated carbon in addition to physical adsorption, there are chemical adsorption. The adsorbability of activated carbon depends on both pore structure and chemical composition.Activated carbon not only contains carbon, but also contains a small amount of chemical binding, functional groups of oxygen and hydrogen, such as carbonyl, carboxyl, phenols, lactones, quinones, ethers. Some of the oxides and complexes contained on these surfaces are derived from the derivatives of raw materials, and some are formed by the action of air or water vapor during and after activation. Sometimes surface sulfides and chlorides are also formed. In the activation, the minerals contained in the raw material are concentrated in the activated carbon to become ash, and the main components of ash are alkali metals and alkali earth metal salts, such as carbonate and phosphate.The ash content can be reduced by washing or pickling.The inorganic composition of activated carbon can be seen from the analysis of four kinds of powdered carbon products in Table 3-1. (Schedule omitted)3, catalyticActivated carbon is associated with catalytic abandonment in many adsorption processes, showing the activity of catalyst. For example, activated carbon adsorbs sulfur dioxide by catalytic oxidation into sulfur trioxide.Due to the presence of specific surface oxygen-containing compounds or complexes, activated carbon has catalytic activity for a variety of reactions, such as the formation of phosgene from chlorine and carbon monoxide.Due to the formation of a complex between activated carbon and the carrier, this complex catalyst greatly increases the catalytic activity, such as palladium salt-holding activated carbon, even if there is no copper salt catalyst, the oxidation reaction of olefin can be catalyzed, and the speed is fast and the selectivity is high.Activated carbon can be used as catalyst carrier because of its well-developed porous structure, huge internal surface area and good heat resistance, acid resistance and alkaline resistance. For example, in the hydrogenation, dehydrogenation cyclation, isomerization and other reactions in organic chemistry, activated carbon is an excellent carrier of platinum and palladium catalysts.4. MechanicalThe following items represent the mechanical properties of activated carbon, which are valued by activated carbon users, especially by a large number of industrial users.(1) Particle size: Using a standard screening method to find the weight of activated carbon left in and through each sieve, indicating the particle size distribution.(2) Static density or pile density: the weight of activated carbon per unit volume of dietary pore volume and intergranular void volume.(3) Bulk density and particle density: the weight of activated carbon per unit volume of the pore volume without the void volume between the particles.(4) Strength: the crushing resistance of activated carbon.(5) Wear resistance: that is, wear resistance or anti-friction performance.These mechanical properties directly affect applications, for example: density affects container size; The thickness of powdered carbon affects the filtration; The distribution of carbon particle size affects the fluid resistance and pressure drop. Fragmentation affects service life and waste carbon regeneration.

The adsorption of solute molecules in water by powdered activated carbon is a complex process, which is the result of the combined action of several forces, including ion attraction, van der Waals force and chemical impurity force. According to the two-rate diffusion theory of adsorption, adsorption is a two-speed process consisting of two stages of rapid diffusion and slow diffusion, and rapid diffusion is completed within a few hours, playing 60% to 80% of the adsorption capacity of activated carbon. Rapid diffusion is a process in which solute molecules are diffused in large pores with low resistance distributed radially in carbon particles. These large pores create radial diffusion resistance. When the molecule diffused further from the macropore into the micropore connected with the macropore, it was extremely slow due to the great resistance generated by the narrow aperture. The micropores are also evenly distributed within the carbon particles, but do not constitute radial diffusion resistance. The factors affecting the adsorption of powdered activated carbon involve the polarity, molecular weight and spatial structure of solute, which depend on the characteristics of water quality. Activated carbon has selective adsorbability to different substance molecules.