The activated carbon appearance is dark and has good adsorption properties. Its chemical properties are stable, strong acid and alkali resistance, high temperature resistance and low density than water. It is a porous hydrophobic adsorbent.
1. Pore structure and distribution of activated carbon.
In the process of manufacturing activated carbon, the volatile organic matter is removed and the space between the lattice is formed, forming many fine hole of different shapes and sizes. The specific surface area generally up to 500-700 ㎡ / g. This is main reason of activated carbon have high adsorption capacity and capacity.
The adsorption capacity of the same substance is sometimes different, which is related to the fine pore structure and pore distribution of activated carbon. According to the radius, there are three types of fine holes:Large and small holes, and the radius is 1000-100000Å;‚Transition hole, radius 20-1000Å; ƒSmall micropores with a radius of less than 20Å.
The small pore volume of activated carbon is 0.15-0.9ml/g, and the surface area accounts for more than 95% of the total surface area of activated carbon. As a result, activated carbon is more developed than other adsorbents. The transition pore volume is generally 0.02 to 0.1ml/g, and the surface area accounts for 5% of the total surface area of activated carbon. Big pore volume average of 0.2 to 0.2 ml/g, surface area is only 0.5 to 2.0 g/㎡, physical adsorption of the liquid phase is not big, but as a catalyst carrier effect is significant.
In the process of adsorption microporous structure determine the adsorption capacity. Almost all surfaces are made of micropores. Coarse and fine adsorption channels are used in coarse and transitional pores, and their presence and distribution influence the rate of adsorption and desorption to a considerable extent.
2. Surface chemical properties of activated carbon.
The adsorption properties of activated carbon are not only affected by the pore structure but also by the chemical properties of activated carbon.
Carbon accounts for 70 to 95 percent of active carbon. There are also hydrogen and oxygen, which are present in the raw material, or are not completely carbonized in the process of the carbonized and remain in the active carbon, or in combination with chemical bonds when activated. Ash constitutes the inorganic part of activated carbon. The content and composition of ash content are different from that of activated carbon, and the ash content of coconut shell is about 3%, and the ash content of coal charcoal is as high as 20% to 30%. The ash of activated carbon has catalytic effect on some electrolytes and non-electrolytes in activated carbon adsorption.
The hydrogen and oxygen in activated carbon have great influence on the adsorption and other properties of activated carbon. In the process of carbonization and activation, hydrogen and oxygen are combined with carbon to make the surface of carbon have various organic functional groups of oxide and hydride. The oxide makes the active carbon and the adsorption molecule have the chemical action, shows the choice adsorption. These organic functional groups includ carboxylic acid, phenolic alkyl, ether, carbonic acid anhydrous, cyclic peroxide and so on.
3. Adsorption and adsorption form
The action of aggregating the solute on the solid surface is called adsorption. Activated carbon surface has 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 huge internal surface and pore distribution. Its surface area and surface oxidation state are relatively small, and the surface area can provide a lot of passageways to the internal control point. Surface oxide has surface energy. The adsorption effect is caused by the imbalance of the carbon atoms on the surface of the pore wall, thus causing surface adsorption.
The adsorption form of activated carbon is divided into physical adsorption and chemical adsorption. The physical adsorption is dependent on the adsorption of the molecular forces, namely the weak Van Der Waals forces, which are mainly between the dipoles and the hydrogen bonds. It has enough strength to capture molecules in the liquid. Physical adsorption is caused by molecular attraction, and the adsorption heat is small. Physical adsorption requires activation energy, which can be carried out at low temperature. This adsorption is reversible, and at the same time, the adsorbed molecules are removed from the solid surface due to the thermal exercise, which is called desorption. The combination of chemical adsorption and valence bond force is an exothermic process.
The chemical adsorption process is divided into three stages. The first is that the adsorption material forms the water film diffusion on the surface of activated carbon, which is called membrane diffusion, and then diffuses into the internal pores of the carbon, known as pore diffusion, and finally adsorbed on the pore surface of carbon. Therefore, the adsorption rate depends on the diffusion of the adsorbed to the surface of the activated carbon. In the physical adsorption, the diffusion velocity and the adsorption rate on the surface of the carbon particles are mainly related to the membrane diffusion and pore diffusion.
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