Various performance characteristics of activated carbon
Surface chemical modification of activated carbon: The chemical properties of activated carbon are mainly determined by chemical functional groups on the surface, surface heteroatoms and compounds. Different chemical functional groups, heteroatoms and compounds form different active centers on the surface of activated carbon, which are obvious to different adsorbates. The difference in adsorption. By surface chemical modification to change the surface acidity and alkalinity of the activated carbon, and introducing or removing certain surface functional groups, the activated carbon can have a certain adsorption or catalytic performance. As mentioned above, oxygen-containing functional groups and nitrogen-containing functional groups exist on the surface of activated carbon, and oxygen-containing functional groups act as centers of surface modification, which play an important role in chemisorption.
The oxygen-containing functional group can be further divided into an acidic functional group and a basic functional group, and the acidic functional group includes a shuttle group, a phenol hydrocarbon group, a awake-type sulfhydryl group, an inner ketone group and a cyclic peroxy group, among which a shuttle group, a phenol hydrocarbon group and a vine group are mainly Acidic functional group. At present, researchers have different views on basic functional groups, and are generally considered to be the Lewis base surface formed by the benzoin or ketone-containing structural group and the surface acidic oxygen-containing group. The acidic oxide makes the activated carbon have a polar property, which is favorable for adsorbing various polar compounds; the basic compound is easy to adsorb weak or non-polar substances. For example, the adsorption amount of phenol on activated carbon decreases with the increase of the amount of phenolic hydrocarbon groups on the activated carbon, and the adsorption amount of benzoic acid is related to the total amount of acidic groups on the surface of activated carbon.
Factors Affecting Liquid Phase Adsorption of Activated Carbon
Influencing factors of liquid phase adsorption of activated carbon: Adsorbable solute has a large adsorption range for inorganic substances. One extreme is dissociated salts, such as potassium chloride and sodium sulfate. These salts can be regarded as not being charcoal in application. At the other extreme, iodine is one of the most readily adsorbed substances known. Between these two extremes, various substances have different degrees of adsorption. The adsorption of some substances is accompanied by chemical changes. The published literature on the adsorption of organic matter from solution fully demonstrates that molecular structure is an important factor affecting the adsorption of activated carbon. The trend of influence is:
a. Aromatic compounds are generally more readily adsorbed than aliphatic compounds having similar molecular weights;
b. Branched compounds Generally, this linear compound is easily adsorbed;
c. The change of stereoisomers is inconsistent, the adsorption of fumaric acid (trans-maleic acid) is larger than that of acid (cis-maleic acid), but trans-diphenylethylene glycol Adsorbability is smaller than cis;
d. The right-handed and left-handed optical isomers have the same adsorptivity.
Activated carbon has a developed pore structure
Activated carbon has a developed pore structure, and the pore size distribution range is wide except for activated carbon molecular sieves. Therefore, various substances having different molecular sizes can be adsorbed, but the selective adsorption separation effect is poor. When the size of the adsorbate molecule is compatible with the pore size of the activated carbon, it is favorable for adsorption. Generally speaking, when the pore radius of the activated carbon is 2 to 4 times larger than the radius of the adsorbate molecule, it is most favorable for adsorption. "Compared with other adsorbents, activated carbon has a large specific surface area and particularly developed micropores. Usually, the specific surface area of ​​activated carbon is as high as 500~1700 m2/g, which is the main reason for the strong adsorption capacity of activated carbon and large adsorption capacity. Carbon with the same surface area sometimes has different adsorption capacities for the same substance, which is related to the pore structure and distribution of activated carbon and surface chemistry. Generally, the pore volume of activated carbon is about 0.15~0.9Ml/g, and the surface area accounts for the total surface area. 95% or more; transition pore volume is about 0.02~0.lmL/g, except for special activation method, the surface area does not exceed 5% of the total surface area; the large pore volume is about 0.2~0.5mL/g. The surface area is only 0.2~ 0.5m2 / g. In the liquid phase adsorption, the diameter of the adsorbate molecules is larger, such as the molecular diameter of the coloring component is more than 3 × 10-9, which has little effect on the micropores, and the adsorption capacity mainly depends on the transition pores.
Generally, the activated carbon with developed transition hole is favorable for liquid phase adsorption, because the liquid molecules are relatively large, and the activated carbon with micropores is favorable for gas phase adsorption. Since the pore structure is developed before and after the nanometer order, the low-concentration vapor which is exposed to the activated carbon is condensed by capillary condensation in the pores of the activated carbon, and becomes liquid filled in the pores. The use of heating, reducing the concentration of the gas phase in contact with the activated carbon, or impregnation with a solvent; the substance adsorbed on the activated carbon can undergo reversible desorption.
Effect of the size of adsorbate molecules on activated carbon
The effect of the size of the adsorbate molecules on the activated carbon: The adsorption amount of the activated carbon on the same group of compounds at low pressure (pressure less than 133.3 Pa) increases with the increase of the molecular weight of the compound. For example, the adsorption amount of activated carbon to ethers is as follows: methyl ether, ether, and propyl ether. However, when the pressure is increased to a certain value, the order of the adsorption amount is completely reversed, and it is changed to dipropyl ether, diethyl ether, and methyl ether.
When evaluating the relative adsorption of steam, it is related to the unit of measurement. If the amount of adsorption is calculated in grams, the adsorption amount of CCL14 is larger than that of chloroform. However, if the amount of the substance is based on the amount of the substance, The amount of adsorbed material (moi) is approximately the same.
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