The activated carbon has crystal structure and pore structure inside, and the surface of activated carbon also has certain chemical structure. The adsorption performance of activated carbon not only depends on the physical (pore) structure of activated carbon, but also depends on the chemical structure of activated carbon surface. During the preparation of activated carbon, the chemical bonds at the edges of the aromatic sheets formed in the carbonization stage are broken to form edge carbon atoms with unpaired electrons. These edge carbon atoms have unsaturated chemical bonds and can react with heterocyclic atoms such as oxygen, hydrogen, nitrogen and sulfur to form different surface groups. The existence of these surface groups undoubtedly affects the adsorption performance of activated carbon. X-ray studies show that these heterocyclic atoms combine with carbon atoms at the edge of the aromatic sheet to produce surface compounds containing oxygen, hydrogen and nitrogen. When these edges become the main adsorption surfaces, these surface compounds change the surface characteristics and properties of activated carbon. The surface groups of activated carbon can be divided into acid, basic and neutral. The acidic surface functional groups include carbonyl, carboxyl, lactone, hydroxyl, ether, phenol, etc., which can promote the adsorption of basic substances on activated carbon; The basic surface functional groups mainly include pyranone (cyclic ketone) and its derivatives, which can promote the adsorption of acidic substances on activated carbon.
The surface of activated carbon prepared by acidic activators such as phosphoric acid is dominated by acidic groups, and it has good adsorption to alkaline substances; The surface of activated carbon prepared by KOH, K2CO3 and other basic activators is mainly composed of basic groups, which is suitable for adsorbing acidic substances; However, the surface functional groups of activated carbon prepared by physical activation methods such as CO2 and H2O are generally neutral.
