Activated carbon possesses both a crystalline structure and an intricate pore architecture, alongside a distinct chemical composition on its surface. The adsorption capacity of activated carbon is influenced not only by its physical (pore) characteristics but also by the chemical nature of its surface. During the preparation process of activated carbon, the edge bonds of aromatic sheets that form during the carbonization phase break, resulting in edge carbon atoms with unpaired electrons. These edge carbon atoms exhibit unsaturated chemical bonds and can interact with heteroatoms such as oxygen, hydrogen, nitrogen, and sulfur to generate various surface functional groups. The presence of these functional groups significantly impacts the adsorption performance of activated carbon. X-ray studies have demonstrated that these heteroatoms bond with the edge carbon atoms in the aromatic sheets to create surface compounds containing oxygen, hydrogen, and nitrogen.
When these edges become the main adsorption surfaces, these surface compounds alter the surface characteristics and properties of activated carbon. The surface groups of activated carbon are divided into three types: acidic, alkaline, and neutral. The acidic surface functional groups include carbonyl, carboxyl, lactone, hydroxyl, ether, phenol, etc., which can promote the adsorption of alkaline substances on activated carbon; The basic surface functional groups mainly include Pyrone (cyclic ketone) and its derivatives, which can promote the adsorption of acidic substances on activated carbon.
The surface of activated carbon prepared with acidic activators such as phosphoric acid is mainly composed of acidic groups, which adsorb alkaline substances well; The surface of activated carbon prepared with alkaline activators such as KOH and K2CO3 is mainly composed of alkaline groups, which are suitable for adsorbing acidic substances; The surface functional groups of activated carbon prepared by physical activation methods such as CO2 and H2O are generally neutral.