Activated carbon is a porous carbon-containing substance. As a kind of adsorbent, it is currently used throughout the world. Activated carbon has a microcrystalline structure, but the arrangement is completely irregular: the height of the crystallite is 0.9-1.2 nm; the width is about 2.0-2.3 nm. Due to the high temperature treatment, its size will be significantly increased; meanwhile, micropores, transition pores or macropores will come into being during the activation process. According to the regulations of the International Union of Pure and Applied Chemistry (IUPAC 1972), the effective radius of micropores is less than 2nm; the effective radius of transition holes is 2-50nm; the effective radius of macropores is greater than 50nm. As we all know, the adsorption capacity of activated carbon is highly admired, so what is the adsorption principle of activated carbon?
The adsorption principle of activated carbon focuses on 2 main factors. The following information can be your reference.
Activated carbon has a developed internal pore structure, with a large number of micropores invisible to the naked eye. 1 gram of activated carbon material has micropores, and the surface area can be as high as 800-1500 square meters after unfolding. In addition, there are many pores on the surface and inside of the activated carbon that are connected to each other. There are adsorption forces in the entire space of the pores. These adsorption forces can bring those adsorbed molecules into the pores. It is these highly developed, human capillary-like pore structures that make activated carbon possess excellent adsorption properties.
- The mutual attraction force between molecules
Molecules have mutual attraction force, also called "van der Waals gravity". When a molecule is captured by the inner pores of the activated carbon and enters the pores of the activated carbon, due to the mutual attraction between the molecules, more molecules will be attracted continuously until the pores of the activated carbon are filled. Therefore, activated carbon has a strong adsorption capacity.
The adsorption of activated carbon can be divided into physical adsorption and chemical adsorption. Physical adsorption mainly occurs when activated carbon removes impurities in liquid and gas phase. The porous structure of activated carbon provides a large amount of surface area, making it extremely easy to absorb and collect impurities. Just like magnetism, all molecules have mutual attraction. Based on this, a large number of molecules on the pore wall of activated carbon can generate a strong attraction. It must be pointed out that the molecular diameter of these adsorbed impurities must be smaller than the pore size of the activated carbon. This is why we constantly change the raw materials and activation conditions to create activated carbons with different pore structures, which are suitable for various impurity absorption applications.
In addition to physical adsorption, chemical reactions often occur on the surface of activated carbon. Activated carbon not only contains carbon, but also contains a small amount of oxygen and hydrogen in the form of chemical bonds and functional groups, such as carboxyl groups, hydroxyl groups, phenols, lactones, quinones, and ethers. The oxides or complexes contained on these surfaces can chemically react with the adsorbed substances, and thus combine with the adsorbed substances to aggregate on the surface of the activated carbon. In summary, the adsorption of activated carbon is the result of the combined effect of the above two kinds of adsorption.