According to the different adsorption capacity of the solid surface, the adsorption type of activated carbon can be divided into three types: physical adsorption, chemical adsorption and ion exchange adsorption:
Physical adsorption: The adsorption of adsorbent and adsorbate (solute) through molecular forces is called physical adsorption. This is a common adsorption appearance. Its characteristic is that the molecules of the adsorbent are not attached to fixed points on the surface of the adsorbent, but can move freely on the interface. Because adsorption is caused by molecular forces, the heat of adsorption is small, and physical adsorption does not require activation energy and can be performed under low temperature conditions. This kind of adsorption is reversible, and the molecules that are adsorbed together will be separated from the solid surface by the thermal motion of the sons. This appearance is called desorption. Physical adsorption can form a single-molecule adsorption layer or a multi-molecule adsorption layer. Because intermolecular forces are universal, one adsorbent can adsorb many kinds of substances, but because the properties of adsorbates (solutes) are different, the amount of adsorption is also different. This adsorption appearance is closely related to the surface area and pore dispersion of the adsorbent.
Chemical adsorption: The effect of the chemical bond between the adsorbent and the adsorbate (solute) causes a chemical reaction to form a strong connection between the adsorbent and the adsorbate (solute). Because the chemical reaction requires a lot of activation energy, it usually needs to be carried out at a higher temperature, and the heat of adsorption is larger. Chemical adsorption is a kind of selective adsorption, that is, an adsorbent only has adsorption effect on certain or specific substances. Because chemical adsorption is carried out by the direct chemical bond between the adsorbent and the adsorbate, chemical adsorption can only form a single layer. The adsorption is relatively stable and difficult to desorb. This adsorption is directly related to the chemical properties of the adsorbent. It is related to the chemical properties of the adsorbate.
AC adsorption: The ions of a substance gather on the charged points on the surface of the adsorbent because of electrostatic attraction. During the adsorption process, there is an exchange of equal ions, that is, each ion of adsorbate (solute) is adsorbed. The adsorbent must release an equal amount of ions together, that is, ion exchange. The charge of the ion is the determinant of AC adsorption. If the concentration of adsorbate (solute) is the same, the more charge the ion carries, the stronger its adsorption force on the counter-charge points on the surface of the adsorbent. Regarding ions with the same charge, the smaller the hydration radius, the more closely they can approach the adsorption point, which is conducive to adsorption. Physical adsorption, chemical adsorption and ion exchange adsorption often exist together. In the water treatment process of activated carbon adsorption method, three kinds of adsorption are used to achieve the purpose of removing pollutants. Regarding different adsorption substances, the effects of the three adsorptions are different.