Adsorption also refers to the phenomenon of a substance (mainly solids) absorbing molecules or ions from the surrounding medium (liquid or gas) on its surface. Activated carbon adsorption is the process of removal from a liquid or gas by the adhesion of contaminants or impurities to the surface of activated carbon particles. Activated carbon is known for its high adsorption capacity due to its porous structure and large surface area.
Adsorption of activated carbon occurs in two main ways.
Physical adsorption #
This type of adsorption occurs when the force of attraction between the adsorbent molecules (the contaminant being removed) and the activated carbon surface is stronger than the force of the adsorbent molecules themselves. Physical adsorption is usually a reversible process in which the substance does not change its original properties during the adsorption process, so the adsorption energy is small and the adsorbed material can easily be detached again, e.g. by adsorbing a gas on activated carbon, the adsorbed gas can be expelled from the surface of the activated carbon by increasing the temperature.
Chemical adsorption #
In this case, a chemical reaction or bond is formed between the molecules of the adsorbate and the surface of the activated carbon or its functional groups. This type of adsorption is usually more specific and irreversible than physical adsorption and it often involves stronger forces such as covalent bonding, hydrogen bonding or electrostatic interactions.
Physisorption and chemisorption do not occur in isolation and often go hand in hand. In wastewater treatment technology, most adsorption is often the result of a combination of several adsorptions.
Factors that can influence the adsorption process include the properties of the activated carbon (e.g. pore size distribution, surface area), the properties of the adsorbate molecules (e.g. size, shape, concentration) and the conditions of the process (e.g. temperature, pH and the presence of other compounds).
Activated carbon adsorption has a wide range of applications as it is effective in removing a variety of contaminants such as organic compounds, chlorine, heavy metals and odours.