How does the adsorption take place

Definition of adsorption

The accumulation of a substance on the surface of an adjacent phase is generally referred to as adsorption. In the case of solids (e.g. activated carbon), adsorption can take place both from the gas phase and from surrounding liquids (water). With absorption, molecules penetrate into the interior of the neighboring phase, e.g. storage of hydrogen in metals. Desorption is the reverse of adsorption processes.

In adsorption, a distinction is usually made between physical adsorption and chemisorption:

The physical adsorption is mainly caused by van der Waalsche forces. During this process, the adsorbed compound remains chemically unchanged. The physical adsorption is reversible, i.e. the adsorbed substances can be released from the surface in their original state under certain conditions.

In the Chemisorption a chemical bond occurs between the adsorbed substance and the surface, which changes the chemical nature of the adsorbed molecule. The chemisorption is not directly reversible.

Adsorbable substances

Organic and non-polar substances are generally adsorbed on activated carbon, for example organic substances: solvents (including chlorinated hydrocarbons), dyes and oil. The following are preferably adsorbed: higher molecular weight and non-polar compounds. The general rule is that with decreasing water solubility, volatility and polarity, as well as increasing molecular weight, the adsorbability increases.

Representation of adsorption

In an adsorption process, adsorption isotherms are used to display the level of adsorption as a function of the concentration of the substance to be adsorbed. The adsorption isotherm describes the state of equilibrium between "pollutant" in the liquid or in the air (residual concentration) and "pollutant" adsorbed on the adsorbent. The maximum load capacity can be read from this.

Adsorption in wastewater treatment

Adsorptive cleaning processes play an important role in wastewater technology. In wastewater treatment, adsorption and ion exchange processes are used to remove low concentrations of ingredients from relatively pure water. The recovery of materials also plays a role here. The procedural processes are the same as in cleaning operations for drinking and process water production.