Sorption
Sorption is a collective term for processes that lead to an accumulation of a substance within a phase or on an interface between two phases. The enrichment within a phase called precisely from absorption , which at the interface Ad sorption . In 1909, James William McBain introduced the generic term sorption for processes in which it is not possible to clearly differentiate between adsorption and absorption.
The sorbent is called the sorbent , sorbent or sorbent . The substance that has not yet been sorbed is called sorptive , while after sorption it is called adsorpt or absorptive . The system of added or stored (sorbed) substance together with the sorbent is called sorbate .
Classification
The causes of sorption are usually physical interactions between sorbent and sorbate. Sorption is an equilibrium process between the absorption and release ( desorption ) of a substance. If there is no equilibrium, an equilibrium is sought. Changes in concentration and temperature as well as displacement reactions influence the sorption and shift the position of the equilibrium. Sorption processes in which certain substances are preferentially (selectively) absorbed are of practical importance.
- Absorption describes an equilibrium process of the storage (and removal) of the sorptive from one phase into the volume of another phase. For example, there is an equilibrium between air (a gas phase) and water (a liquid phase), e.g. B. with regard to the oxygen content of both phases.
-
Adsorption is the attachment (and deposition) of a substance from one phase to the surface (more precisely: interface ) of another phase. In contrast to absorption, the internal volume of the sorbent remains unaffected and can be a solid or liquid phase. Important chromatographic processes ( adsorption chromatography ) as well as many gas filters ( respiratory protection filters ) are based on adsorption.
- Physisorption (physical adsorption) is the general case of adsorption and is based on physical interactions between the adsorbate and the interface.
- Chemisorption (chemical adsorption) is a special case of adsorption. Here there is a chemical bond between the adsorbate and the interface. This can lead to further chemical reactions and is an important part of heterogeneous catalysis . Chemisorption usually takes place at much higher temperatures than physisorption and is limited to solid interfaces. The terms physisorption and chemisorption are mostly used when describing certain processes in which a clear distinction should or can be made between chemical and physical reactions.
The terms desorption and desorbate are used both for the release of adsorbed and absorbed substances.
Measurement & description
The distribution of a substance between the sorbent (e.g. a solid) and its solution (e.g. an aqueous phase) can be determined by means of shaking tests. A state of equilibrium is established between sorption and desorption of a substance. The ratio of the substance concentrations in both phases is u. a. strongly dependent on the temperature. The mathematical description is done with sorption isotherms .
The sorption affinity of substances can be described in a simplified way with dimensionless and concentration- independent distribution coefficients , e.g. B. the distribution coefficient between the organic carbon in the soil and water (Koc or log Koc).
Environmental processes
Sorption, especially adsorption, plays an important role in the behavior of pollutants in the environment, e.g. B. of organic pollutants in the air (accumulation on aerosol particles ) or in the subsoil (accumulation on soil constituents or on colloids ). For example, pollutants can be bound to soil particles in the long term through sorption. Poorly water-soluble and / or poorly volatile pollutants can be transported over long distances by sorption on dust particles in the atmosphere or on suspended particles in the water (including soil seepage and groundwater).
Technical application
Sorption is used technically in absorption chillers and adsorption chillers . These chillers play z. B. in gas-powered camping fridges and solar air conditioning play a role. Sorption is also used in air drying with water-absorbing substances.