Anionenstelle

Anionenstelle is a positively charged adsorption site on a solid that can bind and exchange anions in solution. The term is used in the context of ion exchange materials, including synthetic resins and certain minerals or modified surfaces. Such sites arise from functional groups bearing a permanent positive charge, for example quaternary ammonium groups in anion-exchange resins, or from positively charged surfaces at favorable pH ranges on some oxide minerals.

Mechanism and properties: Anions in solution can replace other bound anions at the anionenstelle, with the exchange driven by electrostatic forces and the relative selectivity of the material. The capacity of these sites is described by the anion-exchange capacity (Anionenaustauschkapazität) and is usually given in equivalents per gram. Selectivity depends on the resin or surface chemistry, the charge and hydration shell of the ions, and the solution composition and pH. Regeneration restores the original charged form of the site and releases the target anions, commonly using a strong base (for example NaOH) for resins that exchange anions.

Comparison and applications: Anionenstellen contrast with cationenstellen, which exchange cations and are typical of many natural clays and ion-exchange resins with negatively charged functional groups. Anion exchange is applied in water treatment and demineralization to remove nitrate, sulfate, phosphate, arsenate, and other anions, and is also used in chemical processing and food industry. In natural systems, positively charged surface sites on minerals or adsorbents can contribute to limited anion exchange under specific pH conditions.

Notes: The term may be used alongside related concepts such as Anionenaustauschstellen or anion-exchange resins, with specific chemistry varying by material.