ionadsorption

Ion adsorption is the adhesion of ions from an aqueous or other liquid phase to the surface of a solid. It is a surface phenomenon and is distinct from absorption, which implies uptake into the bulk of the material. Adsorption of ions can be driven by electrostatic attraction to charged surfaces, by specific chemical bonding to surface functional groups, or by ligand exchange that forms inner-sphere complexes.

Mechanisms include outer-sphere adsorption, where ions remain hydrated and interact through electrostatic forces, and inner-sphere adsorption, where ions form direct bonds with surface atoms by displacing surface groups. The balance between these mechanisms depends on the surface charge, ionic strength, pH, and the presence of competing ions or organic matter. Ion exchange on clays and metal oxides is a common manifestation of ion adsorption in natural and engineered systems.

Common adsorbents include activated carbon, zeolites, layered clay minerals (such as montmorillonite), metal oxides (alumina, ferric oxide), and various nanomaterials. Adsorption is often described by isotherm models such as Langmuir and Freundlich, and kinetics by pseudo-first or pseudo-second order models. The process is influenced by temperature and is often described by adsorption capacity, selectivity, and affinity for specific ions.

Applications include water treatment and remediation (removal of heavy metals, arsenates, and nitrates), soil fertility management through cation exchange capacity, sensors, and catalysis. In soils, ion adsorption regulates nutrient availability and contaminant mobility. The term is closely related to electrostatic adsorption in electrochemical interfaces and to ion exchange processes.