cationexchange

Cation exchange refers to the reversible exchange of cations between a solid phase bearing negatively charged sites and the surrounding aqueous solution. The process is central to many natural and engineered systems, especially soils, clays, organic matter, and ion-exchange resins.

On minerals such as layered clays, negative charges arise from isomorphous substitution in the crystal structure, producing permanently charged surfaces. Exchangeable cations in solution (for example Ca2+, Mg2+, K+, Na+) adsorb to these sites, while cations on the surface are displaced into solution. The exchange reaction can be summarized as X-Na+ (solid) plus Ca2+ (solution) ⇌ X-Ca2+ (solid) plus Na+ (solution). Multivalent cations are typically held more strongly, and selectivity depends on the exchanger type and solution composition.

Cation exchange capacity (CEC) measures how many cations can be held by the solid per unit mass, expressed in cmol(+)/kg. CEC increases with minerals rich in permanent negative charge (such as certain clays and organic matter) and decreases as pH lowers. Techniques to determine CEC and extractable cations involve replacing exchangeable cations with a fixed cation such as ammonium and measuring the displaced ions.

Applications include soil fertility management, where buffering of nutrients and lime response are influenced by CEC; water treatment and softening, where resins exchange undesirable Ca2+ and Mg2+ for Na+ or H+. Limitations include saturation of exchange sites, dependence on ionic strength, and regeneration needs for resins.