Definition of Ion-Exchange:
Ion-exchange is defined as a reversible process by which cations and anions are exchanged between solid and liquid phases, and between solid phases it in close contact with each other. For the exchange of cations and anions the term is used as cation and anion exchange respectively.
Adsorption is defined as a phenomenon by which an increase in concentration or an accumulation of an ion species on a solid occurs due to ion exchange or other reactions. Desorption is a phenomenon by which the replacement or release of an adsorbed ion species occurs.
Chemical and physical processes are intimately related with ion exchange phenomenon which includes weathering of minerals, nutrient adsorption by plants, swelling and shrinkage of clay and leaching of electrolytes. Ion exchange particularly cation exchange, therefore, is considered as the most important process in soils.
Site for Ion Exchange:
The ion exchange property of a soil is almost entirely to the clay and silt fractions and the organic matter. So it is well known that the colloidal material of the soil with effective particle diameters of less than 20 microns being the most important site for causing the ion exchange phenomena.
The soil particles are in amphoteric nature and so soil particles have the power to hold both cations and anions. It is also evident that the charge density and the potential are higher on edges and corners and in furrows and cavities as compared to flat (plane) surfaces and consequently ion exchange phenomena are probably concentrated more in those sites.
Kinds of Exchangeable Ions:
The electric charge on the soil particles is neutralized by an equivalent amount of oppositely charged ions known as exchangeable, counter ions, which are held to the surface mainly by coulomb forces and van der Waals forces.
In most of the soils exchangeable cations are Ca2+, Mg2+, H+, K+, Na+ and NH4 of which Ca2+ is the dominant exchangeable ion. In strong acid soils Al(OH)2 may act as a dominant exchangeable ion. Whereas in alkali soils sodium ion (Na+) becomes dominant and the most common anions are SO42-, CI–, and HCO3– etc.
Factors Affecting Ion-Exchange:
There are various factors which affect ion exchange namely, nature and amount of clay, organic matter content mineralogical composition of the soil, soil reaction and liming etc. Ion exchange phenomena also affected by the irregular shape of the clay particles as well as non-uniform distribution of charges throughout the particles which changes the surface charge density of colloidal clays.
Clay minerals having 2: 1 expanding lattice type generally have high ion-exchange especially cation exchange than that of 1: 1 type of clay minerals (low surface charge density). The application of liming material in the acid soil decreases the rate of ion exchange reactions because of varying degree of ionization.
Organic matter contains different types of functional groups like â€”COOH (carboxyl) group, phenolic group (-C6 H4OH) and enolic groups (â€”COH = CH) etc. with varying amounts. So organic matters in soils containing more -COOH groups have the capacity of causing high ion-exchange.
The participation of different functional groups of organic matter in ion exchange reaction varies with the pH and the nature of the ions present in soils.
In addition, soils containing high amount of organic colloids have high ion exchange properties because of its high surface charge density that affects ion exchange phenomena. Sometimes it is found that the rate of ion exchange was found to be reduced due to organic matter and clay interactions.
The ion exchange capacity particularly cation exchange capacity of soils increases with the increase in soil pH (decreased activity of H+ ions in soil) because part of the exchange positions on the soil colloidal materials are pH dependent.
At low pH values, the numerous hydrogen ions in soil solution suppress the dissociation of hydrogen from most of the pH dependent sites, thus no exchange between these hydrogen’s and the displacing ions occurs.
At higher pH values, there are a very few hydrogen ions in soil solution, more of the hydrogen dissociates from the pH dependent exchange sites in the soil and the displacing ions become exchangeable form.