Ion exchange phenomena

Ion-exchange phenomena

The clay minerals and the humus in the soil have the anion and the cation exchangeable capacity. Ion exchange process includes anion and cation process. The clay and the humus change to the colloidal state and expose a relatively large surface area for the adoption of the water and the ions. Thus, the process by which these ions are exchangeable between solid and liquid phase or between the solid phases, if in close contact with each other is termed as the ion exchange process. The common exchangeable cations are Ca⁺⁺,Mg⁺⁺, H⁺, K⁺and NH₄⁺ and the common exchangeable anions are SO4^--, Cl^-, PO₄^3-, NO₃^-. The cation exchange consists of the interchange between the cations absorbed on the surface of the soil and cations in the soil solution or those released by the plant roots.

Cation-exchange capacity

The total number of the negative sites in the given amount of the soil refers the cation exchange capacity of the soil which exchange sites for the adsorb and hold mostly Ca²⁺, Mg²⁺, K⁺ in the soil of pH more than 6.5. In this, the more acidic sites are occupied by the Al(OH)²⁺. Such adsorbed cations resist the leaching of the water but can be replaced by other cations. The strength of adsorption increases as the valency of cation increases or the strength of sites when negative charge increases.

The more the cation exchange capacity of the soil strength prevent the leaching of the nutrient and becomes available for the plant uptake. The cation exchange site adsorbed many heavy metals like Cd²⁺, Zn²⁺, Ni²⁺, Pb²⁺might be present in the waste water. The adsorption removes them from the percolating thereby cleaning the water that drains the ground water. The cation exchange capacity is expressed as the exchangeable cations per unit of the dry soil.

Mechanisms of the cation exchange

The clay colloids carry the negative charges as the cations are attracted to the clay particles. Such negatively charged particles and the organic matter or the clay is known as micelles. Micelles can be considered as the poly- anions as they carry negative charges. The cation exchange takes place when any cation is added to the soil as Ca⁺⁺, Mg⁺⁺, H⁺, K⁺ or NH₄⁺ through lime, potassium fertiliser or the ammonium sulphate respectively. Such phenomenon can also be observed if a small amount of the soil is leached by a solution of normal ammonium acetate. The filtrate contains exchangeable cations such as Ca⁺⁺, Mg⁺⁺,Na⁺since NH₄⁺ ions have replaced all these cations.

E.g. Ca (micelle) + 2H⁺→ H (micelle) + Ca⁺⁺

Factors affecting Cation Exchange Capacity

The cation exchange capacity in the given textural group the CEC also depends on the organic matter content and the kind of the clay. The cation exchange capacity in most of the soil increases with the pH increases. If the pH of the soil is low then, the permanent charges of the clay and the small portion of the charges of the organic colloids hold ions that can be replaced by the cation exchange.

Anion Exchange

Similar to the cations, the anions are adsorbed on the surface of the soil colloids and one anion may be replaced by the another one. In this, the anion exchange site arises from the protonation of the hydroxyl on the surface of the clay. The hydroxyl takes the hydrogen atom and gets protonated which can also adsorb many anions. The multi-charge positive ion like Fe and Al, have the hydroxyl that can be exchanged with the sulphate, phosphate, molybdate, nitrate, chloride etc. The highest anion capacity occurs in the amorphous clay (ash), aluminium, iron, hydroxide clay and the lesser in the silicate clay.

The soil anions are Cl^-,SO₄^--, Cl^-, PO_4^3-, NO₃^- and some heavy metals are MnO₄^2-, Cr₂O₇^2- and the charge associated with hydroxides of Fe²⁺, Al³⁺ give rise to the adsorption of the anion. The exchange of the phosphate and the sulphate is more complex due to the specific relation between the anion and the soil constituents. For the large cation exchange capacity, the AEC increases when the soil pH decreases.

Soil pH

pH is generally considered as the negative logarithm of the H⁺. The acidity (Concentration of the H⁺) and the basicity (Concentration of the OH^-) is measured in the pH unit and the range of the pH scale goes from 0 to 14. The range from 0 to 6.9 falls in the acid range and the range from 7.1 to 14 falls in the basic range and the pH 7 is known as the neutral point.

The cations found in or adsorbed in the exchange site of the soil colloid may be classified as the acidic cations giving the meaning of the acid forming and the basic cation giving the meaning of the base forming. The soil which is dependent on the basic cation saturation percentage is available on the exchange sites of the soil colloids. The soil alkalinity occurs when there is comparatively higher saturation percentage of the aluminium and the hydrogen ions, on the exchange sites of the soil colloids and in the soil solution. The hydrolysis of the carbonate produces the excess of the hydroxyl ion making the soil alkaline.

CaCO₃ + 2 H₂O → Ca²⁺ + 2OH^- + H₂CO₃

Here, the hydrolysis of the calcium carbonate is sufficient to account for the pH as high as 8.3. If the soil contains the sodium carbonate, there is greater hydrolysis and production of the hydroxyl and pH of the soil go as high as 10. The calcareous soil is saturated with the basic cations as the carbonates are leached out. This exchangeable basic cation also reacts with the water and produces hydroxyls by the hydrolysis.

Ca⁺⁺ (micelle) + 2H₂O → 2 H⁺(micelle) + Ca²⁺ +2OH^-

In the aluminium saturated soil, most of the exchangeable aluminium ion is strongly adsorbed to the sites, and it maintains the equilibrium with the soil solution. Al3+ in the solution hydrolyse to produce the hydrogen ion.

Al³⁺ + H₂O → Al(OH)²⁺ + H⁺

Al (OH)₂+ + H₂O → Al(OH)²⁺ + H⁺

So Al(OH)²⁺ and Al(OH)²⁺thus, formed may be adsorbed to the exchange site and in turn, may also hydrolyze.

Al(OH)₂⁺ + H₂O → Al(OH)₃ + 3H⁺

Importance of the soil pH

The pH of the soil affects the solubility of the mineral. Most of the minerals are more soluble in the acidic solution soils than in the neutral or slightly basic solution. Not only this, the soil pH effect on the activity of the beneficial microorganisms. Most of the nitrogen-fixing bacteria are not very active in the strongly acidic soil. The heterotrophs bacteria are also obstructed by the strong acidity. The strongly acidic soils have the high and toxic concentration of soluble Al and Mn. The soil basicity may also be undesirable for the plant. The plants on the soil having greater pH usually have reduced growth or even may die. It also reduces the solubility of the micronutrients.

References:

Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd.

T., R. (2008). Towards a Sustainable Future . India: PHI (p)Limited.

Keller, E.A.Environmental Geology. Columbus, Ohio: Charles E. Miller Publishing Company,Belland Howell Company, 1985.