Anion Exchange: Meaning, Factors and Importance | Soil Colloids

After reading this article you will learn about:- 1. Meaning of Anion Exchange 2. Factor Affecting Anion Exchange 3. Importance.

Meaning of Anion Exchange:

Anion exchange on clay minerals and soils has not been studied like that of cation exchange. The effect of concentration, mole fraction and complementary ion on the distribution of exchangeable anions (H₂PO₄^– HPO₄⁼, CI^–, SO₄⁼, NO₃^–, MoO₄⁼) seems to be similar to that for cations.

Anion-exchange capacity has been defined as the capacity of a soil to adsorb or release anions under normal soil conditions. The cation to anion exchange capacities of different clay minerals were found to be:

Clay – CEC/AEC

Montmorillonite – 6.7

Illite – 2.3

Kaolinite – 0.5

The anion exchange is known to increase with adsorption of cations like zinc as Zn (OH)⁺ or ZnCl⁺ or both. The concentration of anions in the soil solution is affected by the solid phase. The overall negative charge of the exchange complex causes anions to be negatively adsorbed.

Under certain conditions hydrous oxides of iron and aluminium, allophone and even kaolinite show evidence of having positive charges on their crystal surfaces.

These charges arise from the following two sources:

(i) The protonation or adding of hydrogen ions to hydroxyl groups on the edge of these minerals.

At high pH values (left hand side) the hydrogen ion tends to dissociate from the oxygen, leaving a negative charge on the surface. As the pH is lowered (right hand side), an additional hydrogen ion associates with the hydroxyl, leaving a net positive charge.

(ii) The exchange of the hydroxyl (-OH) groups for other anions, e.g. phosphates and other similar anions (Anion exchange).

This reaction also occurs primarily at low pH values, the negative hydroxyl ion being replaced from positively charged aluminium ions in the crystal.

Factor Affecting Anion Exchange:

(i) Soil pH:

Contrary to cation exchange, the capacity for retaining anions increases with a decrease in soil pH.

(ii) Valency of Ions:

Anion exchange increases with the valence of the anion but decreases with increasing valence of the cation.

(iii) Type and Amount of Clay Colloid:

Anion exchange is much greater in soils high in 1: 1 type clay minerals and those containing hydrous oxides of iron and aluminium then that of soils containing high amounts of 2: 1 type of clay minerals. Clay minerals in the montmorillonite group of expansible layer silicates usually have anion exchange capacities of the less than 5 me/100 g. On the other hand, kaolinites can have an anion exchange capacity as high as 43 me/100 g at an acidic equilibrium pH of 4.7.

(iv) Salt Concentration:

Anion exchange generally decreases with increasing salt concentration. Addition of salts suppresses the unequal ion distribution.

In general the relative order of anion exchange is OH^–> H₂PO^–₄> SO₄^2-> NO₃^–.

Importance of Anion Exchange:

Among various anions, the exchangeable phosphate is of most importance since sulphates or nitrates are not retained in sufficient quantities. So the phenomenon of anion exchange is most important for the release of fixed phosphate in the soil and thereby increases its availability to plants. The anion exchange is carried out mainly by the replacement of OH ions of the clay minerals.

The reaction is as follows:

Such type of phosphate adsorption renders phosphate unavailable to plants. Since the anion exchange is reversible phenomenon a part of the adsorbed phosphate ions again become readily available when soils are dominated with OH ions i.e. means under high pH conditions (may be induced with liming).

The origin of OH ions is from silicate clay minerals and hydrous oxides of iron and aluminium that are present in the soil.

Large quantities of soluble phosphate fixed by these minerals as follows:

In this reaction takes place under slightly acidic soil conditions, the exchange phenomenon is reversible, and soluble phosphate is further released when hydroxy phosphate compound (insoluble) comes in contact with —OH ions.

In this reaction occurs under moderate to strong acid conditions, the phosphate ions are irreversibly fixed and not available to plants.