Ion Exchange

Ion exchange in soil

Colloids are primarily responsible for chemical reactivity in soil. Each colloid has a net negative charge, making it possible for the colloid to attract and hold positively charged particles (cations) such as $Na^+$, $H^+$, $Ca^{2+}$, and $Mg^{2+}$. These cations are attached to colloids and in the soil solution. When one of the cations in the soil solution replaces one of the cations on the soil colloids, cation exchange takes place. This exchange occurs only when the cations in the soil solution are not in equilibrium with those on the soil colloid.

Definition

Ion exchange: Ion exchange is a reversible reaction involving an interchange of ions between ions in the soil solution and ions on the surface of soil colloids. It can be anion exchange or cation exchange.

Cation exchange: This refers to the interchange between a cation in a solution and another cation on the surface of negatively charged materials such as clay or organic matter in the soil.

Anion exchange: Since soil colloids are negatively charged, they cannot attract and hold negatively charged particles (like $SO_4^{2-}$ and $NO_3^-$). Nitrate ($NO_3^-$), being negatively charged like soil colloids, is leached more from the soil, while ammonium ($NH_4^+$), being positively charged, is attracted and held by soil colloids.

Mechanism of ion exchange

Ion exchange in the soil is explained by the electron-kinetic theory of ion exchange. According to this theory, the negative and positive charges associated with soil colloids (clay minerals and organic matter) are balanced by electrostatic attraction of cations and anions. The balancing ions are called exchangeable cations or anions.

Exchangeable cations and anions form outer sphere complexes with charged surfaces, where water of hydration exists between the charged ion and the oppositely charged colloids. These adsorbed ions undergo oscillations, and in the presence of a solution of an electrolyte, an ion from the soil solution enters between the inner charged layer and the outer oscillating ion, thus initiating ion exchange.

Factors affecting composition of exchangeable ions

1. Strength of adsorption
2. Relative concentration of the ion in the soil solution

Cation exchange capacity (CEC)

Cation Exchange Capacity (CEC) is the maximum quantity of total cations that a soil can hold at a given pH value, available for exchange with the soil solution. CEC is a measure of the number of negatively charged sites on soil surfaces capable of retaining positively charged ions by electrostatic force.

Significance of CEC

CEC is useful for determining soil fertility, nutrient retention capacity, and the capacity of soil to protect groundwater from cation contamination. Soils with a higher CEC have better fertility and nutrient retention and are more resistant to cation leaching.

Expression of CEC value

CEC is expressed in terms of the number of equivalents (or milliequivalents) of cations per 100 grams of dry soil. For example, 100g of soil can have CEC values expressed as meq/100g.

Factors affecting CEC

1. Amount of clay in soil: High-silicate clay soils hold more exchangeable cations than low-silicate clay soils, resulting in a higher CEC value.
2. Type of clay: Different clay types have different CEC values due to differences in surface area.
3. Soil organic matter: Organic matter is negatively charged and retains more cations, leading to a higher CEC.
4. pH of the soil: At low pH, excess $H^+$ ions reduce the ionization of colloids, lowering CEC. At high pH, more negative charge is generated on colloids, increasing CEC.

Percentage base saturation

Base saturation refers to the fraction of exchangeable cations that are base cations (such as $Ca^{2+}$, $Mg^{2+}$, $K^+$, and $Na^+$), which neutralize soil acidity. The percentage base saturation is calculated as:

Percentage base saturation = (Base cations / Total exchangeable cations) x 100%

The relationship between base saturation and acid saturation is:

Percentage base saturation + Percentage acid saturation = 100%

Sample problem: calculation of percentage base saturation

Given: A soil sample has a CEC of 25 meq per 100g. After shaking with 40 cm³ of 0.1M HCl and titrating the filtrate with 24.0 cm³ of 0.1 M NaOH, calculate the percentage base saturation.

Solution:

Remaining HCl reacts with NaOH, indicating HCl was in excess. The number of moles of $H^+$ (which attached to the soil colloids) is calculated as follows:

Remaining HCl = 0.004 – 0.0024 = 0.0016 moles of $H^+$

Now, we know 1 equivalent = 1 mole of $H^+$, so:

0.0016 eq was in 200g of soil
? equivalents in 100g
0.0016 eq = 200g
? = 100g
0.0016 x 1000 = 0.8 meq/100g

Percentage Base Saturation (P.B.S) = 3.2%

Example 4: soil test calculations

Given soil test: Nutrient concentrations are provided as meq/100g:

Ca$^{2+}$: 9.9

Mg$^{2+}$: 2.1

K$^{+}$: 2.0

Al$^{3+}$: 7.6

NH_4$$+: 0.6

Na$^{+}$: 0.1

i. Calculate the CEC of the soil:

CEC = (9.9 + 2.1 + 2.0 + 7.6 + 0.6 + 1.0) = 22.3 meq/100g

ii. Calculate the percentage base saturation:

Base cations = (9.9 + 2.1 + 2.0) = 14.0 meq

Acid cations = (7.6 + 0.6 + 0.1) = 8.3 meq

Percentage Base Saturation = (14.0 / 22.3) x 100 = 62.7%

iii. Calculate the percentage aluminium saturation:

Percentage Aluminium Saturation = (7.6 / 22.3) x 100 = 34.1%

Ion exchange in soil advanced examples

Let's explore some more advanced examples involving ion exchange in soil with detailed calculations.

Example 1: calculation of cation exchange capacity (CEC) from soil analysis

Given soil test: The nutrient concentrations in a soil sample are as follows:

Ca$^{2+}$: 5.0 meq/100g

Mg$^{2+}$: 3.5 meq/100g

K$^{+}$: 4.0 meq/100g

Na$^{+}$: 1.0 meq/100g

Al$^{3+}$: 2.5 meq/100g

NH_4$$+: 1.2 meq/100g

i. Calculate the CEC of the soil:

Solution:

CEC is the sum of the base cations and acid cations. Base cations include Ca$^{2+}$, Mg$^{2+}$, K$^{+}$, and Na$^{+}$, while acid cations include Al$^{3+}$ and NH_4$$+.

Base cations:

5.0 + 3.5 + 4.0 + 1.0 = 13.5 meq/100g

Acid cations:

2.5 + 1.2 = 3.7 meq/100g

CEC:

CEC = Base cations + Acid cations = 13.5 + 3.7 = 17.2 meq/100g

Example 2: calculation of percentage base saturation

Given: A soil sample has a CEC of 17.2 meq/100g. The following exchangeable cations are present:

Ca$^{2+}$: 5.0 meq/100g

Mg$^{2+}$: 3.5 meq/100g

K$^{+}$: 4.0 meq/100g

Na$^{+}$: 1.0 meq/100g

Al$^{3+}$: 2.5 meq/100g

NH_4$$+: 1.2 meq/100g

ii. Calculate the percentage base saturation (PBS):

Solution:

Base cations = (Ca$^{2+}$ + Mg$^{2+}$ + K$^{+}$ + Na$^{+}$) = 5.0 + 3.5 + 4.0 + 1.0 = 13.5 meq/100g

Percentage Base Saturation (PBS) is calculated as follows:

PBS = (Base cations / CEC) x 100
PBS = (13.5 / 17.2) x 100 = 78.49%

Example 3: calculation of percentage acid saturation

Given: The same soil test results as above with a CEC of 17.2 meq/100g. We need to calculate the Percentage Acid Saturation.

iii. Calculate the percentage acid saturation (PAS):

Solution:

Acid cations = (Al$^{3+}$ + NH_4$$+) = 2.5 + 1.2 = 3.7 meq/100g

Percentage Acid Saturation (PAS) is calculated as:

PAS = (Acid cations / CEC) x 100
PAS = (3.7 / 17.2) x 100 = 21.5%

Example 4: calculation of soil calcium concentration in meq/100g

Given: A soil sample weighs 50g and contains 0.3g of calcium (Ca). Calculate the concentration of calcium in meq/100g of soil.

iv. Calculate the concentration of calcium in meq/100g of soil:

Solution:

1 mole of calcium (Ca$^{2+}$) weighs 40.08g. The number of moles of calcium in 0.3g is:

Moles of Ca = 0.3g / 40.08g/mol = 0.00749 mol

Now, we convert moles to equivalents. Since calcium has a 2+ charge, 1 mole of Ca$^{2+}$ is equivalent to 2 equivalents. Therefore, the number of equivalents of calcium is:

Equivalents of Ca = 0.00749 mol x 2 = 0.01498 eq

Next, we calculate the concentration in meq (1 eq = 1000 meq):

Concentration of Ca = 0.01498 eq x 1000 = 14.98 meq

Now, to express the concentration in meq/100g of soil:

Concentration of Ca (meq/100g) = (14.98 meq / 50g) x 100g = 29.96 meq/100g

Example 5: calculation of soil's exchangeable cations

Given: A soil sample has a CEC of 30 meq/100g. The following exchangeable cations are found:

Ca$^{2+}$: 10 meq/100g

Mg$^{2+}$: 5 meq/100g

K$^{+}$: 2 meq/100g

Na$^{+}$: 1 meq/100g

Al$^{3+}$: 8 meq/100g

NH_4$$+: 4 meq/100g

v. Calculate the total amount of exchangeable cations in the soil:

Solution:

The total amount of exchangeable cations is the sum of all the cations present in the soil:

Total exchangeable cations = Ca2+ + Mg2+ + K+ + Na+ + Al3+ + NH4+
Total = 10 + 5 + 2 + 1 + 8 + 4 = 30 meq/100g

The total exchangeable cations equals the CEC value of the soil, which confirms the correctness of the given data.