Cec

Cation Exchange Capacity

Mohsin ZafarLecturer

Basic Structural Units

0.26 nm

oxygen

silicon

0.29 nm

aluminium or magnesium

hydroxyl or oxygen

Clay minerals are made of two distinct structural units.

Silicon tetrahedron Aluminium Octahedron

3

Different Clay Minerals

Different combinations of tetrahedral and octahedral sheets form different clay minerals:

1:1 Clay Mineral (e.g., kaolinite, halloysite):

4

Different Clay Minerals

Different combinations of tetrahedral and octahedral sheets form different clay minerals:

2:1 Clay Mineral (e.g., montmorillonite, illite)

Absorption: interception of radiant energy or sound waves

Adsorption: adhesion in an extremely thin layer of molecules to the surfaces of solid bodies or liquids with which they are in contact.

Buffering capacity (BC): represents the ability of the soil to re-supply an ion to the soil solution.

pH independent charge (permanent)

Isomorphic substitution: substitution of one element for another in ionic crystals without changing the structure of the crystal

a. Substitution of Al+++ for Si++++ in tetrahedral

b. Mg++, Fe++, Fe+++ for Al+++ in octahedral

Leaves a net negative charge (permanent)

pH dependent charge: positive charge developed at low pH and excess negative charge formed at high pH

Gain or loss of H+ from functional groups on the surface of soil solids.

a. Hydroxy (-OH)

b. Carboxyl (-COOH)

c. Phenolic (-C6H4OH)

• Cation exchange- the interchange between a cation in solution and another cation on a soil surface

• Cation exchange capacity (CEC)- the total sum of exchangeable cations that a soil can adsorb.

Importance of CEC

• Chemical behavior in soils

• Fertility

• Liming rates– Buffering capacity

• Pesticides

• Contaminants

• Non-acid cation (Base) Saturation

Ion exchange

• Sources of charge:– In 2:1 clays, charge created mostly by

isomorphous substitution. • Not very pH dependent

– Hydroxyls (OH-) and other functional groups on the surfaces of colloidal particles that cause positive or negative charges based on releasing or accepting H+ ions.

• pH dependent• Common source of charge on humus, Fe and Al

oxides, 1:1 type clays, and non-crystalline silicates

Ion exchange

• Positive and negative

– Anion exchange (negative ions)

– Cation exchange (positive ions)

– Units of : cmolc/kg (centimoles of charge per kg)

The Colloidal Fraction: Seat of Soil Chemical and Physical Activity

Some of the many types:

• Layer silicate clays

• Iron and Aluminum oxide clays

• Organic soil colloids: humus

Colloids are small particles in soil that act like banks:

managing the exchange of nutrient currency in the soil

Different soils, like checking accounts, have different capacities to hold nutrient currency: cations and anions

OF GREAT IMPORTANCE: The influence of clay type on CEC

Figure 8.13  Ranges in the cation exchange capacities (at pH 7) that are typical of a variety of soils and soil materials. The high CEC of humus shows why this colloid plays such a prominent role in most soils, and especially those high in kaolinite and Fe, Al oxides, clays that have low CECs.

Typical CEC

Values

Principles of Ionic Exchange

Reversible Reactions

Charge Balance

Ratio Law

Mass Action

Ion Selectivity

Complementary Cations

Reversible Reactions

Can go forwards or backwards

Example:

micelle

K+

K+

micelle

H+

H+

+ 2K+ + 2H+

Balanced by Charge

micelle

K+

K+

micelle

Ca++

+ 2K+

+ Ca++

Charge for Charge…..

NOT ion for ion

The Ratio of Ions on Exchange Site is Equal to the Ratio of Ions

in the Soil Solution

micelle

H+

H+

H+

H+

+ 3Na+

H+

H+H+

micelle

Na+

H+

Na+

H+ H+

+ Na+ and 2H+

6 H : 3 Na

before

4 H : 2 Na

After on colloid

2H : 1Na

After in soln.

Mass Action

micelle

H+

H+

micelle

Ca++

+ CaCO3

+ H2O + CO2

CO2 is a gas and escapes from the soil easily….

This drives the reaction to the right.

Ion Selectivity

Al+3 > Ca+2 > Mg+2 > K+ = NH4+ > Na+

Held tightly ---------------------------------- Held loosely

Based on Valence Charge and Hydrated Ionic Radius

Selectivity = Charge of ion

Size

The Effects of Neighboring Cations

• pH influences what cations are adsorbed to the exchange complex

• At lower pH values, more H+ and Al3+ ions are adsorbed to the exchange complex holds than non-acid nutrient cations

• Acid cations: H+ and Al3+

• Non-acid (or base) cations: Ca2+, Mg2+, K+, Na+ (plant nutrients)

pH influences nutrient holding capacity: Cation Exchange Capacity

Figure 8.14  Influence of pH on the cation exchange capacity of smectite and humus. Below pH 6.0 the charge for the clay mineral is relatively constant. This charge is considered permanent and is due to ionic substitution in the crystal unit. Above pH 6.0 the charge on the mineral colloid increases slightly because of ionization of hydrogen from exposed hydroxyl groups at crystal edges. In contrast to the clay, essentially all of the charges on the organic colloid are considered pH dependent. [Smectite data from Coleman and Mehlich (1957); organic colloid data from Helling et al. (1964)]

Sources of Charge

and their influence on CEC

pH and pOHpH = -log{H+}

Acid cations replacing non-acid cations on soil colloids

What About Anion Exchange ?

First we need to know about:

Soil pH

And Variable Charge

Cl- chlorine

NO3- nitrate

SO4-2 sulfate

PO4-3 phosphate

Essential

Plant

Nutrients

Figure 8.16  (Left) Effect of increasing the pH of subsoil material from an Ultisol from Georgia on the cation and anion exchange capacities. Note the significant decrease in anion exchange capacity associated with the increased soil pH. When a column of the low-pH material (pH = 4.6) was leached with Ca(NO3)2 (right), little sulfate was removed from the soil. In contrast, similar leaching of a column of the soil with the highest pH (6.56), where the anion exchange capacity had been reduced by half, resulted in anion exchange of NO32 ions for SO42 ions and significant leaching of sulfate from the soil. The importance of anion adsorption in retarding movement of specific anions or other negatively charged substances is illustrated. [Data from Bellini et al. (1996)]

CEC vs AEC

• pH

• Texture

• Organic matter content

• Types of clay present

Liming requirements to raise pH to 6.5

Clay minerals and organic matter influence CEC most substantially

Field Estimates of CEC

Uses Soil Texture and Organic Matter Content

to predict the CEC of a soil

How much of a Soil Colloid (%) ?

What type or types of Colloids present ?

A soil contains 20% smectite, 5% Fe/Al oxides, and 4% humus. Calculate its CEC.

(5% = 0.05 kg per 1 kg soil)

Visit Table 8.3: pH of 7 is neutral; smectite CEC = 100 cmolc/kg

Organic Matter CEC = 200 cmolc/kg

Gibbsite/Goethite (Fe/Al oxide) CEC = 4 cmolc/kg

From the clays: 0.2 kg x 100 cmolc/kg = 20 cmolc

From O.M.: .04 kg x 200 cmolc/kg = 8 cmolc

From oxides: 0.05 kg x 4 cmolc/kg = 0.2 cmolc

Sand does not carry a charge, so…

Total CEC of the soil = 20 + 8 + 0.2 = 28.2 cmolc/kg soil

Example