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Transcript of 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