Cation Exchange Capacity By Mohsin Zafar Lecturer
Basic Structural UnitsClay minerals are made of two distinct structural units. hydroxyl or oxygen oxygen aluminium or silicon magnesium 0.26 nm 0.29 nm Silicon tetrahedron Aluminium Octahedron
Different Clay MineralsDifferent combinations of tetrahedral and octahedral sheets form different clay minerals:1:1 Clay Mineral (e.g., kaolinite, halloysite): 3
Different Clay MineralsDifferent combinations of tetrahedral and octahedral sheets form different clay minerals:2:1 Clay Mineral (e.g., montmorillonite, illite) 4
Absorption: interception of radiant energy or sound wavesAdsorption: adhesion in an extremely thin layer of molecules to the surfaces ofsolid bodies or liquids with which they are in contact.Buffering capacity (BC): represents the ability of the soil to re-supply an ion tothe soil solution.
pH independent charge (permanent)Isomorphic substitution: substitution of one element for another in ionic crystals without changing the structure of the crystala. Substitution of Al+++ for Si++++ in tetrahedralb. Mg++, Fe++, Fe+++ for Al+++ in octahedralLeaves a net negative charge (permanent)pH dependent charge: positive charge developed at low pH and excess negative charge formed at high pHGain 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 ActivitySome of the many types:• Layer silicate clays• Iron and Aluminum oxide clays• Organic soil colloids: humusColloids 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
Typical CEC ValuesFigure 8.13 Ranges in the cation exchangecapacities (at pH 7) that are typical of avariety of soils and soil materials. The highCEC of humus shows why this colloid playssuch a prominent role in most soils, andespecially those high in kaolinite and Fe, Aloxides, clays that have low CECs.
Reversible Reactions Can go forwards or backwards Example: K+ H+ micellemicelle + 2K+ + 2H+ K+ H+
Balanced by Charge Charge for Charge….. NOT ion for ion Ca++ K+ micellemicelle + Ca++ + 2K+ K+
The Ratio of Ions on Exchange Site is Equal to the Ratio of Ions in the Soil Solution6 H : 3 Na 4 H : 2 Na 2H : 1Nabefore After on colloid After in soln. H+ H+ Na+H+ Na+ H+ micelle + Na+ and 2H+micelle + 3Na+ H+ H+ H+ H+H+ H+
Mass Action H+micelle micelle + H2O + CO2 + CaCO3 H+ Ca++CO2 is a gas and escapesfrom the soil easily….This drives the reaction tothe right.
Ion Selectivity Al+3 > Ca+2 > Mg+2 > K+ = NH4+ > Na+Held tightly ---------------------------------- Held loosely Based on Valence Charge and Hydrated Ionic Radius Charge of ion Selectivity = Size
pH influences nutrient holding capacity: Cation Exchange Capacity• 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)
Sources of Charge and their influence on CECFigure 8.14 Influence of pH on the cation exchangecapacity of smectite and humus. Below pH 6.0 thecharge for the clay mineral is relatively constant. Thischarge is considered permanent and is due to ionicsubstitution in the crystal unit. Above pH 6.0 thecharge on the mineral colloid increases slightlybecause of ionization of hydrogen from exposedhydroxyl groups at crystal edges. In contrast to theclay, essentially all of the charges on the organiccolloid are considered pH dependent. [Smectite datafrom Coleman and Mehlich (1957); organic colloid datafrom Helling et al. (1964)]
Acid cations replacing non-acid cations on soil colloids
What About Anion Exchange ? Cl- chlorine - Essential NO3 nitrate Plant SO4-2 sulfate Nutrients PO4-3 phosphateFirst we need to know about:Soil pHAnd Variable Charge
CEC vs AECFigure 8.16 (Left) Effect of increasing the pH of subsoil material from an Ultisol from Georgia on the cation andanion exchange capacities. Note the significant decrease in anion exchange capacity associated with theincreased soil pH. When a column of the low-pH material (pH = 4.6) was leached with Ca(NO3)2 (right), littlesulfate 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 ionsfor SO42 ions and significant leaching of sulfate from the soil. The importance of anion adsorption in retardingmovement of specific anions or other negatively charged substances is illustrated. [Data from Bellini et al.(1996)]
Liming requirements to raise pH to 6.5 • pH • Texture • Organic matter content • Types of clay presentClay minerals and organic matter influence CEC most substantially
Field Estimates of CECUses Soil Texture and Organic Matter Content to predict the CEC of a soilHow much of a Soil Colloid (%) ?What type or types of Colloids present ?
ExampleA 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/kgOrganic Matter CEC = 200 cmolc/kgGibbsite/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