Introduction to Cation Exchange Capacity (CEC) – Tutorial

Introduction to Cation Exchange Capacity (CEC) – 3D Models and Shockwave Movies

Introduction
In the previous tutorials we discussed primary and secondary minerals that are commonly found in soil environments: Primary Mineralogy and Clay Mineralogy. Many clay minerals found in soils have the ability to develop a net negative charge which is satisfied through the electrostatic adsorption of cations. This interaction is similar to how a magnet attracts iron filings. Although the filings are held by the magnet one can easily remove them. Similarly the cations adsorbed to colloid surfaces are exchangeable or available for plants and microorganisms. We can define Cation Exchange Capacity or CEC as “the sum total of the exchangeable cations that a soil can adsorb”. Cation exchange capacity is an extremely important property of soils from both an agricultural and environmental standpoint. For example, without CEC essential nutrient cations such as potassium and calcium would have to be continually supplied as inorganic fertilizers throughout a growing season. In this tutorial we will discuss negative charge development on clay minerals, the distribution of cations around negatively charged clay surfaces, and cation exchange reactions that supply nutrient cations to plants and microorganisms.

 

Computer Requirements

The 3D models and CEC demos listed on this page require the use of Quick Time or Shockwave Plug-ins. If your computer does not have these plug-ins, they can be downloaded by clicking Quicktime or Shockwave . Follow the directions for downloading. Once the download is complete you may have to reboot your computer. If you are having difficulties e-mail me or talk to someone at the computer help desk by dialing 231-4357.


Charge Development on Clay Surfaces
The clay fraction of the soil consists of secondary minerals that are extremely small in size. These particles are too small to be seen with an ordinary light microscope and must be viewed using electron microscopes. Most clay particles are smaller than 2 um (2 millionths of a meter). Because of their small size soil colloids have a very high surface area or surface per unit mass. Soil colloids small size, high surface area, and net negative charge make their surfaces extremely reactive. Below are electron micrographs of the clay minerals montmorillonite (left) and kaolinite (right). The white and black bars in the pictures represent a length of 2 uM.

 

Surface charge on soil colloids is developed in two ways: isomorphic substitution (permanent charge) and deprotonation of surface functional groups (pH dependent charge). pH dependent charge occurs on the edges of layer silicates, on variable charge minerals such as oxides of Fe and Al, and organic matter. It is called pH dependent charge because it increases in magnitude as the pH of the aqueous soil environment increases. Most of the pH dependent charge associated with agricultural soils is due to the deprotonation of organic functional groups. As the pH of the soil environment increases weak acid functional groups such as the carboxylic acid donate a proton and generate negative charge:

COOH + OH = COO + H2O

One practical way to increase the CEC of agricultural soils is to increase the organic matter content through tillage practices and increase the pH by adding lime.

Isomorphic substitution is the replacement of one atom by another of similar size in a crystal lattice without disrupting or changing the crystal structure of the mineral. If you remember from the clay and primary mineral tutorials, cations are coordinated to oxygen or hydroxyl anions in mineral structures. The negative charge of the anions is balanced by the positive charge of the cations that are coordinated to it. Net negative charge is developed when a cation of similar size and less positive charge substitutes for one of higher positive charge. Isomorphic substitution can also take place between cations of the same charge or a cation of higher positive charge. In the case of isomorphic substitution between cations of the same charge no charge is developed. In the case of isomorphic substitution between a cation of higher positive charge with one of lower positive charge a net positive charge is developed. The important thing to remember is that isomorphic substitution only occurs between cations of similar ionic radii. In the tutorials below we will be strictly dealing with permanent charge developed through isomorphic substitution.

Use the table below to identify the atoms and molecules which make up the clay minerals:
Color Key for Clay Mineral Models
Silicon Atom
Aluminum Atom
Magnesium Atom

EXAMPLE: Isomorphic substitution of Mg2+ for Al3+ in the octahedral layer of a 2:1 clay mineral. (Click on the buttons and use the mouse to rotate the mineral models.)

In this case Mg has a 2+ charge, Al has a 3+ charge, O has a -2 charge. If the rectangles below represent the octahedral layer in a 2:1 clay mineral then the substitution of one Mg2+ for one Al3+ will give rise to one negative charge. Add the charges generated by each aton to get the overall charge.

Al2O2OH2 (no charge)
AlMgO2OH2 (minus 1 charge)

Now let’s take a 2:1 dioctahedral soil clay mineral. In this mineral there is no isomorphic substitution — Si in the tetrahedral layer and Al in the octahedral layer pyrophyllite . Now let’s take the same mineral and isomorphically substitute 6 Mg+2 atoms for 6 Al+3 atoms in each of the octahedral layers. Each one of these Mg+2 cations will give rise to one negative charge montmorillonite . In soil science we express CEC as cmolc per Kg (that’s centimols of charge). Each negative charge generated by isomorphic substitution gives rise to 1 molc. So based on the number of brown octahedrons (18 molc/mineral) and the overall molecular weight of the mineral picture (calculated by adding up each Si, Al, Mg, and O and multiplying each one by their molecular weight = 8741.1 g/mineral) this mineral will have an overall negative charge or CEC of:

18 molc
mineral
X
mineral
8741 g
X
1000 g
kg
X
100 cmolc
molc
=
206 cmolc
kg

This concept will become clearer when we talk about CEC and view the CEC demos.


Electrical Double Layer
The net negative charge generated by clay particles will be balanced or neutralized by adsorbing cations. Additionally some of this negative charge will be used to exclude or repel anions from the area adjacent to the negatively charged clay surface. This repulsion or exclusion is often called negative adsorption. Therefore there will be a greater concentration of cations adjacent to the clay surface and an area farther away where there is a greater concentration of anions. Eventually, the distribution of cations and anions will be the same as the bulk solution. The combination of the negatively charged clay surface and unequal distribution of cations and anions (compared to the bulk solution) adjacent to the clay particle is called the electrical double layer (EDL) or diffuse double layer (DDL). Remember that we are talking about a phenomenon that occurs on the microscopic level, therefore the actual thickness and volume of the electrical double layer is extremely small. When viewing the shockwave demo for EDL notice that the 18 molc developed in the clay due to isomorphic substitution is balanced through the electrostatic attraction of cations and repulsion of anions. The EDL encompasses the negatively charged clay particle and the aqueous environment that is influenced by its charge.

COLOR KEY FOR CATIONS AND ANIONS IN SHOCKWAVE DEMOS
H+
K+
Ca+2
Cl
Al+3

In all of the demos below we scaled ions similar to their hydrated radii in aqueous environments. The hydrated radii of the cations used to scale the ions are listed below:

Ion
Hydrated radii in nm*
Al3+ 90
Ca2+ 40
K+ 30
Cl 30
H+ 46
*From Lindsay (1979)

When you click on the demo make sure that you observe the cations and anions that are influenced by the negatively charged clay particle. Also, notice that in the bulk solution there are an equal amount of cations and anions — this is the concept of electroneutrality which must occur in soil environments.

Electrical Double Layer Demo


Cation Exchange Capacity
So far we have talked about charge development in soil colloids and the distribution of ions around these charged surfaces. Since cations adsorbed to mineral surfaces are held via weak electrostatic interactions they are available to plants and microorganisms through exchange reactions. For example, plants can secrete protons (H+) that can exchange or displace a nutrient cation such as potassium on a colloid surface. The colloids negative charge is satisfied by the proton and the K+ enters the aqueous environment which can be absorbed by the plant via mass flow. The are several important characteristics of cation exchange:

1. The exchange reaction is rapid

2. The exchange reaction is diffusion controlled

3. The exchange reaction is reversible

4. The exchange reaction is stoichiometric

5. Selectivity of one ion over another

The first three points are pretty straight forward and we will discuss the fifth point in the next section. The forth point is important and merits further explanation. The negative charge generated on a soil colloid via isomorphic substitution can be satisfied by mono-, di-, or trivalent cations. In the case of trivalent cations such as Al+3, 1 cmol can satisfy 3 cmolc on the colloid surface. Similarly 1 cmol of Ca+2 can satisfy 2 cmolc while 1 cmol of K+ can satisfy 1 cmolc on the colloid surface. Or in other words it will take 1/3 cmol of Al+3 or 1/2 of a cmol of Ca+2 or 1 cmol of K+ to satisfy 1 cmolc on a colloid surface. This is what is meant by stoichiometry of the exchange reaction (point number 4). The important thing to understand here is the difference between cmol and cmolc. The cmolc of charge is used to quantify CEC. It is a fundamental unit used to normalize cations of different valence that can adsorb to colloid surfaces. So 1 cmolc of Al+3 = 1cmolc of Ca+2 = 1 cmolc of K+. This is the fundamental unit and puts cations of different valence on an equal playing field allowing soil scientists to quantify CEC.

This can get a little confusing so let’s demonstrate this point using some models. The colloid surface in the models is the same surface used to demonstrate isomorphic substitution — all of the brown polyhedra are Mg+2 cations that have isomorphically substituted for Al+3 in the octahedral layers. Therefore, the mineral shown in the model has a charge due to isomorphic substitution of 18 molc. If we expressed this as cmolc/kg (similar to the previous calculation for the section on isomorphic substitution) this mineral would have a CEC of 206 cmolc/kg. This amount of isomorphic substitution is greater than what is found in naturally occurring montmorillonites but is in the range found in naturally occurring trioctahedral vermiculites. We used this high CEC to demonstrate the concept of cation exchange in our demos. If you scroll up you will find the color key for the ions used in these models. When you click on the CEC models you will notice cations in the interlayers of the clay mineral as well as cations in the bulk solution. If you remember from the EDL demo some of the negative charge on the surface is used to repel anions (negative adsorption). Also, cations in the bulk solution need to be neutralized by anions (concept of electroneutrality). Again for the purposes of our model all the negative charge of the colloid is satisfied by cations adsorbed in the interlayer. Also, only cations (not associated anions) are shown in the bulk solution that will be involved in exchange reactions.

One more concept and we will get to the CEC demo — Base saturation. This is a fairly simple concept — the percent of the exchange complex that is occupied by base cations. Base cations are calcium, potassium, magnesium, and sodium. They are called base cations because they can be associated with strong bases (i.e NaOH or KOH). Acid cations are H+ and Al+3. Al+3 is considered an acid cation because it can generate protons through hydrolysis reactions:

[Al(H2O)6]+3 + H2O = [Al(OH)(H2O)5]+2 + H3O+

When you view the CEC demos pay particular attention to the nature of the exchange complex. If you want to calculate base saturation add up the molc of base cations adsorbed in the interlayers by the molc of acid cations and multiply by 100 — you should come up with about 22%. This is typical for highly weathered Southeastern soils that have not been managed (i.e. lime additions). We will discuss ion selectivity and the nature of the exchange complex after we view our first 2 models. The first model (monovalent exchange) demonstrates a proton displacing potassium from the exchange complex — a +1 cation exchanging for a +1 cation. In the second model (monovalent-Divalent exchange) Ca+2 is exchanged by 2 +1 protons illustrating point number 4 above (stoichiometry).

Mono-valent Exchange Demo      Mono-valent for Divalent Exchange Demo


Nature of the Exchange Complex
Now let’s finish by talking about point #5 above. Some cations will have a preference over others for the negative surface charge of a soil colloid. In general this will be related to the charge of the cation and its hydrated radii. For example the following order of preference is observed for cations with a different valence (mixed series):

Al3+ > Ca2+ = Mg2+ > K+ = NH4+ > Na+

In the case of cations with the same charge or valence the order of preference follows the order of decreasing hydrated radii:

Cs+ > Rb+ > K+ > Na+ > Li+

The above orders of preference can be summed up using Coulombs law which states “The force of attraction between opposite charges is directly proportional to the charges on the ions and inversly proportional to the square of the distance between the charges”. In other words the greater the charge the greater the force of attraction and the greater the distance between the charges the lesser the force of attraction. In highly weathered soils such as the Southeastern USA and the tropics the high rainfall leaches monovalent and divalent cations and the exchange complex becomes dominated by Al+3 (low base saturation). In arid and semi-arid climates base cations are conserved and the exchange complex will be dominated by mono and di-valent cations (high base saturation). Therefore, in order for many highly weathered soils to be productive from an agricultural standpoint we must lime them to increase their base saturation. The first farmer in the USA to recognize the importance of lime was a Virginia Farmer named Edmond Ruffin who used oyster shells on his fields. Today there are a variety of liming sources available to farmers. When lime is applied to soils it dissolves releasing a cation (usually Ca+2) that displces Al+3 from the exchange complex. Although Al+3 is preferred the concentration of Ca+2 in the soil solution is so great that it can overwhelm or displace the Al+3 by mass action. Once the Al+3 enters the soil solution it can hydrolyze (see above reaction) releasing protons that are neutralized by the carbonate and bicarbonate ions generated from the lime.

The demo below demonstrates how Ca2+ can displace Al3+ from the exchange complex. Pay careful attention to the stoichiometry of the reaction.

Di-valent for Tri-valent Exchange Demo

That’s it!! — a relatively simple introduction to charge development and CEC on soil colloids. Remember that the actual reactions represented in the above demos are much more complex in natural environments. Understanding charge development and CEC of soils allows us to better manage our soils for crop production and understand the bioavailability and mobility of contaminants in the vadose zone.

This web page was constructed by M.J. Eick and R.W. Burgholzer from Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0404. If you have comments or suggestions, email us at eick@vt.edu or rburghol@vt.edu

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Welcome to my Environmental Soil Chemistry blog.  This blog will be used primarily to house tutorials and links to bu used for students enrolled in my Environmental Soil Chemistry Course.  While I may make posts periodically most course information will be located on the Scholar course site.

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Matt