This lab deals with the attraction of cations to colloidal surfaces. First, the phenomenon of cation exchange will be introduced, with some additional information on its practical significance. Then I will cover some material on flocculation and dispersion of clay, which is not covered thoroughly in your text. You will then carry out three investigations that vividly demonstrate several of the points I will make. Finally, there are a few thought-provoking questions for you to answer.

The adsorption of cations and their exchange, together with that of anions on the surface of colloids, is probably second in importance only to photosynthesis. So, your understanding of cation exchange should be thorough. An understanding of the cation exchange properties of soil requires a knowledge of the source of negative charges effective for adsorption and desorption of cations. Two fundamental units provide the constituents of clay materials. These are the tetrahedron and the octahedron.

The tetrahedron has four sides and four corners. It can be thought of as a pyramid with a triangular base. A tetrahedron results when the centers of the four spheres such as oxygen atoms closely packed together are connected by lines. The space between the oxygen atoms can be occupied by a small cation such as silicon to form a silicon-oxygen tetrahedron of clay materials. Additional tetrahedra form by oxygen ions being shared with silicon ions in adjacent tetrahedra. The resulting tetrahedral layer is a 'lattice' with every second line of oxygens having every second oxygen ion missing. This provides an opening for another ion of similar size such as potassium to be incorporated in the lattice.

The octahedron has eight sides and six corners. It can be thought of as a double pyramid with a square base an apex both above and below the base. An octahedron results when the centers of six spheres such as oxygen atoms closely packed together are connected by lines. (Actually the oxygen atoms may have very small hydrogen ions attached forming hydroxyl ions.) The space between the oxygen atoms of an octahedron is a little larger than that in a tetrahedron and is generally occupied by an aluminum atom forming the aluminum-hydroxyl octahedron.

Kaolinite clay is composed of one tetrahedral layer and one octrahedral layer. Hence it is called a 1:1 clay. The middle oxygen-hydroxyl layer is shared as part of both the tetrahedral and octrahedral layers.

Two kaolinite layers are bonded together by O-H-O bonds, called Hydrogen bonding which makes the separation of the two units of kaolinite relatively difficult. The characteristic C-axis spacing of the kaolinite clay, i.e., the vertical distance from one repeating unit to the next, is 7.2 A.

Illite clay is composed of one tetrahedral layer on both the top and bottom of the clay with an octahedral layer sandwiched between. Hence it is called a 2:1 layer clay. The two middle oxygen-hydroxyl layers are shared as part of both the tetrahedral and octrahedral layers both top and bottom. Note some substitution of aluminum for silicon in the tetrahedral layer. This type of clay may also be referred to as hydrous mica.

Two Illite layers are bonded together by O-K-O bonds, the K+ having gone into the structure along with Al+++ to replace Si++++ normally found in the tetrahedra. This K bonding prevents the swelling of the illite clay. The C-axis spacing of Illite is 10 A.

Montmorillonite clay is also a 2:1 clay, but it has some substitution of Mg++ for Al+++ in the octrahedral level.

Unlike illite clay, montmorillonite has less K+ remaining to cement the layers together, hence it swells apart upon wetting, exposing internal surfaces where negative charges adsorb and desorb cations from the solution. These negative charges are those from the oxygen ions not neutralized by tetrahedral or octrahedral cations. Since these charges are effective for cation exchange at all pH levels, they are called permanent charges. Cation exchange capacity resulting from these charges is called permanent cation exchange capacity. Prior to weathering of K+ from the clay, these charges were most likely neutralized by K ions. The montmorillonite clay has a variable C-axis spacing depending upon how much water or other polar liquid is present to swell it apart.

You may also want to read Section 8.5 of your text on page 248 to 253. Study especially Figures 8.4, 8.5, 8.6, 8.7, and 8.8. These repeat much of the information covered here in the text and the slides.

The source of permanent charge cation exchange capacity, previously mentioned, is illustrated. The unsubstituted pyrophyllite mineral is used as a reference to indicate the precise balance of (-) and (+) charges and the shared layers of oxygen-hydroxyls in the tetrahedral and octrahedral layers. The substitutions of one Al+++ and one K+ for one Si++++ in both silica layers balance the charge initially. But upon weathering, the K+ becomes free so that other cations can exchange for them, and a net negative charge of 2 results (permanent charge).

The source of permanent charge cation exchange capacity: substitution of Mg++ and K+ for Al+++ to form montmorillonite is shown. The substitution of one or more Mg++ and an equal number of K+ in the alumina layer balances the charge initially. But upon weathering K+ becomes free so that other cations can exchange for them. A net negative charge of one or more results (permanent charge).
In addition to permanent charges previously considered, pH-dependent charges result when H ions dissociate from hydroxyl groups of the mineral. Since these OH groups are attached to different cations or combinations thereof causing different degrees of bonding of the H to the O, the H ions dissociate at different pH levels. Even different individuals of a given group dissociate progressively through a considerable pH range. The pK1 values indicated are the approximate pH levels where half the H+ is dissociated from that particular group. Because of the wide range in pK1 values and progressive dissociation within each group, additional pH-dependent charges gradually increase through a wide range of pH.

pH-dependent charges also result from the dissociation of H+ from the OH groups in organic matter. Several of the more common functional groups illustrate the pronounced pH-dependency of cation exchange capacity in organic matter. Since no permanent charge CEC exists in organic matter, the effects of pH-dependent CEC are generally much more striking than in clay materials.

The CEC's CEC's of whole soils and the contributions of clay and organic matter at different pH's are shown in slide 14. The change in CEC of the clay increased 1.7 times in the pH range from 2.5 to 8.0, while that of the organic matter increased nearly 6 fold. As a consequence, organic matter which contributed only 19% to the CEC of the whole soils at pH 2.5 contributed 45% to the total pH 8.0.

Thus, we can conclude that clay and organic matter particles act as giant anions, being covered over their surfaces with net negative charges. Cations are attracts to the surface and exchange takes place as illustrated in the equation given on page 1 in the lab manual. This all comes about through the existence of the Helmholz double layer, illustrated in (a) of Figure 9.1 of the lab manual. The first layer of charges are the negative charges on the clay surface. The second layer is composed of the mixture of cations that swarm about the clay surface, with the cations closest to the surface as the actual second layer.

Note that the concentration of swarming cations is reduced with increased distance from the surface. Further, the efficiency with which ions replace each other is determined by (a) relative concentration (or numbers) of the cations, (b) the valences of the cations, (c) and the diffusing of the different cations. Stated differently, the first is simply mass action, the second is that higher valence ions are more efficient in satisfying a grater number of charges for each ion when adsorbed, and the third, how fast it moves. The latter is a function of its size, the smallest moving the fastest. However, migration speeds do not follow in the order of the ionic size because of hydration of ions which increases the effective ionic diameters. Hydration of ions is simply the attraction of water molecules to the ions, and when they move in a solution, such as soil water, the water molecules move as if they were part of the ion.

Here is a cation hydrated with six water molecules. The attraction of the water molecules to the ion is caused by the dipolar moment of the water. That is, the hydrogen and 2 oxygens of the molecules are oriented in a way such that the negative charges are located towards one end of the molecule and the positive charges (the Hydrogens) towards the other. The negative end is attracted is attracted to the positively charged ion. The number of water molecules attracted to a cation is dependent upon the charge density of the ion. Figure 9.1 (b) in the lab manual illustrates the effects of hydration. Monovalent cations are more highly hydrated than divalent or polyvalent cations. Within ions of the same valence, the smaller ions attract more water molecules.

Degree of hydration of ions decreases in the following order: Li, Na, K, Rb, Mg, Ca, Al and Ti. According to this series Na could be replaced by K under equal concentrations, and so on. Mass action or differences in concentrations could reverse this tendency for replacement. The reason for this replacement series lies in the fact that Na does a poorer job than K of satisfying a negatively charged surface because the Na ion cannot get as close to the source of the charge as can the K ion. Carrying this reasoning to its conclusion, Al, one of the least hydrated ions gets closer to the source of the negative charge than the other ions, and at the same time, it satisfies three charges rather than the just one as in the case of Na and K. Once adsorbed, Al would be difficult to replace except in ion mass action where the Al might be overwhelmed by sheer numbers of another cation or cations. This is illustrated in (c) of Figure 9.1.

Preferential bonding of one cation over another by exchange sites is demonstrated when a relatively high proportion of the one cation is adsorbed from a two-cation leaching solution passed through columns of soil. Here preferential bonding of Ca over Sr is manifested by an Ap horizon soil with organic matter present while slight preference of Sr over Ca is evidenced by a BA horizon.

It should be evident to you by now that several factors influence the amount of cation exchange, or better, the cation exchange capacity, in a soil. These include: the amount and type of clay, the organic matter content, and soil reaction or pH. A fine-textured soil has a higher cation exchange capacity than a sandy soil; hence, the greater fertility of the fine-textured soil. Secondly, soils with the same texture differ in cation exchange capacity depending on their respective organic matter contents and type of clay present. These relationships are illustrated very nicely in Tables 8.7 (p. 265) and 8.8 (p 267) of your textbook.

Looking at the soil profile, we expect the A horizon to have high CEC compared to the remainder of the profile on the basis of its high organic matter content together with any clay that may be present. The B2 horizon with its high clay content would also exhibit high CEC, but not as great as the A horizon. A well-developed E horizon has low CEC since it has little organic matter and little or no clay. The practical significance of all of these facts is that soils with high CEC provide plants with large amounts of nutrients, especially the metallic cations, if present in the soil. Also, the nutrient loss during fertilization on high CEC soils is low. The nutrients are protected against leaching by being adsorbed on the sites.

CEC is always expressed as centimoles of positive charge per kilogram of soil. If you are not familiar with the term, the list of statements and questions at the end of today's exercise are designed to aid your understanding of this term and the calculations connected with it. This term is important in soil chemical reactions, so don't slide over the material.

Turning to another subject related to surface activity of colloids, we need to consider the concept of dispersion and flocculation. In a colloidal suspension the colloidal particles are dispersed about the liquid. Each particle is independent of the others and moves freely. The smallness of the particles contributes to this independence. However, it is primarily due to the negative charge and the degree of hydration of the particles themselves. You should now refer to Figure 9.2 in the lab manual.

First, like-charged particles repel each other, and unlike-charged particles attract each other as shown in 9.2(a). Secondly, highly hydrated ions adsorbed on the particles increase the particle net negative charge and are repelled by each other as in 9.1(b). But when low hydrated ions are adsorbed, as in Figure 9.1(c) the net negative charge is decreased and the particles become attracted to each other, or more properly, they are less repelled. It is important to remember that the net negative charges are increased or decreased; the permanent negative charges remain unchanged. When the electronegativity of particles is high, the particles are dispersed. We took advantage of this phenomenon in the texture exercise. If you recall, you added a dispersing agent, Na-hexametaphosphate, to the suspension before mixing. The purpose was to disperse the clay from the surfaces of sand and silt particles and from clay particles themselves so as to hold the clay in suspension while the sand and silt settled to the bottom of the cylinder. Better discrimination of the texture analysis results.

The practical significance of dispersion from soil is two-fold. First, dispersion needs to occur before clays are translocated from the upper horizons to the subsoil as a pedogenic process. Secondly, clays are dispersed in soils that have high sodium or potassium contents, as in soils of semi-arid or arid regions. Dispersion prevents the formation of structure and so these soils are easily puddled. The addition of salty irrigation water as in Arizona, California, or Colorado has the same effect. To reduce the effects of salt water, irrigation farmers must use valuable underground freshwater to flush the salts out of the soil. This is a costly use of potable water.

Remembering our discussion on the degree of hydration and the size and valence of the ion, you will recall that divalent or trivalent cations are not as highly hydrated as the monovalent cations. So, if we introduce Ca++ or Al+++ to exchange with the monovalent sodium, not only are we replacing 2 or 3 Na ions and satisfying 2 or 3 negative charges on the particle, the bond is a stronger one because the distance between the polyvalent cations and the charge source is smaller than it was for Na. The net negativity of the particle is reduced sufficiently so that the clay particles tend to be attracted to each other through the action of Van derWaal's forces.

Thus, we have particle collisions and coagulation, the particles flocculate into large aggregates, forming the basis of structure. In a laboratory cylinder, the floccules settle to the bottom of the cylinder after coagulating. Flocculation of clay in the soil forms clay coatings or skins on ped surfaces. Calcium is a strong flocculating agent. The clay coats promote coherence of sand and silt particles which have no surface activity, and enhances the formation of clay aggregates as cementing agents, all together contributing to structure formation. Where this process predominates in the presence of organic matter, granular structure is readily formed. The binding effects of roots, freezing and thawing cycles and wetting and drying cycles also contribute to structure formation by causing shrinking and swelling of the soil. Microbial gums and slimes act as cementing agents for structural aggregates as well.

Finally, the soil cation exchange capacity is a reservoir for plant-available nutrients. The action is at the interface where soil colloid meets root colloid in a bath of soil solution! Our challenge is to focus on the action by some type of instant replay with the aim of developing a better understanding of the game being played. This exercise is an important one in the course, so you should attempt to master the material.