The smaller the size of a particle in a given mass of soil, the greater the surface area exposed for adsorption, catalysis, precipitation, microbial colonisation and other surface phenomenon (Brady and Weil, 2002, p 317). Some of the clays also possess an extensive internal surface area. The total surface area of clay minerals ranges from the T:O layer clays such as kaolinte, with external surface areas of around 10 metres square per gram to more than 800 metres square per gram for the T:O:T clays such as montmorillonite with large internal as well as external surface area.
Cation Exchange Capacity (CEC)1
The Adsorption of ions to clay minerals is the process of ions finding charge deficiencies in the clay external surface by which ions of the opposite charge are attracted to. Each colloid particle attracts thousands of Al 3+, Ca2+, Mg2+, K+, H+ and Na+ ions and other less common cations. Most cations in the soil solution exist in a hydrated state, that is surrounded by water molecules. These ions in the adsorbed layer are also known as exchangeable cations.The cation exchange capacity measures the two fundamental properties of clays, the surface area and the charge on the surface area.(Velde,1992,p.34). To understand how cation exchange works it is first essential to define the internal and external surfaces of a clay particle and to determine how they attract cations and anions. The internal layers are the two planes of atoms on either side of the interlayer space at the base of each tetrahedral sheet, external surfaces are the outside edge of a layer. The inter-layer has no broken bonds and generally has an overall negative charge due to the substituted anions (Ag3+, Fe2+) that the internal layers generally attract cations (K+, Na+, Ca2+). It des not matter what is present in the inter layer as long as they are cations and atomic or molecular in size. The external layer generally attracts negatively charged anions because of cations such as the Si and Al are generally closest to the surface and require the negative charge to compensate. Figure 1 gives an idea of the different surface area between the 1:1 and 2:1 layer clays.
Table 1 below shows the difference in CEC, note that montmorillonite has a large CEC compared to kaolinite. The expanding 2:1 clays, generally have a mixture of cation in the internal layers. (remembering that in a hydrated state, each one of the cations in the inter layer is also surrounded by water).(top)
Table 1: Comparison of the CEC
for selected clay mineral
The amount of stacking within each clay micelle is also imprortant in determining surface area. For example, this stacking is extensive in kaolinite, which is very blocky in nature; stacks range from: 0.05 to 2 microns2 in thickness, and 0.1 to 4 microns across the main face. With crystal edges comprising 10 to 20 percent of the total crystal surface area kaolinite only has a specific surface area of 10 to 20 square metres per gram of dry clay, the lowest surface-to-volume ratio of all clays. The greater proportion of edge area in kaolinite causes these edges to have a stronger influence on electrochemical behaviour (CEC) than for other clays; crystal edges in montmorillonite, by comparison, make up only 2 to 3 percent of total area. Edge charges appear to be pH dependent, hence kaolinite has a low CEC at low pH (and vice versa). Interestingly, the Anion Exchange Capacity AEC is higher for kaolinite than most clay minerals (Goldman et al, 1990).
The inherent swelling potential of aggregates of clay minerals is closely related to the total external and internal surface areas of clay-mineral particles. Clay minerals are capable of adsorbing water on their outer surfaces, which will cause small amount of swelling related to enlargement of the capillary films, that is the pages of the book get water in between them. Some clay minerals, however, such as montmorillonite, in particular bentonite (the Na-exchanged montmorillonite), and beidellite, are capable of absorbing appreciable amounts of water into the interlayer's, between the individual silicate layers of the structural lattice (the tetrahedral and octahedral layers), which results in a high swelling potential. Vermiculite, attapulgite, nontronite, and degraded mica (illite) and chlorite, though less common, are also capable of absorbing appreciable amounts of water (US Geological Survey).
The charges on the internal and external surface layers of the colloids attract the oppositely charged end of the polar water molecule. Some water is also attracted to the exchangeable cations in the interlayer, each of which can be hydrated with a shell of water molecules. Water absorbed between the crystal layers can cause the layers to move apart, making the clay more plastic and swelling its volume. As a clay dries, water in the inter layers are removed, and the layers are bought closer together. Generally, the greater the surface area, the greater the amount of water that can be drawn into and onto the clay layers from when it is air-dry.
• Crystalline water-part of the clay structure (OH-) (very difficult to remove)
• Interlayer water – absorbed water (medium difficult to remove)
• Surface water – adsorbed water (easiest
The double layer model is used to visualize the ionic environment in the vicinity of a charged colloid and explains how electrical repulsive forces occur as shown in figure 2. Initially, attraction from the negative colloid causes some of the positive ions to form a firmly attached layer around the surface of the colloid; this layer of counter-ions is known as the Stern layer. Additional positive ions are still attracted by the negative colloid, but now they are repelled by the Stern layer as well as by other positive ions that are also trying to approach the colloid. This dynamic equilibrium results in the formation of a diffuse layer of counter ions. They have a high concentration near the surface which gradually decreases with distance, until it reaches equilibrium with the counter-ion concentration in the solution. In a similar, but opposite, fashion there is a lack of negative ions in the neighbourhood of the surface,because they are repelled by the negative colloid. Negative ions are called co-ions because they have the same charge as the colloid. Their concentration will gradually increase with distance, as the repulsive forces of the colloid are screened out by the positive ions, until equilibrium is again reached.
The diffuse layer can be visualized as a charged atmosphere surrounding the colloid. The charge density at any distance from the surface is equal to the difference in concentration of positive and negative ions at that point. Charge density is greatest near the colloid and gradually diminishes toward zero as the concentration of positive and negative ions merge together. The attached counter-ions in the Stern layer and the charged atmosphere in the diffuse layer are what is called the double layer. The thickness of this layer depends upon the type and concentration of ions in solution. The point where the Stern layer and the diffuse layer meet is called the slip plane. The Stern layer is considered to be rigidly attached to the colloid, while the diffuse layer is not. The electrical potential at the slip plane in a basic sense, is related to the mobility of the particles and is called the zeta potential. (Zeta-Meter,1997)(top)
Figure 2: Two Ways to Visualize the Double Layer: The left view shows the change in charge density around the colloid. The right shows the distribution of positive and negative ions around the charged colloid. (Source: Zeta-Meter, Inc. )(top)
Soils high in sodium have been long recognized to benefit from applications of gypsum in improving their physical properties. On dispersible, highly weathered soils gypsum significantly increases water infiltration and reduces crusting associated with dispersion-induced sealing (Miller, 1998). Clays can be converted from a dispersed to flocculated state by changing the adsorbed ions from mono-valent to di- or even trivalent ions (Bouwer, 1978, p.20). Clays in a dispersed state as already mentioned, have a tendency to cause higher surface runoff rates and lower hydraulic conductivities. This is due to the particles causing blockages of the pores in the soil matrix and hence lowering the potential infiltration velocity. Consider a dispersed clay due to NaCl in the soil. The Na ions in the soil solution are attracted to the negative clay particles , so the clay particle is now surrounded by a ‘shell’ of Na ions (the stern layer) and a diffuse layer of initially Cl ions, and so on, until a neutral charge is again reached in the soil solution. The number of Na ions that can bond to the clay to satisfy the clay charge creates a thick cushioning effect. This thick cushioning effect means that the repulsion forces are greater than the weak di-polar attractant forces, hence the clay will remain dispersed.
Now consider the addition to the soil of a chemical, of a greater opposite charge than the clay surface, such as Ca or Al ions. These ions can exchange with the Na, still keeping an overall neutral charge on the clay surface. The number of ions, in this case, required to satisfy the clay surface charge will be less than if the Ca ion were in fact a Na ion. This reduces the thickness of the double layer and hence reduces the cushioning ability of the colloid. When this occurs, di-polar forces overcome the repulsion forces making them ‘sticky’ so as they can coalesce and form larger particles called aggregates or flocs; this process is known as coagulation, or flocculation. Specifically, the charge neutralisation is called coagulation and the building of larger ‘flocs’ from smaller ones is called flocculation (Pierce, Weiner and Vesilind, 1998). However, this is very much concentration dependent, and can be explained to some extent using the zeta-potential as shown in figure 3. The relation between the value of the zeta potential and flocculation or dispersion is not exactly determined yet. However, it has been empirically determined that at low zeta potential values flocculation, and at high values of zeta potential dispersion occurs.(Orhan, 1997).(top)
Gypsum is commonly used in agricultural situations to
restore soil structure. In flocculating soil clays into micro-aggregates
it can help to delay or decrease surface crust formation. Provided that
ionic strength of the soil solution is the primary factor responsible
for de-flocculation. For surface clay flocculation to occur, with the
addition of Gypsum, two things must happen. Firstly, the gypsum must be
slightly dissolved in solution to provide ions of a size small enough
to facilitate flocculation and secondly, the clay particles must be bought
together by some kind of mechanical energy to facilitate correct orientation
and configuration. When used in agriculture, rainfall provides the dissolution
media and mechanical energy due to the impact velocity, supplying the
clay particles with impetus. Hence, a greater chance to strike each other
in such a configuration as to be able to flocculate into larger particles.
This maintains a more porous surface layer and reduces runoff associated
with pore collapse (Miller, 1998).(top)
Clogging can be caused by swelling of clays and /or the dispersion or flocculation of clays. Clogging is the blocking of the pore spaces of the soil matrix ultimately preventing or reducing flow. For flocculated clay to cause clogging means that the clay in the dispersed form is very likely to pass through the matrix without any effect. So predominantly, the cause of clogging in agricultural situations can be mostly attributed to dispersed clays and hydrated clays of the 2:1 type that form a consolidated impermeable soil surface. This consolidated crust allows little infiltration, increasing run-off and hence erosion.(top)
1 The standard measure for CEC is usually expressed as milliequivelents per 100 grams of dried clay. Since CEC is based on the capacity of clay to hold or store a charge, it is convenient to use a unit that equates the total charge of different samples to enable direct evaluation of how much of each substance will be retained by the clay. The standard definition used in Chemistry for concentrations is molarity (M) with a sub-multiple of millimolar concentration (mM), but a given volume of one molar sodium chloride has only half the total ionic charge of a similar volume of one molar calcium chloride. The equivalent weight of a substance is defined as its formula weight which is the molecular weight divided by Z where Z is 1) the absolute value of the ion charge; 2) the number of H+ or OH- ions a species can react with for neutralization in an acid-base reaction; or 3) the absolute value of change in valence occurring in an oxidation-reduction (redox) reaction. A major use of the equivalent concept is that one equivalent of one ion or molecule is chemically equivalent to one equivalent of a different ion or molecule – hence we know that 1.0 ml of reagent A will react exactly with 1.0 ml of reagent B. Historically, concentrations of solutions (for volumetric analysis) were defined in terms of normality where a one normal solution is defined as one that contains one equivalent weight of a substance per litre of solution. Hence one molar (1M) NaCl solution is also one normal (1N), and one litre of this solution contains one equivalent, however, one molar (1M) CaCl2 solution is two normal (2N), and one litre of this solution contains two equivalents since Calcium is divalent. Since the Chemistry Profession discontinued the use of normalities, the modern term is the equivalent (eq) or its sub-multiple the milli-equivalent (meq), calculated on the abovementioned basis. One equivalent =1000 meq, also 1eq/Litre = 1000 meq/Litre or may be derived by expressing the equivalent weight in mg (Sawyer et al, 2003). (back).
2 A micron is one millionth of a metre, one thousandth of a mm) or 10-6 metres, the term is convenient and relates well to clay-sized particles which are, more or less, 2 microns or less in extent across the major face of the platelet. Their thickness is considerably smaller than this, ranging from one half to one thousandth of a micron.(back).
This web site was constructed by Benjamin.K. Galton-Fenzi. Last Updated: 6 September, 2003 6:08 PM