Charges in clays are of two types:

Mobile layers (expandable)
• Among the clays of this type are montmorrillonites, smectites, bentonites and Zeolites.
• Expandible: Absorbs water & nutrients

STRUCTURE OF A TRIOCTAHEDRAL V.S. DIOCTAHEDRAL

CEC AND SOURCE OF CHARGES

PH
Acid pH binding in the larger intestine.
Alkaline pH binding in the smaller intestine

PARTICLE SIZE
< 300 mesh º less surface
300 to 400 mesh º more surface
size > 400 mesh º respiratory problems to humans

DRYING TEMPERATURES
Drying at 110 - 120 ⁰C for 25 to 30 minutes it activates and pausterizes the clay

1) Smaller CEC: Larger surface activity, therefore more binding sites

Clays are made of two or more mineral-oxide layers. These layers are stacked parallel units of silica and alumina sheets. The silica form tetrahedral sheets and the alumina forms octahedral sheets. Some of these clay particles have the ability to absorb moisture and will expand while others do not. The difference is due to clay chemistry and the elements (cations) that are components of the layers.

Some of the bonds are weaker and allow expansion of the layers, while others are stronger and do not allow the layers to be forced apart by water between the layers. Sodium bentonite, a montmorillonite is an example of a clay that expands with water addition. Kaolinite is a non-expanding clay which has its unit layers tightly bound together by hydrogen bonding.

These parallel layers contain large amounts of aluminum and silicon. The particle stacks may be neutrally charged by having an equal number of negative and positive charges or they may have negative charges in abundance.

When there are substitutions to elements in these layers, or whole layers are changed, we get different layer bonding and different clays being formed. Substituting a divalent cation (Mg⁺⁺ or Fe⁺⁺) for a trivalent element (Al⁺⁺⁺) will create a negative charge that can distribute itself principally over 10 surface oxygen atoms of the four silica tetrahedra that are associated through their apexes with a single octahedron in the layer.

This distribution of negative charge enhances the base character of the ditrigonal cavity and makes it possible to form complexes with cations and dipolar molecules. Substituting aluminum (Al⁺⁺⁺ for silicon (Si⁺⁺⁺⁺) in the tetrahedral sheet will create a negative charge that is distributed primarily over just three surface oxygen atoms of one tetrahedron. Much stronger complexes are formed with cations and dipolar molecules because of this localization of charge. This negative charge is compensated for by adsorbing a cation on either the interior or exterior of the stack.

These compensating cations which are adsorbed on the particle surfaces may be exchanged for other cations. Hence they are called exchangeable cations of the clay. The amount per unit weight of the clay is the Cation Exchange Capacity (CEC) measured in millequivalents (Meq) per 100 grams of dry clay. The following cation order is generally accepted and is arranged in decreasing preference:

⁺>Al⁺⁺⁺>Ca⁺⁺>Mg⁺⁺>K⁺>NH4⁺>Na⁺>Li⁺

Cations on the left will replace any cation to the right. To replace a cation to the left with one to the right requires a much higher concentration of the cation to the right. Temperature may have an effect on the cation exchange because of solubility-to-temperature relationships.

For instance, calcium salts, such as calcium sulfate, decrease in solubility at higher temperatures while sodium salts are more soluble at higher temperatures.

For a clay particle to adsorb or bind an organic molecule such as a feed mycotoxin, there must be opposite electrical charges that attract. Clays with a high cation exchange capacity (CEC) have a large number of negative charges on their surfaces. Those clays with intermediate or low CEC have mixed positive and negative charges.

Clay particles may be electrically neutral with an equal number of positive and negative charges. Since aflatoxin B1 has been shown to bind with aluminosilicates or bentonites, that contain a large number of negative charges (high CEC), the aflatoxin molecule must contain positive charges or be able to absorb a positive charge. Several of these clay products do not bind to mycotoxins other than aflatoxin, which may be due to the polarity of the electrical charges on the clay particles, to the location of those electrical charges or the sequence of the locations on the clay surface.

For secure binding to occur, it may take multiple electrical sites to hold the mycotoxin molecule even when the correct electrical charges are present. The surface shape of the clay particles, pore size and acidity (pH) may effect the binding as well. More chemically reactive organic molecules (free-radicals) would be expected to form more stable bonds than lower reactive molecules. The clays have several different types of reactions which can cause binding of minerals or organic molecules to the clay particles. Protonation is one type where the organic molecule accepts a proton to become basic. As water content decreases, proton donating abilities of the clay surface increases.

The second type of clay-organic interaction is the formation of coordinate covalent bonds. This is where exchange cations are of the transition metal type or an electron donor such as "N" or "O" on functional groups of organic molecules are available as ligands that will still interact with polar molecules through ion-dipole processes. The third type of interaction is hydrogen bonding of which there are several examples.

One example is a ketone-H bonding with water directly coordinated to an exchange metal cation. A second example is where an organic cation on an exchange site interacts with another organic molecule. A third example is where a protonated amine on an exchange site is H-bonded with a carbonyl group.

If instead of a carbonyl group, one has a second molecule on the same amine, the proton may be shared equally between the two amines in what is called a "symmetrical H Bond". Hydrogen bonding with oxygen of the silicate sheets of clay minerals has been shown as a site of interaction between organics and clays, which are relatively weak compared to ion-dipole interactions.

These bonds are usually referred to as outer-sphere surface complexes because they have one water molecule interposed between the functional group and the bound molecule. These complexes are less stable than inner-sphere surface complexes which involve either ionic or covalent bonding, or a combination of the two. The most abundant and most reactive surface group in sorbent clays is the hydroxyl group exposed on the outer periphery of a mineral particle.

Pores or cavities are formed in the clay particles in the tetrahedral sheets by the functional groups forming a six cornered silica tetrahedra. This six cornered cavity has a diameter of about 0.26nm and is bordered by six sets of lone-pair electron orbitals emanating from the surrounding ring of oxygen atoms. Depending upon the type of clay and the differences within the clay formation, pore size may vary from 0.26nm to 100nm in diameter. The pore size may have an effect on binding of organic molecules as well as the surface bonding.

Phillips, Kubena and Harvey published a procedure in the Poultry Science Journal in 1988 for in-vitro binding studies. Since 1988, several binding studies have been reported using this procedure. This analytical procedure uses 100mg sorbent to 10ug aflatoxin B1 (mycotoxin), which is a 10,000 to 1 ratio. The procedure can give an indication of ability to bind, but with a 10,000 to 1 ratio there is quite an excess of sorbent. That is not practical when conducting in-vivo studies.

A ratio of 1,000 to 1 would be 0.25% binder and 2.5 ppm mycotoxin in the diet and should provide adequate binder to mycotoxin for proper binding. Thorough mixing in the diet and a large number of binder particles is necessary to be sure the binder is in close proximity to the mycotoxin. Any product used as a mycotoxin binder should have a very small particle size to provide a large binder surface area. It is preferred that the particles between 300-400 mesh. Particles smaller than 500 mesh would be too small and would be a problem to handle in a feed mill environment and particles bigger than 300 would diminish their capacity because there would be less surface to bind.

High cation exchange materials such as bentonite and some HSCAS products can have detrimental effects in the animal diet. Many bentonites interfere with drug assay procedures for medicated feeds. A high cation exchange will interfere with mineral absorption in the digestive tract. Reduced calcium or phosphorus absorption and lower bone density have been observed when some mycotoxin binders are included in poultry diets. Inorganic trace mineral absorption (zinc, manganese, copper and iron), may be reduced by a high cation exchange product in the diet. Organic mineral sources (chelates) should bypass that type of interference.

Some companies marketing products for binding feed mycotoxins have claimed their product has the ability to bind mycotoxins because their product was categorized with products that have successful animal binding data. This extrapolation can not be assumed to be true. There are many hydrated sodium calcium aluminosilicate (HSCAS) products. Clays do not perform in the same way for binding feed mycotoxins.

One major company in this field has tested many clay products and found wide differences in their ability to bind specific mycotoxins. The only data of importance to the user of these type of products is the animal test data on a specific product for its ability to bind a specific mycotoxin. A literature search of ochratoxin and tricothecene mycotoxins shows that there are no products that have efficacy to bind them. Only one product has University data showing binding to ochratoxin and that work has not been published.

Table 1 shows analysis of various products that have been used to bind mycotoxins. These products are in general order of cation exchange capacity (CEC) by type of product. I have categorized the products under 30 CEC, products with 35 to 55 CEC and those products with 60 or more CEC.

Many of the products above 65 CEC are sodium or potassium bentonites. Products from 35 to 60 CEC are generally HSCAS products. Products below 29 CEC contain proportionately less content of calcium, magnesium, or sodium. As a comparison good crop soils have a CEC of 7 to 10, while excellent soils have a CEC of 14 to 16 CEC. A few of the products below 20 CEC may not be wholly clay products. Some of the products below 20 meg CEC do not appear to be good mycotoxin binders.

Two products with low CEC (<20 meg) have been shown to not bind vomitoxin in at least two field tests. One product with a CEC of 20 to 25 had zero binding of aflatoxin in a laboratory binding test.

Mycotoxins in the diet of animals can have devastating effects to animal performance. The binding of clay particles to organic molecules (mycotoxins) is a very complex process. Using clay products having a high CEC, can have undesirable nutritional consequences to the animal by binding to mineral components in the diet such as trace minerals. Research on products with a high CEC show that they do not bind mycotoxins other than aflatoxin, while products with a low CEC show low mycotoxin binding ability to aflatoxin and other mycotoxins. Binding of multiple mycotoxins by a single product has been shown by University tests in only one product on the market.

BIBLIOGRAPHY

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