IMPROVED MILBOND-TX® CLAY-BASED MYCOTOXIN BINDERS “REASONS WHY IMTX IS AN EFFICIENT AFB1 BINDER”
In the previous issue of Milwhite’s Journal a simple to understand general overview of clays was presented with regards to their origin, nomenclature, formation and some of their chemical and physical properties which are responsible for certain clays being efficient AFB1 enterosorbents (i.e., binders). Improved Milbond-TX® (IMTX) is a Smectite clay, classified as a “hydrated sodium calcium aluminosilicate” (HSCAS) with a physical crystalline structure consisting of layers resembling “plates or flakes” stacked on top of one another. It is this attribute along with the fact that Smectite clays are associated with a high cation exchange capacity (CEC) that makes IMTX such an efficient AFB1 binder. Aluminosilicate clays are formed in various locations in the earth’s crust from weathered volcanic ash rock and have a high concentration of alumina (Al2O3) and silica (SiO2) along with the presence of basic cations such as Na+, K+, Ca++, Mg++ and Fe++. Smectite, is also referred to as “Fullers Earth Clay” or simply as “Fullers Earth” which, historically, has been used for thousands of years for cleaning. It was especially popular for cleaning and removing the waxy-oily component of wool called “lanolin”. The process of cleaning the wool was called “fulling” and the individuals doing the cleaning were called “fullers”. In fact, the term “Smectite” is Greek in origin and arises from the word “smēchein” which means “to clean”. Other names which are commonly used by the animal feed industry to identify Smectite types of clays are “Bentonite” and “Montmorillonite” and because they have very similar chemical and physical properties, the names Smectite, Bentonite and Montmorillonite are often used interchangeably. Any of these names is acceptable to use when describing an aluminosilicate clay that has a high binding affinity for AFB1. The qualities of IMTX that are responsible for its ability to sequester AFB1 in the animal’s digestive tract are discussed in this issue of Milwhite’s Journal.
IMTX is an “expanding” or “swelling clay” because its structural properties allow water molecules to enter into the spaces between its many layers. Smectite clays, with regards to their capacity to bind AFB1, are able to do so because of an ability to expand to many times their original volume, but all clays do not possess this property. The amount of interlayer expansion in Smectite clays is dependent on which cation is present. When the major interlayer cation is Na+ more expansion occurs because atomic bonding between the interlayers is not as strong allowing more water molecules to enter. When Ca++ is the major interlayer cation less expansion occurs due to stronger interlayer atomic bonding. For instance, Kaolinite is an aluminosilicate clay, just like IMTX, but is non-expandable. The reason is because Kaolinite is considered a 1:1 clay, whereas a Smectite clay, like IMTX, is a 2:1 clay. In order to understand what this means we must remember that the two basic components of clays are the elements Si and Al. As stated earlier, these two elements when combined with oxygen form molecules or “groups” with specific shapes.
For Si the shape is always a tetrahedron and for Al the shape is always an octahedron. These specific molecules (or “groups”) link together by the thousands to form long tetrahedral sheets or long octahedral sheets. When talking about a “clay layer” a 1:1 clay layer always consists of one tetrahedral sheet bound to one octahedral sheet. (Note: An easy way to visualize this is to think of the tetrahedral sheet and the octahedral sheet as two sheets of paper on top of one another forming the layer). Accordingly, in a 2:1 clay each layer will always be formed by two tetrahedral sheets with one octahedral sheet sandwiched between them. Of course, the sheets in a 1:1 and a 2:1 clay must be held together to provide structural integrity to each layer. This is accomplished by the atomic bonding of elements “cations” associated with the sheet’s negatively charged surface located between the layers as well as on the edges and outer surface of each sheet. Once stacked on top of one another the layers in a 1:1 clay (e.g., Kaolinite) do not expand because the atomic bonding (especially hydrogen bonding) holding the two layers together is very strong and does not allow water molecules to enter. This is why Kaolinite has a small amount of surface area and a low cation exchange capacity (CEC). Therefore, because of its properties Kaolinite is excellent for making ceramics, but is not an effective AFB1 binder. In fact, the clay got its name from the Kao-Ling village in the Jiangxi province of China noted for high quality ceramic material known as “kaolin” which is the clay used to produce highly prized porcelain or “fine china”. On the other hand, IMTX is an effective AFB1 binder, but is not the clay of choice for use by the ceramics industry. In a 2:1 clay the atomic bonding (oxygen-oxygen and cation-oxygen) holding the layers together is weak and easily broken and allows water molecules to enter the spaces between the layers. It has been estimated that the average size of the unexpanded interlayer spaces in Smectites is between 1 to 2 nanometers, whereas in a 1:1 clay the size is only 0.7 nanometers.
Once expanded, the water-rich interlayer spaces of IMTX provide an enormous amount of surface area for binding and trapping AFB1 molecules that enter these spaces. Also, the negative charges associated with the interlayer surfaces are balanced by the various cations within the interlayer spaces. Because of this, IMTX is known to possess a high CEC which is also responsible for attracting and binding AFB1 molecules. Since a 1:1 clay does not have as much surface area and the binding of cations and AFB1 is limited solely to the negative charges along the edges and outer area of each layer. It has been estimated that the effective surface area for one gram of 1:1 clay is only 10-30 square meters, whereas in a 2:1 clay like IMTX, the effective surface area is from 650-800 square meters/gm of clay.
Smectite clays, when expanded, provide sufficient space for AFB1 molecules to enter the interlayer spaces. A molecule of AFB1 has a flat-planar configuration with an estimated size of 12.8 X 10.4 angstroms which makes it smaller than many other mycotoxins. Clays containing a crystalline structure with interlayer spaces smaller than these dimensions are, therefore, limited in their effectiveness as binders of AFB1 since effective binding in these clays is limited only to the clay’s external surface and not the interlayers. The bonding between divalent cations and AFB1 occurs at the carbonyl (C=O) groups of AFB1 molecules (see figure). The high CEC and large surface area associated with Smectite clays are the two main driving forces behind their ability to attract and bind AFB1 and other toxins and contaminants.
So, putting all of this in perspective, when a sufficient concentration of IMTX is present in the liquid medium within an animal’s digestive tract, in the presence of AFB1, the chemical and physical properties associated with the crystalline structure of IMTX promote expansion of its interlayers providing spaces for AFB1 molecules to enter more easily. Once within these water-rich spaces formed by these interlayers adsorption of AFB1 occurs with specific atoms associated with its crystalline structure just as adsorption of AFB1 occurs on the outer surfaces and edges of IMTX. Once adsorbed to the outer surfaces and trapped between the interlayers of IMTX, the AFB1 molecules will eventually be excreted in an animal’s feces, thus resulting in no decrease in animal performance which is known to be associated with aflatoxicosis.
Note: The information in this issue of Milwhite’s Journal was compiled by Dr. Orlando Osuna, Director of Health Science at Milwhite, Inc. and Dr. Richard Miles, Professor Emeritus, University of Florida, Gainesville, FL, USA.