CLAYS. PHYSICOCHEMICAL CHARACTERISTICS

Electric Charges

The adsorption of mycotoxins through the utilization of mycotoxin binders basically consists of the neutralization of electric charges. When the electric charges of the
mycotoxin and those of the adsorbent neutralize each other, the toxin will be adsorbed on the surface of the adsorbent.
This process is similar to the mechanism by which a magnet attracts a metal. In other words, attraction occurs as a result of the electric charge difference. For a clay particle to adsorb or bind organic molecules, such as mycotoxins in feeds, opposite electric charges attracting each other must exist.

Clay electric charges can be either polar or bipolar. Polar clays only contain negative charges, so that they can only adsorb mycotoxins with a strong positive charge (i.e.: aflatoxins). Bentonites are a good example of clays that only adsorb aflatoxins, because of the type of charge contained in bentonite. On the contrary,
bipolar clays have both positive and negative charges, allowing them to adsorb not only aflatoxin, but also other types of mycotoxins.

Cationic Exchange Capacity (CEC)

Cationic Exchange capacity is a measure of the amount of cations (positively-charged ions) that a clay can catch. CEC is important since, for irreversible mycotoxin retention to be performed by the absorbent, it is necessary that after the initial attraction occurs between opposite electric charges in the intestine, the presence of multiple electric sites is required for mycotoxin molecules to be retained, even if the Cationic Exchange Capacity (CEC) adequate electric charges are present. A milliequivalent (mEq) is the unit used to measure CEC. It is considered that clays with >60 mEq have a high CEC, and this might interfere with nutrient absorption, particularly minerals. Bentonites are included in this clay group. Clays with <19 mEq (saturation point) make the clay to have a neutral charge, which results in a limited mycotoxin absorption capacity. Coalinites and zeolite are included in this category.

pH

pH is one of the factors with the strongest impact on the mycotoxin retention ability of a clay, after adherence has occurred.
During the mycotoxin adsorption process, binding is not permanent and it can be reversible. A low (acidic) pH means excess positive charges, as a result of the presence of acidic protons (H+). On the other hand, a high (basic or alkaline) pH stands for the presence of more negative charges (OH-).
pH can alter the electric charges present in both the mycotoxins and in the absorbent, resulting in alterations in the link that maintains both molecules together. This effect can apparently occur in the GI tract, so that a low pH can promote mycotoxin adsorption, while the presence of a higher pH can result in mycotoxin release.
Numerous scientists consider that clays should have a slightly basic pH for them to work in the stomach, before mycotoxins are absorbed to the bloodstream. Clays with an acidic pH tend to work better at the end of the GI tract, where the adsorption effect is no longer important, since mycotoxins have already been absorbed by the animal.

Composition

Clays are constituted by 2 or more layers of mineral oxide. These layers are parallel units piled up in silica and alumina lamellae.
Silica forms tetrahedral lamellae, while alumina forms octahedral sheets. Some of these clay particles have the ability of absorbing moisture and expand; others do not. Some of the bonds are weaker, so that they allow for layer expansion. Other bonds are stronger and prevent layers from being separated by water among them. Sodium bentonite a montmorillonite is an example of a clay that expands when water is added.
On the other hand, caolinite is a nonexpandable clay. Caolinite has units of layers strongly bound by hydrogen bonds.
Depending on the type of clay, and on the differences in their constitution, pore size can vary from 0.26 nm to 100 nm in diameter, a trait that can have an effect on binding the organic molecules (i.e. mycotoxins) as well as the surface bond.

Drying Temperature

Temperature can have an effect on cationic exchange, because of the interaction between the solubility and temperature. A 100 150 0C drying temperature allows for a better activation of the clay and improved CEC. In addition to the clay activation effect, drying temperature eliminates the contamination with pathogens that could
infect animals. Currently, this topic is extremely important, considering the potential risk of avian influenza virus spread among commercial birds.

Expansibility

Expansibility is the opening ability of the layers that constitute the clay, depending on their chemical composition. Non expansible clays have fixed layers so that they cannot absorb water or nutrients, and they have a low CEC (<60 mEq). This group includes but is not limited to caolinites, ilites, and chlorites. Expansible clays have the trait that they can absorb water and nutrients, and they have a high CEC (>60 mEq.)

Particle Size

Ideally, a mycotoxin adsorbent particle size should be 35 to 50 microns (300 - 400 mesh.) If particle size is >50 microns, the binder will not have enough exposure surface area to act on mycotoxins. On the contrary, if particle size is too small, the product will be too dusty, making it difficult to use in the feed mill.


Graph 3. CEC based classification of various minerals used as commercial binders and identified with different letters. The CEC value is on the right of each mineral. Products with 35 - 60 mEq typically belong to the HSCAS group.


mEq = Milliequivalents per 100 g clay.

Results in this chart are based on analyses performed by Perry Agricultural Laboratories, USA, using their own equipment calibration standard.