There are different types of clays like Bentonite, Clinoptilolite,
Kaolinite etc. in the market, which are used as toxin binders. Each caly binds aflatoxins
and some ochratoxins. But each clay has its own properties and drawbacks.
In this blog we will study the properties and flaws of all
clays which are used in the industry as toxin binder.
Bentonite:
Bentonites are the clay rocks
altered from glassy igneous material such as a volcanic ash or tuff. Bentonites
have been used as palletizing iron ores, foundry bond clay, ceramic, drilling
mud, sealant, animal feed bond, bleaching clay, agricultural carrier, cat box adsorbent,
adhesive, catalyst and catalyst support, desiccant, emulsion stabilizer,
cosmetic, paint, pharmaceutical, civil engineering, clay organoclay and
polymer-clay nanocomposites.
Bentonites
are greatly affected from the acid activation, ion exchange, heating and
hydrothermal treatments, and some other physicochemical processes. For example,
physicochemical properties such as strength, swelling, plasticity, cohesion,
compressibility, particle size, cation-exchange capacity, pore structure,
surface area surface acidity and catalytic activity as well as the mineralogy
can change considerably by modifications.
Commercial importance of bentonites depends on the contents
of their clay and nonclay minerals. Dominant clay minerals in bentonites are
smectites such as montmorillonite, beidellite, saponite, nontronite and
hectrorite. Bentonites are seldom found as monomineralic clays and may contain
other clay and nonclay minerals and also some organic impurities.
Structures of Smectite:
Smectite is a 2:1 layer clay mineral and has two silica
tetrahedral bonded to a central alumina octahedral (O) sheet as seen in Fig. 2:
The net negative charge of the 2:1 (TOT) layers arising from
the isomorphic substitution in the octahedral sheets of Fe2+ and Mg2+
for Al3+ and in tetrahedral sheets of Al3+ for Si4+
is balanced by the exchangeable cations such as Na+, and Ca2+
located between the layers and around the edges. The mineral is called Na-
smectite (NaS) or Ca-smectite (CaS) corresponding to the exchangeable cation
which in Na+ or Ca2+. Industrial bentonites predominantly
contain either Na-montmorillonite or Ca-montmorillonite. The equivalent amount
of exchangeable cations in one kilogram smectite as well as other clay minerals
and clays is defined cation exchange capacity (CEC).
Swelling:
The physical
state of smectite and corresponding bentonite may be changed with increasing
water content, from anhydrous solid to a hydrated material, semi-rigid plastic,
gel and suspension respectively.
The change
in the physical state of a bentonite from an anhydrous solid to gel is called
swelling. The swelling of smectites occurs between the 2:1 (TOT) layers in
agglomerated particles.
Porous Structure:
Smectites are porous clay minerals. The voids in a solid
located amoung and within particles are caleed pore. The shape of the pores
having different sizes can be cylindrical, parallel-sided slit, wedge, cavity
and ink-bottle.
Adsorption Non-Polar Molecules:
Smectites
and other clay minerals do not adsorb non-polar molecules such as nitrogen,
argon and ethane at elevated temperatures. These molecules can be adsorbed on
smecitites at their liquefaction temperatures but cannot penetrate between the
2:1 (TOT) layers.
Adsorption Polar Molecules:
A
montmorillonite consists of 15-20 elements. Between the different elements,
there are cations (next to crystal water), which are easy to be exchanged by
other cations or positively charged molecules.
The cation exchange capacity (CEC Value) describes the
capacity of clay minerals to exchange cations.
Clinoptilolite (Zeolite):
Clinoptilolite is a natural zeolite
composed of a microporous arrangement of silica and alumina tetrahedral. Among a
high number of natural zeolites, clinoptilolite is best known. Zeolites are
classified as tectoaluminosilicates with large pores and channels in the
structures containing loosely bound water molecules (so called zeolite water)
and alcalic cations (Na+, K+, Li+ Cs+,
Ca2+, Mg2+, Ba2+, Sr2+). They can
selectively adsorb gas and steam molecules, reversibly adsorb and desorb water,
or based on ion selectivity they can exchange own cations for other ones.
Zeolites
are also used as effective adsorbents of toxic agents, particularly aflatoxins
from the feed. They effectively minimize adverse effects of aflatoxins on feed
intake, performance and nutrient conversion, and reduce mycotoxin concentration
in the livers of affected animals. A lower concentration (15g/kg) of zeolite in
the diet seems more effective than a higher concentration (25g/kg) as it was describe
by Oguz snf Kurtoglu.
Zeolite
supplemented diets are well tolerated by the animals; they support biomas
production and improve the health status of the animals. Clinoptilolite in the diet of layer hens
(50g/kg) increased the number of laid eggs, stability of eggshell and
efficiency of food utilization; however, neither the onset of the egg lay
cycle, nor egg weight were affected (Olver, 1997).
Kaolin:
Kaolin is a plastic raw material, particularly consisting of
the clay mineral kaolinite. The chemical formula is Al2O3.2SiO2.2H2O
(39.5% Al2O3, 46.5% SiO2, 14.0% H2O).
Kaolinite ranks among phyllosilicates, are classified into the main groups
according to the type of the layers, interlayer content, charge of the layers
and chemical formulas.
Structure of Kaolinite clay
Physical Properties:
The 1:1
platelets of kaolinite are held together strongly via hydrogen bonding between
the OH of the octahedral layer and the O of the tetrahedral layer. Due to this strong
attraction these platelets do not expand when hydrated and kaolinite only has
external surface area. Also, kaolinite has very little isomorphic substitution
of Al for Si in the tetrahedral layer. Accordingly, it has a low cation
exchange capacity. Kaolinite easily adsorbs water and forms a plastic, paste-like
substance.
