A Review about different clays as toxin Binders

 

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.