The Effect of Simulated Gastric Fluid, Smectite Layer Charge and Bentonite Characteristics on Aflatoxin Adsorption

Aflatoxins are toxic secondary fungal metabolites that are present in a variety of crops. Numerous animal experiments have demonstrated the effectiveness of smectite clay in adsorbing aflatoxin and in reducing the toxicity of the aflatoxin to animals, yet a 100% recovery from toxicity has not been achieved. The incomplete protection from toxicity can be due to competition of nutrients with aflatoxin molecules for adsorption sites in smectite. Furthermore, many physical and chemical properties of the smectite samples can affect the adsorption of aflatoxin. The present study was divided in three sections: 1) To address the effect of pH, proteins and vitamins present in simulated gastric fluid on the aflatoxin adsorption capacity by smectites. Aflatoxin adsorption isotherms were performed in water and simulated gastric fluid for smectites saturated with Na, Ca and Ba. The interaction of smectite and vitamins (B1, D and E) was also analyzed using adsorption isotherms. Both pepsin and vitamin B1 competed with aflatoxin molecules for adsorption sites on smectite. In the interlayer space of smectite, large molecules such as pepsin can block the hydrophobic sites required for aflatoxin adsorption. Vitamin B1 was absorbed in the interlayer space of smectite, too. Vitamin B1 competed with pepsin for adsorption sites, and it enhanced the adsorption of aflatoxin by increasing the accessibility to the hydrophobic domains on smectite surface. 2) To determine the effects of the layer charge origin and octahedral cations on the selectivity and adsorption capacity for aflatoxin. Six smectite samples with different layer charge density and octahedral cation composition were evaluated for aflatoxin adsorption. Adsorption and binding affinity for aflatoxin of high charge density smectites was improved by reducing the layer charge density. Octahedral charged smectites increased the interlayer accessibility of aflatoxin molecules. The octahedral cation composition had a negligible effect on the aflatoxin adsorption capacity of smectites. 3) To analyze the mineralogical properties of natural Texas bentonites for the selection of aflatoxin binders. Bentonite and partially altered volcanic ash samples were collected from six locations and characterized with XRD, FTIR, SEM and TEM. All the samples contained montmorillonite as the dominant clay mineral. The major mineralogical differences were in the sand and silt composition. The octahedral structural composition indicated some isomorphic substitutions of Al by Mg, indicating a high layer charge density in some montmorillonite samples that may reduce the aflatoxin adsorption effectiveness. The presence of octahedral Fe in some montmorillonite samples has negligible effect on their aflatoxin adsorption. The majority of the Texas bentonites samples collected are potentially good aflatoxin adsorbents due to high dioctahedral charge origin smectite and moderate content of Mg in the octahedral sheet