Mineralogy Factors Determining Bentonite Effectiveness In Aflatoxin Detoxification.

Frequent occurrence of aflatoxins in feed and food appear to be unavoidable. Adding bentonites to aflatoxin-contaminated feed has improved animal performance in many studies. Incorporating bentonites into human diet has also shown promising results in reducing the bioavailability of aflatoxins. Despite the encouraging results from animal and human trials, no regulatory agency has approved the use of bentonite as aflatoxin amendment agents due in part to lack of confidence and poor understanding of the detoxification mechanisms. The objectives of this study are to describe: (1) critical mineralogical properties of bentonites that determine their adsorption selectivity and capacity for aflatoxins and (2) research needs to improve the clay's detoxification effectiveness in vivo. Smectite in the bentonite dominates the adsorption of aflatoxin. Adsorption of aflatoxin can occur on both external surfaces and in the interlayer space of smectite. Up to 0.6 mol/kg (or 20% by mass) of aflatoxin can be adsorbed by smectites that have relatively low cation exchange capacities (about 100 cmol/kg or less). The adsorption appears to be irreversible. Only < 0.5% desorption by water washing was observed. The type of exchange cation on smectite plays a critical role in determining its affinity and adsorption capacity for aflatoxin. Divalent cation saturated smectites have much higher adsorption affinity than monovalent cations. These results have led us to conclude that size and polarity match between aflatoxin molecules and the adsorbing domains are key factors in determining selectivity of the clay and binding capacity for aflatoxin. Both theoretical computation and adsorption experiments after cation exchange have supported this conclusion. Well-defined concepts and evidences are needed on the interactions between bentonites and aflatoxins in vivo.