Evaluate the Effect of Smectite's Layer Charge Density and Origin On Its Adsorption for Aflatoxin.

Mycotoxins contaminate a wide variety of crops. Aflatoxins are the most toxic and carcinogenic secondary metabolites produced by fungi Aspergillus flavus and Aspergillus parasiticus. An effective and low cost measure to reduce the bioavailability of aflatoxin in animals is the addition of clays to animal feed. Screening of local and international bentonite samples has shown a range of aflatoxin adsorption from 1.8 to 21.1 % (w/w). Aflatoxin molecules are adsorbed in the interlayer space of the smectites. The aflatoxin adsorption variation among bentontite samples results from the difference in their physical and chemical properties.

Recent experiments have confirmed that the exchangeable cation strongly influences the amount of aflatoxin that can be adsorbed. As the swell/shrink properties of smectites are affected by charge density and charge origin, we expect that the charge origin might influence aflatoxin adsorption. The objective of this study was to determine the effects of the charge origin and octahedral cations on the selectivity and adsorption capacity for aflatoxin.

Six smectite samples with different layer charge sources and octahedral type were selected. The clay fractions of a beidellite, a hectorite, a montmorillonite (4TX), a nontronite, and two saponites (Spain and Australia) samples were evaluated for their aflatoxin adsorption capacity and affinity. Mica was present in the clay fraction of saponite Spain and hectorite, kaolinite in saponite Spain and beidellite, opal CT in saponite Asutralia, and smectite was the dominant mineral in all samples.

The clay fraction was saturated with Na, Ca and Ba to evaluate the adsorption performance. All the samples showed higher adsorption when Ba was the interlayer cation. Ba-saturated hectorite had the highest adsorption maxima of 0.86 mol/kg followed by montmoritllonite 4TX with ~0.5 mo/kg. The charge origin of these two samples is from the octahedral layer.

The samples also showed variation in the cation exchange properties. Nontronite had a CEC of 107.9 cmol/kg. This sample was treated with Li to reduce the layer charge. A reduction of the CEC to ~94 cmol/kg increased the adsorption capacity from ~0.4 mol/kg to 0.71 mol/kg, improving the adsorption affinity too.

Preliminary results corroborate that the adsorption capacity is strongly affected by the exchangeable cations, and the layer charge. Charge origin appears to have less influence in the adsorption capacity but optimal CEC values might depend on the charge origin.