Influence of Layer Charge and Charge Origin of Smectites On Selectivity and Adsorption of Aflatoxin.

Influence of Layer Charge and Charge Origin of Smectites on Selectivity and Adsorption of Aflatoxin A.L. Barrientos-Velázquez1, A. Marroquín-Cardona2, J.B. Dixon1 and Y. Deng1. 1 Soil & Crop Sciences Department, Texas A&M University 2 Veterinary Medicine & Biomedical Sciences, Texas A&M University Every year, corn, peanuts, cotton seeds, tree nuts, and a variety of crops are contaminated by mycotoxins. The most toxic and carcinogenic mycotoxins are aflatoxins produced by fungi Aspergillus flavus and Aspergillus parasiticus. Adding clays to animal feeds is an effective and low cost measure in reducing the bioavailability of aflatoxins to animals. We have investigated several bentonite samples from the USA and other countries and found that their AfB1 adsorption capacity varied from 1.8 to 21.1 % (w/w). Our analysis has demonstrated that the major adsorption site for aflatoxin is the interlayer space in smectites. It appears to be critical to preserve the interlayer the accessibility of aflatoxin molecules, which can be affected by many physical and chemical properties of the samples. More recently, experiments have confirmed that the exchangeable cation strongly influences the amount of aflatoxin that can be adsorbed. It also appears that the octahedral structural cations might influence the adsorption. 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. Natural bulk materials and the clay fractions of a beidellite, a hectorite, two montmorillonite (Novasil and 4TX), a nontronite, and a saponite (Spain and Australia) samples were evaluated for their aflatoxin adsorption capacity and affinity. The maximum adsorption capacity for the unfractionated samples showed a variation among the samples. Montmorillonite 4TX had the highest adsorption of 0.4 mol/kg, followed by montmorillonite Novasil with 0.35mol/kg and saponite Spain with 0.35 mol/kg. Beidellite, saponite Australia and hectorite showed and intermediate adsorption of about 0.20mol/kg. The lowest adsorbent was nontronite with 0.17 mol/kg. The variations among the unfractionated samples appear to be determined by other minerals in the samples. The x-ray diffraction patterns showed quartz and feldspars in the samples. Additionally the saponites contained mica and hectorite calcite. The clay fraction confirmed the presence of mica in both saponites while smectite was the dominant mineral in the other samples. The clay fraction was saturated with Na, Ca and Ba to evaluate the adsorption performance. Preliminary results suggest that the selectivity or adsorption capacity of the smectites for aflatoxin was less infuenced by the origin of the charge on the smectites or the type of octahedral cations, rather, the charge density of the minerals and the type of exchange cations play more important roles.