Influence of layer charge origin and layer charge density of smectites on their aflatoxin adsorption

Abstract The aflatoxin adsorption efficiency of a bentonite depends strongly on the physical, chemical, and mineralogical properties of the smectite, the dominant clay mineral in bentonites. The objective of this study was to determine the effects of the layer charge origin, octahedral cation composition, and layer charge density on the smectites' selectivity and adsorption capacity for aflatoxin. Six smectite samples with different layer charge densities and octahedral cation compositions were evaluated: a montmorillonite (4TX) and a hectorite, which represent smectites with octahedral charge origin; and a beidellite, two saponites from Australia and Spain, and a nontronite, which represent smectites with tetrahedral charge origin. The clay fraction of the smectites was saturated with Na, Ca, and Ba, respectively. Aflatoxin adsorption isotherms were conducted for these smectites. The CEC of one high-charge montmorillonite (5OK) and the nontronite were reduced by Li-saturation and heating. Similar effect of the exchangeable cations was observed in all smectites, but significant differences in the aflatoxin adsorption capacity were observed among the smectites. The octahedrally charged smectites showed higher aflatoxin adsorption than the tetrahedrally charged smectites. The aflatoxin-smectite bonding mechanism was not affected by the layer charge origin or density. Optimal layer charge density was required to have sufficient non-polar sites on the smectite for aflatoxin molecules. High-charge-density smectites had low adsorption capacity and binding affinity for aflatoxin, but aflatoxin adsorption on these smectites can be improved by reducing the layer charge density of the clay mineral. Smectites with octahedral charge increased the interlayer accessibility of aflatoxin molecules. The octahedral cation composition had a negligible effect on the aflatoxin adsorption capacity of smectites.