The role of geological origin of smectites and of their physico-chemical properties on aflatoxin adsorption

Since 2013, bentonite in the form of dioctahedral smectite is an additive authorised in the EU as a substance for the reduction of the contamination of feed by aflatoxins. Several studies indicate a big difference in the effectiveness of smectites in sequestering aflatoxins. A clear correlation between mineralogical and physico-chemical properties of smectites and aflatoxin adsorption has not been well established. In the effort to identify the most critical mineralogical, chemical, and physical properties that affect aflatoxin adsorption by smectites, 29 samples of bentonites obtained from different sources around the world were evaluated. "As received" samples were divided into two main groups, i.e. hydrothermal (n=14) and sedimentary (n=15) bentonites depending on their geological origin. The characterization studies showed that all samples contained dioctahedral smectite as major mineral; a moderate CEC value (60-116 cmol/kg); the presence of iron; a small organic matter content; a near-neutral pH; and a fine and uniform particle size (<45?m). They differed substantially in their sodium, calcium and magnesium contents, and in the swelling properties depending on the geological origin. Several in vitro adsorption studies showed that they also differed in a significant manner in adsorbing aflatoxin B (AFB). A correlation between geological origin and AFB adsorption capacity was found (p<0.001), being sedimentary smectites significantly more effective than hydrothermal ones in adsorbing the toxin at different pH values. The extent of AFB adsorption by all samples was negatively and linearly correlated to the extent of desorption, and sedimentary smectites were significantly more effective than hydrothermal smectites in keeping bound the adsorbed fraction of the toxin (p < 0.001). In addition, correlation studies using the Pearson statistical method showed a significant relationship among some physico-chemical properties of smectites and the amounts of adsorbed toxin. In particular, AFB adsorption by smectites correlated positively with sodium content and swell index, but negatively with d001-value, magnesium and calcium contents. In conclusion, it seems that the geological origin of smectite is a useful guide for the selection of bentonites for AFB detoxification. Sedimentary bentonites containing sodium/swelling-smectite should be preferred to hydrothermal samples as potential aflatoxin binders. Taking into account the geographical origin of our samples, this approach should be applicable to bentonites worldwide.