Abstract
In the study discussed in the chapter, Bacillus thuringiensis toxins were found to adsorb onto pure clay minerals and onto humic acids extracted from soils, and the complexes formed from these interactions were less available for microbial degradation. They retained their pesticidal activity longer than free forms. The aim of this study was to analyze the adsorption of the toxin from Bacillus thuringiensis subsp. tenebrionis (Btt) to surface active soil constituents. Clay-sized fractions were separated from a vertisol, an alfisol, and an oxisol after complete dispersion of the soils by mechanical means and the toxin was purified from a Btt commercial preparation. Adsorption isotherms were established by the difference method in deionized water or in phosphate buffer (pH 6.8). Adsorption of Btt toxin was in the order vertisol > alfisol > oxisol, and it was mostly irreversible, except for the alfisol. Adsorption could be explained by physicochemical conditions and the nature of clay constituents—that is, the dominant clay mineralogy: montmorillonite (vertisol) and kaolinite (alfisol, oxisol). The results show that Bt toxins released into the soil may be retained by soil colloids.
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