Adsorption and Insecticidal Activity of Toxin from Bacillus thuringiensis on Rectorite*

The adsorption and desorption of the toxin from Bacillus thuringiensis strain WG-001 on rectorite were studied at different toxin and/or rectorite concentrations, pH values and temperatures. The insecticidal activity of the adsorbed toxin was evaluated by determining the lethal concentration to kill 50% of the larvae of Heliothis armigera (LC50). The adsorption of the toxin on rectorite in sodium carbonate buffer (pH 9) reached equilibrium within 0.5–1.0 h and the adsorption isotherm of the toxin followed the Langmuir equation (R2 > 0.99). In the pH range from 9 to 11 (carbonate buffer), the adsorbed toxin decreased with increasing pH. The adsorption amounts decreased with increasing rectorite:toxin ratio. The adsorption was not significantly affected by the temperature between 10 and 50 °C. The X-ray diffraction analysis indicated occurrence of the intercalation of the rectorite by the toxin. The infrared absorption spectrum showed that the binding of the toxin did not alter its structure. The LC50 values of the adsorbed toxin were smaller than those of the free toxin. The rectorite protected the toxin from ultraviolet irradiation damage. The desorption of the adsorbed toxin in water ranged from 37.5% to 56.4% and from 27.4% to 41.8% in a carbonate buffer. The desorption percentage also decreased with increasing rectorite:toxin ratio.