Adsorption of the insecticidal protein of Bacillus thuringiensis on montmorillonite, kaolinite, silica, goethite and Red soil

The adsorption and desorption of the insecticidal protein (toxin) of Bacillus thuringiensis strain WG-001 on montmorillonite, Red soil, goethite, kaolinite and silica were studied. The adsorption isotherms of toxin in sodium carbonate buffer (pH 9) followed Langmuir equation (R2 > 0.97) and the curves belonged to L type. The amounts of toxin adsorbed were in the order: montmorillonite > Red soil > goethite > kaolinite > silica. At pH from 6 to 8 (phosphate buffer), the maximum adsorption of toxin on Red soil, kaolinite and goethite occurred at pH 6, while that on montmorillonite and silica were observed at pH 7. In the range of pH from 9 to 11 (carbonate buffer), the toxin adsorbed decreased with increasing pH. The adsorption of toxin on montmorillonite and Red soil reached equilibrium within 0.5 h, and in 1-3 h for goethite, kaolinite and silica systems. The adsorption was not significantly affected by the temperature between 10 °C and 50 °C. Both free and adsorbed toxin were toxic to the larvae of Heliothis armigera. The LC50 value of adsorbed toxin was smaller than that of free toxin. The desorption of adsorbed toxin in water ranged from 24.7% to 40.5% and were from 16.3% to 38.8% in carbonate buffer.