Enhanced pesticide adsorption by thermally modified bentonites

Wyoming bentonite was calcined at 350–550° for 1, 3, and 12 h, dispersed in water and freeze-dried. Calcination at 350–450°C for 12 h decreased the specific surface area strongly. At higher calcination temperatures, the surface area became similar to the value of the uncalcined bentonite (24 m2/g). The micropore (diameter <2 nm) volume of the calcined samples was very small (<0.2 μl). In contrast, the mesopore (diameter 2–50 nm) volume increased sharply when the bentonite was calcined at >450°C. The total specific surface area mainly comprised the mesopore surface area. The Wyoming bentonite used in these experiments adsorbed considerable amounts of metolachlor from aqueous solutions, and the adsorption was enhanced by calcinating the bentonite. The metolachlor molecules, very likely, interact with aluminum ions or oligomeric hydroxoaluminum cations enriched on the edges of the silicate layers as a consequence of the thermal attack to the edges. A pronounced adsorption–desorption hysteresis is advantageous for using these bentonites in slow-release pesticide formulations.