Use of organosmectites to reduce leaching losses of acidic herbicides

The modification of smectitic clays with organic cations via cation-exchange reactions produces sorbents with an increased sorption capacity for organic compounds such as acidic herbicides. These organoclays (OCls) could be used as carriers in controlled release formulations of herbicides to decrease their contamination potential. Various OCls and two acidic herbicides (bentazone [3-isopropyl-1H-2,1,3-benzothiadiazin-4 (3H) one 2,2-dioxide] and dicamba [2-methoxy-3,6-dichlorobenzoic acid]) were selected and herbicide-OCl complexes were prepared by either sorbing the herbicides on the OCl from solution or by dry mixing of both components. Those preparations were assayed controlled-release formulations under static (water solution) and dynamic (soil column leaching) conditions. Herbicide release in closed (static) systems was fast and reached a maximum concentration after 10 to 20 h. The total. herbicide released ranged from 20 to 100% of the active ingredient initially incorporated in the complex depending on the sorption capacity of the OCl for the herbicide and the strength of herbicide-OCl interaction (aging time). Complexes releasing <50% of the herbicide, corresponding to the most sorptive OCls, may not be appropriated for weed control. Total leaching losses in soil columns were reduced from 94% for free technical bentazone to 55 to 90% for bentazone-OCl complexes, and from 100% for technical dicamba to 50 to 100% for dicamba-OCl complexes. Maximum concentrations in the leaching profiles of the herbicide-OCl complexes were much smaller than for the technical compounds. Bioassays of dicamba-OCl complexes as preemergence herbicide showed the same efficiency as the technical compound. These results suggest OCls as possible carriers in controlled release formulations for very mobile and persistent acidic herbicides, thereby decreasing their potential for surface and ground water contamination.