FATE OF METRIBUZIN FROM ALGINATE CONTROLLED-RELEASE FORMULATIONS IN ASHARKEY SOIL - 1 - DESORPTION

Desorption/release of metribuzin -6-tert-butyl-3-(methylthio)-as-triaz in-5(4H)-one] from alginate-kaolin controlled release formulations (CR Fs) with time, and its kinetic retention behavior in soil, were measur ed. All formulations were prepared to contain Na-alginate (1%), kaolin (10%), charcoal (0.2%), Tween 20 (0.5%), and metribuzin. Linseed oil was added at a rate of 0, 1, and 4% in an attempt to control metribuzi n release further. A modified batch desorption technique was used to q uantify the metribuzin release kinetics from the CRFs. For CRFs with 0 % linseed oil, metribuzin release was completed (99%) after 6 days, wi th 80% of the total released during the first day of desorption. In co ntrast, CRFs with 1 and 4% linseed oil exhibited strong kinetics, with continued metribuzin release with time. After 59 days, total amounts of metribuzin released from CRFs containing 0, 1, and 4% linseed oil w ere 100, 68, and 12.4%, respectively. It is apparent that linseed oil acts to significantly decrease metribuzin release from the fommulation , particularly at higher rates. This is related to a combination of ef fects, including formation of a polymeric coating by the linseed oil t hat would serve to decrease diffusion and chemical partitioning betwee n the metribuzin and linseed oil. Batch results for a range of concent rations (up to 100 mg/L of metribuzin in solution) in a Sharkey clay s oil (very-fine, montmorillonitic, nonacid, thermic, Vertic Haplaquept) indicated that adsorption kinetics was lacking. The metribuzin adsorp tion isotherm was linear and reached near equilibrium within 2 hours. This finding is consistent with metribuzin release from CRF with 0% li nseed oil.