Fate of metribuzin from alginate controlled release formulations in a Sharkey soil: 1. Desorption

Desorption/release of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5(4H)-one] from alginate-kaolin controlled release formulations (CRFs) with time, and its kinetic retention behavior in soil, were measured. 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 metribuzin release further. A modified batch desorption technique was used to quantify the metribuzin release kinetics from the CRFs. For CRFs with 0% linseed oil, metribuzin release was completed (99%) after 6 days, with 80% of the total released during the first day of desorption. In contrast, 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 were 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 effects, including formation of a polymeric coating by the linseed oil that would serve to decrease diffusion and chemical partitioning between the metribuzin and linseed oil. Batch results for a range of concentrations (up to 100 mg/L of metribuzin in solution) in a Sharkey clay soil (very-fine, montmorillonitic, nonacid, thermic, Vertic Haplaquept) indicated that adsorption kinetics was lacking. The metribuzin adsorption isotherm was linear and reached near equilibrium within 2 hours. This finding is consistent with metribuzin release from CRF with 0% linseed oil.