DEGRADATION OF GLYPHOSATE IN THE AQUATIC ENVIRONMENT - AN ENZYMATIC KINETIC-MODEL THAT TAKES INTO ACCOUNT MICROBIAL-DEGRADATION OF BOTH FREE AND COLLOIDAL (OR SEDIMENT) PARTICLE ADSORBED GLYPHOSATE

The kinetics of the degradation of the herbicide glyphosate in distill ed water and river water containing river sediment were investigated o ver a period of 72 days. No appreciable degradation of glyphosate was observed in distilled water, while rapid degradation occurred in the r iver water plus sediment from the outset, suggesting that the degradat ion is mainly microbial. An immediate 35 % loss from solution of glyph osate due to adsorption to suspended sediment particles and deposition to the bottom sediment was observed in the river water plus sediment experiment. Subsequently, two linear rates of degradation were observe d in the water phase of this experiment: an initial rapid degradation followed by a slower breakdown. An enzymatic kinetic model is presente d showing that the rate of degradation of glyphosate (G) is given by - d(DELTAG)/dt = k2[G(B)] + k6[GC(B)], where k6 and k2 are the rate cons tants for sediment or colloidal particle absorbed glyphosate (GC) and the unadsorbed glyphosate (G), respectively, and the subscript B denot es microflora-bound.