Hydrolysis of triasulfuron, metsulfuron-methyl and chlorsulfuron in alkaline soil and aqueous solutions

The hydrolysis of triasulfuron, metsulfuron-methyl and chlorsulfuron in aqu eous buffer solutions and in soil suspensions at pH values ranging from 5.2 to 11.2 was investigated. Hydrolysis of all three compounds in both aqueou s buffer and soil suspensions was highly pH-sensitive. The rate of hydrolys is was much faster in the acidic pH range (5.2-6.2) than under neutral and moderately alkaline conditions (8.2-9.4), but it increased rapidly as the p H exceeded 10.2. All three compounds degraded faster at pH 5.2 than at pH 1 1.2. Hydrolysis rates of all three compounds could be described well with p seudo-first-order kinetics. There were no significant differences (P=0.05) in the rate constants (k, day(-1)) of the three compounds in soil suspensio ns from those in buffer solutions within the pH ranges studied. A functiona l relationship based on the propensity of nonionic and anionic species of t he herbicides to hydrolyse was used to describe the dependence of the 'rate constant' on pH, The hydrolysis involving attack by neutral water was at l east 100-fold faster when the sulfonylurea herbicides were undissociated (a cidic conditions) than when they were present as the anion at near neutral pH. In aqueous buffer solution at pH > 11, a prominent degradation pathway involved O-demethylation of metsulfuron-methyl to yield a highly polar degr adate, and hydrolytic opening of the triazine ring. It is concluded that th ese herbicides are not Likely to degrade substantially through hydrolysis i n most agricultural alkaline soils. (C) 2000 Society of Chemical Industry.