Effects of montmorillonite on the chemical degradation kinetics of metsulfuron methyl in aqueous media

The degradation kinetics of the sulfonylurea herbicide metsulfuron methyl (MM) has been studied in batch experiments by UV-VIS spectroscopy, and the effects of pH and the presence of a clay mineral (Na-montmorillonite) are reported. The proposed UV-VIS method gives kinetic results that are comparable to those obtained with chromatographic methods. In aqueous solutions, the chemical degradation of MM is undetectable at pH 6.5-9.5, but increases about 500 times by decreasing the pH from 6 to 2, and 300 times by increasing pH from 9.5 to 11.5. For example, the half-life of MM is shorter than 1 d at pH 2 and 11.5, but longer than a year at pH 6. Na-montmorillonite has an inhibitory effect on MM degradation. A combination of adsorption-desorption studies and degradation kinetic measurements demonstrates that in the presence of Na-montmorillonite only MM that remains in solution is subject to degradation; adsorbed MM is protected by the clay from being degraded. However, since adsorption only occurs at low pH (around 3) it can be envisaged that montmorillonite and supposedly other phyllosilicate clays do not affect significantly the chemical degradation rate of MM in soils and other natural systems.