Analysis of the Photodegradation of the Imidazolinone Herbicides Imazamox, Imazapic, Imazaquin, and Imazamethabenz-methyl in Aqueous Solution.

The photodegradation of the imidazolinone herbicides imazamox, imazapic, imazaquin, and imazamethabenz-methyl has been investigated in phosphate-buffered solutions and buffered solutions containing natural organic matter (NOM). The hydrolysis of imazamethabenz-methyl, the only imidazolinone herbicide susceptible to hydrolysis, was also examined. The rate of hydrolysis of imazamethabenz-methyl increased with increasing pH, with the para isomer degrading more rapidly than the meta isomer. All photodegradation rate constants increased with pH and plateaued after pH 5.2. All imidzaolinones degraded more quickly under 253.7 nm lamps as compared to degradation under 310 nm lamps. Imazamox and imazapic degraded more rapidly than imazaquin at all pH values and had higher quantum yields. In addition, imazamox and imazapic quantum yields increased as a function of pH, whereas imazaquin quantum yields showed no trend as a function of pH. Photodegradation reaction rate constants decreased as the concentration of NOM was increased in the solutions due to the effect of light screening. Formulas for the proposed photoproducts for imazamox, imazapic, and imazaquin in pH 7 phosphate buffers were identified, and structures for the photoproducts are proposed.