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

Abstract

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.

Behavior of the Chiral Herbicide Imazamox in Soils: Enantiomer Composition Differentiates between Biodegradation and Photodegradation

“…In sunlight exposed soil, however, the diacid reached only 1%, and the acid-hydroxy metabolite was not detectable at all. These findings are in line with studies on the direct photolysis of imazamox in water, where these two metabolites were also not detected. − …”

“…Note, that photodegradation of imidazolinone herbicides in aqueous solutions was also slower in the presence of natural organic matter, which was primarily explained by the light screening effect of organic matter. 6,10,24,25 The first two considerations were implemented in the numerical model. In sand, light penetration was extended to the whole layer of 3 mm, and adsorption (and microbial degradation) of imazamox was neglected.…”

“…In more recent field studies with this compound, soils were covered with a 2–5 mm thick sand layer with the presumed intent to exclude potential photodegradation. In water, imazamox undergoes fast direct photolysis. − When irradiated with a xenon-arc lamp (>290 nm), a half-life of only 5 h was determined at pH 7; whereas no degradation was observed in the dark within 15 d…”

Direct photolysis mechanism of pesticides in water

“…162) Rapid direct photolysis of imazamox by using a Xe lamp mainly proceeded via the hydrolysis of the C=N bond of the imidazolinone ring, 4) while two N-alkenyl pyridinecarboximidamides having the C=C bond at different positions, probably formed via ring opening with decarbonylation, were estimated by MS analysis with exposure to UV light at >290 nm. 163,164) Irrespective of the primary photoproducts differently proposed, the herbicide was finally degraded to 2-carbamoyl or 2-carboxypyridine. Similar uncertainty in product identification was reported for other imidazolinone herbicides.…”

“…Similar uncertainty in product identification was reported for other imidazolinone herbicides. [164][165][166][167] Further studies using authentic standards of photoproducts are necessary to determine their reaction pathways. In any case, both S* and T* are likely to be involved in the photolysis of these herbicides, based on LFP and quenching studies on imazethapyr.…”