Pesticides are commonly applied on foliage post-emergence and can form dry deposits on the cuticular wax of crop leaves. Although leaf surfaces can be the first reaction environment for these pesticides after application, their transformation on these lipophilic surfaces is typically not considered in current fate and transport model. Some pesticides were recently reported to undergo enhanced photolysis on leaf surfaces, but several high-use, albeit less photo-labile, pesticides have not been evaluated. Previously, we showed that the sunlight photolysis of six phenoxyacetic acid herbicides (i.e., 2,4-D and its structural analogues) proceeded faster in non-polar solvents (e.g., <i>n</i>-heptane) than in polar solvents (e.g., methanol). Photolysis of these compounds on paraffin wax surface proceeded at similar rates as that in non-polar solvents, but 3-9 times slower than that on quartz. In this work, we focused on elucidating the mechanisms for the photolysis of 2,4-D herbicides. Measured pseudo-first-order photolysis rate constants followed the order: quartz &gt; wax ≈ <i>n</i>-heptane &gt; 2-propanol &gt; water &gt; methanol ≈ acetonitrile. The quantum yields followed the same order with similar magnitude of relative difference. Quantum yields for 2,4-D herbicides on quartz ranged from 0.03-0.12, which are much higher than that in organic solvents. Product analyses show that photoreductive dechlorination accounted for all parent compound decay in organic solvents. Therefore, this postulated radical pathway is likely favored in non-polar environments. On surfaces, however, products of photoreductive dechlorination only accounted for less than 60% of the parent compound decay. The rapid photolysis on surface was likely attributable to differences in reaction mechanisms unique to surfaces. The estimated photolysis half-life of 2,4-D herbicides on wax was 0.4-2.2 d, within the range of their half-lives for biodegradation, the previously considered major dissipation pathway.

Transformation products of 2,4-D sunlight photolysis in simulated leaf surface systems

256th National Meeting (2018)

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