Effects of surfactants and relative humidity on solid/gas desorption of chloroacetanilide and dinitroaniline herbicides

Date of Completion

January 2006

Keywords

Environmental Sciences|Engineering, Environmental

Degree

Ph.D.

Abstract

Contamination of the environment by herbicides used in agriculture is currently a significant concern. Many of the herbicides are endocrine disruptors and carcinogenic compounds that are threatening both human health and the environment. The atmosphere is a major pathway by which herbicides are transported and deposited in the environment. Thus, understanding the processes controlling herbicide exchange between the soil, where they are applied, and the atmosphere is important for investigating the environmental fate of these compounds. It has to be pointed out that, in the field, herbicides are typically applied as formulation mixtures, not pure compounds, in order to ensure uniform application and improve biocide performance. Although more than 80% of the commercial herbicides contain surfactants, little is known about the effects of these formulated surfactants on herbicide exchange between soil and the atmosphere. This study was designed to test the hypothesis that the desorption of herbicides to the gas phase after soil application is enhanced by both increases in ambient relative humidity and by the presence of surfactants in formulation mixtures. Particle/gas and soil/air desorption experiments were performed for seven herbicides from the chloroacetanilide and dinitroaniline families with and without model anionic-nonionic surfactant mixtures under a range of relative humidity conditions (3 to 66% RH) on two selected soils. A desorption fraction F48 was defined to evaluate the relative partitioning tendencies of the study herbicides. With increasing RH, increases in F48 were observed in both the particle/gas desorption of pure herbicides and the soil/air desorption of herbicides associated with surfactants. In the presence of surfactants, slightly enhanced desorption of herbicides from soil to the gas phase was found as the concentration of the surfactant mixture increased. For chloroacetanilide herbicides, F48 always decreased in the order propachlor > alachlor > metolachlor and butachlor; and for dinitroaniline herbicides, F 48 always decreased in the order trifluralin > pendimethalin > prodiamine. The differences in desorption behavior of herbicides can be explained by the differences in their substituent structures for each herbicide family. Based on the experimental results, statistical models were generated using multiple linear regression method to characterize the influence of surfactant concentration, RH and herbicide property (KH or KOA) on F48 for these relative polar herbicides. ANOVA analysis indicated that all these factors were statistically significant for F48 prediction, although the surfactant effect is relatively minor compared to the effects of changes in RH and individual herbicide properties. The current study is the first comprehensive investigation of the effects of surfactants on herbicide solid/gas partitioning processes and the resulting models are helpful to estimate the atmospheric losses of real-world herbicides after field application. ^

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