Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 29, 1997
Publication Date: N/A
Interpretive Summary: Herbicides are an important component of modern agriculture, but there is concern about their impact on the environment. Because many of these chemicals are volatile, accurate estimation of their movement requires that we know their phase composition in the soil; i.e. - how much of the chemical is in solution, how much is in the gas phase, and how much is adsorbed to soil particles. The partitioning of the chemical between the liquid and gas phases is determined by a property of the chemical known as the Henry's constant, Hc. Unfortunately, for most agricultural chemicals the Hc is unknown, and hence must be indirectly estimated on the basis of other known chemical properties, specifically solubility and vapor pressure. Our goal was to construct a system to permit direct measurement of Hc, to obtain values needed to predict chemical movement and to test the validity of indirect estimation techniques. The system we used is a dynamic method known as batch air stripping. With it we measured Hc for a widely used herbicide, EPTC, that is known to be volatile. The system performed reliably and repeatedly, and revealed that the true Hc for EPTC is nearly 11 times larger than previous indirect methods had indicated. This information will be valuable for scientists and regulators in predicting the movement of EPTC, and the system will be useful for determining Hc for other volatile pesticides.
Technical Abstract: Many pesticides are susceptible to volatilization, resulting in decreased pest control and offsite deposition that may cause adverse environmental impacts. Prediction of such losses requires knowledge of pesticide partition coefficients, including Henry's Constant (Hc); however accurate data are lacking. In this experiment, the batch air stripping technique was used to determine Hc for EPTC (S-Ethyl N,N-di-n-propylthiocarbamate). At 25 degrees C, measured Hc in pure water was 0.0107, 11 times greater than the indirectly estimated values reported in the literature. The measured heat of volatilization for EPTC, used to describe the temperature dependence of Hc, was 37.12 kJ mol-1. Hc measured with soil extract was 700 percent greater than that measured in pure water, presumably due to effects of dissolved salts and organic acids.