Cosolvent Effects of Ethanol and Dissolved Organic Matter on the Aqueous Solubility and Partitioning of Organochlorine Pesticides

Abstract

In the effort to promote ethanol/gasoline blends as an alternative fuel, potential negative impacts on the environment have been overlooked. Ethanol is produced in the Midwest and transported to the coasts. With the increased usage and the long distances of transport, a spill is reported on the average of every two days. The Midwest is also a location of high pesticide usage. The coincidence of ethanol spills and high pesticide usage in agricultural watersheds raises the concern that ethanol might enhance the concentration of regulated organochlorine pesticides in surface waters. Thus, the objective of this research was to examine the aqueous solubility and partitioning behavior of organochlorine pesticides in the presence of ethanol. The work reported here showed increases in the aqueous solubility of the organochlorine pesticides, dieldrin, lindane, and chlorpyrifos of about 7.46, 3.31, and 6.61 orders of magnitude, respectively, as the ethanol mass fraction increased from 0.0 to 1.0. A two-part log-linear model, with the breakpoint occurring at an ethanol mass fraction of approximately 0.35 was adequate to describe the relationship between the aqueous solubility of these compounds and the ethanol mass fraction, for all the pesticides examined. The breakpoint seems to be independent of the solute and dependent only on the ethanol-water interactions. The increase in the aqueous solubility of pesticides in the presence of ethanol can affect the partitioning behavior of such compounds between an organic liquid phase (e.g. iso-octane and gasoline) and water. This research showed that, in a water/ethanol/organic liquid system, ethanol displays a high preference for the aqueous phase. The aqueous solubility of the organic liquid also increased by orders of magnitude, but its concentration was still considerably lower than that of ethanol. The high concentration of ethanol resulted in an increased concentration of the organochlorine pesticides in the aqueous phase, causing a decrease in the partition coefficient by 4.83, 2.03, and 4.82 orders of magnitude for dieldrin, lindane, and chlorpyrifos, respectively. A log-log relationship between the partition coefficient versus the aqueous solubility of the pesticide was observed, with a slope of -1 and a y-intercept independent of the specific pesticide. Additionally, river sediment contains organic matter that, if dissolved, can further increasing the aqueous solubility of the pesticides. This study showed that the presence of DOM can further increase the aqueous solubility of lindane by over one order of magnitude. The results suggest that both the composition and concentration of DOM are important factors in determining the extent of the impact on the aqueous solubility, with DOM extracted from contaminated river sediments, like that from the Anacostia River, having a greater effect relative to that extracted from natural soils dominated by humic materials. Moreover, the solute’s polarity compatibility with the DOM was an important factor. In particular, a greater enhancement was noted for lindane, since its smaller size and more symmetric configuration made it more compatible with the bonding structure of the DOM.