Henry's law constants for pesticides measured as a function of temperature and salinity

A wetted wall column device was used to determine air-water distribution coefficients [Henry's law constants (HLCs)] for pesticides: chlorpyrifos, trifluralin, endosulfans I and II, methyl parathion, metolachlor, and 2,4-D. The measured HLCs were all significantly higher than values calculated from published vapor pressure and solubilities except those for technical endosulfan and 2,4-D. With 2,4-D, however, the HLC was higher under acidic conditions. The effects of temperature and salinity were investigated using chlorpyrifos, trifluralin, and endosulfan I. Simulated seawater increased the HLCs from 1.5 to 2.4 times, which suggests a salting out effect. Natural water HLC values that were determined on Chesapeake Bay and Bering/Chukchi Sea water samples of microlayer and subsurface water were all lower than predicted values even after correction for temperature and salinity. Therefore, some as yet undefined factors in the aqueous phase appear to be responsible for these lowered HLCs. There was generally a linear increase in the log HLC versus increase in temperature for all of the pesticides in both distilled water and salt water except for trifluralin, which was curvilinear in simulated seawater.