Vapor pressure of pesticides

Moran, D., Suffolk County Department of Health, Hauppauge, New York 11788, personal communication, March 1983. Hamaker, J.W. Kerllnger, H.O. In "Vapor Pressure of Pesticides In Pestlcldal Formulations Research Physical and Colloidal Chemical Aspects" Wallenburg, J.W., Ed. Adv. Chem. Ser. 86, American Chemical Society Washington, D.C., 1969. 

In comparing the factors governing volatilization from soil and plant surfaces, the dominant effect of adsorption that reduces the vapor pressure of pesticides adsorbed on dry soil becomes apparent ( ). Vapor pressures of pesticides are greatly decreased by their interaction with soil, mainly due to adsorption. 

Seiber, J.N., Woodrow, J.E., Sanders, P.F. (1981) Estimation of ambient vapor pressures of pesticides from gas chromatographic retention data. Abstract, 183rd Am. Chem. Soc. Meeting, New York. 

Therefore, any program for measuring vapor pressures must be able to handle a wide range of values—many mm. Hg to 10"5 mm. Hg and less. At present, no one method is capable of covering such a range, and several must be used. Published work in this area shows that virtually all available methods have been used at one time or another for measuring vapor pressures of pesticides. The reader is referred to standard works on this subject (4, 10, 17), but the following brief survey is offered for orientation. 

Hamaker JW, Kerlinger HO. 1969. Vapor pressures of pesticides. Adv Chem Series 86, 39-54. 

J. W. Hamaker and H. O. Kerlinger, Vapor Pressure of Pesticides , in R. F. Gould s, Ed., Pesticidal Formulations Research, Physical and Colloidal Chemical Aspects, ACS Advances in Chemistry Series No. 86, American Chemical Society, Washington D.C., 1969, pp. 39-54. 

The vaporous collar contains a relatively high-vapor-pressure liquid pesticide mixed throughout the collar. The pesticide is slowly released and fills the atmosphere adjacent to the animal s surface with a vapor of pesticide that kills the pest but is innocuous to the animal. 

At the fundamental level of equilibrium modeling the advantages are many. The model can combine a number of compartments through simple relationship to describe a realistic environment within which chemicals can be ranked and compared. Primary compartments that chemicals will tend to migrate toward or accumulate in can be identified. The arrangement of compartments and their volumes can be selected to address specific environmental scenarios. Data requirements are minimal, if the water solubility and vapor pressure of a chemical are known, other properties can be estimated, and a reasonable estimate of partitioning characteristics can be made. This is an invaluable tool in the early evaluation of chemical, whether the model be applied to projected environmental hazard or evaluation of the behavior of a chemical in an environmental application, as with pesticides. Finally, the approach is mathematically very simple and can be handled on simple computing devices. 

Dobbs, A. J., Grant, C. (1980) Pesticide volatilization rate a new measurement of the vapor pressure of pentachlorophenol at room temperature. Pestic. Sci. 11, 29-32. 

In recent studies, pesticides such as atrazine have been found in precipitation. Therefore volatilization and subsequent transport in the gaseous phase is an important environmental pathway. Vaporization rates of pesticides deposited on surface of soil and plant leaves depend on the physical-chemical properties of the substance. A useful physicochemical criterion is Henry s constant, Ku, which is defined as the equilibrium air-to-water partial pressure ratio of the substance (see Chapter 7). 

Despite endrin s low vapor pressure of 2.0xlCl7 mm Hg (EPA 198la), initial volatilization of 20-30% after agricultural application to soil has been reported to be rapid (Nash 1983). Within 11 days, however, further volatilization was no longer detected (Nash 1983). Unlike some other chlorinated pesticides, endrin volatilization was not enhanced after a rainfall. Small amounts of endrin in soil may also be transported to the air by dust particles. 

Endrin is relatively nonvolatile with a vapor pressure of 2.0xl0 7 mm Hg (EPA 198 la Worthing and Walker 1983). Despite its low volatility, initial loss of agriculturally applied endrin through volatilization was found to be comparable to more volatile pesticides (Nash 1983). No recent data on atmospheric concentrations of endrin could be found in the available literature. Endrin was detected in air samples collected at 4 of the 102 NPL sites where endrin has been detected in some environmental medium however, concentrations were not available (HazDat 1996). 

Sites suitable for conventional SVE have certain typical characteristics. The contaminating chemicals are volatile or semivolatile (vapor pressure of 0.5 mm Hg or greater). Removal of metals, most pesticides, and PCBs by vacuum is not possible because their vapor pressures are too low. The chemicals must be slightly soluble in water, or the soil moisture content must be relatively low. Soluble chemicals such as acetone or alcohols are not readily strippable because their vapor pressure in moist soils is too low. Chemicals to be removed must be sorbed on the soils above the water table or floating on it (LNAPL). Volatile dense nonaqueous liquids (DNAPLs) trapped between the soil grains can also be readily removed. The soil must also have sufficiendy high effective porosity (permeability) to allow free flow of air through the impacted zone. 

Due to the movement of the pesticides to the bed surface, air samples were taken to determine any volatilization and subsequent concentration In the air along the berm on the downwind side of the bed. In most Instances, the top of the berm was only about 12 vertical Inches above the bed surface. Spencer and Farmer ( ) have reviewed the literature on the transfer of pesticides Into the atmosphere. Even though pesticide volatility Is related to vapor pressure of the chemical, there are many factors Influencing the effective vapor pressure from soil and water surfaces. 

There are a few data that suggest that pesticides can undergo reactions indoors. For example, Wallace et al. (1996) observed that the aldrin levels inside a home decreased with time, whereas those of dieldrin did not. Dieldrin had been applied with aldrin but is also an oxidation product of aldrin. One of the reasons for the lack of change in dieldrin may be that it was being formed as the aldrin decayed however, this could not be differentiated from the effects of a lower vapor pressure of dieldrin, which could lead to lower overall removal rates. In the same study, pentachloroanisole was also measured inside the home and attributed to formation by degradation of pentachlorophenol, which is used as a wood preservative and termiticide. 

Demeton, or Systox, was one of the most interesting and challenging pesticides studied. Demeton consists of two isomers, Demeton-S and Demeton-0. The vapor pressures of both isomers are reported to be nearly the same. When gas chromatographed, the isomers are completely resolved and easily quantitated separately. 

Tables 1 and 2 show the chemical name, molecular formula, water solubility, and vapor pressure of representative carbamate and urea pesticides.

The length of time pesticides persist in the forest floor and soil bears strongly on the probability they will be lost by volatilization (28-31). The phenoxy herbicides are commonly applied to forests as the low-volatile esters. These esters are readily hydrolyzed to their respective acids in soil or on the forest floor. For example, Smith (32) reported that no traces of 2,4,5-T and 2,4-D esters were observed in any of four moist soils after 48 and 72 hours, respectively, and most of them were hydrolyzed in less than 24 hours. The vapor pressures of the acids are much lower than the esters and this hydrolysis, along with subsequent degradation of the acids, results in a very low potential for volatilization of these materials from soil. 

Kemme, R.H., Kreps, S.I. (1969) Vapor pressure of primary n-alkyl chlorides and alcohols J. Chem. Eng. Data 14, 98-102. Kenaga, E.E. (1980) Predicted bioconcentration factors and soil sorption coefficients of pesticides and other chemicals. Ecotoxicol. Environ. Saf. 4, 26-38. 

A value of 6 tons/acre-day as the potential evaporation rate from wet soil is conservative (low). If the pesticide is fully exposed—not dissolved, or adsorbed—its rate of loss will be less than expected by multiplying the 6 tons by the vapor pressure of the pesticide and the square root of its molecular weight and dividing by half this product for water (the half allowing for average 50% humidity of the air initially). 

Vapor pressure is essentially the solubility of a compound in air. Permanent gases, such as methane, have high vapor pressures in fact, they have a vapor pressure of 1 atmosphere (atm) or 760 Torr. Some pesticides have medium vapor pressures for example, hexa-chlorobenzene has a vapor pressure of about 10 7 atm. Some compounds, such as decachlorobiphenyl, have vapor pressures that are so low that they are essentially nonvolatile (10 1(1 atm). For our purposes, the interesting range is 10 4 to 10 x atm. 

Pesticides applied indoors vaporize from treated surfaces (e.g. carpets and baseboards) and can be resuspended into air on particles. Many pesticides are semivolatile (saturation vapor pressures between 10 kPa and 10 kPa at 25 °C) and tend to vaporize from treated indoor surfaces. The rate of volatilization will depend on the vapor pressure of the compound, the formulation (solvent, surfactants, microencapsulation, etc.), the ambient and surface temperatures, indoor air movement and exchange rates (ventilation), the type of surface treated and the elapsed time after application. The vapor pressure data for pure pesticides is frequently available and may be of value for assessing the relative importance... 

---