Pesticide volatilization

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. 

Steichen J, Koelliker J, Grosh D, et al. 1988. Contamination of farmstead wells by pesticides, volatile organics and inorganic chemicals in Kansas. GWMR 8 153-160. 

Temperature and moisture content are other important factors that control volatilization of organic contaminants in the subsurface. Spencer and Cliath (1969, 1973) showed that a temperature increase from 20°C to 40°C led to an increase in dieldrin vapor density from 45 ng/L to 700ng/L (Fig. 8.7a). It also may be observed that a reduction in the soil moisture content caused a large reduction in the dieldrin vapor densities, even when the pesticide concentration in the moist soil was high enough to yield vapor densities approaching those of the pure compound. These results explain why reduction in pesticide volatilization in dry soils was observed over many years. 

Sparks DL (ed) (1986) Soil physical chemistry. CRC Press, Boca Raton, Florida Sparks DL (1989) Kinetics of soil processes. Academic Press, San Diego Sparks DL, Huang PM (1985) Physical chemistry of soil potassium. In Munson RE (ed) Potassium in agriculture, ASA, Madison, Wisconsin, pp 201-276 Sparks DL, Jardine PM (1984) Comparison of kinetic equations to describe K-Ca exchange in pure and mixed systems. Soil Sci 138 115-122 Spencer WF, Cliath MM (1969) Vapor densities of dieldrin. Environ Sd Technol 3 670-674 Spencer WF, Chath MM (1973) Pesticide volatilization as related to water loss from soil. J Environ Qual 2 284-289... 

Once In an evaporation bed, a pesticide can adsorb to a soil colloid, undergo chemical or microbial degradation, or escape from the bed by volatilization. An evaporation bed has the potential advantage over an open pond of decreasing pesticide volatilization while allowing for Increased degradation through microbial and soil-catalyzed reactions. 

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. 

Despite the advantages, there is concern over the use of such containment methods because the fate of pesticides put into such sites is not well known ( 1 ). One such fate process is volatilization from the disposal site. Organophosphorus pesticide volatilization from water and soil is relatively unlnvestlgated, and if this route of loss occurs to an appreciable extent from disposal sites, a local respiratory hazard may exist. 

In this paper, the volatilization of five organophosphorus pesticides from model soil pits and evaporation ponds is measured and predicted. A simple environmental chamber is used to obtain volatilization measurements. The use of the two-film model for predicting volatilization rates of organics from water is illustrated, and agreement between experimental and predicted rate constants is evaluated. Comparative volatilization studies are described using model water, soil-water, and soil disposal systems, and the results are compared to predictions of EXAMS, a popular computer code for predicting the fate of organics in aquatic systems. Finally, the experimental effect of Triton X-100, an emulsifier, on pesticide volatilization from water is presented. 

Diazinon, methyl parathion, and parathion, with binding constants to soil considerably greater than 1, showed a decrease in the percent pesticide volatilized in one day as the soil/water... 

The percent pesticide volatilized in one day from wet soil correlated positively with the factor [vapor pressure/(water solubility X binding constant)]. This factor has been reported to be linearly related to the volatilization rate of chemicals from soil surfaces (27). For pesticides with Henry s law constants and soil binding constants within the range studied, the factor is also approximately proportional to the fraction of chemical in soil air at equilibrium (28). In the present study, it was found that four of the pesticides had low factors, and less than 1% volatilized in 1 day (Table III). Diazinon, on the other hand, had a higher factor, and 2% of it volatilized. The use of this factor therefore does seem to have some merit for qualitative prediction. 

Table III. Pesticide Volatilization from Wet Soil Correlated with a Soil Volatilization Factor ...

In order to more closely represent the volatilization environment that would be encountered in an evaporation pond, Triton X-100, a non-ionic emulsifier similar to those used in some pesticide formulations, was added to prepared pesticide solutions at 1000 ppm. The presence of this emulsifier caused a decrease in the percent pesticide volatilized in one day in all cases except for mevinphos (Table VI). Three mechanisms are probably in operation here. First, Triton X-100 micelles will exist in solution because its concentration of 1000 ppm is well above its critical micelle concentration of 194 ppm (30). Pesticide may partition into these micelles, reducing the free concentration in water available for volatilization, which will in turn reduce the Henry s law constant for the chemical (31). Second, the pesticides may exhibit an affinity for the thin film of Triton that exists on the water surface. One can no longer assume that equilibrium exists across the air-water interface, and a Triton X-100 surface film resistance... 

Pesticide % Volatilized Triton Absent 11 Std. Dev. Triton Present... 

Clendening, L.D., W.A. Jury, and F.F. Ernst (1990). A field mass balance study of pesticide volatilization, leaching, and persistence. In D.A. Kurtz, ed., Long Range Transport of Pesticides. Chelsea, MI Lewis Publishers, pp. 47-60. 

These studies showed that volatilization rates from plant or moist soil surfaces can be very large with losses approaching 90% within 3 days for more volatile pesticides. Volatilization losses from dry soil or from incorporated chemicals are much less. 

Measurements of pesticide volatilization in the field have been made by several researchers using microclimate techniques. Vapor... 

Figure 4 shows pesticide volatilization as affected by soil depths of 1, 5, and 10 cm with water evaporation (E) equal to 0.25 cm/d. Since the concentration is inversely proporational to the depth of soil containing the 1 kg/ha of pesticide, the ratio of the concentrations roughly explains the initial relative volatilization rates. The very water soluble (Category III) compounds appear to approach a constant volatilization rate regardless of depth because their volatilization is controlled by diffusion of the chemical through the boundary layer above the soil surface as well as by the rate of movement upward to the soil surface. 

Extrapolating to the forest environment from field measurements of pesticide volatilization in agricultural environments, along with output from the screening model using benchmark properties, we conclude that volatilization from the canopy foliage will be relatively high for the more commonly used forest pesticides. 

The process of pesticide volatilization from a leaf surface is considered first in terms of the component physical processes of sublimation and molecular diffusion through a saturated boundry layer. Predicted volatilization rates based solely on pesticide vapour pressures often bear little relation to field observations due to myriad interactions of the pesticide with the leaf and the surrounding microenvironment. Observed pesticide fluxes above sprayed agricultural fields together with microclimatological characteristics of coniferous forests are then used to predict general patterns of pesticide volatilization from a treated coniferous stand. 

Atmospheric vapor. Vapor results from pesticide volatilization during spray release, vaporization from leaf deposits, and, over a longer time period, desorption from the soil and litter of the forest floor. Several reviews of this subject appear elsewhere in this Symposium volume. 

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