Volatilization rate

The Henry s law constant value of 2.Ox 10 atm-m /mol at 20°C suggests that trichloroethylene partitions rapidly to the atmosphere from surface water. The major route of removal of trichloroethylene from water is volatilization (EPA 1985c). Laboratory studies have demonstrated that trichloroethylene volatilizes rapidly from water (Chodola et al. 1989 Dilling 1977 Okouchi 1986 Roberts and Dandliker 1983). Dilling et al. (1975) reported the experimental half-life with respect to volatilization of 1 mg/L trichloroethylene from water to be an average of 21 minutes at approximately 25 °C in an open container. Although volatilization is rapid, actual volatilization rates are dependent upon temperature, water movement and depth, associated air movement, and other factors. A mathematical model based on Pick s diffusion law has been developed to describe trichloroethylene volatilization from quiescent water, and the rate constant was found to be inversely proportional to the square of the water depth (Peng et al. 1994). 

Smith JH, Bomberger DC Jr, Haynes DL. 1980. Prediction of the volatilization rates of high-volatility chemicals from natural water bodies. Environmental Science and Technology 14 1332-1337. 

Many models are available in the literature, and some of these models can be applied only to specific environmental situations and only for chemicals for which they were developed. Obviously, all models do not provide the same numerical results when employed to provide answers to a particular problem, so care must be taken in choosing an appropriate unsaturated zone model, or when specifying a volatilization rate. For modeling algorithms, and numerical examples the reader is referred to the work of Lyman et al. (6), Bonazountas Wagner (5) and others listed in these references. 

The level III calculation (Fig. 5) shows that the air-water volatilization rate constraint reduces air advective loss to 1.038 m mol/h and other reaction processes assume greater importance. 

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. 

Volatilization volatilization rate constant Iq = 0.03 d-1, microcosm exptl. (Wakeham et al. 1986). 

Mackay, D. Shiu, W.Y., Chau, E. (1983) Calculation of diffusion resistance controlling volatilization rates of organic contaminants from water. Can. J. Fish. Aqua. Sci. 40, 295-303. 

Hexachloroethane released to water or soil may volatilize into air or adsorb onto soil and sediments. Volatilization appears to be the major removal mechanism for hexachloroethane in surface waters (Howard 1989). The volatilization rate from aquatic systems depends on specific conditions, including adsorption to sediments, temperature, agitation, and air flow rate. Volatilization is expected to be rapid from turbulent shallow water, with a half-life of about 70 hours in a 2 m deep water body (Spanggord et al. 1985). A volatilization half-life of 15 hours for hexachloroethane in a model river 1 m deep, flowing 1 m/sec with a wind speed of 3 m/sec was calculated (Howard 1989). Measured half-lives of 40.7 and 45 minutes for hexachloroethane volatilization from dilute solutions at 25 C in a beaker 6.5 cm deep, stirred at 200 rpm, were reported (Dilling 1977 Dilling et al. 1975). Removal of 90% of the hexachloroethane required more than 120 minutes (Dilling et al. 1975). The relationship of these laboratory data to volatilization rates from natural waters is not clear (Callahan et al. 1979). 

The volatilization rate at the surface is further influenced by temperature and by the thickness of the stagnant air layer over the surface. Furthermore, random micrometeorological conditions of the crop stand are also very important (turbulence, thickness of boundary layer, humidity, and wind velocity). Special models are available to calculate the volatilization and vaporization rates of pesticides (Richter, 1999). 

Few studies have been carried out on the volatization rates of organics in land-applied sludges. The presence of organic matter in the sludge may increase the sorption of organic compounds and thus reduce volatilization rates. 

Kilzer L, Scheunert I, Geyer H, et al. 1979. Laboratory screening of the volatilization rates of organic chemicals from water and soil. Chemosphere 10 751-761. 

Vapor pressure The vapor pressure of a substance is defined as the pressure exerted by the vapor (gas) of a substance when it is under equihbrium conditions. It provides a semi-quantitative rate at which it will volatilize from soil and/or water. The vapor pressure of a substance is a required input parameter for calculating the air-water partition coefficient (see Henry s law constant), which in turn is used to estimate the volatilization rate of compounds from groundwater to the unsaturated zone and from surface waterbodies to the atmosphere. 

A volatilization rate constant of 1.1 x lO Vsec was determined when pyrene on a glass surface was subjected to an air flow rate of 3 L/min at 24 °C (Cope and Kalkwarf, 1987). 

Hamelink, J.L., Simon, P.B., and Silberhorn, E.M. Henry s law constant, volatilization rate, and aquatic half-life of octamethylcyclotetrasiloxane, Environ. Sci. Technol, 30(6) 1946-1952, 1996. 

Kaczmar, S.W., D ltri, F.M., Zabik, M.J. Volatilization rates of selected haloforms from aqueous environments. Environ. Toxicol. Chem., 3(l) 31-35,1984. 

Meikle, R.W., Kurihara, N.H., and DeVries, D.H. The photocomposition rates in dilute aqueous solution and on a surface, and the volatilization rate from a surface, Arch. Environ. Contam. Toxicol, 12(2) 189-193, 1983. 

It is known that, in a water phase, immiscible liquids such as gasoline or other petroleum products may form multicomponent droplets of various forms and sizes, under dispersive conditions. These droplets are transported by convection and diffusion, which contributes to the contamination of fresh water systems. However, during droplet transport, more volatile substances partition to the gas phase at the droplet surface, leaving less volatile material that volatilizes more slowly. More volatile material still exists in the droplet interiors, and it tends to diffuse toward the surface because of concentration gradients created by prior volatilization. Different components in a droplet have different volatilization rates, which may vary significantly during droplet transport, and as a result, the contamination of fresh water is affected accordingly. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

Wolters et al. (2003) observed that volatilization kinetics of the fungicide fenpropimorph express a clear correlation between volatilization rates and soil moisture content. Volatilization rates reached a maximum 24 hr after application... 

Triton X-100 decreased volatilization rates of low solubility pesticides from water. 

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. 

In the present work, it was desired to 1) verify the prediction that the Henry s law constant controlled variations in the experimental volatilization rate constants under constant environmental conditions, and 2) compare experimental volatilization rate constants to predicted constants using the two methods for estimating kg for water and the molecular weight adjustment procedure of Liss and Slater as discussed above. 

Samples were prepared and analyzed as reported previously (18). Because of slow concentration decreases with time, low volatilization rates relative to hydrolysis rates in some cases, and small artificial losses of pesticide due to repeated water sampling, the most accurate method of determining volatilization rate constants was to divide the average pesticide concentration for that day into the average volatilization rate over the same period (Equation 1). Rate constants for the seven days were averaged. The entire experiment was performed in triplicate. 

Experimental and predicted volatilization rate constants for the five pesticides are listed in Table II. It should be noted that, despite low H values for the pesticides, experimental volatilization rates for diazlnon and parathlon are fairly rapid from water under the conditions of our tests (t> of 4.2 and 9.6 days, respectively). When compared to their hydrolysis rate constants (Table I), volatilization can be seen to be a more important route of loss than hydrolysis for diazlnon, parathlon, and methyl parathlon. The relative volatilization rates reported here for diazlnon and parathlon are in good agreement with those reported by Lichtenstein (14). 

It is apparent from Table II that variations in the experimental rate constants (k) are essentially controlled by the Henry s law constant, in agreement with the two-film theory prediction. A plot of kys. H for the five pesticides gave an intercept of 5.4 x 10 hr, a slope of 6.9 x 10 mol/(hr atm m" ), and a correlation coefficient of 0.969. Thus, it seems that Henry s law values could be used to predict relative volatilization rates of the pesticides, and an absolute volatilization rate for one pesticide can be calculated if the volatilization rate is known for another and Henry s law constants are known for both ... 

Except for mevinphos, agreement between experimental volatilization rate constants and rate constants predicted using k ... 

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. 

In the present study, EXAMS was used to calculate volatilization rate constants from water, wet soil, and a water-soil mixture. EXAMS uses the two-film theory to calculate volatilization rates from the 10 cm wind speed as discussed above. EXAMS requires as a minimum environment at least one littoral (water) and one benthic (sediment) compartment. A very small benthic compartment for the water system and a very small littoral compartment for the wet soil system (7.09 x 10 m3 volume and 1 x 10 8 m depth in both cases) was used, so that these compartments and their input parameters had a negligible effect on the calculated rates. For the water-soil system, the same proportions were used as in the laboratory experiment. Transfer rates between soil and water were assumed to be rapid relative to volatilization rates, and were set as recommended in the EXAMS manual (24). The input data needed by EXAMS in order to calculate volatilization rates from a water-soil system, using parathlon as an example, are shown in Table IV. 

Percents volatilized in one day for the various media were calculated using initial pesticide amounts and the overall volatilization rate constants, obtained from the half life for volatilization as output by EXAMS. Mevlnphos results are not included here, for as discussed previously, methods for calculation used in EXAMS are not appropriate for water miscible compounds. The experimental and computer predicted percents volatilized in one day are qualitatively similar (Figure 2). Quantitatively, experimental and predicted percents volatilized agreed within a factor of three for diazlnon, methyl parathion, and malathion, and within a factor of five for parathion. Considering the fact that EXAMS was not intended for use with wet soil systems, these results are encouraging. 

Table IV. EXAMS Volatilization Rate Constant Calculation for a Water-Soil System Input Data for Parathlon ...

A simple environmental chamber is quite useful for obtaining volatilization data for model soil and water disposal systems. It was found that volatilization of low solubility pesticides occurred to a greater extent from water than from soil, and could be a major route of loss of some pesticides from evaporation ponds. Henry s law constants in the range studied gave good estimations of relative volatilization rates from water. Absolute volatilization rates from water could be predicted from measured water loss rates or from simple wind speed measurements. The EXAMS computer code was able to estimate volatilization from water, water-soil, and wet soil systems. Because of its ability to calculate volatilization from wind speed measurements, it has the potential of being applied to full-scale evaporation ponds and soil pits. 

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