Volatilization

The loss of volatile organics from a water surface can be described using firstorder kinetics, because it is assumed that the concentration in the atmosphere above the water surface is essentially zero. Equation 3.17 is the basic kinetic equation, and Equation 3.18 can be used to determine the "half-life" of the contaminant of concern (see Chapter 9 for further discussion of the half-life concept and its application to sludge organics):

where

kvol = Volatilization mass transfer coefficient (cm/hr) = (k)(y).

k = Overall rate coefficient (hr-1).

where t1/2 is the time when concentration Ct = (1/2)(C0) (hr), and the other terms are as defined previously.

The volatilization mass transfer coefficient is a function of the molecular weight of the contaminant and the air/water partition coefficient as defined by the Henry's law constant, as shown by Equation 3.19:

y where kvol = Volatilization coefficient (hr-1).

M = Molecular weight of contaminant of concern (g/mol).

The coefficients B1 and B2 are specific to the physical system of concern. Dilling (1977) determined values for a variety of volatile chlorinated hydrocarbons at a well-mixed water surface:

Jenkins et al. (1985) experimentally determined values for a number of volatile organics on an overland flow slope:

The coefficients for the overland-flow case are much lower because the flow of liquid down the slope is nonturbulent and may be considered almost laminar flow (Reynolds number = 100 - 400). The average depth of flowing liquid on this slope was about 1.2 cm (Jenkins et al., 1985).

Using a variation of Equation 3.19, Parker and Jenkins (1986) determined volatilization losses from the droplets at a low-pressure, large-droplet wastewater sprinkler. In this case, the y term in the equation is equal to the average droplet radius; as a result, their coefficients are valid only for the particular sprinkler system used. The approach is valid, however, and can be used for other sprinklers and operating pressures. Equation 3.20 was developed by Parker and Jenkins for the organic compounds listed in Table 3.3:

TABLE 3.3

Volatile Organic Removal by Wastewater Sprinkling

TABLE 3.3

Volatile Organic Removal by Wastewater Sprinkling

Calculated kO for

Substance

Equation 3.20 (cm/min)

Chloroform

0.188

Benzene

0.236

Toluene

0.220

Chlorobenzene

0.190

Bromoform

0.0987

m-Dichlorobenzene

0.175

Pentane

0.260

Hexane

0.239

Nitrobenzene

0.0136

m-Nitrotoluene

0.0322

PCB 1242

0.0734

Naphthalene

0.114

Phenanthrene

0.0218

Source: Parker, L.V. and Jenkins, T.F., Water Res, 20(11), 1417-1426, 1986. With permission.

Source: Parker, L.V. and Jenkins, T.F., Water Res, 20(11), 1417-1426, 1986. With permission.

Volatile organics can also be removed by aeration in pond systems. Clark et al. (1984a) developed Equation 3.21 to determine the amount of air required to strip a given quantity of volatile organics from water via aeration:

A J = (76.4)11 - C J (S)037(V)-045( M )-018(0.33)s (3.21) where

S = Saturated condition of the compound of concern equal to 0, for unsaturated organics; 1, for saturated compounds). V = Vapor pressure (mmHg). M = Molecular weight (g/mol). s = Solubility of organic compound (mg/L).

The values in Table 3.4 can be used in Equation 3.21 to calculate the air-to-water ratio required for some typical volatile organics.

TABLE 3.4

Properties of Selected Volatile Organics for Equation 3.21

TABLE 3.4

Properties of Selected Volatile Organics for Equation 3.21

Chemical

M

s

s

Trichloroethylene

132

1000

0

1,1,1-Trichloroethane

133

5000

1

Tetrachloroethlyene

166

145

0

Carbon tetrachloride

154

B00

1

cis-1,2-Dichloroethylene

9l

3500

0

1,2-Dichloroethane

99

8l00

1

1,1-Dichloroethylene

9l

40

0

Source: Love, O.T. et al., Treatment of Volatile Organic Chemicals in Drinking Water, EPA 600/8-83-019, U.S. Environmental Protection Agency, Municipal Engineering Research Laboratory, Cincinnati, OH, 1983.

Source: Love, O.T. et al., Treatment of Volatile Organic Chemicals in Drinking Water, EPA 600/8-83-019, U.S. Environmental Protection Agency, Municipal Engineering Research Laboratory, Cincinnati, OH, 1983.

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