8.4: Multimedia partition calculations

Multimedia Partition Calculations. The environment, of course, contains all of the media that we have discussed air, water, sediment, particles, living organisms, etc. The goal of multimedia partition calculations is to determine where the chemicals placed in the environment will eventually go. We will perform a multimedia calculation in Example 8.4. A sealed fish tank is chosen because it is a microcosm and does not require boundary conditions, such as air concentration. 

In the multimedia models used in this series of volumes, an air-water partition coefficient KAW or Henry s law constant (H) is required and is calculated from the ratio of the pure substance vapor pressure and aqueous solubility. This method is widely used for hydrophobic chemicals but is inappropriate for water-miscible chemicals for which no solubility can be measured. Examples are the lower alcohols, acids, amines and ketones. There are reported calculated or pseudo-solubilities that have been derived from QSPR correlations with molecular descriptors for alcohols, aldehydes and amines (by Leahy 1986 Kamlet et al. 1987, 1988 and Nirmalakhandan and Speece 1988a,b). The obvious option is to input the H or KAW directly. If the chemical s activity coefficient y in water is known, then H can be estimated as vwyP[>where vw is the molar volume of water and Pf is the liquid vapor pressure. Since H can be regarded as P[IC[, where Cjs is the solubility, it is apparent that (l/vwy) is a pseudo-solubility. Correlations and measurements of y are available in the physical-chemical literature. For example, if y is 5.0, the pseudo-solubility is 11100 mol/m3 since the molar volume of water vw is 18 x 10-6 m3/mol or 18 cm3/mol. Chemicals with y less than about 20 are usually miscible in water. If the liquid vapor pressure in this case is 1000 Pa, H will be 1000/11100 or 0.090 Pa m3/mol and KAW will be H/RT or 3.6 x 10 5 at 25°C. Alternatively, if H or KAW is known, C[ can be calculated. It is possible to apply existing models to hydrophilic chemicals if this pseudo-solubility is calculated from the activity coefficient or from a known H (i.e., Cjs, P[/H or P[ or KAW RT). This approach is used here. In the fugacity model illustrations all pseudo-solubilities are so designated and should not be regarded as real, experimentally accessible quantities. 

Multimedia models and field data indicate that this is a dominant process for the transfer of persistent organic pollutants to lakes (e.g., Bidleman McConnell, 1995 Mackay Wania, 1995). Fluxes of air-water gas exchange are typically calculated using the two-film resistance model (Schwartzenbach et al 1993). Fluxes can be in either direction and depend upon air and water concentrations, partition coefficients, and air and water resistivities. 

In addition, two multimedia models were developed one which takes into account the flows between surface water, lower troposphere, and soil as they result from the three single-medium models EXWAT, EXAIR and EXSOL, and another one which is based on Mackay s fugacity approach for calculating the partitioning of a chemical between the three media (Mackay et al. 1983a 1983b). 

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