Fugacity model partitioning

Equilibrium. Equilibrium between compartments can be expressed either as partition coefficients K.. (i.e. concentration ratio at equilibrium) or in the fugacity models as fugacity capacities and Z. such that K.. is Z./Z., the relationships being depicted in Figur 1. Z is dellned as tfte ratio of concentration C (mol/m3) to fugacity f (Pa), definitions being given in Table I. 

The earliest or Level I fugacity models simulate the simple situation in which a chemical achieves equilibrium between a number of phases of different composition and volume. The prevailing fugacity is simply/ = M/Y.V, x Z where M is the total quantity of chemical (mol), V, is volume (m3), and Z, is the corresponding phase Z value (mol Pa-1 m-3). Although very elementary and naive, this simulation is useful as a first indication of where a chemical is likely to partition. It is widely used as a first step in chemical fate assessments. 

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. 

Tetrahydrofuran is a liquid at room temperature and boils at 66 °C. The fugacity model predicts that tetrahydrofuran will be found in the environment where it is released. Photodegradation by hydroxyl radicals in air is estimated to be rapid and the hydroxyl radical reaction half-life is estimated at 7.3 h. Tetrahydrofuran released to water could partition to the water compartment and readily biodegrade, but not hydrolyze. Tetrahydrofuran has a very low bioaccumulation potential as evidenced by its low octanol/water partition coefficient. 

Fugacity modeling does not allow any new calculations to be made that cannot already be made with the partition coefficients described in the previous three sections. However, a comparison of the fugacity capacity of a chemical in different phases permits a direct assessment of which phase will have the highest chemical concentration at equilibrium. For further details, the reader is referred to Mackay and Paterson (1981) and Schwarzenbach et al. (1993). 

The US Environmental Protection Agency s (EPA s) EPISuite software, for example, contains a Level III fugacity model based on Dr. Mackay s EQC model [65]. EPISuite allows the user to estimate how a chemical partitions between compartments and its overall persistence in the environment. The model represents four main compartments air, water, sediment, and soil. The software essentially solves a series of equations that represent advection... 

LEV3EPI Level in multimedia fugacity model predicts partitioning of chemicals among air, soil, sediment, and water under steady-state conditions for a default model "environment"... 

Overall persistence (Pov) Characteristic time (days) for disappearance of a chemical, based on the modeled partitioning between media (using a Level III fugacity model) and the degradation rate in each medium. The benchmark value, 195 days, is based on the persistence of a-HCH. 

Simple models are used to Identify the dominant fate or transport path of a material near the terrestrial-atmospheric Interface. The models are based on partitioning and fugacity concepts as well as first-order transformation kinetics and second-order transport kinetics. Along with a consideration of the chemical and biological transformations, this approach determines if the material is likely to volatilize rapidly, leach downward, or move up and down in the soil profile in response to precipitation and evapotranspiration. This determination can be useful for preliminary risk assessments or for choosing the appropriate more complete terrestrial and atmospheric models for a study of environmental fate. The models are illustrated using a set of pesticides with widely different behavior patterns. 

Riederer, M.(1990) Estimating partitioning and transport of organic chemicals in the foliage/atmosphere system Discussion of a fugacity-based model. Environ. Sci. Technol. 24, 829-837. 

Possible fate in the environment. An industrial chemical that has been released into the environment will exist in differing concentrations in the various environmental compartments. The concentrations of a substance in air, water, soil and other media following release can be modelled using the concept of fugacity.2 At its simplest, this involves only the use of standard physico-chemical data to estimate the partitioning between the various media. 

Mention has already been made of mathematical models which simulate partitioning in the environment. This has been facilitated by the introduction of fugacity principles to environmental modelling, which simplifies the linking of complex partition and rate constants in many of the current multimedia environmental models. A detailed explanation of the ideas involved, and their application, has recently been published by Mackay.39... 

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