Partitioning Among Phases

Under natural conditions, contaminants often reach the earth s surface as a mixture of (potentially) toxic chemicals, having a range of physicochemical properties that affect their partitioning among the gaseous, liquid, and solid phases. As a consequence, contaminant retention properties in the subsurface are highly diverse. Contaminants may reach the subsurface from the air, water, or land surface. 

Berkowitz et al. Contaminant Geochemistry Transport and Fate in the Subsurface Environment. 

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On the other hand, when dealing with heterogeneous systems (e.g., suspension or emulsion polymerizations), it is important not to confuse thermodynamic effects of monomer partitioning among phases with variations in reactivity ratios. For the calculation of these, the concentrations of the monomers at the reaction site should be considered (at the particles) instead of global concentrations in the system. 

Fugacity modeling provides an alternate way of calculating chemical partitioning among phases. Also, fugacity capacities make it easy to predict 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. (2003). 

Many contaminants exist as immiscible nonaqueous phase liquids (NAPLs) in soil. These liquids do not fully solubilize in water and exist as a separate phase due to physical and chemical differences from water. NAPLs can be classified as light (less dense than water) nonaqueous phase liquids (LNAPLs) and dense (more dense than water) nonaqueous phase liquids (DNAPLs). A list of typical NAPLs and their important properties is presented in Table 8.8. As described elsewhere in this book (Chapter 15), NAPLs may solubilize, volatilize, and otherwise partition among phases. This section focuses on the advective transport of pure-phase NAPL. 

After emission, contaminants may be partitioned among the terrestrial, aqnatic, and various atmospheric phases, and those of sufficient volatility or associated with particles may be transported over long distances. This is not a passive process, however, since important transformations may take place in the troposphere during transit so that attention should also be directed to their transformation products. 

Fate and exposure analyses. The multimedia transport and transformation model is a dynamic model that can be used to assess time-varying concentrations of contaminants that are placed in soil layers at a time-zero concentration or contaminants released continuously to air, soil, or water. This model is used for determining the distribution of a chemical in the environmental compartments. An overview of the partitioning among the liquid, solid and/or gas phases of individual compartments is presented in Fig. 7. The exposure model encompasses... 

Changes of Manganese Partitioning Among Solid-phase Components in Arid-zone Soils Pathways and Short- and Intermediate-term Kinetics... 

A simple phase space model can be used to compute the CO product vibrational energy distribution as a function of the available energy,12-14 Eav. The maximum energy which can be partitioned among the products degrees of freedom is the reaction exoergicity, Ex = hv-DH°[(CO)5W-CO]. For a 351 nm photolysis,... 

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

After reaching the subsurface, contaminants are partitioned among the solid, liquid, and gaseous phases. A fraction of the contaminated gaseous phase is transported into the atmosphere, while the remaining part may be adsorbed on the subsurface solid phase or dissolved into the subsurface water. Contaminants dissolved in the subsurface aqueous phase or retained on the subsurface solid phase are subjected, over the course of time, to chemical, biochemical, and surface-induced degradation, which also lead to formation of metabolites. 

The retention of contaminants in the snbsnrface, controlled by properties of both chemicals and snbsnrface constituents as well as contaminant partitioning among the solid, aqneons, and gaseous phases, are the focus of Part III. Chapter 5 deals... 

Jafvert C.T. (1991). Sediment- and saturated-soil-associated reactions involving an anionic surfactant (dodecyl sulfate). 2. Partition of PAH compounds among phases. Environmental Science and Technology 25 1039-1045. 

Takacs-Novak, K., et al., Relationship Study Between Reversed Phase HPLC Retention and Octanol/Water Partition Among Amphoteric Compounds. J. Liquid Chromatogr., 1995 18, 807-825. 

While it might seem reasonable to use a generic marker such as polyethylene glycol, which is completely eliminated without absorption (used to verify integrity of the epithelial barrier), it is important for the marker to have physical properties similar to the nutrient in question because of the complexity of the postprandial intestinal milieu - a thick slurry of mixed micelles, oil and water phases, and suspended particles. The marker should partition among the phases similarly to the analyte of interest and should have similar intestinal transit times. Thus, sugars must be used to trace sugars, sterols to trace sterols, etc. 

Figure 3.28 Illustrative section from the phase prism of a mixture of oil, water, and surfactant. This section is for constant surfactant concentration (T is temperature). The section shows a middle-phase microemulsion phase existing together with oil (upper) and water (lower) phases. The surfactant is partitioned among all of the phases. The cross-hatching shows how the microemulsion can be O/W (to the left), or W/O (to the right), or bicontinuous (centre). From Schwuger et al. [226]. Copyright 1995, American Chemical Society.

While in the air compartment, the contaminant solubilizes in the vapor-liquid phase or is associated with aerosol particles by adsorption. It is also prone to desorption from the aerosol particles into the vapor phase. Relevant properties of the air used to model transport of partitioning of a contaminant in the air compartment include temperature, turbulence, wind speed, size and composition of aerosol particles, etc.16,19 Relevant properties of the contaminant that measure its tendency to partition among the vapor, liquid, and solid phases in the air include its aqueous solubility (Saq), vapor pressure (VP), Henry s constant... 

Although the rare-earth elements (REEs) have similar geochemical behavior, since they are all large-ion lithophile elements and most of them partition among melts and mineral phases as a smooth function of ionic radius (with the exception of europium, which, commonly being... 

Much of this progress has been achieved using basic techniques that were developed in the early 1980s. New techniques and improved interpretations await discovery and development. A coordinated study at the basin to global scale of the distribution of natural radionuclides, as well as their partitioning among dissolved, colloidal, and particulate phases, would improve our ability to exploit these tracers. 

Mackay et al. (1997) provide detailed examples of fugacity calculations to illustrate how variations in the physical and chemical properties of pesticides affect their partitioning among environmental media. Figure 3 displays the results from some of these calculations for three of the pesticides listed in Table 1. Consistent with the expectations described above, these computations predict that following their release into the hydro-logic system, the relatively water-soluble herbicide atrazine will come to reside mostly in the aqueous phase, the more hydrophobic insecticide... 

Now that solubility and vapor pressure have been defined, consider how a volatile chemical partitions, or distributes itself, between water and air phases at equilibrium. In general, a partition coefficient is the ratio of the concentrations of a chemical in two different phases, such as water and air, under equilibrium conditions. The Henry s law constant, H (or KH), is a partition coefficient usually defined as the ratio of a chemical s concentration in air to its concentration in water at equilibrium. [Occasionally, a Henry s law constant is interpreted in an inverse fashion, as the ratio of a chemical s concentration in water to its concentration in air see, e.g., Stumm and Morgan (1981, p. 179). Note that in that table, KH is equivalent to 1/H as H is defined above ] Values of Henry s law constants are tabulated in a variety of sources (Lyman et al, 1990 Howard, 1989, 1991 Mackay and Shiu, 1981 Hine and Mookerjee, 1975) Table 1-3 lists constants for some common environmental chemicals. When H is not tabulated directly, it can be estimated by dividing the vapor pressure of a chemical at a particular temperature by its aqueous solubility at that temperature. (Think about the simultaneous equilibrium among phases that would occur for a pure chemical in contact with both aqueous and gas phases.) Henry s law constants generally increase with increased temperature, primarily due to the significant temperature dependency of chemical vapor pressures as previously mentioned, solubility is much less affected by the changes in temperature normally found in the environment. 

A stoppered flask at 25°C contains 250 ml of water, 200 ml of octanol, and 50 ml of air. An unknown amount of o-xylene is added to the flask and allowed to partition among the phases. After equilibrium has been established, 5.0 mg of o-xylene is measured in the water. What is the total mass of o-xylene present in the flask ... 

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