Partitioning in the Environment

The persistent organochlorine compounds, once released, will partition between environmental media according to their physical and chemical properties. Steady state equilibrium partitioning between these media has been considered as the simplest model simulating their behaviour. Over recent years, modelling the environmental partitioning and fate of these compounds has led to a broad 

To understand the mechanisms of pesticide behaviour in the environment, simple relationships and theoretical models have been derived to describe partitioning in a myriad of field and laboratory experiments. Some of the more important areas of research are briefly discussed below. 

For the more volatile compounds such as HCB and the HCHs, evaporation will be the major loss mechanism over a relatively short period of time. For compounds with higher adsorption, such as DDT, residual quantities may 

Matthiessen, D. Sheahan. R. Harrison, M. Kirby, R. Rycroft, A. Turnbull, C. Volkner and R. 

The partition of compounds between water and air is given by the Henry s law constant, H, which may be defined as the ratio of a compound s vapour pressure to its solubility, as indicated in equation (1), or alternatively as the dimensionless equilibrium concentration in air over the corresponding concentration in water, Xaw (equation 2). 

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Lyman, W , Prediction of Chemical Partitioning in the Environment, U.S. Environmental Protection Agency, Final Draft Report, May 1981. 

The advantages of developing such correlations is that once any of the parameters is known it is then a simple process to estimate the others. This is particularly useful in early evaluation of chemical partitioning in the environment. From a limited amount of information on a chemical, for example, its vapor pressure, water solubility and melting point, other partitioning parameters can be estimated and used in simple ecosystem models to evaluate the chemical s expected environmental distribution. 

The physical properties of -hexane (see Table 3-2) that affect its transport and partitioning in the environment are water solubility of 9.5 mg/L log Kow (octanol/water partition coefficient), estimated as 3.29 Henry s law constant, 1.69 atm-m3 mol vapor pressure, 150 mm Hg at 25 °C and log Koc in the range of 2.90 to 3.61. As with many alkanes, experimental methods for the estimation of the Koc parameter are lacking, so that estimates must be made based on theoretical considerations (Montgomery 1991). 

Hamer T. 2001. Measurements on octanol-air partition coefficients (KOA) for polybrominated diphenyl ethers (PBDEs) Predicting partitioning in the environment. BFR 55-58. 

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... 

Health effects data also are available for some petroleum fractions or process streams that are less heterogeneous. These materials are more representative of the fractions that may partition in the environment and are more useful for assessing health effects of intermediate and chronic exposure to petroleum hydrocarbons. These products are discussed further in Section 6.2. Additional discussion of these and also the more heterogeneous products is presented in Section 6.3. 

Indeed, OCPs, once released into the environment, are distributed into various environmental compartments (e.g., water, soil, and biota) as a result of complex physical, chemical, and biological processes. In order to perform appropriate exposure and risk assessment analyses, multimedia models of pollutant partitioning in the environment have been developed. Properties which are at the base of such a partitioning are water solubility (WS), octanol-water partition coefficient (Ko ), soil adsorption (K ), and bioconcentration factors (BCFs) in aquatic organisms, following these four equilibriums ... 

The toxicological or cumulative effect of illicit drugs on the ecosystems has not been studied yet. Moreover, their fate and transport in the environment is to a big extent still unknown. Due to their physical-chemical properties (octanol-water partition coefficient, solubility, etc.) some of them, such as cannabinoids, are likely to bioaccumulate in organisms or concentrate in sediments whereas the rest, much more polar compounds, will tend to stay in aqueous environmental matrices. However, continuous exposure of aquatic organisms to low aquatic concentrations of these substances, some of them still biologically active (e.g., cocaine (CO), morphine (MOR) and MDMA) may cause undesirable effects on the biota. 

The properties of these systems depend strongly on the interfacial potentials created at the interface. They arise from oriented molecular dipoles, from ionization of the surfactant hydrophylic groups, and from the partition and adsorption of ions presented in the environment. 

Such a relationship describes how a chemical will partition between water and the atmosphere under equilibrium conditions and is appropriate only for dilute solutions which are typically observed in the environment. Certain hydrocarbons despite possessing relatively low vapor pressures, may tend to partition significantly toward the air. This is largely a result of their correspondingly low water solubilities which result in low values for Kw. Therefore, chemicals which have low values for Kw have a greater tendency to partition towards the air and volatilize from solution. 

A number of mathematical models have been developed in recent years which attempt to predict the behavior of organic water pollutants. >2>3 Models assume that compounds will partition into various compartments in the environment such as air, water, biota, suspended solids and sediment. The input to the models includes the affinity of the compound for each of the compartments, the rate of transfer between the compartments, and the rates of various degradation processes in the various compartments. There is a growing body of data, however, which indicates that the models to date may have overlooked a small but significant interaction. A number of authors have suggested that a portion of the compounds in the aqueous phase may be bound to dissolved humic materials and are not therefore truly dissolved. 

The data presented here indicates that the extent of binding for a particular compound is related to the octanol/water partition coefficient for that compound. This is very similar to the sorption of compounds from water to sediment. Compounds with log Kow values less than four (such as Lindane) will probably not be bound to an appreciable extent in the environment. Compounds with very high log Kow values (DDT and DEHP) may be bound to a significant extent. The extent of binding will depend on both the concentration of humic material and on the nature of the humic material. The humic materials used in this research showed dramatically different affinities for DDT. The reasons for this are poorly understood and deserve further study. 

Physical and Chemical Properties. Most of the important physical-chemical properties of acrylonitrile have been determined (see Chapter 3). However, the partitioning of acrylonitrile between the air and water has been evaluated by using an estimated value for a Henry s law constant. This general approach assumes that the concentration of the chemical in water is low. Because acrylonitrile is relatively soluble in water, this approach may not be accurate. Experimental measurement of the partition coefficient for acrylonitrile at water-air interfaces would be useful in refining models on the behavior of acrylonitrile in the environment. 

It is now clear that the fate of chemicals in the environment is controlled by a combination of three groups of factors. First are the prevailing environmental conditions such as temperatures, flows and accumulations of air, water and solid matter and the composition of these media. Second are the properties of the chemicals which influence partitioning and reaction tendencies, i.e., the extent to which the chemical evaporates or associates with sediments, and how fast the chemical is eventually destroyed by conversion to other chemical species. Third are the patterns of use, into which compartments the substance is introduced, whether introduction is episodic or continuous and in the case of pesticides how and with which additives the active ingredient is applied. 

The major differences between behavior profiles of organic chemicals in the environment are attributable to their physical-chemical properties. The key properties are recognized as solubility in water, vapor pressure, the three partition coefficients between air, water and octanol, dissociation constant in water (when relevant) and susceptibility to degradation or transformation reactions. Other essential molecular descriptors are molar mass and molar volume, with properties such as critical temperature and pressure and molecular area being occasionally useful for specific purposes. A useful source of information and estimation methods on these properties is the handbook by Boethling and Mackay (2000). 

Solubility in water and vapor pressure are both saturation properties, i.e., they are measurements of the maximum capacity that a solvent phase has for dissolved chemical. Vapor pressure P (Pa) can be viewed as a solubility in air, the corresponding concentration C (mol/m3) being P/RT where R is the ideal gas constant (8.314 J/mol.K) and T is absolute temperature (K). Although most chemicals are present in the environment at concentrations well below saturation, these concentrations are useful for estimating air-water partition coefficients as ratios of saturation values. It is usually assumed... 

Leo, A.J. (1975) Calculation of partition coefficients useful in the evaluation of relative hazards of various chemicals in the environment. In Symposium on Structure-Activity Correlations in Studies of Toxicity and Bioconcentration with Aquatic Organisms. G.D. Veith and D.E. Konasewich, Editors, International Joint Commission, Ontario, Canada. 

Another example of the importance of the nature of the stimuli in the environment with respect to the effects of LSD on measures of locomotion is provided by the work of Hughes (93). He placed rats in compartmentalized square boxes for 60 min with a partition separating the novel from the familiar half. The rats were removed for 30 min and immediately injected, or... 

Physical and Chemical Properties. Data on physical and chemical properties are essential for estimating the partitioning of a chemical in the environment. The data on known physical and chemical properties form the basis of many of the input requirements for environmental models that predict the behavior of a chemical under specific conditions including those in hazardous waste landfills. Most of the necessary data on physical and chemical properties are available for 72-hexane. 

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