Chemical Partitioning

The transfer of chemicals between two or more environmental compartments or phases can be described by equilibrium partitioning, and knowledge of this partitioning is essential for understanding and describing chemical fate in the environment. Chemical partitioning takes place between adjacent phases such as between a solid and a liquid (dissolution), a liquid and a gas (volatilisation), a solution and a solid surface (adsorption) or a solution and an immiscible liquid (solvent 

The term partitioning constant or coefficient refers to one chemical species in each phase (for example, ionisable chemicals present in water may exist in both neutral and dissociated forms and therefore each would have a separate partitioning coefficient). For well-defined phases, such as pure water or the pure liquid state of the chemical, then partitioning with another equally well-defined phase such as air results in the use of the term partitioning constant. The Henry s Law constant describing chemical partitioning between pure air and water is one example. Distribution ratios on the other hand, such as the soil-water 

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Allelopathic inhibition of mineral uptake results from alteration of cellular membrane functions in plant roots. Evidence that allelochemicals alter mineral absorption comes from studies showing changes in mineral concentration in plants that were grown in association with other plants, with debris from other plants, with leachates from other plants, or with specific allelochemicals. More conclusive experiments have shown that specific allelochemicals (phenolic acids and flavonoids) inhibit mineral absorption by excised plant roots. The physiological mechanism of action of these allelochemicals involves the disruption of normal membrane functions in plant cells. These allelochemicals can depolarize the electrical potential difference across membranes, a primary driving force for active absorption of mineral ions. Allelochemicals can also decrease the ATP content of cells by inhibiting electron transport and oxidative phosphorylation, which are two functions of mitochondrial membranes. In addition, allelochemicals can alter the permeability of membranes to mineral ions. Thus, lipophilic allelochemicals can alter mineral absorption by several mechanisms as the chemicals partition into or move through cellular membranes. Which mechanism predominates may depend upon the particular allelochemical, its concentration, and environmental conditions (especially pH). 

The physical behavior of a chemical determines how the chemical partitions among the various environmental media and has a large effect on the environmental fate of a substance. For example, the release into soil of two different acids (with similar chemical behavior) may result in one chemical mainly volatilizing into the air and the other chemical becoming mainly sorbed to the organic fraction of the soil. The physical behavior of a substance therefore can have a large effect on the environmental fate of that substance. 

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. 

Hickey M.G., Kittrick J.A. Chemical partitioning of cadmium, copper, nickel, and zinc in soils and sediments containing high levels of heavy metals. J Environ Qual 1984 13 372-376. 

Physical processes acting in the subsurface determine how a chemical partitions in the subsurface media. ... 

Spurlock, F.C. and Biggar, J.W. Thermodynamics of organic chemical partition in soils. 2. Nonlinear partition of substituted phenylureas from aqueous solution. Environ. Sci Technol., 28(6) 996-1002, 1994. 

Simulations for Ni-based superalloys have been carried out by Saunders (1995, 1996b), Chen et al. (1994) and Boettinger et al. (1995). Saunders (1995) used a straightforward Scheil simulation to predict chemical partitioning and the formation of interdendritic phases in a modified single crystal U720 alloy containing... 

Spurlock, F. C., and J. W. Biggar, Thermodynamics of organic chemical partition in soils. 3. Nonlinear partition from water-miscible cosolvent solutions , Environ. Sci. Technol., 28,1002-1009 (1994b). 

Chiou, C. T. (1995). Comment on Thermodynamics of organic chemical partition in soils . Environmental Science Technology, 29, 1421—2. 

As a contaminant moves through soil and groundwater, chemical processes will affect both contaminant concentration and overall hydrogeochemistry (Schoonen, 1998) of the system. Different adsorption mechanisms cause pollutants to adsorb onto the soil, volatilize, precipitate, and be part of the oxidation-reduction processes. Adsorption is loosely described as a process in which chemicals partition from a solution phase into or onto the surfaces of solid-phase materials. Adsorption at particle surfaces tends to retard contaminant movement in soil and groundwater. 

Chemical equilibrium is achieved when chemical is distributed among environmental media (including organisms) according to the chemical s physico-chemical partitioning behavior. Thermodynamically, an equilibrium is defined as "a condition where the chemical s potentials (also chemical activities and chemical fugacities) are equal in the environmental media." At equilibrium, chemical concentrations in static environmental media remain constant over time. 

Hamelink et al. (1971) first proposed the hypothesis that bioconcentration is essentially a chemical partitioning process. Since then, several relationships between the bioconcentration factor and Kow have been reported. Meylan et al. (1999) reported the following correlation for 694 substances in fish ... 

If the rate constant for metabolic transformation (km) in Equation (15) is significant relative to the rate of chemical elimination to water (k2), the steady-state bioconcentration factor will be k, /(k2 + km) and hence lower than k2 /k2, which reflects the situation where the chemical substance is chemically partitioning between the organism and the water. 

Kow Kqc chemical partition coefficient between octanol and water (KoW), and organic carbon and water (KoC)... 

Li, X., Coles, B.J., Ramsey, M.H. and Thornton, I. (1995b) Chemical partitioning of the new NIST standard reference materials (SRM 2709-2711) by sequential extraction using ICPAES. Analyst, 120, 1415-1419. 

Patsakis, N., Sawides, C., Haralambous, K.J. and Loizidou, M. (1998) Chemical partitioning of metals in thermally treated sewage sludge. Environ. Technol, 19, 331-337. 

Unlike elemental concentrations, isotopic compositions are only affected a little by chemical differentiation processes. Mass-dependent isotopic fractionations can arise in chemical partitioning (cf. Section 2.9), of course, but on the scale of interest in the present context, plausible fractionation effects are small, especially at the high temperatures prevalent in the mantle. We can thus be much more confident that a noble gas isotopic composition measured in a mantle-derived sample is indeed characteristic of its mantle source. Representative mantle ranges for selected isotopic ratios are presented in Table 6.2. 

Octanol is a partitioning medium just as water is a partitioning media. While there is nothing inherently special about octanol with respect to other organic liquids, the extent that an organic chemical partitions to octanol from water has become a standard for evaluating hydrophobicity (i.e., chemicals that partition more to octanol from water are more hydrophobic). Since HOPs that are more hydrophobic accumulate more in body tissues, partition more strongly to soils and sediments, and are typically more easily removed by adsorption from water, the extent that HOPs partition to octanol from water is a very important environmental indicator. 

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