Equilibrium model for organic materials

GROVES EL-ZOOBI Equilibrium Model for Organic Materials in Water 487... 

Solubilities of 1,3-butadiene and many other organic compounds in water have been extensively studied to gauge the impact of discharge of these materials into aquatic systems. Estimates have been advanced by using the UNIFAC derived method (19,20). Similarly, a mathematical model has been developed to calculate the vapor—liquid equilibrium (VLE) for 1,3-butadiene in the presence of steam (21). 

The type of model to be used in a given situation depends on the nature of the release and on the properties of the ecosystem. When releases of metals or radionuclides are continuous and steady-state conditions are present, an equilibrium model such as a concentration factor model can be used. In this case,the important parameter needed for the model is the concentration factor, the ratio of the concentration in the animal to that in the water. The concentration in the animal is determined then by multiplying the concentration in the water by the concentration factor. In the case of accidental episodic releases, a dynamic situation exists in which organisms accumulate the material for a relatively short period and then lose it with a characteristic time constant. In this case,the important parameters needed for the model are biological turnover rate constants and concentration factors. 

For the pH values of leachate in these experiments (6.5 to 7.5), the As(V) species in reactions 6 and 7 predominate. Arsenate concentrations were below detection limits throughout the experiments however, after reactive organic carbon concentrations in the core decreased to the level where reduction of O2 was incomplete, geochemical modeling predicted oxidation of As(lll) to As(V). Because As(V) was not detected in core effluent, it was not possible to derive equilibrium constants for these adsorption reactions on this core material, and the equilibrium constants listed in Dzombak and Morel (1990) were used in the model. For reaction 6, the Log Kas(v)i was 23.51 and for reaction 7, the Log Kas(v)2 was 10.58. Arsenite adsorption was modeled by ... 

Reductions of P-keto esters with yeasts have been performed quite often it seems generally accepted that the enantioselectivity of the reduction of cyclic esters seems to be higher than those of open chain (3-keto esters substituted at C-2. Recently, a neuronal network model for these reactions was developed [267,268]. The influence of organic solvents has been investigated [269]. The reduction of six-membered cyclic p-keto esters is rather widespread thus, the reduction of the simplest compound in this context 211 (Fig. 54) gave 65-85% of ethyl(+)(iR,2S)-2-hydroxycyclohexane carboxylate (212) in 86-99% e.e. [270-272] and a diastereoselectivity of 76-99% [270]. Due to an equilibrium by enolization of the starting material followed by a kinetic resolution only one of the possible diastereomers is produced in excess. 

An elegant way to avoid the low yields and the need for recycling half of the material in the case of kinetic resolutions is a dynamic kinetic resolution (DKR). The dynamic stands for the dynamic equilibrium between the two enantiomers that are kinetically resolved (Scheme 6.6A). This fast racemisation ensures that the enzyme is constantly confronted with an (almost) racemic substrate. At the end of the reaction an enantiopure compound is obtained in 100% yield from racemic starting material. Mathematical models describing this type of reaction have been published and applied to improve this important reaction [32, 33]. There are several examples, in which the reaction was performed in water (see below). In most cases the reaction is performed in organic solvents and the hydrolase-catalysed reaction is the irreversible formation of an ester (for example see Figs. 9.3, 9.4, 9.6, 9.12) or amide (for example see Figs. 9.13, 9.14, 9.16). 

For sediment-dwelling organisms, one important factor that determines the degree of exposure to xenobiotics in the sediment phase is the partitioning from the true sediment phase into interstitial water from which the xeno-biotic may then be accumulated by biota. Exposure of sediment biota to xenobiotics is, however, a complex process, since uptake may proceed either via particulate material or via interstitial water, or by both routes. In the equilibrium partition model the concentration of a xenobiotic in the interstitial water (Ciw) is given by the following relation ... 

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