In dealing with the passive transport of organic contaminants from soils to plants (including crops), a partition-limited model is proposed in which (i) the maximum (equilibrium) concentration of a contaminant in any location in the plant is determined by partition equilibrium with its concentration in the soil interstitial water, which in turn is determined essentially by the concentration in the soil organic matter (SOM) and (ii) the extent of approach to partition equilibrium, as measured by the ratio of the contaminant concentrations in plantwater and soil interstitial water, alphapt (< or = 1), depends on the transport rate of the contaminant in soil water into the plant and the volume of soil water solution that is required for the plant contaminant level to reach equilibrium with the external soil-water phase. Through reasonable estimates of plant organic-water compositions and of contaminant partition coefficients with various plant components, the model accounts for calculated values of alphapt in several published crop-contamination studies, including near-equilibrium values (i.e., alphapt approximately equals 1) for relatively water-soluble contaminants and lower values for much less soluble contaminants; the differences are attributed to the much higher partition coefficients of the less soluble compounds between plant lipids and plant water, which necessitates much larger volumes of the plant water transport for achieving the equilibrium capacities. The model analysis indicates that for plants with high water contents the plant-water phase acts as the major reservoir for highly water-soluble contaminants. By contrast, the lipid in a plant, even at small amounts, is usually the major reservoir for highly water-insoluble contaminants.
studiese of silver chloride solutions in pyridine which yielded a value of 8.4 X for K A~c~, on the assumption of simple dissociation only, are not corroborated in the present work.
Acknowledgment.Liquid-phase adsorption isotherms at 25' on an activated carbon have been determined, over a wide range of concentrations, for the following systems: Sudan I11 (benzeneazo-p-benzeneazo-&naphthol) in acetone, cyclohexane, carbon tetrachloride, benzene, and carbon disulfide, and Butter Yellow (p-dimethylaminoazobenzene) in methanol, acetonitrile, acetone, 2-propanol, cyclohexane, heptane, benzene, and carbon disulfide. Except for the high capacity range, most of the data can be fitted to a correlation curve determined for the same carbon from gas-phase adsorption measurements, as predicted by the Polanyi adsorption potential theory. The experimental link between liquid and gas-phase adsorption and the relative constancy of the solvent effect, on adsorption (measured in appropriate units) appear to introduce a measure of predictability to at least some liquid-phase adsorption isotherms. Adsorption tends to be weakest in solvents of highest refractive index.(1) (a) Professor,
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