Sorption coefficients

Permeability P, can be expressed as the product of two terms. One, the diffusion coefficient, reflects the mobility of the individual molecules in the membrane material the other, the Henry s law sorption coefficient, reflects the number of molecules dissolved in the membrane material. Thus equation 9 can also be written as equation 10. 

Fig. 9. Parametric simulation using the MSD model of water flux vs feed concentration as a function of organic sorption coefficient, where for A,...

Because many studies have shown a direct relationship between pesticide sorption and organic carbon content of sod, attempts have been made to develop a universal sorption coefficient based on sorption of the pesticide to sod organic carbon (44). Sorption based on sod organic carbon is expressed as C, where is pesticide sorbed per unit mass sod organic carbon, and C is pesticide solution concentration after equdibration. If. is the fraction of organic carbon, can be obtained from i in the equation. Assumptions in the use of this approach include... 

Indirect methods of estimating sorption have been used when actual measurement of sorption isotherm is impossible (44). For instance, sorption coefficients have been estimated from soil organic carbon and a specific surface of soil, and from semiempidcal equations using pesticide properties. 

It appears that pesticides with solubiHties greater than 10 mg/L are mainly transported in the aqueous phase (48) as a result of the interaction of solution/sediment ratio in the mnoff and the pesticide sorption coefficient. For instance, on a silt loam soil with a steep slope (>12%), >80% of atra2ine transport occurs in the aqueous phase (49). In contrast, it has been found that total metolachlor losses in mnoff from plots with medium ground slopes (2—9%) were <1% of appHed chemical (50). Of the metolachlor in the mnoff, sediment carried 20 to 46% of the total transported pesticide over the monitoring period. 

Plasticization and Other Time Effects Most data from the literature, including those presented above are taken from experiments where one gas at a time is tested, with Ot calculated as a ratio of the two permeabihties. If either gas permeates because of a high-sorption coefficient rather than a high diffusivity, there may be an increase in the permeabihty of all gases in contact with the membrane. Thus, the Ot actually found in a real separation may be much lower than that calculated by the simple ratio of permeabilities. The data in the hterature do not rehably include the plasticization effect. If present, it results in the sometimes slow relaxation of polymer structure giving a rise in permeabihty and a dramatic dechne in selectivity. 

Kenaga EE. 1980. Predicted bioconcentration factors and soil sorption coefficients of pesticides and other chemicals. Ecotoxicol Environ Safety 4 26-38. 

Doucette, W.J. (2003) Quantitative Structure-Activity Relationships for Predicting SoU-Sediment Sorption Coefficients for Organic Chemicals. Environmental Toxicology and Chemistry, 22(8), 1771-1788. 

ECD Electron capture detection Ko, Kqc Soil sorption coefficients... 

Vapor pressure (VP), water solubility ( w), and soil sorption coefficients Koc) are key properties that govern volatilization of agrochemicals from soil. Volatile compounds such as 5 -ethyl dipropylthiocarbamate (EPTC) (VP 4.5 Pa,... 

Sabljia, A., On the prediction of soil sorption coefficients of organic pollutants from molecular structure application of molecular topology model, Environ. Sci. Technol., 21, 358-366, 1987. 

Table 2. The aliphaticity and concentration-dependent organic carbon normalized sorption coefficient (log Koc) for four humic acids (unpublished data by Kang and Xing, 2005)... 

There is a continuing effort to extend the long-established concept of quantitative-structure-activity-relationships (QSARs) to quantitative-structure-property relationships (QSPRs) to compute all relevant environmental physical-chemical properties (such as aqueous solubility, vapor pressure, octanol-water partition coefficient, Henry s law constant, bioconcentration factor (BCF), sorption coefficient and environmental reaction rate constants from molecular structure). 

Bahnick, D. A., Doucette, W. J. (1988) Use of molecular connectivity indices to estimate soil sorption coefficients for organic chemicals. Chemosphere 17, 1703-1715. 

Meylan, W. M., Howard, P. H., Boethling, R. S. (1992) Molecular topology/fragment contribution for predicting soil sorption coefficient. Environ. Sci. Technol. 26, 1560-1567. 

Delle Site, A. (2001) Factors affecting sorption of organic compounds in natural sorbent/water systems and sorption coefficients for selected pollutants. A review. J. Phys. Chem. Ref. Data 30, 187-439. 

De Maagd, P.G.-J., Sinnige, T.L., Schrap, M., Opperhuizen, A., Sijm, T.H.M. (1998) Sorption coefficients of polycyclic aromatic hydrocarbons for two lake sediments Influence of the bactericide sodium azide. Environ. Toxicol. Chem. 17, 1977-1907. 

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