Estimation of the volatilization of organic compounds from soil surfaces.

Several simple models for the estimation of the half-life (t(1/2)) for the depletion of an organic chemical from a soil surface to air were examined. For moist surfaces, two models are proposed: the first requires knowledge of the soil/organic carbon partition coefficient (K(oc)) and the Henry's law constant (H) and the second the vapor pressure (P(s)) of the chemical involved. Due to uncertainties in the experimental K(oc) values those ones predicted by the group-contribution model of Meylan et al. [Environ. Sci. Technol. 26 (1992) 1560]-and proposed by the U.S. Environmental Protection Agency (EPA)-should be used. If reliable experimental P(s) values are not available, the first model is proposed, where in cases when H values are not available, predicted ones by the Bond-Contribution method of Meylan and Howard [Environ. Toxicol. Chem. 10 (1991) 1283]-and also proposed by EPA-can be used. In general, the agreement of the predicted t(1/2) values with the measured ones is within a factor of 3-5. Similar expressions, but with somewhat poorer results, are presented for dry field soils. In all cases, the obtained results represent a substantial improvement over those obtained with the currently used Dow method: t(1/2) = 1.58 x 10(-8)((K(oc) x S)/P(S)), where S is the solubility of the compound in water.

Prediction of the bioaccumulation of persistent organic pollutants in aquatic food webs.

Predictive correlations of the bioaccumulation factor of persistent organic pollutants in aquatic biota are presented as functions of their octanol/water partition coefficient. The correlations demonstrate the importance of differentiating among the different levels in the food web and of accounting for the pollutant's bioavailability by considering the amount freely dissolved in water instead of the total concentration. They also reveal the significance of the pollutant's octanol/water partition coefficient value on its biomagnification along the levels of the trophic chain. Prediction results, finally, demonstrate that the correlations provide reasonably accurate estimates of bioaccumulation, typically within an order-of-magnitude.

Critical Micelle Concentrations of Nonionic Surfactants in Organic Solvents: Approximate Prediction with UNIFAC.

The objective of this study was to model the critical micelle concentration (cmc) of nonionic surfactants in nonaqueous systems using the UNIFAC group contribution method. For the prediction of the cmc the phase separation approach was used, where the micellar phase is approximated as a second liquid phase resulting from the liquid-liquid equilibrium between the solvent and the surfactant, with the necessary activity coefficients predicted by UNIFAC. The limited amount of literature data for reverse micelle formation in nonaqueous systems was used to test the predictions, varying surfactant type, solvent, and temperature. The most promising model was the modified UNIFAC of B. L. Larsen, P. Rasmussen, and A. Fredenslund (Ind. Eng. Chem. Res. 26, 2274 (1987)). Since most nonionic surfactants contain oxyethylene chains, a new set of parameters was evaluated for this group, leading to satisfactory predictions. The average deviation between the predicted and the experimental cmc's was about 0.1 log units. Copyright 2001 Academic Press.