Fugacity models are widely adopted for the environmental exposure assessment of organic chemicals but are inconvenient for nonvolatile substances, such as ionizable chemicals. The activity approach is a robust alternative to the fugacity concept and provides the thermodynamically exact equations to describe the behavior of neutral and ionizable molecules in nonideal systems. A multimedia activity model applicable to neutral and ionizable molecules (MAMI) was developed and tested for 2,4-dichlorophenoxyacetic acid and the bases aniline and trimethoprim. The model features pH and ionic strength dependence and species-specific estimations of partition coefficients from physicochemical properties. Sorption estimates consider both lipophilic and electrical interactions. A realistic regional exposure scenario was simulated for the three test compounds, and model results were compared with results obtained with a conventional fugacity model and with monitoring data. The better performance of MAMI indicates that the activity approach can enlarge the applicability domain and improve model predictions of existing regional models. Model results, supported by experimental evidence, showed the importance of dissociation, electrical interactions in solids, humidity in air, and to a lesser extent salinity in seawater to describe the environmental fate of ionizable organic chemicals. Environ. Toxicol. Chem. 2010;29:789–799. © 2009 SETAC
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