Activity-Based Concept for Transport and Partitioning of Ionizing Organics

Ionizing chemicals, including pesticides, pharmaceuticals, and personal care products, are care products, are widely used chemicals of commerce and have been detected in the environment in large numbers. These “ionics” are subject to a variety of processes, such as dissociation, ion trap, and electrical interactions with organic matter and biota. Conventional chemodynamic concepts and models designed to treat neutral compounds do not necessarily address these processes. A new system of equations, based on activity and analogous to the fugacity approach, is suggested to describe the fate of organic ionics. The total concentration of all molecule species in a bulk compartment is determined from the product of activity ‘a’ and a bulk activity capacity ‘B’. The concentration ratio between compartments in equilibrium depends on the activity ratio and the capacity ratio. Changes in partitioning due to pH, ionic strength, and the ion trap effect are quantified. The calculation is illustrated for two pharmaceuticals, namely the monovalent acid ibuprofen and the monovalent base trimethoprim, in a multimedia lake system. Trimethoprim is neutral at high pH but ionized at low pH, while ibuprofen exhibits the opposite. The concentration ratios of air and biota to water are shown to depend on pH. The activity approach may be used to describe transport and partitioning of multivalent ionizable organic compounds and to build multimedia fate models.