Kinetic Delay in Partitioning and Parallel Particle Pathways: Underappreciated Aspects of Environmental Transport

We outline the general conditions under which chemicals of high hydrophobicity or, more generally, high partition ratios (HPRs) in water or air, may experience unexpectedly long kinetic delays in approaching equilibrium conditions with organic media. Such a "hydrophobic delay" in the case of K _OW or an "aerophobic" delay in the case of K_OA may be misinterpreted as being caused by a change in partitioning behavior or mechanism, resulting in development of nonlinear regression models describing intermedia partitioning. In fact, the partitioning is fundamentally linear but is distorted by a kinetic delay in partitioning. To illustrate this concept, we first compile the fundamental equations describing the diffusive equilibration processes, including a complementary transport mechanism termed a parallel particle pathway (PPP). Such a mechanism may occur simultaneously, shortening the HPR delay and complicating interpretation. Second, we describe five examples in which the HPR delay explains the observed and occasionally difficult-to-interpret environmental behavior of chemicals, namely studies of air-aerosol partitioning, chemical accumulation in indoor dust and surfaces, air-vegetation partitioning, internal transport in organisms, and fish bioaccumulation and toxicity. We believe that the general HPR delay and PPP issues deserve exposure as a commonly occurring and often underappreciated process.