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

Area of Science:

• Environmental Chemistry
• Chemical Kinetics
• Environmental Science

Background:

• Chemicals with high partition ratios (HPRs) in water or air can exhibit unexpectedly long kinetic delays in reaching equilibrium with organic media.
• These delays, termed 'hydrophobic delay' (K_OW) or 'aerophobic' delay (K_OA), can be mistaken for nonlinear partitioning behavior.
• A parallel particle pathway (PPP) can also influence these processes, potentially shortening delays and complicating interpretations.

Purpose of the Study:

• To explain the phenomenon of kinetic delays in the partitioning of hydrophobic chemicals.
• To differentiate true nonlinear partitioning from kinetic delays.
• To highlight the role of the parallel particle pathway (PPP) in chemical partitioning.

Main Methods:

• Compilation of fundamental equations governing diffusive equilibration processes.
• Inclusion of the parallel particle pathway (PPP) as a complementary transport mechanism.
• Analysis of five case studies illustrating the impact of HPR delays on chemical behavior.

Main Results:

• Hydrophobic delays can distort the perception of chemical partitioning, leading to the erroneous development of nonlinear models.
• The parallel particle pathway (PPP) can occur simultaneously, affecting the duration of HPR delays.
• Observed environmental behaviors of chemicals, including air-aerosol partitioning, indoor accumulation, air-vegetation partitioning, organismal transport, and fish bioaccumulation, are explained by HPR delays.

Conclusions:

• The general hydrophobic delay and parallel particle pathway (PPP) are common, often underappreciated processes in environmental chemistry.
• Misinterpreting kinetic delays as nonlinear partitioning can lead to inaccurate models and conclusions.
• Understanding these kinetic effects is crucial for accurate assessment of chemical behavior in the environment.