ABSTRACT
Molecular diffusion is an inherent and ubiquitous mass transport process by which chemical species move within and between environment phases. All environmental mass transfer coefficients, in one way or another, reflect the molecular diffusivity of the chemical species in the environmental “solvents” (i.e., air, water, organic matter, nonaqueous liquids, solids, etc.). It is therefore the fundamental transport parameter for molecular mass transport. It is important to keep inmind that diffusion is generally only used to describe molecular motion in the absence of mechanical mixing or advection. The material in this chapter provides a brief, theoretical introduction to diffusivities and follows this with estimation techniques, developed using empirical data, for chemicals in the significant environmental media compartments. The chapter concludes with example calculations.
