Transport and Fate of Chemicals in the Environment, Introduction
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Estimating the fate and transport of chemicals released into the environment is an interesting and challenging task. The environment can rarely be approximated as well mixed, and the chemicals in the environment often are not close to equilibrium. Thus, chemical fate and transport in the environment requires a background in the physics of fluid flow and transport, chemical thermodynamics, chemical kinetics, and the biology that interacts with all of these processes. The goal is to follow chemicals as they move, diffuse, and disperse through the environment. These chemicals will inevitably react to form other chemicals, in a manner that approaches, but rarely achieves, a local equilibrium. Many times, these reactions are biologically mediated, with a rate of reaction that more closely relates to an organism being hungry, or not hungry, than to first-order kinetics.
KeywordsTurbulent Diffusion Sorption Site Diffusive Transport Transport Path Resistance Unit
The quantity of a compound or chemical per unit volume, unit mass, or unit mole, where 1 mole = 6.02 × 1023 molecules of the chemical or compound. In this text, concentration in mass or moles per volume of water, mass per mass of solid, and moles per mole of gas will be discussed, depending upon the media of interest.
The movement of a constituent with the movement of the fluid.
Total mass per unit volume.
The spreading of fluid constituents through the motion inherent to atoms and molecules.
- Diffusion coefficient
A coefficient that describes the tendency of molecules to spread a constituent mass.
The mixing of a more concentrated solution with one that is less concentrated. The adage “The solution to pollution is dilution” is still used, sometimes appropriately, for many pollution and mitigation processes.
- Kinematic viscosity
The fluid viscosity divided by the fluid density, resulting in units that are similar to a diffusion coefficient, or length squared per time.
- Turbulent diffusion
The mixing of chemicals by turbulence, such that a turbulent diffusion coefficient can be defined separately from the temporal mean convection.