Transport phenomena are among the most important processes in natural systems. Chemical compounds, the constituents of biogeochemical systems, are in continual motion in all parts of the earth. The thermal motion of atoms and molecules is perceived on the macroscopic level as molecular diffusion i.e., as the slow but persistent movement “down along the concentration gradient.” Although the average speed of the atoms is on the order of tens to hundreds of meters per second, the net transport is small, because the molecules do not maintain the same direction long enough. Thus, typical molecular diffusion coefficients of solutes in water are approximately 10-9 m2s - 1 corresponding to characteristic annual transport distances of approximately 20 cm. In solids the diffusion coefficients even drop to values as low as 10-14m2s-1 or less.
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