Molecular and Ionic Diffusion

Diffusion is a continuous process of species migration that tends to decrease concentration gradients both within sediment porewaters and between those waters and overlying waters. ionic diffusion refers to the diffusion of charged species that interact electrostatically, while molecular diffusion is usually used to describe the migration ofneutral species.

The flux, Fd (mol cm-2s-1), produced by the molecular/ionic diffusion in porewaters follows Fick's law of diffusion, which can be described in a one-dimensional (1D) model:

SC Sx where Ds is the effective diffusion coefficient of the solute inside the pores in cm s- , ^ the porosity, C the solute concentration in moll-1, and x the position, or distance, in cm. The diffusion ofsolutes in water can be described with a simplified version ofFick's law using the molecular diffusion coefficient in free water, Dw, which accounts for temperature and mass effects, with diffusion rates increasing with increasing temperature and decreasing mass ofthe solute. However, due to the porosity ofsediments and the added tortuosity of the path taken by an ion or molecule around sediment grains, to apply Fick's law to solute diffusion in sediments requires a correction be made to the diffusion coefficient. This is done by the use of Ds, which for sediments with high porosity is commonly derived from Dw via Archie's law (Ds = ^ Dw).

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