Molecular diffusivity in water

Box 18.4 Temperature Dependence of Molecular Diffusivity in Fluids Illustrative Example 18.3 Estimating Molecular Diffusivity in Water... 

Illustrative Example 18.3 Estimating Molecular Diffusivity in Water... 

A method of estimating molecular diffusivity in water is represented by the Stokes-Einstein equation which describes the theoretical relationship between a chemical s diffusivity in water and its molecular size ... 

Qui H-N (1996) " °Ar- Ar dating of the quartz samples from two mineral deposits in western Yuiman (SW China) by crashing in vacuum. Chem Geol 127 211-222 Rebour V, Billiotte J, Deveughele M, Jambon A, le Guen C (1997) Molecular diffusion in water saturated rocks a new experimental method. J Cont Hydrol 28 71-93 Sano Y, Wakita H, Huang CW (1986) Hehum flux in a continental land area from He/" He ratio in northern Taiwan. Nature 323 55-57... 

For diffusion dominated processes, D is the molecular diffusivity in water of the contaminant of interest multiplied by the porosity and divided by the tortuosity (or hindrance factor, ffp) of the sediment (D = Dv,s/Hp). The Millington and Quirk model (as referenced in Palermo [1]) suggests Hp which for a cap with 40% porosity is about 1.4. The molecular diffusion coefficient in water is a function of temperature and can be estimated by the methods defined by Lyman [4]. In the presence of advection D is increased due to mechanical dispersion in the medium (typically modeled as some dispersivity, a, multiphed by the interstitial velocity, v). The dispersiv-ity is an empirical parameter but is related to the mean particle size in nearly... 

FIGURE 5.3 Correlation of molecular diffusivities in water at 25°C with molecular weight. Rn shown as an outlier (not included in correlation). (Values compiled by Thibodeaux, L.J. 1996. Emironmental Chemodynamics Movement of Chemicals in Air, Water, and Soil, Wiley, New York and measurements by Gustafson, K.E. and Dickhut, R.M. 1994a. Journal of Chemical and Engineering Data 39, 281-285.)... 

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