Measurement of Effective Knudsen Diffusion Coefficients for Powder Beds Used in Heterogeneous Uptake Experiments

The effective Knudsen diffusion coefficients for characteristic oxide powder beds used in heterogeneous uptake experiments have been measured using countercurrent diffusion and transient pressure drop techniques. Room-temperature thermal-velocity-normalized effective Knudsen diffusion coefficients are found to lie in the 0.15 to 0.35 μm range for magnesium silicate, aluminum oxide, and iron oxide powder beds. Measured values are compared with theoretical estimates and are consistent with low bed tortuosities (below 3) expected for media with open porosity above 0.5. The impact of uncertainties in effective diffusion coefficients on corrections of measured uptake coefficients is discussed. The value of careful uptake measurements in both the low and high sample mass limits is reinforced, as this allows uptake corrections independent of explicitly measured or estimated diffusion coefficient values. It is suggested that correction procedures requiring tortuosity values greater than 3 are suspect.