Diffusion of nonreactive and reactive solutes through fine-grained barrier materials

1984 ◽  
Vol 21 (3) ◽  
pp. 541-550 ◽  
Author(s):  
R. W. Gillham ◽  
M. J. L. Robin ◽  
D. J. Dytynyshyn ◽  
H. M. Johnston
Keyword(s):  
Diffusion Coefficient ◽  
Distribution Coefficient ◽  
Bulk Density ◽  
Molecular Diffusion ◽  
Effective Diffusion ◽  
Fickian Diffusion ◽  
Principal Mechanism ◽  
Barrier Materials ◽  
Fine Grained ◽  
Reactive Solutes

In many cases where fine-grained geologic materials are used as barriers to the migration of contaminated fluids, the principal mechanism of contaminant transport is molecular diffusion. Thus the effective molecular diffusion coefficient is the parameter of greatest importance when predicting migration rates and contaminant fluxes. Diffusion coefficients were measured for two non-reactive solutes (36Cl and 3H) and one reactive solute (85Sr) in seven mixtures of bentonite and silica sand ranging from 0 to 100% bentonite by weight. Tortuosity factors were calculated from the results of the nonreactive diffusion experiments, and retardation factors for the reactive solute from measured distribution coefficient (Kd), bulk density, and porosity values. The results showed the diffusive transport of both the reactive and nonreactive solutes to be consistent with a Fickian diffusion equation. For practical purposes, and at the low values of bulk density used in the experiments, the effective diffusion coefficient of the reactive solute could be calculated with a reasonable degree of certainty from the measured retardation factor and an estimated value of tortuosity. The results showed that because of the interaction between the distribution coefficient, bulk density, and porosity, an increase in clay content beyond about 5–10% did not result in a further reduction of the diffusion coefficient of the reactive solute. Key words: diffusion, adsorption, retardation, tortuosity, clay liners.

scholarly journals Macroscopic water vapor diffusion is not enhanced in snow

2020 ◽  
Author(s):  
Kévin Fourteau ◽  
Florent Domine ◽  
Pascal Hagenmuller
Keyword(s):  
Diffusion Coefficient ◽  
Water Vapor ◽  
Effective Diffusion Coefficient ◽  
Molecular Diffusion ◽  
Pore Space ◽  
Effective Diffusion ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Vapor Diffusion ◽  
Water Vapor Diffusion

Abstract. Water vapor transport in dry snowpacks plays a significant role for snow metamorphism and the mass and energy balance of snowpacks. The molecular diffusion of water vapor in the interstitial pores is usually considered as the main or only transport mechanism, and current detailed snow physics models therefore rely on the knowledge of the effective diffusion coefficient of water vapor in snow. Numerous previous studies have concluded that water vapor diffusion in snow is enhanced relative to that in air. Various field observations also indicate that for vapor transport in snow to be explained by diffusion alone, the effective diffusion coefficient should be larger than that in air. Here we show using theory and numerical simulations on idealized and measured snow microstructures that, although sublimation and condensation of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice. The interaction of water vapor with the ice results in water vapor diffusing more than inert molecules in snow, but still less than in free air, regardless of the value of the accommodation coefficient of water on ice. Our results imply that processes other than diffusion, probably convection, play a preponderant role in water vapor transport in dry snowpacks.

Enhanced Diffusion Due to Swimming Bacteria

2003 ◽  
Author(s):  
Min Jun Kim ◽  
Kenneth S. Breuer
Keyword(s):  
Escherichia Coli ◽  
Molecular Weight ◽  
Diffusion Coefficient ◽  
Diffusion Process ◽  
Effective Diffusion Coefficient ◽  
Effective Diffusion ◽  
Flow Cell ◽  
Fickian Diffusion ◽  
Enhanced Diffusion ◽  
Swimming Bacteria

The effect of bacterial motion of the diffusion of a high molecular weight molecule is studied by observing the mixing of two streams of fluid in a micro-fabricated flow cell. The presence of Escherichia coli (E.coli) in one of the streams results in an increase in the effective diffusion coefficient of Dextran, which rises linearly with the concentration of bacteria from a baseline value of 2 × 10−7 cm2/s to 8 × 10−7 cm2/s at a concentration of 2.1 × 109 /ml (approximately 0.5 % by volume). The diffusion process is also observed to undergo a change from standard “Fickian” diffusion to a superdiffusive behavior in which the diffusion exponent rises from 0.5 to 0.55 as the concentration of bacteria rises from 0 to 2.1 × 109/ml.

Impurity Diffusion in Silicon Annealed by Semi-Continuous Laser

1982 ◽  
Vol 13 ◽  
Author(s):  
Jean-Eric Bouree ◽  
Claude Leray ◽  
Michel Rodot
Keyword(s):  
Diffusion Coefficient ◽  
Single Crystals ◽  
Effective Diffusion ◽  
Boron Doping ◽  
Laser Pulses ◽  
Diffusion Time ◽  
Impurity Diffusion ◽  
Continuous Laser ◽  
Fine Grained ◽  
A Value

ABSTRACTThe diffusion of an impurity into a solid which is irradiated by laser pulses of some milliseconds duration has been analysed in terms of effective diffusion time and temperature. In the case of Fe in Si, the diffusion coefficient is found to be similar to that measured by COLLINS and CARLSON, and independent of boron doping. In the case of Al in Si, a value of 2.10–10 cm2/ at 1400°C has been found both for single crystals and for fine-grained polycrystals.

scholarly journals COMPARATIVE ANALYSIS OF METHODS FOR DETERMINING THE EFFECTIVE CO2 DIFFUSION COEFFICIENT IN FINE-GRAINED CONCRETE OF DIFFERENT DENSITIES

2020 ◽  
Vol 10 (1) ◽  
pp. 63-70
Author(s):  
Pavel A. FEDOROV
Keyword(s):  
Diffusion Coefficient ◽  
Comparative Analysis ◽  
Effective Diffusion ◽  
Low Permeability ◽  
Test Results ◽  
Indirect Methods ◽  
Fine Grained ◽  
Co2 Diffusion ◽  
Direct And Indirect Methods ◽  
Membrane Methods

Direct and indirect methods for determining the effective diffusion coefficient of CO2 in concrete are considered. The features of the diffusion process in a capillary-porous body are described. Based on the test results of samples of fine-grained concrete with different densities, a comparative analysis of the coefficients obtained by the main methods was carried out. The criteria for comparison were the dependence of the water-cement ratio on diffusion, as well as the rate of carbonization on time. The presence of significant deviations in the low permeability concretes of the coefficients obtained by the membrane methods of 72 % and the electrical conductivity of concrete saturated with electrolyte 85 % in the low permeability concrete from the values obtained by the carbonization method was established.

scholarly journals Macroscopic water vapor diffusion is not enhanced in snow

2021 ◽  
Vol 15 (1) ◽  
pp. 389-406
Author(s):  
Kévin Fourteau ◽  
Florent Domine ◽  
Pascal Hagenmuller
Keyword(s):  
Diffusion Coefficient ◽  
Water Vapor ◽  
Effective Diffusion Coefficient ◽  
Molecular Diffusion ◽  
Pore Space ◽  
Effective Diffusion ◽  
Vapor Transport ◽  
Water Vapor Transport ◽  
Vapor Diffusion ◽  
Water Vapor Diffusion

Abstract. Water vapor transport in dry snowpacks plays a significant role for snow metamorphism and the mass and energy balance of snowpacks. The molecular diffusion of water vapor in the interstitial pores is usually considered to be the main or only transport mechanism, and current detailed snow physics models therefore rely on the knowledge of the effective diffusion coefficient of water vapor in snow. Numerous previous studies have concluded that water vapor diffusion in snow is enhanced relative to that in air. Various field observations also indicate that for vapor transport in snow to be explained by diffusion alone, the effective diffusion coefficient should be larger than that in air. Here we show using theory and numerical simulations of idealized and measured snow microstructures that, although sublimation and deposition of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice. The interaction of water vapor with the ice results in water vapor diffusing more than inert molecules in snow but still less than in free air, regardless of the value of the sticking coefficient of water molecules on ice. Our results imply that processes other than diffusion play a predominant role in water vapor transport in dry snowpacks.

Diffusion of Heavy Oil in SiO2 Model Catalyst and FCC Catalyst

2012 ◽  
Vol 550-553 ◽  
pp. 158-163 ◽  
Author(s):  
Zi Yuan Liu ◽  
Sheng Li Chen ◽  
Peng Dong ◽  
Xiu Jun Ge
Keyword(s):  
Diffusion Coefficient ◽  
Pore Size ◽  
Effective Diffusion Coefficient ◽  
Pore Diameter ◽  
Effective Diffusion ◽  
Model Catalyst ◽  
Average Pore ◽  
Diffusion Factor ◽  
Fcc Catalyst ◽  
Fcc Catalysts

Through the measured effective diffusion coefficients of Dagang vacuum residue supercritical fluid extraction and fractionation (SFEF) fractions in FCC catalysts and SiO2model catalysts, the relation between pore size of catalyst and effective diffusion coefficient was researched and the restricted diffusion factor was calculated. The restricted diffusion factor in FCC catalysts is less than 1 and it is 1~2 times larger in catalyst with polystyrene (PS) template than in conventional FCC catalyst without template, indicating that the diffusion of SFEF fractions in the two FCC catalysts is restricted by the pore. When the average molecular diameter is less than 1.8 nm, the diffusion of SFEF fractions in SiO2model catalyst which average pore diameter larger than 5.6 nm is unrestricted. The diffusion is restricted in the catalyst pores of less than 8 nm for SFEF fractions which diameter more than 1.8 nm. The tortuosity factor of SiO2model catalyst is obtained to be 2.87, within the range of empirical value. The effective diffusion coefficient of the SFEF fractions in SiO2model catalyst is two orders of magnitude larger than that in FCC catalyst with the same average pore diameter. This indicate that besides the ratio of molecular diameter to the pore diameter λ, the effective diffusion coefficient is also closely related to the pore structure of catalyst. Because SiO2model catalyst has uniform pore size, the diffusion coefficient can be precisely correlated with pore size of catalyst, so it is a good model material for catalyst internal diffusion investigation.

Sign in / Sign up

Export Citation Format

Share Document