WO1997033152A1 - Method and device for determining the molecular diffusion coefficient in a porous medium - Google Patents

Method and device for determining the molecular diffusion coefficient in a porous medium Download PDF

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Publication number
WO1997033152A1
WO1997033152A1 PCT/BE1997/000026 BE9700026W WO9733152A1 WO 1997033152 A1 WO1997033152 A1 WO 1997033152A1 BE 9700026 W BE9700026 W BE 9700026W WO 9733152 A1 WO9733152 A1 WO 9733152A1
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medium
porous medium
movement
diffusion coefficient
molecular diffusion
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PCT/BE1997/000026
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French (fr)
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Hugo Moors
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Studiecentrum Voor Kernenergie, Instelling Van Openbaar Nut
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Publication of WO1997033152A1 publication Critical patent/WO1997033152A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
    • G01N15/08—Investigating permeability, pore-volume, or surface area of porous materials
    • G01N15/082—Investigating permeability by forcing a fluid through a sample
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/02—Column chromatography
    • G01N30/26—Conditioning of the fluid carrier; Flow patterns
    • G01N30/28—Control of physical parameters of the fluid carrier
    • G01N2030/285—Control of physical parameters of the fluid carrier electrically driven carrier
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/89—Inverse chromatography

Definitions

  • the present invention concerns a method for determining the molecular diffusion coefficient in a porous medium, whereby a number of the charged molecules whose molecular diffusion coefficient in the medium is to be determined are put in contact with said medium, and the movement of these charged molecules, i.e. the distance covered by these molecules over a certain length of time, is measured so as to calculate the molecular diffusion coefficient.
  • Knowing the molecular diffusion coefficient of the soil, in particular of clay, may be important, for example in view of storing various forms of waste in geological layers, for example the storage of radioactive materials in these layers.
  • the molecular diffusion coefficient is determined by means of diffusion tests, whereby a number of the molecules whose molecular diffusion coefficient is to be determined are put in contact with the porous medium, and whereby after a waiting period the diffusion of said molecule is determined, which of course is very time-consuming.
  • the molecular diffusion coefficient is determined by means of percolation tests, whereby one tries to accelerate the natural diffusion process by making medium water under pressure flow through the porous medium. Also this method is still time-consuming.
  • the above-mentioned known methods may take several years before a reliable molecular diffusion coefficient can be determined.
  • the invention aims a method to determine the molecular diffusion coefficient providing a quicker outcome than the known methods.
  • This aim is reached according to the invention by applying a voltage gradient to an amount of the porous medium, so that the diffusion process through this medium is accelerated and the movement of the above-mentioned charged molecules is measured.
  • the invention can only be applied to measure the diffusion coefficient of charged molecules, in other words ions, but in practice, most molecules whose diffusion coefficient in a porous medium are to be measured are ions.
  • the voltage gradient is obtained by providing a DC field.
  • a radioactive isotope As molecules whose movement as a function of time is followed can be used a radioactive isotope, or the molecules can be marked by such an isotope. Their presence in the porous medium can then be established by means of a detector specially designed for said isotope. Preferably, the amount of the porous medium is placed in a holder having a buffer on both ends in which the electrodes to provide the potential gradient are placed.
  • the invention also concerns a device with which the method according to any of the preceding embodiments can be easily carried out.
  • the invention concerns a device for measuring the molecular diffusion coefficient, preferably containing a longitudinal holder for the porous medium, a buffer receptacle on two ends of the holder situated opposite one another, two electrodes, namely one in each buffer receptacle, and a source to generate a potential gradient between the electrodes.
  • This holder may consist of a non-conductive sleeve and, on both ends thereof, a non-conductive, for example a ceramic filter element.
  • figure 1 schematically represents a device according to the invention for measuring the molecular diffusion coefficient.
  • figure 2 shows a diagram of the movement of charged molecules during the application of the method.
  • a number of these charged molecules are put in contact with a quantity or a sample of said medium, and a potential gradient, in particular a DC field, is applied to this medium. Then, the movement of the charged molecules over the sample is measured so as to calculate the molecular diffusion coefficient of the charged molecules in the medium.
  • Said potential gradient is provided by means of an external power supply.
  • charged molecules i.e. ions
  • electromigrate through the porous medium
  • the natural diffusion process in particular the movement of the charged molecules, will be amplified.
  • the direction and speed of the movement of the charged molecules depends on their charge: positively charged molecules, cations, will move to the negative pole, i.e. the pole with the lowest potential, whereas negatively charged molecules, anions, will move to the positive pole or the pole with the highest potential.
  • the speed of the ions is not only influenced by the charge and the size of the molecules, but also by the physical and chemical nature of the porous medium.
  • Each charged molecule or ion will develop its own individual speed.
  • the speed of the charged molecule which was put into contact with the medium and whose molecular diffusion coefficient is to be determined, is measured.
  • the molecule is marked with an appropriate radioactive isotope, or if .the charged molecule is a single ion, by replacing the natural ion partly or entirely by a radioactive isotope of this ion, insofar that this can be measured in a simple way.
  • the movement of the marked molecule or the radioactive ion is equal to the movement of the natural molecule or the natural ion.
  • the ionic mobility constant u itself is equal to the ionic speed (in m/s) , divided by the effective electric field (in V/m) .
  • the porous medium consists of a stack of charged particles, which is usually the case with sedimented geological mediums such as sand or clay, the physical structure of the porous medium itself will be altered due tc the electrophoresis, i.e. the movement of charged particles under the influence of the electric gradient.
  • Said electrophoresis of the porous medium is practically avoided by enclosing the porous medium in non-conductive holder.
  • This holder holds the sample of porous medium together and prevents that charged particles characteristic of the medium leave said sample, so that the structure of the porous medium does not change during the determination of the molecular diffusion coefficient.
  • the electro-osmosis i.e. a water flow as a result of the electric gradient, solely depends on the physical and chemical qualities of the porous medium and can hardly be avoided.
  • the measured speed of movement of the charged molecule whose molecular diffusion coefficient is to be measured must be corrected.
  • the speed of the water through electro- osmosis must, depending on the direction of the water flow, be subtracted from or added to the measured ion speed in order to obtain the nett ion speed.
  • the speed through electro-osmosis can usually be determined by means of the equation of Helmholtz- Smoluchow ⁇ ki, but it is preferably determined in an experimental manner.
  • the speed through electro-osmosis (m/s) is equal to the permittivity of the interstitial medium water (C.V ⁇ . ⁇ r 1 ) multiplied by the zetapotential of the charged porous medium (V) and multiplied by the effective electric field (V/m) and divided by the viscosity thereof (N.s.m "2 ) .
  • this electro-osmotic flow is so small that it is to be ignored when determining the molecular diffusion coefficient.
  • Nemst-Einstein equation takes the temperature into account, one only has to measure the equilibrium temperature of the medium during the determination. Naturally, the temperature in the porous medium may not exceed the boiling point of the medium water, so that the magnitude of the maximally applicable potential gradient is restricted.
  • the heating up may possibly be restricted by cooling the porous medium and by thus for example mounting a cooling system around the above-mentioned holder.
  • both electrodes do no longer make contact with the porous medium in a direct manner, but via buffer solutions which will neutralize the protons and hydroxyl ions before they penetrate into the porous medium.
  • the buffer solutions must equal the chemical composition of the medium water as well as possible, so that other chemical disturbances are avoided.
  • the effective medium water is therefore used as buffer water, whose poising action may possibly be intensified by solving buffer agents in it.
  • the above-mentioned buffer solutions also prevent that the ions in the medium make contact with the electrodes and thus oxidize on the anode and reduce on the cathode. These phenomenons occur in the buffer solutions, so that the ions which are present in the porous medium itself are not hindered by said phenomenons during their movement through the porous medium.
  • the above-described method for determining the molecular diffusion coefficient may practically be carried out by means of the device which is schematically represented in figure 1.
  • This device contains the above-mentioned holder which consists of a preferably cylindrical sleeve 1 of non- conductive material, preferably plexiglass, and of two filter elements 2 which close off the ends of the sleeve 1 and are also made of a non-conductive material and consist for example of porous discs of ceramic glass.
  • buffer receptacle 5 On both ends of the holder 1-2 is connected a buffer receptacle 5. These buffer receptacles may form one piece with the sleeve 1 or they may be separate elements which are fixed to the end of the sleeve 1, for example screwed, by means of a seeling ring. Opposite to the filter elements 2, the wall of the buffer receptacles 5 is open.
  • the holder 1-2 is erected for example horizontally, in which case the buffer receptacles 5 may be open on top.
  • the cathode 6 is erected in one buffer receptacle 5, whereas the anode 7 is mounted in the other buffer receptacle 5.
  • Both electrodes for example made of platinum, are connected to the negative and positive pole respectively of a potential gradient source 9, for example a DC source, via lines 8.
  • This potential gradient source 9 is capable of generating a constant potential gradient, a constant current or a constant power.
  • the invention is further illustrated by means of a practical example which was carried out by means of the above-described device.
  • a first lump of Boom clay 10 was pressed in the loosened sleeve 1 by means of a hand press.
  • the iodide ion was provided on one end of the lump 10 in a liquid form and marked as 131 I.
  • the initial place of said ion was marked by placing a filter paper 11 on the first lump of clay 10.
  • Both ends of the sleeve 1 were closed off by the filter elements 2, and the buffer receptacles 5 were fixed on the holder 1-2 and filled with clay water in which the buffer agents had been solved.
  • This buffer to Boom clay was composed as follows: 1 mol of sodium bicarbonate and 0.1 mol of sodium carbonate per litre of clay water.
  • Full metal needles were pressed in the lumps 10 and 12 via the openings 3, and a voltmeter was connected in between two needles to measure the voltage gradient in the medium.
  • the voltage field was interrupted, after which the clay lumps 10 and 12 were pushed out of the holder 1-2 and cut in thin slices of a certain thickness.
  • the amount of iodine 131 isotope was measured in each slice by means of a Nal-gamma detector.

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Abstract

Device for measuring the molecular diffusion coefficient, characterized in that it contains a holder (1-2) of non-conductive material for the porous medium (4), a buffer receptacle (5) on two ends of the holder (1-2) situated opposite one another, two electrodes (6 and 7), namely one in each buffer receptacle, and a source to generate a potential gradient (9) between the electrodes (6 and 7). The method is characterized in that a potential gradient is applied to an amount of the porous medium, so that the diffusion process is accelerated.

Description

Method and device for determining the molecular diffusion coefficient in a porous medium.
The present invention concerns a method for determining the molecular diffusion coefficient in a porous medium, whereby a number of the charged molecules whose molecular diffusion coefficient in the medium is to be determined are put in contact with said medium, and the movement of these charged molecules, i.e. the distance covered by these molecules over a certain length of time, is measured so as to calculate the molecular diffusion coefficient.
Knowing the molecular diffusion coefficient of the soil, in particular of clay, may be important, for example in view of storing various forms of waste in geological layers, for example the storage of radioactive materials in these layers.
According to a known embodiment, the molecular diffusion coefficient is determined by means of diffusion tests, whereby a number of the molecules whose molecular diffusion coefficient is to be determined are put in contact with the porous medium, and whereby after a waiting period the diffusion of said molecule is determined, which of course is very time-consuming.
According to a somewhat faster method, the molecular diffusion coefficient is determined by means of percolation tests, whereby one tries to accelerate the natural diffusion process by making medium water under pressure flow through the porous medium. Also this method is still time-consuming.
Depending on the physicochemical qualities of the porous medium and the interactions of the molecules with the porous medium, the above-mentioned known methods may take several years before a reliable molecular diffusion coefficient can be determined.
The invention aims a method to determine the molecular diffusion coefficient providing a quicker outcome than the known methods.
This aim is reached according to the invention by applying a voltage gradient to an amount of the porous medium, so that the diffusion process through this medium is accelerated and the movement of the above-mentioned charged molecules is measured.
It is true that the invention can only be applied to measure the diffusion coefficient of charged molecules, in other words ions, but in practice, most molecules whose diffusion coefficient in a porous medium are to be measured are ions.
In particular, the voltage gradient is obtained by providing a DC field.
As molecules whose movement as a function of time is followed can be used a radioactive isotope, or the molecules can be marked by such an isotope. Their presence in the porous medium can then be established by means of a detector specially designed for said isotope. Preferably, the amount of the porous medium is placed in a holder having a buffer on both ends in which the electrodes to provide the potential gradient are placed.
The invention also concerns a device with which the method according to any of the preceding embodiments can be easily carried out.
Thus, the invention concerns a device for measuring the molecular diffusion coefficient, preferably containing a longitudinal holder for the porous medium, a buffer receptacle on two ends of the holder situated opposite one another, two electrodes, namely one in each buffer receptacle, and a source to generate a potential gradient between the electrodes.
This holder may consist of a non-conductive sleeve and, on both ends thereof, a non-conductive, for example a ceramic filter element.
In order to better explain the characteristics of the invention, the following preferred embodiment of a method and device for determining the molecular diffusion coefficient in a porous medium according to the invention is described as an example only without being limitative in any way, with reference to the accompanying drawings, in which:
figure 1 schematically represents a device according to the invention for measuring the molecular diffusion coefficient. figure 2 shows a diagram of the movement of charged molecules during the application of the method. In order to determine the molecular diffusion coefficient of a charged molecule or an ion in a water-saturated porous medium, a number of these charged molecules are put in contact with a quantity or a sample of said medium, and a potential gradient, in particular a DC field, is applied to this medium. Then, the movement of the charged molecules over the sample is measured so as to calculate the molecular diffusion coefficient of the charged molecules in the medium.
Said potential gradient is provided by means of an external power supply.
Due to the potential gradient, charged molecules, i.e. ions, will move, namely electromigrate through the porous medium, and thus the natural diffusion process, in particular the movement of the charged molecules, will be amplified.
The direction and speed of the movement of the charged molecules depends on their charge: positively charged molecules, cations, will move to the negative pole, i.e. the pole with the lowest potential, whereas negatively charged molecules, anions, will move to the positive pole or the pole with the highest potential.
As the process takes place in a porous medium, the speed of the ions is not only influenced by the charge and the size of the molecules, but also by the physical and chemical nature of the porous medium.
Each charged molecule or ion will develop its own individual speed. The speed of the charged molecule which was put into contact with the medium and whose molecular diffusion coefficient is to be determined, is measured.
This can be done by means of a chemical as well as a radioactive detection method. In the latter case, the molecule is marked with an appropriate radioactive isotope, or if .the charged molecule is a single ion, by replacing the natural ion partly or entirely by a radioactive isotope of this ion, insofar that this can be measured in a simple way. The movement of the marked molecule or the radioactive ion is equal to the movement of the natural molecule or the natural ion.
By dividing the distance over which the ion has moved by the time of the DC voltage applied, the speed of this ion is obtained. This speed, divided by the actually provided DC field, results in the ionic mobility constant, after which the molecular diffusion coefficient is calculated, for example with the known "Nernst- Einstein" relation represented below.
Di = u.K.T / Z.e = u.R.T. / Z.F
whereby
Ω = the diffusion coefficient in m2/s K = the constant of Boltzmann (1.38054 . 10-23 J/K) T = the absolute temperature in K Z = the valency of the ion e = the elementary charge (1.6021 . 10"19 C)
R = the universal gas constant (8.3143 J.mol^.K"1) F = the constant of Faraday (96 487 C/mol) u = the ionic mobility constant (m.s^.V1) . Although this equation applies in fact to the diffusion coefficient in pure water with an infinite dilution, experiments have shown that this equation also goes for a porous medium, provided that the physical and chemical disturbance of the porous medium due to the application of a potential gradient is avoided, so that the chemical form of the charged molecule does not alter.
The ionic mobility constant u itself is equal to the ionic speed (in m/s) , divided by the effective electric field (in V/m) .
By applying the potential gradient, in particular the DC field, other phenomenons than the ion movement may occur in the porous medium, however, which have an influence on the determination of the molecular diffusion coefficient, namely electrokinetic phenomenons such as electrophoresis and electro-osmosis, and physicochemical phenomenons such as heating due to the Joule effect, electrolysis and oxidation-reduction reactions on the electrodes.
These are undesired effects which must be avoided if possible in practice. If this is not possible, they have to be taken into account when calculating the molecular diffusion coefficient.
If the porous medium consists of a stack of charged particles, which is usually the case with sedimented geological mediums such as sand or clay, the physical structure of the porous medium itself will be altered due tc the electrophoresis, i.e. the movement of charged particles under the influence of the electric gradient.
Said electrophoresis of the porous medium is practically avoided by enclosing the porous medium in non-conductive holder. This holder holds the sample of porous medium together and prevents that charged particles characteristic of the medium leave said sample, so that the structure of the porous medium does not change during the determination of the molecular diffusion coefficient.
The electro-osmosis, i.e. a water flow as a result of the electric gradient, solely depends on the physical and chemical qualities of the porous medium and can hardly be avoided.
Depending on the size of the electro-osmotic flow, the measured speed of movement of the charged molecule whose molecular diffusion coefficient is to be measured, must be corrected. The speed of the water through electro- osmosis must, depending on the direction of the water flow, be subtracted from or added to the measured ion speed in order to obtain the nett ion speed.
The speed through electro-osmosis can usually be determined by means of the equation of Helmholtz- Smoluchowεki, but it is preferably determined in an experimental manner.
According to the above-mentioned equation, the speed through electro-osmosis (m/s) is equal to the permittivity of the interstitial medium water (C.V^.πr1) multiplied by the zetapotential of the charged porous medium (V) and multiplied by the effective electric field (V/m) and divided by the viscosity thereof (N.s.m"2) .
For some porous mediums, this electro-osmotic flow is so small that it is to be ignored when determining the molecular diffusion coefficient.
By applying a voltage field, an electric current starts to flow through the porous medium. As a result, the medium and medium water will be heated.
As the above-mentioned Nemst-Einstein equation takes the temperature into account, one only has to measure the equilibrium temperature of the medium during the determination. Naturally, the temperature in the porous medium may not exceed the boiling point of the medium water, so that the magnitude of the maximally applicable potential gradient is restricted.
The heating up may possibly be restricted by cooling the porous medium and by thus for example mounting a cooling system around the above-mentioned holder.
When a potential gradient, for example a DC field, is applied to a water-saturated porous medium, electrolysis of the water takes place and this water will disintegrate in H* protons on the anode and in OH" hydroxyl ions on the cathode. When these ions meet, they will recombine into water, which will take place in the middle of the porous medium if the anode and the cathode are divided by this porous medium.
During their movement through the porous medium, said H+ and OH" ions will change the chemical conditions in the porous medium. The protons will acidulate the porous medium from the anode to the cathode, whereas the hydroxyl ions will make the porous medium alkaline from the cathode to the anode. Thus is created an acidity gradient through the porous medium which may influence the chemical stability of the moving ions. Depending on the acidity, the chemical form of chemical compounds will spontaneously change so that they obtain other qualities, whereas the charge and consequently also the direction of movement of the ions may change when the acidity changes, so that the speed of the ions is changed.
This disturbing action of the protons and hydroxyl ions is avoided by mounting the anode and the cathode in pH- poising solutions.
Thus, both electrodes do no longer make contact with the porous medium in a direct manner, but via buffer solutions which will neutralize the protons and hydroxyl ions before they penetrate into the porous medium.
The buffer solutions must equal the chemical composition of the medium water as well as possible, so that other chemical disturbances are avoided. Preferably, the effective medium water is therefore used as buffer water, whose poising action may possibly be intensified by solving buffer agents in it.
The above-mentioned buffer solutions also prevent that the ions in the medium make contact with the electrodes and thus oxidize on the anode and reduce on the cathode. These phenomenons occur in the buffer solutions, so that the ions which are present in the porous medium itself are not hindered by said phenomenons during their movement through the porous medium.
The above-described method for determining the molecular diffusion coefficient may practically be carried out by means of the device which is schematically represented in figure 1.
This device contains the above-mentioned holder which consists of a preferably cylindrical sleeve 1 of non- conductive material, preferably plexiglass, and of two filter elements 2 which close off the ends of the sleeve 1 and are also made of a non-conductive material and consist for example of porous discs of ceramic glass.
Spread over the longitudinal direction of the sleeve 1 are provided small openings 3 at varying distances in which metal needles can be stuck during the measuring to measure the effective electric field in the medium 4 with which the holder is filled by means of a voltmeter.
On both ends of the holder 1-2 is connected a buffer receptacle 5. These buffer receptacles may form one piece with the sleeve 1 or they may be separate elements which are fixed to the end of the sleeve 1, for example screwed, by means of a seeling ring. Opposite to the filter elements 2, the wall of the buffer receptacles 5 is open.
The holder 1-2 is erected for example horizontally, in which case the buffer receptacles 5 may be open on top.
The cathode 6 is erected in one buffer receptacle 5, whereas the anode 7 is mounted in the other buffer receptacle 5. Both electrodes, for example made of platinum, are connected to the negative and positive pole respectively of a potential gradient source 9, for example a DC source, via lines 8.
This potential gradient source 9 is capable of generating a constant potential gradient, a constant current or a constant power.
The invention is further illustrated by means of a practical example which was carried out by means of the above-described device.
A first lump of Boom clay 10 was pressed in the loosened sleeve 1 by means of a hand press.
The iodide ion was provided on one end of the lump 10 in a liquid form and marked as 131I.
In order to be able to know the initial position of the iodide ion afterwards, the initial place of said ion was marked by placing a filter paper 11 on the first lump of clay 10.
Subsequently, a second lump of Boom clay 12 was pressed on said filter paper 11 in the sleeve 1, so that the iodide is initially situated right in between the two lumps of clay 10 and 12 on the place of the filter paper 11.
Both ends of the sleeve 1 were closed off by the filter elements 2, and the buffer receptacles 5 were fixed on the holder 1-2 and filled with clay water in which the buffer agents had been solved.
This buffer to Boom clay was composed as follows: 1 mol of sodium bicarbonate and 0.1 mol of sodium carbonate per litre of clay water.
Full metal needles were pressed in the lumps 10 and 12 via the openings 3, and a voltmeter was connected in between two needles to measure the voltage gradient in the medium.
One platinum electrode 6, 7 respectively was placed in each buffer receptacle 5. The voltage gradient formed by a DC source 9 was connected onto these electrodes 6 and 7.
By measuring the potential gradient between each time two of the above-mentioned needles with the voltmeter, the development in time of the size of the DC field was measured on different spots of the clay lumps 10 and 12. The weighted average resulted in an effective electric field of 431 Volt/m.
After a certain time, the voltage field was interrupted, after which the clay lumps 10 and 12 were pushed out of the holder 1-2 and cut in thin slices of a certain thickness.
Of each slice, the distance in relation to the initial position of the iodide-ion, i.e. from the filter paper 11, was noted down.
The amount of iodine 131 isotope was measured in each slice by means of a Nal-gamma detector.
The measured values in counts per second per gram Boom clay as a function of the distance in relation to the initial position of the iodide-ion are represented in figure 2 for the different slices.
The distance of the filter paper 11 to the slice in which the peak value of iodine isotope was measured, and thus the distance between the initial source position (the filter paper 11) and the peak of the curve on the graph of figure 2 is the distance covered by the iodide-ion as a result of the DC-field. This distance amounted to 43.6 mm.
By dividing this distance by the duration of the voltage field, the speed of the iodide-ion was obtained. This amounted to 3.09 . 10"6 m/s.
In a first estimate, the electro-osmosis was not taken into account.
By dividing the measured iodide-ion speed by the effective DC field, the ionic mobility constant for iodide in Boom clay was obtained: u = 7.2 . 10"9 ma.s"1.V"1.
On the basis thereof was then determined the molecular diffusion coefficient for iodide in Boom clay via the above-mentioned Nernst-Einstein equation.
In this manner was obtained a molecular diffusion coefficient of 1.8 . 10"ε cm2/ε, comparable to the molecular diffusion coefficient obtained according to the conventional method. From this can be derived that, as mentioned before, the Nernst-Einstein equation also counts for porous mediums.
Only was the molecular diffusion coefficient obtained according to the invention 100 times faster than with the conventional method.
By repeating the above-mentioned example a second time, practically the same molecular diffusion coefficient was obtained, which indicates that the method is reproducible and reliable.
The present invention is by no means restricted to the above-described embodiments represented in the accompanying drawings; on the contrary, such a method and device can be made in all sorts of variants while still remaining within the scope of the invention.

Claims

Claims .
1. Method for determining the molecular diffusion coefficient in a porous medium, whereby a number of the charged molecules whose molecular diffusion coefficient in the medium is to be determined are put in contact with said medium, and the movement of these charged molecules, i.e. the distance covered by these molecules over a certain length of time, is measured so as to calculate the molecular diffusion coefficient, characterized in that an electric potential gradient is applied to an amount of the porous medium, so that the diffusion process throug this medium is accelerated and the movement of the above-mentioned charged molecules is measured.
2. Method according to claim 1, characterized in that the potential gradient is obtained by applying a DC field to an amount of the porous medium.
3. Method according to claim 1 or 2, characterized in that the movement of the charged molecules is determined by placing a number of these molecules on an initial place in the porous medium and, after having applied the potential gradient for a certain length of time, by determining how many of these charged molecules are situated at varying distances from said initial position, whereby the distance at which the majority of these charged molecules are situated, divided by the above- mentioned time, is the speed of movement of the molecule.
4. Method according to claim 3, characterized in that the ionic mobility constant is determined by dividing the speed of movement of the charged molecules by the effective potential gradient, and in that the molecular diffusion coefficient is determined on the basis thereof by means of the Nernst-Einstein equation.
5. Method according to claim 3 or 4, characterized in that the speed of movement of the charged molecules as a result of electro-osmosis is determined, and in that the measured speed of movement of the molecules is reduced or increased with the speed of movement as a result of electro-osmosis.
6. Method according to any of the preceding claims, characterized in that as charged molecules whose movement is followed as a function of time, radioactive or radioactively marked molecules are used whose presence in the porous medium is determined by means of a suitable detector.
7. Method according to any of the preceding claims, characterized in that the influence of electrophoresis is eliminated by enclosing the medium in a non-conductive holder.
8. Method according to any of the preceding claims, characterized in that protons and hydroxyl ions in the medium, the oxidation on the anode, and the reduction on the cathode are avoided by applying the electric tension for the potential gradient between electrodes which are mounted in pH-poising solutions and by placing the medium between these solutions.
9. Method according to any of the preceding claims, characterized in that the porous medium is cooled.
10. Device for measuring the molecular diffusion coefficient, characterized in that it contains a holder (1-2) of non-conductive material for the porous medium (4) , a buffer receptacle (5) on two ends of the holder (1-2) situated opposite one another, two electrodes (6 and 7) , namely one in each buffer receptacle, and a source to generate a potential gradient (9) between the electrodes (6 and 7) .
11. Device according to claim 10, characterized in that the holder contains a sleeve (1) and both ends thereof a filter element (2) .
12. Device according to claim 11, characterized in that the sleeve (1) is provided with openings (3) to put in metal needles for measuring the potential gradient in different places.
PCT/BE1997/000026 1996-03-05 1997-03-04 Method and device for determining the molecular diffusion coefficient in a porous medium WO1997033152A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
BE9600194A BE1010056A3 (en) 1996-03-05 1996-03-05 METHOD AND DEVICE FOR THE DETERMINATION OF THE MOLECULAR diffusion coefficient in a porous medium.
BE9600194 1996-03-05

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FR2832218A1 (en) * 2001-11-13 2003-05-16 France Etat Ponts Chaussees METHOD FOR DETERMINING THE HYDRAULIC PERMEABILITY OF A MATERIAL
ES2351822A1 (en) * 2007-01-16 2011-02-11 Mondragon Goi Eskola Politeknikoa S.Co "a molecular diffusion device for determining the coefficient of molecular diffusion of liquid mixtures at atmospheric pressure and constant temperature and a method of obtaining the molecular diffusion coefficient" (Machine-translation by Google Translate, not legally binding)
CN105181556A (en) * 2015-09-25 2015-12-23 山东科技大学 Diffusivity testing method for gas in coal on triaxial stress condition
CN105241778A (en) * 2015-11-05 2016-01-13 中国石油大学(北京) Spontaneous imbibition measurement apparatus and experiment method thereof
CN108387485A (en) * 2018-03-29 2018-08-10 安徽理工大学 Solute molecule diffusion coefficients experimental provision based on layering porous media
CN114563320A (en) * 2022-01-14 2022-05-31 重庆永渝检验检测技术有限公司 Method for detecting and judging permeability of surface concrete

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Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2832218A1 (en) * 2001-11-13 2003-05-16 France Etat Ponts Chaussees METHOD FOR DETERMINING THE HYDRAULIC PERMEABILITY OF A MATERIAL
WO2003042688A2 (en) * 2001-11-13 2003-05-22 Laboratoire Central Des Ponts Et Chaussees Method for determining hydraulic permeability of a material
WO2003042688A3 (en) * 2001-11-13 2003-12-11 France Etat Ponts Chaussees Method for determining hydraulic permeability of a material
ES2351822A1 (en) * 2007-01-16 2011-02-11 Mondragon Goi Eskola Politeknikoa S.Co "a molecular diffusion device for determining the coefficient of molecular diffusion of liquid mixtures at atmospheric pressure and constant temperature and a method of obtaining the molecular diffusion coefficient" (Machine-translation by Google Translate, not legally binding)
CN105181556A (en) * 2015-09-25 2015-12-23 山东科技大学 Diffusivity testing method for gas in coal on triaxial stress condition
CN105181556B (en) * 2015-09-25 2017-10-20 山东科技大学 Diffusibility of gases method of testing in coal under a kind of condition of triaxial stress
CN105241778A (en) * 2015-11-05 2016-01-13 中国石油大学(北京) Spontaneous imbibition measurement apparatus and experiment method thereof
CN108387485A (en) * 2018-03-29 2018-08-10 安徽理工大学 Solute molecule diffusion coefficients experimental provision based on layering porous media
CN108387485B (en) * 2018-03-29 2023-09-22 安徽理工大学 Experimental device for determining diffusion coefficient of solute molecules based on layered porous medium
CN114563320A (en) * 2022-01-14 2022-05-31 重庆永渝检验检测技术有限公司 Method for detecting and judging permeability of surface concrete

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