Related Experiment Video
Updated: May 7, 2026
08:14
Atom Probe Tomography Analysis of Exsolved Mineral Phases
Published on: October 25, 2019
Seeming steady-state uphill diffusion of 22Na+ in compacted montmorillonite
Martin A Glaus1, Martin Birgersson, Ola Karnland
1Laboratory for Waste Management, Paul Scherrer Institut , CH-5232 Villigen, Switzerland.
Environmental Science & Technology
|September 25, 2013
Summary
Seemingly uphill diffusion of sodium-22 tracer was observed in compacted sodium montmorillonite. This ion transport is driven by pore water chemistry, not just external concentration gradients, impacting solute transport models.
Area of Science:
- Geochemistry
- Materials Science
- Physical Chemistry
Background:
- Solute transport in porous media is typically diffusion-driven by external concentration gradients.
- Dense clays and clay rocks present complexities due to fixed negative surface charges, complicating ionic species transport.
- Fick's laws are often insufficient for describing ion diffusion in charged porous materials.
Purpose of the Study:
- To investigate the diffusion mechanisms of ionic species in compacted sodium montmorillonite under specific conditions.
- To explore the phenomenon of apparent uphill diffusion of sodium ions.
- To identify the primary driving forces for tracer diffusion in clay nanopores.
Main Methods:
- Experiments using a (22)Na(+) tracer in compacted sodium montmorillonite.
- Utilizing a gradient in the background electrolyte across the clay sample.
- Employing equal initial tracer concentrations on both sides, differing from classical through-diffusion.
Main Results:
- Observed apparent uphill diffusion of (22)Na(+) tracer, moving from low to high concentration.
- Demonstrated that diffusion is driven by the concentration gradient of exchangeable cations within the nanopores.
- Highlighted the limitations of models based solely on external bulk water concentration gradients.
Conclusions:
- The concentration gradient of exchangeable cations in nanopores is the dominant driver for diffusion in compacted sodium montmorillonite.
- Standard diffusion models may inaccurately predict the magnitude and direction of solute fluxes in such systems.
- Understanding pore-scale chemical gradients is crucial for accurate modeling of solute transport in clays.
More Related Videos
Related Concept Videos
01:31Transport Number
The transport number is the fraction of the total current carried by an ion in an electrolyte solution. It is defined as the ratio of the current carried by a specific ion to the total current flowing through the solution. The transport number, t, is central to understanding ionic mobility, which describes how fast an ion moves under the influence of an electric field. This link connects the physical behavior of ions in solution to the chemical processes that occur during electrochemical...
01:12Diffusion
Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
01:21Diffusion
Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
01:17Reabsorption and Secretion in the Loop of Henle
The thick ascending limb of the nephron loop has Na+–K+–2Cl− symporters in the apical membranes of its cells. These symporters simultaneously reclaim one sodium ion, one potassium ion, and two chloride ions from the tubular fluid. Sodium ions are actively transported into the interstitial fluid at the base and sides of the cell, diffusing into the vasa recta. Chloride ions move through leakage channels in the basolateral membrane into the interstitial fluid and then into the vasa recta.
02:34Water and Mineral Acquisition
Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.