Electro-osmosis transport

Pamukcu S, Khan LI, Fang H-Y. (1990). Zinc detoxification of soils by electro-osmosis. Transportation Research Record 1288 41 6. 

Electro osmosis often accompanies electrophoresis. It is the transport of Hquid past a surface or through a porous soHd, which is electricaHy charged but immovable, toward the electrode with the same charge as that of the surface. Electrophoresis reverts to electroosmotic flow when the charged particles are made immovable if the electroosmotic flow is forcibly prevented, pressure builds up and is caHed electroosmotic pressure. 

Electrically assisted transdermal dmg deflvery, ie, electrotransport or iontophoresis, involves the three key transport processes of passive diffusion, electromigration, and electro osmosis. In passive diffusion, which plays a relatively small role in the transport of ionic compounds, the permeation rate of a compound is deterrnined by its diffusion coefficient and the concentration gradient. Electromigration is the transport of electrically charged ions in an electrical field, that is, the movement of anions and cations toward the anode and cathode, respectively. Electro osmosis is the volume flow of solvent through an electrically charged membrane or tissue in the presence of an appHed electrical field. As the solvent moves, it carries dissolved solutes. 

Electrokinetic soil treatment is a commercially available in situ technology for the removal of metals and organic compounds. The application of direct current (DC) in a porous medium leads to two transport mechanisms electromigration and electro-osmosis. The combination of these two transport phenomena results in the movement of contaminant ions toward either the cathode or anode. Nonionic contaminants are transported by electro-osmosis alone. 

In the Nemst-Planck equations used the activity coefficients were neglected a term accounting for the electro-osmosis, however, is present. Calculated and measured concentration profiles could be made to inter-correspond by adapting the term for water transport. The values indirectly determined by electro-osmotic flow were now found to agree with those measured directly. 

We can observe electro-osmosis directly with an optical microscope using liquids, which contain small, yet visible, particles as markers. Most measurements are made in capillaries. An electric field is tangentially applied and the quantity of liquid transported per unit time is measured (Fig. 5.13). Capillaries have typical diameters from 10 fim up to 1 mm. The diameter is thus much larger than the Debye length. Then the flow rate will change only close to a solid-liquid interface. Some Debye lengths away from the boundary, the flow rate is constant. Neglecting the thickness of the electric double layer, the liquid volume V transported per time is... 

In PEMFC systems, water is transported in both transversal and lateral direction in the cells. A polymer electrolyte membrane (PEM) separates the anode and the cathode compartments, however water is inherently transported between these two electrodes by absorption, desorption and diffusion of water in the membrane.5,6 In operational fuel cells, water is also transported by an electro-osmotic effect and thus transversal water content distribution in the membrane is determined as a result of coupled water transport processes including diffusion, electro-osmosis, pressure-driven convection and interfacial mass transfer. To establish water management method in PEMFCs, it is strongly needed to obtain fundamental understandings on water transport in the cells. 

In the standard condition with 80% RH (Fig. 6b), a water content, X [H20/S03H], of around 8 in the membrane and partial dehydration at the anode were observed at a current density of 0.2 A/cm2. This suggested that electro-osmosis was influential, even at this current density, because the membrane (Nation 1110, du Pont, 254 Hm) was thicker than usual. In the dry condition (40% RH), the water content in the membrane was around 3 (Fig. 6c) and the water content profile was flat. This can be attributed to molecular diffusion in the membrane, as fast chemical diffusion at this water content has been reported by Zawodzinski et al.41 It was also noted in both cases of 80% RH and 40% RH that the water generated in the cathode catalyst layer was not sufficiently transported to the membrane. This means that little water generated at the cathode condensed and was exhausted to the cathode gas channel as liquid. In the case of 92% RH shown... 

In 2001, the Netherlands Organization for Scientific Research (NWO) started a project called Chemically and electrically coupled transport in clayey soils and sediments to quantify the role of chemically and electrically coupled transport in clayey soils and to assess its relevance for the distribution and emission of contaminants and water. The project involves three Ph.D. students working on field and laboratory experiments and modelling of chemical-and electro-osmosis. 

Water transport in electrodialysis from the diluate to the concentrate process stream can affect the process efficiency significantly. If a convective flux as a result of pressure differences between flow streams can be excluded there are still two sources for the transport of water from the diluate to the concentrate solution. The first one is the result of osmotic-pressure differences between the two solutions, and the second is due to electro-osmosis that results from the coupling of water to the ions being transported through the membrane due to the driving force of an electrical potential. 

Current utilization In practical application electrodialysis is affected by incomplete current utilization. The reasons for the incomplete current utilization are poor membrane permselectivity, parallel current through the stack manifold, and water transport by convection and due to osmosis and electro-osmosis. In a well-designed stack with no pressure difference between diluate and the concentrate convective water transport is negligibly low and also the current through the manifold can be neglected. Under these conditions the overall current utilization is given by ... 

In the case of electro-osmosis through the pores of a diaphragm, the volume V of liquid transported electro-osmotically per second is equal to qu, where q is the total area of cross section of all the pores in the diaphragm, the assumption being made that the thickness of the double lay jr is negligible in comparison with the pore diameter. On substituting v/q for u in equation (11) the result is... 

Iv) Ion transport, which in the bulk proceeds by conduction, is considerably modified by the presence of the particle. When, as will be assumed here, the particle is kept stationary, the electric force exerted on the countercharge sets this charge into motion. By viscous traction liquid is entrained. This phenomenon is called electro-osmosis-, it will be treated in sec. 4.3b. 

This equation describes how rapidly (positive or negative) excess neutral electrolyte is created. As discussed before, this model is representative of ion transport in double layers as occurs in electrophoresis and electro-osmosis. It is recalled that the error function erf b is defined as... 

Another class of "difflcult" particles Involves systems which are porous. Then they may no longer be classified as dielectric. Ion transport and electro-osmosis may then take place inside the particles. Miller et al. ) found theoretically that porosity tends to increase or decrease the mobility for high and low respectively. Such studies may also be Interesting for the Interpretation of the electrophoresis of aggregates of particles. 

Finally, electrodialysis may be mentioned, a process widely used to de-salt aqueous solutions. An electric field is applied across a stack of alternating cation-exchange and anion-exchange membranes. Ions in the electrolyte solutions between these membranes are transported till they meet a membrane of the same sign, so that electrolyte-rich and electrol)rte-freed solutions are created. The process involves conduction and electro-osmosis. Obviously, irreversible thermod mamics appears very suitable to describe the various flows... 

Safe and effective delivery of peptides has also been successfully demonstrated in human studies using iontophoresis, a technique that uses mild electric current to facilitate transport of molecules across the skin. ° Iontophoresis works primarily by a combination of two forces, electro-repulsion of charged drug molecule away from the electrode and into the skin, and electroosmosis, a convective solvent flow in the direction of the counter-ion transport. In general, cationic proteins and peptides are delivered more efficiently than anionic molecules because electro-osmosis works in the same direction as electro-migration for cationic species. 

The flow of liquid caused by electro-osmosis displays a pluglike profile because the driving force is uniformly distributed along the capillary tube. Consequently, a uniform flow velocity vector occurs across the capillary. The flow velocity approaches zero only in the region of the double layer very close to the capillary surface. Therefore, no peak broadening is caused by sample transport carried out by the electro-osmotic flow. This is in contrast to the laminar or parabolic flow profile generated in a pressure-driven system, where there is a strong pressure drop across the capillary caused by frictional forces at the liquid-solid boundary. A schematic representation of the flow profile due... 

Other methodologies, such as electro-osmosis, have been reported to be successful on certain objects. In this case, electrodes were introduced in areas where the paint layer was already lost completely to reach beneath the remaining parts of the paintings. Migration of anions and cations of detrimental salts was induced via an electrical field. Whereas cations were transported toward earth, anions were attracted by the iron anode. The system is reported to be under observation, with repeated exchange of the iron at the end of its service life. The ions reacting with the iron can be removed from the system and analysed further. A drop in the current of about 300 mA was observed during salt reduction. However, up to now the method has only been applied to a small number of objects so the results will have to be studied carefully. 

In electrokinetic processes, there are two major transport mechanisms electromigration and electro-osmosis. Generally, in an electrical field, electromigration causes cationic metals such as cadmium, zinc, lead, nickel, and copper to move from the anode toward the cathode in electro-osmosis, the direction of movement of the pore water is toward the cathode when the zeta potential of the soil surface is negative. This can result in an enhanced removal of metals because the direction of transport of the ions in both mechanisms is the same. However, the direction of electromigration for anionic pollutants is toward the anode and that for electroosmosis is from anode to cathode, as stated previously. The opposite direction of movement means that the removal rate of anionic pollutants could be reduced. 

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