Electroosmotic Water Transport Research Articles

Properties of electrolyte-filled glass microelectrodes: a model analysis

A novel dynamic mathematical microelectrode model (a model of solvent and solute kinetics in electrolyte-filled microelectrodes) was deduced from experimental observations made on standard (single-barrelled, 3.0 M KCl-filled, ≈10 MΩ) electrodes using (a) electrodiffusion, electro-osmosis, and continuity equations that were placed into the constraints of electrode geometry, and (b) handbook/textbook parameter values, only. The model proved to be able to faithfully reproduce all observed electrochemical and electrical electrode properties, i.e. even those that constituted no part of the model's experimental basis. In theoretical tests, the model shows, for the standard electrode that (a) inside the electrode, any profiles in electrical potential and electrolyte concentration are occurring at the most distal part (≈50 μm) of the tip region, (b) asymmetrical shifts in electrolyte concentration just inside the electrode tip opening are the true cause of the electrode's current rectification, and (c) strong transelectrode currents are producing water flows across the electrode orifice that may affect the volume of smaller and medium-sized cells. In further tests, the model shows, among other things, for non-standard electrodes that (a) decreasing the electrode electrolyte concentration will give rise to marked decreases in electrolyte leakage from the electrode, but only very minor changes in tip potential, and (b) increasing the surface charge of the electrode glass (increases in ζ potential) and/or decreasing the electrode electrolyte concentration will produce increases in electro-osmotic water transport, which may be desirable for the intracellular injection of water-soluble (electro-neutral) substances.

Lactic acid production by electrodialysis Part I: Experimental tests

Lactic acid, the production of which from cheap raw materials is attractive due to its great potential in the production of biodegradable polymers, can be obtained from fermentation broths using electrodialysis. This work discusses the feasibility of this process using commercial membranes, together with the influence of several operating parameters on the electrodialysis stack performance. It was found that it is possible to operate with high current efficiency values, while the overall recovery of sodium lactate depends on the current density. Electroosmotic water transport limits the maximum concentration value achievable using this technique.

Electrochemical properties of cellulosic ion-exchange membranes II. Transport numbers of ions and electro-osmotic flow

Transport numbers of counter-ions inn cellulosic cation- and anion-exchangemembranes have been determined by both emf and Hittorf's methods for various (1:1)-electrolytes at different concentrations. The transport number derived from the emf data, in general, is lower than the value obtained by the Hittorf method, for any given external electrolyte concentration. This difference is attributed to transport of water occurring across the membrane. Electro-osmotic water transport across the membranes in equilibrium with several electrolytes has been determined at different concentrations. The data obtained have been interpreted in terms of the hydrated ion size of the counter-ions. Volume flow and electrical conductance across the membranes in the presence of various electrolytes have been measured at several electrical potential differences. The experimental results have been interpreted in terms of linear phenomenological equations. These relationships have been used to estimate the magnitudes of the phenomenological coefficients for volume flow and conductance across the cationic and anionic membranes. The validity of the phenomenological equation in volume flow across the membranes has been tested by comparison with the data obtained by direct measurements.

Efficient dewatering of waterworks sludge by electroosmosis

Electroosmotic dewatering of water treatment process sludge, in which a relatively high specific resistance causes difficulties with regard to water removal, is described. The dewatering can be both accelerated and made economically more attractive by additives which increase the absolute value of the ζ potential but do not lead to stable suspensions. An increase in ζ potential means an increase in electroosmotic water transport; simultaneously electric charges on the filter repel particles with electrical charges of the same sign, which causes a decrease in the clogging of the filtration material. Best results show an increase in final dry solid content from 4% (by weight) by pressure filtration only, to 16% (by weight) by a combination of electroosmosis with pressure filtration.

Electroosmosis in filter cakes of activated sludge

An experimental apparatus has been developed which fulfils the conditions for measuring the electroosmotic transport of water in filter cakes of activated sludge. A number of different experiments were made on activated sludge treated in different ways, with a dry matter content of approx. 10–30%. The electroosmotic coefficient, i.e. the factor of proportionality between volume flow and applied electric field strength, was found to be independent of, for example, dry matter content or conditioning of the sludge. The experimental results can be explained by assuming a picture of the filter cake as an exopolymer matrix with sludge particles embedded and residual pore solution in isolated pores.

A mobile system for treating low-salinity low-activity liquid wastes

of UPM P membrane in one equipment 7.7 m2), with the concentrate returned to the settler B 1. The filtrate passes under excess pressure of 0.3-0.4 MPa through the membranes and is collected in the intermediate tank B2, and is then pumped through the sodium cation exchange filters F2 and F3 of volume 0.5 m 3 each, which contain KU-2-8 resin in the Na + form (to soften the water before electrodialysis), after which it enters the dialysate tank B3 in the electrodyalizer E1 type E-400.01. The latter operates in flow mode with partial dialysate recirculation, which enables one to purify the wastes continuously. The final decontamination to the level set by health standards is provided by passing the total flux dialysate through the filter F4 of volume 0.5 m 3, which is filled with KU-2-8 cation-exchange resin in the H + form, after which the solution leaves the system. Stable operation is provided in the electrodialysis unit by keeping the salt content and radioactivity in the concentrate in the E1 electrodyaliser at a constant low level by means of a concentrate system based on the E2 electrodialyser and concentrator of type EDK-2789-000 located in a bypass loop, which contains stagnant brine chambers. The concentrate formed there by electroosmotic water transport through ion-exchange membranes is a brine with salt content not less than 100 g/liter.

Electroosmotic transport of water in polyelectrolyte networks

The phenomenon of contraction of polyelectrolyte gels under the influence of direct current is studied experimentally. It is shown that the number of water molecules per one ion of the electric current is very large N 103. This effect can be explained as electroosmotic transport of water molecules through the gel, which can be represented as an effective fine-porous medium with the diameter of pores equal to the mesh size of a network.

The characterisation of the polyethylene‐polystyrene‐divinylbenzene interpolymer cation‐exchange membranes in the electrolysis of borax solutions

AbstractThe synthesis and the characterization of the PE‐PS‐DVB interpolymer cationexchange membranes in the electrolysis of borax solutions to produce boric acid and sodium hydroxide simultaneously was carried out. The characterization of the teflon based “Nafion 324” and polystyrene‐DVB based “Permaplex C‐20” membranes was also performed in the same system for comparison.The DVB contents were varied between 3 – 12% (by wt. in total monomers) and the increase of the cross‐links resulted in the decrease of the water contents and ionexchange capacities of the membranes. The PE present in the membrane acted as a barrier for electroosmotic water transport. The membranes with high DVB contents showed better electrolysis performance due to their low water contents and low electroosmotic water transport properties. The use of the interpolymer membrane containing 11.6% DVB resulted in high current efficiency and high sodium cation dynamic transport number and worked satisfactorily at the process conditions of borax electrolysis.

The effect of electro-osmotic water transport on current efficiency and cell performance in chlor-alkali membrane electrolysis

A theoretical study is made of the transport processes taking place in the ion-exchange membrane serving as the “separator” in a chlor-alkali electrolysis cell. Special attention is paid to the effects of electro-osmotic water transport (EOWT). Starting from the extended Nernst—Planck flux equations, we derive expressions for characteristic quantities such as the current efficiency of alkali production, n OH, and the voltage drop over the membrane as functions of current density, external concentrations of sodium chloride and sodium hydroxide, and electro-osmotic water transport number, t o. It appears that EOWT has a favourable effect on n OH and that the effect on ohmic resistance and voltage drop is slight, whereas the transport of chloride ions to the catholyte increases. The effect of EOWT on n OH can be represented by a single parameter k o, which is proportional to t o (0≦ k o ≦1). Three regions of k o values can be distinguished, the boundaries of which depend on the external concentrations; in each region the maximum current efficiency depends in a different way on the external concentrations: in the lowest region of k o values η OH, increases linearly with k o and decreases with increasing values of C OH (concentration of NaOH in the membrane on the cathodic side); in the highest region up to η OH = 1, η OH increases linearly with k o and increases with increasing values of C Cl (concentration of NaCl in the membrane on the anodic side), whereas in the intermediate region η OH depends on k only. The concentration of NaOH in the membrane, which at maximum efficiency is constant through-out the membrane, also differs in the three regions of k o values (see Fig. 6). Making use of non-published experimental results on the uptake of water, alkali and sodium chloride by Nafion 170 and 427 membranes, we calculate values of η OH as a function of the external alkali concentration for different values of t o and compare with experimental results obtained in an electrolysis cell. It appears that t o ≌ 1 gives the best agreement between theory and experiment. The same result is deduced from experimental values of the total water transport number, by applying corrections for water of hydration carried along by the migrating ions. Calculation shows that with Nafion 170 membranes under practical conditions, i.e., concentrated alkali (≧ 10 M) and 5 M NaCl, EOWT caused the maximum current efficiency to increase from 0.45 at t o = 0 to about 0.75 or 0.80 at t o = 1; the value of k o lies in the “Intermediate region” and the (constant) concentration of NaOH in the membrane has a value about halfway between C Cl and C OH. At a current density of 3 kA/m 2 the maximum value of η OH is attained within about 2%.

Electrokinetic studies on cholesterol-carbohydrate systems

The hydrodynamic and electro-osmotic transport of water and aqueous solutions of d-fructose, d-glucose, sucrose, and urea through cholesterol-coated membrane has been investigated. The results have been examined from the viewpoint of non-equilibrium thermodynamics. Zeta potentials have been estimated in order to characterize the membrane-permeant interface.

Ion and water transport through permselective membranes

Abstract Electroosmotic transport of water accompanying ion transport across an anion-selective membrane has been measured at 25°C using 0.01 N NaC1 solution. Also electrical conductance, self-diffusion coefficient of counterion, and hydrodynamic permeability of the membrane have been measured. These data and others, such as water content and exchange capacity, have been used in the relations derived by Scattergood and Lightfoot employing the generalized Stefan-Maxwell equations to obtain values for the diffusivity coefficients D 1*1, D 13, D 14, and D34. The isotope interaction term D 1*1 was the lowest. Assuming the applicability of the Nernst-Einstein relation to the membrane system, the number of moles of water transported by electroconvection has been evaluated. The difference between the moles of total water transport and the moles of water moved electro-convectively has been assigned as the moles of water firmly associated with a mole of the moving ion. This way a value of 0.58 mole has been derive...

Temperature dependence of electroosmotic water transport through cation-exchange membranes. The effect of proton jumping.

Temperature dependence of electroosmotic water transport through cation-exchange membranes. The effect of proton jumping.

Diffusional interaction in an ion-exchange membrane

The conductivity, electro-osmotic water transport, hydrodynamic permeability, and “self-diffusion” coefficient were measured for a cation exchange membrane in the Ag+ and Na+-forms. Refinements in experimental technique were made. The phenomenological coefficients were calculated and it was shown that some commonly neglected interactions can be important. The suitability of the pseudo-binary expressions was also investigated.

The Electroosmotic Effects Arising from the Interaction of the Selectively Anion and Selectively Cation Permeable Parts of Mosaic Membranes

It has been previously shown, theoretically and in model system experiments, that mosaic membranes composed of anion-selective (electropositive) and cation-selective (electronegative) parts interposed between electrolytic solutions of different concentrations give rise to local electrical circuits. In this work with model systems it is shown that these currents produce electroosmosis. In systems with permselective electronegative membranes and KCl solutions, the electroosmotic water transport was 16 moles/faraday. With the permselective electronegative membrane replaced by more porous electronegative membranes, the electroosmotic effects were about twice as high. With Li salts, the water transport was considerably larger. A system with a permselective electropositive membrane of 50 cm(2) effective area and an electronegative membrane of 120 cm(2) gave internally generated currents up to 20 ma. In extrapolating from the results with macromodels to effects with true mosaics, i.e. microsystems, it is stressed that current depends on the linear distance over which membranes interact. In true mosaic membranes, the current pathways will be of the same order as the dimensions of individual membrane microelements; the sum of all local microcurrents will be correspondingly larger than the current in the macromodel, and the electroosmotic effects will be proportionately greater. Electroosmotic effects with true charge-mosaic membranes may be of the same order or larger than the liquid transport by normal and anomalous osmosis which might occur across the individual parts of the charge-mosaic.

Transport of Bicarbonate, Carbonate, and Chloride Ions through Ion Exchange Membranes

The integral transport numbers of chloride and bicarbonate ions in the commercial anion‐exchange membranes “Permutit 3148,” “AMF 101” (American Machine and Foundry Company) and “Asahi Anionic,” and the concomitant water transport were determined. The cathode compartment contained mixed solutions of sodium bicarbonate and chloride in different proportions and of total normality 0.05. It was found that in all three membranes preferential transport of chloride takes place, that the electroosmotic transport of water with bicarbonate is higher than with chloride and that the electroosmotic water transport varies in the same order as the water content of the chloride forms of the membranes expressed in terms of moles per equivalent Cl−. Competitive transport of carbonate and chloride in “AMF 101” was also measured. The preferential transport of chloride is even more pronounced in this case than in chloride‐bicarbonate systems.

Electro-Osmosis in Soils

Summary The present paper shows that, from a practical point of view, the electro-osmotic transport of water may be assumed to be constant for all soils. The magnitude of the osmotic permeability was found to be ke=5 × 10−5 cm. per sec. for for 1 volt per cm. By comparing ke, with the hydraulic permeability the heights to which the water can be shifted by electro-osmosis were computed and also measured by laboratory tests. For this purpose a special apparatus was constructed. The fact that with colloidal materials agreement between theoretical equilibrium heads and test results exist only for the short interval of time immediately after applying an electric potential was attributed to the development of cracks within the mass of material. The possibility of diverting the flow of pore-water away from cuttings is discussed and illustrated by practical examples. Some indications are given of the magnitude of the required consumption of energy for a few arrangements of electrodes. An explanation is also offered for the favourable effect of electro-osmosis on the consolidation of soft soil deposits.