R. Simons

Trace element removal from ash dam waters by nanofiltration and diffusion dialysis

Abstract Liquid waste streams in coal-fired power plants contain a wide spectrum of trace elements, most of which originate in the coal and remain in the fly ash or bottom ash when the coal is burned. Electric power authorities seek methods for removing toxic elements from such streams. The use of nanofiltration and diffusion dialysis for removing boron and fluoride from ash dam water is described in this article. Negatively charged Nitto Denko NTR-7410 and Filmtec NF40 nanofiltration membranes gave better results than amphoteric Toray UTC-20HF and UTC-60 membranes. Membrane performance depended on pH, the best results being obtained when the pH of the ash dam water was in the 3–4.5 range. Diffusion dialysis membranes with negatively charged functional groups gave better results than membranes with positively charged functional groups.

The origin and elimination of water splitting in ion exchange membranes during water demineralisation by electrodialysis

Abstract Water splitting in anion exchange membranes containing quaternary ammonium groups is due to the presence of tertiary alkyl amino groups in the surface regions. It may be eliminated by methylation, however it reappears during current flow because quaternary ammonium groups in the surface regions are then converted to the tertiary form. The quaternary ammonium groups are more stable in Negev Institute A than in AMF A1OO membranes. By contrast water splitting is not manifested by cation exchange membranes with sulphonic acid groups if the system is sufficiently clean.

Preparation of a high performance bipolar membrane

Abstract A method for preparing low resistance bipolar membranes is described. The membranes are prepared by pressing together manually, separate anion and cation exchange membranes which have been pretreated by immersion in an alkaline solution of a metallic salt. The operating voltage for the bipolar membrane is about 1.0 V, when it separates 1 molar acid and alkali solutions and the current is 100 mA-cm−2. The current efficiency for water splitting is close to 100%. The electrical characteristics are stable under a continuous applied current, for over a year.

Anomalous dielectric dispersion in bimolecular lipid membranes.

Abstract The response of lipid membranes to alternating electric fields has been investigated. The electrical measurements yield the equivalent parallel combination of conductance and capacitance seen by the a.c. bridge. An attempt has been made to determine the film parameters through the use of equivalent circuits for the system. Both the film conductance and capacitance showed a dispersion suggestive of the presence of thin water layers adjacent to the membranes where the electrical properties differ from those of bulk water.

Electrodialysis for clean-up of strongly alkaline samples in ion chromatography

Abstract An electrodialysis method is described for the off-line neutralization of strongly alkaline samples containing trace levels of common inorganic anions. This method uses an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes. These compartments comprise an anode compartment housing, a platinum wire anode and 10 ml of a suitable hydrogen ion donating medium, a sample compartment which contains 1 ml of the alkaline sample, and a cathode compartment housing a platinum wire cathode and a dilute solution of sodium hydroxide. During electrodialysis at either constant applied current or constant applied power, hydrogen ions from the anode compartment displace sodium ions from the sample, thereby effecting neutralization. Experimental parameters, such as the magnitude of the applied current or power, the type of cation-exchange membrane used and the design of the cell have been studied and optimum results were obtained using a Neosepta CM-2 membrane, of area 616 mm 2 supported between two perspex discs, with an applied current of 150 mA or applied power of 3 W. Under these conditions, a 1 ml sample of 1 M sodium hydroxide could be neutralized in 11 min. The most effective hydrogen ion donating medium consisted of a 2:1 (w/v) slurry of BioRad AG 50W-X8 (200–400 mesh, H + form) cation-exchange resin in 1 m M octanesulfonic acid. Recoveries of solute anions (3–10 μg/ml) from the dialysed solution were close to quantitative, except for fluoride and nitrite, which gave recoveries of less than 60%. It is suggested that low recoveries for these ions are due to formation of neutral, protonated species within the membrane with subsequent loss by diffusion.

A mechanism for water flow in bipolar membranes

Abstract When water splitting occurs in a bipolar membrane a steady state is reached in which the water which leaves the interface of the anion and cation exchange regions in the form of hydrogen and hydroxyl ions, is replaced by fresh water from the boundary solutions. Since the boundary solutions may comprise concentrated acid and alkali while the water at the interface is deionised, the flow of the water towards the interface occurs against an osmotic pressure difference which may exceed 100 atmospheres. There must therefore be other forces on the water which combine to exceed the osmotic force and oppose it. The net water flow depends on the gradients in osmotic, Maxwell and hydrostatic pressures. In this article we show that the difference in Maxwell pressure for water between the outside solutions and the interface may exceed the osmotic pressure difference between the regions. We point out that the hydrostatic pressure at the interface can be higher than in the external solutions.

Energy of formation of bimolecular lipid membranes.

Abstract A method for measuring the energy of formation, W , of artificial, bimolecular lipid membranes, in aqueous solutions, is described. The method is based on the determination of the radius of curvature of the membrane as a function of the pressure difference across it. This is accomplished by the use of capacitance measurements on bowed membranes. The method allows W to be determined as a function of the film extension. The energy of formation of the membranes under a variety of ionic solutions was found to be 3.4 erg·cm−2.

The steady and non-steady state properties of bipolar membranes

Abstract The theory for the electrical characteristics of bipolar fixed charge membranes has been extended to cover the response of the membrane to a step change in potential for the case where there are two mobile ions. When both ions have the same mobility the expression for the time constant is identical to that which characterises Maxwell Wagner relaxation between the depletion layer and abutting regions. Two time constants result if the ions have different mobilities. It is shown that a dispersion in the membrane impedance at frequencies of about 70 Hz may be predicted for Chara australis on the bipolar leaflet model.

Pervaporation and evaporation characteristics of a new type of ion exchange membrane

Abstract Microporous Celgard K273 films were irradiated with γ-rays in a grafting solution containing 4-vinyl pyridine. Electron micrographs, grafting level, ion exchange capacity and water content data strongly suggest that the pores of the Celgard films become occluded with crosslinked homopolymer during the grafting and are converted to hydrophilic anion exchange regions. The treated films have remarkably high water fluxes when the downstream side is maintained under a partial vacuum. They also have a modest, i.e. ∼2, water/ethanol permselectivity.

A thermodynamic analysis of particle flow through biological membranes

Abstract The methods of irreversible thermodynamics are applied to the problem of particle flow through biological membranes. Formulae are derived for the unidirectional flux and flux ratio of a permeant species under conditions where there is a coupling between flows and between flows and metabolism. The flux ratio formula is shown to be useful under appropriate conditions, for providing evidence antagonistic to the hypothesis of active transport of the permeant species. A relation is derived which specifies the conditions under which the use of tracers should lead to accurate estimates of unidirectional fluxes. When the treatment is applied to the problem of water flow through an inert semipermeable membrane a particularly simple formula is obtained for the ratio of the permeability coefficients for water as determined osmotically and isotopically.