Salt Filtering by Ion Exchange Grains and Membranes

McKelvey, J. G.; Spiegler, K. S.; Wyllie, M. R. J.

doi:10.1021/j150548a011

Cited by 33 publications

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References 1 publication

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“…This theory was supported by experimental work showing that: (a) salt rejection of cellulose acetate film was relatively poor a t low pressures, increased rapidly in the interval of 150 to 250 psi, then approached 100% asymptotically with further increase in pressure; (b) the dielectric constant of moist film increased more rapidly under compres-sioii than that of dry film; (c) semipermeability was correlated, by means of electrical measurements, with resistance of these membranes to passage of ions incapable of forming hydrogen bonds, but not of ions which may participate in hydrogen bonding; and (d) transference numbers of sodium and chloride ions were such that semipermeability could not be attributed to an ion exchange reaction with the membrane, comparable to that described by McKelvey et al 4 The magnitude and sign of the activation entropy, according to the absolute reaction rate theory,6 will depend upon the relative order associated with the reactant and the activated state. An increase in order in passing from the reactant state to the activated state will be indicated by negative activation entropy.…”

supporting

confidence: 75%

Physical characteristics of osmotic membranes of organic polymers

Reid

Kuppers

1959

J of Applied Polymer Sci

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MTRODUCTIONThe use of synthetic osmotic membranes for desalting saline waters was proposed by Reid1 in 1953. This process involves reversal of osmotic flow from a solvent cell to a solution cell across a semipermeable membrane. Pressure more than sufficient to counterbalance the osmotic pressure of the solution applied to the solution cell will reverse the direction of flow. The feasibility of the process depends upon the discovery or the synthesis of a suit,able semipermeable membrane. Such a membrane must: (a) be semipermeable with respect to an ionic solution; (b) resist chemical reaction with the solution; and (c) possess a physical structure which permits a reasonable rate of water diffusion.Breton2 investigated the semipermeability of films prepared from a variety of polymers and concluded that, among those tried, cellulose acetate film possessed the most desirable features as a semipermeable membrane for sodium chloride solutions: 96y0 of the ions were rejected from solutions ranging in concentration from 0.1M to 0.6M. Nevertheless, cellulose acetate could not conceivably be used in an economic application of this process. The efficient flow rate was of the order of 25~1. l./hr. cm.2 for film one-half mil thick in contact with 0.1M sodium chloride under a pressure of 600 psi. Furthermore, the selectivity of the film failed after six to fourteen days. This failure was shown by Kuppers3 to be associated with the hydrolysis of acetyl groups from polymer chains.Although cellulose acetate was not suited to practical application of osmotic desalting of saline waters, it did offer a means of studying the mechanism by which water is selectively transported across such membranes. It was hoped that this mechanism could be elucidated to the extent that a more practical membrane could be synthesized. Hence, Breton2 initiated an investigation from which stemmed the following theory."Those ions and molecules that cannot enter into hydrogen bonding with the membrane are transferred by hole-type diffusion. The rate of diffusion appears to be governed by a water-cellulose acetate structure. The reaction between water and the cellulose acetate polymers to form bound water regions is induced by compressing the membrane. As pressure is applied on the membrane, more bound water is produced which causes the rate of hole-type diffusion to decrease." "On the other hand, those ions and molecules that can associate with the membrane through hydrogen bonding actually combine with the membrane and are transported through it by alignmenttype diffusion. The formation of the watercellulose acetate structure does not appreciably diminish the diffusion rate of water through the membrane."Hole-type diffusion was interpreted as a process with low activation energy wherein molecules migrated through free-volume between polymer chains with a diffusion coefficient differing little from the self-diffusion coefficient of the diffusing fluid. Alignment-type diffusion, on the other hand, was visualized as a process in which molecules migrated by d...

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supporting

confidence: 75%

Physical characteristics of osmotic membranes of organic polymers

Reid

Kuppers

1959

J of Applied Polymer Sci

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“…Because of the electroneutrality requirement, however, a streaming potential will immediately arise preventing also the passage of the exchanger ion. Experiments with a cationic exchange membrane [19] did show such a desalting effect. The effect was all too low for sea water desalination, however, and an overpressure of 15 times the osmotic pressure had to be applied due to the low water permeability.…”

Section: Discussionmentioning

confidence: 94%

Membrane Charge and Donnan Distribution of Ions by Electromotive Force (EMF) Measurements on a Homogeneous Cellulose Acetate Membrane

Sørensen¹,

Jensen²

1984

Journal of Non-Equilibrium Thermodynamics

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The EMF has been determined in concentration cells with homogeneous cellulose acetate (CA) membranes as separators. The electrolytes studied are HC1, NaCl, KC1 and CaCl 2 in water at 25 °C. The concentration range is from KT 4 mol/dm 3 to 3mol/dm 3 .It is shown by means of Onsager's reciprocal relations that the apparent transference numbers calculated at various values of the variable concentration (from the slope of the EMF vs. log c curves) are identical to the real transference numbers of the cations at the same concentrations with a correction for water transference.Nernst-Planck equations are derived in the low concentration limit of some more general transport equations derived from friction theory and a generalization of Kohlrausch's law of independent ion mobility. Nernst-Planck equations in combination with a simplified treatment of the Donnan distribution of ions seem to be sufficient to explain the data. The water transference is apparently negligible.From the dependence of the EMF on concentration, the ratio between the ionic diffusion coefficients in the membrane and the membrane charge divided by a complex of ionic distribution constants can be determined. The measurements support the assumption that CA-membranes are weak cationic exchangers because of the presence of sparsely distributed glucuronic acid groups along the cellulose chains. The carboxylic groups are partially titrated by HC1, so the charge density becomes lowered. The charge density is also lowered by specific binding of Ca + + ions. The cations K + , Na* and a fortiori Ca + + are much more restricted in their motion than QT, probably due to electrostatic interactions Brought to you by | Purdue University Libraries Authenticated Download Date | 6/16/15 1:50 AM 2 T. Smith S0rensen, J. Birger Jensen with the negative fixed charges and the oxygen lone pairs. The mobility of H + seems to be almost unaffected by the presence of the membnme.The findings explain, why salt rejection in desalination experiments is strongly dependent on pH at low concentrations of NaCl. Today reverse osmosis with CA-membranes is only economically feasible for brackish waters, but we give some suggestions how to change the properties of CA-membranes, so that they might be used for sea water desalination.Inserting Eq. (23) into (22), we obtain an equation for the laboratory flux / x . The equation for J 2 is derived similarly, and we have:

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“…The effect opposite to Osmosis was experimentally discovered in the 1950's in the studies of McKelvey et al [9,10] who observed the salt concentration changes produced in the adjacent solutions while imposing a pressure-driven flow through a membrane separating the compartments. This discovery initiated thousands of publications and widely employed technologies of water desalination [11].…”

Section: Introduction: Mechano-chemical Effects In Charged Porous Mediamentioning

confidence: 93%