The effect of chelation on the selective transport of alkaline-earth metal ions in asymmetric cellulose acetate hyperfiltration membranes

The rejection of calcium and/or magnesium ion by asymmetric cellulose acetate hyperfiltration membranes is increased significantly by formation of the corresponding alkaline-earth metal chelate. Typically solute fluxes are reduced by a factor of 5 consequent to chelation with ethylenediaminetetraacetic acid (EDTA) at pH 6.0. Selective chelation and, in turn, selective transport of magnesium is observed when equimolar solute mixtures corresponding to 1:1:1 magnesium:calcium:EDTA are hyperfiltered. Under these conditions, calcium successfully competes for the stoichiometrically limiting EDTA, and the rejection of magnesium is lower than the rejection observed for the hyperfiltration of the MgEDTA^2? complex in the absence of competitive calcium. Alternatively, the rejection of the CaEDTA^2? complex is increased under these identical conditions, presumably as a consequence of specific interactions between the available free magnesium and the cellulose acetate membrane. The effects reported here all seem to be related to reductions in solute diffusivity associated with the increased size of the alkaline-earth metal ion complex.     

The structure of cellulose acetate membranes II. The physical and transport characteristics of the porous layer of anisotropic membranes

By a technique involving the removal of successive layers of an anisotropic cellulose acetate membrane, and the measurement of solute and volume flow through the remaining structure, it is possible from the results to calculate some of the properties of each decrement which characterizes its structure. These are: average pore diameter, area ratio of pores to total surface, and pore volume fraction.     

Movements of univalent ions in cellulose acetate membranes

Membrane potentials across the asymmetric membranes of cellulose acetate with various salt rejection properties have been measured for univalent ions. The behavior of ions in the membranes is discussed from the viewpoint of relative ionic mobilities calculated from the membrane potentials. The relationship between ionic mobilities and ionic radii in the membranes having salt rejections lower than 80% is almost the same as that in aqueous solutions. This implies that the ions in these membranes behave as if they exist in bulk water. However, the ionic mobilities in the membranes having salt rejections higher than 86% differ significantly depending on the ionic radii. It seems probable that the bound water influences the ionic mobilities in these membranes.     

The electret effect in cellulose acetate reverse osmosis membranes

The electret potentials developed by reverse osmosis electret membranes help control the undesirable deposition of charged colloidal particles on the membrane surfaces during membrane desalination. These antifouling electret membranes should help prevent the costly flux declines normally associated with deposition of colloidal iron oxides on the reverse osmosis membrane surfaces. Homocharge and heterocharge behavior of cellulose acetate membrane electrets have been studied. Asymmetric reverse osmosis membranes and dense membrane films were studied. The homocharge and heterocharge of cellulose acetate reverse osmosis electret membranes have been explained.     

Performance of cellulose acetate butyrate membranes in hyperfiltration of sodium chloride and urea feed solution

Cellulose acetate butyrate (CAB) membranes gave high salt and urea rejection with a water flux of about 3 gfd (gallons/ft² · day) during hyperfiltration at 600 psig. Evidence was obtained which indicated that the CAB membranes used in this work were asymmetric. Membrane heat treatment increased urea rejection significantly while salt rejection was invariant, and water flux decreased. An increase in feed solution temperature caused a significant increase in water flux and a small decrease in urea and salt rejection. Increasing the pressure increased water flux and urea and salt rejection. During a 400-hr life test, the water flux decreased by about 25% while urea rejection increased and salt rejection was invariant. The influence of pressure, membrane heat treatment, and compaction during CAB membranes life testing on urea and salt rejection provided evidence that these two solutes were rejected by somewhat different mechanisms. Salt rejection was consistent with a solution–diffusion mechanism for membrane transport and uncoupled flow while changes in urea rejection with pressure, membrane heat treatment, and compaction during life testing suggested that urea was at least partially rejected by membrane exclusion resulting from geometric factors.     

The permeation behavior of metal complex solutions through cellulose acetate membranes

Transport phenomena of several kinds of metal complexes were investigated with cellulose acetate membranes annealed at 65°–76°C. In reverse osmosis experiments, the rejections of metal complexes involving organic sequestering agents such as EDTA or citric acid were much higher than those of the corresponding metal ions. While, in the case of metal complexes involving small inorganic ligands such as NH₃ or SCN^-, their rejections did not necessarily increase with the increase in the coordination numbers of the metal ions. To more precisely understand such transport behaviors, the distribution and the diffusion coefficients of metal complexes were obtained by desorption-rate measurements with dense cellulose acetate membranes. The results revealed that the distribution of a metal ion to the membrane was largely depended on the coexisting ligands. Attempts were also made to explain the distribution coefficient from the microscopic point of view by using Glueckauf's equation.     

The role of silver nanoparticles on mixed matrix Ag/cellulose acetate asymmetric membranes

Mixed matrix asymmetric membranes were prepared by the addition of silver nanoparticles to cellulose acetate/acetone/formamide casting solutions with ratios acetone/formamide varying from 1.44 to 2.77 to prepare ultrafiltration/nanofiltration membranes covering a wide range of hydraulic permeabilities. Binding of the silver nanoparticles to the polymer matrix is revealed through comparison of the FTIR spectra of the cellulose acetate and the Ag/cellulose acetate membranes. In the later, there is a decrease of the ratio between the bands intensities at 2,000–2,500 cm⁻¹. Membrane surface charge of the mixed matrix membranes varies with the pore size and pH, and when compared with cellulose acetate membranes there is a decrease of the negative surface charge densities. The silver nanoparticles in all mixed matrix membranes results in an enhancement of the hydraulic permeabilities, ranging from 10.8 kg m⁻² h⁻¹ bar⁻¹ to 67.1 kg m⁻² h⁻¹ bar⁻¹. POLYM. COMPOS., 38:32–39, 2017. © 2015 Society of Plastics Engineers     

Ion exchange properties of zirconium phosphate phosphite with alkaline earth metal ions.

The ion exchange properties of zirconium phosphate phosphite, a layered compound with asymmetrical layers, towards alkaline earth cations were investigated and compared with those of a-zirconium hydrogenphosphate.

It was found that these cations were easily taken up in the phosphate regionsof_ a-Zr(HPO₄)_{0 7}(HPO₃),.O.5H₂O whereas only Ca²⁺ and Sr²⁺ are directly exchanged from a-Zr(HP0₄)₂.H₂O. The ion exchange mechanism involves a single pnase transition and the solubility of the exchanging cations in the already formed phases. A wide solubility range was found for barium exchange. The interlayer distances and water content of the exchanged forms of zirconium phosphate-phosphite and those of the corresponding phases of zirconium phosphate were compared and discussed on the basis of the structural features of the two hosts.

Zirconium phosphate-phosphite has a relatively low ion exchange capacity (2.5 meq/g) compared to that of the zirconium phosphate (6.64 meq/g)but it is a more facile exchanger for large and hydrated cations.     

The effect of pressure on the bulk polymer microstructure in cellulose acetate reverse osmosis membranes

The results of the investigations of the pressure effect on the bulk polymer microstructure using Differential Scanning Calorimetry (DSC) are presented. The samples of both “dense” CA-398-3 membranes and Kesting dry asymmetric CA-398-3 membranes have been investigated. The results show that the pressure treatment is also a kind of an “annealing” process that orders the polymer microstructure. The effect takes place both in the case of the originally more ordered (from a DSC point of view) polymer microstructure of “dense” CA membranes and the originally less ordered (almost completely amorphous) polymer microstructure of the dry asymmetric CA membranes investigated. DSC curves on the thermoanalytical diagrams of the pressure treated “dense” CA membranes show mainly a shift of the endothermic peak (melting process?) to higher temperatures, and DSC curves of the pressure treated Kesting dry asymmetric membrane samples show both a shift of the endothermic peak to higher temperatures and quite an enlargement of the area under the peak. It could be inferred from the above that pressure action orders (densifies) the bulk polymer microstructure of both the skin and porous layer of an asymmetric CA reverse osmosis membrane.     

The states of water in cellulose acetate membranes

Differential scanning calorimetric melting endotherms of wet and half-dried cellulose acetate membranes and salt distribution coefficients were studied to clarify the states of water in membranes. We have suggested that (a) there are four states of water in cellulose acetate membranes; (b) these states of water are those of completely free water, free water very weakly interacting with polymer, bound water which can contain salts, and bound water which rejects salts; (c) the semipermeability of membranes depends on the ratio of four states of water in membranes.     

The effect of short air exposure periods on the performance of cellulose acetate membranes from casting solutions with high cellulose acetate content

Highly productive cellulose acetate membranes were cast under conditions of very short air exposure periods from cellulose acetate–acetone–formamide casting solutions having a high cellulose acetate (CA) content and lying close to the phase boundary. Air exposure periods as short as 0.05 sec were used with CA content up to 32 wt-%. Membranes from a casting solution containing 30 wt-% cellulose acetate (E-398-3), 45 wt-% acetone, and 25 wt-% formamide perform as well as membranes from other compositions at all salt rejection levels for a 0.5 wt-% NaCl feed at 600 psig. Partial replacement of acetone by dioxane in the casting solution substantially increases the water flux from membranes cast with short air exposure periods at any given salt rejection level below 96% salt rejection. Addition of small amounts of ZnCl₂ to nondioxane casting solutions with 32 wt-% CA improves membrane performances remarkably for lower salt rejection levels, while the improvement in performance of membranes from 30 wt-% CA casting solutions with dioxane due to ZnCl₂ addition is marginal. Variation in air exposure from 0.05 to 2 sec results in minor performance variations in the membranes having any of these compositions. With air exposure periods beyond 2–3 sec, membrane fluxes drop drastically. The concept of a thinner skin satisfactorily explains the improvement in mixed solvent systems, whereas ZnCl₂ acts as a swelling salt. A Kimura-Sourirajan-type membrane performance plot indicates that for a 0.5 wt-% NaCl feed at 600 psig, membranes of the present work perform as well as the best performing membranes reported in the literature for conversion of brackish water.     

The influence of mineral fillers on the membrane properties of high flux asymmetric cellulose acetate reverse osmosis membranes

To improve the compaction resistance of high flux cellulose acetate membranes, fillers were incorporated into the casting solution. Several filler types — silicium oxides, aluminium oxides, montmorrilonite with or without an organic coating and with different physical properties — were used. During short-term reverse osmosis tests (ca. 7 hours) the water permeability-pressure relationship and the rejection properties of the membranes were investigated. During long-term reverse osmosis tests (100 hours), the flux decline at constant pressure as a function of time has been investigated. A correlation was sought between the pressure- dependency (loss of water permeability as a function of pressure) and the compaction resistance (loss of flux at constant pressure as a function of time) of the membranes tested.     

The effect of precipitating media on the performance of porous cellulose acetate reverse osmosis membranes

A two-stage method of making semi-permeable high flux reverse osmosis membranes was developed using water-ethanol mixtures to precipitate the cellulose acetate. This eliminated the need for heat treatment and produced membranes with fluxes up to 5 m³/m² day and sodium chloride rejections up to 85 %. Their properties are compared with membranes made by the three-stage method involving heat treatment and show more sensitivity to the effects of pressure.     

Thermodynamic and mechanistic characterization of water sorption in homogeneous and asymmetric cellulose acetate membranes

The water sorption isotherms for homogeneous and asymmetric cellulose acetate membranes have been measured at different temperatures. Subtle differences between the water sorption isotherms for asymmetric and homogeneous membranes have been interpreted by suggesting that capillary condensation contributes significantly to sorption in asymmetric membranes at high activities and also to an intriguing excess sorption observed in homogeneous membranes at intermediate activities. This model has been supported by the experimentally determined values of the enthalpy and entropy changes associated with sorption.     

The thermodynamic state of water in cellulose acetate membranes

Using experimental sorption data and corresponding experimental results from calorimetric investigations, the state of water in cellulose acetate (CA) membranes is discussed by applying a theoretical treatment of sorption reported previously (1–3). The sorption of water can be attributed to a gain in surface energy at the polymer/vapor interface. Using differential thermodynamic potentials of sorbed water together with experimentally determined heat capacities of sorbed water, the thermodynamic potentials G, H, and S of sorbed water are estimated for the temperature interval ?40 to + 40°C. At constant temperature, each thermodynamic potential depends on the water content. The resulting distribution function of G indicates that the sorbed water exists in different states. Comparing the Gibbs free energy of sorbed water with that of ice or liquid water at the same temperature leads to the conclusion that none of the sorbed water freezes to ice within the temperature interval used. Based on the Gibbs free energy of water in electrolyte solutions and the distribution function of G for sorbed water, partition coefficients of salts within CA membranes may be estimated. The results are in good agreement with experimentally determined partition coefficients which are available from the literature. As the partition coefficient of a salt is directly related to the salt rejection of the membrane, this provides a method of estimating the desalination performance of a membrane from its water sorption isotherm.     

Studies on the radiolytic degradation of cellulose acetate membranes

The effect of γ-irradiation on the performance of wet cellulose acetate membranes in the dose range of 2.5–10 Mrads was investigated using a ⁶⁰Co source. Changes in transport properties and inherent viscosity of the membranes suggested continued degradation as a result of irradiation. Solubility and specific gravity changes accompanying irradiation indicated some sort of structural aggregation occuring at higher doses. Consumption of dissolved oxygen during irradiation and the extent of deacetylation of the membrane polymer were determined to study the kinetics of the degradative process. Analysis of the end products of irradiation was attempted by UV spectroscopy. ESR spectra of membrane polymer after irradiation were analyzed to identify the free radicals generated. A tentative mechanism of radiolytic degradation causing the observed performance failure is proposed.     

Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions

Certain organic solutes, including phenol, undergo anomalous enrichment when hyperfiltered through cellulose acetate membranes: the solute concentration is higher in the permeate than in the feed solution. A number of existing theoretical approaches describing hyperfiltration phenomena are presented and their merits and limitations upon application to the transport of phenol discussed. A new two-parameter transport relationship is derived based on an extension of the solution–diffusion model. The enrichment, or negative solute rejection by the membrane, is predicted to occur whenever the pressure-induced solute permeation velocity exceeds that of water. By acknowledging and incorporating the effect of pressure on the chemical potential of the solute, the present extended solution–diffusion model relationship successfully describes hyperfiltration data of phenol in homogeneous and asymmetric cellulose acetate membranes provided the contribution of convective flow to the overall solute transport is insignificant. In addition to the transport parameters of the extended solution–diffusion model, the transport parameters of the phenomenological, Kedem–Spiegler, and combined viscous flow–frictional relationship are evaluated from hyperfiltration data obtained with 0.05 and 0.1 wt % phenol feed solutions and homogeneous cellulose acetate membranes of different acetyl content.     

Water and solute transport across cellulose acetate membranes in the treatment of soybean whey by reverse osmosis

In processing full-fat soy flour to produce an acid-precipitated lipid protein concentrate, there results a by-product whey fraction which, because of its high biological oxygen demand, represents a serious disposal problem. Processing of food waste streams by reverse osmosis has received considerable attention because of its low theoretical energy requirement, since no phase change is involved. A series of statistically designed and analyzed experiments were conducted on a pilot-plant reverse osmosis unit to study the effect of the operating parameters on solute and solvent transport in cellulose acetate membranes. Sucrose and sodium chloride solutions were tested in addition to soybean whey to relate the mixed solute system in whey to that of single-solute organic and inorganic feed solutions. Water flux was shown to have an Arrhenius dependency on temperature, and some membrane compaction was observed with the more porous membrane. Concentration polarization for sucrose and sodium chloride solutions increased linearly with water flux. Solute flux for soybean whey solutions decreased with molarity and was independent of pressure, whereas solute rejection increased with temperature and pressure and was independent of molarity. Good agreement was obtained using the derived parameters A, B, and τ for soy whey in the diffusion transport model when compared to the observed experimental values.