Blood-gas equilibria,kinetics and transport

The kinetics and equilibria of the chemical reactions underlying blood respiratory function are reviewed, together with additional physico-chemical factors governing in vivo and extracorporeal blood O₂/CO₂ exchange. The role of red cell 2,3-diphosphoglycerate (DPG) in regulating oxygen-hemoglobin interaction is discussed at length. In addition, the coupling of hemoglobin's reactions with oxygen, carbon dioxide, hydrogen ions and DPG; the biochemical regulation of DPG concentration; and DPG's influence, through a Donnan phenomenon, upon the difference between erythrocyte and plasma pH, are also examined.The kinetics of blood equilibration with carbon dioxide are reviewed, with emphasis upon the enzyme carbonic anhydrase, the exchange of bicarbonate and chloride ions across the red cell membrane and hemoglobin carbamate formation. The discrepancy between the kinetics of oxygen reactions with turbulently flowing hemoglobin solutions and whole blood is discussed in light of recent evaluations of extracellular mass transfer resistance.The significance of blood-gas equilibria and kinetics in CO₂ release in the lungs and O₂ release to tissue is discussed with particular emphasis upon transfusion of blood with modified hemoglobin oxygen affinity. Convection/diffusion-limited O₂ transport is treated in the contexts of O₂ uptake in gas exchange devices and release to tissue. A brief discussion of blood substitute media is also appended.     

混凝土中氯盐-硫酸盐耦合侵蚀的化学-损伤-传输模型研究进展

服役于氯盐和硫酸盐耦合环境的钢筋混凝土结构,很容易因为氯盐和硫酸盐的侵蚀引起性能退化。本文从氯盐和硫酸盐的侵蚀机理出发,系统地分析了硫酸盐侵蚀造成混凝土损伤前后氯离子和硫酸根离子的扩散-结合过程。基于经典的扩散-反应模型以及化学-损伤-传输理论框架,详细阐述离子活度、环境温湿度、微观结构、钙溶蚀等因素对化学-损伤-传输体系的影响,并从热力学角度介绍了氯离子和硫酸根离子耦合传输的热力学模型。基于上述分析,对目前研究存在的问题进行了展望。     

Phenol transport in cellulose acetate membranes

Past detailed studies of solute transport through reverse-osmosis membranes have been conducted only with simple salts. The present work with phenol was undertaken largely because of the practical observation that the transport of low molecular weight organics is much more rapid than that of the salts. Studies of phenol sorption from dilute aqueous solution indicate that the diffusion coefficient for phenol in water-saturated 39.8 wt.-% acetyl cellulose acetate is 9.6 × 10^-10 cm.²/sec., and the equilibrium distribution coefficient between the acetate phase and water is 42. Thus, the diffusion coefficient is quite close to that measured for sodium chloride, and the higher permeability of the membranes to phenol can be attributed entirely to their greater sorption of this solute. In direct osmosis experiments performed with significant water flow a measurable interaction or positive coupling between water and phenol flows has been observed. Further evidence of flow coupling is derived from reverse osmosis experiments in which significant negative solute rejection is observed; i.e., the permeate is enriched in phenol by as much as 20%. It is shown that a solution-diffusion transport model is not adequate to rationalize the results, and a more complex transport model is apparently required.     

Water transport in ion exchange membranes

Water transport across ion exchange membranes has been studied experimentally. Water transference numbers were obtained from streaming potential measurements for cation and anion exchange membranes. At low salt concentration the water transference number reaches a limiting value which for the cation exchange membranes seems to be closely correlated with the cation—water friction in infinite dilute solutions.The observed water transference number for a given anion exchange membrane does not seem to be much dependent on the type of coion even at concentrations where the membrane is no longer perfectly permselective.     

Graphene-based membranes for pervaporation processes

Two-dimensional graphene and its derivatives exhibiting distinct physiochemical properties are intriguing building blocks for researchers from a large variety of scientific fields. Assembling graphene-based materials into membrane layers brings great potentials for high-efficiency membrane processes. Particularly, pervaporation by graphene-based membranes has been intensively studied with respect to the membrane design and preparation. This review aims to provide an overview on the graphene-based membranes for pervaporation processes ranged from fabrication to application. Physical or chemical decoration of graphene-based materials is elaborated regarding their effects on the microstructure and performance. The mass transport of pervaporation through graphene-based membranes is introduced, and relevant mechanisms are described. Furthermore, performances of state-of-the-art graphene-based membranes for different pervaporation applications are summarized. Finally, the perspectives of current challenges and future directions are presented.     

Electron transport processes in conductor-filled polymers

This article contends that three distinct physical processes control electron transport in conductor-filled polymer systems. Percolation is required to explain the macroscopic conduction in the disordered medium. Quantum mechanical tunneling (possibly quantum fluctuation augmented) is needed to describe conduction between adjacent conductive particles at the microscopic level. And, thermal expansion is invoked to deduce constituent volume densities and microscopic tunnel lengths. Each of these mechanisms is given a mathematical form. The ansatz is used to predict resistivity vs. temperature and resistivity vs. conductor-filling fraction functions. Successes and deficiencies of the theory are discussed with respect to experimental data and theoretical considerations.     

Silica transport through charged membranes

The possibility of using the electrodialysis process for the desalination of brackish waters having a high silica content, was investigated. The apparent transport numbers of silica through an anion membrane, under controlled hydrodynamic and chemical experimental conditions, were measured.Results indicate a direct dependence of the silica transport number on the SiO (OH^-₃^- ion concentration in the solution. It was found that the silica transport number decreases by increasing current density if chloride ions are present in the solution and that it is negligible with current density higher than 4 mA/cm² if bicarbonate ions are present.Some desalination tests were carried out by electrodialyzing brackish water with silica concentration of about 150 mg/l as SiO₂.Results showed that both transport and scaling effects of silica in the membrane were negligible with feed solutions containing HCO^-₃ ions and with brine acidified at pH ? 4, in accordance with transport number determinations.     

Piezodialytic transport across charge-mosaic membranes

Three membrane series differing in capacity and crosslinkage of both cation- and anion-exchange resins have been studied to determine the apparent diffusion coefficient of salts as well as the osmotic transport of water through the membranes. The study have covered pressure tests to obtain hydrodynamic permeability coefficient of the membranes, salt enrichment coefficient, volume flux and salt flux for NaCl solution in concentration over the range of 0.05 to 0.25 M. Permeate concentration was 2.75 to 3.95 times higher than that of feed solution.     

Piezodialytic transport across charge-mosaic membranes

The study reported in this paper is a continuation of a study presented earlier'*/Proceedings of the Internat, Congress on Desalination and water Re-use, Nice, France, Oct. 21-27, 1979, Water for life, Vol. III Part 3/. The title membranes were prepared on the basis of polyethylene modified with the copolymers of styrene, 4-vinyl-pyridine and 2-vinyl-benzene. They were designed for the piezodialytic desalination and concentration of brakish waters and seawater. Symmetric and asymmetric electrolytes, KCl, MgCl₂,Mncl₂ CaCl₂ and Na₂So₄, /of an initial concentration between 0.05 and 0.5 M/ contained in brackish waters were concentrated, and the results obtained are discussed. The investigations were carried out in a laboratory system which permitted a recirculation of the liquid under study, and a mixing in the dialysate cell. The pressure at which the electrolytes were concentrated varied from 10.3 to 15.3 Pa /100 to 150 atm/ and was kept constant throughout each measurement.The applicability of the membranes in question to a dialytic concentration was estimated, in terms of the coefficient α which describes the concentration effect. The concentration effect depends on the degree of crosslinking in the ion-exchange elements, on the type concentration of the electrolyte and the value of pressure difference. For electrolytes of an initial concentration of 0.5 M no concentration effect was obtained. In the case of asymmetric electrolytes /but only for an initial concentration of 0.05 M/ the concentration effect was insignificantThe experimental results indicate that the concentration effect decreases with the increase in electrolyte concentration and in the pressure applied.     

Polyorganophosphazene membranes: preparation and transport properties

Polyphenoxyphosphazene, polycumylphenoxyphosphazene, polybutylphenoxyphosphazene and their copolymers were synthesized. Asymmetric and homogeneous membranes for liquid-liquid and gas separations were prepared. The ultrafiltration membranes were characterized by a cut-off higher than 100,000 Daltons. The gas separation membranes, tested with O₂, N₂, CO₂ and CH₄, were characterized by permeabilities similar to the ones of rubbery polymers and selectivities similar to the ones of glassy polymers.     

Water vapor and CO2 transport through amine-containing facilitated transport membranes

Amine-containing CO₂ facilitated transport membranes have great potential to be applied for hydrogen purification from synthesis gas. In some applications, the humidity of the retentate stream is required as well as the purity of hydrogen. The membranes are highly hydrophilic, and they exhibit not only high CO₂ permeance but also high water vapor permeance. In this work, the transport of water vapor and CO₂ through the membranes composed of an amine-containing selective layer and a microporous polysulfone substrate was investigated. From the experiments conducted, water vapor permeance appeared to be independent of the selective layer thickness, indicating that the substrate is the controlling factor of the mass transfer resistance to water vapor transport. Moreover, water vapor permeance appeared to reduce linearly with increasing the number of the substrate layers. But, CO₂ permeance and CO₂/H₂ selectivity did not change significantly as the number of the substrate layers increased. These results indicated that the CO₂ separation performance is governed by the selective layer as expected. In addition, the membranes synthesized from Lupamin^® containing 34% polyvinylamine and 66% salt (sodium formate) demonstrated better CO₂ separation performance than those from pure polyvinylamine, presumably due to better water retention capability of the salt than polyvinylamine.     

Water vapor sorption and transport in dense polyimide membranes

The sorption and transport of water vapor in five dense polyimide membranes were studied by thermogravimetry. The sorption isotherms of water vapor in the polyimides could be successfully interpreted by both the dual‐mode sorption model and the Guggenheim–Anderson–de Boer equation. The water vapor diffusion behavior was found to be nearly Fickian at higher water vapor activities, whereas non‐Fickian diffusion was observed at lower water activities. The phenomena could be well described by the mechanism of combined Fickian and time‐dependent diffusion. The diffusion coefficient and water vapor uptake in the polyimides were strongly dependent on the polymer molecular structure. Except for the polyimide prepared from 3,3′,4,4′‐diphenylsulfone tetracarboxylic dianhydride and 1,3‐bis(4‐aminophenoxy) benzene, the permeability of water vapor in the dense polyimide membranes predicted from the sorption measurement at 30°C corresponded well with the water vapor permeability measured at 85°C. Among the polyimides studied, pyromellitic dianhydride–4,4′‐diaminophenylsulfone (50 mol%)/4,4′‐oxydianiline (50 mol%) showed both high water sorption and diffusion and, therefore, high water vapor permeability, which for vapor permeation membranes is necessary for the separation of water vapor from gas streams. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 87: 2306–2317, 2003     

Imprinted membranes for sustainable separation processes

The rapid industrial growth and the necessity of recovering and recycling raw materials increased the interest in the production of highly selective and efficient separation tools. In this perspective, a relevant input was given by the membrane-based technology and the production of imprinted membranes, which possess specific recognition properties at molecular and ionic level, offers the possibility of developing sustainable and green processes. Furthermore, the integration of imprinted membranes with traditional or membrane-based approaches is a promising strategy in the logic of process intensification, which means the combination of different operations in a single apparatus. This work discusses the concept and separation mechanisms of imprinted membranes. Furthermore, it presents an overview of their application in organic solvent nanofiltration, for the removal of toxic agents and recovery solvent, as well as valuable compounds. The recent advances in water treatment, such as pesticide removal and recovery of metal ions, are also discussed. Finally, potential applications of imprinted membranes in hybrid processes are highlighted, and a look into the future of membrane separations for water treatment and recovery of critical raw materials is offered.     

New cation-exchange membranes for hyperfiltration processes

A new route for the preparation of cation exchange membranes from polystyrene–polyisoprene–polystyrene (SIS) block copolymers has been studied, using N-chlorosulfonyl isocyanate. At temperatures of 0° to 20°C, N-chlorosulfonyl isocyanate reacts readily with the olefin group in polyisoprenes, resulting in a β-lactam-N-sulfonyl chloride group. Films of this product can be cast which are hydrolyzed afterwards with aqueous ammonia at room temperature to give a membrane with ionic sulfonate and neutral carbamoyl groups. Homogeneous membranes are prepared with an SIS block copolymer as starting material and with mole ratios of N-chlorosulfonyl isocyanate/isoprene between 15% and 45%. In hyperfiltration experiments at 40 atmospheres, both NaCl and Na₂SO₄ are rejected up to 82%, while fluxes of 0.25 to 0.30 cm³/cm²·hr are obtained. From permeation and hyperfiltration experiments, it is concluded that the weight fraction of membrane water has a large influence on the flux. The water content in the membrane during the hyperfiltration process is primarily determined by the applied pressure, the type of salt, and its concentration.     

Water binding and irreversible dehydration processes in cellulose acetate membranes

The relative amounts of freezing and nonfreezing water in various water-wet cellulose acetate (CA) membranes were determined by NMR techniques, from the initial heights of the water component in the free induction decay (MNR intensity). The results suggest that (1) a significant fraction of the water in various wet CA membranes does not freeze, probably because of strong interaction with the polymer; (2) the relaxation times T₂ of the nonfreezing water are of the order of milliseconds indicating that they are still highly mobile compared with ice; (3) all the water contained in dense CA films or in membranes equilibrated at relative humidity of 0.93 does not freeze upon cooling the membranes from room temperature to ?60°C; (4) the amounts of nonfreezing bound water in membranes is higher than the total amount of water absorbed from liquid water by a dense film of the same polymer. However, the amounts of nonfreezing water in various CA membranes as calculated from the “relative NMR intensities” is substantially lower than those calculated from DSC melting endotherms by assuming the heat of fusion of water in membranes to be identical to that of pure water. Various possible reasons for this discrepancy are discussed. Measurements on the first desorption-adsorption cycle of wet CA membranes have also been performed. They suggest that during the first dehydration process, irreversible changes are induced in the structure of the membrane which result in a significantly lower accessibility of the polymer to interact with water. The extent of these irreversible changes in membrane structure is dependent on the details of the dehydration process being more pronounced at higher temperatures.     

Sorption equilibria and diffusion coefficients of ethanol and water in the PVA membranes containing cyclodextrin

Effects of cyclodextrin (CD) on the pervaporation characteristics for water/ethanol through the PVA/CD membranes (PVA membranes containing β-CD oligomer) have been investigated in terms of sorption equilibria and diffusion coefficients based on the sorption–diffusion theory. The increase in water selectivity through the pervaporation by CD was due mainly to the changes in the diffusion coefficients by CD, which depended on the feed composition and the cross-linking time. The water selectivity through the sorption equilibria was not increased by the addition of CD, and the ethanol-sorption amount was increased by CD. These effects of CD were interpreted by the inclusion strength in the CD cavity and the cross-linking density of the PVA phase. © 1994 John Wiley & Sons, Inc.     

Gas transport properties of polyaniline membranes

The transport properties of He, H₂, CO₂, O₂, N₂, and CH₄ gases in solvent cast, HCl doped, and undoped polyaniline (PANi) membranes were determined. Measurements were carried out at 40 psi pressure from 19°C to 60°C. An excellent correlation was found between the diffusion coefficients and the molecular diameters of gases. The solubility coefficients of gases were found to correlate with their boiling points or critical temperatures. The sepa-ration factors for CO₂/N₂ and CO₂/CH₄ are dominated by the high solubility of CO₂. These correlations enable us to predict the permeability, diffusion, and solubility coefficients of other gases. After the doping-undoping process, the fluxes of gases with kinetic diameters smaller than 3.5 Å increased but those of larger gases decreased. This results in a higher separation factor for a gas pair involving a small gas molecule and a larger one. © 1996 John Wiley & Sons, Inc.     

Mass transport mechanisms within pervaporation membranes

Pervaporation is an energy-efficient membrane technology for separating liquid molecules of similar physical properties, which may compete or combine with distillation separation technology in a number of applications. With the rapid development of new membrane materials, the pervaporation performance was significantly improved. Fundamental understanding of the mass transport mechanisms is crucial for the rational design of membrane materials and efficient intensification of pervaporation process. Based on the interactions between permeate molecules and membranes, this review focuses on two categories of mass transport mechanisms within pervaporation membranes: physical mechanism (solutiondiffusion mechanism, molecular sieving mechanism) and chemical mechanism (facilitated transport mechanism). Furthermore, the optimal integration and evolution of different mass transport mechanisms are briefly introduced. Material selection and relevant applications are highlighted under the guidance of mass transport mechanisms. Finally, the current challenges and future perspectives are tentatively identified.