Concentration of basic amino acids by electrodialysis

The specific features of concentration of basic amino acids by electrodialysis with bipolar membranes have been studied. The relationships of the process efficiency, described by the concentration factor, with the current density, the concentration of the initial solution, and the nature and characteristics of cationexchange and bipolar membranes used in the system have been obtained. It has been shown that electroosmotic transport of water along with amino acid ions through cation-exchange membranes limits the concentration efficiency. The influence of the size and hydration of the side groups of amino acids on the magnitude of electroosmotic flux has been revealed.     

Modified PTFE–PANI membranes for the recovery of oil products from aqueous oil emulsions

Composite membranes have been prepared by modifying a polytetrafluoroethylene membrane with aniline hydrochloride, and the effect of polymerization time on the efficiency of the recovery of petroleum products from aqueous oil emulsions was studied. It has been found that the modification leads to an increase in the degree of oil removal from oil-in-water emulsions by 29%. The specific productivity of the original and modified membranes has been determined with distilled water and water–oil emulsions. It has been found that the specific productivity of the membranes decreases with the time of membrane treatment with ammonium persulfate. The surface of the modified membranes has been studied with the aid of a scanning electron microscope, and the elemental composition of composite membrane surfaces has been examined by X-ray fluorescence analysis. It has been determined that the modification of a polytetrafluoroethylene membrane with aniline hydrochloride leads to an increase in the carbon content and the appearance of nitrogen, oxygen, and sulfur atoms.     

Effect of the pH of emulsion on ultrafiltration of oil products and nonionic surfactants

The structure and properties of a water–oil emulsion have been studied. The ultrafiltration of the water–oil emulsion has been performed with the use of spiral wound and hollow fiber membranes to separate the emulsion into a filtrate and a concentrate. The effect of pH on the following performance characteristics of the ultrafiltration process has been studied: the efficiency and the degree of separation of oil products and nonionic surfactants. It was found that an increase in the pH value of the emulsion decreased the efficiency of membranes and the degree of separation of nonionic surfactants. It has been found that a Raifil UF membrane effectively rejects oil products from acidic emulsion and, on the contrary, an EMU 45-300 membrane is effective in an alkaline medium. This behavior has been associated with to a positive surface charge of the EMU 45-300 membrane. The maximum efficiencies and degrees of separation of oil products and nonionic surfactants from emulsions with the use of the Raifil UF ultrafiltration membrane and the EMU 45-300 membrane have been attained in the pH ranges from 2.1 to 2.9 and from 2.2 to 2.5, respectively. After ultrafiltration, the size of particles in the filtrate increased by a factor of about 18 due to the coalescence of oil particles in the near-membrane layer because of polarization effects. The increase in the particle size of the dispersed phase in the filtrate can also be explained by a positive surface charge of polysulfonamide membranes.     

Structure and transport properties of pervaporation membranes based on polyphenylene oxide and heteroarm star polymers

Thin-film membranes based on polyphenylene oxide composites with a varying concentration of heteroarm star-shaped polymers (1, 3, and 5 wt %) comprising arms of polystyrene and poly(2-vinylpyridine)- block-poly(tert-butylmethacrylate) diblock copolymer grafted onto a common fullerene C₆₀ core have been developed. The transport properties of the membranes have been studied in the pervaporation separation of a methanol–ethylene glycol mixture. An increase in the star-shaped polymer content in the membrane leads to an increase in the flux and separation factor of the membranes. Sorption studies have revealed that the sorption activity of methanol in the membranes is higher than that of ethylene glycol. The introduction of star-shaped polymer additives into the membrane composition leads to an increase in the degree of equilibrium sorption of the two components of the mixture subjected to separation. The formation of transport channels in pervaporation membranes during sorption in deuterated methanol has been first studied using the small-angle neutron scattering method. Comparative analysis of the data on neutron scattering on the original dry samples, the samples saturated with deuterated methanol, and the samples dried after sorption has shown that the structural uniformity of the composite membranes is higher than that of the matrix polymer. According to scanning electron microscopy, the morphology of the composite membranes is a system of closed cells.     

Hybrid Gas Separation Membranes Containing Star-Shaped Polystyrene with the Fullerene (C<Subscript>60</Subscript>) Core

A physicochemical study of novel hybrid polymer membranes based on polyphenylene oxide with a star-shaped modifier incorporated into the matrix has been conducted, and the transport properties of the membranes in the gas separation process have been studied. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) has been selected as the polymer matrix because of the low cost and high mechanical strength of this material. Star-shaped macromolecules (up to 5 wt %) containing six polystyrene arms grafted onto a fullerene(C₆₀) central core have been used as the filler. The structure and physical properties of the resulting membranes have been characterized by scanning electron microscopy, membrane density measurements, differential scanning calorimetry, and thermogravimetric analysis. Film surface has been studied by contact angle measurements. The gas separation properties of the membranes have been studied by the barometric method for the following individual gases: H₂, O₂, N₂, and CH₄. Data on the separation properties have been plotted as a Robeson diagram to compare with published data. It has been shown that the incorporation of star-shaped polystyrene into the PPO matrix leads to an improvement of the separation efficiency for selected gas pairs and an increase in selectivity compared with that of the unmodified membrane.     

A novel hybrid material based on polytrimethylsilylpropyne and hypercrosslinked polystyrene for membrane gas separation and thermopervaporation

To improve the membrane permeability and separation properties in gas separation processes and thermopervaporative (TPV) recovery of butanol from model fermentation mixtures, hybrid membranes based on polymers with an extremely high free fractional volume—polytrimethylsilylpropyne (PTMSP) and hypercrosslinked polystyrene (HCL-PS)—have been first prepared and experimentally studied. The composite membranes have been fabricated using the commercial sorbent Purolite Macronet MN-200 exhibiting high sorption capacity for organic solvents. It has been found that in the hybrid membranes, HCL-PS sorbent particles are nonuniformly distributed throughout the volume: they are located in the surface layer of the membrane. It has been shown that the introduction of a small amount of a modifying component (0.5–1.0 wt %) into the PTMSP matrix improves the time stability of transport properties and increase by a factor of 1.5–2 the permeability coefficients of the material to light gases (N₂, O₂, CO₂, CH₄) and butane vapor. It has been found that hybrid PTMSP/HCL-PS membranes have higher separation factors than those of PTMSP membranes in the TPV separation of a butanol/water binary mixture.     

New approach to the formation of polyimide ultrafiltration membranes involving modified polyacrylonitrile

A new approach to the formation of asymmetric ultrafiltration membranes based on insoluble rigid-chain aromatic polyimide (PI) and involving polyacrylonitrile modified by partial cyclization (m-PAN) has been studied. It has been found that m-PAN exerts a favorable effect on the membrane formation and composition: it (i) promotes fast precipitation at the stage of wet formation; (ii) acts as an imidization catalyst that allows for decreasing temperature at the stage of solid-phase thermal imidization; and (iii) it remains as a full-fledged component in the PI/m-PAN membrane. It was shown that up to 20 wt % m-PAN can be effectively introduced into the membrane composition. The membranes have been calibrated using a model mixture of proteins with different molecular weights. The structure, permeability to water, and thermomechanical properties of the membranes have been studied.     

Diffusion characteristics of surface-modified ion-exchange membranes based on MK-40, MF-4SK, and polyaniline

Asymmetric membrane materials based on MK-40 membranes modified with a thin layer of MF-4SK perfluorinated ion-exchange resins with incorporated polyaniline have been studied. The diffusion characteristics of the resulting composites have been analyzed. It has been proved that the membranes with polyaniline undergo a decrease in diffusion permeability, which indicates an increase in selectivity. The modified membranes exhibit a higher cation transport rate. The diffusion coefficients of individual cations in mixed cationic membranes have been estimated. It has been shown that the introduction of small amounts of polyaniline leads to an increase in the proton transport rate.     

Influence of polyvinylpyrrolidone on morphology, hydrophilicity, and performance of polyethersulfone microfiltration membranes

The influence of an admixture of polyvinylpyrrolidone (PVP) with different molecular weights on the morphology and properties of polyethersulfone (PES) membranes prepared via water vapor-induced phase inversion followed by immersion in water has been studied. Membrane structure alteration by PVP addition to the casting solution has been observed by means of SEM. The contact angle on monolithic PES films varies from 78° to 54° depending on the PVP content. By Fourier-transform IR spectroscopy, it has been revealed that an increase in molecular weight of PVP leads to an increase in its residual content in the membranes. It has been shown that the addition of PVP with a molecular weight close to that of PES hardly has a significant effect on the permeability and tensile strength of the membranes regardless of the PVP content. However, the introduction of high-molecular-weight PVP leads to a decrease in the pore size and permeability of the membranes as a result of change in viscosity of the solution.     

Performance testing of seawater desalination by nanofiltration

Nanofiltration membranes of the ERN, NF90, and SR90 brands have been studied in use for the treatment of model solutions simulating seawater with the varying salt content. It has been shown that the overall selectivity and efficiency for all types of the membranes increase with increasing pressure; the highest selectivity for anions and cations increases in the order ERN > NF90 > SR90, whereas the membrane efficiency grows as NF90 > SR90 > ERN. The measurement results also allow for the conclusion that the selectivity is slightly dependent on the permeate withdrawal rate and the salinity, the total hardness concentration is reduced by 90?C95%, and the total dissolved salts (TDS) is reduced by 30?C40%. With a TDS of 35 g/L, the operating pressure does not exceed 20 atm, which is more than twice lower than that for reverse osmosis membranes. Thus, these membranes can be used with a high efficiency for softening seawater and brackish water with various salt contents.     

The influence of catalytic additives on electrochemical properties of bipolar membranes

The influence of organic, inorganic, and hybrid catalytic additives on the current–voltage characteristics of the bipolar region of industrial MB-1 and MB-3 bipolar membranes and analogues of an MB-2 bipolar membrane is investigated by the method of frequency spectrum of electrochemical impedance. It is shown that the introduction of compounds accelerating water dissociation reaction into the bipolar region makes it possible to obtain bipolar membranes with lower operating voltage when compared to industrial bipolar membranes.     

Intensification of separation of oil-in-water emulsions using polysulfonamide membranes modified with low-pressure radiofrequency plasma

Effect of low-pressure radiofrequency capacitively-coupled plasma parameters on the separation of model “oil-in-water” emulsions using UPM-20 polysulfonamide membranes with a pore size of 0.01 μm has been studied. Atmospheric air has been used as a plasma gas. The plasma treatment time is 1, 5, 4, and 7 min at a plasma torch anode voltage of U _a = 1.5–7.5 kV. The separated medium is a 3% emulsion based on the industrial oil I-20A; the stabilizer is the surfactant Kosintol-242. The highest performance is observed in the case of membranes treated with plasma for 7 min. It is found that plasma treatment leads to an increase in the emulsion separation efficiency from 90 to 99%. It has been shown that the plasma treatment contributes to the surface hydrophilization of the membranes owing to the formation of oxygen-containing functional groups. The membrane exhibits a contact angle of water of α = 59.6° before treatment and α = 19.5° after a 4-min plasma treatment at U _a = 7.5 kV. Atomic force microscopy reveals that the plasma treatment leads to a change in the surface structure of the membranes, namely, to a decrease in their roughness. The internal structure of the membranes also undergoes changes which result in an increase in their crystallinity.     

Asymmetrical track-etched membranes prepared by double-sided irradiation on the DC-60 cyclotron

Asymmetric track-etched membranes based on polyethylene terephthalate have been prepared using two-sided irradiation on the heavy-ion accelerator DC-60. A process for cyclotron adjustment to bombardment of a polymer material has been described. A procedure for preparing asymmetric track-etched membranes with different pore lengths and sizes has been specified. The strength characteristics of the resulting membrane have been evaluated. It has been shown that the membrane can be used in processes in which the driving force is hydraulic or osmotic pressure.     

Asymmetric track membranes: Relationship between nanopore geometry and ionic conductivity

The revealing of the “diodelike” properties of electrolyte-filled asymmetric nanopores in track membranes has given significant impetus to a detailed study of the properties of “track” nanocapillaries. Studying the behavior of electrolyte solutions in nanovolumes of a given geometry is very important for many applications, such as nanofluid technology, the resistive pulse method for detecting colloidal particles and molecules, modeling of biological membranes, etc. An attempt to find a quantitative relationship between the geometric shape of asymmetric nanopores and asymmetry in electrical conductivity has been made in this paper. The method of chemical etching in the presence of a surfactant was used for the formation of nanopores with different profiles. The pore structure was studied by electron microscopy. It has been found that the rectification ratio increases with the membrane thickness and depends strongly on the curvature of the pore profile in the selective layer. The maximum of the rectification has been observed in a 0.05–0.1M KCl. Simulation of the ionic conductivity of asymmetric nanopores by the Poisson-Nernst-Planck equation qualitatively explains the observed behavior. The effect of the asymmetry of electrical conductivity is well expressed even in cases when the pore radius in the selective layer is substantially greater than the Debye length. The modification of the pore surface by grafting of aminopropyltriethoxysilane results in the sign inversion of electric charge and a sharp change in the current-voltage characteristics of the membranes.     

Structure,morphology, and transport characteristics of profiled bilayer membranes

A procedure for the fabrication of profiled cation-exchange bilayer membranes with the homogenized surface based on the commercial membrane MK-40 has been developed. The surface morphology and membrane microstructure have been studied by atomic-force microscopy, electron microscopy, and standard contact porosimetry. The concentration dependences of the electrical conductivity and diffusion permeability of the profiled and bilayer profiled membranes have been studied. It has been shown that the application of an MF-4SK film on the surface of the profiled membrane results in a decrease in its diffusion permeability and some increase in specific conductivity. Based on the data obtained, the transport and structure parameters have been calculated in terms of the microheterogeneous model to assess the influence of the modification on the properties of the support membrane. The current–voltage characteristics of the membranes have been measured in sodium chloride solutions, and it has been shown that profiling leads to an increase in the limiting current by 40%. The investigation of mass transfer of ions in the channels formed by the support and modified membranes has shown that under intense current regimes, the mass transfer coefficient through the profiled bilayer membrane is one and a half times that through the initial profiled membrane.     

Concentration history of pervaporation

The permeability and selectivity of the separation of aqueous organic solutions by pervaporation have been studied on semicrystalline poly(ethylene terephthalate) membranes and asymmetric amorphous polyvinyltrimethylsilane membranes in the presence of structural rearrangements initiated by the development of relaxation processes. Attention was focused on the concentration history, which lies in the fact that the permeability and the permeate composition depend on the concentration of the previous solution passed through the membrane at a fixed composition of the feed solution. It has been shown that micropores are formed in both membranes under certain pervaporation conditions and mass transfer occurrs through two competing transport channels. As a result, the oscillatory kinetics of pervaporation and the inversion of separation selectivity in a steady state are observed experimentally.     

Fractionation of multicomponent solutions by electrodialysis with bipolar membranes

The process of electrodialysis with bipolar membranes and the performance characteristics of plants that use this process are described. The first results of the laboratory tests of an electrodialysis plant with bipolar membranes are given; these results show that the salt wastes of nuclear power plants can be practically utilized with the manufacture of useful products (acids, alkalis, and boric acid) for the repeated use at the plant. The use of modified ion-exchange membranes and a two-compartment electrodialyzer design with bipolar membranes for the removal of ionic impurities from the solutions of acids and alkalis, in particular, for the production of pure boric acid, is proposed.     

Fabrication of Hollow Fiber Membrane from Polyarylate–Polyarylate Block Copolymer for Air Separation

A process has been developed for spinning asymmetric hollow fiber membranes from a polyarylate–polyarylate block copolymer having a separation factor for pure gases in the oxygen–nitrogen pair of α(O₂/N₂) = 6.3 and an oxygen permeance coefficient of 2.4 Barrer. The effect of the polymer molecular weight, the composition of the dope solution, and the spinning parameters on the gas transport properties of the hollow fiber membrane and its geometry has been studied. The use of a polymer with a molecular weight of 67 kDa, as well as the introduction of surfactants into the dope solution, has made it possible to prepare samples of defect-free membranes with an oxygen permeance of 120 L(STP)/(m² h bar) and a separation factor of α(O₂/N₂) = 6.5, which correspond to the selective layer thickness of 60 nm.     

Hybrid membranes based on polybenzimidazole and hydrated zirconia

Hybrid membranes based on polybenzimidazole and hydrated zirconia doped with phosphoric acid have been synthesized. The effect of synthesis conditions on the properties of the resulting materials has been studied. It has been shown that the modification of the membranes makes it possible to increase their proton conductivity. The introduction of hydrated zirconia decreases the amount of phosphoric acid leached out from the membrane exposed to water vapor. The diffusion permeability of composite membranes in solutions of various salts has been studied. Model tests of some samples under fuel cell operating conditions have been performed.     

Asymmetry of ion transport in hybrid MF-4SC membranes with a gradient distribution of hydrated zirconia

Results of studies of the properties of hybrid MF-4SC membranes with a gradient distribution of hydrated zirconia nanoparticles through the thickness prepared by layer-by-layer casting from a polymer solution are described. The effect of the dopant concentration on the properties of the membranes is studied. At a low oxide concentration, the water uptake and ionic conductivity of the resulting samples increase compared to the original MF-4SC membrane. It is found that the diffusion permeability of HCl and NaCl solutions across these membranes exhibits an asymmetric behavior. It is shown that the diffusion permeability is higher in the case of diffusion of the solutions from the unmodified side of the membrane. The maximum asymmetry coefficient is obtained for a membrane containing 10% ZrO₂ in the modified layer for the diffusion of 0.1 M HCl solution (38%). The causes of the diffusion permeability asymmetry are discussed.