Membranes and Membrane Technologies

The reagent-free electromembrane process of removing carbonates, bicarbonates, and carbonic acid from softened natural carbonate water using an electrodialysis synthesizer EDS-01 with a two-cell unit cell formed by a bipolar membrane and a cation-exchange membrane has been studied. MB-2M membranes modified with an ionopolymer containing phosphoric acid groups catalytically active in a water-splitting reaction have been used as bipolar membranes, while heterogeneous membranes Ralex CMH (Mega a.s., Czech Republic) have been used as cation-exchange membranes. The decarbonization process has been carried out in two stages. At the first stage, a reagent-free correction of pH of the solution treated has been carried out. The value of pH in acid compartments has been adjusted to be 2.5–4.0. At the second stage, this solution has been deaerated with air purified from carbon dioxide. For a quantitative description of the process, a previously developed model has been adapted to describe the electrodialysis process with bipolar and cation-exchange membranes. It is shown that the electrodiffusion transfer of anions through the cation-exchange membrane and bipolar membrane is practically absent, and the change in the concentrations of carbonate ions, bicarbonates, and carbonic acid is due to the quasi-equilibrium chemical reactions. The deaeration of acidified softened water reduces the total carbon content from 5 to 1 mmol/L. The decarbonization of softened water is accompanied by a decrease in the concentration of sodium cations and total mineralization. With an EDS-01 electrodialysis synthesizer performance of 100 L/h, the specific energy consumption is in the range from 0.16 to 6.12 kW h/m³ depending on the current density.

A “green” method to synthesize new composite membranes from a cellulose solution in phosphoric acid on various ultrafiltration substrates is proposed. The method can be used in industry; it differs from the conventional viscose method for producing cellophane and other known methods for synthesizing cellulose-based gas separation membranes by the absence of gaseous emissions and wastewater. The structure of the synthesized samples is studied by electron microscopy, X-ray diffraction, and thermal analysis (DSC). Analysis of the mechanical properties of the samples shows that the new membranes have better mechanical characteristics than those of homogeneous pure cellulose films synthesized in this study and commercial cellophane films. The gas transport properties of new membranes with respect to O₂, N₂, CO₂, CH₄, and He are studied. It is found that the proposed membrane synthesis method provides the formation of uniform dense gas separation layers of cellulose; the membranes show a three orders of magnitude higher gas permeability than that of cellophane films. It is shown that the highest ideal selectivity is exhibited by membranes with a gas separation layer of cellulose on viscose fabric substrates.

Knowledge of the mechanism of deposition of sparingly soluble salts (scaling) in reverse osmosis membrane devices is extremely important for the selection of means of controlling the scaling and increasing the recovery. This study has made it possible to take a fresh look at the mechanism of scaling and the role of antiscalants in the inhibition of this process. The development of the experimental procedure is based on the idea that the first phase of crystallization—nucleation—is homogeneous; that is, it occurs in the “dead” areas in the bulk of the concentrate at high values of calcium carbonate supersaturation. After formation, the crystals are removed from the “dead” areas and deposited on the membrane surface, like other suspended particles contained in the feed water. The paper also presents the results of a study of the adsorption of polymeric antiscalants on crystal surfaces during nucleation and crystal growth on the membrane. The experimentally revealed relations of the adsorption rates of antiscalants to the antiscalant dose, the calcium carbonate formation rate, the nucleation rate, and the total surface of the seed crystals formed are presented. An experimental procedure is described that makes it possible to determine the concentration of dissolved salts in the dead areas and calculate the supersaturation values corresponding to the onset of crystallization from antiscalant-free water and water in the presence of scale inhibitors in various amounts.

The structure and properties of ultrafiltration membranes synthesized from bicomponent solutions in N,N-dimethylformamide using five commercial acrylonitrile copolymers of various compositions and molecular masses have been studied. The molecular mass characteristics of the copolymers (M_w, M_w/M_n) have been determined; the viscosity properties of dilute and concentrated solutions have been studied. It has been shown that depending on the chemical composition and molecular mass of the copolymer, the concentration dependence of the water flux is different: for copolymers with a molecular mass of M_w of 76 000–81 000 g/mol, with an increase in the copolymer concentration in solution from 12 to 16%, the water flux of the membranes decreases from 300–500 to 40–150 L/(m² h) depending on the copolymer composition. For samples with a higher molecular mass (M_w = 99 000 and 107 000 g/mol), the water flux of the membranes hardly depends on the copolymer concentration in the casting solution; it is 150 and 75 L/(m² h), respectively. The rejection factor of the membranes with a molecular mass of 40 000 g/mol for polyvinylpyrrolidone increases with increasing copolymer concentration in the casting solution regardless of the chemical composition and molecular mass of the copolymer. Scanning electron microscopy studies of the membrane structure have shown that membranes synthesized from copolymers with a higher molecular mass have a denser structure and a thicker selective layer than the respective parameters of membranes synthesized from acrylonitrile copolymers with a lower molecular mass, which are characterized by the presence of large macrovoids in the membrane matrix. These differences in the membrane structure are attributed to different viscosities of the casting solutions at identical copolymer concentrations.