Electrodialysis (ED) of calcium-containing solutions is a widely applied process in water treatment, dairy industry etc. The lower performance of ion-exchange membranes in these processes (in comparison, e.g. with ED of sodium-containing solutions) is known, but the reasons for this effect are still unclear. We study the behavior of two sulfonated cation-exchange membranes: a CMX (Astom, Japan) and a CJMC-5 (Hefei Chemjoy Polymer Materials, China). For both membranes, the bulk properties (conductivity, diffusion permeability, counterion permselectivity) change when the membrane is soaked in a 0.02 N CaCl2 solution, but these changes are reversible: when the membranes are again equilibrated with a NaCl solution, the bulk properties restore and become the same as before. The exposure of the membranes to an overlimiting current does not affect the bulk properties. However, it is not the case for the properties controlled by the membrane surface. In particular, the length of the limiting current plateau of the current-voltage curve increases and electroconvection (visualized using Rhodamine 6G) decreases with increasing the time of the membrane exposure to overlimiting current in a CaCl2 solution. The zeta potential is about -30 mV for a "fresh " membrane, and +40 mV after exposure of this membrane to an overlimiting current in a 0.02 N CaCl2 solution; the both measurements were made in the same NaCl solution. The changes in surface-controlled properties appear to be irreversible. This effect is explained by specific adsorption of Ca2+ ions on the surface of sulfonated membranes promoted by a strong electric field at the interface. This explanation is supported by simulations of the distribution of the electric potential and ion concentrations in the interfacial electrical double layer.

Chronopotentiometry using pulses of a constant current of density j is a powerful method for characterizing ionexchange membranes (IEMs). We report on the influence of coion transport on the shape of chronopotentiogram (ChP) and the consequent new possibility of quantifying coion transport based on ChP analysis. We show that in the case where the bathing solution contains Ca2+ or Mg2+ ions, the ChPs of the homogeneous (CMX) and heterogeneous (MK-40) cation-exchange membranes at overlimiting current densities have a maximum (a peak), which appears a few seconds after the transition time. The time required to reach a stationary state is of the order of d(2)/(D) over bar (2) (where d is the membrane thickness and (D) over bar (2) is the coion diffusion coefficient in the membrane); this time under our experiment conditions is about 300-400 s. We show that the cause of the maximum is the increase in coion transfer caused by the current-induced concentration polarization of the bathing solution. This increase in coion transfer results in increasing the limiting current density j(lim), which at j = const leads to a reduction in the resistance of the depleted diffusion layer over time and the appearance of a maximum on the ChP. 1D mathematical modeling is based on the Nernst-Planck-Poisson equations. The main assumption inspired by the works of Levich and Amatore is that the apparent electrolyte diffusion coefficient in the depleted solution increases with increasing electroconvection. The stationary value of this diffusion coefficient is found from the I-V curve. The only fitting parameter is the critical potential difference, which refers to the onset of intensive electroconvection.

Ultrafiltration and nanofiltration membranes that are traditionally used in baromembrane processes have become increasingly frequently applied in electromembrane and baroelectromembrane processes. In the latter case, two driving forces, an electric field and a pressure gradient, are simultaneously used. This study presents data on the specific electrical conductivity, diffusion permeability, and transport numbers of ions as well as current–voltage characteristics and chronopotentiograms of AMN-P and OPMN-P weakly ionized nanofiltration membranes and a number of track membranes in solutions of NaCl and CaCl 2 . The relationship of obtained characteristics with the structure and exchange capacity of membranes is discussed. It is shown that both nanofiltration and track membranes can exhibit a quite high selectivity towards the electrical transport of ions with a certain charge sign. Such selectivity is especially high in respect of the transport of Ca 2+ ions. In the case of an AMN-P membrane, the transport number of Ca 2+ reaches 0.98, while it is noticeably lower for Na + ions. This result correlates well with a known fact about a higher rejection coefficient of doubly charged ions in comparison with singly charged ions in baromembrane processes. Track membrane #115 is also distinguished by high selectivity in respect of cations, the transport number of sodium ions in it is close to 0.96.

Ion-exchange membranes modified with polyelectrolyte layers with alternating charges on fixed groups provide high selectivity among monovalent ions and are therefore promising for use in fractionation of multicomponent solutions by electrodialysis. The structure of such membranes features bipolar boundaries that are capable of raising the system’s resistance and lead to establishing the function of H + and OH − ion generation, which is not always a desirable trait in fractionation by electrodialysis. Here, by analyzing potential transients, we estimate the electrical conductivity of MK-40 cation-exchange membranes coated with a layer of polyethylenimine, which is an anion-exchange substance, and the generation of H + and OH − ions at the formed bipolar boundary. We show that the membrane acquires a high resistance (which can be twice as high) due to coating it with such a layer, and the generation of H + and OH − ions, though it emerges already under the conditions of sub-limiting current, does not become a dominant mass transfer process and leads to a change in pH of the diluate by no more than 1.5.

Recently developed and produced by Hefei Chemjoy Polymer Material Co. Ltd., homogeneous CJMC-3 and CJMC-5 cation-exchange membranes (CJMCED) are characterized. The membrane conductivity in NaCl, Na2SO4, and CaCl2 solutions, permeability in respect to the NaCl and CaCl2 diffusion, transport numbers, current–voltage curves (CVC), and the difference in the pH (ΔpH) of the NaCl solution at the desalination compartment output and input are examined for these membranes in comparison with a well-studied commercial Neosepta CMX cation-exchange membrane produced by Astom Corporation, Japan. It is found that the conductivity, CVC (at relatively low voltages), and water splitting rate (characterized by ΔpH) for both CJMCED membranes are rather close to these characteristics for the CMX membrane. However, the diffusion permeability of the CJMCED membranes is significantly higher than that of the CMX membrane. This is due to the essentially more porous structure of the CJMCED membranes; the latter reduces the counterion permselectivity of these membranes, while allowing much easier transport of large ions, such as anthocyanins present in natural dyes of fruit and berry juices. The new membranes are promising for use in electrodialysis demineralization of brackish water and natural food solutions.

Structural (volume fractions of the gel phase and the intergel solution) and transport (electrical conductivity, diffusion permeability, transport numbers of counterions and coions) characteristics of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes in NaCl, CaCl2, and Na2SO4 solutions have been studied. The investigated membranes have the same chemical nature of the ion-exchange matrix and similar values of ion-exchange capacity, but they differ in the degree of heterogeneity and chemical nature of the reinforcing materials. The difference in the properties between heterogeneous (MK-40 and MA-41) and (conventionally) homogeneous (CMX and AMX) membranes is due to the fact that the heterogeneous membranes have macropores, whereas the homogeneous membranes do not have such pores. It has been shown that the largest macropores, which basically determine the high diffusion permeability of heterogeneous membranes, are formed at the boundaries of reinforcing fabric threads and the composite material. Regarding the influence of the electrolyte nature, the sorption of coions of the membrane gel phase (not containing macropores) is of primary importance; the sorption of coions, as well as diffusion permeability and the transport number of coions, increase in the order: 1 : 2 < 1 : 1 < 2 : 1, where the first numeral is the charge of the counterion and the second one is that of the coion. An important role, especially in the case of heterogeneous membranes, is played by the electrolyte diffusion coefficients in the electroneutral solution that fills the central part of meso- and macropores.