Permselectivity of cation-exchange membranes between different cations in aqueous alcohol mixtures

Cation Exchange Membranes and Process Optimizations in Electrodialysis for Selective Metal Separation: A Review

The selective separation of metal species from various sources is highly desirable in applications such as hydrometallurgy, water treatment, and energy production but also challenging. Monovalent cation exchange membranes (CEMs) show a great potential to selectively separate one metal ion over others of the same or different valences from various effluents in electrodialysis. Selectivity among metal cations is influenced by both the inherent properties of membranes and the design and operating conditions of the electrodialysis process. The research progress and recent advances in membrane development and the implication of the electrodialysis systems on counter-ion selectivity are extensively reviewed in this work, focusing on both structure-property relationships of CEM materials and influences of process conditions and mass transport characteristics of target ions. Key membrane properties, such as charge density, water uptake, and polymer morphology, and strategies for enhancing ion selectivity are discussed. The implications of the boundary layer at the membrane surface are elucidated, where differences in the mass transport of ions at interfaces can be exploited to manipulate the transport ratio of competing counter-ions. Based on the progress, possible future R&D directions are also proposed.

Investigation of Calcium and Magnesium Removal by Donnan Dialysis According to the Doehlert Design for Softening Different Water Types

In this study, calcium and magnesium were removed from Tunisian dam, lake, and tap water using Donnan Dialysis (DD) according to the Doehlert design. Three cation-exchange membranes (CMV, CMX, and CMS) were used in a preliminary investigation to establish the upper and lower bounds of each parameter and to more precisely pinpoint the optimal value. The concentration of compensating sodium ions [Na⁺] in the receiver compartment, the concentration of calcium [Ca²⁺] and magnesium [Mg²⁺] in the feed compartment, and the membrane nature were the experimental parameters. The findings indicate that the CMV membrane offers the highest elimination rate of calcium and magnesium. The Full Factorial Design makes it possible to determine how the experimental factors affect the removal of calcium and magnesium by DD. All parameters used had a favorable impact on the response; however, the calcium and magnesium concentration were the most significant ones. The Doehlert design's Response Surface Methodology (RSM) was used to determine the optimum conditions ([Mg²⁺] = 90 mg·L^-1, [Ca²⁺] = 88 mg·L^-1, [Na⁺] = 0.68 mol·L^-1) allowing a 90.6% hardness removal rate with the CMV membrane. Finally, we used Donnan Dialysis to remove calcium and magnesium from the three different types of natural water: Dam, Lake, and Tap water. The results indicate that, when compared to lake water and tap water, the removal of calcium and magnesium from dam water is the best. This can be linked to the water matrix's complexity. Therefore, using Donnan Dialysis to decrease natural waters hardness was revealed to be suitable.