Monovalent Selective Cation Exchange Membranes Research Articles

High-Performance Monovalent Selective Cation Exchange Membranes with Ionically Cross-Linkable Side Chains: Effect of the Acidic Groups.

Inspired by the charge-governed protein channels located in the cell membrane, a series of polyether ether ketone-based polymers with side chains containing ionically cross-linkable quaternary ammonium groups and acidic groups have been designed and synthesized to prepare monovalent cation-selective membranes (MCEMs). Three acidic groups (sulfonic acid, carboxylic acid, and phenolic hydroxyl) with different acid dissociation constant (pKa) were selected to form the ionic cross-linking structure with quaternary ammonium groups in the membranes. The ionic cross-linking induced the nanophase separation and constructed ionic channels, which resulted in excellent mechanical performance and high cation fluxes. Interesting, the cation flux of membranes increased as the ionization of acidic groups increase, but the selectivity of MCEMs did not follow the same trend, which was mainly dependent on the affinity between the functional groups and the cations. Carboxyl group-containing MCEMs exhibited the best selectivity (9.01 for Li+/Mg2+), which was higher than that of the commercial monovalent cation-selective CIMS membrane. Therefore, it is possible to prepare stable MCEMs through a simple process using ionically cross-linkable polymers, and tuning acidic groups in the membranes provided an attractive approach to improving the cation flux and selectivity of MCEMs.

Removal of total ammoniacal nitrogen from reject water through selective electrodialysis reversal and bipolar electrodialysis

The removal of ammonium and ammonia, represented as total ammoniacal nitrogen (TAN), from reject water through electro-dialysis (ED) and bipolar membrane electrodialysis (BPMED) encounters challenges such as organic fouling, NH3 back-diffusion, and high energy consumption. The efficacy of electrodialysis reversal (EDR) combined with bipolar membrane electrodialysis using cation-exchange membranes (BPC) was assessed as a more practical configuration (EDR + BPC). Additionally, a novel configuration involving monovalent selective cation-exchange membranes (MSCEMs) in an EDR + BPC setup (SEDR + BPC) was investigated. Comparisons were made among BPMED, EDR + BPC, and SEDR + BPC under three load ratios (LN) of 0.8, 1, and 1.3 during continuous operation. The innovative SEDR + BPC configuration, with an LN of 0.8, exhibited the lowest energy consumption for transported TAN (ETAN) at 4.4 MJ·kgN−1 removal and achieved the highest TAN removal efficiency of 78 % with an LN of 1.3. In contrast to conventional BPMED, SEDR + BPC allowed for the recovery of potentially back-diffused NH3 into the acid chamber, minimizing transport losses. Furthermore, scaling in the base chamber was reduced due to the contribution of MSCEMs when applying an LN of 0.8. The MSCEMs increased the molar ratio of TAN over (Mg2+ + Ca2+) in the concentrate and decreased it in the diluate. EDR + BPC and SEDR + BPC configurations exhibited stable and lower cell resistance throughout the operation compared to BPMED, attributed to their ability to generate higher concentration gradients. The results clearly demonstrated the feasibility of low-energy TAN removal from real reject water from sludge anaerobic digestion using the SEDR + BPC setup.

Flexible lithium selective composite membrane for direct lithium extraction from high Na/Li ratio brine

Monovalent selective ion-exchange membranes have been widely used for lithium extraction from brine with a high Mg/Li ratio. However, extracting Li+ directly from raw brine with high Na/Li ratio remains a significant challenge. This study introduces a flexible lithium selective composite membrane by incorporating lithium-ion conductor with a quaternized polymer. The lithium-ion conductor provides pathways for lithium transmission, while the quaternized polymer effectively hinders the leakage of impurity cations. As a result, with the applied voltage of 2.0 V, the membrane achieved a high Li+/Na+ separation coefficient of 35.48 with a Li+ flux of 8.33 × 10−10 mol cm−2 s−1 in the simulated Yiliping brine, which are 56.3 and 2.8 times higher than that of commercial monovalent selective cation exchange membrane (CIMS). We proposed a lithium transport mechanism involving an adsorption-dehydration-diffusion process through the membrane. Theoretical calculations indicate that the combined effects of significant adsorption energy (−0.47 eV), low dehydration energy (515 kJ mol−1), and low diffusion barrier (0.37 eV) leads to the high selectivity for lithium. These findings provide promising prospects for the large-scale preparation of lithium ion-selective membranes, enabling the direct lithium extraction from raw brine.

Harnessing ion resource recovery: Design of selective cation exchange membranes via a synergistic ionic control method

Effective recovery of valuable ion resources such as lithium is gaining increasing importance. Monovalent selective cation exchange membranes (MCEM) used in electrodialysis have shown great potential, but their development is hampered by the classical trade-offs related to membrane permeability, selectivity and energy efficiency. We reported a new route to design high performance MCEM, utilizing the synergy of ionic control (IC) through cation-π bonding of polydopamine (PDA) and alternating current (AC), which has not been reported before. A set of deliberately controlled membrane fabrication conditions were chosen, with different ionic control (i.e., no cation, K+, Li+) and with/without AC for systematic comparison. Combining with morphology & electrochemical measurements, the membrane with electro-assisted K+ control, i.e., PDAM-K+/AC, exhibited respective 72 % and 51 % higher Li+ and K+ transport rate, higher permselectivity of 9.54 (Li+/Mg2+) and 17.86 (K+/Mg2+), and 3.3-fold lower surface electrical resistance (SER) compared to other modified membranes (e.g., PDAM). Also, PDAM-K+/AC exhibited no sign of scaling, supported by its much increased limiting current density. The results supported the hypothesis of the synergistic effect between cation-π bonding and AC electro-deposition, which altered the polymerization dynamics and produced more ordered membrane structure. The control strategy showed a viable route to fabricate ion exchange membranes for imparting monovalent selectivity, reducing scaling and avoiding energy penalty.

Reclamation of acid with electrodialysis process: Influence of selective ion-exchange membranes

Electrodialysis (ED) could concentrate salt solutions, but the leakage of H+ ions across the anion-exchange membrane (AEM) obstructed their application in waste acid recovery. Selective electrodialysis (SED) with acid blocking AEM and monovalent selective cation-exchange membrane (CEM) has the ability of reclaiming acid with high concentration efficiently. This study investigated the individual effect of the two selective ion-exchange membranes by comparing the acid reclamation performance of configurations with standard AEM and CEM (A-C), acid blocking AEM and standard CEM (SA-C) and acid blocking AEM and monovalent selective CEM (SA-SC) for etching solution treatment after diffusion dialysis. Substituting acid blocking AEMs for the standard ones could significantly increase the recovered acid concentration. An acid concentration plateau was reached quickly, but Cu2+ ions transfer through the CEMs continuously due to the low H+/Cu2+ ratio in the dilute of SA-C configuration. The monovalent selective CEM obstructed the migration of Cu2+ through the CEM and broke the balance of H+ migration into and out of the dilute through CEM and selective AEM individually. The configuration of SA-SC has the highest current efficiency and the configuration of A-C has the lowest. The utilization of selective membranes increased the purity of the recovered acid, and decreased the specific energy consumption simultaneously. The acid concentration could be further improved by higher current or enlarged dilute volume in SA-SC configuration. During the ED process, H+ concentration difference between the concentrate and the dilute plays a crucial role on the current efficiency for acid recovery.

Membrane Design Principles for Ion-Selective Electrodialysis: An Analysis for Li/Mg Separation.

Selective electrodialysis (ED) is a promising membrane-based process to separate Li+ from Mg2+, which is the most critical step for Li extraction from brine lakes. This study theoretically compares the ED-based Li/Mg separation performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale using a unified mass transport model, i.e., a solution-friction model. We demonstrated that monovalent selective CEMs with a dense surface thin film like a polyamide film are more effective in enhancing the Li/Mg separation performance than those with a loose but highly charged thin film. Polyamide film-coated CEMs when used in ED have a performance similar to that of polyamide-based NF membranes when used in NF. NF membranes, when expected to replace monovalent selective CEMs in ED for Li/Mg separation, will require a thin support layer with low tortuosity and high porosity to reduce the internal concentration polarization. The coupon-scale performance analysis and comparison provide new insights into the design of composite membranes used for ED-based selective ion-ion separation.

Selective recovery of silver from copper impurities by electrodialysis: Tailoring monovalent selective cation exchange membranes by monomolecular layer deposition

Selective recovery of silver from secondary resources enriched with copper impurities is a well-known challenge but is urgently needed by the industry. This study addresses the challenge by developing a highly efficient monovalent selective cation exchange membrane (CEM) with a specifically tailored polyelectrolyte deposition, enabling efficient Ag+/Cu2+ separation in electrodialysis. Based on the Ag+/Cu2+ separation mechanism in electrodialysis, a selected polyelectrolyte, polyethyleneimine (PEI), was deposited on a standard CEM as a monomolecular layer with precisely controlled polymer chain stretch patterns and optimized morphologies. The effects of deposition conditions, such as solution pH and ionic strength, were studied to ensure the desired surface properties. The selectivity performance of the developed membranes was tested using an equimolar binary mixture; a high Ag+/Cu2+ selectivity of > 20 was documented, exhibiting superior selectivity performance compared to commercial monovalent selective CEMs.

Electrodialysis for Removal of Impurities in Silver Electrowinning

In this work, we investigate the potential of electrodialysis to separate copper from silver electrowinning solution. We further explore and discuss optimization of the electrodialysis process design.Ag is in a high global demand, driven mainly by its use in electronics, jewelry and as investment. The recycling potential for Ag is large and will increase in the coming years due to Ag usage in solar panels. One route of recycling Ag from End-of-life products is batch-electrowinning. In preparation, the products are melted and reshaped into scrape plates which are anodically dissolved in nitric acid solution. Typically, silver is found in alloys with less noble metals like copper, zinc, and iron. While silver is deposited on the stainless-steel cathode surface, the impurities accumulate in the electrolyte. To maintain a high purity of the recycled silver, the contaminated nitric acid solution must be replaced regularly. We investigate the potential of implementing a continues electrodialysis stage parallel to the electrowinning bath to remove copper and other divalent cations from the solution.In electrodialysis, anion- and cations-exchange membranes are stacked alternatingly between two electrodes. When a current is drawn, the cations move towards the cathode, passing the cation-exchange membrane and vice versa for the anions. Since the membranes are selective for either anions or cations, they strongly retard the passage of the opposite ion. Therefore, cations and anions are accumulated in every second compartment, while every other chamber is desalinated. By using monovalent-selective ion-exchange membranes, separation between monovalent (f. ex., Ag+) and multivalent ions (f. ex., Cu2+, Zn2+, Fe2+, Fe3+) can be achieved [1].If the concentration of impurities can be held at a constant level, the nitric acid solution can be recycled continuously rather than replaced as a batch. The only consumable required by the process is electricity. We evaluate the viability of the process and investigate the influence of pH on the process efficiency.We determined the optimal operating current for separating Cu2+ from Ag+ in an equimolar concentration of 10 mM by doing amperodynamic sweeps and recording current-voltage curves. We then tested the desalination performance of an electrodialysis stack with different commercially available anion-/cation-exchange membrane pairs. The single membrane area was 36 cm2, and 9 membranes were used in total (five anion-exchange membranes and four monovalent-selective cation-exchange membranes). The Cu2+ and Ag+ concentrations in the diluate and concentrate compartments were determined by atomic mass spectrometry, and the membranes were analyzed with scanning electrode microscopy to detect adsorbed silver. Since scaling and fouling are dominant drawbacks in industrial applications of electrodialysis, we studied the influence of pH on the adsorption of silver in the membrane.Commercial monovalent-selective cation-exchange membranes by Neosepta showed high selectivity for silver over copper. The limiting current density was 10.4 A/m2 for neutral 10 mM AgNO3 solution at a flow rate of 100 mL/min and increased by almost 10 times when we adjusted the pH to 1 using nitric acid. In neutral solution, silver was recovered to almost 100 % after two hours of electrodialysis at 10 mA, while copper leakage was 7.5 %. At pH 1, silver recovery and copper leakage were 83 % and 9 %, respectively, after two hours electrodialysis at 100 mA. Consequently, the percentage silver adsorbed in the membranes were close to 0 % in neutral solution and 17 % at pH 1. The work input was significantly higher at pH 1 due to electric current consumed by mass transport of protons. As a next step, we investigate the long-term effect on the membranes with regards to Ag and Cu adsorption, which could be stabilizing at a certain concentration of Ag and Cu or increasing over time.We conclude that electrodialysis is a promising technology for removal of impurities in Ag electrowinning from End-of-life products, justifying large room for future research on development of membranes and operational design.[1] Zimmermann, P., Tekinalp, Ö., Deng, L., Forsberg, K., Wilhelmsen, Ø., Burheim, O. (2020). Electrodialysis in Hydrometallurgical Processes. In: Azimi, G., Forsberg, K., Ouchi, T., Kim, H., Alam, S., Baba, A. (eds) Rare Metal Technology 2020. The Minerals, Metals & Materials Series. Springer, Cham.

Polyaniline encapsulated sulphonated graphene oxide based cation exchange membrane for electrodialytic separation of mono- and bi-valent ions

Polyaniline encapsulated sulphonated graphene oxide (SGO-PANI) was prepared by ion exchange process. The SGO-PANI was incorporated in sulphonated poly(ether ether ketone) (SPEEK) matrix to fabricate the mono-valent selective cation exchange membrane. The structural morphology of SGO-PANI was studied by FTIR, XPS, SEM and XRD spectra. Resultant SPEEK@SGO-PANI membrane exhibited excellent stabilities and physicochemical and electrochemical properties suitable for electrodialysis (ED) applications. Encapsulation of strong acid with a weak base seems to improve electrostatic repulsion between membrane surfaces and of bi-valent ions, which are responsible for significant flux reduction. Chronopotentiometric and current–voltage responses of SPEEK@SGO-PANI membrane confirmed its ion-conducting nature along with 3.32 mA cm−2 limiting current density (Ilim). Reported SPEEK@SGO-PANI membrane showed a high electrodialytic flux of Na+ compared to bi-valent ions (Mg2+, Ca2+, and Cd2+). Extremely low energy consumption and high current efficiency (81.18%) at 6.0 V applied voltage for Na+ (0.10 M) revealed the promising nature of the reported membrane. Observed mono-valent ion selectivity aroused due to improved electrostatic repulsion of bi-valent ions with the membrane surface.

High-performance nanofiltration concentrate treatment by a five-chamber bioelectrochemical system

A combination of bioelectrochemical systems and electrodialysis has been considered an effective strategy for removing salts from the nanofiltration (NF) concentrate of electroplating wastewater; however, the recovery efficiency of multivalent metals is low. Herein, a new process based on microbial electrolysis desalination and chemical-production cell with five chambers (MEDCC-FC) has been proposed for the simultaneous desalination and recovery of the multivalent metals from NF concentrate. The MEDCC-FC was found to be significantly superior to the MEDCC with the monovalent selective cation exchange membrane (MEDCC-MSCEM) and MEDCC with the cation exchange membrane (MEDCC-CEM), in terms of the elevated desalination efficiency, multivalent metal recovery efficiency, current density, and coulombic efficiency, and decreased energy consumption and membrane fouling. Within 12 h, the MEDCC-FC provided the desirable outcome, indicated by a maximum current density of 6.88 ± 0.06 A/m2, desalination efficiency of 88 ± 10%, metals recovery efficiency of >58%, and total energy consumption of 1.17 ± 0.11 kWh for the per kg total dissolved solids removal. Mechanistic studies revealed that the integration of CEM and MSCEM in the MEDCC-FC promoted the separation and recovery of multivalent metal. These findings revealed that the proposed MEDCC-FC was promising in treating NF concentrate of electroplating wastewater towards advantages of effectiveness, economic viability, and flexibility.

Fe3+ ions induced rapid co-deposition of polydopamine-polyethyleneimine for monovalent selective cation exchange membrane fabrication

Selective monovalent cation transport is crucial for the concentration of target ion(s) using electro-membrane processes prior to precipitation. Co-deposition of biomimetic adhesive polydopamine (PDA) and the positively charged polyethyleneimine (PEI) has emerged as an effective approach for building monovalent perm-selectivity functional coatings for cation exchange membranes (CEMs). Here we report a facile strategy to fabricate monovalent selective cation exchange membranes through quick co-deposition of PDA-PEI induced by ferric (Fe3+) ions. With CEM substrate ion exchanged with Fe3+ ions, the polymerization of DA on the membrane surface was accelerated. The mechanism of the promoting effect of Fe3+ ions on the PDA-PEI co-deposition was explored by UV spectroscopy, SEM, AFM, zeta potential and area resistance measurements. In this approach, the characteristics of the membrane surface were easily manipulated by varying the Fe3+ concentration. The selective CEM fabricated through 0.0001 M FeCl3 pretreatment and only 30-min PDA-PEI deposition showed remarkably enhanced Na+/Mg2+ selectivity of 8.3, low electrical resistance and excellent stability. This study provides a simple and scalable technique for time-saving and environmentally friendly construction of PDA based coating for membrane modifications.

Sustainable K+/Na+ monovalent-selective membranes with hot-pressed PSS-PVA saloplastics

Monovalent selective cation exchange membranes could play an important role in balancing the K+/Na+ ratio in agricultural feed streams to prevent the toxic effects of excess Na+ in the plant and soil systems, especially in greenhouses and dry areas. A polyelectrolyte complex of polystyrenesulfonate and polyvinylamine in the monomer ratio 1:2.5 is hot-pressed to form a dense saloplastic. The plastic takes up 42% w/w water when equilibrated, while ion-exchange capacity measurements show that it is negatively charged with a net ion-exchange capacity of 1.1 ± 0.4. Resistance measurements show a very promising preferred conductivity for K+ over Na+. This was confirmed by measuring K+ and Na+ transport through the membrane under diffusive conditions from an aqueous mixture of KCl and NaCl. Commercial membranes show resistance-based selectivities of 1.32 ± 0.1 to 1.19 ± 0.1, and diffusion based selectivities of 0.99 ± 0.1 to 0.78 ± 0.1. In contrast, the selectivities for the newly developed saloplastic membrane were 1.80 ± 0.33 for the resistance-based selectivity while the diffusion-based selectivity was 1.91 ± 0.1. The procedure is green as toxic solvents and/or halogenating agents, typically used to make cation exchange membranes, are not needed. This work thus highlights how monovalent selective membranes with a relevant K+/Na+ selectivity can be prepared by a simple and sustainable hot-pressing approach.

The effect of impurity ions on the pH adjustment for vanadium-bearing acid leaching solution by selective electrodialysis

Leaching of vanadium shale with sulfuric acid results in a vanadium acid leaching solution with low pH and many impurity ions. Monovalent selective electrodialysis was employed to adjust the pH of the acid leaching solution by recovering the sulfuric acid. The results demonstrated that CIMS monovalent selective cation exchange membrane had good performance in separating H+ and VO2+ for various vanadium shale acid leaching solutions without Fe3+. The effect of the concentration of impurity ions was investigated. Under the optimum concentration of impurity ions, The treatment time was 125 min, the V rejection ratio reached 98.06%, the water loss rate was controlled at 5.25%, and the pH was adjusted from 0.08 to 1.8. Mg2+ had the best optimization effect on vanadium rejection. Al3+ had the best optimization effect in reducing water loss. F− had the best optimization effect on the treatment time. Meanwhile, the effect of impurity ions on the pH adjustment of acid leaching solution was discussed qualitatively. SEM-EDS analysis confirmed that Fe3+ fouled the membrane and caused the loss of vanadium. Since Fe3+ and PO43− in the acid leaching solution formed FePO4 precipitates. Selective electrodialysis was used to treat vanadium-bearing acid leaching solution, the pH could meet the requirements of separation and purification without consuming any reagent. This process is clean and efficient, and the recycled acid is reused without producing waste water or residue.

Seawater desalination using the microbial electrolysis desalination and chemical-production cell with monovalent selective cation exchange membrane

The objective of this study was to investigate the feasibility of seawater desalination using the microbial electrolysis desalination and chemical-production cell (MEDCC) with monovalent selective cation exchange membrane (MSCEM) (MEDCC-MSCEM). With dissolved aquarium sea salts as artificial seawater, the maximum current density in the MEDCC-MSCEM reached 19.6 ± 0.3 A/m2, which was 43.1% higher than that in the MEDCC with cation exchange membrane (i.e., the traditional MEDCC as the control). The desalination efficiency within 24 h was 76 ± 7% in the MEDCC-MSCEM. The harvested acids (mainly HCl and H2SO4) and alkali (NaOH with 96% purity) concentrations reached 0.28 ± 0.02 and 0.26 ± 0.02 M, respectively. The total energy consumption within 24 h was much lower in the MEDCC-MSCEM than in the control (3.46 ± 0.41 vs. 4.99 ± 0.46 kWh/kg·TDS). The separation efficiency of Na+:Ca2+ and Na+:Mg2+ in the MSCEM was in the range of 42% - 72% and 53% - 87%, respectively. Effective limitation of Ca2+ and Mg2+ by MSCEM resulted in low precipitation of Ca (OH)2 and Mg(OH)2 in the membrane and cathode, which significantly improved the performance of MEDCC. The MEDCC-MSCEM has great potential in seawater desalination with resource recovery.

Brine management from desalination plants for salt production utilizing high current density electrodialysis-evaporator hybrid system: A case study in Kuwait

The potential of producing salt and low salinity water by treating synthetic brine was investigated in a pilot-scale electrodialysis-evaporator hybrid system as part of brine management. An electrodialyzer with 25 cell pairs of monovalent selective anion exchange membranes (AEMs) and monovalent selective cation exchange membranes (CEMs) were used. Effect of three different current densities (300 to 500 A/m2) and two different flowrates (400 L/h and 450 L/h) on brine treatment were evaluated in the current study. The results indicated that the aforementioned membranes were efficient in separating monovalent ions over divalent ions. Moreover, higher current density operation was favoured during the electrodialysis (ED) operation as the operation time for producing salt was shorter. Flowrate is a crucial parameter for the operation of the ED system, where higher flowrate (by 12.5%) reduced the operation time (by 30 min), reduced energy consumption (by 32%) and reduced system resistance (by 32.42%) leading to an increase in current efficiency (by 31.4%). The concentrated brine effluent from ED was fed to an evaporator to produce coarse salt with 84.75% salt recovery and low salinity water of 1526 ppm. Thus, ED-evaporator hybrid system was found to be a good option to treat reject brine from desalination plants.

Integration of selectrodialysis and selectrodialysis with bipolar membrane to salt lake treatment for the production of lithium hydroxide

Extracting lithium from salt lakes and preparing lithium hydroxide could provide important commodities for the economic development in future. In this work, a complete system was divided into two steps of selectrodialysis (SED) and selectrodialysis with bipolar membrane (BMSED) to treat salt lakes for the production of lithium hydroxide. The effects of the applied current density, the commercial monovalent selective cation-exchange membranes (CIMS and CSO), the monovalent selective anion-exchange membranes (ACS and ASV) and the voltage drop in the SED process were investigated, respectively. The experimental results indicated that under the current density of 12 mA/cm2 can effectively avoid membrane scaling. The separation performance for Mg2+ and Ca2+ by using CIMS/ACS membrane stack was demonstrated to be superior to CSO/ASV membrane stack. Additionally, the SED stack had proven to be feasible for concentrating lithium and removing divalent ions simultaneously. In BMSED process, the purpose was to combine the monovalent selective ion-exchange membrane with bipolar membrane (FBM) inside the membrane stack to remove divalent ions and simultaneously produced LiOH. The results showed that the concentration of LiOH and current efficiency were optimized at the current density of 6 mA/cm2. Furthermore, different cation-exchange membranes (CIMS, CSO and CMX) and anion-exchange membranes (ACS, ASV and AMX) were used to evaluate the BMSED process. Compared to the FBM/ASV/CSO/FBM membrane stack and FBM/AMX/CMX/FBM membrane stack, FBM/ACS/CIMS/FBM membrane stack exhibited superior current efficiency and separation performance for Mg2+ and Ca2+.

Evaluation of the ideal selectivity and the performance of selectrodialysis by using TFC ion exchange membranes

The current methods for determination of the membrane specific permselectivity between two competing counter-ions are dependent on various factors, which hinder the comparison of ion selectivity of ion selective membranes. In this work, a new characteristic, “ideal selectivity” is suggested to be used for the evaluation of the specific permselectivity of monovalent selective membranes. Ideal selectivity is determined in a single electrolyte solution containing only one of the competing counterions. We show that this characteristic exhibits a stable value, which is independent from the molar fraction of ions in a solution. Thus, it may be directly used to predict the purity of a salt produced in a dynamic selectrodialysis (SED) process with a monovalent selective membrane. To elaborate this, thin film composite (TFC) cation exchange membranes were prepared with piperazine and trimesoyl chloride by interfacial polymerization. These membranes were compared with a commercial monovalent selective cation exchange membrane CIMS (ASTOM, Japan) in view of their permselectivity (Na+/Mg2+) and the performance in SED. The results show that when the TFC membranes were applied in SED, the purity of Mg2+ in the product reached 75%, which was higher than in the case of CIMS. Furthermore, ideal selectivity of those TFC and CIMS membranes were calculated and fitted with the experimental Mg2+ purity. It was found that ideal selectivity linearly correlates with the product final purity. The extrapolation of the fitting line predicts that when ideal selectivity approaches to 2, SED process the purity could theoretically approach to 100% in the same experimental conditions. Therefore, ideal selectivity can be used to predict the separation efficiency and the final product purity of SED process.

Cationic selectrodialysis for magnesium recovery from seawater on lab and pilot scale

In view of its multiple applications the selective recovery of Mg2+ from seawater can be very interesting. For this purpose, cationic selectrodialysis (cSED), which uses a conventional electrodialysis stack that is supplementary equipped with monovalent selective cation-exchange membranes, was used. Initially, various parameters were investigated on lab scale using synthetic seawater. The results pointed out that a lower current density resulted in a higher Mg2+ concentration in the product. A higher unwanted Mg2+ concentration was obtained in the brine when the initial concentration of NaCl in the product was lowered. Subsequently, it was demonstrated that the treatment of real North Sea water with cSED was technologically feasible during a short-term batch experiment on lab scale. In a next phase, a cSED pilot plant was operated in a stable way in a long-term feed and bleed experiment in which the Mg2+ content of the North Sea water that was used as initial product was at least doubled. However, the Ca2+ concentration in the product also increased. This might reduce the applicability of the recovered product. From this research it is concluded that cSED is a promising and attractive technology to recover Mg2+ from seawater.

Efficient lithium extraction by membrane capacitive deionization incorporated with monovalent selective cation exchange membrane

• A monovalent selective cation exchange membrane was assembled in MCDI system (MSCDI) to separate lithium from magnesium. • MSCDI shows excellent separation effect of magnesium and lithium, with a low energy consumption. • A custom-made large module was adopted for the selective removal of lithium. Membrane capacitive deionization (MCDI) is an effective water purification technology. In this work, a monovalent selective cation exchange membrane (CIMS) was assembled in MCDI system (denoted as MSCDI) to separate lithium from magnesium. The effects of feed water quality and operating conditions on the lithium removal selectivity were systematically investigated. Specifically, in optimized conditions, the lithium selectivity coefficient reached 2.95. Furthermore, a custom-made large MSCDI module was applied to the selective removal of lithium. The removal rates of Li + and Mg 2+ in large module reached 38.4% and 19.2%, respectively. It is worth noting that the specific energy consumption was 0.0018 kWh mol −1 , which is much lower than that of electrodialysis (0.04–0.27 kWh mol −1 ). Therefore, relying on its low energy consumption and environmental friendliness, MSCDI has considerable economic benefits for the efficient lithium extraction.

Fractionating magnesium ion from seawater for struvite recovery using electrodialysis with monovalent selective membranes

As the consumption of global phosphorus reserves accelerates, recovering phosphorus as struvite (MgNH4PO4·6H2O) from wastewater is an important option for phosphorus recycling. However, magnesium source is one of the major limiting factors for struvite recovery. In this work, different from previous studies where seawater was used directly as magnesium source in struvite precipitation, an electrodialysis stack equipped with monovalent selective cation-exchange membranes was designed to fractionate Mg2+ from seawater for struvite recovery. Results revealed that Mg2+ fractionation was achieved effectively. The comparison on applying the driving force for ionic transport showed that constant voltage was more preferable than constant current due to its higher Mg2+ separation efficiency, current efficiency and lower energy consumption. Increasing voltage from 7 V to 13 V would improve Mg2+ permeation ratio from 72.9% to 80.5% into the product stream but simultaneously increased the energy consumption from 5.40 (kWh/kg MgCl2) to 11.69 (kWh/kg MgCl2). In addition, the investigation on the influence of Ca2+ co-existence and further struvite recovery experiments revealed that the variation of Ca2+ concentrations in seawater did not influence Mg2+ fractionation significantly, nevertheless it might reduce struvite recovery efficiency through forming calcium phosphate.