Monovalent Selective Ion-exchange Membranes Research Articles

Assessment of salt-free electrodialysis metathesis: A novel process for brine management in brackish water desalination using monovalent selective ion exchange membranes

Since reverse osmosis (RO) is often limited to a water recovery of 70–85 %, implementing a secondary process like electrodialysis metathesis (EDM) to treat concentrate can increase system water recovery. This study explores salt-free electrodialysis metathesis (SF-EDM) as an alternative to conventional EDM to investigate its performance in the zero discharge desalination (ZDD) process. SF-EDM utilizes monovalent-selective ion exchange membranes, eliminating the need for sodium chloride (NaCl) as a substitution solution. This approach generates one diluate stream and two highly soluble concentrate streams enriched in calcium and sulfate, respectively. In this article, a series of lab-scale tests based on synthetic and real RO concentrate were performed to investigate how SF-EDM performs operating at high water recovery with a feedwater with high scaling potential. We critically analyze the performance in terms of salinity reduction, selectivity, and energy consumption. Results of the study show that without the addition of NaCl, the process was able to selectively separate sulfate and calcium into two concentrate streams achieve a salinity reduction of 93 % and a total system water recovery (RO and SF-EDM) of 90 %. A technoeconomic analysis found SF-EDM to be 80 % of the cost of conventional EDM making it a competitive technology for brackish water RO concentrate management.

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.

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.

In-situ growth of anionic covalent organic frameworks efficaciously enhanced the monovalent selectivity of anion exchange membranes

The development of monovalent selective ion exchange membranes with excellent performance is required owing to the continuous refinement of electrodialysis technology. Here, a tailored strategy for the in-situ growth of a sulfonate-rich covalent organic framework (sCOF) on the anion exchange membrane (AEM) was proposed to construct an ideal platform (sCOF/aAEM3) for superior anion separation. Specifically, sCOF and AEM were covalently linked (-C=N-) to ensure the high stability of the modified layer, which is an important application parameter. The sCOF layer, with a thickness of approximately 2.97 μm, was formed using the Bottom-Up in-situ growth method. The appearance of sulfonate in the pores of sCOF induced a strong electronegativity of −38.85 eV at pH = 7. The periodic structure of sCOF achieved a regularly sulfonate arrangement, which maximized the utilization of the functional group, causing strong electrostatic repulsion on SO42−. The electrodialysis process showed that sCOF/aAEM3 exhibited a superior perm-selective performance (PSO42−Cl− =18.92). In addition, the unique open 1D channel of sCOF endowed sCOF/aAEM3 with an enhanced ion flux of Cl− from 2.96 to 3.35 × 10−8 mol cm2 s−1. This facile strategy has been used for two other commercial AEMs having elevated perm-selectivity, which indicates the great potential for extensive applications.

Dependence of Electrochemical Properties of MK-40 Heterogeneous Membrane on Number of Adsorbed Layers of Polymers.

The creation of monovalent selective ion exchange membranes benefits the desalination of surface waters by removing interfering monovalent ions while preserving polyvalent ionic nutrients. Studies of a promising method of layer-by-layer adsorption of polymers for the creation of monovalent selective coatings note a significant effect of the number of formed layers and of the nature of the external layer on the properties of the resulting membranes. This article reports the changes in properties of layer-by-layer coated heterogeneous membranes occurring at increasing numbers of layers that are attributed to the supposed intermixing of polymers between the layers, namely dependence of limiting current densities determined from i-V curve, enhanced electroconvection that was attributed to the appearing electrical heterogeneity of the surface, and the decreasing monovalent selectivity in electrodialysis of mixed NaCl + CaCl2 solution (from 1.33 to about 1) between the samples with five and six to eight layers of polymers.

Silane functionalized graphene oxide-bound polyelectrolyte layers for producing monovalent cation permselective membranes

This study reports a novel layer-by-layer (LbL) strategy for producing a monovalent cation perm-selective membrane with an improved antifouling property. LbL architectures were fabricated by using the polyvinyl chloride (PVC) based heterogeneous cation exchange membrane as the substrate and chitosan (CS) and polyacrylic acid (PAA) as the polycation and polyanion, respectively. The coating layers on the substrate consist of 1.5 bilayers with CS as the initiating and terminating layer and PAA blended with silane functionalized graphene oxide (S-f-GO) as the middle layer. Molecular electrostatic potential (MEP) of the utilized materials proved the availability of appropriate reactive sites in their structures. FTIR spectra, EDX analysis, zeta-potential measurement, and FESEM images verified successful LbL assembly of CS and PAA blended with S-f-GO. The presence of S-f-GO in the anionic layer of the LbL architectures caused more compactness of the terminating cationic layer and increased the porosity of the anionic layer. LbL membranes containing S-f-GO possessed lower surface roughness and more hydrophilic surface and provided higher antifouling property, monovalent selectivity, and electrical conductivity than LbL architecture without additive. Results showed that utilizing 2 wt% S-f-GO in the anionic layer led to increasing permselectivity (PMg2+Na+)from 1.03 to 6. Also, the incorporation of S-f-GO was found to enhance the regeneration ability of the LbL membranes. This novel strategy for producing the MCPM can open up a new insight into designing advanced monovalent selective ion exchange membranes.

Sulfonated Ti3C2Tx lamellar membrane for efficient monovalent anion selectivity

Monovalent selective ion-exchange membranes with characteristics of interception of multivalent ions and passage of monovalent ions, have aroused widespread research interest. However, most of monovalent selective ion-exchange membranes are dense polymer membranes, which suffer from low ion flux. Herein, a highly aligned two-dimensional (2D) thin anion lamellar composite membrane was developed successfully to address this weakness by assembling sulfonated Ti3C2Tx(STi3C2Tx) nanosheets on quaternized-chitosan (QCS) modified commercial Nylon-66 microfiltration membrane (N66). The QCS modified N66 that provides continuous sites and the STi3C2Tx lamellar ion channel that endows electrostatic repulsion and ion steric effect synergistically enhance the flux and selectivity of the composite membrane. Concretely, the resultant 2D thin anion lamellar composite membrane (N66-QCS/STi3C2Tx) achieves a high Cl−/SO42− selectivity and Cl− flux of 14.67 and 20.35 × 10−8 mol s−1 cm−2, which are 1.94 and 3.08 times higher than the commercial Neosepta ACS (7.56 and 6.60 × 10−8 mol cm−2 s−1). Furthermore, the negatively charged N66-QCS/STi3C2Tx shows favorable hydrophilicity and sufficient antifouling potential. The strategy of fabricating sulfonated Ti3C2Tx anion-exchange membrane provides fundamental information for the rational design of separation membranes that target to acquire high comprehensive performance.

Mathematical modelling of transport of single and double charged ions through layered ion exchange membranes

Monovalent selectivity of ion exchange membranes is important for such areas as hardness removal, stabilization of mining waters and fining of foods. The modern approach to the creation of monovalent selective ion exchange membranes is layer-by-layer adsorption of polyelectrolytes onto a membrane surface, which showed significant improvement in comparison with previously used approaches, most notably in comparison with the creation of a single thick layer of polyelectrolyte. Despite the practical importance, the mechanism of improvement in selectivity is unknown. Knowledge of the distribution of ions within the membrane and adjacent layers of solution would answer the question of the mechanism, but such information is unobtainable through the experiment. Hence we develop the 1D mathematical model based on Nernst-Planck-Poisson equations to describe the fluxes and the concentrations of single and double charged ions, as well as potential drop / current density through the ion exchange membrane. In this paper we formulate the problem and list experimentally determined parameters. Results of calculations will be compared with experimentally recorded I-V and V-t curves.

Removal of fluoride from multicomponent water solutions with the use of monovalent selective ion-exchange membranes

Fluorine is a chemical element which is often present in natural environment. According to the World Health Organization (WHO) standards fluoride content in drinking water cannot be higher than 1.5 mg F−/L. Nitrate was also found in many regions in the world. According to guidelines its content in drinking water cannot exceed 50 mg NO3−/L. Ingestion of fluoride and nitrate in excess leads to various health issues. There are many methods of fluoride removal, e.g. reverse osmosis, ion exchange or electrodialysis (ED). The aim of the research was to evaluate the influence of nitrate on fluoride separation by electrodialysis with monovalent selective ion-exchange membranes (PC-MVA and PC-MVK). During experiments the laboratory installation PCCell BED-1-System was applied. The ED tests were conducted at constant current density (0.78, 1.72, and 2.34 mA/cm2). Model solutions containing fluoride (5, 10, 15 mg F−/L), nitrate (15, 30, 45 mg NO3−/L) and sodium chloride (0.5 g NaCl/L) were applied. The obtained results showed that the ED process with monovalent selective ion-exchange membranes allowed to decrease fluoride content in product water below the WHO guidelines. It was observed that competition between nitrate and fluoride ions occurred during their transport through the anion-exchange membranes. The energy demand was dependent on the current density as well as on the salt concentration in treated solutions.

Design of Monovalent Ion Selective Membranes for Reducing the Impacts of Multivalent Ions in Reverse Electrodialysis.

Reverse electrodialysis (RED) represents one of the most promising membrane-based technologies for clean and renewable energy production from mixing water solutions. However, the presence of multivalent ions in natural water drastically reduces system performance, in particular, the open-circuit voltage (OCV) and the output power. This effect is largely described by the “uphill transport” phenomenon, in which multivalent ions are transported against the concentration gradient. In this work, recent advances in the investigation of the impact of multivalent ions on power generation by RED are systematically reviewed along with possible strategies to overcome this challenge. In particular, the use of monovalent ion-selective membranes represents a promising alternative to reduce the negative impact of multivalent ions given the availability of low-cost materials and an easy route of membrane synthesis. A thorough assessment of the materials and methodologies used to prepare monovalent selective ion exchange membranes (both cation and anion exchange membranes) for applications in (reverse) electrodialysis is performed. Moreover, transport mechanisms under conditions of extreme salinity gradient are analyzed and compared for a better understanding of the design criteria. The ultimate goal of the present work is to propose a prospective research direction on the development of new membrane materials for effective implementation of RED under natural feed conditions.

Separating and recovering lithium from brines using selective-electrodialysis: Sensitivity to temperature

This work was to investigate the lithium ion fractionation from various binary cation systems at various operating temperatures by electrodialysis (ED) with monovalent selective ion exchange membranes (usually called selective-electrodialysis, S-ED). Then a ternary cation system was selected to verify the results obtained from the binary cation systems. In Li+–Mn+ (Mn+: Na+, K+, Mg2+, Ca2+) brines containing only one kind of anion-Cl−, higher efficient recovery of lithium was obtained at higher temperature than at lower one, and the separation coefficients (FM–Li) at the same lithium recovery ratio were found to be changed slightly with temperature ranging from 10°C to 30°C. The results indicate the relationship between transport enhancement/interionic separation effects and temperature could be preliminarily expressed by Arrhenius-type equation. Moreover, the pore size of membrane and the cationic hydration number were analyzed with temperature increase, and then that FM–Li had little variation caused by temperature was explained. It should attribute to the swelling of membrane and a small reduction of cationic hydration numbers. Further verification was done in Li+–Na+–Mg2+ cation system, and the same results were obtained. The investigation experimentally demonstrates that S-ED technology is feasible for separating and recovering lithium from brines in different regions with different environment temperatures.

Selectrodialysis with bipolar membrane for the reclamation of concentrated brine from RO plant

Reverse osmosis (RO) is increasingly becoming the dominant strategy for seawater desalination owing to its excellent desalination performance over the past few decades. It produces a considerable amount of concentrated brine, which is a threat to marine ecosystems accompanying freshwater production. Electrodialysis has been demonstrated to be a feasible process for brine treatment. In this work, a novel process called Selectrodialysis with Bipolar Membranes (BMSED) was firstly proposed for treating concentrated brine. BMSED uses bipolar membranes and monovalent selective ion exchange membranes inside the ED stack and thus combines the selectrodialysis (SED) and bipolar membrane electrodialysis (BMED) processes into a single step. BMSED can desalinate concentrated brine, selectively regenerate monovalent ions and produce acids/bases in a single route. The feasibility of the process was investigated by using different commercial membranes. The factors, including the current density and brine concentrations, were investigated. The results indicated that brine was successfully treated by using BMSED and that a HCl and NaOH solution (up to an approximate 2 mol/L concentration) with high purity was eventually produced. Therefore, BMSED is a pertinent environmentally and economically friendly process for RO concentrated brine management.

Prefractionation of LiCl from concentrated seawater/salt lake brines by electrodialysis with monovalent selective ion exchange membranes

Abstract Exploring the lithium resources in concentrated seawater/salt lake brine could provide necessary support for the sustainable development in future. Selective-electrodialysis (S-ED, equipped with monovalent selective ion-exchange membranes) is considered as an effective way to reduce the ratio of magnesium to lithium in concentrated seawater/salt lake brines. However, the effectiveness of the prefractionation of lithium chloride from brine is not clearly investigated, because both seawater and salt lake brines are complex mixtures. Nowadays, only simple systems with binary or ternary cations system have been investigated. Based on a clean production process for the utilization of concentrated seawater/salt lake brine, the prefractionation of LiCl from concentrated seawater/salt lake brines by S-ED was investigated in this work. From the concentrated seawater experiments, it is beneficial to improve the RLi (recovery ratio of Li+) at a higher voltage, but an excessively high working voltage is adverse to the preliminary separation between Li+ and Mg2+; a bigger VC/VD (initial volume ratio of concentrating and desalting solution, VD = 2.5 L in this paper) is favorable to increase RLi and reduce ESEC (specific energy consumption of Li+). At the optimal voltage of 7 V and VC/VD of 0.6, the mole ratio of LiCl: MgCl2: MgSO4 increased from 1: 2.227: 2.463 to 1: 1.461: 0.085. For the salt lake brines, the optimal voltage for the LiCl prefractionation in the selected brine system was 10 V, which had a higher RLi of 76.45% and an appropriate ESEC of 0.66 kWh/(mol Li). Finally, the parameters of RLi, ESEC and separation effect of LiCl were discussed for the salt lakes of West Taijinar, East Taijinar and Yiliping in China. It is found that the East Taijinar salt lake brine was more suitable to obtain LiCl at a lower ESEC.

Application of selectrodialysis for the removal of As from metallurgical process waters: Recovery of Cu and Zn

Abstract In this work, selectrodialysis (SED) was used to separate arsenic (As(V)) from copper (Cu(II)) and zinc (Zn(II)) of acidic metallurgical process streams by integrating non-selective and selective membranes. The separation process is determined by the chemical speciation of the involved elements. In this case, As(V) is mainly present as anionic species (H2AsO4−), while Cu(II) and Zn(II) are mainly present as cationic species although partially complexed as neutral complexes (CuSO4 and ZnSO4). A lab set-up was used to conduct the experimental tests with different type of standard and monovalent selective ion-exchange membranes. The results obtained showed that by SED configuration it was possible to recover around 80 ± 0.2% of Cu(II), 87 ± 0.2% of Zn(II) and 95 ± 0.3% of As(V) from the feed solution to the rich-product streams with an energy consumption of 2.6 ± 0.2 kWh/kg CuSO4+ZnSO4. Furthermore, a Cu/Zn-rich stream with a purity of both divalent cations of 99.8% (0.02% of As(V)) was achieved by means of SED. Overall, the results herein gathered suggest that SED is an efficient separation technology for Cu(II) and Zn(II) recovery from mining and metallurgical acidic streams.

Development of recovering lithium from brines by selective-electrodialysis: Effect of coexisting cations on the migration of lithium

The process for separating and purifying lithium in brines based on electrodialysis with monovalent selective ion exchange membranes was investigated in our previous work. The migration of coexisting cations in brines was competitive with lithium ions, especially monovalent cations (Na+ and K+). The aim of this study was to examine the influence of major coexisting cations in brines on lithium recovery in the selective-electrodialysis (S-ED) process. Considering the factors of coexisting cations, such as concentration and type, some migration laws of lithium ion were found as follows: the concentration of coexisting cations had negative effect on the migration of lithium ion; the influence order of coexisting cations on lithium migration was contrary to their hydrated radius sequence: K+>Na+>Ca2+>Mg2+. In order to characterize the migration process of cations through monovalent selective cation exchange membrane in microcosmic theory, a partial dehydration conceptual model based on charge capillary column theory and ionic potential was proposed. And the model was used to characterize the ions migration process. Simultaneously, considering the hydration potential which indicates how strongly an ion would attract water molecules, the influence sequence of coexisting cations was explained legitimately. These observations might provide some theoretical basis and technological support for the relevant research of recovering lithium from brines.

Separation of divalent ions from seawater concentrate to enhance the purity of coarse salt by electrodialysis with monovalent-selective membranes

In this study, an electrodialysis (ED) system which was divided into three-stage operation was designed to treat seawater concentrate. The experiment was carried using a laboratory ED-cell with an effective area of 189cm2. Two types of monovalent selective ion-exchange membranes were investigated: CIMS/ACS and CSO/ASV. The effect of applied current density during ED process was also studied. The experimental results indicate that the separation performance for divalent ions (i.e., Ca2+, Mg2+) with CIMS/ACS membranes stack was superior to CSO/ASV membranes stack; furthermore, a lower current density can increase the selectivity in monovalent ions to divalent ions with either the CIMS membrane or the CSO membrane. The current efficiency and energy consumption were optimal at a current density of 4mA/cm2 by using CIMS/ACS membranes stack as the first stage of system in this experiment. Furthermore, the desalination rate (70%) was chosen as the experimental operation endpoint of the first-stage ED operation based on the experimental results. Moreover, the latter two-stage operation was used to concentrate brine to produce coarse salt after evaporation process. Finally, the repeated batch experiments confirmed the system feasibility for treating seawater concentrate to produce coarse salt with the purity of ~85% under continuous operation.

Ion-fractionation of lithium ions from magnesium ions by electrodialysis using monovalent selective ion-exchange membranes

Salt lake brine is an abundant lithium resource and has great developing value and potentiality. But the Mg/Li ratio of some salt lakes in China and Dead Sea in Jordan is extremely high. In this study, electrodialysis (ED) with monovalent selective ion-exchange membranes was used to investigate the separation performance of Li+/Mg2+ from synthetic multinary mixture. The effects of operating conditions on the lithium recovery, permselectivity, and Mg/Li ratio of the product were evaluated. The results indicate that ED with monovalent selective ion-exchange membranes has a significant separation effect on Li+/Mg2+. When the optimized parameters were used in ED, the Mg/Li mass ratio of the product stream was reduced to 8.0 (18.8 times compared to a feed Mg/Li ratio of 150), and simultaneously, the Li+ recovery reached to 95.3%. Compared to nanofiltration, ED exhibited superiority both technically and economically for the fractionation of Li+/Mg2+. It was also verified that the presence of co-existing cations would not be detrimental to the feasibility of lithium extraction using ED. Meanwhile, the separation mechanism of ED process was discussed qualitatively.

Fractioning electrodialysis: a current induced ion exchange process

In desalination often multi ionic compositions are encountered. A preferential removal of multivalent ions over monovalent ions can be of interest to prevent scaling in the desalination process. Recently, a novel fractionating electrodialysis stack is described by Zhang et al., 2012 (in Sep. purify. Technol. 88). In the present work a small modification to such a stack was made, to create a current induced ion exchange process, in which no longer desalination occurs. This was done by building a membrane stack in which monovalent-selective ion-exchange membranes and standard grade ion-exchange membranes, with similar charge sign (so either anion or cation exchange groups), were placed alternatingly between an anode and a cathode to form a membrane stack. A proof of principle of the fractioning electrodialysis technology is given. Ternary mixtures, with a divalent-monovalent ion ratio similar to seawater, were used as feed water. For a cation and an anion fractioning stack, maximum fractionations of divalent ions were obtained of approximately 90 and 60%. At higher applied current density, ions can be fractionated to a larger extent than at lower applied current density. For both stacks the water recovery was ∼50%. Coulombic efficiency of both processes decrease rapidly after the start of the experiment. This leads to relatively large volumetric energy consumptions.

Separation of nutrient ions and organic compounds from salts in RO concentrates by standard and monovalent selective ion-exchange membranes used in electrodialysis

To allow water recycling and nutrient recovery from reverse osmosis (RO) concentrates from a food industry plant, separation of salts from the organic fraction and from nutrient anions is required. In this study, the use of ion-exchange membranes in electrodialysis was investigated to this purpose. Two types of anion-exchange membranes from PCA, Germany, were investigated: a nonselective membrane (SA) and a membrane selective for monovalent anions (MVA). Different approaches including lowering the initial current density and increasing the initial pH were applied. The transport properties of different anions and different small charged organic compounds through ion-exchange membranes were discussed. The separation efficiency, which represents the selectivity, was compared under different conditions. The results show that separation of non-nutrient anions from nitrate and phosphate was difficult, whereas separation of salts from the organic fraction was feasible provided that the organic solutes are similar to uncharged test solutes used in the experiments; it was shown that a higher molar mass of the organic solute has a positive effect on the separation. Lowering the current density can increase the separation efficiency of monovalent/multivalent anions with either the SA membrane (a nonselective membrane) or the MVA membrane (a monovalent selective membrane). When the initial pH was increased, the separation efficiency of monovalent/multivalent anions can be improved for the MVA membrane, however, no obvious change was found for the SA membrane. Finally, experiments on real RO concentrates proved that the separation of salts from organics by electrodialysis is similar to the separation of salts from uncharged solutes and therefore feasible.

Electrodialysis for chloride removal from the chemical recovery cycle of a Kraft pulp mill

The selective removal of chloride from a solution with high ionic strength containing multiple divalent anions, using electrodialysis with monovalent selective ion-exchange membranes is described. The process is characterized in terms of limiting current density, selectivity, and current efficiency. The correlation between limiting current density, chloride concentration and average fluid velocity is discussed. Using actual electrostatic precipitator dust from the chemical recovery cycle of a Kraft pulp mill the fouling behavior of the membranes was investigated in batch desalination experiments. Long term behavior over nearly 100h was studied. The feed contained organic contamination and a large number of inorganic components at low concentrations. The process performed excellently, despite the fact that no pre-treatments were applied to the feed.