Low-concentration Solutions Research Articles

Methane hydrate phase equilibrium considering dissolved methane concentrations and interfacial geometries from molecular simulations.

Natural gas hydrates, mainly existing in permafrost and on the seabed, are expected to be a new energy source with great potential. The exploitation technology of natural gas hydrates is one of the main focuses of hydrate-related studies. In this study, a large-size liquid aqueous solution wrapping a methane hydrate system was established and molecular dynamics simulations were used to investigate the phase equilibrium conditions of methane hydrate at different methane concentrations and interfacial geometries. It is found that the methane concentration of a solution significantly affects the phase equilibrium of methane hydrates. Different methane concentrations at the same temperature and pressure can lead to hydrate formation or decomposition. At the same temperature and pressure, in a system reaching equilibrium, the size of spherical hydrate clusters is coupled to the solution concentration, which is proportional to the Laplace pressure at the solid-liquid interface. Lower solution concentrations reduce the phase equilibrium temperature of methane hydrates at the same pressure; as the concentration increases, the phase equilibrium temperature gradually approaches the actual phase equilibrium temperature. In addition, the interfacial geometry of hydrates affects the thermodynamic stability of hydrates. The spherical hydrate particles have the highest stability for the same volume. Through this study, we provide a stronger foundation to understand the principles driving hydrate formation/dissociation relevant to the exploitation of methane hydrates.

Influence of Contact Pressure and Flow Rate on the Performance of Anion Exchange Membrane Electrolysis

Hydrogen production by electrolysis is a key aspect of a climate-neutral economy. However, there are still weak points in the established low-temperature technologies such as high material costs in case of PEM electrolysis and poor dynamic operations in case of alkaline electrolysis. The emerging electrolysis technology, the anion exchange membrane (AEM) electrolysis, addresses these weaknesses and combines their advantages. Due to the solid polymer electrolyte and the alkaline environment, AEM water electrolysis is featured by a compact zero-gap cell design and inexpensive materials like non-noble metal catalysts. Hence, it offers the potential to a cost-effective and efficient electrolysis technology.To enable the breakthrough of this technology, we contribute by conducting performance evaluations of membrane electrode assemblies (MEAs) under application-oriented conditions. One focus is on non-noble metal MEAs with an active area of 25 cm2 operated with pure water and low concentrated potassium hydroxide solution.To ensure comparability of different MEAs, it is crucial to compare them at their performance optimum. This is achieved by determining the optimal set-up parameters like contact pressure and operation parameters like flow rate for each MEA type. Otherwise, deviation from the optimal operating point by up to 50 % may occur.The influence of the parameters contact pressure and flow rate on the MEA performance is discussed in this presentation. To investigate their impact, we analysed the MEAs in an electrolyser test station using polarisation curves, electrochemical impedance spectroscopy and load tests at constant current and constant voltage.Some of the results presented are obtained within the project “REVAL – reversible anion exchange membrane electrolysis” (funding code 03ZZ0732D) funded by the German Federal Ministry of Education and Research (BMBF).

Fabrication of multifunction-integrated adsorbent based on zwitterion functionalized cellulose with high adsorption performance for anionic/cationic dyes and heavy metal ions

A multifunction-integrated adsorbent based on zwitterion functionalized cellulose (MCC@PGMA-IAA) was synthesized by surface grafting polymerization and then modification with polyethyleneimine and sodium chloroacetate in turn. The adsorbent exhibited high functional group contents, switchable surface charge and chelating structure of iminoacetic acid. With the methylene blue, azophloxine, Ni(II) and Cr(VI) as the models of cationic/anionic dyes and heavy metal ions, batch experiments were conducted. The material showed strong adsorption abilities for dyes and heavy metal ions, with maximum adsorption capacities up to 440.5 mg/g, 320.5 mg/g, 59.3 mg/g, and 127.9 mg/g for methylene blue, azophloxine, Ni(II) and Cr(VI), respectively. Especially, the adsorption capacities of methylene blue and Ni(II) after zwitterionic functionalization increased by 50 times and 95.5% than those after polyethyleneimine modification. The adsorption equilibria were achieved within 10–120 min, and the adsorbent exhibited excellent removal efficiency. Furthermore, the adsorbed pollutants were conveniently desorbed by low concentration of HCl or NaOH solution, and the adsorbent had good reusability. Mechanism studies proved that electrostatic attraction and chelation were main driving forces. These results showed that MCC@PGMA-IAA had the characteristics of extensive sources, simple preparation and multifunctional integration, which was poised to develop as universal adsorbent for removing heavy metal ions and dyes.

Terahertz Liquid Biosensor Based on A Graphene Metasurface for Ultrasensitive Detection with A Quasi-Bound State in the Continuum.

The concept of a quasi-bound state in a continuum (QBIC) has garnered significant attention in various fields such as sensing, communication, and optical switching. Within metasurfaces, QBICs offer a reliable platform that enables sensing capabilities through potent interactions between local electric fields and matter. Herein, a novel terahertz (THz) biosensor based on the integration of QBIC with graphene is reported, which enables multidimensional detection. The proposed biosensor is distinctive because of its ability to discern concentrations of ethanol and N-methylpyrrolidone in a wide range from 100% to 0%, by monitoring the changes in the resonance intensity and maximum wavelet coefficient. This approach demonstrates an excellent linear fit, which ensures robust quantitative analysis. The remarkable sensitivity of our biosensor enables it to detect minute changes in low-concentration solutions, with the lowest detection concentration value (LDCV) of 0.21pgmL-1 . 2D wavelet coefficient intensity cards are effectively constructed through continuous wavelet transforms, which presents a more accurate approach for determining the concentration of the solution. Ultimately, the novel sensing platform offers a host of advantages, including heightened sensitivity and reusability. This pioneering approach establishes a new avenue for liquid-based terahertz biosensing.

Permeability enhancement by CO2 injection and chelating agent stimulation for creating geothermal reservoirs in granite

Existing research indicates that to create geothermal reservoirs using CO2 injection, additional stimulation methods are necessary. N, N-bis(carboxymethyl)-L-glutamic acid (GLDA) injection has been predicted to increase the permeability of CO2 injection-induced cloud-fracture networks (CFNs) and could serve as an additional stimulation method. Nevertheless, the influence of differential stress, flow geometry, and scale on the characteristics of permeability enhancement by GLDA injection is yet to be clarified. Accordingly, this study experimentally elucidated the permeability enhancement characteristics of injecting a chelating agent in fractured granite under differential stress conditions as an additional method for creating geothermal reservoirs using CO2 injection. GLDA injection experiments were conducted on fractured-granite samples under conventional- and true-triaxial stress states under varying differential stress and pH conditions. Regardless of the differential stress and pH conditions, rock deformation and acoustic emission (AE) were negligible during the chelating agent flow-through experiments on the fractured samples, whereas similar permeability enhancement factors were achieved within the same duration. Thus, stress did not affect the permeability enhancement by chelating agent injections. The permeability enhancement factors were inferred to be high near the injection borehole because of the high viscosity of the solution. Therefore, reservoir stimulation should be conducted using low-concentration chelating agent solutions at constant injection pressures. The study provides insights into the stimulation strategies for creating geothermal reservoirs using CO2 injection.

CLUSTERING NACL SOLUTION CONCENTRATION BASED ON IMPEDANCE, CONDUCTIVITY, AND TDS PARAMETERS USING A PCA APPROACH WITH K-MEANS CLUSTERING

A research study requires an approach to implement research data more optimally and be easily interpretable. The PCA approach with K-means clustering is an appropriate method for clustering NaCl solution concentrations based on impedance, TDS, and conductivity parameters. The results of this method indicate that the first and second principal components with eigenvalues of 2.7208 and 1.2728, respectively, represent 68.0% and 31.8% of the total variability. Cumulatively, these two principal components account for 99.8% of the total variance. This suggests that the approach used aligns well with the tested parameters. Therefore, it can be concluded that the PCA approach with K-means clustering can be used to cluster NaCl solution concentrations, distinguishing between low and high concentrations of NaCl solutions

Coion exclusion properties of cation exchange membranes in 2:1 divalent salt solutions

The selectivity of ion exchange membranes hinges on their capacity to exclude coions. Yet, understanding how they reject coions in multivalent salt solutions remains challenging. This study presents a new molecular theory to understand coion exclusion. It examines counterion and coion condensation, including electrosteric effects. Supported by experimental data [Gokturk et al., Nat. Commun., 13 (2022)], we analyze cation exchange membrane potential in a 2:1 salt solution, revealing insights into coion selectivity and exclusion mechanisms. Notably, our theory deviates from earlier research by accommodating condensed divalent counterions that bind to one or two fixed-charged monomers. Additionally, we account for many-body electrostatic interactions, allowing the former to bind to sorbed coions. Our findings highlight the importance of coion condensation, especially in low-concentration salt conditions where coions are mainly condensed. This effect becomes less significant as the external solution concentration rises. These findings clarify the suitability of the ideal Donnan model in high-concentration salts, but its accuracy decreases in low-concentration situations. To summarize, our study emphasizes the need to account for counterion and coion condensation to understand membrane selectivity in low-concentration multivalent salt solutions.

Study on the effect of electrolyte on surface tension of surfactant solution and wettability of coal dust

It is valuable to study the effect of electrolytes on the surface tension and wettability of surfactant solutions for industrial production. Surface tension test, coal dust sink test, and molecular dynamics simulation are employed to investigate the influence of four electrolytes (NaCl, NaAc, Na2CO3, MgCl2) on nonionic surfactant AEO-9 and anionic surfactant SDBS. The study showed that AEO-9 had a more extraordinary ability to reduce surface tension when the surfactant was applied alone. The surface tension achieved by SDBS solution was lower when compounded with electrolytes. The influence degree of electrolytes on the surface tension of SDBS solutions was shown as MgCl2>NaCl≈Na2CO3>NaAc. In contrast, the influence on the surface tension of AEO-9 solutions was insignificant. In addition, adding electrolytes to the low-concentration surfactant solution can markedly reduce the surface tension. When selecting the electrolytes compounded with anionic surfactants, the electrolyte cation is recommended to choose elements with a larger main groups number in the same periodic element from the periodic table and electrolyte anions that can form more hydrogen bonds with water molecules. The results of the coal dust sink test are different from the results of the surface tension measurement. Regardless of whether the electrolyte is added, the sink time of coal dust in the AEO-9 solution is shorter than that in the SDBS solution, and the wettability is better at the same concentration. The results of this study may guide the selection and design of dust suppression solutions.

A hybrid process combining ion exchange resin and bipolar membrane electrodialysis for reverse osmosis remineralization

A new reverse osmosis (RO) permeate remineralization process combining ion exchange resin and bipolar membrane electrodialysis (BMED) was developed. Its feasibility for hardness ions recovery and RO permeate remineralization was investigated. The effect of several operation conditions on the efficiency of the combined remineralization process was studied. Highly efficient cation exchange resin loading was achieved at a low flow rate and low feed solution concentration. The recovered calcium purity and yield considerably improved under gradient elution methods in comparison with commonly applied conventional isocratic elution methods using the same eluent quantity. The purity of the produced acid and base using BMED dropped noticeably with increasing feed NaCl concentration, presumably related to decreased permselectivity of the ion-exchange membranes. The drop in the purity of the calcium recovered when eluting the cation exchange resin with BMED-produced HCl in comparison with commercially available acid at 50 % yield was shown not to affect the remineralization process, where a dilution factor could be applied. This study confirmed the technical feasibility of the developed process for RO permeate remineralization. However, its application can be limited by the water source characteristics, the energy-intensive bipolar membrane process, and applied operational conditions, where more investigation is still needed.

Synthesis of sodium hypochlorite solutions in coaxial flow cells in current reverse mode

Electrodes made of platinized titanium with a surface platinum content of 2–3 mg/cm2 can be used in non-diaphragm flow- and accumulative-type electrolyzers for the electrolysis of low-concentrated NaCl solutions in order to obtain pure NaClO solutions. If electrolysis of 0.15 M NaCl solution is carried out on platinized titanium electrodes in the usual mode at current densities of 20–40 mA/cm2, then the anode surface passes into an oxidized passive state. In this case, the current efficiency of hypochlorite does not exceed 40%, and the current efficiency of chlorate is more than 20%. During a short electrolysis on a preliminarily reduced surface of platinized titanium, the current efficiency of hypochlorite reaches 90% with almost no accumulation of chlorate. Carrying out the long-term electrolysis of low-concentrated NaCl solutions in the regime of periodic polarity reverse makes it possible to significantly (up to 10 times) reduce the content of chlorate in the resulting sodium hypochlorite solutions. The most promising is the synthesis of sodium hypochlorite solutions in flow-type electrochemical reactors consisting of several series-connected electrochemical modules with an undivided electrode space in the mode of periodic current reverse. The electrolyzer of two series-connected cells in the mode without current reverse allows obtaining a solution that contains 500 mg/L of sodium hypochlorite and 130 mg/L of sodium chlorate. Carrying out the electrolysis in the reverse current mode every 30 s reduces the content of sodium chlorate to 25 mg/L, which makes it possible to produce high-purity NaClO solutions.

Identification of Aggregation Processes in Hexamethylenetetramine Aqueous Solutions: A Comprehensive Raman and Acoustic Spectroscopic Study Combined with Density Functional Theory Calculations

Raman scattering has been employed to study in detail the concentration dependence of the vibrational modes for hexamethylenetetramine (HMTA) aqueous solutions. The formation of protonated and/or aggregated species has been clarified by comparing the experimental with the theoretically predicted vibrational spectra by means of quantum mechanical calculations. The analysis has shown that the vibrational modes of the solutions arise from a contribution of the vibrational modes of the HMTA self-aggregates and hetero-aggregates of HMTA with water molecules that are formed in the low- and intermediate-concentration regions, respectively. The protonation of HMTA is ruled out due to the large differences between the experimental and the theoretically calculated spectra of the protonated molecules of HTMA in the fingerprint region. In the low-concentration solutions, the hetero-aggregation reaction of HMTA with water is the dominant mechanism, while at higher concentrations, a self-aggregation mechanism occurs. Ultrasonic absorption and velocity measurements were carried out for hexamethylenetetramine aqueous solutions. The acoustic spectra reveal the presence of only one single Debye-type relaxation process that is assigned to the aggregation mechanism of HMTA. The sound absorption data follow two different dependencies on the HMTA mole fraction. The crossover 0.018 mole fraction signifies two separate regions with distinct structural characteristics. The relaxation mechanism observed in dilute solutions was attributed to hetero-association of HMTA with water molecules, while at higher concentrations, the observed relaxation process was assigned to the self-association reaction of HMTA molecules. This structural transformation is also reflected in several physicochemical properties of the system, including the kinematic viscosity, the mass density, the sound speed and the adiabatic compressibility of the HMTA aqueous solutions. The combination of vibrational and acoustic spectroscopies with molecular orbital calculations allowed us to disentangle the underlying processes and to elucidate the observed relaxation mechanism in the HMTA aqueous solutions.

Mushroom alcohol(1-octen-3-ol)and other 7 aroma compounds selected from Chinese dry-cured hams can enhance saltiness perception

Reducing NaCl content in food while maintaining acceptability poses a significant challenge. Odor-induced saltiness enhancement (OISE) emerges as a promising solution. This study utilized gas chromatography-olfactory (GC-O) in conjunction with gas chromatography–mass spectrometry (GC–MS) to identify 37 key volatile compounds in three representative Chinese dry-cured hams. These compounds had an odor activity value (OAV) of ≥1 or a modification frequency (MF) of ≥30%. Subsequently, quantitative descriptive analysis (QDA) identified eight odorants associated with saltiness. These included 1-octen-3-ol, nonanal, heptanal, 2-methylbutanal, 3-methyl-butanal, benzaldehyde, octanal, and 2,6-dimethylpyrazine. Remarkably, these odorants significantly intensified saltiness (P < 0.05) when added to a low-concentration NaCl solution (0.3%), compared to zero or high concentrations (0.75% and 0.8%). As a result, traditional Chinese salty meat products offer a promising source of odorants for enhancing saltiness, compensating for reduced NaCl content through OISE.

Ratiometric Fluorescence Biosensing of Tandem Biemissive Ag Clusters Boosted by Confined Catalytic DNA Assembly.

The reaction kinetics and yield of traditional DNA assembly with a low local concentration in homogeneous solution remain challenging. Exploring confined catalytic DNA assembly (CCDA) is intriguing to boost the reaction rate and efficacy for creating rapid and sensitive biosensing platforms. A rolling circle amplification (RCA) product containing multiple tandem repeats is a natural scaffold capable of guiding the periodic assembly of customized functional probes at precise sites. Here, we present a RCA-confined CCDA strategy to speed up amplifiable conversion for ratiometric fluorescent sensing of a sequence-specific inducer (I*) by using string green-/red-Ag clusters (sgAgCs and srAgCs) as two counterbalance emitters. Upon recognition of I*, CCDA events are operated by two toehold-mediated strand displacements and localized in repetitive units, thereby releasing I* for recycled signal amplification in the as-grown RCA concatemer. The local concentration of reactive species is increased to facilitate rapider dsDNA complex assembly and more efficient input-output conversion, on which the clustering template sequences of sgAgCs and srAgCs are blocked and opened, enabling srAgCs synthesis but opposite to sgAgCs. Thus, the fluorescence emission of srAgCs goes up, while sgAgCs go down. With the resultant ratio featuring inherent built-in correction, rapid, sensitive, and accurate quantification of I* at the picomolar level is achieved. Benefiting from efficient RCA confinement to enhance reaction kinetics and conversion yield, this CCDA-based strategy provides a new paradigm for developing simple and diverse biosensing methodologies.

Compatible solutes determine the heat resistance of conidia

BackgroundAsexually developed fungal spores (conidia) are key for the massive proliferation and dispersal of filamentous fungi. Germination of conidia and subsequent formation of a mycelium network give rise to many societal problems related to human and animal fungal diseases, post-harvest food spoilage, loss of harvest caused by plant-pathogenic fungi and moulding of buildings. Conidia are highly stress resistant compared to the vegetative mycelium and therefore even more difficult to tackle.ResultsIn this study, complementary approaches are used to show that accumulation of mannitol and trehalose as the main compatible solutes during spore maturation is a key factor for heat resistance of conidia. Compatible solute concentrations increase during conidia maturation, correlating with increased heat resistance of mature conidia. This maturation only occurs when conidia are attached to the conidiophore. Moreover, conidia of a mutant Aspergillus niger strain, constructed by deleting genes involved in mannitol and trehalose synthesis and consequently containing low concentrations of these compatible solutes, exhibit a sixteen orders of magnitude more sensitive heat shock phenotype compared to wild-type conidia. Cultivation at elevated temperature results in adaptation of conidia with increased heat resistance. Transcriptomic and proteomic analyses revealed two putative heat shock proteins to be upregulated under these conditions. However, conidia of knock-out strains lacking these putative heat shock proteins did not show a reduced heat resistance.ConclusionsHeat stress resistance of fungal conidia is mainly determined by the compatible solute composition established during conidia maturation. To prevent heat resistant fungal spore contaminants, food processing protocols should consider environmental conditions stimulating compatible solute accumulation and potentially use compatible solute biosynthesis as a novel food preservation target.

Enhancing oil repellency of electron beam‐grafted polyethylene terephthalate fabric with 2‐(perfluorohexyl) ethyl acrylate monomer via pre‐irradiation

AbstractElectron beam (EB)‐induced graft polymerization is advantageous for the surface modification of fabrics. We investigated the effect of monomer concentration and the addition of alkyl groups on the oil repellency of polyethylene terephthalate (PET) fabrics treated with monomers containing fluoroalkyl groups through EB‐induced graft polymerization via pre‐irradiation. We use 2‐(perfluorohexyl) ethyl acrylate (FEA) and stearyl acrylate (SA(C18)) with long alkyl chains as vinyl monomers to induce reaction with radicals generated from EB irradiation. The weight gain and surface morphology of the PET fabrics change with the FEA monomer concentration. The uniformity of the EB‐grafted PET fabric surface is determined at low monomer solution concentrations. Results of X‐ray photoelectron spectroscopy analysis show that adding 0.1 mol/L of FEA monomer to the EB‐grafted PET fabric yields the highest dodecane contact angle of 93.4° and a surface fluorine concentration of 39.8%. The addition of SA(C18) monomer to the FEA monomer decreases the dodecane contact angle by 77.5° and yields a surface fluorine concentration of 19.1%. EB graft polymerization via pre‐irradiation results in a uniformly treated surface, and stable oil repellency is achieved when using solely the FEA monomer at a lower monomer concentration than that used in a similar irradiation method reportedly previously.

Highly porous NiFe-mixed metal oxides derived from calcinated layered double hydroxide for efficient antibiotics removal

Tetracycline (TC), a widely employed broad-spectrum antibiotic with global prevalence, possesses the ability to infiltrate soil matrices and potentially reach groundwater systems, thereby posing a consequential human health risk. To address this concern, a novel NiFe-mixed metal oxides (MMO) composite was synthesized and employed within quartz sand as a strategic measure to regulate the downward migration of TC. In-depth investigations of TC transport behavior in quartz sand, complemented by NiFe-MMO, were conducted via a series of rigorous batch and saturation-filled column experiments. The impacts of NiFe-MMO dosage, ionic strength, cationic types, solution pH, and flow rates on TC transport were systematically and comprehensively scrutinized. Our findings reveal that even minute quantities of NiFe-MMO prompt substantial TC adsorption, facilitated by electrostatic interactions and surface complexation, notably in low-concentration background solutions. Additionally, the presence of Ca2+ engenders a bridging effect, further enhancing TC adsorption onto the sand. These significant observations bear far-reaching implications in elucidating the fate of antibiotics within the intricate interplay of soil and groundwater domains. The strategic amalgamation of NiFe-MMO and quartz sand offers innovative prospects for managing TC dissemination, thereby augmenting endeavors toward sustainable environmental protection.

Self-Assembly of Alkanethiols on an Oxide-Free Surface of a Copper Electrode from Alkaline Solutions under Electrochemical Control

Using voltammetry and chronoamperometry, the formation process and properties of insulating nanofilms of alkanethiols with different chain lengths (butane-, octane-, dodecanethiol) obtained on an oxide-free copper surface were studied. The electrochemical method for modifying the copper surface includes the removal of the oxide layer by its cathodic reduction, the adsorption of a thiol under electrochemical control, followed by studying the properties of the resulting nanofilm by voltammetry in one solution. It is shown that, with this approach, a dense thiol film is formed, with its blocking properties depending on the adsorption potential, the time of contact of the electrode with the thiol-containing solution, the thiol concentration, and the presence of dissolved oxygen in the solution. The introduction of ethanol into an aqueous alkali solution leads to a significant acceleration of the process of self-assembly of dodecanethiol, but greatly inhibits the process of self-assembly of butanethiol. The approach proposed in this work makes it possible to use aerated low-concentration thiol-containing solutions to obtain alkanethiol films on the Cu surface with good blocking properties.

Prewetting Electron Beam Irradiated and Native Sheep Wool Can Affect Their Sorptivity.

In this work, the effect of prewetting native and electron beam-modified wool on the resulting sorption of Cu(II) from wool solutions was studied. The following conditions and combinations were applied: 6 mM and 50 mM solutions, prewetting time 0-24 h, contact time 1-24 h. The sorption results showed that wetting the wool before sorption from the low concentrated solution can fundamentally improve the efficiency of the separation process. The opposite result was achieved when applying a more concentrated solution; that is, prewetting slightly reduced the sorptivity. The reasons for such results are discussed. The application of these findings can be used to optimize the separation process in technological practice, however, will require solute specification.

Optimizing the synthesis process for Lithium-Ion sieve adsorbents: Effect of calcination temperature and heating rate on reaction efficiency and performance

Processing-structure–property relationships play a crucial role in tuning the performance of materials for a given application in order to attain a suitable set of optimal conditions, and it is therefore imperative to evaluate the parameters of either the synthesis, delithiation, adsorption, and desorption process. Through an orthogonal test design of the solid-state synthesis process, it was determined that a heating rate of 1 ℃/min had consistently higher reaction efficiencies of 68.1 %, 68.6 %, and 72.3 % at the calcination temperatures of 650 ℃, 700 ℃, and 750 ℃ respectively compared to heating rates of 4 ℃/min and 7 ℃/min. Moreover, a decrease in delithiation efficiency with increase in calcination temperature (99 % at 650 ℃, 96.1 % at 700 ℃, and 94.2 % at 750 ℃) at the 1 ℃/min heating rate was observed which was attributed to an increase in the average crystallite size. Adsorption performance of the synthesis-optimized zinc-doped lithium metatitanate showed consistently high adsorption efficiency (>90 %) despite the low adsorption capacity (4.9 mg/g) which was attributed to the low initial lithium-ion concentration of the lithium chloride solution and high adsorbent dosage.

Overlimiting mechanisms of heterogeneous cation- and anion-exchange membranes: A side-by-side comparison

Overlimiting ionic transfer attainable at ion-exchange membranes is a potential way to intensify electromembrane separation processes, such as electrodialysis. The primary mechanisms responsible for overlimiting current are electroconvection, natural convection, and water splitting. For the first time, the present study experimentally investigates all three mechanisms on heterogeneous anion- and cation-exchange membranes, using direct membrane observation and indirect measurement of pH changes in 0.01 M KCl solution. Although the anion- and cation-exchange systems are designed to exhibit the same behavior with different counterions, our results indicate significant differences in the overlimiting ionic transfer. The strong polarization of cation-exchange membranes produced an intensive electroconvective vortex with characteristic velocities in hundreds of μm/s superimposed by an upward electrolyte motion caused by natural convection. No pH changes associated with the water-splitting reaction were observed. The same conditions applied to the anion-exchange membrane resulted in profound pH changes, weaker electroconvection, and natural convection, reaching similar characteristic velocities to the other studied system. Generally, anion-exchange systems were characterized by more significant fluctuations in measured characteristics and were susceptible to fouling by one of the negatively charged markers. The obtained data provide a unified description of phenomena that may occur in the electrodialysis of low-concentrated ionic solutions.