Concentration Polarization Effects Research Articles

Influence of microbubble two-phase feed flow on reverse osmosis desalination using different salt concentrations

This study aims to implement microbubble two-phase flow in the feed stream to improve mass transfer and achieve improved reverse osmosis (RO) process performance. The swirl-flow microbubble generator (MBG), which can effectively generate microbubbles, is integrated into the conventional RO system and has experimentally proven its performance under various operating conditions. To investigate the influence of microbubbles in the feed stream on the RO process depending on the feed concentration, real seawater and two types of artificial brackish water were used as the feed solution. The formation of microbubble two-phase flows in the feed stream had a positive impact on the transmembrane flux and distillate water quality of the RO process; this demonstrates that improved mass transfer due to microbubbles introduced into the feed can effectively attenuate the concentration polarization effect in the RO process. MBG-RO performance using seawater as the feed solution was found to be more pronounced at lower feed pressures and higher airflow rates, with an improvement in permeation flux of up to 16 %. Meanwhile, the MBG contribution to RO performance was lower when using artificial brackish water with relatively low concentration than that when using seawater.

A seaweed-inspired separator for high performance Zn metal batteries: Boosting kinetics and confining side-reactions

Uncontrolled dendrite growth, sluggish reaction kinetics, and drastic side reactions on the anode-electrolyte interface are the main obstacles that restrict the application prospect of aqueous zinc-ion batteries. Traditional glass fiber (GF) separator with chemical inertness is almost ineffective in restricting these challenges. Herein, inspired by the ionic enrichment behavior of seaweed plants, a facile biomass species, anionic sodium alginate (SA), is purposely decorated on the commercial GF separator to tackle these issues towards Zn anode. Benefiting from the abundant zincophilic functional groups and superior mechanical strength properties, the as-obtained SA@GF separator could act as ion pump to boost the Zn2+ transference number (0.68), reduce the de-solvation energy barrier of hydrated Zn2+, and eliminate the undesired concentration polarization effect, which are verified by experimental tests, theoretical calculations, and finite element simulation, respectively. Based on these efficient modulation mechanisms, the SA@GF separator can synchronously achieve well-aligned Zn deposition and the suppression of parasitic side-reactions. Therefore, the Zn||Zn coin cell integrated with SA@GF separator could yield a prolonged calendar lifespan over 1230 h (1 mA cm−2 and 1 mAh cm−2), exhibiting favorable competitiveness with previously reported separator modification strategies. Impressively, the Zn-MnO2 full and pouch cell assembled with the SA@GF separator also delivered superior cycling stability and rate performance, further verifying its practical application effect. This work provides a new design philosophy to stabilize the Zn anode from the aspect of separator.

Development of charged membranes for the ultrafiltration of siRNA

RNA interference (RNAi) is an exciting new therapeutic approach, using small interfering RNA (siRNA) to silence specific genes for treating various diseases. Ultrafiltration has been widely used for the concentration and purification of monoclonal antibodies and other biotherapeutics; however, its application in the downstream processing of double-stranded oligonucleotides remains underexplored. This study investigates the use of ultrafiltration in the final formulation for concentrating a siRNA drug substance. Initial experiments were conducted with Ultracel™ composite regenerated cellulose membranes over a range of molecular weight cutoffs (MWCO) at different siRNA feed concentrations. Results demonstrated significant effects of both fouling and concentration polarization, with the maximum achievable concentration falling well below the target required for subcutaneous injection. Novel surface-modified ultrafiltration membranes were made by reaction of the cellulose with bromopropanesulfonic acid. These negatively charged membranes showed improved siRNA retention, reduced fouling, and a large increase in the maximum achievable siRNA concentration (from 52 to >180 mg/mL). These findings offer critical insights into novel approaches for developing high performance ultrafiltration processes for siRNA concentration.

Functional Protective Coatings Based on Polysaccharides and Single-Ion Conducting Polymers for Li Metal Batteries

This work aims to study a protective coating based on cellulose and P(LiMTFSI) single-ion conductor polymer and its effects on Li metal anode stability and reversibility. The strategy used involves the physical mixture of the aforementioned polymers to enhance the electrochemical performance through Li-ion flux homogenization at the Li metal surface, promoting the formation of a dense plating. Single-ion conducting polymers (SICPs) are incorporated due to their beneficial effects such as enhancing lithium ion transport, boosting lithium transference number (𝑡𝐿𝑖 +), and diminishing the effects of concentration polarization, avoiding the initiation and propagation of high surface area lithium (HSAL). Furthermore, a quasi-solid-state Li metal battery system is explored which brings additional advantages over the typically used organic electrolytes. In this sense, during this study we propose using the protective coating based on cellulose and P(LiMTFSI) to tailor an adequate interphase that creates a good adhesion, by taking advantage of the polymers film formation ability, and therefore improving the lifespan and safety of the battery. The coating effectiveness is tested by casting the solution on an activated lithium surface and performing lithium platting/stripping protocols in Li/Li and Li/Cu pouch cells under selected current densities (0.5, 1 and 2 mA/cm2). Complementary electrochemical tests are carried out to study transport, conductivity parameters, and its performance in full cell coupled with LFP cathode. Finally, we analyze the correlation with the physicochemical properties and electrochemical performed based on spectroscopic techniques and microscopy to study the most relevant parameters influencing a high Coulombic Efficiency. Acknowledgments This work has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska – Curie Grant Agreement N° 945357, and co-financing from the Slovenian Research and Innovation Agency (ARIS) is fully acknowledged.

Nanoarchitectonics in Advanced Membranes for Enhanced Osmotic Energy Harvesting.

Osmotic energy, often referred to as "blue energy", is the energy generated from the mixing of solutions with different salt concentrations, offering a vast, renewable, and environmentally friendly energy resource. The efficacy of osmotic power production considerably relies on the performance of the transmembrane process, which depends on ionic conductivity and the capability to differentiate between positive and negative ions. Recent advancements have led to the development of membrane materials featuring precisely tailored ion transport nanochannels, enabling high-efficiency osmotic energy harvesting. In this review, ion diffusion in confined nanochannels and the rational design and optimization of membrane architecture are explored. Furthermore, structural optimization of the membrane to mitigate transport resistance and the concentration polarization effect for enhancing osmotic energy harvesting is highlighted. Finally, an outlook on the challenges that lie ahead is provided, and the potential applications of osmotic energy conversion are outlined. This review offers a comprehensive viewpoint on the evolving prospects of osmotic energy conversion.

Advanced electrochemical membrane technologies for near-complete resource recovery and zero-discharge of urine: Performance optimization and evaluation

The depletion of nutrient sources in fertilizers demands a paradigm shift in the treatment of nutrient-rich wastewater, such as urine, to enable efficient resource recovery and high-value conversion. This study presented an integrated bipolar membrane electrodialysis (BMED) and hollow fiber membrane (HFM) system for near-complete resource recovery and zero-discharge from urine treatment. Computational simulations and experimental validations demonstrated that a higher voltage (20 V) significantly enhanced energy utilization, while an optimal flow rate of 0.4 L/min effectively mitigated the negative effects of concentration polarization and electro-osmosis on system performance. Within 40 min, the process separated 90.13% of the salts in urine, with an energy consumption of only 8.45 kWh/kgbase. Utilizing a multi-chamber structure for selective separation, the system achieved recovery efficiencies of 89% for nitrogen, 96% for phosphorus, and 95% for potassium from fresh urine, converting them into high-value products such as 85 mM acid, 69.5 mM base, and liquid fertilizer. According to techno-economic analysis, the cost of treating urine using this system at the lab-scale was $6.29/kg of products (including acid, base, and (NH4)2SO4), which was significantly lower than the $20.44/kg cost for the precipitation method to produce struvite. Excluding fixed costs, a net profit of $18.24/m3 was achieved through the recovery of valuable products from urine using this system. The pilot-scale assessment showed that the net benefit amounts to $19.90/m3 of urine, demonstrating significant economic feasibility. This study presents an effective approach for the near-complete resource recovery and zero-discharge treatment of urine, offering a practical solution for sustainable nutrient recycling and wastewater management.

Effects of Varying Spiral-Ring Pitches on CO2 Absorption by Amine Solution in Concentric Circular Membrane Contactors.

The CO2 absorption flux while using monoethanolamide (MEA) solution in a spiral-wired channel was significantly enhanced by optimizing both the descending and ascending spiral ring pitch configurations within the filled channel. In this study, two distinct spiral ring pitch configurations were integrated into concentric circular membrane contactors to augment CO2 absorption flux. Spiral rods were strategically inserted to mitigate concentration polarization effects, thereby reducing mass transfer boundary layers and increasing turbulence intensity. A theoretical one-dimensional model was developed to predict absorption flux and concentration distributions across varying MEA absorbent flow rates, CO2 feed flow rates, and inlet CO2 concentrations in the gas feed. Theoretical predictions of absorption flux improvement were validated against experimental results, demonstrating favorable agreement for both ascending and descending spiral ring pitch operations. Interestingly, the results indicated that descending spiral ring pitch operations achieved higher turbulent intensity compared to ascending configurations, thereby alleviating concentration polarization resistance and enhancing CO2 absorption flux with reduced polarization effects. Specifically, under conditions of a 40% inlet CO2 concentration and 5 cm3/s MEA feed flow rate, a notable 83.69% enhancement in absorption flux was achieved compared to using an empty channel configuration. Moreover, a generalized expression for the Sherwood number was derived to predict the mass transfer coefficient for CO2 absorption in concentric circular membrane contactors, providing a practical tool for performance estimation. The economic feasibility of the spiral-wired module was also assessed by evaluating both absorption flux improvement and incremental power consumption. Overall, these findings underscore the effectiveness of optimizing spiral ring pitch configurations in enhancing CO2 absorption flux, offering insights into improving the efficiency and economic viability of CO2 capture technologies.

Explicit expression for water permeation flux in forward osmosis desalination process

Forward osmosis (FO) is a membrane process of water separation and purification. FO uses the osmotic pressure difference across a semipermeable membrane. The effective osmotic pressure at the membrane–solution interface on both the feed and permeate sides of the membrane is the main driving force for the generation of the water flux. The major hindrance to the permeation of the water flux is the prevalence of concentration polarization on both sides of the membrane. Concentration polarization inhibits permeate flux by increasing the osmotic pressure at the membrane active layer interface on the feed side of the membrane. This work focused on the development of a mathematical model for water flux in the FO process. Combined film theory model and diffusion transport through the membrane were utilized. The effect of internal concentration polarization and external concentration polarization on the flux was studied. Both internal and external concentration polarization were taken into consideration in both membrane orientations, i.e., active layer facing the feed solution and active layer facing the draw solution. The obtained explicit expression for water permeation flux in forward osmosis desalination process shows excellent agreement with the literature data.

Influence of pore size distribution and applied cross-flow on ion rejection and separation

To improve the understanding of the separation mechanism of ions through membrane filtration, ion transport through a membrane with non-uniform pore sizes is investigated theoretically by solving the coupled Poisson-Nernst-Planck and Navier-Stokes equations under various conditions. The model focuses on a membrane with pore sizes following a log-normal distribution and considers the flow interaction between pores. The rejection performance for different pore arrangements is addressed for a rational assessment of the best ion rejection and selectivity in membrane filtration processes. In addition to the dead-end setting, a cross-flow is applied on the feed side to alleviate the concentration polarization effect. In the presence of positively charged nanopore surfaces, both the cases of a single salt (NaCl) and mixed salts (NaCl, MgCl2) present in the liquid are considered. We show that the uniform pore system outperforms non-uniform ones in terms of the overall rejection and selectivity between salts. When pore size polydispersity is present, the arrangement among the larger and smaller pores impacts the local rejection and volumetric flux of pores. Moreover, the competitive transport between cationic co-ions makes Mg2+ demonstrate generally higher rejection in all the pore arrangements considered. As uniform pores show lower flow rates, the separation efficiency of ions may be improved by pore size polydispersity at the same concentration and pressure difference.

Surface polarity modulation enables high-performance polyamide membranes for separation of polar/non-polar organic solvent mixtures

Achieving highly selective separation of organic mixtures is crucial to modern fine chemicals and pharmaceutical industries. The membrane-based organic solvent reverse osmosis (OSRO) technology is expected to break through this barrier. The key to achieving the separation of organic mixed liquids is to design the OSRO membrane as needed based on the physical and chemical properties of the target separation solvent. However, this poses a huge challenge to traditional OSRO membranes. In this work, we report composite polyamide (PA) OSRO membranes with ultrahigh polar/non-polar organic mixture selectivity. The surface polarity of the composite PA membrane is increased by coating a layer of Fe3+/tannic acid (Fe3+/TA) layer on the surface of the PA membrane. The enhanced surface polarity increases the interaction difference between polar and non-polar solvents and the membrane. And a polarization layer is formed on the membrane surface, which weakens the concentration polarization effect on the membrane surface. Under high-concentration feed liquid (the mass ratio of methanol/hexane is 7:3), the separation factor of the prepared membrane reaches 218, which is the highest value of all OSRO membranes reported so far. For the first time, we establish a partial flux analysis for the OSRO process. This analysis unravels the pressure, concentration dependence, and sensitivity of separation performance. It provides novel insights into the design of high-performance OSRO membranes.

A preliminary model of water and salt transport in a laboratory scale pressure-retarded osmosis (PRO) module

Nowadays, more and more attention is being paid to the production of green energy. Recently, energy from salinity gradients has also attracted interest. One such process that can generate useful work is the pressure-retarded osmosis (PRO), which uses a semi-permeable membrane to separate the feed and pressurized draw solutions. The semi-permeable membrane allows the transport of solvent (water) from the feed solution (diluted low pressure) side to the draw solution (concentrated high pressure) side, and thus, the excess of pressurized water on the draw side can be used to generate power, i.e., by expansion in the turbine. This work presents a preliminary numerical model developed to study the water and NaCl salt transport through the semi-preamble membrane and in the PRO module designed to perform experimental studies. The model was first verified for simple 2D module geometry. It was then used to study the flow through a simplified 3D module, mimicking the real one used in laboratory-scale experiments. The influence of the hydrodynamic pressure and the NaCl draw solution’s mass flow rate in the module on the energy generation efficiency was examined. The maximum power density obtained for half the osmotic pressure of the NaCl draw solution (i.e., for 28 bar) was found to be equal to approximately 4 W/m2. An increase in the flow rate of the draw solution causes a decrease in the thickness of the boundary layer at the membrane, which reduces the effect of the external concentration polarization. This results in an increase in the concentration difference on either side of the membrane, contributing to a non-linear increase in power density depending on this mass flow rate. Increasing the average velocity from 0.02 m/s to 0.1 m/s increases the power density by up to 80%, from approximately 2.6 to 4.7 W/m2.

Li6.28La3Zr2Al0.24O12-reinforced Single-ion Conducting Composite Polymer Electrolyte for Room-Temperature Li-Metal Batteries

Composite polymer electrolytes composed of inorganic fillers and organic polymers are promising electrolyte candidates for Li metal batteries, with benefits of improved safety and suppressed lithium dendrite growth. However, a severe concentration polarization effect often occurs when using conventional dual-ion electrolytes, and the increase in internal impedance during cycling results in decreased lifespan of the battery. To address this challenge, a plasticized single-ion conducting composite polymer electrolyte (SICE) was designed and fabricated by polymerizing the monomers of lithium (4-styrenesulfonyl) (trifluoromethanesulfonyl) imide (LiSTFSI) and poly(ethylene glycol) methyl ether acrylate (PEGMEA), crosslinker poly(ethylene glycol) diacrylate (PEGDA), silane-modified Li6.28La3Al0.24Zr2O12 nanofibers (s@LLAZO NFs), along with a PEG-based plasticizer tetraethylene glycol dimethyl ether (TEGDME), by heat-initiation. The anions were restrained and delocalized so that only Li cation migration occurred during the charging/discharging process, leading to a superior lithium-ion transference number. The s@LLAZO NFs enabled direct monomer grafting with the polymer matrix, resulting in controlled formation of an organic-inorganic network with increased filler content and improved filler distribution in the SICE system. The SICE membrane exhibited high ionic conductivity at room temperature, reduced activation energy and excellent oxidation stability. Most importantly, the all-solid-state Li-metal batteries assembled with the fabricated SICE demonstrated stable long-term cycling performance and remarkable rate capability at room temperature.

Highly efficient and direct recovery of low-pressure hydrogen isotopes from tritium extraction gas by PdY alloy membrane permeator

Low-pressure hydrogen recovery is crucial for the fusion reactor deuterium-tritium fuel cycle, and the usual treatment is achieved through multiple steps of enrichment and separation, which has the drawbacks of complex processes and high energy consumption. In this work, a membrane permeator including 12 sets of Pd92Y8 at% alloy tubes has been investigated for the permeability and extraction of hydrogen in low-pressure mixtures. The hydrogen permeation through dense Pd-based membrane mechanism and corresponding Sieverts’ exponents have been verified. The concentration polarization effect is deducted from the results. Through pure hydrogen permeation test, the cold-rolling 80 μm thick Pd92Y8 at% membrane has hydrogen permeability at 400 °C of 3.43×10−8 mol∙m−1∙s−1∙Pa−0.5, which is higher than that of PdAg alloy membrane under the same conditions. Highly efficient recovery efficiency has been achieved for low hydrogen pressure mixtures, and the PdY alloy membrane permeator with 678 cm2 effective area could reach around 95% hydrogen extraction recovery for 0.1 mol% hydrogen-helium mixture and 97.74% recovery for 0.33 mol% at the rate of 21 standard liter per minute (SLM). It indicates that the PdY alloy membrane permeation can be an alternative solution for efficiently recovering low-pressure hydrogen isotopes in tritium extraction gas.

PPS/TA-PEI/β-FeOOH membranes with powerful photo-Fenton self-cleaning capability for efficient oil–water emulsion separation

Polyphenylene sulfide (PPS) microporous membranes have garnered extensive research enthusiasm concerning their outstanding permeation performance and separation accuracy in treating oily wastewater. Nevertheless, the inherent hydrophobic chemical structure, along with the noticeable concentration polarization effect, predisposes them to severe membrane contamination during the separation process. Accordingly, it is desirable to design a PPS self-cleaning membrane with superior anti-fouling capability, fast transport, and remarkable flux recovery. This study marks the first combination of PPS membrane materials with the photo-Fenton process. In detail, the polyphenol (tannic acid, TA)-polycation (polyethyleneimine, PEI)-metal ion (Fe3+) double cross-linking coatings were first assembled on the PPS substrate surface. Subsequently, β-FeOOH photocatalysts were synthesized through in-situ mineralization to further strengthen the hydrophilicity of the membranes. The obtained PPS/TA-PEI/β-FeOOH composite membrane achieved exceptional separation of diverse oil-in-water (O/W) emulsions. The PPS/TA-PEI/β-FeOOH membranes exhibited in-air superhydrophilic and under-water superoleophobicity, exhibiting under-water oil contact angles greater than 151° for diverse oil agents. The fabricated composite membranes displayed high permeability (a flux of the petroleum ether-in-water (P/W) emulsion reached 3400.5 L m−2h−1 bar−1 driven by a pressure of 0.05 MPa) along with ideal oil rejection (98.6 %). Significantly, the PPS/TA-PEI/β-FeOOH membrane possessed powerful photo-Fenton self-cleaning capability, and its flux recoveries of the contaminated membrane exceeded 90.3 % for varied O/W emulsions after being cleaned by visible light for 10 min. We believe that our research presents a novel approach for the exploitation of PPS-based photo-Fenton self-cleaning membranes, which also exhibit great applicative potential in oily wastewater treatment.

Hydrogen Flux Inhibition of Pd-Ru Membranes under Exposure to NH3.

The hydrogen flux inhibition of Pd-Ru membranes under exposure to 1-10% NH3 at 673-773 K was investigated. The Pd-Ru membranes were characterized by XRD, SEM, XPS, and hydrogen permeation tests. The results show that when exposed to 1-10% NH3 at 723 K for 6 h, the hydrogen flux of Pd-Ru membranes sharply decreases by 15-33%, and the decline in hydrogen flux becomes more significant with increasing temperatures. After the removal of 1-10% NH3, 100% recovery of hydrogen flux is observed. XPS results show that nitrogenous species appear on the membrane surface after NH3 exposure, and the hydrogen flux inhibition may be related to the competitive adsorption of nitrogenous species. By comparing the hydrogen flux of Pd-Ru membranes exposed to 10% NH3 with 10% N2, it is indicated that the rapid decrease in hydrogen flux is due to the concentration polarization and competitive adsorption of nitrogenous species. The competitive adsorption effect is attenuated, while the concentration polarization effect becomes more pronounced with increasing temperature.

Locational enrichment of low-abundance analytes through force-environment-modulation microsystem with ion concentration polarization

Ion concentration polarization (ICP) effect in micro-nanofluidic devices can concentrate low-concentration molecules. In this paper, we proposed a novel ICP-based enrichment system with horizontal barriers that adjust the force environment of charged particles, to achieve specific locational analytes enrichment. Mechanism of analyte enrichment through force modulation is revealed through numerical simulation using microchannels with varied channels widths, which is further validated through physical experiment. Overall, a specific target-oriented structure and load regulation method was proposed, under which the low-abundance analytes could be locationally concentrated by ∼ 4000 times. This study is expected to develop an innovative method for specifically locational ion enrichment in terms of scientific principles, providing theoretical and technical support for concentration and detection of charged substances in both research and industry such as biology, environment, chemistry studies, etc.

Predicting forward osmosis performance with synthesized polyamide-based membrane: An integrated machine learning (MATLAB and ANN) and economic analysis framework

Forward Osmosis (FO) has emerged as a highly promising membrane-based water treatment process owing to its intrinsic advantages, such as low energy requirements and the potential for high water recovery. The lab-prepared highly efficient thin film composite (TFC) FO membranes that were used in this research work. The resultant polyamide-based FO membrane showed exceptional performance, notably achieving a high-water flux of 60.94 ± 3.5 L/m2.h and a constrained solute flux of 1.52 ± 0.08 g/m2.h. Machine learning methodologies are strategically employed in this study to construct predictive models for FO membrane performance. Notably, the accuracy of the van't Hoff equation's linear assumption is significantly enhanced by introducing an osmotic pressure calculation based on water activity, considering the nuanced effects of external and internal concentration polarization. The research employs MATLAB software alongside an artificial neural network (ANN) to develop predictive models. MATLAB facilitates the creation of a comprehensive theoretical model, offering insights into forecasting water flux through FO membranes by systematically analyzing diverse membrane parameters and their impact on performance. Concurrently, the ANN model was employed to predict the permeate flux for the laboratory-scale FO membrane system, utilizing the input parameters within the network's training range (R2 > 0.91). An economic assessment is also conducted to estimate the projected membrane cost, a pivotal determinant of system viability. Collectively, these investigations yield valuable insights into the intricacies of FO membrane design and optimization, elucidating their performance dynamics in water treatment and allied applications. This research significantly contributes to the ongoing quest to develop more efficient and precise membrane processes.

Theoretical Analysis of the Influence of Spacers on Salt Ion Transport in Electromembrane Systems Considering the Main Coupled Effects.

This article considers a theoretical analysis of the influence of the main coupled effects and spacers on the transfer of salt ions in electromembrane systems (EMS) using a 2D mathematical model of the transfer process in a desalting channel with spacers based on boundary value problems for the coupled system of Nernst-Planck-Poisson and Navier-Stokes equations. The basic patterns of salt ion transport have been established, taking into account diffusion, electromigration, forced convection, electroconvection, dissociation/recombination reactions of water molecules, as well as spacers located inside the desalting channel. It has been shown that spacers and taking into account the dissociation/recombination reaction of water molecules significantly change both the formation and development of electroconvection. This article confirms the fact of the exaltation of the limiting current studied by Harkatz, where it is shown that the current (flux) of salt ions increases when the dissociation reaction begins by a certain value called the exaltation current, which is proportional to the flow of water dissociation products. A significant combined effect of electroconvection and dissociation/recombination reactions as well as the spacer system in the desalting channel on the transport of salt ions are shown. The complex, nonlinear, and non-stationary interaction of all the main effects of concentration polarization and spacers in the desalting channel are also considered in the work.

A finite element model for concentration polarization and osmotic effects in a membrane channel

AbstractIn this article, we study a mathematical model that represents the concentration polarization and osmosis effects in a reverse osmosis cross‐flow channel with dense membranes at some of its boundaries. The fluid is modeled using the Navier–Stokes equations and the solution‐diffusion is used to impose the momentum balance on the membrane. The scheme consist of a conforming finite element method with the velocity–pressure formulation for the Navier–Stokes equations, together with a primal scheme for the convection–diffusion equations. The Nitsche's method is used to impose the permeability condition across the membrane. Several numerical experiments are performed to show the robustness of the method. The resulting model accurately replicates the analytical models and predicts similar results to previous works. It is found that the submerged configuration has the highest permeate production, but also has the greatest pressure loss of all three configurations studied.

Ion concentration polarization causes a nearly pore-length-independent conductance of nanopores.

There has been a great amount of interest in nanopores as the basis for sensors and templates for preparation of biomimetic channels as well as model systems to understand transport properties at the nanoscale. The presence of surface charges on the pore walls has been shown to induce ion selectivity as well as enhance ionic conductance compared to uncharged pores. Here, using three-dimensional continuum modeling, we examine the role of the length of charged nanopores as well as applied voltage for controlling ion selectivity and ionic conductance of single nanopores and small nanopore arrays. First, we present conditions where the ion current and ion selectivity of nanopores with homogeneous surface charges remain unchanged, even if the pore length decreases by a factor of 6. This length-independent conductance is explained through the effect of ion concentration polarization (ICP), which modifies local ionic concentrations, not only at the pore entrances but also in the pore in a voltage-dependent manner. We describe how voltage controls the ion selectivity of nanopores with different lengths and present the conditions when charged nanopores conduct less current than uncharged pores of the same geometrical characteristics. The manuscript provides different measures of the extent of the depletion zone induced by ICP in single pores and nanopore arrays, including systems with ionic diodes. The modeling shown here will help design selective nanopores for a variety of applications where single nanopores and nanopore arrays are used.