Salt Concentration Polarization Research Articles

Brain-inspired computing with fluidic iontronic nanochannels

The brain's remarkable and efficient information processing capability is driving research into brain-inspired (neuromorphic) computing paradigms. Artificial aqueous ion channels are emerging as an exciting platform for neuromorphic computing, representing a departure from conventional solid-state devices by directly mimicking the brain's fluidic ion transport. Supported by a quantitative theoretical model, we present easy-to-fabricate tapered microchannels that embed a conducting network of fluidic nanochannels between a colloidal structure. Due to transient salt concentration polarization, our devices are volatile memristors (memory resistors) that are remarkably stable. The voltage-driven net salt flux and accumulation, that underpin the concentration polarization, surprisingly combine into a diffusionlike quadratic dependence of the memory retention time on the channel length, allowing channel design for a specific timescale. We implement our device as a synaptic element for neuromorphic reservoir computing. Individual channels distinguish various time series, that together represent (handwritten) numbers, for subsequent in silico classification with a simple readout function. Our results represent a significant step toward realizing the promise of fluidic ion channels as a platform to emulate the rich aqueous dynamics of the brain.

Calcium carbonate scaling and seeding induced precipitation in FO treating textile reverse osmosis concentrate

Forward osmosis (FO) technology has been applied in the treatment and reclamation water from reverse osmosis concentrate. The performance of treating simulated textile reverse osmosis (sROC) was investigated with a focus upon the effect of calcium carbonate scaling and seeding induced precipitation on flux behavior. The effects of DS dilution & reverse salt flux (α), FS concentration & concentration polarization (β) and carbonate calcium scaling (γ) on flux decline were quantified based on the same water recovery at different DS (NaCl) concentration and FS salinity. The calcium carbonate scaling was found to contribute more to the flux decline when the FS salinity was 5000 ± 500 mg/L TDS at higher DS concentration. When the salinity of FS was increased to 10,000 ± 500 and 15,000 ± 500 mg/L TDS, the water flux was found higher under the situation of scaling than those of no scaling due to the fact that CaCO3 precipitation could inhibit the increase of osmosis pressure in FS in some degree. CaCO3 seeding induced precipitation in FO process promoted water flux and thus alleviated the calcium carbonate scaling on FO membrane. This work proposed a new view on the membrane scaling in FO process treating feed solution like reverse osmosis concentrate (ROC), and the seeding induced precipitation turned out to be an efficient strategy to alleviate calcium scaling in FO process and improve performance.

A free-piston batch reverse osmosis (RO) system for brackish water desalination: Experimental study and model validation

Batch RO is designed to achieve high energy efficiency and high recovery in desalination. However, so far relatively few experiments on batch RO have been reported. Here we present an extensive experimental study of a single-acting, free-piston batch RO system using an 8-inch spiral wound membrane. The system was tested in the laboratory with brackish feed water containing up to 5 g/L NaCl. The objective was to quantify system performance in terms of Specific Energy Consumption (SEC), recovery, rejection, and output. Sensitivity to permeate flux and recirculation flow rate was also investigated. Performance was compared against the predictions of a theoretical model that accounts for salt retention, concentration polarization, and longitudinal concentration gradient in the RO module. For the first time, osmotic backflow was measured and incorporated into the model. For feed concentrations ranging from 1 to 5 g/L and recovery of 0.8, hydraulic SEC was measured in the range 0.22–0.48 kWh/m3 and electrical SEC in the range 0.48–0.83 kWh/m3. With improvements to the membrane permeability from 4.4 to 8 LMH/bar, selection of more efficient pumps, and reduction of valve friction losses, the model predicts that hydraulic SEC will be lowered to 0.14–0.39 kWh/m3.

Numerical and experimental investigation of a DC-powered RO system for Sri-Lankan villages

High operation and maintenance costs, and ‘last-mile’ issues for populations not connected to municipal water infrastructure, have limited the utility of centralized reverse osmosis (RO) systems to alleviate water availability issues for rural communities. To address these challenges, a decentralized RO module which employs tubular RO membranes was designed and investigated. The system represents an appropriate technology because it requires minimal pretreatment and it fits in self-contained, low-profile suitcase. This compact desgin also incorporated innovative 3D printed static mixers to minimize salt concentration polarization and to improve performance. Since grid electricity may also be discontinuous in the target use cases, the system was tested with two direct current (DC) power solutions: a photovoltaics-battery configuration and an alternator-battery. The final prototype weighs less than 15 kg and was found to have a production capacity of ∼8 L/h, consuming ∼100 W of pumping power. Experiments of the prototype module revealed a 96% rejection rate of salt from a 1000–2000 ppm synthetic groundwater solution and a >99% rejection coefficient for bovine serum albumin protein and humic acid. From these performance metrics, it was estimated that 36 sets of this decentralized system (with a total CapEx of US$23,400) could provide drinking water for ∼600 rural households at a unit cost of water of 1 LKR/L (0.005 USD/L) in Sri Lanka. Overall, this study demonstrates how computational modeling and 3D printing can be leveraged to develop a compact and cost-effective decentralized RO package that could be rapidly deployed to water-stressed rural areas and in disaster relief applications.

Nanofiltration of saline oil-water emulsions: Combined and individual effects of salt concentration polarization and fouling by oil

This study employed NF270 nanofiltration membranes and SDS-stabilized hexadecane to identify mechanisms of membrane fouling by highly saline oil-water emulsions. Concentration dependencies of NaCl reflection coefficient σ and NaCl permeability coefficient B were measured and used to determine the separate contributions of osmotic pressure and fouling to the overall flux decline. The NaCl permeability coefficient asymptotically converged to the same steady state value across a range of feed salinities and concentration polarization conditions. The measured near-hyperbolic dependence of the reflection coefficient on the transmembrane concentration differential (σΔπm≈ const) negated the effect of concentration polarization on permeate flux. Oil caused an abrupt decrease in permeate flux but only for more saline feeds (seawater level and higher), which was interpreted as a result of membrane surface sealing by coalesced oil. Headloss analysis showed that over the longer term, the additional hydraulic resistance due to a layer of oil droplets on the membrane surface became the dominant fouling mechanism.

Draw solution solute selection for a hybrid forward osmosis-membrane distillation module: Effects on trace organic compound rejection, water flux and polarization

The integrated forward osmosis membrane bioreactor and membrane distillation (OMBR-MD) is an emerging technique for wastewater reclamation and trace organic compound (TrOC) removal. The selection of draw solution (DS) for this system should consider the particularities of both forward osmosis and membrane distillation. The effect of five different draw solutions on forward osmosis and TrOC rejection in a hybrid submerged forward osmosis – membrane distillation (FO-MD) module was investigated in this study. The roles of each solution on FO and MD water flux, FO reverse salt flux, temperature, concentration polarization and rejection of 7 TrOCs were explored. The salt used in the draw solution influenced both FO water flux (NaCl = MgOAc2 (4.8 L.m−2.h−1) > NaOAc (4.6 L.m−2.h−1) > MgCl2 (3.6 L.m−2.h−1) > EDTA-Na2 (2.8 L.m−2.h−1)) and MD water flux (NaOAc (4.0 L.m−2.h−1) > NaCl (3.8 L.m−2.h−1) > MgCl2 (3.3 L.m−2.h−1) > MgOAc2 (3.1 L.m−2.h−1 > EDTA-Na2 (3.0 L.m−2.h−1)). MgCl2 draw solution produced the lowest FO reverse salt flux, demonstrating its potential to prevent salinity build-up when applied in OMBR-MD. MgCl2 and NaCl were the only salts completely rejected by the distillation membrane, which salt losses during draw solution reconcentration. Regardless of the draw solution salt, the integrated FO-MD module could effectively remove 6 of the 7 TrOCs. MgCl2 and NaOAc afforded the highest TrOC rejections, while the lowest rejections were observed when MgAOc2 was used. MgCl2 was selected as the best DS salt for OMBR-MD by using the technique for order preference by similarity to ideal solution (TOPSIS) tool.

Thickness‐Dependent Impedance of Composite Battery Electrodes Containing Ionic Liquid‐Based Electrolytes

AbstractLithium‐ion battery models often neglect the salt concentration polarization inside the electrolyte‐filled pores of the composite electrodes. However, this concentration polarization causes a significant impedance, in particular in the case of electrolytes with low Li+ transference numbers. Here, we analyze in detail measured and calculated impedance spectra of composite electrodes containing a solvate ionic liquid‐based electrolyte and an ionic liquid‐based electrolyte, respectively, in comparison to a conventional carbonate‐based electrolyte. For calculating spectra, we use a recently published model by Huang and Zhang. We find that the impedance at 10−4 Hz, which is relevant for battery cycling rates around 1 C to 2 C, increases in the order carbonate‐based electrolyte<ionic liquid‐based electrolyte<solvate ionic liquid‐based electrolytes, but exhibits a remarkably weak thickness dependence, when the electrode thickness exceeds 50–100 μm. This suggests that electrodes considerably thicker than the conventional 80 μm can be used in batteries without significantly deteriorating battery power.

Mass transport modeling of natural organic matter (NOM) and salt during Nanofiltration of inorganic colloid-NOM mixtures

The behavior of mixtures of inorganic nanoparticles and natural organic matter (NOM) during nanofiltration (NF) was investigated in a lab-scale crossflow filtration system to identify how interactions between these foulants influenced deposition and membrane performance in terms of NOM and salt selectivity. NOM selectivity was reduced substantially when the colloid concentration was sufficient to produce a thick deposited colloidal cake, as a result of enhanced concentration polarization phenomena. This increased the concentration and hence the driving force for NOM. It also increased salt concentration, which screened NOM charge. In addition to reducing the double layer thickness, charge screening has also been shown to reduce NOM effective size, increasing the membrane permeation coefficient. On the other hand, the presence of NOM in the cake layer matrix substantially attenuated the enhancement of salt concentration polarization. A multilayer solute transport model was applied to describe these phenomena and simulate solute selectivity for both NOM and salt in colloid-NOM mixtures. Good agreement of model calculations with experimental data was observed. According to this model, the properties of the first deposited colloidal cake layer is critical in determining the quality of the permeate product in terms of NOM or salt retention.

Colloidal Fouling of Nanofiltration Membranes: Development of a Standard Operating Procedure

Fouling of nanofiltration (NF) membranes is the most significant obstacle to the development of a sustainable and energy-efficient NF process. Colloidal fouling and performance decline in NF processes is complex due to the combination of cake formation and salt concentration polarization effects, which are influenced by the properties of the colloids and the membrane, the operating conditions of the test, and the solution chemistry. Although numerous studies have been conducted to investigate the influence of these parameters on the performance of the NF process, the importance of membrane preconditioning (e.g., compaction and equilibrating with salt water), as well as the determination of key parameters (e.g., critical flux and trans-membrane osmotic pressure) before the fouling experiment have not been reported in detail. The aim of this paper is to present a standard experimental and data analysis protocol for NF colloidal fouling experiments. The developed methodology covers preparation and characterization of water samples and colloidal particles, pre-test membrane compaction and critical flux determination, measurement of experimental data during the fouling test, and the analysis of that data to determine the relative importance of various fouling mechanisms. The standard protocol is illustrated with data from a series of flat sheet, bench-scale experiments.

Study of the Reverse Salt Diffusion in pressure retarded osmosis: Influence on concentration polarization and effect of the operating conditions

Pressure retarded osmosis (PRO) suffers Reverse Salt Diffusion (RSD) and concentration polarization (CP), which reduce its performance. The current work investigates the effect of the operating conditions (concentrations, solutions, pressures, flow rates, and temperatures) on RSD and, as a consequence, on the produced power. The results show that the salt diffusion is severely affected by the operating conditions, which should be carefully controlled to reduce the power losses in osmotic power plants based on PRO. The relationship between CP and RSD is also studied, showing how RSD aggravates the internal concentration polarization (ICP).

Process engineering optimization of nanofiltration unit for the treatment of textile plant effluent in view of solution diffusion model

A nanofiltration (NF) based separation process is used to treat the effluent from a textile plant to allow for water reuse and fulfill environmental standards. The wastewater effluent contains reactive black (RB5)- and disperse (DR60) dyes. A NF-unit model E2 series with HL 2521 TF spiral wound module was used to carry out experiments. 90 & 93% color removal and COD reduction for RB5 and 98 & 95% for DR60 were achieved. A parametric study of the separation process is undertaken to characterize the effects of the operating variables, e.g., trans-membrane pressure, dye/salt concentration in the feed, temperature, and cross flow velocity. The solution diffusion model was used to develop power correlations to calculate the permeate side solute mass transfer coefficient as a function of effective cross-flow Reynolds number. In contrast to the commonly assumed constant hydraulic solvent permeability, a non-linear relationship was developed over the applied trans-membrane net driving pressure. The latter correlates exponentially with salt permeability for both dyes. The effects of feed salt-content on solute mass transfer coefficient, water and salt permeability, concentration polarization, dye hydrophobicity and ionic strength were studied. Results were used to assess engineering specifications of a commercial size NF-plant (500m3/d capacity).

Poly(arylene ether)-Based Single-Ion Conductors for Lithium-Ion Batteries

Single-ion conducting electrolytes present a unique alternative to traditional binary salt conductors used in lithium-ion batteries. It has been shown theoretically that single-ion electrolytes can eliminate the salt concentration gradient and polarization loss in the cell that develop in a binary salt system, resulting in substantial improvements in materials utilization for high power and energy densities. Here, we describe synthesis and characterization of a class of single-ion electrolytes based on aromatic poly(arylene ether)s with pendant lithium perfluoroethyl sulfonates. The microporous polymer film saturated with organic carbonates exhibits a nearly unity Li+ transference number, very high conductivities (e.g., > 10–3 S m–1 at room temperature) over a wide range of temperatures, great electrochemical stability, and outstanding mechanical properties. Excellent cyclability with almost identical charge and discharge capacities has been demonstrated at ambient temperature in the batteries assembled f...

Limits of power production due to finite membrane area in pressure retarded osmosis

Dimensionless analytical expressions for the power attainable from an ideal counterflow pressure retarded osmosis (PRO) system model are developed using a one-dimensional model that accounts for streamwise variations in concentration. This ideal PRO system has no salt permeation or concentration polarization. The expressions show that the optimal hydraulic pressure difference, for which the maximum power is produced, deviates significantly from the classical solution of one-half of the trans-membrane osmotic pressure difference, Δπ/2, as the dimensionless membrane area (MTUπ) increases and the ratio of draw to feed mass flow rates (MR) varies. The overall maximum power attainable from a PRO membrane is found to occur in the limit of infinitely large MTUπ (an effectiveness of unity) and infinite MR. For an ideal PRO system which mixes seawater (35g/kg) and river water (1.5g/kg), the overall maximum power of 1.57kJ/kg of feed can be attained at roughly MTUπ of 15, an MR of 10, and a pressure of 0.83Δπ. Due to economic considerations, a PRO system in practice will have limited membrane area and will operate at an effectiveness of less than unity. The present work can be used to estimate the operating conditions and area required for a PRO system of given performance. The effect of concentration polarization on optimal hydraulic pressure difference and maximum power performance is also investigated using a numerical model.

The effect of low-molecular-weight poly(ethylene glycol) (PEG) plasticizers on the transport properties of lithium fluorosulfonimide ionic melt electrolytes.

The influence of low-molecular-weight poly(ethylene glycol) (PEG, Mw ≈ 550 Da) plasticizers on the rheology and ion-transport properties of fluorosulfonimide-based polyether ionic melt (IM) electrolytes has been investigated experimentally and via molecular dynamics (MD) simulations. Addition of PEG plasticizer to samples of IM electrolytes caused a decrease in electrolyte viscosity coupled to an increase in ionic conductivity. MD simulations revealed that addition of plasticizer increased self-diffusion coefficients for both cations and anions with the plasticizer being the fastest diffusing species. Application of a VTF model to fit variable-temperature conductivity and fluidity data shows that plasticization decreases the apparent activation energy (Ea) and pre-exponential factor A for ion transport and also for viscous flow. Increased ionic conductivity with plasticization is thought to reflect a combination of factors including lower viscosity and faster polyether chain segmental dynamics in the electrolyte, coupled with a change in the ion transport mechanism to favor ion solvation and transport by polyethers derived from the plasticizer. Current interrupt experiments with Li/electrolyte/Li cells revealed evidence for salt concentration polarization in electrolytes containing large amounts of plasticizer but not in electrolytes without added plasticizer.

Salt concentration polarization of liquid electrolytes and determination of transport properties of cations, anions, ion pairs and ion triples

Specifically blocking of cations or anions in an electrochemical polarization experiment is a standard procedure to determine transport numbers. This is non-trivial in the case that internal interaction (ion pairs, ion triples etc.) have to be taken account of. Besides the trivial concentration redistribution owing to mass action laws, it is the dynamics that is affected in a more complex way. The steady state behavior of such a polarization is calculated using the concept of Conservative Ensembles. By the same procedure also the relaxation time of the galvanostatic transient is addressed. Finally it is shown how the individual transport parameters of cations, anions, ion pairs and ion triples can be obtained by a combination of polarization, impedance spectroscopy and cationic as well as anionic tracer (or PFG-NMR) experiments.

Development and testing of a transparent membrane biofouling monitor

A modified version of the membrane fouling simulator (MFS) was developed for assessment of (i) hydraulic biofilm resistance, (ii) performance parameters feed-channel pressure drop and transmembrane pressure drop, and (iii) in situ spatial visual and optical observations of the biofilm in the transparent monitor, e.g. using optical coherence tomography. The flow channel height equals the feed spacer thickness enabling operation with and without feed spacer. The effective membrane surface area was enlarged from 80 to 200 cm2 by increasing the monitor width compared to the standard MFS, resulting in larger biomass amounts for analysis. By use of a microfiltration membrane (pore size 0.05 μm) in the monitor salt concentration polarization is avoided, allowing operation at low pressures enabling accurate measurement of the intrinsic hydraulic biofilm resistance. Validation tests on e.g. hydrodynamic behavior, flow field distribution, and reproducibility showed that the small-sized monitor was a representative tool for membranes used in practice under the same operating conditions, such as spiral-wound nanofiltration and reverse osmosis membranes. Monitor studies with and without feed spacer use at a flux of 20 L m−2 h−1 and a cross-flow velocity of 0.1 m s−1 clearly showed the suitability of the monitor to determine hydraulic biofilm resistance and for controlled biofouling studies.

The evaluation on concentration polarization for effective monitoring of membrane fouling in seawater reverse osmosis membrane system

One of the critical issues for the successful application of seawater reverse osmosis (SWRO) in the desalination processes is membrane fouling. However, it is difficult to evaluate fouling by the flux decline since recently commercialized SWRO membranes have better permeability than the initial membranes and the two-way coupling between salt concentration polarization and fouling influences permeate flux. In this study, we evaluated membrane performance using various factors, namely the permeation coefficient (Lp) and concentration polarization factor (fcp). The values of Lp and fcp were obtained according to the changing conditions of feed water total dissolved solids (TDS) concentration and operating pressure. As a result, Lp and fcp increased with increasing operating pressure at the same feed water TDS, on the other hand, they decreased with increasing feed water TDS at the same operating pressure. The reasons for these results are that concentration polarization is enhanced with increasing operating pressure at the same feed water TDS and concentration polarization is weakened with increasing feed water TDS concentration due to osmotic pressure at the same operating pressure. Based on these factors and the equation combined with osmotic cake filtration theory and thermodynamic model, we could calculate the resistance by NaCl (Rcp), which is constant with time like membrane resistance (Rm). Thus, we would identify the fouling rate through fouling resistance (Rc) by calculating Lp and fcp according to time after spiking organic foulants.

Fabrication of thin film composite poly(amide)-carbon-nanotube supported membranes for enhanced performance in osmotically driven desalination systems

The search for lower energy consumption desalination systems has been driving research in the past decade towards the investigation of osmotically driven membrane processes, such as forward osmosis (FO) or osmotic distillation (OD). Despite similarities with reverse osmosis (RO) membranes, thin film composite (TFC) for FO membranes require careful design to reduce salt concentration polarization formation within the large pores composing the supporting layer. An investigation of a novel, highly stable, robust support made solely of carbon nanotubes (CNTs), which could find applications in both RO and FO was undertaken. TFC membranes were fabricated by interfacially polymerizing for the first time a dense poly(amide) (PA) layer on self-supporting bucky-papers (BPs) made of hydroxyl-functionalized entangled CNTs. These hydrophilic supports exhibited low contact angle with water (<20°), high water uptake capacity (17wt%), large porosity (>90%), making it a promising material when compared with poly(sulfone) (PSf), the traditional polymer used to fabricate TFC membrane supports in RO. In addition, the impact of the support hydrophilicity on the stability of the interfacially polymerized film and on water adsorption was investigated by oxygen-plasma treating various potential support materials, exhibiting similar geometrical properties. The morphology and salt diffusion of both CNT BP and PSf supports were investigated, and the novel BP–PA composite membranes were found to be superior to commercially available TFC membranes.

Organic fouling development in a long channel RO membrane cell

This study was to experimentally investigate organic fouling development in a 1-m long RO membrane channel using alginate as a model organic compound. Five parallel local permeate fluxes with a distance interval of 20 cm along the channel were monitored continuously during the organic filtration tests. It was found that organic fouling became more severe towards the outlet of the channel. This might be mainly attributed to the salt concentration polarization formation along the channel. The higher salt concentration downstream increased the interactions involved in organic fouling such as charge-screening and alginate-calcium bridging, which intensively promoted organic fouling formation in the downstream. A higher feed flow was a common option to mitigate fouling at most lab-scale RO research work, but not the case in this long membrane channel. A higher feed flow changed the organic fouling development profile along the channel, but would not eliminate organic fouling.

Migration of ion-exchange particles driven by a uniform electric field

A cation-selective conducting particle is suspended in an electrolyte solution and is exposed to a uniformly applied electric field. The electrokinetic transport processes are described in a closed mathematical model, consisting of differential equations, representing the physical transport in the electrolyte, and boundary conditions, representing the physicochemical conditions on the particle boundary and at large distances away from it. Solving this mathematical problem would in principle provide the electrokinetic flow about the particle and its concomitant velocity relative to the otherwise quiescent fluid.Using matched asymptotic expansions, this problem is analysed in the thin-Debye-layer limit. A macroscale description is extracted, whereby effective boundary conditions represent appropriate asymptotic matching with the Debye-scale fields. This description significantly differs from that corresponding to a chemically inert particle. Thus, ion selectivity on the particle surface results in a macroscale salt concentration polarization, whereby the electric potential is rendered non-harmonic. Moreover, the uniform Dirichlet condition governing this potential on the particle surface is transformed into a non-uniform Dirichlet condition on the macroscale particle boundary. The Dukhin–Derjaguin slip formula still holds, but with a non-uniform zeta potential that depends, through the cation-exchange kinetics, upon the salt concentration and electric field distributions. For weak fields, an approximate solution is obtained as a perturbation to a reference state. The linearized solution corresponds to a uniform zeta potential; it predicts a particle velocity which is proportional to the applied field. The associated electrokinetic flow is driven by two different agents, electric field and salinity gradients, which are of comparable magnitude. Accordingly, this flow differs significantly from that occurring in electrophoresis of chemically inert particles.