Adsorption Of Salt Ions Research Articles

Data‐Driven Strategies for Accelerated Structural Exploration of High‐Performance 2D Carbon‐Based Seawater Desalination Membranes

The insufficiency of freshwater supplies has posed a serious threat to sustainable socioeconomic growth, and seawater desalination is considered to be the most promising solution to alleviate such pressure. Currently, 2D carbon membranes are identified as deserving candidates due to their high permeability and multiple tunable properties. However, they remain challenging to systematically uncover the potential relationships between structures and properties in various 2D carbon materials. For this, a machine learning (ML) model based on feature datasets of 2D carbon materials effecting desalination properties is trained. The results suggest that structures with a maximum pore size of 10–12 atoms and atomic densities between 0.28 and 0.41 are more likely to achieve high properties. Cml‐MOR based on MOR‐type mordenite zeolite for validation is selected. Further, Cml‐MOR is demonstrated to feature remarkable salt ion adsorption. The effective water flux of Cml‐MOR is 113.51 L cm−2 day−1 MPa−1, and the salt rejection at 110 MPa can reach 98.9%. This work is expected to apply this efficient method to investigate the structure and properties of 2D carbon membranes with great structural diversity; this will attract more people to focus on them and explore their important potential for practical applications.

Regulation Mechanism of Special Functional Groups Contained in Polymer Molecular Chains on the Tribological Properties of Modified Ti6Al4V.

Polymer coatings can effectively improve the surface tribological properties of human implant materials, thereby increasing their service life. In this study, poly(vinylsulfonic acid, sodium salt) (PVS), poly(acrylic acid) (PAA) and poly(vinylphosphonic acid) (PVPA) were used to modify Ti6Al4V surfaces. Experimental analyses were combined with molecular simulation to explore the regulation mechanism of special functional groups contained in polymer molecular chains on the tribological properties of modified surfaces. In addition, the bearing capacities and velocity dependence of different polymer modified surfaces during friction were also explored. The PVS coating, due to physical adsorption, can have an anti-friction effect under NaCl solution lubrication, but is not durable under long-term or repeated usage. Both PAA and PVPA molecular chains can form chemical bonds with Ti6Al4V. Phosphate acid groups can firmly bind to the substrate, and the adsorption of salt ions and water molecules can form a hydrated layer on the PVPA coating surface, achieving ultra-low friction and wear. The adsorption of salt ions would aggravate the surface wear of the PAA-modified Ti6Al4V due to the unfirm binding of carboxyl groups to the substrate, resulting in a high friction coefficient. This study can provide effective guidance for the design of modified polymer coatings on metals.

Molten salt synthesis, morphology modulation, and lithiation mechanism of high entropy oxide for robust lithium storage

High entropy oxides (HEOs) with ideal element tunability and enticing entropy-driven stability have exhibited unprecedented application potential in electrochemical lithium storage. However, the general control of dimension and morphology remains a major challenge. Here, scalable HEO morphology modulation is implemented through a salt-assisted strategy, which is achieved by regulating the solubility of reactants and the selective adsorption of salt ions on specific crystal planes. The electrochemical properties, lithiation mechanism, and structure evolution of composition- and morphology-dependent HEO anode are examined in detail. More importantly, the potential advantages of HEOs as electrode materials are evaluated from both theoretical and experimental aspects. Benefiting from the high oxygen vacancy concentration, narrow band gap, and structure durability induced by the multi-element synergy, HEO anode delivers desirable reversible capacity and reaction kinetics. In particular, Mg is evidenced to serve as a structural sustainer that significantly inhibits the volume expansion and retains the rock salt lattice. These new perspectives are expected to open a window of opportunity to compositionally/morphologically engineer high-performance HEO electrodes.

Great truths are always simple: Preoxidation-tuned cellulose-derived carbon fibers achieving ultrahigh areal desalination capacity in flow-through capacitive deionization

Constructing thick electrodes with both high strength and porous characteristics is challenging but critical to improve the desalination performance of flow-through capacitive deionization (FTCDI). Different from the most reported electrodes composed of carbon nanomaterials, micron-sized carbon fibers are more advantageous due to the innate excellent mechanical stability and macroscopic porous fibrous structure. Herein, we reported the preoxidation-tuned cellulose-derived carbon fibers (POCF) with improved specific surface area, pore volume, hydrophilicity, and carbon defect degree by precisely-controlled preoxidation process, resulting in enhanced electrochemical activity and sites. Further studies of the cellulose pyrolysis mechanism disclose that preoxidation temperature has a crucial influence on the carbon yield and electrical conductivity, which significantly affect the desalination capacity of the integrated (current collector-free and binder-free) POCF electrode. Compared with the conventional activated carbon-coated electrode and commercial activated carbon fibers cloth electrode, the POCF electrode exhibits outstanding desalination performance due to the synergistic effect of high active materials loading and good electrical conductivity. Most importantly, the desalination capacity of POCF electrode realizes the linear growth with 4-fold enhancement by increasing the thickness and remains almost unchanged even up to a large size of 120 cm2, indicating unlimited ion diffusion and excellent scalability. Impressively, an ultrahigh areal salt ion adsorption capacity of 0.76 mg cm−2 is achieved for the 2.4 mm thick POCF electrode. Our work successfully validates the feasibility and superiority of the POCF in constructing thick electrodes for achieving high areal desalination performance.

Data-Driven Design of High-Performance Graphene-Based Seawater Desalination Membranes

Freshwater scarcity has emerged as a prominent bottleneck for global sustainable development, and all countries are sharing a serious reality. Currently, the search for a high-performance, two-dimensional (2D) desalination membrane remains a tremendous challenge. A high-throughput screening involving density functional theory–machine learning (DFT-ML) framework is considered to be an essential avenue to tackle this dilemma. Here, desalination membranes with desirable salt ion retention, water flux, mechanical properties, and service life are selected by particle swarm algorithms. Subsequently, a typical screened-out structure is verified by multiple simulation technology. The results of DFT and ML show that the 2D desalination membranes can be ingeniously singled out by energy density (8.4–8.8 eV/atom), adsorption energy (<−0.5 eV), and mechanical properties parameters (C(θ) > 32 GPa, v(θ) > 0.25). To test the effectiveness of this high-throughput screening method, a 2D desalination membrane (named Dadri-C) was investigated in detail by first-principles and molecular dynamics. We found that Dadri-C has favorable mechanical, thermodynamic, and dynamical stabilities. Dadri-C features sufficient salt ion adsorption sites and flexible electronic structure and confirms excellent selectivity. Further, it exhibits 100% Cl– salt rejection efficiency in the simulation, while achieving a 98.2% Na+ rejection efficiency even at 90 MPa. The maximum water flux obtained while maintaining efficient salt rejection amounts to 525.9 L·cm–2·day–1·MPa–1. Through elaborate screening, we also discovered that the mechanical properties of Dadri-C are acceptable. It also features a self-cleaning function conducive to recycling, indicating that the screening strategy fulfilled the intended target. This proposed research could introduce innovative ideas for efficient screening and design of desalination membrane, which is expected to confer significant opportunities in promoting desalination technology.

Electronic-level deciphering of the desalination mechanism of high-performance graphenylene membranes

The insufficiency of available freshwater has surged as a major global concern in recent years, and the issue is raising ever more awareness. Meanwhile, the recent experimental synthesis of graphenylene has inspired researchers to pursue its applicative merits. The related properties regarding lithium-ion storage electrodes and gas separation membranes of graphenylene have been reported. However, the suitability of graphenylene as a desalination membrane has yet to be fully established. In the present study, with a view to delineating the action mechanism of graphenylene desalination membrane, we have dissected its structure and properties through first-principles and molecular dynamics. The results illustrate that graphenylene comprising dodecagonal, hexagonal, and tetragonal carbon rings contains 0.55 nm nanopores, which should permit outstanding desalination properties. In addition, the periodic pore distribution should support stress dispersion in service to preserve membrane integrity. The low water-molecule free-energy barrier, remarkable salt ion adsorption properties, and favorable anti-agglomeration ability endow graphenylene with desirable permeability and selectivity. Furthermore, we demonstrate that graphenylene is a 2D Dirac semi-metallic material. The interlinked electronic structure of which confers it with self-cleaning behavior, facilitating the restoration of water flux. The present research has elucidated the mechanism whereby effective desalination may be achieved on graphenylene membranes. Further, it has clarified the potential applicability of such membranes. They show good prospects for the advancement of seawater desalination technology.

Phase stability of aqueous mixtures of bovine serum albumin with low molecular mass salts in presence of polyethylene glycol

The stability of bovine serum albumin (BSA) solutions against phase separation caused by cooling the system is studied under the combined influence of added poly(ethylene glycol) (PEG) and alkali halide salts in water as solvent. The phase stability of the system depends on the concentration of the added PEG and its molecular mass, the concentration of the low molecular mass electrolyte and its nature, as also on the pH of the solution. More specifically, the addition of NaCl to the BSA-PEG mixture promotes phase separation at pH=4.0, where BSA carries the net positive charge in aqueous solution, and it increases the stability of the solution at pH=4.6, i.e., near the isoionic point of the protein. Moreover, at pH=4.6, the cloud-point temperature decreases in the order from NaF to NaI and from LiCl to CsCl. The order of the salts at pH=4.0 is exactly reversed: LiCl and NaF show the weakest effect on the cloud-point temperature and the strongest decrease in stability is caused by RbCl and NaNO3. An attempt is made to correlate these observations with the free energies of hydration of the added salt ions and with the effect of adsorption of salt ions on the protein surface on the protein–protein interactions. Kosmotropic salt ions decrease the phase stability of BSA-PEG-salt solutions at pH<pI, while exactly the opposite is true at pH=pI.

Exceptional capacitive deionization desalination performance of hollow bowl-like carbon derived from MOFs in brackish water

As a bright technology for desalinating brackish water, the desalination ability of capacitive deionization (CDI) has been limited by the electrode materials. However, it was still a challenge to adjust the construction of CDI electrode material to improve the salt removal ability. Here, nitrogen-doped hollow bowl-like carbon (HBC) has been successfully constructed in ZIF-8 by facile templating approaches. Physical measurements displayed that HBC had a hollow bowl-like structure, a certain number of nitrogen-containing groups and high specific surface area (1244 m2 g−1). These unique features made HBC have high capacitance (210.7F g−1), remarkable cycle stability and low charge transfer resistance in electrochemical double layer capacitors, indicating its promising application in CDI desalination. Due to the abundant ion accumulation active sites and rapid ion diffusion of HBC materials, provided by unique characteristics of regular hollow bowl-like structure, microporous dominated structure, interconnected channels and nano-scale wall thickness, HBC showed high salt removal ability of 28.06 mg g−1 with 1.2 V in 0.5 g L-1 NaCl solution. Besides, HBC exhibited excellent average salt ion adsorption rate (1.56 mg g−1 min−1) and initial salt adsorption capacity of 90% after 20 cycles.

Radial Movement Optimization Based Optimal Operating Parameters of a Capacitive Deionization Desalination System

The productivity of the capacitive deionization (CDI) system is enhanced by determining the optimum operational and structural parameters using radial movement optimization (RMO) algorithm. Six different parameters, i.e., pool water concentration, freshwater recovery, salt ion adsorption, lowest concentration point, volumetric (based on the volume of deionized water), and gravimetric (based on salt removed) energy consumptions are used to evaluate the performance of the CDI process. During the optimization process, the decision variables are represented by the applied voltage, capacitance, flow rate, spacer volume, and cell volume. Two different optimization techniques are considered: single-objective and multi-objective functions. The obtained results by RMO optimizer are compared with those obtained using a genetic algorithm (GA). The results demonstrated that the RMO optimization technique is useful in exploring all possibilities and finding the optimum conditions for operating the CDI unit in a faster and accurate method.

Supercritical CO2 Effects on Calcite Wettability: A Molecular Perspective

The wettability behavior of reservoir rocks plays a vital role in determining CO2 storage capacity and containment security. Several experimental studies characterized the wettability of CO2/brine/rock systems for a wide range of realistic conditions. To develop a fundamental understanding of the molecular mechanisms responsible for such observations, the results of molecular dynamics simulations, conducted at atomistic resolution, are reported here for representative systems in a wide range of pressure and temperature conditions. Several force fields are considered, achieving good agreement with experimental data for the structure of interfacial water but only partial agreement in terms of contact angles. In general, the results suggest that, at the conditions chosen, water strongly wet calcite, resulting in water contact angles either too low to be determined accurately with the algorithms implemented here or up to ∼46°, depending on the force field implemented. These values are in agreement with some, but not all experimental data available in the literature, some of which report contact angles as high as 90°. One supercritical CO2 droplet was simulated in proximity of the wet calcite surface. The results show pronounced effects due to salinity, which are also dependent on the force field implemented to describe the solid substrate. When the force field predicts complete water wettability, increasing NaCl salinity seems to slightly increase the calcite affinity for CO2, monotonically as the NaCl concentration increases, because of the preferential adsorption of salt ions at the water–rock interface. When the other force field was implemented, it was not possible to quantify salt effects, but the simulations suggested strong interactions between the supercritical CO2 droplet and the second hydration layer on calcite. The results presented could be relevant for predicting the longevity of CO2 sequestration in geological repositories.

Towards Electrochemical Water Desalination Techniques: A Review on Capacitive Deionization, Membrane Capacitive Deionization and Flow Capacitive Deionization.

Electrochemical water desalination has been a major research area since the 1960s with the development of capacitive deionization technique. For the latter, its modus operandi lies in temporary salt ion adsorption when a simple potential difference (1.0–1.4 V) of about 1.2 V is supplied to the system to temporarily create an electric field that drives the ions to their different polarized poles and subsequently desorb these solvated ions when potential is switched off. Capacitive deionization targets/extracts the solutes instead of the solvent and thus consumes less energy and is highly effective for brackish water. This paper reviews Capacitive Deionization (mechanism of operation, sustainability, optimization processes, and shortcomings) with extension to its counterparts (Membrane Capacitive Deionization and Flow Capacitive Deionization).

Seawater Desalination Using MOF-Incorporated Cu-Based Alginate Beads without Energy Consumption.

The demand for fresh water is gradually and globally increasing due to the growth of population and water contamination. To meet this global demand, we fabricated metal-organic framework (MOF)-incorporated Cu-based alginate beads (Cu-MOF-Alg beads) for effective removal of water-dissolved salt ions from seawater. Alginic acid formed a matrix for preconfined Cu coordination. In this matrix, the MOFs were successfully in situ synthesized with organic ligands. The as-prepared Cu-MOF-Alg beads exhibited exceptional salt ion adsorption with the extraction of sedimented MOF particles. The adsorption characteristics of the fabricated Cu-MOF-Alg beads exhibited a linear isotherm according to the concentration of Na+ ions. In addition, the beads could be applied over a wide concentration range of target solutions and maintained their ion adsorption capacity after three repeated uses. The beads exhibited rapid adsorption kinetics, and their salt ion removal rate was approximately 94.3% through a multistage adsorption process. The MOF-treated seawater, which was desalinated to a low concentration of 1000 ppm, was filtered by a mangrove-inspired membrane, which yielded a total ion removal rate of 98.1%. Considering the low material cost compared to other adsorption-based desalination techniques and the absence of external energy supply, the proposed hybrid desalination system can produce purified water at an extremely low cost. Thus, this desalination technique could be economically and ecofriendly utilized for practical seawater desalination in a facile manner.

Capacitance of electrical double layer formed inside a single infinitely long cylindrical pore

By using classical density functional theory, we investigate the capacitance of a primitive model and an extended primitive model electrical double layer (EDL) formed in a single infinitely long cylindrical pore. We obtain dependencies of the EDL differential capacitance on the parameters characterizing the electrolyte-electrode system. It is shown that (i) high electrical valence and small size of counter-ions help to raise the and the two factors have a synergistic effect; the increases with the co-ion size at high bulk concentrations, but is not sensitive to the co-ion size at lower concentrations. (ii) The curve of the versus (surface charge density) moves upwards completely with the cylindrical pore radius regardless of the electrolyte types and bulk concentrations and whether the curve is a camel-shaped or bell-shaped curve. (iii) At low concentrations, the curve is camel shaped regardless of the electrolyte types and pore sizes, and a transition from a camel-shaped to bell-shaped curve will eventually occur when the bulk concentration rises to an appropriate value. Moreover, the high electrical valence tends to delay the transition and raise the transition concentration. (iv) In the case of the camel-shaped curve, the tends to decrease with the after the curve goes beyond its maximal value; the surface charge strength (corresponding to the maxima) increases with the counter-ion valence and pore size, but decreases with the bulk concentration. However, the concentration effect is very small in the presence of only one valence counter-ion, whereas it will become obvious with the presence of two valence counter-ions. (v) In determining the shape of the curve, increasing the strength of an attractive interionic neutral and non-HS interaction has the same effect as decreasing the bulk concentration, i.e. it makes the characteristics of the camel-shaped curve more obvious. Conversely, the incorporation of a repulsive interionic neutral and non-HS interaction transforms an originally not obvious camel-shaped curve into a bell-shaped curve. The above observations can be explained self-consistently by analyzing the adsorption of salt ions and the resulting screening effect and their influencing factors.

Electrowetting on 2D dielectrics: a quantum molecular dynamics investigation

Electrowetting on dielectrics (EWOD) is widely used to manipulate the spreading of a conductive liquid on a dielectric surface by applying an electric field. 2D hydrophobic dielectrics are promising candidates for EWOD applications. In this study, extensive quantum molecular dynamics (MD) simulations are performed to investigate the electrowetting behavior of salty water on hexagonal boron nitride (h-BN) monolayer. The proximal adsorption of salt ions and the associated realignment of the dipole moments of interfacial water with the applied electric field are found to be the physical origin of the electrowetting behavior. At low salt concentration and low electric fields, the proximal adsorption and the realignment follow the applied electric field, and the cosine of the water contact angle (WCA) follows a quadratic dependence on the applied electric field. At high salt concentration and high electric fields, the proximal adsorption saturates, which restricts further realignment and causes a saturation of the WCA. This case study provides physical insights into the much debated mechanism that underlies the contact angle saturation (CAS) found in macroscopic electrowetting phenomena and also provides an avenue for further studies of electrowetting at the atomic scale.

Characteristics of Nitric Acid‐Modified Carbon Nanotubes and Desalination Performance in Capacitive Deionization

AbstractMultiwalled carbon nanotubes (MWCNTs) were treated by nitric acid to probe the effect of nitric acid modification on their properties and desalination performance in capacitive deionization (CDI). The nitric acid modification exerts a slight influence on the morphologies, specific surface area, and pore properties of the MWCNTs but can increase the amount of oxygen‐containing functional groups and then obviously enhance the specific capacitance of the electrodes. Thus, the desalination efficiency can be significantly improved as the modified MWCNTs serve as electrodes in CDI. The adsorption of salt ions on the original and modified MWCNT electrode is fitted well by the Freundlich isotherm other than the Langmuir isotherm.

Parameter-based performance evaluation and optimization of a capacitive deionization desalination process

Capacitive deionization (CDI) is an emerging alternative desalination technology that electrochemically purifies brackish water using electrically polarized capacitive electrodes. This research focuses on the performance of the CDI system. A number of performance criteria were used to assess the desalination system based on the requirements. The performance of the CDI system was assessed in terms of lowest effluent water concentration during the deionization process (mM), specific energy consumption either per gram of salt adsorbed (kJ/g) or per liter of fresh water recovered (J/L), accumulated desalinated water concentration (ppm), salt ions adsorbed in electrodes (g), and the volume of freshwater recovered (L) for different operating parameters. Furthermore, the performance of the desalination system was optimized based on operating parameters of flow rate, applied voltage, cell volume, and capacitance of the CDI cell. Three optimization techniques were suggested according to desalination requirements. Single-objective and multi-objective genetic algorithms (GA) were used to optimize the performance of the CDI system subject to constrained decision variables. The feasible solution obtained through GA optimization showed significant improvement in CDI system performance. Furthermore, the optimized results suggest different optimal solutions based on specific needs, such as maximum salt ion adsorption, lowest desalination energy consumption, high volume of desalinated water, or purest water extracted.

Capacitive neutralization deionization with flow electrodes

Capacitive neutralization deionization with flow electrodes (FCND) was proposed by combining neutralization dialysis (ND) and flow-electrode capacitive deionization (FCDI). Its key property is that by employing flow electrodes, capacitive adsorption of salt ions occurs simultaneously during the neutralization dialysis process. To compare the desalination performance between FCND and FCDI, a series of experiments were conducted by changing salt concentration and applied electrical voltage. The hybrid exhibits a promising average salt adsorption rate (ASAR) and salt removal efficiency (SRE) compared to either ND or FCDI process. The ASAR and SRE of FCND are 203.27mmolm−2min−1 and 72.20% in 0.1mM NaCl solution at 1.2V after 120min, whereas the values of FCDI are 120.86mmolm−2min−1 and 42.92%, respectively. In order to facilitate the understanding of the ion transportation in this complex system, a theory was also set up based on simplified Nernst-Planck equation and Donnan equilibrium, which helps to understand the experimental data.

Controlling swelling/deswelling of stimuli-responsive hydrogel nanofilms in electric fields.

The swelling/deswelling transition of pH-sensitive, electrode-grafted, hydrogel nanofilms when exposed to electric fields is studied by theoretical analysis. In acidic conditions, the response of these films to changes in pH is dominated by network-surface interactions, while intra-network electrostatic repulsions, which are highly modulated by the adsorption of salt ions, determine material response at a higher pH. Film thickness is a non-monotonic function of solution pH and displays a local maximum, a local minimum or both, depending on the salt concentration and the applied voltage. We suggest the use of these materials in the development of biosensors and control of enzyme activity.

Effects of Pore Structure on the High-Performance Capacitive Deionization Using Chemically Activated Carbon Nanofibers

Capacitive deionization (CDI) electrodes were constructed from activated carbon fibers prepared using electrospinning and chemical activation. The CDI efficiencies of these electrodes were studied as a function of their specific surface areas, pore volumes and pore sizes via salt ion adsorption. The specific surface areas increased approximately 90 fold and the pore volume also increased approximately 26 fold with the use of greater amounts of the chemical activation agent. There was a relative increase in the mesopore fraction with higher porosity. A NaCI solution was passed through a prepared CDI system, and the salt removal efficiency of the CDI system was determined by the separation of the Na+ and Cl- ions toward the anode and cathode. The CDI efficiency increased with greater specific surface areas and pore volumes. In addition, the efficiency per unit pore volume increased with a reduction in the micropore fraction, resulting in the suppressed overlapping effect. In conclusion, the obtained improvements in CDI efficiency were mainly attributed to mesopores, but the micropores also played an important role in the high-performance CDI under conditions of high applied potential and high ion concentrations.

Solution processable ionic p-i-n phosphorescent organic light-emitting diodes

We report efficient light-emission from solution-processed single-layered phosphorescent organic light-emitting diodes (PHOLEDs) that were doped with ionic salt and treated with simultaneous electrical and thermal annealing. Because the simultaneous annealing causes the adsorption of salt ions at the electrode surfaces, the energy levels of the organic molecules are bent by the electric fields due to the adsorbed ions; i.e., the annealing can induce the proper formation of an ionic p-i-n structure. As a result, an ionic p-i-n PHOLED with a peak luminescence of over ∼35000cd∕m2 and a power efficiency of 42lm∕W was achieved through increased and balanced carrier injections.