Desalination Mechanism Research Articles

Simultaneous desalination and ammonia recovery using microbial electrolysis desalination and chemical-production cell: A feasibility study of alkaline soil washing wastewater

The high concentrations of ions and ammonia nitrogen (NH3−N) in soil washing wastewater can remarkably affect ecosystem balance. This study aims to prove the feasibility of using alkaline soil washing wastewater as inlet water of microbial electrolysis desalination and chemical-production cell (MEDCC) to desalinate and recover NH3-N, from the views of performance and mechanism. The system could obtain desalination, and NH3-N removal efficiencies of 73 ± 7% and 85 ± 3%, respectively, within 12 h at the maximum current density of 9.8 ± 0.1 A/m2. Importantly, 82.5% of the removed NH3-N could be recovered in the form of NH3·H2O. The addition of electrochemically active bacteria in the anode chamber of MEDCC improved desalination and NH3-N recovery efficiencies by strengthing electric field. Bioenergy provided 58% of the total energy consumption, forming an energy-saving method for soil washing wastewater treatment. Alkalization pretreatment and acid immersion could reduce precipitation from membranes to improve desalination performance and reduce energy consumption. From the standpoint of the mechanism of desalination and NH3-N recovery, the MEDCC is expected to have the potential to simultaneously complete desalination and NH3-N recovery in alkaline wastewater with low energy consumption.

Three-dimensional hierarchical Na3Fe2(PO4)3/C with superior and fast sodium uptake for efficient hybrid capacitive deionization

Using Faradaic electrodes instead of carbon electrodes to assemble hybrid capacitive deionization (HCDI) is an effective strategy to solve the shortcomings of traditional carbon-based CDI with low salt adsorption capacity (SAC). Herein, high HCDI desalination performance of Na3Fe2(PO4)3/Carbon-X (NFeP/C-X) is attested by designing and preparing three-dimensional (3D) hierarchical porous architecture via a sol-gel-calcination method. The NFeP nanoparticles with small size are uniformly embedded in interconnected carbon nanosheets, and then a convenient nanochannels and 3D conductive carbon framework is formed. This fine structure can not only improve the interface area between electrode and electrolyte, but also shorten the ion transmission distance and ensure the rapid transfer of ions and electrons, thus achieving ultra-high desalination performance. As expected, the as-prepared NFeP/C-750 shows an ultrahigh SAC of 94.5 mg g−1, an ultrarapid desalination rate of 22.5 mg g−1 min−1 and a stable regeneration desalination ability. In addition, the desalination mechanism and ion diffusion kinetics of the NFeP/C-750 was deeply studied by ex-situ structure analysis, electrochemical characterization and density functional theories (DFT) calculations. These results demonstrate that the NFeP/C-750 with excellent desalination ability is a feasible material and offers great potential for HCDI desalination.

Spinel ZnCo2O4 Electrode for Capacitive Desalination: Understanding the Enhanced Capacitive Desalination Performance of Spherical ZnCo2O4 Electrode (Adv. Mater. Interfaces 11/2021)

The ball-like spinel ZnCo2O4 (ZCO) electrode is proposed for capacitive desalination in article number 2100125 by Wei Si and Haibo Li. It exhibits prominent sodium-ion adsorption capacity, such as ultra-high specific capacity and remarkable cyclic stability. Most significantly, the desalination mechanism as well as the preferential removal behavior of Na+, K+, and Mg2+ by ZCO electrode is explored.

Understanding the Enhanced Capacitive Desalination Performance of Spherical ZnCo2O4 Electrode

AbstractIn this work, the spinel ZnCo2O4 (ZCO) prepared by a simple strategy is employed for capacitive deionization (CDI) towards enhanced desalination performance. The ZCO reveals typical spherical structure that is beneficial in providing 3D tunnels for ions diffusion. On the basis of electrochemical measurements, the ZCO electrode possesses specific capacitance of 307.9 F g−1 in 1 m NaCl solution at the scan rate of 1 mV s−1. Remarkably, the equivalent series resistance and the Warburg impedance of ZCO electrode is only 0.54 and 0.66 Ω, respectively. Such low resistivity is conducive to improve the efficiency of charge transfer and the driven energy utilization. Regarding the desalination, the ZCO electrode illuminates the salt removal capacity of 39.4 mg g−1 in 2000 µS cm−1 NaCl solution at 1.2 V. Under the same experimental conditions, the ZCO electrode demonstrates a high desalination capacity of 73.39 mg g−1 with charge efficiency of 76.8% at a constant current of 10 mA g−1. Moreover, the preferential removal of Na+, K+, and Mg2+ enabled by ZCO electrode is investigated, suggesting the radius of hydration ions plays a significant role. Beyond that, the analysis on the morphology, crystal phase, and valance state of ZCO electrode at different desalination/salination state are explored to propose the possible desalination mechanism.

Engineering of 3D NaxCoO2 nanostructures for enhanced capacitive deionization: performance and mechanism

The morphology and capacitive desalination mechanism of 3D Na0.6CoO2.

Effect of hydrophilicity on ion rejection of sub-nanometer pores

Ion rejection by (sub)nanometer-sized pores enables membrane-based desalination. Narrowing the pore size is long considered as the primary choice to increase the ion rejection rate, which is unfortunately at much sacrifice of the water permeance. Using the uniform sub-nanometer pore channels in covalent organic frameworks (COFs) as the model system, we herein demonstrate that hydrophilicity of pore walls plays a significant role in determining ion rejections, and ion rejections can be significantly improved by enhancing hydrophilicity with less loss in water permeance. Via non-equilibrium molecular dynamics simulation, the augmented in-pore effect caused by significant pore-wall-involved hydration effect is discovered to dominate the ion rejections. Furthermore, the desalination mechanism of hydrophilic nanopores is proposed: strong hydrophilicity can make cations enter nanopores more easily than anions by compensation of ionic hydrations, but all ions are not able to transport inside nanopores due to the extreme in-pore effect; hence membranes consequently carry positive net charges after being saturated adsorption by ions and then in turn exclude cations by electrostatic repulsions. These findings and understandings on desalination mechanisms can also be applicable to other kinds of nanoporous materials and thus could provide guidelines for the experimental design of next-generation desalination membranes.

Elucidation of the desalination mechanism of solvent extraction method through molecular modeling studies

Desalination by solvent extraction is a simple and cost-effective technology to replace current ones. Although desalination solvents are continually developed, there is still a lack of understanding of the mechanism. In this study, we elucidate the solvent extraction desalination mechanism at the atomic level using molecular dynamics(MD) simulations. Three organic solvents octylamine(8A), dibutylamine(DBA), and 2-ethylhexylamine(EHA), which all have the same chemical composition but the amine group at different positions, were used. We simulated the desalination process from the brine-and-organic-solvents-mixing step to the brine-separation step. The MD simulations showed that DBA and EHA formed planar clusters while 8A formed gel clusters. By analyzing MD trajectories, we identified two types of water recovery mechanism, in which DBA and EHA clusters selectively absorb water and 8A traps brine by forming a gel structure. The polar interactions between water molecules and solvents are important driving forces for the absorption of water. To explain the desalination performances of DBA and EHA, morphological characteristics and surface polarity of the clusters were measured. It was found that a higher surface polarity facilitates more water absorption. Partial recovery of the salt ions was attributed to the surface polarity of the organic solvent clusters.

Enabling Superior Sodium Capture for Efficient Water Desalination by a Tubular Polyaniline Decorated with Prussian Blue Nanocrystals.

The application of electrochemical energy storage materials to capacitive deionization (CDI), a low-cost and energy-efficient technology for brackish water desalination, has recently been proven effective in solving problems of traditional CDI electrodes, i.e., low desalination capacity and incompatibility in high salinity water. However, Faradaic electrode materials suffer from slow salt removal rate and short lifetime, which restrict their practical usage. Herein, a simple strategy is demonstrated for a novel tubular-structured electrode, i.e., polyaniline (PANI)-tube-decorated with Prussian blue (PB) nanocrystals (PB/PANI composite). This composite successfully combines characteristics of two traditional Faradaic materials, and achieves high performance for CDI. Benefiting from unique structure and rationally designed composition, the obtained PB/PANI exhibits superior performance with a large desalination capacity (133.3 mg g-1 at 100 mA g-1 ), and ultrahigh salt-removal rate (0.49 mg g-1 s-1 at 2 A g-1 ). The synergistic effect, interfacial enhancement, and desalination mechanism of PB/PANI are also revealed through in situ characterization and theoretical calculations. Particularly, a concept for recovery of the energy applied to CDI process is demonstrated. This work provides a facile strategy for design of PB-based composites, which motivates the development of advanced materials toward high-performance CDI applications.

MXene as a Cation-Selective Cathode Material for Asymmetric Capacitive Deionization.

Capacitive deionization (CDI) has become a promising method to solve the shortage of freshwater resources recently. However, the co-ion expulsion effect obviously hinders electrosorption capacity and charge efficiency of CDI. In this work, an asymmetric CDI cell is assembled in which Na+-intercalated Ti3C2Tx (NaOH-Ti3C2Tx) serves as a cation-selective cathode, while the activated carbon (AC) serves as the anode. The NaOH-Ti3C2Tx with negatively charged surface groups (-OH, -O, and -F) is adopted to weaken the co-ion expulsion effect. Benefited from the synergistic effect of the reduced co-ion expulsion effect and expanded interlayer space, the asymmetric CDI cell achieves a higher electrosorption capacity of 12.19 mg g-1 and a higher charge efficiency of 0.826 compared with the symmetric one composed of AC (4.55 mg g-1 and 0.306) in 100 mg L-1 NaCl solution. High cyclic stability of the as-prepared asymmetric CDI cell is also observed. The improved desalination performance indicates that NaOH-Ti3C2Tx is a promising alternative as cation-selective cathode material for asymmetric CDI cells. The desalination mechanism is discussed in detail to lay the foundation for further improvement of the CDI performance of other 2D materials like MXene.

Electric field assisted desalination of water using B- and N-doped-graphene sheets: A non-equilibrium molecular dynamics study

The possibility and mechanism of water desalination using newly designed doped graphene sheets is reported. It is hereby demonstrated, through molecular dynamics (MD) simulations, that an electric field-assisted selective ion separation is possible using boron- and nitrogen-doped nano-porous graphene sheets. The sheets are shown to behave as nano-sponges, with potential future applications in water desalination by electrodialysis, for example. Ion separation is found to increase as the extent of ion hydration decreases under increasing field strengths. Boron- and nitrogen-doped graphene sheets are found to exhibit acceptable mechanical properties and, simultaneously, good ion selectivities and that they may be candidates for future technological applications in desalination and other refining processes.

Numerical Simulation of Seawater Desalination Effect in The Micro-Nanochannel

In recent years, the study on the micro-nanochannel in the microfluidic technology has been made significant progress for which when inner walls charged can function similarly as ion permselective membrane. This article constructed one symmetric model with three parallel microchannels connected with two sets of nanochannel arrays and explained the mechanism of desalination within the channel by using numerical simulation method. For example, when the voltage difference, Vcn, across the nanochannel array is equal to 10 VT, the desalting efficiency can have reached nearly 90%; as the Vcn is increased continuously to 20VT, the desalination rate will definitely exceed 99% and as the Vcn increases the rate increases more slowly.

Bio-Precipitation of Calcium and Magnesium Ions through Extracellular and Intracellular Process Induced by Bacillus Licheniformis SRB2

Removal of calcium and magnesium ions through biomineralization induced by bacteria has been proven to be an effective and environmentally friendly method to improve water quality, but the process and mechanism are far from fully understood. In this study, a newly isolated probiotic Bacillus licheniformis SRB2 (GenBank: KM884945.1) was used to induce the bio-precipitation of calcium and magnesium at various Mg/Ca molar ratios (0, 6, 8, 10, and 12) in medium with 30 g L−1 sodium chloride. Due to the increasing pH and HCO3− and CO32− concentrations caused by NH3 and carbonic anhydrase, about 98% Ca2+ and 50% Mg2+ were precipitated in 12 days. The pathways of bio-precipitation include extracellular and intracellular processes. Biominerals with more negative δ13C values (−16‰ to −18‰) were formed including calcite, vaterite, monohydrocalcite, and nesquehonite with preferred orientation. The nucleation on extracellular polymeric substances was controlled by the negatively charged amino acids and organic functional groups. The intracellular amorphous inclusions containing calcium and magnesium also contributed to the bio-precipitation. This study reveals the process and mechanism of microbial desalination for the removal of calcium and magnesium, and provides some references to explain the formation of the nesquehonite and other carbonate minerals in a natural and ancient earth surface environment.

Silica gel-based adsorption cooling cum desalination system: Focus on brine salinity, operating pressure, and its effect on performance

In this study, a prototype of an adsorption desalination (AD) system was designed and manufactured using commercially available alumina silica gel to evaluate and improve its performance. To develop the AD system, the thermophysical properties of the adsorbent were first investigated based on adsorption isotherms of nitrogen and water vapor on the adsorbent; thereby, its applicability as a candidate adsorbent was verified. The performance of the AD system was then evaluated for different types of brine feeders in the evaporator (i.e., perforated plate and spray nozzles) using tap water as a feed over a wide range of key operating conditions such as hot-, cooling-, and chilled-water temperatures. Moreover, the performance of the AD process was assessed to verify the desalination mechanism and consistent desalting performance of the AD process; the process recovered more freshwater from seawater than conventional desalination technologies. The performance of the AD system was demonstrated to be independent of the brine concentration of the evaporator; moreover, the chemical quality of the freshwater recovered was revealed to be comparable to that of deionized water. Spray-assisted evaporation exhibited high water vapor uptake owing to the effect of pressurization during the adsorption process, and hence, exhibited remarkable performance.

Desalination mechanism of modified activated carbon/carbon nanotubes composite electrode

Abstract Modified activated carbon/carbon nanotubes (AC*/CNT*) composite electrode was used as the electrode in a capacitive deionization (CDI) process for desalination in this study. The morphology and electrochemical characteristics of the modified electrode were discussed, and the results showed that after modification, the specific surface area of AC* reached 672.48 m2/g, increased by 29.43%; while the specific surface area of CNT* was 117.39 m2/g, reduced by 9.94% due to the strong oxidation of the mixed acid, the pore volume of CNT* increased by 48.28%. The electrode regeneration test proved that the electrode had good cycling stability. The pseudo-first-order kinetic model could better describe the adsorption rate of the electrodes for ions and the desalination ratio of the AC*/CNT* electrode reached 7.11 mg/g; the Langmuir model could well describe the adsorption mechanism of capacitive deionization, and indicated that the adsorption process of CDI was near to single ion layer adsorption; the change trend of electric mobility with migration time could be well fitted by exponential equations. This study explored a novel composite electrode coating, and initially explored the behavioral characteristics and trends of CDI technology.

Demulsification using an electric field after crude oil desalination by ethylene glycol extraction

ABSTRACTSolvent desalination with preferable economic efficiency is becoming an emerging technology. However, emulsion is still a tricky problem. This paper tried to apply electric field in crude oil desalination with ethylene glycol for the purpose of ameliorating emulsion and promoting a desalination rate. It was found that the salt content after primary electric desalination was 7.5 mg/L and the desalination rate approached 84% at 120 °C and 1,000 V/cm. The determination of rheological property and light transmittance rate was done to investigate the effect of field intensity and temperature on demulsification. The analysis of desalination mechanism indicated that dipolar and dielectrophoretic force played important roles in demulsification.

New insights into the mechanism of flow-electrode capacitive deionization

We report on Faradaic reactions producing H+ (anode) and OH– (cathode) in flow-electrode capacitive deionization (FCDI) operated at 1.2V. These reactions underline an additional electrodialytical desalination mechanism within capacitive deionization, which proceeds in parallel to the known electrosorption mechanism. Examination of flow-electrodes (100ml each, 5% (wt) activated carbon) during FCDI (121cm2 effective membrane area) of 150ml, 4g/l NaCl solution revealed that significant amounts of Na+ and Cl− ions (up to 50% and 30% of Cl− and Na+, respectively) were not adsorbed in the activated carbon particles but were rather dissolved in the aqueous phase of the flow-electrodes. Production of acid (resulting in pH≈1.5) and base (pH≈12.5) in the flow-anode and -cathode solutions was observed during the operation. Reverse pH behaviors were obtained during the regeneration of the flow-electrodes by potential reversal. pH neutralization of the flow-electrode solutions resulted in a sharp increase in both the desalination rate and the electric current of the FCDI cell. Reacting NaOH and HCl in a short-circuited FCDI cell resulted in NaCl production in the water compartment and pH neutralization of both flow-electrodes.Apparently reversible Faradaic reactions that occur on the flow electrodes in the FCDI can be dependent on the properties of the carbon material, electrolyte composition and applied operational parameters (e.g. cell potential) and need to be studied in further detailed investigations.

Computer simulation of water desalination through boron nitride nanotubes.

Development of high-efficiency and low-cost seawater desalination technologies is critical to solving the global water crisis. Herein we report a fast water filtering method with high salt rejection by boron nitride nanotubes (BNNTs). The effect of the radius of BNNTs on water filtering and salt rejection was investigated by molecular dynamics (MD) simulation. Our simulation results demonstrate that fast water permeation and high salt rejection could be achieved by BNNT(7,7) under both high pressure and low pressure. The potential of mean force (PMF) of Na+ ion and water molecule through BNNT(7,7) further revealed the mechanism of seawater desalination by BNNT(7,7). Using BNNT(7,7) array, a 10 cm2 nanotube membrane with 1.5 × 1013 pores per cm2 will produce freshwater with a flow rate of 98 L per day per MPa under 100 MPa. Our study shows the potential application of BNNTs membrane for fast and efficient desalination.

Molecular mechanisms underlying active desalination and low water permeability in the esophagus of eels acclimated to seawater.

Marine teleosts can absorb imbibed seawater (SW) to maintain water balance, with esophageal desalination playing an essential role. NaCl absorption from luminal SW was enhanced 10-fold in the esophagus of SW-acclimated eels, and removal of Na+ or Cl- from luminal SW abolished the facilitated absorption, indicating coupled transport. Mucosal/serosal application of various blockers for Na+/Cl- transporters profoundly decreased the absorption. Among the transporter genes expressed in eel esophagus detected by RNA-seq, dimethyl amiloride-sensitive Na+/H+ exchanger (NHE3) and 4,4'-diisothiocyano-2,2'-disulfonic acid-sensitive Cl-/[Formula: see text] exchanger (AE) coupled by the scaffolding protein on the apical membrane of epithelial cells, and ouabain-sensitive Na+-K+-ATPases (NKA1α1c and NKA3α) and diphenylamine-2-carboxylic acid-sensitive Cl- channel (CLCN2) on the basolateral membrane, may be responsible for enhanced transcellular NaCl transport because of their profound upregulation after SW acclimation. Upregulated carbonic anhydrase 2a (CA2a) supplies H+ and [Formula: see text] for activation of the coupled NHE and AE. Apical hydrochlorothiazide-sensitive Na+-Cl- cotransporters and basolateral Na+-[Formula: see text] cotransporter (NBCe1) and AE1 are other possible candidates. Concerning the low water permeability that is typically seen in marine teleost esophagus, downregulated aquaporin genes (aqp1a and aqp3) and upregulated claudin gene (cldn15a) are candidates for transcellular/paracellular route. In situ hybridization showed that these upregulated transporters and tight-junction protein genes were expressed in the absorptive columnar epithelial cells of eel esophagus. These results allow us to provide a full picture of the molecular mechanism of active desalination and low water permeability that are characteristic to marine teleost esophagus and gain deeper insights into the role of gastrointestinal tracts in SW acclimation.

Toward high permeability, selectivity and controllability of water desalination with FePc nanopores.

Nanoporous materials exhibit promising potential in water transportation applications, especially in ocean water desalination. It is highly desired to have great permeability, selectivity and controllability in the desalination performance of these nanopores. However, it is still a challenge to achieve all three features in one material or device. Here, we demonstrate efficient and controllable water desalination with a nanoporous 2D Fe phthalocyanine (FePc) membrane using molecular dynamics simulations. We find the FePc membrane not only conducts fast water flow, but it also suppresses ion permeation. The selectivity is attributed to a mechanism distinct from the traditional steric exclusion: cations are excluded due to electrostatic repulsion, whereas anions can be trapped in the nanopore and induce the reorganization of ions in the vicinity of the nanopore, which in turn creates a tendency for the trapped anions to move back into the saline reservoir. More interestingly, we find such mechanism is largely due to the sufficiently strong electrostatic interaction of the charged nanopore region with ions and is not restricted to the FePc nanopore. In addition, the number of protonated nitrogen atoms in FePc pores can be modulated by adjusting the pH value of the solution. The extent of the anion occupancy can thus be regulated, giving rise to control of the water flow. Taken together, great permeability, selectivity and controllability can be achieved with this nanosheet system. Moreover, our study suggests there is an alternative mechanism of water desalination which may be realized by intrinsically nanoporous materials such as FePc membranes.

Pyridinic nitrogen doped nanoporous graphene as desalination membrane: Molecular simulation study

Functionalized nanoporous graphene has shown great promise as emerging membrane. Herein, we computationally demonstrate the separation performance using pyridinic-like nitrogen doped nanoporous graphene as reverse osmosis desalination membrane. The water permeation and salt rejection of the functionalized graphene membranes with various nitrogen-doping levels were simulated and characterized. We show that all functionalized graphenes investigated in this work exhibit higher water flux and acceptable salt rejection. In particular, the NOH graphene membrane with partial hydroxyl group inclusion shows excellent desalination efficiency. The interfacial properties of water and ions, as well as their free energy landscapes in passing through the graphene nanopores have been simulated in order to explore the desalination mechanism. The moderate free energy barriers for water passages confirm larger water fluxes in the functionalized graphene membranes. It was revealed that the salt rejection for the functionalized graphenes is the pore size exclusion of hydrated ions and the charged repulsion from pore surfaces is not the main factor. Overall, our results indicate that pyridinic-like nitrogen doped graphenes have a significant potential as nanostructured desalination membranes.