Desalination Tests Research Articles

MXene nanocomposite nanofiltration membrane for low carbon and long-lasting desalination

Current membranes have limitations in permeability and selectivity, and require long-time stability, which drive the development of high-performance nanofiltration (NF) membranes. MXene is a new type of highly hydrophilic and negatively charged two-dimensional (2D) nanosheets material, especially its supernatant has good dispersibility and stability. The MXene supernatant containing several layers was treated as aqueous solvent for interfacial polymerization, consequently the prepared MXene nanocomposite membrane could maintain high permeselectivity even under low pressures required for low carbon. The characterization results showed that the MXene nanocomposite membrane had better hydrophilicity and higher negative charge. At the same time, the Na2SO4 rejection was increased to 97.6% and the pure water flux was improved by about 1.7 times compared with the pristine membrane. In the BSA antifouling test and 58-day persistent desalination test, the MXene nanocomposite membrane showed the better antifouling property, high rejection and good stability. As an excellent 2D modifier, MXene provided fresh ideas and guidance for low-carbon and persistent NF membranes with excellent separation performance.

Stepwise hollow Prussian blue/carbon nanotubes composite as a novel electrode material for high-performance desalination

As a promising intercalation material for capacitive deionization (CDI), Prussian blue (PB) and its analogues (PBAs) have the superiority of high theoretical capacity and easy synthesis. But they often suffer from low conductivity and severe crystal phase transition, resulting in inferior desalination capacity and poor cycling stability. Herein, the dual strategy of structural optimization and carbon-based materials introduction is proposed to enhance the desalination performance of PBAs. Stepwise hollow structure formed by surface etching has been proved to be more outstanding than cubic structure. Enlarged the specific surface area, the contact area with the electrolyte increases, therefore, more active sites are exposed. Besides, the etching of external surfaces provides more buffer space, improves the tolerance to crystal phase transition, and enhances the cycling stability. The introduction of carbon nanotubes brings high conductivity. Specifically, the desalination test shows that stepwise hollow Prussian blue/carbon nanotubes composite delivers a high desalination capacity of 103.4 mg g−1 with outstanding cycling stability. Moreover, the low energy consumption of 0.23 Wh g−1 is also suitable for practical application. The dual strategy opens a window to design advanced electrode materials for CDI.

Na3(VO)2(PO4)2F nanocuboids/graphene hybrid materials as faradic electrode for extra-high desalination capacity

Capacitive deionization (CDI) is considered as a promising desalination technology due to its low energy consumption and no two-second pollution. But the development of traditional CDI is limited by its two drawbacks, which are low deionization capacity and unavoidable parasitic reactions. Hybrid capacitive deionization (HCDI), which is composed of a faradic electrode and an electrical-double-layer electrode, effectively solves the above problem. Herein, we report a typical NASICON material Na3(VO)2(PO4)2F and modify it with rGO, then apply it in HCDI firstly and receive a superior desalination performance. Five samples are prepared by adding different contents GO solution and we choose the best one (NVOPF-4) with the lowest resistance for the desalination tests according to electrochemical performance. The result of desalination shows a high desalination capacity of 175.94 mg·g−1, low energy consumption of 0.35 kWh·kg-NaCl−1, and the energy recovery is 20% at a current density of 25 mg·g−1. NVOPF@rGO displays a promising ability for desalination in capacitive deionization, further confirming NASICON be a suitable material type for HCDI electrode materials.

A Hollow and Compressible 3D Photothermal Evaporator for Highly Efficient Solar Steam Generation without Energy Loss

Solar steam generation offers a sustainable strategy to mitigate global clean water scarcity. To this end, 3D photothermal evaporators have attracted increasing research interest since they can significantly improve both evaporation rate and energy efficiency. However, compared to the 2D evaporators, the 3D ones consume more raw materials and occupy more storage space, which limits their applications for practical portable solar steam generation. To address this issue, a 3D hollow and compressible photothermal evaporator is designed and fabricated which can be compressed to less than one third of its original volume, thus enabling easier transport and storage. Moreover, under 1.0 sun illumination, all evaporation surfaces of this 3D evaporator are lower in temperatures than the surrounding environment, thus providing the unique advantage of zero energy loss to the environment during solar evaporation. Due to the all‐cold evaporation surfaces, during solar evaporation, the evaporator is able to harvest massive energy from both the surrounding air and bulk water, delivering an extremely high evaporation rate of up to 7.6 kg m−2 h−1 under 1.0 sun irradiation. Furthermore, seawater desalination tests demonstrate that the device has great potential for portable solar thermal desalination by delivering clean water with a salinity well below 50 ppb.

Suppressing the oxygen-related parasitic reactions in NaTi2(PO4)3-based hybrid capacitive deionization with cation exchange membrane

The parasitic reactions leading to capacity fading and charge loss remain a serious issue for capacitive deionization (CDI). NaTi2(PO4)3 (NTP) has recently emerged as a promising faradaic cathode in hybrid CDI (HCDI) with high Na+ uptake capacity and good Na+ selectivity, but it is still challenged by serious parasitic reactions. Although the irreversible faradaic reactions on carbon electrode are raising growing attention in CDI research field, the parasitic reactions on faradaic materials are seldom studied in HCDI by now. In this work, we evaluated the parasitic reactions of NTP-reduced graphene oxide (rGO) electrode in both three-electrode mode and full-cell HCDI mode. By using deaired electrolyte, the coulombic efficiency of NTP-rGO is significantly enhanced from 75.0% to 98.2% in 3rd cycle, and the capacity retention rate is promoted from 37.5% to 80.3% at the low current density of 0.1 mA g−1 in 100 cycles, suggesting that electrochemical reduction of oxygen and its derived reactions are the main parasitic reactions in NTP-based HCDI. In full-cell HCDI desalination tests, by introducing cation exchange membrane to block the penetration of dissolved oxygen, the parasitic reactions and pH fluctuations are successfully suppressed. The study here provides an insight into understanding and suppressing the parasitic reactions in HCDI, and should be of value to the development of efficient and stable HCDI for practical applications.

Membrane distillation process application using a novel ceramic membrane for Brackish water desalination

Membrane distillation (MD) is a useful technology for water desalination possible at low temperatures using inexpensive membranes offering high salt rejection. Therefore, in the present study, economically and eco-friendly Saudi red clay, tetraethyl orthosilicate, ammonia, and sodium alginate powder as a binder were used to fabricate a ceramic membrane for membrane distillation using an extrusion technique. The prepared membrane was tested using a vacuum membrane distillation (VMD) process and showed promising permeate flux and salt rejection results. The membrane was characterised by SEM, mechanical testing, contact angle, pore size distribution analysis, and VMD tests were performed using deionized water, magnesium sulphate solution, sodium chloride solutions, and hot raw well water. An average flux of 13.10 kg/(m2·h) with 98.96% rejection was obtained using raw well water. During continuous well water desalination tests, the membrane exhibited a stable performance for 4 h. The average flux decreased by 16.5% after 10 h, possibly owing to fouling because of salts and suspended solids. The membrane performance was recovered after the salt water tests by applying a known cleaning procedure.

Dual-functional superwettable nano-structured membrane: From ultra-effective separation of oil-water emulsion to seawater desalination

Freshwater contamination and scarcity have progressively exacerbated the worldwide water crisis. Developing a feasible method to remediate polluted water as well as converting the abundant seawater to freshwater has become a matter of urgency. Here we demonstrate a nanofibrous poly(vinyl alcohol) based membrane (NPM) with dual functions: oil/water separation and solar water desalination. Interconnected polypyrrole (PPY) nanoparticles were decorated on the NPM as solar light absorber as well as enabling the membrane with superwettability. The underwater superoleophobic property endows the NPM with excellent anti-fouling performance and capacity to treat surfactant-stabilized industrial wastewater containing oil or various non-polar organic solvents with a separation efficiency of more than 99%. Mechanical robustness of the membrane is enhanced by mixing graphene oxide (GO) into NPM. Meanwhile, the efficient solar thermal conversion enables NPM to further desalinize the seawater under one sun solar radiation with excellent efficiency and durability with an evaporation rate of 2.87 kg m−2h−1. In the on-site desalination test, the solar vapor system demonstrated a daily solar water purification yield of 14.3 L m−2 under natural sunlight irradiation, suggesting a promising “green” application as mass manufacture seawater purification system without extra energy source. The outstanding performances of NPM for oil-in-water emulsion separation and the high-rate seawater desalination suggest its great potential for industrial applications in the future.

A win-win strategy of β-cyclodextrin and ion-doped polypyrrole composite nanomaterials for asymmetric capacitive deionization

The hollow carbon sphere (HCS) coated with polypyrrole (PPy) layer was synthesized using β-cyclodextrin as the precursor compound with ion-doped (Cl-, DBS-) polypyrrole for the capacitive removal of salt ions from brackish water. HCS derived from β-cyclodextrin was firstly prepared using Pluronic F-127 as the soft template. The combination of HCS and ion-doped PPy effectively exerts the advantages of both to achieve a win-win effect. The supporting effect of HCS alleviates the agglomeration of PPy and enables the composite material to have a larger specific surface area. The electrochemical tests show that the HCS@PPy-Cl and HCS@PPy-DBS composites have improved electrochemical performance, with specific capacitances of 65.4F g−1 and 101.9F g−1, which are 1.62 times and 2.52 times that of HCS, respectively. Furthermore, the desalination tests indicate that the HCS@PPy-Cl||HCS@PPy-DBS (H-PC||H-PD) asymmetric cell exhibits a high electrosorption capacity of 31.77 mg g−1, which is much higher than that of the HCS||HCS (H||H) symmetric cell (26.35 mg g−1) in 500 mg L-1 NaCl solution. And the charge efficiency of H-PC||H-PD cell can reach 76.2%, which means a great improvement compared to 60.5% of the H||H cell. The improved performances indicate that ion-doped PPy can effectively improve the adsorption capacity of the composite material by providing pseudo-capacitance and suppressing the co-ion effect, while the total specific surface area can effectively increase by HCS and thereby improving the adsorption site.

Facile synthesis and enhanced desalination performance of a novel layered Na4Mn14O27 made from earth-abundant element in capacitive deionization

Capacitive deionization (CDI) is currently restricted by inferior desalination performance. To overcome this limitation, exploring novel intercalation materials is ongoing. A major research interest has been focused on the layered compounds due to their open structures which facilitate ions insertion/extraction. Herein, based on the earth-abundant element, we report a novel layered Na4Mn14O27 via a facile method of one-step calcination. The as-obtained sample exhibits a high desalination capacity of 76.6 mg/g in 500 mM NaCl solution with a voltage of 1.0 V. Long-term desalination test and GCD measurement reveal the as-obtained Na4Mn14O27 sample with excellent stability and recyclability. In additional, high charge efficiency of 0.73~0.98 is attained as the voltage alters from 0.6 to 1.4 V and the salt concentration varies from 5 to 500 mM. Such remarkable performances indicate the Na4Mn14O17 with a considerable promise for high-performance desalination.

A novel graphene oxide-based ceramic composite as an efficient electrode for capacitive deionization

In this study, N-doped and TiO2-decorated graphene oxides were developed as efficient nonprecious electrodes for capacitive deionization. The activity of this new material was evaluated in situ and in vivo. The performance of the synthesized material was measured in different saline solutions (0.1, 0.5 and 1.0 M NaCl) as an electrolyte. The results showed that the new material exhibits very good performance (157 F/g at 5 mV/s and 1.0 M NaCl compared to 19.5 F/g for pure graphene oxide). In the desalination test, which was performed in batch mode, the salt adsorption capacity and the efficiency of salt removal were 9.2 mg/g and 98%, respectively. To check the stability, the desalination test was repeated several times, and no change in the performance was observed. The results provide evidence that the newly synthesized material is a potential electrode material for CDI water desalination with satisfactory salt removal ability.

UV-assisted desalination of seawater using titanium dioxide nanotube doped polyether block amide membrane

Abstract In this study, a UV-assisted desalination system was prepared with a non-porous TiO2-doped Pebax 1657 membrane. The membranes were characterized and desalination tests were performed. The effects of UV light and TiO2 ratio on the performance of desalination were investigated. According to the results, TiO2 incorporation increased the membrane hydrophilicity, increased the membrane swelling values, enhanced the membrane flux and improved the salt rejection. Moreover, UV treatment has a positive effect on desalination performance. The best improvement was achieved in the results of 10 wt.% TiO2-doped membrane. It was found that the flux value of the UV-treated membrane having 10 wt.% TiO2 concentration was 8.2 kg/m2·h and the salt rejection value was 99.97%. It was found that the prepared membrane showed excellent desalination performance.

Obtainment and Characterization of Hydrophilic Polysulfone Membranes by N-Vinylimidazole Grafting Induced by Gamma Irradiation.

Polysulfone (PSU) film and N-vinylimidazole (VIM) were used to obtain grafted membranes with high hydrophilic capacity. The grafting process was performed by gamma irradiation under two experiments: (1) different irradiation doses (100–400 kGy) and VIM 50% solution; (2) different concentration of grafted VIM (30–70%) and 300 kGy of irradiation dose. Characteristics of the grafted membranes were determined by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), contact angle, swelling degree, desalination test, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both experiments indicated that the absorbed dose 300 kGy and the VIM concentration, at 50% v/v, were effective to obtain PSU grafted membranes with 14.3% of grafting yield. Nevertheless, experimental conditions, 400 kGy, VIM 50% and 300 kGy, VIM 60–70% promoted possible membrane degradation and VIM homopolymerization on the membrane surface, which was observed by SEM images; meanwhile, 100–200 kGy and VIM 30–50% produced minimal grafting (2 ± 0.5%). Hydrophilic surface of the grafted PSU membranes by 300 kGy and VIM 50% v/v were corroborated by the water contact angle, swelling degree and desalination test, showing a decrease from 90.7° ± 0.3 (PSU film) to 64.3° ± 0.5; an increment of swelling degree of 25 ± 1%, and a rejection-permeation capacity of 75 ± 2%. In addition, the thermal behavior of grafted PSU membranes registered an increment in the degradation of 20%, due to the presence of VIM. However, the normal temperature of the membrane operation did not affect this result; meanwhile, the glass transition temperature (Tg) of the grafted PSU membrane was found at 185.4 ± 0.5 °C, which indicated an increment of 15 ± 1%.

Salt extraction from lime-based mortars: An experimental study using different poultice formulations

The extractive capability of soluble salts in poultices treatments currently used in mortar conservation (ArbocelTM plus sepiolite; ArbocelTM, sepiolite and sand; Westox-Cocoon®) were tested to evaluate their efficiency and compatibility with specific mortar substrate. The pore size distribution of the experimental mortar substrate and of the blank dried poultices were preliminarily accomplished by mercury intrusion porosimetry (MIP). Desalination tests were carried out on specifically-formulated mortars after capillary absorption of Na2SO4 0.3 M solution. The efficiency of salt extraction was measured by means of electrical conductivity meter and ion chromatography. Among the tested products, Westox-Cocoon® showed the best performance in terms of the ability to extract soluble salts. It is the only poultice with a porosity distribution characterised by predominantly large macropores (20–30 µm), which guarantees it an extraordinary efficiency in the first phase of the extraction process (wetting/dissolving). This last aspect provides a useful term of comparison for conservators who actually use these products and a base of operational protocol more suitable to the needs of the restoration site.

Evaluation of the optimal activation parameters for almond shell bio-char production for capacitive deionization

A study on a possible new biomass waste to be used as electrode material for capacitive deionization (CDI) processes was performed. Raw almond shells were pyrolyzed at 800, 900 and 1000 °C and then activated through CO2. Carbon activation is used to develop porosity inside the material, increasing the specific surface area and the adsorption performances. In this work, authors tried to correlate the effects of pyrolysis and activation temperature on the ion storage capacity. Results from the desalination tests indicated that the best performance in terms of ion adsorption was obtained when the bio-char was activated at the temperature of 900 °C. Brunauer-Emmet-Teller (BET) and Barret-Joyner-Halenda (BHJ) analysis confirmed the trend of the CDI tests, reporting the highest surface area and share of micropore sites for the 900 °C samples. Salt adsorption capacity was found to be in the range of 13.7 to 19.2 mg g−1.

Thin-film nanocomposite membranes incorporated with UiO-66-NH2 nanoparticles for brackish water and seawater desalination

In this work, UiO-66-NH2 nanoparticles, a type of metal-organic frameworks (MOFs) with an average size of ~100 nm, were incorporated into the polyamide-based thin-film nanocomposite (TFN) RO membranes. Their inherent porous structure, suitable triangular aperture size, hydrophilic nature, and high chemical and water stability endow the TFN membranes with greater surface hydrophilicity, lower degree of cross-linking and preferential pathways for water molecules across the selective layers. Herein, we introduce UiO-66-NH2 nanoparticles in the TFN layer via spray assisted pre-disposition. The UiO-66-NH2 TFN membranes with a spray duration of 3 min show a ~50% increase in pure water permeance while maintaining a similar rejection compared with the thin-film composite (TFC) membranes in brackish water desalination tests. The former also displays higher chlorine resistance than the latter. In the stimulated seawater desalination tests, TFN membranes also exhibit a higher water flux and salt rejection than the benchmark membranes.

Enhancing the Desalination Performance of Capacitive Deionization Using a Layered Double Hydroxide Coated Activated Carbon Electrode

Capacitive deionization (CDI) is a promising desalination technology because of its simple, high energy efficient, and eco-friendly process. Among several factors that can affect the desalination capacitance of CDI, wettability of the electrode is considered one of the important parameters. However, various carbon materials commonly have a hydrophobic behavior that disturbs the ion transfer between the bulk solution and the surface of the electrode. In this study, we fabricated a layered double hydroxide (LDH) coated activated carbon electrode using an in-situ growth method to enhance the wettability of the surface of the carbon electrode. The well-oriented and porous LDH layer resulted in a better wettability of the activated carbon electrode, attributing to an enhanced capacitance compared with that of the uncoated activated carbon electrode. Furthermore, from the desalination tests of the CDI system, the LDH coated carbon electrode showed a higher salt adsorption capacity (13.9 mg/g) than the uncoated carbon electrode (11.7 mg/g). Thus, this enhanced desalination performance suggests that the improvement in the wettability of the carbon electrode by the LDH coating provides facile ion transfer between the electrode and electrolyte.

Seawater Desalination by Using Nanofiltration (NF) and Brackish Water Reverse Osmosis (BWRO) Membranes in Sequential Mode of Operation

In this study, the applicability of nanofiltration (NF) membranes as a pretreatment prior to reverse osmosis (RO) in seawater desalination was investigated. The membranes used wereNF270 and NF90 as the NF membranes, while the brackish water (BW) RO membrane BW30 was used as the RO membrane. In desalination tests, permeates of the NF membraneswere collected and used as the feed to the BW30 membrane. The calculated permeate fluxes were 6.7 L/h.m2, 11.3 L/h.m2, 24.3 L/h.m2, and 36.6 L/h.m2 for single BW30-35 bar,NF270-30 bar + BW30-35 bar, NF90-30 bar + BW30-25 bar and NF90-30 BW30-35 bar, respectively. The calculated water recovery and rejected salt values were 51.6%, 41.4%,24.8%, 15.4% and 98.2%, 98.2%, 96.0%, 91.0% for NF90-30 bar + BW30-35 bar, NF90-30 bar + BW30-25 bar, NF270-30 bar + BW30-35 bar and single BW30-35 bar, respectively.The qualities of the product waters of integrated systems (NF+BWRO) and the single BWRO system were also investigated. Boron rejection was fairly well with average boronrejections of 59.3% and 60.2% by NF90-30 bar + BW30-25 bar and NF90-30 bar + BW30-35 bar combinations, respectively while single BW30-35 bar gave an average rejection of49.6%. The results obtained showed that the quality of product water obtained using single BWRO did not comply with the irrigation standards, while the integrated systems providedtotal compliance to irrigation standards with the exception of boron.

Cheap, facile, and upscalable activated carbon-based photothermal layers for solar steam generation.

Solar-to-steam generation characterized by nanostructured photothermal materials and interfacial heating is developed based on various carbon nanostructures such as graphene, reduced graphene oxide, CNT, or their combinations. However, multiple and sophisticated synthetic steps are required to generate macroscopic porosity in photothermal devices for the efficient mass transport of water and generated steam. Additionally, the fabrication of photothermal layers on a practical scale constitutes the main hurdle for real applications toward solar-driven desalination. Herein, we report on the development of highly efficient photothermal layers with a commercially available low-cost material, activated carbon (AC), by using facile filtration and spray coating methods, which lead to the generation of intraparticle porous structure without any additional processing. The AC-based photothermal layers generated 1.17 kg m−2 h−1 of steam under 1 sun, and 4.7 wt% of polyethyleneimine coating on AC enhanced steam generation by 8.5% under 1 sun, corresponding to 1.27 kg m−2 h−1 of the water evaporation rate and 85.66% of the photothermal conversion efficiency. This was due to improvements in light absorption and water uptake properties with the additional advantage of mechanical robustness. The outdoor solar-to-steam generation test with the spray-coated A4-sized photothermal layer in conjunction with the desalination test demonstrated the potential for practical desalination application with upscalability.

Cryogenic Energy for Indirect Freeze Desalination—Numerical and Experimental Investigation

Renewed interest in freeze desalination has emerged due to its advantages over other desalination technologies. A major advantage of the freeze desalination process over evaporative methods is its lower energy consumption (latent heat of freezing is 333.5 kJ/kg and latent heat of evaporation is 2256.7 kJ/kg). Cryogenic fluids like LN2/LAir are emerging as an effective energy storage medium to maximise utilisation of intermittent renewable energy sources. The recovery of this stored cold energy has the potential to be used for freeze desalination. Computational Fluid Dynamics (CFD) modelling was developed to simulate the evaporation of liquid nitrogen to simultaneously conduct freeze desalination to investigate the feasibility of using cryogenic energy for freeze desalination. This integrated CFD model was validated using experimental heat exchanger test facility constructed, to evaporate liquid nitrogen to supply the cooling required for freezing. Parametric study on the LN2 flow rate to observe the volume of ice obtained was also examined using CFD, where increasing the velocity of LN2 by 6 times, increased the volume of ice obtained by 4.3 times. A number of freezing stages were required in order to reduce the ice salinity from 1.5% down to 0.1% as regarded by the World Health Organisation (WHO) as safe to drink. In the cryogenic desalination test rig, approximately 1.35 L of liquid nitrogen was required to reduce the ice salinity from 1.5% to less than 0.1%. Furthermore, the above results illustrate the potential of using the cold energy of cryogenic fluids such as Liquified Natural Gas (LNG) and LN2/LAir for freeze desalination applications as most cold energy during LNG regasification has been unexploited today.

The application feasibility of graphene oxide membranes for pressure-driven desalination in a dead-end flow system

Graphene oxide (GO) membranes have been demonstrated to possess promising prospects in pressure-driven desalination, in which the desalting performance is generally investigated in a dead-end flow system by following the test procedure for polymers. However, there exists a blind zone in desalination test whether the as-reported “retention” is reliable, as there is no experimental evidence to support that the stable salt rejection has been achieved. On this basis, for the first time, we investigated the long-term desalting performance of GO membranes on five different salts, including Na2SO4, NaCl, KCl, MgSO4, and MgCl2, in a conventional dead-end flow device by following the two typical experiment processes for GO membranes. Surprisingly, it was found that the salt rejection of GO membranes dropped down to 0 with the prolongation of testing time. The result indicated that GO membranes were not capable of rejecting salts steadily under pressure-driven conditions. The “salt rejection” in present reports, is only the initial highest value based on their test method. In this study, the underlying mechanism was also elucidated. With extending test time, the metal cations in salt solution would be gradually adsorbed on GO membranes, which shields the negative surface potential, thus leading to the loss of desalination performance. This finding not only rationalizes the challenge of GO membranes in pressure-driven desalination in a dead-end flow system but also prompts us to reassess the feasibility of GO membranes in desalination applications based on Donnan exclusion.