Desalination Capacity Research Articles

Water hyacinth as a green and sustainable material for carbon aerogel electrodes utilized in membrane capacitive deionization

AbstractWater hyacinth is a substantial and quick‐growing plant that can strongly invade and negatively affect the aquatic ecosystem, causing many problems to the water environment by consuming nutrients and oxygen from surface water. However, the hierarchical porous carbon derived from water hyacinth can be employed for the removal of oil spills, heavy metals, or wastewater nutrients. Recently, water‐hyacinth‐derived carbon aerogel has been utilized as electrode for the desalination of brackish water using membrane capacitive deionization (MCDI) technology. In this research, lignocellulose aerogels were synthesized from water hyacinth, which were then pyrolyzed to obtain carbon aerogel and then treated with KOH to acquire activated carbon aerogel, with the corresponding surface area of 51 and 100 m2 g−1. The desalination capacity of the MCDI device was also evaluated using a 200 ppm NaCl feed water solution with an applied potential of 1.2 V. A high salt adsorption capacity value of 14.69 mg g−1 was achieved after 3000 s. These results will lead a new research area of biomass and bio‐waste conversion to fabricate CDI‐utilized carbon aerogel electrodes.

Conductive carbon additives: Friend or foe of capacitive deionization with activated carbon?

Capacitive deionization with activated carbon (AC) electrodes has been widely applied for removing charged ions from aqueous solutions. Such carbon electrodes commonly contain a minor polymer binder and a minor carbon additive content. The choice of carbon additives is rarely investigated in depth regarding the performance enhancement/deterioration they might bring. In this work, we explored the influence of various carbon types, namely, onion-like carbon, carbon black, and micro-mesoporous carbons, on the desalination capacity and rate. Based on the cycling performance of 100 cycles, we draw relationships between the physicochemical properties of different carbon types and their results on electrochemical desalination performance. The results indicate that the direct use of the activated carbon electrode without additives leads to a higher desalination capacity of approximately 10 mg/g in early cycles, though at the cost of a lower desalination rate of 6 μg/g/s. The larger AC particles limit the intraparticle ion transportation due to the increased diffusion path length. The highest desalination rate (20 μg/g/s) is enabled by the incorporation of small and less porous additives, as it shortens the ion diffusion path length due to the increased size dispersion, hence improving the overall ion transport and desalination rates.

Dual-ion electrochemical deionization system based on binder-free Bi2O3@MXene-Bi@MXene electrodes

Dual-ion electrochemical deionization (DEDI) system is a promising strategy for increasing the performance of capacitive deionization (CDI). We presented a simple strategy to simultaneously prepare chloride and sodium ion storage film electrode by combing MXene with Bi2O3. The sandwich-like three-dimensional (3D) structure Bi2O3@MXene film electrode can well storage sodium ions by intercalation reaction. Interestingly, the Bi@MXene film electrode can be prepared from Bi2O3@MXene film simply by reducing Bi2O3 to Bi, which can capture chloride ions by the electrochemical reaction of Bi and BiOCl. The MXene provides outstanding conductivity and flexibility for the composite film electrodes, and the introducing of Bi2O3 limits the re-stacking of MXene nanosheets while the Bi enhances the chloride ions adsorption of MXene by conversion reaction. When assembled as a DEDI device, it obtained a high desalination capacity of 86.8 mg/g at the current of 20 mA/g, and the fastest desalination rate of 3.21 mg/g/min under the current of 200 mA/g. Moreover, a strong power source built by two DEDI devices in series can drive an LED more than 5 min during the regeneration process. Such a simple and effective strategy can promote the development of binder-free electrode DEDI system.

Walnut shell-derived porous carbon with MgSO4 modification for high-performance capacitive deionization

Capacitive deionization (CDI) is considered as a promising desalination technology, and the fabrication of electrode materials with high specific surface area, good hydrophilicity and hierarchical pore structure is beneficial to improve CDI performances. Herein, porous carbon (MAWC-700) with a high specific surface area of 3074.8 m2 g−1 and superior hydrophilicity is derived using walnut shell as precursor, KOH as activation agent, and MgSO4 as modification. The MAWC-700 demonstrates a capacitance of 214.3 F g−1 at 5 mV s−1, and the corresponding CDI delivers a high desalination capacity of 27.79 mg g−1 and salt adsorption rate of 5.56 mg g−1 min−1 at 1.2 V. Hence, the synthesis of biomass-based porous carbon activated with KOH along with MgSO4 is an effective way to improve desalination performances for CDI applications.

MnO2-modified soybean root derived porous carbon with excellent capacity deionization

Developing high-efficient capacitive deionization (CDI) systems with low-cost environmentally friendly electrode materials is one of the most effective ways to address freshwater shortage. Meanwhile, large number of soybean roots are discarded as waste with large production of soybean and contribute to enormous pressure on the environment. In this work, soybean roots are adopted to prepare activate carbon (SRKPC) by a carbonization and KOH activation process. A composite of [email protected]2 with MnO2 nanoparticles supported on SRKPC is synthesized by a low temperature hydrothermal reaction, which exhibits excellent electrochemical properties with high specific capacitance (597.9 F g−1 at 2 mV s−1) and bimode sodium storage characteristics. The dual-mode CDI system based on [email protected]2 delivers an effective desalination performance with a desalination capacity of 42.90 mg g−1, a desalination rate of 2.86 mg g−1 min−1 and relatively good desalination recoverability. Therefore, this work not only exemplifies the simple fabrication of low-cost, environmentally friendly, and efficient dual-mode CDI electrode materials for capacitive desalination, but also provides a sustainable way to turn biomass waste into functional materials.

Synthesis of Low-Crystalline MnO2/MXene Composites for Capacitive Deionization with Efficient Desalination Capacity

MXene has drawn widespread attention as a potential material for electrode use in capacitive deionization (CDI). However, the applications of MXene are limited by its property of low electrical capacity. Herein, a MnO2/MXene composite was firstly evaluated in a capacitive deionization system, in which the MnO2 acts as intercalation-type pseudocapacitive electrodes to enhance the electrical capacity, and MXene provides an electron conduction highway network that improves the charge transfer of the MnO2. The result showed that the low-crystallinity MnO2 with irregular particles was well-distributed on the surface of the MXene. The desalination capacity of 30.5 mg·g−1 is achieved at a voltage window of 1.2 V, which was higher than that of the reported pure MXene and MnO2. The electrical double-layer (EDL) capacitive and the diffusion-controlled processes are the main charge storage mechanisms, and the EDL contribution provides 50.3% to the total capacitance. This result suggests a promising direction for further applying a MnO2/MXene composite in CDI.

Dual-confinement effect in metal oxide nanoparticles/MXene-reduced graphene oxide for high capacitive deionization performance

Metal oxide nanoparticles are one kind of important electrode materials for capacitive deionization (CDI) application. Unfortunately, most metal oxide materials usually exhibit poor electrical conductivity and often experience slow dissolution of metallic species, thereby resulting in limited desalination performance. Overcoming these obstacles still remains challenging due to the lack of synthetic approaches. Taking it into consideration, herein we proposed a dual confinement strategy to promote the CDI application of metal oxide nanoparticles. Nb2O5 was synthesized as a typical example for designing metal oxide/MXene-reduced graphene oxide (rGO) derived from Nb2CTx MXene/graphene oxide (GO) under hydrothermal condition. In the hybrid, Nb2CTx and rGO serve as both the confinement agent and conductive support for Nb2O5 nanoparticles, endowing Nb2O5/Nb2CTx-rGO with improved electronic conductivity and structural stability. As a result, the hybrid shows an outstanding desalination performance, including a NaCl adsorption capacity of 41.07 mg g−1, an ultra-fast average NaCl adsorption rate of 4.14 mg g−1 min−1 and excellent long-term cycling stability of 50 cycles (81 % desalination capacity retention rate). This work is expected to bring new insights in the synthesis of metal oxide materials for electrochemical desalination.

Ag-decorated monolithic carbon sponge with porous directional frameworks for chloride removal in flow-through capacitive deionization

The structure and composition of monolithic electrodes have offered a great advantage for the flow-through capacitive deionization (FT-CDI) process. However, developing a high-performance monolithic electrode with both high desalination capacity and favorable mechanical properties is an enormous challenge to solving the problem of water pollution. Herein, we fabricate an Ag-decorated monolithic carbon sponge (MCS) with directional pore channels (Ag@DMCS) to serve as an FT-CDI electrode by employing NH2-rich poly(m-phenylenediamine)@Fungi (PmPD@Fungi) as a matrix. Ag ions were effectively loaded onto the matrix due to the strong affinity of the NH2 for Ag. Moreover, the cold field effects through the directional ice templating allows the MCS electrodes to construct a unique porous structure with a directional arrangement, further facilitating the fluid flow through the electrode. The results indicated that the Ag@DMCS achieves a superior Cl- adsorption capacity of 68.74 mg g−1 and rapid adsorption of 0.19 mg g−1 s−1 due to the directional pore channels and the uniform loading of Ag nanoparticles. Pressure-drop tests and computational fluid dynamics (CFD) revealed that the directional pore channels in the Ag@DMCS electrodes enabled the system to lower fluid resistance and more uniform pressure distribution during the FT-CDI process. The Ag@DMCS electrode is anticipated to be a promising alternative material for extracting Cl- by the FT-CDI system.

Structural/Compositional-Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization.

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38mg g-1 , a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodiumions (Na+ ) over divalent calcium (Ca2+ ) and magnesim (Mg2+ ) ions, allowing it to be a promising platform for preferential capturing Na+ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.

Space-Confined Metal Ion Strategy for Carbon Materials Derived from Cobalt Benzimidazole Frameworks with High Desalination Performance in Simulated Seawater.

Various metal ions with different valence states (Mg2+ , Al3+ , Ca2+ , Ti4+ , Mn2+ , Fe3+ , Ni2+ , Zn2+ , Pb2+ , Ba2+ , Ce4+ ) are successfully confined in quasi-microcube shaped cobalt benzimidazole frameworks using a space-confined synthesis strategy. More importantly, a series of derived carbon materials that confine metal ions are obtained by high-temperature pyrolysis. Interestingly, the derived carbon materials exhibited electric double-layer and pseudocapacitance properties because of the presence of metal ions with various valence states. Moreover, the presence of additional metal ions within carbon materials may create new phases, which can accelerate Na+ insertion/extraction and thus increase electrochemical adsorption. Density functional theory results showed that carbon materials in which Ti ions are confined exhibit enhanced insertion/extraction of Na+ resulting from the presence of the characteristic anatase crystalline phases of TiO2 . The Ti-containing materials have an impressive desalination capacity (62.8mgg-1 ) in capacitive deionization (CDI) applications with high cycling stability. This work provides a facile synthetic strategy for the confinement of metal ions in metal-organic frameworks and thus supports the further development of derived carbon materials for seawater desalination by CDI.

Selective and continuous ion recovery using flow electrode capacitive deionization with polymer multilayers functionalized ion exchange membrane

Flow electrode capacitive deionization (FCDI) is a newly developed salt removal technology that overcomes the limitations of conventional CDI by eliminating the need for a discharging process, thus achieving much higher desalination capacity and a continuous desalination operation. In this study, we aimed to achieve selective and continuous sodium recovery from Na+/Ca2+ mixtures through FCDI by utilizing polymer multilayer functionalized cation-exchange membranes (CEM). A layer-by-layer method was used to deposit polymer multilayers consisting of PAH (poly allylamine hydrochloride) and PSS (poly styrene sulfonate) on the CEM. Changes in the concentration of feed electrolyte, number of coated polymer layers, and type of outermost layer were investigated in terms of their effect on the selective ion separation. The functionalized CEM switches from a divalent ion affinity to a monovalent ion affinity due to their different ion sizes and charge densities. As the number of polymer layers increased, the selectivity of Na+/Ca2+ increased, reaching up to 3.26 from 0.16 for pristine CEM. We believe that our approach can provide insight into the selective recovery of valuable materials including lithium-ion battery recycling, the recovery of noble metals, and the separation of toxic ions using the FCDI system.

Selective removal of ammonium ions with transition metal hexacyanoferrate (MHCF) electrodes

This study investigated the potential of using transition metal hexacyanoferrates (MHCF) to selectively capture NH4+ over Na+ in brine for nutrient recovery from wastewater. The electrochemical characteristics of five different MHCF (M is Ni, Cu, Fe, Co or Zn), known as Prussian blue analogues (PBA), was investigated using a hybrid capacitive deionization device, and their desalting capacity, stability and energy consumption were evaluated. By adjusting the mass ratio of electrodes, applied voltage, and ion concentration, the relationship between NH4+ removal and selectivity was explored. Results showed that all five MHCF electrodes exhibited high selectivity (> 4) for NH4+ over Na+, but the selectivity is not stable due to comprehensive factors (such as the structural stability of electrode, solution composition, and current density, etc.). Among them, the NiHCF electrode showing the best performance in terms of specific desalination capacity (∼1.23 mmol/g), charge efficiency (>80%), and stability (82.5% capacity retention after 50 cycles). This study demonstrates that PBA has promising potential as NH4+ selective electrodes for nutrient removal and recovery in wastewater.

Performance analysis of nuclear powered desalination unit based on MED-TVC: a case study for Saudi Arabia

Abstract Nuclear desalination has been identified as an alternative option with much lower carbon dioxide emissions to provide fresh water by driving high capacity desalination plants. This work considers a theoretical analysis of using nuclear desalination to provide fresh water in three selected Saudi Arabian cities. It presents a theoretical model that integrates the characteristics of nuclear reactor, power cycle and desalination blocks. The power block includes three steam turbines and five feed water heaters. It is coupled via low pressure turbine to a multiple effect desalination unit integrated with a thermal vapor compressor encompassing eight effects, seven feed preheaters and a down condenser. The output includes work generated as function of fuel mass and reactor type, enrichment percentage, power and water production with different nuclear reactor type. Desalination performance indicators such as the fresh water production rate, gain output ratio (GOR), specific energy consumption (SEC) and specific cooling water mass flow rate have been evaluated and analyzed as function of sea water temperature for three specific Saudi cities. It was found that these indicators reflect better performance along a year for Jizan city than for Jubail and Tabuk. The case of Jizan city gives over the whole year more uniform values of water production rates, gain output ratio, specific energy consumption and cooling water mass flow rates.

Enhanced Capacitance and Desalination Performance with Plasma‐Activated Biochar Electrodes

Derived from electric double‐layer capacitors (EDLCs), capacitive deionization (CDI) using symmetric carbon‐based electrodes is an emerging approach to purifying brackish water into drinking water. The deionization performance is mainly determined by the properties of the electrode materials. This work reports a green and efficient plasma treatment for activating raw biochar, which is subsequently used as the electrode material for both EDLCs and CDI. Oxygen plasma activation effectively modulates the surface charge, morphology, and functional groups. Biochar treated with oxygen plasma exhibits pronounced improvement in supercapacitor specific capacitance from 80 to 97.5 F g−1 and 1.2 times higher desalination capacity in comparison with the raw biochar. Plasma activation can be an effective strategy to improving the capacitance and deionization performance of biochar.

Controlled fabrication of nitrogen-doped porous carbon foam with refined hierarchical architectures for desalination via capacitive deionization

Porous carbon materials have been regarded as a promising alternative to activated carbon for desalination via capacitive deionization (CDI) due to refined architectures and functionalities. However, it is still challenging to obtain a controlled hierarchical pore structure and considerable nitrogen-doped content by convenient method. Herein, nitrogen-doped hierarchical porous carbon foams (NHCFs) with different microstructural features, nitrogen contents and nitrogen species were successfully fabricated via a stepwise pyrolysis carbonization strategy using easily available melamine foam. Due to the synergistic effect of hierarchical porous structure and doped nitrogen, the optimized NHCF sample carbonized at 800℃ (NHCF-800) exhibited a maximum desalination capacity of 30.1 mg g−1 at the optimal operating parameters (500 mg/L NaCl solution, 1.2 V) and an excellent regeneration performance after 50 continuous adsorption–desorption cycles. Furthermore, density functional theory (DFT) was also conducted to elaborate the disparity of sodium adsorption energy among the nitrogen species for in-depth understanding, and it mainly benefits from the ascendency of the pyrrolic-N and pyridinic-N over the graphitic-N dopant. This work paves the way of rational regulation of nitrogen-doped process and hierarchical porous structure carbon as CDI electrode materials for desalination.

Fouling-resistant surface modification of forward osmosis membranes using MoS2-Ag nanofillers

The mitigation of organic fouling is a main challenge for forward osmosis (FO) technology. In order to enhance the fouling resistance and desalination efficiency of the FO membrane, various studies have suggested the incorporation of nanomaterials into thin-film composite (TFC) membranes, which are defined as thin-film nanocomposite (TFN) membranes. In this study, we fabricated MoS2-Ag nanofillers by photochemical deposition method and incorporated them into TFC membranes for FO water treatment. After incorporation of MoS2-Ag nanofillers into TFC membranes, we could observe significant change in the surface of membranes. Especially, active layer surface of TFN membrane was smoothened, which was represented by decrease of roughness from 40.09 ± 1.39 nm of TFC membrane to 15.53 ± 0.17 nm of TFN membrane. Furthermore, according to the hydrophilic surface modification, contact angle of membranes was also decreased from 79.0 ± 2.0° of TFC membrane to 52.2 ± 0.7° of TFN membrane. The surface modification of the TFN membrane resulted in improvement of desalination capacity in terms of 35.2% improved water flux. Finally, the resistance to alginate fouling of TFN membrane was realized and it exhibited almost 4 times lower water flux loss than TFC membrane. This study presents that photochemically synthesized nanofillers could modify the surface of TFN membrane, therefore, this technique can be a feasible approach to future membrane-based technologies.

Enhanced desalination performances by using porous polyaniline-activated carbon composite flow-electrodes in capacitive deionization system

Flow electrodes capacitive deionization (FCDI) is a promising CDI variant, and has been used in various fields like brackishwater desalination and nutrients recovery. Activated carbon (AC), a prevalent FCDI electrode, suffers from low ion adsorption capacity, poor electric conductivity, and large viscosity, which restrict their large-scale applications. In this study, situ polymerization was utilized to prepare porous polyaniline and activated carbon composite (pPANI-AC) electrode with the assistance of ice template. This innovative electrode took advantages of the pseudocapacitive effects of porous polyaniline and the ion channels was increased by exposing the pores and surface of the porous activated carbon. The new composite electrode has an increase in specific surface area from 213.5 m2 g−1 to 616.1 m2 g−1, and an increase in capacitance from 93 to 178 F g−1, and the viscosity flow electrodes decreases from 127 to 21 Pas. Reduced viscosity and intrinsic impedance of the flow electrode result in an increase in flow electrode conductivity and ionic capacity of the FCDI system that has a 70 % increase in desalination capacity. This study developed a low-cost and environmentally friendly active material for high-efficient flowable electrodes, which will pave way for the real applications of FCDI technology.

Hybrid of pyrazine based π-conjugated organic molecule and MXene for hybrid capacitive deionization

In this work, we report a hybrid material of pyrazine based π-conjugated organic molecule, hexaazatrinaphthalene (HATN), and MXene. In this hybrid, MXene serves as a conductive substrate and HATN provides multiple electroactive sites, which are jointly combined to build a unique 3D structure with more ion transport channels for accelerating the charge transfer and ion diffusion rates. The as-obtained HATN/MXene composite displays a superior desalination capacity of 57.5 mg g−1 with an excellent desalination rate of 13.2 mg g−1 min−1 and good long-term stability. This work provides a meaningful guidance for designing and applying organic molecules in capacitive deionization field.

Two-dimensional bimetallic cobalt-copper metal organic framework for improved desalination performance of capacitive deionization

Capacitive deionization (CDI) technology has recently attracted much attention due to its distinct advantages, which include energy efficiency, eco-friendly method, and simple process. Capacitive materials, such as activated carbon, carbon fiber, and carbon aerogel, have mostly been used. However, the conventional carbon-based materials still suffer limitations, such as low electrosorption capacity, slow desalination rate, and insufficient desalination capacity for high saline concentration. Herein, the 2 Dimensional-Cobalt-Copper sulfur linker-based MOF material was prepared by a solvothermal method to develop the performance of electrode material, namely 2D-CoCu sMOF. We applied the synthesized material with porous carbon material (AC) as an electrode material in the CDI desalination system. Owing to the fast and efficient electron transfer in the electrode layer and the lower interfacial resistance between electrode surface and saline electrolyte originating from the redox reactions of MOF structure, 2D − CoCu sMOF exhibited a much higher desalination performance, compared to the pristine AC. CDI experimental results showed that the salt adsorption capacity (SAC) of electrode using 2D − CoCu sMOF was significantly improved from 4.18 to 7.55 mg/g without any changes in the long-term stability test, which was over an 80% increase in desalination performance. This approach provides an effective and simple method for the preparation of MOF-based CDI electrode material, which could have potential for high-performance green technology including desalination applications.

The effect of electrode thickness and electrode/electrolyte interface on the capacitive deionization behavior of the Ti3C2Tx MXene electrodes

2D MXene sheets have been regarded as promising capacitive deionization (CDI) electrodes owing to their high conductivity, hydrophilic surface, excellent adsorption capacity, and reversible intercalation of metal cations. The 2D structure characteristic enables MXene sheets to provide massive spaces to catch Na+ and Cl- ions during the CDI process. In this work, the CDI behavior of Ti3C2Tx MXene has been comprehensively investigated by constructing a symmetrical device. The effect of electrode thickness and electrode/electrolyte on the CDI behavior of the Ti3C2Tx MXene electrodes is deeply investigated. The thin electrode could facilitate the electrolyte (NaCl solution) penetration in the electrode, provide more electrode/electrolyte interface contact, and shorten the ion diffusion pathway from the electrolyte to the internal electrode, leading to faster ion diffusion and better desalination capacity. The optimized MXene‖MXene CDI device with an electrode thickness of 18.41 µm exhibits a high salt removal capacity of 100.9 mg g−1 in NaCl solution at 1.2 V.