Continuous Desalination Process Research Articles

Tailoring the electrode material and structure of rocking-chair capacitive deionization for high-performance desalination.

With the gradually increasing requirement for freshwater, capacitive deionization (CDI) as a burgeoning desalination technique has gained wide attention owing to its merits of easy operation, high desalination efficiency, and environmental friendliness. To enhance the desalination performance of CDI, different CDI architectures are designed, such as membrane CDI, hybrid CDI, and flow-electrode CDI. However, these CDI systems have their own drawbacks, such as the high cost of membranes, capacity limitation of carbon materials and slurry blockage, which severely limit their practical application. Notably, rocking-chair CDI (RCDI) composed of symmetric electrode materials delivers excellent desalination performance because of its special dual chamber structure, which can not only break through the capacity limitations of carbon materials, but also deliver a continuous desalination process. Although RCDI showcases high promise for efficient desalination, few works systematically summarize the advantages and applications of RCDI in the desalination field. This review offers a thorough analysis of RCDI, focusing on its electrode materials, structure designs and desalination applications. Furthermore, the desalination performances of RCDI and other CDI architectures are compared to demonstrate the advantages of RCDI and the prospect of RCDI is elucidated.

Carbon Material-Based Flow-Electrode Capacitive Deionization for Continuous Water Desalination

Flow-electrode capacitive deionization (FCDI) offers an electrochemical, energy-efficient technique for water desalination. In this work, we report the study of carbon-based FCDI, which consists of one desalination chamber and one salination chamber and applies a carbon nanomaterials-based flow electrode that circulates between the cell anode and cathode, to achieve a fast, continuous desalination process. Five different carbon nanomaterials were used for preparing the flow electrode and were studied for the desalination performance, with properties including average salt removal rate (ASRR), salt removal efficiency (SRE), energy consumption (EC) and charge efficiency (CE) being quantitatively determined for comparation. Different FCDI parameters, including carbon concentration and flow rate of the flow electrode and cell voltage, were investigated to examine the influences on the desalination. Long-term operation of the carbon-based FCDI was evaluated using the optimal results found in the conditions of 1.5 M concentration, 1.5 V cell voltage, and 20 mL min−1 flow rate of electrode and water streams. The results showed an ASRR of 63.7 µg cm−2 min−1, EC of 162 kJ mol−1, and CE of 89.3%. The research findings validate a good efficiency of this new carbon-based FCDI technology in continuous water desalination and suggest its good potential for real, long-term application.

Sandwich-structured MXene/wood aerogel with waste heat utilization for continuous desalination

Solar-driven interfacial evaporation technology has shown promising prospect in desalination. However, the weakened energy efficiency caused by intermittent solar irradiation and salt deposition during continuous desalination process is the great obstacle in the application. Here, we report a sandwich-structured MXene/wood aerogel coupled with phase change materials (PCMs) evaporator for high-efficiency and continuous desalination. The multi-stage water transport channel of DWA and the enhanced thermal localization effect of MXene and DWA optimize the performance of the evaporator. The PCMs block as a waste heat recovery module largely reduces heat loss. Under one solar irradiation, the evaporation rate of 2.0 kg·m−2·h−1 and energy efficiency of 92.6 % are achieved. The production of fresh water can reach 15 kg·m−2·day−1. The evaporation rate under intermittent solar irradiation can also reach 1.77 kg·m−2·h−1. There is no clear decrease in evaporation performance as well as salt accumulation for 5 days of continuous evaporation. The energy balance, water transport, thermal localization and salt resistance mechanisms of solar-driven interfacial evaporation are analyzed. This work shows an excellent way to break the limitations of conventional solar interfacial evaporation and achieve stable desalination under intermittent solar irradiation.

Centrifugal reverse osmosis (CRO) − a novel energy-efficient membrane process for desalination near local thermodynamic equilibrium

Seawater reverse osmosis (SWRO) can address the global water crisis that is being exacerbated by climate change, industrialization, pollution, and population growth. However, conventional single-stage RO (SSRO) incurs a high energy cost owing to pressurizing the feed to that required for the desired recovery. This cost could be reduced if SWRO were operated at a progressively increasing pressure dictated by thermodynamic equilibrium for the local transmembrane salt-concentration difference. This is approached by the semi-batch closed-circuit RO (CCRO) process that progressively increases the transmembrane pressure (TMP); however, it incurs cumulative energy losses until the pressure is reached for the desired recovery. The centrifugal reverse osmosis (CRO) process uses rotation of a module to create a centrifugal force that increases the TMP differentially with increasing radial distance from the axis-of-symmetry. Unlike CCRO, CRO is a continuous desalination process. A mathematical model and preliminary design are advanced for a CRO module. CRO can reduce the net specific energy consumption relative to SSRO by 31% for 50% recovery at 56 bar from a 35 g/L seawater feed. A TMP progressively increasing from 28 to 56 bar can be achieved with a CRO module having a radius of 0.72 m rotating at 1000 rpm.

Low energy consumption flow capacitive deionization with a combination of redox couples and carbon slurry

Flow-electrode capacitive deionization (FCDI) is a new sustainable desalination technology where continuous desalination can be achieved by the electrodialysis coupling method. However, its development is hindered owing to high energy consumption and low salt removal rate. Herein, by combining ferri-/ferrocyanide redox couple with flow activated carbon (AC)/carbon black (CB) slurry, continuous desalination process is achieved with a high salt removal rate of 1.31 μg cm−2 s−1 and low energy consumption of 102.68 kJ mol−1 at the current density 2.38 mA cm−2 (50 mA current for a 21 cm2 active area). The operating voltage plateau can be reduced to 0.69 V when 10 wt% AC/CB (mass ratio of 9:1) is mixed with 20 mM/20 mM ferri-/ferrocyanide as the flow electrodes, compared with more than 3 V for only carbon flow or redox medium alone. The influences of carbon content and current densities are further investigated to so that the performances can be controlled. This work enables the development of energy-saving desalination systems by coupling FCDI with redox desalination technique.

Low energy consumption and mechanism study of redox flow desalination

Low-energy desalination technologies capable of continuous throughput at reasonable operating and maintenance cost are now critically needed. Herein, we present a continuous desalination process with average energy consumption as low as 2.14 kJ mole−1 salt removed achievable when using a 10 mM/10 mM ferri-/ferrocyanide solution that flows continuously between anode and cathode chambers. The salt feed at ~3000 ppm can be continuously desalted to a potable level of less than 150 ppm during the desalination process. At the anode, ferrocyanide is oxidized with electrons released to the external circuit and cations captured to achieve charge balance. Ferricyanide is reduced at the cathode, resulting in the constant transfer of anions from the dilute stream to the concentrate stream. The recycling of the redox mediators maintains an extremely low operating potential (~100 mV) and continuous salt extraction with unlimited removal capacity. Comparison of the use of ferri-/ferrocyanide, pure ferricyanide and ferrocyanide solutions indicate that the ultra-low energy consumption can be ascribed to the excellent electrochemical properties of the ferri-/ferrocyanide couple and the extremely low overpotential of the redox reactions owing to the synergistic effect of sufficient simultaneous ferrocyanide oxidation and ferricyanide reduction in the flow electrolyte streams. Our research provides a route for ultra-low energy desalination based on voltage suppression via use of facile redox couple mediators.

Dual-Zinc Electrode Electrochemical Desalination.

Continuous and low‐energy desalination technologies are in high demand to enable sustainable water remediation. Our work introduces a continuous desalination process based on the redox reaction of a dual‐zinc electrode. The system consists of two zinc foils as redox electrodes with flowing ZnCl2 electrolyte, concentrated and diluted salt streams with three anion‐ and cation‐exchange membranes (AEM and CEM) separated configuration (AEM|CEM|AEM). If a constant current is applied, the negative zinc electrode is oxidized, and electrons are released to the external circuit, whereas the positive zinc electrode is reduced, causing salt removal in the dilution stream. The results showed that brackish water can be directly desalted to 380.6 ppm during a continuous batch‐mode process. The energy consumption can be as low as 35.30 kJ mol−1 at a current density of 0.25 mA cm−2, which is comparable to reverse osmosis. In addition, the dual‐zinc electrode electrochemical desalination demonstrates excellent rate performance, reversibility, and batch cyclability through electrode exchange regeneration. Our research provides a route for continuous low‐energy desalination based on metal redox mediators.

Experimental investigation on novel desalination system via gas hydrate

Freshwater scarcity has been troubling the high-quality development of many countries and regions, and seawater desalination is a vital source of freshwater. Recently, gas hydrate based desalination (GHBD) technology attracts much attention because of bringing no extra pollution to water. We developed a novel multifunctional desalination apparatus via gas hydrate with various operation modes and separation methods. In this study, the desalination characters under different separation modes were conducted using simulated seawater, and the hydrate-purging method with a desalination efficiency of more than 80% was selected to desalt the natural seawater sample. In addition, the purging method can greatly improve the desalination effects of both squeezed hydrate column and loose hydrates. The experimental results indicate that the removal efficiencies of different ions in the seawater were similar and their difference was related to the strength of ionic hydration. And above all, we designed and conducted a continuous desalination process including multiple injection, separation, washing and purging operations. The ultimate desalination efficiency using hydrate-purging method was over 80%, and the freshwater recovery was above 30% with 200 mL initial seawater in this study. The experimental proposal and results of this study are of great significance to the development of GHBD technology.

Influence of hexagonal boron nitride nanosheets as the additives on the characteristics and performance of PVDF for air gap membrane distillation

In this study, composite membranes were fabricated through embedment of hexagonal boron nitride (h-BN) in the polyvinylidene fluoride (PVDF). Exfoliated h-BN nanosheets were characterized by transmission electron microscope (TEM). Then, different concentrations of h-BN were incorporated into PVDF to explore their impacts on the membrane performance. Various analyses comprising Fourier transform-infrared spectroscopy (FT-IR), energy dispersive X-ray (EDS), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were applied to identify the membrane characteristics. The composite membrane properties including thermal and mechanical stability, liquid entry pressure (LEP), hydrophobicity, flux, and salt rejection were also investigated. Results showed that PVDF/h-BN membranes compared with PVDF had higher roughness, hydrophobicity, flux, and salt rejection. Applying operating temperature at 80 °C, M2 (contained 0.5 wt% h-BN) as the best membrane indicated flux of 7.1 kg/m2·h (43% increase compared to PVDF) and 99% salt rejection during 60 h continuous desalination process. These improvements in desalination performance were due to increase in pore size, porosity, and van-der-waals interactions between water and h-BN atoms that facilitated fast transport of water through membrane. Moreover, polar nature of BN bonds in h-BN induced high affinity with water that contributed to the selective separation of water through passing from the h-BN nanochannels.

A hydrophilic-oleophobic chitosan/SiO2 composite membrane to enhance oil fouling resistance in membrane distillation

To develop an inexpensive and simple technology and increase anti-oil fouling resistance for membrane distillation applications, a hydrophilic/oleophobic nanocomposite membrane was fabricated by using SiO2/Chitosan (CT) sol solution coating with different volume ratios (0.5 : 1, 1 : 1 and 2 : 1 v/v) on PVDF membrane surface. The formation of SiO2/CT layer on membrane surface was confirmed by Fourier transform infrared (FTIR) spectroscopy and energy-dispersive X-ray spectroscopy (EDX). The influence of hydrophilic nanocomposite layer on the characteristics of membranes, including in-air water contact angle, morphology, porosity, liquid entry pressure of water (LEPw) and direct contact membrane distillation (DCMD) performance, was investigated. The results show that the composite membrane (SiO2/CT (1 : 1 v/v)- PVDF membrane) by adding of 0.5 and 1 g/L gasoline concentrations not only incurred fouling but also a higher flux with respect to the neat membrane in each gasoline concentration. During 8 hours continuous desalination process of saline gasoline emulsion solution (20 gr/L NaCl solution containing 0.5 gr/L gasoline), it was found that all modified membranes had high performance stability in comparison with the neat membrane, the modified membrane showed high performance stability and flux without decreased salt rejection (99.9%). At the end, we conducted performance comparison between the prepared membranes in current work and presser based process.

Highly permeable PVDF membrane with PS/ZnO nanocomposite incorporated for distillation process.

In order to enhance the flux and wetting resistance of PVDF membranes for MD applications, we have developed a novel PVDF blend nanocomposite membrane using a polystyrene/ZnO (PS/ZnO) hybrid nanocomposite. The PS/ZnO nanocomposite was synthesized by free radical polymerization of styrene in the presence of vinyltrimethoxysilane (VTMS) grafted on the surface of ZnO nanoparticles. The blend nanocomposite membrane is fabricated via the phase inversion method and we examined the effects of the PS/ZnO nanocomposite on porosity, mechanical properties, hydrophobicity, LEPw, morphology, surface roughness and MD performance. It was found that the addition of the PS/ZnO hybrid nanocomposite (0.25, 0.5 and 0.75%) resulted in an increase in porosity (>70%), which is attributed to increased pore size and reduction of the spongy layer thickness. Furthermore, the addition of the nanocomposite also improved the surface roughness and contact angle. Comparison between the neat and modified membrane shows that with incorporation of the PS/ZnO nanocomposite, the desalination flux of 30 g L−1 saline aqueous solution significantly increased and rejection reached 99.99%. Meanwhile, during 100 hours continuous desalination process, the membranes composed of 0.75% PS/ZnO hybrid nanocomposite exhibited high performance stability (15.79 kg m−2 h−1) compared with the neat PVDF membrane.

Promising Performance Indicators for Water Desalination and Aqueous Capacitors Obtained by Engineering the Electric Double Layer in Nano-Structured Carbon Electrodes

It is well-known that the electric double layer plays important roles in a variety of applications, ranging from biology to materials sciences. Many studied the electric double layer using a variety of techniques, and as a result our understanding is mature, although not complete. Based on detailed understanding, we expect that by manipulating the electric double layer we could advance tremendously applications in the water-energy nexus. This is particularly true for electric double layer capacitors and capacitive desalination devices. However, such manipulation is not straightforward because of a competition of phenomena that occur within the electric double layer itself, including solvation effects, excluded volume phenomena, and ion–ion correlations. Using molecular dynamics simulations, we designed a composite graphene-based electrode to manipulate structural and dynamical properties of the electric double layer. Our design favors the formation of the compact Helmholtz layer. Inherent to our design is that the compact Helmholtz layer not only is atomically thick, but it is also highly mobile in the direction parallel to the charged surface. We suggest here how to exploit the properties of the engineered electric double layer toward developing a new continuous desalination process that combines the advantages of membrane and capacitive desalination processes, reducing their shortcomings. Insights on the molecular mechanisms relevant to the water-energy nexus are provided.