Continuous Water Desalination Research Articles

Single modular flow-electrode capacitive deionization using modified Prussian blue analogues as cation intercalation electrode for continuous water desalination.

Ion back-diffusion hinders the practical application of conventional flow-electrode capacitive deionization (FCDI) under long-term operational conditions. To address this challenge, the present study integrated cation intercalation deionization (CID) with FCDI. A novel PFCDI-CID system was developed by utilizing a modified Prussian blue analogues owing to their enhanced rheological and electrochemical properties. The PFCDI-CID system achieved a high charge efficiency of 89.77% and an energy-normalized removal salt of 0.69 mol kJ-1 in single-cycle (SC) mode with the flow electrodes mass fraction of 2% and a desalinized water chamber-to-concentrated saline water chamber ratio of 2:1. Furthermore, under continuous operation for 12 h in SC mode, the PFCDI-CID system maintained stable desalination performance within the first 2 h. Over an extended duration, the average charge efficiency of the PFCDI-CID system was maintained at 88.44%, with an average energy-normalized removal salt of 0.65 mol kJ-1. The mechanism revealed that the desalination process involving the Prussian blue analogues primarily involves Na+ intercalation, accompanied by a small amount of electro-sorption process. This system exhibits the characteristics of conventional FCDI while enabling desalination and concentration of simulated saline water during brine discharge, thereby mitigating the impact of ion back-diffusion and broadening the application scope of FCDI.

New insights into the performance analysis of flow-electrode capacitive deionization using ferri/ferrocyanide redox couples for continuous water desalination

Flow Electrode Capacitive Deionization (FCDI) is a promising technology that uses carbon materials as flow electrodes in aqueous conditions to enable continuous desalination. In this work, the parametric investigation of the FCDI cell was carried out for low and high surface area activated carbon. When 1 mM/1 mM ferri-/ferrocyanide redox couple in the 1 M NaCl supporting electrolyte containing the 5 wt% high surface area activated carbon and 0.5 wt% MWCNT is circulated as flow electrode at the flow rate of 8 mL/min in the electrode chamber and 0.1 mM NaCl feed water is circulated in single-pass mode at 8 mL/min, the FCDI system achieved a significant improvement in desalination performance up to 76.38 % of SRE, with a high average salt removal rate of 43.925 µg/cm2s @ 2 V, charge efficiency of 20.11 % and average energy consumption of 0.27 kWh/mol, through a thorough parametric investigation. When the FCDI cell was tested with tap water, the average salt removal rate, average energy consumption, and salt removal efficiency were calculated at 6.11 μg/cm2s, 2.46 kWh/mol, and 46.61 % respectively, resulting in the reduction of total dissolved solids of 600 mg/L at the inlet to 365 mg/L at the outlet. The combination of microporous high surface area carbon and ferro-ferri redox couple propelled the desalination performance in comparison with the macroporous low surface area carbon, maximizing the ion removal rate, which further reduced the energy consumption and operating voltage for the process.

High-performance and scalable wood-based solar-driven interfacial evaporator with corrugated structure for continuous desalination

Interfacial solar-driven evaporation of saline water is a promising technique that enables worldwide upscaling of freshwater with a minimum of carbon footprint. For this purpose, wood is demonstrating a desirable substrate due to its high porosity and low thermal conductivity. Despite much effort has been dedicated to improving the performance of wood evaporators, their scalability and reliability still restrict applications. Here we present a large-scale and highly-efficient wood-based evaporator with corrugated sheet structures using a facile mechanical processing strategy that utilizes the excellent processability of wood. The corrugated structure of the longitudinal wood block, transports water in the longitudinal direction and thereby increases the evaporation area. With the inclusion of a Chinese ink layer and hydrophobic treatment, the prepared wood-based evaporators exhibit a high evaporation rate (1.77 kg m−2 h−1 for spruce wood, 1.42 kg m−2 h−1 for beech wood) and efficiency (111.53% for spruce wood, 88.73% for beech wood) at 1 sun irradiation and continuous water desalination, which is superior to both reported longitudinal and most transverse wood-based evaporators. This work demonstrates a universal and economy strategy for preparing large-sized wood-based solar evaporators with processability, stable performance and high efficiency.

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.

A Multiscale Porous 3D‐Fabric Evaporator with Vertically Aligned Yarns Enables Ultra‐Efficient and Continuous Water Desalination

AbstractSolar steam generation has emerged as an environment‐friendly and promising strategy to overcome the freshwater and energy crisis. However, simultaneously attaining the satisfied evaporation rate, durability and energy utilization in a single system is challengeable, but crucial to the practical application. Herein, inspired by the carpet weaving technique and benefiting from “fiber–yarn–fabric” hierarchical structures, a 3D fabric evaporator with vertical hemp‐yarn arrays is innovatively constructed, featuring a combined structure of multiscale pores from tunable spacing between adjacent fibers or yarns, the vertically aligned architecture coated with superhydrophilic MXene‐sandwiched layer and the adjustable side height/areas. These structures work cooperatively, surprisingly achieving the integrated functions of excellent light capture, the balanced evaporation area and vapor escape space, the additional energy adsorption from the environment and the salt exchange along or between micro/macro pores. Consequently, the 3D evaporator exhibits an outstanding evaporation rate (3.95 kg m−2 h−1 under one‐sun illumination) and extremely high outdoor evaporation (47.04 kg m−2 for 8 h), accompanied with outstanding salt‐resistance (continuous 120 h in 14% brine without salt accumulation), anti‐oilfouling and anti‐bacterial (efficiency over 99.9%) performances. Finally, through rationally assembling multi‐function modules with a condenser, an all‐in‐one “desalination–thermoelectricity–irrigation” platform is built, realizing the maximum utilization of steam condensation enthalpy.

Manipulating hydropathicity/hydrophobicity properties to achieve anti-corrosion copper-based membrane toward high-efficient solar water purification

The development of corrosion-resistant and highly effective solar-thermal evaporators for water purification is of key importance to alleviate freshwater scarcity around the world. Herein, we propose a novel evaporator by integrating polydopamine and n-dodecanethiol-decorated Cu@Cu2O foam into an organic and inorganic hybrid membrane (PTDC). A high solar absorbance of 92% was observed from the PTDC under 1 sun irradiation, leading to an efficient localized photothermal conversion. The intrinsic hydrophilicity of PTDC allows it to exhibit outstanding performance for salts, dyes, and heavy metals removal in both simulated seawater and wastewater, with a water evaporation rate of 1.58 kg m−2 h−1 and high rejection ratios of over 99%. More crucially, the surface chemical treatment of copper-based foam demonstrated excellent corrosion-resistant ability, enabling the PTDC membrane to be utilized for continuous water desalination and decontamination. Taken together, the PTDC membrane offers great potential for the future application of long-term practical solar-energy-driven water purification.

Upscaling 3D Engineered Trees for Off-Grid Desalination.

More than 70% of the population without access to safe drinking water lives in remote and off-grid areas. Inspired by natural plant transpiration, we designed and tested in this study an array of scalable three-dimensional (3D) engineered trees made of natural wood for continuous water desalination to provide affordable and clean drinking water. The trees took advantage of capillary action in the wood xylems and lifted water more than 1 foot off the ground with or without solar irradiation. This process overcame some major challenges of popular solar-driven water evaporation and water harvesting, such as intermittent operation, low water production rate, and system scaling. The trade-off between energy transfer and system footprint was tackled by optimizing the interspacing between the trees. The scaled system has a ratio of surface area (vapor generation) to project area (water transport) up to 118, significantly higher than the prevailing flat-sheet design. The extensive surface area evaporated water at a temperature cooler than the surrounding air, drawing on multiple environmental energy sources including solar, wind, or ambient heat in the air and realized continuous operation. The total energy for evaporation reached over 300% of the one-sun irradiance, enabling a freshwater production rate of 4.8 L m-2 h-1 from an array of 16 trees in an enclosed room and 14 L m-2 h-1 under a 3 m/s airflow. Furthermore, we found that the ambient heat in the air contributed 60%-70% of the total latent heat of vaporization when energy sources were decoupled. During long-term desalination tests, the engineered trees demonstrated a self-cleaning mechanism with daily cycles of salt accumulation and dissolution. Combining the quantification from an evaporation model and meteorology data covering the globe, we also demonstrated that the 3D engineered trees can be of particular interest for sustainable desalination in the Middle East and North Africa (MENA) regions.

High-Performance Salt-Rejecting Solar Vapor Generator from Natural Luffa Vine for Continuous Water Desalination

High-Performance Salt-Rejecting Solar Vapor Generator from Natural Luffa Vine for Continuous Water Desalination

Dual-channel membrane capacitive deionization based on asymmetric ion adsorption for continuous water desalination

Capacitive deionization (CDI) systems are well known for their ability to efficiently desalinate water by adsorbing ions onto two electrodes through the application of an electrical potential difference. In this study, we report a continuous desalination device composed of a novel dual-channel CDI cell architecture containing three ion exchange membranes (CAC-MCDI). The adsorption of ions and electrode regeneration from the desorption ions were performed separately in the two channels and enabled continuous water desalination by the application of an intermittent voltage reversal. Additionally, the novel cell architecture performed an electrodialysis-like process when the adsorption of ions on the electrode reached saturation, demonstrating a synergistic effect. For the first time, a CDI system is modified for the asymmetric adsorption of cations and anions, to reveal the effect of the migration rate difference of ions. Fully making use of ion diffusion differences as well as the synergistic effect can effectively improve the performance of the system and surprisingly increase the charge efficiency above 100%. The parameters for the desalination process were first optimized by response surface methodology (RSM), which predicted that the salt removal efficiency and charge efficiency could reach 69.29% and 103.60%, respectively. Both the experimental results and the predicted values from the model indicated that asymmetric ion adsorption capabilities of CAC-MCDI allowed the system to achieve excellent performance and provide continuous water desalination.

Design and synthesis of two novel carbon aerogels using citric and tartaric acids as catalysts for continuous water desalination

Design and synthesis of two novel carbon aerogels using citric and tartaric acids as catalysts for continuous water desalination

A MXene‐Based Hierarchical Design Enabling Highly Efficient and Stable Solar‐Water Desalination with Good Salt Resistance

AbstractA solar‐thermal water evaporation structure that can continuously generate clean water with high efficiency and good salt rejection ability under sunlight is highly desirable for water desalination, but its realization remains challenging. Here, a hierarchical solar‐absorbing architecture is designed and fabricated, which comprises a 3D MXene microporous skeleton with vertically aligned MXene nanosheets, decorated with vertical arrays of metal–organic framework‐derived 2D carbon nanoplates embedded with cobalt nanoparticles. The rational integration of three categories of photothermal materials enables broadband light absorption, efficient light to heat conversion, low heat loss, rapid water transportation behavior, and much‐improved corrosion and oxidation resistance. Moreover, when assembling with a hydrophobic insulating layer with hydrophilic channel, the MXene‐based solar absorber can exhibit effective inhibition of salt crystallization due to the ability to advect and diffuse concentrated salt back into the water. As a result, when irradiating under one sun, the solar‐vapor conversion efficiency of the MXene‐based hierarchical design can achieve up to ≈93.4%, and can remain over 91% over 100 h to generate clean vapor for stable and continuous water desalination. This strategy opens an avenue for the development of MXene‐based solar absorbers for sustainable solar‐driven desalination.

Chinese ink enabled wood evaporator for continuous water desalination

Solar-driven water desalination is considered as an efficient and sustainable approach to produce clean water. In this work, a bilayered evaporator composed of black Chinese ink layer and wood sheet substrate for continuous desalination is rationally designed. Millimeter-sized hole arrays were drilled within the wood substrate. The ink coating on the wood surface offers strong and broad light absorption. The hierarchical channels in the wood itself functions as thermal insulator and promotes water transport. In addition, the drilled hole arrays provide salt-rejection pathways and enable the wood evaporator with antifouling property. The wood evaporator (H-C-Wood) shows a stable solar thermal conversion efficiency of 74% and an evaporation rate of 1.6 kg m−2 h−1 under simulated 1 sun irradiation. Such ink deposited solar steam generation device is low cost and has a self-regenerating ability.

Performance evaluation of continuous solar still water desalination system

In this paper, the experimental study aims to enhance a distillate water productivity of solar stills. To achieve this objective, solar still with continuous water desalination along 24 h was designed, structured, and operated in the city of Tanta, Gharbia Governorate, Egypt. For the solar stills, the temperatures of the condensing surface are very low throughout the night and during the early hours of the morning. This advantage makes the uses of the solar stills with continuous desalination along 24 h to be more effective. To achieve the continuous water desalination, the electric water heater (EWH) was installed in the basin of solar still to heat the basin water in the night and during the early hours of the morning. The EWH is powered by solar batteries that are charged by PV panels during sunrise. The experimental results presented that the accumulated distillate water productivity for using the continuous water desalination along 24 h varied between 10,631 and 12,087 mL m−2 day−1 after period use of 2 May to 11 August. The improvement in accumulated productivity of solar still with continuous water desalination along 24 h varied between 159.3 and 177.9% as compared to solar still working without EWH. Also, the improvement in the overall efficiency of solar still with continuous water desalination varied between 27.9 and 31.3%.

Measurements of the Electric Conductivity of MWCNT Suspension Electrodes with Varying Potassium Bromide Electrolyte Ionic Strength

Suspension electrodes are under intense investigation due to their use in electrochemical systems such as redox flow batteries, capacitive deionization cells and flow supercapacitors. They provide novel functionalities not available with static electrodes, such as spatial power-energy decoupling for flow hybrid batteries, and continuous water desalination by capacitive deionization cells. Yet their electric conductivity is low relative to static electrodes made of the same material. It was previously found that suspension electrodes employing multi-walled carbon nanotubes (MWCNTs) demonstrate among the highest electric conductivities in deionized water, yet the effect of electrolyte ionic strength on their electric conductivity of was not elucidated. We here measure the electric conductivity of MWCNTs suspensions in a KBr electrolyte under flow using two- and four-electrode setups with alternative and direct current (DC) techniques. Only the two-electrode DC technique measured solely electronic conductivity, while all the other techniques measured the combined ionic and electronic conductivity. The electronic conductivity increased with MWCNT concentration in accordance with percolation theory power law. It increased also with electrolyte concentration by one to two orders of magnitude, with the critical exponent value transitioning from ∼2 to ∼1 with the increasing KBr concentration, which was explained in terms of the Derjaguin-Landau-Verwey-Overbeek theory.

A High-Performance Self-Regenerating Solar Evaporator for Continuous Water Desalination.

Emerging solar desalination by interfacial evaporation shows great potential in response to global water scarcity because of its high solar-to-vapor efficiency, low environmental impact, and off-grid capability. However, solute accumulation at the heating interface has severely impacted the performance and long-term stability of current solar evaporation systems. Here, a self-regenerating solar evaporator featuring excellent antifouling properties using a rationally designed artificial channel-array in a natural wood substrate is reported. Upon solar evaporation, salt concentration gradients are formed between the millimeter-sized drilled channels (with a low salt concentration) and the microsized natural wood channels (with a high salt concentration) due to their different hydraulic conductivities. The concentration gradients allow spontaneous interchannel salt exchange through the 1-2 µm pits, leading to the dilution of salt in the microsized wood channels. The drilled channels with high hydraulic conductivities thus function as salt-rejection pathways, which can rapidly exchange the salt with the bulk solution, enabling the real-time self-regeneration of the evaporator. Compared to other salt-rejection designs, the solar evaporator exhibits the highest efficiency (≈75%) in a highly concentrated salt solution (20 wt% NaCl) under 1 sun irradiation, as well as long-term stability (over 100 h of continuous operation).

Modeling of a continuous water desalination process using directional solvent extraction

Directional Solvent Extraction (DSE) is a novel alternative desalination technology to the traditional evaporation and membrane based desalination processes. This new process takes advantage of the fact that water is soluble in some solvents and its solubility increases with temperature. Furthermore, these solvents are insoluble in water and salts do not dissolve in them. These special characteristics of some “directional solvents” have been utilized in this new process to extract fresh water from seawater.The objective of this paper is to investigate the technical feasibility of a continuous water desalination process using decanoic and octanoic acid as directional solvents. Process modeling was used to predict thermal and electrical energy consumptions for two DSE processes, with and without heat recovery. The effects of highest process temperature, recovery ratio and heat exchanger effectiveness on the thermal energy consumption were studied. The results show that heat recovery significantly reduces the thermal energy consumption. The results also show that octanoic acid has lower thermal and electrical energy consumptions than decanoic acid. Compared to the Multi Stage Flashing (MSF) and Multi Effect Distillation (MED), DSE water desalination process results in higher energy consumption.In conclusion, the results reveal the need to identify more suitable directional solvents with higher product water yield and lower water solubility in order to facilitate the application of DSE as a potential water desalination process.

Mathematical modeling assisted investigation of forward osmosis as pretreatment for microbial desalination cells to achieve continuous water desalination and wastewater treatment

Coupling forward osmosis (FO) with microbial desalination cells (MDCs) can lead to some synergistic benefits, i.e. simultaneous water recovery, desalination and removal of COD (chemical oxygen demand). Herein, an FO-MDC system was developed and examined in a continuously-operated mode. It achieved 70.6% COD removal from a synthetic wastewater and 94.0% conductivity decrease in a salt solution containing 35gL−1 NaCl. The flow rates of both wastewater and salt water exerted strong effects on the system performance. At a draw flow rate of 0.02mLmin−1, the effluent TDS (total dissolved solids) reached 315mgL−1, lower than the maximum contaminant levels of the National Secondary Drinking Water Regulations. A mathematical model was developed for the first time through integrating FO model with MDC model. This model could well predict the key performance parameters such as current generation and desalination effectiveness and rate. When further tested with actual domestic wastewater amended with glucose and NaHCO3, the FO-MDC system generated a desalinated effluent containing 1130mgTDSL−1. The results of this study suggest that proper coordination of the treatment capacity of the two membrane-based treatment processes could create a potentially effective system for wastewater reuse and seawater desalination.