Brackish water desalination using reverse osmosis and capacitive deionization at the water-energy nexus

Can capacitive deionization outperform reverse osmosis for brackish water desalination?

Desalination and disinfection of inland brackish ground water in a capacitive deionization cell using nanoporous activated carbon cloth electrodes

Research progress of sodium super ionic conductor electrode materials for capacitive deionization

Both brackish water desalination and seawater desalination processes are well established and in common use around the globe to create new water supply sources. The farther the location of the source water from the ocean or seashore, the lower the salinity (TDS) of the water and the lower the osmotic pressure that needs to be overcome when desalinated water is produced. This is one of the major reasons that brackish desalination is often considered less costly than seawater desalination. A number of project considerations, however, indicate that seawater desalination can be beneficial and more cost-effective than brackish water desalination. To make a fair comparison, we need to properly compare all major aspects of both types of projects to define the best and most appropriate desalination technology. While brackish water has less feed water TDS, it is more challenging to dispose of the produced concentrate. Also, although brackish water desalination needs less energy to overcome osmotic pressure, it usually requires more energy to draw the water from the well than it takes to pump seawater from the open ocean intake. Another factor is that the temperature of the brackish well water may be lower than the temperature of ocean water, giving seawater desalination an advantage in energy demand. In comparing brackish to seawater desalination, these major aspects should be evaluated: (1) Locations of seawater and brackish water plants, relative to the major consumers of the desalinated water, (2) Transportation (pumping and disposal) costs of the feed water and produced water, (3) Potential colocation of a seawater plant with a large industrial user (e.g., power plant) of the seawater for cooling or other purposes, (4) Produced quality of brackish water and seawater desalination in terms of major minerals and emerging contaminants, (5) Sustainability of the water source: capacity and depth of the brackish water wells, as well as the type of soil. (6) Technical and economic aspects of produced concentrate disposal, (7) Permitting process costs for brackish and seawater desalination, and (8) The economics of both brackish and seawater desalination treatment processes: capital costs, operational and maintenance (O&M) costs, lifetime water cost, and total water cost (TWC). This paper discusses the major evaluation criteria and considerations involved in properly comparing the economic and technical aspects of brackish and seawater desalination to determine the more favorable desalination technology for a given desalination project.

Reverse osmosis pilot plants performance in brackish water desalination

Although the energy efficiency of brackish water capacitive deionization (CDI) and reverse osmosis (RO) have been extensively compared, their relative costs remain poorly defined. We develop a parametric model to estimate the levelized cost of water (LCOW) of three CDI configurations (CDI, membrane CDI, and flow electrode CDI) and compare it with the LCOW of brackish water RO calculated using a process-based optimization model. We find significant deviations between cost-optimal and energy-optimal RO design and operation, highlighting the importance of LCOW in comparative evaluations of desalination technologies. Our results suggest that material (including electrode and ion exchange membrane) costs are the largest cost component for CDI processes. As such, the economic viability of CDI critically depends on the component lifespan, with lifespans longer than 1 year (105 cycles for 5 min cycle duration) required to reduce brackish water desalination costs relative to RO. Finally, sensitivity analyses indicate that CDI processes are unlikely to be cost-competitive against RO for feedwater concentrations greater than 2 g/L. Future research to enhance the economic feasibility of CDI processes should focus on developing more durable electrodes, increasing cost-normalized electrode capacitance, and developing low-cost ion exchange membranes and coatings.

Comprehensive Summary Access to safe and clean water is fundamental to human health and economic development. While the practical impact of emerging technologies depends on their successful demonstration at large scales, capacitive deionization (CDI) has garnered significant attention as a promising approach for efficient desalination of seawater and brackish water. Among the various 2D materials explored for CDI (e.g., graphene, MXenes, covalent organic frameworks), their derived 2D/2D heterostructures, with unique lamellar morphology and interfacial engineering, offer an ideal platform for effectively modulating charge transfer behavior and ion diffusion. Despite a variety of 2D/2D heterostructures with diverse construction modes have been developed as CDI electrodes in recent years, a dedicated review focusing on the design strategies, synergistic effects, water desalination performance, and prevailing challenges remains lacking. In this review, we highlight the cutting‐edge research progress of 2D/2D heterostructures for CDI applications. After an overview of 2D materials and synthetic strategies of 2D/2D heterostructures, the relationships between the morphology/structure/composition and the water desalination performance are discussed in detail. Thereafter, we discuss current limitations and propose future directions for the rational design of 2D/2D heterostructures. This review will promote exploitation of 2D/2D heterostructures with an ideal performance of CDI towards water remediation. Key Scientists Significant progress has been made in the development of 2D/2D heterostructures for capacitive deionization (CDI) applications towards versatile ion capture. This collection of pioneering work underscores a clear trajectory in the field: the strategic construction of 2D/2D heterostructures is a powerful and versatile paradigm for advancing CDI. By intelligently combining different 2D materials, researchers have successfully engineered heterointerfaces with enhanced ion adsorption capacity, superior selectivity, and improved stability, paving the way for next‐generation, high‐performance desalination and water remediation technologies.

Desalination of brackish water by nanofiltration and reverse osmosis

Developing cost-effective brackish water and seawater desalination technology is crucial. Capacitive deionization (CDI) and inverted capacitive deionization (i-CDI) have been recognized as a promising desalination technology with low energy consumption for brackish water.Both systems use an electric field between two porous electrodes to remove and release ions in water reversibly.Recently, CDI has started using pseudocapacitive or battery electrode materials to enhance the desalination performance.Different from the desalination principle of electric double layers, the energy storage mechanisms of pseudocapacitive and battery materials generally involve ion intercalation/adsorption or compound formation for charge balance, leading to the faradaic desalination.Low cost and high theoretical capacity make PBAs and conducting polymers be the potential materials for the faradaic desalination application.In our previous studies, two dissimilar pseudocapacitive materials show a memory effect during brackish water desalination. This allows them to retain ion capturing or releasing states without an electric field, aiding water purification and resource recovery.Based on above viewpoints, two materials with fundamentally different electrochemical properties are demonstrated to construct a high-capacity, hybrid, faradaic deionization system with CuHCF (battery type) as the positive electrode and PPy (pseudocapacitive type) as the negative electrode.The deionization performance of this CuHCF//PPy cell can be improved by reversing the appropriate cell voltage during the discharge process.The plot of specific SRC against time for the above CuHCF//PPy cell with variations in the charging cell voltage but a fixed discharging cell voltage of 0 V. In addition, Fig. 1(b) shows the plot of specific SRC against time for the same cell with variations in the discharging cell voltage but a fixed charging cell voltage of 1.2 V. From Fig. 1(a), at all charging voltages, the SRC generally decreases with the charging time, indicating the ion repelling process. The order of charging cell voltage with respect to increasing the SRC value is: 0.6 V < 0.8 V < 1.0 V < 1.2 V, revealing the impact of the charging cell voltage. From Fig. 1(b), the SRC obviously increase with prolonging the discharging time, suggesting the ion capturing process. However, the order of discharging cell voltage with respect to increasing the SRC value is: -0.4 V < 0 V < −0.1 V< −0.2 V < −0.3 V. Note that a little inverted cell voltage leads to a higher salt-removing capacity and rate in comparison with a discharge cell voltage of 0 V. However, when the inverted voltage is set at −0.4 V, the SRC profile exhibits the unstable performance, probably due to the presence of certain irreversible reactions at this cell voltage.In the stability test, Fig 2 shows the CuHCF//PPy cell still maintained more than 90% of its original SRC after 50 cycles of testing.The mean SRC values of this CuHCF//PPy system obtained from the 8, 15, and 30 mM solutions reached 35.536, 58.824, and 101.84 mg g−1, respectively.This positive correlation between SRC and solution concentration reveals the higher utilization of the electroactive materials in more concentrated solutions and the very high SRC of the hybrid faradaic CuHCF//PPy system.From Fig 3 shows the SRC values of a CuHCF//PPy cell with the charging/discharging times of 30/30 min at the charging/discharging cell voltages of 1.2/−0.2 V in the 8, 15, and 30 mM NaCl solutions.The memory effect of electrochemically active materials can further extend the application of this system to concentrating valuable ions while purifying water, showing another advantage.The results of ion-removing and salt-concentrating experiment with the discharge/charge times = 10 min/10 min for 10 cycles. In this 10-cycle test, the total amount of salts transferred is up to 152.9 mg g−1, revealing the dual function of purifying water and concentrating salts through this hybrid battery//pseudo-capacitive system.This hybrid cell showed high salt removal capacities in the media containing various monovalent and divalent cations. In this work, the suitable working potential windows of both CuHCF and PPy were systematically evaluated by CV and GCD methods with the charge balance application. Moreover, this cell provides the ability in capturing other cations such as Mg2+ and Ca2+, further broadening its future potential applications. This methodology is a promising strategy for constructing a high-performance desalination cells consisting of various active materials. Figure 1

Desalination of high salinity brackish water by an NF-RO hybrid system

A nanofibrous composite reverse osmosis (RO) membrane with a polyamide barrier layer containing interfacial water channels was fabricated on an electrospun nanofibrous substrate via an interfacial polymerization process. The RO membrane was employed for desalination of brackish water and exhibited enhanced permeation flux as well as rejection ratio. Nanocellulose was prepared by sequential oxidations of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and sodium periodate systems and surface grafting with different alkyl groups including octyl, decanyl, dodecanyl, tetradecanyl, cetyl, and octadecanyl groups. The chemical structure of the modified nanocellulose was verified subsequently by Fourier transform infrared (FTIR), thermal gravimetric analysis (TGA), and solid NMR measurements. Two monomers, trimesoyl chloride (TMC) and m-phenylenediamine (MPD), were employed to prepare a cross-linked polyamide matrix, i.e., the barrier layer of the RO membrane, which integrated with the alkyl groups-grafted nanocellulose to build up interfacial water channels via interfacial polymerization. The top and cross-sectional morphologies of the composite barrier layer were observed by means of scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) to verify the integration structure of the nanofibrous composite containing water channels. The aggregation and distribution of water molecules in the nanofibrous composite RO membrane verified the existence of water channels, demonstrated by molecular dynamics (MD) simulations. The desalination performance of the nanofibrous composite RO membrane was conducted and compared with that of commercially available RO membranes in the processing of brackish water, where 3 times higher permeation flux and 99.1% rejection ratio against NaCl were accomplished. This indicated that the engineering of interfacial water channels in the barrier layer could substantially increase the permeation flux of the nanofibrous composite membrane while retaining the high rejection ratio as well, i.e., to break through the trade-off between permeation flux and rejection ratio. Antifouling properties, chlorine resistance, and long-term desalination performance were also demonstrated to evaluate the potential applications of the nanofibrous composite RO membrane; remarkable durability and robustness were achieved in addition to 3 times higher permeation flux and a higher rejection ratio against commercial RO membranes in brackish water desalination.

Desalination technologies are expected to play an important role in producing clean water in the future, resulting a surge in the research and development of energy efficient and cost effective technologies for desalination of seawater and brackish water. Membrane-based desalination technologies such as reverse osmosis (RO) are the most commercially used desalination methods for seawater and treatment of agricultural water, but are highly energy intensive and the future development of desalination is dependent on finding more energy efficient technologies. Capacitive deionization (CDI) is an emerging technology for water desalination, and is based on the phenomenon of ion electrosorption. Simple CDI is an energy efficient technology and its applications for desalination of low molar concentration streams, like brackish water, is demonstrated. However, to expand the applications, research is focused on improving the efficiency and salt removal capacity of CDI systems. To this end, the important requirements are finding electrode materials with higher capacities and CDI systems with higher efficiencies. Also, the large-scale application of CDI for personal and industrial applications is dependent on designing CDI systems with potential to be implemented at different capacities and length scales. In this work, we have studied the CDI for removal of salt and nutrients from agricultural water. Various high surface area electrode materials were considered and the efficiency of CDI for removal is measured via downstream analyses on batch experiments and reported, allowing development of preliminary parameters for optimizing CDI systems for high-strength agricultural wastewaters.

The enhanced performance of NaFe2PO4(SO4)2/C electrode materials in the desalination of brackish water by capacitive deionization

Modeling technologies for desalination of brackish water — toward a sustainable water supply

Side effects of antiscalants on biofouling of reverse osmosis membranes in brackish water desalination