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•Novel positively charged nanofiltration membrane is developed for heavy metal removal.•The prepared BTC-PEI membrane has a high permeability and selectivity.•The relationship between ...the removal efficiency and the properties of metal ions has been fitted.
The toxic heavy metals produced by the discharge of industrial wastewater pose a serious threat to the ecological environment and human health. Nanofiltration (NF) membrane separation technology is widely used in fields such as water softening, heavy metal removal and dye separation due to its environmental friendliness and low cost. Herein, a novel positively charged aliphatic polyamide NF membrane (PEI-BTC) has been developed by using 1,2,3,4-cyclobutane tetracarboxylic acid chloride (BTC) monomer bearing a stereoscopic structure which undergoes classic interfacial polymerization (IP) with polyethyleneimine (PEI) on the Polyether sulfone (PES) support membrane. The physicochemical properties revealed that the PEI-BTC membrane had a larger mean effective pore size (0.285 nm), a thinner separation layer (40 nm) and a stronger positively charged membrane surface (IEP = 7.25) than the traditional PEI-TMC membrane. Compared with previously reported PEI-based and commercial NF membranes, the optimized PEI-BTC membrane exhibits a higher MgCl2 (2000 ppm) rejection of 97.53% and pure water flux of 156.85 kg·m−2·h−1 at 1.0 MPa. Moreover, the prepared PEI-BTC NF membrane shows excellent toxic heavy metal (1000 ppm) removal efficiency in the order of Mn (98.78%) > Zn (98.32%) > Ni (97.74%) > Cu (95.67%) > Cd (90.49%). The results demonstrate that the prepared positively charged aliphatic polyamide NF membrane (PEI-BTC) has a unique industrial production potential for water softening and heavy metal removal.
In recent decades, the environmentally benign electrochemical softening process has been gaining widespread interest as an emerging alternative for water softening. But, in spite of decades of ...research, the fundamental advances in laboratory involving electrolytic cell design and treatment system development have not led to urgently needed improvements in industrially practicable electrochemical softening technique. In this review, we firstly provide the critical insights into the mechanism of the currently widely used cathode precipitation process and its inherent limitations, which seriously impede its wide implementation in industry. To relieve the above limitations, some cutting-edge electrochemically homogeneous crystallization systems have been developed, the effectiveness of which are also comprehensively summarized. In addition, the pros and cons between cathode precipitation and electrochemically homogeneous crystallization systems are systematically outlined in terms of performance and economic evaluation, potential application area, and electrolytic cell and system complexity. Finally, we discourse upon practical challenges impeding the industrial-scale deployment of electrochemical water softening technique and highlight the integration of strong engineering sense with fundamental research to realize industry-scale deployment. This review will inspire the researchers and engineers to break the bottlenecks in electrochemical water softening technology and harness this technology with the broadened industrial application area.
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•The applications of CDI from ion-selectivity to resource recovery.•Mechanisms for selective removal of Ca2+ and Mg2+ were defined and categorized.•Recent advances in electrode ...materials for Ca2+ and Mg2+ removal in CDI.•Clarified the impact of operating conditions and ionic properties on ion-selectivity.•Future perspectives and challenges for Ca2+ and Mg2+ selective removal were raised.
Hardness problems caused by Ca2+ and Mg2+ significantly impact human production and life, greatly hindering the effective use of water resources. Meanwhile, the effective extraction of mineral resources such as calcium and magnesium from industrial waste brine has triggered widespread concern. Due to its sustainability, cost-effectiveness, and low energy consumption, CDI in water treatment applications began to be constantly expanded in recent years, and its research in Ca2+ and Mg2+ selective removal showed a trend of increasing year by year. However, no comprehensive review offers a thorough analysis of this topic. Therefore, our review critically assesses CDI research advances related to calcium and magnesium ion elimination. This article reviews and categorizes general aspects of CDI and selective calcium and magnesium removal mechanisms, summarizing relevant electrode materials and highlighting factors influencing process performance. Following this, this review summarizes the current scenarios for selective application and envisions potential future applications. Finally, the challenges and perspectives in implementing CDI for the selective removal of calcium and magnesium ions have been discussed. The insights from this review will promote the further development of CDI in the field of Ca2+ and Mg2+ selective removal, as well as advance the in-depth research of CDI in ion-selective removal.
Capacitive deionization (CDI) for removal of water hardness was investigated for water softening applications. In order to examine the wettability and pore structure of the activated carbon cloth and ...composites electrodes, surface morphological and electrochemical characteristics were observed. The highly wettable electrode surface exhibited faster adsorption/desorption of ions in a continuous treatment system. In addition, the stack as well as unit cell operations were performed to investigate preferential removal of the hardness ions, showing higher selectivity of divalent ions rather than that of the monovalent ion. Interestingly, competitive substitution was observed in which the adsorbed Na ions were replaced by more strongly adsorptive Ca and Mg ions. The preferential removal of divalent ions was explained in terms of ion selectivity and pore characteristics in electrodes. Finally, optimal pore size and structure of carbon electrodes for efficient removal of divalent ions were extensively discussed.
Desalination and softening of sea, brackish, and ground water are becoming increasingly important solutions to overcome water shortage challenges. Various technologies have been developed for salt ...removal from water resources including multi-stage flash, multi-effect distillation, ion exchange, reverse osmosis, nanofiltration, electrodialysis, as well as adsorption. Recently, removal of solutes by adsorption onto selective adsorbents has shown promising perspectives. Different types of adsorbents such as zeolites, carbon nanotubes (CNTs), activated carbons, graphenes, magnetic adsorbents, and low-cost adsorbents (natural materials, industrial by-products and wastes, bio-sorbents, and biopolymer) have been synthesized and examined for salt removal from aqueous solutions. It is obvious from literature that the existing adsorbents have good potentials for desalination and water softening. Besides, nano-adsorbents have desirable surface area and adsorption capacity, though are not found at economically viable prices and still have challenges in recovery and reuse. On the other hand, natural and modified adsorbents seem to be efficient alternatives for this application compared to other types of adsorbents due to their availability and low cost. Some novel adsorbents are also emerging. Generally, there are a few issues such as low selectivity and adsorption capacity, process efficiency, complexity in preparation or synthesis, and problems associated to recovery and reuse that require considerable improvements in research and process development. Moreover, large-scale applications of sorbents and their practical utility need to be evaluated for possible commercialization and scale up.
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•In-depth analysis of progresses in adsorption for desalination are provided.•Governing mechanisms along with associated isotherms are described.•Synthesis, preparation and modification of adsorbent materials are elaborated.•Trends of diverse adsorption-based processes for desalination are addressed.•Principles and operations of large-scale adsorption processes are discussed.
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•The pH split phenomenon spontaneously occurs in bioelectrochemical electrolytes.•Energy consuming BES can exploit the pH split to boost the process performance.•The pH split promotes ...the anode acidification and the cathode alkalization.•The anode acidification can be used for carboxylic acid concentration.•The cathode alkalization can be used for biogas upgrading and ammonium recovery.
Typical reactions in bioelectrochemical systems (BESs) promote the phenomenon of the pH split between anode and cathode. The pH split results in an undesirable phenomenon which has stimulated several technological solutions to limit its effects, particularly for energy-producing bioelectrochemical systems (BESs). On the other hand, several applications of energy-consuming BESs exploited the pH split to integrate different operations using the bioelectrochemical reactions. Those additional operations, which are directly related to the electric field generated by the bioelectrochemical interphases, include target products extraction, concentration, and recovery. This review offers a comprehensive overview of the different bioelectrochemical applications in which the pH split is used for the integration of bioelectrochemical reactions with products concentration and recovery. By discussing the phenomenon of the pH split in BESs, this paper presents an alternative view to stimulate new niches of applications for the bioelectrochemical processes.
Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water ...reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518−521 ), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.
•A sulfonated activated carbon cathode enhances divalent cation selectivity.•Perfect selectivity achieved with calcium electrosorbed and not sodium.•The separation process consumes low energy of less ...than 0.1 kWh/m3.
Capacitive deionization (CDI) is an emerging membraneless water desalination technology based on storing ions in charged electrodes by electrosorption. Due to unique selectivity mechanisms, CDI has been investigated towards ion-selective separations such as water softening, nutrient recovery, and production of irrigation water. Especially promising is the use of activated microporous carbon electrodes due to their low cost and wide availability at commercial scales. We show here, both theoretically and experimentally, that sulfonated activated carbon electrodes enable the first demonstration of perfect divalent cation selectivity in CDI, where we define “perfect” as significant removal of the divalent cation with zero removal of the competing monovalent cation. For example, for a feedwater of 15 mM NaCl and 3 mM CaCl2, and charging from 0.4 V to 1.2 V, we show our cell can remove 127 μmol per gram carbon of divalent Ca2+, while slightly expelling competing monovalent Na+ (-13.2 μmol/g). This separation can be achieved with excellent efficiency, as we show both theoretically and experimentally a calcium charge efficiency above unity, and an experimental energy consumption of less than 0.1 kWh/m3. We further demonstrate a low-infrastructure technique to measure cation selectivity, using ion-selective electrodes and the extended Onsager-Fuoss model.
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•Hierarchical porous carbon (HPC) was obtained by simple carbonization and pickling.•HPC exhibits large specific surface area (1509 m2/g) with high mesoporosity.•HPC surface possesses ...strong electronegativity, demonstrated by PZC.•HPC delivers a preferable and fast adsorption to hardness ions with high capacity.•Purification of actual brackish water is achieved with HPC cathode.
Capacitive deionization (CDI), as a promising desalination technology, has been widely applied for water purification, heavy metal removal and water softening. In this study, the hierarchical porous carbon (HPC) with extremely large specific surface area (∼1636 m2 g−1), high mesoporosity and negative surface charges, was successfully prepared by one-step carbonization of magnesium citrate and acid etching. HPC carbonized at 800 ℃ exhibited an excellent specific capacitance (207.2 F g−1). The negative surface charge characteristic of HPC was demonstrated by potential of zero charge test. With HPC-800 as a CDI cathode, the super high adsorption capacity of hardness ions (Mg2+: 472 μmol g−1, Ca2+: 425 μmol g−1) with ultrafast adsorption rate was realized, attributed to its abundant mesoporous structure and negative surface charges. The priority order of ion adsorption on HPC in the multi-component salt solution was Mg2+ > Ca2+ > K+ ≈ Na+. The desalination and softening of the actual brackish water have been simultaneously achieved by three-cell CDI stack after four times of adsorption, with 63% decrease of total dissolved solids and 76% reduction of hardness. The current HPC material with outstanding adsorption performance for hardness ions shows great potential in brackish water purification.
While flow-electrode capacitive deionization (FCDI) is an emerging desalination technology, reduction in water hardness using this technology has so far received minimal attention. In this study, ...treatment of influents containing both monovalent and divalent cations using FCDI was carried out with flow-electrodes operated in short-circuited closed-cycle (SCC) configuration. Divalent Ca2+ cations were selectively removed compared to monovalent Na+ with the selectivity becoming dominant when the FCDI unit was operated at lower current densities and hydraulic retention times. Results showed that SCC FCDI operation was much more energy-efficient for brackish water softening compared to operation in isolated closed-cycle (ICC) mode, particularly with implementation of energy recovery. This finding was largely ascribed to (i) charge neutralization of the flow-electrodes in SCC configuration and (ii) regeneration of the active materials to maintain pseudo “infinite” capacity during electrosorption. In addition, mixing of the flow-electrodes in SCC operation significantly inhibited pH excursion in the flow-electrode with resultant alleviation of calcium precipitation on the carbon surface.
