Electrochemical removal of amphoteric ions

Exploring critical pathways using robust strategies: Nanodiamond electrocatalysts for promoting boron removal via electrosorption

This study presents the first instance of a crucial route for the efficient removal of boron from effluents using a strategically applied electrosorption technology using nanodiamonds annealed under argon (denoted as A-NDs). We demonstrate a significant enhancement in adsorption capacity for boron removal facilitated by a flow-through electrosorption cell, and outline the results of surface characterization and electrochemical activity tests of the fabricated nanodiamond (ND) anodes (e.g., Pristine ND and A-NDs annealed at 800 and 1200 ℃). To identify the role of DO in the electrosorption system, we compared the results obtained in the natural state (without gas purging) with those obtained with ambient air and N2 gas purging. In particular, the degree of electrode deterioration (change in the cathode carbon compositional ratio) during the charging process was characterized using X-ray photoelectron spectroscopy. Overall, our system exhibits a favorable boron removal capability (sorption capacity reached 10.5 μmol/g) and energy consumption of <3.4 kWh g-B. Finally, we developed a prediction model for effluent properties using time-series machine learning algorithms based on various electrosorption variables (e.g., DO, pH dynamics, charging/discharging modes and times, and voltage), Through post-process of constructed ML model, voltage showed significant predictive importance. Additionally, the necessity of sequential modeling was emphasized by SHAP analysis. The application of ML algorithms provided a novel approach for the system optimization of electrified water treatment.

Photovoltaic-Driven Battery Deionization System for Efficient and Sustainable Seawater Desalination

Seawater desalination via electrochemical battery deionization (BDI) has shown significant potential for freshwater production. However, its widespread application has been limited by the high energy costs involved. To facilitate the commercialization of BDI technology, it is crucial to develop innovative integrated BDI systems that utilize sustainable energy sources and assess their practical performance for desalination of natural seawater. In this study, we construct the first photovoltaic-driven battery deionization system, termed PV-BDI, capable of continuously and simultaneously removing multiple ions from natural seawater. The system successfully produced freshwater with a total dissolved solids (TDS) level of 704 mg L-1, meeting the maximum acceptable TDS limits recommended by the World Health Organization (WHO) for drinking water standards, which specify a maximum TDS limit of 1000 mg L-1. The mass-specific energy consumption for salt removal to obtain drinking water from natural seawater via this system has been reduced to 0.036 kW·h kg-1, surpassing the performance of other state-of-the-art PV-driven electrochemical-based desalination technologies such as electrodialysis and capacitive deionization (0.068-2.100 kW·h kg-1). This work presents a pioneering proof-of-concept integrated PV-BDI system and demonstrates its practical performance for desalinating natural seawater, thereby laying the foundation for expanding BDI systems in the near future for environmentally friendly and sustainable industrial-scale seawater desalination.

Flow-through electrochemically assisted reverse-osmosis: A new process towards low-chemical desalination

Two-pass reverse osmosis (RO) process is prevailing in seawater desalination, but each process must consume considerable amounts of chemicals to secure product water quality. Caustic soda is used to raise the pH of the first-pass RO permeate (also the second-pass RO feed) to ensure adequate removal of boron in the subsequent second-pass RO, while antiscalants and disinfectants such as hypochlorite are added in the feed seawater for scaling and biofouling control of the first-pass RO membranes. Here, we report for the first time a flow-through electrochemically assisted reverse osmosis (FT-EARO) module system used in the first-pass RO, aiming to dramatically reduce or even eliminate chemical usage for the current RO desalination. This novel system integrated an electroconductive permeate carrier as cathode and an electroconductive feed spacer as anode on each side of the first-pass RO membrane. Upon applying an extremely low-energy (< 0.005 kWh/m3) electrical field, the FT-EARO module could (1) produce a permeate with pH >10 with no alkali dosage, ensuring sufficient boron removal in the second-pass RO, and (2) generate protons and low-concentration free chlorine near the membrane surface, potentially discouraging membrane scaling and biofouling while maintaining satisfactory desalination performance. The current study further elucidated the high scalability of this novel electrified high-pressure RO module design. The low-chemical manner of FT-EARO presents an attractive practical option towards green and sustainable seawater desalination.

Unlocking Bimetallic Active Centers via Heterostructure Engineering for Exceptional Phosphate Electrosorption: Internal Electric Field-Induced Electronic Structure Reconstruction

Development of electrode materials exhibiting exceptional phosphate removal performance represents a promising strategy to mitigate eutrophication and meet ever-stricter stringent emission standards. Herein, we precisely designed a novel LaCeOx heterostructure-decorated hierarchical carbon composite (L8C2PC) for high-efficiency phosphate electrosorption. This approach establishes an internal electric field within the LaCeOx heterostructure, where the electrons transfer from Ce atoms to neighboring La atoms through superexchange interactions in La-O-Ce coordination units. The modulatory heterostructure endows a positive shift of the d band of La sites and the increase of electron density at Fermi level, promoting stronger orbital overlap and binding interactions. The introduction of oxygen vacancies during the in situ nucleation process reduces the kinetic barrier for phosphate-ion migration and supplies additional active centers. Moreover, the hierarchical carbon framework ensures electrical double-layer capacitance for phosphate storage and interconnected ion migration channels. Such synergistically multiple active centers grant the L8C2PC electrode with high-efficiency record in phosphate electrosorption. As expected, the L8C2PC electrode demonstrates the highest removal capability among the reported electrode materials with a saturation capacity of 401.31 mg P g-1 and a dynamic capacity of 91.83 mg P g-1 at 1.2 V. This electrochemical system also performs well in the dephosphorization in natural water samples with low concentration that enable effluent concentration to meet the first-class discharge standard for China (0.5 mg P L-1). This study advances efficient dephosphorization techniques to a new level and offers a deep understanding of the internal electric field that regulates metal orbitals and electron densities in heterostructure engineering.

Rapid, Selective, and Chemical-Free Removal of Dissolved Silica from Water via Electrosorption: Feasibility and Mechanisms

The unavoidable and detrimental formation of silica scale in engineered processes necessitates the urgent development of effective, economic, and sustainable strategies for dissolved silica removal from water. Herein, we demonstrate a rapid, chemical-free, and selective silica removal method using electrosorption. Specifically, we confirm the feasibility of exploiting local pH dynamics at the electrodes in flow-through electrosorption, achieved through a counterintuitive cell configuration design, to induce ionization and concomitant electrosorption of dissolved silica. In addition, to improve the feasibility of silica electrosorption under high-salinity solutions, we developed a silica-selective anode by functionalizing porous activated carbon cloths with aluminum hydroxide nanoparticles (Al(OH)3-p-ACC). The modification markedly enhances silica sorption capacity (2.8 vs 1.1 mgsilica ganode-1) and reduces the specific energy consumption (13.3 vs 19.8 kWh kgsilica-1). Notably, the modified electrode retains remarkable silica sorption capacity even in the presence of high concentrations of co-occurring ions (up to 100 mM NaCl). The mechanisms underlying the superior silica removal stability and selectivity with the Al(OH)3-p-ACC electrode are also elucidated, revealing a synergistic interaction involving outer-sphere and inner-sphere complexation between dissolved silica and Al(OH)3 nanoparticles on the electrodes. Moreover, we find that effective regeneration of the electrodes may be achieved by applying a reverse potential during discharge, although complete regeneration of the modified electrodes may necessitate alternative materials or process optimization. We recommend the adoption of feedwater-specific designs for the development of future silica-selective electrodes in electrosorption capable of meeting silica removal demands across a wide range of engineered systems.

Efficient removal of As(III) from groundwaters through self-alkalization in an asymmetric flow-electrode electrochemical separation system

It remains a great challenge to efficiently remove As(III) from groundwater using traditional technologies due to its stable electroneutral form. This study constructed an asymmetric flow-electrode electrochemical separation (AFES) system, which overcomes the drawback of H+ release from anodic carbon oxidation and achieves continuous self-alkalization function and highly efficient removal of As(III) from groundwater. At the applied voltage of 1.2 V and initial pH 7.5, the system could rapidly decrease the total As (T-As) concentration from 150.0 to 8.9 μg L-1 within 90 min, with an energy consumption of 0.04 kWh m-3. The self-alkalization was triggered by the generation of H2O2 from dissolved oxygen reduction and the adsorption of H+ on the cathode in the feed chamber, which significantly promoted the dissociation and oxidation of As(III), resulting in the removal of T-As predominantly in the form of As(V). The removal performance of T-As was slightly affected by the initial pH and coexisting ions in the feed chamber. The AFES system also exhibited considerable stability after 20 cycles of continuous experiments and superior performance in treating As-containing real groundwater. Moreover, the pH of the alkalized solution can be restored to the initial level by standing or aeration operation. This work offers a novel and efficient pathway for the detoxication of As(III)-contaminated groundwaters.

Electrosorption Integrated with Bipolar Membrane Water Dissociation: A Coupled Approach to Chemical-free Boron Removal

Boron removal from aqueous solutions has long persisted as a technological challenge, accounting for a disproportionately large fraction of the chemical and energy usage in seawater desalination and other industrial processes like lithium recovery. Here, we introduce a novel electrosorption-based boron removal technology with the capability to overcome the limitations of current state-of-the-art methods. Specifically, we incorporate a bipolar membrane (BPM) between a pair of porous carbon electrodes, demonstrating a synergized BPM-electrosorption process for the first time. The ion transport and charge transfer mechanisms of the BPM-electrosorption system are thoroughly investigated, confirming that water dissociation in the BPM is highly coupled with electrosorption of anions at the anode. We then demonstrate effective boron removal by the BPM-electrosorption system and verify that the mechanism for boron removal is electrosorption, as opposed to adsorption on the carbon electrodes or in the BPM. The effect of applied voltage on the boron removal performance is then evaluated, revealing that applied potentials above ∼1.0 V result in a decline in process efficiency due to the increased prevalence of detrimental Faradaic reactions at the anode. The BPM-electrosorption system is then directly compared with flow-through electrosorption, highlighting key advantages of the process with regard to boron sorption capacity and energy consumption. Overall, the BPM-electrosorption shows promising boron removal capability, with a sorption capacity >4.5 μmol g-C^-1 and a corresponding specific energy consumption of <2.5 kWh g-B^-1.

Ti3 C2 -MXene Partially Derived Hierarchical 1D/2D TiO2 /Ti3 C2 Heterostructure Electrode for High-Performance Capacitive Deionization

Constructing faradaic electrode with superior desalination performance is important for expanding the applications of capacitive deionization (CDI). Herein, a simple one-step alkalized treatment for in situ synthesis of 1D TiO₂ nanowires on the surface of 2D Ti₃ C₂ nanosheets, forming a Ti₃ C₂ -MXene partially derived hierarchical 1D/2D TiO₂ /Ti₃ C₂ heterostructure as the cathode electrode is reported. Cross-linked TiO₂ nanowires on the surface help avoid layer stacking while acting as the protective layer against contact of internal Ti₃ C₂ with dissolved oxygen in water. The inner Ti₃ C₂ MXene nanosheets cross over the TiO₂ nanowires can provide abundant active adsorption sites and short ion/electron diffusion pathways. . Density functional theory calculations demonstrated that Ti₃ C₂ can consecutively inject electrons into TiO₂ , indicating the high electrochemical activity of the TiO₂ /Ti₃ C₂ . Benefiting from the 1D/2D hierarchical structure and synergistic effect of TiO₂ and Ti₃ C₂ , TiO₂ /Ti₃ C₂ heterostructure presents a favorable hybrid CDI performance, with a superior desalination capacity (75.62 mg g^-1 ), fast salt adsorption rate (1.3 mg g^-1 min^-1 ), and satisfactory cycling stability, which is better than that of most published MXene-based electrodes. This study provides a feasible partial derivative strategy for construction of a hierarchical 1D/2D heterostructure to overcome the restrictions of 2D MXene nanosheets in CDI.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Boron Removal from Reverse Osmosis Permeate Using an Electrosorption Process: Feasibility, Kinetics, and Mechanism

Boron is present in the form of boric acid (B(OH)₃ or H₃BO₃) in seawater, geothermal waters, and some industrial wastewaters but is toxic at elevated concentrations to both plants and humans. Effective removal of boron from solutions at circumneutral pH by existing technologies such as reverse osmosis is constrained by high energy consumption and low removal efficiency. In this work, we present an asymmetric, membrane-containing flow-by electrosorption system for boron removal. Upon charging, the catholyte pH rapidly increases to above ∼10.7 as a result of water electrolysis and other Faradaic reactions with resultant deprotonation of boric acid to form B(OH)₄^- and subsequent removal from solution by electrosorption to the anode. Results also show that the asymmetric flow-by electrosorption system is capable of treating feed streams with high concentrations of boron and RO permeate containing multiple competing ionic species. On the basis of the experimental results obtained, a mathematical model has been developed that adequately describes the kinetics and mechanism of boron removal by the asymmetric electrosorption system. Overall, this study not only provides new insights into boron removal mechanisms by electrosorption but also opens up a new pathway to eliminate amphoteric pollutants from contaminated source waters.