Electrochemical nitrate reduction by using a novel Co3O4/Ti cathode

Effects of Ionic Interferents on Electrocatalytic Nitrate Reduction: Mechanistic Insight

Nitrate, a prevalent water pollutant, poses substantial public health concerns and environmental risks. Electrochemical reduction of nitrate (eNO3RR) has emerged as an effective alternative to conventional biological treatments. While extensive lab work has focused on designing efficient electrocatalysts, implementation of eNO3RR in practical wastewater settings requires careful consideration of the effects of various constituents in real wastewater. In this critical review, we examine the interference of ionic species commonly encountered in electrocatalytic systems and universally present in wastewater, such as halogen ions, alkali metal cations, and other divalent/trivalent ions (Ca2+, Mg2+, HCO3-/CO32-, SO42-, and PO43-). Notably, we categorize and discuss the interfering mechanisms into four groups: (1) loss of active catalytic sites caused by competitive adsorption and precipitation, (2) electrostatic interactions in the electric double layer (EDL), including ion pairs and the shielding effect, (3) effects on the selectivity of N intermediates and final products (N2 or NH3), and (4) complications by the hydrogen evolution reaction (HER) and localized pH on the cathode surface. Finally, we summarize the competition among different mechanisms and propose future directions for a deeper mechanistic understanding of ionic impacts on eNO3RR.

Nitrate reduction to nitrogen in wastewater using mesoporous carbon encapsulated Pd-Cu nanoparticles combined with in-situ electrochemical hydrogen evolution

The conversion of NO3--N to N2 is of great significance for zero discharge of industrial wastewater. Pd-Cu hydrogenation catalysis has high application prospects for the reduction of NO3--N to N2, but the existing form of Pd-Cu, the Pd-Cu mass ratio and the H2 evolution rate can affect the coverage of active hydrogen (*H) on the surface of Pd, thereby affecting N2 selectivity. In this work, mesoporous carbon (MC) is used as support to disperse Pd-Cu catalyst and is applied in an in-situ electrocatalytic H2 evolution system for NO3--N removal. The Pd-Cu particles with the average size of 6 nm are uniformly encapsulated in the mesopores of MC. Electrochemical in-situ H2 evolution can not only reduce the amount of H2 used, but the H2 bubbles can also be efficiently dispersed when PPy coated nickel foam (PPy/NF) is used as cathode. Moreover, the mesoporous structure of MC can further split H2 bubbles, reducing the coverage of *H on Pd. The highest 77% N2 selectivity and a relatively faster NO3--N removal rate constant (0.10362 min-1) can be achieved under the optimal conditions, which is superior to most reported Pd-Cu catalytic systems. The prepared catalyst is further applied to the denitrification of actual deplating wastewater. NO3--N with the initial concentration of 650 mg L-1 can be completely removed after 180 min of treatment, and the TN removal can be maintained at 72%.

Pd Clusters Loaded with Multivalent Cu Foam for Superior Electrochemical Nitrate Reduction and Selective N≡N Bond Formation

The electrochemical denitrification of nitrate (NO3 -) in actual wastewater to nitrogen (N2) is an effective approach to reversing the current imbalance of the nitrogen cycle and the eutrophication of water. However, electrostatic repulsion between NO3 - and the cathode results in the low efficiency of NO3 - reduction reaction (NO3RR). Here, density functional theory (DFT) calculations are used as a theoretical guide to design a Pd cluster-loaded multivalent Cu foam (Pd/Cu2O-CF) electrocatalyst, which achieves a splendid 97.8% NO3 - removal rate, 97.9% N2 selectivity, 695.5 mg N g-1 Pd h-1 reduction efficiency, and 60.0% Faradaic efficiency at -1.3 V versus SCE. The projected density of states (pDOS) indicates that NO3 - and Pd/Cu2O-CF are bonded via strong complexation between the O 2p (in NO3 -) and Cu 3d (in Cu2O) with the input of voltage, which reduces the electrostatic repulsion and enhances the enrichment of NO3 - on the cathode. In-situ characterizations demonstrate that Pd[H] can reduce Cu2O to Cu, and subsequently Cu reduces NO3 - to nitrite (NO2 -) accompanied by in situ reconfiguration of multivalent Cu foam. NO2 - is then transferred to the surface of Pd clusters by the cascade catalysis and accelerates the breaking of N─O bonds to form Pd─N, and eventually achieves the N≡N bond formation.

Electrochemical Nitrate Reduction: Ammonia Synthesis and the Beyond

Natural nitrogen cycle has been severely disrupted by anthropogenic activities. The overuse of N-containing fertilizers induces the increase of nitrate level in surface and ground waters, and substantial emission of nitrogen oxides causes heavy air pollution. Nitrogen gas, as the main component of air, has been used for mass ammonia production for over a century, providing enough nutrition for agriculture to support world population increase. In the last decade, researchers have made great efforts to develop ammonia processes under ambient conditions to combat the intensive energy consumption and high carbon emission associated with the Haber-Bosch process. Among different techniques, electrochemical nitrate reduction reaction (NO3RR) can achieve nitrate removal and ammonia generation simultaneously using renewable electricity as the power, and there is an exponential growth of studies in this research direction. Here, a timely and comprehensive review on the important progresses of electrochemical NO3RR, covering the rational design of electrocatalysts, emerging CN coupling reactions, and advanced energy conversion and storage systems is provided. Moreover, future perspectives are proposed to accelerate the industrialized NH3 production and green synthesis of chemicals, leading to a sustainable nitrogen cycle via prosperous N-based electrochemistry.

Defect-induced triple synergistic modulation in copper for superior electrochemical ammonia production across broad nitrate concentrations

Nitrate can be electrochemically degraded to produce ammonia while treating sewage while it remains grand challenge to simultaneously realize high Faradaic efficiency and production rate over wide-range concentrations in real wastewater. Herein, we report the defect-rich Cu nanowire array electrode generated by in-situ electrochemical reduction, exhibiting superior performance in the electrochemical nitrate reduction reaction benefitting from the triple synergistic modulation. Notably, the defect-rich Cu nanowire array electrode delivers current density ranging from 50 to 1100 mA cm-2 across wide nitrate concentrations (1-100 mM) with Faradaic efficiency over 90%. Operando Synchrotron radiation Fourier Transform Infrared Spectroscopy and theoretical calculations revealed that the defective Cu sites can simultaneously enhance nitrate adsorption, promote water dissociation and suppress hydrogen evolution. A two-electrode system integrating nitrate reduction reaction in industrial wastewater with glycerol oxidation reaction achieves current density of 550 mA cm-2 at -1.4 V with 99.9% ammonia selectivity and 99.9% nitrate conversion with 100 h stability, demonstrating outstanding practicability.

Enhanced recovery of phosphorus from hypophosphite-laden wastewater via field-induced electro-Fenton coupled with anodic oxidation

Electrochemical recovered ferric phosphate (FePO4) precipitates from hypophosphite-laden wastewater were shown to be an efficient method for phosphorus (P) recovery. However, the influence of chloride ions (Cl-) coexisting commonly in wastewater is not known for this treatment. Herein, a field-induced electro-Fenton coupled with anodic oxidation electrochemical system consisting of a Ti-RuO2 anode, an Fe inductive electrode and an activated carbon fiber (ACF) cathode, namely Ti-RuO2/Fe/ACF(NaCl) system, was established to recover phosphorus (P) as FePO4 from hypophosphite-laden wastewater in the presence of Cl-. This system enabled a hypophosphite (H2PO2-, 1.0 mM) removal ratio of ~100% and all P was recovered within 30 min at 5.0 V under the initial solution pH of 3.0. The Faradaic efficiency and energy consumption of P recovery achieved the maximum value (~94%) and the lowest value (~16 kW h kg-1 P), respectively. Reactive oxygen species including 1O2, FeIVO2+, •O2- and •OH contribute to convert H2PO2- to PO43-, which immediately formed FePO4 with the generated Fe3+ at the optimized conditions. Therein, the contribution of non-radical 1O2 was very considerable. This system exhibited good stability. The efficiency and cost for treatment of actual hypophosphite-laden wastewater were addressed to check its applicability for P recovery.

Simultaneous removal of nitrate nitrogen and orthophosphate by electroreduction and electrochemical precipitation

Electrochemical methods can effectively remove nitrate nitrogen (NO3-N) and orthophosphate phosphorus (PO4-P) from wastewater. This work proposed a process for the simultaneous removal of NO3-N and PO4-P by combining electroreduction with electrochemically-induced calcium phosphate precipitation, and its performance and mechanisms were studied. For the treatment of 100 mg L-1 NO3-N and 5 mg L-1 PO4-P, NO3-N removal of 60-90% (per cathode area: 0.25-0.38 mg h-1 cm-2) and 80-90% (per cathode area: 0.33-0.38 mg h-1 cm-2) could be acquired within 3 h in single-chamber cell (SCC) and dual-chamber cell (DCC), while P removal was 80-98% (per cathode area: 0.10-0.12 mg h-1 cm-2) in SCC after 30 min and 98% (per cathode area: 0.37 mg h-1 cm-2) in DCC within 10 min. The faster P removal in DCC was due to the higher pH and more abundant Ca2+ in the cathode chamber of DCC, which was caused by the cation exchange membrane (CEM). Interestingly, NO3-N reduction enhanced P removal because more OH- can be produced by nitrate reduction than hydrogen evolution for an equal-charge reaction. For 10 mg L-1 PO4-P in SCC, when the initial NO3-N was 0, 20, 100, and 500 mg L-1, the P removal efficiencies after 1 h treatment were < 10%, 45-55%, 86-99%, and above 98% respectively. An increase in Ca2+ concentration also promoted P removal. However, Ca and P inhibited nitrate reduction in SCC at the relatively low initial Ca/P, as CaP on the cathode limited the charge or mass transfer process. The removal efficiency of NO3-N in SCC after 3 h reaction can reduce by about 17%, 40%, and 34% for Co3O4/Ti, Co/Ti, and TiO2/Ti. The degree of inhibition of P on NO3-N removal was related to the content and composition of CaP deposited on the cathode. On the cathode, the lower the deposited Ca and P, and the higher the deposited Ca/P molar ratio, the weaker the inhibition of P on NO3-N removal. Especially, P had little or even no inhibition on nitrate reduction when treated in DCC instead of SCC or under high initial Ca/P. It is speculated that under these conditions, a high local pH and local high concentration Ca2+ layer near the cathode led to a decrease in CaP deposition and an increase in Ca/P molar ratio on the cathode. High initial concentrations of NO3-N might also be beneficial in reducing the inhibition of P on nitrate reduction, as few CaP with high Ca/P molar ratios were deposited on the cathode. The evaluation of the real wastewater treatment was also conducted.

Sustainable conversion of alkaline nitrate to ammonia at activities greater than 2 A cm-2

Nitrate (NO3‒) pollution poses significant threats to water quality and global nitrogen cycles. Alkaline electrocatalytic NO3‒ reduction reaction (NO3RR) emerges as an attractive route for enabling NO3‒ removal and sustainable ammonia (NH3) synthesis. However, it suffers from insufficient proton (H+) supply in high pH conditions, restricting NO3‒-to-NH3 activity. Herein, we propose a halogen-mediated H+ feeding strategy to enhance the alkaline NO3RR performance. Our platform achieves near-100% NH3 Faradaic efficiency (pH = 14) with a current density of 2 A cm-2 and enables an over 99% NO3--to-NH3 conversion efficiency. We also convert NO3‒ to high-purity NH4Cl with near-unity efficiency, suggesting a practical approach to valorizing pollutants into valuable ammonia products. Theoretical simulations and in situ experiments reveal that Cl-coordination endows a shifted d-band center of Pd atoms to construct local H+-abundant environments, through arousing dangling O-H water dissociation and fast *H desorption, for *NO intermediate hydrogenation and finally effective NO3‒-to-NH3 conversion.

Highly selective electrochemical reduction of nitrate via CoO/Ir-nickel foam cathode to treat wastewater with a low C/N ratio

Thorough nitrate removal from reclaimed water by biological techniques without carbon sources is difficult. Flexible, controllable electrochemical nitrate reduction is widely researched. Herein, ultrathin CoO nanosheets were constructed through amino group induction and orientation. The interfacial electron transfer resistance of two-dimensional CoO was 43.4% lower than that of one-dimensional nanoparticles, resulting in higher current density and improved nitrate reduction efficiency. Nickel foam and IrO2-nickel foam electrodes have almost no effect on nitrate reduction. It is worth noting that iridium loading on CoO (nanosheet) regulated the electronic band structure and generated active atomic H* . The nitrate removal rate increased from 45.1% (CoO (nanoparticle)-nickle foam) and 63.8% (CoO (nanosheet)-nickle foam) to 94.64% (CoO/Ir10 wt%-nickle foam). The proton enhancement effect improved indirect nitrate reduction by atomic H* and increased the NO3--N removal rate to 99.8%. Active chlorine species generated by Cl- in the wastewater selectively converted more than 99% of nitrate to N2, exceeding previous Co-based cathode results. In situ DEMS indicated that electrochemical reduction of nitrate included deoxidation (NO3-→*NO2-→*NO→*N/*N2O→N2) and hydrogenation (*NH2→*NH3→NH4+). The NO3--N removal rate of CoO/Ir10 wt% exceeded 65% during treatment of wastewater treatment plant effluents, verifying the feasibility of electrochemical nitrate reduction with the CoO/Ir10 wt% cathode. A strategy for designing electrochemical nitrate reduction electrocatalysts with excellent potential for full-scale application to treat wastewater treatment plant effluent is provided.

Electrochemical denitrification by a recyclable cobalt oxide cathode: Rapid recovery and selective catalysis

Cathodic aging and fouling have presented significant challenges in the realm of electrochemical denitrification for engineering applications. This study focused on the development of an economical and recyclable nanoporous Co3O4/Co cathode through anodization for nitrate reduction. What distinguished our cathode was its exceptional sustainability. Cobalt from the inactive catalyst could be reclaimed onto the substrate, enabling the regeneration of a new Co3O4 layer. This innovative approach resulted in an exceptionally low Co catalyst consumption, a mere 1.936 g/1 kg N, making it the most cost-effective choice among all Co-based cathodes. The Co3O4 catalyst exhibited a truncated octahedron structure, primarily composed of surface Co2+ ions. Density functional theory calculations confirmed that the bonding between the O atom in NO3- ions and the Co atom in Co3O4 was thermodynamically favorable, with a free energy of - 0.89 eV. Co2+ ions acted as "electron porters" facilitating electron transfer through a redox circle Co2+-Co3+-Co2+. However, the presence of two energy barriers (*NH2NO to *N2 and *N2 to N2) with respective heights of 0.83 eV and 1.17 eV resulted in a N2 selectivity of 9.84% and an NH3 selectivity of 90.02%. In actual wastewater treatment, approximately 78% of TN and 93% of NO3- were successfully removed after 3 h, consistent with the prediction kinetic model. This anodization-based strategy offers a significant advantage in terms of long-term cost and presents a new paradigm for electrode sustainability.

Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Electron-rich Au nanocrystals/Co3O4 interface for enhanced electrochemical nitrate reduction into ammonia

Solar-driven electrochemical NO3- reduction reaction (NO3-RR) is a clean and sustainable strategy that can convert pollutant NO3- in wastewater to value-added NH3. In recent years, cobalt oxides-based catalysts have shown their intrinsic catalytic properties toward NO3-RR but still have room for improvement through catalyst design. Coupling metal oxides with noble metal has been demonstrated to improve electrochemical catalytic efficiency. Here, we use Au species to tune the surface structure of Co3O4 and improve the efficiency of NO3-RR to NH3. The obtained Au nanocrystals-Co3O4 catalyst exhibited an onset potential of 0.54 V vs RHE, NH3 yield rate of 27.86 µg/h·cm2, and Faradaic efficiency (FE) of 83.1% at 0.437 V vs RHE in an H-cell, which is much higher than Au small species (Au clusters or single atoms)-Co3O4 (15.12 µg/h·cm2) and pure Co3O4 (11.38 µg/h·cm2), respectively. Combined experiments with theory calculations, we attributed the enhanced performance of Au nanocrystals-Co3O4 to the reduced energy barrier of *NO hydrogenation to the *NHO and suppression of HER, which originated from the charge transfer from Au to Co3O4. Using an amorphous silicon triple-junction (a-Si TJ) as the solar cell and an anion exchange membrane electrolyzer (AME), an unassisted solar-driven NO3-RR to NH3 prototype was realized with a yield rate of 4.65 mg/h and FE of 92.1%.

A Novel Integrated Flow-Electrode Capacitive Deionization and Flow Cathode System for Nitrate Removal and Ammonia Generation from Simulated Groundwater

Electrochemical reduction of nitrate is a promising method for the removal of nitrate from contaminated groundwater. However, the presence of hardness cations (Ca2+ and Mg2+) in groundwaters hampers the electroreduction of nitrate as a result of the precipitation of carbonate-containing solids of these elements on the cathode surface. Thus, some pretreatment process is required to remove unwanted hardness cations. Herein, we present a proof-of-concept of a novel three-chambered flow electrode unit, constituting a flow electrode capacitive deionization (FCDI) unit and a flow cathode (FC) unit, which achieves cation removal, nitrate capture and reduction, and ammonia generation in a single cell without the need for any additional chemicals/electrolyte. The addition of the FCDI unit not only achieves removal of hardness cations but also concentrates the nitrate ions and other anions, which facilitates nitrate reduction in the subsequent FC unit. Results show that the FCDI cell voltage influences electrode stability but has a minimal impact on the overall nitrate removal performance. The concentration of coexisting anions influences the nitrate removal due to competitive sorption of anions on the electrode surface. Our results further show that stable electrochemical performance was obtained over 26 h of operation. Overall, this study provides a scalable strategy for continuous nitrate electroreduction and ammonia generation from nitrate contaminated groundwaters containing hardness ions.

Pd-Doped Co3 O4 Nanoarray for Efficient Eight-Electron Nitrate Electrocatalytic Reduction to Ammonia Synthesis

Ammonia (NH3 ) is an indispensable feedstock for fertilizer production and one of the most ideal green hydrogen rich fuel. Electrochemical nitrate (NO3 - ) reduction reaction (NO3 - RR) is being explored as a promising strategy for green to synthesize industrial-scale NH3 , which has nonetheless involved complex multi-reaction process. This work presents a Pd-doped Co3 O4 nanoarray on titanium mesh (Pd-Co3 O4 /TM) electrode for highly efficient and selective electrocatalytic NO3 - RR to NH3 at low onset potential. The well-designed Pd-Co3 O4 /TM delivers a large NH3 yield of 745.6 µmol h-1 cm-2 and an extremely high Faradaic efficiency (FE) of 98.7% at -0.3 V with strong stability. These calculations further indicate that the doping Co3 O4 with Pd improves the adsorption characteristic of Pd-Co3 O4 and optimizes the free energies for intermediates, thereby facilitating the kinetics of the reaction. Furthermore, assembling this catalyst in a Zn-NO3 - battery realizes a power density of 3.9 mW cm-2 and an excellent FE of 98.8% for NH3 .

Bimetallic Cu-Fe catalysts on MXene for synergistically electrocatalytic conversion of nitrate to ammonia

NO₃^- is a common water pollutant that can serve as a potential nitrogen source for electrocatalytic NH₃ production. However, an efficient and complete removal of low NO₃^- concentrations remains a challenge. Fe₁Cu₂@MXene bimetallic catalysts were constructed on two-dimensional Ti₃C₂T_x MXene carriers via a simple solution-based synthetic method and used for the electrocatalytic reduction of NO₃^-. The combination of the rich functional groups, high electronic conductivity on the MXene surface, and the synergistic effect between the Cu and Fe sites enabled the composite to effectively catalyse NH₃ synthesis, with a 98% conversion of NO₃^- in 8 h and a selectivity for NH₃ of up to 99.6%. In addition, Fe₁Cu₂@MXene showed excellent environmental and cyclic stability at various pH values and temperatures over multiple (14) cycles. Semiconductor analysis techniques and electrochemical impedance spectroscopy confirmed that the synergistic effect provided by the dual active sites of the bimetallic catalyst enabled fast electron transport. This study provides new insights into the synergistic promotion of NO₃^- reduction reactions using bimetals.

Nitrogen-containing wastewater fuel cells for total nitrogen removal and energy recovery based on Cl•/ClO• oxidation of ammonia nitrogen

The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH₃-N/NH₄⁺-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO₄•^-) and chlorine radicals (Cl•, ClO•, Cl₂•^-), for nitrogen removal. Nevertheless, HO• shows low reactivity and N₂ selectivity towards NH₃-N/NH₄⁺-N oxidation, and SO₄•^- also demonstrates unsatisfactory NH₃-N/NH₄⁺-N removal. It has been shown that Cl•/ClO• can efficiently remove NH₃-N/NH₄⁺-N with high N₂ selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH₃-N/NH₄⁺-N and nitrate nitrogen (NO₃^--N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater.

In Situ/Operando Methods for Understanding Electrocatalytic Nitrate Reduction Reaction

With the development of industrial and agricultural, a large amount of nitrate is produced, which not only disrupts the natural nitrogen cycle, but also endangers public health. Among the commonly used nitrate treatment techniques, the electrochemical nitrate reduction reaction (eNRR) has attracted extensive attention due to its mild conditions, pollution-free nature, and other advantages. An in-depth understanding of the eNRR mechanism is the prerequisite for designing highly efficient electrocatalysts. However, some traditional characterization tools cannot comprehensively and deeply study the reaction process. It is necessary to develop in situ and operando techniques to reveal the reaction mechanism at the time-resolved and atomic level. This review discusses the eNRR mechanism and summarizes the possible in situ techniques used in eNRR. A detailed introduction of various in situ techniques and their help in understanding the reaction mechanism is provided. Finally, the current challenges and future opportunities in this research area are discussed and highlighted.

Electrocatalytic reduction of nitrate by carbon encapsulated Cu-Fe electroactive nanocatalysts on Ni foam

Electrocatalytic denitrification is an attractive and effective method for complete elimination of nitrate (NO₃^-). However, its application is limited by the activity and stability of the electrocatalyst. In this work, a novel bimetallic electrode was synthesized, in which N-doped graphitized carbon sealed with Cu and Fe nanoparticles and immobilized them on nickel foam (CuFe NPs@NC/NF) without any chemical binder. The immobilized Cu-Fe nanoparticles not only facilitated the adsorption of the reactant but also enhanced the electron transfer between the cathode and NO₃^-, thus promoting the electrochemical reduction of NO₃^-. Therefore, the as-prepared electrode exhibited enhanced electrocatalytic activity for NO₃^- reduction. The composite electrode with the Cu/Fe molar ratio of 1:2 achieved the highest NO₃^- removal (79.4 %) and the lowest energy consumption (0.0023 kW h mg^-1). Furthermore, the composite electrode had a robust NO₃^- removal capacity under various conditions. Benefitting from the electrochlorination on the anode, this electrochemical system achieved nitrogen (N₂) selectivity of 94.0 %. Moreover, CuFe NPs@NC/NF exhibited good stability after 15 cycles, which should be attributed to the graphitized carbon layer. This study confirmed that CuFe NPs@NC/NF electrode is a promising and inexpensive electrode with long-term stability for electrocatalytic denitrification.

Recent Developments of Light-Harvesting Excitation, Macroscope Transfer and Multi-Stage Utilization of Photogenerated Electrons in Rotating Disk Photocatalytic Reactor

The rotating disk photocatalytic reactor is a kind of photocatalytic wastewater treatment technique with a high application potential, but the light energy utilization rate and photo quantum efficiency still need to be improved. Taking photogenerated electrons as the starting point, the following contents are reviewed in this work: (1) Light-harvesting excitation of photogenerated electrons. Based on the rotating disk thin solution film photocatalytic reactor, the photoanodes with light capture structures are reviewed from the macro perspective, and the research progress of light capture structure catalysts based on BiOCl is also reviewed from the micro perspective. (2) Macroscope transfer of photogenerated electrons. The research progress of photo fuel cell based on rotating disk reactors is reviewed. The system can effectively convert the chemical energy in organic pollutants into electrical energy through the macroscopic transfer of photogenerated electrons. (3) Multi-level utilization of photogenerated electrons. The photogenerated electrons transferred to the cathode can also generate H2O2 with oxygen or H2 with H+, and the reduction products can also be further utilized to deeply mineralize organic pollutants or reduce the nitrate in water. This short review will provide theoretical guidance for the further application of photocatalytic techniques in wastewater treatment.

In Situ Reconstruction of Metal Oxide Cathodes for Ammonium Generation from High-Strength Nitrate Wastewater: Elucidating the Role of the Substrate in the Performance of Co3O4-x

In situ electrochemical reconstruction is important for transition metal oxides explored as electrocatalysts for electrochemical nitrate reduction reactions (ENRRs). Herein, we report substantial performance enhancement of ammonium generation on Co, Fe, Ni, Cu, Ti, and W oxide-based cathodes upon reconstruction. Among them, the performance of a freestanding ER-Co₃O_4-x/CF (Co₃O₄ grown on Co foil subjected to electrochemical reduction) cathode was superior to its unreconstructed counterpart and other cathodes; e.g., an ammonium yield of 0.46 mmol h^-1 cm^-2, an ammonium selectivity of 100%, and a Faradaic efficiency of 99.9% were attained at -1.3 V in a 1400 mg L^-1 NO₃^--N solution. The reconstruction behaviors were found to vary with the underlying substrate. The inert carbon cloth only acted as a supporting matrix for immobilizing Co₃O₄, without appreciable electronic interactions between them. A combination of physicochemical characterizations and theoretical modeling provided compelling evidence that the CF-promoted self-reconstruction of Co₃O₄ induced the evolution of metallic Co and the creation of oxygen vacancies, which promoted and optimized interfacial nitrate adsorption and water dissociation, thus boosting the ENRR performance. The ER-Co₃O_4-x/CF cathode performed well over wide ranges of pH and applied current and at high nitrate loadings, ensuring its high efficacy in treating high-strength real wastewater.

Defect engineering of two-dimensional materials for advanced energy conversion and storage

In the global trend towards carbon neutrality, sustainable energy conversion and storage technologies are of vital significance to tackle the energy crisis and climate change. However, traditional electrode materials gradually reach their property limits. Two-dimensional (2D) materials featuring large aspect ratios and tunable surface properties exhibit tremendous potential for improving the performance of energy conversion and storage devices. To rationally control the physical and chemical properties for specific applications, defect engineering of 2D materials has been investigated extensively, and is becoming a versatile strategy to promote the electrode reaction kinetics. Simultaneously, exploring the in-depth mechanisms underlying defect action in electrode reactions is crucial to provide profound insight into structure tailoring and property optimization. In this review, we highlight the cutting-edge advances in defect engineering in 2D materials as well as their considerable effects in energy-related applications. Moreover, the confronting challenges and promising directions are discussed for the development of advanced energy conversion and storage systems.

Application of electrochemical oxidation for the enhancement of antibiotic resistant bacteria removal in stormwater bioretention cells

Recently, increasing attention has been paid to the removal of antibiotic resistant bacteria (ARB) during electrochemical advanced oxidation processes. However, there is still no available literature about the application of electrochemical oxidation (EO) to enhance ARB removal in stormwater bioretention cells. Batch experiments were conducted to investigate target ARB (E. coli K-12 carrying blaTEM, tetR and aphA) removals in bioretention cells with different current densities and ratios of air to water (A/W). ARB removals for bioretention cells with 17.6 μA/m² of current density and 24:1 of A/W ratio was the largest with 5.28 log reduction, which was obviously higher than the one (3.68 log reduction) in the control (without EO). H₂O₂ production could be responsible for ARB removals in the used bioretention cells, where H₂O₂ levels increased at first and then decreased with the increase of current densities and A/W ratios. The evaluation for the application of EO implied that the highest antibiotic resistance (AR) conjugation frequency (3.8 × 10^-3) at 3.5 μA/m² of current density and 48:1 of A/W ratios was 124.5 % of the one in the control, while the largest AR transformation frequencies at 17.6 μA/m² of current density and 48:1 of A/W ratios was 366.9 % (tetR) and 216.2 % (aphA) of the corresponding in the control, and there were still stable for both dominant microflora and metabolic activities in bioretention cells with electricity and aeration, suggesting that EO could be promising for the enhancement of ARB removals in bioretention cells.

Boosting generation of reactive oxygen and chlorine species on TNT photoanode and Ni/graphite fiber cathode towards efficient oxidation of ammonia wastewater

Photoelectrocatalytic (PEC) process combining the merits of photocatalysis and electrocatalysis is considered as a promising ammonia oxidation technology for water treatment. However, some key issues, such as the limited in situ generation of oxidants on photoanode, slow mass transfer problem and generation of nitrate/nitrite by-products hinder the further application of PEC process in the treatment of ammonia pollutant. In this study, the graphite felt (GF) cathodes modified by different transition metals (Ni, Fe, Mn, Co, Cu) were screened by physicochemical and photoelectrochemical characterizations. The results show that the Ni-GF cathode with more Ni⁰ uniformly distributed on the GF surface had the best electrocatalytic activity to generate H₂O₂. The PEC system composed of 10.0 wt% Ni-GF cathode and optimized titania nanotubes (TNTs) photoanode selectively converted about 96.1% ammonia to N₂ within 90 min. Compared with the single TNTs photoanode system, the ammonia oxidation reaction rate constant of the synergistic PEC oxidation system was increased by about two times, which demonstrated the role of the oxidants simultaneously generated on both anode and cathode. The in situ generated reactive oxygen-based oxidants and chlorine-based oxidants interacted together, and ClO• acted a leading role in the ammonia oxidation which were confirmed by quenching and probe experiments. In addition, the contributions of •OH and ClO• were significantly improved in the synergistic PEC oxidation system, compared with the single TNTs photoanode system. Furthermore, the nitrate by-products generated by the ammonia oxidation were further reduced on the Ni-GF cathode. The large amount of active chlorine and active oxygen generated on the electrode diffused into the bulk, effectively overcoming the mass transfer limitation of direct oxidation. Therefore, the developed TNTs photoanode/Ni-GF cathode system can continuously and efficiently convert ammonia to N₂ without the formation of nitrate/nitrite by-products.

Enhanced biological S0 accumulation by using signal molecules during simultaneous desulfurization and denitrification

A high rate of elemental sulfur (S⁰) accumulation from sulfide-containing wastewater has great significance in terms of resource recovery and pollution control. This experimental study used Thiobacillus denitrificans and denitrifying bacteria incorporated with signal molecules (C6 and OHHL) for simultaneous sulfide (S^2-) and nitrate (NO₃^-) removal in synthetic wastewater. Also, the effects on S⁰ accumulation due to changes in organic matter composition and bacteria proportion through signal molecules were analyzed. The 99.0% of S^2- removal and 99.3% of NO₃^- was achieved with 66% of S⁰ accumulation under the active S^2- removal group. The S⁰ accumulation, S^2- and NO₃^- removal mainly occurred in 0-48 h. The S⁰ accumulation in the active S^2- removal group was 2.0-6.3 times higher than the inactive S^2- removal groups. In addition, S⁰/SO₄^2- ratio exhibited that S⁰ conversion almost linearly increased with reaction time under the active S^2- removal group. The proportion of Thiobacillus denitrificans and H⁺ consumption showed a positive correlation with S⁰ accumulation. However, a very high or low ratio of H⁺/S⁰ is not suitable for S⁰ accumulation. The signal molecules greatly increased the concentration of protein-I and protein-II, which resulted in the high proportion of Thiobacillus denitrificans. Therefore, high S⁰ accumulation was achieved as Thiobacillus denitrificans regulated the H⁺ consumption and electron transfer rate and provided suppressed oxygen environment. This technology is cost-effective and commercially applicable for recovering S⁰ from wastewater.

Electrochemical Mechanisms and Optimization System of Nitrate Removal from Groundwater by Polymetallic Nanoelectrodes

Zn-Cu-TiO₂ polymetallic nanoelectrodes were developed using Ti electrodes as the substrate. The reaction performance and pollutant removal mechanism of the electrodes were studied for different technological conditions by analyzing the electrochemical properties of the electrodes in the electrochemical system, using Ti, TiO₂, Cu-TiO₂, and Zn-Cu-TiO₂ electrodes as cathodes and Pt as the anode. The Tafel curve was used for measuring the corrosion rate of the electrode. The Tafel curve resistance of the Zn-Cu-TiO₂ polymetallic nanoelectrode was the smallest, so the Zn-Cu-TiO₂ nanoelectrode was the least prone to corrosion. The electrode reaction parameters were determined using cyclic voltammetry (CV). Zn-Cu-TiO₂ polymetallic nanoelectrodes have the lowest peak position and the highest electrochemical activity. The surface area of the electrode was determined by the time-current (CA) method, and it was found that the Zn-Cu-TiO₂ polymetallic nanoelectrode had a larger surface area and the highest removal rate of nitrate. The Ti, TiO₂, Cu-TiO₂, and Zn-Cu-TiO₂ electrodes also had higher removal rates for real groundwater, and the differences between the removal rates of nitrates for deionized water and real groundwater decreased as removal time increased. The Zn-Cu-TiO₂ polymetallic nanoelectrode exhibited the highest removal rate for real groundwater. This study reveals the reaction mechanism of the cathode reduction of nitrate, which provides the basis for constructing electrochemical reactors and its application in treating nitrate-contaminated groundwater. A mathematical model of optimized working conditions was created by the response surface method, and optimum time, NaCl concentration, and current density were 93.39 min, 0.22 g/L, and 38.34 mA/cm², respectively. Under these optimal conditions, the nitration removal rate and ammonium nitrogen generation in the process solution were 100% and 0.00 mg/L, respectively.

Highly selective electrocatalytic reduction of nitrate to nitrogen in a chloride ion-free system by promoting kinetic mass transfer of intermediate products in a novel Pd-Cu adsorption confined cathode

The mass transfer on the catalyst surface has a great influence on the selectivity of electrocatalytic nitrate reduction to nitrogen. In this study, a Pd-Cu adsorption confined nickel foam cathode is designed in the absence of both proton exchange membranes and chloride ions. The repulsion of the cathode enables intermediate products such as nitrite to accumulate in the confined region, resulting in an increase in the possibility of a second-order reaction to form nitrogen. The system can obtain more than 92% continuous N₂ selectivity when it is used to treat 200 mg L^-1 NO₃^--N under a current density of 8 mA cm^-2, which is not only higher than those of semiconfined and nonconfined systems but also significantly better than the results obtained by Pd-Cu directly modified cathodes prepared by electrodeposition or impregnation. It is found that a high initial nitrate concentration and low current density are more beneficial for the accumulation of intermediates on Pd-Cu catalysts, thus improving the formation of nitrogen. A mechanism study reveals that the intermediates can completely occupy the active sites on the surface of Pd, avoiding the generation of active hydrogen, and therefore inhibiting the first-order reaction to produce ammonia. Moreover, the reducibility of Pd-Cu can also be gradually improved under the function of the cathode so that the system exhibits good stability. This study demonstrates an environmentally friendly and promising method for total nitrogen removal from industrial wastewater with high conductivity.

New insight into electrochemical denitrification using a self-organized nanoporous VO-Co3O4/Co cathode: Plasma-assistant oxygen vacancies catalyzed efficient nitrate reduction

A novel self-organized nanoporous V_O-Co₃O₄/Co cathode was prepared via anodization and plasma treatment and obtained a significant nitrate reduction efficiency. In the anodization, an oxide layer with the nano-sized pore structure initially grew in-situ on the Co substrate and showed a better surface area. Subsequently, He-plasma increased surface oxygen vacancies (V_O) from 24 % to 57 %. Electrons in vacancies were charged into empty e_g orbital of low-spin Co³⁺(O_h, octahedral) and firstly generated high-spin Co²⁺(O_h) with the configuration of t_2g⁶e_g¹, accounting for 71.7 % of cobalt species. Accordingly, two original mechanisms (Vo-catalyzed and Co²⁺(O_h)-catalyzed) were concluded in this study. Oxygen vacancies increased the charge intensity and served as absorption sites in nitrate reduction. Meanwhile, massive Co²⁺(O_h) provided electrons in the e_g orbital with a higher energy state and mediated the faster electron transfer through a Co²⁺-Co³⁺-Co²⁺ redox cycle, compared with Co²⁺ (T_d, tetrahedral). Ultimately, a faster reaction kinetic of 0.0220 min^-1 was achieved by V_O-Co₃O₄ than other cathodes e.g., Co₃O₄ (0.0150 min^-1). Such V_O-Co₃O₄/Co cathode-based denitrification strategy displayed great advantages in engineering application and completely removed 90 % of TN from actual wastewater.

Synthesis, Adsorption Isotherm and Kinetic Study of Alkaline- Treated Zeolite/Chitosan/Fe3+ Composites for Nitrate Removal from Aqueous Solution-Anion and Dye Effects

In the present study, alkaline-treated zeolite/chitosan/Fe³⁺ (ZLCH-Fe) composites were prepared and analyzed using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and pH of zero point of charge (pH_zpc) to remove nitrates from water. The process was carried out using an adsorption method with a varied initial pH, adsorbent dosage, initial nitrate concentration and contact time. The pH_zpc demonstrated that the ZLCH-Fe surface had a positive charge between 2 and 10, making it easier to capture the negative charge of nitrate. However, the optimal pH value is 7. After 270 min, the maximum adsorption capacity and percent removal reached 498 mg/g and 99.64%, respectively. Freundlich and pseudo-second-order were fitted to the adsorption isotherm and kinetic models, respectively. An evaluation was conducted on the effects of anions-SO₄^2- and PO₄^3--and dyes-methylene blue (MB) and acid red 88 (AR88)-upon nitrate removal. The results indicated that the effect of the anion could be inhibited, in contrast to dye effects. However, the optimal pH values were changed to 10 for MB and 2 for AR88, resulting in a hydrogel formation. This might be indicated by the protonation of hydroxyl and amino groups resulting from a chitosan nitrate reaction in the AR88 solution.

Study and actual application of the electrochemical reactor in flow-through mode based on channel confinement

The method of enhancing mass transfer and improving reaction efficiency by confinement has attracted much attention in the electrochemical research field. In this research, to make low diffusion-limited electrochemical reactors fieldable, a new electrochemical reactor in flow-through mode was established with the mass-produced Ti/RuO₂-IrO₂ felt fibers as the electrodes. The effects of voltage, current, and electrode thickness were explored in this study. When the flow mode was switched from flow-by to flow-through, the single-pass degradation effect of rhodamine B rose from 4.4% to 74.8% under the same operating conditions. Meanwhile, a mass transfer model was established based on the results of removal efficiency and electrode channel parameters. The model was in good agreement with the new electrode parameters verification (R² > 0.970). With this model, it could derive specific results on the effect of pore size change on the treatment effect. The impact of enhancing mass transfer by confining the pore sizes is most clearly gained at a certain range (less than 100 μm). Furthermore, a pilot-scale electrochemical reactor in flow-through mode was built, and excellent performance was shown in the treatment of actual waste leachate. The removal efficiencies of total nitrogen, ammonia, and nitrate were 80.9%, 88.6%, and 64.5% in 30 min, respectively. It will be a promising technology with good prospect.

Efficient electrocatalytic nitrate reduction in neutral medium by Cu/CoP/NF composite cathode coupled with Ir-Ru/Ti anode

In this work, a three-dimensional self-supporting copper/cobalt phosphide/nickel foam (Co/CoP/NF) composite was fabricated and employed as the cathode for electrochemical nitrate removal from surface water with the assistance of a commercial Ir-Ru/Ti anode. The experimental results demonstrate that the introduction of Cu nanoparticles on CoP nanosheets is favorable for the electrocatalytic nitrate reduction. The influences of operating parameters (pH value, current density and initial nitrate concentration) on the nitrate reduction were assessed with the presence of Cl^-. At the optimized conditions, the removal of nitrate exhibits an efficiency ca. 100% via the coupling electrochemical reduction and oxidation processes. Moreover, the nitrogen selectivity is found to be as high as 98.8% within 210 min, accompanied with a promising test endurance (>94.0% for total nitrogen (TN) and NO₃^- removal efficiencies after an electrochemical run of 24.5 h). Importantly, as for the treated actual surface water, the concentration of TN is smaller than 1.5 mg L^-1, in accordance with the limit of Ⅳ-level standard of the surface water environmental quality in China (GB 3838-2002). The efficient removal of nitrate can be attributed to the synergistic effect of Cu and CoP microparticles to enhance the reduction activity, as well as the subsequent chloride oxidation for the major intermediate of ammonium.

Overcoming barriers for nitrate electrochemical reduction: By-passing water hardness

Water matrix composition impacts water treatment performance. However, matrix composition impacts have rarely been studied for electrochemical water treatment processes, and the correlation between the composition and the treatment efficiency is lacking. This work evaluated the electrochemical reduction of nitrate (ERN) using different complex water matrices: groundwater, brackish water, and reverse osmosis (RO) concentrate/brine. The ERN was conducted using a tin (Sn) cathode because of the high selectivity towards nitrogen evolution reported for Sn electrocatalysts. The co-existence of calcium (Ca2+), magnesium (Mg2+), and carbonate (CO32-) ions in water caused a 4-fold decrease in the nitrate conversion into innocuous nitrogen gas due to inorganic scaling formation on the cathode surface. XRF and XRD analysis of fouled catalyst surfaces detected brucite (Mg(OH)2), calcite (CaCO3), and dolomite (CaMg(CO3)2) mineral scales formed on the cathode surface. Surface scaling created a physical barrier on the electrode that decreased the ERN efficiency. Identifying these main sources of ERN inhibition was key to devising potential fouling mitigation strategies. For this reason, the chemical softening pre-treatment of a real brackish water was conducted and this significantly increased nitrate conversion and faradaic efficiency during subsequent ERN treatment, leading to a lower electric energy consumption per order. Understanding the ionic foulant composition responsible for influencing electrochemically-driven technologies are the first steps that must be taken to move towards niche applications such as decentralized ERN. Thus, we propose either direct ERN implementation in regions facing high nitrate levels in soft waters, or a hybrid softening/nitrate removal system for those regions where high nitrate and high-water hardness appear simultaneously.

Novel Ni foam catalysts for sustainable nitrate to ammonia electroreduction

Electrochemical nitrate reduction (NO₃^-RR) is considered a promising approach to remove environmentally harmful nitrate from wastewater while simultaneously producing ammonia, a product with high value. An important consideration is the choice of catalyst, which is required not only to accelerate NO₃^-RR but also to direct the product selectivity of the electrolysis toward ammonia production. To this end, we demonstrate the fabrication of novel Ni foam catalysts produced through a dynamic hydrogen bubble template assisted electrodeposition process. The resulting foam morphology of the catalyst is demonstrated to crucially govern its overall electrocatalytic performance. More than 95% Faradaic efficiency of ammonia production was achieved in the low potential range from -0.1 to -0.3 V vs. RHE. Hydrogen was found to be the only by-product of the nitrate reduction. Intriguingly, no other nitrogen containing products (e.g., NO,N₂O, or N₂) formed during electrolysis, thus indicating a 100% selective (nitrate→ammonia) conversion. Therefore, this novel Ni foam catalyst is a highly promising candidate for truly selective (nitrate→ammonia) electroreduction and a promising alternative to mature copper-based NO₃^-RR benchmark catalysts. Excellent catalytic performance of the novel Ni foam catalyst was also observed in screening experiments under conditions mimicking those in wastewater treatment.

Improved mineralization and total nitrogen reduction by combination of electro-reduction and electro-oxidation for nitrophenol removal

In this work, p-Nitrophenol (p-NP) was electro-chemically removed by using a prepared Co₃O₄/Ti cathode and a BDD anode to achieve the simultaneous reduction of total organic carbon (TOC), total nitrogen (TN) and toxicity. The prepared Co₃O₄/Ti cathode showed higher electro-activity than the Ti cathode towards p-NP reduction with the removal rate higher than 90.6% but without mineralization. The electro-oxidation removed 84.3% of TOC but none of TN. To develop an optimized process for mineralization and TN removal during p-NP electrolysis, the combination of electro-oxidation and electro-reduction were evaluated by using a dual-chamber cell and a single-chamber cell, respectively. As a result of the re-oxidation and re-reduction in the single-chamber cell, the typically used mode of the simultaneous redox, showed a lower removal of TOC and TN than the combination processes as well as an increased toxicity. The TN removal for both combined modes (21.0%-32.9%) was all higher than that of the mode of reduction because the produced inorganic nitrogen such as ammonia and nitrate could be partially oxidized or reduced to nitrogen gas. The results suggested that the combination process could significantly improve the mineralization and TN reduction for p-NP removal, accompanied with 60.3% decrease of acute toxicity for the reduction after oxidation mode.

Iron-Doped Cobalt Phosphide Nanohoops for Electroreduction of Nitrate to Ammonia

Heteroatom doping can effectively tune the electronic structure of an electrocatalyst to accelerate the adsorption/desorption of reaction intermediates, which sharply increases their intrinsic electroactivity. Herein, we successfully prepare iron (Fe)-doped cobalt phosphide (CoP) nanohoops (Fe/CoP NHs) with different Fe/Co atomic ratios as highly active electrocatalysts for the nitrate electrocatalytic reduction reaction (NIT-ERR). Electrochemical measurements reveal that appropriate Fe doping can improve the electroactivity of cobalt phosphide nanohoops for the NIT-ERR. In a 1 M KOH electrolyte, the Fe/CoP NHs with the optimized chemical composition can achieve an efficient ammonia (NH₃) generation rate of 27.6 mg h^-1 mg_cat^-1 for the conversion of NO₃^- into NH₃ and a Faradaic efficiency of 93.3% at a -0.25 V potential, which exceed the values of various previously reported nanomaterials in an alkaline electrolyte.

Electrocatalytic Upcycling of Nitrate Wastewater into an Ammonia Fertilizer via an Electrified Membrane

Electrochemically upcycling wastewater nitrogen such as nitrate (NO₃^-) and nitrite (NO₂^-) into an ammonia fertilizer is a promising yet challenging research topic in resource recovery and wastewater treatment. This study presents an electrified membrane made of a CuO@Cu foam and a polytetrafluoroethylene (PTFE) membrane for reducing NO₃^- to ammonia (NH₃) and upcycling NH₃ into (NH₄)₂SO₄, a liquid fertilizer for ready-use. A paired electrolysis process without external acid/base consumption was achieved under a partial current density of 63.8 ± 4.4 mA·cm^-2 on the cathodic membrane, which removed 99.9% NO₃^- in the feed (150 mM NO₃^-) after a 5 h operation with an NH₃ recovery rate of 99.5%. A recovery rate and energy consumption of 3100 ± 91 g-(NH₄)₂SO₄·m^-2·d^-1 and 21.8 ± 3.8 kWh·kg^-1-(NH₄)₂SO₄, respectively, almost outcompete the industrial ammonia production cost in the Haber-Bosch process. Density functional theory (DFT) calculations unraveled that the in situ electrochemical conversion of Cu²⁺ into Cu¹⁺ provides highly dynamic active species for NO₃^- reduction to NH₃. This electrified membrane process was demonstrated to achieve synergistic nitrate decontamination and nutrient recovery with durable catalytic activity and stability.

Achieving high-performance electrocatalytic reduction of nitrate by N-rich carbon-encapsulated Ni-Cu bimetallic nanoparticles supported nickel foam electrode

The cathode with low-energy consumption and long-term stability is pivotal to achieve the conversion of nitrate (NO₃^-) to nitrogen (N₂) by electrocatalytic denitrification. Herein, a binder-free electrode was synthesized by directly immobilizing N-doped graphitized carbon layer-encapsulated NiCu bimetallic nanoparticles on nickel foam (NF) (NiCu@N-C/NF) and served as the cathode for electrocatalytic NO₃^- reduction. Morphological characterization indicated that Ni and Cu nanoparticles were encapsulated by the N-doped graphitized carbon layer and well-dispersed on the surface of NF. Compared with monometallic composite cathode (Cu@N-C/NF and Ni@N-C/NF), NiCu@N-C/NF exhibited better NO₃^- removal performance (98.63 %) and lower energy consumption (0.007 kW·h mmol^-1), which should be attributed to its strong adsorption ability to NO₃^- and excellent electron transfer property. Meanwhile, its electrocatalytic performance could be maintained in wide initial NO₃^- concentration (1.79-7.14 mM) and solution pH (3-11). With the assistance of electrochlorination, the N₂ selectivity of electrochemical system was up to 99.89 % in the presence of 0.028 M Cl^-. More importantly, NiCu@N-C/NF electrode displayed an ultra-high stability during ten recycling experiments. This study indicated that the binderless composite cathode NiCu@N-C/NF had great potential in electrocatalytic NO₃^- removal from wastewater.

Building dual active sites Co3O4/Cu electrode to break scaling relations for enhancement of electrochemical reduction of nitrate to high-value ammonia

The electrochemical reduction of nitrate to ammonia (ERNA) has become a green treatment technology for nitrate wastewater. However, strong adsorption for product caused high activation energy for product desorption in scaling relations, blocking activity of electrocatalyst in ERNA. Density functional theory (DFT) calculations indicated that constructing dual active sites (Cu and Co₃O₄) could achieve strong adsorption of nitrate (-2.91 eV) with low desorbing energy barrier of ammonia (0.13 eV), breaking scaling relations. This dual active sites Co₃O₄/Cu electrode achieved a high ammonia yield rate of 684 µg mg_cat^-1 h^-1 with 94.6% faradic efficiency, surpassing single active site Co₃O₄ and Cu electrodes. For mechanism, In-situ electrochemical characterization found the vital intermediates of *NH and *NH₂, demonstrating that indirect reduction was the main pathway on Co₃O₄/Cu electrode. Thus, it is believed that building dual active sites on electrode is a reliable strategy to enhance ERNA for purifying industrial wastewater polluted by nitrate.

Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst

Electrochemically converting nitrate ions, a widely distributed nitrogen source in industrial wastewater and polluted groundwater, into ammonia represents a sustainable route for both wastewater treatment and ammonia generation. However, it is currently hindered by low catalytic activities, especially under low nitrate concentrations. Here we report a high-performance Ru-dispersed Cu nanowire catalyst that delivers an industrial-relevant nitrate reduction current of 1 A cm^-2 while maintaining a high NH₃ Faradaic efficiency of 93%. More importantly, this high nitrate-reduction catalytic activity enables over a 99% nitrate conversion into ammonia, from an industrial wastewater level of 2,000 ppm to a drinkable water level <50 ppm, while still maintaining an over 90% Faradaic efficiency. Coupling the nitrate reduction effluent stream with an air stripping process, we successfully obtained high purity solid NH₄Cl and liquid NH₃ solution products, which suggests a practical approach to convert wastewater nitrate into valuable ammonia products. Density functional theory calculations reveal that the highly dispersed Ru atoms provide active nitrate reduction sites and the surrounding Cu sites can suppress the main side reaction, the hydrogen evolution reaction.

Electrocatalytic Reduction of Nitrate via Co3O4/Ti Cathode Prepared by Electrodeposition Paired With IrO2-RuO2 Anode

Nitrate pollution is already a global problem, and the electrocatalytic reduction of nitrate is a promising technology for the remediation of wastewater and polluted water bodies. In this work, Co₃O₄/Ti electrodes were prepared by electrodeposition for the electrocatalytic reduction of nitrate. The morphology, chemical, and crystal structures of Co₃O₄/Ti and its catalytic activity were investigated. Then, the electrocatalytic nitrate reduction performance of Co₃O₄/Ti as the cathode was evaluated by monitoring the removal efficiencies of nitrate (NO₃⁻-N) and total nitrogen (TN), generation of reduction products, current efficiency (CE), and energy consumption (EC) at different operating conditions. Under the catalysis of Co₃O₄/Ti, NO₃⁻ was reduced to N₂ and NH₄⁺, while no NO₂⁻ was produced. After the introduction of chloride ions and IrO₂-RuO₂/Ti as the anode, NH₄⁺ was selectively oxidized to N₂. The removal efficiencies of NO₃⁻-N (at 100 mg/L) and TN after 2 h were 91.12% and 60.25%, respectively (pH 7.0; Cl⁻ concentration, 2000 mg/L; current density, 15 mA/cm²). After 4 h of operation, NO₃⁻-N and TN were completely removed. However, considering the EC and CE, a 2-h reaction was the most appropriate. The EC and CE were 0.10 kWh/g NO₃⁻N and 40.3%, respectively, and electrocatalytic performance was maintained after 10 consecutive reduction cycles (2 h each). The cathode Co₃O₄/Ti, which is prepared by electrodeposition, can effectively remove NO₃⁻-N, with low EC and high CE.

CO2 bubble-assisted in-situ construction of mesoporous Co-doped Cu2(OH)2CO3 nanosheets as advanced electrodes towards fast and highly efficient electrochemical reduction of nitrate to N2 in wastewater

The development of high-efficient and cost-effective electrocatalysts is crucial to remove nitrate pollutant in wastewater. Herein, we design and prepare mesoporous Co-doped Cu₂(OH)₂CO₃ malachite nanosheets as an electrocatalyst toward highly efficient nitrate reduction using a facile CO₂ bubble-assisted coprecipitation synthesis. The electrocatalytic performance is subject to the Co/Cu ratio of this malachite. Remarkably, compared with the pristine monometal Cu or Co-based electrocatalyst, the optimal electrocatalyst, 0.3Co@Cu₂(OH)₂CO₃, displays fast and highly efficient removal capacity of nitrate with an impressive high total nitrogen (TN) removal of 8628.99 mg N g^-1_CoCu (398.79 mg N g_cat^-1 h^-1), N₂ selectivity of 97.11% as well as negligible nitrite product at 100 mg L^-1 NO₃^--N and 2000 mg L^-1 Cl^- neutral electrolyte. Above all, high total nitrogen removal efficiency (81.92%) and chemical oxygen demand (73.74%) in actual wastewater. Its excellent electrocatalytic performance is achieved by regulating the electronic structure and the adsorption/desorption of the intermediate. This study discovers a new type of electrode materials for nitrate removal in wastewater.

Preparation of three dimensional bimetallic Cu-Ni/NiF electrodes for efficient electrochemical removal of nitrate nitrogen

It still remains a hotspot and great challenge to efficiently remove the nitrate nitrogen from high salt wastewater. Herein, a novel three dimensional porous bimetallic copper-nickel alloy electrode was fabricated with Ni foam (NiF) as substrate. The physicochemical and electrochemical characterization results showed Cu-Ni/NiF electrode possessed the smaller particle size (0.3-1.0 μm) and electrode film resistance comparing with Ni/NiF and Cu/NiF electrodes. Besides, higher double layer capacitance (C_dl) for Cu-Ni/NiF electrode indicated more electrochemical active sites could be used in the electrochemical nitrate nitrogen (NO₃^--N) removal. The electrochemical experiments showed the Cu-Ni/NiF electrode had the optimal NO₃^--N reduction ability and almost 100% NO₃^--N removal could be achieved with 30 min. All NO₃^--N removal processes were in accord with the pseudo-first-order reaction kinetics completely. The gaseous nitrogen selectivity for Cu-Ni/NiF electrode could reach 80.9% within 300 min. Stability assessment experiments indicated the Cu-Ni/NiF electrode all kept an excellent stability with Na₂SO₄ or NaCl electrolyte and the Cl^- addition could significantly improve the gaseous nitrogen selectivity. Finally, a possible removal mechanism of NO₃^--N was proposed. This work offered a direction for designing non-noble bimetallic electrodes for nitrate removal.

Nitrogen Removal Characteristics of a Cold-Tolerant Aerobic Denitrification Bacterium, Pseudomonas sp. 41

Nitrogen pollution of surface water is the main cause of water eutrophication, and is considered a worldwide challenge in surface water treatment. Currently, the total nitrogen (TN) content in the effluent of wastewater treatment plants (WWTPs) is still high at low winter temperatures, mainly as a result of the incomplete removal of nitrate (NO3−-N). In this research, a novel aerobic denitrifier identified as Pseudomonas sp. 41 was isolated from municipal activated sludge; this strain could rapidly degrade a high concentration of NO3−-N at low temperature. Strain 41 completely converted 100 mg/L NO3−-N in 48 h at 15 °C, and the maximum removal rate reached 4.0 mg/L/h. The functional genes napA, nirS, norB and nosZ were successfully amplified, which provided a theoretical support for the aerobic denitrification capacity of strain 41. In particular, the results of denitrification experiments showed that strain 41 could perform aerobic denitrification under the catalysis of NAP. Nitrogen balance analysis revealed that strain 41 degraded NO3−-N mainly through assimilation (52.35%) and aerobic denitrification (44.02%), and combined with the gene amplification results, the nitrate metabolism pathway of strain 41 was proposed. Single-factor experiments confirmed that strain 41 possessed the best nitrogen removal performance under the conditions of sodium citrate as carbon source, C/N ratio 10, pH 8, temperature 15–30 °C and rotation speed 120 rpm. Meanwhile, the bioaugmentation test manifested that the immobilized strain 41 remarkably improved the denitrification efficiency and shortened the reaction time in the treatment of synthetic wastewater.

Ambient Ammonia Synthesis via Electrochemical Reduction of Nitrate Enabled by NiCo2 O4 Nanowire Array

NiCo₂ O₄ nanowire array on carbon cloth (NiCo₂ O₄ /CC) is proposed as a highly active electrocatalyst for ambient nitrate (NO₃ ^- ) reduction to ammonia (NH₃ ). In 0.1 m NaOH solution with 0.1 m NaNO₃ , such NiCo₂ O₄ /CC achieves a high Faradic efficiency of 99.0% and a large NH₃ yield up to 973.2 µmol h^-1  cm^-2 . The superior catalytic activity of NiCo₂ O₄ comes from its half-metal feature and optimized adsorption energy due to the existence of Ni in the crystal structure. A Zn-NO₃ ^- battery with NiCo₂ O₄ /CC cathode also shows a record-high battery performance.