Nitrate and nitrite electrocatalytic reduction on Rh-modified pyrolytic graphite electrodes

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.

Direct Synthesis of Ammonia from Nitrate on Amorphous Graphene with Near 100% Efficiency

Ammonia is an indispensable commodity in the agricultural and pharmaceutical industries. Direct nitrate-to-ammonia electroreduction is a decentralized route yet challenged by competing side reactions. Most catalysts are metal-based, and metal-free catalysts with high nitrate-to-ammonia conversion activity are rarely reported. Herein, it is shown that amorphous graphene synthesized by laser induction and comprising strained and disordered pentagons, hexagons, and heptagons can electrocatalyze the eight-electron reduction of NO₃ ^- to NH₃ with a Faradaic efficiency of ≈100% and an ammonia production rate of 2859 µg cm^-2 h^-1 at -0.93 V versus reversible hydrogen electrode. X-ray pair-distribution function analysis and electron microscopy reveal the unique molecular features of amorphous graphene that facilitate NO₃ ^- reduction. In situ Fourier transform infrared spectroscopy and theoretical calculations establish the critical role of these features in stabilizing the reaction intermediates via structural relaxation. The enhanced catalytic activity enables the implementation of flow electrolysis for the on-demand synthesis and release of ammonia with >70% selectivity, resulting in significantly increased yields and survival rates when applied to plant cultivation. The results of this study show significant promise for remediating nitrate-polluted water and completing the NO_x cycle.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFO_chem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Atomic Hydrogen in Electrocatalytic Systems: Generation, Identification, and Environmental Applications

Electrochemical reduction has emerged as a viable technology for the removal of a variety of organic contaminants from water. Atomic hydrogen (H*) is the primary species generated in electrochemical reduction processes. In this work, identification and quantification for H* are reviewed with a focus on methods used to generate H* at different positions. Additionally, we present recently developed proposals for the surface chemistry mechanisms of H* on the most commonly used cathodes as well as the use of H* in standard electrochemical reactors. The proposed reaction pathways in different H* systems for environmental applications are also discussed in detail. As shown in this review, the key hurdles facing H* reduction technologies are related to i) the establishment of systematic and practical synthetic methods; ii) the development of effective identification approaches with high specificity; and, iii) an in-depth exploration of the H* reaction mechanism to better understand the reaction process of H*.

Optimisation and stability of Rh particles on noble metal films immobilised on H + conducting solid polymer electrolyte in attaining efficient nitrate removal

During the electrocatalytic reduction of nitrate, nitrite is often evolved as a product along with ammonia due to the sluggish nitrite to ammonia conversion process compared to the nitrate to nitrite conversion step. Rhodium metal has been proven to enhance nitrite to ammonia conversion rates, yielding ammonia as the only final product. In the present article, we have shown how effectively Rh nanoparticles immobilized on Pt and Pd films deposited on H +  conducting Nafion-117 membranes eliminate intermediate nitrite ions during the progress of the nitrate reduction reaction in a flow type reactor. In this research, we also demonstrated the optimization of Rh nanoparticles on the cathode surface to attain effective nitrate reduction along with a reproducibility check. The dissolution of loosely held Rh nanoparticles on the cathodic surface was observed, which tends to redeposit during cathodic electrolysis, causing stable performance. Finally, Tafel analysis was performed to show the relative kinetic feasibility of the Rh modified Pt and Pd electrodes in attaining nitrate reduction reactions in neutral medium.

Insight into Hydrogenation Selectivity of the Electrocatalytic Nitrate-to-Ammonia Reduction Reaction via Enhancing the Proton Transport

The electrocatalytic nitrate-to-ammonia reduction reaction route (NARR) is one of the emerging routes toward green ammonia synthesis, and its conversion efficiency is controlled mainly by the hydrogenation selectivity. This study proposed a likely NARR route feasible and effective even in a neutral condition. Its high catalytic selectivity and efficiency were achieved by a switch of the sulfate solution to the phosphate buffer solution (PBS), while conditions of NO₃ ^- concentration, pH, and applied potential were maintained unchanged. Specifically, the faradaic efficiencies toward NH₃ (FE NH 3 ) in Na₂ SO₄ were as low as 9.8, 19.8, and 11.4 % versus remarkably jumping to 82.8, 90.5, and 89.5 % in PBS under -0.75, -1.0, and -1.25 V, respectively. The corresponding faradaic efficiencies toward NO₂ ^- (FE NO 2 - ), 77.0, 69.2, and 73.7 % in Na₂ SO₄ , significantly dropped to10.8, 7.4, and 4.4 % in PBS, evidencing an unexpected selectivity reversal of the nitrate reduction from NO₂ ^- to NH₃ . This insight was further revealed by the visualization of the pH gradient near the electrode surface during NARR and confirmed by density functional theory calculations; PBS notably facilitated the proton transport and active mitigation over the proton transfer barrier. The use of PBS resulted in a maximal partial current density toward NH₃ (J NH 3 ) and NH₃ formation rate (r NH 3 ) up to 133.5 mA cm^-2 and 1.74×10^-7  mol s^-1  cm^-2 in 1.0 m KNO₃ at -1.25 V.

Electrocatalytic reduction of nitrate - a step towards a sustainable nitrogen cycle

Nitrate enrichment, which is mainly caused by the over-utilization of fertilisers and industrial sewage discharge, is a major global engineering challenge because of its negative influence on the environment and human health. To solve this serious problem, many technologies, such as the activated sludge method, reverse osmosis, ion exchange, adsorption, and electrodialysis, have been developed to reduce the nitrate levels in water bodies. However, the applications of these traditional techniques are limited by several drawbacks, such as a long sludge retention time, slow kinetics, and undesirable by-products. From an environmental perspective, the most promising nitrate reduction technology is enabled to convert nitrate into benign N₂, and features low cost, high efficiency, and environmental friendliness. Recently, electrocatalytic nitrate reduction has been proven by satisfactory research achievements to be one of the most promising methods among these technologies. This review provides a comprehensive account of nitrate reduction using electrocatalysis methods. The fundamentals of electrocatalytic nitrate reduction, including the reaction mechanisms, reactor design principles, product detection methods, and performance evaluation methods, have been systematically summarised. A detailed introduction to electrocatalytic nitrate reduction on transition metals, especially noble metals and alloys, Cu-based electrocatalysts, and Fe-based electrocatalysts is provided, as they are essential for the accurate reporting of experimental results. The current challenges and potential opportunities in this field, including the innovation of material design systems, value-added product yields, and challenges for products beyond N₂ and large-scale sewage treatment, are highlighted.

Strategic management of nitrate pollution from contaminated water using viable adsorbents: An economic assessment-based review with possible policy suggestions

Groundwater contaminated with nitrate has prompted a flurry of research studies around the world in the recent years to address this burning environmental issue. The common presence of nitrates in groundwater, wastewater, and surface waters has thrown an enormously critical challenge to the global research communities to provide safe and clean drinking water to municipalities. As per WHO, the maximum permissible limit of nitrate in drinking water is 10 mg/L and in groundwater is 50 mg/L; exceeding the limits, several human health problems are observed. Adsorption, ion-exchange processes, membrane-based approaches, electrochemical and chemical procedures, biological methods, filtration, nanoparticles, etc. have been well investigated and reviewed to reduce nitrate levels in water samples in the recent years. Process conditions, as well as the efficacy of various approaches, were discovered to influence different techniques for nitrate mitigation. But, because of low cost, simple operation, easy handling, and high removal effectiveness, adsorption has been found to be the most suitable and efficient approach. The main objectives of this review primarily focuses on the creation of a naturally abundant, cost-effective innovative abundant material, such as activated clay particles combined with iron oxide. Oxide-clay nanocomposite materials, effectively remove nitrate with higher removal efficiency along with recovery of nitrate concentrated sludge. Such methods stand out as flexible and economic ways for capturing stabilized nitrate in solid matrices to satisfy long-term operations. A techno-economic assessment along with suitable policy suggestions have been reported to justify the viability of the brighter processes. Indeed, this kind of analytical review appears ideal for municipal community recommendations on abatement of excess nitrate to supply of clean water.

Au@Carbon quantum Dots-MXene nanocomposite as an electrochemical sensor for sensitive detection of nitrite

A highly sensitive electrochemical sensor was developed through a one-pot green synthesis method for nitrite detection based on the electrochemical technique. Xylan-based carbon quantum dots (CQDs) were used as green in situ reducing agent to prepare CQDs capped gold nanoparticles (Au@CQDs). MXene of good electrical conductivity was used as the immobilized matrix to fabricate Au@CQDs-MXene nanocomposites with the advantages of good electrical conductivity and electrocatalysis. An electrochemical sensor for nitrite monitor was obtained by loading the Au@CQDs-MXene on a glassy carbon electrode. The sensor presents high sensitivity, good stability, wide linear range, and excellent selectivity due to the high catalytic activity of AuNPs and CQDs, the large specific surface area of MXene, and exceptional electrical conductivity of AuNPs and MXene. Under the optimal condition, the linear detection range of the sensor was from 1 μM to 3200 μM with a detection limit of 0.078 μM (S/N = 3), which was superior to most reported sensors using differential pulse voltammetry (DPV) method. Furthermore, this sensor was successfully applied to detect nitrite in tap water and salted vegetables with satisfactory recoveries. This modified electrocatalytic sensor shows a new pathway to fabricate nitrite detection sensor with feasibility for practical application.

Electrochemical nitrate reduction of brines: Improving selectivity to N2 by the use of Pd/activated carbon fiber catalyst

Contamination of water by nitrate has become a worldwide problem, being high levels of this ion detected in the surface, and groundwater, mainly due to the intensive use of fertilizers, and to the discharge of not properly treated effluents. This study aims to evaluate the electrocatalytic process, carried out in a cell divided into two compartments by a cation exchange membrane, and with a copper plate electrode as cathode, identifying the effects of current density, pH, the use of a catalyst in the nitrate reduction, and the production of gaseous compounds. The highest nitrate reduction was obtained with a current density of 2.0 mA cm^-2, without pH adjustment and, in this condition, nitrite ion was mainly formed. The application of activated carbon fibers with palladium (1% wt. and 3% wt.) in an alkaline medium presented an increase in gaseous compounds formation. With 2.0 mA cm^-2, pH adjustment, and applying 3% wt. Pd catalyst, the highest selectivity to gaseous compounds was obtained (95%) with no nitrite detection. These results highlight the viability of using the process developed at this work for the treatment of nitrate contaminated waters.

ZIF-8 engineered bismuth nanosheet arrays for boosted electrochemical reduction of nitrate

Removal of nitrate in wastewater is of great importance to environmental protection and humanity. However, the competitive reaction of hydrogen evolution (HER), which could occupy most active sites of the electrocatalyst, is one of the big challenges for nitrate removal. In this study, a novel zeolitic imidazolate framework-8 film engineered bismuth nanosheet electrocatalyst (ZIF-8/Bi-CC) was designed and synthesized for the electrochemical reduction of nitrate. The water contact angle and electrochemical tests demonstrated that the construction of the hydrophobic ZIF-8 film effectively weakened the competition of HER. And the nitrate removal efficiency and ammonium selectivity increased by 25.9% and 34.2% respectively after bismuth nanosheets were embedded into the ZIF-8 film. Besides, the bismuth concentration detection results indicated that the ZIF-8 film as the protective shell could effectively prevent the leaching of bismuth into the solution. More importantly, the final nitrate removal rate of ZIF-8/Bi-CC was close to 90% after 5 h when treating actual garbage fly ash wastewater, the NITRR efficiency stability and the obtained product were confirmed by five electrochemical cycles. The metal-organic framework film engineered electrocatalyst is a promising strategy for designing a new catalyst for the removal of nitrate in industrial wastewater.

Unveiling Potential Dependence in NO Electroreduction to Ammonia

Recently, electrochemical NO reduction (eNORR) to ammonia has attracted enormous research interests due to the dual benefits in ammonia synthesis and denitrification fields. Herein, taking Ag as a model catalyst, we have developed a microkinetic model to rationalize the general selectivity trend of eNORR with varying potential, which has been observed widely in experiments, but not understood well. The model reproduces experiments well, quantitatively describing the selectivity turnover from N₂O to NH₃ and from NH₃ to H₂ with more negative potential. The first turnover of selectivity is due to the thermochemical coupling of two NO* limiting the N₂O production. The second turnover is attributed to the larger transfer coefficient (β) of HER than NH₃ production. This work reveals how electrode potential regulate the selectivity of eNORR, which is also beneficial to understand the commonly increasing HER selectivity with the decrease of potential in some other electroreduction reactions such as CO₂ reduction.

PEM Electrolysis-Assisted Catalysis Combined with Photocatalytic Oxidation towards Complete Abatement of Nitrogen-Containing Contaminants in Water

Electrolysis-assisted nitrate (NO₃ ^- ) reduction is a promising approach for its conversion to harmless N₂ from waste, ground, and drinking water due to the possible process simplicity by in-situ generation of H₂ /H/H⁺ by water electrolysis and to the flexibility given by tunable redox potential of electrodes. This work explores the use of a polymer electrolyte membrane (PEM) electrochemical cell for electrolysis-assisted nitrate reduction using SnO₂ -supported metals as the active cathode catalysts. Effects of operation modes and catalyst materials on nitrate conversion and product selectivity were studied. The major challenge of product selectivity, namely complete suppression of nitrite (NO₂ ^- ) and ammonium (NH₄ ⁺ ) ion formation, was tackled by combining with simultaneous photocatalytic oxidation to drive the overall reaction towards N₂ formation.

Au-modified organic/inorganic MWCNT/Cu/PANI hybrid nanocomposite electrode for electrochemical determination of nitrate ions

A new electrochemical sensor is reported for the based on the application of noble bimetal nanoparticles (gold and copper) to polymeric-carbon-modifiers for the reduction of nitrate. This sensor was designed for nitrate ion measurement at the surface of pencil graphite electrode modified by a nanocomposite. The modification was the electrosynthesis of gold nanoparticles on the MWCNT/copper-polyaniline (Cu-PANI) nanocomposite. Physicochemical properties of the synthesized hybrid nanocomposites and their surface performance efficiency are characterized using microscopic, spectroscopic, and electrochemical techniques. At optimized pH, the nitrate peak current (at working potential of 1084 mV versus Ag/AgCl reference electrode) was linear in the concentration range 0.8-30.0 μM with a detection limit of 0.09 μM using differential pulse voltammetry. Modified sensor was successfully implemented to quantify nitrate ions in wastewater resulting from the production line for industrial barium chromate and an example of aqueduct water with appropriate recovery levels. • Aniline was polymerized in phosphoric acid solution using peroxydisulfate as an initiator. • MWCNT@CuNPs@PANNSs@AuNPs nanocomposite on PGE electrode was revealed specific recognition for NO₃^-. • The electrochemical sensor displayed high selectivity and sensitivity for the detection of NO₃^-.

Electrocatalytic nitrate reduction on rhodium sulfide compared to Pt and Rh in the presence of chloride

Chloride poisoning is a serious problem for the electrocatalytic reduction of aqueous nitrate (NO3−) and improved electrocatalysts are needed.

Performance of an Integrated Membrane Process with Electrochemical Pre-Treatment on Poultry Slaughterhouse Wastewater Purification

Industrial activities produce a variety of pollutants that may not be easily treated using centralized wastewater treatment systems based on a single treatment unit. The variability of the pollutants brings the importance of industrial-specific integrated wastewater treatment plants such as integrated membrane filtration systems. However, the performance of a membrane filtration process can be highly affected by the presence of high amounts of suspended particles in the raw wastewater. Therefore, proper selection of a pre-treatment unit prior to a membrane filtration wastewater treatment system is a key aspect of its performance. This study investigated the performance of an integrated membrane filtration treatment system connected to an electrochemical process (pre-treatment) on the purification of a poultry slaughterhouse wastewater toward achieving a high-quality effluent. The industrial-scale treatment plant installed at the Izhevsk Production Corporative (PC) poultry farm in Kazakhstan is composed of an electrochemical, ultrafiltration (UF), and reverse osmosis (RO) as the main treatment units. From the analysis results, the electrochemical pre-treatment unit was observed to be highly effective for the removal of some physicochemical parameters such as turbidity, color, total suspended solids, total iron, aluminum, chemical oxygen demand, and biochemical oxygen demand; with removal efficiency ranging from 71 to 85%. The low removal efficiency of the pre-treatment system was also observed from free and total chlorine, nitrites, nitrates, phosphates, and ammonium nitrogen; with removal efficiency ranging from 4 to 45%. While in general, the overall treatment train was observed to be highly efficient for some physicochemical parameters such as turbidity, color, total suspended solids, as well as chemical and biochemical oxygen demand; maintaining almost 100% removal efficiency throughout the study period. Also, the high removal efficiency of the electrochemical pre-treatment processes led to a relatively low rate of cake formation on the membrane filters.

Electrochemical analysis of anionic analytes in weakly supported media using electron transfer promotion effect: a case study on nitrite

In this study, a simple technique was developed for the electrochemical detection of anionic analytes in weakly supported media. This was conducted by the use of electrochemical paper-based analytical devices (ePADs). A sensing platform was modified with nereistoxin and used to determine nitrite as a case study. The electrochemical response was improved due to the accelerated electron transfer between the sensing platform and the nitrite through the electrostatic interaction of the amino group of nereistoxin and the nitrite. The electrocatalytic current of the nitrite in the presence of nereistoxin was enhanced in the weakly supported media. By using nereistoxin as a signal enhancer, 97% of the electrochemical signal was obtained at the low ionic strength of the electrolyte, while less than 35% of this signal was obtained in the absence of nereistoxin. The limit of detection was as low as 20 nM using an ePAD. Generally, the proposed ePAD serves as a promising, efficient and low-cost device for sensing applications in weakly supported media.

Electrochemical reduction of nitrate on boron-doped diamond electrodes: Effects of surface termination and boron-doping level

This study is among the first to systematically study the electrochemical reduction of nitrate on boron-doped diamond (BDD) films with different surface terminations and boron-doping levels. The highest nitrate reduction efficiency was 48% and the highest selectivity in the production of nitrogen gas was 44.5%, which were achieved using a BDD electrode with a hydrogen-terminated surface and a B/C ratio of 1.0%. C-H bonds served as the anchor points for attracting NO₃^- anions close to the electrode surface, and thus accelerating the formation of NO₃^-_(ads). Compared to oxygen termination, hydrogen-terminated BDD exhibited higher electrochemical reactivity for reducing nitrate, resulting from the formation of shallow acceptor states and small interfacial band bending. The hydrophobicity of the hydrogen-terminated BDD inhibited water electrolysis and the subsequent adsorption of atomic hydrogen, leading to increased selectivity in the production of nitrogen gas. A BDD electrode with a boron-doping level of 1.0% increased the density of acceptor states, thereby enhancing the conductivity and promoting the formation of C-H bonds after the cathodic reduction pretreatment leading to the direct reduction of nitrate.

Ultrathin Ni-MOF Nanobelts-Derived Composite for High Sensitive Detection of Nitrite

In this paper, the Ni/NiO ultrathin nanobelts were successively synthesized by a facile in suit conversion process using pre-synthesized Ni-based metal-organic frameworks (MOFs) nanobelts as parent materials to detect the nitrite (NaNO2). The synthesized Ni/NiO composites have the advantages in structure, as follows: (I) Interleaved 3D reticulated structure has strong mechanical stability; (II) Ultrathin nanobelt structures allow more active sites to be exposed and make the transfer of charge faster; (III) A large number of ultrafine Ni nanoparticles decorate the building blocks of the NiO nanobelt and enhance the electrical conductivity. Ni/NiO/GCE has an obvious oxidation peak at 0.78 V, when the concentration is between 0.5 and 1000 μM, the oxidation peak current of NaNO2 is linearly related to the concentration, and the sensitivity is 1.5319 μA mM-1 cm-2 (S/N = 3). Moreover, the experimental results also concluded that the Ni/NiO ultrathin nanobelts not only indicated wonderful reproducibility in the determination of NaNO2 in the pickled pork samples, but also could be well-recovered and keep stable for a long time.

Iridium Nanotubes as Bifunctional Electrocatalysts for Oxygen Evolution and Nitrate Reduction Reactions

One-dimensionally (1D) hollow noble meal nanotubes are attracting continuous attention because of their huge potential applications in catalysis and electrocatalysis. Herein, we successfully synthesize hollow iridium nanotubes (Ir NTs) with the rough porous surface by the 1-hydroxyethylidene-1, 1-diphosphonic acid-induced self-template method under hydrothermal conditions and investigate their electrocatalytic performance for oxygen evolution (OER) and nitrate reduction reactions (NO₃^-RR) in an acidic electrolyte. The unique 1D and porous structure endow Ir NTs with big surface areas, high conductivity, and optimal atom utilization efficiency. Consequently, Ir NTs exhibit significantly enhanced activity and durability for acidic OERs compared with commercial Ir nanocrystals (Ir c-NCs), which only require the overpotential of 245 mV to deliver the current density of 10 mA cm^-2. Meanwhile, Ir NTs also show higher electrocatalytic activity for NO₃^-RR than that of Ir c-NCs, such as a Faraday efficiency of 84.7% and yield rate of 921 μg h^-1 mg_cat^-1 for ammonia generation, suggesting that Ir NTs are universally advanced Ir-based electrocatalysts.

Denitrification with non-organic electron donor for treating low C/N ratio wastewaters

Denitrification with non-organic electron donors for treating low C/N ratio wastewater has attracted growing interests. Hydrogen, reduced sulfur compounds and ferrous ions are mainly used in autotrophic denitrification, holding promise for achieving practical applications. Recently, the development of autotrophic denitrification-based processes, such as bioelectrochemically-supported hydrogenotrophic denitrification and sulfur-/iron-based denitrification assisted multi-contaminant removal, provide opportunities for applying these processes in wastewater treatment. Exploration of the autotrophic denitrification process in terms of contaminant removal mechanism, interaction among functional microorganisms, and potential full-scale applications is thus of great importance. Here, an overview of the commonly used non-organic electron donors, e.g., hydrogen, reduced sulfur compounds and ferrous ions, in denitrification for treating low C/N ratio wastewater is provided. Also, the feasibility of applying the combined processes based on autotrophic denitrification with the compounds is discussed. Furthermore, challenges and future possibilities as well as concerns about the practical applications are envisaged in this review.

Heterostructural CuO-ZnO Nanocomposites: A Highly Selective Chemical and Electrochemical NO2 Sensor

A simple one-step chemical method is employed for the successful synthesis of CuO(50%)-ZnO(50%) nanocomposites (NCs) and investigation of their gas sensing properties. The X-ray diffraction studies revealed that these CuO-ZnO NCs display a hexagonal wurtzite-type crystal structure. The average width of 50-100 nm and length of 200-600 nm of the NCs were confirmed by transmission electron microscopic images, and the 1:1 proportion of Cu and Zn composition was confirmed by energy-dispersive spectra, i.e., CuO(50%)-ZnO(50%) NC studies. The CuO(50%)-ZnO(50%) NCs exhibit superior gas sensing performance with outstanding selectivity toward NO2 gas at a working temperature of 200 °C. Moreover, these NCs were used for the indirect evaluation of NO2 via electrochemical detection of NO2 - (as NO2 converts into NO2 - once it reacts with moisture, resulting into acid rain, i.e., indirect evaluation of NO2). As compared with other known modified electrodes, CuO(50%)-ZnO(50%) NCs show an apparent oxidation of NO2 - with a larger peak current for a wider linear range of nitrite concentration from 20 to 100 mM. We thus demonstrate that the as-synthesized CuO(50%)-ZnO(50%) NCs act as a promising low-cost NO2 sensor and further confirm their potential toward tunable gas sensors (electrochemical and solid state) (Scheme 1).

2,4-Dichlorophenol removal from water using an electrochemical method improved by a composite molecularly imprinted membrane/bipolar membrane

Low efficiency is often a problem in electrochemical reductive hydrodechlorination (ERHD) to remove chlorinated compounds such as 2,4-dichlorophenol (24DCP) from water. In this study, a composite molecularly imprinted membrane (MIM)/bipolar membrane (BPM) was introduced onto a palladium-coated titanium mesh electrode (BPM/MIM@Pd/Ti) to increase the concentration of 24DCP on the surface of electrode and ERHD efficiency. The efficiency of ERHD of 24DCP increased from 70 to 88% by introduction of the two membranes, from 71 to 89% by increasing current density from 5.0 to 30 mA/cm², and from 80 to 94% by increasing the electrolyte concentration from 0.25 to 1.00 mol/L. Treatment with Fenton's reagent after ERHD achieved 100% 24DCP removal, with chemical oxygen demand and total organic carbon reductions of 91 and 87%, respectively. Notably, these reductions were greater than obtained from the direct oxidation of the 24DCP solution by Fenton's reagent alone (i.e., 98, 84, and 72%, respectively). No products were detected in solution by GC-MS after treatment with the proposed combination technology. The mechanism of 24DCP removal and degradation involved adsorption, electrochemical hydrodechlorination via H_ads, and Fenton oxidation. Results show the process has high potential for removing 24DCP from aqueous solution.

Stimulation impact of electric currents on heterotrophic denitrifying microbial viability and denitrification performance in high concentration nitrate-contaminated wastewater

Electric current stimulation has been shown to have a positive influence on heterotrophic denitrifying microbial viability and has the potential to improve wastewater denitrification performance. This study investigated the effects of varying current densities on microbial activity and NO₃^- removal efficiency under heterotrophic conditions.NO₃^- removal rate was highest at an applied current density of 400 mA/m². However, the optimum removal efficiency of total inorganic nitrogen (TIN; 99%) was achieved when the current density was fixed at 200 mA/m². Accumulation of NH₄⁺-N and NO₂^--N byproducts were also minimized at this current density. The activity of heterotrophic denitrifying microorganisms was much higher at both 200 and 400 mA/m². Moreover, the average adenosine-5'-triphosphate (ATP) content (an indicator of cell metabolism) at a current density of 1600 mA/m² was lower than that under no current, indicating heterotrophic denitrifying microbial activity can be inhibited at high current densities. Hence, direct electrical stimulation on the activity of heterotrophic denitrifying microorganisms in the developed system should be lower than 1600 mA/m². This study improves the understanding of electric current influence on heterotrophic denitrifying microorganisms and promotes the intelligent application of direct electrical stimulation on wastewater treatment processes.

Comparison of performance between boron-doped diamond and copper electrodes for selective nitrogen gas formation by the electrochemical reduction of nitrate

The electrochemical nitrate reduction by using boron-doped diamond (BDD) and copper (Cu) electrodes was investigated at various potentials. Product selectivity of nitrate reduction was strongly dependent on the applied potential for both electrodes. The highest selectivity of nitrogen gas production was obtained at -2.0 V (vs. Ag/AgCl) by using a BDD electrode with a faradaic efficiency as high as 45.2%. Compared with Cu electrode, nitrate reduction on BDD electrode occurred at more positive potential, and the production of nitrogen gas was larger. The transformation of surface-adsorbed nitrate into molecular nitrogen would be accelerated on BDD electrode with hindering nitrite production. In addition, low concentration of surface-adsorbed hydrogen on the BDD would also retard the ammonia generation, leading to increase in the selectivity of nitrogen gas formation. Meanwhile, BDD electrode could hinder the hydrogen evolution reaction, which enhanced the efficiency for nitrate reduction and decreased energy consumption. BDD electrode has excellent stability to remain better performance for reducing nitrate during electrolysis without any variation of surface morphology or chemical components.

Cr(VI) Effect on Tc-99 Removal from Hanford Low-Activity Waste Simulant by Ferrous Hydroxide

Here, Cr(VI) effects on Tc-immobilization by Fe(OH)₂(s) are investigated while assessing Fe(OH)₂(s) as a potential treatment method for Hanford low-activity waste destined for vitrification. Batch studies using simulated low-activity waste indicate that Tc(VII) and Cr(VI) removal is contingent on reduction to Tc(IV) and Cr(III). Furthermore, complete removal of both Cr and Tc depends on the amount of Fe(OH)₂(s) present, where complete Cr and Tc removal requires more Fe(OH)₂(s) (∼200 g/L of simulant), than removing Cr alone (∼50 g/L of simulant). XRD analysis suggests that Fe(OH)₂(s) reaction and transformation in the simulant produces mostly goethite (α-FeOOH), where Fe(OH)₂(s) transformation to goethite rather than magnetite is likely due to the simulant chemistry, which includes high levels of nitrite and other constituents. Once reduced, a fraction of Cr(III) and Tc(IV) substitute for octahedral Fe(III) within the goethite crystal lattice as supported by XPS, XANES, and/or EXAFS results. The remaining Cr(III) forms oxide and/or hydroxide phases, whereas Tc(IV) not fully incorporated into goethite persists as either adsorbed or partially incorporated Tc(IV)-oxide species. As such, to fully incorporate Tc(IV) into the goethite crystal structure, additional Fe(OH)₂(s) (>200 g/L of simulant) may be required.

Electrochemical synthesis of gold nanoparticles decorated flower-like graphene for high sensitivity detection of nitrite

In this paper, the spherical Au nanoparticles and 3D flower-like structure graphene were successively deposited on glassy carbon electrode (GCE) (Au/f-GE/GCE) via a facile and two-step electrodeposition method for the detection of nitrite ions (NaNO₂). The morphology and composition elements were confirmed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction measurements (XRD). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the electrochemical behaviors of NaNO₂ on the as-prepared electrode. Compared to f-GE/GCE and Au/GCE, Au/f-GE/GCE showed a sharp and obvious oxidation peak at 0.78V. The oxidation peak current of NaNO₂ was linearly proportional to its concentration in the range from 0.125 to 20375.98μM, with a detection limit of 0.01μM (at S/N=3). Furthermore, the experiment results also showed that the as-prepared electrode exhibited excellent reproducibility and long-term stability, as well as good recovery when applied to the determination of NaNO₂ in pickled pork samples.