Electrochemical removal of nitrate ion from aqueous solution by pulsing potential electrolysis

Pulsed Nitrate-to-Ammonia Electroreduction Facilitated by Tandem Catalysis of Nitrite Intermediates

Electroreduction of nitrate to ammonia offers a promising pathway for nutrient recycling and recovery from wastewater with energy and environmental sustainability. There have been considerable efforts on the regulation of reaction pathways to facilitate nitrate-to-ammonia conversion over the competing hydrogen evolution reaction but only with limited success. Here, we report a Cu single-atom gel (Cu SAG) electrocatalyst that produces NH₃ from both nitrate and nitrite under neutral conditions. Given the unique mechanism of NO₂^- activation on Cu SAGs with spatial confinement and strengthened kinetics, a pulse electrolysis strategy is presented to cascade the accumulation and conversion of NO₂^- intermediates during NO₃^- reduction with the prohibited competition from the hydrogen evolution reaction, thus substantially enhancing the Faradaic efficiency and the yield rate for ammonia production compared with constant potential electrolysis. This work underlines the cooperative approach of the pulse electrolysis and SAGs with three-dimensional (3D) framework structures for highly efficient nitrate-to-ammonia conversion enabled by tandem catalysis of unfavorable intermediates.

Evaluation of electrochemical and O3/UV/H2O2 methods at various combinations during treatment of mature landfill leachate

In this study, electrochemical treatment and application of O₃/UV/H₂O₂ in various combinations were evaluated in respect to their efficiency to depurate mature landfill leachate. Based on preliminary experiments, electrochemical treatment using stainless-steel electrodes at 2 cm gap was performed optimally at 50 mA/cm² and pH 6, while application of O₃ at 120 L/h, UV at 991 J/cm² and H₂O₂ concentration of 1 g/L was carried out. Electrochemical treatment and O₃/UV/H₂O₂ under optimal conditions were applied as follows: I) electrochemical treatment, followed by O₃/UV/H₂O₂ and solids precipitation, II) electrochemical treatment, followed by precipitation and then by O₃/UV/H₂O₂ treatment, and III) O₃/UV/H₂O₂, followed by electrochemical treatment. A low performance was observed when O₃/UV/H₂O₂ preceding electrochemical treatment. Solids, TKN and total COD (tCON) removal was primarily achieved through electrocoagulation, whereas color and soluble COD (sCOD) reduction was mainly attributed to electrochemical oxidation. Experimental setup I was the most efficient treatment scheme, resulting in tCOD, sCOD, TKN, TSS, SAC_UV254nm and color number reduction of 73%, 80%, 76%, 79%, 94% and 98%, respectively. Indeed, O₃/UV/H₂O₂ step could be omitted since its effectiveness was restricted during landfill leachate treatment. In conclusion, electrochemical treatment followed by precipitation could result in effective reduction of nutrients and color.

Enhanced nitrate reduction reaction via efficient intermediate nitrite conversion on tunable CuxNiy/NC electrocatalysts

Electroreduction of nitrate (NO₃^-) to value-added ammonia (NH₃) provides an alternative to NH₃ production industry and remediation of NO₃^--containing wastewater. This study reports a series of Cu-Ni catalysts with component-controllable Cu_xNi_y nanoparticles encapsulated in N-doped carbon film (Cu_xNi_y/NC), and disclosure of the associated mechanism for NO₃^- reduction reaction (NO₃^-RR). Cu_0.43Ni_0.57/NC achieves a better NO₃^--N removal proportion of 89% in comparison with the reference catalysts, including Cu/NC (73%) and Cu_xNi_y/NC with other compositions (Cu_0.79Ni_0.21/NC, 83%; Cu_0.26Ni_0.74/NC, 62%; Ni/NC, 20%). The experimental results and density functional theory calculations demonstrate that the lowered energy barriers of *NO₂-to-*NO derived from appropriate Ni atom alloying plays a key role in the enhanced catalytic activity. Auxiliary porous substrate further contributes to the exposure of active sites and the durability of catalyst structure. These findings offer a mechanistic understanding of catalyst structure on the NO₃^-RR activity and valuable insights toward rational design of other catalysts for enhanced NO₃^-RR.

A three-dimensional Cu nanobelt cathode for highly efficient electrocatalytic nitrate reduction

Water treatment techniques for destructive removal of nitrates by reducing them to harmless N₂ have recently begun to emerge. In this study, we present a novel three-dimensional (3D) Cu nanobelt cathode for efficient electrochemical nitrate reduction. Upon an applied potential of -1.4 V vs. Ag/AgCl, the removal efficiency of nitrates by the 3D Cu nanobelt electrode reaches 100% at 60 min, compared to 2.6% for the Cu foam electrode under the same conditions. Based on the mass balance on nitrogen atoms, the major product is determined to be ammonia. In the simulated wastewater containing NaCl, the as-generated ammonia ions are simultaneously oxidized into harmless N₂ by the in situ generated ClO^- ions from the Pt anode, resulting in the complete removal of inorganic nitrogen (nitrate, nitride and ammonia) from wastewater. The mechanism for the improvement of electrocatalytic activity is systematically investigated. Firstly, the large surface area of the 3D Cu nanobelt electrode facilitates the mass transfer of nitrates, resulting in accelerated electrochemical kinetics. Secondly, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements confirm that the 3D Cu nanobelt electrode exhibits improved charge transfer ability. Also, further investigations demonstrate that the 3D Cu nanobelt electrode preferentially reacts with nitrates, compared to the pristine Cu foam electrode readily reacting with the dissolved oxygen (DO) to generate H₂O₂. This study might expand the prospects of electrocatalytic techniques towards the destructive removal of inorganic nitrogen pollutants in wastewater.

Simultaneous removal of nitrate and nitrite using electrocoagulation/floatation (ECF): A new multi-response optimization approach

This study aims to remove both nitrate and nitrite from wastewater as well as modeling and simultaneous optimizing the electrocoagulation/floatation (ECF) process with 3 responses, namely, the residual nitrate, the residual nitrite and the operating costs; so that all responses meet the standard limitations. For this purpose, 57 experiments designed by the response surface method (RSM) were carried out. The effect of selected variables, including initial pH, current intensity, initial nitrate concentration, number of electrodes, reaction time and their interactions were evaluated. The analysis of variance (ANOVA) confirmed that the predicted equations were in reasonable agreement with the experimental data for three responses. To reach a new multi-response optimization approach, a code was developed in MATLAB software, which was applied to optimize the responses all together. Eight optimized conditions were obtained in accordance with the residual nitrate and the residual nitrite of less than 50 mg/L and 10 mg/L, respectively, and the limited operating costs to 10 ± 0.05 US$/(kg NO₃^-removed).

Efficient nano titanium electrode via a two-step electrochemical anodization with reconstructed nanotubes: Electrochemical activity and stability

A two-step anodization method was used to prepare an efficient nano Ti electrode (ENTE), based on the nano Ti electrode (NTE) that was synthesized by the traditional anodization method. The result of FESEM showed there were many nanopores and nanoparticles on the surface of the ENTE. Compared with Ti electrode, the ENTE exhibited an increased electrochemical activity of nitrate reduction, attributing to its ∼1.09-fold higher reduction peak current density. Values of average current efficiency towards nitrate reduction indicated that the electrochemical properties of different electrodes were raised in the order of ENTE (0.36) > NTE (0.25) > Ti electrode (0.15). The ENTE exhibited a ∼3.33-fold higher electroactive surface area than that of Ti electrode. The higher current density throughout the 1000 s and the ∼1.27-fold higher final current density at 1000 s suggested that the ENTE had a higher stability for nitrate electroreduction. The nitrate reduction efficiency increased with the increasing of initial nitrate-nitrogen concentration and temperature. Similar effect was obtained from current density below 50 mA cm^-2. And under the neutral condition, a higher nitrate reduction efficiency was achieved. The curved surface and higher surface area due to the nanopores of the ENTE increased the nitrate concentration in the EDL and enhanced the potential of individual nitrate ions in the diffuse layer exponentially. This research provided a new route to assess a nano-electrode with high stability and a clear reaction mechanism in EDL.

Solution-Plasma-Mediated Synthesis of Si Nanoparticles for Anode Material of Lithium-Ion Batteries

Silicon anodes have attracted considerable attention for their use in lithium-ion batteries because of their extremely high theoretical capacity; however, they are prone to extensive volume expansion during lithiation, which causes disintegration and poor cycling stability. In this article, we use two approaches to address this issue, by reducing the size of the Si particles to nanoscale and incorporating them into a carbon composite to help modulate the volume expansion problems. We improve our previous work on the solution-plasma-mediated synthesis of Si nanoparticles (NPs) by adjusting the electrolyte medium to mild buffer solutions rather than strong acids, successfully generating Si-NPs with <10 nm diameters. We then combined these Si-NPs with carbon using MgO-template-assisted sol-gel combustion synthesis, which afforded porous carbon composite materials. Among the preparations, the composite material obtained from the LiCl 0.2 M + H₃BO₃ 0.15 M solution-based Si-NPs exhibited a high reversible capacity of 537 mAh/g after 30 discharge/charge cycles at a current rate of 0.5 A/g. We attribute this increased reversible capacity to the decreased particle size of the Si-NPs. These results clearly show the applicability of this facile and environmentally friendly solution-plasma technique for producing Si-NPs as an anode material for lithium-ion batteries.

Solution plasma synthesis of Si nanoparticles

Silicon nanoparticles (Si-NPs) were directly synthesized from a Si bar electrode via a solution plasma. In order to produce smaller Si-NPs, the effects of different electrolytes and applied voltages on the product were investigated in the experiments detailed in this paper. The results demonstrated that the use of an acidic solution of 0.1 M HCl or HNO3 produced Si-NPs without SiO2 formation. According to the transmission electron microscopy and electron energy-loss spectroscopy, the obtained Si-NPs contained both amorphous and polycrystalline Si particles, among which the smaller Si-NPs tended to be amorphous. When an alkaline solution of K2CO3 was used instead, amorphous SiO2 particles were synthesized owing to the corrosion of Si in the high-temperature environment. The pH values of KCl and KNO3 increased during electrolysis, and the products were partially oxidized in the alkaline solutions. The particle size increased with an increasing applied voltage because the excitation temperature of the plasma increased.

Recovery of hydrogen and removal of nitrate from water by electrocoagulation process

The present study provides an optimization of electrocoagulation process for the recovery of hydrogen and removal of nitrate from water. In doing so, the thermodynamic, adsorption isotherm, and kinetic studies were also carried out. Aluminum alloy of size 2 dm(2) was used as anode and as cathode. To optimize the maximum removal efficiency, different parameters like effect of initial concentration, effect of temperature, pH, and effect of current density were studied. The results show that a significant amount of hydrogen can be generated by this process during the removal of nitrate from water. The energy yield calculated from the hydrogen generated is 3.3778 kWh/m(3). The results also showed that the maximum removal efficiency of 95.9% was achieved at a current density of 0.25 A/dm(2), at a pH of 7.0. The adsorption process followed second-order kinetics model. The adsorption of NO3(-) preferably fitting the Langmuir adsorption isotherm suggests monolayer coverage of adsorbed molecules. Thermodynamic studies showed that adsorption was exothermic and spontaneous in nature. The energy yield of generated hydrogen was ~54% of the electrical energy demand of the electrocoagulation process. With the reduction of the net energy demand, electrocoagulation may become a useful technology to treat water associated with power production. The aluminum hydroxide generated in the cell removes the nitrate present in the water and reduced it to a permissible level making the water drinkable.

Treatment of nitrate contaminated water using an electrochemical method

Treatment of nitrate contaminated water which is unsuitable for biological removal using an electrochemical method with Fe as a cathode and Ti/IrO(2)-Pt as an anode in an undivided cell was studied. In the absence and presence of 0.50 g/L NaCl, the nitrate-N decreased from 100.0 to 7.2 and 12.9 mg/L in 180 min, respectively, and no ammonia and nitrite by-products were detected in the presence of NaCl. The nitrate reduction rate increased with increasing current density, with the nitrate reduction rate constant k(1) increasing from 0.008 min(-1) (10 mA/cm(2)) to 0.016 min(-1) (60 mA/cm(2)) but decreasing slightly with increasing NaCl concentration. High temperature favoured nitrate reduction and the reaction followed first order kinetics. The combination of the Fe cathode and Ti/IrO(2)-Pt anode was suitable for nitrate reduction between initial pH values 3.0 and 11.0. e.g. k(1)=0.010 min(-1) (initial pH 3.0) and k(1)=0.013 min(-1) (initial pH 11.0). Moreover, the surface of all used cathodes appeared rougher than unused electrodes, which may have increased the nitrate reduction rate (4-6%).

Optimisation of electro-Fenton denitrification of a model wastewater using a response surface methodology

Electro-Fenton denitrification of a model wastewater was studied using platinized titanium electrodes in a batch electrochemical reactor. The model wastewater was prepared from components based on the real aquaculture effluent with nitrate concentrations varying from 200 to 800 mg L(-1). The technical as well as scientific feasibility of the method was assessed by the relationship between the most significant process variables such as various Fenton's reagent to hydrogen peroxide ratios (1:5; 1:20 and 1:50) and current densities (0.17 mA cm(-2), 0.34 mA cm(-2) and 0.69 mA cm(-2)) and their response on denitrification efficiency in terms of nitrate degradation using central composite Box-Behnken experimental design was determined. The goodness of the model was checked by the coefficient of determination R(2) (0.9775), the corresponding analysis of variance P>F and a parity plot. The ANOVA results indicated that the proposed model was significant and therefore can be used to optimize denitrification of a model wastewater. The optimum reaction conditions were found to be 1:20 Fenton's reagent/hydrogen peroxide ratio, 400 mg L(-1) initial nitrate concentration and 0.34 mA cm(-2) current density. Treatment costs in terms of electricity expenditure at 0.17, 0.34 and 0.69 mA cm(-2) was 7.6, 16 and 41.8 euro, respectively, per kilogram of nitrates and 1, 2 and 4 euro, respectively, per cubic meter of wastewater.

Simultaneous reduction of nitrate and oxidation of by-products using electrochemical method

Electrochemical denitrification was studied with an objective to enhance the selectivity of nitrate to nitrogen gas and to remove the by-products in an undivided electrochemical cell, in which Cu-Zn cathode and Ti/IrO(2)-Pt anode were assembled. In the presence of 0.50 g/L NaCl as supporting electrolyte, the NO(3)(-)-N decreased from 100.0 to 9.7 mg/L after 300 min electrolysis; no ammonia and nitrite were detected in the treated solution. The surface of the cathode was appeared to be rougher than unused after electrolysis at initial pH 6.5 and 12.0. After electrolysis of 5h at the initial pH 3.0, passivation of the Cu-Zn cathode was observed. The reduction rate slightly increased with increasing current density in the range of 10-60 mA/cm(2) and temperatures had little effect on nitrate reduction. Nitrate could be completely removed by the simultaneous reduction and oxidation developed in this study, which is suitable for deep treatment of nitrate polluted water.

Electrochemical regeneration of zeolites and the removal of ammonia

The electrochemical regeneration of zeolites was investigated with the objective of removing ammonia from water harmlessly and reusing the regeneration solution in an undivided electrochemical cell assembled with a Ti/IrO(2)-Pt anode and a Cu/Zn cathode. Zeolites could be completely regenerated through the electrochemical method in this study. With NaCl as a supporting electrolyte, the conversion rate of ammonia adsorbed by the zeolites into nitrogen gas was more that 96%, while the conversion rate to nitrate was less than 4%; no ammonia or nitrite was detected in the solution after electrolysis. The surface of the cathode appeared to be rougher after electrolysis than before. More nitrate was produced when the amount of NaCl was raised or when the current density was increased to the range of 20-60 mA/cm(2). The regeneration solution can be repeatedly reused over a long period of time with the proper amount of NaCl added to the solution. Even after the solution was reused for five times, it could still completely regenerate the zeolites, saving both water resources and the chemical reagent.