Nitrate reduction via micro-electrolysis on Zn-Ag bimetal combined with photo-assistance

Electrochemical production of hydroxylamine from nitrate on metal electrodes: A comparative study of selectivity and efficiency

Despite the great potential of electrochemical nitrate reduction as a hydroxylamine production method, this strategy has not been sufficiently examined, and the effects of electrode material type on the selectivity and efficiency of this reduction remain underexplored. To bridge this gap, the present study evaluated six metals (Ag, Cu, Ni, Sn, Ti, and Zn) as cathode materials for the electrochemical reduction of nitrate to hydroxylamine, showing that the selectivity of hydroxylamine production was maximal for Sn, while the corresponding faradaic and energy utilization efficiencies were maximal for Ti. Although all tested materials favored nitrate reduction over hydrogen evolution, the disparity in the onset potentials of these reactions did not adequately explain the variations in nitrate removal efficiency, which was found to be influenced by material resistance and charge-transfer properties. The rate constants of elementary nitrate reduction steps determined from the time-dependent concentrations of nitrate and its reduction products (nitrous acid, hydroxylamine, and ammonium) were used to calculate the selectivity and efficiency of hydroxylamine production for each electrode. In turn, these selectivities and efficiencies were correlated with the density functional theory-computed adsorption energies of a key hydroxylamine precursor on different electrodes to afford a volcano-type plot with Ti and Sn at its pinnacle. Thus, this study introduces valuable descriptors and methods for the further screening of electrocatalysts for hydroxylamine generation and the establishment of more environmentally friendly hydroxylamine production techniques utilizing sustainable electricity.

Enhanced reduction of nitrate by TDER packed with surface-modified plastic particles electrodes

Based on Iron cathodes, nitrate could be selectively decomposed into other lower-valence nitrogen compounds, including ammonia, nitrogen gas, nitrite and nitric oxide, but the removal efficiencies of nitrate and total nitrogen (TN), are affected significantly by the synergistic effects of anodes, chloride electrolyte and conductive plastic particles electrodes. In this work, the base material Titanium (Ti) metal plates and plastic particles which surfaces were mainly coated with Ru-Sn oxidizing compounds, were applied as plates anodes and conductive particles electrodes in Three Dimensional Electrode Reactors (TDER). The Ti/RuSn plate anodes showed excellent performance on degrading nitrate, more nitrogen gas (83.84%) and less ammonia (15.51%) was produced, less TN and Iron ion (0.02 mg/L) was left in the wastewater, and less amount of chemical sludge (0.20 g/L) was produced. Furthermore, the removal efficiencies of nitrate and TN were further increased by the surface-modified plastic particles, which were cheap, reusable, corrosion-resistance, easy to obtain as manufactured materials and light to be suspended in waters. The degradation of nitrate and its intermediates was enhanced possibly by the continuous synergistic reactions initiated by hydrogen radicals, which was generated on the countless surficial active Ru-Sn sites of Ti/RuSn metal plate anodes and plastic particles electrodes, among residual nitrogen intermediates, most of ammonia was selectively converted to gaseous nitrogen by hypochlorite from chloride ion reaction.

Micro-electrolysis based nitrate reduction from aqueous solution by CNTs-Al-Cu composite under alkaline environment

CNTs-Al was prepared by ball milling combined with sintering process and then used for CNTs-Al-Cu synthesis with chemical deposition method. The obtained CNTs-Al-Cu composite was systematically characterized and its NO₃^--N reduction performance under alkaline condition was also evaluated. As indicated by the reduction batch experiment, 80.2% of NO₃^--N removal efficiency was obtained in 90 min at pH of 9. The product of the reduction process was dominated by NO₂^--N, which was further reduced to harmless N₂. The reusability of CNTs-Al-Cu composite was evaluated, and the experiment results showed that 68.1% of NO₃^--N removal efficiency was maintained after 3 cycles of regeneration. Finally, based on the characterization results and kinetic analysis, it was concluded that micro-electrolysis was mainly responsible for the removal of NO₃^--N by CNTs-Al-Cu.

Selective photo-reduction of NO2- to N2 in the presence of Fe2+ and citric acid

The photo-reduction of NO₂^- has received increasing attention due to its high photo-activity. However, the intermediate products of NO₂^- photo-reduction might contain NO_x, which are also toxic pollutants. Herein, a novel strategy to selectively photo-reduce NO₂^- to N₂ was proposed using Fe²⁺ and citric acid (H₃Cit) as assistant to eliminate the formation of NO_x. In this strategy, NO₂^- was firstly reduced to NO by the combination of photon, Fe²⁺ and H₃Cit; the generated NO was then immediately captured by Fe²⁺-H₃Cit to form Fe²⁺-H₃Cit-NO complex; finally, H₃Cit was activated by Fe³⁺ and •OH in Fe²⁺/H₃Cit/UV/NO₂^- system to produce carbon dioxide anion radical (CO₂•^-), which could reduce the NO in Fe²⁺-H₃Cit-NO complex to N₂ with high efficiency and selectivity. The removal efficiencies of NO₂^- and TN were 98.6% and 87.5%, respectively, and the selectivity of N₂ was 81.6% in Fe²⁺/H₃Cit/UV/NO₂^- system after 60-min reaction at initial pH of 2.2, Fe²⁺ dosage of 3.0 mmol·L^-1 and H₃Cit dosage of 3.0 mmol·L^-1. Based on the experimental results and spectral analysis, the mechanism of NO₂^- selective reduction in Fe²⁺/H₃Cit/UV/NO₂^- system was proposed. Our finding provides a new way for wastewater denitrification and water purification.

Selective reduction of nitrate to nitrogen by Fe0-Cu0-CuFe2O4 composite coupled with carbon dioxide anion radical under UV irradiation

Zero-valent iron (Fe⁰) has been widely used for the reduction of nitrate, but the end reduction product is mainly ammonium. Here, a novel strategy for selective reduction of nitrate (NO₃^-) to nitrogen gas (N₂) with high efficiency and N₂ selectivity was investigated using Fe-based material (Fe⁰-Cu⁰-CuFe₂O₄) combined with citric acid (CA) and ultraviolet (UV) irradiation. In this strategy, the nitrate was firstly reduced to nitrite (NO₂^-) by Fe⁰-Cu⁰-CuFe₂O₄/UV process, and then the produced NO₂^- could be further reduced to N₂ by carbon dioxide anion radicals (CO₂^•-) which was generated from CA that was added later. In this process, the selective reduction of NO₃^- to NO₂^- was a key step. For this purpose, we synthesized Fe⁰-Cu⁰-CuFe₂O₄ composite by simple chemical replacement and in-situ growth process, which made it have a delicate structure with good contact between Cu and Fe and CuFe₂O₄. The selective reduction of NO₃^- to NO₂^- in Fe⁰-Cu⁰-CuFe₂O₄/UV process was due to that the Cu⁰ was the electron enrichment center and the photo-generated hole could suppress the NO₃^- reduction to NH₄⁺ by Fe²⁺. In this proposed strategy, 100% NO₃^- removal efficiency and 96.3% N₂ selectivity were achieved when the initial NO₃^- concentration was 30 mg N/L and the reduction time was 60 min. The denitrification mechanism of the Fe⁰-Cu⁰-CuFe₂O₄/UV/CA system was proposed.

A critical review of various adsorbents for selective removal of nitrate from water: Structure, performance and mechanism

Nitrate is ubiquitous pollutant due to its high water solubility, usually contributing to eutrophication, and posing a threat to aquatic ecosystem and human health. Adsorption approach has been widely used for nitrate removal because of the simplicity, easy operation, and low cost. Adsorbent plays a key role in the adsorptive removal of nitrate. The adsorption performance and adsorption mechanism are determined by the structural feature of adsorbent that is dependent on the preparation method. In this review, various types of adsorbents for nitrate removal were systematically summarized, their preparation, characterization, and adsorption performance were evaluated; the factors influencing the nitrate adsorption performance were discussed; the adsorption isotherm models, kinetic models and thermodynamic parameters were examined; and the possible adsorption mechanisms responsible for nitrate adsorption were categorized; the possible correlation of adsorbent structure to adsorption performance and adsorption mechanism were explained; the potential applications of adsorbents were discussed; finally, the strategies for improving adsorption capacity and selectivity towards nitrate, the challenges and future perspectives for developing novel adsorbent were also proposed. This review will deepen the understanding of nitrate removal by adsorption process and help the development of high-performance adsorbents for selective nitrate removal from water and wastewater.

A new supported Cu/Pd bimetallic nanoparticles composites prestoring reductant for nitrate removal: high reactivity and N2 selectivity

Catalytic hydrogen reduction appears to be a promising strategy for nitrate removal. However, the danger and low utilization of H₂ are the disadvantages of catalytic hydrogen reduction. Sodium borohydride (NaBH₄), considered a potential candidate for hydrogen storage, has been investigated as an electron source for the catalytic reduction of contaminants. However, extensive use of NaBH₄ makes commercialization costly and causes environmental pollution. In this study, we prepared supported Cu/Pd bimetallic nanoparticles that could prestore hydrogen. No additional reducing agent was required during the nitrate reduction process. The performance and mechanism of Cu/Pd bimetallic nanoparticles for nitrate reduction are discussed. Good performance was obtained with high reactivity (99.04% nitrate removal efficiency) and high selectivity for N₂ (94.71%). The Cu/Pd bimetallic catalyst could be recovered by NaBH₄ for 5 cycles. Moreover, a 97.49% nitrate removal efficiency was obtained for actual wastewater, indicating good prospects for nitrate reduction applications.

A novel strategy for sequential reduction of nitrate into nitrogen by CO2 anion radical: Experimental study and DFT calculation

In order to expand the application of CO₂ anion radical (CO₂^-), as a novel green reductant in the control of environmental pollution, CO₂^- radical was induced into the reduction of nitrate. The reduction efficiency, products and mechanism of nitrate or nitrite by CO₂^- radical were investigated based on the results of batch experiments and theoretical calculation using density functional theory (DFT) methods, respectively. It was found that: (1) the efficiency of nitrate reduction by CO₂^- radical from the HCOOH/UV system was far lower than that of nitrite under the same reaction conditions, (2) the rate-control step of nitrate reduction by CO₂^- radical was the transformation process of nitrate into nitrite with an activation energy of 23.9 kcal/mol, (3) the final products of nitrate reduction were mainly composed of nitrogen (N₂). On this basis, a novel strategy of rapid reduction of nitrate into N₂ using CO₂^- radical was proposed. Specifically, nitrate was firstly reduced into nitrite with the assistance of Zn/Ag bimetal, and then nitrite was further reduced into N₂ by CO₂^- radical. In this way, the removal efficiency of nitrate was all achieved nearly 100% in the initial nitrate concentration ranging from 25 to 100 mg (N)/L, while the highest N₂ selectivity could reach 97.5%. This work provided a promising approach for the reduction of nitrate into nitrogen with high efficiency and high N₂ selectivity by CO₂^- radical.

Enhancement in the rate of nitrate degradation on Au- and Ag-decorated TiO2 photocatalysts

The solar-driven reduction of nitrate to nitrogen has been studied in the presence of a formate hole scavenger, over a series of Au- and Ag-decorated TiO₂ catalysts.