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

Influence of modified biochar supported sulfidation of nano-zero-valent-iron (S-nZVI/BC) on nitrate removal and greenhouse gas emission in constructed wetland

In this study, the biochar (BC) produced from sawdust, sludge, reed and walnut were used to support sulfidation of nano-zero-valent-iron (S-nZVI) to enhance nitrate (NO₃^--N) removal and investigate the impact on greenhouse gas emissions. Batch experiment results showed the S-nZVI/BC_{sawdust (2:1, 500)}, S-nZVI/BC_{sludge (2:1, 900)}, S-nZVI/BC_{reed (2:1, 700)}, and S-nZVI/BC _{walnut (2:1, 700)} respectively improved NO₃^--N removal efficiencies by 22%, 20%, 3% and 0.1%, and the selectivity toward N₂ by 22%, 25%, 22% and 18%. S-nZVI uniformly loaded on BC provided electrons for the conversion of NO₃^--N to N₂ through Fe⁰. At the same time, FeS_x layer was formed on the outer layer of ZVI in the sulfidation process to prevent iron oxidation, so as to improve the electrons utilization efficiency After adding four kinds of S-nZVI/BC into constructed wetlands (CWs), the NO₃^--N removal efficiencies could reach 100% and the N₂O emission fluxes were reduced by 24.17%-36.63%. And the average removal efficiencies of TN, COD, TP were increased by 21.9%, -16.5%, 44.3%, repectively. The increasing relative abundances of denitrifying bacteria, such as Comamonas and Simplicispira, suggested that S-nZVI/BC could also improve the process of microbial denitrification. In addition, different S-nZVI/BC had different effects on denitrification functional genes (narG, nirk, nirS and nosZ genes), methanotrophs (pmoA) and methanogenesis (mcrA). This research provided an effective method to improve NO₃^--N removal and reduce N₂O emission in CWs.

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.

Reductive Removal and Recovery of As(V) and As(III) from Strongly Acidic Wastewater by a UV/Formic Acid Process

The removal of arsenic (As(V) and As(III)) from strongly acidic wastewater using traditional neutralization or sulfuration precipitation methods produces a large amount of arsenic-containing hazardous wastes, which poses a potential threat to the environment. In this study, an ultraviolet/formic acid (UV/HCOOH) process was proposed to reductively remove and recover arsenic from strongly acidic wastewater in the form of valuable elemental arsenic (As(0)) products to avoid the generation of hazardous wastes. We found that more than 99% of As(V) and As(III) in wastewater was reduced to highly pure solid As(0) (>99.5 wt %) by HCOOH under UV irradiation. As(V) can be efficiently reduced to As(IV) (H₂AsO₃ or H₄AsO₄) by hydrogen radicals (H^•) generated from the photolysis of HCOOH through dehydroxylation or hydrogenation. Then, As(IV) is reduced to As(III) by H^• or through its disproportionation. The reduction of As(V) to H₄AsO₄ by H^• and the disproportionation of H₄AsO₄ are the main reaction processes. Subsequently, As(III) is reduced to As(0) not only by H^• through stepwise dehydroxylation but also through the disproportionation of intermediate arsenic species As(II) and As(I). With additional density functional theory calculations, this study provides a theoretical foundation for the reductive removal of arsenic from acidic wastewater.

Characterization of TiO2 and an as-prepared TiO2/SiO2 composite and their photocatalytic performance for the reduction of low-concentration N-NO3- in water

Excessive N-NO₃^- water pollution has become a widespread and serious problem that threatens human and ecosystem health. Here, a TiO₂/SiO₂ composite photocatalyst was prepared via the sol-gel/hydrothermal method. TiO₂ and TiO₂/SiO₂ were characterized by X-ray diffraction (XRD), UV-Vis differential reflectance spectroscopy (DRS), Fourier infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Afterward, the photocatalytic performance of TiO₂ and TiO₂/SiO₂ to reduce low nitrate concentrations (30 mgN L^-1) under UV light was evaluated and the effects of different factors on this process were investigated, after which the reaction conditions were optimized. Removal rates of up to 99.93% were achieved at a hole scavenger (formic acid) concentration of 0.6 mL L^-1, a CO₂ flow rate of 0.1 m³ h^-1, and a TiO₂ concentration of 0.9 g L^-1. In contrast, TiO₂/SiO₂ at a 1.4 g L^-1 concentration and a TiO₂ load rate of 40% achieved a removal rate of 83.48%, but with more than 98% of nitrogen generation rate. NO₂^- and NH₄⁺ were the minor products, whereas N₂ was the main product.

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.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.