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.

New insights into iron/nickel-carbon ternary micro-electrolysis toward 4-nitrochlorobenzene removal: Enhancing reduction and unveiling removal mechanisms

The ternary micro-electrolysis material iron/nickel-carbon (Fe/Ni-AC) with enhanced reducibility was constructed by introducing the trace transition metal Ni based on the iron/carbon (Fe/AC) system and used for the removal of 4-nitrochlorobenzene (4-NCB) in solution. The composition and structures of the Fe/Ni-AC were analyzed by various characterizations to estimate its feasibility as reductants for pollutants. The removal efficiency of 4-NCB by Fe/Ni-AC was considerably greater than that of Fe/AC and iron/nickel (Fe/Ni) binary systems. This was mainly due to the enhanced reducibility of 4-NCB by the synergism between anode and double-cathode in the ternary micro-electrolysis system (MES). In the Fe/Ni-AC ternary MES, zero-iron (Fe⁰) served as anode involved in the formation of galvanic couples with activated carbon (AC) and zero-nickel (Ni⁰), respectively, where AC and Ni⁰ functioned as double-cathode, thereby promoting the electron transfer and the corrosion of Fe⁰. The cathodic and catalytic effects of Ni⁰ that existed simultaneously could not only facilitate the corrosion of Fe⁰ but also catalyze H₂ to form active hydrogen (H*), which was responsible for 4-NCB transformation. Besides, AC acted as a supporter which could offer the reaction interface for in-situ reduction, and at the same time provide interconnection space for electrons and H₂ to transfer from Fe⁰ to the surface of Ni⁰. The results suggest that a double-cathode of Ni⁰ and AC could drive much more electrons, Fe²⁺ and H*, thus serving as effective reductants for 4-NCB reduction.

Synthesis and applications of various bimetallic nanomaterials in water and wastewater treatment

Bimetallic nanoparticles are the complex combination of two different metal constituents in nanoscale. Water and wastewater treatment utilizing bimetallic particles is an emerging research area. When two metals are combined, it can show not only the properties of its constituents but also new and enhanced properties derived by the synergy of the combination. These properties of bimetallic nanoparticles inevitably depend on the size, structure, and morphology of the particles. Thus the adopting synthesis strategy is very crucial to achieve desired results. Here in this review, the various bimetallic synthesis strategies are compared. The bimetallic nanoparticles decontaminate water through adsorption and/or catalysis mechanism. The various degradation pathways, specifically, adsorption, reduction, oxidation, and advanced oxidation processes are discussed in detail in this review.

Removal of contaminants by activating peroxymonosulfate (PMS) using zero valent iron (ZVI)-based bimetallic particles (ZVI/Cu, ZVI/Co, ZVI/Ni, and ZVI/Ag)

In this study, four different ZVI/M-PMS systems (e.g., ZVI/Cu, ZVI/Co, ZVI/Ni and ZVI/Ag) were fabricated to investigate the removal of contaminants (Rhodamine B, 2,4-dichlorophenol, bisphenol A, bisphenol F, levofloxacin, and chloramphenicol).

Reduction of nitrate by zero valent iron (ZVI)-based materials: A review

Zero valent iron (ZVI) and ZVI-based materials have been widely used for the reduction of nitrate, a major contaminant commonly detected in groundwater and surface water. The reduction of nitrate by ZVI is influenced by various factors, such as the physical and chemical characteristics of ZVI and the operational parameters. There are some problems for the nitrate reduction by ZVI alone, for example, the formation of iron oxides on the surface of ZVI at high pH condition, which will inhibit the further reduction of nitrate; in addition, the end reduction product is mainly ammonium, which itself needs to be concerned. Several strategies, such as the optimization of the structure of ZVI composites and the addition of reducing assistants, have been proposed to increase the reduction efficiency and the selectivity of end product of nitrate reduction in a wide range of pH, especially under neutral pH condition. This review will mainly focus on the high efficient reduction of nitrate by ZVI-based materials. Firstly, the reduction of nitrate by ZVI alone was briefly introduced and discussed, including the influence of physical and chemical characteristics of ZVI and some operational parameters on the reduction efficiency of nitrate. Then, the strategies for enhancing the reduction efficiency and the N₂ selectivity of the reductive products of nitrate were systematically analyzed and evaluated, especially the optimization of the structure of ZVI composites (e.g., doped ZVI composite, supported ZVI composite and premagnetized ZVI), and the addition of reducing assistants (e.g., metal cations, ligand, hydrogen gas and light) were highlighted. Thirdly, the mechanisms and pathways of nitrate reduction were discussed. Finally, concluding remarks and some suggestions for the future research were proposed.

Role and application of iron in water treatment for nitrogen removal: A review

It is crucial to have a review on the role of iron in water treatment for the guidance towards the selection of appropriate processes, content of iron, and application conditions, as there are few reviews available at present and the systematic information is lacking for both researchers and engineers. The objectives of this review are to summarize the state of arts with respect to iron applied in nitrogen removal, discuss chemical and biological or bio-chemical combined nitrogen removal pathways and processes coupled with iron, and to reveal reaction mechanisms as well as providing references or even solutions to pertinent the practical engineering application of nitrate removal coupling with iron. The following information have been summarized and discussed in details: (1) iron based materials with varieties of preparations and forms, (2) major coupling ways of nitrogen removal methods or processes with iron application, (3) chemical reaction equations about a variety of chemical and biological or bio-chemical combined processes and the main mechanisms. In addition, challenges and/or drawbacks during the nitrogen removal processes will also be discussed in this paper, which is aimed to seek better practical engineering applications of nitrate removal coupling with iron.

Porous Fe0/C ceramsites for removal of aqueous Pb(ii) ions: equilibrium, long-term performance and mechanism studies

This study reports the equilibrium, long-term performance and mechanisms in removing Pb(ii) ions by metallic iron/carbon (Fe0/C) ceramsites (FCC). The Pb(ii) removal equilibrium data was analyzed using the Langmuir, Freundlich and Dubinin-Radushkevich isotherms. At the FCC dosage of 1.14 g L-1, 95.97% of Pb(ii) ions were removed from 50 mg L-1 Pb(ii) solution at initial pH 6.0. The Langmuir isotherm could fit well with the data at initial pH 3.0 with a maximum monolayer adsorption capacity of 112.36 mg g-1 at 25 °C, while the data obtained at initial pH 6.0 could be described by the Freundlich model, indicating multilayer adsorption of Pb species on the FCC. Column tests demonstrated that FCC achieved the highest Pb(ii) removal of 65.86% after 12 days' run compared to 32.35% for Fe0/activated carbon couples and only 1.24% for activated carbon. The X-ray diffraction and X-ray photoelectron spectroscopy analysis revealed that the PbO (dominant Pb species), Pb0, asisite and plumbojarosite appeared after Pb(ii) removal. Scanning electron microscopy with energy dispersive X-ray spectroscopy showed that PbO particles with numerous structures were deposited on the FCC surface in a high amount. The decrease of the Fe/C mass ratio from 7.5 : 1 to 0.298 : 1 revealed that microscale Fe0 could been readily corroded by forming galvanic couples between Fe0 and carbon. The mechanisms of Pb(ii) removal by the FCC were proposed.