Performance and mechanism of chelating resin (TP-207) supported Pd/Cu bimetallic nanoparticles in selective reduction of nitrate by using ZVI (zero valent iron) as reductant

A critical review of nitrate reduction by nano zero-valent iron-based composites for enhancing N2 selectivity

Due to the highly reductive capacity of nano zero-valent iron (nZVI) nanoparticles, the reduction of nitrate (NO3--N) is prone to produce ammonia nitrogen (NH4+-N) as a by-product and has low selectivity for nitrogen gas (N2). Water and dissolved oxygen (DO) in the solution consume electrons from nZVI, decreasing the efficiency of NO3--N reduction. In order to overcome the drawbacks of plain nZVI being used to remove NO3--N pollution, nZVI-based multifunctional materials have been constructed to realize the selective conversion of NO3--N to N2 as well as the efficient removal of NO3--N. Therefore, advanced research on the reduction of NO3--N by nZVI-based composites has been comprehensively reviewed. Strategies to improve NO3--N reduction efficiency and N2 selectivity are proposed. Moreover, the shortcomings of iron-based nanomaterials in NO3--N pollution control have been summarized, and some suggestions for future research directions provided.

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.

Removal of Nitrate Nitrogen in Groundwater by Attapulgite Loaded with Nano-Zero-Valent Iron

Nano-zero-valent iron (nZVI) can be used to remove nitrate nitrogen (NO3-N) from groundwater. However, it has low reduction efficiency owing to its oxidation and aggregation characteristics. Thus, nZVI-loaded material is used to alleviate these drawbacks. In this study, nZVI-coated attapulgite (ATP) was prepared for the removal of NO3-N from groundwater. ATP-nZVI was prepared using the chemical liquid deposition-coreduction method. The prepared materials were characterized by SEM, XRD, and XPS analyses, which confirmed that the aluminum silicate particles in the ATP structure are effective carriers of nZVI and effectively inhibit self-consumption caused by the oxidation and aggregation of nZVI. The batch experiments examined experimental samples containing 30 mg/L nitrate and analyzed the effects of various parameters, including the material, mass ratio, initial pH, initial temperature, and coexisting anions on the NO3-N removal efficiency. The results showed that the optimal removal rate of the composite was 78.61%, which is higher than that using the same amount of ATP, iron powder, and nZVI. When the mass ratio of ATP to nZVI was 1 : 1, the NO3-N removal efficiency was the highest. When the pH value increased from 3 to 9, the NO3-N removal rate decreased, while an increase in the reaction temperature promoted NO3-N removal. The order of the inhibitory effect of coexisting anions on NO3-N removal by various nanoions was PO43–>CO32–>SO42–>Cl–. The adsorption kinetic model fitting results indicated that the chemisorption of electron exchange between ATP and nZVI in NO3-N removal was the main rate-limiting step in the reaction. This study demonstrates the potential of the prepared ATP-nZVI composite for NO3-N removal from groundwater.

Selective reduction of nitrite to nitrogen by polyaniline-carbon nanotubes composite at neutral pH

The selective reduction of nitrite (NO₂^-) to nitrogen by chemical reductant is a desirable strategy to remove NO₂^- from polluted water and wastewater. However, the residue and reuse of chemical reductant are two main issues to be addressed. Herein, a novel polyaniline-carbon nanotubes composite (PANI-CNTs) was developed by in-situ polymerization to selectively reduce NO₂^- to nitrogen gas (N₂). The used PANI-CNTs could be reused after regeneration with NaBH₄. The PANI-CNTs could reduce NO₂^- with 93.9% N₂ selectivity at initial pH of 6.8. The NO₂^- removal efficiency only decreased by 12.08% after five cycles of reduction/regeneration. The interconversion between imine nitrogen (-N) and amine nitrogen (-NH-) groups induced the chemical reduction of NO₂^- and regeneration of PANI-CNTs. PANI-CNTs exhibited an excellent performance for the removal of NO₂^- in the presence of competitive ions and in actual water and wastewater samples. This new PANI-CNTs composite may have great potential for water purification and wastewater denitrification.

Feasibility of reduced iron species for promoting Li and Co recovery from spent LiCoO2 batteries using a mixed-culture bioleaching process

The recovery of metals from spent LiCoO₂ batteries (SLBs) is essential to avoid resource wastage and the production of hazardous waste. However, the major challenge in regard to recovering metals from SLBs using traditional bioleaching is the low Co yield. To overcome this issue, a mixed culture of Acidithiobacillus caldus and Sulfobacillus thermosulfidooxidans was designed for use in SLBs leaching in this study. With the assistance of Fe²⁺ as a reductant, 99% of Co and 100% of Li were leached using the above mixed-culture bioleaching (MCB) process, thus solving the problem of low metal leaching efficiency from SLBs. Analysis of the underlying mechanism revealed that the effective extraction of metals from SLBs by the Fe²⁺-MCB process relied on Fe²⁺-releasing electrons to reduce refractory Co(III) to Co(II) that can be easily bioleached. Finally, the hazardous SLBs was transformed into a non-toxic material after treatment utilizing the Fe²⁺-MCB process. However, effective SLBs leaching was not achieved by the addition of Fe⁰ to the MCB system. Only 25% Co and 31% Li yields were obtained, as the addition of Fe⁰ caused acid consumption and bacterial apoptosis. Overall, this study revealed that reductants that cause acid consumption and harm bacteria should be ruled out for use in reductant-assisted bioleaching processes for extracting metals from SLBs.

Enhancing nitrate removal efficiency of micro-sized zero-valent iron by chitosan gel balls encapsulating

The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO₃^--N in the absence of organic carbon sources and neutral pH. The NO₃^--N removal rate of MZVI@CS was 0.37 mg-N·L^-1·d^-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L^-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (E_a) of MZVI@CS with nitrate was 38.23 kJ·mol^-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L^-1). A stable denitration rate (0.37-2.28 mg-N·L^-1·d^-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N₂, and only a small amount was converted to NH₄⁺ or NO₂^-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO₃^--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO₃^--N deep removal of water bodies with neutral pH and low C/N ratios.

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.