Fine structure characterization of zero-valent iron nanoparticles for decontamination of nitrites and nitrates in wastewater and groundwater

Abstract

The main objectives of the present study were to investigate the chemical reduction of nitrate or nitrite species by zero-valent iron nanoparticle (ZVIN) in aqueous solution and related reaction kinetics or mechanisms using fine structure characterization. This work also exemplifies the utilization of field emission-scanning electron microscope (FE–SEM), transmission electron microscopy (TEM), and x-ray diffraction (XRD) to reveal the speciation and possible reaction pathway in a very complex adsorption and redox reaction process. Experimentally, ZVIN of this study was prepared by sodium borohydride reduction method at room temperature and ambient pressure. The morphology of as-synthesized ZVIN shows that the nearly ball and ultrafine particles ranged of 20–50 nm were observed with FE–SEM or TEM analysis. The kinetic model of nitrites or nitrates reductive reaction by ZVIN is proposed as a pseudo first-order kinetic equation. The nitrite and nitrate removal efficiencies using ZVIN were found 65–83% and 51–68%, respectively, based on three different initial concentrations. Based on the XRD pattern analyses, it is found that the quantitative relationship between nitrite and Fe(III) or Fe(II) is similar to the one between nitrate and Fe(III) in the ZVIN study. The possible reason is due to the faster nitrite reduction by ZVIN. In fact, the occurrence of the relative faster nitrite reductive reaction suggested that the passivation of the ZVIN have a significant contribution to iron corrosion. The extended x-ray absorption fine structure (EXAFS) or x-ray absorption near edge structure (XANES) spectra show that the nitrites or nitrates reduce to N₂ or NH₃ while oxidizing the ZVIN to Fe₂O₃ or Fe₃O₄ electrochemically. It is also very clear that decontamination of nitrate or nitrite species in groundwater via the in-situ remediation with a ZVIN permeable reactive barrier would be environmentally attractive.     

Synthesis of MgO powders from molten salts

The reactions of anhydrous MgSO₄ and MgCl₂ in different molten nitrates and nitrites which produce fine MgO powders were studied. The stoichiometries of the reactions were established and the effects of various reaction conditions on the properties of the MgO powders produced were investigated. The addition of Lux-Flood bases to nitrate melts can lower the reaction temperatures and further lower the crystallite size of the MgO powders produced. The results showed that very fine MgO powders with high purity can be precipitated from molten nitrates and nitrites at temperatures below 600°C, and these powders can be obtained with soft agglomeration if the powders precipitated are properly processed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Direct synthesis of mesoporous Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> through citric acid-assisted solid thermal decomposition

A simple and inexpensive approach to synthesizing mesoporous Fe₃O₄ is developed by using citric acid-assisted solid thermal decomposition of ferric nitrate. The structure and magnetic property of mesoporous Fe₃O₄ were characterized by XRD, FT–IR, N₂ adsorption–desorption isotherms, TEM, and vibrating sample magnetometer. It was shown that the decomposition of citric acid results in the formation of the mesoporous structure and narrow pore-size distribution. The reducing atmosphere which created by the decomposition of the ferric nitrate–citric acid complex caused the partial reduction of Fe(III) to Fe(II) and in turn the formation of Fe₃O₄. Moreover, the strength of the coordination between carboxyl group and Fe³⁺ also affected the phase composition of the iron oxides.     

Low temperature preparation and characterization of intermetallics—Fe2W,FeMo and some ternary Fe-W-Mo systems

Monophase Fe₂W and FeMo intermetallics have been prepared by hydrogen reduction of Fe₂WO₆ and FeMoO₄ oxides respectively below 1050 K. The ternary intermetallics Fe₂W_0·9Mo_0·1 and FeMo_0·9W_0·1 have also been prepared by a similar method from the respective substituted oxides. The oxides and the intermetallics have been characterized by x-ray powder diffraction, x-ray photoelectron and Mössbauer spectroscopies. The observed negative isomer shift of Fe in the intermetallics is attributed to an increase in the electron density in Fe by electron transfer from W or Mo to Fe.     

Bulk and surface structures of iron doped zirconium oxide systems: Influence of preparation method

Three different Fe-Zr oxide systems were prepared using firstly classical impregnation of iron nitrates on calcined ZrO₂ (Fe_x/ZrO₂, x represents Fe/Zr ratio = 0.01 and 0.11), secondly impregnation of iron nitrates on dried zirconium hydroxide ZrO(OH)₂ (Fe_x/ZrO(OH)₂) and finally hydrolysis of aqueous suspension of iron and zirconium salts to coprecipitate iron and zirconium hydroxides (Fe_x-Zr). Thermal decomposition study of dried samples evidenced a delay in the temperature crystallization of zirconia for Fe_x-Zr and Fe_x/ZrO(OH)₂, the more the iron content in the sample, the more important the delay. For these samples, the formation and the stabilization of different phases were evidenced by several characterization techniques : X-Ray Diffraction (XRD), Raman spectroscopy and Electron Paramagnetic Resonance (EPR).The interaction of iron species with zirconia was different in accordance with different preparations. A bulk dispersion of the coprecipitated sample was observed and as a consequence Zr^{3 +} defects in the solid were not produced. In the case of Fe_x/ZrO₂ sample, production of surface Zr^{3 +} ions was established at low temperature of calcination (up to 600^∘C) and explained by the reaction of NO₃⁻ with Zr^{4 +} on the zirconia surface. However such interaction did not occur for Fe_x/ZrO(OH)₂ since a low dispersion of iron species was observed by X-ray Photoelectron Spectroscopy (XPS), deposited phase (Fe₂O₃) forming preferentially blocks. Temperature Programmed Reduction (TPR) showed that the reduction of small particles of Fe₂O₃ and bulk Fe₂O₃ present in the impregnated samples was easier than that of iron species well dispersed in the bulk of the coprecipitated solid.     

Preparation of zirconia powders from molten nitrites and nitrates

Finely dispersed ZrO₂ powders with high purity have been prepared by the reactions of Zr(SO₄)₂ in molten nitrites and nitrates. The effects of the starting materials of -Zr(SO₄)₂ and -Zr(SO₄)₂, the different nitrate and nitrite melts, and reaction conditions such as temperature and time, on the structure and crystallite size of the ZrO₂ powders produced were investigated. The processing of the prepared powders was also studied under different conditions; the results showed that very fine ZrO₂ powders with soft agglomeration could be produced if the proper treatment of the powders was applied.     

Investigation of the reduction kinetics of wustite fine powders

The reduction of wustite (FeO) by hydrogen was investigated over the temperature range 723 to 873 K. The reduction mechanism was found to depend on temperature relative to an effective eutectoid decomposition temperature of 803 K. Above this temperature the reaction proceeds directly according to: FeO + H₂ = Fe + H₂O while below this temperature the reduction is accomplished through the following two sequential steps: 4FeO = Fe + Fe₃O₄ and Fe + Fe₃O₄ + 4H₂ = 4Fe + 4H₂O. Approximate activation energies for reduction above and below 803 K were determined to be 51 and 58 kJ mol^–1, respectively. Investigations were also conducted on the oxidation of the iron resulting from the reduction of FeO.     

Nitrate reduction over nanoscale zero-valent iron prepared by hydrogen reduction of goethite

Nitrate reduction with nanoscale zero-valent iron (NZVI) was reported as a potential technology to remove nitrate from nitrate-contaminated water. In this paper, nitrate reduction with NZVI prepared by hydrogen reduction of natural goethite (NZVI-N, -N represents natural goethite) and hydrothermal goethite (NZVI-H, -H represents hydrothermal goethite) was conducted. Besides, the effects of reaction time, nitrate concentration, iron-to-nitrate ratio on nitrate removal rate over NZVI-H and NZVI-N were investigated. To prove their excellent nitrate reduction capacities, NZVI-N and NZVI-H were compared with ordinary zero-valent iron (OZVI-N) through the static experiments. Based on all above investigations, the mechanism of nitrate reduction with NZVI-N was proposed. The result showed that reaction time, nitrate concentration, iron-to-nitrate ratio played an important role in nitrate reduction by NZVI-N and NZVI-H. Compared with OZVI, NZVI-N and NZVI-H showed little relationship with pH. And NZVI-N for nitrate composition offers a higher stability than NZVI-H because of the existence of Al-substitution. Furthermore, NZVI-N, prepared by hydrogen reduction of goethite, has higher activity for nitrate reduction and the products contain hydrogen, nitrogen, NH₄⁺, a little nitrite, but no NOx, meanwhile NZVI-N was oxidized to Fe²⁺. It is a relatively easy and cost-effective method for nitrate removal, so NZVI-N reducing nitrate has a great potential application in nitrate removal of groundwater.     

Phase Composition of the Products of Fe Oxidation in Fe + C + NaCl + H2O + O2 Exothermic Mixtures

Using thermodynamic analysis of the Fe–C–NaCl–H₂O–O₂ system and experimental studies (x-ray diffraction and Mössbauer spectroscopy) of exothermic mixtures containing Fe metal, activated carbon, water, and NaCl, we identified the state of Fe and determined the phase composition of the reaction products at different stages of oxidation with atmospheric oxygen. The calculation and experimental results are in reasonable agreement. Under the conditions of restricted access for air, the main oxidation product is magnetite, Fe₃O₄. Free access for air leads to the formation of hydrous ferric oxide, Fe₂O₃ · nH₂O. The most stable phase under the conditions of interest is goethite, Fe₂O₃ · H₂O (-FeOOH). Storage of incompletely oxidized samples away from air for 7–14 days leads to partial reduction of iron(III) oxide phases to Fe₃O₄ and -Fe.     

Mössbauer,infrared and magnetic characterization of iron/cobalt species in cage structure alumino-silicates (zeolites)

Magnetic, Mössbauer, and I.R. studies on S.P. (superparamagnetic) or M.D. (multidomain) particles of Fe and Co species dispersed in cage structure aluminosilicates in relation to syngas (CO + H₂) Fischer-Tropsch conversion are reported. The difference in the catalytic activity of such species has been shown to depend upon their degree of dispersion. The carbonyl impregnation gave ultra-fine S.P. Fe₃O₄, whereas the nitrate impregnation gave M.D. Fe₃O₄ or α-Fe₂O₃. The active Fe₅C₂ component was converted to Fe₃C during the above reaction.     

Greenly synthesised silver‐alginate nanocomposites for degrading dyes and bacteria

The environmentally friendly synthesis of silver nanoparticles (AgNPs) has been achieved employing silver nitrate and sodium alginate (SA) without using other chemicals except for sodium hydrate. In the synthesis process, SA functions as both reductive and stabilising agent. The as‐synthesised AgNPs size can be controlled just changing the reactive parameters such as the concentration of silver nitrate and SA, the solution pH, the reaction temperature and time. Formation of AgNPs was observed by the colour change in the reaction medium which was further established with UV–Vis spectroscopy. The characterisation of AgNPs infers that the as‐synthesised AgNPs with an average size of 8.2 nm were spherical in shape and a face cubic crystal structure. The AgNPs‐SA beads were easily prepared using AgNPs‐SA nanocomposites due to SA crosslinking with metal ions. The catalytic efficiency of the resulting AgNPs beads is evaluated for the reduction of dyes such as 4‐nitrophenol, methylene blue and reactive red in the presence of NaBH₄. Antibacterial efficacy of AgNPs was analysed against gram‐negative Escherichia Coli and gram‐positive Staphylococcus aureus by measuring the zones of inhibition on the solid growth medium. The as‐synthesised AgNPs have shown efficient inhibitory activity against the tested bacterial strains.Inspec keywords: nanocomposites, dyes, filled polymers, silver, nanoparticles, nanofabrication, pH, ultraviolet spectra, visible spectra, catalysis, dissociation, microorganisms, nanomedicine, reduction (chemical), antibacterial activityOther keywords: greenly synthesised silver‐alginate nanocomposites, dye degradation, environmentally friendly synthesis, sodium alginate, sodium hydrate, reductive agent, stabilising agent, reactive parameters, silver nitrate concentration, solution pH, reaction temperature, reaction time, colour change, reaction medium, UV‐visible spectroscopy, face cubic crystal structure, metal ions, catalytic efficiency, dye reduction, 4‐nitrophenol, methylene blue, reactive red, antibacterial efficacy, gram‐negative Escherichia Coli, gram‐positive Staphylococcus aureus, inhibition zones, solid growth medium, inhibitory activity, bacterial strains, Ag     

Emerging alternative for artificial ammonia synthesis through catalytic nitrate reduction

Artificial catalytic synthesis of ammonia has become a hot research frontier in recent years.It is regarded as a promising approach that may replace the Haber-Bosch process and reduce global carbon dioxide emission.However,it is extremely difficult for the cleavage of nitrogen molecules under ambient con-ditions.Thus the ammonia yield rate is still low and the study is still limited in lab scale.If nitrites or nitrates are used as nitrogen sources,rather than nitrogen gas,the catalytic efficiency can be signifi-cantly improved,and the residual nitrate and nitrite contaminations in water systems can be efficiently eliminated and converted to energy sources at the same time.It is an emerging alternative for artificial ammonia synthesis,while there is not enough focus on the reduction of nitrate and nitrite.Herein,we systematically compared the differences between the reduction of nitrogen and nitrates,as well as listed the challenges in this area.The total conversion rate and energy efficiency of catalytic nitrate reduction are much higher than nitrogen gas reduction due to the much higher solubility and better converting pathway,which might be further enhanced by employing catalysts improvement strategies.Further,we also proposed suitable materials as well as a few future researches needs that may help boost the development of artificial ammonia synthesis using nitrate.     

Iron species in fusion-cast,glassy-phase-containing corundum materials; EPR and susceptibility analyses

By means of EPR, susceptibility, EMP, light-microscopic, thermal and chemical methods the influence of production conditions and subsequent treatments on glassy-phase-containing corundum materials were studied. Melting of the system (Al₂O₃, SiO₂, Na₂O, Fe₃O₄) under reductive conditions leads to a reduction of Fe³⁺ species contained to Fe²⁺ and even to Fe⁰ clusters with ferromagnetic behaviour. Both species markedly influence the mechanical properties of the material by increase of their volumes in consequence of oxidation in subsequent thermal processes. The following model with regard to the localization of the iron species in the system ensues: Fe(III) in corundum, Fe₂O₃, Fe₃O₄ and (scarcely) in the glassy phase; Fe(II) in the glassy phase, FeAl₂O₄ (hercynite) as a solid solution in corundum, and Fe₃O₄; (Fe⁰) clusters in corundum. It is therefore not surprising that grinding of the compact material considerably alters the magnetic properties of the samples.     

Chemiluminescence analyzer of NOx as a high-throughput screening tool in selective catalytic reduction of NO

A chemiluminescence-based analyzer of NO_x gas species has been applied for high-throughput screening of a library of catalytic materials. The applicability of the commercial NO_x analyzer as a rapid screening tool was evaluated using selective catalytic reduction of NO gas. A library of 60 binary alloys composed of Pt and Co, Zr, La, Ce, Fe or W on Al₂O₃ substrate was tested for the efficiency of NO_x removal using a home-built 64-channel parallel and sequential tubular reactor. The NO_x concentrations measured by the NO_x analyzer agreed well with the results obtained using micro gas chromatography for a reference catalyst consisting of 1 wt% Pt on γ-Al₂O₃. Most alloys showed high efficiency at 275 °C, which is typical of Pt-based catalysts for selective catalytic reduction of NO. The screening with NO_x analyzer allowed to select Pt-Ce_(X) (X=1–3) and Pt–Fe₍₂₎ as the optimal catalysts for NO_x removal: 73% NO_x conversion was achieved with the Pt–Fe₍₂₎ alloy, which was much better than the results for the reference catalyst and the other library alloys. This study demonstrates a sequential high-throughput method of practical evaluation of catalysts for the selective reduction of NO.     

Formation of Al3Ti/Mg composite by powder metallurgy of Mg–Al–Ti system

An in situ titanium trialuminide (Al₃Ti)-particle-reinforced magnesium matrix composite has been successfully fabricated by the powder metallurgy of a Mg–Al–Ti system. The reaction processes and formation mechanism for synthesizing the composite were studied by differential scanning calorimetry (DSC), x-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). Al₃Ti particles are found to be synthesized in situ in the Mg alloy matrix. During the reaction sintering of the Mg–Al–Ti system, Al₃Ti particles are formed through the reaction of liquid Al with as-dissolved Ti around the Ti particles. The formed intermetallic particles accumulate at the original sites of the Ti particles. As sintering time increases, the accumulated intermetallic particles disperse and reach a relatively homogeneous distribution in the matrix. It is found that the reaction process of the Mg–Al–Ti system is almost the same as that of the Al–Ti system. Mg also acts as a catalytic agent and a diluent in the reactions and shifts the reactions of Al and Ti to lower temperatures. An additional amount of Al is required for eliminating residual Ti and solid-solution strengthening of the Mg matrix.     

Facile synthesis of water-soluble and superparamagnetic Fe3O4 dots through a polyol-hydrolysis route

Monodisperse Fe₃O₄ dots with a mean size of about 2.3 nm were successfully synthesized via a polyol-hydrolysis route without adding any dispersant. Inorganic iron nitrate was used as the metal source and triethylene glycol (TEG) was used as the polyol solvent. The Fe₃O₄ dots were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selective area electron diffraction (SAED), Fourier transform infrared (FTIR) spectroscopy, N₂ adsorption–desorption, and magnetization measurements. The as-synthesized Fe₃O₄ dots can not only be coagulated from the polyol by ethanol and acetone, but also easily redispersed in water by ultrasonication, resulting in a clear Tyndall effect. The obtained Fe₃O₄ dots exhibited superparamagnetism at room temperature and the saturation magnetization is much lower than those reported in previous works. The formation mechanism of the Fe₃O₄ dots was proposed to be the hydrolysis of iron nitrates and subsequent dehydration and partial reduction of Fe³⁺ to Fe²⁺ at elevated temperatures in TEG.     

Green synthesis of magnetically recoverable Fe3 O4 /HZSM‐5 and its Ag nanocomposite using Juglans regia L. leaf extract and their evaluation as catalysts for reduction of organic pollutants

In this work, an Fe₃ O₄ /HZSM‐5 nanocomposite was synthesised in the presence of Juglans regia L. leaf extract. Then, silver nanoparticles (Ag NPs) were immobilised on the surface of prepared magnetically recoverable HZSM‐5 using selected extract for reduction of Ag⁺ ions to Ag NPs and their stabilisation on the surface of the nanocomposite. The reduction of Ag⁺ ions occurs at room temperature within a few minutes. Characterisation of the prepared catalysts has been carried out using fourier transform infrared (FT‐IR), X‐ray diffraction, field‐emission scanning electron microscopy (FESEM), energy‐dispersive spectroscopy, Brunauer–Emmett–Teller method, and a vibrating sample magnetometer. According to the FESEM images of the nanocomposites, the average size of the Ag NPs on the Fe₃ O₄ /HZSM‐5 surface was >70 nm. The Ag/Fe₃ O₄ /HZSM‐5 nanocomposite was a highly active catalyst for the reduction of methyl orange and 4‐nitrophenol in aqueous medium. The utilisation of recycled catalyst for three times in the reduction process does not decrease its activity.Inspec keywords: silver, X‐ray chemical analysis, X‐ray diffraction, nanocomposites, reduction (chemical), nanofabrication, nanoparticles, transmission electron microscopy, catalysts, Fourier transform infrared spectra, iron compounds, field emission scanning electron microscopy, zeolites, magnetometry, particle sizeOther keywords: Ag‐Fe₃ O₄ , temperature 293 K to 298 K, green synthesis, catalyst material, 4‐nitrophenol reduction, methyl orange reduction, particle size, vibrating sample magnetometry, Brunauer–Emmett–Teller method, field‐emission scanning electron microscopy, X‐ray diffraction, FT‐IR spectroscopy, silver nanoparticles, Juglans regia L. leaf extract, organic pollutant reduction, magnetically recoverable nanocomposites, energy‐dispersive spectroscopy     

Synthesis and processing of nanostructured M50 type steel

A nanostructured FeCrMoVM50 type steel was prepared via a chemical route. The process involved the thermal decomposition of organometallic precursors Fe(CO)₅, Cr(CO)₆, Mo(CO)₆ and V(CO)₆, at 150°C for the formation of nanostructured M50 type steel powders. In addition to the thermal decomposition of these carbonyls, the results of the reduction of respective metal halides for the production of the same steel are also presented. The nanostructured steel powders obtained were also consolidated samples, were characterized using x-ray synthesized powders, as well as the consolidated samples, were characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high resolution electron microscopy (HRTEM). Possible mechanisms for the formation of nanostructured particles are also discussed.     

Synthesis of Molybdenum- and Vanadium-Containing Mixed Oxides in Polymer–Salt Systems

Rare-earth alkaline-earth mixed oxides containing transition metals (Mo, V) were prepared via pyrolysis in polymer–salt systems. The products were characterized by thermal analysis, resistance measurements, dilatometry, optical microscopy, and x-ray diffraction. The introduction of polyvinyl alcohol into the system containing lanthanum or strontium nitrate and ammonium molybdate was found to have a significant effect on the thermal decomposition process, testifying to changes in the bonding configurations of the constituent components in the systems studied, capable of forming stable gels, which are then used as precursors to synthesize oxide materials. The temperatures of different stages of dehydration were shown to be lower in the polymer-containing systems. The effect of solution acidity was assessed by examining thermal decomposition in systems containing a polymer and Mo or W salts and acidified with nitric acid. The reaction of nitrates (oxidants) with the polymer was accompanied by an exotherm at 170°C, corresponding to the melting of ammonium nitrate, resulting from an exchange reaction. The exothermic reaction was found to reduce the decomposition temperatures of the salts involved. The use of polymer–salt systems allowed the mixed oxides SrMoO₄ and La₂(MoO₄)₃ to be synthesized at lower temperatures in comparison with the coprecipitation of poorly soluble compounds. The method was also shown to be suitable for preparing perovskite oxides in the La_{1 – x} Sr _x Co_{1 – z} M _z O_{3 ± y} (M = Mo, V) systems.     

Solvothermal‐assisted green synthesis of hybrid Chi‐Fe3 O4 nanocomposites: a potential antibacterial and antibiofilm material

In this present study, a hybrid Chi‐Fe₃ O₄ was prepared, characterised and evaluated for its antibacterial and antibiofilm potential against Staphylococcus aureus and Staphylococcus marcescens bacterial pathogens. Intense peak around 260 nm in the ultraviolet–visible spectrum specify the formation of magnetite nanoparticles. Spherical‐shaped particles with less agglomeration and particle size distribution of 3.78–46.40 nm were observed using transmission electron microscopy analysis and strong interaction of chitosan with the surface of magnetite nanoparticles was studied using field emission scanning microscopy (FESEM). X‐ray diffraction analysis exhibited the polycrystalline and spinel structure configuration of the nanocomposite. Presence of Fe and O, C and Cl elements were confirmed using energy dispersive X‐ray microanalysis. Fourier transform infrared spectroscopic analysis showed the reduction and formation of Chi‐Fe₃ O₄ nanocomposite. The antibacterial activity by deformation of the bacterial cell walls on treatment with Chi‐Fe₃ O₄ nanocomposite and its interaction was visualised using FESEM and the antibiofilm activity was determined using antibiofilm assay. In conclusion, this present study shows the green synthesis of Chi‐Fe₃ O₄ nanocomposite and evaluation of its antibacterial and antibiofilm potential, proving its significance in medical and biological applicationsInspec keywords: visible spectra, particle size, magnetic particles, nanocomposites, nanoparticles, X‐ray diffraction, nanofabrication, transmission electron microscopy, X‐ray chemical analysis, nanomagnetics, microorganisms, antibacterial activity, iron compounds, ultraviolet spectra, biomedical materials, field emission scanning electron microscopy, Fourier transform infrared spectra, filled polymers, crystal growth from solution, polymer structureOther keywords: potential antibacterial material, antibiofilm potential, magnetite nanoparticles, solvothermal‐assisted green synthesis, hybrid Chi‐Fe₃ O₄ nanocomposites, staphylococcus aureus, staphylococcus marcescens, bacterial pathogens, ultraviolet–visible spectrum, spherical‐shaped particles, particle size, transmission electron microscopy, FESEM, field emission scanning electron microscopy, X‐ray diffraction, spinel structure, polycrystalline structure, energy dispersive X‐ray microanalysis, Fourier transform infrared spectroscopic analysis, deformation, bacterial cell walls, Fe₃ O₄