Enhanced reduction of perchlorate by elemental iron at elevated temperatures

Kinetics of perchlorate reduction by elemental iron was examined at elevated temperatures using microwave heating and conventional block heating. It was hypothesized that increasing the solution temperature may accelerate the reduction of perchlorate by overcoming the high activation energy barrier. Results from microwave heating study showed that 98% of aqueous perchlorate was removed in 1 h at 200 degrees C. Similar results observed in control experiments with a block heater indicated that the enhancement in the extent and rate of perchlorate removal by elemental iron was mostly due to heat energy at high temperature. The rapid and complete reduction of perchlorate by elemental iron at elevated temperatures suggests that iron reduction process at elevated temperature may be an option to consider for complete removal of perchlorate from industrial discharges.     

Carbon solubilities in iron at elevated temperatures analysed by statistical thermodynamics

Abstract

Carbon solubilities in iron at elevated temperatures (>1000 K) are analysed on the basis of statistical thermodynamics. A parameter Q which refers to the extent of stabilization of a carbon atom in the iron lattice is estimated for α-, γ-, and molten FeC_x phases. The results suggest that the carbon atom is most stable in molten FeC_x and least stable in (α-FeC_x. In addition, the value of Q_C^α for (α-FeC_x at high temperatures appears to be different from that at lower temperatures. This observation can be interpreted as the effect of the magnetic transition of the iron atom around the Curie temperature. The values for Q estimated for the other interstitial elements X (X = H, N, P, S) in iron lattices are compared.

MST/586     

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.     

Template-free synthesis of flower-shaped zero-valent iron nanoparticle: Role of hydroxyl group in controlling morphology and nitrate reduction

Well-dispersed single phasic flower-like zero valent iron nanoparticles have been synthesized under aerobic conditions using a facile approach without the addition of any additives or templates. The role of hydroxyl groups of polyhydroxy alcohols in controlling surface morphology of nanoparticles has been thoroughly investigated. The obtained nanoparticles have been characterized by TEM, FE-SEM, XRD and BET surface area analyzer. Electron microscopy analyses reveal that the solvent plays a pivotal role in determining the morphology of the particles. With increase in viscosity of the solvent, formations of ‘petal-like’ structures, which are joined at the center are formed. The nitrate removal efficiency of the iron nanoparticles synthesized in different solvents has been studied and it is seen that the “flower-like” iron nanoparticles were most active in the removal of nitrate. Experiments have been done by varying (i) nitrate concentrations, (ii) nanoparticle dose, and (iii) type of nanoparticles. The results conclude that highest removal efficiency (～100%) was achieved when the nanoparticle dose was 2.88 g/L, even for high nitrate concentrations up to 400 mg/L. The major highlight of this work is the fact that even though the nanoparticles synthesized in glycerol-water mixture have larger size in comparison to the other nanoparticles, still they remove the nitrates with highest efficiency.”     

Aqueous Cr(VI) reduction by electrodeposited zero-valent iron at neutral pH: acceleration by organic matters

This work investigated the effect of co-existing organic matters on aqueous Cr(VI) reduction by electrodeposited zero-valent iron (ED Fe(0)) at neutral pH. The ED Fe(0) prepared in a solution containing mixture of saccharin, L-ascorbic acid and sodium dodecyl sulfate showed higher activity in reducing the aqueous Cr(VI) at neutral pH than that prepared without any organic presence. XRD and SEM indicated that the structure of ED Fe(0) was significantly improved to nano-scale by the presence of organic mixture in the preparation solution. Further, the ED Fe(0) activity in the Cr(VI) reduction at neutral pH was increased by the co-existence of citric acid or oxalic acid in the chromate solution. Electrochemical impedance spectroscopy (EIS) demonstrated that the corrosive current increased with the concentration of organic matter in the reaction solution. With the co-existing organic matters in the preparation solution, the ED Fe(0) corroded more rapidly due to its nano-size, thus the Cr(VI) reduction by the ferrous iron was accelerated. With the co-existing organic matters in the reaction solution, the Cr(VI) reduction was accelerated by a Fe(II) complex as the main electron donor, and a prevention of the passivation due to the Fe(III) and Cr(III) complexes also accelerated the Cr(VI) reduction.     

Mechanism study of nitrate reduction by nano zero valent iron

This study investigates the fate of nitrogen species during nitrate reduction by nano-scale zero valent iron (NZVI) and related reaction mechanisms. The NZVI used for the experiments was prepared by chemical reduction without a stabilizing agent. NZVI has great ability to reduce nitrate. However, the question of what end-product results from nitrate reduction by NZVI has sparked controversy. Establishing nitrogen mass balance by quantitative analysis of aqueous phase and gas-phase nitrogen species, this study clearly determines that nitrate was converted to ammonium ion followed by ammonia stripping under a strong alkaline condition, which leads to a decrease in the total aqueous nitrogen amount. Moreover, some of the major reactions, which consisted of nitrate reduction, ammonia production, and ammonia stripping were modelled by pseudo first-order kinetics. According to the model estimation results, additional reaction mechanisms would exist in an early stage of reaction. This might be due to the adsorption and desorption reaction which could be explained by the core-shell structure model.     

Detoxification of PAX-21 ammunitions wastewater by zero-valent iron for microbial reduction of perchlorate

US Army and the Department of Defense (DoD) facilities generate perchlorate (ClO₄⁻) from munitions manufacturing and demilitarization processes. Ammonium perchlorate is one of the main constituents in Army's new main charge melt-pour energetic, PAX-21. In addition to ammonium perchlorate, hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and 2,4-dinitroanisole (DNAN) are the major constituents of PAX-21. In order to evaluate microbial perchlorate reduction as a practical option for the treatment of perchlorate in PAX-21 wastewater, we conducted biodegradation experiments using glucose as the primary sources of electrons and carbon. Batch experiments showed that negligible perchlorate was removed in microbial reactors containing PAX-21 wastewater while control bottles containing seed bacteria and glucose rapidly and completely removed perchlorate. These results suggested that the constituents in PAX-21 wastewater may be toxic to perchlorate reducing bacteria. A series of batch toxicity test was conducted to identify the toxic constituents in PAX-21 and DNAN was identified as the primary toxicant responsible for inhibiting the activity of perchlorate reducing bacteria. It was hypothesized that pretreatment of PAX-21 by zero-valent iron granules will transform toxic constituents in PAX-21 wastewater to non-toxic products. We observed complete reduction of DNAN to 2,4-diaminoanisole (DAAN) and RDX to formaldehyde in abiotic iron reduction study. After a 3-day acclimation period, perchlorate in iron-treated PAX-21 wastewater was rapidly decreased to an undetectable level in 2 days. This result demonstrated that iron treatment not only removed energetic compounds but also eliminated the toxic constituents that inhibited the subsequent microbial process.     

Preparation of micaceous iron oxide by the oxidation of iron with pressurized oxygen in concentrated sodium hydroxide solution at elevated temperatures

Iron powders were oxidized in NaOH solutions of 5–25 mol kg^–1 at 403–563 K and 5 MPa of oxygen partial pressure. Various types and morphologies of iron compounds such as fine particles of Fe₃O₄, micaceous -Fe₂O₃, and coagulated particles of -NaFeO₂ were formed depending on the experimental conditions. The observed critical concentrations of NaOH above which -NaFeO₂ was formed was in good agreement with those thermodynamically calculated for the hydrolysis equilibrium of -NaFeO₂.     

Removal of arsenic from water by zero-valent iron

Batch and column experiments were conducted to investigate the effect of dissolved oxygen (DO) and pH on arsenic removal with zero-valent iron [Fe(0)]. Arsenic removal was dramatically affected by the DO content and the pH of the solution. Under oxic conditions, arsenate [As(V)] removal by Fe(0) filings was faster than arsenite [As(III)]. Greater than 99.8% of the As(V) was removed whereas 82.6% of the As(III) was removed at pH 6 after 9h of mixing. When the solution was purged with nitrogen gas to remove DO, less than 10% of the As(III) and As(V) was removed. High DO content and low solution pH also increased the rate of iron corrosion. The removal of arsenic by Fe(0) was attributed to adsorption by iron hydroxides generated from the oxic corrosion of Fe(0). The column results indicated that a filtration system consisting of an iron column and a sand filter could be used for treatment of arsenic in drinking water.     

Embrittlement of Armco iron by indium at high temperatures

We determined the relative elongation () and ultimate strength (_u) of hollow Armco iron specimens and Armco iron specimens filled with indium at high temperatures in a vacuum. It was shown that indium embrittles Armco iron in the temperature range 850–950°C. The lowest values of were obtained at a temperature of 925°C, where a small amount of the -phase is observed in the structure of strained specimens. The high-temperature liquid metal embrittlement of Armco iron is caused by an indiummelt-induced decrease in the flow stress, which localizes strains in the -phase and initiates its premature cracking. On the basis of metallographic investigations, we made the conclusion that corrosion is not responsible for the high-temperature liquid metal embrittlement of Armco iron.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 6, pp. 41–45, November–December, 1993.     

Enhanced crack closure performance of microbial mortar through nitrate reduction

Microbial self-healing of concrete has been widely investigated, yet the suggested microbial pathways are limited to ureolysis and the aerobic oxidation of carbon sources. Each of these pathways has certain environment and process related drawbacks which arise a need for an alternative pathway to proceed further. This study presents the NO₃⁻ reduction as an alternative microbial self-healing strategy. In the tests, we used previously described NO₃⁻ reducing bacteria, and two different porous protective carriers. The highest crack width healed by the bacteria was 370 ± 20 μm in 28 days and 480 ± 16 μm in 56 days. Water tightness regain up to 85% was achieved at the end of 56 days for 465 ± 21 μm crack width. Precipitates were identified as forms of CaCO₃ and were abundant in microbial specimens particularly on the inner crack surface. The findings evidence the potential of the NO₃⁻ reduction pathway for development of microbial self-healing concrete.     

Solid lubricants for applications at elevated temperatures

Relative motion between mating surfaces at elevated temperatures often causes substantial material degradation due to friction and wear. Conventionally, solid lubricants have been used to reduce wear damage and friction drag under extreme conditions where liquid lubricants do not function properly. The recent trend towards higher operating temperatures in advanced power generating systems, i.e. turbomachinery, gas turbines, and hot adiabatic diesel engines, has imposed severe limitations on the currently available solid lubricants. The unusually aggressive conditions in these systems phased out most conventional solid lubricants and gave impetus to the search for more efficient materials. This paper discusses the lubricating characteristics of four different groups of materials known to provide lubricity under elevated temperature conditions. These groups are polymers, laminar solids, metal fluorides and metal oxides. Polymer lubricants are efficient lubricants within the range from room temperature to about 300 °C. Laminar solids extend that range to about 450 °C. Graphite, also a laminar solid, is an exception since it can offer excellent lubricity beyond 450 °C in the form of gaseous oxidation products. Stable fluorides and metal oxides are useful lubricants between 500 and 1000 °C, though their performance is rather poor at lower temperatures.     

Solubility of tributyl phosphate and its degradation products in concentrated uranyl nitrate solutions and of uranyl nitrate in tributyl phosphate at elevated temperatures

Solubility of UO₂(NO₃)₂, TBP, and its degradation products in the organic and aqueous phases of the system TPB-H₂O-HNO₃-UO₂(NO₃)₂ at 25–128°C over the uranyl nitrate concentration range 200–1200 g 1^–1 is studied. The solubility of TBP and its degradation products in uranyl nitrate hexahydrate melt decreases in the order H₂MBP > HDBP > TBP > H₃PO₄. At the boiling point of the melt, their solubility ranges from 100 to 0.5 g l^–1. The solubility of uranyl nitrate in the TBP phase under the same conditions is caused by formation of complexes with a composition close to that of the monosolvate UO₂(NO₃) ₂ TBP.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 436–439.Original Russian Text Copyright © 2004 by Usachev, Markov.     

Coupled acidification and ultrasound with iron enhances nitrate reduction

Contaminated soils, especially when pollutant concentrations are high, pose potentially serious threats to surface and groundwater quality, when there are spills, discharges, or leaking storage tanks. For in situ remediation of nitrate in groundwater, the use of zero-valent iron (Fe(0)) is suggested. The formation of passivating scales on Fe(0) over time may limit the long-term reduction potential of Fe(0). The aim of this study was to investigate the effect of ultrasound and pH on the destruction of passive oxide film. Batch tests were conducted in a specially designed protocol using ultrasound, and changing iron (commercial iron powder of micro-scale grain size) loading and pH. The results showed deactivation of the degradation process by Fe(0) with combined ultrasound/iron system and with ultrasound alone. However, the degradation rate increases with decrease in pH. The degradation rate was 45% for pH 2 and 20% for pH 4. The combination of iron, acidification, and ultrasound showed excellent degradation efficiency, and the degradation rate was 99%. Acidification could destroy passive oxide film and activate iron, thus, hastening the reaction between Fe(0) and nitrate. Ultrasound was helpful in destroying or preventing the formation of passive oxide film under acidification.     

Fatigue strength of Inconel 718 at elevated temperatures

The strength of Inconel 718 under rotary bending fatigue is investigated at room temperature, 300, 500 and 600 °C in air. It is found that in the long-life region, the fatigue strength of a plain specimen is much higher at elevated temperatures than at room temperature, though the static strength decreases with the increase in temperature. The effect of temperature on the fatigue strength is examined in terms of the initiation and early growth behaviour of a small crack. The results are discussed in relation to the competition between the softening of the nickel matrix ( phase) and the surface oxidation at elevated temperatures.     

Photoconductivity of ZnTe thin films at elevated temperatures

Photoconductivity of thermally evaporated ZnTe thin films was studied at different elevated temperatures. A gap type cell configuration with Al electrodes on glass substrates was used. The conductivity was found to obey two distinct conduction mechanisms within the region of applied fields. At low fields the photoconduction is ohmic and at high fields it is of Poole-Frenkel type. With increase of ambient temperatures, the Poole-Frenkel conductivity regions were found to extend to lower fields. The temperature dependence of dark conductivity also was found to be of similar nature. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.