Effects of Selected Good’s pH Buffers on Nitrate Reduction by Iron Powder

The effects of three selected Good’s pH buffers on the performance of an Fe0/nitrate/H2O system were evaluated. The Good’s pH buffer itself did not reduce nitrate directly. Nitrate reduction by iron powder at near-neutral pH was negligible in an unbuffered system, but it was greatly enhanced with the presence of the buffer. A significant amount of aqueous Fe2+ (or Fe3+) was released after adding the Good’s pH buffer into the Fe0/H2O system with or without nitrate. In general, the pH of the buffered solution increased from the initial pH ( = ～ 4.6–5.3, depending on buffer’s pKa) to near-neutral pH. After the initial pH hiking, the pH in the system was more or less stable for a period of time ( ～ 5–10?h, usually concurrent with a fairly stable aqueous Fe2+). The pH then drifted to ～ 7.1 to 8.6, depending on the buffer’s initial concentration, the buffer’s pKa, and the consumption of Fe2+ concurrent with nitrate reduction. While a common assumption made by researchers is that Good’s pH buffers do not directly participate in reaction processes involved in contaminant remediation, this study shows that as side effects, the Good’s pH buffer may react with iron powder.     

Effects of Surface-Bound Fe2+ on Nitrate Reduction and Transformation of Iron Oxide(s) in Zero-Valent Iron Systems at Near-Neutral pH

Nitrate reduction in an iron/nitrate/water system with or without an organic buffer was investigated using multiple batch reactors under strict anoxic conditions. Nitrate reduction was very limited (<10%) at near-neutral pH in the absence of the organic buffer. However, nitrate reduction was greatly enhanced if the system: (1) had a low initial pH ( ～ 2–3); (2) was primed with adequate aqueous Fe2+; or (3) was in the presence of the organic buffer. In Cases (1) and (3), nitrate reduction usually was involved in three stages. The first stage was quick, and H+ ions directly participated in the corrosion of iron grains. The second stage was very slow due to the formation of amorphous oxides on the surface of iron grains, while the third stage was characterized by a rapid nitrate reduction concurrent with the disappearance of aqueous Fe2+. Results indicate that reduction of nitrate by Fe0 will form magnetite; Fe2+ (aq.) can accelerate reduction of nitrate and will be substoichiometrically consumed. Once nitrate is exhausted in the system, no more Fe2+ will be consumed. In the presence of nitrate, Fe2+ (aq) will be adsorbed onto the surface of iron grains or iron oxides; the surface-complexed Fe(II) (extracted by acetate with pH = 4.1) might be oxidized and become structural Fe(III), resulting in a steadily increasing ratio of Fe(III)/Fe(II) in the oxides formed. The transformation of nonstoichiometric amorphous iron oxides into crystalline magnetite, a nonpassive oxide, triggers the rapid nitrate removal thereafter.     

Reduction of Nitrate, Bromate, and Chlorate by Zero Valent Iron (Fe0)

Oxo-anions occur in drinking waters, pose potential health risks, and should be controlled. It may be possible to incorporate zero-valent iron (Fe0) into water treatment processes to remove oxo-anions. Under near neutral pH (～7) and aerobic conditions, the three oxo-anions studied (NO3?, BrO3?, ClO3?) were electrochemically reduced by Fe0 in batch and continuous-flow packed column experiments. Mass balances provided strong evidence that ammonia is the primary reduction by-product from nitrate, chloride from chlorate, and bromide from bromate. Protons were consumed during the reaction, resulting in an increase in pH (i.e., production of hydroxide). Oxo-anion removal rates decreased as follows: BrO3?>ClO3?>NO3?. Differing rates of oxo-anion removal between batch and continuous flow column tests suggested that higher solid (Fe0) to liquid ratios increase oxo-anion electrochemical reduction, and scaling up of batch kinetic data to larger scale must consider the solid–liquid ratios. The atomic structure (atomic radii, electron orbital configuration, electron affinity) of nitrogen, chlorine, and bromine elements of the oxo-anions, and the bond dissociation energy between these elements and oxygen, were good indicators for the relative rates of reduction by Fe0.     

Mass Transfer Effects on Kinetics of Dibromoethane Reduction by Zero-Valent Iron in Packed-Bed Reactors

Mass transfer effects on the kinetics of 1,2-dibromoethane (EDB) reduction by zero-valent iron (ZVI) in batch reactors, a laboratory scale packed-bed reactor, and a pilot scale packed-bed reactor are described. EDB was debrominated by ZVI to ethylene and bromide. EDB sorption to the cast iron surface was nonlinear and was described by a Langmuir equation. Laboratory scale column studies showed a nonlinear dependence of EDB removal on flow rate and initial EDB concentration. A nonequilibrium model of EDB sorption and reaction dependent on mass transfer was constructed using the laboratory scale data. The model was verified using data from a larger pilot scale packed-bed reactor that was used to remove EDB from contaminated groundwater. The data showed two distinct removal processes, an initial rapid phase dominated by mass transfer followed by a slower phase where surface reactions dominated. The model successfully predicted the transition from mass transfer controlled to surface reaction controlled conditions in the pilot scale data.     

Enhancement of Nitrate Reduction in Fe0-Packed Columns by Selected Cations

Tests were conducted in Fe0-packed columns to investigate the effects of adding selected cations on nitrate removal by Fe0. Due to a rapid passivation of Fe0, only negligible nitrate was reduced in the columns without adding the selected cation. However, adding certain selected cations (Fe2+, Fe3+, or Al3+) in feed solution can significantly enhance nitrate reduction. Extending hydraulic retention time (HRT) increased nitrate removal by the columns, but the increase was not linearly proportional to HRT. Decreases in columns’ hydraulic conductivity (K) were monitored in an 8?month operating period. A modest decrease in K was recorded in the upper and the middle section of the media bed, whereas a significant decrease in K occurred in the inlet section. X-ray diffraction analyses indicate that magnetite (Fe3O4) was the dominant species of the iron corrosion products in the entire height of the column media under anoxic and other test conditions. In the inlet section, however, lepidocrocite and goethite were also identified. Cementation was found to occur only in the inlet section, suggesting that lepidocrocite and goethite, rather than magnetite, might be responsible for the cementation and thereby cause the hydraulic clogging. The magnetite coating would not necessarily cause clogging of the media.     

高磷铁矿直接还原动力学研究

通过TG,应用非等温法和等温法研究高磷铁矿直接还原的动力学规律,为工业试验提供理论依据。高磷铁矿直接还原分三个阶段进行,相界面反应、一维扩散、二维扩散。主体反应阶段是一维扩散。动力学方程、表观活化能和指前因子在等温和非等温法研究直接还原动力学中的结果一致。     

Smelting Reduction of Iron Ore‐Coal Composite Pellets

Smelting reduction of iron ore‐coal composite pellets has been carried out in an induction furnace. The pellets, tied with tungsten wire, were immersed into the liquid metal bath. The experiments were carried out in the temperature range of 1713 K to 1733 K. For 16‐18 mm diameter pellets, it was observed that (i) the time required for complete dissolution in liquid metal bath is 83‐90 seconds, (ii) the fraction of reduction for 40 seconds of immersion varied from 0.68‐0.87 for Jharia coal pellets and 0.73‐0.92 for Bhilai coal pellets, and (iii) the fraction of reduction increases with decreasing Fe/C ratio and increasing immersion period. X‐ray diffraction (XRD) of reduced pellets showed that the reduction occurred topochemically. Scanning electron microscopic (SEM) examination of partially reduced pellets evinced the structural changes in pellets. The present investigation aims to assess the effect of Fe/C ratio in pellet, volatile matter in coal, and time of pellet immersion into liquid metal bath on reduction of iron ore‐coal composite pellets.     

Kinetics of Arsenite Oxidation by Chemoautotrophic Thiomonas arsenivorans Strain b6 in a Continuous Stirred Tank Reactor

As(III) oxidation by a chemoautotrophic bacterium, Thiomonas arsenivorans strain b6, was evaluated in a continuous stirred tank reactor (CSTR) under a range of influent As(III) concentrations (2,000–4,000 mg/L) and hydraulic retention times (HRTs) (21.7–74.9 h). Five steady states were obtained after the CSTR was continuously operated for 115 days with over 99% As(III) oxidized under the optimal growth conditions for strain b6 at pH 6 and 30°C. The culture exhibited strong resilience by recovering from an As(III) overloading of 4,847.4±290.9?mg/day/L operated at a HRT of 21.7 h. Arsenic mass balance analysis revealed that As(III) was mainly oxidized to As(V), with unaccounted arsenic well within the analytical error of measurement. The best estimates of biokinetic parameters for As(III) oxidation were obtained using the steady-state data and the Monod expression based model [k = 5?mg As(III)/mg dry cell weight/h; Ks = 20.1?mg/L; kd = 0.008?h?1; and Y = 0.011?mg cell dry weight/mg As(III)]. The Monod model and the reactor mass balance successfully simulated both the steady-state and transient phases of CSTR operation. Sensitivity analyses defined Y and k to be the most sensitive to model predictions, whereas kd and Ks were least sensitive to model simulations of As(III) oxidation under steady-state conditions.     

Development of a New Zero-Valent Iron Zeolite Material to Reduce Nitrate without Ammonium Release

Previous studies have revealed that the application of zero-valent iron (ZVI) in reducing nitrate is limited by ammonium production and the requirement for adequate pH control. The current study focused on developing a new material potentially applicable in permeable reactive barriers, which can reduce nitrate without ammonium release under unbuffered pH. The new material, referred to as ZanF, is derived from zeolite modified by Fe(II), followed by borohydride reduction. The pseudo-first-order rate constant (kobs) of ZanF in the early period of nitrate reduction was 10 times higher than that of the ZVI used in this study. However, the kobs of ZanF decreased in the reaction period that followed. Even though both ZVI and ZanF produced ammonium as a product of nitrate reduction, ZanF removed it to below detection limits via adsorption, whereas ZVI did not remove it to any significant extent. ZanF maintained its high reactivity even under an initial pH of 6.2 without buffer. The higher ZanF/solution ratio increased the removal rate of ZanF as well as the removal efficiency.     

Development of a Response Surface for Prediction of Nitrate Removal in Sulfur–Limestone Autotrophic Denitrification Fixed-Bed Reactors

Sulfur–limestone autotrophic denitrification (SLAD) processes are very efficient for treatment of ground or surface water contaminated with nitrate. However, detailed information is not available on the interaction among some major variables on the design and performance of the SLAD process. In this study, the response surface method was used by designing a rotatable central composite test scheme with 12 SLAD column tests. A polynomial linear regression model was set up to quantitatively describe the relationship of the effluent and influent nitrate–nitrogen concentration and hydraulic retention time (HRT) in the SLAD column reactors. This model may be used for estimating the effluent nitrate–nitrogen concentration when the influent nitrate–nitrogen concentration ranges between 20 and 110?mg/L and the HRT ranges between 2 and 9?h. Based on our model and the requirement for nitrite control, we recommend that the HRT of the SLAD column reactor be kept ≥ 6?h and the nitrate loading rate less than 200 g NO3?–N/day?m3 media to achieve high nitrate removal efficiency (>99%) and prevent nitrite accumulation from being >1?mg/L NO2?–N.     

Pyrolysis Kinetics of Rice Husk in Different Oxygen Concentrations

The effects of oxygen on the pyrolysis of rice husk were investigated with a thermal gravimetric analysis (TGA) reaction system. Pyrolysis experiments of rice husk were carried out in N2, 10% O2, and air at the heating rates of 2, 5, and 10?K/min. The TGA curves indicated two principal reactions, distinguished by significant and distinct mass changes over the experimental range. Oxygen enhanced the pyrolysis reactions of intermediates, which was produced from the first stage reaction of rice husk. A simple kinetic model was proposed for the pyrolysis of rice husk in different oxygen concentrations. The corresponding activation energies, preexponential factors, and reaction orders were determined. The experimental results were satisfactorily fitted by the proposed kinetic model.     

Modeling of Nitrate Adsorption and Reduction in Fe0-Packed Columns through Impulse Loading Tests

A conventional tracer study using Li+ and Cl? was conducted on four Fe0-packed column reactors for nitrate removal. Both Li+ and Cl? showed strong adsorption onto iron media and thus were not ideal tracers for the study. Tests using an impulse loading of nitrate were then innovated to investigate the transport and reduction of nitrate in the reactors. The impulse loading was superposed on a continuous constant feeding of nitrate which generated a steady effluent baseline. A multivariable model incorporating hydraulic dispersion, adsorption/desorption, and reduction of nitrate was developed and numerically solved. Both Langmuir adsorption and linear adsorption isotherms were separately applied to describe nitrate adsorption on the reactive surface. The parameters of the model were estimated by fitting the model with the response curves from the impulse loading tests. These estimated parameters were consistent with previous studies. Specifically, the modeling results suggest a significant adsorption of nitrate by the iron media, causing an evident retardation effect. The research may lead to new methods for studying the fate of contaminants in porous reactive environments.     

Evaluation for Biological Reduction of Nitrate and Perchlorate in Brine Water Using the Hydrogen-Based Membrane Biofilm Reactor

Whereas ion exchange is an attractive technology for treating perchlorate and nitrate in drinking water, a major disadvantage is that the resin must be regenerated using a brine, producing wastes with high concentrations of nitrate, perchlorate, and salt. This study investigates the potential for simultaneous nitrate and perchlorate reductions in high-salt conditions using the H2-based membrane biofilm reactor (MBfR). The autotrophic biological reductions produce harmless N2 and Cl?, making the brine safe for reuse or disposal. A very high-strength brine ( ～ 15% salt) from a commercial ion-exchange membrane, Purolite, supported biofilm accumulation and allowed slow reduction rates for nitrate and perchlorate. Reduction rates increased significantly when the Purolite brine was diluted by 50% or more. A synthetic high-strength salt medium containing nitrate, perchlorate, or both supported more rapid reduction rates for as high as 20?g/L ( ～ 2%) NaCl, while 40?g/L NaCl slowed reduction by 40% or more, confirming that the microorganisms in the MBfR were inhibited by high salt content. An increase of H2 pressure gave higher fluxes for 20?g/L NaCl, demonstrating that H2 availability controlled the reduction kinetics when the system was not salt-inhibited.     

Chloride Interactions with Iron Surfaces: Implications for Perchlorate and Nitrate Remediation Using Permeable Reactive Barriers

Perchlorate (ClO4?) can be reduced by iron surfaces, suggesting that permeable reactive barriers may represent a useful groundwater remediation strategy. However, chloride produced by the reaction inhibits further perchlorate removal. Adsorption of chloride on iron filings was investigated as a potential mechanism of chloride interference. The effect of chloride on the removal of nitrate, another oxyanion reactive at iron surfaces, was also investigated to draw more general conclusions about anion competition when target compounds adsorb electrostatically. A triple layer adsorption model was used to describe chloride sorption isotherms on the iron filings using magnetite as the model surface and defining a single type of surface hydroxyl sorption site. The model considered electrostatic attraction, specific sorption, and the effect of adsorbed Fe2+ on chloride sorption. Experimental and modeling results indicate that chloride competition is probably not of concern for nitrate reduction in permeable reactive barriers. However, perchlorate reduction is significantly inhibited by chloride in both buffered and unbuffered solutions, possibly because the reactive sorbed Fe2+ sites may be preferentially occupied by chloride.     

直接还原铁在电弧炉炼钢中的应用

对直接还原铁 (DRI)性能及其在电弧炉冶炼的特性作了较全面的论述。了解和掌握其特性是发挥其优越性的关键 ,是发展特殊钢和纯净钢的重要途径。     

2,4,6-Trinitrotoluene Transformation Using Spinacia Oleracea: Saturation Kinetics of the Nitrate Reductase Enzyme

A series of aqueous phase and soil-slurry phase microcosm studies were conducted on 2,4,6-trinitrotoluene (TNT) to obtain kinetic data for optimizing a treatment protocol using an enzyme extract from spinach (Spinacia oleracea). Crude extract was obtained by homogenization of fresh leaves with a buffered protease inhibitor, and employed as phytoremediation agent. Aqueous phase microcosms containing 20?mg/L TNT and soil–slurry microcosms containing 1 g of a characterized sandy loam soil contaminated with 2,500?mg/kg TNT and 1,000?mg/kg hexahydro-1,3,5-trinitro-1,3,5-triazine were dosed with fixed aliquots of extract and analyzed for TNT transformation over time. The TNT concentration was monitored using a colorimetric method for nitroaromatic compounds based on EPA Method 8515. Nitrate reductase activity of the applied crude extract was simultaneously quantified. The transformation of TNT was described by a pseudofirst-order reaction. Coupling kinetic rate information with enzyme activity allowed for estimation of a second-order rate constant with respect to activity. A rectangular hyperbola function normalized for enzyme activity described observed kinetic data based on enzyme saturation, similar to a Michaelis–Menten relationship. Pseudofirst-order rate constants for the aqueous phase and soil–slurry phase experiments were fit to this function. The maximum rate of reaction (kmax) for TNT transformation was 0.50 and 0.04?h?1 for aqueous phase and soil–slurry phase experiments, respectively, while respective half-saturation constants (Ksat*) were comparable in value at 0.63 and 0.28?U/μmol–NO2, respectively. A Hanes–Woolf plot of reaction velocity versus TNT concentration with and without soil suggests an uncompetitive inhibition mechanism may be affecting overall nitrate reductase efficacy. Temperature effects for both aqueous phase and soil–slurry phase microcosms followed the Arrhenius relationship with estimated activation energies of 54.7 and 26.1?kJ/mol, respectively.     

氧化物杂质对铁氧化物还原动力学的影响

系统分析了氧化物杂质对铁氧化物还原动力学的影响及其机理。发现：各种杂 特点与杂质浓度、存在开矿和／或加入方法,样品的初始化学组成和物理性状,还原剂的种类、反应温度、还原分数等多种因素有关。     

碳、硅铁及碳化硅对白钨矿还原动力学的影响

在实验室用15 kW碳管炉进行碳粉、硅铁粉(75％Si)、碳化硅粉对白钨矿粉(67．25％WO3)还原动力学影响的研究。结果表明,碳还原白钨矿的反应级数为二级,反应表观活化能为234．6 kJ／mol;碳在较低温度下,反应性能差,当温度达到1 400℃时,反应剧烈;随温度升高,硅铁的还原性能比较平稳,反应产生SiO₂,使渣量增加;碳化硅高温反应性能好(≥1 400℃);碳和硅铁适合较低合金化率(3％W),碳化硅适宜用于较高的合金化率(≥5％W)。     

低温下氢气还原氧化铁的动力学研究

 用热重分析法研究了低温下不同粒度氧化铁的氢还原动力学,得出在同一温度下,铁矿粉粒度从107.5 μm降到2.0 μm后,由于粉体的表面积大幅度增加,提高了粉气接触面积,从而使得化学反应的速度提高了8倍左右,还原反应的表观活化能从78.3 kJ/mol降低到36.9 kJ/mol;当反应速度相同时, 粒度6.5 μm的粉体的反应温度比107.5 μm的降低了80 ℃左右。同时,通过理论推导和实验结果表明,当反应扩散层厚度相同时,铁矿粉粒度越小,反应扩散层厚度越薄,其还原率越高。     

Reduction Behaviour of Olivine Iron Ore Pellets in the Experimental Blast Furnace

An experimental study was conducted to determine the reduction behaviour of olivine iron ore pellets and associated reduction mechanisms in the experimental blast furnace (EBF) located at Luleå. Two sets of EBF samples, namely slowly annealed excavated samples and rapidly quenched probe samples of olivine bearing iron ore pellets were examined in detail. Pellet samples were analysed using SEM, XRD and SIROQUANT analysis to quantitatively determine iron ore phase transformations during descent in the EBF. In the tested EBF campaign, up to 75% of reduction occurred at less than 1100°C, i.e. before the pellet reached the cohesive zone while rest of 25% reduction was completed when pellets reached a temperature of 1300°C and hence within the cohesive zone. The reduction degree of pellets was found to have a linear correlation with distance from the stock line of the EBF. This study showed that the presence of olivine did not have a significant effect on reduction degree for temperatures less than 1100°C in the upper zone of the EBF. However, olivine increased the reduction rate in the final stage of reduction for temperatures in excess of 1100°C in the cohesive zone, which was attributed to the formation of an increased amount of molten FeO containing slag within the pellet. This study is expected to make important contributions towards further improvements in the pellet design as well as the optimization of blast furnace operation and efficiency.