Reduction Of HNO3 Research Articles

Thermo kinetic studies on chemical denitration of simulated waste solution

Denitration of high level liquid waste (HLLW) generated during the aqueous reprocessing of spent fuels of nuclear reactors facilitates reduction of waste volume by evaporation and minimize corrosion in waste storage tanks. Chemical denitration of HLLW using formaldehyde results in the production of only gaseous products NOx, CO2, and H2O. The reaction between HNO3 and formaldehyde is exothermic and can lead to uncontrolled process conditions due to the induction period before starting the reaction and pressure development owing to the release of all gaseous products simultaneously. Reduction of HNO3 in the concentration range 1–4 M as well as a simulated waste solution by formaldehyde was investigated using an adiabatic calorimeter to measure the increase in pressure and temperature during the reaction, which are responsible for the unsafe process. Thermo-kinetic parameters such as the enthalpy change and activation energy were derived for the denitration reaction.

Electrochemical mechanism of uranium mononitride dissolution in aqueous solutions of nitric acid

Abstract It was shown that the dissolution of UN with metallic conduction follows an electrochemical mechanism when it proceeds in contact with an electrically conductive medium (HNO3 solution). According to this mechanism, the oxidation of UN (at the anode) passes an electron into the UN matrix, which is a conductor, and can then reduce nitric acid in a parallel reaction a short distance away at another exposed surface of the UN (at the cathode). As a result, the reduction of HNO3 affords NO and NO2, while oxidation of uranium mononitride affords NH3, N2, and N2O. The occurrence of these two separate processes accounts for the composition and yields of the products formed from UN and HNO3 as well as for the nitrogen isotope distribution between them when UN and HNO3 were labeled with 14N or 15N. A mathematical equation describing the dependence of N2 and N2O yields on HNO3 concentration was derived. It was also shown that the calculated value of standard electromotive force of the galvanic pair formed on the UN surface during its dissolution in HNO3 is high enough to initiate and support the electrochemical mechanism of its dissolution in nitric acid.

Impact of environmental variables on the reduction of nitric acid by proxies for volatile organic compounds emitted by motor vehicles

Recent work has identified nitric acid (HNO3) as a potential precursor of nitrous acid (HONO), which is an important source of oxidants that regulate ozone and particulate pollution. Recent work in our laboratory has indicated that the reduction of HNO3 to HONO can occur homogeneously in the presence of surrogates for volatile organic compounds (VOCs) emitted by motor vehicles. This study focuses on the impact of environmental variables on the rate of formation of HONO in this process. The observed base case (25.0 °C and ∼20.0% relative humidity (RH)) HONO formation rate was 0.54 ± 0.09 ppb h−1, values comparable to enhancements observed in HONO during morning rush hour in Houston, TX. The rate was enhanced at lower temperatures of ∼20.0 °C, but the rate remained statistically similar (1σ) for experiments conducted at temperatures of 25 °C, 30 °C, and 35 °C. The assumption that multiple reactive components of the VOC mixture react with HNO3 is supported by this observation, and the relative importance of each reactive species in the reaction may vary with temperature. The enhanced rate at lower temperatures could make the proposed reaction mechanism more important at night. The formation rate of HONO does not change substantially when initial HNO3 concentration is varied between 400 and 4600 ppt, suggesting that the concentration of reactive VOCs was the limiting factor. The reduction of HNO3 to HONO appears not to occur heterogeneously on the aerosol surfaces tested. The presence of ∼120 ppb of ammonia has no observable impact on the reaction. However, it is likely that UV irradiation (λ = 350 nm) decreases the formation rate of HONO either by consuming the reactive VOCs involved or by directly interfering with the reaction. The “renoxification” of less reactive HNO3 to more reactive HONO has significant implications for daytime ozone and particulate pollution.

Release of Nitrogen Oxides into the Gas Phase Accompanying the Dissolution of Uranium Nitride in Nitric Acid

The release of nitrogen oxides (N2O, NO, NO2) accompanying the dissolution of UN in HNO3 with concentration 3–16 mol/liter and the ratio S:L = 1:7.5 and in 7.2 mol/liter HNO3 with S:L varying from 3 to 30 in atmospheric air or argon is studied. In the indicated HNO3 concentration range N2O is formed mainly as a result of the oxidation of UN and secondarily as a result of the reduction of HNO3. The presence or absence of oxygen in the atmosphere has no effect on the amount of N2O formed. The formation of NO2 is due to the reduction of HNO3. The presence of oxygen increases the amount of NO2 formed as a result of the oxidation of NO. The amount of nitrogen monoxide NO released into atmospheric air is higher than the amount released into an argon atmosphere. The amount of NO formed increases with increasing nitric acid concentration. The release of nitrogen oxides from UN as well as from nitric acid results in considerable dilution of nitride nitrogen, which casts doubt on the possibility of recycling 15N in the case where U15N is used as fuel.

The reduction of HNO3 by volatile organic compounds emitted by motor vehicles

Nitric acid (HNO3) was reduced in a flow tube by volatile organic carbon compounds (VOCs) generated from engine oil vapor. The primary reaction product was believed to be HONO. The reaction was not enhanced when Teflon® Raschig rings were added to the flow tube to increase surface area, thereby showing the reaction to be homogeneous under the conditions studied. The HONO formation observed ranged between 0.1 and 0.6 ppb h−1, with a mean of 0.3 ± 0.1 ppb h−1, for typical HNO3 concentrations of 4–5 ppb and estimated concentrations of the reactive components in the engine oil vapor between 200 and 300 ppt. The observations in this study compare well to a recently published field study conducted in Houston that observed average formation rates of 0.6 ± 0.3 ppb h−1. Water vapor was found to decrease the HONO formation rate by ∼0.1 ppb h−1 for every 1% increase in the water mixing ratio.

Nitric Acid Reduction on 316L Stainless Steel under Conditions Representative of Reprocessing

Steels comprise the largest class of metal-based materials encountered on nuclear sites. An understanding of how process steels interact with HNO3 in spent fuel treatment plant environments is required to enable informed decisions to be made about the design and effective application of different steel types within nuclear environments. Stainless steels readily passivate in nitric acid. However, increasing the oxidising power of the media can lead to passive film dissolution, resulting in rapid transpassive corrosion. The corrosion of steels in nitric acid is further complicated by the autocatalytic reduction of HNO3 to aqueous HNO2 which attacks the steel surface. This paper describes the effect of this behaviour on process steels in stagnant and/or flowing conditions using electrochemical and microgravimetric based methods. We describe linear sweep voltammetry studies performed on a 316L stainless steel rotating disk electrodes in varying concentrations of nitric acid and rotation speeds and provide a qualitative interpretation of the results and what these imply about the mechanism of HNO3 reduction. These findings will be used in follow on studies to determine the kinetic parameters of the nitric acid reduction reaction at the surface of 316L stainless steel.

Mass and Electron Balance for the Oxidation of Silicon during the Wet Chemical Etching in HF/HNO3 Mixtures

The stoichiometry of the wet-chemical etching of silicon in concentrated HF/HNO3 mixtures has been studied. By quantifying the major reaction products in solution, the established model that 3 mol of Si are oxidized by 4 mol of HNO3 to yield 4 mol of NO could not be confirmed. In HNO3-rich HF/HNO3 mixtures, approximately 1.1 mol of HNO3 are required to oxidize 1 mol of Si. Excess HNO3 leads to massive accumulation of N(III) species in the etchants and massive formation of nitrous oxides due to incomplete reduction of the HNO3. An excess of HNO3 leads to higher consumption and poorer utilization indicated by the massive accumulation of N(III) species in the etchants. In HF-rich mixtures, only 0.9 mol of HNO3 are needed to oxidize 1 mol of Si yielding a lower accumulation of N(III) species and a higher utilization of the HNO3. Two parallel pathways contribute to the oxidation of silicon in such solutions: (i) via the oxidation by HNO3 and reactive intermediates generated by the reduction of HNO3 and (ii) vi...

Analysis of gaseous reaction products of wet chemical silicon etching by conventional direct current glow discharge optical emission spectrometry (DC-GD-OES)

Major parts of the mechanism of the wet chemical etching of Si using mixtures of nitric and hydrofluoric acid are still in question. One of them is the issue about the formation of H and its relevance in the oxidation of Si. In the present work H evolution during acid etching of Si was estimated as a function of the HF/HNO3 mixing ratio and the temperature. A commercial GD-OES instrument with an unchanged Grimm type glow discharge chamber was used to determine the amount of formed H. A small fraction of the reaction gases, originated by etching of Si in a PTFE apparatus under Ar atmosphere, was fed by a constant Ar flow via a drying column into the discharge chamber. The gases were excited by the plasma of a continuously DC sputtered Fe sample. Numerous elements like Si, N, O, H, and even B from the B-doped Si were detected. However, only the H emissions at 121.5 nm and 656.3 nm were used for the estimation of the released H. The obtained results show that H is preferentially generated in HNO3-poor etch solutions (HNO3 < 40% (v/v)). In the range from −10 to 35 °C the temperature of the etch solution shows a negligible influence on the H generation, but affects the amount of nitrous gases. This indicates that particularly in HNO3-poor etch solutions the oxidation of Si proceeds to a considerable extent via the formation of H parallel to the already known pathway via the reduction of HNO3.

Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Nitroxyl (HNO), the one-electron reduced and protonated congener of nitric oxide (NO), is a chemically unique species with potentially important biological activity. Although HNO-based pharmaceuticals are currently being considered for the treatment of chronic heart failure or stroke/transplant-derived ischemia, the chemical events leading to therapeutic responses are not established. The interaction of HNO with oxidants results in the well-documented conversion to NO, but HNO is expected to be readily reduced as well. Recent thermodynamic calculations predict that reduction of HNO is biologically accessible. Herein, kinetic analysis suggests that the reactions of HNO with several mechanistically distinct reductants are also biologically feasible. Product analysis verified that the reductants had in fact been oxidized and that in several instances HNO had been converted to hydroxylamine. Moreover, a theoretical analysis suggests that in the reaction of HNO with thiol reductants, the pathway producing sulfinamide is significantly more favorable than that leading to disulfide. Additionally, simultaneous production of HNO and NO yielded a biphasic oxidative capacity.

The effect of active carbon on the reduction of concentrated nitric acid by HCOOH

In nuclear industry, removal of nitric acid from solutions is required in the course of chemical separation and waste treatment procedure as well as in chemical conversion steps. The reduction of HNO 3 by HCOOH to gaseous products such as nitrogen, nitrogen oxides, and carbon dioxide is an attractive way to accomplish the denitration. A typical problem for the denitration is the existence of the induction period. The induction period has been explained as the time necessary to increase the concentration of HNO 2, which is an important reaction intermediate, above a threshold value. In this study, adsorption sites on the surface of active carbon were found to promote HNO 2 formation and efficiently suppress the induction period. Induction time was shortened by increasing the amount of active carbon in the solution. When the solution contains 3 M HNO 3 and 1 M HCOOH, 10 g/L of active carbon was enough to eliminate the induction period at 50 °C. The catalytic effect exhibited by active carbon was similar to that reported for Pt/SiO 2. Therefore, on the surface of active carbon, there is a redox cycle, where HNO 3 is reduced to HNO 2 and then the oxidized surface site will be reduced by HCOOH.

Electrochemical Behavior of Nickel in HNO&lt;SUB&gt;3&lt;/SUB&gt; and the Effect of Chloride Ions

The electrochemical behavior of nickel in HNO3 solutions of varying concentrations was examined using the cyclic voltammetry and potentiodynamic anodic polarization techniques. The anodic branch of the cyclic voltammogram is characterized by one anodic dissolution peak and a passivation region before oxygen evolution. The cathodic branch shows only one cathodic reduction peak corresponding to the reduction of HNO3. Analysis of the anodic polarization data shows features of both reversible and irreversible reactions pointing to the complexity of the system. CT− ions enhance the active dissolution of nickel in HNO3 due to the adsorption on the bare metal surface and cause destruction of the passive film and initiation of pitting corrosion.

Wells–Dawson-derived tetrameric complexes {K 28H 8[P 2W 15Ti 3O 60.5] 4} electrochemical behaviour and electrocatalytic reduction of nitrite and of nitric oxide

At low pH, trivacant polyoxometalate α-[P 2W 15O 56] 12- reacts with titanyl and forms tetrameric {K 28H 8[P 2W 15Ti 3O 60.5] 4} ( 1). The purity of [P 2W 15Ti 3O 60.5] 4 36− has been confirmed by IR spectroscopy, elemental analysis, and 31P NMR spectrometry. The electrochemical behaviour of the tetrameric complexes in aqueous solution has been investigated at various pHs. [P 2W 15Ti 3O 60.5] 4 36− exhibits successive reduction processes of the W atoms in a negative potential range. All waves are pH dependent at pH < 4. The anion showed an electrocatalytic effect on the reduction of HNO 2, but this effect is greatly influenced by pH. At low pH (pH < 3.5) the cathodic peak current of the first W VI/V couple is enhanced while its anodic couterpart disappears. When pH > 3.5 no catalytic effect is observed. On the contrary, upon saturation of the POM solution with NO, even at pH > 3.5, electrocatalytic reduction of NO was observed. The tetrameric tri-Ti IV-substituted Wells–Dawson polyoxometalate [P 2W 15Ti 3O 60.5] 4 36− was not degraded by repeated cycling between its oxidized and reduced states.

Differences between Weekday and Weekend Air Pollutant Levels in Southern California

Ambient air quality data were analyzed to empirically evaluate the effects of reductions of volatile organic compounds (VOCs) and oxides of nitrogen (NOx) emissions on weekday and weekend levels of ozone (O3; 1991–1998) and particulate NO3 - (1980–1999) in southern California. Despite significantly lower O3 precursor levels on weekends, 20 of 28 South Coast Air Basin (SoCAB) sites (28 of all 78 southern California sites) showed statistically significant higher mean O3 levels on Sundays than on weekdays (p < 0.01); 49 of the remaining 50 sites showed no significant differences between mean weekday and Sunday peak O3 levels. We also observed no statistically significant differences between mean weekday and weekend concentrations of particulate NO3 - or nitric acid (HNO3, the precursor of particulate NO3 -). Averaged over sites, the mean Sunday NOx and nonmethane hydrocarbon concentrations were 25–41% and 16–30% lower, respectively, than on weekdays. Site-to-site differences between weekend and weekday mean peak hourly O3 levels were related to whether O3 formation was limited by the availability of NOx. A thermodynamic equilibrium model predicts that particulate NO3 - levels would decrease in response to a reduction of HNO3, and that particulate ammonium NO3 - formation was not limited by the availability of ammonia. The similarity of mean weekday and weekend levels of NO3 - therefore did not result from limitations on the formation of particulate NO3 - from its precursor, HNO3.

Satellite observation of dehydration in the Arctic Polar stratosphere

We report the first space‐borne observation of dehydration in the Arctic polar stratosphere. In January 1997, the Improved Limb Atmospheric Spectrometer (ILAS) observed up to ∼3 ppmv water vapor reduction during ice cloud formation and ∼2 ppmv permanent removal of water vapor, mostly at altitudes between 23 and 26 km. In some cases, the dehydrated air was downwind from mountain wave induced Polar Stratospheric Cloud (PSC) events. Furthermore, simultaneous observations of HNO3 and H2O show that the gas phase reduction of HNO3 in the Arctic (January 1997) was much smaller than that observed in the Antarctic (June 1997) when a similar level of water vapor reduction occurred.

HNO 3 analyzer by scrubber difference and the NO–ozone chemiluminescence method

A fast response analyzer for HNO 3 in highly polluted air is described. The time resolution attainable was 12 s. The method is based on the difference in a technique for HNO 3-scrubbed and non-scrubbed air and the reduction of HNO 3 to NO with the use of a line of catalytic converters and a method for the subsequent NO-ozone chemiluminescence. A sample air stream, in which particulates are removed with a Teflon filter, is divided into two channels. CH-1 is directly connected to the converter line, and CH-2 contains a HNO 3 scrubber packed with a nylon fiber that goes to another converter line. Each converter line is composed of a hot quartz-bead converter (QBC) and a molybdenum converter (MC) in a series. A QBC reduces HNO 3 to (NO+NO 2), which is called NO x . The MC reduces the NO x to NO. For CH-1, the analyzer detects most compounds that typically comprise NO y (J. Geophys. Res. 91 (1986) 9781). These CH-1 compounds are called NO y ′ hereafter (NO y -particulate nitrate) because the particulates are removed by the filter. A difference in the detector signal for the two channels indicates HNO 3. For a blank test, atmospheric air in which HNO 3 was pre-scrubbed by an extra nylon fiber was introduced to the analyzer. Variations in the blank value were 0.38±0.42 and 0.34±0.55 ppb during the high readings (NO y ′-HNO 3 ) (called NO y * hereafter) (111±12 ppb, N=180), and low NO y * readings (62±8 ppb, N=180), respectively, indicating that the lowest detection limit of the analyzer is 1.1 ppb (2 σ). When the data obtained with the analyzer is compared to the data using the denuder method, a linear correlation with the regression of Y=0.973 X+0.077 ( r 2=0.916 ( N=20)) in the range of 0–6.5 ppb HNO 3 is obtained, which is an excellent agreement. Atmospheric monitoring was carried out at Kobe. Although the average concentration of HNO 3 was 2.6±1.3 ppb, ca.10 ppb for a HNO 3 concentration was occasionally observed when the NO y * concentration was high, i.e., more than 100 ppb.

Partitioning of NOy species in the summer Arctic stratosphere

Volume mixing ratio profiles of the quantitatively significant NOy species NO, NO2, HNO3, HNO4, ClNO3 and N2O5 were measured remotely from 8 to 38 km by the JPL MkIV FTIR solar absorption spectrometer during balloon flights from Fairbanks, Alaska (64.8°N, 147.6°W) on May 8 and July 8, 1997. The observed ratio of NOx (NO + NO2) to NOy (total reactive nitrogen) is 10 to 30% greater than calculated by a steady state model using standard photochemistry constrained by MkIV measurements of long lived precursors (e.g., H2O, CH4, CO and N2O) and SAGE II aerosol surface area. The persistence of this discrepancy to 38 km altitude suggests that processes involving aerosols, such as the reduction of HNO3 on the surface of soot particles, cannot be the sole explanation. The most likely resolution to this discrepancy is that the rate of NO2 + OH + M → HNO3 + M (the dominant sink of NOx in the Arctic stratosphere during times of near continuous solar illumination) is significantly slower than the currently recommended rate.

Carbon aerosols and atmospheric photochemistry

Carbon aerosols are produced by all combustion processes. This paper investigates some possible effects of heterogeneous reduction of atmospheric constituents on carbon aerosols. Reduction of HNO3, NO2, and O3 on carbon aerosols may be an important effect of increased air traffic that has not been considered to date. It is shown that if HNO3, NO2 and O3 are heterogeneously reduced on atmospheric amorphous carbon aerosols, then a significant, lower stratospheric ozone loss mechanism could exist. This ozone loss mechanism is almost independent of temperature and does not require the presence of sunlight. The mechanism can operate at all latitudes where amorphous carbon aerosols are present. The relative importance of the mechanism increases with nightlength. The reduction of HNO3 on carbon aerosols could also be a significant renoxification process wherever carbon aerosols are present. Owing to the very different soot levels in the two hemispheres, this implies that there should be a hemispheric assymetry in the role of these mechanisms. The renoxification leads to simulated tropospheric HNO3/NOx ratios that are close to those observed. In contrast to the stratospheric response, the tropospheric production of NOx due to the reduction of HNO3 would lead to tropospheric ozone production.

Equilibrium of particle nitrite with gas phase HONO: Tropospheric measurements in the high Arctic during polar sunrise

Gas phase HONO(g) and nitrite in particles of &lt;5‐μm size were measured in the troposphere during the Polar Sunrise Experiment at Alert, Northwest Territories, Canada, during January 19 to April 20, 1992, using denuder‐filter pack sampling and IC‐UV detection. The measurements indicated that HONO(g) existed at concentrations of up to 70 ppt before polar sunrise but gradually decreased to 5–10 ppt after sunrise. The calculated OH formation rate from HONO(g) photolysis was greater than from the photolysis of both O3 and CH2O by more than one order of magnitude during the sunlit period and led to moderately high levels of OH, e.g., 3×105 molecules cm−3 OH at noontime on April 5. Particle nitrite measurements showed a gradual increase in concentrations with increasing solar insolation, but the concentrations were generally less than 10 ppt. The pH and the sulfate molar concentrations of the particles and the water vapor mixing ratio indicate that the particles were highly acidic being approximately 70% (W/W) H2SO4 solution. In such highly concentrated H2SO4 solution, most particle nitrite should exist as hydrated nitrosonium ion H2ONO+. Taking this into consideration, the particle nitrite was in an approximate equilibrium with the measured HONO(g). This equilibrium, with HONO(g) rapidly photolyzed, was a good indication that the particles were effective sources of HONO(g) and implied rapid production of particle N(+III) during this period. Two possible pathways leading to the formation of particle N(+III) species are suggested, i.e., reduction of HNO3(aq) by SO2(g) and reduction of NO3−; (aq) by Br− (aq). However, N2O5 reaction with NaBr cannot be ruled out as the alternative HONO(g) formation mechanism which bypasses the equilibrium.

Photoreduction of nitric acid by electronically excited diplatinum(II,II) pyrophosphite, [Pt2(P2O5H2)4]4?

The reduction of HNO3(0.1M) to NO2– by an electronically excited platinum complex is described; the intermediate products, [Pt2II,III(P2O5H2)4]3– and HNO3–, have been identified by transient absorption spectroscopy and quenching studies.

ChemInform Abstract: Catalytic Activity of a Tungsten Carbide Electrocatalyst in the Reduction of HNO3, HNO, and NH2OH by Molecular Hydrogen.

AbstractFür die durch WC katalysierten Reduktionsprozesse A‐C werden die Rollen verschiedener, die Gesamtreaktionsgeschwindigkeiten beeinflussender Faktoren wie Temp. und Konzentration untersucht.