Simulation of radiation-chemical transformations in aqueous nitric oxide and nitrite solutions

A kinetic model of radiation-chemical transformations of nitrogen oxide and nitrites in aqueous solutions is proposed. It includes the previously developed reaction scheme for water and H₂, H₂O₂, and O₂ solutions complemented by the reactions of water radiolysis products with NO and NO₂⁻. It has been shown that the model describes well experimental data on the decomposition of the compounds and the buildup of products depending on the absorbed dose in aqueous solutions at different pH values.     

Deuterium exchange in the systems of H2O+/H2O and H3O+/H2O

The reactions of H₂O⁺, H₃O⁺, D₂O⁺, and D₃O⁺ with neutral H₂O and D₂O were studied by tandem mass spectrometry. The H₂O⁺ and D₂O⁺ ion reactions exhibited multiple channels, including charge transfer, proton transfer (or hydrogen atom abstraction), and isotopic exchange. The H₃O⁺ and D₃O⁺ ion reactions exhibited only isotope exchange. The variation in the abundances of all ions involved in the reactions was measured over a neutral pressure range from 0 to 2 × 10⁻⁵ Torr. A reaction scheme was chosen, which consisted of a sequence of charge transfer, proton transfer, and isotopic exchange reactions. Exact solutions to two groups of simultaneous differential equations were determined; one group started with the reaction of ionized water, and the other group started with the reactions of protonated water. A nonlinear least-squares regression technique was used to determine the rate coefficients of the individual reactions in the schemes from the ion abundance data. Branching ratios and relative rate coefficients were also determined in this manner.A delta chi-squared analysis of the results of the model fitted to the experimental data indicated that the kinetic information about the primary isotopic exchange processes is statistically the most significant. The errors in the derived values of the kinetic information of subsequent channels increased rapidly. Data from previously published selected ion flow tube (SIFT) study were analyzed in the same manner. Rigorous statistical analysis showed that the statistical isotope scrambling model was unable to explain either the SIFT or the tandem mass spectrometry data. This study shows that statistical analysis can be utilized to assess the validity of possible models in explaining experimentally observed kinetic behaviors.     

On the Mechanism of Formation of Intratrack Yields of Water Radiolysis Products upon Irradiation with Fast Electrons and Positrons: 2. Concentration and Time Dependences of Yields

A mathematical model for the formation of main transient and final radiolysis products generated in tracks of fast electrons and positrons in water and aqueous solutions was constructed and described in terms of equations of inhomogeneous chemical kinetics in part 1 of this study. The model takes into account the reactions of a solute with epithermal electrons, thermal, and hydrated electrons; the ambipolar character of diffusion of charged intratrack particles; and new pathways of the formation of hydrogen and positronium due to the appearance of weakly bound states of electrons. In the present paper, the model was quantitatively fitted to experimental data on both time variation in the yields of radiolytic products (H₃O⁺, e _aq ⁻ , H, OH, OH⁻, H₂O₂) in pure water and the yields of hydrogen (H₂, H), hydrated electron (e _aq ⁻ ) and positronium (Ps) in various dilute and concentrated aqueous solutions.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 5, 2005, pp. 330–338.Original Russian Text Copyright © 2005 by Stepanov, Byakov.     

Non-catalytic remediation of aqueous solutions by microwave-assisted photolysis in the presence of H2O2

Advanced oxidation processes have emerged as potentially powerful methods to transform organic pollutants in aqueous solutions into non-toxic substances. In this work, a comparison of degradation dynamics of five aromatic compounds (phenol, chlorobenzene, nitrobenzene, 4-chlorophenol, and pentachlorophenol) in aqueous solutions by non-catalytic UV, MW, and combined MW/UV remediation techniques in the presence of H₂O₂ is presented. Relative degradation rate constants have been monitored and the major products were identified. The combined degradation effect of UV and MW radiation was found larger than the sum of isolated effects in all cases studied. It is concluded that such an overall efficiency increase is essentially based on a thermal enhancement of subsequent oxidation reactions of the primary photoreaction intermediates. Optimizations revealed that this effect was particularly significant in samples with a low concentration of H₂O₂, however, a larger excess of H₂O₂ was essential to complete the destruction in most experiments. The absence of heterogeneous catalysts was in no doubt an additional advantage of the technique applied.     

On-Demand Synthesis of H2O2 by Water Oxidation for Sustainable Resource Production and Organic Pollutant Degradation

H₂O₂ is a versatile and environmentally friendly chemical involved in water treatment, such as advanced oxidation processes. Anthraquinone oxidation is widely used for large-scale production of H₂O₂, which requires significant energy input and periodic replacement of the carrier molecule. H₂O₂ production should be customized considering the specific usage scenario. Electrochemical synthesis of H₂O₂ can be adopted as alternatives to traditional method, which avoids concentration, transportation, and storage processes. Herein, we identified Bi₂WO₆:Mo as a low-cost and high-selectivity choice from a series of Bi-based oxides for H₂O₂ generation via two-electron water oxidation reaction. It can continuously provide H₂O₂ for in situ degradation of persistent pollutants in aqueous solution. Clean energy from H₂ can also be produced at the cathode. This kind of water splitting producing sustainable resources of H₂O₂ and H₂ is an advance in environmental treatment and energy science.     

Specific features of the radiolysis of water and aqueous solutions of H2 and O2 by mixed n,γ-radiation with a high portion of the neutron component

A previously unknown feature of the kinetics of the radiolysis of water and hydrogen, oxygen, and hydrogen peroxide solutions has been discussed. By calculation, it has been revealed that concentration oscillations of the radiolysis products can appear during irradiation of the solution with fast neutrons or mixed n,γ-radiation with a high portion of the neutron component. The period and amplitude of the oscillations depend on the temperature, the dose rate, and the ratio of n/γ radiation components. It has been shown that oscillations cannot be excited during γ-radiolysis under any conditions. It is suggested that the mechanism of the oscillations is similar to the Belousov-Zhabotinsky reaction mechanism. A chain reaction proceeds in the irradiated system, in which the reactants H₂O₂ (“reducing agent”), “oxidizing agent” OH radicals initiating the chain, and the “catalyst” are introduced from the outside. Hydrogen molecules produced by the action of radiation play the role of the “catalyst”, and H₂O molecules formed in the secondary reactions are the “deactivated form of the catalyst”. Hydrogen atoms and hydrated electrons propagate the chain. Oxygen formed in both spurs and the secondary reactions is the “inhibitor” terminating the chain reaction.     

Radiolysis of iodotyrosines in aqueous-ethanol solutions

⁶⁰Co-gamma radiolysis of 3-iodotyrosine and 3,5-diiodotyrosine in aqueous-ethanol solutions has shown that the chemical effects are mainly determined by the interaction of radicals from the radiolysis of solvent and controlled by the composition of the solution. The influence of varying solvent composition and radiation dose on the amount of iodoamino acid converted and on the yields of the radiolysis products (I₂, I^–, IO ₃ ^– and H₂O₂) formed in aerated solutions at room termperature were investigated. The formation of I₂ is dependent upon the acidity of the solution and is mainly produced as an after-effect due to the interaction of H₂O₂ with I^– ions, both being radiolysis products. The variation of radiation-chemical yields with the solute and solvent composition, and the probable mechanisms for formation of the radiolysis products are discussed.     

About the OH yield in the radiolysis of an aqueous/H2O2 system. Its optimisation for water treatment

Unless the radiolytic reducing species are neutralised or converted into oxidising species, an EB remediation system cannot be considered a true Advanced Oxidation Processes (AOP). A water/H₂O₂ system irradiated by UVC mercury lamps constitutes a widely used OH production method. Employing H₂O₂ in radiolysis as well, an enhancement of the oxidative efficiency of an EB treatment can be obtained. Pulse radiolysis measurements of an aerated aqueous/H₂O₂/KSCN system have been systematically undertaken to assess the optimal H₂O₂ concentration. By linearly fitting a competition kinetics relationship, it is found that the scavengeable extra-yield of OH is ΔG(OH)=0.24 μmol J^?1 (R=0,9958), while the maximum experimental yield is measured G(OH)_max=(0.52±0.02) μmol J^?1 when [H₂O₂]=5–10 mM. Exceeding these concentrations the OH yield drops off.     

Direct Synthesis of H2O2 by a H2/O2 Fuel Cell

Direct synthesis of H₂O₂ solutions by a fuel cell method was reviewed. The fuel cell reactor of [O₂, gas-diffusion cathode electrolyte solutions Nafion membrane electrolyte solutions gas-diffusion anode, H₂] is very effective for formation of H₂O₂. The three-phase boundary (O₂(g)–electrode(s)–electrolyte(l)) in the gas-diffusion cathode is essential for efficient formation of H₂O₂. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk electrolyte solutions are essential for H₂O₂ accumulation. The maxima H₂O₂ concentrations of 1.2 M (3.5 wt%) and 2.4 M (7.0 wt%) were accomplished by the heat-treated Mn-OEP/AC electrocatalyst with H₂SO₄ electrolyte and by the VGCF electrocatalyst with NaOH electrolyte, respectively, under short circuit conditions.     

Anthraquinone Redox Relay for Dye‐Sensitized Photo‐electrochemical H2O2 Production

Anthraquinone (AQ) redox mediators are introduced to metal‐free organic dye sensitized photo‐electrochemical cells (DSPECs) for the generation of H₂O₂. Instead of directly reducing O₂ to produce H₂O₂, visible‐light‐driven AQ reduction occurs in the DSPEC and the following autooxidation with O₂ allows H₂O₂ accumulation and AQ regeneration. In an aqueous electrolyte, under 1 sun conditions, a water‐soluble AQ salt is employed with the highest photocurrent of up to 0.4 mA cm^?2 and near‐quantitative faradaic efficiency for producing H₂O₂. In a non‐aqueous electrolyte, under 1 sun illumination, an organic‐soluble AQ is applied and the photocurrent reaches 1.8 mA cm^?2 with faradaic efficiency up to 95 % for H₂O₂ production. This AQ‐relay DSPEC exhibits the highest photocurrent so far in non‐aqueous electrolytes for H₂O₂ production and excellent acid stability in aqueous electrolytes, thus providing a practical and efficient strategy for visible‐light‐driven H₂O₂ production.     

Anthraquinone Redox Relay for Dye-Sensitized Photo-electrochemical H2O2 Production

Anthraquinone (AQ) redox mediators are introduced to metal-free organic dye sensitized photo-electrochemical cells (DSPECs) for the generation of H₂O₂. Instead of directly reducing O₂ to produce H₂O₂, visible-light-driven AQ reduction occurs in the DSPEC and the following autooxidation with O₂ allows H₂O₂ accumulation and AQ regeneration. In an aqueous electrolyte, under 1 sun conditions, a water-soluble AQ salt is employed with the highest photocurrent of up to 0.4 mA cm⁻² and near-quantitative faradaic efficiency for producing H₂O₂. In a non-aqueous electrolyte, under 1 sun illumination, an organic-soluble AQ is applied and the photocurrent reaches 1.8 mA cm⁻² with faradaic efficiency up to 95 % for H₂O₂ production. This AQ-relay DSPEC exhibits the highest photocurrent so far in non-aqueous electrolytes for H₂O₂ production and excellent acid stability in aqueous electrolytes, thus providing a practical and efficient strategy for visible-light-driven H₂O₂ production.     

The effect of interfacial mass transfer on steady-state water radiolysis

The effect of aqueous–gas interfacial transfer of volatile species on the γ-radiolysis of water was studied as a function of gas-to-liquid volume ratio at various solution pHs and cover gas compositions. Water samples with cover-gas headspace were irradiated at an absorbed dose rate of 2.5 Gy s^?1 and the radiolytic productions of H₂ in the cover gas and H₂O₂ in the water phase were monitored as a function of irradiation time. The experimental results were compared with computer simulations using a water radiolysis kinetics model that included primary radiolysis, subsequent reactions of the primary radiolysis products in the aqueous phase, and aqueous–gas interfacial transfer of the volatile species H₂ and O₂. This study shows that the impact of the interfacial mass transfer strongly depends on pH. At pH≤8 (lower than the pKa of ?H of 9.6) the effect of aqueous-to-gas phase transfer of the volatile species on the steady-state concentrations of the other radiolysis products is negligible. At higher pHs (≥8), radiolytic production of O₂ is slow but considerable, which results in significant increase in the steady-state concentrations of H₂ and H₂O₂ compared to those at lower pHs. Thus, in the presence of headspace, the interfacial transfer of both H₂ and O₂ becomes significant, and the aqueous concentrations of H₂ and O₂ are no longer independent of the interfacial surface area and water volume. Nevertheless, the accumulated mass of H₂(g) in the headspace is proportional to the aqueous concentration of H₂ at all pHs, and the gaseous concentration of H₂ in the headspace can be used to infer the aqueous concentration of H₂.     

Calculated and measured transient product yields in pulse radiolysis of aqueous solutions: Concentration dependence

Pulse radiolysis of aqueous solutions was modeled by using 54 equations for the reaction of water radiolysis intermediates with carefully selected rate coefficients. Yields of products formed in the hydrated electron+solute and hydroxyl radical+solute reactions were calculated and compared with the measured yields in wide concentration range. These reactions are in competition with the reactions of the water radiolysis intermediates with each other and with H₂O, H⁺ and H₂O₂. An empirical equation was developed for the calculation of scavenged product yields that can be used in cases when due to low rate coefficient, low solubility or very high absorbance, low solute concentrations are applied and a considerable fraction of the water radiolysis intermediates does not react with the solute.     

Simulation of the radiation-chemical transformations of acetic acid in aqueous solutions

A general model for the radiolysis of acetic acid and its aqueous solutions is proposed. The model adequately describes experimental data on the degradation of the acid and the formation of gases (H₂, CO₂, and CH₄) in aqueous solutions at various pH values.     

Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure

This paper presents results from lean CO/H₂/O₂/NO_x oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high‐pressure laminar flow reactor designed to conduct well‐defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H₂/O₂, CO/CO₂, and NO_x chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS‐QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO₂ ? H₂O + O₂, CO + OH ? [HOCO] ? CO₂ + H, HOCO + OH ? CO + H₂O₂, NO₂ + H₂ ? HNO₂ + H, NO₂ + HO₂ ? HONO/HNO₂ + O₂, and HNO₂(+M) ? HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H₂/O₂, H₂/O₂/NO₂, and CO/H₂O/O₂ at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H₂O₂ → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008     

Measurement and correlation of the solubility of MnSO4·H2O with MgSO4·7H2O in MeOH + H2O solution

Data on the solubility of manganese sulphate monohydrate in water, and in aqueous alcohols is essential for salting-out crystallization studies. The solubilities for the quaternary system MnSO₄·H₂O + MgSO₄·7H₂O + H₂O + MeOH solution were determined in the temperature ranges 293.2–308.2 K over the mole fraction methanol ranges of 0.00–0.16. The solubility data were used for modelling with the modified extended electrolyte non-random two-liquid (NRTL) equation. The present extension uses ion-specific parameters instead of the electrolyte-specific NRTL binary interaction parameters. This approach has feasibility for many electrolytes and mixed aqueous solution systems principally. The model was found to correlate the solubility data satisfactory.     

Enhanced adsorption of radioactive strontium ions from aqueous solution by H2O2-modified attapulgite

Effective adsorption of Sr(II) onto H₂O₂-modified attapulgite in aqueous solution was investigated about kinetics and isothermal equilibrium adsorption. The adsorption equilibrium process of Sr(II) on adsorbents reached about 8 h at 40 °C. The adsorption kinetics followed the pseudo-second order equation and the isothermal adsorption data were fit well with the Langmuir isotherm model. The enhanced adsorption mechanism of H₂O₂-modified attapulgite for Sr(II) in aqueous solution was expatiated in detail. The H₂O₂ treatment for attapulgite is effective and as-made adsorbents can be applied for removal of Sr(II) in radioactive waste water.

     

Contact-electro-catalysis for Direct Synthesis of H2O2 under Ambient Conditions

Hydrogen peroxide (H₂O₂) is an indispensable basic reagent in various industries, such as textile bleach, chemical synthesis, and environmental protection. However, it is challenging to prepare H₂O₂ in a green, safe, simple and efficient way under ambient conditions. Here, we found that H₂O₂ could be synthesized using a catalytic pathway only by contact charging a two-phase interface at room temperature and normal pressure. Particularly, under the action of mechanical force, electron transfer occurs during physical contact between polytetrafluoroethylene particles and deionized water/O₂ interfaces, inducing the generation of reactive free radicals (⋅OH and ⋅O₂⁻ ), and the free radicals could react to form H₂O₂, yielding as high as 313 μmol L⁻¹ h⁻¹. In addition, the new reaction device could show long-term stable H₂O₂ production. This work provides a novel method for the efficient preparation of H₂O₂, which may also stimulate further explorations on contact-electrification-induced chemistry process.