ISSN 00181439, High Energy Chemistry, 2014, Vol. 48, No. 4, pp. 230–238. © Pleiades Publishing, Ltd., 2014. Original Russian Text © A.V. Gordeev, B.G. Ershov, A.V. Safonov, 2014, published in Khimiya Vysokikh Energii, 2014, Vol. 48, No. 4, pp. 272–280.
RADIATION CHEMISTRY
Simulation of the RadiationChemical Yields of Hydrogen and Hydrogen Peroxide in Concentrated Solutions A. V. Gordeev, B. G. Ershov, and A. V. Safonov Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31 (bldg. 4), Moscow, 119071 Russia email:
[email protected] Received February 18, 2013; in final form, December 10, 2013
Abstract—A method based on the use of descriptive kinetics of elementary reactions has been proposed for calculating the yields of H2 and H2O2 in concentrated solutions. The influence of scavengers on the primary yields was taken into account by introducing into the scheme of radiationchemical transformations addi tional reactions with the radical ion products of water radiolysis that compete with the H2 and H2O2 forma tion reactions in “spurs.” The proposed calculation scheme allows the change in the initial yields in the pres ence of scavengers to be directly included in the general scheme of radiationchemical transformations. The scheme satisfactorily describes the observed yields in concentrated nitrate and nitrite solutions, which form the base medium of liquid radioactive waste to be disposed in geological repositories. DOI: 10.1134/S0018143914040055
In model calculations of radiationchemical trans formations, the radiationchemical yield (G, mole cule/100 eV) is a quantity that is as important as reac tion rate constants. Together with the dose rate (P, Gy/s), yields determine the intensity of generation of species in a solution under irradiation. In radiation chemistry, several types of yield (initial, primary, and observed) are distinguished [1]. For engineering cal culations of radiation impact on a system, it is more convenient to use observed yields; their values depend on the composition of the system and the buildup rate of substance X is defined as G(X)P. In the absence of measured values of observed yields, they can be calcu lated using a model of radiationchemical transforma tions. In this case, primary yields are used, which refer to the time of escape of the products from “spurs” (“blobs”, “tracks”) into the homogeneous medium, i.e., about ~10–7 s after their generation. However, the values of the primary yields can be conventionally assumed constant in dilute solutions only. In concen trated solutions, their inspur formation is affected by the solute. Like observed yields, primary yields can be calculated with the use of values for the initial yield. But the model in this case should include diffusion processes in addition to homogeneous kinetics reac tions. For modeling the radiolysis of systems in which component concentrations are subjected to significant changes, these additional processes make it necessary to include an additional independent module in the calculation procedure, thereby strongly slowing down the computation. This delay is particularly noticeable when simulation of radiolytic transformations is an element in a more general model, for example, the fil
tration model of a liquid radioactive waste (LRW) dis posal site. In this paper, we propose a procedure for approxi mate description of the intraspur processes, leading to the formation of the molecular yields of H2 and H2O2 through the chemical reactions used for the sim ulation of radiationchemical processes in a homoge neous medium. This approach makes it easy to inte grate the calculation of primary radiationchemical yields into the general scheme of radiationchemical transformations of solutions and, thus, simplifies the adjustment of the yields to a significant change in the composition of the medium. CALCULATION PROCEDURE The effect of a solute (X) on the formation of the yields can be formally described by two types of rela tionships that give straight lines in the G(X)–[X]1/3 coordinates at concentrations below ~0.1 mol/L or the 1/G(X)–[X] coordinates at higher concentrations [2]. The diffusionkinetics concepts of the yield for mation mechanism and the distribution of reactive species in the spurs permit the both relationships to be explained in terms of the same standard model [3]. It is currently assumed [4], on the basis of picosecond radiolysis data for aqueous solutions containing differ ent scavengers [5, 6], that the formation of the primary yields of products in the spurs follows two steps. In the early stage ( 106 L mol–1s–1 or rearrang ing the expression as follows:
kSe1+ X =
αSe1ke⎯ + X aq
ke⎯ + X + 1 × 10
6
+ 3.7 × 10 −8 ke⎯ + X , aq
(17) αSOH1kOH+ X −8 kSOH1+ X = + 4.3 × 10 kOH+ X. kOH+ X + 1 × 106 Figure 4b shows the kSe2+X–ke+X and kSOH2+X–kOH+X plots. It is evident that there is no distinct linear rela tionship in this case, especially for kSe2+X. Nonethe less, using data for the simplest systems in which the mechanism of transformations is understood to the greatest extent (NO 2−, N2O, Br–, COO–, etc.), we found that the kSe2+X–ke+X and kSOH2+X–kOH+X rela tions can be expressed by: aq
SIMULATION OF THE H2O2 YIELD As is the case of H2 yield, two components can be distinguished in the magnitude of the primary yield of hydrogen peroxide (G H 2O 2 ): the yield of H2O2 directly from excited and ionized water molecules Gw0, H2O2 and the yield resulting from the intraspur reactions. Unlike the yield G w0, H2 , whose value can be determined in the presence of a large amount of scavenger, the value of Gw0, H2O2 is much more difficult to determine because of the low stability of hydrogen peroxide. The value of Gw0, H2O2 is estimated at
HIGH ENERGY CHEMISTRY
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