Solubility of Niobium(V) under Cementitious Conditions: Importance of Ca-niobate
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CC8.31.1
Solubility of Niobium(V) under Cementitious Conditions: Importance of Ca-niobate Caterina Talerico1, Michael Ochs1, and Eric Giffaut2 BMG Engineering Ltd., Ifangstrasse 11, CH-8952 Schlieren-Zürich, Switzerland, [email protected] 2 ANDRA, Parc de la Croix Blanche, 1/7 rue Jean Monnet, F-92298 Châtenay-Malabry, France 1
ABSTRACT The solubility of niobium was investigated for Ca and pH conditions relevant for cementequilibrated solutions. For the pH range considered (9.5-13.2), the dissolved Nb concentration decreases with increasing pH. Overall, experiments lead to Nb concentrations between 2·10-5 M and 2·10-9 M. For all pH values, the dissolved Nb concentration also decreases systematically with increasing Ca concentration. X-ray diffraction measurements of selected experiments confirmed the presence of a solid Ca-Nb-oxide phase, with CaNb4O11·8H2O (hochelagaite) being the most likely composition. On the basis of these findings an empirical regression model for the prediction of Nb solubility data as a function of pH and Ca concentration was derived. This empirical relation is consistent with the presence of a solubility limiting Ca-Nb solid phase and permits to predict aqueous Nb solubility values in cementitious environments over a relatively wide range of conditions. Predicted values are in good agreement with independent experimental results. INTRODUCTION The solubility of radionuclides under conditions relevant for nuclear waste repositories is an important parameter for safety analyses. For Nb(V), an important element for various radioactive waste types, very little is known about its chemistry under alkaline conditions expected in the near-field of a repository containing cement-based material. This is compounded by the generally poor knowledge on the solubility of solid Nb(V) phases and the hydrolysis of Nb(V). On the basis of the few thermodynamic data available for Nb(V) in the literature, calculated Nb concentrations at pH values above 11 would indicate extremely high solubility values (≥10-2 M) [1]. Based on these data, very high concentrations of Nb would have to be expected in the nearfield of a cement-based repository. At the same time, experimental studies on Nb(V) solubility under highly alkaline conditions are sparse in the literature. However, the few published experiments conducted with cementequilibrated solutions [2, 3, 4] indicate that the solubility of Nb under such conditions is about 1·10-8 M, i.e. many orders of magnitude lower than the calculated values. An explanation for this discrepancy is still lacking to date. The goals of the present study are to: 1) provide reliable data for estimating maximum Nb concentrations in the near-field of a cement-based repository; 2) resolve the apparent discrepancy between measured and calculated Nb solubility limits; and 3) identify the key factors influencing the Nb solubility in cementitious environments.
CC8.31.2
EXPERIMENTAL METHODS The experiments were designed to obtain data on the solubility of Nb as a function of pH and Ca co
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