PNNL

Abstract

Zirconium is an important element in the nuclear fuel cycle. Thermodynamic data and models to reliably predict Zr-OH system behavior in various conditions including high ionic strengths are required and currently are unavailable. Most available experimental data are rather old, obtained using inadequate methodologies, and provide equilibrium constant values that differ by many orders of magnitude. Previous reviews have recommended values based on available data. These reviews used all of the available data, including poor quality data, in a global fit to determine these values. This has resulted in recommended thermodynamic models with a large number of polynuclear species and a number of mononuclear species with values of thermodynamic constants for the solubility product of ZrO2(am) and Zr-OH hydrolysis constants that are many orders of magnitude different than those for the reliable analogous Hf reactions. In this critical review, we have evaluated the quality of the available data, selected only those data that are of high quality, and reinterpreted all of the high quality data using SIT and Pitzer models for applications to high ionic strength solutions. Herein for 25°C we 1) present formation constant values for ZrOH3+, Zr(OH)22+, Zr(OH)4(aq), Zr(OH)5-, and Zr(OH)62-, and the solubility product for ZrO2(am) which are consistent with the Hf system, 2) report a revised value for the formation of Ca3Zr(OH)64+, 3) show that several hypothetical polynuclear species (Zr3(OH)93+, Zr4(OH)15+, and Zr4(OH)16(aq)) proposed in previous reviews are not needed, and 4) show that polynuclear species (Zr3(OH)48+ and Zr4(OH)88+) are not important in a very extensive H+ concentration range (0.1 to 10-15.4 mol·kg-1). Our review has also resulted in SIT and Pitzer ion-interaction parameters applicable to as high ionic strength solutions as 5.6 mol·kg-1 in NaCl, 2.11 mol·kg-1 in CaCl2, and 23.5 mol·kg-1 in NaOH.