Abstract
The solubilities of zirconium hydroxide and oxide after aging at 278, 313, and 333 K were measured at 278, 298, 313, and 333 K in the pHc range of 0.3–7 in a 0.5 M ionic strength solution of NaClO4 and HClO4. Size distributions of the colloidal species were investigated by ultrafiltration using membranes with different pore sizes, and the solid phases were examined by X-ray diffraction. The apparent solubility of zirconium amorphous hydroxide (Zr(OH)4(am)), prepared by the oversaturation method, decreased with increasing aging temperature (Ta), and the size distributions obtained after aging at elevated temperatures indicated the growth of the colloidal species. We, therefore, suggested that agglomeration of the colloidal species and dehydration and crystallization of Zr(OH)4(am) as the solubility-limiting solid phase occurred over the course of aging at elevated temperatures. For sample solutions of the crystalline oxide (ZrO2(cr)), the aging temperature had no significant effect on the solubility, but the solubility data at lower temperatures were found to be slightly higher than those at higher temperatures, implying a small fraction of the amorphous components. In the analysis of different solid phases (Zr(OH)4(s,Ta), Ta = 278, 313, and 333 K) depending on the aging temperatures, the solubility products (Ksp, Ta) were determined at different measurement temperatures, and the enthalpy change (ΔrH○) for Zr4++ 4OH– ↔ Zr(OH)4(s,Ta) was calculated using the van't Hoff equation. The solid-phase-transformation process at elevated temperatures was also analyzed based on the obtained Ksp, Ta and ΔrH○ values.
Acknowledgement
This work was supported by the JSPS Grant-in-Aid for Scientific Research (B) Grant Number 24360393.
©2015 Walter de Gruyter Berlin/Boston