Fundamental investigation of Zr(IV) solubility and surface processes in alkaline systems: a combined solubility, spectroscopic and theoretical study

Zirconium is an element of interest in the context of nuclear waste management and forms sparingly soluble ZrO2(am, hyd). The ageing or exposure to elevated temperatures expectedly results in a transition of amorphous ZrO2(am, hyd) into the thermodynamically stable crystalline phase ZrO2(cr) and thus in a decreased solubility in aqueous systems. The main objective of this study is to investigate this expected phase transformation process. Amorphous ZrO2(am, hyd) was precipitated, aged at 80°C and investigated with solid phase characterization methods (XRD, TEM, TG-DTA,…), undersaturation solubility experiments in alkaline 0.5 M NaCl-NaOH, 0.02 and 0.2 M CaCl2-Ca(OH)2 solutions and supporting theoretical calculations. The solid phase characterization results indicate, that a transformation process from amorphous towards crystalline ZrO2(s), accompanied by increasing particle size, decreasing water content and decreasing surface area is induced by ageing at 80°C. The solubility experiments show that this process also leads to a decreasing solubility, but additionally a decreasing solubility with ageing time and also for ageing at 22°C was observed and attributed to changes in the surface properties.