Effect of Ca2+ and Zn2+ on UO2 dissolution rates

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Uranium dioxide (UO2) dissolution is critical for uranium mobilization in subsurface environments.
• Understanding the impact of common groundwater cations on UO2 stability is essential for remediation and waste disposal.

Purpose of the Study:

• To investigate the effect of calcium (Ca2+) and zinc (Zn2+) on UO2 dissolution rates.
• To elucidate the mechanisms of inhibition, including adsorption and precipitation reactions.

Main Methods:

• Experiments conducted in a continuously stirred tank reactor (CSTR) under anoxic and oxic conditions.
• Solution chemistry and Extended X-ray Absorption Fine Structure (EXAFS) analyses.
• Investigation of UO2 solids recovered from experiments.

Main Results:

• Zn2+ exhibited a significantly greater inhibitory effect on UO2 dissolution than Ca2+.
• Under oxic conditions, UO2 dissolution rates decreased 7-fold with Ca2+ and 1450-fold with Zn2+.
• Precipitation of Ca-U(VI) and Zn-based (hydrozincite) phases on UO2 surfaces was observed, inhibiting dissolution.

Conclusions:

• Divalent cations, particularly Zn2+, can form passivating layers on UO2, substantially reducing dissolution rates.
• These findings have critical implications for the long-term stability of UO2 in nuclear waste repositories and contaminated sites.
• The precipitation of cation-dependent phases controls uranium mobility in groundwater systems.