EXPERIMENTAL MEASUREMENTS OF THE SOLUBILITY OF CEO2 IN NANO3 SOLUTIONS: HYDROLYSIS OF CE(IV) AND COMPLEXATION BY ORGANIC LIGANDS

To establish the effect of pH and ionic strength on the hydrolysis of Ce(IV), experiments on the solubility of CeO₂ were conducted at room temperature, in acidic (pH=2.0 to 4.0), and high ionic strength (0.1, 1.0 and 3.0 molal in NaNO₃) solutions. Additional experiments were conducted to investigate the effect of organic ligands  (citrate, EDTA and oxalate: 100 to 500 mmol) on the solubility of CeO₂ at the same temperature-pH-ionic strength conditions. Data on the hydrolysis of Ce(IV) and solubility of the CeO₂ in the presence of organic ligands are of critical importance the safe disposal and treatment of high-level nuclear waste: Ce (IV) is considered to be a chemical analogue of tetravalent actinides such as Pu(IV).

Approach to equilibrium was from undersaturation for all experiments and was indicated by the attainment of steady-state concentrations after 60 days. Solubilities obtained as a function of pH and ionic strength in the absence of organic ligands are:

	pH=2	pH=3	pH=4
I=0.1	1.64E-04	8.15E-05	5.18E-05
I=1.0	9.43E-05	4.80E-05	4.11E-05
I=3.0	1.15E-04	4.17E-05	2.96E-05

Plots of log m_Ce vs. pH conform to a straight line with a slope of 0.25 suggesting that Ce(OH)₃⁺ and Ce(OH)₄⁰ are the predominant species under these conditions.

The solubility of CeO₂ increases significantly (by as much as an order of magnitude) in the presence of organic ligands at the concentrations studied at pH=2.0.  Experiments involving organic ligands at higher pH are still in progress.