An Experimental Investigation of U(IV) Organic Aqueous Complexation with Citric Acid, NTA and EDTA

Waste from nuclear energy production and mining can result in discharge of UO₂ into the environment. Within anoxic near surface aqueous settings where UO₂ can be released an opportunity to encounter and complex with organic acids may occur. Reactions between UO₂ and ligands can promote the solubility and mobility of uranium. Published results describing complexation of U(IV) and organic ligands are extremely limited and inconsistent.  This study aims to identify the resulting U(IV)-ligand species formed during complexation reactions, to derive K_sp values from the reactions and to contribute to the overall understanding of how ligands may enhance the solubility of synthetic UO_2(cr). Organic ligands investigated in this study include citric acid, nitrilotriacetic acid (NTA) and ethylene diamine tetraacetic acid (EDTA). Solubility studies were conducted at 25°C inside a glove box anoxic chamber containing a mixed N₂/H₂ atmosphere. Batch solubility experiments were carried out in 50 mL centrifuge tubes with pH ranging from 5 to 10 and as a function of acid concentration. Acid concentrations ranged from 1 nM to100 mM, with 0.01 M NaCl used as background electrolyte. Eu(II)CL₂ was added to solutions to suppress oxidation of  U(IV) to U(VI). Analysis of the solubility data revealed that exposure to citric, NTA, and EDTA had little effect on UO₂ solubility. Solubility varied within an order of magnitude when compared to hydrolysis of the UO_2(cr) mineral used in this study. The log activity of the UO_2(cr) hydrolysis yielded a value of -5.43. Under all measured conditions of organic ligand concentration, the data consistently show an increase in uranium concentration to a median log U activity of -4.69.  The observed solubility of UO_2(cr) in the presence of chelating organic ligands is inconsistent with expected values based on literature data. Potential reasons for this discrepancy are under investigation.