Studies on Metal Gluconic Acid Complexes
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Studies on Metal Gluconic Acid Complexes Peter Warwick,1 Nick Evans1 and Sarah Vines2 1 Department of Chemistry, Loughborough University, Loughborough, Leics., LE11 3TU, UK 2 United Kingdom Nirex Limited, Curie Avenue, Harwell, Didcot, Oxon. OX11 0RH, UK ABSTRACT The presence of organic complexants, such as gluconic acid, in an intermediate-level radioactive-waste (ILW) repository may have a detrimental effect on the sorption of radionuclides, by forming organic complexes in solution. In order to assess this, stability constants are required for the complexes formed with radionuclides at high pH. This study reports the stability constants for the reactions of some divalent metals with gluconic acid (Gl). The metals studied were Cd, Co, Fe(II), and U(VI) at pH 13.3; and Co and U(VI) at pH 7. The constants were measured by the Schubert (ion-exchange) or solubility product methods. Stoichiometries of the complexes were also determined. At pH 7 each complex was of the form M1Gl1, with log β values suggestive of salt formation. At high pH, log β values were between 13 and 20. The constants have enabled speciation calculations to be performed showing the effect of gluconic acid on the metal’s solubility, with data for Ni included. Solubility is predicted to increase in the presence of Gl from pH 9 to 13.5, suggesting that it may have an impact on radionuclide behaviour. The largest solubility increases are for Cd and Co, the smallest, U(VI). INTRODUCTION The Nirex Phased Geological Repository Concept involves the use of a cementitious backfill to provide a chemical barrier to radionuclide migration from the near field [1]. The backfill chemically conditions the porewater to provide a high pH under which the solubility of many radionuclides is low. The backfill also provides a large surface area for radionuclide sorption. The presence of organic complexants may increase the solubility and decrease the sorption of radionuclides by forming soluble organic complexes [2], thereby increasing their mobility in the near field and alkaline-disturbed zones. Gluconic acid, CO2H(CHOH)4CH2OH, is a polyhydroxylated carboxylic acid with potentially labile hydroxyl protons that can be displaced during metal complexation at high pH. Its possible presence in a cementitious ILW repository as a cement additive has been reported elsewhere [3]. Stability constants are required for the reactions of radionuclides with gluconic acid at high pH, in order to calculate the effect on radionuclide migration. To date the NiGl [4] constant has been reported. The work reported here adds constants for Cd, Co, Fe(II), and U(VI) at pH 13.3; and Co and U(VI) at pH 7. Some of these elements will have radionuclides that will be present in the repository, and some are useful analogues or “model” elements for important radionuclides. All constants except for U(VI) at high pH were measured by the Schubert method [4]. The solubility product method [2] was used for measuring the constant for U(VI) at pH 13.3. The stoichiometries of the complexes, and the number of
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