Complexation of Actinides with Amide Derivatives of Oxydiacetic Acid
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0893-JJ08-06.1
Complexation of Actinides with Amide Derivatives of Oxydiacetic Acid Linfeng Rao and Guoxin Tian Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, CA 94720, U.S.A. ABSTRACT Complexation of Np(V), U(VI) and Nd(III) with dimethyl-3-oxa-glutaramic acid (DMOGA) and tetramethyl-3-oxa-glutaramide (TMOGA) was studied in comparison with the complexation with oxydiacetic acid (ODA). Stability constants and enthalpy of complexation were determined by potentiometry, spectrophotometry and calorimetry. Thermodynamic parameters, in conjunction with structural information of solid compounds, indicate that DMOGA and TMOGA form tridentate complexes with the ether-oxygen participating in bonding with actinide/lanthanide ions. The trends in the stability constants, enthalpy and entropy of complexation are discussed in terms of the difference in the hydration of the amide groups and carboxylate groups and the difference in the charge density of the metal ions. INTRODUCTION Alkyl-substituted amides have been studied in recent years as promising extractants for actinide separation from nuclear wastes. The amide ligands have a few advantages over traditional organophosphorus compounds used in actinide separations (e.g., tributylphosphate, TBP). For example, the products of radiolytic and hydrolytic degradation of amides are less detrimental to separation processes and stripping of actinides from the amide-containing organic solvents is relatively easy. Besides, the amide ligands are completely incinerable so that the amount of secondary wastes generated in nuclear waste treatment could be significantly reduced. One group of alkyl-substituted oxa-diamides, including the symmetrical tetraoctyl-3-oxaglutaramide [1] and tetraisobutyl-oxa-glutaramide [2], and asymmetrical N,N'-dimethyl-N,N'dihexyl-3-oxaglutaramide [3], have been shown to be effective in the solvent extraction of actinides from nuclear wastes. Distribution ratios of actinides under different conditions have been determined in these studies, but the underlying thermodynamic principles governing the complexation of actinides with oxa-diamides remain unrevealed. To improve the efficiency of amides as extractants to separate actinides, systematic studies are needed to obtain thermodynamic and structural information on the complexes. Thermodynamic parameters (e.g., enthalpy and entropy) and structural factors (e.g., denticity and steric hindrance) that affect the binding strength need to be determined. Since the oxa-amide ligands can be considered as derivatives of oxydiacetic acid (ODA) and thermodynamic data on the complexation of ODA with actinides are available in the literature [4,5,6,7], we have chosen to conduct a comparative study between ODA and two methyl-substituted amides, dimethyl-3-oxa-glutaramic acid (DMOGA) and tetramethyl-3-oxa-glutaramide (TMOGA) in aqueous solutions (Figure 1). Using the small methyl groups on the amides gives the ligands substantial solubility in aqueous solutions, so that multiple thermodynamic techniques (pote
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