The effect of isosaccharinic acid and gluconic acid on the retention of Eu(III), Am(III) and Th(IV) by calcite

Abstract

Calcite is an important component of many potential host rocks currently under consideration for the disposal of radioactive wastes. Even in the chemically disturbed zone formed around a cementitious repository, this mineral remains largely unaffected by the hyper-alkaline waters migrating out of the near field. Thus, due to its abundance and geochemical stability, calcite could play an important role in the retardation of radionuclides released from a repository for nuclear wastes. Actinides are an important class of elements present in almost all radioactive waste streams, and for this reason, investigations of their retention behaviour under representative chemical conditions are particularly relevant to assessing safe disposal in the long term. Organic ligands originating from the degradation of cellulosic materials in the repository or present as cement additives could possibly reduce the retardation of tri- and tetravalent actinides due to the formation of stable metal–ligand complexes in solution. In this study, isosaccharinic acid (ISA) and gluconic acid (GLU) have been taken as representatives of cellulose degradation products and concrete admixtures, respectively. Batch-type sorption experiments have been conducted to investigate the effect of ISA and GLU on the retardation of 152Eu, 241Am and 228Th by calcite. 152Eu and 241Am are representatives of the trivalent lanthanides and actinides, respectively, and 228Th is a representative of the tetravalent actinides. High ISA and GLU concentrations in solution were found to significantly affect the sorption of the radionuclides. R d values for Eu(III) and Am(III) decreased significantly at ISA concentrations above 10 −5 M and at GLU concentrations above 10 −7 M. The critical concentration limits were similar for Th(IV), that is 2 × 10 −5 M in the case of ISA and 10 −6 M in the case of GLU. The effects of ISA and GLU on the immobilisation of Eu(III), Am(III) and Th(IV) were interpreted in terms of complex formation in solution. In the case of Eu(III) and Am(III) in ACW, the metal–ligand complexes were found to have a 1:1 stoichiometry. Complexation constants of these aqueous ISA and GLU complexes with Eu(III) and Am(III) were determined and discussed in connection with the presently unclear situation concerning the stability constant of the Eu ( OH ) 3 0 species. In the case of Th(IV) in ACW, it was assumed that a Th(IV)–ISA–Ca complex and a Th(IV)–GLU–Ca complex are formed, both having a 1:2:1 stoichiometry. The complexation constants of these complexes were determined and compared with the literature data. Assuming maximum concentrations for ISA and GLU in the pore water of the disturbed zone of a cementitious repository based on representative near-field conditions ([ISA] aq = 3 × 10 −6 M, [GLU] aq = 10 −7 M), it is predicted that the formation of aqueous ISA and GLU complexes would not significantly affect Eu(III), Am(III) and Th(IV) sorption on calcite.