Interaction of U(VI) with α-isosaccharinic acid: structural elucidation of the formed complexes and implications for the retention of U(VI) on bentonite

Low and intermediate level waste contains considerable amounts of cellulosic materials, which will be degraded relatively fast under alkaline conditions, with isosaccharinic acid (ISA), a polyhydroxy-carboxylic acid, being the main degradation product. It has been shown that the α-form is a stronger complexant for certain radionuclides compared to the β-form and that the complex formation affects the sorption as well as the solubility adversely.[1]
In the particular case of U(VI) the number of studies concerning the speciation in the presence of ISA is small. The excellent spectroscopic properties of the uranyl-entity were used to determine the speciation by UV-vis, luminescence, ATR-FTIR and EXAFS spectroscopy properly under acidic conditions. To understand the complex formation mechanism on a molecular level, the behavior of the ligand was simultaneously investigated by ATR-FTIR and NMR spectroscopy and the results were compared to theoretical data from DFT-calculations, whereby a dominant chelate binding motif via the carboxylic and the α-hydroxy-group was identified.
Whereas hydrolysis of U(VI) and carbonate-complexation can be neglected under acidic conditions, they have to be carefully considered under neutral and alkaline conditions as competitive reactions in addition to the complex formation with ISA. In this context the influence of ISA on the retention of U(VI) on bentonite was investigated. Sorption experiments were performed under anaerobic (carbonate-free) and aerobic (with carbonate) conditions between pH 8 and 13 in the presence of ISA. Time-resolved laser-induced fluorescence spectroscopy was used to determine the aqueous speciation of U(VI) and the results will be compared to measurements without ISA.

[1] Van Loon, L. R., et al., Radiochimica Acta, 1999, Vol. 86, https://doi.org/10.1524/ract.1999.86.34.183.