Complexation of Th(IV) and Eu(III) by α-isosaccharinic acid under alkaline conditions

The complexation of Th(IV) and Eu(III) by α-isosaccharinic acid (ISA) has been studied in the pH range from 10.7 to 13.3 by batch sorption experiments, and the influence of Ca on the complexation was investigated. Sixteen data sets – each determined at variable ISA concentrations – are used to determine the stoichiometry of the complexation reactions and the stability constants. Based on best-fit analysis of the sorption data, it is postulated that 1:1 Th:ISA complexes are formed in the absence of Ca according to the complexation reaction: Th+ISA↔(ThISA)_-4H+4H with log K=-10.1 at I=0.3 M. In the presence of Ca, the sorption data can be interpreted best by a mixed-metal complex, according to the complexation reaction: Th+2ISA+Ca↔(Th(ISA)₂Ca)_-4H+4H with logK^Ca=-3.6 at I=0.3 M. There are no indications that Ca participates in complexes of ISA with Eu. The sorption data suggest that 1:1 Eu:ISA complexes are dominant in the pH range from 10.7 to 13.3 according to the complexation reaction: Eu+ISA↔(EuISA)_-4H+4H with logK=-30.6 at I = 0.3 M. It is important to note that the stoichiometric numbers and stability constants proposed here are not independent of the hydrolysis reactions of the two metal cations. Thus, the same hydrolysis data as used here have to be applied in speciation calculations with the ISA complexation data for Eu(III) and Th(IV).