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

Abstract

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 concentrationsare 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) 2 Ca) −4H + 4H with log K 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 log K = −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).

Thermodynamics of Np(IV) complexes with gluconic acid under alkaline conditions: sorption studies

“…A cation exchange resin Bio-Rad 50W-X2 (Bio-Rad Laboratories, Hercules, USA) was used as sorbent material. Before use, this resin was washed and conditioned following a procedure described elsewhere [4]. Gluconic acid was available as a Na salt (Merck AG, Dietikon, Switzerland).…”

“…This wall sorption has been previously shown to depend on the complexation with organic ligands such as GLU [4]. Therefore, in order to evaluate the sorption of 239 Np on the tubes walls, the centrifuge tubes were emptied after sampling, rapidly rinsed with 10 mL of a solution identical to the one used in the preceding sorption test but without 239 Np tracer and then filled with 20 mL 1 M HCl solution to allow desorption of the radionuclide from the walls.…”

“…GLU concentrations in cement could be as high as 10 −2 M on the assumption that GLU is entirely released into the pore water, although sorption by cement surfaces may reduce this concentration to values below 10 −7 M [3]. The formation of very stable complexes of An(III) and An(IV) with ISA and GLU has been reported in the literature [4][5][6][7]. The stability of An(IV)-GLU complexes seems to be further increased in the presence of Ca, where ternary Ca-An(IV)-GLU complexes have been described for Th (IV).…”

“…The Schubert method [e.g. 1,4,6,8] was applied to determine the stoichiometry and the stability constants of the Np(IV)-GLU complexes. The resulting thermodynamic data were compared with data available for other An(IV)-GLU complexes.…”

Thermodynamic Model for ThO2(am) Solubility in Isosaccharinate Solutions

“…Only very limited reliable data are available for isosaccharinate (ISA -) complexes of thorium. Two types of data that are available for this system are 1) those that are based on sorption experiments under alkaline conditions [2][3][4][5][6] and 2) those based on liquid-liquid extractions [7,8]. The data based on sorption experiments have recently been reviewed by Hummel et al [1] who concluded that the entire data can be represented by [Th(OH) 4 (aq) + ISA -= Th(OH) 4-x (ISA -xH ) +xH 2 O; Th(OH) 4 (aq) + 2ISA -+ Ca 2+ = CaTh(OH) 4-2y (ISA -yH )(aq) + 2yH 2 O] and that the values of x and y in the above reactions cannot be determined by standard thermodynamic methods.…”