f-element complexation in solutions to high ionic strengths.

摘要

The complexation of f-elements with ligands in brine is of considerable interest since the release of actinides into the salt-bed environment is considered the primary hazard of radioactive wastes in a long-term geological disposal. To predict the actinide migration in the environment, the aqueous complexes formed between trivalent europium or hexavalent uranyl cations with chloride, nitrate, fluoride and acetate were studied in a wide range of ionic strength, from 0.1 M to 9.0 M.; Ligand competition experiments were employed to measure the stability constants of these f-element complexes: solvent extraction, titrations with pH electrode or ion selective electrode, as well as other techniques.; Based on measured equilibrium constants and literature data, a Parabolic model was developed in this study to correlate equilibrium constants (pKa of acids, log {dollar}betasb{lcub}rm i{rcub}{dollar} of metal complexes, etc) with ionic strength. This correlation was compared with that by the SIT and the Pitzer models. Possibilities of decomposing the coefficients {dollar}Deltavarepsilon{dollar} and {dollar}Deltadelta{dollar} into {dollar}varepsilonsb{lcub}rm i{rcub}{dollar} and {dollar}deltasb{lcub}rm i{rcub}{dollar} were analyzed with a revision of the Pitzer's subequations.; The speciation diagrams of Eu(III), U(VI) in brines at I = 0.7 M and 5.0 M show that the f-element inorganic complexes studied in this work are less important species for neutral brines at high ionic strengths owing to increased hydrolysis or carbonate complex formation. The influence of ionic strength on f-element speciation in brine solutions is reflected by the increased total concentration of chloride and the variation of log {dollar}betasb{lcub}rm MLi{rcub}{dollar} and PKa{dollar}sb{lcub}rm HL{rcub}{dollar} values with ionic strength.