Effect of ionic strength on complexation of Pu(IV) with humic acid

Successful geochemical modelling of the migration of radioactive materials, such as the transuranic elements, from nuclear waste repositories is dependent upon an understanding of their interaction with biogeopolymers such as humic acids, the most likely complexing agents in groundwaters. An established silica/humic acid composite has been evaluated as a model substrate for naturally occurring humate- coated minerals that are likely to be present in the vicinity of the repositories. The binding of Pu(IV), the highly likely oxidation station, by the silica/humic substrate was examined at pH4 in the range 0.02 to 3.00 M NaClO₄ by the titration method. Pu(IV)-humate conditional stability constants have been evaluated from data obtained from these experiments by using non-linear regression of binding isotherms. The results have been interpreted in terms of complexes of 1 : 1 stoichiometry. Analysis of the complex formation dependency with ionic strength shows that the effect of ionic strength on humate complexation of Pu(IV) is not dramatically pronounced. The complexation constants are evaluated for the humate interaction with Pu4+ and Pu(OH)₃+ at pH 4. The complexation constants are found, respectively, to be log HA/β(Pu4+) = 16.6 ± 0.3 and log β0_1.3. =46.6 ± 2.3. The estimations through analogy from previous results are in agreement with these new experimental data.