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Influence of naturally-occurring heterogeneous complex-forming materials on the migration behavior of actinides in the geosphere (III)

Tochiyama, Osamu*

To estimate the polyelectrolyte effect and the effect of the heterogeneous composition of humic acids, the complex formation constants of Eu(III) and Ca(II) with Aldrich humic acid and polyacrylic acid were obtained, for Eu(10$$^{-8}$$ to 10$$^{-5}$$ M) by solvent extraction with TTA and TBP in xylene, for Ca (10$$^{-10}$$M) with TTA and TOPO in cyclohexane and for Ca(10$$^{-4}$$M) by using ion-selective electrode. By defining the apparent formation as $$beta_{alpha}$$ = [MR$$_{m}$$]/([M][R]), where [R] denotes the concentration of dissociated functional group, [M] and [MR$$_{m}$$] denote the concentration of free and bound metal ion and pcH is defined as-log[H], the values of log$$beta_{alpha}$$ have been obtained at pcH 4.8 - 5.5 in 0.1 - 1.0M NaClO$$_{4}$$ and NaCl. Log$$beta_{alpha}$$ of Eu-humate varied from 5.0 to 9.3 and that of Ca-humate from 2.0 to 3.4..For both humate and polyacrylate, log$$beta_{alpha}$$ increased with pcH or with the degree of dissociation. The increase in the ionic strength O.1 to 1.0 M decreased the log$$beta_{alpha}$$, the decrease in log$$beta_{alpha}$$ of Eu(III)-humate is 1.6, that of Eu(III), polyacrylate 0.7, that of Ca(II)-humate 1.9 and that of Ca(II)-polyacrylate 1.2. While the increase in the metal ion produced no effect on log$$beta_{alpha}$$ of polyacrylate, log$$beta_{alpha}$$ of humate decreased. Depending on the concentration of Eu(III), the coexistence of Ca(II) reduced log $$beta_{alpha}$$ of humate by 0 to 0.8. The dependence of log$$beta_{alpha}$$ of humate on the metal ion concentration suggests the coexistence of strong and weak binding sites in the hmnic acid.

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