Complexation of Cm(III) with aqueous silicic acid

The complexation of 248Cm(III) with aqueous silicic acid is investigated in the pH range of 1.5-9.0 in 0.03 M NaCl with varying the silicate concentration under and over saturation in reference to the solubility of amorphous silica in order to characterize the reaction with monosilicic and polysilicic acid. Speciation is made by time-resolved laser fluorescence spectroscopy (TRLFS) in combination with radiometric quantification. Three different complexation products are observed: Cm-silicate(I), Cm-silicate(II) and Cm-silicate(III). The first and second species are formed in under-saturation of silicic acid, whereas all three species are produced in over-saturation of silicic acid. The formation of Cm-silicate(I) following the reaction of Cm3+ with H3SiO4- appears in both under and over-saturation of silicic acid only as a minor fraction at pH = 4-7. Cm-silicate(II) and Cm-silicate(III) are found to be colloidal. Cm-silicate(II) shows its spectroscopic characteristics varying with the experimental condition, whereas Cm-silicate(III) formed exclusively with polysilicic acid remains consistent and stable. The experimental results indicate that a species conversion takes place as Cm-silicate(I) → Cm-silicate(II) → Cm-silicate(III) with increasing pH and silicon concentration. Only the reaction leading to the formation of Cm-silicate(I) can be quantified for its stability constant as log β = 7.74 ± 0.08. (orig.)