Effect of humic acid on the sorption of Cm(III) onto<i>γ</i>-Al₂O₃studied by the time-resolved laser fluorescence spectroscopy

Abstract

Summary Sorption of Cm(III) onto γ -alumina coated with humic acid (HA) is studied by the Time Resolved Laser Fluorescence Spectroscopy (TRLFS). The experiments are performed at 0.1 M NaClO 4 , 0.44 g/L γ -Al 2 O 3 , 10 mg/L HA and at a metal ion concentration of 2×10 -7 mol/L. At the investigated pH range (4 to 10) HA is completely sorbed to γ-Al 2 O 3 . The excitation spectrum of Cm(III) bound to HA/ γ -Al 2 O 3 in the wavelength range 370-400 nm exhibits broad flat bands very different from those obtained for the Cm(III) aquo ion and the Cm(III)- γ -Al 2 O 3 surface complex, respectively. The spectrum, lacking distinctive structure due to intramolecular energy transfer processes, points to the predominant binding of the Cm(III) to surface-bound HA. TRLFS experiments performed at two different excitation wavelengths (λ ex =355 and 396.6 nm) allow for a differentiation of humic-bound and non-humic-bound Cm(III). Differences in fluorescence spectra obtained at the different excitation wavelengths are found at pH&lt;6.9. They are due to the presence of the non-complexed Cm(III) aquo ion which is not detected in the indirect excitation mode (λ ex =355 nm). At pH≥7, the fluorescence spectra obtained by indirect and direct excitation become congruent and again point to the existence of only humic-bound Cm(III) species. Comparison of peak maxima and fluorescence lifetimes for Cm(III)-HA and Cm(III)-HA/ γ -Al 2 O 3 , however, reveal differences. The results clearly indicate a contribution of the γ -Al 2 O 3 surface to the Cm(III) binding and, thus, suggest the formation of ternary complexes such as &gt;Al-O-Cm(III)(HA).