Optical spectroscopic studies of the sorption of UO 2+ 2 species on a reference smectite

Details Optical spectroscopic studies of the sorption of UO 2+ 2 species on a reference smectite Optical spectroscopic studies of the sorption of UO 2+ 2 species on a reference smectite

Sep 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3613m&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 17, pp.3613-3623

Computer Science

9

Scientific paper

The speciation of UO 2+ 2 (uranyl) on a reference smectite (SAz-1 from Cheto, Arizona, USA) has been investigated by electronic emission and Raman vibrational spectroscopies. The spectroscopic studies have been done on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from ~2.5 to 7 and high initial ionic strength (~0.1-0.3 M). The uranyl loading levels in these samples ranged from ~0.1% to ~53% of the reported cation exchange capacity (~1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra vary significantly in intensity and band-shape as a function of uranyl concentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emission spectrum is a composite of spectra from multiple uranyl emitters. At the lowest loading levels a uranyl sorption complex with an apparent vibronic spacing of ~750 cm -1 dominates the spectra. At intermediate loading levels an additional uranyl sorption complex also having an apparent vibronic spacing of ~ 750 cm -1 is present at an approximately constant concentration ratio to the species in the most dilute samples. At the highest loading levels, a uranyl sorption complex with a vibronic spacing of ~ 850 cm -1 dominates the spectra. Raman spectra have been obtained for the more concentrated uranyl/clay samples. Two distinct bands (855 cm -1 and 883 cm -1 ) are seen in the spectral region of the totally symmetric uranyl stretch. The 855 cm -1 band correlates with the dominant high-coverage species, while the 883 cm -1 band arises from an additional sorption complex. Comparison of these results with aqueous solution spectral data suggests that monomeric uranyl moieties are responsible for the observed spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several structurally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant components in the emission spectra in the low and intermediate coverage clay sample are sorbed to amphoteric edge site(s). The uranyl species responsible for the dominant component in the emission spectrum and the 855 cm -1 Raman band in the high-coverage clays is proposed to be exchanged into the fixed charge site(s). The additional complex identified by the Raman band at 883 cm -1 is also proposed as a sorption complex at fixed-charge sites.

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