OPTICAL SPECTROSCOPIC STUDIES OF THE SORPTION OF UO2(2+) SPECIES ON AREFERENCE SMECTITE

The speciation of UO22+ (uranyl) on a reference smectite (SAz-1 from C heto, Arizona, USA) has been investigated by electronic emission and R aman vibrational spectroscopies. The spectroscopic studies have been d one on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from similar to 2.5 to 7 and high initial ioni c strength (similar to 0.1-0.3 M). The uranyl loading levels in these samples ranged from similar to 0.1% to similar to 53% of the reported cation exchange capacity (similar to 1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra va ry significantly in intensity and band-shape as a function of uranyl c oncentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emi ssion spectrum is a composite of spectra from multiple uranyl emitters . At the lowest loading levels a uranyl sorption complex with an appar ent vibronic spacing of similar to 750 cm(-1) dominates the spectra. A t intermediate loading levels an additional uranyl sorption complex al so having an apparent vibronic spacing of similar to 750 cm(-1) is pre sent at an approximately constant concentration ratio to the species i n the most dilute samples. At the highest loading levels, a uranyl sor ption complex with a vibronic spacing of similar to 850 cm(-1) dominat es the spectra. Raman spectra have been obtained for the more concentr ated uranyl/clay samples. Two distinct bands (855 cm(-1) and 883 cm(-1 )) are seen in the spectral region of the totally symmetric uranyl str etch. The 855 cm(-1) band correlates with the dominant high-coverage s pecies, while the 883 cm(-1) band arises from an additional sorption c omplex. Comparison of these results with aqueous solution spectral dat a suggests that monomeric uranyl moieties are responsible for the obse rved spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several str ucturally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant componen ts in the emission spectra in the low and intermediate coverage clay s ample are sorbed to amphoteric edge site(s). The uranyl species respon sible for the dominant component in the emission spectrum and the 855 cm(-1) Raman band in the high-coverage clays is proposed to be exchang ed 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 a t fixed-charge sites.