Optical spectroscopic studies of the sorption of UO22+ species on a reference smectite

The speciation of UO22+ (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