Do Perchlorate and Triflate Anions Bind to the Uranyl Cation in an Acidic Aqueous Medium? A Combined EXAFS and Quantum Mechanical Investigation

Molecular characterization of uranium(VI) sorption complexes on iron(III)-rich acid mine water colloids

“…Out-ofplane coordination of water molecules should weaken the UAO ax bonds and lengthen them. In fact, the measured UAO ax bond distance of 1.80-1.81 Å is substantially larger than the UAO ax bond distance of 1.76 Å of aqueous species such as [UO 2 (H 2 O) 5 ] 2+ (Sémon et al, 2001). According to Kannappan et al (2004), the origin of an enlarged UAO ax bond length of 1.805 Å is a hydrogen bond OAH Á Á Á O ax with the length of 2.831 Å .…”

“…The total NO 3 À concentrations were held below 0.02 M to avoid ternary complex formation with the adsorbed UO 2 2þ . According to reported EXAFS data, the ions of a 0.1 M UO 2 (NO 3 ) 2 solution fully dissociate and form the pentahydrated [UO 2 (H 2 O) 5 ] 2+ species (Sémon et al, 2001). To cross-check this, we prepared additional samples using the salts of chloride or perchlorate instead of nitrate, because these ions are excluded from complex formation with UO 2 2þ at the given electrolyte concentration (Sémon et al, 2001;Hennig et al, 2005).…”

“…Considering the above mentioned bidentate coordination of uranyl to the ferrihydrite surface, three O eq atoms are assumed to be coordinated with water molecules from the initial hydration sphere. Therefore, three O eq atoms were fixed with R UAO eq of 2.41 Å and r 2 of 0.007 Å 2 , representing average values of pentahydrated UO 2ðaqÞ 2þ (Allen et al, 1997;Sémon et al, 2001). The other two O eq atoms fit to radial distances of $2.30 Å and r 2 of $0.0055 Å 2 (Table 4), suggesting a splitting of the equatorial oxygen shell into two subshells.…”

“…Despite the absence of nitrate and great care to exclude carbonate, this peak also appeared in spectra of U(VI) sorbed schwertmannite and goethite (a-FeOOH) at a pH as low as 4.2 (Walter et al, 2003), which would further reduce the solubility of carbonate if present. The contribution of chlorine is unlikely because the chosen perchlorate background concentration was much below the level needed to form innersphere complexes with uranyl (Sémon et al, 2001). Hence within these systems the FT peak at 2.4 Å is more likely attributed to oxygen than to carbon, nitrogen, or chlorine.…”

Uranyl(VI) Nitrate Salts: Modeling Thermodynamic Properties Using the Binding Mean Spherical Approximation Theory and Determination of “Fictive” Binary Data

“…Uranyl perchlorate, chloride, and nitrate osmotic coefficients were taken from Goldberg selection, considering Robinson et al measurements 7,8 (up to 3.24 mol‚kg -1 for UO 2 -(NO 3 ) 2 ) but also the most reliable values from other authors. 49 According to experimental and theoretical results, 50 for uranyl perchlorate, we assumed that no complexes are formed:…”