Perrhenate Complexation by Uranyl in Traditional Solvents and in Ionic Liquids: A Joint Molecular Dynamics/Spectroscopic Study

Abstract

The complexation of perrhenate (ReO(4)(-)) anions by the uranyl (UO(2)(2+)) cation has been investigated by joint molecular dynamics simulations and spectroscopic (UV-vis, TRLFS, and EXAFS) studies in aqueous solution, acetonitrile, and three ionic liquids (ILs), namely, [Bmi][Tf(2)N], [Me(3)BuN][Tf(2)N], and [Bu(3)MeN][Tf(2)N] that are based on the same Tf(2)N(-) anion (bis(trifluoromethylsulfonyl)imide) and either Bmi(+) (1-butyl,3-methylimidazolium), Me(3)BuN(+), or Bu(3)MeN(+) cations. They show that ReO(4)(-) behaves as a weak ligand in aqueous solution and as a strong ligand in acetonitrile and in the ILs. According to MD simulations in aqueous solution, the UO(2)(ReO(4))(2) complex quickly dissociates to form UO(2)(H(2)O)(5)(2+), while in acetonitrile, a stable UO(2)(ReO(4))(5)(3-) species forms from dissociated ions. In the ILs, the UO(2)(ReO(4))(n)(2-n) complexes (n = 1 to 5) remained stable along the dynamics, and to assess their relative stabilities, we computed the free energy profiles for stepwise ReO(4)(-) complexation to uranyl. In the two studied ILs, complexation is favored, leading to the UO(2)(ReO(4))(5)(3-) species in [Bmi][Tf(2)N] and to UO(2)(ReO(4))(4)(2-) in [Bu(3)MeN][Tf(2)N]. Furthermore, in both acetonitrile and [Bmi][Tf(2)N] solutions, MD and PMF simulations support the formation of dimeric uranyl complexes [UO(2)(ReO(4))(4)](2)(4-) with two bridging ReO(4)(-) ligands. The simulation results are qualitatively consistent with spectroscopic observations in the different solvents, without firmly concluding, however, on the precise composition and structure of the complexes in the solutions.

Exploring Actinide Materials Through Synchrotron Radiation Techniques

“…For other complexed anions, such as the ReO 4 − , Br − , Tf 2 N − , CF 3 SO 3 − , and ClO 4 − , their interactions with uranyl ions have also been studied extensively. Chaumont et al studied the interaction of ReO 4 − with uranyl ions in Tf 2 N − ‐based ionic liquids. The EXAFS results showed that centered uranium was surrounded by 5 equatorial oxygen atoms at average U‐O distances of 2.43 Å, which decreased slightly with the Re/U ratio up to 10.…”

Environmental modeling of uranium interstitial compositions of non-stoichiometric oxides: experimental and theoretical analysis

“…and [UO 2 (CH 3-CN) 5 ] 2? dications is also evaluated (Chaumont et al 2012;Kulyukhin and Kamenskaya 2010). As the quantitation by ESI-MS and MALDI-MS method is conceptually different, due to different, virtually continuous and pulsed ionization techniques, the sensitivity and reproducibility are discussed comprehensively.…”

“…The environmental speciation, thereby, has involved complex (bio)geochemical reactions in solution, sorption/desorption processes onto minerals, biological processes causing distribution of uranium various oxidation states and chemical compositions (Conradson et al 2004;Malliakas et al 2012;Lee et al 2009;Chaumont et al 2012;Park et al 1990;Van Horn and Huang 2006). In parallel, the minerals themselves near to geosphere, when they are exposed to the effect of aerobic atmospheric conditions are oxidized and/or decomposed yielding different oxidative products, thus causing different uptake behavior of heavy metals in the environment, particularly reflecting in mind uranium and other radionuclides (Scott et al 2007).…”

“…Furthermore, the mainly studied equilibrium sorption models onto environmental subsurfaces describing uranium transport, relevant to large number of mineralogical, hydrological and biogeochemical processes have shown, in fact, a non-equilibrium kinetic control of the sorption of uranium species onto mineral surfaces (Um et al 2010;Guerra et al 2010). The aqueous speciation depends mainly on carbon dioxide, uranium concentration levels, pH, pressure, and temperature (Conradson et al 2004;Malliakas et al 2012;Lee et al 2009;Chaumont et al 2012;Park et al 1990;Van Horn and Huang 2006). For the purpose of nuclear waste management, knowledge of coordination ability of uranium including various phases, solvent effect causing different complexation behavior and formation of different coordination compounds with the inclusion of solvent molecules is great importance, too (Conradson et al 2004;Malliakas et al 2012;Lee et al 2009;Chaumont et al 2012;Park et al 1990;Van Horn and Huang 2006).…”

“…The aqueous speciation depends mainly on carbon dioxide, uranium concentration levels, pH, pressure, and temperature (Conradson et al 2004;Malliakas et al 2012;Lee et al 2009;Chaumont et al 2012;Park et al 1990;Van Horn and Huang 2006). For the purpose of nuclear waste management, knowledge of coordination ability of uranium including various phases, solvent effect causing different complexation behavior and formation of different coordination compounds with the inclusion of solvent molecules is great importance, too (Conradson et al 2004;Malliakas et al 2012;Lee et al 2009;Chaumont et al 2012;Park et al 1990;Van Horn and Huang 2006). Therefore, from analytical viewpoint, uranium O-containing compositions as analytical objects are among most complex coordination system known.…”