title: pH dependence of uranyl retention in a quartz/solution system: an XPS study.
abstract: We have investigated the pH dependence of U(VI) retention in quartz/10(-4) M uranyl solution systems, under conditions favoring formation of polynuclear aqueous species and of colloids of amorphous schoepite as U(VI) solubility-limiting phases. X-ray photoelectron spectroscopy was used to gain insights into the coordination environments of sorbed/precipitated uranyl ions in the centrifuged quartz samples. The U4f XPS spectra made it possible to identify unambiguously the presence of two uranyl components. A high binding energy component, whose relative proportion increases with pH, exhibits the U4f lines characteristic of a reference synthetic metaschoepite. Such a high binding energy component is interpreted as a component having a U(VI) oxide hydrate character, either as polynuclear surface oligomers and/or as amorphous schoepite-like (surface) precipitates. Its pH dependence suggests that a binding of polynuclear species at quartz surfaces and/or a formation of amorphous schoepite-like (surface) precipitates is favored when the proportion of aqueous polynuclear species increases. A second surface component exhibits binding energies for the U4f core levels at values significantly lower (DeltaE(b)=1.2 eV) than for metaschoepite, evidencing uranyl ions in a distinct coordination environment. Such a low binding energy component may be attributed to monomeric uranyl surface complexes on the basis of published EXAFS data. Such a hypothesis is supported by a major contribution of the low binding energy component to the U4f XPS spectra of reference samples for uranyl sorbed on quartz from very acidic 10(-3) M uranyl solutions where UO(2)(2+) ions predominate.
authors.affiliation: Institut de Recherches Subatomiques, ULP/CNRS/IN2P3, UMR 7500, 67037 Cedex 2, Strasbourg, France. annick.froideval@ires.in2p3.fr
authors.forename: A
authors.initials: A
authors.lastname: Froideval
authors.forename: M
authors.initials: M
authors.lastname: Del Nero
authors.forename: R
authors.initials: R
authors.lastname: Barillon
authors.forename: J
authors.initials: J
authors.lastname: Hommet
authors.forename: G
authors.initials: G
authors.lastname: Mignot