| Abstract
| - The mechanisms of the uranium(VI) sorption on schwertmannite and goethite in acid sulfate-rich solutionswere studied by Extended X-ray Absorption Fine Structure(EXAFS) spectroscopy. The samples were preparedunder N2 atmosphere and initial uranium(VI) concentrationsof 1 × 10-5 (pH 6.5) to 5 × 10-5 M (pH 4.2). The ionicstrength was adjusted using 0.01 M Na2SO4 or 0.01 M NaClO4,respectively. The EXAFS structural parameters for uranium(VI) sorbed on goethite in sulfate-rich, acid and near-neutral solutions indicate that uranium(VI) forms an inner-sphere, mononuclear, bidentate surface complex. Thiscomplex is characterized by a uranium-ferric-iron distanceof approximately 3.45 Å. Uranium(VI) sorbed ontoschwertmannite in acid and sulfate-rich solution iscoordinated to one or two sulfate molecules with a uranium−sulfur distance of 3.67 Å. The EXAFS results indicateformation of binuclear, bidentate surface complexes andpartly of mononuclear, monodentate surface complexescoordinated to the structural sulfate of schwertmannite. Theformation of ternary uranium(VI)−sulfate surface complexescould not be excluded because of the uncertainty inassigning the sulfate either to the bulk structure or toadsorption reactions. The uranium(VI) adsorption ontoschwertmannite in perchlorate solution occurs predominantlyas a mononuclear, bidentate complexation with ferriciron due to the release of sulfate from the substrate.
|
