Characterization of U(VI)-carbonato ternary complexes on hematite: EXAFS and electrophoretic mobility measurements

We have measured U(VI) adsorption on hematite using EXAFS spectroscopy and electrophoresis under conditions relevant to surface waters and aquifers (0 .01 to 10 mu M dissolved uranium concentrations, in equilibrium with air, p H 4.5 to 8.5). Both techniques suggest the existence of anionic U(VI)-carbo nato ternary complexes. Fits to EXAFS spectra indicate that U(VI) is simult aneously coordinated to surface FeO6 octahedra and carbonate (or bicarbonat e) ligands in bidentate fashions, leading to the conclusion that the ternar y complexes have an inner-sphere metal bridging (hematite-U(VI)-carbonato) structure. Greater than or equal to 50% of adsorbed U(VI) was comprised of monomeric hematite-U(VI)-carbonato ternary complexes, even at pH 4.5. Multi meric U(VI) species were observed at pH greater than or equal to 6.5 and aq ueous U(VI) concentrations approximately an order of magnitude more dilute than the solubility of crystalline beta-UO2(OH)(2). Based on structural con straints, these complexes were interpreted as dimeric hematite-U(VI)-carbon ato ternary complexes. These results suggest that Fe-oxide U(VI)-carbonato complexes are likely to be important transporting-limiting species in oxic aquifers throughout a wide range of pH values. Copyright (C) 2000 Elsevier Science Ltd.