Structural characterization of U(VI) surface complexes on kaolinite in the presence of humic acid using EXAFS spectroscopy.

To determine the influence of humic acid (HA), pH, and presence of atmospheric CO2 on the sorption of U(VI) onto kaolinite, the structure of the surface complexes was studied by U L III-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The best fits to the experimental EXAFS data were obtained by including two uranium coordination shells with two axial (O ax) and five equatorial (O eq) oxygen atoms at 1.77+/-0.02 and 2.34+/-0.02 A, respectively, and two coordination shells with one Al/Si atom each at 3.1 and 3.3 A. As in the case of the binary system U(VI)-kaolinite, uranium forms inner-sphere surface complexes by edge sharing with aluminum octahedra and/or silicon tetrahe…