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

• Published in: Journal of colloid and interface science 2008-03, Vol.319 (1), p.40-47
• Main Authors: Křepelová, Adéla, Reich, Tobias, Sachs, Susanne, Drebert, Jakob, Bernhard, Gert
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Exact sciences and technology; EXAFS; General and physical chemistry; Humic acid; Kaolinite; Sorption; Surface complexes; Surface physical chemistry; Uranium(VI); XPS
• ISSN: 0021-9797; 1095-7103
• DOI: 10.1016/j.jcis.2007.11.010

To determine the influence of humic acid (HA), pH, and presence of atmospheric CO 2 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   Å , respectively, and two coordination shells with one Al/Si atom each at 3.1 and 3.3 Å. 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 tetrahedra. HA and atmospheric CO 2 as well as pH had no influence on the EXAFS structural parameters in the pH range of 5–8. Despite the presence of HA, U(VI) prefers to sorb directly onto kaolinite and not to HA that is bound to the clay surface. X-ray photoelectron spectroscopy (XPS) measurements of kaolinite particles that had been exposed to HA suspensions showed that significant parts of the kaolinite surface are not covered by HA. Raw U L III-edge k 3 -weighted EXAFS data (left) and corresponding Fourier transforms (right) for U(VI) sorbed onto kaolinite under different experimental conditions.