TITLE
X-Ray Absorption Fine Structure and Time-Differential-Perturbed Angular
Correlation Study of Hafnium(IV) sorbed onto Amorphous Silica*
TEXT
M. A. DENECHE[a], H. GECKEIS[a], C. POHLMANN[a], D. DEGERING[b]
[a]Forschungszentrum Karlsruhe, Institut fur Nukleare
Entsorgungstechnik, P.O. Box 3640, 76021, Karlsruhe, Germany
[b]Technische Universitat Bergakademie Freiberg, Institut fur
Angewandte Physik, Bernhard-von-Cotta-Str. 4, 09596, Freiberg, Germany
Sorption of Hf(IV) onto amorphous silica at pH values between 1 and 7
is investigated by time-differential-perturbed angular correlation
(TDPAC) and x-ray absorption fine structure (XAFS) spectroscopy.
Hafnium(IV) is studied as a chemical analogue for understanding the
speciation of tetravalent actinides[1]. In addition, $^181$Hf is a
frequently investigated TDPAC probe. Results from TDPAC measurements
show that there are two similar surface binding sites for Hf(IV) at
low pH values. At a pH above 3, a third, unspecific surface binding
site for Hf(IV) begins to dominate. The TDPAC signal of all three
sorption sites differs from that for polynuclear hydrolysis product
formed in Hf(IV) solutions at pH > 3 and are, therefore, different
from precipitated hydrolysis species. It is therefore assumed that
sorbed Hf(IV) species are present. XAFS investigations of the
sorption samples are performed on the Hf L$_3$ edge, in order to
elucidate Hf(IV) species structure for the sorption sites and to
detect any presence of polynuclear surface species. Analysis of
extended x-ray absorption fine structure (EXAFS) spectra show that
sorption samples exhibit changing Hf coordination with sample
preparation pH. Changes in the x-ray absorption near-edge structure
(XANES) for the sorbed samples, compared to reference samples,
indicate a direct interaction of Hf(IV) cations with the silica
substrate. Short Hf-O distances obtained from theoretical fits to
EXAFS oscillations also indicate formation of an inner-sphere surface
complex.
*Work supported by the Deutsche Forschungsgemeinschaft
[1]C. Pohlmann, D. Degering, H. Geckeis, W. G. Thies, Hyperfine
Interactions, vol. 121, p. 313 (1999).
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