Y. Takahashi et al., Direct observation of tetravalent cerium in ferromanganese nodules and crusts by X-ray-absorption near-edge structure (XANES), GEOCH COS A, 64(17), 2000, pp. 2929-2935
X-ray-absorption near-edge structure (XANES) was successfully applied for t
he diner determination of Ce valence in hydrogenetic ferromanganese nodules
(two samples) and crust tone sample). Rare earth element (REE) patterns of
the hydrogenetic ferromanganese nodules and crust recovered from the Pacif
ic Ocean show large positive Ce anomalies, especially when normalized to se
awater. Despite some disadvantages in the spectroscopic analysis of Ce in n
atural samples (e.g., low concentration and interference from other element
s), intense synchrotron radiation and measurement of fluorescent XANES with
semiconductor detectors having high energy resolution enabled us to detect
XANES at Ce L-III edge in the ferromanganese nodule or crust. In the XANES
spectra of the nodule or crust, large double peaks were observed, showing
that the oxidation state of Ce in the nodule or crust is predominantly tetr
avalent with little contribution from Ce(III). This is the first in-situ qu
antitative analysis of Ce(IV) among total Ce in natural samples as a trace
element as far as we know. The results obtained by XANES seem to be consist
ent with the percentage of Ce(IV) estimated from seawater-normalized REE pa
tterns, suggesting that seawater is a precursor of REE in the marine deposi
ts. The present results also confirm that Ce enrichment relative to other R
EEs in hydrogenetic ferromanganese deposits is due to oxidative removal of
Ce from seawater to the ferromanganese nodules and crust in the marine envi
ronment. Comparison of XANES spectra of Ce sorbed on synthetic manganese ox
ide and CeO2 vs. those of the ferromanganese nodules and crust suggests tha
t the initial reaction of Ce with the ferromanganese nodules and crust is t
he sorption of dissolved Ce(III) to the manganese oxide surface. Copyright
(C) 2000 Elsevier Science Ltd.