DIRECT OBSERVATION OF TETRAVALENT CERIUM IN FERROMANGANESE NODULES AND CRUSTS BY X-RAY-ABSORPTION NEAR-EDGE STRUCTURE (XANES)

Abstract

X-ray-absorption near-edge structure (XANES) was successfully applied for the direct determination of Ce valence in hydrogenetic ferromanganese nodules (two samples) and crust (one sample). Rare earth element (REE) patterns of the hydrogenetic ferromanganese nodules and crust recovered from the Pacific Ocean show large positive Ce anomalies, especially when normalized to seawater. Despite some disadvantages in the spectroscopic analysis of Ce in natural samples (e.g., low concentration and interference from other elements), intense synchrotron radiation and measurement of fluorescent XANES with semiconductor detectors having high energy resolution enabled us to detect XANES at Ce LIII 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 tetravalent with little contribution from Ce(III). This is the first in-situ quantitative 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 consistent with the percentage of Ce(IV) estimated from seawater-normalized REE patterns, suggesting that seawater is a precursor of REE in the marine deposits. The present results also confirm that Ce enrichment relative to other REEs in hydrogenetic ferromanganese deposits is due to oxidative removal of Ce from seawater to the ferromanganese nodules and crust in the marine environment. Comparison of XANES spectra of Ce sorbed on synthetic manganese oxide and CeO2 vs. those of the ferromanganese nodules and crust suggests that the initial reaction of Ce with the ferromanganese nodules and crust is the sorption of dissolved Ce(III) to the manganese oxide surface.