17:15 〜 18:30
[MZZ46-P04] Local structure of rare earth elements in marine ferromanganese oxides by extended X-ray absorption fine structure analysis
キーワード：希土類元素、広域X線吸収端微細構造、スペシエーション
Introduction
Marine ferromanganese oxides (MFMO), which is a mixture of nanoparticles of iron oxides and manganese oxides, formed as nodules or crusts at seafloor has been also of great interest as promising materials as metal resources. Among various elements enriched to MFMO, total abundance of rare earth elements (REE) enriched to MFMO reachs 2000 mg/kg. However, the extraction of REE other than by acidic solutions has not been reported for MFMO. In particular, high extraction rate of the target ions by aqueous solution at circumneutral pH is preferred, considering its possible influence on environment and ease to handle it for the refining processes.
In this study, speciation of REE, or yttrium (Y), in MFMO was conducted by extended X-ray absorption fine structure (EXAFS) analysis to reveal REE species adsorbed at the surface of MFMO or incorporated into MFMO. The species is highly important to understand their enrichment to the materials and also their extraction rate under various conditions. More specifically, formation of inner- and outer-sphere complexes at the solid-water interface is a most important factor controlling the extraction rate.
Sample and experimental
Typical marine ferromanganese nodules (D535) at the seafloor (water depth: 5222 m) employed in this study was collected in the Central Pacific, which is most likely formed by precipitation directly from seawater. This hydrogenetic process was also supported by iron and manganese ratio and other geochemical and mineralogical characteristics reported previously (Takahashi et al., 2007). Total REE abundances in D535 were 1920 mg/kg (Y: 130 mg/kg). EXAFS for Y at K-edge was employed to clarify the local structure of Y, or REE, in D535 at beamline BL01B1 in SPring-8.
Results and discussion
EXAFS spectra in k space for hydrated Y3+, Y adsorbed on MnO2 and goethite, and Y in D535 showed that the spectrum of Y in D535 is very similar to that of Y adsorbed on MnO2. More quantitative analysis was conducted by the fitting of each shell in the spectra in R space for Y on MnO2 and Y in D535 using backscattering and phase shift parameters given by FEFF. The fitting results quantitatively revealed that Y forms inner-sphere complex on MnO2. It is suggested thta inner-sphere complexation of Y3+ in MnO2 is essential as the host phase of trace elements under seawater condition with high salinity, because high ionic strength inhibits formation of outer-sphere complex as adsorbed species due to the ion-exchange reaction with other abundant cations in seawater.
Marine ferromanganese oxides (MFMO), which is a mixture of nanoparticles of iron oxides and manganese oxides, formed as nodules or crusts at seafloor has been also of great interest as promising materials as metal resources. Among various elements enriched to MFMO, total abundance of rare earth elements (REE) enriched to MFMO reachs 2000 mg/kg. However, the extraction of REE other than by acidic solutions has not been reported for MFMO. In particular, high extraction rate of the target ions by aqueous solution at circumneutral pH is preferred, considering its possible influence on environment and ease to handle it for the refining processes.
In this study, speciation of REE, or yttrium (Y), in MFMO was conducted by extended X-ray absorption fine structure (EXAFS) analysis to reveal REE species adsorbed at the surface of MFMO or incorporated into MFMO. The species is highly important to understand their enrichment to the materials and also their extraction rate under various conditions. More specifically, formation of inner- and outer-sphere complexes at the solid-water interface is a most important factor controlling the extraction rate.
Sample and experimental
Typical marine ferromanganese nodules (D535) at the seafloor (water depth: 5222 m) employed in this study was collected in the Central Pacific, which is most likely formed by precipitation directly from seawater. This hydrogenetic process was also supported by iron and manganese ratio and other geochemical and mineralogical characteristics reported previously (Takahashi et al., 2007). Total REE abundances in D535 were 1920 mg/kg (Y: 130 mg/kg). EXAFS for Y at K-edge was employed to clarify the local structure of Y, or REE, in D535 at beamline BL01B1 in SPring-8.
Results and discussion
EXAFS spectra in k space for hydrated Y3+, Y adsorbed on MnO2 and goethite, and Y in D535 showed that the spectrum of Y in D535 is very similar to that of Y adsorbed on MnO2. More quantitative analysis was conducted by the fitting of each shell in the spectra in R space for Y on MnO2 and Y in D535 using backscattering and phase shift parameters given by FEFF. The fitting results quantitatively revealed that Y forms inner-sphere complex on MnO2. It is suggested thta inner-sphere complexation of Y3+ in MnO2 is essential as the host phase of trace elements under seawater condition with high salinity, because high ionic strength inhibits formation of outer-sphere complex as adsorbed species due to the ion-exchange reaction with other abundant cations in seawater.