2:30 PM - 2:45 PM
[MTT38-04] Elucidation of the reduction of uranium in biotite by advanced X-ray spectroscopy
Keywords:Transition edge sensor, HERFD-XANES, biotite, uranium
[Introduction] In the old uranium (U) mine of Ningyo-Toge and Tono, biotite had been reported as one of the host phases of U (Honda et al., 1998). However, chemical species and the influence of coexisting elements during U adsorption in biotite have not been discussed. The mechanism of U retention by biotite remains unsolved. It is necessary to determine the chemical species of U in biotite and to clarify the U adsorption sites in biotite. X-ray absorption spectroscopy is a powerful tool to determine the chemical species of trace elements in environmental samples. High-sensitivity measurement of elements up to several ppm in samples is possible by analyzing the X-ray fluorescence when X-rays are incident on samples. However, biotite contains large amounts of rubidium (Rb) as interlayer cations, which interfere with the measurement of U in X-ray fluorescence analysis (XRF). The accurate determination of chemical species and distribution of U in biotite is difficult with a semiconductor detector (SDD). Therefore, (1) we applied high-energy-resolution fluorescence detection-X-ray absorption near-edge structure spectroscopy (HERFD-XANES) using a X-ray emission spectrometer to bulk biotite samples to identify U chemical species in biotite. In addition, (2) The U retention spots in thinned biotite samples were identified by using a Transition Edge Sensor (TES) as a detector in microbeam X-ray fluorescence analysis (μ-XRF).
[Experiments] Biotite samples were collected from boring core samples of an old uranium mine.
(1) The bulk biotite samples were sealed in polyethylene film. Fluorescence XANES measurements using a silicon drift detector (SDD) and HERFD-XANES measurements using a X-ray emission spectrometer were performed at BL39XU, SPring-8.
(2) After biotite was resin-filled, both sides were polished using lapping film to prepare a biotite thin sample with a thickness of about 100 μm. Microbeam X-rays with a beam size of 1 μm were used for the measurement at SPring-8 BL37XU. The SDD and TES detectors were used to obtain the fluorescence X-ray intensities of the U La line at 13.612 keV and the Rb Ka line at 13.395 keV with an incident energy of 17.2 keV.
[Results and Discussion]
(1) Estimation of chemical species in bulk biotite samples by HERFD-XANES
The HERFD-XANES spectra of bulk biotite samples were compared with the conventional XANES spectra using the SDD. The absorption peak of conventional XANES spectrum was not observed by Rb interference, making it difficult to distinguish the chemical species of U in the biotite. The absorption peak of HERFD-XANES spectrum of U in the bulk biotite sample was observed with a high-energy resolution of X-ray emission spectrometer. The chemical species of U in biotite were U(IV) and U(VI), indicating that biotite retained partially reduced U as U(IV).
(2) TES-μ-XRF-XANES analysis of U distribution state in thinned biotite samples
By mapping analysis using microbeam X-rays, U retention spots in the biotite thin sample were detected. It is difficult to separate the U Lα line from the Rb Kα line in the XRF spectrum with an energy resolution of approximately 220 eV of the SDD. On the other hand, using TES, the U Lα line and the Rb Kα line was fully deconvoluted. The accurate U and Rb mapping analysis indicated that the intensity of Rb was low at U retention spots. The chemical species of the U retention spots were analyzed by μ-XRF-XANES, which revealed that U was partially present in reduced species as U(IV). These results suggest that U retention and reduction in biotite proceeds at sites where interlayer cation were lost due to weathering.
[Reference] Honda et al. J. Nucl. Sci. Technol. 31(8), 803-812.
[Experiments] Biotite samples were collected from boring core samples of an old uranium mine.
(1) The bulk biotite samples were sealed in polyethylene film. Fluorescence XANES measurements using a silicon drift detector (SDD) and HERFD-XANES measurements using a X-ray emission spectrometer were performed at BL39XU, SPring-8.
(2) After biotite was resin-filled, both sides were polished using lapping film to prepare a biotite thin sample with a thickness of about 100 μm. Microbeam X-rays with a beam size of 1 μm were used for the measurement at SPring-8 BL37XU. The SDD and TES detectors were used to obtain the fluorescence X-ray intensities of the U La line at 13.612 keV and the Rb Ka line at 13.395 keV with an incident energy of 17.2 keV.
[Results and Discussion]
(1) Estimation of chemical species in bulk biotite samples by HERFD-XANES
The HERFD-XANES spectra of bulk biotite samples were compared with the conventional XANES spectra using the SDD. The absorption peak of conventional XANES spectrum was not observed by Rb interference, making it difficult to distinguish the chemical species of U in the biotite. The absorption peak of HERFD-XANES spectrum of U in the bulk biotite sample was observed with a high-energy resolution of X-ray emission spectrometer. The chemical species of U in biotite were U(IV) and U(VI), indicating that biotite retained partially reduced U as U(IV).
(2) TES-μ-XRF-XANES analysis of U distribution state in thinned biotite samples
By mapping analysis using microbeam X-rays, U retention spots in the biotite thin sample were detected. It is difficult to separate the U Lα line from the Rb Kα line in the XRF spectrum with an energy resolution of approximately 220 eV of the SDD. On the other hand, using TES, the U Lα line and the Rb Kα line was fully deconvoluted. The accurate U and Rb mapping analysis indicated that the intensity of Rb was low at U retention spots. The chemical species of the U retention spots were analyzed by μ-XRF-XANES, which revealed that U was partially present in reduced species as U(IV). These results suggest that U retention and reduction in biotite proceeds at sites where interlayer cation were lost due to weathering.
[Reference] Honda et al. J. Nucl. Sci. Technol. 31(8), 803-812.