Characterization of arsenic accumulating minerals in the iron sediments by SR μ-XRF imaging, μ-XAFS and XRD.

Department of Chemistry, Tokyo University of Science* Department of Geosystem Engineering, University of Tokyo, Japan** Research & Utilization Division, JASRI, JAPAN***
â—‹Satoshi Endo* Izumi Nakai* Akiko Hokura* Yasuhiro Kato** Yasuko Terada***

When the arsenic containing minerals are decomposed with mine wastewater during the mining activity, arsenic is leached from the minerals to the environments. Recently, some researchers have found phenomena where arsenic concentration in the wastewater decreased without artificial treatments and the quality of the wastewater could be controlled by natural factors. To understand how arsenic is removed from wastewater, we have analyzed the iron precipitates collected from the floor of water stream by utilizing synchrotron radiation (SR) X-ray techniques and revealed elemental distribution and identified the minerals.
Rocks composed of the iron precipitates were collected from the stream floor located in Gunma pref. The samples were embedded with resin and fixed to the quartz glass plate, and then grounded to 10 microns in thickness. For SEM observation, the samples were further treated by etching with 3% HF and with ultrasonic cleaner for 10 minutes. The samples were analyzed by SR μ-XRF(X-ray fluorescence) imaging and μ-XAFS (X-ray absorption fine structure) at BL37XU, SPring-8. The beam size was ca. 1 x 1μm2. The samples were also subjected to SR-XRD (X-ray diffraction) analysis at BL-4A, Photon Factory and SEM observation.
The SR-μ-XRF imaging showed that high level of arsenic accumulated in the iron precipitates as dots with typical radius of ca. 20 microns. The μ-XAFS results showed that arsenic exists as arsenate form and is coordinated by four oxygen atoms at 1.7 Å. It was observed that arsenic-accumulating points have peculiar spherical structures. Furthermore, SR-XRD patterns showed that these structures consisted of strengite (FePO4.2H2O). Consequently, it has been found that arsenic accumulated in strengite via ion exchange with PO4.