Chemical Speciation of Arsenic-Accumulating Mineral in a Sedimentary Iron Deposit by Synchrotron Radiation Multiple X-ray Analytical Techniques

Abstract

The comprehensive characterization of As(V)-bearing iron minerals from the Gunma iron deposit, which were probably formed by biomineralization, was carried out by utilizing multiple synchrotron radiation (SR)-based analytical techniques at BL37XU at SPring-8. SR microbeam X-ray fluorescence (SR-mu-XRF) imaging showed a high level of arsenic accumulation in the iron ore as dots of ca. 20 microm. Based on SEM observations and SR X-ray powder diffraction (SR-XRD) analysis, it was found that arsenic is selectively accumulated in strengite (FePO4 x 2H2O) with a concentric morphology, which may be produced by a biologically induced process. Furthermore, the X-ray absorption fine structure (XAFS) analysis showed that arsenic in strengite exists in the arsenate (AsO4(3-)) form and is coordinated by four oxygen atoms at 1.68 angstroms. The results suggest that strengite accumulates arsenic by isomorphous substitution of AsO4(3-) for PO4(3-) to form a partial solid-solution of strengite and scorodite (FeAsO4 x 2H2O). The specific correlation between the distribution of As and biominerals indicates that microorganisms seems to play an important role in the mineralization of strengite in combination with an arsenic-accumulating process.

Study of arsenopyrite weathering products in mine wastes from abandoned tungsten and tin exploitations

“…P exists in scorodite by isomorphous substitution of PO 4 3− for AsO 4 3− or by simultaneous incorporation into the crystal lattice, and is mineralogically expressed as a solid solution of scorodite and strengite, though they form only a partial solid-solution series. No continuous solid solution is produced due to the size difference between AsO 4 3− and PO 4 3− [26]. The electron backscatter images also suggest such Table 1 Chemical analyses of the arsenopyrite weathering products in the Barruecopardo mine wastes (Scms, Sc sf , Sc cl and Sccry: scorodite microcrystalline massive, spherulitic, colloform and in crystals, and R-b ph: Red-blackish phases).…”

Antimony(V) Incorporation into Synthetic Ferrihydrite, Goethite, and Natural Iron Oxyhydroxides

“…A few grams of the soil were dispersed in a high purity resin (Eposet, Maruto Co., Ltd), glued onto a glass plate with a wax product, and polished with abrasive α-Al 2 O 3 paper. The epoxy-resin was selected in this study because it was reported to be inert for metal oxidation states (Manceau et al, 2004) and was often used in similar μ-XAFS studies (Reguer et al, 2005;Takahashi et al, 2007;Endo et al, 2008) for embedding natural samples. After polishing to a thickness of about 40 μm, the thin section was removed from the glass plate by dissolving the wax (Takahashi et al, 2007).…”

Structural Incorporation of As5+into Phosphosiderite by a Strengite/Scorodite-like Arrangement

“…1 ). (2) 1 0.00702 (7) 0.00716 (10) 0.00517 (10) 0.00873 (11) 0.00036 (6) 0.00017 (7) 0.00011 (5) P/As -0.08690 (6) 0.14892 (3) 0.18386 (4) 0.9777(16)/ 0.00612 (10) 0.00672 (16) 0.00423 (15) 0.00742 (16) -0.00039 (10) 0.00023 (11) 0.00026 (9) 0.0223 ( 0.0269 (7) 0.0266 (7) 0.0102 (5) 0.0005 (…”

“…[14] Nevertheless, the most likely incorporation mechanism for As 5 + into phosphosiderite remains the homovalent substitution of P 5 + , a mechanism already investigated and demonstrated for strengite. [15] In comparison to heterovalent substitution reactions which rely on balance reactions (e. g., [16,17]), or the incorporation of structural clusters where whole parts of the host structure are replaced (e. g., [18]), homovalent substitutions are considered to be relatively simple. In our study, however, we show that even the homovalent substitution of P 5 + by As 5 + can be relatively complex and best described by a combination of long-and short-range order methods.…”