Spectroscopic study on the local structure of sulfate (SO42-) incorporated in scorodite (FeAsO4 center dot 2H(2)O) lattice: Implications for understanding the Fe(III)-As(V)-SO42--bearing minerals formation

The incorporation of sulfate (SO42-) into the scorodite (FeAsO4 center dot 2H(2)O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of SO42- speciation and local structure in scorodite lattice is still lacking. In this study, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), sulfur K-edge X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopic analyses in combination with density functional theory (DFT) calculations were used to determine the local coordination environment of SO42- in the naturally and hydrothermally synthesized scorodite. The SO42- retention in natural scorodite and the effect of pH value and initial Na' concentration on the incorporation of SO42- in synthetic scorodite were investigated. The results showed that trace amounts of SO42- were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that SO42- was homogeneously distributed inside the natural and synthetic scorodite particles, and its content in the synthetic scorodite increased slightly with the initial Na' concentration at pH of 1.2 and 1.8. The FTIR features and XANES results indicated that the coordination number (CN) of Fe0 6 octahedra around SO42- in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 angstrom with an average of similar to 1.47 angstrom, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 angstrom with an average of similar to 3.33 angstrom. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) angstrom with a CNS-Fe = 4 for SO42- in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that SO42- in the scorodite lattice may be in the form of a Fe-2(SO4)(3)-like local structure. The present study is significant for understanding the formation mechanism of scorodite in natural environments and hydrometallurgical unit operations for waste sulfuric acid treatment.