Spectroscopic study on the local structure of sulfate (S O 4 2 −) incorporated in scorodite (FeAsO4·2H2O) lattice: Implications for understanding the Fe(III)-As(V)-S O 4 2 −-bearing minerals formation.

The incorporation of sulfate ( S O 4 2 − ) into the scorodite (FeAsO4·2H2O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of S O 4 2 − speciation and local structure in scorodite lattice is still lacking. In this study, X‑ray difraction (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 S O 4 2 − in the naturally and hydrothermally synthesized scorodite. The S O 4 2 − retention in natural scorodite and the effect of pH value and initial Na+ concentration on the incorporation of S O 4 2 − in synthetic scorodite were investigated. The results showed that trace amounts of S O 4 2 − were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that S O 4 2 − 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 FeO6 octahedra around S O 4 2 − in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 Å with an average of ~1.47 Å, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 Å with an average of ~3.33 Å. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) Å with a CNS-Fe = 4 for S O 4 2 − in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that S O 4 2 − in the scorodite lattice may be in the form of a Fe2(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. [ABSTRACT FROM AUTHOR]