Three crystalline ferric arsenate phases: (1) scorodite; FeAsO 4 ·2H 2 O, (2) ferric arsenate sub-hydrate (FAsH; FeAsO 4 ·0.75H 2 O) and (3) basic ferric arsenate sulfate (BFAS; Fe[(AsO 4 ) 1−x (SO 4 ) x (OH) x ]·wH 2 O) synthesized by hydrothermal precipitation (175-225 • C) from Fe(III)-AsO 4 3− -SO 4 2− solutions have been investigated via Raman and infrared spectroscopies. The spectroscopic nature of these high-temperature Fe(III)-AsO 43− -SO 4 2− phases has not been extensively studied despite their importance to the hydrometallurgical industrial processing of precious metal (Au and Cu) arsenic sulfidic ores. It was found that scorodite, FAsH and BFAS all gave rise to very distinct arsenate, sulfate and hydroxyl vibrations. In scorodite and FAsH, the distribution of the internal arsenate modes was found to be distinct, with the factor effect being more predominant in the crystal system. For the crystallographically unknown BFAS phase, vibrational spectroscopy was used to monitor the arsenate ↔ sulfate solid solution behavior that occurs in this phase where the molecular symmetry of arsenate and sulfate in the crystal structure is reduced from an ideal T d to a distorted T d or C 2 /C 2v symmetry. With the new collected vibrational data of the pure phases, the use of attenuated total reflectance infrared (ATR-IR) spectroscopy was finally extended to investigate the nature of the arsenate in an industrial residue generated by pressure oxidation of a gold ore, where it was found that the arsenate was present in the form of BFAS.

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The nature of synthetic basic ferric arsenate sulfate (Fe(AsO 4 ) 12x (SO 4 ) x (OH) x ) and basic ferric sulfate (FeOHSO 4 ): their crystallographic, molecular and electronic structure with applications in the environment and energy3

BACKGROUND: Aluminum (III) hydroxy-gels find important applications in areas such as paint pigments, pharmaceuticals and water treatment or toxic metal sequestration. Since the method of preparation may affect their properties and performance, in this work we prepare aluminum hydroxy-gels from either chloride or sulphate salts and subject them to comparative characterization. RESULTS: Aluminum (III) hydroxy-gels were produced by partial quick neutralization of 2 mol L −1 AlCl 3 or Al(SO 4 ) 1.5 salt solutions with 5 N NaOH at room temperature. The gels were found, following ageing and water washing, to consist of 60-70 wt% Al(OH) 3 , 5-18 wt% Cl or SO 4 and ∼20 wt% water. Both gel materials upon drying were seen to be highly porous formed from aggregates of very fine particles nucleated during the fast neutralization process. The Al(SO 4 ) 1.5 -derived gel was found to differ significantly from the AlCl 3 -derived gel both in terms of surface area (38 m 2 g −1 vs. 18 m 2 g −1 ) and chemical features. The aluminum chloride gel material is probably composed of chains of aluminum octahedra (Al n (OH) 2.5 Cl 0.5n (H 2 O) 3n ) while the aluminum sulphate gel of SO 4 -stabilized Keggin Al 13 structure: AlO 4 Al 12 (OH) 24 (SO 4 ) 3.5 (H 2 O) 12 . CONCLUSION: The distinct molecular structure of the aluminum sulphate-derived gel may provide an effective matrix for hazardous metal containment.Figure 9. Schematic representations of (a) the dimer Al 2 (OH) 2 (H 2 O) 8 4+ ion 34 and (b) the Keggin Al 13 structure 33 . Adapted with permission from Inorganic Chemistry.

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