Vibrational spectroscopy study of hydrothermally produced scorodite (FeAsO₄·2H₂O), ferric arsenate sub‐hydrate (FAsH; FeAsO₄·0.75H₂O) and basic ferric arsenate sulfate (BFAS; Fe[(AsO₄)_1−x(SO₄)_x(OH)_x]·wH₂O)

Abstract

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.

Vibrational Spectroscopy of Complex Synthetic and Industrial Products

“…In general, OH stretches at lower wavenumbers are stronger in bond strength (smaller H-bond bond distances) then those at higher wavenumbers. In the case of arseniosiderite two hydroxyl stretches (3100, 3576 cm-1) are observed, almost identical to that found in Scorodite (Hawthorne, 1976;Gomez et al, 2010bGomez et al, , 2011b. These two OH stretches in arseniosiderite are indicative of two distinct types of H-bonding environments (one strong and one weak); yukonite on the other hand exhibits only a diffuse band (3111-3215 cm -1 ) indicative of a disordered type of H-bonding, typical of its glassy disordered nature.…”

“…Elemental composition of the three main phases of interest found for our work ) in comparison to the other previous (Swash & Monhemius, 1994) and "new" reported phases (Dutrizac & Jambor, 2007). (Gomez et al, 2010b. The FTIR spectra of scorodite, Type 1 (same as FAsH) and Type 2 (same as BFAS) using transmission collection mode and KBr pellets (Swash, 1996;Ugarte and Monhemius,1992).…”

“…It is noted here that the mode of collection is very important to consider as different samples give rise to better signals for detection in different IR collection modes. In our work (Gomez et al, 2010b care was taken to analyze these opaque types of powders in both transmission as well was reflection modes; after careful evaluation of the spectra for each of these phases it was decide that the Attenuated Total Reflectance (ATR) mode would give us enough of an energy range as well as the required spectra resolution to be able to extract useful molecular information of these phases. The ATR mode is especially suited for opaque powders, thin films and solutions, while the transmission mode often tends to 'smear' vibrations due to the less light transmission or detection in opaque solids, (i.e.…”

“…The infrared hydroxyl region (3000-4000 cm -1 ) has the advantage of giving us insights into the intermolecular hydrogen bond nature of minerals (Sumin de Portilla, 1974;Hawthorne, 1976). For example in Scorodite, where two distinct types of intra-molecular hydrogen bonding occurs between the crystal water molecules of metal octahedra and the arsenate groups of the form MO -H---OAs; it is possible to relate each specific water (OH) stretching to a particular vibrational band of the two distinct crystallographic water sites (Hawthorne, 1976;Gomez et al, 2010b). The relative strength of H-bonding interactions between the two Raman (left-bottom) and ATR-IR (right-bottom) spectra of yukonite and arseniosiderite is also shown below.…”

“…In these studies, the new phases and results were found to be completely different than in the studies of Swash & Monhemius (1994). Therefore since the true characteristics, conditions/mechanisms of formation, and long term arsenic environmental stability had not been unequivocally established or determined; a series of synthetic, characterization and environmental arsenic stability tests were conducted and published via the use of several techniques with vibrational spectroscopy being particularly important as will be shown below (Gomez et al, 2010b.…”