Abstract
Basic ferric sulfate-arsenates [FeSAsOH, Fe(SO4)x(AsO4)y(OH)z·nH2O] were prepared and characterized to study their potential fixation of arsenic in the oxidizing and acidic environment through a dissolution for 330d. The synthetic solids were well-shaped monoclinic prismatic crystals. For the dissolution of the sample FeSAsOH–1 [Fe(SO4)0.27(AsO4)0.73 (OH)0.27·0.26H2O] at 25–45°C and initial pH 2, all constituents preferred to be dissolved in the order of AsO43− > SO42− > Fe3+ in 1–3 h, in the order of SO42− > AsO43− > Fe3+ from 1–3 h to 12–24 h, and finally in the order of SO42− > Fe3+ > AsO43−. The released iron, sulfate, and arsenate existed dominantly as Fe3+/Fe(OH)2+/FeSO4+, HSO4−/SO42−/FeSO4+, and H3AsO40/H2AsO4−, respectively. The higher initial pHs (6 and 10) could obviously inhibit the release of Fe3+ from solid into solution, and the solid components were released in the order of SO42− > AsO43− > Fe3+. The crystal tops were first dissolved, and the crystal surfaces were gradually smoothed/rounded until all edges and corners disappeared. The dissociations were restricted by the Fe-O(H) breakdown in the FeO6 octahedra and obstructed by the OH− and AsO4 tetrahedra outliers; the lowest concentration of the dissolved arsenic was 0.045 mg/L. Based on the dissolution experiment at 25°C and pH 2, the solubility products (Ksp) for the basic ferric sulfate-arsenate [Fe(SO4)0.27(AsO4)0.73 (OH)0.27·0.26H2O], which are equal to the ion activity products (logˍIAP) at equilibrium, were calculated to be -23.04 ± 0.01 with the resulting Gibbs free energies of formation (ΔGfo) of −914.06 ± 0.03 kJ/mol.
Highlights
Contrary to the poorly crystalline Fe(III)AsO4 compounds coprecipitated during usual neutralization of hydrometallurgical effluents [12], the controlled or Journal of Chemistry autoclave processing resulted in the formation of wellcrystallized precipitates [3]. e basic ferric sulfate-arsenate [FeSAsOH, Fe(SO4)x(AsO4)1−x(OH)x·wH2O] [2] was one of the three crystalline ferric sulfate-arsenates, which were detected to crystallize when the arsenic-containing minerals as the raw materials were treated in the autoclave under the hydrothermal condition of the Fe(III)-SO4-AsO4 solution (150–230°C), on which recent characterizations indicated that arsenic was immobilized in the FeSOH [Basic ferric sulfate]–FeSAsOH solid solution [4]
To determine the bulk compositions, 50 mg of each basic ferric sulfate-arsenate was digested in 20 mL 6 M hydrochloric acid that was diluted to 50 mL by using HNO3 solution. e iron, sulfur, and arsenic concentrations were analyzed by a Perkin-Elmer Optima 7000DV inductively coupled plasma-optical emission spectrometer (ICP-OES) with the proper reference standards. e H2O contents were estimated by the mass balance based on the thermal analytic results, which were obtained from 30°C to 1135°C in nitrogen gas using a Netzsch STA 409 thermogravimetric analyzer (TGA)
All of the prepared solids before and after dissolution were studied by an X’Pert PRO X-ray diffractometer (XRD) with Cu-Kα radiation of 1.540598 A (40 mA and 40 kV) in the 2θ range from 5° to 90° at the scan step of 0.0263° and the scan rate of 5.3333°/min and recognized by comparing the recorded XRD spectra to literature references. e functional groups and the morphologies of the basic ferric sulfate-arsenates were analyzed by a Nicolet Nexus 470 Fourier transform infrared spectrophotometer (FT-IR) over the spectral range from 400 to 4000 cm−1 and a Jeol JSM7900F field emission scanning electron microscope (FE-SEM) with an energy dispersive spectrometer (EDS), respectively
Summary
Arsenic is an extremely toxic byproduct of the mining and smelting of precious and nonferrous metals [1,2,3,4,5] and a common metalloid element in mineral feedstocks, which could be mobilized/discharged during the metallurgical operation, and it results in a serious environmental problem [6, 7]. E crystalline “Phase 3” [Fex(SO4, AsO4) (OH)y·nH2O] was a monoclinic polytype of the basic ferric sulfate and it formed at 175–225°C through the isomorphic replacement of AsO4 for SO4 with the OH decrease to keep the charge balances, for example, the Fe(SO4)0.6(AsO4)0.4(OH)0.6 ·0.4H2O solid solution [4], its structure was suggested to be triclinic (pseudoorthorhombic) in the further research. Until now, most researches were carried out mainly on the forming conditions, structural characterization, and leachability of FeSAsOH [2, 4, 13], and little information about the dissolution mechanism, solubility, and stability of the basic ferric sulfate-arsenates is nowadays accessible. E dissolution mechanism, long-term solubility, and stability of the FeSAsOH solids at different solution pHs and temperatures are examined. The crystalline FeSAsOH solids from Fe(III)-SO4-AsO4 solutions are prepared by a simple hydrothermal method. e dissolution mechanism, long-term solubility, and stability of the FeSAsOH solids at different solution pHs and temperatures are examined. e structural and morphological variations of the synthetical FeSAsOH phases before and after dissolution are examined using various instruments; besides, the potential for arsenic fixation is discussed
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
