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Pantuzzo F.L.,Federal University of Minas Gerais | Pantuzzo F.L.,National Institute of Science and Technology on Minerals Resources | Pantuzzo F.L.,Golder Associates | Santos L.R.G.,Golder Associates | And 2 more authors.
Hydrometallurgy | Year: 2014

The solubility-product constant of an Al(III)-As(V) amorphous phase synthesized at 25 C was determined. The phase was found to dissolve congruently at pH 2.7 and incongruently above this pH value. From the congruent dissolution data corrected by activity coefficients, a solubility-product constant (Ksp) of 10- 18.06 ± 0.05 (25 C, 1 atm) was determined. The standard Gibbs free energy of formation (ΔG f) calculated for this phase was - 2068.37 kJ·mol- 1. The solid was identified by the chemical formula AlAsO4·3.5H2O, according to a combination of chemical and thermogravimetric analyses. The thermodynamic data on amorphous aluminum arsenate is relevant to the investigations of arsenic immobilization in environments where As-Al association occurs, such as those occurring in mining sites, contaminated soils and sediments. © 2014 Elsevier B.V. Source


Freitas E.T.F.,Federal University of Minas Gerais | Freitas E.T.F.,University of Queensland | Freitas E.T.F.,National Institute of Science and Technology on Minerals Resources | Stroppa D.G.,International Iberian Nanotechnology Laboratory | And 8 more authors.
Chemosphere | Year: 2016

The nature of As-Al-Fe co-precipitates aged for 120 days are investigated in detail by High Resolution Transmission Electron Microscopy (HRTEM), Scanning TEM (STEM), electron diffraction, Energy Dispersive X-Ray Spectroscopy (EDS), Electron Energy-Loss Spectroscopy (EELS), and Energy Filtered Transmission Electron Microscopy (EFTEM). The Al present in magnetite is shown to favour As incorporation (up to 1.10 wt%) relative to Al-free magnetite and Al-goethite, but As uptake by Al-magnetite decreases with increasing Al substitution (3.53-11.37 mol% Al). Arsenic-bearing magnetite and goethite mesocrystals (MCs) are formed by oriented aggregation (OA) of primary nanoparticles (NPs). Well-crystalline magnetite likely formed by Otswald ripening was predominant in the Al-free system. The As content in Al-goethite MCs (having approximately 13% substituted Al) was close to the EDS detection limit (0.1 wt% As), but was below detection in Al-goethites with 23.00-32.19 mol% Al. Our results show for the first time the capacity of Al-magnetite to incorporate more As than Al-free magnetite, and the role of Al in favouring OA-based crystal growth under the experimental conditions, and therefore As retention in the formed MCs. The proposed mechanism of As incorporation involves adsorption of As onto the newly formed NPs. Arsenic is then trapped in the MCs as they grow by self-assembly OA upon attachment of the NPs. We conclude that Al may diffuse to the crystal faces with high surface energy to reduce the total energy of the system during the attachment events, thus favouring the oriented aggregation. © 2016 Elsevier Ltd. Source


Duarte G.,Federal University of Minas Gerais | Duarte G.,National Institute of Science and Technology on Minerals Resources | Ciminelli V.,Federal University of Minas Gerais | Gasparon M.,University of Queensland | And 4 more authors.
Understanding the Geological and Medical Interface of Arsenic, As 2012 - 4th International Congress: Arsenic in the Environment | Year: 2012

Arsenic (As) stability in gold mining tailings has been evaluated. The tailings consist of a sulfide flotation concentrate containing predominately pyrite, arsenopyrite and minor scorodite, which have been submitted to cyanidation. This material was disposed of in tanks prepared with an (iron) Fe and aluminium (Al)- enriched oxisol used as a liner and barrier for As remobilization. After approximately 10 y of disposal, both the sulfide tailings and the liner have been collected and analyzed. Sequential extraction analyses were combined with Electronic Microscopy, X-ray Diffraction, and X-ray Absorption Spectroscopy analyses, aiming at identifying the constituents of the tailings and As speciation. The results indicated that arsenic is present in the tailings as arsenopyrite, and also as As(V) species, possibly scorodite. This finding together with the concentration of the remaining sulfur indicates that no significant oxidation of the pyrite/arsenopyrite concentrates has taken place over the ten-year period. Finally, the identification of As(V) in the soil samples combined with measurements of As concentration in the monitoring wells in the surrounding area confirmed the soil's role as an effective chemical barrier to prevent As mobilization and transport to the environment. © 2012 Taylor & Francis Group. Source


Duarte G.,Federal University of Minas Gerais | Duarte G.,National Institute of Science and Technology on Minerals Resources | Ciminelli V.S.T.,Federal University of Minas Gerais | Ciminelli V.S.T.,National Institute of Science and Technology on Minerals Resources | And 10 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

The complexation of aqueous As(III) species on gibbsite was investigated as a function of pH. Theoretical calculations and X-ray absorption fine structure spectroscopy (XAFS) were combined to elucidate the structure of arsenite surface complexes on synthetic gibbsite. Several adsorption sites were evaluated using the self-consistent charge corrected density-functional based tight-binding (SCC-DFTB) method. The formation of bidentate-binuclear, bidentate-mononuclear, monodentate-mononuclear, and monodentate-binuclear complexes by means of both acid-base and non-dissociative mechanisms was studied in detail. The SCC-DFTB calculations showed the bidentate-binuclear/acid-base complex as the most thermodynamically stable geometry for As(III) bonding to gibbsite surface, estimating As-O and As-Al distances of 1.75 and 3.24å, respectively. EXAFS results also demonstrated As(III) complexation to three oxygen atoms in the first shell, at a distance of 1.77å, and to aluminum in the second shell at a distance of 3.21å, characteristic of bidentate-binuclear configuration, at pH 5.0, 7.0 and 9.0. Another As-Al interaction, attributed to the monodentate-binuclear complex due to its distance of 3.49å, was shown from EXAFS results to provide a minor contribution to As(III) sorption on gibbsite. Therefore, results from theoretical calculations and experimental measurements confirmed the occurrence of inner-sphere complexation during the As(III) adsorption on gibbsite, in a pH range of 5-9. Hence, the higher As(III) mobility in the environment, when compared to As(V), was suggested to be related to the protonation of the As(III) adsorbed complexes. This protonation would restore the neutral H 3AsO 3 molecule, which could be then released from the mineral surface. These results might be useful to predict and control arsenic mobility in aqueous environments, particularly where Al oxy-hydroxides are often found. © 2011 Elsevier Ltd. Source

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