Mineralogical and chemical variations of ochreous precipitates from acid sulphate waters (asw) at the Roşia Montană gold mine (Romania)

Abstract

The mineralogical and chemical variations of ochreous precipitates forming from acid sulphate waters discharged from the lowest mine adit (“Sf. Cruci din Orlea”) of the Roşia Montană Gold Mine (Romania) were investigated by a multianalytical approach (XRPD, IR, TEM, ICP) applied to surface precipitates and associated waters. The mineralogy of the precipitates changed significantly as a consequence of the variations in the chemical parameters of the circulating solutions (mainly pH, Eh, and sulphate concentrations) which were mainly controlled by mixing with unpolluted waters of Roşia River. Ochreous precipitates are characterized by high concentrations of potentially toxic elements (PTEs; in particular Cr, Co, Ni, Cu, Zn, As, Cd, and Pb) and consist of a mixture, in variable proportion, of jarosite and schwertmannite, which represent the stable secondary minerals along the investigated transect of Roşia River. Particular regard is given to the ability of authigenic phases to selectively scavenge selected PTEs from contaminated solutions during their genesis and minerogenetic evolution. Furthermore, laboratory kinetic batch experiments on natural heterogeneous samples of ochreous precipitates were carried out to investigate the release processes involving PTEs and to verify the type and the amount of elements that can be temporarily/permanently trapped by the solid phase from the contaminated solutions. The comparative analysis of the precipitates and waters of the Roşia Montană mining area indicated that the role of secondary minerals as “mitigating agents” can be limited because even minor pH–Eh oscillations would cause mineralogical transformations that could lead to trace elements mobilization in the environment

Chalcopyrite Dissolution at 650 mV and 750 mV in the Presence of Pyrite

“…However, stoichiometric Cu to Fe dissolution of chalcopyrite at 650 mV is expected (Figure 1b), regardless of the presence of pyrite, as this Eh is less than the rest potential of pyrite but greater than that of chalcopyrite [38] and little dissolution of pyrite occurs. These findings are of significance in providing better understanding of the factors affecting dissolution rate of mixed chalcopyrite and pyrite systems in AMD systems within the most often observed Eh range of 500-800 mV (SHE) [39][40][41]. In particular the findings suggest that metalliferous Cu-containing drainage may be of importance in this Eh range and that acidic drainage from pyrite dissolution may be partially inhibited if the Eh remains below pyrite's rest potential.…”

Kinetics and roles of solution and surface species of chalcopyrite dissolution at 650 mV

“…Although a wide range of solution http://dx.doi.org/10.1016/j.gca.2015.04.012 0016-7037/Crown copyright Ó 2015 Published by Elsevier Ltd. All rights reserved. redox potentials (E h ), from negative values to more than 1000 mV (SHE), have been reported for AMD from differing sites (Nordstrom, 2000;Bednar et al, 2005;García et al, 2005;Cheng et al, 2009;Favas, 2013;Hatar et al, 2013), E h in the range of 600-700 mV are most often observed (Costa et al, 2008;Borkowski et al, 2013;Azzali et al, 2014). AMD with pH below 2.0 has also been frequently observed (Dubrovsky et al, 1985;Morin et al, 1988;Alpers et al, 1992;Druschel et al, 2004;Bednar et al, 2005;Costa and Duarte, 2005;Costa et al, 2008;Lu et al, 2011) with, in one extreme case, AMD with negative pH being recorded at Iron Mountain, California (Nordstrom et al, 1999).…”

“…Acidic drainage with pH less than 3 and high sulfate concentrations can dissolve and transport environmentally toxic elements such as Zn, Cr, Cu, Ni, Co, Cd, Pb and As, derived from both sulfide and gangue minerals (Azzali et al, 2014), causing serious downstream environmental degradation. Although a wide range of solution http://dx.doi.org/10.1016/j.gca.2015.04.012 0016-7037/Crown copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.…”

Mineralogical characteristics of sediments and heavy metal mobilization along a river watershed affected by acid mine drainage

“…Researches on the ochreous precipitates from AMD have shown that the deposits contain a diverse group of iron oxy-hydroxide minerals, in particular, the poorly ordered hydroxysulfates (e.g., jarosite and schwertmannite) that are powerful sorbents for trace metals and oxyanions [ 2 ]. Additionally, the release of heavy metals into the groundwater is associated with Fe(III) oxyhydroxide minerals by dissolution or transformation [ 3 – 7 ]. Due to the potential ecological and human health risks [ 8 – 9 ] associated with such a release, the growing awareness of the environmental protection calls for a better understanding of the mineralogical characteristics and transport of metals under extremely acidic conditions of AMD.…”