Mineralogical control on arsenic release during sediment-water interaction in abandoned mine wastes from the Argentina Puna

The role of efflorescent salts associated with sulfide-rich mine wastes in the short-term cycling of arsenic: Insights from XRD, XAS, and µ-XRF studies

The evaporation of As-rich leachates generated by the weathering of sulfide-rich mine wastes accumulated in abandoned tailing dams of the La Concordia mine, triggers the widespread precipitation of saline crusts and efflorescences. Because these salts are highly soluble, they may release high concentrations of arsenic after rainfall events. Thus, the goal of this work is to assess the solid speciation of As in these efflorescences, which may help to understand the short-term cycling of As in the site. The results reveal that As is present only as As(V), while its capacity to be retained in the salts highly depends on their mineralogical composition. Hydrous sulfates, such as gypsum and epsomite show a very low capacity to scavenge As, while copiapite retains the highest concentrations of this element. The spectroscopic evidences suggest that in this mineral, As(V) is included within the lattice, substituting sulfate in the tetrahedral sites. Because copiapite is highly soluble, it may be considered as one of the most important transient reservoirs of As in the site that can release high concentrations of this hazardous pollutant during the occasional rainfall events produced during the wet season.

Origin, distribution, and geochemistry of arsenic in the Altiplano-Puna plateau of Argentina, Bolivia, Chile, and Perú

Elevated concentrations of arsenic in water supplies represent a worldwide health concern. In at least 14 countries of South America, high levels have been detected relative to international standards and guidelines. Within these countries, the high plateau referred to as the "Altiplano-Puna", encompassing areas of Argentina, Bolivia, Chile, and Perú, exhibits high arsenic concentrations that could be affecting 3 million inhabitants. The origins of arsenic in the Altiplano-Puna plateau are diverse and are mainly natural in origin. Of the natural sources, the most important correspond to mineral deposits, brines, hot springs, and volcanic rocks, whereas anthropogenic sources are related to mining activities and the release of acid mine drainage (AMD). Arsenic is found in all water types of the Altiplano-Puna plateau over a wide range of concentrations (0.01 mg·L^-1 < As in water > 10 mg·L^-1) which in decreasing order correspond to: AMD, brines, saline waters, hot springs, rivers affected by AMD, rivers and lakes, and groundwater. Despite the few studies which report As speciation, this metalloid appears mostly in its oxidized form (As[V]) and its mobility is highly susceptible to the influence of dry and wet seasons. Once arsenic is released from its natural sources, it also precipitates in secondary minerals where it is generally stable in the form of saline precipitates and Fe oxides. In relation to human health, arsenic adaptation has been detected in some aboriginal communities of the Puna together with an efficient metabolism of this metalloid. Also, the inefficient methylation of inorganic As in women of the Altiplano might lead to adverse health effects such as cancer. Despite the health risks of living in this arsenic-rich environment with limited water resources, not all of the Altiplano-Puna is properly characterized and there exists a lack of information regarding the basic geochemistry of arsenic in the region.

Characterization of secondary products in arsenopyrite-bearing mine wastes: influence of cementation on arsenic attenuation

The secondary products of an arsenopyrite-bearing mine waste dump were characterized in order to ascertain their mineralogical, chemical and environmental features and to appraise their role in the abatement of As in the environment. To this purpose, representative surface samples of weathered sulfides (including cemented phases) and hardpan samples were collected and studied by X-ray powder diffraction (XRD), polarized light microscopy, electron microprobe analysis (EMPA), micro-Raman spectroscopy and digestion, extraction and leaching methods. Scorodite, amorphous ferric arsenates (AFA), elemental sulfur, hydronium jarosite, goethite, hematite and hydrous ferric oxides were the secondary products identified in the mine wastes. The hardpan was mainly constituted by gangue minerals, including sulfides (arsenopyrite and pyrite/marcasite) with different weathering degrees, cemented by cracked yellow phases corresponding to AFA with Fe/As molar ratios of 1.14-1.85 and elemental sulfur. These cracked compounds were also the binding agent in the other cemented phases. Hydronium jarosite and Fe (oxyhydr)oxides were enriched in As, showing values of 0.19-3.98 and 0.81-7.49 wt.% As₂O₅, respectively. The As leachability and lability from hardpan and cemented phases were not decreased as compared to those from the other weathered phases not showing cementation in the mine waste dump.

Synchrotron XAS study on the As transformations during the weathering of sulfide-rich mine wastes

This paper describes the weathering processes that occurred across two tailing dumps in the Concordia mine (Puna of Argentina) with the purpose of evaluating the formation of secondary As-bearing minerals due to arsenopyrite alteration. After 30 years of exposure, the gradual oxidation of the wastes produced a number of secondary mineral phases containing As in different chemical arrangements. Synchrotron-based X-ray absorption spectroscopy was used to determine both, As and Fe solid speciation and to identify the formed As-bearing minerals. The results reveal that in the first stages of oxidation, As released from arsenopyrite is adsorbed/substituted in the jarosite structure partially inhibiting its dissolution. When pH values in the system slightly increase As-jarosite transforms into schwertmannite, where the released As could be re-adsorbed or co-precipitated. When the available adsorption sites become oversaturated with As, the precipitation of amorphous ferric arsenates may occur. The latter, likely constitute the more labile As fractions in the sediments and are therefore the main phases contributing As to the nearby environments. These amorphous and labile phases are more abundant in the uppermost layers of the profiles, where oxidation has taken place for a more prolonged time-lapse. The described transformations are enhanced by the acidic pH, the absence of minerals attenuating the acidity and the high sulfate and As concentrations in pore water.

Defects and their behaviors in mineral dissolution under water environment: A review

Mineral dissolution is a spontaneous process that takes indispensible role in the determination of water quality in a specific water body. Deep insights into defects as a result of characterization technique development have greatly improved our understanding of their significances and behaviors in the dissolution within the mineral-water interface. Based on the progresses from previous decades, this review attempts to re-elaborate the molecular-scale process of dissolution. Material flow within the mineral/water interface is updated, with emphasis on the function of defect sites. A brief introduction of defect properties is presented, including the microscopic appearances and typical physicochemical characteristics. Feasible strategies that have been adopted to increase the defect abundance are inferred, which maybe enlightening for hydrometallurgy. The merits and drawbacks of the techniques that could be employed for the qualitative and quantitative determination of defect presence are introduced, although relatively satisfactory performances are noted. With the aid of these techniques, it is concluded that screw dislocation is the main defect type responsible for surface topography evolution as a result of dissolution. Finally, this review identifies the current knowledge gaps and future research needs for comprehensively identifying the significance of defects in mineral dissolution.

Long term metal release and acid generation in abandoned mine wastes containing metal-sulphides

The sulphide-rich mine wastes accumulated in tailing dumps of La Concordia Mine (Puna of Argentina) have been exposed to the weathering action for more than 30 years. Since then, a series of redox reactions have triggered the generation of a highly acidic drainage -rich in dissolved metals-that drains into the La Concordia creek. The extent of metal and acid release in the site was analysed through field surveys and laboratory experiments. Static tests were conducted in order to predict the potential of the sulphidic wastes to produce acid, while Cu-, Zn-, Fe- and Pb-bearing phases present in the wastes were identified by XRD, SEM/EDS analysis and sequential extraction procedures. Finally, the release of these metals during sediment-water interaction was assessed in batch experiments carried out in a period of nearly two years. Field surveys indicate that the prolonged alteration of the mine wastes led to elevated electrical conductivity, pH values lower than 4 and metal concentrations that exceed the guide values for drinking water in the La Concordia stream regardless of the dominating hydrological conditions. The highly soluble Fe and Mg (hydrous)sulphates that form salt crusts on the tailings surfaces and the riverbed sediments play an important role in the control of metal mobility, as they rapidly dissolve in contact with water releasing Fe, but also Cu and Zn which are scavenged by such minerals. Another important proportion of the analysed metals is adsorbed onto Fe (hydr)oxides or form less soluble hydroxysulfates. Metals present in these phases are released to water more slowly, thus representing a potential long term source of heavy metal pollution. The obtained results are a contribution to the understanding of long term metal transformations and mobility in mine waste-impacted sites.