Franco Frau

Geochemical controls on arsenic distribution in the Baccu Locci stream catchment (Sardinia, Italy) affected by past mining

Abstract The Baccu Locci stream catchment (Sardinia, Italy) is affected by serious As contamination as a consequence of past mining. The presence of both point and widespread sources of contamination (waste-rock dumps and flotation tailings, respectively) strongly affects surface water chemistry, and produces high As concentrations (hundreds of μg l −1 ) in stream waters. Water chemistry of the Baccu Locci stream changes considerably over a distance of about 10 km as a consequence of various, locally concomitant, processes acting along the stream course: (1) mixing with metal-rich SO 4 waters; (2) dissolution/precipitation of metal-bearing phases; (3) mixing with HCO 3 -dominated lake waters; (4) gypsum dissolution coupled with calcite precipitation; (5) mixing with dilute surface and/or ground waters. In contrast to metals (e.g. Pb, Cu, Zn and Cd), whose dissolved concentrations rapidly decrease downstream of the mined area through (co-)precipitation/adsorption mechanisms, As concentrations tend to gradually increase (up to 0.9 mg l −1 ) along the stream course as far as the alluvial plain, though significant variations are locally observed. This behaviour is mainly due to the higher mobility of As than metals under the near neutral-oxidative conditions occurring in the Baccu Locci stream waters. Results of a leaching test indicate that part of the As contained in the flotation tailings occurs as As(III), which is more mobile and less strongly sorbed than As(V). The As released to the waters by various mechanisms (i.e. release/desorption from the Fe(III)-hydroxides coatings of silicate grains, oxidation of residual arsenopyrite, decomposition of scorodite) tends to remain in solution and to be transported long distances. As a consequence of the widespread presence of highly As-contaminated flotation tailings all over the medium-lower Baccu Locci stream catchment, long-term As contamination is expected.

Rare earth elements in waters from the albitite-bearing granodiorites of Central Sardinia, Italy

With the aim of contributing to the knowledge of the geochemical behaviour and mobility of the rare earth element (REE) in the natural water systems, the ground and surface waters of the Ottana–Orani area (Central Sardinia, Italy) were sampled. The study area consists of albititic bodies included in Hercynian granodiorites. The waters have pH in the range of 6.0–8.6, total dissolved solid (TDS) of between 0.1 and 0.6 g/l, and major cation composition dominated by Ca and Na, whereas predominant anions are Cl and/or HCO3. The pH and the major-element composition of the waters are the factors affecting the concentration of REE in solution. The concentrations of ∑REE+Y in the samples filtered at 0.4 μm vary between 140 and 1600 ng/l, with La of between 14 and 314 ng/l, and Yb of between <6 and 12 ng/l. A negative Ce anomaly, especially marked at high pH, is observed in the groundwaters. The surface waters show lower REE concentrations, which are independent of pH, and negligible Ce anomaly. Speciation calculations, carried out with the EQ3NR computer program, showed that the complexes with the CO32− ligand are the dominant REE species at pH in the range of 6.7–8.6. The REE3+ ions dominate the speciation at pH <6.7 and only in the light REE (LREE). The relative concentrations of REE in water roughly reflect those in the aquifer host rocks. However, when concentrations of REE in water are normalised relative to the parent rocks, a preferential fractionation of heavy REE (HREE) into the water phase can be observed, suggesting the greater mobility and stability of HREE in aqueous solution.

Mineralogical controls on arsenic mobility in the Baccu Locci stream catchment (Sardinia, Italy) affected by past mining

Abstract Mineralogical-chemical techniques (XRD, SEM/EDX, WDXRF) and a sequential selective extraction procedure were applied to mine-waste materials and stream sediments from the Baccu Locci stream catchment (Sardinia, Italy) affected by serious As contamination as a consequence of past mining. Results indicate that solid-state speciation of As is mainly dominated by the presence of Fe(III) hydroxides (arsenical ferrihydrites with various Fe/As molar ratios) occurring as coatings of silicate grains, in which As is contained as sorbed or co-precipitated species. Scorodite (FeAsO4·2H2O) is common too, whereas arsenopyrite is generally subordinate but, owing to its relatively rapid oxidation, environmentally significant. Moreover, some unidentified arsenates of Ca-Fe or K-Fe were also detected. Arsenic contained in these phases is slowly, but continuously, released in relatively small amounts through three main mechanisms: (1) oxidation of residual arsenopyrite to scorodite; (2) decomposition of scorodite into a hydroxide or oxide of Fe(III); (3) desorption/release from Fe(III) hydroxides. Decomposition of the unidentified arsenates is also probable, e.g. Ca-Fe arsenate → calcite + Fe(III) hydroxide + As release. The flotation tailings are widely scattered and distributed in the middle-lower Baccu Locci stream catchment, and represent the most dangerous As-generating contamination source in the study area.

Trace element and stable isotope data from a flowstone in a natural cave of the mining district of SW Sardinia (Italy): evidence for Zn²⁺-induced aragonite precipitation in comparatively wet climatic conditions

INTRODUCTION Many natural caves in the Lower Cambrian limestones and dolostones of SW Sardinia (Italy) have been discovered during mine excavations in several Pb-Zn sulphide deposits. These “mine caves” host a wide variety of speleothems, both from a morphological and mineralogical point of view. Besides the rather uncommon cave minerals such as hemimorphite, hydrozincite, barite and cerussite, most of the speleothems are composed of aragonite and calcite. Another anomaly of the region is that aragonite is found here, in the stability field of calcite, in much larger quantities than in normal karstic settings. Many authors agree that the principal cause of metastable precipitation of aragonite in cave environments is related to the presence of Mg2+ in the percolating fluids (Hill & Forti, 1997 and references

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

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.

Legacy of Cyanide and ARD at a Low-Scale Gold Mine (Furtei, Italy)

A gold deposit was exploited from 1997 to 2003 at Furtei, Sardinia (Italy). Gold and silver were recovered from oxidized ores by cyanidation; copper concentrates were separated from the sulfide ores by flotation. Following unplanned closure, sulfide-rich materials derived from mining and processing residues were left on site. During operation, cyanide solutions were disposed of in a tailings impoundment. The cyanide underwent natural degradation so that by 2011, cyanide concentrations in the tailings impoundment and seeps were below the Italian limit for industrial effluents. However, during the rainy season, sulfide-rich materials in the waste rock dumps produce extreme acidic solutions and concentrations of some dissolved contaminants, especially ammonium, aluminum, arsenic, copper, iron and manganese, still exceed discharge criteria in the tailings impoundment and seeps. Highly contaminated drainage flows downstream from the mine and poses a hazard to agricultural areas.ZusammenfassungBei Furtei auf Sardinien (Italien) wurde zwischen 1997 und 2003 eine Goldlagerstätte abgebaut. Gold und Silber wurden durch Cyanidlaugung der oxidierten Erze gewonnen; Kupferkonzentrate wurden durch Flotation aus den Sulfiderzen separiert. Nach der unplanmäßigen Schließung verblieben sulfidreiche Bergematerialien sowie Aufbereitungsrückstände am Standort zurück. Während des Betriebes wurden die Cyanidlösungen in einen Schlammteich verbracht. Cyanid unterlag dem natürlichen Abbau, sodass in 2011 die Cyanidkonzentrationenen im Schlammteich sowie in den Sickerwässern den italienischen Grenzwert für Industrieabwässer unterschritten. Jedoch bilden die sulfidreichen Bergehalden während der Regenzeit extreme Sauerwässer, sodass die Konzentrationen einiger Wasserinhaltsstoffe, speziell die von Ammonium, Aluminium, Kupfer, Eisen und Mangan, im Schlammteich und in den Sickerwässern noch immer über den Einleitwerten liegen. Der hoch kontaminierte Abstrom des Bergwerks stellt eine Gefahr für landwirtschaftlich genutzte Flächen dar.ResumenUn depósito de oro fue explotado desde 1997 hasta 2003 en Furtei, Cerdeña (Italia). Oro y plata fueron recuperados de minerales oxidados por cianuración; los concentrados de cobre fueron separados por flotación. Tras el cierre no planificado, los materiales ricos en sulfuros derivados de los residuos de las operaciones mineras y de procesamiento, fueron dejados en el sitio. Durante la operación, las soluciones cianuradas fueron dispuestas en embalses de relaves. El cianuro fue sometido a degradación natural de modo que hacia 2011, las concentraciones de cianuro en los embalses de relaves y en los filtrados, fueron menores que los limites italianos para efluentes industriales. Sin embargo, durante la estación lluviosa, los minerales ricos en sulfuros en las escombreras de mineral estéril, producen soluciones extremadamente ácidas y con concentraciones especialmente de amonio, aluminio, arsénico, cobre, hierro y manganeso que exceden los criterios de descargo en los embalses de relaves y en las filtraciones. Ese drenaje altamente contaminado flujo corriente abajo desde la mina y representa un peligro para las áreas agrícolas.抽象意大利撒丁岛Furtei金矿开采时间为1997-2003年。该金矿采用氰化物氧化含金矿物的方法实现金、银提取,通过浮选法从硫化物中提取铜。当金矿闭坑时,采矿和选矿过程中产生的富含硫化物废矿石遗留于采场,残留氰化物溶液仍被圈闭于尾矿坝。至2011年,氰化物经过自然降解在尾矿坝渗出液中的浓度已经降至意大利工业废水排放标准以下。但是,在雨季,富含硫化物废矿石仍能产生强酸废水,尾矿坝渗出液溶解性污染物如铵、铝、砷、铜、铁和锰等仍然超标。从矿区向下游排放的高度污染废水对下游农业构成严重威胁。

Enargite by XPS

X-ray photoelectron spectroscopy was used for characterizing the enargite surface. Freshly cleaved samples were analyzed at liquid nitrogen temperature. Enargite is a copper arsenic sulfide of formula Cu3AsS4; it is used as a minor ore of copper. Enargite is a potential source of arsenic and may create environmental problems through the release of toxic elements upon oxidation.

Otavite from Montevecchio, Sardinia, Italy

Abstract The rare mineral otavite CdCO3 was found at the abandoned Pb-Zn mine of Montevecchio (SW Sardinia, Italy). Otavite occurs in the oxidised gossan, in association with aurichalcite. SEM imaging shows that otavite is composed of lenticular hollow structures containing aggregates of tiny rhombohedral crystals. Oxidation of primary Cd-bearing sulphides (sphalerite and greenockite) releases up to 24 ppm of Cd to drainage waters of the Montevecchio area. These waters are generally saturated with respect to otavite. Therefore, this mineral might play an important role in controlling Cd mobility in the environment.

Acid production by FeSO4•nH2O dissolution: Comment

Abstract Dissolution of natural and commercial melanterite generates a significant decrease in pH that is not thermodynamically explained by Fe2+ hydrolysis. It has been recently proposed that the production of acidity is actually caused by hydrolysis of Fe3+ occurring in trace amounts in melanterite. Following this finding, the experiments of melanterite dissolution previously conducted by the author have been reviewed and modeled with PHREEQC. Without invoking oxidation of Fe2+ to Fe3+, modeling results indicate that the amount of Fe3+ needed to significantly decrease solution pH is low (0.16-0.20 wt%) and already contained in melanterite in the form of contaminant

The Impact of Saharan Dust on Trace Metal Solubility in Rainwater in Sardinia, Italy

A set of trace metals (Si, Al, Fe, Mn, Sr, Ba, Rb, Co, Ni, V, Zn, Cd, Cu and Pb) was analysed in rainwater from Southern Sardinia. Crust-Poor Rain (CPR) and Crust-Rich Rain (CRR) groups of events were distinguished. The CPR EFcrust value of non-marine strontium (cSr), chosen as the new “cut-off” value, allowed to discriminate crustal metals (Al, Fe, Si, Rb, Mn, Co and Ba) from anthropogenic metals (Cu, Pb, Cd and Zn). The contents of crustal metals in rainwater increase in proportion to their solubility as a consequence of mineral dissolution, while Saharan dust appears to act as a “sink” for some anthropogenic metals in rainwater.