Geochemical and mineralogical datasets on waters and stream precipitates from an abandoned mining site: Montevecchio-Ingurtosu district, Rio Irvi (SW Sardinia)

Geochemical modelling data and Powder X-Ray Diffraction data on samples collected along Rio Irvi (Montevecchio-Ingurtosu mining district, SW Sardinia, Italy) are reported in this paper. The data show the results of data processing to calculate water chemical speciation of ions and saturation indices of relevant mineral phases. These data are related with the research article: De Giudici G. et al (2018), Application of hydrologic-tracer techniques to the Casargiu adit and Rio Irvi (SW-Sardinia, Italy): Using enhanced natural attenuation to reduce extreme metal loads, Applied Geochemistry, vol.96, 42–54. The comparison of the calculated saturation indices of relevant Fe-bearing phases with the PXRD data of samples collected along the stream confirm the quality of the SI dataset and the good correlation between the calculations and the observed data. The comparison of this dataset with others can help to deeper understand and quantify the impact of past and current mining activity on water bodies, contributing to implement the scientific background for the application of remediation actions

Magnetic Materials: Novel Monitors of Long-Term Evolution of Engineered Barrier Systems

Most safety cases for the deep geological disposal of radioactive waste are reliant on the swelling of bentonite in the engineered barrier system as it saturates with groundwater. Assurance of safety therefore requires effective monitoring of bentonite saturation. The time- and fluid-dependent corrosion of synthetic magnets embedded in bentonite is demonstrated here to provide a novel and passive means of monitoring saturation. Experiments have been conducted at 70 °C in which neo magnets, AlNiCo magnets, and ferrite magnets have been reacted with saline (NaCl, KCl, CaCl2) solutions and alkaline fluids (NaOH, KOH, Ca(OH)2 solutions; pH = 12) in the presence of bentonite. Nd-Fe-B magnets undergo extensive corrosion that results in a dramatic change from ferromagnetic to superparamagnetic behaviour concomitant with bentonite saturation. AlNiCo magnets in saline solutions show corrosion but only limited decreases in their magnetic intensities, and ferrite magnets are essentially unreactive on the experimental timescales, retaining their initial magnetic properties. For all magnets the impact of their corrosion on bentonite swelling is negligible; alteration of bentonite is essentially governed by the applied fluid composition. In principle, synthetic magnet arrays can, with further development, be designed and embedded in bentonite to monitor its fluid saturation without compromising the integrity of the engineered barrier system itself

PRELIMINARY STUDIES ON BIOPRECIPITATION PROCESSES MEDIATED BY SULFATE REDUCING BACTERIA (SRB) AND METAL IMMOBILIZATION IN MINE IMPACTED ENVIRONMENTS.

Mining activity often leaves a critical legacy represented by huge volumes of mine wastes and residues, usually made up of highly reactive materials, which lead to the mobilization and dispersion of harmful elements in soils and waters. Although these extreme environments are adverse to the development of living organisms, it has been observed that some microorganisms are able to adapt, playing a role in metal mobility, and becoming part of the resilience of the system itself. The Iglesiente and Arburese (SW Sardinia, Italy) mine districts, now abandoned, have been exploited for centuries by mining activities aimed at Pb-Zn extraction from sulfides and non-sulfides (calamine) deposits. Here, biogeochemical barriers naturally occur as an adaptation of the ecosystem to environmental stresses. Studies, from macroscale to microscale, showed that sulfate-reducing bacteria (SRB) may influence metal mobility by mediating the precipitation of secondary authigenic metal sulfides under reducing conditions. Specifically, framboids of Zn sulfides and Fe sulfides have been observed in the sections of stream sediments core characterized by the presence of abundant organic matter, especially residues of vegetal tissues (e.g. roots and stems of Juncus acutus and Phragmites australis). Laboratory-scale experiments were performed to better understand the bioprecipitation processes. For this purpose, anaerobic batch tests were carried out using high polluted mining waters (Zn and sulfate concentrations up to 102 and 103 mg/l, respectively) inoculated with native selected sulfate-reducing bacteria from stream sediments collected in the investigated areas. Dramatic decrease (up to 100%) in Zn and sulfate was observed in solutions. Moreover, scanning electron microscopy - energy dispersive spectroscopy (SEM-EDS) analysis, performed on solids recovered at the end of the experiments, showed the presence of precipitates characterized by a tubular morphology and made up by S and Zn. SRB inocula were studied by next-generation sequencing (NGS) approach, with the aim to compare the microbial diversity of the different SRB communities and to search for indigenous novel metal-tolerant sulfidogenic microorganisms. These findings represent a valuable step forward to plan effective bioremediation strategies for reducing metal mobility and dispersion. Also, bioprecipitation mediated by SRB can have great potentialities for metal recovery and our results can help to develop biomining techniques. The authors acknowledge CESA (E58C16000080003) from RAS and RAS/FBS (F72F16003080002) grants, and the CeSAR (Centro Servizi d'Ateneo per la Ricerca) of the University of Cagliari, Italy, for SEM analysis

Natural attenuation can lead to environmental resilience in mine environment

Four streams flowing in the Iglesiente and Arburese mine districts (SW Sardinia, Italy), exploited for zinc (Zn) and lead (Pb) extraction from sulphides and secondary non-sulphide mineralization (calamine ores), have been studied combining investigations from the macroscale (hydrologic tracer techniques) to the microscale (X-ray powder diffraction, scanning electron microscopy, X-ray absorption spectroscopy). In the investigated area, concerns arise from release of metals to water during weathering of ore minerals and mine-waste. Specifically, Zn is observed at extremely high concentrations (10s of mg/L or more) in waters in some of the investigated catchments. The results from synoptic sampling campaigns showed marked differences of Zn loads, from 6.3 kg/day (Rio San Giorgio) to 2000 kg/day (Rio Irvi). Moreover, natural attenuation of metals was found to occur i) through precipitation of Fe compounds (Fe oxy/hydroxides and “green rust”), ii) by means of the authigenic formation of metal sulphides promoted by microbial sulphate reduction, iii) by metal intake in roots and stems of plants (Phragmites australis and Juncus acutus) and by immobilization in the rhizosphere, and iv) by cyanobacterial biomineralization processes that lead to formation of Zn-rich phases (hydrozincite and amorphous Zn-silicate). The biologically mediated natural processes that lead to significant abatement and/or reduction of metal loads, are the response of environmental systems to perturbations caused from mine activities, and can be considered part of the resilience of the system itself. The aim of this study is to understand the effect of these processes on the evolution of the studied systems towards more stable and, likely, resilient conditions, e.g. by limiting metal mobility and favouring the improvement of the overall quality of water. The understanding of how ecosystems adapt and respond to contamination, and which chemical and physical factors control these natural biogeochemical barriers, can help to plan effective remediation actions

Assessment of origin and fate of contaminants along mining-affected Rio Montevecchio (SW Sardinia, Italy): A hydrologic-tracer and environmental mineralogy study

Hydrologic tracer techniques were applied to Rio Montevecchio (SW Sardinia, Italy), a stream affected by mine drainage, allowing the calculation of discharge and contaminant loads. Discharge along the stream showed a constant increase throughout the 2.7 km-long study reach, up to 13.6 l/s at the last synoptic point. Calculated loads of mine-related constituents were large, reaching values of 1780 kg/day for, 340 kg/day for Zn, 47 kg/day for Fe, and 50 kg/day for Mn. The difference of the cumulative instream metal loads between the first and the last synoptic sampling points indicated gains of 421 kg/day for Zn, 2080 kg/day for, 56 kg/day for Mn, and 50 kg/day for Fe. The source areas critical for contaminants loading were almost all concentrated in the first 800 meters of the stream, with the exception of Pb, whose loading occurs evenly along the whole study reach. Precipitation of secondary minerals along the streambed was responsible for a very high attenuation of Al and Fe loads (66% and 77%) and affected also and Zn loads, though less effectively. Rio Montevecchio has the second highest metal load among the rivers investigated with tracer techniques in SW Sardinia. In comparison with Rio Irvi, which has one order of magnitude higher metal loads, natural attenuation processes limit the loads in Rio Montevecchio. Results are useful to clarify the hydrogeochemical paths involved in the release and attenuation of pollutants, improving our understanding of stream responses to contamination and aiding development of site-specific remediation actions

Development of a novel magnetic monitoring system for Engineered Barriers of Geological Disposal Facilities

The UK Committee on Radioactive Waste Management (CoRWM) recommended, in 2006, that geological disposal coupled with safe and secure interim storage should have been the way forward for the long-term management of the UK’s higher activity wastes. The design of the underground repository contemplates the presence of bentonite plugs to seal access galleries and deposition boreholes and hence the interaction between the clay-based backfill material and the underground water. Remote monitoring of the fluid saturation of the barrier, the waste canisters and of the surrounding subsurface Geological Disposal Facility environment assumes a relevant importance to guarantee the safety of the repository and to increase the confidence and the reliance of the communities living in areas potentially affected by the repository over time. This remote monitoring of the Engineered Barrier System represents a technical challenge due to the unsuitability of some of the traditional geotechnical techniques or to the intrinsic unreliability of many geophysical prospecting techniques in providing information about the evolution of the Thermo-Hydro-Mechanical-Chemical coupling of the system over long timescales up to and including post-closure evolution. In this project, I offer an initial approach to an innovative way of using mineral magnetism, and, in particular, I analyse the possible exploitation of corrosion-induced variations of the magnetic properties of several magnetic materials to monitor water saturation in the Engineered Barrier System and its evolution through time. Initially the reactivity of several natural and synthetic materials is tested under different “extreme” conditions to analyse the feasibility of the research concept and identify the materials more adapt to carry out the job. The effects that the corrosion of the magnetic materials has on the clay matrix is also analysed in detail throughout all the thesis work. The initial tests lead to the identification of specific transitions in the hysteretic behaviour of three of the initial candidates (Nd-Fe-B, AlNiCo and SmCo alloys). These three materials are subsequently tested under conditions closer to a real “evolved” Barrier System, where the groundwater interacts, with cementiferous grout producing hyperalkaline leachates. The final tests consider the temporal evolution (after 4, 8 and 12 months) of the magnetic properties of these materials in a dysoxic environment under imposed fluid-flow. The results show a clear change in the hysteretic properties of the three materials analysed and the feasibility of the monitoring of the Barrier fluid saturation in the short-term. Furthermore, the corrosion of the magnets, under the conditions applied, did not cause formation of non-swelling clays

Following the amphibolite to greenschist metamorphic path through the structural parameters of spinels from amsaga (Mauritania)

We investigated the crystal and structural behavior of several Cr-bearing spinels from the Archean chromitites of Amsaga (Mauritania). The chemical and structural data testified a retrograde metamorphism from amphibolite to greenschist facies, witnessed by relative changes in the amount of all the major oxides (Cr, Al, Mg, Fe2+, Fe3+), the relative proportion of Fe3+/Fetot as well as the structural parameters, including the cell edge and the oxygen coordinate. The general agreement between electron microprobe and Mössbauer data indicates that the analyzed spinels are stoichiometric. The structural data revealed that the oxygen positional parameter of amphibole-bearing samples is the highest observed among Cr-bearing spinels with similar Cr# and Mg#. Consequently, it is suggested that a structural study of detrital Cr-spinels could be important in discriminating an amphibole-chromitite source from an ophiolite source

Sources and dispersion of antimony at the abandoned mine of Su Suergiu (SE Sardinia)

Antimony (Sb) pollution linked to present and past mining activities is a current environmental concern. Typical Sb concentrations in natural unpolluted waters are less than 1 μg/L, but can increase up to 103104 μg/L near mining and smelting sites. This study investigates the processes involved in the Sb release and dispersion in the abandoned Sb mine area of Su Suergiu (SE Sardinia). For this purpose, mineralogical (XRPD) and minerochemical (SEMEDS) analyses have been performed on outcropping rocks, mine wastes and foundry slags; moreover, the Sb contents in waters draining the mine area and surroundings have been determined. Waters flowing at Su Suergiu show slightly alkaline pH and oxidizing conditions, and their chemical composition is Ca(Na)sulfate. Waters of adits, spring and streams sampled in the mine area show variable Sb content, between 48 and 4,020 μg/L (median: 323 μg/L). The drainage waters collected downstream the foundry slag heaps reaches 104 μg/L of Sb concentration, indicating that the residues of Sb metallurgy, especially metallic Sb and Sb2O3 (valentinite/sénarmontite) dumped in the foundry slag heaps, are the main Sb pollution sources at Su Suergiu. Of particular interest is the presence of mopungite, NaSb(OH)6, that acts as a temporary, highly soluble sink for Sb. Waters draining the mine area flow into the Riu Ciurixedda stream, which in turn flows into the Flumendosa River, a major regional source of water for agricultural and domestic uses. After the Riu Ciurixedda confluence, the Sb concentration in the Flumendosa waters increases above drinking water limits set by the World Health Organization (20 μg/L) and European Community (5 μg/L). Results show that Sb(V), stable under oxidizing conditions as dissolved Sb(OH)6 in a wide range of pH, prevails over the less mobile Sb(III) in the studied waters, explaining Sb dispersion up to several kilometers downstream the mine area, where natural Sb attenuation occurred mainly by dilution

Effect of bioprecipitation of secondary minerals mediated by sulphate reducing bacteria (SRB) on metal mobility in mine impacted environment: preliminary data

Mine and related activities are sources of huge volumes of mine wastes, tailings and residues of metallurgy, often characterized by high contents of metals and semimetals such as Zn, Pb, Fe, As, etc. These materials, when exposed to surface/near surface conditions, can be subjected to oxidation processes leading to the mobilization and dispersion of contaminants in soils and waters. Centuries of intensive mine exploitation, mainly addressed to zinc (Zn) and lead (Pb) extraction from sulphide and calamine deposits, left a seriously impacted environment in the Iglesiente and Arburese mine districts (SW Sardinia). Studies performed at the watershed scale, showed high sulphate (SO4 2-) and metal contents (mainly Zn and Fe) in rivers flowing in the area, however significant differences up to three orders of magnitude have been observed among them: from 6 kg/day of Zn load in Rio San Giorgio to 2000 kg/day in Rio Irvi waters. Microscale investigations (X-Ray Powder Diffraction (XRPD), Scanning Electron Microscopy energy dispersive spectroscopy (SEM-EDS), etc.) carried out on streambed sediments, allowed to recognize the presence of biogeochemical barriers, in well-developed hyporheic zone, that significantly affects metals mobility. Of particular interest is the presence of secondary sulphide minerals (e.g., framboidal FeSX and ZnS) the precipitation of which is mediated by sulphate reducing bacteria (SRB), under reducing conditions. With the aim to better understand the bioprecipitation processes, anaerobic batch experiments have been performed at the laboratory scale. Specifically, highly polluted Rio Irvi water (Zn = 550 mg/l) was inoculated with selected native SRB isolated from core sediments sampled along Rio San Giorgio and Rio Naracauli riverbanks. Different SRB microbial communities and novel metal-tolerant sulphidogenic microorganisms have been identified by the next-generation sequencing (NGS) approach. SEM-EDS analysis carried out on solids recovered at the end of experiments, showed the presence of (bio)precipitates having Zn and S composition and tubular morphology. At the same time, up to 100% of Zn removal has been determined by chemical analysis, performed by inductively coupled plasma optical emission spectroscopy (ICP-OES), of recovered solution (Paganin et al., 2021). These findings indicate the effectiveness of SRB in limiting metal mobility, suggest their potential in metal recovery, and highlight the importance of selecting native microbial communities already adapted to extreme environments

Antimony contamination sources and alteration pathways of Sb mineral phases in an abandoned mining area: The role of secondary mopungite [NaSb(OH)6]

Antimony pollution caused by mining activities is a current environmental concern. This study investigates the processes involved in the Sb release and mobility in the abandoned Sb mine of Su Suergiu (SE Sardinia, Italy). Analyses of outcropping rocks, mine wastes and smelting slags by means of X-Ray Powder Diffraction, Scanning Electron Microscopy - Energy Dispersive Spectroscopy and Electron Microprobe – Wavelength Dispersive Spectroscopy provided mineralogical and compositional data which contributed to the discussion about the oxidation pathways of Sb phases. The main Sb sources are metallic Sb and Sb2O3 (valentinite/senarmontite), dumped in the smelting slag heaps as residues of metallurgical processes, and primary stibnite (Sb2S3) found in natural outcrops and mine wastes. These minerals, subjected to weathering processes, release Sb in solution where it is oxidized and remains as dissolved Sb(OH)6- . Carbonates and Na phases, like hydrate NaAl-silicate derived from metallurgical processes, influence the geochemical equilibria of the smelting slag heaps, where the precipitation of the rare mopungite, Na[Sb(OH)6], has been observed. At Su Suergiu, mopungite originates from a dissolution-precipitation process as the last forming mineral of the oxidation pathways, limiting the Sb mobility by bonding the Sb(OH)6- in solution. Among the detected Sb secondary phases (e.g., Sb-oxides, FeSb oxides, etc.), mopungite is the prevalent Sb binder although it acts as a temporary sink because its stability is influenced by the hydrological regime, its solubility, and the water physicochemical parameters. Secondary Sb bearing minerals can control the dispersion of Sb in contaminated area. At Su Suergiu the role of Fe-bearing compounds on Sb mobility is subordinate to that of mopungite due to the specific geochemical conditions related to the metallurgical activities. The relevance of this study arises from the known toxicity of Sb and from its worldwide diffuse mining, that results in the widespread occurrence of Na–Sb-rich residues produced by Sb smelting plants