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•Cross-scale analytical techniques are utilized to delineate mechanisms of sphalerite weathering in a mine waste-rock pile.•Oxidation causes congruent dissolution of ...sphalerite.•Secondary sequestration of Cu occurs locally on the surfaces of sphalerite.•Zinc and Cd migration is controlled by pH-dependent reactions on the surfaces of Fe-oxhydroxides.•Continuous release of Zn and Cd is expected in the seepage in absence of remedial efforts.
Degraded water quality and environmental impacts caused by weathering of sulfide-bearing mine wastes are a legacy remaining at many historical mine sites. Release and mobilization of toxic metals (e.g., Zn and Cd) in mine drainage are often associated with weathering of primary sulfide minerals and precipitation and dissolution of secondary minerals. This study aims to couple field-scale measurements of physicochemical parameters with μm- and nm-scale mineralogical characterization to investigate sphalerite weathering and controls on Zn and Cd migration in an uncovered mine waste-rock pile. Elevated concentrations of Zn and Cd, potentially harmful to the receiving environment, were detected in waste-rock pore water and seepage. A suite of analytical techniques, including scanning electron microscopy – energy dispersive X-ray spectroscopy (SEM-EDS), scanning transmission electron microscopy (STEM) with EDS, electron microprobe analysis (EMPA), and synchrotron-based micro X-ray fluorescence (μ-XRF) mapping and micro X-ray absorption near edge structure (μ-XANES) spectroscopy were utilized in a high-resolution investigation of elemental- and secondary-mineral associations with sphalerite grains. EMPA and μ-XRF elemental maps and STEM investigations show depletion of Zn, Fe, S, and Cd, local enrichment of Cu, and distinct dissolution pits at the margins of sphalerite grains. Using μ-XANES, the speciation of solid phases containing Zn, Cu, Fe, and S indicates the dominance of primary sphalerite with microscopic inclusions of chalcopyrite, along with sparse occurrence of secondary Cu-bearing sulfides formed during weathering. These results suggest sphalerite oxidation leads to congruent dissolution without formation of distinct secondary-mineral coatings. Sphalerite weathering significantly contributes to elevated aqueous concentrations of Zn and Cd. Field-scale observations and mineralogical investigations indicate common occurrence of Fe-oxyhydroxides in the waste-rock pile. Seepage chemistry and surface complexation modeling results suggest that pH-dependent variations in reactions on the surfaces of the Fe-oxyhydroxides control aqueous concentrations of Zn and Cd, and the Zn/Cd ratio in the seepage. This work highlights the importance of understanding the mechanisms of sulfide-mineral oxidation and subsequent surface precipitation and adsorption, such that appropriate remediation action can be implemented to limit mobilization of metals to sensitive ecosystems.
Galena and Pb-bearing secondary phases are the main sources of Pb in the terrestrial environment. Oxidative dissolution of galena releases aqueous Pb and SO4 to the surficial environment and commonly ...causes the formation of anglesite (in acidic environments) or cerussite (in alkaline environments). However, conditions prevalent in weathering environments are diverse and different reaction mechanisms reflect this variability at various scales. Here we applied complementary techniques across a range of scales, from nanometers to 10 s of meters, to study the oxidation of galena and accumulation of secondary phases that influence the release and mobilization of Pb within a sulfide-bearing waste-rock pile. Within the neutral-pH pore-water environment, the oxidation of galena releases Pb ions resulting in the formation of secondary Pb-bearing carbonate precipitates. Cerussite is the dominant phase and shannonite is a possible minor phase. Dissolved Cu from the pore water reacts at the surface of galena, forming covellite at the interface. Nanometer scale characterization suggests that secondary covellite is intergrown with secondary Pb-bearing carbonates at the interface. A small amount of the S derived from galena is sequestered with the secondary covellite, but the majority of the S is oxidized to sulfate and released to the pore water.
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•Cross-scale techniques are integrated to investigate galena oxidation.•Synchrotron-based measurements indicate the formation of covellite on the galena surface.•Lead released by galena oxidation accumulates as cerussite and shannonite in the neutral-pH environment.•Secondary covellite occurs intergrown with Pb-bearing carbonates on the galena surface.•Increasing effluent Pb concentrations are likely over the long term in the absence of remedial actions.
Extracting resources from abandoned mines Bao, Zhongwen; Ptacek, Carol J.; Blowes, David W.
Science (American Association for the Advancement of Science),
08/2023, Letnik:
381, Številka:
6659
Journal Article
Recenzirano
Recovering minerals and metals from abandoned mines could aid decarbonization
The development of environmentally friendly technologies (e.g., electric vehicles, wind turbines, solar panels, and ...lithium-ion batteries) to achieve a low-carbon future will require large quantities of critical minerals and metals (
1
). Mining and extraction of these materials are anticipated to generate substantially larger volumes of mine wastes, including waste rock and tailings (
2
). The release of contaminated water from mine wastes can cause long-term environmental damage and land degradation, posing challenges for pollution control and environmental remediation. Resource extraction from contaminated water, mine wastes, and mine workings (e.g., unexploited open pits and underground tunnels) at abandoned mines could potentially address the increased demands of critical minerals and metals for decarbonization. However, careful planning is required to ensure that negative environmental effects are not exacerbated during resource recovery at abandoned mines.
Parallel Magnetic Resonance (MR) imaging is a well-established acceleration technique based on the spatial sensitivities of array receivers. Eigenvector-based SPIRiT (ESPIRiT) is a new parallel MR ...imaging reconstruction method that combines the advantages of the SENSE and GRAPPA methods. It estimates multiple sets of the sensitivity maps from the calibration matrix that is constructed from the auto-calibration data. To improve the quality of the reconstructed image, we introduced the Total Variation (TV) and ℓ
pseudo-norm Joint TV (ℓ
JTV) regularization terms to the ESPIRiT model for parallel MR imaging reconstruction, which were solved by using the Operator Splitting (OS) method. The resulting denoising problems with the TV and ℓ
JTV regularization terms were solved by exploiting the Majorization Minimization method. Simulation experiments on two in vivo data sets demonstrated that the proposed OS algorithm with the TV regularization term (OSTV) and OS algorithm with the ℓ
JTV regularization term (OSℓ
JTV) outperformed the conventional method with the ℓ
regularization term in terms of SNR and NRMSE. And the OSℓ
JTV algorithm was slightly superior to the OSTV algorithm with the TV regularization term.
Parallel Magnetic Resonance (MR) imaging is a well-established acceleration technique based on the spatial sensitivities of array receivers. Eigenvector-based SPIRiT (ESPIRiT) is a new parallel MR ...imaging reconstruction method that combines the advantages of the SENSE and GRAPPA methods. It estimates multiple sets of the sensitivity maps from the calibration matrix that is constructed from the auto-calibration data. To improve the quality of the reconstructed image, we introduced the Total Variation (TV) and ℓp pseudo-norm Joint TV (ℓpJTV) regularization terms to the ESPIRiT model for parallel MR imaging reconstruction, which were solved by using the Operator Splitting (OS) method. The resulting denoising problems with the TV and ℓpJTV regularization terms were solved by exploiting the Majorization Minimization method. Simulation experiments on two in vivo data sets demonstrated that the proposed OS algorithm with the TV regularization term (OSTV) and OS algorithm with the ℓpJTV regularization term (OSℓpJTV) outperformed the conventional method with the ℓ1 regularization term in terms of SNR and NRMSE. And the OSℓpJTV algorithm was slightly superior to the OSTV algorithm with the TV regularization term.
Tracking the S oxidation pathway in sulfide-bearing mine waste-rock piles is complicated by variations in water content, O2 and Fe3+ concentrations, microbial diversity, mineralogy, the occurrence of ...a variety of S species associated with incomplete oxidation, and nonlinear coupling between physicochemical processes. Synchrotron-based S K-edge X-ray absorption near edge structure (XANES) spectroscopy facilitates the identification and quantification of S species (i.e., S2−, S22−, S8, S2O32−, and SO42−) in a weathered mine waste-rock pile containing pyrite, sphalerite, galena, pyrrhotite, and chalcopyrite. Mineralogy-dependent polysulfide and thiosulfate pathways both affect the S oxidation within the waste-rock pile. Currently, the polysulfide pathway, with S8 as the intermediate S species, generates high concentrations of dissolved metals (from the rapid oxidation of monosulfides including sphalerite, galena, pyrrhotite, and chalcopyrite). As these monosulfides are depleted, the thiosulfate pathway of pyrite oxidation, including intermediate oxidation products (i.e., S8 and S2O32−), will become the dominant oxidation pathway, resulting in the potential for generation of additional acidic drainage. This information highlights the importance of identifying the sulfide-mineral oxidation reaction pathways when attempting to estimate acid generation potential, and when developing remediation strategies for the storage and management of sulfide-bearing waste rock.
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•Geostatistical analyses of C and S contents in waste-rock piles at three mine sites.•Log-normal distributions of C and S contents persist in waste-rock piles at different ...scales.•Carbonate and sulfide minerals in waste-rock piles are randomly distributed.•Hotspots of greater acid generation potential may exist randomly in waste-rock piles.
Social and environmental consequences associated with sulfide-bearing mine wastes (i.e., waste rock and mill tailings) and acid mine drainage constitute growing threats to sustainable mining and development, biodiversity of ecosystems, and global water quality. Spatial heterogeneity of geochemically important parameters in waste-rock piles is well-known and is important to incorporate in environmental impact assessments; however, quantitative characterization of spatial heterogeneity remains under investigation. Here, we show the integrated results from trans-disciplinary waste-rock studies at three mine sites in northern Canada, conducted over the past 20 years. These studies focus on diverse projects, including an experimental-scale test pile study, characterization of medium-scale stockpiles, and an examination of an operational-scale pile. Through geostatistical analyses, we observed that spatial heterogeneity of geochemically important parameters, including the abundances of carbonate and sulfide minerals, in field-scale waste-rock piles can be represented by random distributions using log-normal scaling. Our findings are foundational for forecasting waste-rock weathering and for environmental risk assessments, and can be applied to enhance the development of long-term waste-rock management strategies and improve reclamation measures of waste-rock piles throughout the world.
Sustained oxidation of sulfide minerals in waste rock generates acid mine drainage for time frames of hundreds to thousands of years. Management to minimize long-term degradation of water supplies ...and ecosystem health requires a thorough understanding of geochemical processes occurring within the waste rock. A comprehensive field investigation was conducted on sulfide and carbonate-rich waste-rock dumps at the Faro Mine Complex, south-central Yukon Territory, Canada. This investigation included installation of three highly instrumented boreholes, in situ measurements of physical and geochemical parameters (e.g., water content, temperature, air permeability, and pore-gas O2 and CO2 concentration), and collection of pore-water, pore-gas, and solid-phase samples. Field and laboratory measurements indicate the waste rock is lithologically and mineralogically segregated, with sulfide-rich (>20 wt% S) waste rock dominating in the lower benches (20–50 mbgs) and carbonate-bearing waste rock dominating in the upper benches and other parts of the dumps. The observed segregation strongly influences spatial variability of geochemical parameters. Three distinct geochemical zones are observed within the waste-rock dumps, including a rapid O2-supply zone, with near-atmospheric concentrations of O2 (0–30 mbgs); a strong O2-depletion zone (30–50 mbgs), characterized by abundant sulfide minerals and significant O2 depletion to <5 vol%; and a thermally-induced O2-enriched zone (50 mbgs to the pre-mining surface), where thermally-driven gas transport results in replenishment of O2 concentrations (7–18 vol%). These observations indicate sulfide-mineral oxidation and pH neutralization via dissolution of carbonate and aluminosilicate minerals influence pore-water pH and concentrations of dissolved sulfate and metal(loid)s. High concentrations of dissolved metal(loid)s (e.g., up to Fe 36,900 mg L−1, Zn 14,200 mg L−1, Pb 6.16 mg L−1, Al 8,750 mg L−1, As 63.6 mg L−1, Mn 2,170 mg L−1, Cd 21.7 mg L−1, and Cr 1.53 mg L−1) and SO4 (up to 114,000 mg L−1) in pore water correlate with extreme values of pH (as low as 1.4). In the absence of an effective remediation strategy, the oxidation of still-abundant un-oxidized sulfide minerals will continue through vast portions of the waste-rock dumps.
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•Lithological segregation of the waste rock affects spatial variability of pore-water geochemistry.•Pore-gas transport is caused by diffusion, wind-induced advection, and thermally driven convection.•Site-specific geochemical investigation can facilitate decision-making related to remediation.
Degraded water quality and environmental impacts caused by weathering of sulfide-bearing mine wastes are a legacy remaining at many historical mine sites. Release and mobilization of toxic metals ...(e.g., Zn and Cd) in mine drainage are often associated with weathering of primary sulfide minerals and precipitation and dissolution of secondary minerals. This study aims to couple field-scale measurements of physicochemical parameters with lm-and nm-scale mineralogical characterization to investigate sphalerite weathering and controls on Zn and Cd migration in an uncovered mine waste-rock pile. Elevated concentrations of Zn and Cd, potentially harmful to the receiving environment, were detected in waste-rock pore water and seepage. A suite of analytical techniques, including scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS), scanning transmission electron microscopy (STEM) with EDS, electron microprobe analysis (EMPA), and synchrotron-based micro X-ray fluorescence (mu-XRF) mapping and micro X-ray absorption near edge structure (mu-XANES) spectroscopy were utilized in a high-resolution investigation of elemental-and secondary-mineral associations with sphalerite grains. EMPA and mu-XRF elemental maps and STEM investigations show depletion of Zn, Fe, S, and Cd, local enrichment of Cu, and distinct dissolution pits at the margins of sphalerite grains. Using mu-XANES, the speciation of solid phases containing Zn, Cu, Fe, and S indicates the dominance of primary sphalerite with microscopic inclusions of chalcopyrite, along with sparse occurrence of secondary Cu-bearing sulfides formed during weathering. These results suggest sphalerite oxidation leads to congruent dissolution without formation of distinct secondary-mineral coatings. Sphalerite weathering significantly contributes to elevated aqueous concentrations of Zn and Cd. Field-scale observations and mineralogical investigations indicate common occurrence of Fe-oxyhydroxides in the waste-rock pile. Seepage chemistry and surface complexation modeling results suggest that pH-dependent variations in reactions on the surfaces of the Fe-oxyhydroxides control aqueous concentrations of Zn and Cd, and the Zn/Cd ratio in the seepage. This work highlights the importance of understanding the mechanisms of sulfide-mineral oxidation and subsequent surface precipitation and adsorption, such that appropriate remediation action can be implemented to limit mobilization of metals to sensitive ecosystems.
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