Final Report Summary - CORAGEM (COpper isotopes as indicators of Redox processes during Acid mine drainage GEneration and Mitigation)
The generation of mine waters (acid mine drainage, AMD) containing high concentrations of dissolved metals is the most significant environmental problem deriving from the international mining industry. Uncontrolled discharge of AMD can lead to severe environmental impacts on a number of receptors including soils, water bodies, biosphere, humans and infrastructure. Understanding the magnitude and duration of AMD generation and developing effective remediation strategies is a complex technical challenge. The purpose of this project was to unravel poorly understood geochemical processes in mine sites through the application of novel analytical techniques such as the analysis of heavy stable isotopes in combination with synchrotron based X-Ray Absorption Spectroscopy. A particular focus of this project was the analysis of non-traditional metal stable isotopes such as copper, zinc and nickel. Those elements were selected as the focus for this project because they are essential micronutrients for many organisms, important resources to secure economic growth and common contaminants at mine sites with potentially harmful and ecotoxic effects on (microbial) organisms, vegetation and humans. Isotopes of transition and post-transition elements have been applied in environmental systems such as mineral deposits, chondrites and water bodies. However, the number of studies relating to mine drainage generation, mitigation and remediation have so far been minimal. The main outcomes of this research programme will be highlighted in continuation. For this purpose three selected studies conducted during the fellowship will be explained in detail.
Project I
Accelerated oxidative weathering in a reaction cell was performed over a 33 week period on well characterized, sulfidic mine waste from the Kidd Creek Cu-Zn volcanogenic massive sulfide deposit, Canada. The ASTM D 5744 standard protocol was complemented with the analysis of stable isotope ratios of zinc. Filtered zinc concentrations (< 0.45 µm) in the leachate ranged between 4.5 mg L-1 and 1.9 g L-1 - potentially controlled by pH, mineral solubility kinetics and (de)sorption processes. The zinc stable isotope ratios varied mass-dependently within +0.05 and +0.26 ‰ amu-1 relative to IRMM 3702, and were strongly dependent on the pH (rpH-d66Zn = 0.65 p < 0.005 n = 31). At a pH below 5, zinc mobilization was governed by sphalerite oxidation and hydroxide dissolution – pointing to the isotope signature of sphalerite (+0.05 to +0.08 ‰ amu-1). Over a period characterized by a pH value pH = 6.1 ± 0.6 isotope ratios were significantly more enriched in 66Zn with an offset of ≈ 0.115 ‰ amu-1 compared to sphalerite, suggesting zinc release may have been derived from a second zinc source, such as carbonate minerals, which compose 8 wt. % of the tailings. This study confirms the benefit of applying zinc isotopes alongside standard monitoring parameters to track principal zinc sources and weathering processes in complex multi-phase matrices.
Project II
Leachate from an instrumented pilot-scale waste-rock pile at the Diavik Diamond mine, Northwest Territories, was monitored. The well-characterized waste rock consists of granite, pegmatitic granite and biotite schist with an average total sulfur and carbonate carbon concentration of 0.053 and 0.027 wt. %, respectively. The leachate emerging from the southern basal drain of the waste rock pile has been monitored since 2007. The zinc stable isotope footprint was characterized alongside standard monitoring parameters during two field seasons, May to November 2011 and 2012. The pH ranged between 4.3 and 6.8 and carbonate alkalinity was low or undetectable (< 35 mg L-1 CaCO3 eq). The pH was governed by the oxidation of sulfide minerals and the dissolution of primary carbonate minerals and secondary Al and Fe oxyhydroxysulfates and hydroxides. Dissolved Al and Fe concentrations averaged 6.78 mg L-1 and 175 µg L-1, respectively. The main processes controlling Zn concentrations in the range of 0.4 and 4.7 mg L-1 (average = 2.2 mg L-1) were the oxidative dissolution of sphalerite (ZnS) and the attenuation by secondary Fe and Al hydroxides. Zinc isotopes were fractionated mass dependently. Zinc isotope ratios, ranging between -0.08 and +0.09 ‰ amu-1 (average = +0.025 ‰ amu-1, n = 43) were consistent with values reported for sphalerite from other deposits. The deviations in isotope ratios (Δ = 0.18 ‰ amu-1) were significant in comparison to analytical uncertainties (0.03 ‰ amu-1). Zinc isotope ratios and concentrations were largely uncorrelated, suggesting that the processes affecting Zn mobility had little or no impact on the Zn isotope signature. These data suggest that the Zn isotope ratios of the waste-rock leachate may be used as a fingerprint to track anthropogenic, mine-derived Zn sources in an environment under fluctuating pH, temperatures and ionic strengths.
Project III
Two test cells (TC4, TC7) containing tailings of the Zn-Ag-Au-Pb massive sulfide – sedimentary exhalative deposit Greens Creek, Alaska, were amended with organic substrate to promote sulfidogenesis and limit metal mobilization from the tailings through precipitation as secondary mineral sulfides. The pore waters of the cells were monitored for zinc isotopic ratios and compared to those of a control cell (TC2) without amendment. The upper, oxygenated horizons of the three cells showed signs of enhanced mineral sulfide oxidation with pH ranging between 6.9 to 7.1 and zinc concentrations of up to 324 mg L-1. Isotopic ratios of zinc in this horizon were similar within ±0.05 ‰ (δ66ZnIRMM) in the three cells. As to the limited isotopic fractionation occurring during sphalerite oxidation, an isotopic bulk average value of +0.3 ‰ is proposed. Despite the significantly lower zinc concentrations (1 to 7.2 mg L-1) in the reducing horizon of the control cell (oxidizing horizon: 97 mg L-1), limited isotope fractionation (Δmax = 0.19 ‰) was observed and was potentially controlled by a preferential uptake of heavy zinc isotopes in secondary carbonates. Fractionation of up to 0.35 ‰ was observed in the reducing horizon of the test cells directly underneath the oxidizing horizon at a depth of enhanced microbial activity. We suggest that the increased microbial production of bicarbonate led to an enhanced precipitation of 66ZnCO3 and a concomitant enrichment of 64Zn in the pore waters. In addition, the higher concentrations of hydrogen sulfide in TC4 may have led to an increased isotope fractionation in this cell through a preferential uptake of 64Zn in HS- complexes compared to TC7. In the horizon below, where acid producing bacteria showed maximum abundance, slightly higher zinc concentrations (1 to 2.7 mg L-1) paired with heavier isotope ratios were observed in the pore waters. We suggest, that this increase may be connected to a reductive dissolution of iron hydroxides causing a release in sorbed and co-precipiatated zinc potentially enriched in 66Zn leading to a shift in isotope ratios to values approaching those observed in the oxidizing horizon.
The results of the projects were presented on international conferences and published (or in process to be published) in international scientific journals. The detailed geochemical, microbiological and mineralogical characterization of metal-rich solid and liquid phases from mine wastes paired with reactive transport modelling conducted as part of this research has enabled us to draw more conclusions on principal reaction pathways to occur in these environments that we were unable to described before. The studies led to a better understanding of metal mobilization and attenuation processes that will be a significant contribution to trace environmental pollution deriving from the metal mining industry in the future.
Project I
Accelerated oxidative weathering in a reaction cell was performed over a 33 week period on well characterized, sulfidic mine waste from the Kidd Creek Cu-Zn volcanogenic massive sulfide deposit, Canada. The ASTM D 5744 standard protocol was complemented with the analysis of stable isotope ratios of zinc. Filtered zinc concentrations (< 0.45 µm) in the leachate ranged between 4.5 mg L-1 and 1.9 g L-1 - potentially controlled by pH, mineral solubility kinetics and (de)sorption processes. The zinc stable isotope ratios varied mass-dependently within +0.05 and +0.26 ‰ amu-1 relative to IRMM 3702, and were strongly dependent on the pH (rpH-d66Zn = 0.65 p < 0.005 n = 31). At a pH below 5, zinc mobilization was governed by sphalerite oxidation and hydroxide dissolution – pointing to the isotope signature of sphalerite (+0.05 to +0.08 ‰ amu-1). Over a period characterized by a pH value pH = 6.1 ± 0.6 isotope ratios were significantly more enriched in 66Zn with an offset of ≈ 0.115 ‰ amu-1 compared to sphalerite, suggesting zinc release may have been derived from a second zinc source, such as carbonate minerals, which compose 8 wt. % of the tailings. This study confirms the benefit of applying zinc isotopes alongside standard monitoring parameters to track principal zinc sources and weathering processes in complex multi-phase matrices.
Project II
Leachate from an instrumented pilot-scale waste-rock pile at the Diavik Diamond mine, Northwest Territories, was monitored. The well-characterized waste rock consists of granite, pegmatitic granite and biotite schist with an average total sulfur and carbonate carbon concentration of 0.053 and 0.027 wt. %, respectively. The leachate emerging from the southern basal drain of the waste rock pile has been monitored since 2007. The zinc stable isotope footprint was characterized alongside standard monitoring parameters during two field seasons, May to November 2011 and 2012. The pH ranged between 4.3 and 6.8 and carbonate alkalinity was low or undetectable (< 35 mg L-1 CaCO3 eq). The pH was governed by the oxidation of sulfide minerals and the dissolution of primary carbonate minerals and secondary Al and Fe oxyhydroxysulfates and hydroxides. Dissolved Al and Fe concentrations averaged 6.78 mg L-1 and 175 µg L-1, respectively. The main processes controlling Zn concentrations in the range of 0.4 and 4.7 mg L-1 (average = 2.2 mg L-1) were the oxidative dissolution of sphalerite (ZnS) and the attenuation by secondary Fe and Al hydroxides. Zinc isotopes were fractionated mass dependently. Zinc isotope ratios, ranging between -0.08 and +0.09 ‰ amu-1 (average = +0.025 ‰ amu-1, n = 43) were consistent with values reported for sphalerite from other deposits. The deviations in isotope ratios (Δ = 0.18 ‰ amu-1) were significant in comparison to analytical uncertainties (0.03 ‰ amu-1). Zinc isotope ratios and concentrations were largely uncorrelated, suggesting that the processes affecting Zn mobility had little or no impact on the Zn isotope signature. These data suggest that the Zn isotope ratios of the waste-rock leachate may be used as a fingerprint to track anthropogenic, mine-derived Zn sources in an environment under fluctuating pH, temperatures and ionic strengths.
Project III
Two test cells (TC4, TC7) containing tailings of the Zn-Ag-Au-Pb massive sulfide – sedimentary exhalative deposit Greens Creek, Alaska, were amended with organic substrate to promote sulfidogenesis and limit metal mobilization from the tailings through precipitation as secondary mineral sulfides. The pore waters of the cells were monitored for zinc isotopic ratios and compared to those of a control cell (TC2) without amendment. The upper, oxygenated horizons of the three cells showed signs of enhanced mineral sulfide oxidation with pH ranging between 6.9 to 7.1 and zinc concentrations of up to 324 mg L-1. Isotopic ratios of zinc in this horizon were similar within ±0.05 ‰ (δ66ZnIRMM) in the three cells. As to the limited isotopic fractionation occurring during sphalerite oxidation, an isotopic bulk average value of +0.3 ‰ is proposed. Despite the significantly lower zinc concentrations (1 to 7.2 mg L-1) in the reducing horizon of the control cell (oxidizing horizon: 97 mg L-1), limited isotope fractionation (Δmax = 0.19 ‰) was observed and was potentially controlled by a preferential uptake of heavy zinc isotopes in secondary carbonates. Fractionation of up to 0.35 ‰ was observed in the reducing horizon of the test cells directly underneath the oxidizing horizon at a depth of enhanced microbial activity. We suggest that the increased microbial production of bicarbonate led to an enhanced precipitation of 66ZnCO3 and a concomitant enrichment of 64Zn in the pore waters. In addition, the higher concentrations of hydrogen sulfide in TC4 may have led to an increased isotope fractionation in this cell through a preferential uptake of 64Zn in HS- complexes compared to TC7. In the horizon below, where acid producing bacteria showed maximum abundance, slightly higher zinc concentrations (1 to 2.7 mg L-1) paired with heavier isotope ratios were observed in the pore waters. We suggest, that this increase may be connected to a reductive dissolution of iron hydroxides causing a release in sorbed and co-precipiatated zinc potentially enriched in 66Zn leading to a shift in isotope ratios to values approaching those observed in the oxidizing horizon.
The results of the projects were presented on international conferences and published (or in process to be published) in international scientific journals. The detailed geochemical, microbiological and mineralogical characterization of metal-rich solid and liquid phases from mine wastes paired with reactive transport modelling conducted as part of this research has enabled us to draw more conclusions on principal reaction pathways to occur in these environments that we were unable to described before. The studies led to a better understanding of metal mobilization and attenuation processes that will be a significant contribution to trace environmental pollution deriving from the metal mining industry in the future.