•Dissolved Hg decreases by >90% with high-T biochars (600 and 700°C).•Elevated SO42− (up to 1000mgL−1) is released from manure-derived biochar.•XRF results indicate Hg is distributed heterogeneously ...throughout biochar particles.•S XANES indicates presence of reduced and oxidized S species in biochar.•Hg EXAFS indicate Hg is bound to S atoms in biochar particle when S content is high.
Thirty-six biochars produced from distinct feedstocks at different temperatures were evaluated for their potential to remove mercury (Hg) from aqueous solution at environmentally relevant concentrations. Concentrations of total Hg (THg) decreased by >90% in batch systems containing biochars produced at 600 and 700°C and by 40–90% for biochars produced at 300°C. Elevated concentrations of SO42− (up to 1000mgL−1) were observed in solutions mixed with manure-based biochars. Sulfur X-ray absorption near edge structure (XANES) analyses indicate the presence of both reduced and oxidized S species in both unwashed and washed biochars. Sulfur XANES spectra obtained from biochars with adsorbed Hg were similar to those of washed biochars. Micro-X-ray fluorescence mapping results indicate that Hg was heterogeneously distributed across biochar particles. Extended X-ray absorption fine structure modeling indicates Hg was bound to S in biochars with high S content and to O and Cl in biochars with low S content. The predominant mechanisms of Hg removal are likely the formation of chemical bonds between Hg and various functional groups on the biochar. This investigation provides information on the effectiveness and mechanisms of Hg removal that is critical for evaluating biochar applications for stabilization of Hg in surface water, groundwater, soils, and sediments.
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•Cr(VI) decreased from 50 to <0.02 mg/L by biochar at pH 2 with 8.8% in solution as Cr(III).•Bulk XANES show only Cr(III) in solid phase, whereas micro-XANES show 0–36% Cr(VI).•EXAFS ...indicate removed Cr in Cr(OH)3 form with same atomic structure of goethite.•Confocal µ-XRF redox mapping show only Cr(III) in biochar at early reaction time.•Reaction steps include electrostatic attraction, diffusion, reduction, ion exchange.
Biochar is an effective, environmentally sustainable material for removing Cr(VI) from water. Potential removal mechanisms include surface reactions or reactions within the biochar structure with direct bonding of Cr(VI) or reduction of Cr(VI) and bonding of the reduced Cr forms. Diffusion process and Cr(VI) and Cr(III) distributions in biochar particles have not been elucidated. Aqueous Cr(VI) removal experiments followed by solid-phase analyses were conducted to evaluate the effectiveness of raw and modified oak wood biochar for removing aqueous Cr(VI) and to further determine removal mechanisms. Results showed that concentrations of Cr(VI) decreased from ~50 to <0.02 mg L−1 at initial pH 2 after 1 d using raw oak wood biochar, with 8.8% of the initial aqueous Cr reduced to Cr(III) in the solution. Similarly, effective removal of Cr(VI) was observed using polysulfide-modified biochar; whereas ~54% of initial Cr(VI) was removed using HNO3-treated biochar. Bulk X-ray absorption near-edge structure (XANES) analysis showed Cr is present as Cr(III) within the unmodified biochar, whereas confocal micro-XANES analysis showed the existence of Cr(VI) (0–36%) in selected spots and Cr(0) (43%) in one spot within a biochar sample collected after 30 min. Extended X-ray absorption fine structure (EXAFS) results showed the atomic structure of Cr within the unmodified biochar was similar to Cr(OH)3, with O and Cr in the first and second shells. Confocal micro-X-ray fluorescence imaging (CMXRFI) results indicated total Cr (tCr) was heterogeneously distributed in the imaged area with a higher intensity close to the particle surface. Redox mapping results indicated no Cr(VI) in the unmodified biochar collected at 30 min; Cr(III) was the primary form and also remained close to the surface at later time. The removal mechanisms likely involve electrostatic attraction and diffusion inside the particle, followed by reduction and ion exchange reactions.
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.
The Northwest Tailings Containment Area at the inactive Giant Mine (Canada) contains a complex mixture of arsenic-containing substances, including flotation tailings (84.8 wt%; with 0.4 wt% residual ...S), roaster calcine wastes (14.4 wt% Fe oxides), and arsenic trioxide (0.8 wt%) derived from an electrostatic precipitator as well as As-containing water (21.3 ± 4.1 mg L
As) derived from the underground mine workings. In the vadose zone the tailings pore water has a pH of 7.6 and contains elevated metal(loid)s (2.37 ± 5.90 mg L
As); mineral oxidizers account for 2.5% of total 16S rRNA reads in solid samples. In the underlying saturated tailings, dissolved Fe and As concentrations increase with depth (up to 72 and 20 mg L
, respectively), and the mean relative abundance of Fe(III)-reducers is 0.54% of total reads. The potential for As mobilization via both reductive and oxidative (bio)processes should be considered in Giant Mine remediation activities. The current remediation plan includes installation of an engineered cover that incorporates a geosynthetic barrier layer.
•Geochemistry and mineralogy of sulfide mine tailings are reviewed.•Case studies highlight metal(loid) release, transport and attenuation processes.•Interactions among chemical, biological and ...physical processes are complex.•Effective tailings management requires comprehensive understanding of processes.
Tailings generated during processing of sulfide ores represent a substantial risk to water resources. The oxidation of sulfide minerals within tailings deposits can generate low-quality water containing elevated concentrations of SO4, Fe, and associated metal(loid)s. Acid generated during the oxidation of pyrite FeS2, pyrrhotite Fe(1−x)S and other sulfide minerals is neutralized to varying degrees by the dissolution of carbonate, (oxy)hydroxide, and silicate minerals. The extent of acid neutralization and, therefore, pore-water pH is a principal control on the mobility of sulfide-oxidation products within tailings deposits. Metals including Fe(III), Cu, Zn, and Ni often occur at high concentrations and exhibit greater mobility at low pH characteristic of acid mine drainage (AMD). In contrast, (hydr)oxyanion-forming elements including As, Sb, Se, and Mo commonly exhibit greater mobility at circumneutral pH associated with neutral mine drainage (NMD). These differences in mobility largely result from the pH-dependence of mineral precipitation–dissolution and sorption–desorption reactions. Cemented layers of secondary (oxy)hydroxide and (hydroxy)sulfate minerals, referred to as hardpans, may promote attenuation of sulfide-mineral oxidation products within and below the oxidation zone. Hardpans may also limit oxygen ingress and pore-water migration within sulfide tailings deposits. Reduction–oxidation (redox) processes are another important control on metal(loid) mobility within sulfide tailings deposits. Reductive dissolution or transformation of secondary (oxy)hydroxide phases can enhance Fe, Mn, and As mobility within sulfide tailings. Production of H2S via microbial sulfate reduction may promote attenuation of sulfide-oxidation products, including Fe, Zn, Ni, and Tl, via metal-sulfide precipitation. Understanding the dynamics of these interrelated geochemical and mineralogical processes is critical for anticipating and managing water quality associated with sulfide mine tailings.
Reduction of Se(IV) by sulfur reducing bacteria (SRB) can remove Se from groundwater either by direct respiration or the production of H2S(g) and subsequent abiotic reduction. This study examined ...abiotic Se(IV) reduction by H2S(g) to determine the associated Se isotope fractionation. The extent of fractionation was compared to the results with studies of Se(IV) reduction in systems containing microorganisms to assess whether these processes could be distinguished. A solution containing Na2S was added in increasing concentrations to solutions containing Se(IV) as SeO3 2– to reduce and precipitate Se. Precipitates with three distinct colors were observed. Powder X-ray diffraction (PXRD) results yielded three distinct spectra for each of the three colors of precipitate, which corresponded to Se n S8–n (orange) or Se(0) (red) and S(0) (yellow). The δ82Se values of the residual dissolved Se increased as the aqueous Se concentration decreased. The S/Se in solution affected the isotopic fractionation, with an 82ε of 10.1 ± 0.6‰ observed for solutions with S/Se < 1.7, and of 7.9 ± 0.3‰ for solutions with S/Se > 1.7.
Quantitative forecasts of acid mine drainage (AMD) production are important for remediation planning. Reactive transport simulations corresponding to a detailed sampling location at a covered legacy ...tailings impoundment in northern Ontario, Canada, were conducted to quantitatively assess the predominant hydrogeochemical reactions. The simulations span the period from the end of tailings deposition (circa 1970) to early 2020, 12 years after remediation by a five-layer composite cover. The conceptual model of uncovered tailings weathering and subsequent geochemistry of the covered tailings system was implemented in 1D using the multi-component reactive transport code MIN3P. Transient monthly infiltration, post-cover boundary condition changes, and a dynamic temperature regime were incorporated. The shrinking core model, including parallel O2(aq) and Fe3+ oxidation reactions for the waste rock in the cover and the underlying tailings, was implemented to simulate the oxidation of As-bearing pyrite, chalcopyrite, and sphalerite. Primary carbonate and aluminosilicate host minerals promoted acid-neutralization reactions. Precipitation of secondary phases and sorption/desorption of Cu, Zn, and arsenite were incorporated into the model. The overall agreement between key simulated and field-measured post-cover aqueous geochemical parameters suggests that the conceptual model captured the primary hydrogeochemical processes in the covered tailings. A lack of reliable data on initial tailings mineralogy and pre-cover hydrogeochemistry increased simulation uncertainty. Simulated reaction rates indicate that where intact, the cover decreased sulfide oxidation rates by both O2(aq) and Fe3+ and improved pore-water quality over time. Simulation results indicate that elevated concentrations of Zn and As are likely to persist in the tailings regardless of cover performance, whereas concentrations of Cu and Al are the parameters most sensitive to cover effectiveness.
•Tailings hydrogeochemistry was modelled from deposition to remediation by cover.•Where intact, cover decreased sulfide oxidation rates and improved water quality.•High Zn and As concentrations are likely to persist regardless of cover performance.•Aqueous Cu and Al are the most sensitive to cover effectiveness.
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.
Batch experiments were conducted using granular zerovalent iron (G-ZVI) with either ultrapure water or CaCO3 saturated simulated groundwater to assess the extent of Se isotope fractionation in ...solution under the anaerobic conditions characteristic of many aquifers. G-ZVI is a common remediation material in permeable reactive barriers (PRB) to treat Se-contaminated groundwater, and stable isotopes are a potential tool for assessing removal mechanisms. The solution composition, speciation of Se, and Se isotope ratios were determined during both sets of experiments. Dissolved Se concentrations decreased from 10 to <2 mg L–1 after 3 d in the CaCO3 system and below 0.4 mg L–1 after 2 d in the ultrapure water system. XANES analysis of the solid phase showed spectra consistent with the formation of Se(IV), Fe2(SeO3)3, FeSe, FeSe2, and Se(0) on the G-ZVI. Selenium isotope ratio measurements in solution in the CaCO3 and ultrapure water experiments showed enrichment of δ82/76Se values from −0.94 ± 0.07‰ and −1.93 ± 0.20‰ to maximum values of 6.85 ± 0.52‰ and 5.68 ± 0.20‰ over 72 and 36 h, respectively. The effective fractionations associated with the reduction of Se(VI) were 4.3‰ within the CaCO3 saturated water and 3.0‰ in ultrapure water.
Extremely acidic mine waters with pH values as low as −3.6, total dissolved metal concentrations as high as 200 g/L, and sulfate concentrations as high as 760 g/L, have been encountered underground ...in the Richmond Mine at Iron Mountain, CA. These are the most acidic waters known. The pH measurements were obtained by using the Pitzer method to define pH for calibration of glass membrane electrodes. The calibration of pH below 0.5 with glass membrane electrodes becomes strongly nonlinear but is reproducible to a pH as low as −4. Numerous efflorescent minerals were found forming from these acid waters. These extreme acid waters were formed primarily by pyrite oxidation and concentration by evaporation with minor effects from aqueous ferrous iron oxidation and efflorescent mineral formation.
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