Tailings Pore Water Research Articles

Aqueous and mineralogical sulfur speciation in legacy tailings with variable redox conditions

Sulfide mineral reactivity and dissolved sulfur (S) speciation in tailings systems remain poorly constrained across redox gradients, especially in heterogeneous materials. We characterized the S-bearing mineralogy, prevailing geochemical conditions, and dissolved, intermediate (incompletely oxidized) S speciation, at three legacy tailings in Ontario and two in Nova Scotia, Canada. Tailings in Cobalt, Ontario, contained up to 3.5 wt-% sulfide minerals, mainly authigenic chalcopyrite and cobaltite/gersdorfitte. Sulfate (up to 537 ppm) dominated dissolved S in surface and porewaters from these sites, but significant fractions of thiosulfate and tetrathionate, at >40% of total S, could be detected in tailings porewaters. Tailings at the Goldenville and Montague sites in Nova Scotia contained up to 7.5 wt-% sulfide minerals, mainly pyrite and arsenopyrite, but contained smaller fractions of intermediate S (<15% on average, mostly thiosulfate and sulfite). Across all sites, overlying surface waters contained both lower total S levels, as well as lower intermediate-S fractions, than tailings porewaters. Redox conditions appeared to exert a dominant control on aqueous S speciation, and correlations of intermediate S with dissolved Fe levels and tailings mineralogy suggested the latter presented lesser influences on S dynamics. This work demonstrates that incomplete S oxidation and resultant intermediate S species play an important role for S mobility in sub-oxic tailings systems and at legacy mine sites and should therefore be considered in long-term reactivity and water quality predictions.

Microbial processes with the potential to mobilize As from a circumneutral-pH mixture of flotation and roaster tailings

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−1 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−1 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−1, 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.

Factors controlling tungsten mobility in W[sbnd]Cu skarn tailings

Few studies have addressed the mobility of tungsten in mine waste, which could act as a point source for metal leaching of this potentially toxic metal. This study examines the behavior of tungsten in acidic floodplain tailings and circumneutral impoundment tailings at the Cantung Mine in the Northwest Territories, Canada. In the Cantung deposit, tungsten is present as scheelite (CaWO4), copper is hosted in chalcopyrite (CuFeS2), and gangue mineralogy consists of abundant pyrrhotite (Fe1-xS), silicate minerals, and carbonate minerals. Samples of surface waters, tailings porewaters, and solid tailings samples were collected from Cantung Mine. For surface waters, both filtered (0.45 μm) and unfiltered aliquots were collected to compare dissolved and particulate element concentrations.Tungsten concentrations range from 5.8 to 30.0 μg/L in water samples with pH values between 7.1 and 8.1, whereas samples below pH 7.1 have concentrations below the detection limit of 0.1 μg/L. Tungsten and iron concentrations are both on average 1.7 times higher in unfiltered aliquots compared to filtered aliquots, suggesting that tungsten is transported as dissolved species but is also adsorbed to suspended Fe-oxyhydroxide minerals at neutral pH. Tailings were analyzed by scanning electron microscopy (SEM) paired with automated mineralogy software (MLA), synchrotron-based μX-ray diffraction and fluorescence (μXRD-XRF), and partial leach digestions with inductively coupled plasma mass spectrometry (cold and hot hydroxylamine hydrochloride, aqua regia, Li-metaborate fusion, and 4-acid digestion with HNO3-HClO4-HF-HCl). Samples from the impounded tailings and the highly oxidized Flat River Tailings distributed on the floodplain (FRT) have similar scheelite content (0.15 wt% and 0.21 wt%, respectively), and scheelite shows no evidence of alteration. Synchrotron-based μXRD of Fe-oxyhydroxide minerals in the FRT identified goethite (FeOOH) and lepidocrocite (γ-FeOOH) as products of pyrrhotite oxidation. Rare rims on pyrrhotite from the tailings impoundment have μXRD spectra consistent with hematite (Fe2O3) and maghemite (γ-Fe2O3). The μXRF maps of the hematite-maghemite rims have prominent tungsten peaks, which represent included scheelite grains and possibly structurally incorporated tungsten. The hydroxylamine leaches yield higher tungsten concentrations in tailings samples from the circumneutral impoundments than samples from the acidic FRT, suggesting the tailings impoundments have more tungsten that is associated with poorly crystalline and amorphous Fe-oxyhydroxide phases than the FRT. Over time, labile-hosted tungsten in the FRT may have been washed down the Flat River during Fe-oxyhydroxide recrystallization and high energy flooding events. These results indicate that scheelite has limited solubility in neutral or acid mine waters, and the mobility of the small amount of tungsten released is governed by the present of iron oxyhydroxide minerals, the transport of colloidal material in surface water, and the stability of the depositional environment.

Pilot-scale feasibility study for the stabilization of coal tailings via microbially induced calcite precipitation

Sustainable long-term solutions to managing tailings storage facilities (TSFs) are integral for mines to operate in a safe and environmentally responsible manner. The long-term storage of subaqueous tailings can pose significant safety, environmental, and economic risks; therefore, alternative containment strategies for maintaining geochemical stability of reactive materials must be explored. In this study, the physical and geochemical stabilization of coal tailings using microbially induced calcite precipitation (MICP) was evaluated at a laboratory pilot scale. Three application techniques simulated commonly used agricultural approaches and equipment that could be deployed for field-scale treatment: spraying on treatment solutions with irrigation sprinklers, mixing tailings and treatment solutions with a rototiller, and distributing treatment solutions via shallow trenches using an excavator ripper. Test cells containing 1.0 × 1.0 × 0.5 m of tailings were treated with ureolytic bacteria (Sporosarcina pasteurii) and cementation solutions composed of urea and calcium chloride for 28 days. Penetrometer tests were performed following incubation to evaluate the extent of cementation. The spray-on application method showed the greatest strength improvement, with in an increase in surface strength of more than 50% for the 28-day testing period. The distribution of treatment solution using trenches was found to be less effective and resulted in greater variability in particle size distribution of treated tailings and would not be recommended for use in the field. The use of rototilling equipment provided a homogenous distribution of treatment solution; however, the disruption to the tailings material was less effective for facilitating effective cementation. Bacterial plate counts of soil samples indicated that S. pasteurii cultures remained viable in a tailings environment for 28 days at 18 °C and near-neutral pH. The treatment was also found to stabilize the pH of tailings porewater sampled over the 28-day incubation period, suggesting the potential for the treatment to provide short-term geochemical stability under unsaturated conditions.

Drainage chemistry of mine tailings from a carbonatite-hosted Nb-REE deposit, Oka, Québec, Canada

Potential environmental issues associated with the mining of carbonatites are receiving increased attention due to the importance of critical metals for green technologies. This study investigates the chemistry of tailings seepage at the former Saint Lawrence Columbium mine near Oka, Québec, Canada, which produced pyrochlore concentrate and ferroniobium from a carbonatite-hosted Nb-REE deposit. Detailed field sampling and laboratory methods were used to characterize the hydraulic properties of the tailings, their bulk chemistry, mineralogy, pore water and effluent chemistries. The tailings are composed of REE-enriched calcite (64–89 wt %) and fluorapatite (2–22 wt %), as well as biotite (6–17 wt %) and chlorite (0–7 wt %). Minor minerals include ankerite, pyrite, sphalerite, molybdenite, magnetite and unrecovered pyrochlore. Secondary minerals include gypsum, barite, strontianite and rhodochrosite. Geochemical mass balance modeling, constrained by speciation modeling, was used to identify dissolution, precipitation and exchange reactions controlling the chemical evolution of pore water along its flow path through the tailings impoundment. In the unsaturated zone, these reactions include sulfide oxidation and calcite dissolution with acid neutralization. Below the water table, gypsum dissolution is followed by sulfate reduction and FeS precipitation driven by the oxidation of organic carbon in the tailings. Incongruent dissolution of biotite and chlorite releases K, Mg, Fe, Mn, Ba and F and forms kaolinite and Ca-smectite. Cation exchange reactions further remove Ca from solution, increasing concentrations of Na and K. Fluoride concentrations reach 23 mg/L and 8 mg/L in tailings pore water and effluent, respectively. These values exceed Canadian guidelines for the protection of aquatic life. In the mildly alkaline (pH 8.3) pore waters, Mo is highly mobile and reaches an average concentration of 83 μg/L in tailings effluent, which slightly exceeds environmental guidelines. Concentrations (unfiltered) of Zn reach 1702 μg/L in tailings pore water although values in effluent are usually less than 20 μg/L. At the ambient pH, Zn is strongly adsorbed by Fe–Mn oxyhydroxides. Although U forms mobile complexes in tailings pore water, concentrations do not exceed 16 μg/L due to the low solubility of its pyrochlore host. Adsorption and the low solubility of pyrochlore limit concentrations of Nb to less than 49 μg/L. Cerium, from calcite dissolution, is strongly adsorbed although it reaches concentrations (unfiltered) in excess of 1 mg/L and 100 μg/L in pore water and effluent, respectively. Results of this study show that mine tailings from carbonatite deposits are enriched in a wide variety of incompatible elements with multiple mineral hosts of varying solubility. Some of these elements, such as F and Mo, may represent contaminants of concern because of their mobility in alkaline tailings waters.

Uranium removal from complex mining waters by alginate beads doped with cells of Stenotrophomonas sp. Br8: Novel perspectives for metal bioremediation

Uranium-containing effluents generated by nuclear energy industry must be efficiently remediated before release to the environment. Currently, numerous microbial-based strategies are being developed for this purpose. In particular, the bacterial strain Stenotrophomonas sp. Br8, isolated from U mill tailings porewaters, has been already shown to efficiently precipitate U(VI) as stable U phosphates mediated by phosphatase activity. However, the upscaling of this strategy should overcome some constraints regarding cell exposure to harsh environmental conditions. In the present study, the immobilization of Br8 biomass in an inorganic matrix was optimized to provide protection to the cells as well as to make the process more convenient for real-scale utilization. The use of biocompatible, highly porous alginate beads for Br8 cells immobilization resulted the best alternative when investigating by a multidisciplinary approach (High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), Environmental Scanning Electron Microscopy (ESEM), Fourier Transform Infrared Spectroscopy with Attenuated Total Reflectance, etc.) several consolidated entrapment methods. This biomaterial was applied to complex real U mining porewaters (containing 47 mg/L U) in presence of an organic phosphate source (glycerol-2-phosphate) to produce reactive free orthophosphates through Br8 phosphatase activity. Uranium immobilization rates around 98 % were observed after one cycle of 72 h. In terms of U removal ability as a function of biomass, Br8-doped alginate beads were determined to remove up to 1199.5 mg U/g dry biomass over two treatment cycles. Additionally, optimized conditions for storing Br8-doped beads and for a correct application were assessed. Results for U accumulation kinetics and HAADF-STEM/ESEM analyses revealed that U removal by the immobilized cells is a biphasic process combining a first passive U sorption onto bead and/or cell surfaces and a second slow active biomineralization. This work provides new practical insights into the biological and physico-chemical parameters governing a high-efficient U bioremediation process based on the phosphatase activity of immobilized bacterial cells when applied to complex mining waters under laboratory conditions.

Reactive transport modelling of porewater geochemistry and sulfur isotope fractionation in organic carbon amended mine tailings

Field experiments previously conducted to assess organic carbon (OC) amendments for in situ biological treatment of tailings porewater at the Greens Creek Mine (Alaska, USA) showed sulfate reduction, metal-sulfide precipitation, and decreased fluxes of sulfate and dissolved metals. Here, we develop two reactive transport models using the reactive transport code MIN3P to simulate hydrogeochemical processes and δ34S–SO4 isotope fractionation over four years in test cells containing unamended (control) and amended (5 vol % OC) tailings. These models successfully simulate observed data including pH, SO4, δ34S–SO4, Ca, Fe, K, Mg, Mn, Si, and Zn. The models also indicate that dissolution of carbonate and, to a lesser extent, aluminosilicate minerals neutralize acidic porewater generated from sulfide mineral oxidation and sulfate reduction reactions. Application of a constant kinetic fractionation factor of 0.9820 to simulate measured δ34S–SO4 trends confirms that sulfate removal principally results from microbially-mediated sulfate reduction in conjunction with OC oxidation and subsequent metal-sulfide precipitation. Gypsum precipitation/dissolution and thiosulfate disproportionation have negligible effects on modelled porewater δ34S–SO4 signatures. Our simulations are consistent with the previous findings that metal-sulfide precipitation controls Fe and Zn attenuation in amended tailings and that coprecipitation reactions contribute to metal removal. Overall, these simulations demonstrate that coupled reactive transport modelling incorporating stable isotope fractionation can improve the understanding of hydrogeochemical and biogeochemical controls within in situ treatment systems, further illustrating the benefits and limitations of this technique for improving water quality of mine drainage.

Process network modelling of the geochemical reactions responsible for acid mine drainage emanating from the Witwatersrand tailings facilities

ABSTRACTAcid mine drainage (AMD) and associated metal(loid) and SO42- pollution of soil, surface water and groundwater is ubiquitously associated with tailings material generated by Au mining in the Witwatersrand Basin in South Africa. The individual geochemical processes responsible for the AMD generation in this tailings material are relatively well understood. What is less clear are how these different processes interact as a network within the tailings system. Process network modelling (PNM) is a tool that can be used to study such interactive and complex networks of geochemical processes, especially when stochastic methods, e.g. Monte Carlo simulation, are included in the model development. Secondary mineral phase supersaturation requirements from classical nucleation theory are also built into the model.A PNM was developed for a tailings facility in the Witwatersrand gold basin focussing on pH, Fe(total) and SO42- concentrations in the tailings pore water and the relationship of these parameters to the dissolution of pyrite, O2 diffusion into the tailings, oxidation of Fe2+ and the precipitation of secondary minerals, specifically goethite and jarosite. The model indicated that AMD conditions develop fairly rapidly after the sulphidic material is exposed to the Earth’s oxygenated atmosphere. The concentration of H+, and hence the pH, in the tailings pore water is controlled by a number of feedbacks. The positive feedback, implying addition of H+, is the dissolution of pyrite and the precipitation of the secondary Fe3+-bearing minerals goethite and jarosite. Jarosite precipitation was shown to increase the median H+ addition rate by ~2%. The negative feedback, i.e. decrease in H+, is the oxidation of Fe2+ to Fe3+. This feedback loop produces a net excess of H+. Together with the buffer effect of goethite and jarosite precipitation, system steady-state conditions are eventually achieved with respect to pH.The pore water SO42- concentration is controlled by the positive and negative feedback of pyrite dissolution and jarosite precipitation. This feedback loop produces a large excess of SO42- and steady state conditions can only be achieved if SO42- is physically removed from the tailings system, e.g. seepage to groundwater.Oxidation of the Fe2+ produced by pyrite dissolution to Fe3+ is the only positive feedback for tailings pore water Fe3+ concentrations. The negative feedbacks are precipitation of goethite and jarosite and the oxidation of pyrite by Fe3+. The former effect is delayed as these phases first have to achieve a certain level of supersaturation in the tailings pore water solution before they can form. The precipitation of jarosite and goethite, by removing Fe3+ from solution, decreases the effect of Fe3+ pyrite oxidation causing O2 to remain the dominant oxidation mechanism. This feedback loop produces a small excess of Fe3+ over time, however, the model is very sensitive to other factors, e.g. O2 diffusion deeper into the tailings facility.

Draft genome sequence data of Microbacterium sp. strain Be9 isolated from uranium-mill tailings porewaters.

Microbacterium are Gram-positive, nonspore-forming, rod-shaped bacteria inhabiting a wide range of environments including soil, water, dairy products, other living organisms, etc. Microbacterium sp. strain Be9, isolated from mill tailings porewaters in France, shows a remarkable behavior in presence of uranium under distinct conditions, which is the main reason for the interest in sequencing its genome. In this work, we describe the draft genome sequence of Be9, comprising 4,046,806 bp, with a G+C content of 68.10% and containing 3,947 protein-coding sequences. The preliminary genome annotation analysis identified some genes encoding for resistance to antibiotics and toxic compounds like heavy metals. This draft genome has been deposited at DDBJ/ENA/GenBank under the accession PRJNA590666.

Accelerating Mineral Carbonation in Ultramafic Mine Tailings via Direct CO2 Reaction and Heap Leaching with Potential for Base Metal Enrichment and Recovery

Abstract Accelerated carbonation of ultramafic mine tailings has the potential to offset CO2 emissions produced by mining ores from Cu-Ni-platinum group element, podiform chromite, diamondiferous kimberlite, and historical chrysotile deposits. Treatments such as acid leaching, reaction of tailings with elevated concentrations of gaseous CO2, and optimization of tailings pore water saturation have been shown to enhance CO2 sequestration rates in laboratory settings. The next challenge is to deploy treatment technologies on the pilot and field scale while minimizing cost, energy input, and adverse environmental impacts. Implementation of accelerated tailings carbonation at field scale will ideally make use of in situ treatments or modified ore-processing routes that employ conventional technology and expertise and operate at close to ambient temperatures and pressures. Here, we describe column experiments designed to trial two geochemical treatments that address these criteria: (1) direct reaction of partially saturated ultramafic tailings with synthetic flue gas from power generation (10% CO2 in N2) and (2) repeated heap leaching of ultramafic tailings with dilute sulfuric acid. In the first experiment, we report rapid carbonation of brucite [Mg(OH)2] in the presence of 10% CO2 gas within tailings sampled from the Woodsreef chrysotile mine, New South Wales, Australia. Within four weeks, we observe a doubling of the amount of CO2 stored within minerals relative to what is achieved after three decades of passive mineral carbonation via air capture in the field. Our simulated heap leaching experiments, treated daily with 0.08 M H2SO4, produce high-Mg leachates that have the potential to sequester 21.2 kg CO2 m–2 y–1, which is approximately one to two orders of magnitude higher than the rate of passive carbonation of the Woodsreef mine tailings. Although some nesquehonite (MgCO3 · 3H2O) forms from these leachates, most of the Mg is precipitated as Mg sulfate minerals instead. Therefore, an acid other than H2SO4 could be used; otherwise, sulfate removal would be required to maximize CO2 sequestration potential from acid heap leaching treatments. Reactive transport modeling (MIN3P) is employed to simulate acid leaching experiments and predict the effects of heap leaching for up to five years. Finally, our synchrotron X-ray fluorescence microscopy results for leached tailings material reveal that valuable trace metals (Fe, Ni, Mn, Co, Cr) become highly concentrated within secondary Fe (hydr)oxide minerals at the pH neutralization horizon within our column experiments. This discrete horizon migrates downward, and our reactive transport models indicate it will become increasingly enriched in first-row transition metals in response to continued acid leaching. Acid-leaching treatments for accelerated mineral carbonation could therefore be useful for ore processing and recovery of base metals from tailings, waste rock, or low-grade ores.

Antimony mobility during the early stages of stibnite weathering in tailings at the Beaver Brook Sb deposit, Newfoundland

The aqueous speciation and mineralogy of antimony (Sb) in waters and tailings at Beaver Brook antimony deposit have been analyzed to understand Sb mobility during the initial stages of stibnite (Sb2S3) weathering in a near-surface environment. Dissolution of stibnite in oxidizing conditions releases Sb in drainage water and Sb is incorporated into the mineral structures of several secondary minerals. The most abundant Sb host in Beaver Brook tailings is primary stibnite, which dissolves, releasing Sb(III) to the pore water which rapidly oxidizes to Sb(V). The maximum concentration of Sb in tailings pore water is 26.4 mg/L and only 0.9% is in the form of Sb(III). In all surface water, Sb concentration ranges from 0.01 to 26.1 mg/L (average 9.4 mg/L) and is mostly present in its Sb(V) form (98.9–99.2% of total Sb). The secondary minerals containing Sb formed in tailings impoundment include tripuhyite-like Sb–Fe oxides (FeSbO4) where Sb is an important part of their structure, with variable Fe/Sb ratios and Sb concentrations of up to 37.8% by weight (average of 21.7%). These are important Sb host phases in the top 30 cm of tailings. Iron oxides enriched in Sb, such as goethite (FeOOH), where Sb (average of 3.9% by weight) is adsorbed or incorporated in the structure are common but represent less than 1.3% of the total mass of Sb. The elevated Mg concentrations in tailing ponds and pore water promote the precipitation of brandholzite (Mg[Sb(OH)6]2·6H2O) (in association with gypsum) during dry periods, which is easily dissolved during rainy periods. Brandholzite dissolution may significantly contribute to the concentration of dissolved Sb, together with stibnite dissolution, whereas Sb–Fe oxides are stable in the neutral pH, oxidized surface environment. Arsenic (As) accompanies Sb in all media but its behavior differs from that of Sb. The source of As is arsenopyrite, which decomposes more slowly than stibnite. This may be due to the formation of oxidation rims on arsenopyrite grains composed of Fe, As, S, Sb, and Ca which slow the dissolution, whereas no rims are seen on stibnite. Also, despite similar As and Sb concentration in bulk tailings, the concentration of Sb in drainage water is higher than that of As. In pore water, As(III) is the dominant oxidation state of As suggesting that the oxidation of dissolved As is slower than that of Sb.

Thermal Disturbances in Permafrost Due to Open Pit Mining and Tailings Impoundment

The paper is concerned with thermal disturbances in continuous permafrost due to open pit mining and tailings impoundment in the cold regions of Northern Canada. Numerical simulations were conducted to investigate issues of thermal regime changes and permafrost degradation in both the short term and the long term in connection with the proposed Kiggavik project in Nunavut, Canada. The results of numerical simulations indicate that no open talik would form below the East Zone and Center Zone pits during the estimated mining and milling operation period of 14 years, although a thin thawed zone would develop surrounding the open pits. For the Main Zone pit where the excavation would break through permafrost, the open talik remains following an extended operation period of 25 years with a 5 to 30 m thawed zone along the pit side walls. In the long term, with a plausible climate change scenario of 5 °C increase in the mean annual ground surface temperature during the next 100 years, the permafrost surrounding the in-pit tailings management facilities (TMFs) would reduce greatly in about 500 years. However, an approximately 40 m thick permafrost layer would remain on the top of the TMFs, which is impervious and would prevent any tailings pore water from migrating upward to reach the ground surface.

Hydraulic variations in permafrost due to open-pit mining and climate change: a case study in the Canadian Arctic

The paper deals with numerical computations of hydraulic variations and groundwater flow changes in continuous permafrost due to climate change and open-pit mining in the cold regions of Northern Canada. The work is a case study in connection with the proposed Kiggavik project in Nunavut, Canada. A major challenge in simulating fluid flow through partially frozen porous media is how to define the transient hydraulic conductivity as a function of temperature. An indirect approach based on the similarity between soil freezing and soil–water characteristic curves for unsaturated soils is implemented to define the relation between unfrozen water content and cryogenic suction which is linked to temperature by the Clausius–Clapeyron equation. Richard’s equation is then used to model fluid flow in partially frozen ground conditions. Finite element numerical modelling results of a worst-case climate change scenario indicate that although the permafrost could disappear completely in about 750 years with an increase in $$7\,^{\circ }$$C in the mean annual ground surface temperature during the next 100 years, the groundwater level around tailings facility would be approximately 35 m below ground surface. Hence, tailings pore water would not likely to migrate upwards to reach the ground surface even after the permafrost is completely disappeared. These conclusions are certainly within uncertainties and limitations of the analysis.

Evaluation of the impact of karst depression-type impoundments on the underlying karst water systems in the Gejiu mining district, southern Yunnan, China

The influence of mining activities on groundwater quality is a growing concern due to highly toxic metals in the tailings. Carbonate rocks are common in the surrounding rocks of ore deposits. But such mining districts generally do not have the conditions to construct traditional tailings impoundments due to the different degrees of karstification. This paper utilises field investigations, chemical analyses, and permeability tests, etc. to study the effect of tailings impoundments on the underlying karst water systems in the Gejiu world-class Sn-polymetallic mine located in a typical karst region in southern China, with samples collected from both karst springs deriving from the mining area and tailings porewater. Because of the abundance of sulphide minerals in the ores, acid mine drainage is expected to pose a high risk. However, the results indicate that the tailings porewater is moderately neutral and is characterized by the SO4–Ca–Mg hydrochemical type. This can be attributed to the substantial buffering capacity of carbonate minerals. Pollution elements in the tailings porewater mainly include SO42− and toxic metals including As, Cd, Mn, Pb, and Hg. The type of pollution factors and the degree of contamination are not consistent in the different tailings impoundments. The hydrochemical classification of four karst springs distributed in the south of the mining area is HCO3–Ca–Mg. The concentrations of SO42− and the toxic constituents in the springs are far below the national standards for drinking water. The spring water not being contaminated may be attributed to the operation mode of the tailings impoundments (peripheral discharge and reuse of ponded water), the residual clay between the tailings impoundments and the carbonate bedrocks and the sufficient buffering capacity of the underlying karst systems. The tailings disposal at the Gejiu tin mine within the natural karst depressions is able to provide a reference for the operation of karst mines in China and elsewhere.

Geochemistry and pH control of seepage from Ni-Cu rich mine tailings at Selebi Phikwe, Botswana.

Acid mine drainage from mine tailings at Selebi Phikwe, eastern Botswana, has been investigated using a combination of total decomposition, sequential extraction, X-ray diffraction, Mössbauer spectroscopy, and SEM analyses of solid phase samples, water analyses, isotopic analyses, and geochemical modeling. The principal ferric phases in the seepage stream sediments are jarosite and goethite, which incorporate Ni and Cu. The Mössbauer spectroscopy (MS) indicated exclusively 3+ oxidation state of iron with typical features of ferric hydroxides/sulfates. A fraction of dissolved sulfate is also sequestered in gypsum which precipitates further downstream. Significant portions of Fe, Ni, and Cu are transported in suspension. Values of pH decreased downstream due to H+ generated by the precipitation of jarosite. Values of δ2H and δ18O indicate evaporation of pore water in the mine tailings before seepage. Values of δ34S(SO4) are consistent with the oxidation of sulfides, but sample from the seepage face is affected by dissolution of gypsum. No minerals of Ni and Cu were detected and the principal attenuation processes seem to be adsorption and co-precipitation with jarosite. Higher contents of Cu are sequestered in solid phases compared to Ni, in spite of much higher dissolved Ni concentrations. Based on the speciation calculations, seepage water is undersaturated with respect to all Ni and Cu phases and adsorption and co-precipitation with jarosite seems to be the principal attenuation processes. Direct geochemical modeling was able to reproduce downstream pH trends, thus confirming the precipitation of jarosite as the principal pH-controlling process.

Inter-comparison geochemical modelling approaches and implications for environmental risk assessments: A Witwatersrand gold tailings source term characterisation study

Acid mine drainage (AMD) has long been recognised as a major environmental concern from mines associated with especially Fe-sulphide minerals with the ore and/or gangue. The generation of AMD and its associated geochemical risks, e.g. leaching of toxic metal(loid)s into the environment, is a major concern from such mines, as is the case in the Witwatersrand gold mines. Geochemical modelling, incorporating thermodynamic equilibrium and kinetic in modelling geochemical processes in the mineral waste, forms an important part of this process. However, a concern regarding the geochemical modelling is on the meaning of the results, especially when these models are used to make long-term projections, as complex scientific models cannot be validated. This study shows that inter-model comparison using existing data from specifically gold tailings pore water is a viable method to build confidence in geochemical models used for risk assessments. It also demonstrates for trace elements, specifically As, Co, Ni, Pb and Zn, that sources can be identified and the release rates estimated. The process of model confidence building is the most important in identifying problems with the conceptual model as well as determining which sources are important for specific trace elements. The Pb concentration in uraninite exceeds that of pyrite by 2 orders of magnitude; however, pyrite contains sufficient concentrations of Pb so that its’ more rapid dissolution rate renders it the more important source of Pb in the tailings pore water. The varying release rates for the trace metal(loid)s can also be used to estimate their mode of occurrence within source mineral structures. Quantification of the concentration of trace elements in specific host mineral phases should be a requirement of any environmental geochemical risk assessment and can be done using standard mineralogical analytical tools.

Simulating drainage quality of sulfide mine tailings ponds

The Xiangxi gold mine was taken as an example to illustrate the application of the finite element and updated Lagrangian approach in the simulation and prediction of the quality of tailings effluents. The numerical modeling results suggest that tailings-water interaction at the early stage leads to the acidification and release of heavy metals which are responsible for the persistent pollution to the environment for 30 years. Tailings pore water can be gradually neutralized and contents of polluted species tend to decline markedly with time. The gangue dissolution and organic matter reaction can stimulate acid neutralization and decrease the diffusion rate of oxygen in tailings impoundments. The contents of pore-water species in upper tailings are significantly higher than in lower part and change severely at 6 m depth. Therefore, the hydrological zoning phenomenon in tailings impoundments accounts for geochemical zonation. This model has the potential to predict drainage quality of sulfide mine tailing impoundments.

An in-situ process to consolidate oil sands mine tailings

The oil sands mining tailings inventory is growing and current treatment technologies are costly or unable to recover water effectively and handle the high solids content of the waste. Here we consider in-situ treatment of mature fine tailings (MFT) where additives are injected directly into the tailings to consolidate the solids so that the pond area can be reclaimed. This method eliminates the need for a dilution pre-step in the treatment of MFT. In a series of experiments, the impact of additives (iron(III) chloride and polyacrylamide), mixing speed, gas additives, and process scale on the de-watering performance of treated MFT was examined. It was found that the in-situ treatment method resulted in a better tailings product (higher final solids content) than an ex-situ method with ideal mixing conditions. The injected gases contributed to de-watering, chemically (by CO2 decreasing the pH of the tailings pore water) and physically (by convective mixing). After treatment, a layer of supernatant water continuously accumulates at the top of the tailings layer. The results demonstrate that injection of chemicals into the tailings layer leads to water rejection from the tailings layer leading to a layer with high solid content.

Post-depositional behaviour of mercury and arsenic in submarine mine tailings deposited in Buyat Bay, North Sulawesi, Indonesia

The post-depositional geochemical behaviour of mercury and arsenic in submarine mine tailings from the Mesel Gold Mine in Buyat Bay, North Sulawesi, Indonesia was assessed by in situ sampling of tailings porewaters using dialysis arrays and seawater and fish monitoring. Under steady-state conditions one year after cessation of tailings discharge, the calculated arsenic efflux incrementally added 0.8 μg/L of arsenic to the overlying seawater. The mercury efflux across the tailings-seawater interface was negligible. The arsenic and mercury concentration in seawater bottom samples monitored biannually during a 9-year post-closure program were 1.54 μg/L and <0.05 μg/L, respectively. Analysis of 650 fish tissue samples, from the post-closure monitoring had mean mercury and arsenic concentrations consistently below the FAO/WHO CODEX, and Australian and New Zealand National Food Standards, respectively. The results of the porewater, seawater and fish tissue demonstrate that the arsenic and mercury-bearing bearing compounds in the tailings are geochemically stable.

Arsenic mobility in weathered gold mine tailings under a low-organic soil cover

Historical gold mining operations in Nova Scotia, Canada, resulted in numerous deposits of publicly accessible, arsenic (As)-rich mine waste that has weathered in situ for 75–150 years, resulting in a wide range of As-bearing secondary minerals. The geochemical heterogeneity of this mine waste creates a challenge for identifying a single remediation approach that will limit As mobility. A 30-cm-thick, low-organic content soil cover was evaluated in a laboratory leaching experiment where, to simulate natural conditions, the equivalent of 2 years of synthetic rainwater was leached through each column and two dry seasons were incorporated into the leaching protocol. Each column was a stratigraphic representation of the four major tailings types found at the historical Montague and Goldenville gold mine districts: hardpan tailings, oxic tailings, wetland tailings, and high Ca tailings. Hardpan tailings released acidic, As-rich waters (max 12 mg/L) under the soil cover but this acidity was buffered by surrounding oxic tailings. Leachate from the oxic tailings was circumneutral, with average As concentrations between 4.4 and 9.7 mg/L throughout the experiment. The presence of carbonates in the high Ca tailings resulted in near-neutral to weakly alkaline leachate pH values and average As concentrations between 2.1 and 6.1 mg/L. Oxidation of sulfides in the wetland tailings led to acidic leachate over time and a decrease in As concentrations to values that were generally less than 1 mg/L. This study shows that the use of a low-organic content soil cover does not create reducing conditions that would destabilize oxidized, As-bearing secondary phases in these tailings. However, oxygen penetration through the cover during dry seasons would continue to release As to tailings pore waters via sulfide oxidation reactions.