Sulfide Mine Tailings Research Articles

Soil morphology, genesis, and monolith construction of an acid sulfate soil with silica-cementation in the US Mid-Atlantic Region

Soil monoliths are powerful tools for soil education and for representing soils as they are found in the field. They preserve and display major soil features and are portable. The monolith herein described was collected from an active acid sulfate soil located near the western shore of the Chesapeake Bay in the Coastal Plain physiographic province in Maryland, US. The key features of this profile include a silica-cemented layer, a sulfuric horizon, prominent concentrations of jarosite and iron oxide, glauconite, and sulfidic materials. The silica-cemented layer, similar to a duripan, created a challenge for the collection of a soil monolith from this site. This layer was disassembled, fragments were cut to size, and the layer was reassembled in a soil monolith containing the underlying and overlying soil horizons. This appears to be the first published report of a method for creating a soil monolith in a silica-cemented soil. Silica-cementation in acid sulfate soils is rarely reported in the pedologic literature, though cemented layers are occasionally reported in literature on sulfidic mine tailings. By understanding silica-cementation in acid sulfate soils we may be able to induce it as a management practice in mine tailings and other environmentally hazardous sites. The goals of this study are to report on a method for creating soil monoliths from silica-cemented soils, to present explanations for the genesis of this unusual soil, and to highlight how a pedologic study of this soil could lead to further experiments to mitigate some environmentally hazardous sites.

Grain sorting effects on geochemical characteristics of sulfide mine tailings: a case study

The geochemical evolution of a sulfide mine tailings impoundment in SW Spain was studied. The impoundment was selected because of its small size and its tailings deposition system with a simple discharge point. The objective of this study was to test the hypothesis that mineral segregation associated to hydraulic sorting has significant effects on the geochemical characteristics and the long term weathering. Tailings samples were collected along depth profiles in three sampling points (proximal, central and distal to the point of discharge), and characterized by color, grain size, pH, acid-base account and chemical elements concentration, with the help of routine XRD analysis. Three vertical zones were identified: an upper oxidized zone, a transition intermediate zone, and an unoxidized zone. The analytical results indicate a segregation pattern in the unoxidized tailings based on differences in size and density of tailings grains. Near the discharge point, tailings were coarser and rich in pyrite, whereas the proportion of silicates increased from proximal to distal points. This results in a clear zoning which has consequences on geochemical and mineralogical evolution under weathering, showing substantial differences in the depth of the oxidation front, the acid generation and neutralization capacity, the formation of Fe secondary phases (jarosite) and the total content of the sulfide-related elements (Fe, As, Zn, Cu, Pb and Cd). The results of the study can serve to improve the theoretical bases for the development of conceptual models for predicting environmental impacts associated with sulfide tailings impoundments. Recently, the impoundment has been covered with a soil cover. This fact offers the possibility of new research on its evolution under new conditions.

Mine tailings influencing soil contamination by potentially toxic elements

The potentially hazardous contents of mine tailings can pose a serious threat to the environment. Tailings dispersed around the abandoned Monica mine (Bustarviejo) in the Autonomous Region of Madrid (Central Spain) were studied to determine the concentration of several potential toxic elements and their geochemical impact in the surrounding soils. A total of 17 surface soil samples were collected from both mixed sulfide mine tailings sites and unmined soils, within a radius of 1900 m from the mine entrance. The processing of minerals (basically arsenopyrite, matildite and sphalerite) produced tailings with a pH as low as 2.9. Elements such as As, Cu, Zn, Cd, Pb, W, Ag, Fe were found in very high concentrations, contaminating the soil to varying degrees (these elements were sometimes 10- to 20-times higher in the tailings than in the unmined soils). Given its short distance and accessibility from such a large city as Madrid, it is of undeniable environmental and educational interest. Among other factors, there is a need for improvements to tailings management strategies.

Effectiveness of alkaline amendments in acid mine drainage remediation

Acid mine drainage (AMD) generated from pyrite oxidation in sulfide mine tailings is a major geoenvironmental problem. This AMD is characterized by pH<3.5 and high concentration of trace elements. Thus, liming is a prerequisite for sustainable stabilization and neutralization of acid-producing sulfide mine tailings. The use of agri-food waste and industrial by-products rich calcium carbonate or calcium oxides and hydroxides becomes attractive as an alternative limestone in the remediation of acid mine tailing impoundments. Addition effect of five different types of alkaline amendments into sulfide mine tailing (SMT) was studied. Two series of laboratory experiments were conducted on SMT to evaluate the use of chicken eggshell residue (CES) alone or in combination with neutralizing agents to neutralize acidity and prevent mobilization of trace elements in SMT. This study showed that tailing samples amended with cement or magnesium oxide are more highly buffered and resistant against anthropogenic re-acidification than tailing samples amended with CES, CAL or DOL. Materials rich in oxides, hydroxides and carbonates mixed with eggshells may confer to SMT long-term protection against acid atmospheric deposition or re-acidification of limed SMT. These results may have practical implications in tailing management for the reduction of acid generation and trace metal mobility in sulfide mine drainage system and of water contamination risk.

Biodegradation of MCHM and PPH in River Microcosms and Activated Sludge

A mixture of coal processing agents, namely crude-MCHM (primarily 4-methylcyclohexanemethanol) and stripped-PPH (primarily dipropylene glycol phenyl ether and propylene glycol phenyl ether) spilled into the Elk River near Charleston, West Virginia in January 2014. The spill resulted in a do-not-use ban on drinking water for up to 10 days. Studies were undertaken to determine the potential for aerobic biodegradation of crude-MCHM and PPH in the riverine system and in the sewage treatment plant postflushing of affected water lines. Complete biotransformation of crude-MCHM occurred only when MCHM was initially <6 mg L−1. When both PPH and crude-MCHM were present, biotransformation of both compounds only occurred when MCHM was initially <2.3 mg L−1 and PPH was <3 mg L−1. Limited conversion of PPH was typically observed (<40%). Estimated maximum specific biodegradation rates were 0.087 mg cis-MCHM (mg biomass h)−1 and 0.23 mg trans-MCHM (mg biomass h)−1. The estimated half saturation constants were 2.7 mg cis-MCHM L−1 and 10.5 mg trans-MCHM L−1. Toxicity studies with agar diffusion plates and ToxTrac indicate both MCHM and PPH exhibit toxicity to Elk River microcosm cultures and that microbiological degradation of MCHM and PPH may generate more toxic products. Phylogenetic analysis of sediments and biodegradation microcosm cultures identified methanotrophs, methylotrophs, and organisms associated with xenobiotic biodegradation, acid mine drainage, and sulfidic mine tailings. These studies can be used to understand the likelihood of biological degradation of MCHM and PPH in environmental and treatment systems.

Vegetation successfully prevents oxidization of sulfide minerals in mine tailings

The oxidization of metal sulfide in tailings causes acid mine drainage. However, it remains unclear whether vegetation prevents the oxidization of metal sulfides. The oxidization characteristics and microbial indices of the tailings in the presence of various plant species were investigated to explore the effects of vegetation on the oxidization of sulfide minerals in tailings. The pH, reducing sulfur, free iron oxides (Fed), chemical oxygen consumption (COC) and biological oxygen consumption (BOC) were measured. Key iron- and sulfur-oxidizing bacteria (Acidithiobacillus spp., Leptospirillum spp. and Thiobacillus spp.) were quantified using real-time PCR. The results indicate that vegetation growing on tailings can effectively prevent the oxidization of sulfide minerals in tailings. A higher pH and reducing-sulfur content and lower Fed were observed in the 0–30 cm depth interval in the presence of vegetation compared to bare tailings (BT). The COC gradually decreased with depth in all of the soil profiles; specifically, the COC rapidly decreased in the 10–20 cm interval in the presence of vegetation but gradually decreased in the BT profiles. Imperata cylindrica (IC) and Chrysopogon zizanoides (CZ) profiles contained the highest BOC in the 10–20 cm interval. The abundance of key iron- and sulfur-oxidizing bacteria in the vegetated tailings were significantly lower than in the BT; in particular, IC was associated with the lowest iron- and sulfur-oxidizing bacterial abundance. In conclusion, vegetation successfully prevented the oxidization of sulfide minerals in the tailings, and Imperata cylindrica is the most effective in reducing the number of iron- and sulfur-oxidizing bacteria and helped to prevent the oxidization of sulfide minerals in the long term.

A methodology for ranking potential pollution caused by abandoned mining wastes: application to sulfide mine tailings in Mazarrón (Southeast Spain)

Mining wastes often contain high concentrations of toxic elements, whose mobility and dispersion may pose an environmental hazard for soils, water, ecosystems and people. This article describes the partial application of a methodology designed to evaluate the pollution potential of abandoned mining wastes, using two indices for potential pollution evaluation: an index of contamination (IC) and a hazard average quotient (CPP). Composite samples, consisting of at least 30 subsamples, were taken at three sulfidic tailings impoundments in the mining area of Mazarron (SE Spain). Mineralogical, physical, and chemical characteristics were analyzed: color, particle size, powder XRD analysis, pH, total content of toxic elements, and concentrations in leachates by the standard procedure EN-12457-2. Tailings were extremely acid (pH 2.30–2.52). Some ranges of total content (in mg/kg) were: As (381–565), Pb (2602–4487), Sb (139–170), and Zn (3254–5652). The concentration measured in the EN-12457 leachates (µg/L) was as high as 367,000 for Zn, 2030 for Cu, and 974 for Cd. The IC and CPP values were among the most high of the tailings inventoried in Spain. The combination of the indices provided a good estimation of the potential toxicity of these wastes, and it can be useful to rank abandoned mining waste facilities.

Utilization of sulphidic tailings from gold mine as a raw material in geopolymerization

The mining industry produces a large quantity of sulphidic mine tailings, which cause several environmental issues during waste management. Currently, there is increasing interest in new technologies to recycle and utilize mine tailings more effectively in the future. In this present study, the geopolymerization of mine tailings has been studied. Sulphidic mine tailings from a gold mining site were activated with a NaOH solution and commercial ground granulated blast furnace slag (GGBFS) was used as a co-binder. Characterization of the mine tailings and the mechanical strength of the specimens produced were investigated. In addition, the effects of different NaOH concentrations and the amount of co-binder materials on a matrix were tested. The porosity of the specimens produced was evaluated using water absorption tests, the microstructure of the fractures was analyzed with field emission scanning electron microscope (FESEM), and the crystalline phases were identified by X-ray diffraction. The results show that the unconfined compressive strength (UCS) of the specimens produced from pure mine tailings was in the range of 1.3–3.5MPa. The UCS increased and water permeability decreased with 5% GGBFS content in the mixture. By optimizing the NaOH concentration and GGBFS content, the UCS of the specimens varied from 1.8MPa to 25MPa. The alkali-activation of mine tailings allows binders to be made with sufficient compressive strength, which can be used as a backfill in mining sites or raw material in the construction industry.

Assessing metal transfer to vegetation and grazers on reclaimed pyritic Zn and Pb tailings.

A study of the concentrations of zinc and lead in an engineered soil capping system overlying sulphide mine tailings was undertaken. Tailings geochemistry, soil cover and vegetation were monitored over a 4-year period, and a cattle grazing demonstration exercise was conducted over a 1-year period. Whilst the tailings had a relatively high pyrite content and demonstrated oxidation, a circum neutral pH was observed for the duration of the study period due to the high dolomitic content. No evidence of metal mobility into the soil cover and vegetation was observed over the monitoring period. Relatively high Zn herbage content is attributed to the glacial till component of the soil cover. Similarly, no evidence of metal transfer to grazing cattle was observed through blood and tissue analysis with Zn content not significantly different from control animals. Pb tissue content was below limit of detection.

From lithotroph- to organotroph-dominant: directional shift of microbial community in sulphidic tailings during phytostabilization.

Engineering microbial diversity to enhance soil functions may improve the success of direct revegetation in sulphidic mine tailings. Therefore, it is essential to explore how remediation and initial plant establishment can alter microbial communities, and, which edaphic factors control these changes under field conditions. A long-term revegetation trial was established at a Pb-Zn-Cu tailings impoundment in northwest Queensland. The control and amended and/or revegetated treatments were sampled from the 3-year-old trial. In total, 24 samples were examined using pyrosequencing of 16S rRNA genes and various chemical properties. The results showed that the microbial diversity was positively controlled by soil soluble Si and negatively controlled by soluble S, total Fe and total As, implying that pyrite weathering posed a substantial stress on microbial development in the tailings. All treatments were dominated by typical extremophiles and lithotrophs, typically Truepera, Thiobacillus, Rubrobacter; significant increases in microbial diversity, biomass and frequency of organotrophic genera (typically Nocardioides and Altererythrobacter) were detected in the revegetated and amended treatment. We concluded that appropriate phytostabilization options have the potential to drive the microbial diversity and community structure in the tailings toward those of natural soils, however, inherent environmental stressors may limit such changes.

Biodegradation of Biosolids Under Aerobic Conditions: Implications for Cover Materials for Sulfide Mine Tailings Remediation

Sewage sludge residue (biosolids) was investigated for its potential as a long-term tailings cover. Biosolids may prevent oxygen diffusion into underlying sulfide tailings through microbial aerobic biodegradation of organic matter. Biosolids were investigated at laboratory-, pilot-, and field-scale using analysis of total organic matter (TOM) mass reduction and O2, CO2, CH4 concentrations to quantify the biodegradation rate. A 156-day, open microcosm experiment, in which the loss of biosolids mass over time at differing temperatures, mimicking ambient (20–22 °C), mesophilic (34 °C), and thermophilic (50 °C) conditions, indicated that TOM biodegradation was best in the mesophilic temperature range, with 14.8, 27.2, and 26.7 % mass depletion at ambient, mesophilic, and thermophilic conditions, respectively. The data was correlated to field-scale data that evaluated biodegradation rates via decreasing O2 and increasing CO2 concentrations. Field biodegradation rates were less than laboratory rates because lower mean annual temperatures (0.6–0.7 °C) diminished microbial activity. A calibrated model indicates that 20 % of a field application of biosolids will degrade within 2 years. However, the rate declines with time due to exhaustion of the most readily degradable organic fraction. If biodegradation cannot be maintained, the long-term effectiveness of biosolids as a covering material for mine tailings remains a concern.

Toxic metal(loid) speciation during weathering of iron sulfide mine tailings under semi-arid climate

Toxic metalliferous mine-tailings pose a significant health risk to ecosystems and neighboring communities from wind and water dispersion of particulates containing high concentrations of toxic metal(loid)s (e.g., Pb, As, Zn). Tailings are particularly vulnerable to erosion before vegetative cover can be reestablished, i.e., decades or longer in semi-arid environments without intervention. Metal(loid) speciation, linked directly to bioaccessibility and lability, is controlled by mineral weathering and is a key consideration when assessing human and environmental health risks associated with mine sites. At the semi-arid Iron King Mine and Humboldt Smelter Superfund site in central Arizona, the mineral assemblage of the top 2m of tailings has been previously characterized. A distinct redox gradient was observed in the top 0.5m of the tailings and the mineral assemblage indicates progressive transformation of ferrous iron sulfides to ferrihydrite and gypsum, which, in turn weather to form schwertmannite and then jarosite accompanied by a progressive decrease in pH (7.3–2.3).Within the geochemical context of this reaction front, we examined enriched toxic metal(loid)s As, Pb, and Zn with surficial concentrations 41.1, 10.7, 39.3mmolkg−1 (3080, 2200, and 2570mgkg−1), respectively. The highest bulk concentrations of As and Zn occur at the redox boundary representing a 1.7 and 4.2-fold enrichment relative to surficial concentrations, respectively, indicating the translocation of toxic elements from the gossan zone to either the underlying redox boundary or the surface crust. Metal speciation was also examined as a function of depth using X-ray absorption spectroscopy (XAS). The deepest sample (180cm) contains sulfides (e.g., pyrite, arsenopyrite, galena, and sphalerite). Samples from the redox transition zone (25–54cm) contain a mixture of sulfides, carbonates (siderite, ankerite, cerrusite, and smithsonite) and metal(loid)s sorbed to neoformed secondary Fe phases, principally ferrihydrite. In surface samples (0–35cm), metal(loid)s are found as sorbed species or incorporated into secondary Fe hydroxysulfate phases, such as schwertmannite and jarosites. Metal-bearing efflorescent salts (e.g., ZnSO4·nH2O) were detected in the surficial sample. Taken together, these data suggest the bioaccessibility and lability of metal(loid)s are altered by mineral weathering, which results in both the downward migration of metal(loid)s to the redox boundary, as well as the precipitation of metal salts at the surface.

Long-term mineralogical and geochemical evolution of sulfide mine tailings under a shallow water cover

The long-term influence of a shallow water cover limiting sulfide-mineral oxidation was examined in tailings deposited near the end of operation in 1951 of the former Sherritt-Gordon Zn–Cu mine (Sherridon, Manitoba, Canada). Surface-water, pore-water and core samples were collected in 2001 and 2009 from above and within tailings deposited into a natural lake. Mineralogical and geochemical characterization focused on two contrasting areas of this deposit: (i) sub-aerial tailings with the water table positioned at a depth of approximately 50cm; and (ii) sub-aqueous tailings stored under a 100cm water cover. Mineralogical analyses of the sub-aerial tailings showed a zone of extensive sulfide-mineral alteration extending 40cm below the tailings surface. Moderate alteration was observed at depths ranging from 40 to 60cm and was limited to depths >60cm. In contrast, sulfide-mineral alteration within the submerged tailings was confined to a <6cm thick zone located immediately below the water-tailings interface. This narrow zone exhibited minimal sulfide-mineral alteration relative to the sub-aerial tailings. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy showed results that were consistent with the mineralogical investigation. Pore-water within the upper 40cm of the sub-aerial tailings was characterized by low pH (1.9–4.2), depleted alkalinity, and elevated SO4 and metal concentrations. Most-probable number (MPN) enumerations revealed abundant populations of acidophilic sulfur-oxidizing bacteria within these tailings. Conversely, pore-water in the sub-aqueous tailings was characterized by near-neutral pH, moderate alkalinity, and relatively low concentrations of dissolved SO4 and metals. These tailings exhibited signs of dissimilatory sulfate reduction (DSR) including elevated populations of sulfate reducing bacteria (SRB), elevated pore-water H2S concentrations, and strong δ34S-SO4 and δ13C-DIC fractionation. Additionally, mineralogical investigation revealed the presence of secondary coatings on primary sulfide minerals, which may serve as a control on metal mobility within the sub-aqueous tailings. Results from this study provide critical long-term information on the viability of sub-aqueous tailings disposal as a long-term approach for managing sulfide-mineral oxidation.

Microbial diversity at the moderate acidic stage in three different sulfidic mine tailings dumps generating acid mine drainage

In freshly deposited sulfidic mine tailings the pH is alkaline or circumneutral. Due to pyrite or pyrrhotite oxidation the pH is dropping over time to pH values <3 at which acidophilic iron- and sulfur-oxidizing prokaryotes prevail and accelerate the oxidation processes, well described for several mine waste sites. The microbial communities at the moderate acidic stage in mine tailings are only scarcely studied. Here we investigated the microbial diversity via 16S rRNA gene sequence analysis in eight samples (pH range 3.2–6.5) from three different sulfidic mine tailings dumps in Botswana, Germany and Sweden. In total 701 partial 16S rRNA gene sequences revealed a divergent microbial community between the three sites and at different tailings depths. Proteobacteria and Firmicutes were overall the most abundant phyla in the clone libraries. Acidobacteria, Actinobacteria, Bacteroidetes, and Nitrospira occurred less frequently. The found microbial communities were completely different to microbial communities in tailings at <pH 3 described in the literature.

Influence of disposal configurations on hydrogeological behaviour of sulphidic paste tailings: A field experimental study

Surface paste disposal (SPD) is a new alternative employed by the mining industry for storage of mine tailings at the surface. In comparison with the conventional slurry tailings disposal, SPD could offer operational and environmental advantages, such as a better water management, no need for complex retaining dams, a reduced footprint of the tailings disposal area, and the possibility to use progressive reclamation. This paper describes a field investigation through a large-scale experimental cell to assess an SPD application for sulphidic mine tailings. The work addresses the effect of two disposal configurations (i.e., partially cemented and un-cemented) on their hydrogeological behaviour when submitted to actual climatic conditions, focusing mainly on the implementation challenges as well as on the first results obtained. Tailings were deposited in thin layers (10 layers of 10cm each one) into two experimental cells (D×L×H=8m×15m×2m). Cement was added locally (2wt.% of Portland cement) in the first layer of the cell (CC) to study its effect; the second cell (UC) is cement free. The evolution of volumetric water content Ѳ, suction ψ, oxygen consumption and cracks for each cell was monitored during and after deposition. Results show that the CC provides slightly higher Ѳ and smaller ψ values than the UC, most likely due to its geological properties dictated by the bottom cemented layer.

Evolution of Acid Mine Drainage Formation in Sulphidic Mine Tailings

Sulphidic mine tailings are among the largest mining wastes on Earth and are prone to produce acid mine drainage (AMD). The formation of AMD is a sequence of complex biogeochemical and mineral dissolution processes. It can be classified in three main steps occurring from the operational phase of a tailings impoundment until the final appearance of AMD after operations ceased: (1) During the operational phase of a tailings impoundment the pH-Eh regime is normally alkaline to neutral and reducing (water-saturated). Associated environmental problems include the presence of high sulphate concentrations due to dissolution of gypsum-anhydrite, and/or effluents enriched in elements such as Mo and As, which desorbed from primary ferric hydroxides during the alkaline flotation process. (2) Once mining-related operations of the tailings impoundment has ceased, sulphide oxidation starts, resulting in the formation of an acidic oxidation zone and a ferrous iron-rich plume below the oxidation front, that re-oxidises once it surfaces, producing the first visible sign of AMD, i.e., the precipitation of ferrihydrite and concomitant acidification. (3) Consumption of the (reactive) neutralization potential of the gangue minerals and subsequent outflow of acidic, heavy metal-rich leachates from the tailings is the final step in the evolution of an AMD system. The formation of multi-colour efflorescent salts can be a visible sign of this stage.

Surficial weathering of iron sulfide mine tailings under semi-arid climate

Mine wastes introduce anthropogenic weathering profiles to the critical zone that often remain unvegetated for decades after mining cessation. As such, they are vulnerable to wind and water dispersion of particulate matter to adjacent ecosystems and residential communities. In sulfide-rich ore tailings, propagation to depth of the oxidative weathering front controls the depth-variation in speciation of major and trace elements. Despite the prevalence of surficial mine waste deposits in arid regions of the globe, few prior studies have been conducted to resolve the near-surface profile of sulfide ore tailings weathered under semi-arid climate. We investigated relations between gossan oxidative reaction-front propagation and the molecular speciation of iron and sulfur in tailings subjected to weathering in a semi-arid climate at an EPA Superfund Site in central Arizona (USA). Here we report a multi-method data set combining wet chemical and synchrotron-based X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES) methods to resolve the tight coupling of iron (Fe) and sulfur (S) geochemical changes in the top 2m of tailings. Despite nearly invariant Fe and S concentration with depth (130–140 and 100–120gkg−1, respectively), a sharp redox gradient and distinct morphological change was observed within the top 0.5m, associated with a progressive oxidative alteration of ferrous sulfides to (oxyhydr)oxides and (hydroxy)sulfates. Transformation is nearly complete in surficial samples. Trends in molecular-scale alteration were co-located with a decrease in pH from 7.3 to 2.3, and shifts in Fe and S lability as measured via chemical extraction. Initial weathering products, ferrihydrite and gypsum, transform to schwertmannite, then jarosite-group minerals with an accompanying decrease in pH. Interestingly, thermodynamically stable phases such as goethite and hematite were not detected in any samples, but ferrihydrite was observed even in samples with the lowest pH, indicating its metastable persistence in these semiarid tailings. The resulting sharp geochemical speciation gradients in close proximity to the tailings surface have important implications for plant colonization, as well as mobility and bioavailability of co-associated toxic metal(loid)s.

Impact of Sewage Sludge on Groundwater Quality at a Formerly Remediated Tailings Impoundment

Sewage sludge can be a suitable, organic-rich substrate to promote vegetation of sulfide-mine tailings, but it may contain contaminants, that, when oxidized, can adversely affect underlying groundwater systems. The geochemical impact of a surface application of 12,000 metric tons of anaerobically-digested sewage sludge on the groundwater quality of a remediated sulfide-tailings impoundment in northern Sweden was evaluated to determine if sludge-borne metals and nitrate were released to the underlying groundwater system. Two years of data from a field-scale groundwater monitoring programme initiated just before the sludge application was compared to groundwater data from 1998 to 2006. Grass was successfully established within 2 years. However, until that occurred, elevated concentrations of sludge-borne metals (Cu, Ni, Pb, Zn) were released to the underlying groundwater. In addition, the release of nitrate likely exacerbated metal concentrations by providing an oxidant for pyrite in the underlying tailings. The release was periodic due to the establishment of the grass, which immobilized metals and nitrate in the sludge. Metals bound as organo-metallic complexes, due to dissolved organic carbon released from the sludge, migrated across the tailings impoundment. Model simulations indicate that the plume will take 6 years to exit the groundwater environment. Though the impacts are relatively short-term, this type of application should be reconsidered in the future.

Determination of the reaction rate coefficient of sulphide mine tailings deposited under water

The efficiency of a water cover to limit dissolved oxygen (DO) availability to underlying acid-generating mine tailings can be assessed by calculating the DO flux at the tailings–water interface. Fick's equations, which are generally used to calculate this flux, require knowing the effective DO diffusion coefficient (Dw) and the reaction (consumption) rate coefficient (Kr) of the tailings, or the DO concentration profile. Whereas Dw can be accurately estimated, few studies have measured the parameter Kr for submerged sulphide tailings. The objective of this study was to determine Kr for underwater sulphide tailings in a laboratory experiment. Samples of sulphide mine tailings (an approximately 6 cm layer) were placed in a cell under a water cover (approximately 2 cm) maintained at constant DO concentration. Two tailings were studied: TA1 with high sulphide content (83% pyrite) and TA2 with low sulphide content (2.8% pyrite). DO concentration was measured with a microelectrode at various depths above and below the tailings–water interface at 1 mm intervals. Results indicate that steady-state condition was rapidly attained. As expected, a diffusive boundary layer (DBL) was observed in all cases. An iterative back-calculation process using the numerical code POLLUTEv6 and taking the DBL into account provided the Kr values used to match calculated and experimental concentration profiles. Kr obtained for tailings TA1 and TA2 was about 80 d−1 and 6.5 d−1, respectively. For comparison purposes, Kr obtained from cell tests on tailings TA1 was lower than Kr calculated from the sulphate production rate obtained from shake-flask tests. Steady-state DO flux at the water–tailings interface was then calculated with POLLUTEv6 using tailings characteristics Dw and Kr. For the tested conditions, DO flux ranged from 608 to 758 mg O2/m2/d for tailings TA1 and from 177 to 221 mg O2/m2/d for tailings TA2. The impact of placing a protective layer of inert material over the tailings was also investigated for tailings TA1 (with high sulphide content). A protective layer of only 5 cm reduced the DO flux into the tailings at about 5 mg/m2/d, compared to 608 mg O2/m2/d without a protective layer, or an approximately 99% reduction in flux.

Formation of a hardpan in the co-disposal of fly ash and sulfide mine tailings and its influence on the generation of acid mine drainage

Pilot-scale field experiments were conducted to test a strategy for the treatment of acid mine drainage by addition of an alkaline cover of coal combustion fly ash on two sulfide mine tailings impoundments (Iberian Pyrite Belt, SW Spain). During the pilot-scale treatment, calcium-rich alkaline solutions from fly ash interacted with metal-rich acidic solutions from the mine tailings, leading to the massive precipitation of newly-formed phases mainly in the fly ash close to the interface between both materials. Over time, the interaction between fly ash and pyrite sludge promoted the formation of a chemically cemented zone or hardpan, which was analyzed at high spatial resolution by micro-X-ray diffraction (μ-XRD) and micro-X-ray fluorescence (μ-XRF) based on synchrotron radiation. These micro-characterization techniques extensively identified poorly-crystalline Fe oxyhydroxysulfates, jarosite and gypsum as newly-formed phases in the hardpan, and gypsum in the fly ash. As deduced from Principal Component Analysis of the fluorescence intensity data, these phases seem to exert a significant mineralogical control on element mobility; in particular, jarosite showed high affinity for As, whereas poorly-crystalline Fe-rich assemblages selectively concentrated Mn, Ni and Pb. Sequential extractions indicated that the application of a fly ash cover significantly reduced the bioavailability of most of the elements by modification of their chemical speciation into less mobile forms under typical oxidizing conditions. As an additional advantage, the development of the hardpan hindered the penetration of oxidizing agents to sulfide mine tailings. This effect significantly reduced the sulfide oxidation rates, therefore attenuating the release of potentially pollutant elements to the environment.