Performance of a Geosynthetic-Clay-Liner Cover System at a Cu/Zn Mine Tailings Impoundment

Novel insights into the kinetics and mechanism of arsenopyrite bio-dissolution enhanced by pyrite

Arsenopyrite and pyrite often coexist in metal deposits and tailings, thus simultaneous bioleaching of both sulfides has economic (as well as environmental) significance. Important targets in bio-oxidation operations are high solubilization rates and minimized accumulation of Fe(III)/As-bearing secondary products. This study investigated the role of pyrite bioleaching in the enhancement of arsenopyrite dissolution. At a pyrite to arsenopyrite mass ratio of 1:1, 93.6% of As and 93.0% of Fe were solubilized. The results show that pyrite bio-oxidation can promote arsenopyrite dissolution, enhance S0 bio-oxidation, and inhibit the formation of jarosites, tooeleite, and amorphous ferric arsenate. The dry weight of the pyrite & arsenopyrite residue was reduced by 95.1% after bioleaching, compared to the initial load, while only 5% weight loss was observed when pyrite was absent. A biofilm was formed on the arsenopyrite surface in the presence of pyrite, while a dense passivation layer was observed in the absence of pyrite. As(III) (as As2O3) was a dominant As species in the pyrite & arsenopyrite residue. Novel and detailed findings are presented on arsenopyrite bio-dissolution in the presence of pyrite, and the presented approach could contribute to the development of novel cost-effective extractive bioprocesses. ENVIRONMENTAL IMPLICATION: The oxidation of arsenopyrite presents significant environmental hazards, as it can contribute to acid mine drainage generation and arsenic mobilization from sulfidic mine wastes. Bioleaching is a proven cost-effective and environmentally friendly extractive technology, which has been applied for decades in metal recovery from minerals or tailings. In this work, efficient extraction of arsenic from arsenopyrite bioleaching was presented through coupling the process with bio-oxidation of pyrite, resulting in lowered accumulation of hazardous and metastable Fe(III)/As-bearing secondary phases. The results could help improve current biomining operations and/or contribute to the development of novel cost-effective bioprocesses for metal extraction.

Microbiological and geochemical characterization of As-bearing tailings and underlying sediments

Over the past 100 years, extensive oxidation of As-bearing sulfide-rich tailings from the abandoned Long Lake Gold Mine (Canada) has resulted in the formation of acid mine drainage (pH 2.0-3.9) containing high concentrations of dissolved As (∼400 mg L-1), SO42-, Fe and other metals. Dissolved As is predominantly present as As(III), with increased As(V) near the tailings surface. Pore-gas O2 is depleted to < 1 vol% in the upper 30-80 cm of the tailings profile. The primary sulfides, pyrite and arsenopyrite, are highly oxidized in the upper portions of the tailings. Elevated proportions of sulfide-oxidizing prokaryotes are present in this zone (mean 32.3% of total reads). The tailings are underlain by sediments rich in organic C. Enrichment in δ34S-SO4 in pore-water samples in the organic C-rich zone is consistent with dissimilatory sulfate reduction. Synchrotron-based spectroscopy indicates an abundance of ferric arsenate phases near the impoundment surface and the presence of secondary arsenic sulfides in the organic-C beneath the tailings. The persistence of elevated As concentrations beneath the tailings indicates precipitation of secondary As sulfides is not sufficient to completely remove dissolved As. The oxidation of sulfides and release of As is expected to continue for decades. The findings will inform future remediation efforts and provide a foundation for the long-term monitoring of the effectiveness of the remediation program.

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.

Removal of arsenic and metals from groundwater impacted by mine waste using zero-valent iron and organic carbon: Laboratory column experiments

Acid mine drainage and the associated contaminants, including As and metals, are ongoing environmental issues. Passive remediation technologies have the potential to remove As from mine waste effluents. A series of laboratory column experiments was conducted to evaluate the effectiveness of varying mixtures of organic carbon (OC), zero-valent iron (ZVI), and limestone for the treatment of As, metals, SO₄^2-, and acidity in groundwater from an abandoned gold mine. The onset of bacterially-mediated SO₄^2- reduction was indicated by a decrease in Eh, a decline in aqueous SO₄^2- concentrations coupled with enrichment of δ³⁴S, and the presence of sulfate-reducing bacteria and H₂S. Removal of As was observed within the first 3 cm of reactive material, to values below 10 µg L^-1, representing > 99.9% removal. An increase in pH from 3.5 to circumneutral values and removal of metals including Al, Cu, and Zn was also observed. Synchrotron results suggest As was removed through precipitation of As-crystalline phases such as realgar and orpiment, or through adsorption as As(V) on ferrihydrite. The results indicate the potential for a mixture of OC and ZVI to remove As from acidic, mine-impacted water.

Microbiology of a multi-layer biosolid/desulfurized tailings cover on a mill tailings impoundment

The Strathcona Waste Water Treatment System (SWWTS; Sudbury, ON, Canada) has received mill tailings from Ni/Cu ore processing from 1970 to present. Demonstration-scale, multi-layer cover systems were installed on selected tailings deposition cells at the SWWTS. The cover systems are comprised of an upper layer of organic carbon-rich material, composed of a layer biosolids fertilizer along with composted municipal food and yard waste, then a layer of desulfurized, fine-grained tailings. Organic carbon components used in these covers promote microbial communities that consume O₂, thus decreasing sulfide oxidation rates in the underlying tailings. The aim of this study was to investigate the microbiology of the cover systems and the underlying tailings, using a combination of culture-dependent (most probable number) and culture-independent (16S rRNA gene amplicon sequencing) techniques, and assess the impact of the organic component of the cover system four to six years after implementation. Most tailings samples were characterized by circumneutral bulk pH and low concentrations of dissolved metals. The presence of the organic cover resulted in elevated counts of sulfate-reducers (by two orders of magnitude, compared to control samples) immediately below the organic cover, as well as an increased abundance of heterotrophic species (∼10⁸ cells g^-1) at greater depth (∼4 m) in the tailings profile. Mineral-oxidizing microorganisms were also present in the tailings, with neutrophilic sulfur-oxidizers dominating the samples (mean ∼10⁶ cells g^-1). Relative abundances of sulfur- and/or iron-oxidizers determined by sequencing ranged from 0.5 to 18.3% of total reads (mean ∼5.6% in amended tailings) and indicated the presence of local microenvironments with ongoing sulfide oxidation. This work provides a detailed characterization of the microbiology of a tailings impoundment with an organic cover, highlighting the opportunities associated with monitoring microbial processes in such remediation systems.