首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Hardpans, or cemented layers, form by precipitation and cementation of secondary minerals in mine tailings and may act as both physical and chemical barriers. Precipitation of secondary minerals during weathering of tailings can sequester metal(loid)s, thereby limiting their release to the environment. At Montague Gold Mines in Nova Scotia, tailings are partially cemented by the Fe arsenate mineral scorodite (FeAsO4·2H2O). Previous studies have shown that the formation of scorodite can effectively limit aqueous As concentrations due to its relatively low solubility (<1 mg/L at pH 3–4) and high As content (43–52 wt.% As2O5, this study). Co-existing waters and solids were sampled at Montague Gold Mines to identify present-day field conditions influencing scorodite precipitation and dissolution, and to better understand the mineralogical and chemical relationship between hardpan and tailings. In addition to scorodite, hardpan cements were found to include amorphous Fe arsenate and Fe oxyhydroxide. Nearly all hardpan is associated with historical arsenopyrite-bearing concentrate which provides a source of acidity, As5+ and Fe3+ for secondary mineral precipitation. Pore waters sampled from the hardpan have pH values ranging from 2.43 to 7.06. Waters with the lowest pH values also have the highest As concentrations (up to 35.8 mg/L) and are associated with the most extensive hardpan and greatest amount of weathered sulfide. Samples from areas with discontinuous hardpan and less sulfide have near-neutral pH and lower As concentrations. Detailed petrographic observations indicate that the identity and stability of As-bearing secondary minerals depends on the continued availability of sulfide concentrate. The results of this study are being used to develop remediation strategies for highly weathered, hardpan-bearing tailings that consider the stability of both primary and secondary minerals under various cover scenarios.  相似文献   

2.
The oxidation and subsequent dissolution of sulfide minerals within mine tailings impoundments releases H+, Fe(II), SO4 and trace elements to the tailings pore water. Subsequent pH-buffering and hydrolysis reactions result in the precipitation of secondary phases such as gypsum, goethite and jarosite. In areas of intense precipitation, cemented layers or “hardpans” often form within the shallow tailings. Three cemented layers within pyrrhotite-bearing mine tailings at the Fault Lake, Nickel Rim and East Mine impoundments located near Sudbury, Canada, were examined. The location of the three cemented layers within the tailings stratigraphy varies as does their location relative to the water table. The morphology, mineralogy and chemical composition of the cemented layers also vary between sites. The bulk density within the three cemented layers all showed an increase relative to the surrounding uncemented tailings ranging from 9% to 29%. The porosity of each cemented layer decreased relative to the surrounding uncemented tailings ranging from an 8% to 18% decrease. The cemented layers also showed relative enrichment of total sulfur, carbon and trace elements relative to the surrounding uncemented tailings. Arsenic concentrations showed an enrichment in the cemented layers of up to 132%, Cd up to 99%, Co up to 84%, Cu up to 144%, Ni up to 693% and Zn up to 145% relative to the surrounding uncemented tailings. All the cemented layers studied show an evolution of the secondary phases with time from a gypsum–jarosite-based cement to a goethite-rich cement. The formation of these layers could potentially have a significant effect on the environmental impacts of sulfide-bearing mine waste.  相似文献   

3.
Speciation and colloid transport of arsenic from mine tailings   总被引：2，自引：0，他引：2  
In addition to affecting biogeochemical transformations, the speciation of As also influences its transport from tailings at inoperative mines. The speciation of As in tailings from the Sulfur Bank Mercury Mine site in Clear Lake, California (USA) (a hot-spring Hg deposit) and particles mobilized from these tailings have been examined during laboratory-column experiments. Solutions containing two common, plant-derived organic acids (oxalic and citric acid) were pumped at 13 pore volumes d−1 through 25 by 500 mm columns of calcined Hg ore, analogous to the pedogenesis of tailings. Chemical analysis of column effluent indicated that all of the As mobilized was particulate (1.5 mg, or 6% of the total As in the column through 255 pore volumes of leaching). Arsenic speciation was evaluated using X-ray absorption spectroscopy (XAS), indicating the dominance of arsenate [As(V)] sorbed to poorly crystalline Fe(III)-(hydr)oxides and coprecipitated with jarosite [KFe3(SO4, AsO4)2(OH)6] with no detectable primary or secondary minerals in the tailings and mobilized particles. Sequential chemical extractions (SCE) of <45 μm mine tailings fractions also suggest that As occurs adsorbed to Fe (hydr)oxides (35%) and coprecipitated within poorly crystalline phases (45%). In addition, SCEs suggest that As is associated with 1 N acid-soluble phases such as carbonate minerals (20%) and within crystalline Fe-(hydr)oxides (10%). The finding that As is transported from these mine tailings dominantly as As(V) adsorbed to Fe (hydr)oxides or coprecipitated within hydroxysulfates such as jarosite suggests that As release from soils and sediments contaminated with tailings will be controlled by either organic acid-promoted dissolution or reductive dissolution of host phases.  相似文献   

4.
 The oxidation and the subsequent dissolution of sulfide minerals within the Copper Cliff tailings area have led to the release of heavy metals such as Fe, Ni, and Co to the tailings pore water. Dissolved concentrations in excess of 10 g/l Fe and 2.2 g/l Ni have been detected within the shallow pore water of the tailings, with increasing depth these concentrations decrease to or near analytical detection limits. Geochemical modelling of the pore-water chemistry suggests that pH-buffering reactions are occurring within the shallow oxidized zones, and that secondary phases are precipitating at or near the underlying hardpan and transition zones. Mineralogical study of the tailings confirmed the presence of goethite, jarosite, gypsum, native sulfur, and a vermiculite-type clay mineral. Goethite, jarosite, and native sulfur form alteration rims and pseudo-morphs of the sulfide minerals. Interstitial cements, composed of goethite, jarosite, and gypsum, locally bind the tailings particles, forming hardpan layers. Microprobe analyses of the goethite indicate that it contains up to 0.6 weight % Ni, suggesting that the goethite is a repository for Ni. Other sinks detected for heavy metals include jarosite and a vemiculite-type clay mineral which locally contains up to 1.6 weight % Ni. To estimate the mass and distribution of heavy metals associated with the secondary phases within the shallow tailings, a series of chemical extractions was completed. The experimental design permitted four fractions of the tailings to be evaluated independently. These four fractions consisted of a water-soluble, an acid-leachable, and a reducible fraction, as well as the whole-rock total. Twenty-five percent of the total mass of heavy metals was removed in the acid-leaching experiments, and 100% of the same components were removed in the reduction experiments. The data suggest that precipitation/coprecipitation reactions are providing an effective sink for most of the heavy metals released by sulfide mineral oxidation. In light of these results, potential decommissioning strategies should be evaluated with the recognition that changing the geochemical conditions may alter the stability of the secondary phases within the shallow tailings. Received: 9 April 1997 · Accepted: 21 July 1997  相似文献   

5.
  相似文献   

6.
Mineralogical, geochemical and microbial characterization of tailings solids from the Greens Creek Mine, Juneau, Alaska, was performed to evaluate mechanisms controlling aqueous geochemistry of near-neutral pH pore water and drainage. Core samples of the tailings were collected from five boreholes ranging from 7 to 26 m in depth. The majority of the 51 samples (77%) were collected from the vadose zone, which can extend >18 m below the tailings surface. Mineralogical investigation indicates that the occurrence of sulfide minerals follows the general order: pyrite [FeS2] >> sphalerite [(Zn,Fe)S] > galena [PbS], tetrahedrite [(Fe,Zn,Cu,Ag)12Sb4S13] > arsenopyrite [FeAsS] and chalcopyrite [CuFeS2]. Pyrite constitutes <20 to >35 wt.% of the tailings mineral assemblage, whereas dolomite [CaMg(CO3)2] and calcite [CaCO3] are present at ?30 and 3 wt.%, respectively. The solid-phase geochemistry generally reflects the mineral assemblage. The presence of additional trace elements, including Cd, Cr, Co, Mo, Ni, Se and Tl, is attributed to substitution into sulfide phases. Results of acid–base accounting (ABA) underestimated both acid-generating potential (AP) and neutralization potential (NP). Recalculation of AP and NP based on solid-phase geochemistry and quantitative mineralogy yielded more representative results. Neutrophilic S-oxidizing bacteria (nSOB) and SO4-reducing bacteria (SRB) are present with populations up to 107 and 105 cells g−1, respectively. Acidophilic S-oxidizing bacteria (aSOB) and iron-reducing bacteria (IRB) were generally less abundant. Primary influences on aqueous geochemistry are sulfide oxidation and carbonate dissolution at the tailings surface, gypsum precipitation–dissolution reactions, as well as Fe reduction below the zone of sulfide oxidation. Pore-water pH values generally ranged from 6.5 to 7.5 near the tailings surface, and from approximately 7–8 below the oxidation zone. Elevated concentrations of dissolved SO4, S2O3, Fe, Zn, As, Sb and Tl persisted under these conditions.  相似文献   

7.
Arsenic-bearing stream sediments enter the Upper Isle River, an Au mining-influenced basin (France), by the discharge of mining sites, tailings runoff and weathering of mineralized veins in granites and gneiss. Some fresh ochreous As-rich deposits on the river banks and in floodplains are identified as additional As-rich point sources (As between 0.07 and 6.5 wt.%). The <63 μm fraction of stream sediments contains elevated As bulk concentrations, ranging from 160 to 890 mg/kg, compared to the geochemical background (70 mg/kg on average). It is also enriched in Cd, Hg and W. Spatial variations of these trace elements show 3 significant increases corresponding to the 3 drained mining districts. They decrease down river but are still enriched 30 km downstream of the mining districts due to downstream transportation. Three types of trace element-bearing phases have been identified as: (i) detrital primary sulfides, with high in situ As percentages (up to 43.7 wt.%). They also carry significant amounts of W according to the differences in chemical compositions of the total and light <63 μm fractions. These sulfide particles do not show any sign of alteration in the oxygenated stream sediments; (ii) Secondary Fe–Mn oxyhydroxides,some with very high in situ As2O5 concentrations (up to 59.8 wt.%) and with about 40% of the total Cd composition. They occur as fresh precipitates in the river banks and floodplains and as discrete particles in stream sediments and (iii) Al–Si fine-grained phases. Their major element composition is highly variable with in situ As2O5 concentrations ranging between 430 and 5020 mg/kg. This type of solid phase is also the major carrier of Hg.  相似文献   

8.
Nearly half a century after mine closure, release of As from the Ylöjärvi Cu–W–As mine tailings in groundwater and surface water run-off was observed. Investigations by scanning electron microscopy (SEM), electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (μ-XRD), micro-X-ray absorption near edge structure (μ-XANES) and micro-extended X-ray absorption fine structure (μ-EXAFS) spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mechanisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indicated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As2O5) and Fe(III) arsenates were important secondary controls on As mobility. The μ-XRD, μ-XANES and μ-EXAFS analyses suggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates, including arsenical ferrihydrite, scorodite, kaňkite, and hydrous ferric arsenate (HFA). The pH within 200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite. Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihydrite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings. Arsenic mobility in tailings pore water at the Ylöjärvi mine will depend on continued arsenopyrite oxidation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity of Fe(III) (oxy)hydroxides within this tailings deposit.  相似文献   

9.
Secondary minerals formed in tailings derived from a W-rich deposit were investigated in detail using transmission electron microscopy (TEM). The study focused on secondary minerals that formed in the vicinity of oxidized sphalerite [ZnS] and tennantite [Cu10(Fe,Zn)2As4S13] grains. Samples for TEM analysis were prepared directly from petrographic thin sections using a focused ion beam instrument. This method insured that spatial relationships among primary grains, secondary minerals and the pore spaces were maintained. The results from this study indicate that the secondary coatings associated with sphalerite and tennantite are composed of several discrete phases. The phases identified in this study include an Fe–Zn–As–O phase, secondary sulfides, native Cu, an Fe–Si–O phase, an In–O phase, and wulfenite [PbMoO4]. The Fe–Zn–As–O phase precipitates directly from the pore water and the nearby primary mineral grains act as a source for some of the elements (e.g., Zn from sphalerite, As from tennantite). Secondary Cu sulfides were found at the outer margins of sphalerite and roquesite [CuInS2] grains. It is likely that these Cu sulfides form as a result of interactions between the primary grain and aqueous Cu(II) present in the pore water, similar to what occurs in supergene environments. A secondary sulfide that was composed of variable amounts of Cu, Zn, As, Fe and S was also identified along the outer margins of tennantite. Native Cu was found in association with chalcopyrite [CuFeS2] inclusions that were present in one of the sphalerite grains and probably represents a low-temperature secondary phase. The oxidation of chalcopyrite in the presence of aqueous Si leads to the formation of a nanocrystalline or amorphous Fe–Si–O phase. Roquesite oxidation leads to the formation of a crystalline In–O phase, which is likely dzhalindite [In(OH)3]. Wulfenite was found in the interstitial voids present in the Fe–Zn–As–O phase suggesting that it forms by direct precipitation from the local pore water. The results from this study indicate that secondary coatings consist of complex secondary phases that may only be distinguished at the nanoscale. The TEM investigations reveal details regarding mineralogical sinks and sources for aqueous components that may otherwise be overlooked.  相似文献   

10.
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.  相似文献   

11.
《Applied Geochemistry》2005,20(3):639-659
The oxidation of sulfide minerals from mine wastes results in the release of oxidation products to groundwater and surface water. The abandoned high-sulfide Camp tailings impoundment at Sherridon, Manitoba, wherein the tailings have undergone oxidation for more than 70 a, was investigated by hydrogeological, geochemical, and mineralogical techniques. Mineralogical analysis indicates that the unoxidized tailings contain nearly equal proportions of pyrite and pyrrhotite, which make up to 60 wt% of the total tailings, and which are accompanied by minor amounts of chalcopyrite and sphalerite, and minute amounts of galena and arsenopyrite. Extensive oxidation in the upper 50 cm of the tailings has resulted in extremely high concentrations of dissolved SO4 and metals and As in the tailings pore water (pH < 1, 129,000 mg L−1 Fe, 280,000 mg L−1 SO4, 55,000 mg L−1 Zn, 7200 mg L−1 Al, 1600 mg L−1 Cu, 260 mg L−1 Mn, 110 mg L−1 Co, 97 mg L−1 Cd, 40 mg L−1 As, 15 mg L−1 Ni, 8 mg L−1 Pb, and 3 mg L−1 Cr). The acid released from sulfide oxidation has been extensive enough to deplete carbonate minerals to 6 m depth and to partly deplete Al-silicate minerals to a 1 m depth. Below 1 m, sulfide oxidation has resulted in the formation of a continuous hardpan layer that is >1 m thick. Geochemical modeling and mineralogical analysis indicate that the hardpan layer consists of secondary melanterite, rozenite, gypsum, jarosite, and goethite. The minerals indicated mainly control the dissolved concentrations of SO4, Fe, Ca and K. The highest concentrations of dissolved metals are observed directly above and within the massive hardpan layer. Near the water table at a depth of 4 m, most metals and SO4 sharply decline in concentration. Although dissolved concentrations of metals and SO4 decrease below the water table, these concentrations remain elevated throughout the tailings, with up to 60,600 mg L−1 Fe and 91,600 mg L−1 SO4 observed in the deeper groundwater. During precipitation events, surface seeps develop along the flanks of the impoundment and discharge pore water with a geochemical composition that is similar to the composition of water directly above the hardpan. These results suggest that shallow lateral flow of water from a transient perched water table is resulting in higher contaminant loadings than would be predicted if it were assumed that discharge is derived solely from the deeper primary water table. The abundance of residual sulfide minerals, the depletion of aluminosilicate minerals in the upper meter of the tailings and the presence of a significant mass of residual sulfide minerals in this zone after 70 a of oxidation suggest that sulfide oxidation will continue to release acid, metals, and SO4 to the environment for decades to centuries.  相似文献   

12.
The comparative behaviour of Ni, Cu and Zn in the system “mine tailings–ground water–plants” has been investigated at the Ni–Cu mine site operated by INCO Ltd. Thompson Operations, Thompson, Manitoba. Oxidation of sulphide minerals causes the release of metals from exposed tailings containing Ni ∼2000 ppm, Cu ∼150 ppm and Zn ∼100 ppm to the ground water, which contains 350 mg/L Ni, 0.007 mg/L Cu, and 1.6 mg/L Zn. The metal concentration in the ground water is affected by the relative proportions of sulfide minerals, the rate of oxidation of sulphide minerals (Ni-bearing pyrrhotite > sphalerite > chalcopyrite), and the affinity of the metals for secondary Fe-phases (Ni > Zn > Cu).  相似文献   

13.
A laboratory-based assessment of copper remobilization from Cu-rich mine tailings exposed to anoxic, sulfide rich waters was performed. The results from incubation experiments, conducted over a 20 day period, were compared to thermodynamic modelling calculations of copper speciation in sulfidic waters. The tailings materials were observed to react rapidly with added sulfide, consuming 159 μmol HS− g−1 (dry wt) within a 24 h period. The consumption of sulfide was attributed to a two stage process involving the reduction of Fe-hydroxy phases by sulfide followed by reaction with available Fe2+ and Cu2+ resulting in the formation Fe- and Cu-sulfide phases. During incubation experiments, the dissolved copper concentrations in the absence of sulfide were approximately 0.31 μmol l−1, whereas in the presence of sulfide (0.5–5 mM) concentrations were typically 0.24 μmol l−1. The experiments did not indicate enhanced solubility owing to the formation of soluble copper sulfide species. The predictions (based on the most recent thermodynamic data for aqueous Cu-sulfide and Cu-polysulfide species) did not accurately explain the laboratory observations. Model predictions were greatly influenced by the assumptions made about the oxidation state of copper under anoxic conditions and the solid sulfide phase controlling copper solubility. The study emphasizes the limitations of modelling copper speciation in sulfidic waters and the need for laboratory or field verification of predictions.  相似文献   

14.
A series of laboratory column tests on reactive mine tailings was numerically simulated to study the effect of high water saturation on preventing sulfide mineral oxidation and acid mine drainage (AMD). The approach, also known as an elevated water table (EWT), is a promising alternative to full water covers for the management and closure of sulfidic tailings impoundments and for the long term control of acid mine drainage. The instrumented columns contained reactive tailings from the Louvicourt mine, Quebec, and were overlain by a protective sand cover. Over a 13–19 month period, the columns were exposed to atmospheric O2 and flushed approximately every month with demineralized water. A free draining control column with no sand cover was also used. During each cycle, water table elevations were controlled by fixing the pressure at the column base and drainage water was collected and analyzed for pH and Eh, major ions, and dissolved metals (Fe, Zn, Cu, Pb, and Mg). The columns were simulated using the multi-component reactive transport model MIN3P which solves the coupled nonlinear equations for transient water flow, O2 diffusion, advective–dispersive transport and kinetic geochemical reactions. Physical properties and mineralogical compositions for the material layers were obtained from independent laboratory data. The simulated and observed data showed that as the water table elevation increased, the effluent pH became more neutral and SO4 and dissolved metal concentrations decreased by factors on the order of 102–103. It is concluded that water table depths less than or equal to one-half of the air entry value (AEV) can keep mine tailings sufficiently saturated over the long term, thus reducing sulfide oxidation and AMD production.  相似文献   

15.
Desert mine tailings may accumulate toxic metals in the near surface centimeters because of low water through-flux rates. Along with other constraints, metal toxicity precludes natural plant colonization even over decadal time scales. Since unconsolidated particles can be subjected to transport by wind and water erosion, potentially resulting in direct human and ecosystem exposure, there is a need to know how the lability and form of metals change in the tailings weathering environment. A combination of chemical extractions, X-ray diffraction, micro-X-ray fluorescence spectroscopy, and micro-Raman spectroscopy were employed to study Pb and Zn contamination in surficial arid mine tailings from the Arizona Klondyke State Superfund Site. Initial site characterization indicated a wide range in pH (2.5–8.0) in the surficial tailings pile. Ligand-promoted (DTPA) extractions, used to assess plant-available metal pools, showed decreasing available Zn and Mn with progressive tailings acidification. Aluminum shows the inverse trend, and Pb and Fe show more complex pH dependence. Since the tailings derive from a common source and parent mineralogy, it is presumed that variations in pH and “bio-available” metal concentrations result from associated variation in particle–scale geochemistry. Four sub-samples, ranging in pH from 2.6 to 5.4, were subjected to further characterization to elucidate micro-scale controls on metal mobility. With acidification, total Pb (ranging from 5 to 13 g kg−1) was increasingly associated with Fe and S in plumbojarosite aggregates. For Zn, both total (0.4–6 g kg−1) and labile fractions decreased with decreasing pH. Zinc was found to be primarily associated with the secondary Mn phases manjiroite and chalcophanite. The results suggest that progressive tailings acidification diminishes the overall lability of the total Pb and Zn pools.  相似文献   

16.
Despite its importance within environmental management strategies, little concern is shown to sulfide oxidation and/or hardpan formation at neutral pH where dry condition prevails. Two gold mine tailings in Egypt, El Sid and Barramiya, were studied for their geochemical/mineralogical properties, and climate influence on hardpan formation. The tailings are characterised by homogeneous silt-sized sediments (>42%), have high carbonate, predominantly as calcite for El Sid and dolomite-ankerite for Barramiya, and low-sulfide contents, chiefly as pyrite, galena and sphalerite for El Sid, and arsenopyrite–pyrite for Barramiya. El Sid is characterised by high average concentrations of Pb (2,758 mg/Kg) and Zn (2,314 mg/Kg), its lower part dominated by mafics, overlaid by granitoids. Barramiya has higher As (average 2,836 mg/Kg) content and represents a mixture of mica-schists/mafics-ultramafics. During field investigations, no hardpans were identified, only bassanite and gypsum were found at the surface of El Sid tailings, forming thin layers and desiccation crack fillings. Column experiments showed a thin crust consisting of gypsum, halite and sodium sulfate formed at the top of the column of El Sid tailings after 2 weeks, this was not recognized in the column from Barramiya. The homogenous thickened tailings deposition in both areas did not favour hardpan formation, since the critical amounts of reacting sulfides were never achieved in individual lamina, due to missing mineral/grain size fractionation. The high-temperature/low-water availability, characteristic for desert climate regions did not allow significant sulfides oxidation. Therefore, both tailings will suffer from continuous erosion and spreading out of contaminants to the environment for a prolonged period of time by sporadic flash floods.  相似文献   

17.
Microbial SO42− reduction limits accumulation of aqueous As in reducing aquifers where the sulfide that is produced forms minerals that sequester As. We examined the potential for As partitioning into As- and Fe-sulfide minerals in anaerobic, semi-continuous flow bioreactors inoculated with 0.5% (g mL−1) fine-grained alluvial aquifer sediment. A fluid residence time of three weeks was maintained over a ca. 300-d incubation period by replacing one-third of the aqueous phase volume of the reactors with fresh medium every seven days. The medium had a composition comparable to natural As-contaminated groundwater with slightly basic pH (7.3) and 7.5 μM aqueous As(V) and also contained 0.8 mM acetate to stimulate microbial activity. Medium was delivered to a reactor system with and without 10 mmol L−1 synthetic goethite (α-FeOOH). In both reactors, influent As(V) was almost completely reduced to As(III). Pure As-sulfide minerals did not form in the Fe-limited reactor. Realgar (As4S4) and As2S3(am) were undersaturated throughout the experiment. Orpiment (As2S3) was saturated while sulfide content was low (∼50 to 150 μM), but precipitation was likely limited by slow kinetics. Reaction-path modeling suggests that, even if these minerals had formed, the dissolved As content of the reactor would have remained at hazardous levels. Mackinawite (Fe1 + xS; x ? 0.07) formed readily in the Fe-bearing reactor and held dissolved sulfide at levels below saturation for orpiment and realgar. The mackinawite sequestered little As (<0.1 wt.%), however, and aqueous As accumulated to levels above the influent concentration as microbial Fe(III) reduction consumed goethite and mobilized adsorbed As. A relatively small amount of pyrite (FeS2) and greigite (Fe3S4) formed in the Fe-bearing reactor when we injected a polysulfide solution (Na2S4) to a final concentration of 0.5 mM after 216, 230, 279, and 286 days. The pyrite, and to a lesser extent the greigite, that formed did sequester As from solution, containing 0.84 and 0.23 wt.% As on average, respectively. Our results suggest that As precipitation during Fe-sulfide formation in nature occurs mainly in conjunction with pyrite formation. Our findings imply that the effectiveness of stimulating microbial SO42− reduction to remediate As contamination may be limited by the rate and extent of pyrite formation and the solubility of As-sulfides.  相似文献   

18.
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.  相似文献   

19.
Reactive transport simulations have been applied to investigate possible effects of stratification on the potential of sulfide-bearing mine tailings to form protective cemented layers. The simulations are based on characteristic strata found at a German tailings site, including sulfide-enriched heavy mineral layers, mica-enriched silt layers, and homogeneously mixed layers. The simulated secondary phases (jarosite, gypsum, amorphous ferric arsenate, amorphous Fe hydroxide, alunite, amorphous silica, and kaolinite) are similar to those observed in the field. Using scanning electron microscope analyses of cemented layers, it has been observed that the pore area becomes disconnected if the porosity is decreased to values below 15%, which would indicate a strong decrease in permeability. Stratification was found to play a crucial role in cemented layer formation. Cemented layers are absent or insignificant in systems with a homogeneous distribution of Fe-bearing sulfides. They are extensively developed in systems with (a) an arsenopyrite-rich layer or (b) a mica-enriched layer situated immediately below an Fe-sulfide enriched layer. The modeling results have clearly demonstrated that the key processes operating in scenario (a) are very different from the key processes in scenario (b). In scenario (a), the oxidation of arsenopyrite is followed by the precipitation of amorphous ferric arsenate, which can be solely responsible for significant pore reduction. In scenario (b), the presence of a large amount of reactive aluminosilicates (e.g. biotite and Ca-bearing plagioclase) immediately below the Fe-sulfide rich layer appears to be crucial. Key processes are extensive formation of Acid Rock Drainage (ARD) followed by enhanced (pH-driven) weathering of aluminosilicates, resulting in the accumulation of secondary phases directly below the Fe-sulfide rich layer. In both scenarios, a cemented layer is formed that effectively retards the further downward movement of the oxidation front. The presented details on the role of stratification in the formation of cemented layers could be considered in the construction of mining heaps as a possible measure to stimulate natural attenuation.  相似文献   

20.
The capacity of mine waste to trap CO2 is, in some cases, much larger than the greenhouse gas production of a mining operation. In mine tailings, the presence of secondary carbonate minerals that trap CO2 can therefore represent substantial fixation of this greenhouse gas. The abilities of three methods of quantitative phase analysis to measure trace nesquehonite (MgCO3·3H2O) in samples of processed kimberlite have been assessed: the method of reference intensity ratios (RIR), the internal standard method, and the Rietveld method with X-ray powder diffraction data. Tests on synthetic mixtures made to resemble processed kimberlite indicate that both the RIR and Rietveld methods can be used accurately to quantify nesquehonite to a lower limit of approximately 0.5 wt.% for conditions used in the laboratory. Below this value, estimates can be made to a limit of approximately 0.1 wt.% using a calibration curve according to the internal standard method. The RIR method becomes increasingly unreliable with decreasing abundance of nesquehonite, primarily as a result of an unpredictable decline in preferred orientation of crystallites. For Rietveld refinements, structureless pattern fitting was used to account for planar disorder in lizardite by considering it as an amorphous phase. Rietveld refinement of data collected from specimens that were serrated to minimize preferred orientation of crystallites gives rise to systematic overestimates of refined abundances for lizardite and underestimates for other phases. The resulting pattern of misestimates may be mistaken for the effect of amorphous and/or nanocrystalline material in samples. This effect is mitigated by collecting data from non-serrated specimens, which typically give relative errors on refined abundances for major and minor phases in the range of 5–20%. However, relative error can increase rapidly for abundances less than 5 wt.%. Nonetheless, absolute errors are sufficiently small that estimates can be made for the amount of CO2 stored in secondary nesquehonite using the RIR method or the Rietveld method for abundances ?0.5 wt.% and a calibration curve for abundances <0.5 wt.%. The extent to which C is being mineralized in an active mine setting at the Diavik Diamond Mine, Northwest Territories, Canada, has been investigated. Rietveld refinement results and calibrated abundances for trace nesquehonite are used to estimate the amount of CO2 trapped in Diavik tailings. Results of quantitative phase analysis are also used to calculate neutralization potentials for the kimberlite mine tailings and to estimate the contribution made by secondary nesquehonite.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号