Dissolution Of Pyrite Research Articles

Inhibition of Acid Mine Drainage from a Pyrite-rich Mining Waste Using Industrial By-products: Role of Neo-formed Phases

In the present study, the potential use of the industrial waste residues, such as coal fly ash and clinker dust, was evaluated in inhibiting acid mine drainage generation from pyrite-rich wastes using non-saturated column experiments. Two columns (B and C) were filled with a mixture of industrial residues over pyrite-quartz sand (PS), while one column (A) used as the control was solely filled with PS. Artificial irrigation was maintained in each column by pouring Milli-Q water (pH 6.5). The leachate chemistry and the precipitation of neo-formed phases in the columns were examined. Based on the pH and concentration of SO4 2− in the effluent collected over a period of time, it can be inferred that pyrite dissolution was dominant in column A. In columns B and C, the addition of industrial waste produced a near neutral to alkaline leachate. At this pH, Fe released from pyrite oxidation was immediately depleted by precipitating into Fe-oxyhydroxide phases that likely coated the pyrite grains (termed as microencapsulation) which inhibited oxidation. This was supported from the scanning electron microscope observation and the geochemical modeling results. In addition, the precipitation of other neo-formed phases such as calcium carbonate and gypsum, along with the pozzolanic reactions of industrial wastes, possibly increased the cementation of fly ash particles leading to the development of a compact material, which prevented further oxidation by restricting infiltration and oxygen contact to the pyrite surface. This occurred best in column B; thus, it produced better quality of leachates than column C, though at the end both columns showed a significantly decreased concentration of Fe (up to 99.9 %) and other metals such as Cu, Cr, Pb, Zn, and Mn (lower values than the WHO permissible limits of drinking water) compared to control.

Arsenic in a fractured slate aquifer system, New England, USA: Influence of bedrock geochemistry, groundwater flow paths, redox and ion exchange

Elevated As levels have been reported by the Vermont Geological Survey in groundwater from public and domestic bedrock wells in northwestern New England (USA). The study area in southwestern Vermont is underlain by pyrite-rich, organic-rich slates that were thrusted over carbonate and clastic sedimentary rocks of the continental shelf during the Ordovician Taconian Orogeny, and the distribution of wells with elevated As shows that they were completed in slates. Hydrochemical and bedrock geochemical analysis indicates that elevated As in the aquifer system is controlled by the following: (1) the presence of black slates that are rich in arsenian pyrite (200–2000ppm As); (2) release of As via the dissolution of As-rich pyrite; (3) geochemically-reducing and slightly alkaline conditions, where high As values occur at Eh<200mV and pH>7; and (4) physical hydrogeological parameters that foster low Eh and high pH, particularly long groundwater flow paths and low well yields (i.e. high residence time) which provides high rock to water ratios. Where all four factors affect As contents in groundwater, 72% of wells in a zone of distal groundwater flow/low-relief topography exceed 10μg/L (ppb) and 60% of wells in this zone exceed 25ppb As. Where flow paths are shorter in slates and groundwater has higher Eh and lower pH (i.e. in regions of higher-relief topography closer to recharge zones), only 3% of wells contain >10ppb As and none contain >25ppb.Overall, 28% (50/176) of low-elevation wells (<245meters above sea level [masl]) exceed 10ppb As; only 3% (2/60) of higher-elevation wells (245–600masl) exceed 10ppb As. Over the entire aquifer system, 22% of bedrock wells (52/236) exceed 10ppb and the mean As concentration is 12.4ppb. Strong positive correlations among Fe, SO4 and As in groundwater confirm that dissolution of pyrite is the dominant As source. Positive correlations among SO4, Na and As indicate that, in reducing (Eh<200mV) groundwater, Fe(II) is exchanged for Na on mineral surfaces following pyrite dissolution and As remains in solution; conversely, in oxidizing groundwater (recharge zones), Fe(II) is oxidized to Fe(III) and the subsequent formation of ferrihydrite removes As (V) from solution.

Microbial Ecology in Extreme Acidic Pit Lakes from the Iberian Pyrite Belt (SW Spain)

Abstract. In mining areas in which sulfur-containing ores are exposed, the oxidation of sulfides leads to the formation of acidic mine drainage (AMD) waters. Both traditional and molecular microbial studies have shown that chemolithotrophic sulfur- and iron-oxidizing bacteria are responsible for this activity. The Iberian Pyrite Belt (IPB), located in southwestern Spain represents one of the world’s largest accumulations of mine wastes and AMD waters. Mineralogical and textural characteristics of the IPB ores favor the oxidation and dissolution of pyrite and the subsequent formation of AMD waters. Acidic pit lakes in the IPB are a hotspot for this activity. These pit lakes are former open pit mines where ores have been exposed leading the formation of AMD waters. At present, there are more than 25 of these pit lakes between the provinces of Huelva and Seville (SW Spain). This work reports the physical properties, hydrogeochemical characteristics, and microbial diversity of two pit lakes located in the IPB Nuestra Señora del Carmen (NSC), and Concepción (CN). Both pit lakes are acid (pH 2-4) and showed chemical and thermal stratification with well defined chemoclines. One particular characteristic of NSC is that it has developed a chemocline very close to the surface (2 m depth). Microbial community composition of the water column was analyzed by 16S and 18S rRNA gene cloning and sequencing. The microorganisms detected in NSC are characteristic of acid mine drainage (AMD), including iron oxidizing bacteria (Leptospirillum) and facultative iron reducing bacteria and archaea (Acidithiobacillus ferrooxidans, Actinobacteria, Acidimicrobiales, Ferroplasma) detected in the bottom layer. Diversity in CN was higher than in NSC. Microorganisms known from AMD systems (Acidiphilium,AcidobacteriaandFerrovum) and microorganisms never reported from AMD systems were both identified.

Electrochemical Behavior of Carbon Paste Electrode with Gold-Bearing Pyrite in Bioleaching

The dissolution of gold-bearing pyrite plays an important role in bioleaching of gold. This paper describes a fundamental study on the electrochemical behavior and reaction mechanisms of gold-bearing pyrite leaching in the form of Carbon Paste Electrode (CPE) with and without microorganisms using Cyclic Voltammetry (CV) and polarization curve. A two step process was suggested from Cyclic voltammetry. Electrode passivation by elemental sulphur was observed below 700mV (vs. SCE), elemental sulphur was then oxidized to sulphate when the electrode potential further increased from 700mV. The polarization current density of CPE and the oxidation rate of pyrite are further enhanced by the presence of microorganisms. Analyses of EDS and XPS confirmed the formation of elemental sulphur and sulphate. This electrochemical method successfully showed its simplicity and reliability to measure oxidation rate of gold bearing pyrite.

Bioleaching of Sulphide Minerals with &lt;i&gt;Sulfobacillus acidophilus&lt;/i&gt; at 45°C

Copper sulphide ore bioleaching processes are currently under full development in Chile and worldwide, for treatment of: secondary sulphide in heaps [1], low-grade ores in dumps, and concentrates in reactors at high temperatures [2]. Abundant knowledge is available in the literature and great experience has been gathered about the behavior of mesophilic microorganisms and their application in bioleaching of sulfides in heaps and dumps [, as well as, about the behavior of extreme thermophilic microorganisms in reactors [. However, there is little information about the behavior of some moderately thermophilic microorganisms in the dissolution of chalcopyrite and pyrite at 45oC [. This is the case ofS. acidophilus, a gram-positive rod shaped that often forms endospores and grow at temperature optima, generally between 45 and 55°C. It has been characterized as a chemolithoheterotrophic bacteria that obtains energy from the oxidation of ferrous iron or from reduced sulfur compounds and that utilizes yeast extracts as a carbon source [.S. acidophiluswas isolated by Golovacheva and Karavaiko [ and described by Norris et al. [. Limited literature exists about its participation in bioleaching processes. In this work, we present experimental results obtained from the bioleaching of chalcopyrite and pyrite withS. acidophilusat 45°C, and compare them with those obtained withAt. ferrooxidansat 30°C.

Polysulfides do not cause passivation: Results from the dissolution of pyrite and implications for other sulfide minerals

One of the most prevalent suppositions in hydrometallurgy and geochemistry is that sulfur, polysulfides or metal-deficient layers are responsible for the passivation of the dissolution of mineral sulfides. Hence, the purpose of this work was to determine whether the dominant paradigm, that polysulfides are the cause of passivation, is true. The effect of polysulfides on the rate of dissolution of pyrite was studied specifically under conditions in which passivation and pseudo-passivation were not observed. The argument of this work is that if polysulfides are present on the surface and no passivation is observed, then polysulfides are not the primary passivating agent as previously claimed. In order to do this, pyrite was oxidized electrochemically at different potentials (anodic dissolution) under conditions in which no passivation was observed. The pyrite samples were then analysed by X-ray electron spectroscopy (XPS) and Raman spectroscopy (RS). The thickness of the polysulfide layer was estimated by two methods (i) the gold sputtering technique in conjunction with the XPS analysis, and (ii) a comparison of relative peak heights on the RS. Both techniques showed that the polysulfide thickness increased with potential. However, at the same time the electrical current increased exponentially with potential, indicating that polysulfides are not the causative agent for passivation. Samples were also obtained from experiments in which pyrite was dissolved by ferric ions (oxidative dissolution) and in the presence of bacteria (bacterial dissolution). The thickness of the polysulfide layer was similar in both cases to that of the anodic dissolution experiments, suggesting that neither ferric ions nor bacteria affected these surface coatings. Finally, it is argued that mineral sulfides experience pseudo-passivation, and a more probable explanation for this pseudo-passivation is one based on semiconductor electrochemistry.

Effect of oxygen co-injected with carbon dioxide on Gothic shale caprock–CO2–brine interaction during geologic carbon sequestration

Co-injection of oxygen, a significant component in CO2 streams produced by the oxyfuel combustion process, can cause a significant alteration of the redox state in deep geologic formations during geologic carbon sequestration. The potential impact of co-injected oxygen on the interaction between synthetic CO2–brine (0.1M NaCl) and shale caprock (Gothic shale from the Aneth Unit in Utah) and mobilization of trace metals was investigated at ~10MPa and ~75°C. A range of relative volume percentages of O2 to CO2 (0, 1, 4 and 8%) were used in these experiments to address the effect of oxygen on shale–CO2–brine interaction under various conditions. Major mineral phases in Gothic shale are quartz, calcite, dolomite, montmorillonite, and pyrite. During Gothic shale–CO2–brine interaction in the presence of oxygen, pyrite oxidation occurred extensively and caused enhanced dissolution of calcite and dolomite. Pyrite oxidation and calcite dissolution subsequently resulted in the precipitation of Fe(III) oxides and gypsum (CaSO4·2H2O). In the presence of oxygen, dissolved Mn and Ni were elevated because of oxidative dissolution of pyrite. The mobility of dissolved Ba was controlled by barite (BaSO4) precipitation in the presence of oxygen. Dissolved U in the experimental brines increased to ~8–14μg/L, with concentrations being slightly higher in the absence of oxygen than in the presence of oxygen. Experimental and modeling results indicate the interaction between shale caprock and oxygen co-injected with CO2 during geologic carbon sequestration can exert significant impacts on brine pH, solubility of carbonate minerals, stability of sulfide minerals, and mobility of trace metals. The major impact of oxygen is most likely to occur in the zone near CO2 injection wells where impurity gases can accumulate. Oxygen in CO2–brine migrating away from the injection well will be continually consumed through the reactions with sulfide minerals in deep geologic formations.

Oxidative dissolution of pyrite in acidic media

The pyrite oxidative dissolution in air-saturated (AS), H2O2, and Fe3+ solutions at pH 2.5 and 25 °C was investigated by electrochemical and aqueous batch experiments. The corrosion current density (icorr) increases from AS solution to Fe3+ and H2O2 solutions. For the same oxidant, icorr increases when the concentration of the oxidant increases. Similar variation was observed for the corrosion potential (Ecorr). Electrochemical impedance spectroscopy measurements have indicated that in AS and H2O2 solutions, the charge transfer is the rate determining step of pyrite oxidative dissolution. In the presence of Fe(aq)3+, both the charge transfer process and mass transfer caused by the diffusion of oxidant or reaction products across the interface of electrode control the mineral oxidative dissolution. The corrosion current densities of oxidative dissolution measured by electrochemical methods are higher than those estimated from dissolution rates determined by aqueous bath experiments. The observed differences suggest that the mechanism of polarized electrode oxidation is different by the mechanism of pyrite oxidation under open circuit conditions.

Sources of Sr and S In Aluminum-Phosphate-Sulfate Minerals In Early-Middle Triassic Sandstones (Iberian Ranges, Spain) and Paleoenvironmental Implications for the West Tethys

Aluminum-phosphate–sulfate (APS) minerals, formed during early diagenesis in relation to acid meteoric waters, are the main host of Sr and S in the Early–Middle Triassic continental sandstones of the Iberian Ranges (east of the Iberian Peninsula). The sources of these elements and the effects of paleoenvironmetal changes on these sources and on the formation of APS minerals during Early–Middle Triassic times, were established on the basis of Sr and S isotopic analyses. The S and Sr data (d34S V-CDT = +11 to +14% and 87Sr/86Sr = 0.7099–0.7247, respectively) can be interpreted as resulting from mixing of different sources. Strontium was sourced from the dissolution of pre-existing minerals like K-feldspar and clay minerals inherited from the source areas, causing high radiogenic values. However, the isotopic signal must also be influenced by other sources, such as marine or volcanic aerosol that decreased the total 87Sr/86Sr ratios. Marine and volcanic aerosols were also sources of sulfur, but the d34S was lowered by dissolution of pre-existing sulfides, mainly pyrite. Pyrite dissolution and volcanic aerosols would also trigger the acid conditions required for the precipitation of APS minerals. APS minerals in the study area are found mainly in the Canizar Formation (Olenekian?–Aegian), which has the lowest 87Sr/86Sr ratios. The lower abundance of APS minerals in the Eslida Formation (Aegian–Pelsonian) may indicate change in the acidity of pore water towards more alkaline conditions, while the increased 87Sr/86Sr ratios imply decreased Sr input from volcanic activity and/or marine aerosol inputs during Anisian times. Therefore, the decrease in abundance of APS minerals from the Early to Middle Triassic and the variations in the sources of Sr and S are indicative of changes in paleoenvironmental conditions during the beginning of the Triassic Period. These changes from acid to more alkaline conditions are also coincident with the first appearance of carbonate paleosols, trace fossils, and plant fossils in the upper part of the Canizar Formation (and more in the overlying Eslida Formation) and mark the beginning of biotic recovery in this area. The presence of APS minerals in other European basins of the Western Tethys (such as the German Basin, the Paris Basin and the southeastern France and Sardinia basins) could thus also indicate that unfavorable environmental conditions caused delay in biotic recovery in those areas. In general, the presence of APS minerals may be used as an indicator of arid, acidic conditions unfavorable to biotic colonization.

Probing deep weathering in the Shale Hills Critical Zone Observatory, Pennsylvania (USA): the hypothesis of nested chemical reaction fronts in the subsurface

ABSTRACTWeathering is both an acid‐base and a redox reaction in which rocks are titrated by meteoric carbon dioxide (CO2) and oxygen (O2). In general, the depths of these weathering reactions are unknown. To determine such depths, cuttings of Rose Hill shale were investigated from one borehole from the ridge and four boreholes from the valley at the Susquehanna Shale Hills Observatory (SSHO). Pyrite concentrations are insignificant to depths of 23 m under the ridge and 8–9 m under the valley. Likewise, carbonate concentrations are insignificant to 22 and 2 m, respectively. In addition, a 5–6 m‐thick fractured layer directly beneath the land surface shows evidence for loss of illite, chlorite, and feldspar. Under the valley, secondary carbonates may have precipited.The limited number of boreholes and the tight folding make it impossible to prove that depth variations result from weathering instead of chemical heterogeneity within the parent shale. However, carbonate depletion coincides with the winter water table observed at ~20 m (ridge) and ~2 m depth (valley). It would be fortuitous if carbonate‐containing strata are found under ridge and valley only beneath the water table. Furthermore, pyrite and carbonate react quickly and many deep reaction fronts for these minerals are described in the literature. We propose that deep transport of O2 initiates weathering at SSHO and many other localities because pyrite commonly oxidizes autocatalytically to acidify porewaters and open porosity. According to this hypothesis, the mineral distributions at SSHO are nested reaction fronts that overprint protolith stratigraphy. The fronts are hypothesized to lie subparallel to the land surface because O2 diffuses to the water table and causes oxidative dissolution of pyrite. Pyrite‐derived sulfuric acid (H2SO4) plus CO2 also dissolve carbonates above the water table. To understand how reaction fronts record long‐term coupling between erosion and weathering will require intensive mapping of the subsurface. Copyright © 2013 John Wiley &amp; Sons, Ltd.

The effect of sulphate ions and temperature on the leaching of pyrite. III. Bioleaching

The effect of the total sulphate concentration on the rate of pyrite dissolution was investigated in batch, bioleach tests on a high-grade concentrate at 35°C, 50°C and 65°C.Good microbial activity was achieved for concentrations up to 42g/L sulphate. However, higher concentrations resulted in decreased ferrous oxidation kinetics, with the solution potential profiles showing “lag periods” at the highest sulphate concentration.The bioleach results showed that the rate of pyrite dissolution decreased markedly above an average sulphate concentration of 55g/L for all three temperatures, with the adverse effect on the dissolution kinetics being more pronounced at 50°C and 65°C. As a result of this, increased temperature has no beneficial effect on the rate of dissolution at concentrations of sulphate above 55g/L.High extents of pyrite dissolution (90% to 98%) were achieved at all three temperatures irrespective of sulphate concentration, with almost identical extents of sulphide oxidation.Rates of pyrite dissolution under bioleaching conditions agree well with those derived under similar conditions from abiotic dissolution experiments and electrochemical measurements. This important observation confirms the so-called indirect mechanism for the bioleaching process in the case of pyrite and also that the mixed potential model can be used to quantitatively describe both abiotic dissolution and biotic dissolution of pyrite in acidic solutions.

Dissolution of Pyrite Particles by Chemical Oxidation and Bioleaching

Dissolution of pyrite by chemical oxidation and bioleaching were studied with using short-term batch experiments. The results show that the rate of oxidative dissolution of pyrite increases with the increasing concentration of ferric in ferric sulfate solutions. With the corresponding in the bioleaching, the leaching rate of pyrite is markedly affected by the Eh of the solution. The ferric/ferrous ratio controls the relative rate of the oxidation reactions involved in the process. Additionally, the leaching rate of pyrite is controlled by the pH. The phase analysis of products indicated that S2- 2oxidation can produce S0 and SO2- 4under these conditions. On the basis of predecessors and using these species，the simplest expected oxidation mechanism is S2O2- 3regarded as the intermediate mechanism during the oxidation process.

Effect of pH and Dissolved Silicate on the Formation of Surface Passivation Layers for Reducing Pyrite Oxidation

Acid mine drainage (AMD)and toxic metal release generated by oxidation of sulphide minerals, particularly pyrite, in mine wastes,are a critical environmental issue worldwide.Currently, there are many options to diminish sulphide oxidation including barrier methods that isolate pyrite from oxygen or water, chemical additives and inhibition of iron-oxidizing bacteria. This study focuses on understanding the rolethatsilicate and pH conditions play in the formation and stabilisation ofpyrite surface passivation layers found in lab and field studies.The results from pyrite dissolution tests under various conditions showed that the pyrite oxidation rate has been reduced by up to 60% under neutral pH with additional soluble silicate. Solution speciation calculation predicted that crystalline goethite is formed in the experiment without silicate additionbutan amorphous iron hydroxide surface layer is stabilizedby the addition of the silicate, inhibiting goethite formation and continuing pyrite oxidation.This coherent, continuous amorphous layer has been verified in SEM.

Origin, mobility, and temporal evolution of arsenic from a low-contamination catchment in Alpine crystalline rocks

The reduction to 10μg/l of the limit for arsenic in drinking water led many resource managers to deal with expensive treatments. In the very common case of arsenic levels close to the recommended maximum concentration, knowing the origin and temporal evolution of As has become of great importance. Here we present a case study from an alpine basin. Arsenic speciation, isotopic compositions of pyrite, sulfate and water, and concentrations of major and trace elements demonstrate a geogenic source for arsenic linked to the dissolution of pyrite. We provide new tools to further study As at low concentrations where many processes may be masked. The observed negative correlation between δ34SSO4 and [As] is interpreted as a Rayleigh-type sulfur-isotope fractionation during increasing pyrite dissolution. The observed positive correlation between δ18OSO4 and AsV/AsIII could help to retrieve initial redox conditions. A 3-year long monitoring at high-resolution demonstrated that drought conditions enhance pyrite dissolution whose degradation products are scavenged by recharge water. An increase in As in groundwater may result from droughts due to enhanced oxygen entry in the unsaturated zone. The 2003 European heatwave had a major effect.

Bioleaching of low grade uranium ore containing pyrite using A. ferrooxidans and A. thiooxidans

A process of uranium extraction from ore containing 3.1 % pyrite by bacterial leaching was investigated in shaken flasks during 90 days. The highest uranium recovery amounting to 85.1 % was obtained using binary mixture of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans that was exceeding results obtained by traditional acid leaching technique up to 27 %. High uranium recovery was founded to be due to the high degree of pyrite dissolution that can be readily achieved by bacterial leaching (up to 98.0 %).

Fenton-Like Oxidation of Refractory Chemical Wastewater Using Pyrite

Fenton-like reaction technologies are attracting considerable attention due to the high oxidizing ability, especially in the treatment of refractory chemical contaminants. However, some disadvantages in traditional Fenton reagents limited its wide application. In this study, pyrite, a natural iron bearing mineral, was used as a new kind of Fenton-like catalyst in the treatment of textile wastewater. The catalytic activity of pyrite and the parameters influencing the removal of Chemical Oxygen Demand (COD) and Biochemical Oxygen Demand (BOD5), such as pyrite dosage, H2O2 concentration, pH, and suspended solids (SS) had been systematically examined. Results showed that pyrite exhibited a high catalytic reactivity over a wide pH value range. At the optimal conditions, a COD removal rate of 70% and an increase in the mass ratio of BOD5/COD (B/C) from 0.25 to 0.56 were achieved in the presence of 9.7 mM H2O2 and 10 g/L pyrite at initial solution pH value of 9.0. And what’s more, a rapid decrease in solution pH and a high level of iron were observed when pyrite was added to the wastewater, probably due to the oxidative dissolution of pyrite. Consequently, Apart from the heterogeneous process, it was found that a homogeneous interaction between ferrous iron and H2O2 in a classical Haber-Weiss mechanism was also occurred. The wonderful reactivity of pyrite makes it be used as catalyst for the oxidative technology to treat the original wastewater without the need to pre-adjust the solution pH.

Thermodynamics of carbofuran adsorption on pyrite

Thermodynamic parameters of carbofuran adsorption on pyrite were determined from experimental data. In order to minimize the dissolution of pyrite as well the degradation of carbofuran, all experiments were restricted to (15 to 20)min. The maximum adsorption capacity of carbofuran varied with both pH and temperature in the order: ΓcarbmaxpH1.77>ΓcarbmaxpH2.70>ΓcarbmaxpH4.01. The Dubinin–Radushkevick (D–R) model was used to assess the degree of site heterogeneity. The interactions between carbofuran and surface sites can largely be ascribed to specific chemical interactions.

Oxidative dissolution of pyrite surfaces by hexavalent chromium: Surface site saturation and surface renewal

In-situ reduction of toxic Cr(VI) to nontoxic Cr(III) represents an important natural attenuation process for Cr(VI)-impacted environments. This study investigates the stoichiometry and kinetics of Cr(VI) reduction by pyrite, a reduced iron–sulfur mineral ubiquitous in recent estuarine and marine sediments. Pyrite suspensions at surface loadings of 0.28–2.10m2/L (typical of estuarine or marine sediments) were capable of completely reducing 7–120μM Cr(VI) on the timescale of minutes to days, with the time to reaction completion decreasing with increasing pyrite loading, decreasing initial Cr(VI) concentration, and decreasing suspension pH. Analysis of metal species (Cr and Fe) and sulfur species in solution and at the mineral surface indicated that Cr(VI) oxidatively dissolved the pyrite surface, releasing ferrous iron and sulfate into solution as the reaction progressed. Surface disulfide groups were postulated as the Cr(VI)-reactive surface entity. Net production or consumption of aqueous Fe(II) was shown to depend upon the relative rates of proton-promoted Fe(II) release, Fe(II) release due to oxidative dissolution of pyrite in the presence of Cr(VI), and Fe(II) consumption due to homogeneous reaction with Cr(VI).Kinetics of Cr(VI) reduction by pyrite displayed a biphasic pattern, and the time to reaction completion increased dramatically with increasing initial Cr(VI) concentration. Rapid Cr(VI) removal occurred early in the reaction progress, attributable to Cr(VI) loss under an adsorption-limited regime. Slow, approximately zero-order, Cr(VI) removal occurred over the bulk of the time courses, and corresponded to Cr(VI) removal under surface site saturation conditions. Stoichiometric Cr(VI) reduction was able to proceed under surface site limited conditions owing to regeneration of reactive surface sites following desorption/dissolution of oxidized surface products, as demonstrated in repeat Cr(VI)-spiking experiments. The role of surface passivation was evaluated by comparing rates of Cr(VI) reduction in the presence and absence of the Cr(III)-complexing agent citrate. While citrate addition significantly enhanced Cr(III) solubility, rates of Cr(VI) reduction were only marginally accelerated, suggesting that Cr(OH)3(s) coatings did not completely block access of Cr(VI) to reactive surface sites on pyrite. Given the rapid rates of Cr(VI) reduction with pyrite under pH and surface coverage conditions typical of natural environments, we propose that Cr(VI) reduction by pyrite be considered in fate and transport models for Cr in contaminated sediments.

Pyrite-driven reactive oxygen species formation in simulated lung fluid: implications for coal workers’ pneumoconiosis

The origin of coal worker's pneumoconiosis (CWP) has been long debated. A recent epidemiological study shows a correlation between what is essentially the concentration of pyrite within coal and the prevalence of CWP in miners. Hydrogen peroxide and hydroxyl radical, both reactive oxygen species (ROS), form as byproducts of pyrite oxidative dissolution in air-saturated water. Motivated by the possible importance of ROS in the pathogenesis of CWP, we conducted an experimental study to evaluate if ROS form as byproducts in the oxidative dissolution of pyrite in simulated lung fluid (SLF) under biologically applicable conditions and to determine the persistence of pyrite in SLF. While the rate of pyrite oxidative dissolution in SLF is suppressed by 51% when compared to that in air-saturated water, the initial amount of hydrogen peroxide formed as a byproduct in SLF is nearly doubled. Hydroxyl radical is also formed in the experiments with SLF, but at lower concentrations than in the experiments with water. The formation of these ROS indicates that the reaction mechanism for pyrite oxidative dissolution in SLF is no different from that in water. The elevated hydrogen peroxide concentration in SLF suggests that the decomposition, via the Fenton mechanism to hydroxyl radical or with Fe(III) to form water and molecular oxygen, is initially inhibited by the presence of SLF components. On the basis of the oxidative dissolution rate of pyrite measured in this paper, it is calculated that a respirable two micron pyrite particle will take over 3years to dissolve completely.

Significance of Microbial Communities and Interactions in Safeguarding Reactive Mine Tailings by Ecological Engineering

Pyritic mine tailings (mineral waste generated by metal mining) pose significant risk to the environment as point sources of acidic, metal-rich effluents (acid mine drainage [AMD]). While the accelerated oxidative dissolution of pyrite and other sulfide minerals in tailings by acidophilic chemolithotrophic prokaryotes has been widely reported, other acidophiles (heterotrophic bacteria that catalyze the dissimilatory reduction of iron and sulfur) can reverse the reactions involved in AMD genesis, and these have been implicated in the "natural attenuation" of mine waters. We have investigated whether by manipulating microbial communities in tailings (inoculating with iron- and sulfur-reducing acidophilic bacteria and phototrophic acidophilic microalgae) it is possible to mitigate the impact of the acid-generating and metal-mobilizing chemolithotrophic prokaryotes that are indigenous to tailing deposits. Sixty tailings mesocosms were set up, using five different microbial inoculation variants, and analyzed at regular intervals for changes in physicochemical and microbiological parameters for up to 1 year. Differences between treatment protocols were most apparent between tailings that had been inoculated with acidophilic algae in addition to aerobic and anaerobic heterotrophic bacteria and those that had been inoculated with only pyrite-oxidizing chemolithotrophs; these differences included higher pH values, lower redox potentials, and smaller concentrations of soluble copper and zinc. The results suggest that empirical ecological engineering of tailing lagoons to promote the growth and activities of iron- and sulfate-reducing bacteria could minimize their risk of AMD production and that the heterotrophic populations could be sustained by facilitating the growth of microalgae to provide continuous inputs of organic carbon.