Pyrite Surface Area Research Articles

The effect of cathodic surface areas on galvanic interactions between grinding media, chalcocite and pyrite and subsequent mineral flotation

Sulphide mineral processing often involves grinding media and a number of types of sulphide minerals at various ratios of feed grades, but conventional studies on galvanic interactions for sulphide mineral processing often looked into dual-phase galvanic contacts with an equal surface area of anode and cathode. For the first time, this study investigated the galvanic interaction of a tri-phase galvanic system (GM-Cc-Py) consisting of grinding media, chalcocite and pyrite with different surface area ratios of pyrite to chalcocite (Py/Cc) to reflect the practical nature. The electrochemical studies found that chalcocite acted as a cathode in the galvanic system in the absence of pyrite, but it was switched to an anode after the addition of pyrite at Py/Cc = 1. An increase in pyrite surface area not only further anodised grinding media and chalcocite, but also increased the galvanic interaction and the oxidation of anodic materials. This nature of galvanic interactions was found to impact the pulp chemistry after grinding and also the mineral flotation behaviour. A greater pyrite surface area caused a more reducing pulp condition and higher chalcocite oxidation, both promoting copper activation on pyrite and then pyrite flotation. This study identified a high amount of pyrite recovered in copper mineral flotation at a high pyrite feed grade and suggested solving this problem by altering the cathodic role of pyrite.

Flotation performance of chalcopyrite in the presence of an elevated pyrite proportion

• Chalcopyrite became more anodic after being contacted with pyrite. • A larger surface pyrite/chalcopyrite ratio led to a stronger galvanic interaction. • Chalcopyrite flotation was compromised by 5% pyrite but restored by 25% pyrite. • Pyrite flotation was enhanced with an increase in pyrite proportion. • Chalcopyrite or pyrite flotation was controlled by the galvanic interaction. Recently, the copper industry is facing a challenge in dealing with low-grade copper ores and the flotation performance deteriorates when a copper ore with an elevated pyrite proportion is processed. This study investigated the effect of pyrite proportion on chalcopyrite and pyrite flotation via a tri-mineral system (chalcopyrite-pyrite-quartz) with a fixed Cu head grade while varying the pyrite proportion. Electrochemical studies revealed that the overall corrosion current of chalcopyrite-pyrite galvanic interaction increased with the increase of pyrite surface area, boosting the anodic role of chalcopyrite. Flotation results showed that both Cu grade and Cu recovery decreased by the introduction of 5% pyrite, but Cu recovery was restored by the introduction of 25% pyrite. Collectorless flotation, EDTA extraction and pulp chemistry measurement indicated that chalcopyrite and pyrite flotation behaviours at different pyrite proportions were governed by the galvanic interaction between chalcopyrite and pyrite which controlled chalcopyrite oxidation and copper activation on pyrite.

Does galvanic coupling with pyrite increase the rate of dissolution of chalcopyrite under ambient conditions? An electrochemical study

The role of galvanic coupling of chalcopyrite and pyrite in the enhanced rate of dissolution of chalcopyrite in acidic sulfate and chloride solutions at 30 °C has been defined in a detailed electrochemical study. The results indicate that, under ambient conditions, the effect of galvanic contact with pyrite only marginally increases the rate of dissolution of chalcopyrite in both sulfate and chloride solutions containing iron(III) despite significant increases in the mixed potential of chalcopyrite. This has been shown to be due to the fact that the effect of potential on the rate of anodic oxidation of chalcopyrite is small in the relevant potential under these conditions. This is confirmed by the observation that the mixed potential and, therefore, the galvanic current is largely unaffected by a 4-fold decrease in pyrite surface area. The galvanic current transients compared well with potentiostatic current/time transients for chalcopyrite at the mixed potential observed during coupling. In the case of copper(II) as the oxidant in chloride solutions, the galvanic currents are small due largely to the fact that the increase in chalcopyrite potential and decrease in pyrite potential are small as a result of the lower reduction potential of the copper(II)/copper(I) couple than the iron(III)/iron(II) couple. The galvanic currents with dissolved oxygen as the oxidant are significantly lower than with iron(III) as the oxidant and confirm the well-known fact that oxygen is less effective than iron(III) as an oxidant for chalcopyrite even when galvanic coupled to pyrite. The electrochemical data obtained in this study indicate that the small galvanic currents observed do likely not account for the relatively large increases in rate previously observed in agitated leaching of chalcopyrite/pyrite slurries at elevated temperatures. • The effect of galvanic contact with pyrite on the rate of dissolution of chalcopyrite was studied. • The mixed potential of chalcopyrite increased significantly on contact with pyrite. • Small galvanic currents due to the small effect of potential on the rate of anodic oxidation of chalcopyrite. • The mixed potential and the galvanic current is largely unaffected by a 4-fold decrease in pyrite surface area. • The galvanic current transients compared well with potentiostatic current/time transients.

The galvanic effect of pyrite enhanced (bio)leaching of enargite (Cu3AsS4)

Enargite is a representative arsenic-bearing copper resource, and pyrite is often found co-existed with it. Understanding the effect of pyrite on enargite (bio)leaching is of vital importance for the recovery of copper from enargite. The galvanic interaction between enargite and pyrite in bioleaching and electrochemical systems was investigated. Bioleaching experiment showed that the addition of pyrite significantly promoted the enargite leaching rate due to the galvanic effect. However, the pyrite promotion effect on the enargite leaching rate decreased due to the oxidation of pyrite and the surface area of pyrite reduced in the bioleaching process. The dissolution rate of enargite without pyrite was controlled by diffusion through a product film, suggesting a passivation layer formed around the enargite surface. The presence of pyrite changed the rate-limiting factor of enargite dissolution to the surface chemical control, indicating that the addition of pyrite eliminated the passive effect. Electrochemical studies showed the mixed potentials (EM) and dissolution current densities (Idiss) of enargite electrodes increased when the enargite was in contact with a pyrite electrode, and the Emix and Idis further increased with increasing the surface area of the pyrite electrode. The effect of the pyrite electrode surface area on the dissolution of enargite electrodes was quantified by the kinetics of the half-cell reactions occurring in the pyrite-enargite galvanic couple under mixed potentials with oxygen and ferric ions as oxidants. Electrochemical impedance spectroscopy studies demonstrated a significant decrease in the charge transfer resistance and passivation film resistance of the enargite electrode in contact with pyrite, indicating that the cathodic reduction controlled the dissolution of enargite electrode and galvanically coupled pyrite continued to act as a cathode to facilitate the oxidation of enargite by oxygen and ferric ions even after enargite was passivated.

The effect of gold coupling on the surface properties of pyrite in the presence of ferric ions

Abstract The surface properties of gold-associated pyrite play a determining role in pyrite dissolution in gold extraction. However, the surface properties of pyrite have been always studied without the consideration of gold coupling. In this study, the reactivity, surface speciation and topography of pyrite after its oxidation with and without gold coupling in the presence of Fe3+, a strong oxidant widely used in the industry, were studied and compared using EIS (Electrochemical Impedance Spectroscopy), Cryo-XPS (X-ray Photoelectron Spectroscopy at-145 °C) and AFM (Atomic Force Microscopy), respectively. It was interesting to find that gold could catalyse Fe3+ reduction and gold coupling significantly enhanced pyrite oxidation with more oxidised iron species such as Fe3O4/Fe2O3 and FeOOH and oxidised sulphur species such as Sn2−/S8 and SO32− formed on the pyrite surface. Gold coupling also resulted in a much rougher and larger surface area of pyrite, appreciably increasing the electrochemical reactivity of pyrite for its successive oxidation and dissolution. These findings provide insights into the physicochemical properties of gold-associated pyrite, which can be exploited to improve the gold extraction process.

Kinetic studies of amyl xanthate adsorption and bubble attachment to Cu-activated sphalerite and pyrite surfaces

Kinetic studies of bubble attachment and amyl xanthate (AX) adsorption to CuSO4 (Cu) activated sphalerite and pyrite surfaces were investigated by a novel in-situ approach with high-speed video microscopy (HSVM) and cryogenic X-ray photoelectron spectroscopic (Cryo-XPS). The drainage rate (DR) of thin films between the bubble and the surfaces was linked with the rate of AX adsorption onto the mineral surfaces. The single minerals studies revealed that the drainage (and adsorption) rate on polished pyrite was higher than on polished sphalerite after treatment with AX. However, the AX adsorption rate exhibited a higher value on the Cu-activated sphalerite than on the Cu-activated pyrite, indicating different surface components formed on the minerals surfaces and their different abilities to uptake Cu ions. Modeling AX adsorption rate on the unactivated and activated minerals by a pseudo-first order rate equation revealed the dependence of the drainage (and adsorption) rate on the initial AX concentration and the available active sites on the minerals surfaces. For the two minerals system, AX treatment results showed a significant drop in the adsorption rate of AX on the activated sphalerite with increasing the sphalerite to pyrite surface area ratio from 1:1 to 1:6. It confirmed the dependence of the AX adsorption rate on the available active sites on the minerals. Cryo-XPS confirmed that Cu-xanthate complex on the Cu-activated sphalerite and dixanthogen on the Cu-activated pyrite were the dominant surface components after treating with AX. It showed a selectivity of AX on the activated sphalerite with chemisorbed Cu-xanthate formation. However, consumption of AX with Cu-activated pyrite in the form of dixanthogen as well as Cu- or Fe-xanthate complexes were inevitable, especially when the available pyrite surface area was high in the mixed mineral system.

A comparative study of the attachment of air bubbles onto sphalerite and pyrite surfaces activated by copper sulphate

Pyrite is a gangue mineral in the flotation of sphalerite activated by copper sulphate (CS). In this study, the attachment of air bubbles onto sphalerite and pyrite surfaces activated by CS was comparatively studied using a novel approach with High-Speed Video Microscopy (HSVM) and X-ray photoelectron spectroscopic (XPS). HSVM allowed for the in-situ determination of the liquid film Drainage Rate (DR) and Contact Angle (CA) of the bubble attachment. The results showed that after the activation sphalerite became hydrophobic by the formation of polysulphide as confirmed by XPS while pyrite remained hydrophilic. With increasing activation time and CS concentration, both DR and CA on the sphalerite surface rapidly increased at the beginning and reached constant values, while those on pyrite remained unchanged. The changes in the CA values of the sphalerite was successfully modelled by considering a second-order rate process for the CS activation which was proportional to initial copper concentration and available Zn active sites on the sphalerite surface. If both sphalerite and pyrite were simultaneously exposed to CS solutions (but not in the physical contact), the rate of CS activation of sphalerite remained unchanged when the ratio of sphalerite to pyrite surface areas exposed to CS solutions was equal to 1:1. However, the activation rate of sphalerite significantly dropped with increasing the sphalerite:pyrite surface area ratio from 1:3 to 1:6. The effect of the surface area ratio on the CS activation rate was prominent at activation times higher than 3min. The XPS results for the mixed minerals showed the reduction in sphalerite Cu/Zn exchange and polysulphide/Cu ratios with increasing available pyrite surface area in CS solutions, indicating the significant drop in the CS activation of sphalerite in the presence of pyrite. The outcomes of this study provided further insights into the selective flotation of sphalerite from pyrite.

Hydrothermal synthesized micro/nano-sized pyrite used as cathode material to improve the electrochemical performance of thermal battery

In this paper, single-phase pyrite with different particle sizes is prepared by hydrothermal method with the reactants of Na2S, S, and FeSO4. Compared with natural pyrite powder, synthesized pyrite nanocubes with small particle sizes and fine distribution have lower resistance, higher specific capacity and power density during the discharging process. The enlarging contacting surface area of pyrite with electrolytes will reduce the concentration polarization caused by the discharging current and improve the utilization efficiency of the pyrite materials. When pulse currents are loaded, compared with natural pyrite single-cell thermal battery, synthetic ones have smaller voltage drops and higher power density during the pulse discharging process. Influences of particle sizes on the thermal stability of the pyrite powders have also been investigated.

A Numerical Multi-Component Reactive Model for Pyrite Oxidation and Pollutant Transportation in a Pyritic, Carbonate-Rich Coal Waste Pile in Northern Iran

A one-dimensional numerical finite volume model is presented to simulate pyrite oxidation and reactive transportation of the oxidation products in a pyritic, carbonate-rich, coal waste pile. The proposed model incorporates the shrinking core concept for describing pyrite oxidation, pyrite surface area reduction, oxygen diffusion, and transport of the oxidation products through the waste pile. The model governing equations were solved using the PHOENICS computational fluid dynamics model. The accuracy of the model was verified with field data. Pyrite oxidation was more intense at shallower depths where oxygen decreased almost linearly from the pile surface to an approximate depth of 2 m. The lowest pH, 3.5, was predicted at a depth of 0.5 m. The waste pile has high neutralisation potential due to buffering by carbonate minerals. The maximum concentration of SO4 2−, 31.6 mol/m3, was predicted at an approximate depth of 4 m and to remain constant throughout the rest of waste profile. Simulation of a scenario with a cap shows that iron and sulphate was removed from the upper parts of the pile; their peak concentrations shifted downward due to dilution. Oxygen source removal limited iron and sulphate production. These results will be useful for developing an appropriate remediation scheme.

Reactive oxygen species generated in the presence of fine pyrite particles and its implication in thermophilic mineral bioleaching

In the tank bioleaching process, maximising solid loading and mineral availability, the latter through decreasing particle size, are key to maximising metal extraction. In this study, the effect of particle size distribution on bioleaching performance and microbial growth was studied through applying knowledge based on medical geology research to understand the adverse effects of suspended fine pyrite particles. Small-scale leaching studies, using pyrite concentrate fractions (106-75, 75-25, -25μm fines), were used to confirm decreasing performance with decreasing particle size (D 50 <40μm). Under equivalent experimental conditions, the generation of the reactive oxygen species (ROS), hydrogen peroxide and hydroxyl radicals from pyrite was illustrated. ROS generation measured from the different pyrite fractions was found to increase with increasing pyrite surface area loading (1.79-74.01m(2)L(-1)) and Fe(2+) concentration (0.1-2.8g L(-1)) in solution. The highest concentration of ROS was measured from the finest fraction of pyrite (0.85mM) and from the largest concentration of Fe(2+) (0.78mM). No ROS was detected from solutions containing only Fe(3+) under the same conditions tested. The potential of ROS to inhibit microbial performance under bioleaching conditions was demonstrated. Pyrite-free Sulfolobus metallicus cultures challenged with hydrogen peroxide (0.5-2.5mM) showed significant decrease in both cell growth and Fe(2+) oxidation rates within the concentration range 1.5-2.5mM. In combination, the results from this study suggest that conditions of large pyrite surface area loading, coupled with high concentrations of dissolved Fe(2+), can lead to the generation of ROS, resulting in oxidative stress of the microorganisms.

Phenylalanine as a hydroxyl radical-specific probe in pyrite slurries.

The abundant iron sulfide mineral pyrite has been shown to catalytically produce hydrogen peroxide (H2O2) and hydroxyl radical (.OH) in slurries of oxygenated water. Understanding the formation and fate of these reactive oxygen species is important to biological and ecological systems as exposure can lead to deleterious health effects, but also environmental engineering during the optimization of remediation approaches for possible treatment of contaminated waste streams. This study presents the use of the amino acid phenylalanine (Phe) to monitor the kinetics of pyrite-induced .OH formation through rates of hydroxylation forming three isomers of tyrosine (Tyr) - ortho-, meta-, and para-Tyr. Results indicate that about 50% of the Phe loss results in Tyr formation, and that these products further react with .OH at rates comparable to Phe. The overall loss of Phe appeared to be pseudo first-order in [Phe] as a function of time, but for the first time it is shown that initial rates were much less than first-order as a function of initial substrate concentration, [Phe]o. These results can be rationalized by considering that the effective concentration of .OH in solution is lower at a higher level of reactant and that an increasing fraction of .OH is consumed by Phe-degradation products as a function of time. A simplified first-order model was created to describe Phe loss in pyrite slurries which incorporates the [Phe]o, a first-order dependence on pyrite surface area, the assumption that all Phe degradation products compete equally for the limited supply of highly reactive .OH, and a flux that is related to the release of H2O2 from the pyrite surface (a result of the incomplete reduction of oxygen at the pyrite surface). An empirically derived rate constant, Kpyr, was introduced to describe a variable .OH-reactivity for different batches of pyrite. Both the simplified first-order kinetic model, and a more detailed numerical simulation, yielded results that compare well to the observed kinetic data describing the effects of variations in concentrations of both initial Phe and pyrite. This work supports the use of Phe as a useful probe to assess the formation of .OH in the presence of pyrite, and its possible utility for similar applications with other minerals.

Effect of pH on Aqueous Se(IV) Reduction by Pyrite

Interaction of aqueous Se(IV) with pyrite was investigated using persistently stirred batch reactors under O2-free (<1 ppm) conditions at pH ranging from 4.5 to 6.6. Thermodynamic calculations, an increase in pH during the experiments, and spectroscopic observation indicate that the reduction of aqueous Se(IV) by pyrite is dominated by the following reaction: FeS2+3.5HSeO3−+1.5H+=2SO4(2−)+Fe2++3.5Se(0)+2.5H2O. The released Fe(II) was partitioned between the bulk solution and pyrite surface at pH≈4.5 and 4.8, with the Fe2+ density at pyrite-solution interface about 4 orders of magnitude higher than that in the bulk solution, while iron oxyhydroxide precipitated at pH≈6.6, resulting in the decrease of dissolved iron. In the Se(IV) concentration range of the experiments, aqueous Se(IV) reduction rate follows the pseudofirst order which is in the form of ln mSe(IV)=−k′t+ln mSe(IV)0, where k′ is apparent rate constant combining the rate constant k and pyrite surface area to mass of solution ratio (A/M). And the aqueous Se(IV) reduction rate constant for a standard system (k) with 1 m2 pyrite surface area per 1 kg solution was obtained to be 1.65×10(−4) h(−1), 3.28×10(−4) h(−1), and 4.76×10(−4) h(−1) at pH around 4.5, 4.8, and 5.1, respectively. The positive correlation between reaction rate and pH disagrees with the theories that protons are consumed when HSeO3− is reduced to Se0, and negative charge density on pyrite surface increases as pH increases. Thus, a ferrous iron mediated electron transfer mechanism is proposed to operate during the reduction of aqueous Se(IV) by pyrite. pH and iron concentration affect significantly on Se(IV) reaction rate and reaction product.

Study on the purification property of pyrite and its spectra on the processing of metal-bearing wastewater

The purification property of pyrite was discussed by using in situ attenuated total reflection-Fourier transform infrared spectroscopy. Results showed that there might be dissolution–adsorption precipitation equilibrium of heavy metals on the surface of pyrite, which is dependent on the surface oxidation of pyrite and the neutralization reaction of carbonate within pyrite. If there was excessive carbonate within pyrite, the “dissolution” of metals would be less than that of the “adsorption precipitation,” making pyrite exhibit its purification property. Based on this property, pyrite was used to process simulated wastewater containing Pb2+, Hg2+, Cd2+, Cr(VI) and Cu2+. Results showed that the efficiencies of metal removal exceeded 96%. In addition, reflectance spectroscopy and absorption spectroscopy were also utilized to investigate the simulated metal-bearing wastewater treatment process. Analysis by diffused reflectance infrared Fourier transform spectroscopy confirmed that the superficial hydroxyl groups in pyrite reacted with metal ions during the wastewater treatment process. Reflectance spectroscopy in the visible region was used to characterize the variation in particle size and specific surface area of pyrite during the wastewater treatment process, which explained its increasing activity when reutilized. Further, analysis by absorption spectroscopy and X-ray photoelectron spectroscopy indicated that the process involved when using pyrite for the treatment of Cr(VI)-containing wastewater was an adsorption–precipitation process.

Reduction of chromium(VI) by pyrite in dilute aqueous solutions

Hexavalent chromium is a known toxic and mobile contaminant in the environment. Reduction of Cr(VI) using Fe(II) compounds is one of the most common practices in dealing with Cr(VI) as a contaminant. Among the various Fe(II) salts suggested for Cr(VI) treatment, pyrite, also a potential waste as well as a natural mineral, has been suggested. This study investigated the abiotic reduction of Cr(VI) by pyrite under dark and anaerobic conditions. The effect of pertinent chemical properties such as the initial Cr(VI) concentration, the amount of pyrite, temperature, and pH on the pyrite reduction of Cr(VI) was studied. The results showed that the rate of Cr(VI) reduction were strongly dependent on pH. A maximum rate at pH 3 was observed; at pH > 3.0, the initial rate of Cr(VI) reduction decreased as the pH increased and at pH < 3.0, the rates increased as pH increased. The rates of pyrite oxidation by Cr(VI) from pH 2.0 to 11.0 were found to be zero-order with respect to the concentration of initial Cr(VI) and second order with respect to the ratio of pyrite surface area to solution volume. The activation energy of the Cr(VI) reduction in the temperature range of 10–50 °C was 73 kJ/mol. The relatively high activation energy indicated that surface reaction controlled the reduction rate over this temperature range. Based on the results, it is seen that Cr(VI) reduction reaction took place in two distinct stages each involved with multiple steps. The first stage involved the dissolution of pyrite under acidic conditions that released Fe(II) and S 2 2− ions which were then readily oxidized by Cr(VI) with the production of Cr(III), Fe(III), and SO 4 2− species. In the presence of Cr(VI), direct oxidation of pyrite took place at rate slower than the above redox reactions. Immediately upon the depletion of Cr(VI), reaction between Fe(III) and excessive pyrite resulted in the generation of Fe(II) species, however.

Comparison of rate laws for the oxidation of five pyrites by dissolved oxygen in acidic solution

Pyrites from five localities were oxidized with dissolved oxygen (DO) in acidic solution at 298 K. The objective of this study was to compare the oxidation rates of pyrite samples with different impurity contents. Initial conditions were a solution pH adjusted to approximately 4 and air saturation. DO, Fe2+, Fe(III) (as Fe3+ and Fe(III)-OH complexes), and SO42− concentrations in the experimental solutions were periodically measured along with pH and redox potential (Eh). On the basis of the relationship between DO concentration and reaction duration, the rate laws for the oxidation of the pyrites were obtained. The rate laws differed for the various pyrites and can be expressed as follows: Rate = −(1/fDO) (V/A) (d[DO]/dt) = k[DO]nwhere the unit of oxidation rate is mol m−2 s−1, fDO is the stoichiometric coefficient for DO in the pyrite oxidation reaction, V is the volume (L) of the solution, A is the total surface area (m2) of pyrite, [DO] is the DO concentration (mol L−1), t is time (s), k is a rate constant at pH ∼ 4, and n is the reaction order (0.5-1). The differences in the rate laws were due to differences in the oxidation rates of Fe2+ during the oxidation of the pyrites, and the differences in the oxidation rates of Fe2+ were related to the occurrence of impurities in the pyrites. Oxidation of Fe2+ was accelerated by increases in the total amount of impurities in the pyrite. Precipitation of the Fe3+ produced during the reaction was promoted by increased amounts of arsenic in the pyrites.

Mineralogic and sulfur isotopic effects accompanying oxidation of pyrite in millimolar solutions of hydrogen peroxide at temperatures from 4 to 150 °C

Oxidation of pyrite by hydrogen peroxide (H 2O 2) at millimolar levels has been studied from 4 to 150 °C in order to evaluate isotopic effects potentially associated with radiolytic oxidation of pyrite. Gaseous, aqueous, and solid phases were collected and measured following sealed-tube experiments that lasted from 1 to 14 days. The dominant gaseous product was molecular oxygen. No volatile sulfur species were recovered from any experiment. Sulfate was the only aqueous sulfur species detected in solution, with sulfite and thiosulfate below the detection limits. X-ray diffraction patterns and images from scanning electron microscopy reveal solid residues composed primarily of hydrated ferric iron sulfates and sporadic ferric–ferrous iron sulfates. Hematite was detected only in solid residue produced during high temperature experiments. Elemental sulfur and/or polysulfides are inferred to be form on reacting pyrite surface based on extraction with organic solvents. Pyrite oxidation by H 2O 2 increases in rate with increasing H 2O 2concentration, pyrite surface area, and temperature. Rates measured in sealed-tube experiments at 25°C, for H 2O 2 concentration of 2 × 10 −3 M are 8.8 × 10 −9 M/m 2/sec, which are higher than previous estimates. A combination of reactive oxygen species from H 2O 2 decomposition products and reactive iron species from pyrite dissolution is inferred to aggressively oxidize the receding pyrite surface. Competing oxidants with temperature-dependent oxidation efficiencies results in multiple reaction mechanisms for different temperatures and surface conditions. Sulfur isotope values of remaining pyrite were unchanged during the experiments, but showed distinct enrichment of 34S in produced sulfate and depletion in elemental sulfur. The Δ sulfate–pyrite and Δ elemental sulfur–pyrite was +0.5 to +1.5‰ and was −0.2 to −1‰, respectively. Isotope data from high-temperature experiments indicate an additional 34S-depleted sulfur fraction, with up to 4‰ depletion of 34S, in the hematite. Sulfur isotope trends were not influenced by H 2O 2 concentration, temperature, or reaction time. Results of this study indicate that radiolytically produced oxidants, such as hydrogen peroxide and hydroxyl radicals, could efficiently oxidize pyrite in an otherwise oxygen-limited environment. Although H 2O 2 is generally regarded as being of minor geochemical significance on Earth, the H 2O 2 molecule plays a pivotal role in Martian atmospheric and soil chemistry. Additional experimental and field studies are needed to characterize sulfur and oxygen isotope systematics during radiolytical oxidation of metallic sulfides and elemental sulfur.

Geochemical effects of oxidation products and framboidal pyrite oxidation in acid mine drainage prediction techniques

Acid mine drainage predictive testwork associated with the Australian Mineral Industries Research Association (AMIRA) P387A Project: Prediction and Kinetic Control of Acid Mine Drainage (AMD) has critically examined static acid assessment and kinetic information from acid–base accounting techniques, including net acid production potential (NAPP), net acid generation (NAG) and column leach tests. This paper compares results on two waste rock samples that were obtained from the Kaltim Prima Coal mine (KPC) containing significant quantities of fine-grained framboidal pyrite. In agreement with other research, the authors' results indicated that framboidal pyrite is more reactive than euhedral forms due to the greater specific surface area of framboidal pyrite. This is evidenced by optical microscopy of reacted samples. Importantly, the results showed that NAPP testing is biased by the rapid acid generating oxidation of framboidal pyrite prior to, and during the acid neutralisation capacity (ANC) test. This can result in negative ANC values for samples containing significant framboidal pyrite (often “corrected” to zero kg H 2SO 4/t) when significant ANC is actually present in the sample. NAG testing using H 2O 2 indicated that samples containing a significant quantity of framboidal pyrite can result in the catalytic decomposition of the H 2O 2 prior to complete oxidation of the sulfide minerals present, requiring sequential addition of H 2O 2 for completion. A benefit of the NAG test, however, is that it assesses the net acid generation capacity of the sample without bias towards acid generation as is observed using NAPP methods. The kinetic NAG test also gives information on the reaction sequence of framboidal and euhedral pyrite. Periodic (kinetic) analysis of sub-samples from column leach tests indicated rapid oxidation of the framboidal pyrite compared to the euhedral pyrite, which was correlated with the greater framboidal pyrite surface area. Calculations to determine the sulfide/sulfate acidity derived from the oxidation of framboidal pyrite prior to; and during the ANC test have been developed to provide a better indication of the actual ANC (ANC Actual) of the sample. Paste pH values of <pH 4–5 may be one suitable trigger mechanism for the implementation of this new method. This has led to an improved NAPP estimation of total acid production. Together with NAG and column leach testing this improved methodology has resulted in accurate AMD characterisation of samples containing acidic oxidation products and framboidal pyrite.

SEM and AFM images of pyrite surfaces after bioleaching by the indigenous Thiobacillus thiooxidans.

The bioleaching mechanism of pyrite by the indigenous Thiobacillus thiooxidans was examined with the aid of scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the pyrite surface. The presence of pyrite eliminated the lag phase during growth of this microorganism. This was due to the stimulatory effect on cell growth of the slight amount of Cu2+ that had leached from the pyrite. Zn2+ was found to be much more readily solubilized than Cu2+. The efficiency of bioleaching was four times higher than that of chemical leaching. SEM images provided evidence of direct cell attachment onto the pyrite surface, thereby enhancing the bioleaching rate. Furthermore, extracellular polymeric substances (EPSs) were found on the pyrite surface after 4 days of oxidation. AFM images showed that the pyrite surface area positively correlated with the oxidation rate. A combination of direct and indirect mechanism is probably responsible for the oxidation of pyrite by T. thiooxidans.

Cu(II) adsorption mechanism on pyrite: an XAFS and XPS study

The surface of a <38 µm ground fraction of pyrite was analysed using x-ray photoelectron spectroscopy (xps), x-ray absorption fine structure spectroscopy (XAFS) and inductively coupled plasma atomic emission spectroscopy (ICPAES) prior to and after Cu(II) adsorption under different pH and Cu(II) concentration conditions. Under all conditions copper is found to be coordinated to sulphur on the surface of pyrite with an average bond length of 2.27±0.02 Å. A Cu–O bond with a length of 2.00 Å due to Cu(OH)2 precipitation was also observed for a sample with copper adsorbed at pH 8.5 from a solution initially containing 2.84 × 10−4 mol m−2 Cu (all concentrations given are normalized against the surface area of pyrite in solution). The Cu 2p XPS signals for the samples with adsorbed copper show the presence of Cu(I) ions on the pyrite surfaces in all cases. For the 2.84 × 10−4 mol m−2 Cu solution at pH 8.5 the pyrite was observed to have both a Cu(I) and a Cu(II) XPS component. The ground pyrite surfaces showed increases in the oxidized S22− (Eb = 163.5 eV) signal on copper adsorption with minor increases in Fe(III) oxy- or hydroxy-species observed on pyrite surfaces exposed to long periods of adsorption. It is proposed that the reduction of Cu(II) to Cu(I) on the surface of the pyrite is accompanied by the oxidation of the S22− species present in the pyrite surface. Solution analysis in conjunction with surface analysis has indicated that a 1 : 1 exchange of Fe(II) ions for Cu(II) ions did not occur during copper adsorption and thus an ion-exchange mechanism is ruled out. Copyright © 2000 John Wiley & Sons, Ltd.

Thermal decomposition of acetate: III. Catalysis by mineral surfaces

The kinetics of thermal decarboxylation of aqueous solutions of acetic acid and sodium acetate were evaluated at 335 and 355°C in contact with various surfaces as potential catalysts. Quartz, fused quartz, calcite, natural pyrite, titanium oxide, and Au apparently do not catalyze aqueous decarboxylation reactions, in contrast to Pyrex, Ca-montmorillonite, Fe-bearing montmorillonite, hematite, synthetic pyrite, and magnetite. The dependence of the rate of acetic acid decarboxylation on the surface area of pyrite per unit solution volume was also studied. The results show that the decarboxylation of acetic acid and acetate is catalyzed heterogeneously, with the cleavage of the C-C bond occurring while the acetate molecule is adsorbed onto a surface. Entropies and enthalpies of activation obtained from these experiments are compatible with the isokinetic relationship established previously for acetic acid and acetate under similar experimental conditions, indicating the existence of a common rate-determining step. Experimental evidence indicates that oxidation of acetic acid can occur with hematite and defected magnetite. These oxidative decomposition reactions differ from the decarboxylation reaction in that CO 2 and polycondensates are produced instead of CO 2 and CH 4.