The oxidative dissolution of arsenopyrite (FeAsS) and enargite (Cu3AsS4) by Leptospirillum ferrooxidans

Arsenopyrite (FeAsS) and enargite (Cu₃AsS₄) fractured in a nitrogen atmosphere were characterised after acidic (pH 1.8), oxidative dissolution in both the presence and absence of the acidophilic microorganism Leptospirillum ferrooxidans. Dissolution was monitored through analysis of the coexisting aqueous solution using inductively coupled plasma atomic emission spectroscopy and coupled ion chromatography-inductively coupled plasma mass spectrometry, and chemical changes at the mineral surface observed using X-ray photoelectron spectroscopy and environmental scanning electron microscopy (ESEM). Biologically mediated oxidation of arsenopyrite and enargite (2.5 g in 25 ml) was seen to proceed to a greater extent than abiotic oxidation, although arsenopyrite oxidation was significantly greater than enargite oxidation. These dissolution reactions were associated with the release of ∼917 and ∼180 ppm of arsenic into solution. The formation of Fe(III)-oxyhydroxides, ferric sulphate and arsenate was observed for arsenopyrite, thiosulphate and an unknown arsenic oxide for enargite. ESEM revealed an extensive coating of an extracellular polymeric substance associated with the L. ferrooxidans cells on the arsenopyrite surface and bacterial leach pits suggest a direct biological oxidation mechanism, although a combination of indirect and direct bioleaching cannot be ruled out. Although the relative oxidation rates of enargite were greater in the presence of L. ferrooxidans, cells were not in contact with the surface suggesting an indirect biological oxidation mechanism. Cells of L. ferrooxidans appear able to withstand several hundreds of ppm of As(III) and As(V).     

Selective separation of pyrite from chalcopyrite and arsenopyrite by biomodulation using Acidithiobacillus ferrooxidans

This paper discusses the selective depression of pyrite from chalcopyrite and arsenopyrite by biomodulation using Acidithiobacillus ferrooxidans under natural conditions of pH. The effect of bacteria–mineral interaction on the surface charge of mineral and bacterial cell was studied by microelectrophoresis. Adhesion experiments were conducted to establish the relationship between cell adhesion to specific minerals and the electrokinetic behaviour of the minerals subsequent to interaction with cells. Effect of bacterial interaction on the xanthate-induced flotation of all the minerals was assessed. Adhesion of A. ferrooxidans on pyrite was rapid and tenacious and subsequent to interaction with cells, pyrite remained hydrophilic even in presence of xanthate collector. The collector, on the other hand, was able to render good flotability to chalcopyrite even after interaction with bacterial cells. Copper activated arsenopyrite was able to retain its hydrophobicity in presence of cells due to poor attachment kinetics of cells to the mineral surface. Thus, by suitably conditioning with the cells and collector, it was possible to effectively depress pyrite from chalcopyrite and arsenopyrite.     

Mineralogical,geochemical, and microbial investigation of a sulfide-rich tailings deposit characterized by neutral drainage

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.     

Geochemical,mineralogical and microbiological characterization of a sulphide-bearing carbonate-rich gold-mine tailings impoundment,Joutel, Québec

The results of an integrated geochemical and mineralogical study conducted at the Agnico-Eagle gold-mine tailings impoundment, Joutel, Québec, are correlated with bacterial populations determined from an enumeration of 3 groups of Thiobacilli. The tailings were determined to contain approximately 5 wt.% sulphide–S, predominantly as pyrite, and up to 30 wt.% carbonate minerals, chiefly as dolomite–ankerite and siderite. The objective of the study was to evaluate the potential for the development of acidic drainage and dissolved-metal migration in carbonate-rich tailings impoundments, and to compare the results of the geochemical and microbiological characterization of the tailings. Sulphide-oxidation reactions have proceeded to a depth of 20–100 cm below the tailings surface. Pyrrhotite consistently shows more alteration than pyrite and arsenopyrite. Pyrrhotite is altered mainly through the replacement by goethite. The most abundant Thiobacilli are neutrophilic bacteria of the Thiobacillus thioparus type. The maximum most probable number values for these bacteria occur 20–40 cm below the tailings surface, a depth that coincides with the disappearance of oxide coatings. This observation, coupled with the sharp decline in gas-phase O₂ concentration, suggests that rapid bacterially-mediated S–oxidation is occurring at this depth. The pore-water pH throughout the tailings varies between 6.5 and 8.5; no low-pH waters were observed in the impoundment. These neutral pH conditions are attributed to the effect of acid-consuming carbonate-mineral dissolution reactions, which are also indicated by increased concentrations of Mg and Ca and alkalinity in the shallow zone of the tailings. As a result of these acid-neutralization reactions, dissolved metal concentrations are low.     

Specific affinity for phosphate uptake and specific alkaline phosphatase activity as diagnostic tools for detecting phosphorus-limited phytoplankton and bacteria

We analyzed responses of soluble reactive phosphorus (SRP), bioavailable phosphate (PO₄), particulate phosphorus, turnover time of orthophosphate (T_t), and alkaline phosphatase activity (APA) to varying degrees of nutrient stress. The nutrient stress was evoked by different treatments in concentration and combination of inorganic nitrogen (N) and phosphorus (P), and labile organic carbon (glucose) to mesocosms in experiments carried out in eutrophic southern (Odense Fjord, Denmark) and northern (Tvärminne Archipelago, Finland) coastal zones of the Baltic Sea. Despite seasonal and geographical differences, similar responses were observed in both experiments. Low SRP (<100 nmol l^?1), shortT _t (<10 h), and increased levels of APA were observed in both N+P balanced and P deficient treatments, while the opposite trend was observed in P replete treatments. The shortestT _t and the highest APA were found when glucose was combined with N treatment. Bioavailable PO₄ was estimated usingT _t and P uptake rates as derived from stoichiometric conversion of carbon based primary and bacterial production. With shorterT _t, the PO₄ pool declined to <1 nmol-P l^?1, whereas the SRP background pool (difference between SRP and PO₄) remained relatively constant (c. 50 nmol l^?1). APA was inversely related to PO₄ but not to SRP. Responses of specific APA and specific affinity for PO₄ uptake, which are APA and PO₄ uptake rates (inverse ofT _t), respectively, normalized to the summed P biomass of phytoplankton and bacteria, responded consistently to the pattern and magnitude of nutrient limitation evoked in our experiments. Our results, together with a literature survey, suggest that both parameters can be useful in examining PO₄ availability for the natural phytoplankton and bacteria community in P starved aquatic systems.     

Features of ferric sulfate precipitates formed by different cultivations of Acidithiobacillus ferrooxidans

This study focused on the ferric sulfate precipitates formed during the culture of Acidithiobacillus ferrooxidans (A. ferrooxidans) in a modified 9K medium by applying a potential control on the electrode. X-ray diffraction (XRD), environmental scanning electron microscope (ESEM), Raman spectroscopy (Raman) and Fourier Transform Infrared spectroscopy (FTIR) were carried out to characterize and identify the precipitates which were formed, respectively, in the electrochemical cultivation with a fixed cathode potential (bias-experiment) and in the conventional batch cultivation without cathode potential control (no-bias-experiment). The results indicated that K-jarosite presented in both experiments while NH₄-jarosite and schwertmannite were only found in the no-bias-experiment. The formation of different precipitates could be attributed to the different growth statuses and rates of A. ferrooxidans and the different concentrations of Fe³⁺. In the bias-experiment, external electrons reproduced Fe²⁺ and promoted the growth of A. ferrooxidans, thus resulting in the low Fe³⁺ concentration and the rapid depletion of NH₄ ⁺ as the nitrogen source, in which K-jarosite was preferentially formed. In the no-bias-experiment, the lower concentration of A. ferrooxidans was observed, which was due to the continuous consumption of Fe²⁺ by bacteria, thus resulting in the relatively higher Fe³⁺ and the NH₄ ⁺ concentration in culture. The high concentration of Fe³⁺ favored the precipitation of the solid solution of K-NH₄-H₃O jarosite, and led to the formation of schwertmannite after K⁺ and NH₄ ⁺ were depleted.     

Influence of phosphate on the oxidation kinetics of nanomolar Fe(II) in aqueous solution at circumneutral pH

The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH₂PO₄⁺, FeHPO₄ and FePO₄⁻ have been determined for the first time. FePO₄⁻ was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M⁻¹ s⁻¹. FeH₂PO₄⁺ and FeHPO₄ were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10⁻² M⁻¹ s⁻¹ and (1.2 ± 0.8) × 10⁻¹ M⁻¹ s⁻¹, respectively.     

Carbon isotope fractionation of Thermoanaerobacter kivui in different growth media and at different total inorganic carbon concentration

Chemolithotrophic homoacetogenic bacteria apparently express a characteristic stable carbon isotope fractionation and may contribute significantly to acetate production in anoxic environments. However, fractionation factors (ε) in bacterial cultures have rarely been determined and the effect of substrate availability has not been assessed. We therefore studied the kinetic carbon isotope effect in cultures of Thermoanaerobacter kivui grown at 55 °C. The fractionation factor in HCO₃⁻ buffered medium was ca. 15‰ more negative than that in PO₄³⁻ buffered medium. To test whether the difference was caused by the initial substrate ratio of H₂ and total inorganic carbon (TIC; 0.5 in HCO₃⁻ vs. 4.0 in PO₄³⁻ buffered medium), T. kivui was grown in either [3-(N-morpholino) propanesulfonic acid, MOPS] buffered or PO₄³⁻ buffered media with different HCO₃⁻ concentration. Indeed, the fractionation factor became more negative with increasing HCO₃⁻ concentration and decreasing H₂/TIC ratio. While pH had only a small effect, the fractionation was generally more negative in MOPS buffered than in phosphate buffered media, indicating that the buffer system also affected fractionation. Collectively, the results show that substrate availability and other environmental factors affect the magnitude of isotope fractionation during acetate production by chemolithotrophic homoacetogenesis.     

Bioleaching of high pyrite carbon-rich sphalerite preflotation tailings

In this work, zinc extraction was investigated using bioleaching process. The used samples were carbon-rich preflotation tailings prepared from a lead–zinc mineral processing plant, located in Yazd province, Iran. Two samples were obtained with high amount of pyrite, while the first sample contained high arsenic (As) substitution in the pyrite crystal lattice, and it was about 4–5 times more than that of the second sample. The organic matter in both samples has presented a signature of poorly crystalline carbon. Bioleaching experiments were designed and carried out by a mixed culture of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans in shake flasks. The results showed that in the optimum point of experiments (pH 1.94–2, pulp density 10 %, without initial Fe⁺²), about 95 % of Zn would be extracted within 14 days, while with no use of bacteria, merely 35 % of Zn content was dissolved (under the same condition). Because of the characteristics of the prepared samples, the bacterial medium (9K) was removed in the optimum condition of bioleaching tests. Results showed that even in the absence of 9K medium, bacteria had proper growth too, such that more than 93 % of Zn content was extracted. In fact, the same results were achieved in the absence and presence of 9K medium. Comparison of the obtained results in bioleaching tests under the optimum condition indicated that Zn extraction was the same for both high and low As samples, while Fe extraction from low As content sample was nearly 4 times higher than that of the other sample.     

铁细菌培养过程中低分子有机酸钠盐对含硫酸根铁矿物形成的影响

普遍存在环境中的低分子有机酸盐对氧化亚铁硫杆菌的矿化产物(施氏矿和黄钾铁矾等铁矿物)会产生影响,从而导致环境中有毒重金属迁移转化发生变化。本文探讨了低分子有机酸钠盐对铁细菌HX3成长过程中代谢产物铁矿物的影响,并利用XRD、FTIR、FESEM和EDS对形成的铁矿物进行了表征与分析。研究结果表明,低浓度低分子有机酸钠盐的加入对细菌氧化Fe~(2+)的影响不明显,但可加速黄钾铁矾的形成;苹果酸钠的加入较柠檬酸钠和草酸钠更利于施氏矿向黄钾铁矾转变。高浓度低分子有机酸钠盐(苹果酸钠、柠檬酸钠和草酸钠依次为20、40和40mmol/L)的加入对细菌培养过程中Fe~(2+)的氧化有抑制作用;抑制影响从大到小的顺序为:苹果酸钠柠檬酸钠草酸钠。该研究结果可为含氧化亚铁硫杆菌等铁细菌的酸性矿山废水中铁矿物的形成转化和生物矿化机理提供理论参考。     

Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates

The crystallographic structures of the synthetic cheralite, CaTh(PO₄)₂, and its homolog CaNp(PO₄)₂ have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO₄ (Ln = La to Gd). The space group is P2₁/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO₄)₂ are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R _wp = 4.87%, R _p = 3.69% and R _B = 3.99%. For CaNp(PO₄)₂, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R _wp = 6.74%, R _p = 5.23%, and R _B = 6.05%. The results indicate significant distortions of bond length and angles of the PO₄ tetrahedra in CaTh(PO₄)₂ and to a lesser extent in CaNp(PO₄)₂. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO₄)₂. A comparison with the isostructural compounds LnPO₄ (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca²⁺ and Th⁴⁺ introduces a distortion of the PO₄ tetrahedra.     

Effects of phospholipid on pyrite oxidation in the presence of autotrophic and heterotrophic bacteria

Pyrite oxidation occurring in solutions containing iron oxidizing autotrophic bacteria, Acidithiobacillus ferrooxidans (A. ferrooxidans), and/or heterotrophic bacteria, Acidiphilium acidophilum (A. acidophilum), has been investigated. Under the conditions used, the amount of pyrite oxidized in the presence of both species was similar to the amount oxidized in the presence of A. ferrooxidans alone over a period of 30 days. Pretreatment of pyrite with the phospholipid, [1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (23:2 Diyne PC)], to form an adsorbed organic layer reduced the amount of pyrite oxidation in the absence of bacteria and in the presence of A. ferrooxidans. The addition of lipid to pyrite prior to its exposure to a mixed A. ferrooxidans/A. acidophilum solution also showed initial oxidation suppression. However, after 4-5 days the effectiveness of the lipid in suppressing pyrite oxidation was lost and oxidation of the mineral proceeded at a rate that was similar to lipid-free pyrite in the presence of both microbial populations. If, however, lipid/pyrite was pretreated with UV radiation to induce cross-linking of the lipid tails (via polymerization of diacetylene groups in the tails), the lipid layer showed a strong suppression of pyrite oxidation for up to at least 30 days in the presence of both microbial populations. It was also shown with in situ atomic force microscopy (AFM) that the introduction of lipid to pyrite with colonized A. ferrooxidans led to the displacement of a fraction of surface bound bacteria. This lipid-induced displacement was confirmed by ex situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR).     

Weathering of phlogopite in simulated bioleaching solutions

The purpose of this study was to examine structural alterations of finely ground phlogopite, a trioctahedral mica, when exposed to acid, iron- and sulfate-rich solutions typical of bioleaching systems. Phlogopite suspensions were supplemented with ferrous sulfate and incubated with iron- and sulfur-oxidizing bacteria (Acidithiobacillus ferrooxidans) at 22 °C. As bacteria oxidized ferrous iron, ferric iron thus formed partially precipitated as K-jarosite. K-jarosite precipitation was contingent on the preceding ferrous iron oxidation by bacteria and the release of interlayer-K from phlogopite. This chemically and microbially induced weathering involved alteration of phlogopite to a mixed layer structure that included expansible vermiculite. The extent of phlogopite weathering and structure expansion varied with duration of the contact, concentration of ferrous iron and phlogopite, and the presence of monovalent cations (NH₄⁺, K⁺, or Na⁺) in the culture solution. NH₄⁺ and K⁺ ions (100 mM) added to culture suspensions precipitated as jarosite and thereby effectively prevented the loss of interlayer-K and structural alteration of phlogopite. Additional Na⁺ (100 mM) was insufficient to precipitate ferric iron as natrojarosite and therefore the precipitation was coupled with interlayer-K released from phlogopite. When ferrous iron was replaced with elemental sulfur as the substrate for A. ferrooxidans, the weathering of phlogopite was based on chemical dissolution without structural interstratification. The results demonstrate that iron oxidation and the concentration and composition of monovalent ions can have an effect on mineral weathering in leaching systems that involve contact of phlogopite and other mica minerals with acid leach solutions.     

Magnesia mixture as a regulator in the separation of pyrite from chalcopyrite and arsenopyrite

The aim of this paper is to find an effective method for the separation of the undesirable constituents, namely, chalcopyrite and arsenopyrite from pyrite used for the production of H₂SO₄. A new effective method is developed for co-depressing chalcopyrite with arsenopyrite by AsI₃, followed by the addition of magnesia mixture. This method has been shown to be based on the fact that iron sites exist in the three minerals, whereas copper and arsenic sites exist only in chalcopyrite and arsenopyrite, respectively. This is coupled with the ability of both Cu(I) and Cu(II) to precipitate As(III) in the form of insoluble copper arsenides, namely Cu₃As, Cu₃As₂. In contrast, neither Fe(II) nor Fe(III) form stable arsenides. Consequently, As³⁺ ions are selectively adsorbed onto the surface of chalcopyrite. The facility for oxidizability of As(III) is well known and hence it adsorbs oxygen from the pulp and changes to As(V) of higher valency and smaller size, with ionic potential over 10. Accordingly, it yields a stable complex anion with covalent bonding, namely, [AsO₄]^3?. These newly created arsenate sites on the surface of chalcopyrite, as well as the corresponding original arsenate sites on the surface of arsenopyrite combine with magnesia mixture to form cations leading to the formation of tightly abutting strongly hydrophilic layers of … AsO₄NH₄Mg.6H₂O. The spread of this hydrophilic film on arsenopyrite and chalcopyrite surfaces leads to the screening of their surfaces, making them difficult of access for the collector, ethyl xanthate. Since the pK_a of xanthic acid occurs at pH below 3, xanthate species predominate at pH above 8 and are adsorbed selectively on the pyrite surface in sufficient quantity for its selective flotation and hence for its separation to take place in the pH range 8–9.     

Modification of olivine surface morphology and reactivity by microbial activity during chemical weathering

Prior transmission electron microscope studies showed that the surface geometry of olivine changes dramatically during natural chemical weathering. However, similar morphological evolution has not been reported in laboratory studies of olivine dissolution. In this study, we examined the development of fayalite (Fe₂SiO₄) surface morphology during both abiotic and biotic (using Acidithiobacillus ferrooxidans) laboratory dissolution experiments at an initial pH of 2.0. The fayalite came from Cheyenne Canyon, Colorado (Smithsonian # R 3516) and contains a few percent laihunite (olivine structure with ordered ferric iron and vacancies, ∼Fe_0.8²⁺Fe_0.8³⁺SiO₄). High-resolution field emission low voltage scanning electron microscope (SEM) characterization of all reacted samples showed etch patterns consistent with those reported from naturally reacted olivine. High-resolution transmission electron microscope (HRTEM) data demonstrated pervasive channeling on (001), with channel spacings that range down to < 10 nm. Formation of channels on (001) is probably initiated by preferential removal of cations from olivine M1 sites. Channeling confers at least an order of magnitude increase in surface area. Relict strips of olivine between channels contain laihunite layers that are oriented parallel to channel margins. X-ray diffraction analyses indicated that the relative abundance of laihunite is higher in reacted compared to unreacted samples. This result is consistent with prior studies of naturally weathered olivine that suggest that laihunite is far less readily dissolved than olivine.Samples reacted in the presence of A. ferrooxidans cells that enzymatically oxidized iron, or in solutions where ferric iron was added to simulate biological activity, dissolve at a much slower rate than samples reacted abiotically. We attribute suppression of the olivine dissolution rate to surface adsorption of Fe³⁺. It is probable that ferric iron adsorption is controlled by M2 sites in the underlying olivine structure. If this is coupled with removal of M1 cations during channel formation, then a modified laihunite-like surface will develop (vacancies in laihunite are on M1 sites). Although surface modification might only penetrate a few atomic layers, an inherently unreactive laihunite-like surface structure could explain both the pervasive channeling and the dramatic suppression of the measured dissolution rate.     

Evaluation of phosphate fertilizers for ameliorating acid mine waste

The aim of the study was to determine whether the application of bulk industrial chemicals (potassium permanganate and water-soluble phosphate fertilizer) to partly oxidized, polyminerallic mine wastes can inhibit sulfide oxidation, and metal and metalloid mobility. The acid producing waste rocks were metal (Pb, Zn, Cu) and metalloid (As, Sb) rich and consisted of major quartz, dickite, illite, and sulfide minerals (e.g., galena, chalcopyrite, tetrahedrite, sphalerite, pyrite, arsenopyrite), as well as minor to trace amounts of pre- and post-mining oxidation products (e.g., hydrated Fe, Cu, Pb, and alkali mineral salts). SEM-EDS observations of treated waste material showed that metal, metal–alkali, and alkali phosphate coatings developed on all sulfides. The abundance of phosphate phases was dependant on the fertilizer type and the availability of metal and alkali cations in solution. In turn, the release of cations was dependent on the amount of sulfide oxidation induced by KMnO₄ during the experiment and the dissolution of soluble sulfates. Mn, Ca, Fe, and Pb phosphates remained stable during H₂O₂ leaching, preventing acid generation and metal release. In contrast, the lack of complete phosphate coating on arsenopyrite allowed oxidation and leaching of As to proceed. The mobilized As did not form phosphate phases and consequently, As displayed the greatest release from the coated waste. Thus, the application of KMnO₄ and the water-soluble phosphate fertilizer Trifos (Ca(H₂PO₄)₂) to partly oxidized, polyminerallic mine wastes suppresses sulfide oxidation and is most effective in inhibiting Cu, Pb, and Zn (Sb) release. However, the technique appears ineffective in suppressing oxidation of arsenopyrite and preventing As leaching.     

Geological analogue for circumneutral pH mine tailings: implications for long-term storage,Macraes Mine,Otago, New Zealand

Tailings from the Macraes Au mine cyanidation process are stored in an impoundment about 0.6 km² and 80 m deep whose pH is maintained near 8 by the neutralizing capacity of the gangue minerals. The tailings are sandy (>50 μm particles), have a hydraulic conductivity of about 10⁻² m/day, and contain 0.1–1.0 wt.% S and 0.1–1.5 wt.% graphitic C from the primary deposit. Concentrations of As in the pore water of the mixed tailings, which are a combination of various tailings types, range from 0.1 to 20 ppm, HCO₃^- is 100 to 200 ppm, and dissolved SO₄ is 100–1700 ppm. The mixed tailings will be stored in this impoundment in perpetuity after mining ceases. Confidence in the long-term pH stability of these tailings can be gained from examination of mineralogically and chemically similar geological analogues in the immediate vicinity. A sequence, typically about 5 m thick, of sands and gravels derived from the Macraes mineralized zone 12–28 ka ago contains rounded detrital sulfide mineral grains which are unoxidized despite their close proximity to the surface and the occasional incursion of oxygenated waters. These sediments have a hydraulic conductivity of about 10⁻⁴ m/day. Saturating water pH is currently 7–8. Sands with 0.2–0.8 wt.% organic C host SO₄-reducing bacteria (SRB), and local cementation by authigenic framboidal pyrite has occurred. SRB were found in water-saturated sediments with decreased hydraulic conductivity and alkaline and anoxic conditions. These bacteria are involved in the formation of authigenic framboidal pyrite, reducing the cycling of dissolved Fe in the sediments. Carbon is not a limiting factor in this process as organic matter is present in the sandstone and ground water contains up to 180 ppm HCO₃^-. Comparison of the 28 ka old sediments with the modern tailings suggests that the chemical behaviour of the two will be similar, possibly with the crystallization of authigenic pyrite in the tailings over the long term. As long as the present slightly anoxic and circumneutral pH environmental conditions are maintained in the mixed tailings impoundment, sulfide decomposition and acidification are unlikely.     

A kinetic study of the oxidation of arsenopyrite in acidic solutions: implications for the environment

Arsenopyrite is an important component of many ore deposits and dissolves in the O₂-rich, acidic surface waters that are commonly found in the vicinity of active mines, releasing As, Fe and S to the environment. However, despite the potentially serious effect of this pollution on the human and animal population, the rate at which such oxidation occurs is poorly known. Kinetic experiments were therefore conducted in a mixed flow reactor to investigate the oxidation of arsenopyrite in Fe₂(SO₄)₃ solutions (pH=l.8) having a concentration of l×l0⁻² to 1 ×l0⁻⁵ mol kg⁻¹ at temperatures of 45, 35, 25 and 15 °C. The results of these experiments show that the rate of oxidation of arsenopyrite increases with increasing concentration of dissolved Fe₂(SO₄)₃ and temperature. They also show that As released during the oxidation of arsenopyrite has the form As(III), and that the rate of conversion of As(III) to As(V) is relatively low, although it tends to increase with increasing concentration of dissolved Fe₂(SO₄)₃ and temperature. In the presence of Cl⁻, oxidation of arsenopyrite is accelerated, as is the conversion of As(III) to As(V). These findings indicate that exploitation of arsenopyrite-bearing ores will cause contamination of groundwaters by As at levels sufficient to have a major negative effect on the health of humans and animals.     

Characterisation of the Sarcheshmeh copper mine tailings,Kerman province,southeast of Iran

Mineral processing operation at the Sarcheshmeh porphyry copper mine has produced huge quantities of tailings materials containing sulphide minerals in particular pyrite. These tailings materials were geochemically and mineralogically characterised to assess pyrite and chalcopyrite oxidation, acid mine drainage generation, and trace element mobility to lead development of a proper remediation plan. Five vertical trenches up to 4.2 m deep were excavated from the tailings surface, and 70 solid samples were taken in 0.3 m intervals. The samples were first mineralogically analysed. Pyrite was the main sulphide mineral found in the tailings. The gangue minerals include quartz ± muscovite–illite ± chlorite ± albite ± orthoclase ± halite. The samples were geochemically analysed for total concentrations of 62 elements, paste pH, SO₄ ^2?, CO₃ ^2?, and HCO₃ ^?. The maximum concentrations of SO₄ ^2? (1,300, 1,170, 1,852, 1,960 and 837 mg/L) were observed at a depth of 0.9 m in profiles A, B, C, D and E, respectively. The tailings have a high acid-producing potential and low acid-neutralising potential (pyrite 4–6 wt %, calcite 1 wt %). Fe₂(SO₄)₃, CuSO₄, MgSO₄ and MnSO₄ were the dominant secondary sulphate minerals in the tailings. The lowest pH values (2.9, 3 and 3) were measured at a depth of 0.3 m in the profiles A, B and C, 3.9 at a depth of 0.6 m in the profile D and 3 at a depth of 0.9 m in the profile E. The upper portions of the profiles C (1.8 m) and D (2.1 m) were moderately oxidised, while oxidation in the profiles A, B and E did not extend more than 1.2, 1.2 and 1.5 m beneath the tailings surface. Zn, Pb, Rb, U, Hf, Nd, Zr and Ga show almost a constant trend with depth. Cd, Sr, Th, La and Ce increased with increasing depth of the tailings materials while, Co, V, Ti, Cr, Cu, As, Mn, Ag, Mo and Ni exhibit initially a decreasing trend from tailings surface to the depths that vary between 0.9 and 1.2. They then remained constant with the depth. The results show pyrite and chalcopyrite oxidation at surface layers of the tailings and subsequent leaching of the oxidation products and trace elements by infiltrated atmospheric precipitation.