Pyrite Oxidation

Framboids, spherical aggregates of minute, usually pyrite, grains are the commonest texture of sulfides in sedimentary rocks. They also occur in many other environments, ranging from magmatic rocks to antiquarian books. Framboidal textures are formed by several minerals including copper and zinc sulfides, greigite, magnetite, magnesioferrite or hematite, as the products of both primary deposition and replacement. Special attention is paid to pyrite framboids in this paper, with detailed descriptions of different framboid-related forms: annular framboids, sunflower framboids, micro- and polyframboids. Framboidal forms are often hierarchically structured over three size scales, with complexities ranging from microframboids, to framboids, and to polyframboids. Pyrite in framboids results from inorganic reactions between dissolved iron and sulfide, with a greigite intermediary. The sulfur is usually biogenic in origin. Simple pyrite framboids are formed during aggregation, possibly enhanced by the magnetic properties of the monosulfide precursor. Further processes, including particulation and organically controlled aggregation, result in more complicated forms such as polyframboids. Pyrite framboids can grow to euhedral grains provided the supply of iron and sulfide is not limited. Pyrite framboids often occur in close spatial relationship with organic matter, silica or carbonates, which influence their formation and growth. Replacement of iron sulfide framboid-related forms by other minerals can be indicative of sediment diagenetic conditions, or additional processes such as mineralization in ore deposits. Framboids influence the distribution of many trace elements. Due to their high specific surface areas, framboids can accumulate these trace elements during growth. Recrystallization of framboids can redistribute many trace elements. An overview of the common association between pyrite framboids and fossils is given, stressing both the importance of organic matter in framboid formation, and of the framboids in the processes of mineralization and preservation of fossils. Iron sulfide framboids may have influenced the earliest stages of life formation on the Earth as a source of energy and catalytic action, by accumulating organic compounds, and by acting as reaction chambers and templates which facilitated reproduction and information transfer. Applications from framboid studies are presented, ranging from determination of the redox conditions in water and sediment columns using size and distribution of framboids, to evaluation of deterioration processes in museum specimens.

The low grade wastes produced by coal washing plant often contain pyrite which may be oxidised and produce acid mine drainage (AMD) if it is exposed to the atmosphere. AMD is characterised by high concentrations of iron, high sulphate and low pH and it is ...

Oxidative dissolution experiments were carried out on pyrite from multiple petrogenetic environments (hydro-thermal, sedimentary, and coal-related nodules) to investigate possible variations in the iron isotopic composition of pyrite and the products of pyrite dissolution. The experimental materials were leached under carefully controlled abiotic conditions, and a subset of leachates and starting materials from these experiments was analyzed for 56 Fe/ 54 Fe by multicollector ICP-MS. Bulk pyrite δ 56 Fe values (relative to IRMM-014) ranged from −0.1 to +1.3‰, with hydrothermal bulk pyrite values b+0.5‰ and those of coal and sedimentary nodular pyrite ≥+0.5‰, higher than most previously measured values for Phanerozoic sedimentary pyrite. We suggest that this reflects precipitation of coal pyrite from a high-δ 56 Fe continental source, such as Fe derived from dissolution of Fe(III) oxides. This interpretation is consistent with pyrite rare earth element (REE) patterns. This could allow differentiation of Fe contributed from coal-and shale-related pyrite at abandoned mine drainage sites. Leachates from oxidative dissolution of the pyrite at pH = 3 yielded, with few exceptions, δ 56 Fe values equal to or lower than those of the coexisting bulk pyrite, by up to about 1‰. These shifts are consistent in direction (but not magnitude) with equilibrium isotope fractionation predictions from theory, with possible second order effects from isotopic heterogeneity within individual natural pyrite samples.

Sulfide-rich tailings are a well-known environmental threat due to their production of acid drainage (AD) and release of potential toxic elements (PTE) to the local environment. The presence of heterogeneous materials produces complex environmental signatures and complicates the quantitative prediction of contamination. The present work provides a method of quantifying such heterogeneities, starting from mineral processing data of the Reps, Mirdita (Albania) site. A quantitative flow sheet (QF) method was applied to a selected dump site of the Mirdita copper mining district where secondary pyrite separation had been used in the past. The site is subject to long-lasting (10 3 years) AD processes with significant release of PTE into the local environment. The tailings at the Reps site are divided into two classes based on the sulfide S content, respectively, represented by high-sulfide-content (S [ 10 wt%) materials (hS) and low-sulfide-content (S \ 3 wt%) material (lS). The reconstruction of the QF allowed us to identify the hS tailings as the discharge of single-flotation processing lines. This material accounts for about 82% of the total potential H 2 SO 4 production, even though it represents \ 20% of the entire tailing discard. The QF is a useful tool for the evaluation of heterogeneity and consequently for the modeling of waste management within abandoned sites and in working plants. Given a good quantification, heterogeneity can in fact support the setting of pyrite separation lines or the separate management of pyrite-rich tailing dumps.

You may discover a specimen in your collection that is suffering from pyrite decay and it may already appear to be too late to save it. However, a pile of ash-like substance in a card tray might look like it is destined only for the bin but careful cleaning and simple stabilisation techniques may reveal a useful specimen underneath. If so, record it as fully as possible as soon as possible using either traditional techniques (photography and/or moulding and casting) or by using modern techniques such as photogrammetry, CT scanning, Micro CT scanning or laser scanning  as the specimen will be prone to further deterioration in the future.

Several O isotope studies have shown that SO 2À 4 produced from aqueous pyrite oxidation mainly contains water-derived O and minor atmospherically-derived O 2. However, the incorporation of O 2 into SO 2À 4 has been shown to decrease continuously during pyrite oxidation experiments. Hence, it remains uncertain if (and how) O 2 is permanently incorporated into SO 2À 4 during pyrite oxidation. Abiotic aerobic batch pyrite oxidation experiments in aqueous solutions were performed under acid pH conditions. After 151 days, 18 O-enriched O 2 was injected into the headspace of the reaction vessels. Increasing d 18 O SO 4 values with increasing injection volume of 18 O-enriched O 2 indicated the permanent incorporation of about 9% O 2 into the produced SO 2À 4 during pyrite oxidation from 151 to 201 days. Molecular oxygen may be incorporated into SO 2À 4 by oxidation of the S intermediate species sulfite (and maybe tetrathionate) into SO 2À 4. However, only 4% of the O 2 consumed during the experiments was incorporated into SO 2À 4. Slightly increased d 18 O H 2 O values from experiments with the largest injection of 18 O-enriched O 2 indicated the incorporation of O 2 into water molecules which may proceed during the cathodic reduction of O 2. Thus, O 2 was an important electron acceptor under aerobic acid conditions. The observed e SO 4 ÀO 2 value indicated that the oxidation of dissolved Fe 2+ by O 2 did not play an important role. Furthermore, the lack of 32 S enrichment in SO 2À 4 compared to pyrite indicated that the oxidation of adsorbed Fe 2+ by O 2 should not be a dominant mechanism, although it may be catalyzed onto the pyrite surface. Hence, O 2 should accept electrons predominantly from pyrite.

The low grade wastes produced by coal washing plant often contain pyrite which may be oxidised and produce acid mine drainage (AMD) if it is exposed to the atmosphere. AMD is characterised by high concentrations of iron, high sulphate and low pH and it is considered to be a major cause of long-term poor water quality and the source of many environmental problems. In this study, the pollution problem associated with the wastes (which cover an area of about 1 km2) produced by Alborz Sharghi coal washing plant, at Shahrood, Iran, was investigated by mineralogical and geochemical techniques. Mineralogical analysis indicates the presence of pyrite in the unoxidised waste and its depletion in the oxidised zone. Geochemical analysis of samples taken from different depths over the coal wastes dump up to 2 metres show that pyrite oxidation in the upper 75 cm of the dump has resulted in a peak concentration of sulphate, low pH and high concentrations of metals (pH<3.5, 349 mg/L , 270 ppm F...

For all dumps of closed mining sites within the Lausitz mining district geochemical inventory was determined, considering pyrite oxidation and sulphate reduction. Based on existing groundwater flow models and geochemical inventories, the dissolved mass discharge is calculated using PHT3D. Contaminated area and amount of dissolved sulphate in groundwater is predicted to increase at least until 2025. Therefore good groundwater quality conditions and even trend reversal cannot be expected within the time frame of the EUWater Framework Directive (EU-WFD). Measures that might significantly improve conditions for the whole region are not at hand. Therefore it might be necessary to define less stringent environmental objectives in terms of the legal framework of the EU-WFD.

Although pyrite oxidation has been extensively studied by oxygen and sulfur isotope investigations (as well as by further methods), details of oxidation mechanisms are still under discussion. In this study, the chemical oxidation of pyrite was studied under anaerobic and low oxygen conditions with different amounts of ferric iron in order to investigate oxidation mechanisms and to determine an exact value of the oxygen isotope enrichment factor ε SO 4 –H 2 O. Results showed that pyrite was predominantly oxidized by ferric iron in anaerobic experiments with high ferric iron/pyrite surface ratios. The obtained oxygen isotope enrichment factor ε SO 4 –H 2 O = 2.3‰ was in the range of recently determined values and indicated the preferential incorporation of 18 O from water molecules into sulfate during pyrite oxidation. δ 18 O SO 4 values from anaerobic and low oxygen experiments with low ferric iron/surface ratios indicated a significant proportion of molecular oxygen in sulfate which should be unintentionally adsorbed onto the pyrite surface during placing the pyrite into the experimental flasks. In the presence of water, adsorbed molecular oxygen acted as pyrite oxidant and was incorporated into the produced sulfate. A sulfur isotope enrichment factor ε SO 4 –FeS 2 = − 0.8‰ was observed from pyrite oxidation by ferric iron and may indicate the formation of elemental sulfur. A lack of sulfur isotope enrichment in sulfate relative to pyrite was observed in most of the experiments where pyrite was oxidized by molecular oxygen. However, a sulfur isotope enrichment in sulfate relative to pyrite may also occur during aerobic pyrite oxidation.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t Experiments were conducted to investigate (i) the rate of O-isotope exchange between SO 4 and water molecules at low pH and surface temperatures typical for conditions of acid mine drainage (AMD) and (ii) the O-and S-isotope composition of sulfates produced by pyrite oxidation under closed and open conditions (limited and free access of atmospheric O 2) to identify the O source/s in sulfide oxidation (water or atmospheric molecular O 2) and to better understand the pyrite oxidation pathway. An O-isotope exchange between SO 4 and water was observed over a pH range of 0–2 only at 50 °C, whereas no exchange occurred at lower temperatures over a period of 8 a. The calculated half-time of the exchange rate for 50 °C (pH = 0 and 1) is in good agreement with former experimental data for higher and lower temperatures and excludes the possibility of isotope exchange for typical AMD conditions (T 6 25 °C, pH P 3) for decades. Pyrite oxidation experiments revealed two dependencies of the O-isotope composition of dissolved sul-fates: O-isotope values decreased with longer duration of experiments and increasing grain size of pyrite. Both changes are interpreted as evidence for chemisorption of molecular O 2 to pyrite surface sites. The sorption of molecular O 2 is important at initial oxidation stages and more abundant in finer grained pyrite fractions and leads to its incorporation in the produced SO 4. The calculated bulk contribution of atmospheric O 2 in the dissolved SO 4 reached up to 50% during initial oxidation stages (first 5 days, pH 2, fine-grained pyrite fraction) and decreased to less than 20% after about 100 days. Based on the direct incorporation of molecular O 2 in the early-formed sulfates, chemisorption and electron transfer of molecular O 2 on S sites of the pyrite surface are proposed, in addition to chemisorption on Fe sites. After about 10 days, the O of all newly-formed sulfates originates only from water, indicating direct interaction of hydroxyls from water with S at the anodic S pyrite surface site. Then, the role of molecular O 2 is as proposed in previous studies: acting as electron acceptor only at the cathodic Fe pyrite surface site for oxidation of Fe(II) to Fe(III).

Acid mine drainage (AMD) is an inevitable problem in mining and has adverse effects in water quality. Studying AMD formation will be valuable in controlling the composition of mine waters and in planning the rehabilitation method for a mine. In this research, kinetics of AMD formation of Toledo pyrite was studied using two column experiments. The mechanisms of AMD formation and the effects of various factors on pH drop were first studied. Another column test was done for validation and to study the role of Fe 2+ /Fe 3+ ratio in the change of leachate pH. The first experiment revealed that time and particle size are the most significant factors. It was also observed that the sudden pH drop during the starting hours was due to cracks formed from beneficiation, and the formation of Fe(OH) 3. The laddered behavior of pH thereafter was due to decrease in formation of Fe(OH) 3 , and the precipitates in pyrite surface that lowered the surface area available for pyrite oxidation. The results of the second experiment validated the laddered behavior of pH. It was also observed that particle size distribution and pyrite surface were affected by the change in pH. Fe 2+ /Fe 3+ ratio of leachate generally decreased as pH dropped.

Acid-sulphate soils are of major environmental concern in many wetlands. Severe acidification episodes have occurred worldwide because of the oxidation of iron sulphides to sulphuric acid by human activities, and diverse techniques have been set up to determine the presence of acid-sulphate soils. This paper evaluates the usefulness of four common easy-to-apply field survey tests for potential acid-sulphate diagnosis in some Histosols and Entisols in wetlands: incomplete oxidation by fast air-drying, incubation, fast oxidation with hydrogen peroxide, and the indirect determination of sulphide with lead acetate. Samples of 227 surface-organic and underlying mineral soils of poorly drained Histosols and Entisols of the Orinoco river delta plain were tested. Results showed that for highly organic samples the interpretation of results obtained from the acid-sulphate soil tests may be misleading, because they cannot be unambiguously related to the production of sulphuric acid derived from pyrite oxidation. Mineral samples yielded more reliable results. The incomplete oxidation by fast air-drying test did not induce significant acidification either in organic or in mineral samples; the final pH values were dependent on the original pH values. The fast oxidation with hydrogen peroxide test was effective with mineral samples. During the incubation test, the slower rate of pH decrease and the final values obtained with the organic samples suggested retardation in the rate and amount of acid generation by pyrite oxidation because of the concurrent oxygen consumption by organic matter and bacterial activity. The indirect determination of sulphide with lead acetate yielded only qualitative results in organic samples, but worked well in mineral samples, indicating a higher content of pyrite intermediates. Effective estimation of the actual presence and potential for acidification of soil is important, in order to avoid excessive or inappropriate amelioration techniques to prevent acid production.

Soil Use and Management, March 2008, 24, 60–68
DOI: j.1475-2743.2007.00134.x

This paper reports detailed O 2 measurements of pyrite bearing sediments in a column study and their interpretation based on a hydrogeochemical modelling approach. The research focuses on the quantitative effects of effective diffusion and microbiologic activity on pyrite weathering and acidification. A column experiment was set up and O 2 saturation and moisture contents were monitored over 100 days. The anoxic material used for the column experiment was taken from a sediment core of a mining waste dump in the southern periphery of the Lohsa storage system in the Lusatia region of Germany. The measured O 2 breakthrough curves were modelled using the simulator SAPY, a one-dimensional reactive transport code which considers the kinetics of chemical reactions and the delivery of O 2 into the sediment. The simulation yielded a strong dependence of pyrite oxidation on the moisture content which was quantified by an empirical equation. It was shown that the oxidation rate was catalysed by microbial activity exceeding the rate of diffusive O 2 delivery. In order to develop a management tool for predictive issues the results have already been applied to natural environments in another study using the adapted model.