Oxidation Of FeS Research Articles

Calcite dissolution accompanying early diagenesis in turbiditic deep ocean sediments

The oxidation of marine sedimentary organic carbon (C org) by O 2 diffusing from bottom water produces a characteristic linked loss of C org and CaCO 3. A theoretical treatment is presented, based on carbonate system porewater equilibrium and Redfield C org stoichiometry, which predicts that the molar ratio of the two losses should be 1:1.08. This ratio differs significantly from literature values which ignore porewater equilibria. Calcium carbonate dissolution in well-constrained profiles from vertically-homogenous Atlantic turbiditic sediments exceeds that predicted on the basis of this simple theoretical model, indicating that additional processes must also contribute to CaCO 3 loss. Possible additional diagenetic processes involving Fe 2+, Mn 2+, S, NO 3 −and NH 4 +geochemistry are evaluated to account for this discrepancy. It is demonstrated that (after C org respiration) oxidation of trace FeS 2 is the process most likely to have produced the CaCO 3 loss observed. In addition, loss of alkalinity consequent upon the oxidation of NH 4 + is shown to contribute significantly to CaCO 3 loss. This NH 4 + is supplied to the progressive oxidation front from below by diffusion from a much deeper zone of SO 4 2− reduction and appears quantitatively capable of accounting for the remainder of the observed CaCO 3 loss at a site where a progressive oxidation front is active in a turbidite. In order to investigate later diagenetic effects, three buried turbidite units recovered by ODP drilling were also studied. Units sampled in the depth range 130–230 mbsf (age 3–7 My) still retained clear evidence of the effects of similar fossil oxidation fronts in the solid phase C org and CaCO 3 profiles. Thus the rapid oxidation front process leaves a clear signature which can survive longer-term anoxic C org remineralisation processes.

Acid‐Base Properties of a Limed Pyritic Overburden during Simulated Weathering

AbstractSurface‐mine reclamation is often hindered by the formation of acid minesoil and acid mine drainage from FeS2 oxidation. Surface soils containing FeS2 are often treated with crushed limestone (predominately CaCO3) to prevent acid minesoil formation. The main objective of this study was to evaluate the long‐term effectiveness of liming pyritic minesoil to prevent the formation of acid minesoil and acid mine drainage. Pyritic minesoils that did not receive lime became acidic very rapidly and produced acidic leachate. Almost all of the FeS2 in this treatment oxidized during the first 200 d. The addition of lime at a rate of 25% of the theoretical acid‐base account (ABA) significantly slowed FeS2 oxidation, but rapid oxidation ensued after the added lime was neutralized. Treatments receiving a liming rate of 50% ABA or greater remained neutral to alkaline throughout the study. Acid‐base values and residual FeS2‐CO3 data, however, indicate that the lime was dissolving from the system faster than the FeS2 was oxidizing, and all the treatments would eventually become acidic. The results indicate that the liming of a pyritic overburden to an ABA of 125% is not a sustainable solution to preventing acid minesoil and acid mine drainage.

Kinetics of pyrite formation by the H 2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The rate equation

A kinetic study of the reaction Fe S ( s ) + H 2 S ( aq ) = FeS 2 + H 2 ( g ) where FeS is precipitated FeS, H 2S (aq) is aqueous H 2S, FeS 2 is pyrite, and H 2(g) is hydrogen gas, shows that the rate between 25 and 125°C can be described by the equation d FeS 2 / dt = k ( FeS ) ( c H 2 S ( aq ) ) where k the second order rate constant varies between 1.03 × 10 −4L mol −1 s −1 at 25°C and 3.20 × 10 −3 L mol −1 s −1 at 125°C. The rate constant shows a sigmoidal temperature dependence with an average Arrhenius activation energy of 35 kJ mol −1. The reaction is surprisingly fast at ambient temperatures with up to 50% reaction being completed within one day. The direct dependence of the rate on cH 2S (aq) means that the rate is pH dependent for any fixed total sulfide concentration. In typical sulfidic aquatic systems and sediments 9 × 10 −13 to 9 × 10 −8 mol FeS 2 per L sediment will be formed each day by this process. This is equivalent to approximately 3 × 10 −10 to 3 × 10 −5 mol FeS 2 per g sediment per year. At pH = 7, for the same total sulfide and FeS constraints, the rate of pyrite formation 1.5 × 10 −9 to 1.5 × 10 −4 mol FeS 2 per g sediment per year. In hydrothermal systems, such as deep ocean vents, the rate of pyrite formation by oxidation of FeS by H 2S at 125°C assuming a typical H 2S concentration of 1 mM is 3.2 × 10 −6 mol L −1 s −1 per mol FeS. A 1 million tonne pyrite deposit could form from a solution containing 1 mmol FeS and 1 mmol H 2S by this process in 1000 years at a flow rate of 0.3 Ls −1. The fluid would have a H 2 concentration of 3 × 10 −9 M. The process is by far the most rapid of the pyrite-forming reactions hitherto identified. Alternative pyrite-forming processes involving HS −, rather than H 2S, as the reaction requires an additional oxidising agent to maintain electron balance. These pathways may involve reactants such as polysulfides or intermediaries such as greigite, Fe 3S 4. In natural systems, therefore, the H 2S process will tend to be favored in strictly anoxic environments. In transitional environments, with limited molecular oxygen contents, pyrite formation through the polysulfide pathway or a solid state process may become more important. However, the generally low concentrations of polysulfides and greigite observed in natural pyrite-forming environments might suggest that these processes are always subordinate. The fast nature of the H 2S oxidation process suggest that it makes a significant contribution to the global sulfur cycle. The process provides a straightforward explanation of a number of observations regarding pyrite formation in natural systems, including pyrite formation in strictly anoxic environments and rapid pyrite formation. Hydrogen generation through this process in aquatic and sedimentary systems provides an important alternative metabolite for microorganisms.

Active Site of Iron-Based Catalyst in Coal Liquefaction

Catalytic activity for coal liquefaction of sulfate-promoted iron oxide was investigated by a high-pressure differential thermal analysis technique and an autoclave test. It was found by X-ray photoelectron spectrometry that the main component of the catalyst was iron oxide, but this included 1.1 wt % sulfur. A particular finding from these experiments was that the catalyst showed sufficiently high activity for coal liquefaction without added sulfur and the activity was comparable to the activities of the catalyst with sulfur and FeS2 catalyst in a range of temperatures between 375 and 450 °C. This was very interesting because the sulfate-promoted iron oxide catalyst showed significantly high activity without addition of a promoter. To find possible reasons, temperature-programmed desorption profiles of hydrogen (TPRD) of these catalysts were measured. The sulfate-promoted iron oxide catalyst showed clear TPRD profiles in a wide range of temperatures between 100 and 350 °C. In contrast, FeS did not show any TPRD profile. However, after oxidation of FeS with air at elevated temperatures, it did show TPRD profiles, although these were weak. This suggests that surface-sulfate species can activate hydrogen molecules. It is well-known that sulfur or sulfides are easily oxidized with water or air. It is also well-known that much water is included in the coal liquefaction system. Therefore, the catalysts used in coal liquefaction are always exposed to oxidative agents in the reaction system. Therefore, it was concluded that an active site in working states for coal liquefaction was sulfate species formed during the process on the surface of the iron-based catalysts in the coal liquefaction.

A simple method to study the oxidizing power of rice roots under submerged soil conditions

AbstractA method was developed to visualize and to study the oxidizing power of rice roots growing under submerged soil conditions. The experimental set up consisted of a transparent jar with an inner core containing the submerged soil surrounded by a coarse (500 μm) and a fine (30 μm) meshed nylon net and a plastic folio. The space of about 1 cm between the jar wall and the soil column was either filled with water (before removing the plastic folio) or with agar containing the redox indicator. Three‐week‐old rice (Oryza saliva L.) seedlings grown in a sandy soil under controlled growth chamber conditions were transplanted into the jars between the plastic folio and the fine‐meshed (30 μm) nylon net. The agar medium (0.5% agar) containing 10 ppm leuco melhylene blue redox indicator or 5 mM ferrous sulphate or precipitated ferrous sulfide (10 m M ferrous sulphate + 4 m M Na2S) was filled in the transparent jars immediately after sucking out water. Within 4 h the oxidizing power of rice roots became visible by bluish coloration all along of roots and the agar medium around roots due to oxidation of leuco methylene blue. In case of ferrous sulphate reddish brown coloration was observed after one day around the roots and on the surface of roots because of ferrous iron oxidation. When agar medium blackened by precipitated FeS was used the root zone first became transparent because of oxidation of FeS and after few days the roots became reddish brown indicating iron oxyhydroxide deposition. The use of ferrous sulphate and ferrous sulfide enables to study the oxidizing power of rice roots for extended periods, whereas it is not possible to grow rice plants in leuco methylene blue for more than a few hours. However (results not shown), rice cultivars showed differences in oxidizing power of the roots.

Oxidation in the rhizosphere of mangroveKandelia candelseedlings

Abstract A study of the mechanism of oxidation capacity in mangrove roots was undertaken in Kandelia candel. By applying leuco methylene blue in semi-agar medium, the oxidation power of Kandelia candel roots was found. Oxidation activity was shown by a color change in the rhizosphere. There was substantial activity in the apical section of the root, decreasing progressively through the central and basal sections. Excised root, and the roots of seedlings with the shoots greased with vaseline showed a near failure of oxidation power. Similar result was given in another experiment which the root of K. candel was submerged in FeS precipitate in semi-agar medium. Due to the oxidation of FeS, after a few days the color of the medium changed from black to colorless or yellowish in the rhizosphere. This indicates that oxygen was released from K. candel root to the media. The oxidative power was also evaluated by α-naphthylamine oxidation. Activity was found to be the highest in the apical section of the root, declining progressively toward the basal section, which agrees with the results of the first experiment. However, the oxidation activity in the mangrove roots was much lower than in rice roots. Oxygen released from the roots is thus ascertained to be the major cause of the oxidation power of K. candel.

Simulated Aerobic Pedogenesis in Pyritic Overburden with a Positive Acid—Base Account

AbstractReclamation of surface‐mined land is often hindered by the excess salts and acidity produced by the weathering of pyritic overburden. This study was conducted to document the initial transformations that occur when pyritic overburden containing excess acid neutralizing potential is used as parent material in minesoil construction. An overburden containing 0.8% FeS2 (pyrite) and 1.6% inorganic carbonate (predominantly dolomite) was collected from the highwall of an active lignite surface mine in Panola County, Texas. The overburden was lightly crushed through a 13‐mm sieve and packed into three replicate lysimeters (0.75 by 0.75 by 1.2 m). The lysimeters were leached monthly with 63.5 mm of deionized water for 24 mo. The initial material had a pH of 8.3 and an excess acid neutralizing potential. Progressive FeS2 oxidation released H2SO4 and the pH decreased to 6.8. The dolomite dissolved, neutralizing the acidity, with subsequent release of Ca and Mg ions into solution. Leachate Ca2+ and SO2−4 concentrations exceeded the ion activity product of gypsum in the lower 60 cm of the lysimeters. Thin‐section analysis revealed that gypsum crystals precipitated along margins of residual pyrite particles and in conductive vughs and channels. The continued accumulation of gypsum in minesoil development could eventually lead to the formation of a gypsic or a petrogypsic horizon. A restrictive layer such as this would decrease vertical movement of water and O2, which would reduce vegetative growth, increase runoff and erosion, and thus increase the probability of reclamation failure.

The impact of desiccation of a freshwater marsh (Garcines Nord, Camargue, France) on sediment-water-vegetation interactions

The impact of desiccation on a marsh sediment was studied both in the laboratory and in the field. Changes in the sediment chemistry of a homogenized sediment suspension during desiccation were studied in the laboratory. FeS was oxidized completely. A considerable mineralization of organic phosphate took place, from both the acid soluble organic phosphate fraction and from the residual organic phosphate fraction, but no significant mineralization of organic matter was observed. The o-P formed during the mineralization was recovered partly in the Fe(OOH) ≈P fraction and partly in the CaC3≈P fraction. An upward flux was found. During spring and summer 1990 the water inlet to a shallow permanent freshwater marsh with a surface of about 1.5 hectares was blocked, in order to desiccate the marsh by evaporation. The sediments initially consisted of a black anoxic organic top layer and a less organic anoxic gray layer. During the desiccation of the sediment a brown oxic surface layer was formed from the black layer and an increase of pH and Eh occurred. Subsequent rainfall made the Eh increase further but caused a decrease in pH indicating an increase in bacterial activity. A progressive oxidation of FeS was observed. An increase in Tot-P in the surface layer and a decrease in the gray and the black layer of the sediment occurred, probably due to a capillary upward flux. A mineralization of organic matter was observed in the two deeper layers. In the upper brown layer this mineralization was less evident, probably because it has been masked by the capillary movement. A net C loss of 40% was calculated to have occurred in the layer 0–40 cm. In the deeper layers a decrease in Tot-N was observed, whereas no important increase occurred in the top layer. Over a sediment layer of 40 cm a N loss of 50% was calculated. C- and N losses occurred simultaneously, suggesting the importance of mineralization as a source of inorg-N for denitrification. The chemical and physical changes in the sediment during desiccation affected layers down to 40 cm. This means that not only the top layer of a sediment but also deeper layers are active in systems of which part of the sediment dries occasionally. Fractionation of the surface sediment phosphate showed an increase of Fe(OOH) ≈ P in the top layer due to the oxidation of FeS to Fe(OOH), enlarging the P-adsorption capacity of the sediment. A mineralization of about 50% of acid soluble organic phosphate occurred. After rainfall, a net increase in residual organic phosphate occured presumably due to an increase of bacterial activity. Drying may therefore be utilized as a tool, in wetland management, to eliminate organic nitrogen and carbon from the sediment. In rice culture, it may be used to make part of the organic nitrogen available to the rice.

Removal of inorganic sulfur from coal by Thiobacillus ferrooxidans.

The kinetics of removal of inorganic sulfur (FeS 2 ) in coal by Triobacillus ferroxidans was studied in a we mixed batch reactor. Experiments were made at 30 o C and pH 2.0 on adsorption of the bacteria to coal particles and bacterial desulfurization. In the adsorption experiments, it was found that T. ferrooxidans was selectively adsorbed on inorganic sulfur in coal and that the equilibrium data obeyed the Langmuir isotherm. The coal desulfurization was found to occur by a direct bacterial oxidation of FeS 2 , the chemical oxidation via ferric iron being insignificant. The FeS 2 in coal was completely oxidized to sulfate ion, and the maximum conversion of 82% was achieved within two weeks

Origin of stratiform sediment-hosted manganese carbonate ore deposits: Examples from Molango, Mexico, and TaoJiang, China

Carbonate and sulfide minerals from the Molango, Mexico, and TaoJiang, China, Mn deposits display similar and distinctive δ 34S and δ 13C patterns in intervals of manganese carbonate mineralization. δ 13C-values for Mn-bearing carbonate range from −17.8 to +0.5‰ (PDB), with the most negative values occurring in high-grade ore zones that are composed predominantly of rhodochrosite. In contrast, calcite from below, within and above Mn-carbonate zones at Molango has δ 13 C≈0‰ (PDB). Markedly negative δ 13C data indicate that a large proportion of the carbon in Mn-carbonates was derived from organic matter oxidation. Diagenetic reactions using MnO 2 and SO 2− 4 to oxidize sedimentary organic matter were the principle causes of such 12C enrichment. Pyrite content and sulfide δ 34S-values also show distinctive variations. In unmineralized rocks, very negative δ 34S-values ( avg. < −21‰ CDT ) and abundant pyrite content suggest that pyrite formed from diagenetic, bacteriogenic sulfate reduction. In contrast, Mn-bearing horizons typically contain only trace amounts of pyrite (e.g., <0.5 wt% S with δ 34S-values 34S-enriched, in some cases to nearly the value for contemporaneous seawater. 34S-enriched pyrite from the Mn-carbonate intervals indicates sulfide precipitation in an environment that underwent extensive SO 2− 4 reduction, and was largely a closed system with regard to exchange of sulfate and dissolved sulfide with normal seawater. The occasional occurrence of 34S-depleted pyrite within Mn-carbonate zones dominated by 34S-enriched pyrite is evidence that closed-system conditions were intermittent and limited to local pore waters and did not involve entire sedimentary basins. Mn-carbonate precipitation may have occluded porosity in the surficial sediments, thus establishing an effective barrier to SO 2− 4 exchange with overlying seawater. Similar isotopic and mineralogic characteristics from both the Molango and TaoJiang deposits, widely separated in geologic time and space, suggest they were formed similarly by MnO 2 precipitation at the margins of dysaerobic to anoxic marine basins. Mn-carbonate formed predominantly by early-diagenetic reduction of Mn-oxides via oxidation of organic matter in near-surface sediments. In addition to MnCO 3 precipitation, organic matter oxidation reactions resulted in oxidation of FeS to Fe-oxides such as magnetite, maghemite and hematite. The latter process explains anomalously low pyrite content and abundant Fe-oxide minerals in ore zones dominated by rhodochrosite.

On the attainment of stable autothermal operation in continuous pressure leaching reactors

The autothermal model of operation of a multi-stage oxidative pressure leaching reactor is analysed with the aid of a descriptive mathematical model. For this, the adiabatic heat balance equation of the reactor is solved along with the mass balance equations. The model considers the process system (pressure oxidation) to be controlled simultaneously by the kinetics of the two parallel exothermic leaching reactions (oxidation of FeS 2/FeAsS by O 2), and the gas-liquid mass transfer of oxygen. By maintaining the total pressure of the system constant, that is P tot(=P H 2 O + P O 2 ∗ )= constant , enhanced thermal and kinetic stability is achieved at the autothermal steady-state operating temperature. The combined S/As content of the feed is of crucial importance to the thermal economy of the process. Feeds with sulphur content above 5.5 wt% can be processed without any preheating with either a 4 stage autoclave (equal size compartments), or a 5 stage autoclave having the first double the size of the rest. The higher the sulphur content the lower the concentration of solids in the feed slurry has to be in order to attain autothermal operation. For feeds with low sulphur content (<5 wt%), autoclaves with the first compartment substantially larger than the rest have to be employed for optimum thermal balance results.

Stable Isotope Evidence for the Origin of the Urkut Manganese Ore Deposit, Hungary

ABSTRACT The stratiform sedimentary manganese ore deposit at Urkut, Hungary contains primary ore that is exclusively manganese carbonate. It shares many geological, sedimentological, mineralogical, and geochemical characteristics with other organic-rich, sediment-hosted, marine manganese deposits. Samples from two stratigraphic sequences through the marlstone-hosted Mn carbonate ore at Urkut were studied. Stable isotope compositions of both Mn-rich and Mn-poor carbonates were analyzed and are discussed, together with the geologic background of the deposit, rock and mineral chemistry, and x-ray diffraction mineralogy. The 13C values of carbonates (-1.24 to -30.78) show a negative linear correlation with Mn contents and a negative exponential correlation with total organic carbon contents. These relations suggest that the Urkut deposit was produced by early diagenetic precipitation of manganese carbonate. Data are consistent with models for bacterial metabolic pathways. Thus, mineralization was a consequence of bacterially mediated diagenetic reactions that involved Mn reduction via one or two coupled reactions: the oxidation of organic matter with Mn oxyhydroxide reduction, or the oxidation of FeS produced as a byproduct of seawater sulfate reduction with Mn oxyhydroxide reduction. xygen-deficient water in the depositional basin acted as a carrier and reservoir for Mn+2 before its deposition.

An experimental evaluation of the use of rock phosphate (apatite) for the amelioration of acid-producing coal mine waste

The weathering of coal and coal-associated rocks (AMD) containing iron disulphide minerals produces acid mine drainage. (AMD). This is the most important environmental problem in high-sulphur coal basins. It is generally agreed that the major oxidizing agent of FeS 2 is the ferric ion. Experiments have shown that the average equilibrium voltage for the Fe 3+/FeS 2 system is 0.42 which corresponds to a Fe 3+: Fe 2+ ratio of 10 −6: 1. In an ideal solution, if the Fe 3+: Fe 2+ ratio could be reduced to less than 10 −6: 1, the Fe 3+-oxidation of FeS 2 could not take place. It has been shown that both Fe 2+ and Fe 3+ ions can be precipitated by the phosphate ion under ideal conditions at near-neutral pH-conditions, producing a Fe 3+:Fe 2+ ratio of 10 −11: 1. Experiments utilizing rock phosphate (apatite) as a source of phosphate ion showed that the treatment of coal-associated rock containing iron disulphide minerals not only prevented the production of acid, but also terminated the production of acid from materials already undergoing oxidation. The results of these experiments indicate that rock phosphate (apatite) is an effective AMD ameliorant based upon its ability to reduce the Fe 3+: Fe 2+ ion ratio to below 10 −6: 1 and its ability to reduce the subsequent solution potential to the less than 0.42 V, required to allow the oxidation of FeS 2 by Fe 3+.

Transformations of sulfur compounds in marsh-flat sediments

Measurements were made in mud-flat sediments from Flax Pond salt marsh to characterize the rates and mechanisms of sulfur cycling in an organic-rich coastal marine environment. Approximately 13 mmoles/m 2 of reduced sulfur are generated annually in the mud flat and the dominant-solid-phase product is pyrite. Ion activity products involving dissolved iron and sulfide species indicate approximate saturation with respect to metastable iron sulfide phases, showing that pyrite is not likely to be the first-formed Fe-bearing sulfide. Comparison of ΣH 2S vs. SO −− 4 relationships in anoxic incubation experiments with those occurring in the undisturbed sediment permits evaluation of possible mechanisms involved in the transformation of metastable iron monosulfides to pyrite. Oxidants ( e.g. MnO 2) that are introduced into the surface sediment, either by animal activity or physical events, are apparently necessary to cause major oxidation of FeS and ΣH 2S to pyrite and sulfate. Solid-phase sulfur analyses and net ΣH 2S accumulation in sediment pore waters are consistent with major sulfide oxidation, indicating that approximately 95% of the sulfide generated in the mud flat is reoxidized to sulfate and roughly half of this oxidation involves dissolved sulfide. The major factors limiting reduced sulfur burial are physical and biological disturbances and a low abundance of reactive solid-phase iron (2 wt%).

Cationic surfactants in electrochemical determination of sulfide minerals

Addition of a cationic surfactant (Hyamine-2389) solubilized iron pyrite powder and increased the oxidation potential of water to a value that allowed anodic voltammetric measurements to be made directly on the powders. An oxidation maximum (at 0.69 V vs. SCE) at pH 12.8 is associated with oxidation of FeS 2 to Fe 2O 3. Because adsorption of the pyrite on the electrode provides preconcentration, the peak height is very sensitive to concentration. Measurements above the critical micelle concentration (CMC) were complicated because peak height was not a simple function of pyrite concentration and the calibration curve was shifted by the presence of an insoluble inert material. However, at concentrations of surfactant below the CMC, a relatively linear relation was observed between peak height and the amount of pyrite; thus the technique can be used to determine the iron pyrite in solid ore samples.

Etude electrochimique de la pyrite en milieu acide

Resume The electrochemical behaviour of pyrite has been studied in 1 M sulfuric acid. Voltammetric experiments were carried out with synthetic pyrite, using a carbon paste electrode. In this medium, the anodic oxidation of FeS2 occurs at + 0.98 V vs sce and leads to the formation of Fe3+, S and H2SO4. The reduction of FeS2 to Fe2+ and H2S occurs at −0.50 V.

Pyrite oxidation in acid sulphate soils: The role of miroorganisms

A study has been made of microbial processes in the oxidation of pyrite in aicd sulphate soil material. Such soils are formed during aeration of marine muds rich in pyrite (FeS2). Bacteria of the type ofThiobacillus ferrooxidans are mainly responsible for the oxidation of pyrite, causing a pronounced acidification of the soil. However, becauseThiobacillus ferrooxidans functions optimally at pH values bellow 4.0, its activity cannot explain the initial pH drop from approximately neutral to about 4. This was shown to be a non-biological process, in which bacteria play an insignificant part. AlthoughThiobacillus thioparus andThiobacillus thiooxidans were isolated from the acidifying soil, they did not stimulate oxidation of FeS2, but utilized reduced sulphur compounds, which are formed during the non-biological oxidation of FeS2. Ethylene-oxide-sterilized and dry-sterilized soil inoculated with pure cultures of mixtures of various thiobacilli or with freshly sampled acid sulphate soil soil did not acidify faster than sterile blanks.Thiobacillus thiooxians. Thiobacillus thioparus. Thiobacillus intermedius andThiobacillus perometabolis increased from about 104 to 105 cells/ml in media with FeS2 as energy source. However, FeS2 oxidation in the inoculated media was not faster than in sterile blanks. Attempts to isolate microorganisms other thanThiobacillus ferrooxidans, like metallogenium orLeptospirillum ferrooxidans, which might also be involved in the oxidation of FeS2 were not successful. Addition of CaCO3 to the soil prevented acidification but did not stop non-biological oxidation of FeS2.

Mössbauer spectroscopic and magnetokinetic studies on the oxidation of pyrite (FeS2)

In an earlier publication 1 we reported aspects of magnetokinetic studies on the oxidation of FeS2 carried out as a function of its particle size (90 to 250μm) and temperature (400–500 °C); in this study we considered mostly the formation of α and γ Fe2O3. In the present paper we have considered the formation of other oxidation products such as (a) Fe2 (SO4)3, and (b) superparamagnetic particles of α Fe2O3. Mössbauer spectroscopy has confirmed the presence of (a) and, coupled with X-ray line broadening studies has shown the absence of (b). The yield of Fe2 (SO4), is found to increase with decreasing starting particle size of FeS2. Using our earlier results 1 we have obtained values of (dM/dt) from the initial slopes of the magnetization (M) vs. time curves for particle sizes (180 to 250 μm). From plots of ln (dM/dt) versus 1/T an activation energy of ∼7 Kcal/mole has been obtained for the following predominant reaction: FeS+(11/4)O2→ (1/2) αFe2O3+2SO2. This value compares favorably with values reported by other workers2 who used classical thermogravimetric analysis.

Pyrite oxidation by Thiobacillus ferrooxidans with special reference to the sulphur moiety of the mineral.

Available cultures of Thiobacillus ferrooxidans were found to be contaminated with bacteria very similar to Thiobacillus acidophilus. The experiments described were performed with a homogeneous culture of Thiobacillus ferrooxidans. Pyrite (FeS2) was oxidized by Thiobacillus ferrooxidans grown on iron (Fe2+), elemental sulphur (S0) or FeS2. Evidence for the direct utilization of the sulphur moiety of pyrite by Thiobacillus ferrooxidans was derived from the following observations: a. Known inhibitors of Fe2+ and S0 oxidation, NaN3 and NEM, respectively, partially abolished FeS2 oxidation. b. A b-type cytochrome was detectable in FeS2- and S0-grown cells but not in Fe2+-grown cells. c. FeS2 and S0 reduced b-type cytochromes in whole cells grown on S0. d. CO2 fixation at pH 4.0 per mole of oxygen consumed was the highest with S0, lowest with Fe2+ and medium with FeS2 as substrate. e. Bacterial Fe2+ oxidation was found to be negligible at pH 5.0 whereas both FeS2 and S0 oxidation was still appreciable above this pH. f. Separation of pyrite and bacteria by means of a dialysis bag caused a pronounced drop of the oxidation rate which was similar to the reduction of pyrite oxidation by NEM; indirect oxidation of the sulphur moiety by Fe3+ was not affected by separation of pyrite and bacteria. Bacterial oxidation and utilization of the sulphur moiety of pyrite were relatively more important with increasing pH.

Acidification and Deacidification of Coastal Plain Soils as a Result of Periodic Flooding

AbstractPeriodic flooding of recent coastal plain soils causes either acidification or deacidification of the surface horizons. Acidification of sulfate‐bearing “nonacid” soils (pH &gt; 5) involves formation of FeS and partial loss of alkalinity (HCO3‐) in the flood water during the wet season, followed by oxidation of FeS to ferric oxide and sulfuric acid in the dry season. Deacidification of acid soils (pH &lt; 4.5) takes place during flooding. This involves reduction of ferric oxides to Fe2+, the simultaneous release of adsorbed SO42‐ or hydrolysis of basic sulfates (that function as proton donor during iron reduction at low pH), followed by oxidation of dissolved FeSO4 to ferric oxide and sulfuric acid at the soil‐water interface, and removal of H2SO4 by lateral surface drainage. During soil reduction the actual pH increases and a shift from one process to the other takes place if the pH becomes high enough (4.5 to 5) for the weak acids CO2 and H2S to function as proton donors. Therefore, deacidification beyond pH 4.5 to 5 automatically invokes the release of alkalinity. Conversely, acidification to a pH below 4.5 to 5 during a dry season will be undone by deacidification in the following wet season. This explains why the surface horizons of periodically flooded acid sulfate soils and nonacid marine soils eventually attain the same pH under dry conditions. This pH is fundamentally related to the dissociation constant, the solubility and the partial pressure of CO2.