The role of carbonate ions in pyrite oxidation in aqueous systems

The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO₃⁻, FeCO₃⁰, Fe(CO₃)(OH)⁻ and FeCO₃²⁻) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O₂, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O₂ is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.     

Nonequilibrium thermodynamic model of manganese carbonate oxidation

Manganese carbonate can be converted to many kinds of manganese oxides when it is aerated in air and oxygen.Pure manganese carbonate can be changed into Mn3O4 and γ-MnOOH,and manganese carbonate ore can be converted to MnO2 under the air-aerating and oxygen-aerating circumstances.The oxidation process of manganese carbonate is a changing process of mineral association,and is also a converting process of valence of manganese itself.Not only equilibrium stat,but also nonequilibrium state are involved in this whole process,This process is an irreversible heterogeneous complex reaction,and oberys the nonequilibrium thermodynamic model,The oxidation rate of manganese cabonate is controlled by many factors,especially nonmanganese metallic ions which play an important role in the oxidation process of manganese carbonate.     

The oxidation state of manganese in marine sediments and ferromanganese nodules

Marine sediments and ferromanganese nodules from the Pacific Ocean have been analyzed for the $\text{O}\text{Mn}$ ratio of solid manganese. We tested six chemical methods and concluded that the iodometric and oxalate methods were equivalent and were the best choice in terms of accuracy and precision on natural samples. We choose the iodometric method for most of our analyses because the oxalate procedure is a method of differences.The ferromanganese nodules that we analyzed were all from MANOP site H and had $\text{Mn}\text{Fe}$ ratios that ranged from 5.6 to 70. These nodules were invariably highly oxidized with $\text{O}\text{Mn}$ values ranging from 1.90 to 2.00. Our most precise analyses suggest that less than 1% of the total manganese is present as Mn(II).We also analyzed red clay and hemipelagic sediments from the eastern tropical Pacific (Baja borderland and MANOP site H) and carbonate ooze samples from the equatorial Pacific. These sediments are also highly oxidized ($\text{O}\text{Mn}\text{= 1.90 to 2.00}$) except when Mn(II) appears in the interstitial water. As dissolved Mn(II) increases the value of the $\text{O}\text{Mn}$ ratio in the solid phase decreases. The $\text{O}\text{Mn}$ ratio decreases to values as low as 1.40. This decrease appears to be due to a decrease in oxidized manganese by reduction, however, an increase in reduced manganese in the solid sediments by adsorption or MnCO₃ formation can not be ruled out in all cases.     

The oxidation state of manganese in surface sediments of the deep sea

Iodometric titration of deep sea sediment from cores at MANOP sites M and H indicate a particulate Mn oxidation state between 3.3 and 3.9 in the top 5 cm. Results from site H show a minimum in the Mn oxidation state at 1.5 cm depth indicating Mn reduction is occurring above the zone of pore water Mn oxidation. Using a simple box model, the rate of reduction is calculated to be 19 mg/cm² kyr ? 77 mg/cm² kyr, at least 5 times the flux of Mn to nodules in this region. Although no Mn oxidation state minimum is observed in site M sediments, oxic reduction is inferred from a particulate mass balance, indicating that a large fraction of the Mn rain to this site is not preserved in the sediments and must be remobilized. We suggest that the process of reduction in oxic or suboxic environments near the sediment-water interface may be an important mechanism controlling the concentration of Mn in sediments and provides a mechanism for supplying diagenetic Mn to the nodules at site H.     

Rates of accumulation of manganese nodules and associated sediment from the equatorial Pacific

The accumulation rates and trace element contents of manganese nodules and coexisting sediment in a core collected from the central equatorial Pacific indicate that the nodules are chemically very similar and accumulated at similar rates, but that each stopped growing when it was buried. Elements largely confined to authigenic phases accumulate at comparable rates in nodules and sediment, whereas elements associated with detrital phases accumulate much more rapidly in the sediment.     

Rates of microbial hydrogen oxidation and sulfate reduction in Opalinus Clay rock

Hydrogen gas (H₂) may be produced by the anoxic corrosion of steel components in underground structures, such as geological repositories for radioactive waste. In such environments, hydrogen was shown to serve as an electron donor for autotrophic bacteria. High gas overpressures are to be avoided in radioactive waste repositories and, thus, microbial consumption of H₂ is generally viewed as beneficial. However, to fully consider this biological process in models of repository evolution over time, it is crucial to determine the in situ rates of microbial hydrogen oxidation and sulfate reduction. These rates were estimated through two distinct in situ experiments, using several measurement and calculation methods. Volumetric consumption rates were calculated to be between 1.13 and 1.93 μmol cm⁻³ day⁻¹ for H₂, and 0.14 and 0.20 μmol cm⁻³ day⁻¹ for sulfate. Based on the stoichiometry of the reaction, there is an excess of H₂ consumed, suggesting that it serves as an electron donor to reduce electron acceptors other than sulfate, and/or that some H₂ is lost via diffusion. These rate estimates are critical to evaluate whether biological H₂ consumption can negate H₂ production in repositories, and to determine whether sulfate reduction can consume sulfate faster than it is replenished by diffusion, which could lead to methanogenic conditions.     

The aqueous oxidation of complex sulfide concentrates in hydrochloric acid

Complex sulfides containing sphalerite, galena, chalcopyrite, and small amounts of silver in a matrix of pyrite can be decomposed at 120°C and oxygen pressure of 1000 kPa in 1–2 N HCl for 90 min to yield > 97% of the zinc and > 95% of the copper in solution while about 83% of the lead remains in the residue as PbCl₂ and PbSO₄ and 85% of the silver, together with most of the pyrite. The recovery of elemental sulfur is nearly 100% with respect to ZnS, PbS, and CuFeS₂. Leaching in HCl is faster than in H₂SO₄ at the same acid normality, and the process is diffusion-controlled (strongly dependant on agitation speed and having an activation energy of 3.6 kcal/mole). Lead jarosite, Pb_0.5Fe₃ (SO₄)₂?(OH)₆, is mainly formed when H₂SO₄ is used as a leaching agent.     

Neptunium and manganese biocycling in nuclear legacy sediment systems

Understanding the behaviour of the highly radiotoxic, long half-life radionuclide neptunium in the environment is important for the management of radioactively contaminated land and the safe disposal of radioactive wastes. Recent studies have identified that microbial reduction can reduce the mobility of neptunium via reduction of soluble Np(V) to poorly soluble Np(IV), with coupling to both Mn- and Fe(III)- reduction implicated in neptunyl reduction. To further explore these processes Mn(IV) as δMnO₂ was added to sediment microcosms to create a sediment microcosm experiment “poised” under Mn-reducing conditions. Enhanced removal of Np(V) from solution occurred during Mn-reduction, and parallel X-ray absorption spectroscopy (XAS) studies confirmed Np(V) reduction to Np(IV) commensurate with microbially-mediated Mn-reduction. Molecular ecology analysis of the XAS systems, which contained up to 0.2 mM Np showed no significant impact of elevated Np concentrations on the microbial population. These results demonstrate the importance of Mn cycling on Np biogeochemistry, and clearly highlight new pathways to reductive immobilisation for this highly radiotoxic actinide.     

Adsorption of copper onto goethite in aqueous systems

The adsorption of copper(II) onto goethite was studied as a function of pH, total dissolved copper concentration, surface area of goethite, and ionic strength. The adsorption of copper was similar to that of other hydrolyzable metals. A tenfold increase in goethite surface area had a significant effect on the adsorption edge, but a tenfold increase in the ionic strength of the medium did not effect the adsorption edge. The distribution coefficients increase sharply with increase in pH and ranged from 10 to 60,000 ml/g over a range of two and half pH units, depending on the goethite surface area and copper concentration. A tenfold decrease in ionic strength as well as a tenfold increase in surface area of goethite did not have any effect on the magnitude of distribution coefficients. Distribution coefficients were used to calculate the number of protons released per mole of copper adsorbed during the adsorption process. The average number of protons released per mole of copper adsorbed was estimated to be 1.40 ± 0.10.Managed by Martin Marietta Energy System, Inc., for the U.S. Department of Energy under contract no. DE-AC05-840R21400.     

Influence of extractable soil manganese on oxidation capacity of different soils in Korea

We examined the relationship between soil oxidation capacity and extractable soil manganese, iron oxides, and other soil properties. The Korean soils examined in this study exhibited low to medium Cr oxidation capacities, oxidizing 0.00–0.47 mmol/kg, except for TG-4 soils, which had the highest capacity for oxidizing added Cr(III) [>1.01 mmol/kg of oxidized Cr(VI)]. TG and US soils, with high Mn contents, had relatively high oxidation capacities. The Mn amounts extracted by dithionite-citrate-bicarbonate (DCB) (Mn^d), NH₂OH·HCl (Mn^h), and hydroquinone (Mn^r) were generally very similar, except for the YS1 soils, and were well correlated. Only small proportions of either total Mn or DCB-extractable Mn were extracted by NH₂OH·HCl and hydroquinone in the YS1 soils, suggesting inclusion of NH₂OH·HCl and hydroquinone-resistant Mn oxides, because these extractants are weaker reductants than DCB. No Cr oxidation test results were closely related to total Mn concentrations, but Mn^d, Mn^h, and Mn^r showed a relatively high correlation with the Cr tests (r = 0.655–0.851; P < 0.01). The concentrations of Mn^d and Mn^h were better correlated with the Cr oxidation tests than was the Mn^r concentration, suggesting that the oxidation capacity of our soil samples can be better explained by Mn^d and Mn^h than by Mn^r. The first component in principal components analysis indicated that extractable soil Mn was a main factor controlling net Cr oxidation in the soils. Total soil Mn, Fe oxides, and the clay fraction are crucial for predicting the mobility of pollutants and heavy metals in soils. The second principal component indicated that the presence of Fe oxides in soils had a significant relationship with the clay fraction and total Mn oxide, and was also related to heavy-metal concentrations (Zn, Cd, and Cu, but not Pb).     

Chromium oxidation by manganese (hydr)oxides in a California aquifer

There are increasing concerns with elevated levels of Cr(VI) in the environment because it is a strong oxidant, corrosive, and carcinogenic. The concerns extend to the presence of Cr(VI) in many aquifers in California and elsewhere, where relatively high levels have been attributed to both industrial pollution and natural processes. The authors have, therefore, determined if natural redox processes contribute to the presence of high Cr(VI) concentrations (6–36 μg L⁻¹) in an aquifer in central California relative to non-detectable concentrations (<0.1 μg L⁻¹) in an adjacent aquifer. Specifically, the distribution and the redox speciation of dissolved (<0.45 μm) Cr have been compared with those of particulate Mn and Fe oxy-hydroxides in sediments, using X-ray absorption spectroscopy at the Mn and Fe L-edges. The analyses show a correlation between the presence of dissolved Cr(VI) and Mn (hydr)oxide minerals, which are the only common, naturally occurring minerals known to oxidize Cr(III) in laboratory experiments. This covariance substantiates the results of those experiments and previous field studies that indicate natural oxidation mechanisms might account for the relatively high levels of Cr(VI) in the study site, as well as for elevated concentrations in other aquifers with similar biogeochemical conditions.     

The oxidation of cobalt(II) adsorbed on manganese dioxide

X-ray photoelectron spectroscopy (XPS) measurements of cobalt adsorbed on MnO₂ reveal strong evidence that Co(II) has been oxidized to Co(III). The manganese spectra are characteristic of Mn(IV). Model calculations suggest that Co(II) cannot be oxidized by O₂ to Co(III) in bulk solution at seawater concentrations but that the oxidation can proceed in the presence of the strong electric field at the MnO₂-solution interface. Ni(II), however, cannot be oxidized at the interface except at very high concentrations. These calculations suggest that the oxidation of Co(II) can explain the geochemical separation of cobalt from nickel.     

Environmental oxidation rate of manganese(II): bacterial catalysis

A simple mass balance for dissolved manganese(II) in waters containing low levels of oxygen in Saanich Inlet indicates that the residence time for Mn(II) removal to the solid phase is on the order of a few days. The average oxidation state of Mn in particulate material sampled from the region of Mn removal was 2.3–2.7, and electron micrographs revealed structures characteristic of bacterially formed Mn precipitates. Radiotracer experiments utilizing ⁵⁴Mn(II) indicated that removal of Mn from solution in the region of active uptake was substantially blocked by a poison mixture, demonstrating that Mn(II) binding to particulates is catalyzed by bacteria in this environment.     

The kinetics of iodide oxidation by the manganese oxide mineral birnessite

The kinetics of iodide (I⁻) and molecular iodine (I₂) oxidation by the manganese oxide mineral birnessite (δ-MnO₂) was investigated over the pH range 4.5-6.25. I⁻ oxidation to iodate proceeded as a two-step reaction through an I₂ intermediate. The rate of the reaction varied with both pH and birnessite concentration, with faster oxidation occurring at lower pH and higher birnessite concentration. The disappearance of I⁻ from solution was first order with respect to I⁻ concentration, pH, and birnessite concentration, such that −d[I⁻]/dt = k[I⁻][H⁺][MnO₂], where k, the third order rate constant, is equal to 1.08 ± 0.06 × 10⁷ M⁻² h⁻¹. The data are consistent with the formation of an inner sphere I⁻ surface complex as the first step of the reaction, and the adsorption of I⁻ exhibited significant pH dependence. Both I₂, and to a lesser extent, sorbed to birnessite. The results indicate that iodine transport in mildly acidic groundwater systems may not be conservative. Because of the higher adsorption of the oxidized I species I₂ and , as well as the biophilic nature of I₂, redox transformations of iodine must be taken into account when predicting I transport in aquifers and watersheds.     

Chromite oxidation by manganese oxides in subseafloor basalts and the presence of putative fossilized microorganisms

Chromite is a mineral with low solubility and is thus resistant to dissolution. The exception is when manganese oxides are available, since they are the only known naturally occurring oxidants for chromite. In the presence of Mn(IV) oxides, Cr(III) will oxidise to Cr(VI), which is more soluble than Cr(III), and thus easier to be removed. Here we report of chromite phenocrysts that are replaced by rhodochrosite (Mn(II) carbonate) in subseafloor basalts from the Koko Seamount, Pacific Ocean, that were drilled and collected during the Ocean Drilling Program (ODP) Leg 197. The mineral succession chromite-rhodochrosite-saponite in the phenocrysts is interpreted as the result of chromite oxidation by manganese oxides. Putative fossilized microorganisms are abundant in the rhodochrosite and we suggest that the oxidation of chromite has been mediated by microbial activity. It has previously been shown in soils and in laboratory experiments that chromium oxidation is indirectly mediated by microbial formation of manganese oxides. Here we suggest a similar process in subseafloor basalts.     

On the transformation of struvite into newberyite in aqueous systems

The transformation of struvite MgNH₄PO₄ · 6H₂O into newberyite MgHPO₄ · 3H₂O is studied in aqueous solutions at 25° C and 37° C by following the evolution of solutions, supersaturated with respect to both phases in the concentration range 0.025–0.50 M. The precipitation of struvite as the first phase is accompanied by a sharp decrease of pH, which subsequently remains constant. During this period, newberyite crystals can nucleate whereas struvite is dissolving. Another decrease of pH, due to the newberyite growth, is observed when almost all struvite has disappeared. Finally pH tends asymptotically to a constant value, corresponding to a stable equilibrium. The whole process may take several months. At neither stage were epitaxial relationships observed between struvite and newberyite. The shape of the newberyite crystals is an indicator of their origin in mineralogical and biological systems.     

Electrochemical oxidation of succinic acid in aqueous solutions using boron doped diamond anodes

In this work, the electrochemical oxidation of succinic acid on boron-doped diamond (BDD) anodes was investigated. Voltammetric study had shown that no peaks appeared in the region of electrolyte stability which indicates that succinic acid oxidation can take place at a potential close to the potential region of electrolyte oxidation. Galvanostatic electrolyses achieved total chemical oxygen demand (COD) removals and high mineralization yields under different operating conditions (initial COD, current density and nature of supporting electrolyte). Oxalic, glycolic and formic acids were the main intermediates detected during anodic oxidation of succinic acid on BDD electrode and carbon dioxide as the final product. The mean oxidation state of carbon reached the value of 4 at the end of electrolysis which is indicative of mineralization of almost all organics present in aqueous solution. The exponential profile of COD versus specific electrical charge has shown that mass transfer is the limiting factor for the kinetics of electrochemical process. A simple mechanism was proposed for the mineralization of succinic acid. First, hydroxyl radicals attack of succinic acid leading to formation of glycolic, glyoxylic, fumaric and maleic acids. Then, theses acids undergo rapid and non-selective oxidation by hydroxyl radicals to be transformed into oxalic and formic acids which leads to further oxidation steps to mineralize these acids into carbon dioxide and water.     

Kinetics of reactions between aqueous sulfates and sulfides in hydrothermal systems

The rates of chemical reactions between aqueous sulfates and sulfides are essentially identical to sulfur isotopic exchange rates between them, because both the chemical and isotopic reactions involve simultaneous oxidation of sulfide-sulfur atoms and reduction of sulfate-sulfur. The rate of reaction can be expressed as a second order rate law: R = k·[∑SO₄^2?]·[∑S^2?], where R is the overall rate, k is the rate constant and [∑SO₄^2?] and [∑S^2?] are molal concentrations. We have computed the rate constants from the available experimental data on the partial exchange of sulfur isotopes between aqueous sulfates and sulfides using the rate law established by us: $\text{ln(}\text{α}{}^{\mathrm{e}}\text{? α}\text{α}{}^{\mathrm{e}}\text{? α}{}^0\text{) = ? kt([∑SO}{}_4{}^{\mathrm{2?}}\text{] + [∑S}{}^{\mathrm{2?}}\text{])}$, where t is time and α⁰, α, and α^e are, respectively, the fractionation factors at t = 0 (the initial condition), at the end of experiment, and at equilibrium. The equilibrium fractionation factor can be expressed as: $\text{1000 ln α}{}^{\mathrm{e}}\text{=}\text{6.463 × 10}{}^6\text{T}{}^2\text{+ 0.56 (±.5)}$ (T in Kelvin).The rate constants are strongly dependent on T and pH, but not in as simple a manner as suggested by Igumnov (1976). Our rate constants in Na-bearing hydrothermal solutions decrease by 1 order of magnitude with an increase in pH by 1 unit at pH's less than ~3, remain constant in the pH range of ~4 to ~7, and again decrease at pH >7. The activation energy for the reaction also depends on pH: 18.4 ± 1 kcal/mole at pH = 2, 29.6 ± 1 kcal/mole at pH = 4 to 7, and between 40 and 47 kcal/mole at pH around 9. The observed pH dependence of the rate constant and of the activation energy can be best explained by a model involving thiosulfate molecules as reaction intermediates, in which the intramolecular exchange of sulfur atoms in thiosulfates becomes the rate determining step.The rate constants obtained in this study were used to compute the changes in the isotopic fractionation factors between aqueous sulfates and sulfides during cooling of fluids. Comparisons with data of coexisting sulfate-sulfide minerals in hydrothermal deposits, suggest that simple cooling was not a likely mechanism for coprecipitation of sulfate and sulfide minerals at temperatures below 350°C. Mixing of sulfide-rich solutions with sulfate-rich solutions at or near the depositional sites is a more reasonable process for explaining the observed fractionation.The degree of attainment of chemical equilibrium between aqueous sulfates and sulfides in a hydrothermal system, and the applicability of a_O2-pH type diagrams to mineral deposits, depends on the ∑S content and the thermal history of the fluid, which in turn is controlled by the flow rate and the thermal gradient in the system.The rates of sulfate reduction by non-bacterial processes involving a variety of reductants are also dependent on T, pH, [∑SO₄^2?], and [∑S^2?], and appear to be fast enough to become geochemically important at temperatures above about 200°C.