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1.
Fe(II)–Fe(III) layered double hydroxysalt green rusts, GRs, are very reactive compounds with the general formula, [Fe II(1−x) Fe IIIx (OH) 2] x+·[(x/n) A n−·(m/n) H 2O] x−, where x is the ratio Fe III/Fe tot, and reflects the structure in which brucite-like layers alternate with interlayers of anions A n− and water molecules. Two types of crystal structure for GRs, GR1 and GR2, represented by the hydroxychloride GR1(Cl −) and the hydroxysulphate GR2(SO 42−) are distinguished by X-ray diffraction due to different stacking. By analogy with GR1(Cl −) the structure of the fougerite GR mineral, [Fe II(1−x) Fe IIIx (OH) 2] x+·[x OH −·(1−x) H 2O] x- Fe(OH) (2+x)·(1−x) H 2O, is proposed displaying interlayers made of OH − ions and water molecules (in situ deprotonation of water molecules is necessary for explaining the flexibility of its composition). The space group of mineral GR1(OH −) would be R3̄m, with lattice parameters a≅0.32 and c≅2.25 nm. Stability conditions and the Eh-pH diagram of Fe(OH) (2+x) (the water molecules are omitted) are determined from hydromorphic soil solution equilibria with GR mineral in Brittany (France). Computed Gibbs free energies of formation from soil solution/mineral equilibrium fit well with a regular solid solution model: μ°[Fe(OH) (2+x)]=(1−x) μ°[Fe(OH) 2]+x μ°[Fe(OH) 3]+ RT [(1− x) ln (1− x)+ x ln x]+A 0 x (1− x), where μ°[Fe(OH) 2]=−492.5 kJ mol −1, μ°[Fe(OH) 3]=−641 kJ mol −1 and A0=−243.9 kJ mol −1 at the average temperature of 9±1°C. The upper limit of occurrence of GR mineral at x=2/3, i.e. Fe 3(OH) 8, is explained by its unstability vs. α-FeOOH and/or magnetite; Fe(OH) 3 is thus a hypothetical compound with a GR structure which cannot be observed. These thermodynamic data and Eh-pH diagrams of Fe(OH) (2+x) can be used most importantly to predict the possibility that GR minerals reduce some anions in contaminated soils. The cases of NO 3−, Se(VI) or Cr(VI) are fully illustrated. 相似文献
2.
In order to verify Fe control by solution - mineral equilibria, soil solutions were sampled in hydromorphic soils on granites and shales, where the occurrence of Green Rusts had been demonstrated by Mössbauer and Raman spectroscopies. Eh and pH were measured in situ, and Fe(II) analyzed by colorimetry. Ionic Activity Products were computed from aqueous Fe(II) rather than total Fe in an attempt to avoid overestimation by including colloidal particles. Solid phases considered are Fe(II) and Fe(III) hydroxides and oxides, and the Green Rusts whose general formula is [Fe II1−xFe IIIx(OH) 2] +x· [x/z A −z] −x, where compensating interlayer anions, A −, can be Cl −, SO 42−, CO 32− or OH −, and where x ranges a priori from 0 to 1. In large ranges of variation of pH, pe and Fe(II) concentration, soil solutions are (i) oversaturated with respect to Fe(III) oxides; (ii) undersaturated with respect to Fe(II) oxides, chloride-, sulphate- and carbonate-Green Rusts; (iii) in equilibrium with hydroxy-Green Rusts, i.e., Fe(II)-Fe(III) mixed hydroxides. The ratios, x = Fe(III)/Fe t, derived from the best fits for equilibrium between minerals and soil solutions are 1/3, 1/2 and 2/3, depending on the sampling site, and are in every case identical to the same ratios directly measured by Mössbauer spectroscopy. This implies reversible equilibrium between Green Rust and solution. Solubility products are proposed for the various hydroxy-Green Rusts as follows: log Ksp = 28.2 ± 0.8 for the reaction Fe 3(OH) 7 + e − + 7 H + = 3 Fe 2+ + 7 H 2O; log Ksp = 25.4 ± 0.7 for the reaction Fe 2(OH) 5 + e − + 5 H + = 2 Fe 2+ + 5 H 2O; log Ksp = 45.8 ± 0.9 for the reaction Fe 3(OH) 8 + 2e − + 8 H + = 3 Fe 2+ + 8 H 2O at an average temperature of 9 ± 1°C, and 1 atm. pressure. Tentative values for the Gibbs free energies of formation of hydroxy-Green Rusts obtained are: Δ fG° (Fe 3(OH) 7, cr, 282.15 K) = −1799.7 ± 6 kJ mol −1, Δ fG° (Fe 2(OH) 5, cr, 282.15 K) = −1244.1 ± 6 kJ mol −1 and Δ fG° (Fe 3(OH) 8, cr, 282.15 K) = −1944.3 ± 6 kJ mol −1. 相似文献
3.
A steady state geochemical model has been developed to assist in understanding surface-catalysed oxidation of aqueous Fe(II) by O 2(aq), which occurs rapidly at circumneutral pH. The model has been applied to assess the possible abiotic removal of Fe(II)(aq) from alkaline ferruginous mine water discharges using engineered reactors with high specific-surface area filter media. The model includes solution and surface speciation equilibrium, oxidation kinetics of dissolved and adsorbed Fe(II) species and mass transfer of O 2(g). Limited field data for such treatment of a mine water discharge were available for model development and assessment of possible parameter values. Model results indicate that an adsorption capacity between 10 −6 and 10 −5 mol l −1 is sufficient for complete removal, by oxidation, of the Fe(II)(aq) load at the discharge. This capacity corresponds approximately to that afforded by surface precipitation of Fe(III) oxide onto plastic trickling filter media typically used for biological treatment of wastewater. Extrapolated literature values for microbial oxidation of Fe(II)(aq) by neutrophilic microbial populations to the simulated reactor conditions suggested that the microbially-mediated rate may be several orders-of-magnitude slower than the surface-catalysed oxidation. Application of the model across a range of mine water discharge qualities shows that high Fe(II)(aq) loadings can be removed if the discharge is sufficiently alkaline. Additional reactor simulations indicate that reactor efficiency decreases dramatically with pH in the near acid region, coinciding with the adsorption edge for Fe 2+ on Fe oxyhydroxide. Alkaline discharges thus buffer pH within the range where Fe(II)(aq) adsorbs onto the accreting Fe hydroxide mineral surface, and undergoes rapid catalytic oxidation. The results suggest that the proposed treatment technology may be appropriate for highly ferruginous alkaline discharges, typically associated with abandoned deep coal mines. 相似文献
4.
The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH) 2+ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe 3+. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe 2+/Fe 3+ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10 −4 and 10 −2 mol L −1. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): r py=0.55 c(HAsc) −0.35 r HAsc=3.6 c(HAsc) 0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7. 相似文献
5.
The Fe 3+/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe 2+ analyses and electron microprobe FeO total measurements. The data provide a new estimate for the MORB oxygen fugacity ( fO 2) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe 3+/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO 2 estimate is 0.8 log units more oxidized than the average fO 2 proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe 3+/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe 2+ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe 3+/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe 3+/ΣFe relationships with partial melting proxies such as Na 8.0.Based on the Fe 3+ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe 3+ during partial melting, and in the relatively constant Fe 3+/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe 3+/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe 2O 3 systematics can be accounted for by using a fO 2 of FMQ-1 that is equivalent to the average fO 2 reported for abyssal peridotites. 相似文献
6.
This work focuses on sulfide mineral oxidation rates under oxic conditions in freshly processed pyrite-rich tailings from the ore concentrator in Boliden, northern Sweden. Freshly processed tailings are chemically treated in the plant to kill bacteria and to obtain increased metal yields, resulting in a high pH level of 10–12 in the process water. Different oxidation experiments (abiotic oxidation in untreated tailings, acid abiotic oxidation and acid microbial oxidation), containing the Boliden tailings, were performed at room temperature with dissolved oxygen (0.21 atm O 2) for 3 months. The different pyrite oxidation rates given from the study were 2.4×10 −10 mol m −2 s −1 for the microbial, 5.9×10 −11 mol m −2 s −1 for the acidic abiotic and 3.6×10 −11 mol m −2 s −1 for the untreated experiments. Because of the potential precipitation of gypsum in the batch solutions, these oxidation rates are considered minimum values. The release rates for copper and zinc from chalcopyrite and sphalerite in the acid experiments were also investigated. These rates were normalized to the metal concentration in the tailings, and then compared to the release rate for iron from pyrite. These normalized results indicated that metal release decreased in the order Cu>Zn>Fe, demonstrating that pyrite is more resistant to oxidation than sphalerite and chalcopyrite. Pyrite was also more resistant to acidic dissolution than to microbial dissolution, while a significant fraction of sphalerite and chalcopyrite dissolved in the acid abiotic solutions. 相似文献
7.
Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe 4O 5–Mg 2Fe 2O 5 solid solutions and (2) to explore the phase relations involving (Mg,Fe) 2Fe 2O 5 (denoted as O 5-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O 5-phase coexists with Fe 2O 3, hp-(Mg,Fe)Fe 2O 4, (Mg,Fe) 3Fe 4O 9 or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O 5-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe 3+-bearing phase B. However, (Mg,Fe) 2Fe 2O 5 does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe 2+ and Fe 3+ (at least ~?1.0 cations Fe per formula unit). Fe-L 2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe 3+/Fe tot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si 4+?+?[6]Mg 2+?=?2Fe 3+. The ability to accommodate Fe 2+ and Fe 3+ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg 2Fe 2O 5 have been refined, yielding H° 1bar,298?=???1981.5 kJ mol ??1. Solid solution is complete across the Fe 4O 5–Mg 2Fe 2O 5 binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe 2+ with silicates indicates that (Mg,Fe) 2Fe 2O 5 has a strong preference for Fe 2+. Modelling of partitioning with olivine is consistent with the O 5-phase exhibiting ideal mixing behaviour. Mg–Fe 2+ partitioning between (Mg,Fe) 2Fe 2O 5 and ringwoodite or wadsleyite is influenced by the presence of Fe 3+ and OH incorporation in the silicate phases. 相似文献
8.
Aqueous oxidation of sulfide minerals to sulfate is an integral part of the global sulfur and oxygen cycles. The current model for pyrite oxidation emphasizes the role of Fe 2+-Fe 3+ electron shuttling and repeated nucleophilic attack by water molecules on sulfur. Previous δ 18O-labeled experiments show that a variable fraction (0-60%) of the oxygen in product sulfate is derived from dissolved O 2, the other potential oxidant. This indicates that nucleophilic attack cannot continue all the way to sulfate and that a sulfoxyanion of intermediate oxidation state is released into solution. The observed variability in O 2% may be due to the presence of competing oxidation pathways, variable experimental conditions (e.g. abiotic, biotic, or changing pH value), or uncertainties related to the multiple experiments needed to effectively use the δ 18O label to differentiate sulfate-oxygen sources. To examine the role of O 2 and Fe 3+ in determining the final incorporation of O 2 oxygen in sulfate produced during pyrite oxidation, we designed a set of aerated, abiotic, pH-buffered (pH = 2, 7, 9, 10, and 11), and triple-oxygen-isotope labeled solutions with and without Fe 3+ addition. While abiotic and pH-buffered conditions help to eliminate variables, triple oxygen isotope labeling and Fe 3+ addition help to determine the oxygen sources in sulfate and examine the role of Fe 2+-Fe 3+ electron shuttling during sulfide oxidation, respectively.Our results show that sulfate concentration increased linearly with time and the maximum concentration was achieved at pH 11. At pH 2, 7, and 9, sulfate production was slow but increased by 4× with the addition of Fe 3+. Significant amounts of sulfite and thiosulfate were detected in pH ? 9 reactors, while concentrations were low or undetectable at pH 2 and 7. The triple oxygen isotope data show that at pH ? 9, product sulfate contained 21-24% air O 2 signal, similar to pH 2 with Fe 3+ addition. Sulfate from the pH 2 reactor without Fe 3+ addition and the pH 7 reactors all showed 28-29% O 2 signal. While the O 2% in final sulfate apparently clusters around 25%, the measurable deviations (>experimental error) from the 25% in many reaction conditions suggest that (1) O 2 does get incorporated into intermediate sulfoxyanions (thiosulfate and sulfite) and a fraction survives sulfite-water exchange (e.g. the pH 2 with no Fe 3+ addition and both pH 7 reactors); and (2) direct O 2 oxidation dominates while Fe 3+ shuttling is still competitive in the sulfite-sulfate step (e.g. the pH 9, 10, and 11 and the pH 2 reactor with Fe 3+ addition). Overall, the final sulfate-oxygen source ratio is determined by (1) rate competitions between direct O 2 incorporation and Fe 3+ shuttling during both the formation of sulfite from pyrite and from sulfite to final sulfate, and (2) rate competitions between sulfite and water oxygen exchange and the oxidation of sulfite to sulfate. Our results indicate that thiosulfate or sulfite is the intermediate species released into solution at all investigated pH and point to a set of dynamic and competing fractionation factors and rates, which control the oxygen isotope composition of sulfate derived from pyrite oxidation. 相似文献
9.
Fe L-, S L-, and O K-edge X-ray absorption spectra of natural monoclinic and hexagonal pyrrhotites, Fe 1-xS, and arsenopyrite, FeAsS, have been measured and compared with the spectra of minerals oxidized in air and treated in aqueous acidic solutions, as well as with the previous XPS studies. The Fe L-edge X-ray absorption near-edge structure (XANES) of vacuum-cleaved pyrrhotites showed the presence of, aside from high-spin Fe 2+, small quantity of Fe 3+, which was higher for a monoclinic mineral. The spectra of the essentially metal-depleted surfaces produced by the non-oxidative and oxidative acidic leaching of pyrrhotites exhibit substantially enhanced contributions of Fe 3+ and a form of high-spin Fe 2+ with the energy of the 3d orbitals increased by 0.3–0.8 eV; low-spin Fe 2+ was not confidently distinguished, owing probably to its rapid oxidation. The changes in the S L-edge spectra reflect the emergence of Fe 3+ and reduced density of S s–Fe 4s antibonding states. The Fe L-edge XANES of arsenopyrite shows almost unsplit eg band of singlet Fe 2+ along with minor contributions attributable to high-spin Fe 2+ and Fe 3+. Iron retains the low-spin state in the sulphur-excessive layer formed by the oxidative leaching in 0.4 M ferric chloride and ferric sulphate acidic solutions. The S L-edge XANES of arsenopyrite leached in the ferric chloride, but not ferric sulphate, solution has considerably decreased pre-edge maxima, indicating the lesser admixture of S s states to Fe 3d orbitals in the reacted surface layer. The ferric nitrate treatment produces Fe 3+ species and sulphur in oxidation state between +2 and +4. 相似文献
10.
Despite encrustation by Fe and Al hydroxides, limestone can be effective for remediation of acidic mine drainage (AMD). Samples of water and limestone (CaCO 3) were collected periodically for 1 a at 3 identical limestone-filled drains in Pennsylvania to evaluate the attenuation of dissolved metals and the effects of pH and Fe- and Al-hydrolysis products on the rate of CaCO 3 dissolution. The influent was acidic and relatively dilute (pH<4; acidity <90 mg) but contained 1–4 mg·L −1 of O 2, Fe 3+, Al 3+ and Mn 2+. The total retention time in the oxic limestone drains (OLDs) ranged from 1.0 to 3.1 hr. Effluent remained oxic (O 2>1 mg·L −1) but was near neutral (pH=6.2–7.0); Fe and Al decreased to less than 5% of influent concentrations. As pH increased near the inflow, hydrous Fe and Al oxides precipitated in the OLDs. The hydrous oxides, nominally Fe(OH) 3 and Al(OH) 3, were visible as loosely bound, orange-yellow coatings on limestone near the inflow. As time elapsed, Fe(OH) 3 and Al(OH) 3 particles were transported downflow. The accumulation of hydrous oxides and elevated pH (>5) in the downflow part of the OLDs promoted sorption and coprecipitation of dissolved Mn, Cu, Co, Ni and Zn as indicated by decreased UK concentrations of the metals in effluent and their enrichment relative to Fe in hydrous-oxide particles and coatings on limestone. Despite thick (∼1 mm) hydrous-oxide coatings on limestone near the inflow, CaCO 3 dissolution was more rapid near the inflow than at downflow points within and the OLD where the limestone was not coated. The high rates of CaCO 3 dissolution and Fe(OH 3) precipitation were associated with the relatively low pH and high Fe 3+ concentration near the inflow. The rate of CaCO 3 dissolution decreased with increased pH and concentrations of Ca 2+ and HCO 3− and decreased Pco 2. Because overall efficiency is increased by combining neutralization and hydrolysis reactions, an OLD followed by a settling pond requires less land area than needed for a two-stage treatment system consisting of an anoxic limestone drain an oxidation-settling pond or wetland. To facilitate removal of hydrous-oxide sludge, a perforated-pipe subdrain can be installed within an OLD. 相似文献
11.
Arsenopyrite is an important component of many ore deposits and dissolves in the O 2-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 Fe 2(SO 4) 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 Fe 2(SO 4) 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 Fe 2(SO 4) 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. 相似文献
12.
Synthetic melilites on the join Ca 2MgSi 2O 7 (åkermanite) — Ca 2FeSi 2O 7 (iron åkermanite) with Fe/(Fe+Mg) from 0.0 to 0.7 exhibit, at room temperature, an incommensurate phase with a rectangular modulation of a wavelength of about 19 Å in the [110] direction. Upon increase of temperature, they transform to a commensurate melilite structure at about 80° C for Fe/(Fe+Mg)=0.0 and about 250° C for Fe/(Fe+Mg)=0.6. In addition to the T(2) positions of the melilite structure filled by Si, the incommensurate phase exhibits two distinguishable T(1) sites containing the Mg and Fe 2+. These two sites merge into one site during the phase transition from the incommensurate to the commensurate phase. A structural model for the incommensurate phase is based on the misfit between the tetrahedral (Mg, Fe 2+)Si 2O 7 4? sheets and the Ca 2+ ions. 相似文献
13.
The influence of oxygen fugacity ( fO 2) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T- fO 2 conditions (1100–1420°C and 10 ?12–10 ?0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe 3+/Fe 2+) = k log( fO 2) + q, where k and q are constants depending on melt composition and temperature. The Fe 3+/Fe 2+ ratio decreases with decreasing fO 2 ( T = const) and increasing temperature ( fO 2 = const). The structural state of Fe 3+ depends on the degree of iron oxidation. With increasing Fe 3+/Fe 2+ ≥ 1, the dominant coordination of Fe 3+ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe 3+/Fe 2+. 相似文献
14.
Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053 wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1 year. Sulfate concentrations reached 2000 mg L −1 when ambient temperatures were >10 °C, and decreased as ambient temperatures declined to <0 °C. The pH decreased to <5, concomitant with an alkalinity minimum of <1 mg L −1 (as total CaCO 3), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were <0.36 and <0.11 mg L −1, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13 mg L −1), Ni (<7.0 mg L −1), Co (<1.5 mg L −1), Zn (<0.5 mg L −1), Cd (<0.008 mg L −1) and Cu (<0.05 mg L −1) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite [Fe 1−xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite [CuFeS 2] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO 4, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock. 相似文献
15.
Biotite dissolution experiments were carried out to better understand the dissolution kinetics and Fe behavior under low O 2 conditions, and to give an insight into the Precambrian weathering. Mineral dissolution with a continuous flow-through reactor was employed at 25 °C for up to 65 days varying partial pressure of atmospheric oxygen ( PO 2), pH (6.86 and 3.01) and Fe content in mineral (1.06 and 0.11 mol of Fe per O 10(OH,F) 2 for biotite and phlogopite, respectively) independently for the examination of their effects on biotite dissolution. Low PO 2 conditions were achieved in a newly developed glove box ( PO 2 ? 6 × 10 −4 atm; referred to as anoxic conditions), which was compared to the present, ambient air conditions (0.2 atm of PO 2; oxic conditions). The biotite dissolution rate was slightly faster under anoxic conditions at pH 6.86 while it was not affected by PO 2 at pH 3.01. There was no direct effect of Fe content on dissolution rate at pH 6.86 while there was a small difference in dissolution rate between biotite and phlogopite at pH 3.01. The 1.5 order-of-magnitude faster release rate of Fe under anoxic conditions for biotite dissolution at pH 6.86 resulted from the difference in ratio of Fe 3+ precipitates remaining in the reactor to Fe dissolved (about 60% and 100% under anoxic and oxic conditions, respectively), which is caused mainly by the difference in PO 2. The results infer that the Fe 2+ and Fe 3+ contents in the Paleoproterozoic paleosols, fossil weathering profiles, are reflected by atmospheric oxygen levels at the time of weathering. 相似文献
16.
Iron monosulfide formation and oxidation processes were studied in the extensively drained acid sulfate soil environment of the Tweed River floodplain in eastern Australia. Porewater profiles of pH, Eh, SO 42−, Fe 2+, Fe 3+, Cl −, HCO 3−, and metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) were obtained using in situ dialysis membrane samplers (`peepers'). Concentrations of acid volatile S (AVS), pyrite, total S, reactive Fe, total and organic C, simultaneously extracted metals (SEMs) and total elemental composition by X-ray fluorescence, were determined on sediment samples. The oxidation of pyrite in the surrounding landscape provides a source of acidity, Fe, Al, SO 4 and metals, which are exported into the drainage system where they accumulate in the sediments and porewaters. Negative porewater concentration gradients of SO 42− and Fe 2+, and large AVS concentrations in the sediments, indicate Fe monosulfides form rapidly under reducing conditions and consume acidity and metals. Oxidation of the sediments during previous drought episodes has resulted in the conversion of monosulfides and pyrite to oxidised Fe minerals and the release of acidity, SO 42−, Fe 3+, and metals to the surface waters. These formation and oxidation cycles show that Fe monosulfides play an important role in controlling water quality in the drainage system. 相似文献
17.
Synthetic spinel harzburgite and lherzolite assemblages were equilibrated between 1040 and 1300° C and 0.3 to 2.7 GPa, under
controlled oxygen fugacity ( f
O
2). f
O
2 was buffered with conventional and open double-capsule techniques, using the Fe−FeO, WC-WO 2-C, Ni−NiO, and Fe 3O 4−Fe 2O 3 buffers, and graphite, olivine, and PdAg alloys as sample containers. Experiments were carried out in a piston-cylinder apparatus
under fluid-excess conditions. Within the P-T-X range of the experiments, the redox ratio Fe 3+/ΣFe in spinel is a linear function of f
O
2 (0.02 at IW, 0.1 at WCO, 0.25 at NNO, and 0.75 at MH). It is independent of temperature at given Δlog( f
O
2), but decreases slightly with increasing Cr content in spinel. The Fe 3+/ΣFe ratio falls with increasing pressure at given Δlog( f
O
2), consistent with a pressure correction based on partial molar volume data. At a specific temperature, degree of melting
and bulk composition, the Cr/(Cr+Al) ratio of a spinel rises with increasing f
O
2. A linear least-squares fit to the experimental data gives the semi-empirical oxygen barometer in terms of divergence from
the fayalite-magnetite-quartz (FMQ) buffer:
相似文献
18.
Polarized absorption spectra of natural piemontite (Ca1.802Mn
2+0.178
Mg0.025) (Mn
3+0.829
Fe
3+0.346
Al1.825) [(Si2.992Al0.008) O12OH], viridine (Al1.945Mn
3+0.033
Fe
3+0.063
Mg0.003) [O|Si0.970 O4], and kanonaite (Al1.291Mn
3+0.682
Fe
3+0.019
) [O|Si1.006 O4] were measured at 295 and ca. 100 K. For piemontite, lowering the temperature resulted in a sharpening of broad bands in the 10 000–25 000 cm−1 region supporting their assignment to single ion Mn3+ in M3 non-centrosymmetric sites. Alternatively, in kanonaite, temperature behaviour pointed to a slightly stronger influence of vibronic coupling on strong bands near 16 000 and 22 000 cm−1, which supported an interpretation of Mn3+ in nearly centrosymmetric M1 sites. Measurements at ca. 100 K show pronounced fine structure in the viridine spectra which is attributed to Fe3+. The ɛ values for Mn3+ spin-allowed bands in the three minerals lie in the range 18 to 227 [1·g-atom−1·cm−1]. For the same band and polarisation, ɛ values in Mn3+-bearing andalusite-type minerals viridine and kanonaite are the same, which indicates an absence of strong magnetic coupling effects between Mn3+ ions in the andalusite type structure down to ca. 100 K. In silicates, the high ɛ values for Mn3+ spin-allowed bands, in comparison to those obtained for Fe2+ spin-allowed bands from sites of “similar distortion”, is attributed to a higher degree of covalency in the Mn3+-O bonds compared to the Fe2+-O bonds, as a result of the higher valence state of manganese. 相似文献
19.
During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel?+?nickel oxide (NNO) or fayalite?+?magnetite?+?quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce 4+/Ce 3+ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce 4+/Ce 3+ in zircon. Temperature-dependent lattice strain causes Ce 4+/Ce 3+ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce 4+/Ce 3+ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce 4+/Ce 3+ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce 4+/Ce 3+ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce 4+/Ce 3+ are corroborated by Fe–Ti-oxide-based T–?O 2 trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H 2O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H 2 and/or SO 3 and/or Fe 2+ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H 2O-poor magmas tend to follow a T–?O 2 trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe 3+/Fe 2+ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ?O 2 in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ±?0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ?O 2?≈?NNO?+?4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO 2 activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry. 相似文献
20.
Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes
were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn 2+, and Fe 2+ within the sediments. The O 2 concentrations decreased sharply from about 200 μmol L −1 in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn 2+ was detected below the oxic zones with peak concentrations up to 600 μmol L −1, whereas dissolved Fe 2+ had peak concentrations up to 1,000 μmol L −1 in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N
ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater
discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to
shallow O 2 penetration depths, high dissolved Mn 2+ and Fe 2+ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had
promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with
increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O 2 profiles within the sediments. Based on the O 2 profiles, O 2 fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m −2 day −1, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm −3 a −1. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as
sewage discharge. 相似文献
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