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1.
The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K1.38Na0.05)(Al2.87Mg0.46Fe3+0.39Fe2+0.28Ti0.07)[Si7.02Al0.98]O20(OH)4 was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H2SO4) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K+ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na+ from the solution and K+ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower RAl (dissolution rate calculated from steady state Al release) than RSi (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.  相似文献   

2.
By using accelerator mass spectrometry, we measured 10Be (T1/2 = 1.5 Ma) concentrations in nine Ivory Coast (IVC) tektites, in six soil samples collected near the Bosumtwi impact crater, the likely source region, and in a depth profile taken through a 23 g moldavite. In the core of the moldavite sample we also measured an upper limit on the 36Cl (T1/2 = 0.3 My) concentration. The average 10Be concentration in IVC tektites of (22 ± 11) × 106 atom/g exceeds reasonable limits for a meteoritic component or cosmic-ray production in situ after tektite formation. The 10Be must be meteoric, which implies that IVC tektites formed from soils or sediments. Corrected to the time of formation (ToF) 1.07 Ma ago and for a small in situ component, the average 10Be concentration of (35 ± 7) × 106 atom/g (1 − σ mean) is considerably lower than those of contemporary Bosumtwi soils, ∼250 × 106 atom/g, or of Australasian tektites at their ToF, 0.8 Ma B.P. near Lake Bosumtwi today the soil column is only ∼1 m thick. If the landscape was similar 1.07 Ma ago, then the total thickness of the tektite formation zone probably did not exceed 10 m. With increasing depth below the surface of the moldavite, the 10Be concentrations decrease rapidly owing to the presence of a surface component, probably of recent origin. The main interior mass of the sample contains ∼0.8 × 106 atom 10Be/g and fewer than 0.1 × 106 atom 36Cl/g, little of which can be meteoritic. Although not definitive, consideration of several possible cosmic-ray exposure histories suggests that about half the interior 10Be has a meteoric origin, which if corrected to the time of formation yields a concentration compatible with those measured in typical contemporary soils. The observations are consistent with the formation of three of the four main tektite groups from surface soils or sediments.  相似文献   

3.
Information on the particle size and reactive surface area of natural samples is essential for the application of surface complexation models (SCM) to predict bioavailability, toxicity, and transport of elements in the natural environment. In addition, this information will be of great help to enlighten views on the formation, stability, and structure of nanoparticle associations of natural organic matter (NOM) and natural oxide particles.Phosphate is proposed as a natively present probe ion to derive the effective reactive surface area of natural samples. In the suggested method, natural samples are equilibrated (?10 days) with 0.5 M NaHCO3 (pH = 8.5) at various solid-solution ratios. This matrix fixes the pH and ionic strength, suppresses the influence of Ca2+ and Mg2+ ions by precipitation these in solid carbonates, and removes NOM due to the addition of activated carbon in excess, collectively leading to the dominance of the PO4-CO3 interaction in the system. The data have been interpreted with the charge distribution (CD) model, calibrated for goethite, and the analysis results in an effective reactive surface area (SA) and a reversibly bound phosphate loading Γ for a series of top soils.The oxidic SA varies between about 3-30 m2/g sample for a large series of representative agricultural top soils. Scaling of our data to the total iron and aluminum oxide content (dithionite-citrate-bicarbonate extractable), results in the specific surface area between about 200-1200 m2/g oxide for most soils, i.e. the oxide particles are nano-sized with an equivalent diameter in the order of ∼1-10 nm if considered as non-porous spheres. For the top soils, the effective surface area and the soil organic carbon fraction are strongly correlated. The oxide particles are embedded in a matrix of organic carbon (OC), equivalent to ∼1.4 ± 0.2 mg OC/m2 oxide for many soils of the collection, forming a NOM-mineral nanoparticle association with an average NOM volume fraction of ∼80%. The average mass density of such a NOM-mineral association is ∼1700 ± 100 kg/m3 (i.e. high-density NOM). The amount of reversibly bound phosphate is rather close to the amount of phosphate that is extractable with oxalate. The phosphate loading varies remarkably (Γ ≈ 1-3 μmol/m2 oxide) in the samples. As discussed in part II of this paper series (Hiemstra et al., 2010), the phosphate loading (Γ) of field samples is suppressed by surface complexation of NOM, where hydrophilic, fulvic, and humic acids act as a competitor for (an)ions via site competition and electrostatic interaction.  相似文献   

4.
The speciation of aqueous dissolved sulfur was determined in hydrothermal waters in Iceland. The waters sampled included hot springs, acid-sulfate pools and mud pots, sub-boiling well discharges and two-phase wells. The water temperatures ranged from 4 to 210 °C, the pHT was between 2.20 and 9.30 at the discharge temperature and the SO4 and Cl concentrations were 0.020-52.7 and <0.01-10.0 mmol kg−1, respectively. The analyses were carried out on-site within ∼10 min of sampling using ion chromatography (IC) for sulfate (SO42−), thiosulfate (S2O32−) and polythionates (SxO62−) and titration and/or colorimetry for total dissolved sulfide (S2−). Sulfite (SO32−) could also be determined in a few cases using IC. Alternatively, for few samples in remote locations the sulfur oxyanions were stabilized on a resin on site following elution and analysis by IC in the laboratory. Dissolved sulfate and with few exceptions also S2− were detected in all samples with concentrations of 0.02-52.7 mmol kg−1 and <1-4100 μmol kg−1, respectively. Thiosulfate was detected in 49 samples of the 73 analyzed with concentrations in the range of <1-394 μmol kg−1 (S-equivalents). Sulfite was detected in few samples with concentrations in the range of <1-3 μmol kg−1. Thiosulfate and SO32− were not detected in <100 °C well waters and S2O32− was observed only at low concentrations (<1-8 μmol kg−1) in ∼200 °C well waters. In alkaline and neutral pH hot springs, S2O32− was present in significant concentrations sometimes corresponding to up to 23% of total dissolved sulfur (STOT). In steam-heated acid-sulfate waters, S2O32− was not a significant sulfur species. The results demonstrate that S2O32− and SO32− do not occur in the deeper parts of <150 °C hydrothermal systems and only in trace concentrations in ∼200-300 °C systems. Upon ascent to the surface and mixing with oxygenated ground and surface waters and/or dissolution of atmospheric O2, S2− is degassed and oxidized to SO32− and S2O32− and eventually to SO42− at pH >8. In near-neutral hydrothermal waters the oxidation of S2− and the interaction of S2− and S0 resulting in the formation of Sx2− are considered important. At lower pH values the reactions seemed to proceed relatively rapidly to SO42− and the sulfur chemistry of acid-sulfate pools was dominated by SO42−, which corresponded to >99% of STOT. The results suggest that the aqueous speciation of sulfur in natural hydrothermal waters is dynamic and both kinetically and source-controlled and cannot be estimated from thermodynamic speciation calculations.  相似文献   

5.
Micro-Raman spectroscopy, even though a very promising technique, is not still routinely applied to analyse H2O in silicate glasses. The accuracy of Raman water determinations critically depends on the capability to predict and take into account both the matrix effects (bulk glass composition) and the analytical conditions on band intensities. On the other hand, micro-Fourier transform infrared spectroscopy is commonly used to measure the hydrous absorbing species (e.g., hydroxyl OH− and molecular H2O) in natural glasses, but requires critical assumptions for the study of crystal-hosted glasses. Here, we quantify for the first time the matrix effect of Raman external calibration procedures for the quantification of the total H2O content (H2OT = OH− + H2Om) in natural silicate glasses. The procedures are based on the calibration of either the absolute (external calibration) or scaled (parameterisation) intensity of the 3550 cm−1 band. A total of 67 mafic (basanite, basalt) and intermediate (andesite) glasses hosted in olivines, having between 0.2 and 4.8 wt% of H2O, was analysed. Our new dataset demonstrates, for given water content, the height (intensity) of Raman H2OT band depends on glass density, reflectance and water environment. Hence this matrix effect must be considered in the quantification of H2O by Raman spectroscopy irrespective of the procedure, whereas the parameterisation mainly helps to predict and verify the self-consistency of the Raman results. In addition, to validate the capability of the micro-Raman to accurately determine the H2O content of multicomponent aluminosilicate glasses, a subset of 23 glasses was analysed by both micro-Raman and micro-FTIR spectroscopy using the band at 3550 cm−1. We provide new FTIR absorptivity coefficients (ε3550) for basalt (62.80 ± 0.8 L mol−1 cm−1) and basanite (43.96 ± 0.6 L mol−1 cm−1). These values, together with an exhaustive review of literature data, confirm the non-linear decline of the FTIR absorptivity coefficient (ε3550) as the glass depolymerisation increases. We demonstrate the good agreement between micro-FTIR and micro-Raman determination of H2O in silicate glasses when the matrix effects are properly considered.  相似文献   

6.
Schwertmannite (Fe8O8(OH)6SO4) is a common Fe(III)-oxyhydroxysulfate mineral in acid-sulfate systems, where its formation and fate strongly influence water quality. The present study examines transformation of schwertmannite to goethite (FeOOH), as catalyzed by interactions with Fe(II) in anoxic aquatic environments. This study also evaluates the role of the Fe(II) pathway in influencing the formation of iron-sulfide minerals in such environments. At pH > 5, the rates of Fe(II)-catalyzed schwertmannite transformation were several orders of magnitude faster than transformation in the absence of Fe(II). Complete transformation of schwertmannite occurred within only 3-5 h at pH > 6 and Fe(II)(aq) ? 5 mmol L−1. Model calculations indicate that the Fe(II)-catalyzed transformation of schwertmannite to goethite greatly decreases the reactivity of the Fe(III) pool, thereby favoring SO4-reduction and facilitating the formation of iron-sulfide minerals (particularly mackinawite, tetragonal FeS). Examination of in situ sediment geochemistry in an acid-sulfate system revealed that the rapid Fe(II)-catalyzed transformation was consistent with an abrupt shift from an acidic Fe(III)-reducing regime with abundant schwertmannite near the sediment surface, to a near-neutral mackinawite-forming regime where goethite was dominant. This study demonstrates that the Fe(II) pathway exerts a major influence on schwertmannite transformation and iron-sulfide formation in anoxic acid-sulfate systems. These findings have important implications for understanding acidity dynamics and trace element mobility in such systems.  相似文献   

7.
Raman spectroscopy is a powerful method for the determination of CO2 densities in fluid inclusions, especially for those with small size and/or low fluid density. The relationship between CO2 Fermi diad split (Δ, cm−1) and CO2 density (ρ, g/cm3) has been documented by several previous studies. However, significant discrepancies exist among these studies mainly because of inconsistent calibration procedures and lack of measurements for CO2 fluids having densities between 0.21 and 0.75 g/cm3, where liquid and vapor phases coexist near room temperature.In this study, a high-pressure optical cell and fused silica capillary capsules were used to prepare pure CO2 samples with densities between 0.0472 and 1.0060 g/cm3. The measured CO2 Fermi diad splits were calibrated with two well established Raman bands of benzonitrile at 1192.6 and 1598.9 cm−1. The relationship between the CO2 Fermi diad split and density can be represented by: ρ = 47513.64243 − 1374.824414 × Δ + 13.25586152 × Δ2 − 0.04258891551 × Δ3 (r2 = 0.99835, σ = 0.0253 g/cm3), and this relationship was tested by synthetic fluid inclusions and natural CO2-rich fluid inclusions. The effects of temperature and the presence of H2O and CH4 on this relationship were also examined.  相似文献   

8.
Group B metals, such as Hg, Cu, Ag, Pb and Cd bind strongly to reduced inorganic and organic S(II−) ligands. These S(II−) ligands, stable in oxic waters for significant periods of time, occur at the <1–100 s nM concentrations. It is hypothesized that S(II−) ligands are stabilized as Cu–S molecules associated with organic matter by multi-ligand binding or in nano-pore encapsulations in organic matter. S(II−) ligands are estimated by two methods: purge/trap analysis as Cr-reducible sulfide (CRS), and strong ligand (SLT) from a competitive ligand titration with Ag(I). The CRS/SLT ratio is nearly one for selected samples. CRS correlates reasonably well (r2 ∼ 0.5) with organic C with a slope of 14.6 nM per mg C. The conditional binding constant of Ag–SL is 11.3 for effluent associated with waste-water and decreases for river waters from about 12–8.8 as the strong sites are occupied with Ag(I).  相似文献   

9.
The experiments were conducted in the open CO2 system to find out the equilibrium fractionation between the carbonate ion and CO2(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO2−N2 gases with different δ13C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ13C = − 4.2‰. The ΔCO3T2−−CO2(g) equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of ΔCO3T2−−CO2(g) and ΔHCO3T−−CO2(g) fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.  相似文献   

10.
The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H4SiO40), thereby influencing Si concentration in soil solution. Here we study the impact of H4SiO40 adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0-72 h) and initial concentration (ic) of Si in solution (0.61-1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σSEM). The quantitative adsorption of H4SiO40 by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ30Si = +0.02 ± 0.07‰ (±2σSD)), the solutions were systematically enriched in 30Si reaching maximum δ30Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H4SiO40 adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.  相似文献   

11.
Temporal variations in the concentration and N isotopic ratios of inorganic N (NH4– and NO3–N) as affected by the soil temperature regime together with the input of bird excreta were analyzed in a sedentary soil under a dense colony (1.6 nests/m2) of breeding Black-tailed Gulls (Laruscrassirostris: a ground-nesting seabird). Surface soil samples were taken monthly from mid-March to late July 2005 from Kabushima Island, Hachinohe, northeastern Japan. The spatial concentration of inorganic N in the soils varied considerably on all sampling dates. There may be a statistically significant trend, showing increased NH4–N content from settlement up to early June when the input of fecal N attains its maximum, and then decreases towards the end of breeding activity (early August). Abundant NO3–N was observed in all soils, particularly in the later stage of breeding (up to 3800 mg-N/kg dry soil), refuting earlier claims that nitrification is unimportant in the soils. δ15N values of NH4 in the soils showed unusually high values up to +51‰, reflecting N isotope fractionation due to volatilization of NH3 during the mineralization. Mean δ15N values of the monthly collected totals of NH4 and NO3 were not significantly different at the 5% level based on ANOVA and significant differences were observed only among the three means of NO3–N collected in mid-March (settlement of colony: δ15N = −0.2 ± 3.5‰) and late July (later stages of breeding: δ15N = +22.1 ± 7.0‰, +23.3 ± 7.8‰) at the 1% and 5% levels by t-test, respectively. Such an observation of significantly increased δ15N values for NO3–N in soils from the fledgling stage indicates the integration of denitrification coupled with nitrification under a limited supply of fecal N.  相似文献   

12.
Stable carbon (δ13C) and hydrogen (δD) isotopic compositions of n-alkanes, anteiso-alkanes, n-alkanoic acids, n-alkanols, phytol and sterols in raw leaves of Acer argutum and Acer carpinifolium, their fallen leaves, mold and soils from a natural Acer forest were measured in order to: (1) understand isotopic variation of the plant biomarkers in a plant-soil system and (2) evaluate which biomarker is the most effective recorder of soil vegetation. Long-chain (> C24) n-alkanes, n-alkanoic acids and n-alkanols are gradually enriched in 13C up to 12.9‰ (average of 4.3‰) and depleted in D up to 94‰ (average of 55‰) from raw leaves to soils. However, anteiso-alkanes, phytol and sterols show little variation in both δ13C (< ± 1‰) and δD (< ± 2‰) from raw leaves to soils. These isotope signatures in a plant-soil system indicate that isoprenoid plant biomarkers such as sterols in soils faithfully preserve the isotopic compositions of dominant higher plants growing on the soils without a diagenetic effect upon the isotopic compositions. In contrast, long-chain n-alkyl molecules in soils undergo specific isotopic modification during biodegradation associated with early diagenesis and/or a significant contribution from heterotrophic reworking.  相似文献   

13.
The hexa-aqua complexes [Fe(H2O)6−m−n(OH)n](2−n)+n = 0 → 3, m = 0 → 6 − n; [Fe(H2O)6−m−n(OH)n](3−n)+n = 0 → 4, m = 0 → 6 − n were investigated by ab-initio methods with the aim of determining their ground-state geometries, total energies and vibrational properties by treating their inner solvation shell as part of their gaseous precursor1 (or “hybrid approach”). After a gas-phase energy optimization within the Density Functional Theory (DFT), the molecules were surrounded by a dielectric representing the Reaction Field through an implicit Polarized Continuum Model (PCM). The exploration of several structural ligand arrangements allowed us to quantify the relative stabilities of the various ionic species and the role of the various forms of energy (solute-solvent electronic interaction, cavitation, dispersion, repulsion, liberation free energy) that contribute to stabilize the aqueous complexes. A comparison with experimental thermochemistries showed that ab-initio gas-phase + solvation energies are quite consistent with experimental evidence and allow the depiction of the most stable form in solution and the eventual configurational disorder of water/hydroxyl species around central cations. A vibrational analysis performed on the 54Fe, 56Fe, 57Fe and 58Fe isotopomers indicated important separative effects systematically affected by the extent of deprotonation. The role of the system’s redox state (fO2) and acidity (pH) on the isotopic imprinting of the aqueous species in solution was investigated by coupling the separative effects with speciation calculations. The observed systematics provided a tool of general utility in the interpretation of the iron isotopic signature of natural waters. Applications to the interpretation of isotopic fractionation in solution dictated by redox equilibria and to the significance of the Fe-isotopic imprinting of Banded Iron Formations are given.  相似文献   

14.
Titratable actual acidity (TAA) is a technique commonly used to estimate the existing pool of exchangeable H+ in acid sulfate soils (ASS). A widely adopted version of the TAA method involves titrating a 1M KCl suspension of oven-dry soil (1:40) with NaOH to a known pH endpoint. However, when ASS are subject to long term re-flooding during wetland remediation, former sulfuric horizons can develop substantial quantities of porewater Fe2+, non-sulfidic solid-phase Fe(II) and a variety of reduced inorganic sulfur (RIS) species (e.g. pyrite, mackinawite, greigite and elemental sulfur). For these sediments, an oven-drying approach may induce oxidation of the abundant Fe(II) and/or reactive RIS species, thereby generating H+ and leading to overestimation of existing in situ exchangeable H+. In this study, we compare TAA via the standard approach (1M KCl; 1:40; oven-dry soil, 4 hr extract; TAAD) with an identical O2-free extraction approach using wet-sediment (TAAW). We apply both methods to former sulfuric horizon sediments from freshwater re-flooded ASS wetlands. There are significant (α = 0.01) differences (up to 12×) between TAA measured by the two methods, with the oven-dried standard approach overestimating TAA relative to the wet, O2-free approach in 85% of cases. Despite the fact that all AVS-S and some S(0) was oxidised during the oven-drying process, the increases in TAA (TAAD–TAAW) show very weak correlation(s) with corresponding losses in RIS species or increases in water soluble sulfate and KCl extractable sulfate. However, oven-drying caused substantial loss of 1M KCl exchangeable Fe(II) and 1 M HCl-extractable Fe(II) and led to large increases in 1 M HCl-extractable Fe(III). These changes in Fe fractions displayed strong positive linear correlation (α = 0.01) with increases in TAA. Although this is not evidence of causality, it suggests that oxidation of Fe(II) species are playing an important role in the development of additional exchangeable H+ and may be largely responsible for the contrasting TAA derived by the two methods. The differences in TAA between the two methods are greatest in organic-rich surface sediments and are significantly positively correlated with total organic carbon content. These findings have major implications for accurately assessing TAA in re-flooded ASS wetlands.  相似文献   

15.
Iron (III) oxides are ubiquitous in near-surface soils and sediments and interact strongly with dissolved phosphates via sorption, co-precipitation, mineral transformation and redox-cycling reactions. Iron oxide phases are thus, an important reservoir for dissolved phosphate, and phosphate bound to iron oxides may reflect dissolved phosphate sources as well as carry a history of the biogeochemical cycling of phosphorus (P). It has recently been demonstrated that dissolved inorganic phosphate (DIP) in rivers, lakes, estuaries and the open ocean can be used to distinguish different P sources and biological reaction pathways in the ratio of 18O/16O (δ18OP) in PO43−. Here we present results of experimental studies aimed at determining whether non-biological interactions between dissolved inorganic phosphate and solid iron oxides involve fractionation of oxygen isotopes in PO4. Determination of such fractionations is critical to any interpretation of δ18OP values of modern (e.g., hydrothermal iron oxide deposits, marine sediments, soils, groundwater systems) to ancient and extraterrestrial samples (e.g., BIF’s, Martian soils). Batch sorption experiments were performed using varied concentrations of synthetic ferrihydrite and isotopically-labeled dissolved ortho-phosphate at temperatures ranging from 4 to 95 °C. Mineral transformations and morphological changes were determined by X-Ray, Mössbauer spectroscopy and SEM image analyses.Our results show that isotopic fractionation between sorbed and aqueous phosphate occurs during the early phase of sorption with isotopically-light phosphate (P16O4) preferentially incorporated into sorbed/solid phases. This fractionation showed negligible temperature-dependence and gradually decreased as a result of O-isotope exchange between sorbed and aqueous-phase phosphate, to become insignificant at greater than ∼100 h of reaction. In high-temperature experiments, this exchange was very rapid resulting in negligible fractionation between sorbed and aqueous-phase phosphate at much shorter reaction times. Mineral transformation resulted in initial preferential desorption/loss of light phosphate (P16O4) to solution. However, the continual exchange between sorbed and aqueous PO4, concomitant with this mineralogical transformation resulted again in negligible fractionation between aqueous and sorbed PO4 at long reaction times (>2000 h). This finding is consistent with results obtained from natural marine samples. Therefore, 18O values of dissolved phosphate (DIP) in sea water may be preserved during its sorption to iron-oxide minerals such as hydrothermal plume particles, making marine iron oxides a potential new proxy for dissolved phosphate in the oceans.  相似文献   

16.
Partitioning of Ni in calcite, CaCO3, was evaluated with the aim of collecting data on partition and distribution coefficients and to enhance understanding about the interaction of Ni with the calcite surface and further incorporation into the bulk. This information will aid in the interpretation of geological processes for safety assessment of waste repositories and contamination of groundwater. Coprecipitation experiments were carried out by the constant addition method at 25 °C and pCO2 = 1 and 10−3.5 atm. Ni was moderately partitioned from solution into calcite. For dilute solid solutions (XNi < 0.001), Ni partition coefficients were estimated to be ∼1 and found to be weakly dependent on calcite precipitation rate in the range of 3-230 nmol m−2 s−1. Ni molar fraction in the solid is directly proportional to Ni concentration in the solution. The fit of the data to such a model is good evidence that Ni is taken up as a true solid solution, not simply by physical trapping.  相似文献   

17.
The range in 56Fe/54Fe isotopic compositions measured in naturally occurring iron-bearing species is greater than 5‰. Both theoretical modeling and experimental studies of equilibrium isotopic fractionation among iron-bearing species have shown that significant fractionations can be caused by differences in oxidation state (i.e., redox effects in the environment) as well as by bond partner and coordination number (i.e., nonredox effects due to speciation).To test the relative effects of redox vs. nonredox attributes on total Fe equilibrium isotopic fractionation, we measured changes, both experimentally and theoretically, in the isotopic composition of an Fe2+-Fe3+-Cl-H2O solution as the chlorinity was varied. We made use of the unique solubility of FeCl4− in immiscible diethyl ether to create a separate spectator phase against which changes in the aqueous phase could be quantified. Our experiments showed a reduction in the redox isotopic fractionation between Fe2+- and Fe3+-bearing species from 3.4‰ at [Cl−] = 1.5 M to 2.4‰ at [Cl−] = 5.0 M, due to changes in speciation in the Fe-Cl solution. This experimental design was also used to demonstrate the attainment of isotopic equilibrium between the two phases, using a 54Fe spike.To better understand speciation effects on redox fractionation, we created four new sets of ab initio models of the ferrous chloride complexes used in the experiments. These were combined with corresponding ab initio models for the ferric chloride complexes from previous work. At 20 °C, 1000 ln β (β = 56Fe/54Fe reduced partition function ratio relative to a dissociated Fe atom) values range from 6.39‰ to 5.42‰ for Fe(H2O)62+, 5.98‰ to 5.34‰ for FeCl(H2O)5+, and 5.91‰ to 4.86‰ for FeCl2(H2O)4, depending on the model. The theoretical models predict ferric-ferrous fractionation about half as large (depending on model) as the experimental results.Our results show (1) oxidation state is likely to be the dominant factor controlling equilibrium Fe isotope fractionation in solution and (2) nonredox attributes (such as ligands present in the aqueous solution, speciation and relative abundances, and ionic strength of the solution) can also have significant effects. Changes in the isotopic composition of an Fe-bearing solution will influence the resultant Fe isotopic signature of any precipitates.  相似文献   

18.
Solid phases of silicon dioxide react with water vapor with the formation of hydroxides and oxyhydroxides of silica. Recent transpiration and mass-spectrometric studies convincingly demonstrate that H4SiO4 is the predominant form of silica in vapor phase at water pressure in excess of 10−2 MPa. Available literature transpiration and solubility data for the reactions of solid SiO2 phases and low-density water, extending from 424 to 1661 K, are employed for the determination of ΔfG0, ΔfH0 and S0 of H4SiO4 in the ideal gas state at 298.15 K, 0.1 MPa. In total, there are 102 data points from seven literature sources. The resulting values of the thermodynamic functions of H4SiO4(g) are: ΔfG0 = −1238.51 ± 3.0 kJ mol−1, ΔfH0 = −1340.68 ± 3.5 kJ mol−1 and S0 = 347.78 ± 6.2 J K−1 mol−1. These values agree quantitatively with one set of ab initio calculations. The relatively large uncertainties are mainly due to conflicting data for H4SiO4(g) from various sources, and new determinations of would be helpful. The thermodynamic properties of this species, H4SiO4(g), are necessary for realistic modeling of silica transport in a low-density water phase. Applications of this analysis may include the processes of silicates condensation in the primordial solar nebula, the precipitation of silica in steam-rich geothermal systems and the corrosion of SiO2-containing alloys and ceramics in moist environments.  相似文献   

19.
Zinc isotopes have been studied along two smelter-impacted soil profiles sampled near one of the largest Pb and Zn processing plants in Europe located in northern France, about 50 km south of Lille. Mean δ66Zn values along these two soil profiles range from +0.22 ± 0.17‰ (2σ) to +0.34 ± 0.17‰ (2σ) at the lowest horizons and from +0.38 ± 0.45‰ (2σ) to +0.76 ± 0.14‰ (2σ) near the surface. The δ66Zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background (mean value +0.31 ± 0.38‰), whereas heavier δ66Zn values near the surface of the two soils are related to anthropogenic Zn. This anthropogenic Zn occurs in the form of franklinite (ZnFe2O4)-bearing slag grains originating from processing wastes at the smelter site and exhibiting δ66Zn values of +0.81 ± 0.20‰ (2σ). The presence of franklinite is indicated by EXAFS analysis of the topsoil samples from both soil profiles as well as by micro-XANES analysis of the surface horizon of a third smelter-impacted soil from a distant site. These results indicate that naturally occurring Zn and smelter-derived Zn exhibit significantly different δ66Zn values, which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of Zn in smelter-impacted soils. In addition to a possible influence of additional past sources of light Zn (likely Zn-sulfides and Zn-sulfates directly emitted by the smelter), the light δ66Zn values in the surface horizons compared to smelter-derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and/or abiotic pedological processes (Zn-bearing mineral precipitation, Zn complexation by organic matter, and plant uptake of Zn). This conclusion emphasizes the need for additional Zn isotopic studies before being able to use Zn isotopes to trace sources and pathways of this element in surface environments.  相似文献   

20.
In soils, silicon released by mineral weathering can be retrieved from soil solution through clay formation, Si adsorption onto secondary oxides and plant uptake, thereby impacting the Si-isotopic signature and Ge/Si ratio of dissolved Si (DSi) exported to rivers. Here we use these proxies to study the contribution of biogenic Si (BSi) in a soil-plant system involving basaltic ash soils differing in weathering degree under intensive banana cropping. δ30Si and Ge/Si ratios were determined in bulk soils (<2 mm), sand (50-2000 μm), silt (2-50 μm), amorphous Si (ASi, 2-50 μm) and clay (<2 μm) fractions: δ30Si by MC-ICP-MS Nu Plasma in medium resolution, operating in dry plasma with Mg doping (δ30Si vs. NBS28 ± 0.12‰ ± 2σSD), Ge/Si computed after determination of Ge and Si concentrations by HR-ICP-MS and ICP-AES, respectively. Components of the ASi fraction were quantified by microscopic counting (phytoliths, diatoms, ashes). Compared to fresh ash (δ30Si = −0.38‰; Ge/Si = 2.21 μmol mol−1), soil clay fractions (<2 μm) were enriched in light Si isotopes and Ge: with increasing weathering degree, δ30Si decreased from −1.19 to −2.37‰ and Ge/Si increased from 4.10 to 5.25 μmol mol−1. Sand and silt fractions displayed δ30Si values close to fresh ash (−0.33‰) or higher due to saharian dust quartz deposition, whose contribution was evaluated by isotopic mass balance calculation. Si-isotopic signatures of bulk soils (<2 mm) were strongly governed by the relative proportions of primary and secondary minerals: the bulk soil Si-isotopic budget could be closed indicating that all the phases involved were identified. Microscopic counting highlighted a surface accumulation of banana phytoliths and a stable phytolith pool from previous forested vegetation. δ30Si and Ge/Si values of clay fractions in poorly developed volcanic soils, isotopically heavier and Ge-depleted in surface horizons, support the occurrence of a DSi source from banana phytolith dissolution, available for Si sequestration in clay-sized secondary minerals (clay minerals formation and Si adsorption onto Fe-oxide). In the soil-plant system, δ30Si and Ge/Si are thus highly relevant to trace weathering and input of DSi from phytoliths in secondary minerals, although not quantifying the net input of BSi to DSi.  相似文献   

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