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1.
The surface reactivity of biogenic, nanoparticulate UO2 with respect to sorption of aqueous Zn(II) and particle annealing is different from that of bulk uraninite because of the presence of surface-associated organic matter on the biogenic UO2. Synthesis of biogenic UO2 was accomplished by reduction of aqueous uranyl ions, by Shewanella putrefaciens CN32, and the resulting nanoparticles were washed using one of two protocols: (1) to remove surface-associated organic matter and soluble uranyl species (NAUO2), or (2) to remove only soluble uranyl species (BIUO2). A suite of bulk and surface characterization techniques was used to examine bulk and biogenic, nanoparticulate UO2 as a function of particle size and surface-associated organic matter. The N2-BET surface areas of the two biogenic UO2 samples following the washing procedures are 128.63 m2 g−1 (NAUO2) and 92.56 m2 g−1 (BIUO2), and the average particle sizes range from 5-10 nm based on TEM imaging. Electrophoretic mobility measurements indicate that the surface charge behavior of biogenic, nanoparticulate UO2 (both NAUO2 and BIUO2) over the pH range 3-9 is the same as that of bulk. The U LIII-edge EXAFS spectra for biogenic UO2 (both NAUO2 and BIUO2) were best fit with half the number of second-shell uranium neighbors compared to bulk uraninite, and no oxygen neighbors were detected beyond the first shell around U(IV) in the biogenic UO2. At pH 7, sorption of Zn(II) onto both bulk uraninite and biogenic, nanoparticulate UO2 is independent of electrolyte concentration, suggesting that Zn(II) sorption complexes are dominantly inner-sphere. The maximum surface area-normalized Zn(II) sorption loadings for the three substrates were 3.00 ± 0.20 μmol m−2 UO2 (bulk uraninite), 2.34 ± 0.12 μmol m−2 UO2 (NAUO2), and 2.57 ± 0.10 μmol m−2 UO2 (BIUO2). Fits of Zn K-edge EXAFS spectra for biogenic, nanoparticulate UO2 indicate that Zn(II) sorption is dependent on the washing protocol. Zn-U pair correlations were observed at 2.8 ± 0.1 Å for NAUO2 and bulk uraninite; however, they were not observed for sample BIUO2. The derived Zn-U distance, coupled with an average Zn-O distance of 2.09 ± 0.02 Å, indicates that Zn(O,OH)6 sorbs as bidentate, edge-sharing complexes to UO8 polyhedra at the surface of NAUO2 nanoparticles and bulk uraninite, which is consistent with a Pauling bond-valence analysis. The absence of Zn-U pair correlations in sample BIUO2 suggests that Zn(II) binds preferentially to the organic matter coating rather than the UO2 surface. Surface-associated organic matter on the biogenic UO2 particles also inhibited particle annealing at 90 °C under anaerobic conditions. These results suggest that surface-associated organic matter decreases the reactivity of biogenic, nanoparticulate UO2 surfaces relative to aqueous Zn(II) and possibly other environmental contaminants.  相似文献   

2.
A synergistic experimental-computational approach was used to study the molecular-scale mechanisms of Ni sorption at varying loadings and at pH 6-8 on the biogenic hexagonal birnessite produced by Pseudomonas putida GB-1. We found that Ni is scavenged effectively by bacterial biomass-birnessite assemblages. At surface excess values below 0.18 mol Ni kg−1 sorbent (0.13 mol Ni mol−1 Mn), the biomass component of the sorbent did not interfere with Ni sorption on mineral sites. Extended X-ray absorption fine structure (EXAFS) spectra showed two dominant coordination environments: Ni bound as a triple-corner-sharing (Ni-TCS) complex at vacancy sites and Ni incorporated (Ni-inc) into the MnO2 sheet, with the latter form of Ni favored at high sorptive concentrations and decreased proton activity. In parallel to our spectral analysis, first-principles geometry optimizations based on density functional theory (DFT) were performed to investigate the structure of Ni surface complexes at vacancy sites. Excellent agreement was achieved between EXAFS- and DFT-derived structural parameters for Ni-TCS and Ni-inc. Reaction-path calculations revealed a pH-dependent energy barrier associated with the transition from Ni-TCS to Ni-inc. Our results are consistent with the rate-limited incorporation of Ni at vacancy sites in our sorption samples, but near-equilibrium state of Ni in birnessite phases found in nodule samples. This study thus provides direct and quantitative evidence of the factors governing the occurrence of Ni adsorption versus Ni incorporation in biogenic hexagonal birnessite, a key mineral in the terrestrial manganese cycle.  相似文献   

3.
Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of 90Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of 90Sr in sediment–water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01 mol L−1), pH variation is the main control on sorption. The sorption edge for 90Sr was observed between pH 4 and 6 with maximum sorption occurring (Kd ∼ 103 L kg−1) at pH 6–8. At ionic strengths above 10 mmol L−1, and at pH values between 6 and 8, cation exchange processes reduced 90Sr uptake to the sediment. This exchange process explains the lower 90Sr sorption (Kd ∼ 40 L kg−1) in the presence of artificial Magnox tank liquor (IS = 29 mmol L−1). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr2+ in either HCO3-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ∼9 O atoms at 2.58–2.61 Å after 10 days. This is equivalent to the Sr2+ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ∼80% of solid associated 90Sr was exchangeable with 1 M MgCl2 in all experiments. These results suggest that over long periods, 90Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of 90Sr is to be expected.  相似文献   

4.
Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ66Zn < −0.37); EH3 chondrites (0.15 < δ66Zn < 0.31); EH4 chondrites (0.15 < δ66Zn < 0.27); EH5 chondrites (δ66Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ66Zn < 0.63); the Shallowater aubrite (1.48 < δ66Zn < 2.36); EL6 chondrites (2.26 < δ66Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ66Zn = 0.37).The aubrite Peña Blanca Spring (δ66Zn = −7.04‰) and the EL6 North West Forrest (δ66Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ66Zn < 1‰) is much smaller ( [Luck et al., 2005] and [Herzog et al., 2009]).EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.  相似文献   

5.
We report seawater boron concentration (mg kg−1) and chlorinity (‰) values measured in seawater samples (n = 139) collected at various depths in the North Pacific and North Atlantic oceans and the East/Japan Sea (located in the western temperate North Pacific). Our results indicate that variations in seawater boron concentration are strongly coupled to variations in chlorinity (and salinity), yielding a mean boron to chlorinity ratio of 0.2414 ± 0.0009 mg kg−1 ‰−1 (boron to salinity ratio = 0.1336 ± 0.0005 mg kg−1 ‰−1). This ratio was surprisingly universal throughout the water column in the three marine basins and across widely different ocean surface regimes, but differs from the generally accepted ratio of 0.232 ± 0.005 mg kg−1 ‰−1 determined by Uppström (1974), which was based on only 20 measurements at four sites in the tropical Pacific Ocean. In converting total alkalinity to carbonate alkalinity (and vice versa) for thermodynamic calculations, the difference between these two ratios leads to a difference of 5 μmol kg−1 in estimates for ocean surface waters, where the contribution of borate to total alkalinity is typically greatest. We suggest the use of the new boron to chlorinity ratio for predicting seawater boron concentrations using chlorinity (or salinity) data.  相似文献   

6.
The mobility and solid-state speciation of zinc in a pseudogley soil (pH = 8.2-8.3) before and after contamination by land-disposition of a dredged sediment ([Zn] = 6600 mg kg−1) affected by smelter operations were studied in a 50 m2 pilot-scale test site and the laboratory using state-of-the-art synchrotron-based techniques. Sediment disposition on land caused the migration of micrometer-sized, smelter-related, sphalerite (ZnS) and franklinite (ZnFe2O4) grains and dissolved Zn from the sediment downwards to a soil depth of 20 cm over a period of 18 months. Gravitational movement of fine-grained metal contaminants probably occurred continuously, while peaks of Zn leaching were observed in the summer when the oxidative dissolution of ZnS was favored by non-flooding conditions. The Zn concentration in the <50 μm soil fraction increased from ∼61 ppm to ∼94 ppm in the first 12 months at 0-10 cm depth, and to ∼269 ppm in the first 15 months following the sediment deposition. Higher Zn concentrations and enrichments were observed in the fine (<2 μm) and very fine (<0.2 μm) fractions after 15 months (480 mg kg−1 and 1000 mg kg−1, respectively), compared to 200 mg kg−1 in the <2 μm fraction of the initial soil. In total, 1.2% of the Zn initially present in the sediment was released to the environment after 15 months, representing an integrated quantity of ∼4 kg Zn over an area of 50 m2. Microfocused X-ray fluorescence (XRF), diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy techniques were used to image chemical associations of Zn with Fe and Mn, and to identify mineral and Zn species in selected points-of-interest in the uncontaminated and contaminated soil. Bulk average powder EXAFS spectroscopy was used to quantify the proportion of each Zn species in the soil. In the uncontaminated soil, Zn is largely speciated as Zn-containing phyllosilicate, and to a minor extent as zincochromite (ZnCr2O4), IVZn-sorbed turbostratic birnessite (δ-MnO2), and Zn-substituted goethite. In the upper 0-10 cm of the contaminated soil, ∼60 ± 10% of total Zn is present as ZnS inherited from the overlying sediment. Poorly-crystalline Zn-sorbed Fe (oxyhydr)oxides and zinciferous phyllosilicate amount to ∼20-30 ± 10% each and, therefore, make up most of the remaining Zn. Smaller amounts of franklinite (ZnFe2O4), Zn-birnessite and Zn-goethite were also detected. Further solubilization of the Zn inventory in the sediment, and also remobilization of Zn from the poorly-crystalline neoformed Fe (oxyhydr)oxide precipitates, are expected over time. This study shows that land deposition of contaminated dredged sediments is a source of Zn for the covered soil and, consequently, presents environmental hazards. Remediation technologies should be devised to either sequester Zn into sparingly soluble crystalline phases, or remove Zn by collecting leachates beneath the sediment.  相似文献   

7.
Bulk Cd adsorption isotherm experiments, thermodynamic equilibrium modeling, and Cd K edge EXAFS were used to constrain the mechanisms of proton and Cd adsorption to bacterial cells of the commonly occurring Gram-positive and Gram-negative bacteria, Bacillus subtilis and Shewanella oneidensis, respectively. Potentiometric titrations were used to characterize the functional group reactivity of the S. oneidensis cells, and we model the titration data using the same type of non-electrostatic surface complexation approach as was applied to titrations of B. subtilis suspensions by Fein et al. (2005). Similar to the results for B. subtilis, the S. oneidensis cells exhibit buffering behavior from approximately pH 3-9 that requires the presence of four distinct sites, with pKa values of 3.3 ± 0.2, 4.8 ± 0.2, 6.7 ± 0.4, and 9.4 ± 0.5, and site concentrations of 8.9(±2.6) × 10−5, 1.3(±0.2) × 10−4, 5.9(±3.3) × 10−5, and 1.1(±0.6) × 10−4 moles/g bacteria (wet mass), respectively. The bulk Cd isotherm adsorption data for both species, conducted at pH 5.9 as a function of Cd concentration at a fixed biomass concentration, were best modeled by reactions with a Cd:site stoichiometry of 1:1. EXAFS data were collected for both bacterial species as a function of Cd concentration at pH 5.9 and 10 g/L bacteria. The EXAFS results show that the same types of binding sites are responsible for Cd sorption to both bacterial species at all Cd loadings tested (1-200 ppm). Carboxyl sites are responsible for the binding at intermediate Cd loadings. Phosphoryl ligands are more important than carboxyl ligands for Cd binding at high Cd loadings. For the lowest Cd loadings studied here, a sulfhydryl site was found to dominate the bound Cd budgets for both species, in addition to the carboxyl and phosphoryl sites that dominate the higher loadings. The EXAFS results suggest that both Gram-positive and Gram-negative bacterial cell walls have a low concentration of very high-affinity sulfhydryl sites which become masked by the more abundant carboxyl and phosphoryl sites at higher metal:bacteria ratios. This study demonstrates that metal loading plays a vital role in determining the important metal-binding reactions that occur on bacterial cell walls, and that high affinity, low-density sites can be revealed by spectroscopy of biomass samples. Such sites may control the fate and transport of metals in realistic geologic settings, where metal concentrations are low.  相似文献   

8.
The 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model has been used over the past decade or so to quantitatively describe the uptake of metals with oxidation states from II to VI on 2:1 clay minerals; montmorillonite and illite. One of the main features in this model is that there are two broad categories of amphoteric edge sorption sites; the so called strong (SSOH) and weak (SW1OH) sites. Because of their different sorption characteristics, it was expected that the coordination environments of the surface complexes on the two site types would be different. Zn isotherm data on two montmorillonites, Milos and STx-1, were measured and modelled using the 2SPNE SC/CE sorption model. The results were used to define the most favourable experimental conditions under which Zn sorption was either dominated by the strong (SSOH, ∼2 mmol kg−1) or by the weak sites (SW1OH, ∼40 mmol kg−1). Highly oriented self-supporting films were prepared for polarised extended X-ray absorption fine structure (P-EXAFS) investigations.Montmorillonites often contain Zn incorporated in the clay matrix. The Zn bound in this form was quantified and the results from the analysis of the P-EXAFS spectra were taken into account in the interpretation of the spectra measured at low Zn loadings (∼2 mmol kg−1) and medium Zn loadings (∼30 mmol kg−1). The Zn spectra on the “strong sites” exhibited a pronounced angular dependency and formed surface complexes in the continuity of the Al-octahedral sheets at the montmorillonite edges. In contrast, the Zn “weak site” spectra showed only a weak angular dependency. The spectroscopic evidence indicates the existence of two distinct groups of edge surface binding sites which is consistent with a multi-site sorption model and in particular with the strong/weak site concept intrinsic to the 2SPNE S/CE sorption model.  相似文献   

9.
The soluble and insoluble hydrolysis products of palladium were investigated in aqueous solutions of 0.6 mol kg−1 NaCl at 298.2 K. Potentiometric titrations of millimolal palladium(II) solutions were used to monitor hydrolysis reactions of the mononuclear PdCl3OH2− and species. Spectrophotometric titrations were also used to corroborate the speciation change and to extract the correlative molar absorption coefficients for the PdCl3OH2− species in the 210-320 nm range. Longer-term potentiometric titrations systematically yielded precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments in the 3-11 pH range showed the dominant precipitating phase to be Pd(OH)1.72Cl0.28. EXAFS measurements yielded an average of 3.50 O and 0.50 Cl atoms per Pd atom with a Pd-O distance of 2.012 Å and a Pd-Cl distance of 2.185 Å. Speciation modeling of proton and palladium mass balance data of experiments for palladium concentrations ranging from 0.047 to 10.0  mmol kg−1 required the presence of polynuclear complexes containing 3-9 palladium atoms. The existence of such complexes is moreover supported by previous investigations of palladium hydroxide chains of the type [Pd(OH)1.72Cl0.28]n, that are coiled and/or aggregated into nanometer-sized (15-40 Å) spheroids.  相似文献   

10.
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   

11.
Fractionation of Cu and Zn isotopes during adsorption onto amorphous ferric oxyhydroxide is examined in experimental mixtures of metal-rich acid rock drainage and relatively pure river water and during batch adsorption experiments using synthetic ferrihydrite. A diverse set of Cu- and Zn-bearing solutions was examined, including natural waters, complex synthetic acid rock drainage, and simple NaNO3 electrolyte. Metal adsorption data are combined with isotopic measurements of dissolved Cu (65Cu/63Cu) and Zn (66Zn/64Zn) in each of the experiments. Fractionation of Cu and Zn isotopes occurs during adsorption of the metal onto amorphous ferric oxyhydroxide. The adsorption data are modeled successfully using the diffuse double layer model in PHREEQC. The isotopic data are best described by a closed system, equilibrium exchange model. The fractionation factors (αsoln-solid) are 0.99927 ± 0.00008 for Cu and 0.99948 ± 0.00004 for Zn or, alternately, the separation factors (Δsoln-solid) are −0.73 ± 0.08‰ for Cu and −0.52 ± 0.04‰ for Zn. These factors indicate that the heavier isotope preferentially adsorbs onto the oxyhydroxide surface, which is consistent with shorter metal-oxygen bonds and lower coordination number for the metal at the surface relative to the aqueous ion. Fractionation of Cu isotopes also is greater than that for Zn isotopes. Limited isotopic data for adsorption of Cu, Fe(II), and Zn onto amorphous ferric oxyhydroxide suggest that isotopic fractionation is related to the intrinsic equilibrium constants that define aqueous metal interactions with oxyhydroxide surface sites. Greater isotopic fractionation occurs with stronger metal binding by the oxyhydroxide with Cu > Zn > Fe(II).  相似文献   

12.
The Xunyang Hg mine (XMM) situated in Shaanxi Province is an active Hg mine in China. Gaseous elemental Hg (GEM) concentrations in ambient air were determined to evaluate its distribution pattern as a consequence of the active mining and retorting in the region. Total Hg (HgT) and methylmercury (MeHg) concentrations in riparian soil, sediment and rice grain samples (polished) as well as Hg speciation in surface water samples were measured to show local dispersion of Hg contamination. As expected, elevated concentrations of GEM were found, ranging from 7.4 to 410 ng m−3. High concentrations of HgT and MeHg were also obtained in riparian soils, ranged from 5.4 to 120 mg kg−1 and 1.2 to 11 μg kg−1, respectively. Concentrations of HgT and MeHg in sediment samples varied widely from 0.048 to 1600 mg kg−1 and 1.0 to 39 μg kg−1, respectively. Surface water samples showed elevated HgT concentrations, ranging from 6.2 to 23,500 ng L−1, but low MeHg concentrations, ranging from 0.022 to 3.7 ng L−1. Rice samples exhibited high concentrations of 50–200 μg kg−1 in HgT and of 8.2–80 μg kg−1 in MeHg. The spatial distribution patterns of Hg speciation in the local environmental compartments suggest that the XMM is the source of Hg contaminations in the study area.  相似文献   

13.
The mineralogy of natural ferromanganese coatings on quartz grains and the crystal chemistry of associated trace elements Ni, Zn, Ba, and As were characterized by X-ray microfluorescence, X-ray diffraction, and EXAFS spectroscopy. Fe is speciated as ferrihydrite and Mn as vernadite. The two oxides form alternating Fe- and Mn-rich layers that are irregularly distributed and not always continuous. Unlike naturally abundant Fe-vernadite, in which Fe and Mn are mixed at the nanoscale, the ferrihydrite and vernadite are physically segregated and the trace elements clearly partitioned at the microscopic scale. Vernadite consists of two populations of interstratified one-water layer (7 Å phyllomanganate) and two-water layer (10 Å phyllomanganate) crystallites. In one population, 7 Å layers dominate, and in the other 10 Å layers dominate. The three trace metals Ni, Zn, and Ba are associated with vernadite and the metalloid As with ferrihydrite. In vernadite, nickel is both substituted isomorphically for Mn in the manganese layer and sorbed at vacant Mn layer sites in the interlayer. The partitioning of Ni is pH-dependent, with a strong preference for the first site at circumneutral pH and for the second at acidic pH. Thus, the site occupancy of Ni in vernadite may be an indicator of marine vs. continental origin, and in the latter, of the acidity of streams, lakes, or soil pore waters in which the vernadite formed. Zinc is sorbed only in the interlayer at vacant Mn layer sites. It is fully tetrahedral at a Zn/Mn molar ratio of 0.0138, and partly octahedral at a Zn/Mn ratio of 0.1036 consistent with experimental studies showing that the VIZn/IVZn ratio increases with Zn loading. Barium is sorbed in a slightly offset position above empty tetrahedral cavities in the interlayer. Arsenic tetrahedra are retained at the ferrihydrite surface by a bidentate-binuclear attachment to two adjacent iron octahedra, as commonly observed. Trace elements in ferromanganese precipitates are partitioned at a few, well-defined, crystallographic sites that have some elemental specificity, and thus selectivity. The relative diversity of sorption sites contrasts with the simplicity of the layer structure of vernadite, in which charge deficit arises only from Mn4+ vacancies (i.e., no Mn3+ for Mn4+ substitution). Therefore, sorption mechanisms primarily depend on physical and chemical properties of the sorbate and competition with other ions in solution, such as protons at low pH for Ni sorption.  相似文献   

14.
Industrialization, urbanization, and agricultural practices are 3 of the most important sources of metal accumulations in soils. Concentrations of Cr, Mn, Ni, Cu, Pb, Zn and Cd were determined in surface soils collected under different land uses, including urban (UR), industrial (IN-1 and IN-2), agricultural (AG), abandoned unused (AB), and natural (NA) sites to examine the influence of anthropogenic activities on metals in soils formed in a typical Mediterranean environment. The highest concentrations of Cr, Cd, and Pb observed in the NW industrial area (IN-2) were 63.7, 3.34 and 2330 mg metal kg−1 soil, for each metal, respectively. The SW industrial area (IN-1) contained the highest Zn content at 135 mg kg−1. However, soils with the highest concentrations of Ni and Cu were located in AG sites at 30.9 and 64.9 mg kg−1 soil, respectively. Sampling locations with the highest concentrations of Mn were identified in AB sites. Using the concentrations of metals at the NA sites as the baseline levels, soils collected from all other land uses in the study area exhibited significantly higher total contents of Zn, Mn, Cr and Ni. Metal enrichment was attributed to fertilizer and pesticide applications, industrial activities, and metal deposition from a high volume of vehicular traffic (for Pb and Cd). High concentrations of Mn in some samples were attributed to parent materials. The study demonstrated that anthropogenic activities associated with various land uses contribute to metal accumulation in soils and indicated a need to closely monitor land management practices to reduce human and ecological risks from environmental pollution.  相似文献   

15.
The concentrations of twenty four chemical elements in the surface layer of natural desert soils and the cultivated farmland soils were measured at a desert-oasis ecotone in the middle of Heihe river basin, north-west China. Background values were estimated for (a) major elements (Si 335.3 g kg− 1, Al 49.4 g kg− 1, Fe 19.1 g kg− 1, Ca 29.4 g kg− 1, Mg 8.9 g kg− 1, K 20.1 g kg− 1, Na 17.5 g kg− 1 and P 0.338 g kg− 1), (b) heavy metals and non-metals (Cr 55.8 mg kg− 1, Mn 404.8 mg kg− 1, Ni 17.7 mg kg− 1, Cu 5.1 mg kg− 1, Zn 33.7 mg kg− 1, Pb 15.5 mg kg− 1 and As 5.2 mg kg− 1) and (c) other trace elements (Ti 2.0 mg kg− 1, V 55.3 mg kg− 1, Co 5.7 mg kg− 1, Rb 82.4 mg kg− 1, Sr 232.9 mg kg− 1, Y 14.7 mg kg− 1, Zr 194.9 mg kg− 1, Nb 7.8 mg kg− 1 and Ba 720.6 mg kg− 1). After natural desert soil was cultivated for agricultural use, significant changes in element concentrations occurred under tillage, irrigation and fertilisation management. Compared to natural soil, the for the levels of Si, K, Na, Sr, Zr and Ba decreased, and no changes were observed for Rb, while the values of the other 17 elements increase in agricultural soil from 1.2 to 3.5 times. However, their absolute concentrations are still low, suggesting that the arable soil in this region remains comparatively a clean soil. The increased silt, clay and organic carbon content, under long-term irrigation, enriched the fine-grained materials, and application of fertilisers and manure contributed to the accumulation of most elements in arable soil. The accumulation of elements in agricultural soil increased with increasing cultivation years and extent of soil development.  相似文献   

16.
The Ni geochemistry of limonite and saprolite laterite ores from Pujada in the Philippines has been investigated using a mixture of laboratory and synchrotron techniques. Nickel laterite profiles are typically composed of complicated mineral assemblages, with Ni being distributed heterogeneously at the micron scale, and thus a high degree of spatial resolution is required for analysis. This study represents the first such analysis of Philippine laterite ores. Synchrotron bulk and microprobe X-ray absorption spectroscopy (XAS), comprising both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, together with synchrotron microprobe X-ray fluorescence microscopy (XFM) and diffraction (XRD) have been applied to provide quantitative analysis of the mineral components and Ni speciation.Synchrotron microprobe EXAFS spectroscopy suggests that the limonite Ni is associated with phyllomanganate via adsorption onto the Mn oxide layers and substitution for Mn within these layers. Laboratory scanning electron microscopy, coupled to electron dispersive spectroscopy analyses, indicates that Ni is also associated with concentrated Fe containing particles and this is further confirmed by synchrotron bulk and microprobe investigation. Linear combination fitting of the bulk EXAFS limonite data suggests 60 ± 15% of the Ni is associated with phyllomanganate, with the predominant fraction adsorbed above vacancies in the MnO6 layers with the remainder being substituted for Mn within these layers. The remaining 40 ± 10% of the Ni in the limonite ore is incorporated into goethite through replacement of the Fe. In the saprolite ore, 90 ± 23% of the Ni is associated with a serpentine mineral, most likely lizardite, as a replacement for Mg. The remaining Ni is found within phyllomanganate adsorbed above vacancies in the MnO6 layers.  相似文献   

17.
Pyrite dissolution and interaction with Fe(II), Co(II), Eu(III) and U(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]aq/[Fe]aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm−2. Concentration isotherms for Eu(III) and U(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10−6 mol g−1. In the U(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10−8 mol g−1. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO2(s).Scanning electron microscopy and micro-Raman spectrometry of U(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U(VI).  相似文献   

18.
Sorption of rare earth elements (REEs) and Ce oxidation on natural and synthetic Mn oxides have been investigated by many researchers. Although Mn(II)-oxidizing microorganisms are thought to play an important role in the formation of Mn oxides in most natural environments, Ce oxidation by biogenic Mn oxide and the relevance of microorganisms to the Ce oxidation process have not been well understood. Therefore, in this study, we conducted sorption experiments of REEs on biogenic Mn oxide produced by Acremonium sp. strain KR21-2. The distribution coefficients, Kd(REE), between biogenic Mn oxide (plus hyphae) and 10 mmol/L NaCl solution showed a large positive Ce anomaly and convex tetrad effect variations at pH 3.8, which was consistent with previous works using synthetic Mn oxide. The positive Ce anomaly was caused by oxidation of Ce(III) to Ce(IV) by the biogenic Mn oxide, which was confirmed by analysis of the Ce LIII-edge XANES spectra. With increasing pH, the positive Ce anomaly and convex tetrad effects became less pronounced. Furthermore, negative Ce anomalies were observed at a pH of more than 6.5, suggesting that Ce(IV) was stabilized in the solution (<0.2 μm) phase, although Ce(III) oxidation to Ce(IV) on the biogenic Mn oxide was confirmed by XANES analysis. It was demonstrated that no Ce(III) oxidation occurred during sorption on the hyphae of strain KR21-2 by the Kd(REE) patterns and XANES analysis. The analysis of size exclusion HPLC-ICP-MS showed that some fractions of REEs in the filtrates (<0.2 μm) after sorption experiments were bound to organic molecules (40 and <670 kDa fractions), which were possibly released from hyphae. A line of our data indicates that the negative Ce anomalies under circumneutral pH conditions arose from Ce(III) oxidation on the biogenic Mn oxide and subsequent complexation of Ce(IV) with organic ligands. The suppression of tetrad effects is also explained by the complexation of REEs with organic ligands. The results of this study demonstrate that the coexistence of the biogenic Mn oxide and hyphae of strain KR21-2 produces a specific redox chemistry which cannot be explained by inorganic species.  相似文献   

19.
The presence of PAHs, n-alkanes, pristane, and phytanes in core sediment from the Vossoroca reservoir (Parana, southern Brazil) was investigated. The total concentration of the 16 PAHs varied from 15.5 to 1646 μg kg−1. Naphthalene was present in all layers (3.34–74.0 μg kg−1). The most abundant and dominant n-alkanes were n-C15 and n-C36, with average concentrations of 198.1 ± 46.8 and 522.9 ± 167.7 μg kg−1, respectively. Lighter n-alkanes were distributed more evenly through the layers and showed less variation, specially n-C9, n-C12, and n-C18, with average concentrations of 14.6 ± 3.0, 31.6 ± 1.9, and 95.0 ± 5.2 μg kg−1, respectively; heavier n-alkanes were more unevenly distributed.  相似文献   

20.
CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The 53Cr excesses are correlated with the 53Mn/55Mn ratio and result from the in situ decay of 53Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial 53Mn/55Mn ratios ((53Mn/55Mn)0) of (4.1 ± 1.2) × 10−6 and (5.1 ± 1.7) × 10−6, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial 53Mn/55Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10−6 to (4.8 ± 2.1) × 10−6 for QUE 93005 and from (3.1 ± 1.6) × 10−6 to (1.3 ± 0.5) × 10−5 for ALH 83100. A possible interpretation for the ranges in (53Mn/55Mn)0 could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.  相似文献   

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