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1.
Mineral-associated organic matter (OM) represents a large reservoir of organic carbon (OC) in natural environments. The factors controlling the extent of the mineral-mediated OC stabilization, however, are poorly understood. The protection of OM against biodegradation upon sorption to mineral phases is assumed to result from the formation of strong bonds that limit desorption. To test this, we studied the biodegradation of OM bound to goethite (α-FeOOH), pyrophyllite, and vermiculite via specific mechanisms as estimated from OC uptake in different background electrolytes and operationally defined as ‘ligand exchange’, ‘Ca2+ bridging’, and ‘van der Waals forces’. Organic matter extracted from an Oa forest floor horizon under Norway spruce (Picea abies (L.) Karst) was reacted with minerals at dissolved OC concentrations of ∼5-130 mg/L at pH 4. Goethite retained up to 30.1 mg OC/g predominantly by ‘ligand exchange’; pyrophyllite sorbed maximally 12.5 mg OC/g, largely via ‘van der Waals forces’ and ‘Ca2+ bridging’, while sorption of OM to vermiculite was 7.3 mg OC/g, mainly due to the formation of ‘Ca2+ bridges’. Aromatic OM components were selectively sorbed by all minerals (goethite ? phyllosilicates). The sorption of OM was strongly hysteretic with the desorption into 0.01 M NaCl being larger for OM held by ‘Ca2+ bridges’ and ‘van der Waals forces’ than by ‘ligand exchange’. Incubation experiments under aerobic conditions (initial pH 4; 90 days) revealed that OM mainly bound to minerals by ‘ligand exchange’ was more resistant against mineralization than OM held by non-columbic interactions (‘van der Waals forces’). Calcium bridges enhanced the stability of sorbed OM, especially for vermiculite, but less than the binding via ‘ligand exchange’. Combined evidence suggests that the extent and rate of mineralization of mineral-associated OM are governed by desorption. The intrinsic stability of sorbed OM as related to the presence of resistant, lignin-derived aromatic components appears less decisive for the sorptive stabilization of OM than the involved binding mechanisms. In a given environment, the type of minerals present and the solution chemistry determine the operating binding mechanisms, thereby the extent of OM sorption and desorption, and thus ultimately the bioavailability of mineral-associated OM.  相似文献   

2.
The Po River (Italy) experienced a 100-year flood in October 2000. Surface sediments (0-1 cm) from cross-shelf transects were collected in the Po prodelta area (Adriatic Sea) in December 2000, in order to describe the distribution of organic matter (OM) along the main sediment dispersal system immediately after the flood event. Stations were subsequently reoccupied in October 2001 and April 2002. This sampling program provided a special opportunity to characterize the initial surficial flood deposit and the evolution of its associated OM over the course of 2 years. CuO oxidation, elemental, δ13C, Δ14C, and grain-size analyses were carried out to characterize the source, age, and spatial variability of sedimentary OM. Statistical analysis (PERMANOVA) was then applied to investigate temporal changes in different portions of the Po prodelta area. Isotopic and biomarker data suggest that the sedimentary OM in the flood deposit was initially dominated by aged (Δ14CDec-00 = −298.7 ± 56.3‰), lignin-poor OM (ΛDec-00 = 1.96 ± 0.33 mg/100 mg OC), adsorbed on the fine material (clayDec-00 = 72.1 ± 4.8%) delivered by the flood. In the 2 years following the flood, post-depositional processes significantly increased the content of lignin (ΛOct-01 = 2.19 ± 0.51 mg/100 mg OC; ΛApr-02 = 2.61 ± 0.63 mg/100 mg OC); and coarse material (silt and sand), while decreasing the contributions from aged OC (Δ14COct-01 = −255.7 ± 32.8‰; Δ14CApr-02 = −213.2 ± 30.4‰) and fine fraction (clayOct-01 = 54.8 ± 9.5%; clayApr-02 = 44.6 ± 13.3%). The major changes were observed in the northern and central portions of the prodelta.  相似文献   

3.
Extracellular polymeric substances (EPS) are continuously produced by bacteria during their growth and metabolism. In soils, EPS are bound to cell surfaces, associated with biofilms, or released into solution where they can react with other solutes and soil particle surfaces. If such reaction results in a decrease in EPS bioaccessibility, it may contribute to stabilization of microbial-derived organic carbon (OC) in soil. Here we examined: (i) the chemical fractionation of EPS produced by a common Gram positive soil bacterial strain (Bacillus subtilis) during reaction with dissolved and colloidal Al species and (ii) the resulting stabilization against desorption and microbial decay by the respective coprecipitation (with dissolved Al) and adsorption (with Al(OH)3(am)) processes. Coprecipitates and adsorption complexes obtained following EPS-Al reaction as a function of pH and ionic strength were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The stability of adsorbed and coprecipitated EPS against biodegradation was assessed by mineralization experiments for 1100 h. Up to 60% of the initial 100 mg/L EPS-C was adsorbed at the highest initial molar Al:C ratio (1.86), but this still resulted only in a moderate OC mass fraction in the solid phase (17 mg/g Al(OH)3(am)). In contrast, while coprecipitation by Al was less efficient in removing EPS from solution (maximum values of 33% at molar Al:C ratios of 0.1-0.2), the OC mass fraction in the solid product was substantially larger than that in adsorption complexes. Organic P compounds were preferentially bound during both adsorption and coprecipitation. Data are consistent with strong ligand exchange of EPS phosphoryl groups during adsorption to Al(OH)3(am), whereas for coprecipitation weaker sorption mechanisms are also involved. X-ray photoelectron analyses indicate an intimate mixing of EPS with Al in the coprecipitates, which is not observed in the case of EPS adsorption complexes. The incubation experiments showed that both processes result in overall stabilization of EPS against microbial decay. Stabilization of adsorbed or coprecipitated EPS increased with increasing molar Al:C ratio and biodegradation was correlated with EPS desorption, implying that detachment of EPS from surface sites is a prerequisite for microbial utilization. Results indicate that the mechanisms transferring EPS into Al-organic associations may significantly affect the composition and stability of biomolecular C, N and P in soils. The observed efficient stabilization of EPS might explain the strong microbial character of organic matter in subsoils.  相似文献   

4.
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5.
Organic matter (OM) in mineral-organic associations (MOAs) represents a large fraction of carbon in terrestrial ecosystems which is considered stable against biodegradation. To assess the role of MOAs in carbon cycling, there is a need to better understand (i) the time-dependent biogeochemical evolution of MOAs in soil, (ii) the effect of the mineral composition on the physico-chemical properties of attached OM, and (iii) the resulting consequences for the stabilization of OM. We studied the development of MOAs across a mineralogical soil gradient (0.3-4100 kyr) at the Hawaiian Islands that derived from basaltic tephra under comparable climatic and hydrological regimes. Mineral-organic associations were characterized using biomarker analyses of OM with chemolytic methods (lignin phenols, non-cellulosic carbohydrates) and wet chemical extractions, surface area/porosity measurements (N2 at 77 K and CO2 at 273 K), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The results show that in the initial weathering stage (0.3 kyr), MOAs are mainly composed of primary, low-surface area minerals (olivine, pyroxene, feldspar) with small amounts of attached OM and lignin phenols but a large contribution of microbial-derived carbohydrates. As high-surface area, poorly crystalline (PC) minerals increase in abundance during the second weathering stage (20-400 kyr), the content of mineral-associated OM increased sharply, up to 290 mg C/g MOA, with lignin phenols being favored over carbohydrates in the association with minerals. In the third and final weathering stage (1400-4100 kyr), metastable PC phases transformed into well crystalline secondary Fe and Al (hydr)oxides and kaolin minerals that were associated with less OM overall, and depleted in both lignin and carbohydrate as a fraction of total OM. XPS, the N2 pore volume data and OM-mineral volumetric ratios suggest that, in contrast to the endmember sites where OM accumulated at the surfaces of larger mineral grains, topsoil MOAs of the 20-400-kyr sites are composed of a homogeneous admixture of small-sized PC minerals and OM, which originated from both adsorption and precipitation processes. The chemical composition of OM in surface-horizon MOAs, however, was largely controlled by the uniform source vegetation irrespective of the substrate age whereas in subsoil horizons, aromatic and carboxylic C correlated positively with oxalate-extractable Al and Si and CuCl2-extractable Al concentrations representing PC aluminosilicates and Al-organic complexes (r2 > 0.85). Additionally, XPS depth profiles suggest a zonal structure of sorbed OM with aromatic carbons being enriched in the proximity of mineral surfaces and amide carbons (peptides/proteins) being located in outer regions of MOAs. Albeit the mineralogical and compositional changes of OM, the rigidity of mineral-associated OM as analyzed by DSC changed little over time. A significantly reduced side chain mobility of sorbed OM was, however, observed in subsoil MOAs, which likely arose from stronger mineral-organic bindings. In conclusion, our study shows that the properties of soil MOAs change substantially over time with different mineral assemblages favoring the association of different types of OM, which is further accentuated by a vertical gradient of OM composition on mineral surfaces. Factors supporting the stabilization of sorbed OM were (i) the surface area and reactivity of minerals (primary or secondary crystalline minerals versus PC secondary minerals), (ii) the association of OM with micropores of PC minerals (via ‘sterically’ enhanced adsorption), (iii) the effective embedding of OM in ‘well mixed’ arrays with PC minerals and monomeric/polymeric metal species, (iv) the inherent stability of acidic aromatic OM components, and (iv) an impaired segmental mobility of sorbed OM, which might increase its stability against desorption and microbial utilization.  相似文献   

6.
Sorption of Cm(III) and Eu(III) at trace concentrations onto Ca-montmorillonite (SWy-1) and Na-illite (Illite du Puy) has been studied under anaerobic conditions by batch sorption experiments and time-resolved laser fluorescence spectroscopy (TRLFS). Comparison of the results from spectroscopic and batch sorption experiments with Cm and Eu indicates the existence of outer-sphere complexes at pH <4 in the experiments with Na-illite (0.25 g/L solid; 2.5 × 10−7 mol/L Cm; 0.1 mol/L NaClO4). In the case of Ca-montmorillonite, (0.25 g/L solid, 2.5 × 10−7 mol/L Cm or 10−6 mol/L Eu, 0.066 mol/L Ca(ClO4)2), Cm/Eu outer-sphere complexes do not form at significant levels due to the Ca2+ competition for the clay mineral cation-exchange sites. TRLFS spectra indicate the formation of inner-sphere surface complexes at pH >5 for both clay minerals. Five H2O/OH− molecules remain in the first metal ion coordination sphere of the sorbed Eu/Cm. Measured fluorescence lifetimes of sorbed Eu/Cm and peak deconvolution of Cm-spectra are consistent with the formation of surface complexes of the form ≡S-O-Eu/Cm(OH)x(2−x)(H2O)5−x. At pH ≥ 12 Cm becomes incorporated into a surface precipitate at the Ca-montmorillonite surface presumably composed of Ca(OH)2 or calcium silicate hydrate. A dramatic shift of the fluorescence emission band by more than 20 nm and a clear increase in the fluorescence lifetime suggests the almost complete displacement of coordinated H2O and OH−. The pH dependent Eu sorption data obtained in batch experiments are consistent with spectroscopic data on Eu and Cm within experimental uncertainties thus demonstrating the validity of Eu as a homologue for trivalent actinides. Parameterization of a two-site protolysis nonelectrostatic surface complexation and cation exchange model using the batch sorption data and spectroscopic results is discussed in Part 2 of this work.  相似文献   

7.
Sequestration of organic carbon (OC) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OC sequestration and stability in soils. Short-range-ordered Fe oxides, such as ferrihydrite, demonstrate a high affinity for OC in binary systems. Calcium commonly co-associates with OC and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the co-association for OC sequestration remain unresolved. We explored the effect of calcium (Ca2+) on the sorption of dissolved OC to 2-line ferrihydrite. Sorption experiments were conducted between leaf litter-extractable OC and ferrihydrite at pH 4 to 9 with different initial C/Fe molar ratios and Ca2+ concentrations. The extent of OC sorption to ferrihydrite in the presence of Ca2+ increased across all tested pH values, especially at pH ≥ 7. Sorbed OC concentration at pH 9 increased from 8.72 ± 0.16 to 13.3 ± 0.20 mmol OC g?1 ferrihydrite between treatments of no added Ca2+ and 30 mM Ca2+ addition. Batch experiments were paired with spectroscopic studies to probe the speciation of sorbed OC and elucidate the sorption mechanism. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species that were preferentially bound to ferrihydrite and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. Results from batch to spectroscopic experiments provide significant evidence for the enhancement of dissolved OC sequestration to 2-line ferrihydrite and suggest the formation of Fe–Ca-OC ternary complexes. Findings of this research will inform modeling of environmental C cycling and have the potential to influence strategies for managing land to minimize OM stabilization.  相似文献   

8.
Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing 109Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric PCO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd2+ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R2 = 0.992). It differentiates between two ‘pools’ of sorbed Cd2+ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd2+ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R2 = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ∼175 d, for release to a ‘zero-sink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption.  相似文献   

9.
The sorption of 57Fe(II) onto an Fe-free, mineralogically pure and Ca-saturated synthetic montmorillonite sample (structural formula: Ca0.15(Al1.4Mg0.6)(Si4)O10(OH,F)2), was studied as a function of pH under strictly anoxic conditions (N2 glove box atmosphere, O2 content <1 ppm), using wet chemistry and cryogenic (T = 77 K) 57Fe Mössbauer spectrometry. No Fe(III) was detected in solution at any pH. However, in pH conditions where Fe(II) is removed from solution, a significant amount of surface-bound Fe(III) was produced, which increased with pH from 0% to 3% of total Fe in a pre-sorption edge region (i.e. at pH < 7.5 where about 15% of total Fe is sorbed) to 7% of total Fe when all Fe is sorbed. At low pH, where the pre-sorption edge plateau occurs (2 < pH < 7.5), the total sorbed-Fe amount remained constant but, within this sorbed-Fe pool, the Fe(III)/Fe(II) ratio increased with pH, from 0.14 at pH 2 up to 0.74 at pH 7. The pre-sorption edge plateau is interpreted as cation exchange on interlayer surfaces together with a sorption phenomenon occurring on highly reactive (i.e. high affinity) surface sites. As pH increases and protons are removed from the clay edge surface, we propose that more and more of these highly reactive sites acquire a steric configuration that stabilizes Fe(III) relative to Fe(II), thereby inducing a Fe to clay particle electron transfer. A sorption model based on cation exchange combined with surface complexation and electron transfers reproduces both wet chemical as well as the Mössbauer spectrometric results. The mechanism is fully reversible: sorbed-Fe is reduced as pH decreases (Mössbauer solid-state analyses) and all Fe returned to solution is returned as Fe(II) (solution analyses). This would not be the case if the observed oxidations were due to contaminant oxidizing agents in solution. The present work shows that alternating pH may induce surface redox phenomena in the absence of an electron acceptor in solution other than H2O.  相似文献   

10.
Sorption of Ni(II) onto chlorite surfaces was studied as a function of pH (5–10), ionic strength (0.01–0.5 M) and Ni concentration (10−8–10−6 M) in an Ar atmosphere using batch sorption with radioactive 63Ni as tracer. Such studies are important since Ni(II) is one of the major activation products in spent nuclear fuel and sorption data on minerals such as chlorite are lacking. The sorption of Ni(II) onto chlorite was dependent on pH but not ionic strength, which indicates that the process primarily comprises sorption by surface complexation. The maximum sorption was at pH ∼ 8 (Kd = ∼10−3 cm3/g). Desorption studies over a period of 1–2 weeks involving replacement of the aqueous solution indicated a low degree of desorption. The acid–base properties of the chlorite mineral were determined by titration and described using a non-electrostatic surface complexation model in FITEQL. A 2-pK NEM model and three surface complexes, Chl_OHNi2+, Chl_OHNi(OH)+ and Chl_OHNi(OH)2, gave the best fit to the sorption results using FITEQL. The high Kd values and low degree of desorption observed indicate that under expected groundwater conditions, a large fraction of Ni(II) that is potentially leachable from spent nuclear fuel may be prevented from migrating by sorption onto chlorite surfaces.  相似文献   

11.
Sorptive stabilization of organic matter in soils by hydrous iron oxides   总被引：5，自引：0，他引：5  
Strong correlations between iron oxides (FeOx) and organic matter (OM) in soils have implied the importance of the former in stabilizing the latter. One mechanism thought to be important in this stabilization is sorption. We tested this possibility by reductively dissolving FeOx in a wide variety of soils and measuring the organic carbon (OC) that was solubilized. The OC dissolved from non-FeOx phases via anion exchange was corrected for by parallel control extractions. The resultant pool, reductively soluble OC, made up a minor amount of total soil OC in all but one of these soils, indicating that simple sorption reactions do not stabilize the bulk of soil OC in most mineral soils. OC:Fe ratios in the extracts from 2/3 of these soils were less than 0.22 (wt/wt), consistent with a sorbed state for this OC and showing that OC sorption by FeOx in these soils is limited by the amount of FeOx. The remaining soils had low pH and high OM concentrations; their higher OC:Fe ratios indicate inclusion of precipitated organo-Fe complexes in the extracts, which are likely only partially extracted by our method. The high volumetric ratios of OM to FeOx found in correlations between them from the literature are inconsistent with a dominant sorption control and point instead to stabilization to other mechanisms such as organo-Fe complexes or ternary associations among FeOx, OM and other minerals.  相似文献   

12.
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   

13.
The sorption of Np(V) and Np(IV) onto kaolinite has been studied in the absence and presence of humic acid (HA) in a series of batch equilibrium experiments under different experimental conditions: [Np]0: 1.0 × 10-6 or 1.0 × 10-5 M, [HA]0: 0 or 50 mg/L, I: 0.01 or 0.1 M NaClO4, solid to liquid ratio: 4 g/L, pH: 6–11, anaerobic or aerobic conditions, without or with carbonate. The results showed that the Np(V) sorption onto kaolinite is affected by solution pH, ionic strength, Np concentration, presence of carbonate and HA. In the absence of carbonate, the Np(V) uptake increased with pH up to ∼96% at pH 11. HA further increased the Np(V) sorption between pH 6 and 9 but decreased the Np(V) sorption between pH 9 and 11. In the presence of carbonate, the Np(V) sorption increased with pH and reached a maximum of 54% between pH 8.5 and 9. At higher pH values, the Np(V) sorption decreased due to the presence of dissolved neptunyl carbonate species with a higher negative charge that were not sorbed onto the kaolinite surface which is negatively charged in this pH range. HA again decreased the Np(V) uptake in the near-neutral to alkaline pH range due to formation of aqueous neptunyl humate complexes. The decrease of the initial Np(V) concentration from 1.0 × 10−5 M to 1.0 × 10−6 M led to a shift of the Np(V) adsorption edge to lower pH values. A higher ionic strength increased the Np(V) uptake onto kaolinite in the presence of carbonate but had no effect on Np(V) uptake in the absence of carbonate.  相似文献   

14.
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   

15.
Zinc uptake in suspensions (?3.7 g L−1) of MX80 montmorillonite was investigated at pH 4.0 and 7.3, a total Zn concentration ([Zn]total) of 500 μM, and dissolved Si concentrations ([Si]aq) of ∼70 and ∼500 μM in 0.5 M NaCl, by kinetics experiments and polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy. Differential thermogravimetric analysis verified the cis-vacant character of the montmorillonite. No Zn uptake occurred at pH 4.0, confirming that cation exchange was hampered by the high ionic strength of the suspension. At pH 7.3 and low [Si]aq (∼70 μM), Zn uptake occurred rapidly during the first hour of reaction, and then leveled off to 50 μmol/g montmorillonite at 168 h. The uptake rate is consistent with Zn sorption on pH-dependent edge sites. At pH 7.3 and high [Si]aq (∼500 μM), the initial sorption rate was similar, but Zn sorption continued, reaching 130 μmol/g at 168 h, and was paralleled by Si uptake with a Si/Zn uptake ratio of 1.51(10), suggesting formation of a Zn (hydrous) silicate. P-EXAFS data indicated that the first oxygen coordination shell of sorbed Zn is split into two subshells at 1.97(2) and 2.08(3)-2.12(2) Å for all EXAFS samples. These two distances are assigned to a mixture of tetrahedral (IVZn) and octahedral (VIZn) Zn complexes. The proportion of IVZn was lower in the high [Si]aq samples and decreased with reaction time. Al low [Si]aq and 216 h of reaction, nearest cationic shells of 0.6(4) Al in the film plane and 0.5(4) Si out of the film plane were detected at 3.00(2) and 3.21(2) Å, respectively, and were interpreted as the formation of IVZn and VIZn mononuclear complexes at the edges of montmorillonite platelets, in structural continuity to the (Al, Mg) octahedral sheets. At high [Si]aq, in-plane Zn and Al and out-of-plane Si neighbors were detected at 4 h, indicating the formation of Zn phyllosilicate nuclei at the layer edges. At 313 h, Zn-Al pairs were no longer detected, and Zn atoms were surrounded on average by 3.4(5) in-plane Zn at 3.10(1) Å and 1.7(9) out-of-plane Si at 3.30(2) Å, supporting the precipitation of a Zn phyllosilicate. Thus, dioctahedral Al phyllosilicate may act as a nucleating surface for the heterogeneous formation of trioctahedral Zn phyllosilicate at [Si]aq relevant to natural systems.  相似文献   

16.
Fourier transformed infrared (FTIR) spectroscopy was used to characterize arsenate-ferrihydrite sorption solids synthesized at pH 3-8. The speciation of sorbed arsenate was determined based on the As-O stretching vibration bands located at 650-950 cm−1 and O-H stretching vibration bands at 3000-3500 cm−1. The positions of the As-O and O-H stretching vibration bands changed with pH indicating that the nature of surface arsenate species on ferrihydrite was strongly pH dependent. Sorption density and synthesis media (sulfate vs. nitrate) had no appreciable effect. At acidic pH (3, 4), ferric arsenate surface precipitate formed on ferrihydrite and constituted the predominant surface arsenate species. X-ray diffraction (XRD) analyses of he sorption solids synthesized at elevated temperature (75 °C), pH 3 clearly showed the development of crystalline ferric arsenate (i.e. scorodite). In neutral and alkaline media (pH 7, 8), arsenate sorbed as a bidentate surface complex (in both protonated FeO2As(O)(OH)− and unprotonated forms). For the sorption systems in slightly acidic media (pH 5, 6), both ferric arsenate and surface complex were probably present on ferrihydrite. It was further determined that the incorporated sulfate in ferrihydrite during synthesis was substituted by arsenate and was more easily exchangeable with increasing pH.  相似文献   

17.
Large portions of organic N (ON) in soil exist tightly associated with minerals. Mineral effects on the type of interactions, chemical composition, and stability of ON, however, are poorly understood. We investigated mineral-associated ON along a Hawaiian soil chronosequence (0.3-4100 kyr) formed in basaltic tephra under comparable climatic, topographic, and vegetation conditions. Mineral-organic associations were separated according to density (ρ > 1.6 g/cm3), characterized by X-ray photoelectron spectroscopy (XPS), X-ray absorption near edge fine structure (NEXAFS) and analyzed for amino acid enantiomers and amino sugars. The 14C activity of mineral-bound OC was estimated by accelerator mass spectrometry. The close OC-ON relationship (r = 0.96) and XPS results suggest that ON exists incorporated in bulk mineral-bound OM and likely becomes associated with minerals as part of sorbing OM. The youngest site (0.3 kyr), with soils mainly composed of primary minerals (olivine, pyroxene, feldspar) and with little ON, contained the largest proportion of hydrolyzable amino sugars and amino acids but with a small share of acidic amino acids (aspartic acid, glutamic acid). In soils of the intermediate weathering stage (20-400 kyr), where poorly crystalline minerals and metal(hydroxide)-organic precipitates prevail, more mineral-associated ON was present, containing a smaller proportion of hydrolyzable amino sugars and amino acids due to the preferential accumulation of other OM components such as lignin-derived phenols. Acidic amino acids were more abundant, reflecting the strong association of acidic organic components with metal(hydroxide)-organic precipitates and variable-charge minerals. In the final weathering stage (1400-4100 kyr) with well-crystalline secondary Fe and Al (hydr)oxides and kaolin minerals, mineral-organic associations held less ON and were, relative to lignin phenols, depleted in hydrolyzable amino sugars and amino acids, particularly in acidic amino acids. XPS and NEXAFS analyses showed that the majority (59-78%) of the mineral-associated ON is peptide N while 18-34% was aromatic N. Amino sugar ratios and d-alanine suggest that mineral-associated ON comprises a significant portion of bacterial residues, particularly in the subsoil. With increasing 14C age, a larger portion of peptide N was non-hydrolyzable, suggesting the accumulation of refractory compounds with time. The constant d/l ratios of lysine in topsoils indicate fresh proteinous material, likely due to continuous sorption of or exchange with fresh N-containing compounds. The 14C and the d/l signature revealed a longer turnover of proteinous components strongly bound to minerals (not NaOH-NaF-extractable). This study provides evidence that interactions with minerals are important in the transformation and stabilization of soil ON. Mineral-associated ON in topsoils seems actively involved in the N cycling of the study ecosystems, accentuating N limitation at the 0.3-kyr site but increasing N availability at older sites.  相似文献   

18.
We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ65Cu‰ = ((65Cu/63Cusample/65Cu/63CuNIST 976 standard) − 1) ? 103. Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰.  相似文献   

19.
Diffusion coefficients for oxygen and hydrogen were determined from a series of natural uraninite-H2O experiments between 50 and 700 °C. Under hydrous conditions there are two diffusion mechanisms: (1) an initial extremely fast-path diffusion mechanism that overprinted the oxygen isotopic composition of the entire crystals regardless of temperature and (2) a slower volume-diffusive mechanism dominated by defect clusters that displace or eject nearest neighbor oxygen atoms to form two interstitial sites and two partial vacancies, and by vacancy migration. Using the volume diffusion coefficients in the temperature range of 400-600 °C, diffusion coefficients for oxygen can be represented by D = 1.90e−5 exp (−123,382 J/RT) cm2/s and for temperatures between 100 and 300 °C the diffusion coefficients can be represented by D = 1.95e−10 exp (−62484 J/RT) cm2/s, where the activation energies for uraninite are 123.4 and 62.5 kJ/mol, respectively. Hydrogen diffusion in uraninite appears to be controlled by similar mechanisms as oxygen. Using the volume diffusion coefficients for temperatures between 50 and 700 °C, diffusion coefficients for hydrogen can be represented by D = 9.28e−6 exp (−156,528 J/RT) cm2/s for temperatures between 450 and 700 °C and D = 1.39e−14 exp (−34518 J/RT) cm2/s for temperatures between 50 and 400 °C, where the activation energies for uraninite are 156.5 and 34.5 kJ/mol, respectively.Results from these new experiments have implications for isotopic exchange during natural UO2-water interactions. The exceptionally low δ18O values of natural uraninites (i.e. −32‰ to −19.5‰) from unconformity-type uranium deposits in Saskatchewan, in conjunction with theoretical and experimental uraninite-water and UO3-water fractionation factors, suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals. The low δ18O values have been interpreted as resulting from the low temperature overprinting of primary uranium mineralization in the presence of relatively modern meteoric fluids having δ18O values of ca. −18‰, despite petrographic and U-Pb isotope data that indicate limited alteration. Our data show that the anomalously low oxygen isotopic composition of the uraninite from the Athabasca Basin can be due to meteoric water overprinting under reducing conditions, and meteoric water or groundwater can significantly affect the oxygen isotopic composition of spent nuclear fuel in a geologic repository, with minimal change to the chemical composition or texture. Moreover, the rather fast oxygen and hydrogen diffusion coefficients for uraninite, especially at low temperatures, suggest that oxygen and hydrogen diffusion may impart characteristic isotopic signals that can be used to track the route of fissile material.  相似文献   

20.
The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe2+, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe2+ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe2+, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe2+ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg−1). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H2 species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.  相似文献   

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