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1.
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   

2.
The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH2PO4+, FeHPO4 and FePO4− have been determined for the first time. FePO4− was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M−1 s−1. FeH2PO4+ and FeHPO4 were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10−2 M−1 s−1 and (1.2 ± 0.8) × 10−1 M−1 s−1, respectively.  相似文献   

3.
This paper presents the results of extensive field trials measuring rates of Fe(II) oxidation at a number of Fe-bearing mine drainage discharges in the UK. Batch experiments were carried out with samples taken at regular intervals and Fe(II) concentration determined spectrophotometrically using 2′2-bipyridyl as the complexing agent. Initial concentrations for Fe(II) were 5.65-76.5 mg/L. Temperature, pH and dissolved O2 (DO) were logged every 10 s, with pH at the start of the experiments in the range 5.64-6.95 and alkalinity ranging from 73 to 741 mg/L CaCO3 equivalent. A numerical model based on a fourth order Runge-Kutta method was developed to calculate values for k1, the rate constant for homogeneous oxidation, from the experimental data. The measured values of pH, temperature, [Fe(II)] and DO were input into the model with resulting values for k1 found to be in the range 2.7 × 1014-2.7 × 1016 M−2 atm−1 min−1. These values for k1 are 1-3 orders of magnitude higher than previously reported for laboratory studies at a similar pH. Comparison of the observed Fe(II) oxidation rates to data published by other authors show a good correlation with heterogenous oxidation rates and may indicate the importance of autocatalysis in these systems. These higher than expected rates of Fe oxidation could have a significant impact on the design of treatment schemes for the remediation of mine drainage and other Fe-bearing ground waters in the future.  相似文献   

4.
Sorption and catalytic oxidation of Fe(II) at the surface of calcite   总被引：1，自引：0，他引：1  
The effect of sorption and coprecipitation of Fe(II) with calcite on the kinetics of Fe(II) oxidation was investigated. The interaction of Fe(II) with calcite was studied experimentally in the absence and presence of oxygen. The sorption of Fe(II) on calcite occurred in two distinguishable steps: (a) a rapid adsorption step (seconds-minutes) was followed by (b) a slower incorporation (hours-weeks). The incorporated Fe(II) could not be remobilized by a strong complexing agent (phenanthroline or ferrozine) but the dissolution of the outmost calcite layers with carbonic acid allowed its recovery. Based on results of the latter dissolution experiments, a stoichiometry of 0.4 mol% Fe:Ca and a mixed carbonate layer thickness of 25 nm (after 168 h equilibration) were estimated. Fe(II) sorption on calcite could be successfully described by a surface adsorption and precipitation model (Comans & Middelburg, GCA51 (1987), 2587) and surface complexation modeling (Van Cappellen et al., GCA57 (1993), 3505; Pokrovsky et al., Langmuir16 (2000), 2677). The surface complex model required the consideration of two adsorbed Fe(II) surface species, >CO3Fe+ and >CO3FeCO3H0. For the formation of the latter species, a stability constant is being suggested. The oxidation kinetics of Fe(II) in the presence of calcite depended on the equilibration time of aqueous Fe(II) with the mineral prior to the introduction of oxygen. If pre-equilibrated for >15 h, the oxidation kinetics was comparable to a calcite-free system (t1/2 = 145 ± 15 min). Conversely, if Fe(II) was added to an aerated calcite suspension, the rate of oxidation was higher than in the absence of calcite (t1/2 = 41 ± 1 min and t1/2 = 100 ± 15 min, respectively). This catalysis was due to the greater reactivity of the adsorbed Fe(II) species, >CO3FeCO3H0, for which the species specific rate constant was estimated.  相似文献   

5.
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   

6.
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   

7.
79Se is a potentially mobile long-lived fission product, which may make a dominant contribution to the long-term radiation exposure resulting from deep geological disposal of radioactive waste. Its mobility is affected by sorption on minerals. Selenium sorption processes have been studied mainly by considering interaction with a single mineral surface. In the case of multi-component systems (e.g. soils), it is difficult to predict the radioelement behaviour only from the mineral constituents. This study contributes to the understanding of multi-component controls of Se concentrations towards predicting Se behaviour in soils after migration from a disposal site. This goal was approached by measuring selenite sorption on mono and multi-phase systems physically separated by dialysis membranes. To the best of the authors’ knowledge, very few studies have used dialysis membranes to study the sorption competition of selenite between several mineral phases. Other workers have used this method to study the sorption of pesticides on montmorillonite in the presence of dissolved organic matter. Indeed, this method allows measurement of individual Kd in a system composed of several mineral phases. Dialysis membranes allowed (i) determination of the competition of two mineral phases for selenite sorption (ii) and determination of the role of humic acids (HAs) on selenite sorption in oxidising conditions. Experimental results at pH 7.0 show an average Se(IV) sorption distribution coefficient (Kd) of approximately 125 and 9410 L kg−1 for bentonite and goethite, respectively. The average Kd for goethite decreases to 613 L kg−1 or 3215 L kg−1 in the presence of bentonite or HA, respectively. For bentonite, the average Kd decreases slightly in the presence of goethite (60 L kg−1) and remains unchanged in the presence of HA. The experimental data were successfully modelled with a surface complexation model using the PHREEQC geochemical code. The drastic decrease in Se(IV) sorption on goethite in a multi-phase system is attributed to competition with dissolved silica released by bentonite. As with Si the HA compete with Se for sorption sites on goethite.  相似文献   

8.
The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 × 10−9 atm O2). The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Fe/Tc(IV) form was observed when 11 μmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. Heterogeneous reduction predominated after approximately 0.01 mmol/L of Fe(II) was oxidized. The heterogeneous reaction was more rapid, and was catalyzed by Fe(II) that adsorbed to the Fe/Tc(IV) redox product. Wet chemical and Fe-X-ray absorption near edge spectroscopy measurements (XANES) showed that Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox products after reaction termination. 57Fe-Mössbauer, extended X-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements revealed that the Fe/Tc(IV) solid phase was poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with an octahedral Tc(IV) dimer bound in bidentate edge-sharing mode to octahedral Fe(III) associated with surface or vacancy sites in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2·nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2·nH2O(s) as a result of its intraparticle/intragrain residence. Precipitates of this nature may form in anoxic sediments or groundwaters, and the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.  相似文献   

9.
Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared 56Fe/54Fe ratios of coexisting aqueous iron (Fe(II)aq, Fe(III)aq) and iron oxyhydroxide precipitates (Fe(III)ppt) resulting from the oxidation of ferrous iron under experimental conditions at low pH (<3). Experiments were carried out using both pure cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO42− and Cl− salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, 56Fe/54Fe ratios of residual Fe(II)aq sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor αFe(III)aq-Fe(II)aq ∼ 1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (∼1.0034) and that predicted for isotopic equilibrium between Fe(II)aq and Fe(III)aq (∼1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in 56Fe/54Fe ratios between Fe(III)aq, isolated as a solid by the addition of NaOH to the final solution at each time point under N2-atmosphere, and Fe(II)aq was in most cases and on average close to 2.9‰ (αFe(III)aq-Fe(II)aq ∼ 1.0029), consistent with isotopic equilibrium between Fe(II)aq and Fe(III)aq. The ferric iron precipitation experiments revealed that 56Fe/54Fe ratios of Fe(III)aq were generally equal to or greater than those of Fe(III)ppt, and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record.  相似文献   

10.
Proton binding constants for the edge and basal surface sites of kaolinite were determined by batch titration experiments at 25 °C in the presence of 0.1 M, 0.01 M and 0.001 M solutions of NaNO3 and in the pH range 3-9. By optimizing the results of the titration experiments, the ratio of the edge sites to the basal surface sites was found to be 6:1. The adsorption of Cd(II), Cu(II), Ni(II), Zn(II) and Pb(II) onto kaolinite suspensions was investigated using batch adsorption experiments and results suggested that in the lower pH range the metallic cations were bound through non-specific ion exchange reactions on the permanently charged basal surface sites (X−). Adsorption on these sites was greatly affected by ionic strength. With increasing pH, the variable charged edge sites (SOH) became the major adsorption sites and inner-sphere specifically adsorbed monodentate complexes were believed to be formed. The effect of ionic strength on the extent of adsorption of the metals on the variable charged edge sites was much less than those on the permanently charged sites. Two binding constants, log K(X2Me) and log K(SOMe), were calculated by optimizing these constants in the computer program FITEQL. A model combining non-specific ion exchange reactions and inner-sphere specific surface complexations was developed to predict the adsorption of heavy metals onto kaolinite in the studied pH range. Linear free energy relationships were found between the edge site binding constants and the first hydrolysis constants of the metals.  相似文献   

11.
To assess the dynamics of microbially mediated U-clay redox reactions, we examined the reduction of iron(III)-rich nontronite NAu-2 and uranium(VI) by Shewanella oneidensis MR-1. Bioreduction experiments were conducted with combinations and varied concentrations of MR-1, nontronite, U(VI) and the electron shuttle anthraquinone-2,6-disulfonate (AQDS). Abiotic experiments were conducted to quantify U(VI) sorption to NAu-2, the reduction of U(VI) by chemically-reduced nontronite-Fe(II), and the oxidation of uraninite, U(IV)O2(s), by nontronite-Fe(III). When we incubated S. oneidensis MR-1 at lower concentration (0.5 × 108 cell mL−1) with nontronite (5.0 g L−1) and U(VI) (1.0 mM), little U(VI) reduction occurred compared to nontronite-free incubations, despite the production of abundant Fe(II). The addition of AQDS to U(VI)- and nontronite-containing incubations enhanced both U(VI) and nontronite-Fe(III) reduction. While U(VI) was completely reduced by S. oneidensis MR-1 at higher concentration (1.0 × 108 cell mL−1) in the presence of nontronite, increasing concentrations of nontronite led to progressively slower rates of U(VI) reduction. U(VI) enhanced nontronite-Fe(III) reduction and uraninite was oxidized by nontronite-Fe(III), demonstrating that U served as an effective electron shuttle from S. oneidensis MR-1 to nontronite-Fe(III). The electron-shuttling activity of U can explain the lack or delay of U(VI) reduction observed in the bulk solution. Little U(VI) reduction was observed in incubations that contained chemically-reduced nontronite-Fe(II), suggesting that biologic U(VI) reduction drove U valence cycling in these systems. Under the conditions used in these experiments, we demonstrate that iron-rich smectite may inhibit or delay U(VI) bioreduction.  相似文献   

12.
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   

13.
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   

14.
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   

15.
Microbial Mn(II) oxidation kinetics in response to oxygen concentration were assessed in suboxic zone water at six sites throughout the Black Sea. Mn(II) oxidation rates increased asymptotically with increasing oxygen concentration, consistent with Michaelis-Menten enzyme kinetics. The environmental half-saturation constant, KE, of Mn(II) removal (oxidation) varied from 0.30 to 10.5 μM dissolved oxygen while the maximal environmental rate, VE−max, ranged from 4 to 50 nM h−1. These parameters varied spatially and temporally, consistent with a diverse population of enzymes catalyzing Mn oxide production in the Black Sea. Coastally-influenced sites produced lower KE and higher VE−max constants relative to the Western and Eastern Gyre sites. In the Bosporus Region, the Mn(II) residence time calculated using our KE and VE−max values with 0.1 μM oxygen was 4 days, 25-fold less than previous estimates. Our results (i) indicate that rapid Mn(II) oxidation to solid phase Mn oxides in the Black Sea’s suboxic zone is stimulated by oxygen concentrations well below the 3-5 μM concentration reliably detected by current oceanographic methods, (ii) suggest the existence of multiple, diverse Mn(II)-oxidizing enzymes, (iii) are consistent with shorter residence times than previously calculated for Mn(II) in the suboxic zone and (iv) cast further doubt on the existence of proposed reactions coupling solid phase Mn oxide production to electron acceptors other than oxygen.  相似文献   

16.
The oxidation of Fe(II) with H2O2 at nanomolar levels in seawater have been studied using an UV-Vis spectrophotometric system equipped with a long liquid waveguide capillary flow cell. The effect of pH (6.5 to 8.2), H2O2 (7.2 × 10−8 M to 5.2 × 10−7 M), HCO3− (2.05 mM to 4.05 mM) and Fe(II) (5 nM to 500 nM) as a function of temperature (3 to 35 °C) on the oxidation of Fe(II) are presented. The oxidation rate is linearly related to the pH with a slope of 0.89 ± 0.01 independent of the concentration of HCO3−. A kinetic model for the reaction has been developed to consider the interactions of Fe(II) with the major ions in seawater. The model has been used to examine the effect of pH, concentrations of Fe(II), H2O2 and HCO3− as a function of temperature. FeOH+ is the most important contributing species to the overall rate of oxidation from pH 6 to pH 8. At a pH higher than 8, the Fe(OH)2 and Fe(CO3)22− species contribute over 20% to the rates. Model results show that when the concentration of O2 is two orders of magnitude higher than the concentration of H2O2, the oxidation with O2 also needs to be considered. The rate constants for the five most kinetically active species (Fe2+, FeOH+, Fe(OH)2, FeCO3, Fe(CO3)22−) in seawater as a function of temperature have been determined. The kinetic model is also valid in pure water with different concentrations of HCO3− and the conditions found in fresh waters.  相似文献   

17.
With a half-life of 15.7 Ma, a high mobility and the potential to accumulate in the biosphere, 129I is considered, in safety assessment calculations for radioactive waste repositories, to be one of the main contributors to the radiological dose. Several authors have reported that, at low concentration, I− is weakly retained on argillaceous rocks. This process is not yet well-understood and different hypotheses have been put forward as to whether reactive phases or experimental artifacts (e.g. pyrite oxidation) could be the reason for the retention of I− observed at low concentration. The aim of this study was to investigate the effect on I− mobility of (i) the redox conditions and (ii) the amount of pyrite and natural organic matter (NOM) contents of the rock. These questions were addressed by performing batch sorption, through-diffusion and out-diffusion experiments on rock samples of Toarcian argillaceous rock from Tournemire (Aveyron, France). One of the challenges faced during this study was to distinguish actual transport properties from experimental artifacts. A especially elaborate experimental set-up allowed limiting the (i) oxidation of both argillaceous rock and I−, and (ii) carbonate precipitation. A comparison of the batch sorption results obtained for two Toarcian clay specimens, that differed in their amount of pyrite and NOM, allowed relating I− sorption to pyrite oxidation. However, no evidence was found to associate the I− behavior to the NOM amounts. While the through-diffusion experiments showed a very slight sorption (distribution ratio (Rd) = 0.016 mL g−1) for the lowest I− concentration under oxic conditions, the out-diffusion tests performed after the through-diffusion experiments on the same cells showed significant sorption under both oxic and anoxic conditions, resulting in Rd ranging from 0.02 mL g−1 to 1.25 mL g−1. The range of Rd values was higher for the upstream reservoir under oxic conditions. The discrepancies observed between the through-diffusion and the out-diffusion experiments suggest a kinetic control of the I− uptake by argillaceous rocks under oxic and anoxic conditions.  相似文献   

18.
The potential for reduction of 99TcO4−(aq) to poorly soluble 99TcO2 · nH2O(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO2 · nH2O(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO4−(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h−1), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and 57Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O4− were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO4−(aq) associated with groundwater contaminant plumes.  相似文献   

19.
Methylmercury can accumulate in fish to concentrations unhealthy for humans and other predatory mammals. Most sources of mercury (Hg) emit inorganic species to the environment. Therefore, ecological harm occurs when inorganic Hg is converted to methylmercury. Sulfate- and iron-reducing bacteria (SRB and FeRB) methylate Hg, but the effects of processes involving oxidized and reduced forms of sulfur and iron on the reactivity of Hg, including the propensity of inorganic Hg to be methylated, are poorly understood. Under abiotic conditions, using a laboratory flow reactor, bisulfide (HS−) was added at 40 to 250 μM h−1 to 5 g L−1 goethite (α-FeOOH) suspensions to which Hg(II) was adsorbed (30-100 nmol m−2) at pH 7.5. Dissolved Hg initially decreased from 103 or 104 nM (depending on initial conditions) to 10−1 nM, during which the concentration of Hg(II) adsorbed to goethite decreased by 80% and metacinnabar (β-HgS(s)) formed, based on identification using Hg LIII-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. The apparent coordination of oxygens surrounding Hg(II), measured with EXAFS spectroscopy, increased during one flow experiment, suggesting desorption of monodentate-bound Hg(II) while bidentate-bound Hg(II) persisted on the goethite surface. Further sulfidation increased dissolved Hg concentrations by one to two orders of magnitude (0.5 to 10 nM or 30 nM), suggesting that byproducts of bisulfide oxidation and Fe(III) reduction, primarily polysulfide and potentially Fe(II), enhanced the dissolution of β-HgS(s) and/or desorption of Hg(II). Rapid accumulation of Fe(II) in the solid phase (up to 40 μmol g−1) coincided with faster elevation of dissolved Hg concentrations. Fe(II) served as a proxy for elemental sulfur [S(0)], as S(0) was the dominant bisulfide oxidation product coupled to Fe(III) reduction, based on sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy. In one experiment, dissolved Hg concentrations tracked those of all sulfide species [S(-II)]. These results suggest that S(-II) reacted with S(0) to form polysulfide, which then caused the dissolution of β-HgS(s). A secondary Fe-bearing phase resembling poorly formed green rust was observed in sulfidized solids with scanning electron microscopy, although there was no clear evidence that either surface-bound or mineralized Fe(II) strongly affected Hg speciation. Examination of interrelated processes involving S(-II) and Fe(III) revealed new modes of Hg solubilization previously not considered in Hg reactivity models.  相似文献   

20.
The adsorption of five toxic metallic cations, Cd(II), Cu(II), Ni(II), Pb(II) and Zn(II), onto montmorillonite was investigated as a function of pH and ionic strength and a two-site surface complexation model was used to predict the adsorption data. The results showed that in the lower pH range, 3∼6 for Cd, Cu, Ni and Zn, and 3∼4.5 for Pb, the adsorption was greatly affected by ionic strength, while in the higher pH range, the adsorption was not. In the lower pH range, the metallic cations were mainly bound through the formation of outer-sphere surface on the permanently charged basal surface sites (≡X−), while in the higher pH range the adsorption occurred mainly on the variably charged edge sites (≡SOH) through the formation of inner-sphere surface complexes. Acid-base surface constants and metal binding constants for the two sites were optimized using FITEQL. The adsorption affinity of the five metallic cations to the permanently charged sites of montmorillonite was Pb > Cu > Ni ≈ Zn ≈ Cd, while that to the variable charged sites was Pb ? Cu > Zn > Cd > Ni.  相似文献   

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