Desorption kinetics of radiocesium from subsurface sediments at Hanford Site, USA     

Chongxuan Liu  John M. Zachara  James P. McKinley 《Geochimica et cosmochimica acta》2003,67(16):2893-2912

The desorption of ¹³⁷Cs⁺ was investigated on sediments from the United States Hanford site. Pristine sediments and ones that were contaminated by the accidental release of alkaline ¹³⁷Cs⁺-containing high level nuclear wastes (HLW, 2 × 10⁶ to 6 × 10⁷ pCi ¹³⁷Cs⁺/g) were studied. The desorption of ¹³⁷Cs⁺ was measured in Na⁺, K⁺, Rb⁺, and NH₄⁺electrolytes of variable concentration and pH, and in presence of a strong Cs⁺-specific sorbent (self-assembled monolayer on a mesoporous support, SAMMS). ¹³⁷Cs⁺ desorption from the HLW-contaminated Hanford sediments exhibited two distinct phases: an initial instantaneous release followed by a slow kinetic process. The extent of ¹³⁷Cs⁺ desorption increased with increasing electrolyte concentration and followed a trend of Rb⁺ ≥ K⁺ > Na⁺ at circumneutral pH. This trend followed the respective selectivities of these cations for the sediment. The extent and rate of ¹³⁷Cs⁺ desorption was influenced by surface armoring, intraparticle diffusion, and the collapse of edge-interlayer sites in solutions containing K⁺, Rb⁺, or NH₄⁺. Scanning electron microscopic analysis revealed HLW-induced precipitation of secondary aluminosilicates on the edges and basal planes of micaceous minerals that were primary Cs⁺ sorbents. The removal of these precipitates by acidified ammonium oxalate extraction significantly increased the long-term desorption rate and extent. X-ray microprobe analyses of Cs⁺-sorbed micas showed that the ¹³⁷Cs⁺ distributed not only on mica edges, but also within internal channels parallel to the basal plane, implying intraparticle diffusive migration of ¹³⁷Cs⁺. Controlled desorption experiments using Cs⁺-spiked pristine sediment indicated that the ¹³⁷Cs⁺ diffusion rate was fast in Na⁺-electrolyte, but much slower in the presence of K⁺ or Rb⁺, suggesting an effect of edge-interlayer collapse. An intraparticle diffusion model coupled with a two-site cation exchange model was used to interpret the experimental results. Model simulations suggested that about 40% of total sorbed ¹³⁷Cs⁺ was exchangeable, including equilibrium and kinetic desorbable pools. At pH 3, this ratio increased to 60-80%. The remainder of the sorbed ¹³⁷Cs⁺ was fixed or desorbed at much slower rate than our experiments could detect.  相似文献   

Sorption of Cs to micaceous subsurface sediments from the Hanford site, USA   总被引：1，自引：0，他引：1  

John M Zachara  Steven C SmithChongxuan Liu  James P McKinleyR.Jeffrey Serne  Paul L Gassman 《Geochimica et cosmochimica acta》2002,66(2):193-211

The sorption of Cs⁺ was investigated over a large concentration range (10⁻⁹−10⁻² mol/L) on subsurface sediments from a United States nuclear materials site (Hanford) where high-level nuclear wastes (HLW) have been accidentally released to the vadose zone. The sediment sorbs large amounts of radiocesium, but expedited migration has been observed when HLW (a NaNO₃ brine) is the carrier. Cs⁺ sorption was measured on homoionic sediments (Na⁺, K⁺, Ca²⁺) with electrolyte concentrations ranging from 0.01 to 1.0 mol/L. In Na⁺ electrolyte, concentrations were extended to near saturation with NaNO_3(s) (7.0 mol/L). The sediment contained nonexpansible (biotite, muscovite) and expansible (vermiculite, smectite) phyllosilicates. The sorption data were interpreted according to the frayed edge-planar site conceptual model. A four-parameter, two-site (high- and low-affinity) numeric ion exchange model was effective in describing the sorption data. The high-affinity sites were ascribed to wedge zones on the micas where particle edges have partially expanded due to the removal of interlayer cations during weathering, and the low-affinity ones to planar sites on the expansible clays. The electrolyte cations competed with Cs⁺ for both high- and low-affinity sites according to the trend K⁺ >> Na⁺ ≥ Ca²⁺. At high salt concentration, Cs⁺ adsorption occurred only on high-affinity sites. Na⁺ was an effective competitor for the high-affinity sites at high salt concentrations. In select experiments, silver-thiourea (AgTU) was used as a blocking agent to further isolate and characterize the high-affinity sites, but the method was found to be problematic. Mica particles were handpicked from the sediment, contacted with Cs⁺_(aq), and analyzed by electron microprobe to identify phases and features important to Cs⁺ sorption. The microprobe study implied that biotite was the primary contributor of high-affinity sites because of its weathered periphery. The poly-phase sediment exhibited close similarity in ion selectivity to illite, which has been well studied, although its proportion of high-affinity sites relative to the cation exchange capacity (CEC) was lower than that of illite. Important insights are provided on how Na⁺ in HLW and indigenous K⁺ displaced from the sediments may act to expedite the migration of strongly sorbing Cs⁺ in subsurface environments.  相似文献   

Radiocaesium dispersion and fixation in the lagoon systems of “Ria de Aveiro”, Portugal     

Iuliu Bobos  Fernando Rocha 《Journal of Geochemical Exploration》2006,88(1-3):367

Radiocaesium (¹³⁷Cs) dispersion and Cs⁺ fixation were studied in the sediments collected from the lagoon systems of “Ria de Aveiro”. The Cs⁺ sorption was tested for the fine mica grains and for the < 2 μm clay fractions extracted from silty clays. The Cs⁺ exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs⁺ engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the ¹³⁷Cs were found associated with mica-rich silty clays. The ¹³⁷Cs ranges from 3.2 to 3.9 Bq kg^− 1 in the < 38 μm fractions and from 2.9 to 3.3 Bq kg^− 1 in the < 64 μm fractions.  相似文献   

Modelling of Cs sorption in natural mixed-clays and the effects of ion competition     

《Applied Geochemistry》2014

Cs migration in the environment is mainly controlled by sorption onto mineral surfaces, in particular on clay minerals. With the objective of designing a geochemical reactive barrier to treat ¹³⁷Cs accidental pollution in an industrial waste repository, different natural clayrocks were studied to analyse their capacity to retain Cs.The simple semi-empiric K_d-approach for experimental data analysis, is unsatisfactory to describe the variability of sorption upon chemical changes. Indeed, due to the high salinity of the site, the effects of competitive ions must be evaluated and quantified. Thus, the development of sorption models, capable of reproducing experimental data obtained under conditions representative of the contaminated site, and applicable to reactive transport studies, is needed.In this study, a model for Cs sorption, which takes into account the main mineralogy of the sorbent, the composition of the natural water (and ion competition) was successfully applied to interpret the non-linear Cs sorption under natural conditions.The selectivity coefficients of Cs with respect to the most important cations present in the site water (Na, K, NH₄, Ca) were derived by means of experiments in single clay minerals and synthetic mono-component solutions. Then, these parameters were tested in systems of increasing complexity.Considering the mineralogical composition of raw materials, it was shown that the principal contribution to Cs sorption is given by the mineral illite, while smectite starts to be relevant only at very high Cs loadings. Kaolinite, even in concentrations around 10 wt% of the clayey fraction, played only a minor role.With respect to the solution composition, the model was able to predict Cs sorption in electrolyte concentrations up to twice than that of seawater and up to 500 mg/L NH₄⁺. The effect of highly competing ions, especially NH₄⁺ and K⁺, on Cs retention is more important at low ionic strengths and low Cs loadings, where adsorption is dominated by illite selective frayed edge sites, FES. Divalent cations are not especially relevant as competing cations for Cs.  相似文献   

Thermally induced cation migration in Na and Li montmorillonite     

V. Luca  C. M. Cardile 《Physics and Chemistry of Minerals》1988,16(1):98-103

In order to determine whether Li⁺ cations penetrate into the octahedral layers of montmorillonites upon mild heating (Hofmann-Klemen effect) ⁵⁷Fe Mössbauer spectra of Na⁺ and Li⁺ exchanged montmorillonite were obtained before and after treatment at 220 ° C. The ⁵⁷Fe nucleus was used as a remote probe to detect electronic perturbations which would occur if a Li cation was to move into the octahedral layer from the interlayer after heating. The ambient Mössbauer spectra showed that a high charge density interlayer cation such as Li⁺ is effective in reducing the phonon energy of ^VIFe²⁺. In addition the EFG at octahedral sites can be significantly modified by interlayer cations as evidenced by the larger quadrupole splitting value measured for the Li⁺-exchanged sample with respect to the Na⁺-sample. Interlayer collapse and migration of exchange cations into the montmorillonite lattice after heating to 220 ° C resulted in the oxidation of the ^VIFe²⁺ and a decrease in site distortion for ^IVFe³⁺. Similar spectral parameters for the Fe³⁺ resonances of both Na⁺ — and Li⁺-heated samples suggested the interlayer cations do not penetrate as far as the octahedral layers. In order to utilize the enhanced sensitivity of ^VIFe²⁺ Δ values to changes in EFG the Fe³⁺ in the heated montmorillonites was reduced to Fe²⁺ with hydrazine. Similar spectral parameters for both the Na⁺ — and Li⁺-exchanged montmorillonite were observed giving further evidence that Li cations do not migrate into vacant octahedral sites.  相似文献   

Prediction of solid/liquid distribution coefficients of radiocaesium in soils and sediments. Part three: a quantitative test of a KID predictive equation     

《Applied Geochemistry》1996,11(4):601-603

An equation is presented for predicting solid/liquid distribution coefficients of radiocaesium on soils and sediments (freshwater and marine). The equation is based on the value of the radiocaesium interception potential of the illitic frayed edge sites of the system and liquid phase composition (K, NH₄ and Na). The predictive potential of the equation is tested for some 130 combinations of soil/sediment and water composition covering a three-order-of-magnitude range in K_D^137_Cs values. On average, experimental K_D^137_Cs values exceed predicted values by a factor of 1.62 ± 0.75.  相似文献   

Nature of Co-bearing ferromanganese crusts of the Magellan Seamounts (Pacific Ocean): Communication 2. Ion exchange properties of ore minerals     

G. V. Novikov  S. V. Yashina  M. E. Mel’nikov  I. V. Vikent’ev  O. Yu. Bogdanova 《Lithology and Mineral Resources》2014,49(2):138-164

The results of experimental studies of ion exchange properties of Co-bearing ferromanganese crusts in the Magellan Seamounts (Pacific Ocean) are discussed. Maximum reactivity in reactions with the participation of manganese minerals (Fe-vernadite, vernadite) is typical of Na⁺, K⁺, and Ca²⁺ cations, whereas minimum activity is recorded for cations Pb²⁺ and Co²⁺. The exchange complex of ore minerals in crusts is composed of Na⁺, K⁺, Ca²⁺, Mg²⁺, and Mn²⁺ cations. The exchange capacity of manganese minerals increases from the alkali metal cations to rare and heavy metal cations. Peculiarities of the affiliation of Co²⁺, Mn²⁺, and Mg²⁺ cations in manganese minerals of crusts are discussed. In manganese minerals, Co occurs as Co²⁺ and Co³⁺ cations. Metal cations in manganese minerals occur in different chemical forms: sorbed (Na⁺, K⁺, Ca²⁺, Mn²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺, and Pb²⁺); sorbed and chemically bound (Mg²⁺, Ni²⁺, Y³⁺, La³⁺, and Mo⁶⁺); and only chemically bound (Co³⁺). It is shown that the age of crust, its preservation time in the air-dry state, and type of host substrate do not affect the ion exchange indicators of manganese minerals. It has been established that alkali metal cations are characterized by completely reversible equivalent sorption, whereas heavy metal cations are sorbed by a complex mechanism: equivalent ion exchange for all metal cations; superequivalent, partly reversible sorption for Ba²⁺, Pb²⁺, Co²⁺, and Cu²⁺ cations, relative to exchange cations of manganese minerals. The obtained results refine the role of ion exchange processes during the hydrogenic formation of Co-bearing ferromanganese crusts.  相似文献   

Thermal decomposition of NH<Subscript>4</Subscript><Superscript>+</Superscript>-vermiculite from Santa Olalla (Huelva,Spain) and its relation to the metal ion distribution in the octahedral sheet     

J. L.?Pérez-RodríguezEmail author  J.?Poyato  M. C.?Jiménez de Haro  L. A.?Pérez-Maqueda  A.?Lerf 《Physics and Chemistry of Minerals》2004,31(7):415-420

The investigation of the NH₃ loss in the NH₄⁺-vermiculite (Santa Olalla) by thermogravimetry, evolved gas analysis, chemical analysis, X-ray diffraction and IR spectroscopy is reported here. The mass loss during heating takes place in two steps at about 650 and 825 °C. Additionally, the releases of H₂O and NH₃ occurs simultaneously. The experimental results indicate that the protons remaining in the interlayer space after NH₃ removal trigger the H₂O release. X-ray diffraction shows that during the decomposition of NH₄⁺-vermiculite there are two domains with different interlayer spaces at ~9 and ~10 Å. As the decomposition proceeds, the intensity of the 9 Å peak increases at the expense of the second one. The change in the IR-stretching modes of the structural OH groups during heating indicates that the OH groups surrounded by 3Mg²⁺ or 2Mg²⁺Fe²⁺ are released at lower temperatures than those with environments like 2Mg²⁺Fe³⁺, 2Mg²⁺Al³⁺ or more complex ones.  相似文献   

Torsional mode frequency and elastic anisotropy in alkali vermiculites     

Gupta  H. C.  Mahanti  S. D.  Solin  S. A. 《Physics and Chemistry of Minerals》1988,16(3):291-294

The torsional mode frequency _t in Cs_1–xRb_x-Vermiculite has been determined using an angular force constant model and a virtual crystal approximation. The sensitivity of _t has been examined with respect to the force constants between the Kagome' oxygen frame and the interlayer cations. These force constants were used to calculate the longitudinal elastic constants, C₁₁ and C₃₃ and are consistent with the observed elastic anisotropy in related layer silicates. The observed nonlinear x-dependence of the torsional mode frequency in ternary systems can be related to the polarizability of the interlayer cations (Cs⁺,Rb⁺).  相似文献   

Microelectrode Study of Oxygen Uptake and Organic Matter Decomposition in The Sediments of Xiamen Western Bay   总被引：2，自引：0，他引：2  

Kunming Xu  Liping Zhang  Wenbin Zou 《Estuaries and Coasts》2009,32(3):425-435

Sediment cores were sampled from Xiamen Western Bay at five sites during the summer and winter of 2006 and Hg–Au microelectrodes were used to make on board measurements of the concentration gradients of dissolved oxygen, Mn²⁺, and Fe²⁺ within the sediments. The O₂ concentrations decreased sharply from about 200 μmol L⁻¹ in the bottom seawater to zero within a depth of a few millimeters into the sediment. Dissolved Mn²⁺ was detected below the oxic zones with peak concentrations up to 600 μmol L⁻¹, whereas dissolved Fe²⁺ had peak concentrations up to 1,000 μmol L⁻¹ in deeper layers. The elemental contents of organic carbon and nitrogen within the sediments were analyzed and their C/N ratios were in the range of 9.0 to 10.1, indicative of heavy terrestrial origin. Sediments from two sites near municipal wastewater discharge outlets had higher organic contents than those from the other sites. These high organic contents corresponded to shallow O₂ penetration depths, high dissolved Mn²⁺ and Fe²⁺ concentrations, and negative redox potentials within the sediments. This indicated that the high organic matter content had promoted microbial respiration within the sediments. Overall, the organic content did not show any appreciable decrease with increasing sediment depths, so a quadratic polynomial function was used to fit the curve of O₂ profiles within the sediments. Based on the O₂ profiles, O₂ fluxes across the seawater and sediment interface were estimated to be in the range 6.07 to 14.9 mmol m⁻² day⁻¹, and organic carbon consumption rates within the surface sediments were estimated to be in the range 3.3 to 20.8 mgC cm⁻³ a⁻¹. The case demonstrated that biogeochemistry within the sediments of the bay was very sensitive to human activities such as sewage discharge.  相似文献   

Animated molecular dynamics simulations of hydrated caesium-smectite interlayers     

Rebecca Sutton  Garrison Sposito 《Geochemical transactions》2002,3(1):73

Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs⁺ formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs⁺ within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs⁺ for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs⁺ and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output.  相似文献   

Carbon Cycling and the Coupling Between Proton and Electron Transfer Reactions in Aquatic Sediments in Lake Champlain   总被引：1，自引：0，他引：1  

Wei-Jun Cai  George W. LutherIII  Jeffrey C. Cornwell  Anne E. Giblin 《Aquatic Geochemistry》2010,16(3):421-446

We used fine-scale porewater profiles and rate measurements together with a multiple component transport–reaction model to investigate carbon degradation pathways and the coupling between electron and proton transfer reactions in Lake Champlain sediments. We measured porewater profiles of O₂, Mn²⁺, Fe²⁺, HS⁻, pH and pCO₂ at mm resolution by microelectrodes, and profiles of NO₃ ⁻, SO₄ ²⁻, NH₄ ⁺, total inorganic carbon (DIC) and total alkalinity (TA) at cm resolution using standard wet chemical techniques. In addition, sediment–water fluxes of oxygen, DIC, nitrate, ammonium and N₂ were measured. Rates of gross and net sulfate reduction were also measured in the sediments. It is shown that organic matter (OM) decomposes via six pathways: oxic respiration (35.2%), denitrification (10.4%), MnO₂ reduction (3.6%), FeOOH reduction (9.6%), sulfate reduction (14.9%), and methanogenesis (26.4%). In the lake sediments, about half of the benthic O₂ flux is used for aerobic respiration, and the rest is used for the regeneration of other electron acceptors produced during the above diagenetic reactions. There is a strong coupling between O₂ usage and Mn²⁺ oxidation. MnO₂ is also an important player in Fe and S cycles and in pH and TA balance. Although nitrate concentrations in the overlying water were low, denitrification becomes a quantitatively important pathway for OM decomposition due to the oxidation of NH₄ ⁺ to NO₃ ⁻. Finally, despite its low concentration in freshwater, sulfate is an important electron acceptor due to its high efficiency of internal cycling. This paper also discusses quantitatively the relationship between redox reactions and the porewater pH values. It is demonstrated here that pH and pCO₂ are sensitive variables that reflect various oxidation and precipitation reactions in porewater, while DIC and TA profiles provide effective constraints on the rates of various diagenetic reactions.  相似文献   

Ion exchange properties of oceanic ferromanganese nodules and enclosing pelagic sediments     

G. V. Novikov  I. O. Murdmaa 《Lithology and Mineral Resources》2007,42(2):137-167

Results of the experimental study of ion exchange properties of deep-sea pelagic sediments and related ferromanganese nodules (FMN) are considered. The exchange complex of sediments and nodules includes Na⁺, K⁺, Ca²⁺, and Mg²⁺ cations. The FMNs also include Mn²⁺ cations. Series of reactivity of metal cations during exchange reactions in different types of pelagic clayey sediments and diagenetic-sedimentary FMN are compiled. Series of exchange capacity of the sediments and FMN for alkali and heavy metal cations are also presented. The exchange capacity of FMN is always higher than that of enclosing sediments. Sediments are characterized by reversible equivalent sorption of cations of both alkali and heavy metals. Irrespective of the mineral composition, the FMNs are characterized by the reversible equivalent sorption of alkali metal cations, whereas sorption of heavy metal cations is only partly reversible. More over, alkali metal cations do not replace heavy metal cations. The results obtained refine the role of ion exchange processes in the redistribution of heavy metal cations at the water-bottom sediment interface during the diagenetic-sedimentary formation of ferromanganese nodules.  相似文献   

Structure and dynamics of water on Li-, Na-, K-, Cs-, H₃O-exchanged muscovite surfaces: A molecular dynamics study     

Hiroshi Sakuma  Katsuyuki Kawamura 《Geochimica et cosmochimica acta》2011,75(1):63-23407

The distribution and dynamics of water molecules and monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺, and H₃O⁺) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs⁺ and Cl⁻ ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li⁺ ions near the muscovite surface. For Na⁺, K⁺, Cs⁺, and H₃O⁺ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li⁺, Na⁺, K⁺, and H₃O⁺ ions adsorbed on the muscovite surfaces. The water molecules around Cs⁺ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs⁺ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li⁺ ions and decreased with an increase in the ionic radius.  相似文献   

Impact of highly basic solutions on sorption of Cs to subsurface sediments from the Hanford site, USA     

C.C. Ainsworth  J.M. Zachara  K. Wagnon  S. McKinley  C. Liu  S.C. Smith  H.T. Schaef  P.L. Gassman 《Geochimica et cosmochimica acta》2005,69(20):4787-4800

The effect of caustic NaNO₃ solutions on the sorption of ¹³⁷Cs to a Hanford site micaceous subsurface sediment was investigated as a function of base exposure time (up to 168 d), temperature (10°C or 50°C), and NaOH concentration (0.1 mol/L to 3 mol/L). At 10°C and 0.1 M NaOH, the slow evolution of [Al]_aq was in stark contrast to the rapid increase and subsequent loss of [Al]_aq observed at 50°C (regardless of base concentration). Exposure to 0.1 M NaOH at 10°C for up to 168 d exhibited little if any measurable effect on sediment mineralogy, Cs⁺ sorption, or Cs⁺ selectivity; sorption was well described with a two-site ion exchange model modified to include enthalpy effects. At 50°C, dissolution of phyllosilicate minerals increased with [OH]. A zeolite (tetranatrolite; Na₂Al₂Si₃O₁₀·2H₂O) precipitated in 0.1 M NaOH after about 7 days, while an unnamed mineral phase (Na₁₄Al₁₂Si₁₃O₅₁·6H₂O) precipitated after 4 and 2 days of exposure to 1 M and 3 M NaOH solutions, respectively. Short-term (16 h) Cs⁺ sorption isotherms (10⁻⁹-10⁻² mol/L) were measured on sediment after exposure to 0.1 M NaOH for 56, 112, and 168 days at 50°C. There was a trend toward slightly lower conditional equilibrium exchange constants (Δlog _Na^CsK_c ∼ 0.25) over the entire range of surface coverage, and a slight loss of high affinity sites (15%) after 168 days of pretreatment with 0.1 M base solution. Cs⁺ sorption to sediment over longer times was also measured at 50°C in the presence of NaOH (0.1 M, 1 M, and 3 M NaOH) at Cs⁺ concentrations selected to probe a range of adsorption densities. Model simulations of Cs⁺ sorption to the sediment in the presence of 0.1 M NaOH for 112 days slightly under-predicted sorption at the lower Cs⁺ adsorption densities. At the higher adsorption densities, model simulations under-predicted sorption by 57%. This under-prediction was surmised to be the result of tetranatrolite precipitation, and subsequent slow Na → Cs exchange. At higher OH concentrations, Cs⁺ sorption in the presence of base for 112 days was unexpectedly equal to, or greater than that expected for pristine sediment. The precipitation of secondary phases, coupled with the fairly unique mica distribution and quantity across all size-fractions in the Hanford sediment, appears to mitigate the impact of base dissolution on Cs⁺ sorption.  相似文献   

Post‐deposition diffusion of 137Cs in lake sediment: Implications for radiocaesium dating     

J. KLAMINDER  P. APPLEBY  P. CROOK  I. RENBERG 《Sedimentology》2012,59(7):2259-2267

Peak activities of radiocaesium (¹³⁷Cs) in lake sediments have frequently been used to infer the ages of sediments deposited in the 1960s (¹³⁷Cs derived from nuclear bomb testing) or in 1986 (Chernobyl derived ¹³⁷Cs). Records of the vertical distribution of ¹³⁷Cs in sediments can thus be used to provide accurate dates for a critical period in which palaeoecological reconstructions often overlap contemporary monitoring data. However, knowledge regarding how the distribution of ¹³⁷Cs in sediments is affected by post‐depositional processes is limited to interpretations based on the ¹³⁷Cs distribution in sediments sampled at a single given date. This study assesses the extent to which the ¹³⁷Cs record in annually laminated (varved) lake sediments is affected by post‐depositional diffusion, using 11 archived sediment cores sampled between 1986 and 2007. The sediment record reveals how Chernobyl ¹³⁷Cs incorporated into the 1986 varve diffused downwards in the core at a decreasing rate over time, whereas the surface sediments continued to receive inputs of ¹³⁷Cs mobilized from the catchment soils or lake margin. In spite of these processes, all cores post‐dating the Chernobyl accident had a clear and well‐resolved peak in the 1986 varve, justifying the use of this feature as a fixed chronostratigraphic feature. Because of the very high levels of Chernobyl fallout at this site, downwards migration of Chernobyl ¹³⁷Cs has, however, completely masked the nuclear weapons ¹³⁷Cs fallout peak that had been clearly preserved in the 1964 varve of a pre‐Chernobyl core sampled just three weeks before the Chernobyl accident. In consequence, the weapons fallout marker is likely to be of little use for determining ¹³⁷Cs dates in areas strongly affected by high levels of Chernobyl fallout.  相似文献   

The Effects of Varying Salinity on Ammonium Exchange in Estuarine Sediments of the Parker River,Massachusetts   总被引：1，自引：0，他引：1  

Nathaniel B. Weston  Anne E. Giblin  Gary T. Banta  Charles S. Hopkinson  Jane Tucker 《Estuaries and Coasts》2010,33(4):985-1003

We examined the effects of seasonal salinity changes on sediment ammonium (NH₄ ⁺) adsorption and exchange across the sediment–water interface in the Parker River Estuary, by means of seasonal field sampling, laboratory adsorption experiments, and modeling. The fraction of dissolved NH₄ ⁺ relative to adsorbed NH₄ ⁺ in oligohaline sediments rose significantly with increased pore water salinity over the season. Laboratory experiments demonstrated that small (∼3) increases in salinity from freshwater conditions had the greatest effect on NH₄ ⁺ adsorption by reducing the exchangeable pool from 69% to 14% of the total NH₄ ⁺ in the upper estuary sediments that experience large (0–20) seasonal salinity shifts. NH₄ ⁺ dynamics did not appear to be significantly affected by salinity in sediments of the lower estuary where salinities under 10 were not measured. We further assessed the importance of salinity-mediated desorption by constructing a simple mechanistic numerical model for pore water chloride and NH₄ ⁺ diffusion for sediments of the upper estuary. The model predicted pore water salinity and NH₄ ⁺ profiles that fit measured profiles very well and described a seasonal pattern of NH₄ ⁺ flux from the sediment that was significantly affected by salinity. The model demonstrated that changes in salinity on several timescales (tidally, seasonally, and annually) can significantly alter the magnitude and timing of NH₄ ⁺ release from the sediments. Salinity-mediated desorption and fluxes of NH₄ ⁺ from sediments in the upper estuary can be of similar magnitude to rates of organic nitrogen mineralization and may therefore be important in supporting estuarine productivity when watershed inputs of N are low.  相似文献   

离子色谱法同时测定铀矿浸出液中的阳离子   总被引：1，自引：0，他引：1  

李文娟  王志畅  刘金辉 《岩矿测试》2010,29(6):715-718

对铬铁矿中亚铁的测定方法进行了探讨。首先对已有方法进行了验证和筛选,然后对Li2SO4助熔剂的助溶作用及其反应机理进行了研究,从而推荐了H2SO4-H3PO4-V2O5-V(Ⅳ)-Li2SO4和H2SO4-H3PO4-Ce(SO4)2-Li2SO4两种改进型的溶剂体系用于铬铁矿试样中亚铁的测定,所得结果具有良好的重现性。  相似文献   

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay   总被引：1，自引：0，他引：1  

C.A.J. Appelo  L.R. Van Loon 《Geochimica et cosmochimica acta》2010,74(4):1201-57

Diffusion experiments with HTO, ³⁶Cl⁻, Br⁻, I⁻, ²²Na⁺, ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ at trace concentrations in a single sample of Opalinus Clay are modeled with PHREEQC’s multicomponent diffusion module. The model is used first in a classical approach to derive accessible porosities, geometrical factors (the ratio of pore tortuosity and constrictivity) and sorption behavior of the individual tracers assuming that the clay is homogeneous. The accessible porosity for neutral species and cations is obtained from HTO, the anion exclusion volume from ³⁶Cl⁻ and Br⁻, and the cation exchange capacity from ²²Na⁺. The homogeneous model works well for tritium, the anions and ²²Na⁺. However, the ⁸⁵Sr²⁺ and ¹³⁴Cs⁺ experiments show an early arrival of the tracer and a front-form that suggest a dual porosity structure. A model with 10% dead-end pores, containing 19% of the total exchange capacity, can satisfactorily calculate all the experimental data. The Cs⁺ diffusion model builds on a 3-site exchange model, constructed from batch sorption data. The excellent agreement of modeled and measured data contradicts earlier reports that the exchange capacity for Cs⁺ would be smaller in diffusion than in batch experiments.The geometrical factors for the anions are 1.5 times larger than for HTO, and for the cations 2-4 times smaller than for HTO. The different behavior is explained by a tripartite division of the porespace in free porewater, diffuse double layer (DDL) water, and interlayer water in montmorillonite. Differences between estimated and observed geometrical factors for cations are attributed to increased ion-pairing of the divalent cations in DDL water as a result of the low relative dielectric permittivity. Interlayer and/or surface diffusion contributes significantly to the diffusive flux of Cs⁺ but is negligible for the other solutes. The geometrical factors for anions are higher than estimated, because pore constrictions with overlapping double layers force the anions to take longer routes than HTO and the cations. Small differences among the anions can also be attributed to different ion-pairing in DDL water.  相似文献   

Structure of heavy metal sorbed birnessite. Part III: Results from powder and polarized extended X-ray absorption fine structure spectroscopy     

Alain ManceauBruno Lanson  Victor A. Drits 《Geochimica et cosmochimica acta》2002,66(15):2639-2663

The local structures of divalent Zn, Cu, and Pb sorbed on the phyllomanganate birnessite (Bi) have been studied by powder and polarized extended X-ray absorption fine structure (EXAFS) spectroscopy. Metal-sorbed birnessites (MeBi) were prepared at different surface coverages by equilibrating at pH 4 a Na-exchanged buserite (NaBu) suspension with the desired aqueous metal. Me/Mn atomic ratios were varied from 0.2% to 12.8% in ZnBi and 0.1 to 5.8% in PbBi. The ratio was equal to 15.6% in CuBi. All cations sorbed in interlayers on well-defined crystallographic sites, without evidence for sorption on layer edges or surface precipitation. Zn sorbed on the face of vacant layer octahedral sites (□), and shared three layer oxygens (O_layer) with three-layer Mn atoms (Mn_layer), thereby forming a tridentate corner-sharing (TC) interlayer complex (Zn-3O_layer-□-3Mn_layer). ^TCZn complexes replace interlayer Mn²⁺ (Mn_inter²⁺) and protons. ^TCZn and ^TCMn_inter³⁺ together balance the layer charge deficit originating from Mn_layer⁴⁺ vacancies, which amounts to 0.67 charge per total Mn according to the structural formula of hexagonal birnessite (HBi) at pH 4. At low surface coverage, zinc is tetrahedrally coordinated to three O_layer and one water molecule (^[IV]TC complex: (H₂O)-^[IV]Zn-3O_layer). At high loading, zinc is predominantly octahedrally coordinated to three O_layer and to three interlayer water molecules (^[VI]TC complex: 3(H₂O)-^[VI]Zn-3O_layer), as in chalcophanite (^[VI]ZnMn₃⁴⁺O₇·3H₂O). Sorbed Zn induces the translation of octahedral layers from −a/3 to +a/3, and this new stacking mode allows strong H bonds to form between the ^[IV]Zn complex on one side of the interlayer and oxygen atoms of the next Mn layer (O_next): O_next…(H₂O)-^[IV]Zn-3O_layer. Empirical bond valence calculations show that O_layer and O_next are strongly undersaturated, and that ^[IV]Zn provides better local charge compensation than ^[VI]Zn. The strong undersaturation of O_layer and O_next results not only from Mn_layer⁴⁺ vacancies, but also from Mn³⁺ for Mn⁴⁺ layer substitutions amounting to 0.11 charge per total Mn in HBi. As a consequence, ^[IV]Zn,Mn_layer³⁺, and Mn_next³⁺ form three-dimensional (3D) domains, which coexist with chalcophanite-like particles detected by electron diffraction. Cu²⁺ forms a Jahn-Teller distorted ^[VI]TC interlayer complex formed of two oxygen atoms and two water molecules in the equatorial plane, and one oxygen and one water molecule in the axial direction. Sorbed Pb²⁺ is not oxidized to Pb⁴⁺ and forms predominantly ^[VI]TC interlayer complexes. EXAFS spectroscopy is also consistent with the formation of tridentate edge-sharing (^[VI]TE) interlayer complexes (Pb-3O_layer-3Mn), as in quenselite (Pb²⁺Mn³⁺O₂OH). Although metal cations mainly sorb to vacant sites in birnessite, similar to Zn in chalcophanite, EXAFS spectra of MeBi systematically have a noticeably reduced amplitude. This higher short-range structural disorder of interlayer Me species primarily originates from the presence of Mn_layer³⁺, which is responsible for the formation of less abundant interlayer complexes, such as ^[IV]Zn TC in ZnBi and ^[VI]Pb TE in PbBi.  相似文献