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   

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   

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   

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   

Surface acidity constants of α-Al2O3 between 25 and 70°c     

《Geochimica et cosmochimica acta》1999,63(19-20):3077-3085

The effect of temperature on the adsorption of H⁺ onto corundum was investigated experimentally by conducting potentiometric titrations at 25, 30, 40, 50, 60 and 70°C. These titrations were used to determine apparent acidity constants, given by the product of an intrinsic adsorption constant and a coulombic term (K_i^app = K_i^inte^{(−ΔZFΨ/RT)}) at the above temperatures.First and second intrinsic acidity constants were determined using a constant capacitance model (CCM). These constants and the point of zero charge change linearly with inverse temperature. These data were used to determine thermodynamic constants for the proton adsorption reactions.In the coulombic term, only the capacitance and the surface charge change with temperature and both were determined with the titration data at the various temperatures. Results show that the change in capacitance can be predicted with changes in the dielectric constant of water with increasing temperature. At a given pH, changes in the surface charge with temperature can, in turn, be predicted with a linear regression.With the above model, apparent acidity constants of corundum (including the chemical and electrostatic interactions) can be predicted for any temperatures between 25 and 70°C and possibly higher. These apparent constants change over several orders of magnitude in this temperature range (mainly due to a change in the coulombic term) and small temperature changes could have a strong influence on the stability of surface complexes.  相似文献   

Surface complexation of neodymium at the rutile-water interface: A potentiometric and modeling study in NaCl media to 250°C     

Moira K. Ridley  Michael L. Machesky  Donald A. Palmer 《Geochimica et cosmochimica acta》2005,69(1):63-81

  相似文献   

Diffusion and sorption of Cs, I and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan: Clay-based modeling approach     

Yukio Tachi  Kenji Yotsuji  Yoshimi Seida  Mikazu Yui 《Geochimica et cosmochimica acta》2011,75(22):6742-6759

Diffusion and sorption behaviors of cationic Cs⁺, anionic I⁻ and neutral HTO in samples of the Wakkanai Formation from the Horonobe underground research laboratory (URL), Japan, were investigated as a function of ionic strength (I) of synthetic groundwater by through-diffusion and batch sorption experiments and mechanistic modeling. The effective diffusivities (D_e) measured by through-diffusion experiments showed cation excess and anion exclusion effects, which were strongly dependent on I; D_e for Cs⁺ decreased as I increased, D_e for I⁻ showed the opposite dependency and D_e for HTO showed no dependence. The sorption of Cs⁺ measured by through-diffusion and batch sorption experiments were described by Freundlich isotherms with consistent parameters and decreased with I as a result of competitive ion exchange.Diffusion and sorption behaviors were interpreted by assuming the clay components of illite and smectite control diffusion and sorption mechanisms. The component additive (CA) sorption model, which includes illite and smectite contents and their ion exchange constants, provided a reasonable account of the Cs⁺ sorption trends measured as functions of I and Cs concentration. The diffusion model was developed by coupling the electrical double layer (EDL) model, describing the change of ionic concentrations (cation excess and anion deficit) and viscoelectric effects caused by electrostatic interaction at negatively charged clay surfaces, and a simplified pore model assuming one type of pore shape and includes their size distribution. When averaging the electrostatic effects by using the pore surface area distribution, the model could predict the cation excess and anion exclusion effects, and its dependence on I reasonably well. This result implies the nanoscale pores dominating the pore surface area can strongly impact on ionic diffusion in argillaceous rocks. The clay-based modeling approach described here provides a useful tool to predict ionic diffusion and sorption in argillaceous rocks.  相似文献   

Adsorption of Rb and Sr at the orthoclase (0 0 1)-solution interface     

P. Fenter  C. Park 《Geochimica et cosmochimica acta》2008,72(7):1848-1863

Adsorption of Rb⁺ and Sr²⁺ at the orthoclase (0 0 1)-solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb⁺ adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K⁺ site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å²). These results are consistent with an ion-exchange reaction in which Rb⁺ occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO₃)₂ solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr²⁺ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr²⁺ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb⁺, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr²⁺ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr²⁺ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr²⁺ on orthoclase. Differences in the partitioning of Sr²⁺ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson-Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.  相似文献   

Room temperature Mössbauer analysis of jarosite-type compounds     

A. Leclerc 《Physics and Chemistry of Minerals》1980,6(4):327-334

Powdered samples of some jarosite-type compounds were analysed at room temperature by ⁵⁷Fe Mössbauer spectroscopy. This group of compounds is described by the formula: MFe₃(SO₄)₂(OH)₆, where M can be H₃O⁺, Na⁺, K⁺, Rb⁺, Ag⁺, NH ₄ ⁺ , Tl⁺, 1/2Pb²⁺ or 1/2Hg²⁺. Although all the spectra were very similar, a linear relationship between the quadrupole splitting and the iron content of the samples was observed for the monovalent jarosite-type compounds.  相似文献   

Phase transition of potassium sulfate,K2SO4 (II); dielectric constant and electrical conductivity     

Michihiro Miyake  Mutsumi Matsuo  Mayumi Hata  Shin-ichi Iwai 《Physics and Chemistry of Minerals》1981,7(2):88-90

The phase transition of K₂SO₄ has been investigated by measurements of the dielectric constant and electrical conductivity, correlated with the structural point of view. Using single crystals, the temperature dependences of the dielectric constants and electrical conductivities were measured at frequencies of 0.3, 1, 3, and 5 MHz in the temperature range from 20° to 640 °C. Within this range, the dielectric constant does not reach a maximum, but near the phase-transition temperature at 587° C, the dielectric constant along the c axis shows a larger discontinuity than those along the a and b axes. The temperature dependence of the dielectric constant is consistent with the disordered structure of the high temperature form. Based on the parabolic increase of the dielectric constant in the temperature range from 582° to 587° C, it is likely that the phase transition propagates through an intermediate state. The electrical conductivity coefficients of K₂SO₄ increase with increasing temperature, exhibiting semiconducting character above the phase-transition temperature. In the high-temperature form, the electrical conductivity along the a axis exceeds that along the c axis. Since the electrical conductivity of K₂SO₄ is mainly ionic in character, the migration of K⁺ ions makes a major contribution to the conduction process.  相似文献   

Major ion composition and seasonal variation in the Lesser Himalayan lake: case of Begnas Lake of the Pokhara Valley, Nepal     

Udhab Raj Khadka  AL. Ramanathan 《Arabian Journal of Geosciences》2013,6(11):4191-4206

The Begnas Lake in the Pokhara Valley is one of the threatened habitats in Nepal. The major ion chemistry explains the status of most of the inorganic nutrients and their possible sources. However, the earlier studies mostly cover limnological investigations, and phytoplankton and zooplankton diversity. Thus, the present study has been conducted to investigate the geochemical processes and to examine the seasonal variation of chemical composition within Begnas Lake. The results showed that SO ₄ ^2- , PO ₄ ^3- , and NO ₃ ^- increased compared with the previous values. The domination of Ca²⁺, Mg²⁺, and HCO ₃ ^- explains the influence of carbonate weathering on the major ion concentration. In general, pH and dissolved oxygen decreased with the depth of water-column, while electric conductivity, total dissolved solids, HCO ₃ ^- , Cl^-, H₄SiO₄, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, and Fe increased. Among the cations, the predominance of Ca²⁺ and Mg²⁺ as characterized by high (>0.6) (Ca²⁺ + Mg²⁺)/(Tz⁺) and (>0.8) (Ca²⁺ + Mg²⁺)/(Na⁺ + K⁺) equivalent ratios, also suggests prevalence of carbonate weathering. The low value of (Na⁺ + K⁺)/Tz⁺ ratio shows deficiency of Na⁺ and K⁺, suggesting low contribution of cations via aluminosilicate weathering. The C-ratio suggests a proton source derived both from oxidation of sulfide and dissolution and dissociation of atmospheric CO₂ during different seasons. Though the major hydro-chemical parameters are within permissible limit, the increase in trophic state of the lake suggests that inherent biogeochemical processes make the limiting nutrients available, rendering eutrophic effect. Therefore, further comprehensive studies incorporating sediment–water interaction ought to be carried out to explain the ongoing phenomena and curb the eutrophication process in the lake.  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   

Reversible ion-exchange fixation of cesium-137 leading to mobilization from reservoir sediments     

David W. Evans  James J. Alberts  Roy A. Clark 《Geochimica et cosmochimica acta》1983,47(6):1041-1049

The radioactive fission product, ¹³⁷Cs, has been observed to mobilize from bottom sediments of two South Carolina reservoirs during summer thermal stratification and hypolimnetic anoxia. Mobilization is attributed to ion-exchange displacement of ¹³⁷Cs from sediments by cations such as NH⁺₄, Fe⁺² and Mn⁺² released under anaerobic conditions.Three types of ¹³⁷Cs binding sites to sediment clay minerals are identified: 1) surface and planar sites from which ¹³⁷Cs is generally exchangeable by all cations studied (Na⁺, NH⁺₄, H⁺, Cs⁺, Ca⁺², Mg⁺², Fe⁺², and Mn⁺²); 2) wedge sites where ¹³⁷Cs exchange is sterically limited to cations of similar size and charge (NH⁺₄, Cs⁺, K⁺, and perhaps H₃O⁺); 3) interlayer sites from which ¹³⁷Cs is not readily exchanged. More than 15 years after final ¹³⁷Cs inputs, the reservoir sediments we studied showed the following percentage distribution of sites: 2 to 9% surface sites, 6 to 13% wedge sites, and 78 to 85% interlayer sites. In contrast, lake and stream sediments near Oak Ridge, Tennessee receiving ¹³⁷Cs inputs more than 20 years earlier had greater than 99% of their ¹³⁷Cs associated with non-exchangeable interlayer sites. The difference is attributed to the paucity in the South Carolina sediments of weathered micaceous clay minerals with their abundant interlayer sites. Such interlayer deficient clays are dominant in the Atlantic and Gulf coastal plains of the United States and elsewhere. This suggests that ¹³⁷Cs will be physically and chemically more mobile in such areas as well as more biologically available. Mobility will be enhanced in regimes where cation inputs favoring ¹³⁷Cs exchange occur. Subsurface waste disposal sites where anaerobic conditions develop with NH⁺₄ production and Fe⁺² and Mn⁺² release might be such a regime.  相似文献   

Surface chemistry and relative Ni sorptive capacities of synthetic hydrous Mn oxyhydroxides under variable wetting and drying regimes     

Corrie Kennedy  Lesley A Warren 《Geochimica et cosmochimica acta》2004,68(3):443-454

The surface binding site characteristics and Ni sorptive capacities of synthesized hydrous Mn oxyhydroxides experimentally conditioned to represent three hydrological conditions—MnO_XW, freshly precipitated; MnO_XD, dried at 37°C for 8 d; and MnO_XC, cyclically hydrated and dehydrated (at 37°C) over a 24-h cycle for 7 d—were examined through particle size analysis, surface acid-base titrations and subsequent modelling of the pK_a spectrum, and batch Ni sorption experiments at two pH values (2 and 5). Mineralogical bulk analyses by XRD indicate that all three treatments resulted in amorphous Mn oxyhydroxides; i.e., no substantial bulk crystalline phases were produced through drying. However, drying and repeated wetting and drying resulted in a non-reversible decrease in particle size. In contrast, total proton binding capacities determined by acid-base titrations were reversibly altered with drying and cyclically re-wetting and drying from 82 ± 5 μmol/m² for the MnO_XW to 21 ± 1 μmol/m² for the MnO_XD and 37 ± 5 μmol/m² for the MnO_XC. Total proton binding sites measured decreased by ≈75% with drying from the MnO_XW and then increased to ≈50% of the MnO_XW value in the MnO_XC. Thus, despite a trend of higher surface area for the MnO_XD, a lower total number of sites was observed, suggesting a coordinational change in the hydroxyl sites. Surface site characterization identified that changes also occurred in the types and densities of surface sites for each hydrologically conditioned Mn oxyhydroxide treatment (pH titration range of 2-10). Drying decreased the total number of sites as well as shifted the remaining sites to more acidic pK_a values. Experimentally determined apparent pH_zpc values decreased with drying, from 6.82 ± 0.06 for the MnO_XW to 3.2 ± 0.3 for the MnO_XD and increased again with rewetting to 5.05 ± 0.05 for the MnO_XC. Higher Ni sorption was observed at pH 5 for all three Mn oxyhydroxide treatments compared to pH 2. However, changes in relative sorptive capacities among the three treatments were observed for pH 2 that are not explainable simply as a function of total binding site density or apparent pH_zpc values. These results are the first to our knowledge, to quantitatively link the changes induced by hydrologic variability for surface acid base characteristics and metal sorption patterns. Further, these results likely extend to other amorphous minerals, such as Fe oxyhydroxides, which are commonly important geochemical solids for metal scavenging in natural environments.  相似文献   

Humic substance charge determination by titration with a flexible cationic polyelectrolyte   总被引：1，自引：0，他引：1  

Wen-Feng Tan  Willem Norde  Luuk K. Koopal 《Geochimica et cosmochimica acta》2011,75(19):5749-5761

The anionic charge of humic substances (HS) plays a major role in the interaction of HS with other components. Therefore, the potential of the polyelectrolyte titration technique to obtain the charge density of HS in simple 1-1 electrolyte solutions has been investigated. Titrations are carried out with an automatic titrator combined with the “Mütek particle charge detector” which allows determination of the Mütek potential and the pH as a function of the added amount of titrant which is a solution of poly-diallyldimethylammonium chloride (polyDADMAC), a cationic strong polyelectrolyte. When the Mütek potential reverses its sign the iso-electric point (IEP) of the polyDADMAC-HS complex is reached. The polyDADMAC/HS mass ratio at the IEP gives information on the HS charge density and from the pH changes in solution an estimate of the charge regulation in the HS-polyDADMAC complex can be obtained. In general, for polyDADMAC-HS complexes an increase in the dissociation of the acid groups of HS is found (charge regulation). The charge regulation decreases with increasing concentration of 1-1 background electrolyte. Cation incorporation can be neglected at 1-1 electrolyte concentrations ? 1 mmol L⁻¹ and a 1-1 stoichiometry exists between the polyDADMAC and HS charge. However, at these low salt concentrations the charge regulation is substantial. A detailed analysis of purified Aldrich humic acid (PAHA) at pH 5 and a range of KCl concentrations reveals that the anionic charge of PAHA in the complex increases at 5 mmol L⁻¹ KCl by 30% and at 150 mmol L⁻¹ KCl by 12%. On the other hand, increasing amounts of K⁺ become incorporated in the complex: at 5 mmol L⁻¹ KCl 5% and at 150 mmol L⁻¹ KCl 24% of the PAHA charge is balanced by K⁺. By comparing at pH 5 the mass ratios polyDADMAC/PAHA in the complex at the IEP with the theoretical mass ratios of polyDADMAC/PAHA required to neutralize PAHA in the absence of charge regulation and K⁺ incorporation, it is found that at 50 mmol L⁻¹ KCl the extra negative charge due to the interaction between polyDADMAC and PAHA is just compensated by K⁺ incorporation in the complex. Therefore, a pseudo 1-1 stoichiometry exists at about 50 mmol L⁻¹ 1-1 electrolyte concentration and only at this salt concentration polyDADMAC titrations and conventional proton titrations give identical results. Most likely this is also true for other HA samples and other pH values. For FA further study is required to reveal the conditions for which polyDADMAC and proton titrations give identical results.  相似文献   

Surface-charge properties and UO22+ adsorption of a subsurface smectite     

《Geochimica et cosmochimica acta》1996,60(18):3399-3414

  相似文献   

Adsorption and surface complexation modeling of palladium,rhodium and platinum in surficial semi-arid soils and sediment     

Aboubakar Sako  Lidia LopesAlakendra N. Roychoudhury 《Applied Geochemistry》2009

Sorption isotherms, time-dependent adsorption and surface complexation modeling studies were used to investigate the post-depositional mobility of three of the platinum group-elements (Pd, Rh, and Pt) in semi-arid soil and sediment samples with varying surface properties. The acidity constants (Log K_a1 and Log K_a2), optimized from batch titration data, ranged from 4.69 to 5.34 for Log K_a1 and from −6.51 to −7.61 for Log K_a2, suggesting the occurrence of both protonation and deprotonation reactions on the solid surfaces. Partition coefficients and removal rates of the metals had a general trend of Pd > Pt > Rh. The sediment sample, with the highest clay content and exchangeable cation concentrations, also had the highest affinity for the metals. The times required for sediment to adsorb 63% of the metals were 2.63 h, 4.08 h and 10.64 h for Pd, Pt and Rh, respectively. The FITEQL program successfully optimized the conditional binding constants of the metals on the solids from batch adsorption data. The constants decreased in the order of Pd > Rh > Pt, which was consistent with the observed high affinity of the solids for Pd. The modeling results also showed that aqueous Pd was the least sensitive to pH followed by Rh and Pt. However, metal adsorption below the points of zero net proton charges (ca. pH 6.7) is attributable to the involvement of permanent negatively charged binding sites in the adsorption process. Notably, partition coefficients, removal rates and conditional binding constants all showed a high affinity of Pd for the solids. A similarity between the model outputs and the batch adsorption data indicates the suitability of the model for describing the mobility and retention of the three metals in semi-arid soils and sediments.  相似文献   

Verteilung anorganischer Fremdionen bei der Kristallisation von Alkalichloriden     

Jörg Reichert 《Contributions to Mineralogy and Petrology》1966,13(2):134-160

Investigations were made on the distribution of the foreign cations Na⁺, K⁺, Rb⁺, Cs⁺, NH ₄ ⁺ , Ca⁺⁺, Sr⁺⁺, Ba⁺⁺ and Pb⁺⁺ between NaCl-, KCl- and RbCl-single crystals and their aqueous solutions or melts. The growth conditions were kept as constant as possible: large single crystals grew slowly, that is from melts at constant undercooling and from solutions at low and constant supersaturation. The distribution coefficient D=C _s /C _l was calculated for the different systems (C _s =concentration of the foreign substance in the crystal, C _l =concentration of the foreign substance in the liquid phase, both in Mol-%). The different values for D can be explained qualitatively with the help of the ionic radii or by comparing the gain of energy with the expenditure of energy, which play an important part during the process of crystallization.

---

---

Meinem hochverehrten Lehrer, Prof. Dr. W. v. Engelhardt, gilt mein besonderer Dank für die Überlassung des interessanten Themas und für seine wertvolle Unterstützung und Beratung während dieser Arbeit. Herrn Prof. Dr. S. Haussühl bin ich für viele Ratschläge und Hinweise zu Dank verpflichtet. Ferner danke ich allen wissenschaftlichen und technischen Mitarbeitern des Tübinger Mineralogischen Instituts, die mir mit Rat und Tat bei der Durchführung dieser Arbeit behilflich waren.  相似文献   

Sediment-water interactions of thallium under simulated estuarine conditions   总被引：1，自引：0，他引：1  

Andrew Turner  Amelie Cabon  Andrew S. Fisher 《Geochimica et cosmochimica acta》2010,74(23):6779-6787

Thallium(I) has been added to estuarine sediment suspended in various natural and artificial aqueous samples in order to examine its reactivity under simulated estuarine conditions. In river water and sea water, adsorption of Tl to sediment was so rapid that a period of desorption-relaxation succeeded instantaneous adsorption. Entire time-courses could not be fitted with a conventional kinetic model, but pseudo-first-order forward and reverse rate constants of 0.0044 and 0.30 h⁻¹, respectively, were derived for river water by omitting measurements defining the adsorption “overshoot” observed at the onset of the experiment. The extent of adsorption after a 16 h equilibration period was considerably greater in river water than in sea water, and displayed a quasi-linear increase with increasing pH over the range 2-9 in the former but no clear dependence on pH in the latter. A logarithmic reduction in the sediment-water distribution coefficient, K_D, was observed on estuarine mixing from river water to sea water. Experiments conducted in electrolyte solutions coupled with inorganic equilibrium speciation modeling revealed that the effect was the combined result of a reduction in the activity of Tl⁺, an increase in the proportion of TlCl⁰ and increasing competition for adsorption sites from K⁺ with an increase in salinity. Overall, there was little experimental evidence for either the oxidation of Tl⁺ or its complexation by dissolved organic matter. The findings of the investigation are discussed in terms of the likely behavior of Tl in estuaries.  相似文献   

Acid-base properties of a goethite surface model: A theoretical view     

Adelia J.A. Aquino  Daniel Tunega  Georg Haberhauer  Hans Lischka 《Geochimica et cosmochimica acta》2008,72(15):3587-3602

Density functional theory is used to compute the effect of protonation, deprotonation, and dehydroxylation of different reactive sites of a goethite surface modeled as a cluster containing six iron atoms constructed from a slab model of the (1 1 0) goethite surface. Solvent effects were treated at two different levels: (i) by inclusion of up to six water molecules explicitly into the quantum chemical calculation and (ii) by using additionally a continuum solvation model for the long-range interactions. Systematic studies were made in order to test the limit of the fully hydrated cluster surfaces by a monomolecular water layer. The main finding is that from the three different types of surface hydroxyl groups (hydroxo, μ-hydroxo, and μ₃-hydroxo), the hydroxo group is most active for protonation whereas μ- and μ₃-hydroxo sites undergo deprotonation more easily. Proton affinity constants (pK_a values) were computed from appropriate protonation/deprotonation reactions for all sites investigated and compared to results obtained from the multisite complexation model (MUSIC). The approach used was validated for the consecutive deprotonation reactions of the [Fe(H₂O)₆]³⁺ complex in solution and good agreement between calculated and experimental pK_a values was found. The computed pK_a for all sites of the modeled goethite surface were used in the prediction of the pristine point of zero charge, pH_PPZN. The obtained value of 9.1 fits well with published experimental values of 7.0-9.5.  相似文献