Uranium(6+) sorption on montmorillonite: Experimental and surface complexation modeling study     

Roberto T. Pabalan  David R. Turner 《Aquatic Geochemistry》1996,2(3):203-226

Sorption interactions with montmorillonite and other clay minerals in soils, sediments, and rocks are potentially important mechanisms for attenuating the mobility of U(6+) and other radionuclides through the subsurface environment. Batch experiments were conducted (in equilibrium with atmospheric % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% acbiGaiWiG-bfadaWgaaWcbaacbaGaa43qaiaa+9eadaWgaaqaaiaa% +jdaaWqabaaaleqaaaaa!400D!\[P_{CO_2 } \])to determine the effects of varying pH (2 to 9), solid-mass to solution-volume ratio (M/V = 0.028 to 3.2 g/L), and solution concentration (2 × 10^–7 and 2 × 10^–6 M ²³³U) on U(6+) sorption on SAz-1 montmorillonite. The study focused on U(6+) surface complexation on hydroxylated edge sites as the sorption mechanism of interest because it is expected to be the predominant sorption mechanism at pHs typical of natural waters (pH 6 to 9). Thus, the experiments were conducted with a 0.1 M NaNO₃ matrix to suppress ion-exchange between U(6+) in solution and interlayer cations. The results show that U(6+) sorption on montmorillonite is a strong function of pH, reaching a maximum at near-neutral pH (6 to 6.5) and decreasing sharply towards more acidic or more alkaline conditions. A comparison of the pH-dependence of U(6+) sorption with that of U(6+) aqueous speciation indicates a close correspondence between U(6+) sorption and the predominance field of U(6+)-hydroxy complexes. At high pH, sorption is inhibited due to formation of aqueous U(6+)-carbonate complexes. At low pH, the low sorption values indicate that the 0.1 M NaNO₃ matrix was effective in suppressing ion-exchange between the uranyl (UO₂ ²⁺) species and interlayer cations in montmorillonite. At pH and carbonate concentrations typical of natural waters, sorption of U(6+) on montmorillonite can vary by four orders of magnitude and can become negligible at high pH.The experimental results were used to develop a thermodynamic model based on a surface complexation approach to permit predictions of U(6+) sorption at differing physicochemical conditions. A Diffuse-Layer model (DLM) assuming aluminol (>AlOH) and silanol (>SiOH) edge sites and two U(6+) surface complexation reactions per site effectively simulates the complex sorption behavior observed in the U(6+)-H₂O-CO₂-montmorillonite system at an ionic strength of 0.1 M and pH > 3.5. A comparison of model predictions with data from this study and from published literature shows good agreement and suggests that surface complexation models based on parameters derived from a limited set of data could be useful in extrapolating radionuclide sorption over a range of geochemical conditions. Such an approach could be used to support transport modeling by providing a better alternative to the use of constant K _d s in transport calculations.  相似文献   

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation   总被引：2，自引：0，他引：2  

Jeffrey G. Catalano  Gordon E. Brown Jr. 《Geochimica et cosmochimica acta》2005,69(12):2995-3005

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.  相似文献   

The influence of citric acid,EDTA, and fulvic acid on U(VI) sorption onto kaolinite     

Michelle Barger  Carla M. Koretsky 《Applied Geochemistry》2011

Uranium(VI) sorption onto kaolinite was investigated as a function of pH (3–12), sorbate/sorbent ratio (1 × 10^?6–1 × 10^?4 M U(VI) with 2 g/L kaolinite), ionic strength (0.001–0.1 M NaNO₃), and pCO₂ (0–5%) in the presence or absence of 1 × 10^?2–1 × 10^?4 M citric acid, 1 × 10^?2–1 × 10^?4 M EDTA, and 10 or 20 mg/L fulvic acid. Control experiments without-solids, containing 1 × 10^?6–1 × 10^?4 M U(VI) in 0.01 M NaNO₃ were used to evaluate sorption to the container wall and precipitation of U phases as a function of pH. Control experiments demonstrate significant loss (up to 100%) of U from solution. Although some loss, particularly in 1 × 10^?5 and 1 × 10^?4 M U experiments, is expected due to precipitation of schoepite, adsorption on the container walls is significant, particularly in 1 × 10^?6 M U experiments. In the absence of ligands, U(VI) sorption on kaolinite increases from pH ～3 to 7 and decreases from pH ～7.5 to 12. Increasing ionic strength from 0.001 to 0.1 M produces only a slight decrease in U(VI) sorption at pH < 7, whereas 10% pCO₂ greatly diminishes U(VI) sorption between pH ～5.5 and 11. Addition of fulvic acid produces a small increase in U(VI) sorption at pH < 5; in contrast, between pH 5 and 10 fulvic acid, citric acid, and EDTA all decrease U(VI) sorption. This suggests that fulvic acid enhances U(VI) sorption slightly via formation of ternary ligand bridges at low pH, whereas EDTA and citric acid do not form ternary surface complexes with the U(VI), and that all three ligands, as well as carbonate, form aqueous uranyl complexes that keep U(VI) in solution at higher pH.  相似文献   

Ni(II) sorption on natural chlorite     

Åsa Zazzi  Anna-Maria Jakobsson  Susanna Wold 《Applied Geochemistry》2012

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 (K_d = ∼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 K_d 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   

Impact of Alkaline Earth Metals on Aqueous Speciation of Uranium(VI) and Sorption on Quartz   总被引：2，自引：0，他引：2  

Sreejesh Nair  Broder J. Merkel 《Aquatic Geochemistry》2011,17(3):209-219

The effect of Mg-, Ca-, and Sr–Uranyl-Carbonato complexes with respect to sorption on quartz was studied by means of batch experiments with U(VI) concentration of 0.126 × 10⁻⁶ M in the presence and absence of Mg, Ca, and Sr (each 1 mM) at pH from 6.5 to 9. In the absence of alkaline earth elements, 90% of the U(VI) sorbed on the quartz surface at all pH. In the presence of Mg, Ca, and Sr, the sorption of U(VI) on quartz decreased to 50, 10, and 30%, respectively. Sorption kinetics of U(VI) on quartz is faster in the absence of alkaline earth elements and reached equilibrium after 12 h, whereas in the presence of Mg, Ca and Sr, the kinetics of U(VI) sorption on quartz is pH dependent and attained equilibrium after 24 h. Aqueous speciation calculations for alkaline earth uranyl carbonates were carried out by using PHREEQC with the Nuclear Energy Agency thermodynamic database (NEA_2007) by adding constants for MUO₂(CO₃)₃²⁻ and M₂UO₂(CO₃)₃⁰ (M = Ca, Mg, Sr). This study reveals that alkaline earth elements can have a significant effect on the aqueous speciation of U(VI) under neutral to alkaline pH conditions and subsequently sorption behavior and mobility of U(VI) in aqueous environments.  相似文献   

Evidence for the nucleation and epitaxial growth of Zn phyllosilicate on montmorillonite     

Michel L. Schlegel  Alain Manceau 《Geochimica et cosmochimica acta》2006,70(4):901-917

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

Sorption modelling on illite Part I: Titration measurements and the sorption of Ni, Co, Eu and Sn     

M.H. Bradbury 《Geochimica et cosmochimica acta》2009,73(4):990-1003

In this study the physico-chemical, titration and sorption characteristics of Na-illite du Puy (Na-illite) have been measured and modelled. Samples of illite, collected in the region of le Puy-en-Velay, France, were purified and conditioned to the Na-form and physico-chemically characterised. Potentiometric titrations on suspensions of the Na-illite were carried out using a batch backtitration technique in 0.01, 0.1 and 0.5 M NaClO₄ background electrolytes from pH∼3 to ∼11.5 in an inert atmosphere glove box. The supernatant solutions from each titration experiment in each series were analysed for K, Mg, Ca, Sr, Si, Al, Fe and Mn. The titration data were modelled in terms of the protolysis of two amphoteric edge sites (S^W1OH and S^W2OH) without an electrostatic term. Sorption edges (solid/liquid distribution ratios versus pH at trace sorbate concentrations and constant ionic strength) were determined for the transitions metals Ni(II) and Co(II), the lanthanide Eu(III), and the heavy metal Sn(IV) on Na-illite with NaClO₄ as the background electrolyte under anoxic conditions (CO₂ ? 2 ppm, O₂ ? 2 ppm). The study thus encompasses a broad range of metals with different thermodynamic characteristics and with valence states ranging from II to IV. The results from the modelling of the titration data, in combination with a non electrostatic surface complexation and cation exchange sorption model were applied to quantitatively describe the uptake characteristics of the metals listed above on Na-illite. Since sorption edges were measured at trace concentrations, metal uptake was modelled as occurring on strong type sites (S^SOH) only. This sorption model, the two site protolysis non electrostatic surface complexation and cation exchange model (2SPNE SC/CE model) had been previously developed and used to describe metal uptake on montmorillonite.  相似文献   

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH,ionic strength,carbonate and humic acid     

Katja Schmeide  Gert Bernhard 《Applied Geochemistry》2010

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   

Fe sorption onto nontronite (NAu-2)     

Deb P. Jaisi  Chongxuan Liu  Ruth E. Blake 《Geochimica et cosmochimica acta》2008,72(22):5361-5371

The sorption of ferrous iron to a clay mineral, nontronite (NAu-2, a ferruginous smectite), was investigated under strictly anoxic conditions as a function of pH (3-10), Fe²⁺ concentration (0.01-50 mM), equilibration time (1-35 days), and ionic strength (0.01-0.5 M NaClO₄). The surface properties of NAu-2 were independently characterized to determine its fixed charge and amphoteric site density in order to interpret the Fe²⁺ sorption data. Fe²⁺ sorption to NAu-2 was strongly dependent on pH and ionic strength, reflecting the coupled effects of Fe²⁺ sorption through ion exchange and surface complexation reactions. Fe²⁺ sorption to NAu-2 increased with increasing pH from pH 2.5 to 4.5, remained constant from pH 4.5 to 7.0, increased again with further increase of pH from pH 7.0 to 8.5, and reached a maximum above pH 8.5. The Fe²⁺ sorption below pH 7.0 increased with decreasing ionic strength. The differences of Fe²⁺ sorption at different ionic strengths, however, diminished with increasing equilibration time. The Fe²⁺ sorption from pH 4.5 to 7.0 increased with increasing equilibration time up to 35 days and showed stronger kinetic behavior in higher ionic strength solutions. The kinetic uptake of Fe²⁺ onto NAu-2 is consistent with a surface precipitation mechanism although our measurements were not able to identify secondary precipitates. An equilibrium model that integrates ion exchange, surface complexation and aqueous speciation reactions reasonably well describes the Fe²⁺ sorption data as a function of pH, ionic strength, and Fe²⁺ concentration measured at 24 h of equilibration. Model calculations show that the species Fe(OH)⁺ was required to describe Fe²⁺ sorption above pH 8.0 satisfactorily. Overall, this study demonstrated that Fe²⁺ sorption to NAu-2 is affected by complex equilibrium and kinetic processes, likely caused by surface precipitation reactions.  相似文献   

Sorption of uranium(VI) at the clay mineral–water interface     

Samer Bachmaf  Broder J. Merkel 《Environmental Earth Sciences》2011,63(5):925-934

Batch experiments were conducted to study the sorption of uranium on selected clay minerals (KGa-1b and KGa-2 reference kaolinite, SWy-2 and STx-1b reference montmorillonite, and IBECO natural bentonite) as a function of pH (4–9) and 0.001, 0.01, and 0.025 M NaCl in equilibrium with the CO₂ partial pressure of the atmosphere. Uranium concentrations were kept below 100 μg L⁻¹ to avoid precipitation of amorphous Uranium-hydroxides. Solely PTFE containers and materials were used, because experiments showed significant sorption at higher pH on glass ware. All batch experiments were performed over a period of 24 h, since kinetic experiments proved that the common 10 or 15 min are in many cases by far not sufficient to reach equilibrium. Kaolinite showed much greater uranium sorption than the other clay minerals due to the more aluminol sites available. Sorption on the poorly crystallized KGa-2 was higher than on the well-crystallized KGa-1b. Uranium sorption on STx-1b and IBECO exhibited parabolic behavior with a sorption maximum around pH 6.5. Sorption of uranium on montmorillonites showed a distinct dependency on sodium concentrations because of the effective competition between uranyl and sodium ions, whereas less significant differences in sorption were found for kaolinite. The presence of anatase as impurity in kaolinite enhanced the binding of uranyl-carbonate complexes with surface sites. The kinetic of uranium sorption behavior was primarily dependent on the clay minerals and pH. A multisite surface complexation model without assuming exchange is based on the binding of the most dominant uranium species to aluminol and silanol edge sites of montmorillonite, respectively to aluminol and titanol surface sites of kaolinite. For eight surface species, the log_k was determined from the experimental data using the parameter estimation code PEST together with PHREEQC.  相似文献   

Experimental study and modeling of the sorption of uranium(VI) onto olivine-rock     

《Applied Geochemistry》2002,17(4):399-408

The sorption of U(VI) onto the surface of olivine has been experimentally investigated at 25 °C under an air atmosphere as a function of pH, solid surface to volume ratio and total U concentration. Sorption has been observed to decrease as the extent of carbonate complexation of U(VI) in solution increases, which is attributed to the competition between aqueous and solid ligands for the coordination of U. The experimental results have been interpreted by means of two different approaches: (1)a semi-empirical model, exemplified by the application of a Langmuir isotherm and (2) a non-electrostatic thermodynamic surface complexation model which includes the formation of the surface species: >SO–UO₂⁺ and >SO–UO₂(OH). The following stability constants for these species have been determined from the thermodynamic analysis: K(>SO–UO₂⁺)=289±71 and K(>SO–UO₂(OH))=(3.4±0.4)×10⁻⁶. The comparison of the sorption of U onto olivine with granites of different origin indicate that the use of this mineral as additive to the backfill of deep high level nuclear waste repositories could retard the migration of U from the repository to the geosphere.  相似文献   

Adsorption of molybdenum on to anatase from dilute aqueous solutions     

《Applied Geochemistry》2002,17(5):649-656

Adsorption of Mo on to hydrous TiO₂ (anatase) particles was investigated. Batch experiments were conducted at 19 and 90 °C over a pH range of 2 to 12 and Mo concentrations ranging from approximately 10⁻⁶ to 10⁻⁴ M. The extent of sorption was strongly dependent on pH and surface loading. Maximum sorption was observed in the acidic pH range at low surface loading. Adsorption behavior was described using the empirical Langmuir adsorption model. A constant capacitance surface complexation model was also used to fit the adsorption isotherms using a ligand exchange reaction for a hydroxyl surface site on anatase. Comparison of experimental data at two different temperatures (19 and 90 °C) indicates that Mo sorption in the acidic pH range decreases with increasing temperature.  相似文献   

Removal of phosphorus from water using scallop shell synthesized ceramic biomaterials     

Nan Chen  Weiwu Hu  Chuanping Feng  Zhenya Zhang 《Environmental Earth Sciences》2014,71(5):2133-2142

This study describes the development of scallop shell synthesized ceramic biomaterial for phosphorus removal from water. The synthesized biomaterial was characterized by scanning electron microscope, Brunauer–Emmett–Teller and X-ray diffractometer methods. The influences of contact time, initial phosphate concentration, initial solution pH, co-existing ions and temperature for phosphorus removal were investigated by batch experiments. The results indicated that the equilibrium data can be fitted by the Langmuir isotherm model at temperatures ranging from 15 to 55 °C, with the maximum sorption capacity of 13.6 mg/g. Sorption kinetics followed a pseudo-second-order kinetic equation model. The sorption process was optimal at a wide range of solution pH (above 2.4), with a relatively high sorption capacity level. Phosphorus sorption was slightly impeded by the presence of F^?, HCO₃ ^? and NH₄ ⁺ ions, and significantly inhibited by Cl^?, SO₄ ^2? and NO₃ ^? ions. Sorption process appeared to be controlled by a chemical precipitation processes. The mechanism may be attributed to ion complexation during subsequent sorption of phosphorus on scallop shell synthesized ceramic biomaterial.  相似文献   

Uranyl adsorption and surface speciation at the imogolite-water interface: Self-consistent spectroscopic and surface complexation models   总被引：1，自引：0，他引：1  

Yuji Arai  M. McBeath  J. Joye 《Geochimica et cosmochimica acta》2006,70(10):2492-2509

Macro- and molecular-scale knowledge of uranyl (U(VI)) partitioning reactions with soil/sediment mineral components is important in predicting U(VI) transport processes in the vadose zone and aquifers. In this study, U(VI) reactivity and surface speciation on a poorly crystalline aluminosilicate mineral, synthetic imogolite, were investigated using batch adsorption experiments, X-ray absorption spectroscopy (XAS), and surface complexation modeling. U(VI) uptake on imogolite surfaces was greatest at pH ∼7-8 (I = 0.1 M NaNO₃ solution, suspension density = 0.4 g/L [U(VI)]_i = 0.01-30 μM, equilibration with air). Uranyl uptake decreased with increasing sodium nitrate concentration in the range from 0.02 to 0.5 M. XAS analyses show that two U(VI) inner-sphere (bidentate mononuclear coordination on outer-wall aluminol groups) and one outer-sphere surface species are present on the imogolite surface, and the distribution of the surface species is pH dependent. At pH 8.8, bis-carbonato inner-sphere and tris-carbonato outer-sphere surface species are present. At pH 7, bis- and non-carbonato inner-sphere surface species co-exist, and the fraction of bis-carbonato species increases slightly with increasing I (0.1-0.5 M). At pH 5.3, U(VI) non-carbonato bidentate mononuclear surface species predominate (69%). A triple layer surface complexation model was developed with surface species that are consistent with the XAS analyses and macroscopic adsorption data. The proton stoichiometry of surface reactions was determined from both the pH dependence of U(VI) adsorption data in pH regions of surface species predominance and from bond-valence calculations. The bis-carbonato species required a distribution of surface charge between the surface and β charge planes in order to be consistent with both the spectroscopic and macroscopic adsorption data. This research indicates that U(VI)-carbonato ternary species on poorly crystalline aluminosilicate mineral surfaces may be important in controlling U(VI) mobility in low-temperature geochemical environments over a wide pH range (∼5-9), even at the partial pressure of carbon dioxide of ambient air (p_CO2 = 10^−3.45 atm).  相似文献   

Sorption of Eu on Na- and Ca-montmorillonites: experimental investigations and modelling with cation exchange and surface complexation     

M.H Bradbury  B Baeyens 《Geochimica et cosmochimica acta》2002,66(13):2325-2334

The 2-site protolysis no electrostatics surface complexation and cation exchange (2SPNE/CE) model used in previous work to model the sorption of Ni and Zn on Na- and Ca-montmorillonites was applied to sorption edges and isotherms measured for Eu on these two montmorillonite forms. The aim was to further test the applicability of the sorption model on a trivalent element with a more complex aqueous chemistry. An additional reason for choosing Eu was that it is considered to be a good chemical analogue for other lanthanides and trivalent actinides. With site types, site capacities, and protolysis constants fixed at the values in the Ni/Zn studies, all of the measured sorption edge data could be modelled using cation exchange and the monodentate surface species, ≡S^SOEu²⁺, ≡S^SOEuOH⁺ and ≡S^SOEu(OH)₃⁻, on the strong site type. However, an additional modelling study showed that the same data were almost equally well described by considering bidentate surface complexes, (≡S^SO)₂Eu⁺ and (≡S^SO)₂Eu(OH)₂⁻, and cation exchange. To model the sorption isotherm measurements up to pH = 7.2, only one additional weak site surface complex was required, ≡S^W1OEu²⁺ for the monodentate case and (≡S^W1O)₂Eu⁺ for the bidentate case. Selectivity coefficients are given for Eu³⁺- Ca²⁺ and Eu³⁺- Na⁺ exchange on the planar sites and surface complexation constants for monodentate and bidentate Eu surface species on the edge sites of montmorillonite.  相似文献   

Geochemistry of groundwater in parts of Guntur district, Andhra Pradesh, India   总被引：4，自引：0，他引：4  

N. Subba Rao 《Environmental Geology》2002,41(5):552-562

Hydrogeochemical investigations, which are significant for the assessment of water quality, have been carried out to study the sources of dissolved ions in groundwaters of some rural areas of Guntur district, Andhra Pradesh, India. Groundwaters in the area are mostly brackish. High contents of SiO₂, and Na⁺ and Cl^- ions in groundwater, in comparison with those of seawater, suggests a meteoric origin of groundwater. The high concentration of SiO₂ and various geochemical signatures reflect the weathering of minerals. However, the Na⁺+K⁺ vs Cl^- ratio suggests weathering, has occurred only to some extent. The chemistry of groundwater favours the formation of clay minerals (montmorillonite, illite and chlorite), because of evapotranspiration. The positive saturation index of CaCO₃ and the high signatures of Ma²⁺:Ca²⁺ and Na⁺:Ca²⁺ reveals the occurrence of evaporation. The evaporation enhances the concentration of ions (which occurred originally in the water) in the soils during summer. The very high % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBae % bbnrfifHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr % 0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8 % frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaqabeaadaaakeaacqqG % tbWucqqGpbWtdaqhaaWcbaGaeeinaqdabaGaeeOmaiJaeeyla0caaa % aa!2EC5! SO₄^{2 -} {\rm SO}_{\rm 4}^{{\rm 2 - }} and Cl^- contents in some groundwaters and the occurrence of kankar (CaCO₃) in the area suggest a long history of evaporation. Greater ionic concentration in the groundwaters of post-monsoon compared with pre-monsoon indicates the increasing addition of leachates into the groundwater from the soils in the monsoon and anthropogenic activities, which leads to a deteriorating quality of groundwater. According to the Gibbs' diagrams, rock weathering, to some extent, and evaporation are the dominant phenomena responsible for the higher ionic concentrations found in groundwater. Measures that benefit sustainable management of groundwater quality are suggested in this study.  相似文献   

Sorption modelling on illite. Part II: Actinide sorption and linear free energy relationships     

M.H. Bradbury 《Geochimica et cosmochimica acta》2009,73(4):1004-5168

Sorption edge data for Ni(II), Co(II), Eu(III) and Sn(IV) [Bradbury M. H. and Baeyens B. (2009) Sorption modelling on illite. Part I: titration measurements and sorption of Ni(II), Co(II), Eu(III) and Sn(IV), Part I] on purified Na-Illite du Puy are available from some previous work, and some new measurements for Am(III), Th(IV), Pa(V) and U(VI) are presented here. All of these sorption edge measurements have been modelled with a 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model for which the site types, site capacities and protolysis constants were fixed [Bradbury M. H. and Baeyens B. (2009), Part I]. In addition, two further data sets for the sorption of Am(III) and Np(V) on Illite du Puy, obtained from the literature, were also modelled in this work. Thus, surface complexation constants for the strong sites in the 2SPNE SC/CE sorption model for nine metals with valence states from II to VI have been obtained. A linear relationship between the logarithm of strong site metal binding constants, ^SK_x−1, and the logarithm of the corresponding aqueous hydrolysis stability constant, ^OHK_x, extending over nearly 35 orders of magnitude is established here for illite for these nine metals. Such correlations are often termed linear free energy relationships (LFER), and although they are quite common in aqueous phase chemistry, they are much less so in surface chemistry, especially over this large range. The LFER for illite could be described by the equation: where, “x” is an integer. A similar relationship has been previously obtained for montmorillonite, thus LFERs relating to the sorption on two of the most important clay minerals present in natural systems have been established. Such an LFER approach is an extremely useful tool for estimating surface complexation constants for metals in a chemically consistent manner. It provides a means of obtaining sorption values for radionuclides for which there are no measured values and thus allows gaps in missing sorption data to be filled. An ultimate goal of this approach is to develop a thermodynamic sorption database. This could then be used in radioactive waste management performance assessment studies to calculate sorption in natural systems, and thereby replace the current usage of single solid liquid distribution coefficients (K_d values) to describe radionuclide uptake. Finally, with the data now available, the 2SPNE SC/CE sorption model can be ported into reactive transport models allowing radionuclide migration to be calculated under spatially and temporally changing conditions.  相似文献   

Modelling the sorption of Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) on montmorillonite: Linear free energy relationships and estimates of surface binding constants for some selected heavy metals and actinides     

Michael H. Bradbury  Bart Baeyens 《Geochimica et cosmochimica acta》2005,69(4):875-892

In solution thermodynamics, and more recently in surface chemistry, it is well established that relationships can be found between the free energies of formation of aqueous or surface metal complexes and thermodynamic properties of the metal ions or ligands. Such systematic dependencies are commonly termed linear free energy relationships (LFER). A 2 site protolysis non-electrostatic surface complexation and cation exchange (2SPNE SC/CE) model has been used to model “in house” and literature sorption edge data for eleven elements: Mn(II), Co(II), Ni(II), Zn(II), Cd(II), Eu(III), Am(III), Sn(IV), Th(IV), Np(V) and U(VI) to provide surface complexation constants for the strong sites on montmorillonite. Modelling a further 4 sets of sorption isotherms for Ni(II), Zn(II), Eu(III) and U(VI) provided complexation constants for the weak sites. The protolysis constants and site capacities derived for the 2SPNE SC/CE model in previous work were fixed in all of the calculations. Cation exchange was modelled simultaneously to provide selectivity coefficients. Good correlations between the logarithms of strong ^SK_x−1 and weak ^W1K_x−1 site binding constants on montmorillonite and the logarithm of the aqueous hydrolysis constants ^OHK_x were found which could be described by the following equations: Strong (≡S^SOH) sites: