Mechanism of kaolinite dissolution at room temperature and pressure Part II: kinetic study

Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25°C and in the pH range from 1 to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al–hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10−12.19aH+0.55 + 10−14.36 + 10−10.71aOH−0.75Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. mAn attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10−8.25 [>Al2OH2+] + 10−10.82 [>AlOH2+]0.5 + 10−9.1 [>Al2OH + >AlOH + >SiOH] + 103.78 [>Al2O− + >AlO−]3This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite.     

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10^?5 M, (dissolved uranyl 10^?5 to 10^?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO₃ solutions containing up to 0.01 M NaHCO₃. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10^?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO₂OH⁺ and (UO₂)₃(OH)⁺₅, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO₂OH⁺ surface complex, and a monodentate, bidentate or tridentate (UO₂)₃(OH)⁺₅surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10^?2 and 10^?3 M.     

Calcium adsorption at the rutile-water interface: a potentiometric study in NaCl media to 250°C

Calcium adsorption by rutile was studied potentiometrically from 25 to 250°C, at ionic strengths of 0.03 and 0.30 m in NaCl media, using two complementary experimental methodologies. In the first, net proton adsorption in the presence and absence of Ca²⁺ was monitored, and in the second, samples were periodically withdrawn during the course of a titration to determine Ca²⁺ adsorption directly. These experiments revealed that Ca²⁺adsorption systematically increased with temperature relative to the pH of zero net proton charge in NaCl media alone (pH_znpc(NaCl) – pH). That is, as temperature increased, Ca²⁺ adsorption commenced at progressively more positive pH_znpc(NaCl) – pH values. Increasing ionic strength from 0.03 to 0.30 m NaCl suppressed Ca²⁺ adsorption at all temperatures as a result of either increased competition from Na⁺ or greater complexation of Ca²⁺ by Cl⁻. Finally, there was no apparent trend in the proton stoichiometric ratios (moles H⁺ released / moles Ca²⁺ adsorbed) with increasing temperature. This suggests that the electrostatic and/or chemical processes involved in Ca²⁺ adsorption do not change greatly with increasing temperature. Favorable entropic effects, related to the increasing ease of releasing Ca²⁺ waters of hydration, are believed to be primarily responsible for the increase in adsorption with temperature.     

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

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.     

Adsorption and surface oxidation of Fe(II) on metal (hydr)oxides

The Fe(II) adsorption by non-ferric and ferric (hydr)oxides has been analyzed with surface complexation modeling. The CD model has been used to derive the interfacial distribution of charge. The fitted CD coefficients have been linked to the mechanism of adsorption. The Fe(II) adsorption is discussed for TiO₂, γ-AlOOH (boehmite), γ-FeOOH (lepidocrocite), α-FeOOH (goethite) and HFO (ferrihydrite) in relation to the surface structure and surface sites. One type of surface complex is formed at TiO₂ and γ-AlOOH, i.e. a surface-coordinated Fe²⁺ ion. At the TiO₂ (Degussa) surface, the Fe²⁺ ion is probably bound as a quattro-dentate surface complex. The CD value of Fe²⁺ adsorbed to γ-AlOOH points to the formation of a tridentate complex, which might be a double edge surface complex. The adsorption of Fe(II) to ferric (hydr)oxides differs. The charge distribution points to the transfer of electron charge from the adsorbed Fe(II) to the solid and the subsequent hydrolysis of the ligands that coordinate to the adsorbed ion, formerly present as Fe(II). Analysis shows that the hydrolysis corresponds to the hydrolysis of adsorbed Al(III) for γ-FeOOH and α-FeOOH. In both cases, an adsorbed M(III) is found in agreement with structural considerations. For lepidocrocite, the experimental data point to a process with a complete surface oxidation while for goethite and also HFO, data can be explained assuming a combination of Fe(II) adsorption with and without electron transfer. Surface oxidation (electron transfer), leading to adsorbed Fe(III)(OH)₂, is favored at high pH (pH > ∼7.5) promoting the deprotonation of two Fe_III-OH₂ ligands. For goethite, the interaction of Fe(II) with As(III) and vice versa has been modeled too. To explain Fe(II)-As(III) dual-sorbate systems, formation of a ternary type of surface complex is included, which is supposed to be a monodentate As(III) surface complex that interacts with an Fe(II) ion, resulting in a binuclear bidentate As(III) surface complex.     

X-ray absorption fine structure determination of pH-dependent U-bacterial cell wall interactions

X-ray absorption fine structure (XAFS) measurements was used at the U L3-edge to directly determine the pH dependence of the cell wall functional groups responsible for the absorption of aqueous UO₂²⁺ to Bacillus subtilis from pH 1.67 to 4.80. Surface complexation modeling can be used to predict metal distributions in water-rock systems, and it has been used to quantify bacterial adsorption of metal cations. However, successful application of these models requires a detailed knowledge not only of the type of bacterial surface site involved in metal adsorption/desorption, but also of the binding geometry. Previous acid-base titrations of B. subtilis cells suggested that three surface functional group types are important on the cell wall; these groups have been postulated to correspond to carboxyl, phosphoryl, and hydroxyl sites. When the U(VI) adsorption to B. subtilis is measured, observed is a significant pH-independent absorption at low pH values (<3.0), ascribed to an interaction between the uranyl cation and a neutrally charged phosphoryl group on the cell wall. The present study provides independent quantitative constraints on the types of sites involved in uranyl binding to B. subtilis from pH 1.67 to 4.80. The XAFS results indicate that at extremely low pH (pH 1.67) UO₂²⁺ binds exclusively to phosphoryl functional groups on the cell wall, with an average distance between the U atom and the P atom of 3.64 ± 0.01 Å. This U-P distance indicates an inner-sphere complex with an oxygen atom shared between the UO₂²⁺ and the phosphoryl ligand. The P signal at extremely low pH value is consistent with the UO₂²⁺ binding to a protonated phosphoryl group, as previously ascribed. With increasing pH (3.22 and 4.80), UO₂²⁺ binds increasingly to bacterial surface carboxyl functional groups, with an average distance between the U atom and the C atom of 2.89 ± 0.02 Å. This U-C distance indicates an inner-sphere complex with two oxygen atoms shared between the UO₂²⁺ and the carboxyl ligand. The results of this XAFS study confirm the uranyl-bacterial surface speciation model.     

Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

We have examined the effects of aqueous complexation on rates of dissimilatory reductive precipitation of uranium by Shewanella putrefaciens. Uranium(VI) was supplied as sole terminal electron acceptor to Shewanella putrefaciens (strain 200R) in defined laboratory media under strictly anaerobic conditions. Media were amended with different multidentate organic acids, and experiments were performed at different U(VI) and ligand concentrations. Organic acids used as complexing agents were oxalic, malonic, succinic, glutaric, adipic, pimelic, maleic, citric, and nitrilotriacetic acids, tiron, EDTA, and Aldrich humic acid. Reductive precipitation of U(VI), resulting in removal of insoluble amorphous UO₂ from solution, was measured as a function of time by determination of total dissolved U. Reductive precipitation was measured, rather than net U(VI) reduction to U(IV), to assess overall U removal rates from solution, which may be used to gauge the influence of chelation on microbial U mineralization. Initial linear rates of U reductive precipitation were found to correlate with stability constants of 1:1 aqueous U(VI):ligand and U(IV):ligand complexes. In the presence of strongly complexing ligands (e.g., NTA, Tiron, EDTA), UO₂ precipitation did not occur. Our results are consistent with ligand-retarded precipitation of UO₂, which is analogous to ligand-assisted solid phase dissolution but in reverse: ligand exchange with the U⁴⁺ aquo cation acts as a rate-limiting reaction moderating coordination of water molecules with U⁴⁺, which is a necessary step in UO₂ precipitation. Ligand exchange kinetics governing dissociation rates of ligands from U(VI)-organic complexes may also influence overall UO₂ production rates, although the magnitude of this effect is unclear relative to the effects of U(IV)-organic complexation. Our results indicate that natural microbial-aqueous systems containing abundant organic matter can inhibit the formation of biogenic amorphous UO₂.     

Cadmium and lead interaction with diatom surfaces: A combined thermodynamic and kinetic approach

This work is devoted to the physico-chemical study of cadmium and lead interaction with diatom-water interfaces for two marine planktonic (Thalassiosira weissflogii, TW; Skeletonema costatum, SC) and two freshwater periphytic species (Achnanthidium minutissimum, AMIN; Navicula minima, NMIN) by combining adsorption measurements with surface complexation modeling. Adsorption kinetics was studied as a function of pH and initial metal concentration in sodium nitrate solution and in culture media. Kinetic data were consistent with a two-step mechanism in which the loss of a water molecule from the inner coordination sphere of the metal is rate limiting. Reversible adsorption experiments, with 3 h of exposure to metal, were performed as a function of pH (2-9), metal concentration in solution (10⁻⁹-10⁻³ M), and ionic strength (10⁻³-1.0 M). While the shape of pH-dependent adsorption edge is similar among all four diatom species, the constant-pH adsorption isotherm and maximal binding capacities differ. Measurements of electrophoretic mobilities (μ) revealed negative surface potential for AMIN diatom, however, the absolute value of μ decreases with increase of [Pb²⁺]_aq suggesting the metal adsorption on negative surface sites. These observations allowed us to construct a surface complexation model (SCM) for cadmium and lead binding by diatom surfaces that postulates the Constant Capacitance of the electric double layer and considers Cd and Pb complexation with mainly carboxylic and, partially, silanol groups. In the full range of investigated Cd concentration, the SCM is able to describe the concentration of adsorbed metal as a function of [Cd²⁺]_aq without implying the presence of high affinity, low abundance sites, that are typically used to model the metal interactions with natural multi-component organic substances. At the same time, Cd fast initial reaction requires the presence of “highly reactive sites” those concentration represents only 2.5-3% of the total amount of carboxylic sites. For reversible adsorption experiments, the dominating carboxylic groups, whose concentration is allowed to vary within the uncertainty of experimental acid-base titrations, are sufficient to reproduce the metal adsorption isotherms. Results of this study strongly suggest that laboratory experiments performed in a wide range of metal to biomass ratios, represent robust and relatively simple method for assessing the distribution of metals between aqueous solution and planktonic and periphytic biomass in natural settings.     

Adsorption of rare earth elements onto Carrizo sand: Experimental investigations and modeling with surface complexation

Rare earth element (REE) adsorption onto sand from a well characterized aquifer, the Carrizo Sand aquifer of Texas, has been investigated in the laboratory using a batch method. The aim was to improve our understanding of REE adsorption behavior across the REE series and to develop a surface complexation model for the REEs, which can be applied to real aquifer-groundwater systems. Our batch experiments show that REE adsorption onto Carrizo sand increases with increasing atomic number across the REE series. For each REE, adsorption increases with increasing pH, such that when pH >6.0, >98% of each REE is adsorbed onto Carrizo sand for all experimental solutions, including when actual groundwaters from the Carrizo Sand aquifer are used in the experiments. Rare earth element adsorption was not sensitive to ionic strength and total initial REE concentrations in our batch experiments. It is possible that the differences in experimental ionic strength conditions (i.e., 0.002-0.01 M NaCl) chosen were insufficient to affect REE adsorption behavior. However, cation competition (e.g., Ca, Mg, and Zn) did affect REE adsorption onto Carrizo sand, especially for light rare earth elements (LREEs) at low pH. Rare earth element adsorption onto Carrizo sand can be successfully modeled using a generalized two-layer surface complexation model. Our model calculations suggest that REE complexation with strong surface sites of Carrizo sand exceeds the stability of the aqueous complexes LnOH²⁺, LnSO₄⁺, and LnCO₃⁺, but not that of Ln(CO₃)₂^- or LnPO₄^o in Carrizo groundwaters. Thus, at low pH (<7.3), where major inorganic ligands did not effectively compete with surface sites for dissolved REEs, free metal ion (Ln³⁺) adsorption was sufficient to describe REE adsorption behavior. However, at higher pH (>7.3) where solution complexation of the dissolved REEs was strong, REEs were adsorbed not only as free metal ion (Ln³⁺) but also as aqueous complexes (e.g., as Ln(CO₃)₂^- in Carrizo groundwaters). Because heavy rare earth elements (HREEs) were preferentially adsorbed onto Carrizo sand compared to LREEs, original HREE-enriched fractionation patterns in Carrizo groundwaters from the recharge area flattened along the groundwater flow path in the Carrizo Sand aquifer due to adsorption of free- and solution-complexed REEs.     

The adsorption characteristics of δ-manganese dioxide: a collection of diffuse double layer constants for the adsorption of H+, Cu2+, Ni2+, Zn2+, Cd2+ and Pb2+

Thermodynamic data for all fate-determining processes are needed in order to predict the fate and transport of metals in natural systems. The surface complexation properties of a synthetic MnO₂, δ-MnO₂, have accordingly been investigated using glass electrode potentiometry. Experimental data were interpreted according to the surface complexation model in conjunction with the diffuse double layer model of the solid/solution interface. Adsorption constants were determined using the non-linear optimisation program FITEQL. Surface complexation parameters determined in this way were validated against results obtained from the literature. Best fits of alkalimetric titration data were obtained with a 2-site, 3 surface-species model of the δ-MnO₂ surface. Site concentrations of 2.23×10⁻³ mol g⁻¹ and 7.66×10⁻⁴ mol g⁻¹ were obtained. Corresponding logarithms of formation constants for the postulated surface species are −1.27 (≡XO⁻), −5.99 (≡YO⁻) and 3.52 (≡YOH₂⁺) at I=0.1 M. The surface speciation of δ-MnO₂ is dominated by ≡XO⁻ over the pH range investigated. Metal adsorption was modelled with surface species of the type ≡XOM⁺, ≡XOMOH, ≡YOM⁺, ≡YOMOH (M=Cu, Ni, Zn, Cd and Pb) and ≡XOM₂OH²⁺ (M=Pb). For Cu, Ni and Zn, titration data could be modelled with ≡XOM⁺, ≡XOMOH, ≡YOM⁺ and ≡YOMOH, whereas for Cd, ≡XOM⁺ and ≡YOM⁺ were sufficient. Lead data were best modelled by assuming the dinuclear species ≡XOM₂OH²⁺ to be the only surface species to form. Adsorption constants determined for Ni, Cu and Zn follow the Irving-Williams sequence. The model suggests an adsorption order of (Pb, Cu) > (Ni, Zn) > Cd. The discrepancy between model predictions and published adsorption results is similar to the variability observed in experimental results from different laboratories.     

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

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.     

The adsorption of aquatic humic substances by iron oxides

The interactions of humic substances from Esthwaite Water with hydrous iron oxides (α-FeOOH, α-Fe₂O₃, amorphous Fe-gel) have been examined by measuring adsorption isotherms and by microelectrophoresis. In Na⁺-Cl^?-HCO₃^?at I = 0.002 M (medium I) the extent of adsorption decreases with increasing pH. The results are consistent with a mechanism involving ligand exchange of humic anionic groups with H₂O and OH^?of surface Fe-OH₂⁺and Fe-OH groups respectively, with an increasing degree of protonation of the adsorbed humics as the adsorption density increases at constant pH.At pH 7 in a medium containing Mg²⁺, Ca²⁺ and SO₄^2?, at their Esthwaite Water concentrations and at I= 0.002 M (medium II) the adsorption capacity of goethite (α-FeOOH) is approximately twice that in medium I. Electrophoresis experiments show that the extra capacity is associated with coadsorption of Mg²⁺ and/or Ca²⁺ ions.When the iron oxides are added to samples of Esthwaite Water itself they become negatively charged and plots of electrophoretic mobility against pH for the natural water are identical to those in medium II plus humics.     

Infrared spectroscopic characterisation of arsenate (V) ion adsorption from mine waters,Macraes mine,New Zealand

Processing waters contain up to 10 mg l⁻¹ dissolved As at the Macraes mine, New Zealand, and this is all removed by adsorption as the water percolates through a large earth dam. Laboratory experiments were set up to identify which mineral is the most effective substrate for this adsorption of As. The experiments were conducted using infrared (IR) spectroscopy of thin mineral films adhering to a ZnSe prism. Silicates, including kaolinite, adsorbed only small amounts of As which was readily washed off. Hydrated Fe oxides (HFO) were extremely effective at adsorbing As, particularly the natural amorphous HFO currently being deposited from dam discharge waters at the Macraes mine. An adsorption isotherm determined for this natural material has the adsorption constant, K_ads=(1.9±0.4)×10⁴ M⁻¹, and the substrate becomes saturated with adsorbed As when solution concentrations exceed about 50 mg l⁻¹. Saturation is not being reached at the Macraes mine. Arsenic adsorbed on to natural HFO has a distinctive IR spectrum with the absorption peak varying from 800 cm⁻¹ (alkaline solutions) to 820 cm⁻¹ (neutral to acid solutions). Much of this adsorbed As is strongly bound and difficult to wash off. Arsenate ions adsorb in a bidentate structure which may be a precursor for scorodite crystallisation.     

碳氮源对P.Fluorescens吸附和还原U(Ⅵ)的影响研究

为探讨不同碳氮源对培养基中铀酰离子的络合形态及荧光假单胞菌对铀吸附和还原作用的影响,本文以查氏培养基为基础,将蔗糖替换为葡萄糖或将硝酸钠替代为氯化铵,采用Visual MINTEQ分析研究铀酰离子络合形态。研究结果表明,碳氮源的种类对微生物生长无影响。U(Ⅵ)浓度为10 mg/L时,3种培养基中铀酰离子的主要络合形态均为UO2PO-4。U(Ⅵ)浓度为50~200 mg/L,查氏和葡萄糖碳源培养基中铀酰离子主要络合形态均为UO2(SO4)2-2。氯化铵氮源培养基中,U(Ⅵ)浓度为50 mg/L时铀酰离子的主要络合形态为UO2HPO4(aq),浓度为100~200 mg/L时主要是UO2Cl2(aq)。荧光假单胞菌对U(Ⅵ)耐受浓度高达100 mg/L,当U(Ⅵ)浓度达到200 mg/L时,菌体失活。荧光假单胞菌为活体时,对U(Ⅵ)的吸附率为84.02%~92.59%,还原率为3.32%~10.94%,不同碳氮源对铀吸附和还原的影响较小。荧光假单胞菌为死体时,对铀的吸附率为24.33%~39.05%;非葡萄糖碳源培养基中,对铀的还原率为37.50%~44.58%,含还原性葡萄糖的培养基条件下U(Ⅵ)的还原率为53.12%。还原性葡萄糖与荧光假单胞菌对铀的还原为协同作用。荧光假单胞菌为活体时,培养基成分被微生物充分利用,两者协同作用不明显。     

Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ inner-sphere complexes on variable-charge ≡AlOH sites and as Cu²⁺ on ion exchangeable ≡X--H⁺ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)₆ polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)₂Cu(OH)₂⁰, tridentate (≡Al₃O(OH)₂)Cu₂(OH)₃⁰ and ≡X--Cu²⁺ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions

     

Trace metal–humate interactions. I. Experimental determination of conditional stability constants

The enhancement of mobility of radionuclides in the geosphere through complexation by humic substances is a source of uncertainty in performance assessment of radioactive waste repositories. Only very few data sets are available which are relevant for performance assessment of an underground repository for radioactive waste. Using the equilibrium dialysis-ligand exchange method developed at the Paul Scherrer Institut, conditional stability constants for the formation of complexes of Aldrich humic acid with Ca²⁺, NpO₂⁺, Co²⁺, Ni²⁺, UO₂²⁺ and Eu³⁺ and complexes of Laurentian soil- and Suwannee River fulvic acid with Co²⁺, UO₂²⁺ and Eu³⁺ were measured. pH was varied between 5 and 10 and ionic strength between 0.02 and 0.2 M. The data are presented as equilibrium coefficients that are free from any model assumptions. The equilibrium coefficients increased in the order Ca²⁺≅NpO₂⁺<Co²⁺< Ni²⁺<UO₂²⁺< Eu³⁺. The quality of the data is assessed in an extended discussion of statistical and systematical errors, and by a critical ‘rereview’ of the auxiliary stability constants used for the calculation of the equilibrium coefficients. An approximate overall uncertainty of 0.5 log-units is estimated for the stability data reported. The conditional stability constants were found to increase markedly with increasing pH in the case of Co²⁺, UO₂²⁺ and Eu³⁺. For Ni²⁺, Ca²⁺ and NpO₂⁺ this effect was less pronounced. For all metal ions tested, the influence of ionic strength was of less importance, and the conditional stability constants did not show a significant dependence on the type of humic substances investigated.     

Synthesis,crystallographic data,solubility and electrokinetic properties of meta-zeunerite,meta-kirchheimerite and nickel-uranylarsenate

{M[UO₂¦AsO₄]₂ · nH₂O} with M=Cu²⁺, Co²⁺, Ni²⁺ has been synthesized from reagent grade chemicals and by ion exchange of trögerite {HUO₂AsO₄ · 4 H₂O}. Synthetic meta-zeunerite (M=Cu²⁺), meta-kirchheimerite (M=Co²⁺) and nickel-uranylarsenate are all tetragonal. The cell parameters determined from Guinier-Hägg diffraction data for {Cu[UO₂¦AsO₄]₂ · 8 H₂O} are a=b=7.10 Å and c=17.42 Å, with Z=2 and the measured density 3.70 g cm^?3. The cell parameters for {Co[UO₂¦AsO₄]₂ · 7 H₂O} and {Ni[UO²¦AsO⁴]² · 7 H₂O} are a=b=20.25 Å and c=17.20 Å, with Z=16 and the measured density 3.82 and 3.74 g cm^?3, respectively. The solubility products for synthetic Cu-, Co- and Ni-uranylarsenate at 25° C are 10^?49.20, 10^?45.34 and 10^?45.10, respectively. The zeta-potential remains negative between pH=2 and pH=9 and is strongly affected by the presence of different cations.