Mechanisms of Nd(III) uptake by 11 Å tobermorite and xonotlite

The uptake of Nd(III) by the crystalline C–S–H phases 11 Å tobermorite and xonotlite has been investigated by the combined use of wet chemistry techniques, extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray diffraction (XRD) in combination with Rietveld refinement. The results from XRD and EXAFS allowed the different modes of Nd–Ca substitution in tobermorite and xonotlite to be distinguished from each other. Wet chemistry and EXAFS data showed that the formation of any Nd solid phase with fixed stoichiometry could be ruled out. XRD studies on the samples with high Nd loading (350 μmol Nd/g solid phase) further showed that Nd was bound in the structure of C–S–H phases. The EXAFS data suggested that Nd could form several species on xonotlite and tobermorite at low loadings (7–35 μmol Nd/g solid phase). Neodymium was predominantly bound on the external surface of both crystalline C–S–H phases after 1 day of reaction time and predominantly incorporated in the Ca layers of the crystalline C–S–H phases in the long run (?60 days reaction time). The latter process was faster at low Nd loadings and was apparently controlled by re-crystallization of the C–S–H phases. Neodymium incorporation was accompanied by the release of “zeolitic” water (water molecules in the interlayer of C–S–H) and bridging Si tetrahedra, reflected by the formation of more disordered structures in both C–S–H phases. The Nd retention model proposed in this study helps to improve understanding of the immobilization of trivalent lanthanides and actinides in cementitious materials. This knowledge is essential for long-term predictions of radionuclide retention in conjunction with a more detailed assessment of the safe disposal of actinides in the cementitious near field of a repository for radioactive waste.     

Coordination chemistry and hydrolysis of Fe(III) in a peat humic acid studied by X-ray absorption spectroscopy

The speciation of iron (Fe) in soils, sediments and surface waters is highly dependent on chemical interactions with natural organic matter (NOM). However, the molecular structure and hydrolysis of the Fe species formed in association with NOM is still poorly described. In this study extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry and hydrolysis of Fe(III) in solution of a peat humic acid (5010-49,200 μg Fe g⁻¹ dry weight, pH 3.0-7.2). Data were analyzed by both conventional EXAFS data fitting and by wavelet transforms in order to facilitate the identification of the nature of backscattering atoms. Our results show that Fe occurs predominantly in the oxidized form as ferric ions and that the speciation varies with pH and Fe concentration. At low Fe concentrations (5010-9920 μg g⁻¹; pH 3.0-7.2) mononuclear Fe(III)-NOM complexes completely dominates the speciation. The determined bond distances for the Fe(III)-NOM complexes are similar to distances obtained for Fe(III) complexed by desferrioxamine B and oxalate indicating the formation of a five-membered chelate ring structure. At higher Fe concentrations (49,200 μg g⁻¹; pH 4.2-6.9) we detect a mixture of mononuclear Fe(III)-NOM complexes and polymeric Fe(III) (hydr)oxides with an increasing amount of Fe(III) (hydr)oxides at higher pH. However, even at pH 6.9 and a Fe concentration of 49,200 μg g⁻¹ our data indicates that a substantial amount of the total Fe (>50%) is in the form of organic complexes. Thus, in environments with significant amounts of organic matter organic Fe complexes will be of great importance for the geochemistry of Fe. Furthermore, the formation of five-membered chelate ring structures is in line with the strong complexation and limited hydrolytic polymerization of Fe(III) in our samples and also agrees with EXAFS derived structures of Fe(III) in organic soils.     

Europium retention onto clay minerals from 25 to 150 °C: Experimental measurements, spectroscopic features and sorption modelling

The sorption of Eu(III) onto kaolinite and montmorillonite was investigated up to 150 °C. The clays were purified samples, saturated with Na in the case of montmorillonite. Batch experiments were conducted at 25, 40, 80 and 150 °C in 0.5 M NaClO₄ solutions to measure the distribution coefficients (Kd) of Eu as a trace element (<10⁻⁶ mol/L) between the solution and kaolinite. For the Na-montmorillonite, we used Kd results from a previous study [Tertre, E., Berger, G., Castet, S., Loubet, M., Giffaut, E., 2005. Experimental study of adsorption of Ni²⁺, Cs⁺ and Ln³⁺ onto Na-montmorillonite up to 150 °C. Geochim. Cosmochim. Acta69, 4937-4948] obtained under exactly the same conditions. The number and nature of the Eu species sorbed onto both clay minerals were investigated by time resolved laser fluorescence spectroscopy (TRLFS) in specific experiments in the same temperature range. We identified a unique inner-sphere complex linked to the aluminol sites in both clays, assumed to be AlOEu²⁺ at the edge of the particles, and a second exchangeable outer-sphere complex for montmorillonite, probably in an interlayer position. The Kd values were used to adjust the parameters of a surface complexation model (DLM: diffuse layer model) from 25 to 150 °C. The number of Eu complexes and the stoichiometry of reactions were constrained by TRLFS. The acidity constants of the amphoteric aluminol sites were taken from another study [Tertre, E., Castet, S., Berger, G., Loubet, M., Giffaut, E. Acid/base surface chemistry of kaolinite and Na-montmorillonite at 25 and 60 °C: experimental study and modelling. Geochim. Cosmochim. Acta, in press], which integrates the influence of the negative structural charge of clays on the acid/base properties of edge sites as a function of temperature and ionic strength. The results of the modelling show that the observed shift of the sorption edge towards low pH with increasing temperature results solely from the contribution of the AlOEu²⁺ edge complexes. Finally, we successfully tested the performance of our model by confronting the predictions with experimental Kd data. We used our own data obtained at lower ionic strength (previous study) or higher suspension density and higher starting concentration (TRLFS runs, this study), as well as published data from other experimental studies [Bradbury, M.H., Baeyens, B., 2002. Sorption of Eu on Na and Ca-montmorillonite: experimental investigations and modeling with cation exchange and surface complexation. Geochim. Cosmochim. Acta66, 2325-2334; Kowal-Fouchard, A., 2002. Etude des mécanismes de rétention des ions U(IV) et Eu(III) sur les argiles: influence des silicates. Ph.D. Thesis, Université Paris Sud, France, 330p].     

Zinc sorption to biogenic hexagonal-birnessite particles within a hydrated bacterial biofilm

Biofilm-embedded Mn oxides exert important controls on trace metal cycling in aquatic and soil environments. The speciation and mobility of Zn in particular has been linked to Mn oxides found in streams, wetlands, soils, and aquifers. We investigated the mechanisms of Zn sorption to a biogenic Mn oxide within a biofilm produced by model soil and freshwater Mn^II-oxidizing bacteria Pseudomonas putida. The biogenic Mn oxide is a c-disordered birnessite with hexagonal layer symmetry. Zinc adsorption isotherm and Zn and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy experiments were conducted at pH 6.9 to characterize Zn sorption to this biogenic Mn oxide, and to determine whether the bioorganic components of the biofilm affect metal sorption properties. The EXAFS data were analyzed by spectral fitting, principal component analysis, and linear least-squares fitting with reference spectra. Zinc speciation was found to change as Zn loading to the biosorbent [bacterial cells, extracellular polymeric substances (EPS), and biogenic Mn oxide] increased. At low Zn loading (0.13 ± 0.04 mol Zn kg⁻¹ biosorbent), Zn was sorbed to crystallographically well-defined sites on the biogenic oxide layers in tetrahedral coordination to structural O atoms. The fit to the EXAFS spectrum was consistent with Zn sorption above and below the Mn^IV vacancy sites of the oxide layers. As Zn loading increased to 0.72 ± 0.04 mol Zn kg⁻¹ biosorbent, Zn was also detected in octahedral coordination to these sites. Overall, our results indicate that the biofilm did not intervene in Zn sorption by the Mn-oxide because sorption to the organic material was observed only after all Mn vacancy sites were capped by Zn. The organic functional groups present in the biofilm contributed significantly to Zn removal from solution when Zn concentrations exceeded the sorption capacity of the biooxide. At the highest Zn loading studied, 1.50 ± 0.36 mol Zn kg⁻¹ biosorbent, the proportion of total Zn sorption attributed to bioorganic material was 38 mol%. The maximum Zn loading to the biogenic oxide that we observed was 4.1 mol Zn kg⁻¹ biogenic Mn oxide, corresponding to 0.37 ± 0.02 mol Zn mol⁻¹ Mn. This loading is in excellent agreement with previous estimates of the content of cation vacancies in the biogenic oxide. The results of this study improve our knowledge of Zn speciation in natural systems and are consistent with those of Zn speciation in mineral soil fractions and ferromanganese nodules where the Mn oxides present are possibly biogenic.     

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

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.     

New insight into the structure of nanocrystalline ferrihydrite: EXAFS evidence for tetrahedrally coordinated iron(III)

Ferrihydrite (Fh) is a short-range ordered nanocrystalline iron(III) (oxyhydr)oxide that has been recognized to play an important role in contaminant sequestration and in iron cycling in geological and biological systems. Despite intensive research for the two last decades, the structure of Fh is still a subject of debate. In the present study, we report extended X-ray absorption fine structure (EXAFS) spectroscopy data collected on a large set of ferrihydrites and model compounds samples including especially nano-crystalline maghemite (Mh), goethite (Gt), and akaganeite (Aka). This set of EXAFS data recorded at cryogenic temperature over a wide energy range allows us to precisely determine the Fe-O mean distance (〈Fe-O〉) in the first coordination shell of iron for this large set of iron (oxyhydr)oxides. Our EXAFS analysis includes both classical shell-by-shell fits of Fourier-filtered and unfiltered data as well as analysis of Fe-O distance distribution in the first coordination shell of iron using the Landweber iteration method. 〈Fe-O〉 determined by these complementary EXAFS analyses are similar: 〈Fe-O〉 is shorter in Mh (1.96 ± 0.01 Å) that contains 37.5% of tetrahedral iron, than in Gt (2.01 ± 0.01 Å), Aka (2.00 ± 0.01 Å) and hematite (Hm) (2.01 ± 0.01 Å) that do not contain tetrahedral iron. 〈Fe-O〉 for the five Fh samples investigated (1.97 ± 0.01 Å) was found to be slightly longer than in Mh and significantly shorter than those in Gt, Aka and Hm. This short 〈Fe-O〉 distance in Fh indicates the presence of significant amount of tetrahedrally coordinated iron(III) in all Fh samples studied, which ranges between 20 ± 5% and 30 ± 5% of total iron. In addition, our analysis of Fe-Fe distances observed by EXAFS is consistent with a Keggin-like motif at a local scale (∼5 Å) in the Fh structure.     

Uranyl sorption species at low coverage on Al-hydroxide: TRLFS and XAFS studies

Detailed understanding of the respective roles of solution and surface parameters on the reactions at uranyl solution/Al-(hydr)oxide interfaces is crucial to model accurately the behaviour of U in nature. We report studies on the effects of the initial aqueous uranyl speciation in moderately acidic solutions, e.g. of mononuclear, polynuclear uranyl species and/or (potential) U(VI) colloids, on the sorption of U by large surface areas of amorphous Al-hydroxide. Investigations by Extended X-ray Absorption Fine Structure (EXAFS) and Time-Resolved Laser-induced Fluorescence Spectroscopy (TRLFS) reveal similar U coordination environments on Al-hydroxide for low to moderate U loadings of sorption samples obtained at pH 4-5, independently of the presence of mononuclear or polynuclear aqueous species, or of the potential instability of initial solutions favoring true U-colloids formation. EXAFS data can be interpreted in terms of a dimeric, bidentate, inner-sphere uranyl surface complex as an average of the U surface structures. TRLFS data, however, provide valuable insights into the complex U surface speciation. They indicate multiple uranyl surface species under moderately acidic conditions, as predominant mononuclear and/or dinuclear, inner-sphere surface complexes and as additional minor species having U atoms in a uranyl (hydr)oxide-like coordination environment. The additional species probably occur as surface polymers and/or as adsorbed true U colloids, depending on the aqueous U concentration level (1-100 μM). These results are of importance because they suggest that Al-hydroxide surface characteristics strongly control uranyl surface species in moderately acidic systems.     

Transformations of mercury, iron, and sulfur during the reductive dissolution of iron oxyhydroxide by sulfide

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

Sorption of Eu(III)/Cm(III) on Ca-montmorillonite and Na-illite. Part 2: Surface complexation modelling

Sorption edges and isotherms for Eu(III) uptake on Ca-montmorillonite and Na-illite in 0.066 mol/L Ca(ClO₄)₂ and 0.1 mol/L NaClO₄ background electrolytes, respectively, were modelled using a quasi-mechanistic sorption model (the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model). For both clay minerals the Eu sorption edges could be quantitatively modelled in the pH range ∼3 to ∼10 using cation exchange reactions for Eu³⁺/Na⁺ and Eu³⁺/Ca²⁺ and three surface complexation reactions on the strong sorption sites forming ≡S^SOEu²⁺, ≡S^SOEuOH⁺ and ≡S^SOEu(OH)₂° inner sphere complexes which appear successively with increasing pH. Time resolved laser fluorescence spectroscopy (TRLFS) measurements of Cm(III) loaded Ca-montmorillonite and Na-illite were available from Part 1 of this work. De-convolution of the normalised fluorescence spectra measured at different pH values indicated three distinct Cm surface complexes, Cm complexes 1, 2 and 3 for both clay minerals, in agreement with model predictions, but with different distribution functions for the individual species. Under the assumption that Eu and Cm exhibit essentially the same hydrolysis and sorption behaviour, the Eu surface complexation constants were used to predict surface species distribution functions for Cm under the same experimental conditions used in the TRLFS measurements. Comparison of modelled and experimentally deduced species distributions indicated that for both clay minerals peak heights and widths of the three peaks did not correspond particularly well. It is shown that the calculated species distribution functions are sensitive to the values of the hydrolysis constants used in the calculations, whereas modelling the sorption edge measurements by applying the 2SPNE SC/CE approach is much less sensitive. By modifying the values of the hydrolysis constants within their uncertainty range and re-modelling the sorption edges, considerably better correspondence between the modelled and TRLFS species distribution functions was found. In particular, peak positions, heights and widths for the model predicted peaks for the ≡S^SOCm²⁺ and ≡S^SOCmOH⁺ species distribution, and those for Cm complexes 1 and 2 derived from TRLFS, were found to be very close for both clay minerals. However, discrepancies were still apparent between the profile for the calculated ≡S^SOEu(OH)₂° surface species and the Cm complex 3 species, especially in the case of Na-illite.     

Speciation and solubility of heavy metals in contaminated soil using X-ray microfluorescence, EXAFS spectroscopy, chemical extraction, and thermodynamic modeling

Synchrotron-based X-ray radiation microfluorescence (μ-SXRF) and micro-focused and powder extended X-ray absorption fine structure (EXAFS) spectroscopy measurements, combined with desorption experiments and thermodynamic calculations, were used to evaluate the solubility of metal contaminants (Zn, Cu, Pb) and determine the nature and fractional amount of Zn species in a near-neutral pH (6.5-7.0) truck-farming soil contaminated by sewage irrigation for one hundred years. Zn is the most abundant metal contaminant in the soil (1103 mg/kg), followed by Pb (535 mg/kg) and Cu (290 mg/kg). The extractability of Zn, Pb, and Cu with citrate, S,S-ethylenediaminedisuccinic acid (EDDS), and ethylenediaminetetraacetic acid (EDTA) was measured as a function of time (24 h, 72 h, 144 h), and also as a function of the number of applications of the chelant (5 applications each with 24 h of contact time). Fifty-three percent of the Zn was extracted after 144 h with citrate, 51% with EDDS and 46% with EDTA, compared to 69, 87, and 61% for Cu, and 24, 40, and 34% for Pb. Renewing the extracting solution removed more of the metals. Seventy-nine, 65, and 57% of the Zn was removed after five cycles with citrate, EDDS and EDTA, respectively, compared to 88, 100, and 72% for Cu, and 91, 65, and 47% for Pb. Application to the untreated soil of μ-SXRF, laterally resolved μ-EXAFS combined with principal component analysis, and bulk averaging powder EXAFS with linear least-squares combination fit of the data, identified five Zn species: Zn-sorbed ferrihydrite, Zn phosphate, Zn-containing trioctahedral phyllosilicate (modeled by the Zn kerolite, Si₄(Mg_1.65Zn_1.35)O₁₀(OH)₂ · nH₂O), willemite (Zn₂SiO₄), and gahnite (ZnAl₂O₄), in proportions of ∼30, 28, 24, 11, and less than 10%, respectively (precision: 10% of total Zn). In contrast to Cu and Pb, the same fractional amount of Zn was extracted after 24 h contact time with the three chelants (40-43% of the initial content), suggesting that one of the three predominant Zn species was highly soluble under the extraction conditions. Comparison of EXAFS data before and after chemical treatment revealed that the Zn phosphate component was entirely and selectively dissolved in the first 24 h of contact time. Preferential dissolution of the Zn phosphate component is supported by thermodynamic calculations. Despite the long-term contamination of this soil, about 79% of Zn, 91% of Pb, and 100% of Cu can be solubilized in the laboratory on a time scale of a few days by chemical complexants. According to metal speciation results and thermodynamic calculations, the lower extraction level measured for Zn is due to the Zn phyllosilicate component, which is less soluble than Zn phosphate and Zn ferrihydrite.     

Radium uptake during barite recrystallization at 23 ± 2 °C as a function of solution composition: An experimental Ba and Ra tracer study

High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl₂, Na₂SO₄, NaCl and NaHCO₃) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with ¹³³Ba or ²²⁶Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both ¹³³Ba and ²²⁶Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of ¹³³Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The ¹³³Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of ¹³³Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10⁻¹¹ to 1.9 × 10⁻¹⁰ mol m⁻² s⁻¹ (2.4-16 μmol m⁻² d⁻¹), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10⁻¹⁰ mol m⁻² s⁻¹ or ∼49 μmol m⁻² d⁻¹) were determined.Radium uptake by barite was determined by monitoring the decrease of ²²⁶Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (W_AB = 3720-6200 J/mol, a₀ = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (W_AB = 1240 J/mol, a₀ = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings.     

Incorporation of trivalent actinides into calcite: A time resolved laser fluorescence spectroscopy (TRLFS) study

In order to characterize and quantify the substitution of Ca(II) by Cm(III) (coordination, charge compensation), homogeneous Cm(III) coprecipitated calcite was synthesized in a mixed-flow-through experiment. Two sets of experiments were conducted at pH 8.1 and at pH 12.5.At pH 8.1 two calcites, a calcite with a low Cm³⁺ concentration (LCMpH8.1) and a calcite with a high M³⁺ (Gd³⁺ and Cm³⁺) concentration (HCMpH8.1) were grown and investigated by time resolved laser fluorescence spectroscopy. The Cm(III) emission spectra of LCMpH8.1 and HCMpH8.1 show the same Cm(III) fluorescence signals for two Cm(III) species; Cm(III) species (1) with a peak maximum at 606.2 nm and Cm(III) species (2) with a peak maximum at 620.3 nm. Cm(III) species (1) has a mean lifetime of τ = 386 ± 40 μs and Cm(III) species (2) has a mean lifetime of τ = 1874 ± 200 μs. A lifetime of 386 μs correlates with 1.3 water molecule in the first coordination sphere of the Cm ion whereas a lifetime of 1874 μs indicates the total loss of the Cm(III) hydration sphere. According to the fluorescence emission peak position and the fluorescence emission lifetime, Cm(III) species (1) is identified as a surface sorbed species whereas Cm(III) species (2) is identified as a Cm(III) incorporated into the calcite lattice.Cm(III) fluorescence emission spectra of Cm(III) doped calcite grown at pH 12.5 (LCMpH12.5) show the same peak maxima which are found for LCMpH8.1 and HCMpH8.1 grown at pH 8.1 but an additional emission band at 608.2 nm (3) is found, which can be assigned to a further Cm(III) species. Fluorescence emission lifetime measurements show that this Cm(III) species (3) has a lifetime of τ = 477 ± 25 μs, which correlates with 0.9 water molecules in the first coordination sphere. Cm(III) species (3) is suggested to be a CmOH²⁺ incorporated species.     

X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite

Organic ligands are known to interfere with the polymerization of Fe(III), but the extent of interference has not been systematically studied as a function of structural ligand properties. This study examines how the number and position of phenol groups in hydroxybenzoic acids affect both ferrihydrite formation and its local (<5 Å) Fe coordination. To this end, acid Fe(III) nitrate solutions were neutralized up to pH 6.0 in the presence of 4-hydroxybenzoic acid (4HB), 2,4-dihydroxybenzoic acid (2,4DHB), and the hydroquinone 3,4-dihydroxybenzoic acid (3,4DHB). The initial molar ligand/Fe ratios ranged from 0 to 0.6. The precipitates were dialyzed, lyophilized, and subsequently studied by X-ray absorption spectroscopy and synchrotron X-ray diffraction. The solids contained up to 32 wt.% organic C (4HB ∼ 2,4DHB < 3,4DHB). Only precipitates formed in 3,4DHB solutions comprised considerable amounts of Fe(II) (Fe(II)/Fe_tot ≤ 6 mol%), implying the abiotic mineralization of the catechol-group bearing ligand during Fe(III) hydrolysis under oxic conditions. Hydroxybenzoic acids decreased ferrihydrite formation in the order 4HB ∼ 2,4DHB ? 3,4DHB, which documents that phenol group position rather than the number of phenol groups controls the ligand’s interaction with Fe(III). The coordination numbers of edge- and double corner-sharing Fe in the precipitates decreased by up to 100%. Linear combination fitting (LCF) of Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra revealed that this decrease was due to increasing amounts of organic Fe(III) complexes in the precipitates. Although EXAFS derived coordination numbers of Fe in ferrihydrite remained constant within error, all organic ligands decreased the coherently scattering domain (CSD) size of ferrihydrite as indicated by synchrotron X-ray diffraction analysis (4HB < 2,4DHB ? 3,4DHB). With decreasing particle size of ferrihydrite its Fe(O,OH)₆ octahedra became progressively distorted as evidenced by an increasing loss of centrosymmetry of the Fe sites. Pre-edge peak analysis of the Fe K-edge XANES spectra in conjunction with LCF results implied that ferrihydrite contains on an average 13 ± 3% tetrahedral Fe(III), which is in very good agreement with the revised single-phase structural model of ferrihydrite (Michel, F. M., Barron, V., Torrent, J., Morales, M. P. et al. (2010) Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism. Proc. Natl. Acad. Sci. USA107, 2787-2792). The results suggest that hydroxybenzoic acid moieties of natural organic matter (NOM) effectively suppress ferrihydrite precipitation as they kinetically control the availability of inorganic Fe(III) species for nucleation and/or polymerization reactions. As a consequence, NOM can trigger the formation of small ferrihydrite nanoparticles with increased structural strain. These factors may eventually enhance the biogeochemical reactivity of ferrihydrite formed in NOM-rich environments. This study highlights the role of hydroquinone structures of NOM for Fe complexation, polymerization, and redox speciation.     

Effect of groundwater pH and ionic strength on strontium sorption in aquifer sediments: Implications for Sr mobility at contaminated nuclear sites

Strontium-90 is a beta emitting radionuclide produced during nuclear fission, and is a problem contaminant at many nuclear facilities. Transport of ⁹⁰Sr in groundwaters is primarily controlled by sorption reactions with aquifer sediments. The extent of sorption is controlled by the geochemistry of the groundwater and sediment mineralogy. Here, batch sorption experiments were used to examine the sorption behaviour of ⁹⁰Sr in sediment–water systems representative of the UK Sellafield nuclear site based on groundwater and contaminant fluid compositions. In experiments with low ionic strength groundwaters (<0.01 mol L⁻¹), pH variation is the main control on sorption. The sorption edge for ⁹⁰Sr was observed between pH 4 and 6 with maximum sorption occurring (K_d ∼ 10³ L kg⁻¹) at pH 6–8. At ionic strengths above 10 mmol L⁻¹, and at pH values between 6 and 8, cation exchange processes reduced ⁹⁰Sr uptake to the sediment. This exchange process explains the lower ⁹⁰Sr sorption (K_d ∼ 40 L kg⁻¹) in the presence of artificial Magnox tank liquor (IS = 29 mmol L⁻¹). Strontium K-edge EXAFS spectra collected from sediments incubated with Sr²⁺ in either HCO₃-buffered groundwater or artificial Magnox tank liquor, revealed a coordination environment of ∼9 O atoms at 2.58–2.61 Å after 10 days. This is equivalent to the Sr²⁺ hydration sphere for the aqueous ion and indicates that Sr occurs primarily in outer sphere sorption complexes. No change was observed in the Sr sorption environment with EXAFS analysis after 365 days incubation. Sequential extractions performed on sediments after 365 days also found that ∼80% of solid associated ⁹⁰Sr was exchangeable with 1 M MgCl₂ in all experiments. These results suggest that over long periods, ⁹⁰Sr in contaminated sediments will remain primarily in weakly bound surface complexes. Therefore, if groundwater ionic strength increases (e.g. by saline intrusion related to sea level rise or by design during site remediation) then substantial remobilisation of ⁹⁰Sr is to be expected.     

The structure of monomeric and dimeric uranyl adsorption complexes on gibbsite: A combined DFT and EXAFS study

We investigated the structure of uranyl sorption complexes on gibbsite (pH 5.6-9.7) by two independent methods, density functional theory (DFT) calculations and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U-L_III edge. To model the gibbsite surface with DFT, we tested two Al (hydr)oxide clusters, a dimer and a hexamer. Based on polarization, structure, and relaxation energies during geometry optimization, the hexamer cluster was found to be the more appropriate model. An additional advantage of the hexamer model is that it represents both edges and basal faces of gibbsite. The DFT calculations of (monomeric) uranyl sorption complexes show an energetic preference for the corner-sharing versus the edge-sharing configuration on gibbsite edges. The energy difference is so small, however, that possibly both surface species may coexist. In contrast to the edge sites, sorption to basal sites was energetically not favorable. EXAFS spectroscopy revealed in all investigated samples the same interatomic distances of the uranyl coordination environment (R_U-Oax ≈ 1.80 Å, R_U-Oeq ≈ 2.40 Å), and towards the gibbsite surface (R_U-O ≈ 2.87 Å, R_U-Al ≈ 3.38 Å). In addition, two U-U distances were observed, 3.92 Å at pH 9.7 and 4.30 Å at pH 5.6, both with coordination numbers of ∼1. The short U-U distance is close to that of the aqueous uranyl hydroxo dimer, UO₂(OH)₂, reported as 3.875 Å in the literature, but significantly longer than that of aqueous trimers (3.81-3.82 Å), suggesting sorption of uranyl dimers at alkaline pH. The longer U-U distance (4.30 Å) at acidic pH, however, is not in line with known aqueous uranyl polymer complexes. Based on the EXAFS findings we further refined dimeric surface complexes with DFT. We propose two structural models: in the acidic region, the observed long U-U distance can be explained with a distortion of the uranyl dimer to form both a corner-sharing and an edge-sharing linkage to neighboring Al octahedra, leading to R_U-U = 4.150 Å. In the alkaline region, a corner-sharing uranyl dimer complex is the most favorable. The U-O path at ∼2.87 Å in the EXAFS spectra arises from the oxygen atom linking two Al cations in corner-sharing arrangement. The adsorption structures obtained by DFT calculations are in good agreement with the structural parameters from EXAFS analysis: U-Al (3.394 Å), U-U (3.949 Å), and U-O (2.823 Å) for the alkaline pH model, and U-Al (3.279 Å), U-U (4.150 Å), and U-O (2.743 Å) for the acidic pH model. This work shows that by combining EXAFS and DFT, consistent structural models for uranyl sorption complexes can be obtained, which are relevant to predict the migration behavior of uranium at nuclear facilities.     

Speciation and thermodynamic properties for cobalt chloride complexes in hydrothermal fluids at 35-440 °C and 600 bar: An in-situ XAS study

Aqueous Co(II) chloride complexes play a crucial role in cobalt transport and deposition in ore-forming hydrothermal systems, ore processing plants, and in the corrosion of special Co-bearing alloys. Reactive transport modelling of cobalt in hydrothermal fluids relies on the availability of thermodynamic properties for Co complexes over a wide range of temperature, pressure and salinity. Synchrotron X-ray absorption spectroscopy was used to determine the speciation of cobalt(II) in 0-6 m chloride solutions at temperatures between 35 and 440 °C at a constant pressure of 600 bar. Qualitative analysis of XANES spectra shows that octahedral species predominate in solution at 35 °C, while tetrahedral species become increasingly important with increasing temperature. Ab initio XANES calculations and EXAFS analyses suggest that in high temperature solutions the main species at high salinity (Cl:Co >> 2) is CoCl₄²⁻, while a lower order tetrahedral complex, most likely CoCl₂(H₂O)_2(aq), predominates at low salinity (Cl:Co ratios ∼2). EXAFS analyses further revealed the bonding distances for the octahedral Co(H₂O)₆²⁺ (^octCo-O = 2.075(19) Å), tetrahedral CoCl₄²⁻ (^tetCo-Cl = 2.252(19) Å) and tetrahedral CoCl₂(H₂O)_2(aq) (^tetCo-O = 2.038(54) Å and ^tetCo-Cl = 2.210(56) Å). An analysis of the Co(II) speciation in sodium bromide solutions shows a similar trend, with tetrahedral bromide complexes becoming predominant at higher temperature/salinity than in the chloride system. EXAFS analysis confirms that the limiting complex at high bromide concentration at high temperature is CoBr₄²⁻. Finally, XANES spectra were used to derive the thermodynamic properties for the CoCl₄²⁻ and CoCl₂(H₂O)_2(aq) complexes, enabling thermodynamic modelling of cobalt transport in hydrothermal fluids. Solubility calculations show that tetrahedral CoCl₄²⁻ is responsible for transport of cobalt in hydrothermal solutions with moderate chloride concentration (∼2 m NaCl) at temperatures of 250 °C and higher, and both cooling and dilution processes can cause deposition of cobalt from hydrothermal fluids.     

Two-dimensional pH distributions and dynamics in bioturbated marine sediments

The seafloor is the site of intense biogeochemical and mineral dissolution-precipitation reactions which generate strong gradients in pH near the sediment-overlying water interface. These gradients are usually measured in one-dimension vertically with depth. Two-dimensional pH distributions in marine sediments were examined at high resolution (65 × 65 μm pixel) and analytical precision over areas of ∼150 to 225 cm² using a newly developed pH planar fluorosensor. Dramatic three-dimensional gradients, complex heterogeneity, and dynamic changes of pH occur in the surficial zone of deposits inhabited by macrofauna. pH can vary by ±2 units horizontally as well as vertically over millimeter scales. pH minima zones often form in association with redoxclines within a few millimeters of inner burrow walls, and become more pronounced with time if burrows remain stable and irrigated for extended periods. Microenvironmental pH minima also form locally around decaying biomass and relict burrow tracks, and dissipate with time (∼5 d). H⁺ concentrations and fluxes in sandy mud show complex acid-base reaction distributions with net H⁺ fluxes around burrows up to ∼12 nmol cm⁻² d⁻¹ and maximum net reaction rates varying between −90 (consumption) to 120 (production) μM d⁻¹ (∼90 nmol cm⁻¹ d⁻¹ burrow length). Acid producing zones that surround irrigated burrows are largely balanced by acid titration zones along inner burrow walls and outer radial boundaries. The geometry and scaling of pH microenvironments are functions of diagenetic reaction rates and three-dimensional transport patterns determined by sediment properties, such as diffusive tortuosity, and by benthic community characteristics such as the abundance, mobility, and size of infauna. Previously, undocumented biogeochemical phenomena such as low pH regions associated with in-filled relict biogenic structures and burrowing tracks are readily demonstrated by two-dimensional and time-dependent images of pH and sedimentary structure.     

Rare earth elements (REE) and yttrium in stream waters, stream sediments, and Fe-Mn oxyhydroxides: Fractionation, speciation, and controls over REE + Y patterns in the surface environment

We have collected ∼500 stream waters and associated bed-load sediments over an ∼400 km² region of Eastern Canada and analyzed these samples for Fe, Mn, and the rare earth elements (REE + Y). In addition to analyzing the stream sediments by total digestion (multi-acid dissolution with metaborate fusion), we also leached the sediments with 0.25 M hydroxylamine hydrochloride (in 0.05 M HCl), to determine the REE + Y associated with amorphous Fe- and Mn-oxyhydroxide phases. We are thus able to partition the REE into “dissolved” (<0.45 μm), labile (hydroxylamine) and detrital sediment fractions to investigate REE fractionation, and in particular, with respect to the development of Ce and Eu anomalies in oxygenated surface environments. Surface waters are typically LREE depleted ([La/Sm]_NASC ranges from 0.16 to 5.84, average = 0.604, n = 410; where the REE are normalized to the North America Shale Composite), have strongly negative Ce anomalies ([Ce/Ce^∗]_NASC ranges from 0.02 to 1.25, average = 0.277, n = 354), and commonly have positive Eu anomalies ([Eu/Eu^∗]_NASC ranges from 0.295 to 1.77, average = 0.764, n = 84). In contrast, the total sediment have flatter REE + Y patterns relative to NASC ([La/Sm]_NASC ranges from 0.352 to 1.12, average = 0.778, n = 451) and are slightly middle REE enriched ([Gd/Yb]_NASC ranges from 0.55 to 3.75, average = 1.42). Most total sediments have negative Ce and Eu anomalies ([Ce/Ce^∗]_NASC ranges from 0.097 to 2.12, average = 0.799 and [Eu/Eu^∗]_NASC ranges from 0.39 to 1.43, average = 0.802). The partial extraction sediments are commonly less LREE depleted than the total sediments ([La/Sm]_NASC ranges from 0.24 to 3.31, average = 0.901, n = 4537), more MREE enriched ([Gd/Yb]_NASC ranges from 0.765 to 6.28, average = 1.97) and Ce and Eu anomalies (negative and positive) are more pronounced.The partial extraction recovered, on average ∼20% of the Fe in the total sediment, ∼80% of the Mn, and 21-29% of the REEs (Ce = 19% and Y = 32%). Comparison between REEs in water, partial extraction and total sediment analyses indicates that REEs + Y in the stream sediments have two primary sources, the host lithologies (i.e., mechanical dispersion) and hydromorphically transported (the labile fraction). Furthermore, Eu appears to be more mobile than the other REE, whereas Ce is preferentially removed from solution and accumulates in the stream sediments in a less labile form than the other REEs + Y. Despite poor statistical correlations between the REEs + Y and Mn in either the total sediment or partial extractions, based on apparent distribution coefficients and the pH of the stream waters, we suggest that either sediment organic matter and/or possibly δ-MnO₂/FeOOH are likely the predominant sinks for Ce, and to a lesser extent the other REE, in the stream sediments.     

Characterization of elemental release during microbe-basalt interactions at T = 28 °C

This study used batch reactors to characterize the rates and mechanisms of elemental release during the interaction of a single bacterial species (Burkholderia fungorum) with Columbia River Flood Basalt at T = 28 °C for 36 days. We primarily examined the release of Ca, Mg, P, Si, and Sr under a variety of biotic and abiotic conditions with the aim of evaluating how actively metabolizing bacteria might influence basalt weathering on the continents. Four days after inoculating P-limited reactors (those lacking P in the growth medium), the concentration of viable planktonic cells increased from ∼10⁴ to 10⁸ CFU (Colony Forming Units)/mL, pH decreased from ∼7 to 4, and glucose decreased from ∼1200 to 0 μmol/L. Mass-balance and acid-base equilibria calculations suggest that the lowered pH resulted from either respired CO₂, organic acids released during biomass synthesis, or H⁺ extrusion during uptake. Between days 4 and 36, cell numbers remained constant at ∼10⁸ CFU/mL and pH increased to ∼5. Purely abiotic control reactors as well as control reactors containing inert cells (∼10⁸ CFU/mL) showed constant glucose concentrations, thus confirming the absence of biological activity in these experiments. The pH of all control reactors remained near-neutral, except for one experiment where the pH was initially adjusted to 4 but rapidly rose to 7 within 2 days. Over the entire 36 day period, P-limited reactors containing viable bacteria yielded the highest Ca, Mg, Si, and Sr release rates. Release rates inversely correlate with pH, indicating that proton-promoted dissolution was the dominant reaction mechanism. Both biotic and abiotic P-limited reactors displayed low P concentrations. Chemical analyses of bacteria collected at the end of the experiments, combined with mass-balances between the biological and fluid phases, demonstrate that the absence of dissolved P in the biotic reactors resulted from microbial P uptake. The only P source in the basalt is a small amount of apatite (∼1.2%), which occurs as needles within feldspar grains and glass. We therefore conclude that B. fungorum utilized apatite as a P source for biomass synthesis, which stimulated elemental release from coexisting mineral phases via pH lowering. The results of this study suggest that actively metabolizing bacteria have the potential to influence elemental release from basalt in continental settings.