Ba isotopic signature for early differentiation between Cs and Ba in natural fission reactors

Ba isotopic studies of the Oklo and Bangombé natural fission reactors in east Gabon provide information on the geochemical behavior of radioactive Cs (¹³⁵Cs and ¹³⁷Cs) in a geological medium. Large isotopic deviations derived from fissiogenic Ba were found in chemical leachates of the reactor uraninites. The fissiogenic Ba isotopic patterns calculated by subtracting the non-fissiogenic component are classified into three types that show different magnifications of chemical fractionation between Cs and Ba. In addition, the isotopic signatures of fissiogenic ¹³⁵Ba, ¹³⁷Ba and ¹³⁸Ba suggest an early differentiation between Cs and Ba of less than 20 years after the production of fissiogenic Cs and Ba. On the other hand, only small excesses of ¹³⁵Ba (ε < +1.8) and/or ¹³⁷Ba (ε < +1.3) were identified in some clay samples, which might have resulted from selective adsorption of ¹³⁵Cs and ¹³⁷Cs that migrated from the reactors by differentiation.     

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

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

Interaction of bentonite colloids with Cs, Eu, Th and U in presence of humic acid: A flow field-flow fractionation study

The interaction of Cs(I), Eu(III), Th(IV) and U(VI) with montmorillonite colloids was investigated in natural Grimsel Test Site groundwater over a 3 years period. The asymmetric flow field-flow fractionation combined with various detectors was applied to study size variations of colloids and to monitor colloid association of trace metals. The colloids suspended directly in the low ionic strength (I), slightly alkaline granitic groundwater (I = 10⁻³ mol/L, pH 9.6) showed a gradual agglomeration with a size distribution shift from initially 10-200 nm to 50-400 nm within over 3 years. The Ca²⁺ concentration of 2.1 × 10⁻⁴ mol/L in the ground water is believed to be responsible for the slow agglomeration due to Ca²⁺ ion exchange against Li⁺ and Na⁺ at the permanently charged basal clay planes. Furthermore, the Ca²⁺ concentration lies close to the critical coagulation concentration (CCC) of 10⁻³ mol L⁻¹ for clay colloids. Slow destabilization may delimit clay colloid migration in this specific groundwater over long time scales. Eu(III) and Th(IV) are found predominantly bound to clay colloids, while U(VI) prevails as the UO₂(OH)₃⁻ complex and Cs(I) remains mainly as aquo ion under our experimental conditions. Speciation calculations qualitatively represent the experimental data. A focus was set on the reversibility of metal ion-colloid binding. Addition of humic acid as a competing ligand induces rapid metal ion dissociation from clay colloids in the case of Eu(III) even after previous aging for about 3 years. Interestingly only partial dissociation occurs in the case of Th(IV). Experiments and calculations prove that the humate complexes dominate the speciation of all metal ions under given conditions. The partial irreversibility of clay bound Th(IV) is presently not understood but might play an important role for the colloid-mediated transport of polyvalent actinides over wide distances in natural groundwater.     

Silicon control of strontium and cesium partitioning in hydroxide-weathered sediments

Cation partitioning and speciation in an aqueous soil suspension may depend on the coupling of reaction time, sorbate amount and mineral weathering reactions. These factors were varied in sediment suspension experiments to identify geochemical processes that affect migration of Sr²⁺ and Cs⁺ introduced to the subsurface by caustic high level radioactive waste (HLRW). Three glacio-fluvial and lacustrine sediments from the Hanford Site (WA, USA) were subjected to hyperalkaline (pH > 13), Na-Al-NO₃-OH solution conditions within a gradient field of (i) sorptive concentration (10⁻⁵-10⁻³ m) and (ii) reaction time (0-365 d). Strontium uptake (q_Sr) exceeded that of cesium at nearly all reaction times. Sorbent affinity for both Cs⁺ and Sr²⁺ increased with clay plus silt content at early times, but a prolonged slow uptake process was observed over the course of sediment weathering that erased the texture effect for Sr²⁺; all sediments showed similar mass normalized uptake after several months of reaction time. Strontium became progressively recalcitrant to desorption after 92 d, with accumulation and aging of neoformed aluminosilicates. Formation of Cs⁺ and Sr²⁺-containing cancrinite and sodalite was observed after 183 d by SEM and synchrotron μ-XRF and μ-XRD. EXAFS data for q_Sr ≈ 40 mmol kg⁻¹ showed incorporation of Sr²⁺ into both feldspathoid and SrCO₃(s) coordination environments after one year. Adsorption was predominant at early times and low sorbate amount, whereas precipitation, controlled largely by sediment Si release, became increasingly important at longer times and higher sorbate amount. Kinetics of contaminant desorption at pH 8 from one year-weathered sediments showed significant dependence on background cation (Ca²⁺ versus K⁺) composition. Results of this study indicate that co-precipitation and ion exchange in neoformed aluminosilicates may be an important mechanism controlling Sr²⁺ and Cs⁺ mobility in siliceous sediments impacted by hyperalkaline HLRW.     

Microbial dissolution of calcite at T = 28 °C and ambient pCO2

This study used batch reactors to quantify the mechanisms and rates of calcite dissolution in the presence and absence of a single heterotrophic bacterial species (Burkholderia fungorum). Experiments were conducted at T = 28°C and ambient pCO₂ over time periods spanning either 21 or 35 days. Bacteria were supplied with minimal growth media containing either glucose or lactate as a C source, NH₄⁺ as an N source, and H₂PO₄⁻ as a P source. Combining stoichiometric equations for microbial growth with an equilibrium mass-balance model of the H₂O-CO₂-CaCO₃ system demonstrates that B. fungorum affected calcite dissolution by modifying pH and alkalinity during utilization of ionic N and C species. Uptake of NH₄⁺ decreased pH and alkalinity, whereas utilization of lactate, a negatively charged organic anion, increased pH and alkalinity. Calcite in biotic glucose-bearing reactors dissolved by simultaneous reaction with H₂CO₃ generated by dissolution of atmospheric CO₂ (H₂CO₃ + CaCO₃ → Ca²⁺ + 2HCO₃⁻) and H⁺ released during NH₄⁺ uptake (H⁺ + CaCO₃ → Ca²⁺ + HCO₃⁻). Reaction with H₂CO₃ and H⁺ supplied ∼45% and 55% of the total Ca²⁺ and ∼60% and 40% of the total HCO₃⁻, respectively. The net rate of microbial calcite dissolution in the presence of glucose and NH₄⁺ was ∼2-fold higher than that observed for abiotic control experiments where calcite dissolved only by reaction with H₂CO₃. In lactate bearing reactors, most H⁺ generated by NH₄⁺ uptake reacted with HCO₃⁻ produced by lactate oxidation to yield CO₂ and H₂O. Hence, calcite in biotic lactate-bearing reactors dissolved by reaction with H₂CO₃ at a net rate equivalent to that calculated for abiotic control experiments. This study suggests that conventional carbonate equilibria models can satisfactorily predict the bulk fluid chemistry resulting from microbe-calcite interactions, provided that the ionic forms and extent of utilization of N and C sources can be constrained. Because the solubility and dissolution rate of calcite inversely correlate with pH, heterotrophic microbial growth in the presence of nonionic organic matter and NH₄⁺ appears to have the greatest potential for enhancing calcite weathering relative to abiotic conditions.     

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

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

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: II. Mineral transformation and microscale partitioning

Microscopic and spectroscopic studies were conducted to assess mineral transformation processes in aqueous suspensions of illite (Il), vermiculite (Vm) and montmorillonite (Mt) that were subjected to weathering in a simulated high-level radioactive tank waste leachate (0.05 m Al_T, 2 m Na⁺, 1 m NO₃⁻, pH ∼14, Cs⁺ and Sr²⁺ present as co-contaminants). Time series (0 to 369 d) experiments were conducted at 298 K, with initial [Cs]₀ and [Sr]₀ concentrations from 10⁻⁵ to 10⁻ mol kg⁻. Incongruent clay dissolution resulted in an accumulation of secondary aluminosilicate precipitates identified as nitrate-sodalite, nitrate-cancrinite and zeolite X, by molecular spectroscopy and electron microscopy (XRD, IR, NMR, SEM-EDS and TEM-EDS). Contaminant fate was dependent on competing uptake to parent clays and weathering products. TEM-EDS results indicated that high Il affinity for Cs was due to adsorption at frayed edge sites. The Il system also comprised Sr-rich aluminous precipitates after 369 d reaction time. In Mt systems, Cs and Sr were co-precipitated into increasingly recalcitrant spheroidal precipitates over the course of the experiment, whereas contaminant association with montmorillonite platelets was less prevalent. In contrast, Cs and Sr were found in association with weathered Vm particles despite the formation of spheroidal aluminosilicate precipitates that were comparable to those formed from Mt dissolution.     

Complexation of cadmium to sulfur and oxygen functional groups in an organic soil

Cadmium (Cd) is a toxic trace element and due to human activities soils and waters are contaminated by Cd both on a local and global scale. It is widely accepted that chemical interactions with functional groups of natural organic matter (NOM) is vital for the bioavailability and mobility of trace elements. In this study the binding strength of cadmium (Cd) to soil organic matter (SOM) was determined in an organic (49% organic C) soil as a function of reaction time, pH and Cd concentration. In experiments conducted at native Cd concentrations in soil (0.23 μg g⁻¹ dry soil), halides (Cl, Br) were used as competing ligands to functional groups in SOM. The concentration of Cd in the aqueous phase was determined by isotope-dilution (ID) inductively-coupled-plasma-mass-spectrometry (ICP-MS), and the activity of Cd²⁺ was calculated from the well-established Cd-halide constants. At higher Cd loading (500-54,000 μg g⁻¹), the Cd²⁺ activity was directly determined by an ion-selective electrode (ISE). On the basis of results from extended X-ray absorption fine structure (EXAFS) spectroscopy, a model with one thiolate group (RS⁻) was used to describe the complexation (Cd²⁺ + RS⁻ ? CdSR⁺; log K_CdSR) at native Cd concentrations. The concentration of thiols (RSH; 0.047 mol kg⁻¹ C) was independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. Log K_CdSR values of 11.2-11.6 (pK_a for RSH = 9.96), determined in the pH range 3.1-4.6, compare favorably with stability constants for the association between Cd and well-defined thiolates like glutathione. In the concentration range 500-54,000 μg Cd g⁻¹, a model consisting of one thiolate and one carboxylate (RCOO⁻) gave the best fit to data, indicating an increasing role for RCOOH groups as RSH groups become saturated. The determined log K_CdOOCR of 3.2 (Cd²⁺ +  RCOO⁻ ? CdOOCR⁺; log K_CdOOCR; pK_a for RCOOH = 4.5) is in accordance with stability constants determined for the association between Cd and well-defined carboxylates. Given a concentration of reduced sulfur groups of 0.2% or higher in NOM, we conclude that the complexation to organic RSH groups may control the speciation of Cd in soils, and most likely also in surface waters, with a total concentration less than 5 mg Cd g⁻¹ organic C.     

Diverse nucleosynthetic components in barium isotopes of carbonaceous chondrites: Incomplete mixing of s- and r-process isotopes and extinct Cs in the early solar system

Barium isotopic compositions of chemical leachates from six carbonaceous chondrites, Orgueil (CI), Mighei (CM2), Murray (CM2), Efremovka (CV3), Kainsaz (CO3), and Karoonda (CK4), were determined using thermal ionization mass spectrometry in order to assess the chemical evolution in the early solar system.The Ba isotopic data from most of the leachates show variable ¹³⁵Ba excesses correlated with ¹³⁷Ba excesses, suggesting the presence and heterogeneity of additional nucleosynthetic components for s- and r-processes in the solar system. The isotopic deviations observed in this study were generally small (−1 < ε < +1) except in the case of the acid residues of CI and CM meteorites. Large deviations of ¹³⁵Ba (ε = −13.5 to −5.0) and ¹³⁷Ba (ε = −6.2∼−1.2) observed in the acid residues from one CI and two CM meteorites show significant evidence for the enrichment of s-process isotopes derived from presolar grains. Two models were proposed to estimate the ¹³⁵Cs isotopic abundances by subtraction of the s- and r-isotopic components from the total Ba isotopic abundances in the three CM meteorites, Mighei, Murchison (measured in a previous study), and Murray. The data points show individual linear trends between ¹³⁵Cs/¹³⁶Ba ratios and ¹³⁵Ba isotopic deviations for the three samples. Considering the different trends observed in the three CM meteorites, the Ba isotopic composition of the CM meteorite parent body was heterogeneous at its formation. Chronological information is unclear in the data for Murchison and Murray because of large analytical uncertainties imposed by error propagation. Only the Mighei meteorite data indicate the possible existence of presently extinct ¹³⁵Cs (¹³⁵Cs/¹³³Cs = (2.7 ± 1.6) × 10⁻⁴) in the early solar system. Another explanation of the data for the three CM meteorite is mixing of at least three components with different Ba isotopic compositions, although this is model-dependent.     

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.     

Flocculation improves uptake of Sr and Cs from radioactive effluents

This paper presents the results of investigation on uptake of radioactive species ⁹⁰Sr and ¹³⁷Cs present in the liquid effluents from nuclear processing plants. Chemical precipitation process is adopted to remove radioactivity from the effluents with low and intermediate level of activity. In this process, radioactive ⁹⁰Sr and ¹³⁷Cs are co-precipitated along with copper ferrocyanide, ferric hydroxide and either calcium phosphate or barium sulphate. These precipitates being fine in size require flocculation for enhanced settling rate in clarifier/thickener. The flocculation by some selected high molecular weight polyacrylamide based polymers has improved the uptake of radioactive metal ions. The adsorption of these radioactive species has been found to increase in the presence of these flocculants thereby improving the decontamination factor (DF). While flocculating the precipitates, there may be some complex formation with Sr²⁺/Cs⁺, flocculant and the substrate. This has enhanced the uptake of the radioactive metal ions from the liquid component. The plant trials have indicated the improvement of DF value due to flocculation by cationic flocculant.     

Multicomponent diffusion of a suite of tracers (HTO, Cl, Br, I, Na, Sr, Cs) in a single sample of Opalinus Clay

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

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

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

Chemical and isotopic constraints on water/rock interactions at the Lost City hydrothermal field, 30°N Mid-Atlantic Ridge

Low temperature vent fluids (<91 °C) issuing from the ultramafic-hosted hydrothermal system at Lost City, 30°N Mid-Atlantic Ridge, are enriched in dissolved volatiles (H₂,CH₄) while attaining elevated pH values, indicative of the serpentization processes that govern water/rock interactions deep in the oceanic crust. Here, we present a series of theoretical models to evaluate the extent of hydrothermal alteration and assess the effect of cooling on the systematics of pH-controlled B aqueous species. Peridotite-seawater equilibria calculations indicate that the mineral assemblage composed of diopside, brucite and chrysotile likely dictates fluid pH at moderate temperature serpentinization processes (<300 °C), by imposing constraints on the aCa⁺⁺/a²H⁺ ratios and the activity of dissolved SiO₂. Based on Sr abundances and the ⁸⁷Sr/⁸⁶Sr isotope ratios of vent fluids reported from Lost City, estimated water/rock mass ratios (w/r = 2-4) are consistent with published models involving dissolved CO₂ and alkane concentrations. Combining the reported δ¹⁸O values of vent fluids (0.7‰) with such w/r mass ratios, allows us to bracket subseafloor reaction temperatures in the vicinity of 250 °C. These estimates are in agreement with previous theoretical studies supporting extensive conductive heat loss within the upflow zones. Experimental studies on peridotite-seawater alteration suggest that fluid pH increases during cooling which then rapidly enhances boron removal from solution and incorporation into secondary phases, providing an explanation for the highly depleted dissolved boron concentrations measured in the low temperature but alkaline Lost City vent fluids. Finally, to account for the depleted ¹¹B composition (δ¹¹B ∼25-30‰) of vent fluids relative to seawater, isotopic fractionation between tetrahedrally coordinated aqueous boron species with BO₃-bearing mineral sites (e.g. in calcite, brucite) is proposed.     

Nanoparticles in natural systems I: The effective reactive surface area of the natural oxide fraction in field samples

Information on the particle size and reactive surface area of natural samples is essential for the application of surface complexation models (SCM) to predict bioavailability, toxicity, and transport of elements in the natural environment. In addition, this information will be of great help to enlighten views on the formation, stability, and structure of nanoparticle associations of natural organic matter (NOM) and natural oxide particles.Phosphate is proposed as a natively present probe ion to derive the effective reactive surface area of natural samples. In the suggested method, natural samples are equilibrated (?10 days) with 0.5 M NaHCO₃ (pH = 8.5) at various solid-solution ratios. This matrix fixes the pH and ionic strength, suppresses the influence of Ca²⁺ and Mg²⁺ ions by precipitation these in solid carbonates, and removes NOM due to the addition of activated carbon in excess, collectively leading to the dominance of the PO₄-CO₃ interaction in the system. The data have been interpreted with the charge distribution (CD) model, calibrated for goethite, and the analysis results in an effective reactive surface area (SA) and a reversibly bound phosphate loading Γ for a series of top soils.The oxidic SA varies between about 3-30 m²/g sample for a large series of representative agricultural top soils. Scaling of our data to the total iron and aluminum oxide content (dithionite-citrate-bicarbonate extractable), results in the specific surface area between about 200-1200 m²/g oxide for most soils, i.e. the oxide particles are nano-sized with an equivalent diameter in the order of ∼1-10 nm if considered as non-porous spheres. For the top soils, the effective surface area and the soil organic carbon fraction are strongly correlated. The oxide particles are embedded in a matrix of organic carbon (OC), equivalent to ∼1.4 ± 0.2 mg OC/m² oxide for many soils of the collection, forming a NOM-mineral nanoparticle association with an average NOM volume fraction of ∼80%. The average mass density of such a NOM-mineral association is ∼1700 ± 100 kg/m³ (i.e. high-density NOM). The amount of reversibly bound phosphate is rather close to the amount of phosphate that is extractable with oxalate. The phosphate loading varies remarkably (Γ ≈ 1-3 μmol/m² oxide) in the samples. As discussed in part II of this paper series (Hiemstra et al., 2010), the phosphate loading (Γ) of field samples is suppressed by surface complexation of NOM, where hydrophilic, fulvic, and humic acids act as a competitor for (an)ions via site competition and electrostatic interaction.     

Kinetics of Fe(III) precipitation in aqueous solutions at pH 6.0-9.5 and 25 °C

The kinetics of Fe(III) precipitation in synthetic buffered waters have been investigated over the pH range 6.0-9.5 using a combination of visible spectrophotometry, ⁵⁵Fe radiometry combined with ion-pair solvent extraction of chelated iron and numerical modeling. The rate of precipitation, which is first order with respect to both dissolved and total inorganic ferric species, varies by nearly two orders of magnitude with a maximum rate constant of 16 ± 1.5 × 10⁶ M⁻¹ s⁻¹ at a pH of around 8.0. Our results support the existence of the dissolved neutral species, Fe(OH)₃⁰, and suggest that it is the dominant precursor in Fe(III) polymerization and subsequent precipitation at circumneutral pH. The intrinsic rate constant of precipitation of Fe(OH)₃⁰ was calculated to be allowing us to predict rates of Fe(III) precipitation in the pH range 6.0-9.5. The value of this rate constant, and the variation in the precipitation rate constant over the pH range considered, are consistent with a mechanism in which the kinetics of iron precipitation are controlled by rates of water exchange in dissolved iron hydrolysis species.     

Metal-silicate partitioning of cesium: Implications for core formation

Here we present the first set of metal-silicate partitioning data for Cs, which we use to examine whether the primitive mantle depletion of Cs can be attributed to core segregation. Our experiments independently varied pressure from 5 to 15 GPa, temperature from 1900 to 2400 °C, metallic sulfur content from pure Fe to pure FeS, silicate melt polymerization, expressed as a ratio of non-bridging oxygens to tetrahedrally coordinated cations (nbo/t) from 1.26 to 3.1, and fO₂ from two to four log units below the iron-wüstite buffer. The most important controls on the partitioning behavior of alkalis were the metallic sulfur content, expressed as X_S, and the nbo/t of the silicate liquid. Normalization of X_S to 0.5 yielded the following expressions for D-values as a function of nbo/t: log D_Na = −2.0 + 0.44 × (nbo/t), log D_K = −2.4 + 0.67 × ( nbo/t), and log D_Cs = −3.2 + 1.17 × (nbo/t). Normalization of nbo/t to 2.7 resulted in the following equations for D-values as a function of S content: log D_Na = −4.1 + 6.4 × X_S, log D_K = −7.7 + 13.9 × X_S, and log D_Cs = −12.1 + 23.3 × X_S.There appears to be a negative pressure effect up to 15 GPa, but it should be noted that this trend was not present before normalization, and is based on only two measurements. There is a positive trend in cesium’s metal-silicate partition coefficient with increasing temperature. D_Cs exhibits the largest change and increased by a factor of three over 500 °C. The effect of oxygen fugacity has not been precisely determined but in general, lowering fO₂ by two log units resulted in a rise in all D-values of approximately an order of magnitude. In general, the sensitivity of partition coefficients to changing parameters increased with atomic number.The highest D-value for Cs observed in this study is 0.345, which was obtained at nbo/t of 2.7 and a metal phase of pure FeS. This metallic composition has far more S than has been suggested for any credible core-forming metal. We therefore conclude that the depletion of Cs in Earth’s mantle is either caused by radically different behavior of Cs at pressures higher than 15 GPa or is not related to core formation. Even so, we have shown that a planet with a sufficient S inventory may incorporate significant amounts of alkali elements into its core.     

Sorption of Cs to micaceous subsurface sediments from the Hanford site, USA

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

Size, speciation and lability of NOM-metal complexes in hyperalkaline cave dripwater

Transport of trace metals by natural organic matter (NOM) is potentially an important vector for trace metal incorporation in secondary cave precipitates [speleothems], yet little is known about the size distribution, speciation and metal binding properties of NOM in cave dripwaters. A hyperalkaline cave environment (ca. pH 11) was selected to provide information on colloid-metal interactions in cave waters, and to address the lack of high-pH data in natural systems in general. Colloidal (1 nm-1 μm) NOM in hyperalkaline cave dripwater from Poole’s Cavern, UK, was characterised by flow field-flow fractionation (FlFFF) coupled to UV and fluorescence detectors and transmission electron microscopy (TEM) coupled to X-ray energy-dispersive spectroscopy (X-EDS); trace-metal lability was examined by diffusive gradients in thin films (DGT). Colloidal aggregates and small particulates (>1 μm) imaged by TEM were morphologically heterogeneous with qualitative elemental compositions (X-EDS spectra; n = 41) consistent with NOM aggregates containing aluminosilicates, and iron and titanium oxides. Globular organic colloids, with diameters between ca. 1 and 10 nm were the most numerous colloidal class and exhibited high UV-absorbance (254 nm) and fluorescence intensity (320:400 nm excitation: emission) in optical regions characteristic of humic-like compounds. Metal binding with humic substances was modelled using the WHAM 6.1 (model VI) and visual MINTEQ 3.0 (NICA-Donnan) speciation codes. At pH 11, both models predicted dominant humic binding of Cu (ca. 100%) and minimal binding of Ni and Co (ca. <1-7%). A DGT depletion experiment (7 days duration) with the hyperalkaline dripwater showed that the available proportion of each metal was much lower than its total concentration. Metal availability for DGT in the initial stages (24 h) was consistent with weaker binding of alkaline earth metals by humic substances (Ba > Sr > V > Cu > Ni > Co), compared to the transition metals. Integrated over the entire experiment, the DGT-available proportion of transition metals (Ni > Cu & V >> Co) differed greatly from the expected hierarchy from WHAM and MINTEQ, indicating unusually strong complexation of Co. Total metal concentrations of Cu, Ni, and Co in raw and filtered PE1 dripwater samples (n = 53) were well correlated (Cu vs. Ni, R² = 0.8; Cu vs. Co, R² = 0.5) and were strongly reduced (> ca. 50%) by filtration at ca. 100 and 1 nm, indicating a common colloidal association. Our results demonstrate that soil-derived colloids reach speleothems, despite transport through a karst zone with potential for adsorption, and that NOM is a dominant complexant of trace metals in high pH speleothem-forming groundwaters.