Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment

Time-resolved U(VI) laser fluorescence spectra (TRLFS) were recorded for a series of natural uranium-silicate minerals including boltwoodite, uranophane, soddyite, kasolite, sklodowskite, cuprosklodowskite, haiweeite, and weeksite, a synthetic boltwoodite, and four U(VI)-contaminated Hanford vadose zone sediments. Lowering the sample temperature from RT to ∼ 5.5 K significantly enhanced the fluorescence intensity and spectral resolution of both the minerals and sediments, offering improved possibilities for identifying uranyl species in environmental samples. At 5.5 K, all of the uranyl silicates showed unique, well-resolved fluorescence spectra. The symmetric O = U = O stretching frequency, as determined from the peak spacing of the vibronic bands in the emission spectra, were between 705 to 823 cm⁻¹ for the uranyl silicates. These were lower than those reported for uranyl phosphate, carbonate, or oxy-hydroxides. The fluorescence emission spectra of all four sediment samples were similar to each other. Their spectra shifted minimally at different time delays or upon contact with basic Na/Ca-carbonate electrolyte solutions that dissolved up to 60% of the precipitated U(VI) pool. The well-resolved vibronic peaks in the fluorescence spectra of the sediments indicated that the major fluorescence species was a crystalline uranyl mineral phase, while the peak spacing of the vibronic bands pointed to the likely presence of uranyl silicate. Although an exact match was not found between the U(VI) fluorescence spectra of the sediments with that of any individual uranyl silicates, the major spectral characteristics indicated that the sediment U(VI) was a uranophane-type solid (uranophane, boltwoodite) or soddyite, as was concluded from microprobe, EXAFS, and solubility analyses.     

The influence of Fe-rich coatings on the dissolution of anorthite at pH 2.6

Surface coatings are very common on mineral grains in soils but most laboratory dissolution experiments are carried out on pristine, uncoated mineral grains. An experiment designed to unambiguously isolate the effect of surface coatings on mineral dissolution from any influence of solution saturation state is reported. Two aliquots of 53 to 63 μm anorthite feldspar powder were used. One was dissolved in pH 2.6 HCl, the other in pH 2.6 FeCl₃ solution, both for ∼6000 h in flow-through reactors. An amorphous Fe-rich, Al-, Ca- and Si-free orange precipitate coated the anorthite dissolved in the FeCl₃ solution. BET surface area of the anorthite increased from 0.16 to 1.65 m² g⁻¹ in the HCl experiment and to 3.89 m² g⁻¹ in the FeCl₃ experiment. The increase in surface area in the HCl experiment was due to the formation of etch pits on the anorthite grain surface whilst the additional increase in the FeCl₃ experiment was due to the micro- and meso-porous nature of the orange precipitate. This precipitate did not inhibit or slow the dissolution of the anorthite. Steady state dissolution rates for the anorthite dissolved in the HCl and FeCl₃ were ∼2.5 and 3.2 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ respectively. These rates are not significantly different after the cumulative uncertainty of 17% in their value due to uncertainty in the inputs parameters used in their calculation is taken into account. Results from this experiment support previous theoretical and inference-based conclusions that porous coatings should not inhibit mineral dissolution.     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

Clay mineral weathering and contaminant dynamics in a caustic aqueous system: I. Wet chemistry and aging effects

Caustic high level radioactive waste induces mineral weathering reactions that can influence the fate of radionuclides released in the vicinity of leaking storage tanks. The uptake and release of Cs^I and Sr^II were studied in batch reactors of 2:1 layer-type silicates—illite (Il), vermiculite (Vm) and montmorillonite (Mt)—under geochemical conditions characteristic of leaking tank waste at the Hanford Site in WA (0.05 m Al_T, 2 m Na⁺, 1 m NO₃⁻, pH ∼14, Cs and Sr 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 10⁻³ mol kg⁻¹. Clay mineral type affected the rates of (i) hydroxide promoted dissolution of Si, Al and Fe, (ii) precipitation of secondary solids and (iii) uptake of Cs and Sr. Initial Si release to solution followed the order Mt > Vm > Il. An abrupt decrease in soluble Si and/or Al after 33 d for Mt and Vm systems, and after 190 d for Il suspensions was concurrent with accumulation of secondary aluminosilicate precipitates. Strontium uptake exceeded that of Cs in both rate and extent, although sorbed Cs was generally more recalcitrant to subsequent desorption and dissolution. After 369 d reaction time, reacted Il, Vm and Mt solids retained up to 17, 47 and 14 mmol kg⁻¹ (0.18, 0.24 and 0.02 μmol m⁻²) of Cs, and 0, 27 and 22 mmol kg⁻¹ (0, 0.14 and 0.03 μmol m⁻²) Sr, respectively, which were not removed in subsequent Mg exchange or oxalic acid dissolution reactions. Solubility of Al and Si decreased with initial Cs and Sr concentration in Mt and Il, but not in Vm. High co-contaminant sorption to the Vm clay, therefore, appears to diminish the influence of those ions on mineral transformation rates.     

Early diagenesis of biogenic silica in the Amazon delta: alteration, authigenic clay formation, and storage

Deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques. Rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets. A variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating Amazon delta sediments indicates that both of these general assumptions are unjustified. Apparent lack of significant biogenic silica storage in deltaic environments, particularly in the tropics, may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica. Biogenic silica particles buried in suboxic Amazon delta deposits can be unaltered, partially dissolved, covered with aluminosilicate or metal-rich coatings, or completely reconstituted into authigenic K-Fe-rich aluminosilicate minerals. Pore water (K, Mg, F, Si) and solid-phase distributions, direct observations of particles, laboratory experiments, and depositional context indicate that authigenic clays form rapidly (<1 yr) in the seasonally reworked surface layer (∼ 0.5-2 m) of the delta topset and are disseminated during sediment remobilization. Fe, Al-oxide rich debris derived from the tropical drainage basin is an abundant reactant, and thus the supply of biogenic silica is a major control on the amount of clay formed.The mild 1% Na₂CO₃ alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0.1N HCl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution. Well-crystallized clays are not significantly affected by this modification nor is bulk Amazon River bed sediment. The two-step procedure indicates that ∼90% of the biogenic silica originally present in deposits is converted to clay or otherwise altered, raising the effective quantity of biogenic silica stored from ∼33 to ∼296 μmol Si g⁻¹ (∼1.8% SiO₂). Biogenic Si stored in the delta increases away from the river mouth, across shelf and along the dispersal system where primary production is highest. The K/Si ratio of labile authigenic material is ∼0.19 mol mol⁻¹, far higher than Amazon River suspended matter (∼0.07 mol mol⁻¹). Diagenetic models indicate formation rates in the mobile sediment layer of ∼2.8 μmol K g⁻¹ yr⁻¹ (∼16 μmol Si g⁻¹ yr⁻¹). Inclusion of authigenic alteration products of biogenic silica in estimates of reactive Si burial increases the deltaic storage of riverine Si to ∼22% of the Amazon River input. The rapid formation of aluminosilicates from biogenic SiO₂, seawater solutes, and remobilized Fe, Al-oxides represents a form of reverse weathering. Rapid reverse weathering reactions in tropical muds and deltaic deposits, the largest sediment depocenters on Earth, confirms the general importance of these processes in oceanic elemental cycles.     

Variation in calcite dissolution rates:: A fundamental problem?

A comparison of published calcite dissolution rates measured far from equilibrium at a pH of ∼ 6 and above shows well over an order of magnitude in variation. Recently published AFM step velocities extend this range further still. In an effort to understand the source of this variation, and to provide additional constraint from a new analytical approach, we have measured dissolution rates by vertical scanning interferometry. In areas of the calcite cleavage surface dominated by etch pits, our measured dissolution rate is 10^−10.95 mol/cm²/s (PCO₂ 10^−3.41 atm, pH 8.82), 5 to ∼100 times slower than published rates derived from bulk powder experiments, although similar to rates derived from AFM step velocities. On cleavage surfaces free of local etch pit development, dissolution is limited by a slow, “global” rate (10^−11.68 mol/cm²/s). Although these differences confirm the importance of etch pit (defect) distribution as a controlling mechanism in calcite dissolution, they also suggest that “bulk” calcite dissolution rates observed in powder experiments may derive substantial enhancement from grain boundaries having high step and kink density. We also observed significant rate inhibition by introduction of dissolved manganese. At 2.0 μM Mn, the rate diminished to 10^−12.4 mol/cm²/s, and the well formed rhombic etch pits that characterized dissolution in pure solution were absent. These results are in good agreement with the pattern of manganese inhibition in published AFM step velocities, assuming a step density on smooth terraces of ∼9 μm⁻¹.     

Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO_2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of ∼10⁻⁸ M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO_2.0 to UO_2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO_2.25 (U₄O₉) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions.     

Far from equilibrium enstatite dissolution rates in alkaline solutions at earth surface conditions

Far from equilibrium enstatite dissolution rates both open to atmospheric CO₂ and CO₂ purged were measured as a function of solution pH from 8 to 13 in batch reactors at room temperature. Congruent dissolution was observed after an initial period of incongruent dissolution with preferential Si release from the enstatite. Steady-state dissolution rates in open to atmospheric CO₂ conditions decrease with increase in solution pH from 8 to 12 similar to the behavior reported by other investigators. Judging from the pH 13 dissolution rate, rates increase with pH above pH 12. This is thought to occur because of the increase in overall negative surface charges on enstatite as Mg surface sites become negative above pH 12.4, the pH of zero surface charge of MgO.Steady-state dissolution rates of enstatite increase above pH 10 when CO₂ was purged by performing the experiments in a N₂ atmosphere. This suggests inhibition of dissolution rates above pH 10 when experiments were open to the atmosphere. The dissolved carbonate in these solutions becomes dominantly CO₃²⁻ above pH 10.33. It is argued that CO₃²⁻ forms a >Mg₂-CO₃ complex at positively charged Mg surface sites on enstatite, resulting in stabilization of the surface Si-O bonds. Therefore, removal of solution carbonate results in an increase in dissolution rates of enstatite above pH 10. The log rate of CO₂-purged enstatite dissolution in moles per cm² per s as a function of increasing pH above pH 10 is equal to 0.35. This is consistent with the model of silicate mineral dissolution in the absence of surface carbonation in alkaline solutions proposed earlier in the literature.     

Direct determinations of the rates of rhyolite dissolution and clay formation over 52,000 years and comparison with laboratory measurements

Four porous, glass-dominated rhyolites from Kozushima Island, different in age and extent of weathering, were studied. Because the four rhyolites are homogeneously weathered to considerable depth, and because their initial chemical compositions were equal, the different rock characteristics can provide information about rates of rhyolite dissolution and clay mineral formation over ∼52,000 yr. Because glass surfaces retreat without surface roughening, surface area (measured by Brunauer-Emmett-Teller method; BET) was assumed to be approximately constant over time. The field dissolution rate, as inferred from the rate of loss of Si, was ∼6 × 10⁻¹⁹ mol cm⁻² s⁻¹. The estimated clay mineral formation rate was ∼1 × 10⁻¹⁹ mol cm⁻² s⁻¹. About 20% of dissolved Si precipitated as clays. In order to investigate the factors affecting the field dissolution rate, dissolution experiments that used powdered and block rhyolite samples were conducted. Under relevant field conditions (20°C and pH 6∼7), the rates were ∼5 × 10⁻¹⁷ and ∼5 × 10⁻¹⁸ mol cm⁻² s⁻¹ for powdered rhyolite and blocks, respectively. The dissolution rates obtained in this study decrease in the order powder > block > field. Because all surface areas were directly measured by BET, the differences are not attributable to the errors in surface area. The most plausible explanations of the slower rates are the lower degree of flushing and resultant high-solution saturation states in the pores (both in the field and in the rhyolite blocks used in experiments) plus the formation of alteration/hydrated layers at the glass surface.     

Precipitation kinetics and carbon isotope partitioning of inorganic siderite at 25°C and 1 atm

Siderite was precipitated from NaHCO₃ and Fe(ClO₄)₂ solutions under anaerobic conditions at 25°C and 1 atm total pressure using a modified version of the chemo-stat technique and the free-drift technique. Samples of solution and solid were withdrawn at different time intervals during time course experiments to determine the bulk and isotope composition of the solution and solid, and the morphology and mineralogy of the solid. A series of metastable precursors precipitated and dissolved sequentially, culminating in well-crystallized siderite rhombohedra having an average edge of ∼ 2 μm and a limited size distribution. Siderite precipitation rate ranged from 10^0.23 to 10^2.44 μmol•m⁻²•h⁻¹ for saturation states (with respect to siderite) ranging from near equilibrium to 10^3.53. Calculated carbon isotope fractionation factors (10³lnα) averaged 8.5 ± 0.2 (1σ n = 4) for the siderite-CO_2(g) system and 0.5 ± 0.2 (1σ n = 4) for the siderite-HCO₃⁻_(aq) system.     

Natural attenuation of uranium and formation of autunite at the expense of apatite within an oxidizing environment, south Eastern Desert of Egypt

X-ray diffraction (XRD), back scattered electron imaging (BSE), wavelength-dispersion spectral scan (WDS), X-ray compositional mapping and quantitative electron probe micro analyses (EPMA) have been used to examine a natural attenuation of U during low temperature alteration of the Sela granite, south Eastern Desert of Egypt. The data confirmed that a pre-existing hydroxyapatite was transformed to autunite through an unidentified intermediate phase. The boundaries between these three phases are not sharp and are generally interfering indicative of the replacement of Ca by U. The hydroxyapatite, intermediate phase and autunite show similar chondrite normalized rare earth elements (REE) patterns suggesting a genetic relationship. Alteration processes have enriched the three phases with heavy rare earth elements (HREE) and Eu and caused Ce, Dy and Yb negative anomalies. Based on the pH of the aqueous solutions, two mechanisms may explain the conversion of hydroxyapatite to autunite: (1) the dissolution of hydroxyapatite and precipitation of autunite which would happen when the uranyl bearing solutions were acidic enough (pH = 3–6.8) to be able to dissolve the pre-existing hydroxyapatite and (2) sorption of the uranyl ion on the surface of hydroxyapatite followed by substitution of (UO₂)²⁺ at the expense of Ca²⁺. The latter mechanism would have happened if the pH of the aqueous solutions were near neutral and at low dissolved concentrations of uranyl ion. The genesis of uranyl mineralization in the Sela area supports the use of apatite-based technologies for U remediation in an oxidizing environment.     

Ferrihydrite dissolution by pyridine-2,6-bis(monothiocarboxylic acid) and hydrolysis products

Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10⁻⁶ mol m⁻² h⁻¹. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H₂S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.     

Dissolution of illite in saline-acidic solutions at 25 °C

The dissolution rate of illite, a common clay mineral in Australian soils, was studied in saline-acidic solutions under far from equilibrium conditions. The clay fraction of Na-saturated Silver Hill illite (K_1.38Na_0.05)(Al_2.87Mg_0.46Fe³⁺_0.39Fe²⁺_0.28Ti_0.07)[Si_7.02Al_0.98]O₂₀(OH)₄ was used for this study. The dissolution rates were measured using flow-through reactors at 25 ± 1 °C, solution pH range of 1.0-4.25 (H₂SO₄) and at two ionic strengths (0.01 and 0.25 M) maintained using NaCl solution. Illite dissolution rates were calculated from the steady state release rates of Al and Si. The dissolution stoichiometry was determined from Al/Si, K/Si, Mg/Si and Fe/Si ratios. The release rates of cations were highly incongruent during the initial stage of experiments, with a preferential release of Al and K over Si in majority of the experiments. An Al/Si ratio >1 was observed at pH 2 and 3 while a ratio close to the stoichiometric composition was observed at pH 1 and 4 at the higher ionic strength. A relatively higher K⁺ release rate was observed at I = 0.25 in 2-4 pH range than at I = 0.01, possibly due to ion exchange reaction between Na⁺ from the solution and K⁺ from interlayer sites of illite. The steady state release rates of K, Fe and Mg were higher than Si over the entire pH range investigated in the study. From the point of view of the dominant structural cations (Si and Al), stoichiometric dissolution of illite occurred at pH 1-4 in the higher ionic strength experiments and at pH ?3 for the lower ionic strength experiments. The experiment at pH 4.25 and at the lower ionic strength exhibited lower R_Al (dissolution rate calculated from steady state Al release) than R_Si (dissolution rate calculated from steady state Si release), possibly due to the adsorption of dissolved Al as the output solutions were undersaturated with respect to gibbsite. The dissolution of illite appears to proceed with the removal of interlayer K followed by the dissolution of octahedral cations (Fe, Mg and Al), the dissolution of Si is the limiting step in the illite dissolution process. A dissolution rate law showing the dependence of illite dissolution rate on proton concentration in the acid-sulfate solutions was derived from the steady state dissolution rates and can be used in predicting the impact of illite dissolution in saline acid-sulfate environments. The fractional reaction orders of 0.32 (I = 0.25) and 0.36 (I = 0.01) obtained in the study for illite dissolution are similar to the values reported for smectite. The dissolution rate of illite is mainly controlled by solution pH and no effect of ionic strength was observed on the dissolution rates.     

Dissolution kinetics of synthetic Na-smectite. An integrated experimental approach

The effect of pH and Gibbs energy on the dissolution rate of a synthetic Na-montmorillonite was investigated by means of flow-through experiments at 25 and 80 °C at pH of 7 and 9. The dissolution reaction took place stoichiometrically at 80 °C, whereas at 25 °C preferential release of Mg over Si and Al was observed. The TEM-EDX analyses (transmission electronic microscopy with quantitative chemical analysis) of the dissolved synthetic phase at 25 °C showed the presence of newly formed Si-rich phases, which accounts for the Si deficit. At low temperature, depletion of Si concentration was attributed to incongruent clay dissolution with the formation of detached Si tetrahedral sheets (i.e., alteration product) whereas the Al behaviour remains uncertain (e.g., possible incorporation into Al-rich phases). Hence, steady-state rates were based on the release of Mg. Ex situ AFM measurements were used to investigate the variations in reactive surface area. Accordingly, steady-state rates were normalized to the initial edge surface area (11.2 m² g⁻¹) and used to propose the dissolution rate law for the dissolution reactions as a function of ΔG_r at 25 °C and pH∼9:

     

Uptake of uranium and trace elements in pyrite (FeS2) suspensions

Pyrite dissolution and interaction with Fe^(II), Co^(II), Eu^(III) and U^(VI) have been studied under anoxic conditions by solution chemistry and spectroscopic techniques. Aqueous data show a maximal cation uptake above pH 5.5. Iron (II) uptake can explain the non-stoichiometric [S]_aq/[Fe]_aq ratios often observed during dissolution experiments. Protonation data corrected for pyrite dissolution resulted in a proton site density of 9 ± 3 sites nm⁻². Concentration isotherms for Eu^(III) and U^(VI) sorption on pyrite indicate two different behaviours which can be related to the contrasted redox properties of these elements. For Eu^(III), sorption can be explained by the existence of a unique site with a saturation concentration of 1.25 × 10⁻⁶ mol g⁻¹. In the U^(VI) case, sorption seems to occur on two different sites with a total saturation concentration of 4.5 × 10⁻⁸ mol g⁻¹. At lower concentration, uranium reduction occurs, limiting the concentration of dissolved uranium to the solubility of UO₂(s).Scanning electron microscopy and micro-Raman spectrometry of U^(VI)-sorbed pyrite indicate a heterogeneous distribution of U at the pyrite surface and a close association with oxidized S. X-ray photoelectron spectroscopy confirms the partial reduction of U and the formation of a hyperstoichiometric UO_2+x(s). Our results are consistent with a chemistry of the pyrite surface governed not by Fe^(II)-bound hydroxyl groups, but by S groups which can either sorb cations and protons, or sorb and reduce redox-sensitive elements such as U^(VI).     

Does reactive surface area depend on grain size? Results from pH 3, 25 °C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite

Laboratory determined mineral weathering rates need to be normalised to allow their extrapolation to natural systems. The principle normalisation terms used in the literature are mass, and geometric- and BET specific surface area (SSA). The purpose of this study was to determine how dissolution rates normalised to these terms vary with grain size. Different size fractions of anorthite and biotite ranging from 180-150 to 20-10 μm were dissolved in pH 3, HCl at 25 °C in flow through reactors under far from equilibrium conditions. Steady state dissolution rates after 5376 h (anorthite) and 4992 h (biotite) were calculated from Si concentrations and were normalised to initial- and final- mass and geometric-, geometric edge- (biotite), and BET SSA. For anorthite, rates normalised to initial- and final-BET SSA ranged from 0.33 to 2.77 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 5.74 to 8.88 × 10⁻¹⁰ mol_feldspar m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.11 to 1.65 mol_feldspar g⁻¹ s⁻¹. For biotite, rates normalised to initial- and final-BET SSA ranged from 1.02 to 2.03 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric SSA ranged from 3.26 to 16.21 × 10⁻¹² mol_biotite m⁻² s⁻¹, rates normalised to initial- and final-geometric edge SSA ranged from 59.46 to 111.32 × 10⁻¹² mol_biotite m⁻² s⁻¹ and rates normalised to initial- and final-mass ranged from 0.81 to 6.93 × 10⁻¹² mol_biotite g⁻¹ s⁻¹. For all normalising terms rates varied significantly (p ? 0.05) with grain size. The normalising terms which gave least variation in dissolution rate between grain sizes for anorthite were initial BET SSA and initial- and final-geometric SSA. This is consistent with: (1) dissolution being dominated by the slower dissolving but area dominant non-etched surfaces of the grains and, (2) the walls of etch pits and other dissolution features being relatively unreactive. These steady state normalised dissolution rates are likely to be constant with time. Normalisation to final BET SSA did not give constant ratios across grain size due to a non-uniform distribution of dissolution features. After dissolution coarser grains had a greater density of dissolution features with BET-measurable but unreactive wall surface area than the finer grains. The normalising term which gave the least variation in dissolution rates between grain sizes for biotite was initial BET SSA. Initial- and final-geometric edge SSA and final BET SSA gave the next least varied rates. The basal surfaces dissolved sufficiently rapidly to influence bulk dissolution rate and prevent geometric edge SSA normalised dissolution rates showing the least variation. Simple modelling indicated that biotite grain edges dissolved 71-132 times faster than basal surfaces. In this experiment, initial BET SSA best integrated the different areas and reactivities of the edge and basal surfaces of biotite. Steady state dissolution rates are likely to vary with time as dissolution alters the ratio of edge to basal surface area. Therefore they would be more properly termed pseudo-steady state rates, only appearing constant because the time period over which they were measured (1512 h) was less than the time period over which they would change significantly.     

Silicon isotopic composition of dissolved silicon and suspended particulate matter in the Yellow River, China, with implications for the global silicon cycle

The silicon isotopic composition of dissolved silicon and suspended particulate matter (SPM) were systematically investigated in water samples from the mainstem of the Yellow River and 4 major tributaries. The SPM content of the Yellow River varied from 1.4 to 38,560 mg/L, averaging 3568 mg/L, and the δ³⁰Si of suspended particulate matter (δ³⁰Si_SPM) varied from 0.3‰ to −0.4‰, averaging −0.02‰. The major factors affecting the SPM content and the δ³⁰Si_SPM values in the Yellow River were inferred to be the mineralogical, chemical and isotopic characteristics of the sediments from the Loess Plateau and a combination of the climate and the flow discharge of the river.The major ions in the Yellow River water were Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, SO₄²⁻ and Cl⁻. High salt concentration was observed in samples from the middle and lower reaches, likely reflecting the effects of evaporation and irrigation because the Na⁺, Mg²⁺, SO₄²⁻ and K⁺ concentrations were correlated with the Cl⁻ concentration. The dissolved Si concentration (D_Si) increased downstream, varying from 0.016 to 0.323 mM. The δ³⁰Si of dissolved Si (δ³⁰Si_Diss) varied from 0.4‰ to 2.5‰, averaging 1.28‰. The major processes controlling the D_Si and δ³⁰Si_Diss of the Yellow River are (a) the weathering of silicate rocks, (b) the formation of phytoliths in plants, (c) the evaporation of water from and the addition of meteoric water to the river system, which only affects concentrations, (d) the adsorption and desorption of aqueous monosilicic acid on iron oxide, and (e) the dissolution of phytoliths in soils.The D_Si and δ³⁰Si_Diss values of global rivers vary spatially and temporally in response to changes in climate, chemical weathering intensity and biological activity. The moderately positive δ³⁰Si_Diss values observed in the Yellow River may be attributed to the higher rates of chemical weathering and biological activities that have been observed in this catchment in comparison with those of other previously studied catchments, excluding the Yangtze River. Human activities may also potentially influence chemical weathering and biological activities and affect the D_Si and δ³⁰Si_Diss values of the major rivers of the world. Further river studies should be performed to gain a better understanding of the global Si isotope budget.     

The effect of Si and Al concentrations on the removal of U(VI) in the alkaline conditions created by NH3 gas

Remediation of uranium in the deep unsaturated zone is a challenging task, especially in the presence of oxygenated, high-carbonate alkalinity soil and pore water composition typical for arid and semi-arid environments of the western regions of the U.S. This study evaluates the effect of various pore water constituencies on changes of uranium concentrations in alkaline conditions, created in the presence of reactive gases such as NH₃ to effectively mitigate uranium contamination in the vadose zone sediments. This contaminant is a potential source for groundwater pollution through slow infiltration of soluble and highly mobile uranium species towards the water table. The objective of this research was to evaluate uranium sequestration efficiencies in the alkaline synthetic pore water solutions prepared in a broad range of Si, Al, and bicarbonate concentrations typically present in field systems of the western U.S. regions and identify solid uranium-bearing phases that result from ammonia gas treatment. In previous studies (Szecsody et al. 2012; Zhong et al. 2015), although uranium mobility was greatly decreased, solid phases could not be identified at the low uranium concentrations in field-contaminated sediments. The chemical composition of the synthetic pore water used in the experiments varied for silica (5–250 mM), Al³⁺ (2.8 or 5 mM), HCO₃⁻ (0–100 mM) and U(VI) (0.0021–0.0084 mM) in the solution mixture. Experiment results suggested that solutions with Si concentrations higher than 50 mM exhibited greater removal efficiencies of U(VI). Solutions with higher concentrations of bicarbonate also exhibited greater removal efficiencies for Si, Al, and U(VI). Overall, the silica polymerization reaction leading to the formation of Si gel correlated with the removal of U(VI), Si, and Al from the solution. If no Si polymerization was observed, there was no U removal from the supernatant solution. Speciation modeling indicated that the dominant uranium species in the presence of bicarbonate were anionic uranyl carbonate complexes (UO₂(CO₃)₂⁻² and UO₂(CO₃)₃⁻⁴) and in the absence of bicarbonate in the solution, U(VI) major species appeared as uranyl-hydroxide (UO₂(OH)₃⁻ and UO₂(OH)₄⁻²) species. The model also predicted the formation of uranium solid phases. Uranyl carbonates as rutherfordine [UO₂CO₃], cejkaite [Na₄(UO2)(CO₃)₃] and hydrated uranyl silicate phases as Na-boltwoodite [Na(UO₂)(SiO₄)·1.5H₂O] were anticipated for most of the synthetic pore water compositions amended from medium (2.9 mM) to high (100 mM) bicarbonate concentrations.     

Effects of seawater carbonate ion concentration and temperature on shell U, Mg, and Sr in cultured planktonic foraminifera

We investigate the sensitivity of U/Ca, Mg/Ca, and Sr/Ca to changes in seawater [CO₃²⁻] and temperature in calcite produced by the two planktonic foraminifera species, Orbulina universa and Globigerina bulloides, in laboratory culture experiments. Our results demonstrate that at constant temperature, U/Ca in O. universa decreases by 25 ± 7% per 100 μmol [CO₃²⁻] kg⁻¹, as seawater [CO₃²⁻] increases from 110 to 470 μmol kg⁻¹. Results from G. bulloides suggest a similar relationship, but U/Ca is consistently offset by ∼+40% at the same environmental [CO₃²⁻]. In O. universa, U/Ca is insensitive to temperature between 15°C and 25°C. Applying the O. universa relationship to three U/Ca records from a related species, Globigerinoides sacculifer, we estimate that Caribbean and tropical Atlantic [CO₃²⁻] was 110 ± 70 μmol kg⁻¹ and 80 ± 40 μmol kg⁻¹ higher, respectively, during the last glacial period relative to the Holocene. This result is consistent with estimates of the glacial-interglacial change in surface water [CO₃²⁻] based on both modeling and on boron isotope pH estimates. In settings where the addition of U by diagenetic processes is not a factor, down-core records of foraminiferal U/Ca have potential to provide information about changes in the ocean’s carbonate concentration.Below ambient pH (pH < 8.2), Mg/Ca decreased by 7 ± 5% (O. universa) to 16 ± 6% (G. bulloides) per 0.1 unit increase in pH. Above ambient pH, the change in Mg/Ca was not significant for either species. This result suggests that Mg/Ca-based paleotemperature estimates for the Quaternary, during which surface-ocean pH has been at or above modern levels, have not been biased by variations in surface-water pH. Sr/Ca increased linearly by 1.6 ± 0.4% per 0.1 unit increase in pH. Shell Mg/Ca increased exponentially with temperature in O. universa, where Mg/Ca = 0.85 exp (0.096*T), whereas the change in Sr/Ca with temperature was within the reproducibility of replicate measurements.     

Diagenetic Ge-Si fractionation in continental margin environments: further evidence for a nonopal Ge sink

We present data for dissolved germanic and silicic acids from several settings: sediment pore water profiles collected from the Peru-Chile continental margin, fluxes measured with in situ benthic flux chambers and shipboard whole-core incubations, and water column profiles from the California continental margin. Collectively, these data show that Ge and Si are fractionated in these continental margin sedimentary environments during diagenesis with ∼50% of the Ge released by opal dissolution being sequestered within the sediments. The areal extent of this diagenetic fractionation covers station depths from ∼100 m to >1000 m. Sediments from these sites typically have high pore water Fe²⁺ present in the upper ∼2 cm. At sites with low Fe²⁺ concentrations in the upper pore waters, the Ge:Si benthic regeneration ratio indicates little or no fractionation during diatom dissolution. Consistent with the sedimentary fractionation, water column dissolved Ge:Si ratios along the continental margin are on average lower (0.66 μmol/mol) than the global average ratio (0.72 μmol/mol, Mortlock and Froelich, 1996). This lower “average” ratio is driven by two distinct ΔGe:ΔSi data trends having similar slopes but different intercepts. Data from the upper ∼150 m has a Ge:Si slope of 0.74 ± 0.04 μmol/mol (2σ) and an intercept of 0.5 ± 0.4; whereas below ∼550 m the slope is 0.70 ± 0.06 μmol/mol, but the intercept is −5.0 ± 8.0. Assuming that the sediments sampled here are representative of all reducing marine environments, an assumption requiring further testing, our calculations indicate that sequestration of Ge into a nonopal phase throughout the global ocean in the depth range 0.2-1 km is sufficient to balance the Ge budget. Thus, we tentatively conclude that sequestering of Ge in reducing continental margin sediments is the “missing” Ge sink.