Cadmium deposition and mobility in the sediments of an acidic oligotrophic lake

A dated core from the profoundal zone in a pristine oligotrophic acidic lake was analyzed for Cd as well as for Al, Ca, Fe, Mg, Mn, Pb, Ti and total carbon and nitrogen. Overlying water and porewater samples were also obtained on six occasions at the same site, and yielded vertical profiles of pH and dissolved Cd, Ca, Fe, Mg, Mn, sulfide, SO₄⁻², organic and inorganic carbon concentrations. These extensive porewater and sediment geochemical data were used, together with information on infaunal benthos, to decipher the sedimentary record of Cd contamination. Depth variation of sediment Ca concentrations indicate that the lake suffered from progressive acidification starting about 1950. The present-day accumulation rate of Cd (J_acc^Cd = 5.4 ± 0.4 × 10⁻¹¹ mol cm⁻² yr⁻¹) in the sediments is the sum of the flux of Cd deposited with settling particles (J_S^Cd = 3.3 ± 0.2 × 10⁻¹¹ mol cm⁻² yr⁻¹) and the fluxes of dissolved Cd across the sediment-water interface due to molecular diffusion (J_D^Cd = 1.8 ± 0.3 × 10⁻¹¹ mol cm⁻² yr⁻¹), bioturbation (J_B^Cd = 1.1 ± 0.2 × 10⁻¹⁴ mol cm⁻² yr⁻¹) and bioirrigation (J_I^Cd = 0.27 ± 0.05 × 10⁻¹¹ mol cm⁻² yr⁻¹). Biological mixing of the sediments was negligible. The shape of the vertical profile of total Cd concentration with depth in the sediment appears to be determined more by its input history than by post-depositional mobilization and redistribution in the sediment column.     

Arsenic, iron and sulfur co-diagenesis in lake sediments

Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (ΣS^−II), total zero-valent sulfur (ΣS⁰), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, ²¹⁰Pb and ¹³⁷Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As (). The model defines depth intervals where As is either released to (positive ) or removed from (negative ) the porewaters.At two of the sites, whose bottom water were oxygenated at sampling time, a production zone ( = 12 × 10⁻¹⁸ mol cm⁻³ s⁻¹-71 × 10⁻¹⁸ mol cm⁻³ s⁻¹) is inferred a few cm below the sediment-water interface, coincident with sharp porewater As and Fe peaks that indicate an intense coupled recycling of As and Fe. This process is confirmed by solid As and Fe maxima just below the sediment surface. In these two lakes a zone of As consumption ( = −5 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −53 × 10⁻¹⁸ mol cm⁻³ s⁻¹), attributed to the slow adsorption of As to authigenic Fe oxyhydroxides, occurs just above the production zone. A second-order rate constant of 0.12 ± 0.03 cm³ mol⁻¹ s⁻¹ is estimated for this adsorption reaction.Such features in the porewater and solid profiles were absent from the two other lakes that develop a seasonally anoxic hypolimnion. Thermodynamic calculations indicate that the porewaters of the four lakes, when sulfidic (i.e., ΣS^−II ? 0.1 μM), were undersaturated with respect to all known solid As sulfides; the calculation also predicts the presence of As^V oxythioanions in the sulfidic waters, as suggested by a recent study. In the sulfidic waters, the removal of As ( = −1 × 10⁻¹⁸ mol cm⁻³ s⁻¹ to −23 × 10⁻¹⁸ mol cm⁻³ s⁻¹) consistently occurred when saturation, with respect to FeS_(s), was reached and when As^V oxythioanions were predicted to be significant components of total dissolved As. This finding has potential implications for As transport in other anoxic waters and should be tested in a wider variety of natural environments.     

Self-diffusion of Si and O in diopside-anorthite melt at high pressures

Self-diffusion coefficients for Si and O in Di₅₈An₄₂ liquid were measured from 1 to 4 GPa and temperatures from 1510 to 1764°C. Glass starting powders enriched in ¹⁸O and ²⁸Si were mated to isotopically normal glass powders to form simple diffusion couples, and self-diffusion experiments were conducted in the piston cylinder device (1 and 2 GPa) and in the multianvil apparatus (3.5 and 4 GPa). Profiles of ¹⁸O/¹⁶O and ^29,30Si/²⁸Si were measured using secondary ion mass spectrometry. Self-diffusion coefficients for O (D(O)) are slightly greater than self-diffusion coefficients for Si (D(Si)) and are often the same within error. For example, D(O) = 4.20 ± 0.42 × 10⁻¹¹ m²/s and D(Si) = 3.65 ± 0.37 × 10⁻¹¹ m²/s at 1 GPa and 1662°C. Activation energies for self-diffusion are 215 ± 13 kJ/mol for O and 227 ± 13 kJ/mol for Si. Activation volumes for self-diffusion are −2.1 ± 0.4 cm³/mol and −2.3 ± 0.4 cm³/mol for O and Si, respectively. The similar self-diffusion coefficients for Si and O, similar activation energies, and small, negative activation volumes are consistent with Si and O transport by a cooperative diffusion mechanism, most likely involving the formation and disassociation of a high-coordinated intermediate species. The small absolute magnitudes of the activation volumes imply that Di₅₈An₄₂ liquid is close to a transition from negative to positive activation volume, and Adam-Gibbs theory suggests that this transition is linked to the existence of a critical fraction (∼0.6) of bridging oxygen.     

Diffusion kinetics of Fe and Mg in aluminous spinel: experimental determination and applications

The diffusion coefficients of Fe²⁺ and Mg in aluminous spinel at ∼20 kb, 950 to 1325°C, and at 30 kb, 1125°C have been determined via diffusion couple experiments and numerical modeling of the induced diffusion profiles. The oxygen fugacity, fO₂, was constrained by graphite encapsulating materials. The retrieved self-diffusion coefficients of Fe²⁺ and Mg at ∼20 kb, 950 to 1325°C, fit well the Arrhenian relation, D = D₀exp(−Q/RT), where Q is the activation energy, with D₀(Fe) = 1.8 (±2.8) × 10⁻⁵, D₀(Mg) = 1.9 (±1.4) × 10⁻⁵ cm²/s, Q(Fe) = 198 ± 19, and Q(Mg) = 202 ± 8 kJ/mol. Comparison with the data at 30 kb suggests an activation volume of ∼5 cm³/mol. From analysis of compositional zoning in natural olivine-spinel assemblages in ultramafic rocks, previous reports concluded that D(Fe-Mg) in spinel with Cr/(Cr + Al) ≤0.5 is ∼10 times that in olivine. The diffusion data in spinel and olivine have been applied to the problems of preservation of Mg isotopic inhomogeneity in spinel within the plagioclase-olivine inclusions in Allende meteorite and cooling rates of terrestrial ultramafic rocks.     

Source and movement of helium in the eastern Morongo groundwater Basin: The influence of regional tectonics on crustal and mantle helium fluxes

We assess the role of fracturing and seismicity on fluid-driven mass transport of helium using groundwaters from the eastern Morongo Basin (EMB), California, USA. The EMB, located ∼200 km east of Los Angeles, lies within a tectonically active region known as the Eastern California Shear Zone that exhibits both strike-slip and extensional deformation. Helium concentrations from 27 groundwaters range from 0.97 to 253.7 × 10⁻⁷ cm³ STP g⁻¹H₂O, with corresponding ³He/⁴He ratios falling between 1.0 and 0.26 R_A (where R_A is the ³He/⁴He ratio of air). All groundwaters had helium isotope ratios significantly higher than the crustal production value of ∼0.02 R_A. Dissolved helium concentrations were resolved into components associated with solubility equilibration, air entrainment, in situ production within the aquifer, and extraneous fluxes (both crustal and mantle derived). All samples contained a mantle helium-3 (³He_m) flux in the range of 4.5 to 1351 × 10⁻¹⁴ cm³ STP ³He cm⁻² yr⁻¹ and a crustal flux (J₀) between 0.03 and 300 × 10⁻⁷ cm³ STP ⁴He cm⁻² yr⁻¹. Groundwaters from the eastern part of the basin contained significantly higher ³He_m and deep crustal helium-4 (⁴He_dc) concentrations than other areas, suggesting a localized source for these components. ⁴He_dc and ³He_m are strongly correlated, and are associated with faults in the basin. A shallow thermal anomaly in a >3,000 m deep graben in the eastern basin suggests upflow of fluids through active faults associated with extensional tectonics. Regional tectonics appears to drive large scale crustal fluid transport, whereas episodic hydrofracturing provides an effective mechanism for mantle-crust volatile transport identified by variability in the magnitude of degassing fluxes (³He_m and J₀) across the basin.     

Connections between surface complexation and geometric models of mineral dissolution investigated for rhodochrosite

Mineral dissolution rates have been rationalized in the literature by surface complexation models (SCM) and morphological and geometric models (GM), and reconciliation of these conceptually different yet separately highly successful models is an important goal. In the current work, morphological alterations of the surface are observed in real time at the microscopic level by atomic force microscopy (AFM) while dissolution rates are simultaneously measured at the macroscopic level by utilizing the AFM fluid cell as a classic flow-through reactor. Rhodochrosite dissolution is studied from pH = 2 to 11 at 298 K, and quantitative agreement is found between the dissolution rates determined from microscopic and macroscopic observations. Application of a SCM model for the interpretation of the kinetic data indicates that the surface concentration of >CO₃H regulates dissolution for pH < 7 while the surface concentration of >MnOH₂⁺ regulates dissolution for pH > 7. A GM model explains well the microscopic observations, from which it is apparent that dissolution occurs at steps associated with anisotropic pit expansion. On the basis of the observations, we combine the SCM and GM models to propose a step-site surface complexation model (SSCM), in which the dissolution rates are quantitatively related to the surface chemical speciation of steps. The governing SSCM equation is as follows: R = χ_1/2(k_co + k_ca)[>CO₃H] + χ_1/2(k_mo + k_ma)[>MnOH₂⁺ ], where R is the dissolution rate (mol m⁻² s⁻¹), 2χ_1/2 is the fraction of surface sites located at steps, [>CO₃H] and [>MnOH₂⁺ ] are surface concentrations (mol m⁻²), and k_co, k_ca, k_mo, and k_ma are the respective dissolution rate coefficients (s⁻¹) for the >CO₃H and the >MnOH₂⁺ surface species on obtuse and acute steps. We find k_co = 2.7 s⁻¹, k_ca = 2.1 × 10⁻¹ s⁻¹, k_mo = 4.1 × 10⁻² s⁻¹, k_ma = 3.7 × 10⁻² s⁻¹, and χ_1/2 = 0.015 ± 0.005. The rate coefficients quantify the net result of complex surface step processes, including double-kink initiation and single-kink propagation. We propose that the SSCM model may have general applicability for dissolution far from equilibrium of flat mineral surfaces of ionic crystals, at least those that dissolve by step retreat.     

Rates of weathering rind formation on Costa Rican basalt

Weathering rind thicknesses were measured on ∼ 200 basaltic clasts collected from three regionally extensive alluvial fill terraces (Qt 1, Qt 2, and Qt 3) preserved along the Pacific coast of Costa Rica. Mass balance calculations suggest that conversion of unweathered basaltic core minerals (plagioclase and augite) to authigenic minerals in the porous rind (kaolinite, allophane, gibbsite, Fe oxyhydroxides) is iso-volumetric and Ti and Zr are relatively immobile. The hierarchy of cation mobility (Ca ≈ Na > K ≈ Mg > Si > Al > Fe ≈ P) is similar to other tropical weathering profiles and is indicative of differential rates of mineral weathering (anorthite > albite ≈ hypersthene > orthoclase ? apatite). Alteration profiles across the cm-thick rinds document dissolution of plagioclase and augite and the growth of kaolinite, with subsequent dissolution of kaolinite and precipitation of gibbsite as weathering rinds age. The rate of weathering rind advance is evaluated using a diffusion-limited model which predicts a parabolic rate law for weathering rind thickness, r_r, as a function of time, t(r_r =), and an interface-limited model which predicts a linear rate law for weathering rind thickness as a function of time (r_r = k_appt). In these rate laws, κ is a diffusion parameter and k_app is an apparent rate constant. The rate of advance is best fit by the interface model.Terrace exposures are confined to the lower reaches of streams draining the Pacific slope near the coast where the stream gradient is less than ∼3 m/km, and terrace deposition is influenced by eustatic sea level fluctuations. Geomorphological evidence is consistent with terrace deposition coincident with sea level maxima when the stream gradient would be lowest. Assigning the most weathered regionally extensive terrace Qt 1 (mean rind thickness 6.9 ± 0. 6cm) to oxygen isotope stage (OIS) 7 (ca. 240 ka), and assuming that at time = 0 rind thickness = 0, it is inferred that terrace Qt 2 (r_r = 2.9 ± 0.1 cm) is coincident with stage 5e (ca. 125 ka) and that Qt 3 (r_r = 0.9 ± 0.1 cm) is consistent with OIS 3 (ca. 37 ka). These assignments yield a value of k_app of 8.6 × 10⁻¹³ cm s⁻¹ (R² = 0.99). Only this value satisfies both the existing age controls and yields ages coincident with sea level maxima. Using this value, elemental weathering release fluxes across a weathering rind from Qt 2 range from 6.0 × 10⁻⁹ mol Si m⁻² s⁻¹ to 2.5 × 10⁻¹¹ mol K m⁻² s⁻¹. The rate of rind advance for the Costa Rican terraces is 2.8 × 10⁻⁷ m yr⁻¹. Basalt rind formation rates in lower temperature settings described in the literature are also consistent with interface-controlled weathering with an apparent activation energy of about 50 kJ mol⁻¹. Rates of rind formation in Costa Rica are an order of magnitude slower than reported for global averages of soil formation rates.     

Energetics of anhydrite, barite, celestine, and anglesite: a high-temperature and differential scanning calorimetry study

The thermochemistry of anhydrous sulfates (anglesite, anhydrite, arcanite, barite, celestine) was investigated by high-temperature oxide melt calorimetry and differential scanning calorimetry. Complete retention and uniform speciation of sulfur in the solvent was documented by (a) chemical analyses of the solvent (3Na₂O · 4MoO₃) with dissolved sulfates, (b) Fourier transform infrared spectroscopy confirming the absence of sulfur species in the gases above the solvent, and (c) consistency of experimental determination of the enthalpy of drop solution of SO₃ in the solvent. Thus, the principal conclusion of this study is that high-temperature oxide melt calorimetry with 3Na₂O · 4MoO₃ solvent is a valid technique for measurement of enthalpies of formation of anhydrous sulfates. Enthalpies of formation (in kJ/mol) from the elements (ΔH_f^o) were determined for synthetic anhydrite (CaSO₄) (−1433.8 ± 3.2), celestine (SrSO₄) (−1452.1 ± 3.3), anglesite (PbSO₄) (−909.9 ± 3.4), and two natural barite (BaSO₄) samples (−1464.2 ± 3.7, −1464.9 ± 3.7). The heat capacity of anhydrite, barite, and celestine was measured between 245 and 1100 K, with low- and high-temperature Netzsch (DSC-404) differential scanning calorimeters. The results for each sample were fitted to a Haas-Fisher polynomial of the form C_p(245 K < T < 1100 K) = a + bT + cT⁻² + dT^−0.5 + eT². The coefficients of the equation are as follows: for anhydrite a = 409.7, b = −1.764 × 10⁻¹, c = 2.672 × 10⁶, d = −5.130 × 10³, e = 8.460 × 10⁻⁵; for barite, a = 230.5, b = −0.7395 × 10⁻¹, c = −1.170 × 10⁶, d = −1.587 × 10³, e = 4.784 × 10⁻⁵; and for celestine, a = 82.1, b = 0.8831 × 10⁻¹, c = −1.213 × 10⁶, d = 0.1890 × 10³, e = −1.449 × 10⁻⁵. The 95% confidence interval of the measured C_p varies from 1 to 2% of the measured value at low temperature up to 2 to 5% at high temperature. The measured thermochemical data improve or augment the thermodynamic database for anhydrous sulfates and highlight the remaining discrepancies.     

The hydrolysis of gold(I) in aqueous solutions to 600°c and 1500 bar

The solubility of gold has been measured in aqueous solutions at temperatures between 300 and 600°C and pressures from 500 to 1500 bar to determine the stability and stoichiometry of the hydroxy complexes of gold(I) in hydrothermal solutions. The experiments were carried out using a flow-through autoclave system. The solubilities, measured as total dissolved gold, were in the range 1.2 × 10⁻⁸ to 2.0 × 10⁻⁶ mol kg⁻¹ (0.002 to 0.40 mg kg⁻¹), in solutions of total dissolved sodium between 0.0 and 0.5 mol kg⁻¹, and total dissolved hydrogen between 4.0 × 10⁻⁶ and 4.0 × 10⁻⁴ mol kg⁻¹. At constant hydrogen molality, the solubility of gold increases with increasing temperature and decreases with increasing pressure. The solubilities were found to be independent of pH but increased with decreasing hydrogen molality at constant temperature and pressure. Consequently, gold dissolves in aqueous solutions of acidic to alkaline pH according to the reactionAu(s)+H₂O(l)=AuOH(aq)+0.5H₂(g) K_s,1The solubility constant, logK_s,1, increases with increasing temperature from a minimum of −8.76 (±0.18) at 300°C and 500 bar to a maximum of −7.50 (±0.11) at 500°C and 1500 bar and decreases to −7.61 (±0.08) at 600°C and 1500 bar. From the equilibrium solubility constant and the redox potential of gold, the formation constant to form AuOH(aq) was calculated. At 25°C the complex formation is characterised by an exothermic enthalpy and a positive entropy. With increasing temperature and decreasing pressure, the formation reaction becomes endothermic and is accompanied by a large positive entropy, indicating a greater electrostatic interaction between Au⁺ and OH⁻.     

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.     

Influence of phosphate on the oxidation kinetics of nanomolar Fe(II) in aqueous solution at circumneutral pH

The oxygenation kinetics of nanomolar concentrations of Fe(II) in aqueous solution have been studied in the absence and presence of millimolar concentrations of phosphate over the pH range 6.0-7.8. At each phosphate concentration investigated, the overall oxidation rate constant varied linearly with pH, and increased with increasing phosphate concentration. A model based on equilibrium speciation of Fe(II) was found to satisfactorily explain the results obtained. From this model, the rate constants for oxygenation of the Fe(II)-phosphate species FeH₂PO₄⁺, FeHPO₄ and FePO₄⁻ have been determined for the first time. FePO₄⁻ was found to be the most kinetically reactive species at circumneutral pH with an estimated oxygenation rate constant of (2.2 ± 0.2) × 10 M⁻¹ s⁻¹. FeH₂PO₄⁺ and FeHPO₄ were found to be less reactive with oxygen, with rate constants of (3.2 ± 2) × 10⁻² M⁻¹ s⁻¹ and (1.2 ± 0.8) × 10⁻¹ M⁻¹ s⁻¹, respectively.     

Surface chemistry of disordered mackinawite (FeS)

Disordered mackinawite, FeS, is the first formed iron sulfide in ambient sulfidic environments and has a highly reactive surface. In this study, the solubility and surface chemistry of FeS is described. Its solubility in the neutral pH range can be described by K_s^app = {Fe²⁺} · {H₂S(aq)} · {H⁺}⁻² = 10^+4.87±0.27. Acid-base titrations show that the point of zero charge (PZC) of disordered mackinawite lies at pH ∼7.5. The hydrated disordered mackinawite surface can be best described by strongly acidic mono-coordinated and weakly acidic tricoordinated sulfurs. The mono-coordinated sulfur site determines the acid-base properties at pH < PZC and has a concentration of 1.2 × 10⁻³ mol/g FeS. At higher pH, the tricoordinated sulfur, which has a concentration of 1.2 × 10⁻³ mol/g FeS, determines surface charge changes. Total site density is 4 sites nm⁻². The acid-base titration data are used to develop a surface complexation model for the surface chemistry of FeS.     

Interferometric study of the dolomite dissolution: a new conceptual model for mineral dissolution

The dissolution rate and mechanism of three different cleavage faces of a dolomite crystal from Navarra (near Pamplona), Spain, were studied in detail by vertical scanning interferometry techniques. A total of 37 different regions (each about 124 × 156 μm in size) on the three sample surfaces were monitored as a function of time during dissolution at 25°C and pH 3. Dissolution produced shallow etch pits with widths reaching 20 μm during 8 h of dissolution. Depth development as a function of time was remarkably similar for all etch pits on a given dolomite surface.On the basis of etch pit distribution and volume as a function of time, the calculated dissolution rate increases from near zero to 4 × 10⁻¹¹ mol cm⁻² s⁻¹ over 5 h. The time variation is different for each of the three cleavage surfaces studied. In addition, the absolute dissolution rates of different parts of the dolomite crystal surface can be computed by using a reference surface. The different surfaces yield an “average” rate of 1.08 × 10⁻¹¹ mol cm⁻² s⁻¹ with a standard deviation of 0.3 × 10⁻¹¹ mol cm⁻² s⁻¹ based on about 60 analyses. The mean absolute rate of the dolomite surface is about 10 times slower than the rate calculated from etch pit dissolution alone. On the other hand, earlier batch rate data that used BET surface areas yield rates that are at least 30 to 60 times faster than our directly measured mean dissolution rate for the same pH and temperature.A conceptual model for mineral dissolution has been inferred from the surface topography obtained by the interferometry investigations. In this model, mineral dissolution is not dominated by etch pit formation itself but rather by extensive dissolution stepwaves that originate at the outskirts of the etch pits. These stepwaves control the overall dissolution as well as the dependence on temperature and saturation state.     

Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, C and F NMR spectroscopy

The AlOx_1-3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by ¹³C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx₂⁻ complex (k(298 K) = 5 s⁻¹, ΔH^# = 67 ± 5 kJ mol⁻¹, ΔS^# = −6 ± 6 J mol⁻¹ K⁻¹, the rate determining step could be the breaking of the Al-O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx₃³⁻ complex and the free ligand (k₃₀(298 K) = 6.6 · 10⁻⁵ s⁻¹, ΔH^# = 164 ± 17 kJ mol⁻¹, ΔS^# = 225 ± 51 J mol⁻¹ K⁻¹, D/I_d mechanism) were determined by dynamic 1D and 2D ¹³C NMR measurements. Mixed complexes, AlFOx, AlFOx₂^2-, AlF₂Ox⁻, and AlF₂Ox₂^3-, with overall stability (logβ) of 11.53 ± 0.03, 15.67 ± 0.03, 15.74 ± 0.02, and 19.10 ± 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by ¹³C and¹⁹F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox^2--F⁻ complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.     

Elemental and isotopic fractionation of Type B CAI-like liquids by evaporation

Vacuum evaporation experiments with Type B CAI-like starting compositions were carried out at temperatures of 1600, 1700, 1800, and 1900 °C to determine the evaporation kinetics and evaporation coefficients of silicon and magnesium as a function of temperature as well as the kinetic isotope fractionation factor for magnesium. The vacuum evaporation kinetics of silicon and magnesium are well characterized by a relation of the form J = J_oe^−E/RT with J_o = 4.17 × 10⁷ mol cm⁻² s⁻¹, E = 576 ± 36 kJ mol⁻¹ for magnesium, J_o = 3.81 × 10⁶ mol cm⁻² s⁻¹, E = 551 ± 63 kJ mol⁻¹ for silicon. These rates only apply to evaporation into vacuum whereas the actual Type B CAIs were almost certainly surrounded by a finite pressure of a hydrogen-dominated gas. A more general formulation for the evaporation kinetics of silicon and magnesium from a Type B CAI-like liquid that applies equally to vacuum and conditions of finite hydrogen pressure involves combining our determinations of the evaporation coefficients for these elements as a function of temperature (γ = γ₀e^−E/RT with γ₀ = 25.3, E = 92 ± 37 kJ mol⁻¹ for γ_Si; γ₀ = 143, E = 121 ± 53 kJ mol⁻¹ for γ_Mg) with a thermodynamic model for the saturation vapor pressures of Mg and SiO over the condensed phase. High-precision determinations of the magnesium isotopic composition of the evaporation residues from samples of different size and different evaporation temperature were made using a multicollector inductively coupled plasma mass spectrometer. The kinetic isotopic fractionation factors derived from this data set show that there is a distinct temperature effect, such that the isotopic fractionation for a given amount of magnesium evaporated is smaller at lower temperature. We did not find any significant change in the isotope fractionation factor related to sample size, which we interpret to mean that recondensation and finite chemical diffusion in the melt did not affect the isotopic fractionations. Extrapolating the magnesium kinetic isotope fractionations factors from the temperature range of our experiments to temperatures corresponding to partially molten Type B CAI compositions (1250-1400 °C) results in a value of α_Mg ≈ 0.991, which is significantly different from the commonly used value of .     

Monomethylmercury sources in a tropical artificial reservoir

The distribution and speciation of mercury (Hg) in the water column, the inputs (wet deposition and tributaries) and the outputs (atmospheric evasion and outlet) of an artificial partially anoxic tropical lake (Petit-Saut reservoir, French Guiana) were investigated on a seasonal basis in order to appraise the cycling and transformations of this metal. The total mercury (HgT) concentrations in the oxygenated epilimnetic waters averaged 5 ± 3 pmol L⁻¹ in the unfiltered samples (HgT_UNF) and 4 ± 2 pmol L⁻¹ in the dissolved (HgT_D) phase (<0.45 μm). On average, the monomethylmercury (MMHg) constituted 8%, 40% and 18% of the HgT in the dissolved phase, the particulate suspended matter and in the unfiltered samples, respectively. Covariant elevated concentrations of particulate MMHg and chlorophyll a in the epilimnion suggest that phytoplankton is an active component for the MMHg transfer in the lake. In the anoxic hypolimnion the HgT_UNF averages 13 ± 6 pmol L⁻¹ and the HgT_D 8 ± 4 pmol L⁻¹. The averages of MMHg_P and MMHg_D in hypolimnetic waters were two and three times the corresponding values of the epilimnion, 170 ± 90 pmol g⁻¹ and 0.9 ± 0.5 pmol L⁻¹, respectively. In the long dry and wet seasons, at the flooded forest and upstream dam sampling stations, the vertical profiles of MMHg_D concentrations accounted for two distinct maxima: one just below the oxycline and the other near the benthic interface. Direct wet atmospheric deposition accounted for 14 moles yr⁻¹ HgT_UNF, with 0.7 moles yr⁻¹ as MMHg_UNF, while circa 76 moles yr⁻¹ of HgT_UNF, with 4.7 moles yr⁻¹ as MMHg_UNF, coming from tributaries. Circa 78 moles (∼17% as MMHg) are annually exported through the dam, while 23 moles yr⁻¹ of Hg⁰ evolve in the atmosphere. A mass balance calculation suggests that the endogenic production of MMHg_UNF attained 8.1 moles yr⁻¹, corresponding to a methylation rate of 0.06% d⁻¹. As a result, the Petit-Saut reservoir is a large man-made reactor that has extensively altered mercury speciation in favor of methylated species.     

Records of radionuclide deposition in two salt marshes in the United Kingdom with contrasting redox and accumulation conditions

Single cores from two salt marshes in the United Kingdom located near different nuclear facilities were investigated to compare chronostratigraphic estimates derived from the natural radionuclide ²¹⁰Pb_excess with estimates from the known times of introduction of artificial radionuclides to the environment. Both cores selected had clear visual indications of redox zonation, and evidence for diagenetic redox perturbation of the radionuclide records was also sought. In the core from Beaulieu Marsh on the south coast of England, the redox zonation was revealed by the profiles of the redox-sensitive elements Mn + I, Fe + P + As, and S, and the records of nuclear discharges were entirely contained within oxidized conditions in the upper 40 cm. The constant flux/constant sedimentation ²¹⁰Pb_excess accumulation estimate was 76% of that derived from the 1963 fallout ¹³⁷Cs level (0.35 vs. 0.46 g cm⁻² yr⁻¹ dry mass), but the constant flux ²¹⁰Pb_excess method indicated that accumulation rates were lower at Beaulieu before ∼1950. On any timescale, ¹³⁷Cs appears earlier in the sediment record than its introduction to the environment, but although downward diffusion of ¹³⁷Cs relative to ²⁴¹Am has clearly occurred, the ¹³⁷Cs peak still appeared in place and there was negligible penetration of ¹³⁷Cs into underlying reduced conditions. This core also contained a peak of the ⁶⁰Co discharges from either or both the Winfrith and La Hague nuclear plants that peaked in 1980 and 1984, respectively. The sediments in the core from Wyre Marsh on the eastern coast of the Irish Sea had accumulated more rapidly than those at Beaulieu, and in this case the redox zonation could be established only from Mn and S profiles. Here, the constant initial activity ²¹⁰Pb_excess accumulation rate estimate was 125% of that derived from the ¹³⁷Cs peak correlated with the 1975 Sellafield discharge maximum (0.79 vs. 0.64 g cm⁻² yr⁻¹). Sellafield discharge ¹³⁷Cs swamps fallout or Chernobyl ¹³⁷Cs signals in this core, but the ¹³⁷Cs and ²⁴¹Am sediment records are well separated and remain consistent with the slightly different discharge patterns over time. This is so despite the fact that the maximum activity levels of both isotopes are now located well into reducing conditions out of which Mn must have migrated. The ²¹⁰Pb profile appeared similarly unaffected by the oxidized/reduced boundary in this case. This core was too short to define the limits of any downward ¹³⁷Cs migration. ²¹⁰Pb_excess accumulation rate estimates for salt marshes should be viewed with some caution because of the steady-state assumptions inherent in all ²¹⁰Pb_excess methods and the potential for fluctuating accumulation conditions and open system behavior in salt marshes.     

Coupling between Pbex and organic matter in sediments of a nutrient-enriched lake: An example from Lake Chenghai, China

Sediment cores were collected from deep-water areas of Lake Chenghai, China in June 1997. The vertical profile of ¹³⁷Cs activity gives reliable geochronological results. The results also indicate that sediment accumulation rates in deep-water areas of Lake Chenghai were relatively constant in recent decades, averaging 0.43 g cm^− 2 y^− 1, despite a variable organic carbon influx. ²¹⁰Pb_eq (= ²²⁶Ra) activity was relatively constant also, with an average value of 54.3 ± 3.2 Bq kg^− 1. Vertical profiles of ²¹⁰Pb_ex (= ²¹⁰Pb_total − ²²⁶Ra) decreased exponentially, resulting in somewhat lower sediment accumulation rates (0.3 g cm^− 2 y^− 1). These lower rates are likely less reliable, as the relatively large fluctuations in ²¹⁰Pb_ex activities correlate closely to the organic carbon (C_org) content of the sediments. For example, the vertical profile of ²¹⁰Pb_ex activity displays peaks at mass depths of 3.7-4.7 g cm^− 2 (10-12 cm) and 10-11 g cm^− 2(25-28 cm), similar to the maxima in the vertical profile of C_org. This phenomenon must be related to the delivery of particulate organic matter (POM) from the water to the sediments, or to watershed soil erosion. Since the mean atomic ratios of H_org / C_org and C_org / N_org in Lake Chenghai sediments are 5.5 and 7.0, respectively, indicating that POM was predominantly derived from the remains of authigenic algae, this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by ²¹⁰Pb_ex. Sedimentation fluxes (F(C_org)) of particulate organic carbon since 1970 varied between 60 to 160 g m^− 2 y^− 1, and appeared to closely influence variations in ²¹⁰Pb_ex concentrations. For example, sedimentation fluxes of ²¹⁰Pb_ex (F(²¹⁰Pb_ex)) showed maxima in the years 1972-1974 and 1986-1989, likely reflecting historical variations of lake biological productivity or carbon preservation.     

Influence of uranium (VI) on the metabolic activity of stable multispecies biofilms studied by oxygen microsensors and fluorescence microscopy

The effect of uranium added in ecologically relevant concentrations (1 × 10⁻⁵ and 1 × 10⁻⁶ M) to stable multispecies biofilms was studied by electrochemical oxygen microsensors with tip diameters of 10 μm and by confocal laser fluorescence microscopy (CLSM). The microsensor profile measurements in the stable multispecies biofilms exposed to uranium showed that the oxygen concentration decreased faster with increasing biofilm depth compared to the uranium free biofilms. In the uranium containing biofilms, the oxygen consumption, calculated from the steady-state microprofiles, showed high consumption rates of up to 61.7 nmol cm⁻³ s⁻¹ in the top layer (0-70 μm) and much lower consumption rates in the lower zone of the biofilms. Staining experiments with 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) and 4,6-diamidino-2-phenylindole (DAPI) confirmed the high respiratory activities of the bacteria in the upper layer. Analysis of the amplified 16S rRNA gene fragments showed that the addition of uranium in ecologically relevant concentrations did not change the bacterial diversity in the stable multispecies biofilms and is therefore not responsible for the different oxygen profiles in the biofilms. The fast decrease in the oxygen concentrations in the biofilm profiles showed that the bacteria in the top region of the biofilms, i.e., the metabolically most active biofilm zone, battle the toxic effects of aqueous uranium with an increased respiratory activity. This increased respiratory activity results in O₂ depleted zones closer to the biofilm/air interface which may trigger uranium redox processes, since suitable redox partners, e.g., extracellular polymeric substance (EPS) and other organics (e.g., metabolites), are sufficiently available in the biofilm porewaters. Such redox reactions may lead to precipitation of uranium (IV) solids and consequently to a removal of uranium from the aqueous phase.     

Gold(I) complexing in aqueous sulphide solutions to 500°C at 500 bar

The solubility of gold has been measured in aqueous sulphide solutions from 100 to 500°C at 500 bar in order to determine the stability and stoichiometry of sulphide complexes of gold(I) in hydrothermal solutions. The experiments were carried out in a flow-through system. The solubilities, measured as total dissolved gold, were in the range 3.6 × 10⁻⁸ to 6.65 × 10⁻⁴ mol kg⁻¹ (0.007-131 mg kg⁻¹), in solutions of total reduced sulphur between 0.0164 and 0.133 mol kg⁻¹, total chloride between 0.000 and 0.240 mol kg⁻¹, total sodium between 0.000 and 0.200 mol kg⁻¹, total dissolved hydrogen between 1.63 × 10⁻⁵ and 5.43 × 10⁻⁴ mol kg⁻¹ and a corresponding pH_{T, p} of 1.5 to 9.8. A non-linear least squares treatment of the data demonstrates that the solubility of gold in aqueous sulphide solutions is accurately described by the reactions

Au(s)+H₂S(aq)=AuHS(aq)+0.5H₂(g) K_s,100