A chemical equilibrium model of solution behavior and solubility in the H-Na-K-Ca-OH-Cl-HSO4-SO4-H2O system to high concentration and temperature

This paper describes a chemical model for calculating solute and solvent activities and solid-liquid equilibria in the H-Na-K-Ca-OH-Cl-HSO₄-SO₄-H₂O system from low to high solution concentration within the 0° to 250°C temperature range. The solubility modeling approach of Harvie and Weare (1980) and Harvie et al. (1984), and their implementation of the Pitzer (1973) specific interaction equations are employed. This model expands the variable temperature H-Na-K-OH-Cl-HSO₄-SO₄-H₂O model of Christov and Moller (2004) by evaluating the Ca²⁺-acid/base binary and ternary solution parameters and the chemical potentials of three basic calcium solid phases: Ca(OH)₂(s), CaCl₂.Ca(OH)₂.H₂O(s), and CaCl₂.3Ca(OH)₂.13H₂O and three calcium chloride hydrates (CaCl₂.nH₂O; n = 6, 4, 2). Comparisons of solubility and activity predictions with experimental data used and not used in model parameterization are given. Limitations of the model due to data insufficiencies are discussed.     

Raman spectroscopic study of CO2-NaCl-H2O mixtures in synthetic fluid inclusions at high temperatures

Mixtures of CO₂-NaCl-H₂O contained in synthetic fluid inclusions are studied by laser Raman spectroscopy at high temperatures. With increasing temperature, the band splitting (X) of υ₁-2υ₂ diad of spectrum of CO₂ presents more variations, and the intensity ratios of the hot bands to the υ₁-2υ₂ diad increase. For mixtures of gas phase rich in CO₂ and liquid phase rich in H₂O before homogenization, the strength of hydrogen bonding of water in the liquid phase decreases almost linearly with increasing temperature. When mixtures become homogeneous liquid phases, carbon dioxide content increases significantly as a result of the abrupt decrease in hydrogen bonds. Our results show that the hydrogen bonds change only slightly at higher temperatures above the homogeneous point, and a certain extent of the hydrogen bonds still remains at the highest temperature of 550°C of this work. The study is helpful to Raman spectroscopic analysis of natural fluid inclusions at high temperatures.     

Oxygen isotope exchange kinetics between H2O and H4SiO4 from ab initio calculations

Oxygen isotope exchange between H₂O and H₄SiO₄ was modeled with ab initio calculations on H₄SiO₄ + 7H₂O. Constrained optimizations were performed with the B3LYP/6-31+G(d,p) method to determine reactants, transition states, and intermediates. Long-range solvation was accounted for using self-consistent reaction field calculations. The mechanism for exchange involves two steps, a concerted proton transfer from H₄SiO₄ forming a 5-coordinated Si followed by a concerted proton transfer from the 5-coordinated Si forming another H₄SiO₄. The 5-coordinated Si intermediate is C2 symmetric. At 298K and with implicit solvation included, the Gibbs free energy of activation from transition state theory is 66 kJ/mol and the predicted rate constant is 16 s⁻¹. Equilibrium calculations between 298K and 673K yield α_H4SiO4-H2O that are uniformly less than, but similar to, α_qtz-H2O, and therefore α_qtz-H4SiO4 is expected to be relatively small in this temperature range.     

Pyrite oxidation in moist air

The rate of pyrite oxidation in moist air was determined by measuring, over time, the pressure difference between a sealed chamber containing pyrite plus oxygen and a control. The experiments carried out at 25°C, 96.7% fixed relative humidity, and oxygen partial pressures of 0.21, 0.61, and 1.00 atm showed that the rate of oxygen consumption is a function of oxygen partial pressure and time. The rates of oxygen consumption (r, mol/m²sec) fit the expression

(A)     

Oxygen-exchange pathways in aluminum polyoxocations

Using molecular dynamics simulations and electronic structure methods, we postulate a mechanism to explain the complicated reactivity trends that are observed for oxygen isotope exchange reactions between sites in aluminum polyoxocations of the ε-Keggin type and bulk solution. Experimentally, the molecules have four nonequivalent oxygens that differ considerably in reactivity both within a molecule, and between molecules in the series: Al₁₃, GaAl₁₂, and GeAl₁₂ [MO₄Al₁₂(OH)₂₄(H₂O)₁₂ⁿ⁺(aq); with M = Al(III) for Al₁₃, n = 7; M = Ga(III) for GaAl₁₂, n = 7; M = Ge(IV) for GeAl₁₂, n = 8]. We find that a partly dissociated, metastable intermediate molecule of expanded volume is necessary for exchange of both sets of μ₂-OH and that the steady-state concentration of this intermediate reflects the bond strengths between the central metal and the μ₄-O. Thus the central metal exerts extraordinary control over reactions at hydroxyl bridges, although these are three bonds away.This mechanism not only explains the reactivity trends for oxygen isotope exchange in μ₂-OH and η-OH₂ sites in the ε-Keggin aluminum molecules, but also explains the observation that the reactivities of minerals tend to reflect the presence of highly coordinated oxygens, such as the μ₄-O in boehmite, α-, and γ-Al₂O₃ and their Fe(III) analogs. The partial dissociation of these highly coordinated oxygens, coupled with simultaneous activation and displacement of neighboring metal centers, may be a fundamental process by which metals atoms undergo ligand exchanges at mineral surfaces.     

A stable isotope-based approach to tropical dendroclimatology

We describe a strategy for development of chronological control in tropical trees lacking demonstrably annual ring formation, using high resolution δ¹⁸O measurements in tropical wood. The approach applies existing models of the oxygen isotopic composition of alpha-cellulose (Roden et al., 2000), a rapid method for cellulose extraction from raw wood (Brendel et al., 2000), and continuous flow isotope ratio mass spectrometry (Brenna et al., 1998) to develop proxy chronological, rainfall and growth rate estimates from tropical trees lacking visible annual ring structure. Consistent with model predictions, pilot datasets from the temperate US and Costa Rica having independent chronological control suggest that observed cyclic isotopic signatures of several permil (SMOW) represent the annual cycle of local rainfall and relative humidity. Additional data from a plantation tree of known age from ENSO-sensitive northwestern coastal Peru suggests that the 1997-8 ENSO warm phase event was recorded as an 8‰ anomaly in the δ¹⁸O of α-cellulose. The results demonstrate reproducibility of the stable isotopic chronometer over decades, two different climatic zones, and three tropical tree genera, and point to future applications in paleoclimatology.     

Carbon isotope effects in carbonate systems

Global carbon cycle models require a complete understanding of the δ¹³C variability of the Earth’s C reservoirs as well as the C isotope effects in the transfer of the element among them. An assessment of δ¹³C changes during CO₂ loss from degassing magmas requires knowledge of the melt-CO₂ carbon isotope fractionation. In order to examine the potential size of this effect for silicate melts of varying composition, ¹³C reduced partition functions were computed in the temperature range 275 to 4000 K for carbonates of varying bond strengths (Mg, Fe, Mn, Sr, Ba, Pb, Zn, Cd, Li, and Na) and the polymorphs of calcite. For a given cation and a given pressure the ¹³C content increases with the density of the carbonate structure. For a given structure the tendency to concentrate ¹³C increases with pressure. The effect of pressure (‰/10 kbar) on the size of the reduced partition function of aragonite varies with temperature; in the pressure range 1 to 10⁵ bars the change is given by:

(1)     

Manganese-chromium isotope systematics of enstatite meteorites

We present results of a study of the ⁵³Mn-⁵³Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide ⁵³Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial ⁵³Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic ⁵³Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of ⁵³Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial ⁵³Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic ⁵³Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of ⁵³Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic ⁵³Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ ⁵³Mn-⁵³Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the ¹²⁹I-¹²⁹Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the ¹²⁹I-¹²⁹Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years.     

Reversibility of radiocaesium sorption on illite

Adsorption of trace amounts of radiocaesium on NH₄-, K-, and Na-saturated Fithian illite and subsequent desorption by 1 M NH₄ showed that a substantial amount of radiocaesium (44%, 46%, and 91% for NH₄-, K-, and Na-illite, respectively) cannot be desorbed after only 5 min of adsorption. Our results suggest that this instantaneous fixation is caused by the collapse of the frayed edges of the clay mineral and the relatively high concentration of radiocaesium building up in solution in the batch desorption experiments. Consequently, commonly applied high-NH₄ extractions underestimate truly exchangeable amounts of radiocaesium in soils and sediments containing illitic clay minerals. The rate of desorption of trace amounts of radiocaesium from the solids using high NH₄ or Cs concentrations has a half-life of about 2 yr, reflecting radiocaesium desorption from (partially) collapsed interlayers. Extraction of radiocaesium from illite after 5 min of contact time with a Cs-selective adsorbent or a 1 × 10^-6 M CsCl solution shows that 100% of the bound radiocaesium is readily available. The desorption rate in the presence of a Cs-selective adsorbent has a half-life of about 0.2 yr. Desorption of radiocaesium from illite using different ammonium concentrations shows that radiocaesium partitioning follows reversible ion-exchange theory if the NH₄ concentration is below 1 × 10^-4 M, and sufficient time (weeks) is allowed for the reaction to proceed. Thus, radiocaesium sorption reversibility in the natural environment is much higher than generally assumed, and equilibrium solid/liquid partitioning may be assumed for the long-term modelling of radiocaesium mobility in the natural environment. In the particular case of anoxic freshwater sediments with very high NH₄ concentrations in the pore waters (up to several mmol.L^-1), collapse of the frayed edges of illite may occur, influencing radiocaesium partitioning. If collapse occurs before radiocaesium adsorbs to illite, high caesium sorption reversibility as measured by high-NH₄ extraction can be expected because further collapse of the frayed edges during the extraction procedure will be limited. This effect has indeed been observed earlier in the extraction of radiocaesium from anoxic freshwater sediments with high-NH₄ solutions and was as yet unexplained.     

Thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions

The thermochemistry of jarosite-alunite and natrojarosite-natroalunite solid solutions was investigated. Members of these series were either coprecipitated or synthesized hydrothermally and were characterized by XRD, FTIR, electron microprobe analysis, ICP-MS, and thermal analysis. Partial alkali substitution and vacancies on the Fe/Al sites were observed in all cases, and the solids studied can be described by the general formula K_1-x-yNa_y(H₃O)_xFe_zAl_w(SO₄)₂(OH)_6-3(3-z-w)(H₂O)_3(3-z-w). A strong preferential incorporation of Fe over Al in the jarosite/alunite structure was observed. Heats of formation from the elements, ΔH°_f, were determined by high-temperature oxide melt solution calorimetry. The solid solutions deviate slightly from thermodynamic ideality by exhibiting positive enthalpies of mixing in the range 0 to +11 kJ/mol. The heats of formation of the end members of both solid solutions were derived. The values ΔH°_f = −3773.6 ± 9.4 kJ/mol, ΔH°_f = −4912.2 ± 24.2 kJ/mol, ΔH°_f = −3734.6 ± 9.7 kJ/mol and ΔH°_f = −4979.7 ± 7.5kJ/mol were found for K_0.85(H₃O)_0.15Fe_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, K_0.85(H₃O)_0.15Al_2.5(SO₄)₂(OH)_4.5(H₂O)_1.5, Na_0.7(H₃O)_0.3Fe_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9, and Na_0.7(H₃O)_0.3Al_2.7(SO₄)₂(OH)_5.1(H₂O)_0.9 respectively. To our knowledge, this is the first experimentally-based report of ΔH°_f for such nonstoichiometric alunite and natroalunite samples. These thermodynamic data should prove helpful to study, under given conditions, the partitioning of Fe and Al between the solids and aqueous solution.     

Chemical and isotopic equilibrium between CO2 and CH4 in fumarolic gas discharges: Generation of CH4 in arc magmatic-hydrothermal systems

The chemical and isotopic composition of fumarolic gases emitted from Nisyros Volcano, Greece, and of a single gas sample from Vesuvio, Italy, was investigated in order to determine the origin of methane (CH₄) within two subduction-related magmatic-hydrothermal environments.Apparent temperatures derived from carbon isotope partitioning between CH₄ and CO₂ of around 340°C for Nisyros and 470°C for Vesuvio correlate well with aquifer temperatures as measured directly and/or inferred from compositional data using the H₂O-H₂-CO₂-CO-CH₄ geothermometer. Thermodynamic modeling reveals chemical equilibrium between CH₄, CO₂ and H₂O implying that carbon isotope partitioning between CO₂ and CH₄ in both systems is controlled by aquifer temperature.N₂/³He and CH₄/³He ratios of Nisyros fumarolic gases are unusually low for subduction zone gases and correspond to those of midoceanic ridge environments. Accordingly, CH₄ may have been primarily generated through the reduction of CO₂ by H₂ in the absence of any organic matter following a Fischer-Tropsch-type reaction. However, primary occurrence of minor amounts of thermogenic CH₄ and subsequent re-equilibration with co-existing CO₂ cannot be ruled out entirely. CO₂/³He ratios and δ¹³C_CO2 values imply that the evolved CO₂ either derives from a metasomatized mantle or is a mixture between two components, one outgassing from an unaltered mantle and the other released by thermal breakdown of marine carbonates. The latter may contain traces of organic matter possibly decomposing to CH₄ during thermometamorphism.     

Neodymium isotopic variations in Northwest Pacific waters

Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (?_Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ?_Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ?_Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters.In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ?_Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ?_Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ?_Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.     

History of carbonate ion concentration over the last 100 million years

Instead of having been more or less constant, as once assumed, it is now apparent that the major ion chemistry of the oceans has varied substantially over time. For instance, independent lines of evidence suggest that calcium concentration ([Ca²⁺]) has approximately halved and magnesium concentration ([Mg²⁺]) approximately doubled over the last 100 million years. On the other hand, the calcite compensation depth, and hence the CaCO₃ saturation, has varied little over the last 100 My as documented in deep sea sediments. We combine these pieces of evidence to develop a proxy for seawater carbonate ion concentration ([CO₃²⁻]) over this period of time. From the calcite saturation state (which is proportional to the product of [Ca²⁺] times [CO₃²⁻], but also affected by [Mg²⁺]), we can calculate seawater [CO₃²⁻]. Our results show that [CO₃²⁻] has nearly quadrupled since the Cretaceous. Furthermore, by combining our [CO₃²⁻] proxy with other carbonate system proxies, we provide calculations of the entire seawater carbonate system and atmospheric CO₂. Based on this, reconstructed atmospheric CO₂ is relatively low in the Miocene but high in the Eocene. Finally, we make a strong case that seawater pH has increased over the last 100 My.     

Transient dissolution patterns on stressed crystal surfaces

We present an experimental investigation on the dissolution of uniaxially stressed crystals of NaClO₃ in contact with brine. The crystals are immersed in a saturated fluid, stressed vertically by a piston and monitored constantly in situ with a CCD camera. The experiments are temperature-controlled and uniaxial shortening of the sample is measured with a high-resolution capacitance analyzer. Once the crystal is stressed it develops dissolution grooves on its free surface. The grooves are oriented with their long axis perpendicular to the direction of compressive stress and the initial distance between the parallel grooves is in accordance with the Asaro-Tiller-Grinfeld instability. We observe a novel, transient evolution of this roughness: The grooves on the crystal surface migrate upwards (against gravity), grow in size and the inter-groove distance increases linearly with time. During the coarsening of the pattern this switches from a one-dimensional geometry of parallel grooves to a two-dimensional geometry with horizontal and vertical grooves. At the end of the experiment one large groove travels across the crystal and the surface becomes smooth again. Uniaxial shortening of the crystal by pressure solution creep decays exponentially with time and shows no long term creep within the range of the resolution of the capacitance analyzer (accuracy of 100nm over a period of 14 days). This indicates that, while active, the fast transient processes on the free surface increase the solution concentration and thereby significantly slow down or stop pressure solution at the top of the crystal. This novel feedback mechanism can explain earlier results of cyclic pressure solution creep and demands development of a more complex theory of pressure-solution creep including processes that act on free surfaces.     

A mechanism of mineral replacement: isotope tracing in the model system KCl-KBr-H2O

The solid solution-aqueous solution system KCl-KBr-H₂O is used as a model system to determine the mechanism of the replacement process of one crystalline phase by another. A single crystal of KBr was allowed to react with a saturated KCl solution enriched in ⁴⁰K and the resulting new product phases were analyzed for changes in both anion and K isotope composition, using SEM, EDX, microprobe analysis, mass spectrometry and X-ray diffraction. The results show that the new product, K(Cl,Br), contains elements derived from both the original fluid and solid phases, indicating that both anions and K isotopes are exchanged during the replacement reaction. The interface between the advancing reaction front and the original parent crystal is sharp on a micron scale, showing no reaction profile that could indicate a solid state diffusion mechanism. Macroscopically the new phase is turbid due to the development of porosity that is consistent with a net volume deficit replacement reaction and this porosity may act as an indication that a replacement process has taken place. Single crystal X-ray diffraction patterns show the preservation of the crystallographic orientation during the replacement process. The replacement mechanism is interpreted as a result of a coupled process of dissolution and recrystallization occurring at the fluid-crystal interface.     

Precise and accurate isotopic measurements using multiple-collector ICPMS

New techniques of isotopic measurements by a new generation of mass spectrometers equipped with an inductively-coupled-plasma source, a magnetic mass filter, and multiple collection (MC-ICPMS) are quickly developing. These techniques are valuable because of (1) the ability of ICP sources to ionize virtually every element in the periodic table, and (2) the large sample throughout. However, because of the complex trajectories of multiple ion beams produced in the plasma source whether from the same or different elements, the acquisition of precise and accurate isotopic data with this type of instrument still requires a good understanding of instrumental fractionation processes, both mass-dependent and mass-independent. Although physical processes responsible for the instrumental mass bias are still to be understood more fully, we here present a theoretical framework that allows for most of the analytical limitations to high precision and accuracy to be overcome. After a presentation of unifying phenomenological theory for mass-dependent fractionation in mass spectrometers, we show how this theory accounts for the techniques of standard bracketing and of isotopic normalization by a ratio of either the same or a different element, such as the use of Tl to correct mass bias on Pb. Accuracy is discussed with reference to the concept of cup efficiencies. Although these can be simply calibrated by analyzing standards, we derive a straightforward, very general method to calculate accurate isotopic ratios from dynamic measurements. In this study, we successfully applied the dynamic method to Nd and Pb as examples. We confirm that the assumption of identical mass bias for neighboring elements (notably Pb and Tl, and Yb and Lu) is both unnecessary and incorrect. We further discuss the dangers of straightforward standard-sample bracketing when chemical purification of the element to be analyzed is imperfect. Pooling runs to improve precision is acceptable provided the pooled measurements are shown to be part of a single population. Second-order corrections seem to be able to improve the precision on ¹⁴³Nd/¹⁴⁴Nd measurements. Finally, after discussing a number of potential pitfalls, such as the consequence of peak shape, correlations introduced by counting statistics, and the effect of memory on double-spike methods, we describe an optimal strategy for high-precision and accurate measurements by MC-ICPMS, which involves the repetitive calibration of cup efficiencies and rigorous assessment of mass bias combined with standard-sample bracketing. We suggest that, when these simple guidelines are followed, MC-ICPMS is capable of producing isotopic data precise and accurate to better than 15 ppm.     

An experimental and theoretical determination of oxygen isotope fractionation in the system magnetite-H2O from 300 to 800°C

Oxygen isotope fractionations have been determined between magnetite and water from 300 to 800°C and pressures between 10 and 215MPa. We selected three reaction pathways to investigate fractionation: (a) reaction of fine-grained magnetite with dilute aqueous NaCl solutions; (b) reduction of fine-grained hematite through reaction with dilute acetic acid; and (c) oxidation of fine iron power in either pure water or dilute NaCl solutions. Effective use of acetic acid was limited to temperatures up to about 400°C, whereas oxide-solution isotope exchange experiments were conducted at all temperatures. Equilibrium ¹⁸O/¹⁶O fractionation factors were calculated from the oxide-water experiments by means of the partial isotope exchange method, where generally four isotopically different waters were used at any given temperature. Each run product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and on a limited basis, high-resolution transmission electron microscopy (HRTEM) and Mössbauer spectroscopy. Results from the microscopic examinations indicate the formation of well-crystallized octahedra and dodecahedra of magnetite where the extent of crystallization, grain size, and grain habit depend on the initial starting material, P, T, solution composition, and duration of the run.The greatest amount of oxygen isotope exchange (∼90% or greater) was observed in experiments where magnetite either recrystallized in the presence of 0.5 m NaCl from 500 to 800°C or formed from hematite reacted with 0.5 m acetic acid at 300, 350 and 400°C. Fractionation factors (10³ ln α_mt-H2O) determined from these partial exchange experiments exhibit a steep decrease (to more negative values) with decreasing temperature down to about 500°C, followed by shallower slope. A least-squares regression model of these partial exchange data, which accounts for analytical errors and errors generated by mass balance calculations, gives the following expression for fractionation that exhibits no minimum: 1000lnα_lmt-lw=−8.984(±0.3803)x+3.302(±0.377)x²—0.426(±0.092)x³ with an R² = 0.99 for 300 ≤ T≤ 800°C (x = 10⁶/T²). The Fe oxidation results also exhibit this type of temperature dependence but shifted to slightly more negative 10³ ln α values; there is the suggestion that a kinetic isotope effect may contribute to these fractionations. A theoretical assessment of oxygen isotope fractionation using β-factors derived from heat capacity and Mössbauer temperature (second-order Doppler) shift measurements combined with known β-factors for pure water yield fractionations that are somewhat more negative compared to those determined experimentally. This deviation may be due to the combined solute effects of dissolved magnetite plus NaCl (aq), as well as an underestimation of β_mt at low temperatures. The new magnetite-water experimental fractionations agree reasonably well with results reported from other experimental studies for temperatures ≥ 500°C, but differ significantly with estimates based on quasi-theoretical and empirical approaches. Calcite-magnetite and quartz-magnetite fractionation factors estimated from the combination of magnetite β’s calculated in this study with those for calcite and quartz reported by Clayton and Kieffer (1991) agree very closely with experimentally determined mineral-pair fractionations.     

Reduction of TcO4 by sediment-associated biogenic Fe(II)

The potential for reduction of ⁹⁹TcO₄⁻_(aq) to poorly soluble ⁹⁹TcO₂ · nH₂O_(s) by biogenic sediment-associated Fe(II) was investigated with three Fe(III)-oxide containing subsurface materials and the dissimilatory metal-reducing subsurface bacterium Shewanella putrefaciens CN32. Two of the subsurface materials from the U.S. Department of Energy’s Hanford and Oak Ridge sites contained significant amounts of Mn(III,IV) oxides and net bioreduction of Fe(III) to Fe(II) was not observed until essentially all of the hydroxylamine HCl-extractable Mn was reduced. In anoxic, unreduced sediment or where Mn oxide bioreduction was incomplete, exogenous biogenic TcO₂ · nH₂O_(s) was slowly oxidized over a period of weeks. Subsurface materials that were bioreduced to varying degrees and then pasteurized to eliminate biological activity, reduced TcO₄⁻_(aq) at rates that generally increased with increasing concentrations of 0.5 N HCl-extractable Fe(II). Two of the sediments showed a common relationship between extractable Fe(II) concentration (in mM) and the first-order reduction rate (in h⁻¹), whereas the third demonstrated a markedly different trend. A combination of chemical extractions and ⁵⁷Fe Mössbauer spectroscopy were used to characterize the Fe(III) and Fe(II) phases. There was little evidence of the formation of secondary Fe(II) biominerals as a result of bioreduction, suggesting that the reactive forms of Fe(II) were predominantly surface complexes of different forms. The reduction rates of Tc(VII)O₄⁻ were slowest in the sediment that contained plentiful layer silicates (illite, vermiculite, and smectite), suggesting that Fe(II) sorption complexes on these phases were least reactive toward pertechnetate. These results suggest that the in situ microbial reduction of sediment-associated Fe(III), either naturally or via redox manipulation, may be effective at immobilizing TcO₄⁻_(aq) associated with groundwater contaminant plumes.     

A Donnan potential model for metal sorption onto Bacillus subtilis

In this study, we conducted electrophoretic mobility, potentiometric titration, and metal sorption experiments to investigate the surface charge characteristics of Bacillus subtilis and the electrostatic interactions between metal cations and the cell surface electric field. Electrophoretic mobility experiments performed as a function of pH and ionic strength show an isoelectric point of pH 2.4, with the magnitude of the electrokinetic potential increasing with increasing pH, and decreasing with increasing ionic strength. Potentiometric titration experiments conducted from pH 2.4 to 9 yield an average surface charge excess of 1.6 μmol/mg (dry mass). Corresponding cell wall charge density values were used to calculate the Donnan potential (Ψ_DON) as function of pH and ionic strength. Metal sorption experiments conducted with Ca(II), Sr(II), and Ba(II) exhibit strong ionic strength dependence, suggesting that the metal ions are bound to the bacterial cell wall via an outer-sphere complexation mechanism. Intrinsic metal sorption constants for the sorption reactions were determined by correcting the apparent sorption constant with the Boltzmann factor. A 1:2 metal-ligand stoichiometry provides the best fit to the experimental data with log K₂^int values of 5.9 ± 0.3, 6.0 ± 0.2, 6.2 ± 0.2 for Ca(II), Sr(II), and Ba(II) respectively. Electrophoretic mobility measurements of cells sorbed with Ca(II), Sr(II), and Ba(II) support the 1:2 sorption stoichiometry. These results indicate that electrical potential parameters derived from the Donnan model can be applied to predict metal binding onto bacterial surfaces over a wide range of pH and ionic strength conditions.     

Barite deposition resulting from phototrophic sulfide-oxidizing bacterial activity

Barite (BaSO₄) deposits generally arise from mixing of soluble barium-containing fluids with sulfate-rich fluids. While the role of biological processes in modulating barium solubility has been shown, no studies have shown that the biological oxidation of sulfide to sulfate leads to barite deposition. Here we present an example of microbially mediated barite deposition in a continental setting. A spring in the Anadarko Basin of southwestern Oklahoma produces water containing abundant barium and sulfide. As emergent water travels down a stream to a nearby creek, sulfate concentration increases from 0.06 mM to 2.2 mM while Ba²⁺ concentration drops from 0.4 mM to less than 7 μM. Stable isotope analysis, microbial activity studies, and in situ experiments provide evidence that as sulfide-rich water flows down the stream, anaerobic, anoxygenic, phototrophic bacteria play a dominant role in oxidizing sulfide to sulfate. Sulfate then precipitates with Ba²⁺ producing barite as travertine, cements, crusts, and accumulations on microbial mats. Our studies suggest that phototrophic sulfide oxidation and concomitant sulfur cycling could prove to be important processes regulating the cycling of barium in continental sulfur-containing systems.