Carbonate complexation of yttrium and the rare earth elements in natural waters

Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25°C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M³⁺+nHCO₃⁻?M(CO₃)_n³⁻²ⁿ+nH⁺, where n = 1 or 2. Formation constants written in terms of HCO₃⁻ concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO₃²⁻ concentrations. Formation constants for the above complexation equilibria, _CO3^Hβ₁=[MCO₃⁺][H⁺][M³⁺]⁻¹[HCO₃⁻]⁻¹ and _CO3^Hβ₂=[M(CO₃)₂⁻][H⁺]²[M³⁺]⁻¹[HCO₃⁻]⁻², have very similar dependencies on ionic strength because the reaction MCO₃⁺+HCO₃⁻?M(CO₃)₂⁻+H⁺ is isocoulombic. Potentiometric analyses indicate that the dependence of log_CO3^Hβ₁ and log_CO3^Hβ₂ on ionic strength at 25°C is given as

(A)     

A surface structural model for ferrihydrite II: Adsorption of uranyl and carbonate

The adsorption of uranyl (UO₂²⁺) on ferrihydrite has been evaluated with the charge distribution (CD) model for systems covering a very large range of conditions, i.e. pH, ionic strength, CO₂ pressure, U(VI) concentration, and loading. Modeling suggests that uranyl forms bidentate inner sphere complexes at sites that do not react chemically with carbonate ions. Uranyl is bound by singly-coordinated surface groups present at particular edges of Fe-octahedra of ferrihydrite while another set of singly-coordinated surface groups may form double-corner bidentate complexes with carbonate ions. The uranyl surface speciation strongly changes in the presence of carbonate due to the specific adsorption of carbonate ions as well as the formation of ternary uranyl-carbonate surface complexes. Data analysis with the CD model suggests that a uranyl tris-carbonato surface complex, i.e. (UO₂)(CO₃)₃⁴⁻, is formed. This species is most abundant in systems with a high pH and carbonate concentration. This finding differs significantly from previous interpretations made in the literature. At high pH and low carbonate concentrations, as can be prepared in CO₂-closed systems, the model suggests the additional presence of a ternary uranyl-monocarbonato complex. The binding mode (type A or type B complex) is uncertain. At high uranyl concentrations, uranyl polymerizes at the surface of ferrihydrite giving, for instance, tris-uranyl surface complexes with and without carbonate. The similarities and differences between U(VI) adsorption by goethite and ferrihydrite are discussed from a surface structural point of view.     

Kinetics of reaction between O2 and Mn(II) species in aqueous solutions

The objective of this research is to assess critically the experimental rate data for O₂ oxidation of dissolved Mn(II) species at 25°C and to interpret the rates in terms of the solution species of Mn(II) in natural waters. A species kinetic rate expression for parallel paths expresses the total rate of Mn(II) oxidation as Σk_i a_ij, where k_i is the rate constant of species i and a_ij is the species concentration fraction in solution j. Among the species considered in the rate expression are Mn(II) hydrolysis products, carbonate complexes, ammonia complexes, and halide and sulfate complexes, in addition to the free aqueous ion. Experiments in three different laboratory buffers and in seawater yield an apparent rate constant for Mn(II) disappearance, k_app,j ranging from 8.6 × 10⁻⁵ to 2.5 × 10⁻² (M⁻¹s⁻¹), between pH 8.03 and 9.30, respectively. Observed values of k_app exceed predictions based on Marcus outer-sphere electron transfer theory by more than four orders of magnitude, lending strong support to the proposal that Mn(II) + O₂ electron transfer follows an inner-sphere path. A multiple linear regression analysis fit of the observed rates to the species kinetic rate expression yields the following oxidation rate constants (M⁻¹s⁻¹) for the most reactive species: MnOH⁺, 1.66 × 10⁻²; Mn(OH)₂, 2.09 × 10¹; and Mn(CO₃)₂²⁻, 8.13 × 10⁻². The species kinetic rate expression accounts for the influence of pH and carbonate on oxidation rates of Mn(II), through complex formation and acid-base equilibria of both reactive and unreactive species. At pH ∼8, the greater fraction of the total rate is carried by MnOH⁺. At pH greater than ∼8.4, the species Mn(OH)₂ and Mn(CO₃)₂²⁻ make the greater contributions to the total rate.     

Adsorption of organic matter at mineral/water interfaces: I. ATR-FTIR spectroscopic and quantum chemical study of oxalate adsorbed at boehmite/water and corundum/water interfaces

The types and structures of adsorption complexes formed by oxalate at boehmite (γ-AlOOH)/water and corundum (α-Al₂O₃)/water interfaces were determined using in situ attenuated total reflectance fourier transform infrared (ATR-FTIR) spectroscopy and quantum chemical simulation methods. At pH 5.1, at least four different oxalate species were found at or near the boehmite/water interface for oxalate surface coverages (Γ_ox) ranging from 0.25 to 16.44 μmol/m². At relatively low coverages (Γ_ox < 2.47), strongly adsorbed inner-sphere oxalate species (IR peaks at 1286, 1418, 1700, and 1720 cm⁻¹) replace weakly adsorbed carbonate species, and a small proportion of oxalate anions are adsorbed in an outer-sphere mode (IR peaks at 1314 and 1591 cm⁻¹). IR peaks indicative of inner-sphere adsorbed oxalate are also observed for oxalate at the corundum/water interface at Γ_ox = 1.4 μmol/m². With increasing oxalate concentration (Γ_ox > 2.47 μmol/m²), the boehmite surface binding sites for inner-sphere adsorbed oxalate become saturated, and excess oxalate ions are present dominantly as aqueous species (IR peaks at 1309 and 1571 cm⁻¹). In addition to these adsorption processes, oxalate-promoted dissolution of boehmite following inner-sphere oxalate adsorption becomes increasingly pronounced with increasing Γ_ox and results in an aqueous Al(III)-oxalate species, as indicated by shifted IR peaks (1286 → 1297 cm⁻¹ and 1418 → 1408 cm⁻¹). At pH 2.5, no outer-sphere adsorbed oxalate or aqueous oxalate species were observed. The similarity of adsorbed oxalate spectral features at pH 2.5 and 5.1 implies that the adsorption mechanism of aqueous HOx⁻ species involves loss of protons from this species during the ligand-exchange reaction. As a consequence, adsorbed inner-sphere oxalate and aqueous Al(III)-oxalate complexes formed at pH 2.5 have coordination geometries very similar to those formed at pH 5.1.The coordination geometry of inner-sphere adsorbed oxalate species was also predicted using quantum chemical geometry optimization and IR vibrational frequency calculations. Geometry-optimized Al₈O₁₂ and Al₁₄O₂₂ clusters with the reactive surface Al site coordinated by three oxygens were used as model substrates for corundum and boehmite surfaces. Among the models considered, calculated IR frequencies based on a bidentate side-on structure with a 5-membered ring agree best with the observed frequencies for boehmite/oxalate/water samples at Γ_ox = 0.25 to 16.44 μmol/m² and pH 2.5 and 5.1, and for a corundum/oxalate/water sample at Γ_ox = 1.4 μmol/m² and pH 5.1. Based on these results, we suggest that oxalate bonding on boehmite and corundum surfaces results in 5-coordinated rather than 4- or 6-coordinated Al surface sites.     

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.     

Solidsolution interaction: The effect of carbonate alkalinity on adsorbed thorium

Elevated activities of dissolved Th have been found in Soap Lake, an alkaline lake in Eastern Washington. Dissolved ²³²Th ranges from less than 0.001 to 4.9 dpm/L compared to about 1.3 × 10⁻⁵ dpm/ L in sea water. The enhanced activity in the lake coincides with an increase in carbonate alkalinity. Experiments were conducted to evaluate the effect of pH, ionic strength and carbonate alkalinity on Th adsorption on goethite. Thorium (10⁻¹³ M total) in the presence of 5.22 mg/L α-FeOOH and 0.1 M NaNO₃ has an adsorption edge from pH 2–5. At pH 9.0 ± 0.6 the percent Th absorbed on the solid began to decrease from 100% at 100 meq/L carbonate alkalinity and exhibited no adsorption above 300 meq/L. The experimental data were modeled to obtain the intrinsic adsorption equilibrium constants for Th hydrolysis species. These adsorption constants were incorporated in the model to interpret the observed effect of carbonate alkalinity on Th adsorption. There are two main effects of the alkalinity. To a significant degree the decrease in Th adsorption is due to competition of HCO⁻₃ and CO²⁻₃ ions for surface sites. Dissolved Th carbonate complexes also contribute to the increase of Th in solution.     

Metal sorption to dolomite surfaces

Potential human intrusion into the Waste Isolation Pilot Plant (WIPP) might release actinides into the Culebra Dolomite where sorption reactions will affect of radiotoxicity from the repository. Using a limited residence time reactor the authors have measured Ca, Mg, Nd adsorption/exchange as a function of ionic strength, P_CO2, and pH at 25°C. By the same approach, but using as input radioactive tracers, adsorption/exchange of Am, Pu, U, and Np on dolomite were measured as a function of ionic strength, P_CO2, and pH at 25°C. Metal adsorption is typically favored at high pH. Calcium and Mg adsorb in near-stoichiometric proportions except at high pH. Adsorption of Ca and Mg is diminished at high ionic strengths (e.g., 0.5M NaCl) pointing to association of Na⁺ with the dolomite surface, and the possibility that Ca and Mg sorb as hydrated, outer-sphere complexes. Sulfate amplifies sorption of Ca and Mg, and possibly Nd as well. Exchange of Nd for surface Ca is favored at high pH, and when Ca levels are low. Exchange for Ca appears to control attachment of actinides to dolomite as well, and high levels of Ca²⁺ in solution will decrease K_ds. At the same time, to the extent that high P_CO2s increase Ca²⁺ levels, K_ds will decrease with CO₂ levels as well, but only if sorbing actinide-carbonate complexes are not observed to form (Am-carbonate complexes appear to sorb; Pu-complexes might sorb as well. U-carbonate complexation leads to desorption). This indirect CO₂ effect is observed primarily at, and above, neutral pH. High NaCl levels do not appear to affect to actinide K_ds.     

Solubility of plutonium(VI) carbonate in saline solutions

Among the plutonium oxidation states found to form in the environment, mobile plutonium(VI) can exist under oxidizing conditions and in waters with high chloride content due to radiolysis effects. We are investigating the solubility and speciation of plutonium(VI) carbonate under conditions relevant to natural waters and brines such as those found near some geologic radioactive waste repositories. The solid Pu(VI) phase PuO₂CO₃(s) was prepared and its solubility was measured in NaCl and NaClO₄ solutions in a CO₂ atmosphere as a function of pH and ionic strength (0.1-5.6 m). The concentration of soluble plutonium in solution was calculated from spectroscopic data and liquid scintillation counting. Spectroscopic measurements also revealed the plutonium oxidation state. The apparent solubility product of PuO₂CO₃(s) was determined at selected electrolyte concentrations to be, log K_s,0 = −13.95 ± 0.07 (0.1 m NaCl), log K_s,0 = −14.07 ± 0.13 (5.6 m NaCl), and log K_s,0 = −15.26 ± 0.11 (5.6 m NaClO₄). Specific ion interaction theory was used to calculate the solubility product at zero ionic strength, .     

The oxidation of carbonate green rust into ferric phases:solid-state reaction or transformation via solution

The oxidation of carbonate green rust, GR(CO₃²⁻), in NaHCO₃ solutions at T = 25°C has been investigated through electrochemical techniques, FTIR, XRD, TEM and SEM. The used GR(CO₃²⁻) samples were made of either suspended solid in solution or a thin electrochemically formed layer on the surface of an iron disc. Depending on experimental conditions, oxidation occurs, with or without major modifications of the GR(CO₃²⁻) structure, suggesting the existence of two pathways: solid-state oxidation (SSO) leading to a ferric oxyhydroxycarbonate as the end product, and a dissolution-oxidation-precipitation (DOP) mechanism leading to ferric oxihydroxides such as lepidocrocite, goethite, or ferrihydrite. A formula was proposed for this ferric oxyhydroxycarbonate, Fe₆^IIIO_(2+x)(OH)_(12-2x)(H₂O)_x(CO₃), assuming that the solid-state oxidation reaction is associated to a deprotonation of the water molecules within the interlayers, or of the hydroxyl groups in the Fe(O,H) octahedra layers. The DOP mechanism involves transformation via solution with the occurrence of soluble ferrous-ferric intermediate species. A discussion about factors influencing the oxidation of carbonate green rust is provided hereafter. The ferric oxyhydroxycarbonate can be reduced back to GR(CO₃²⁻) by a reverse solid-state reduction reaction. The potentiality for a solid-state redox cycling of iron to occur may be considered. The stability of the ferric oxyhydroxycarbonate towards thermodynamically stable ferric phases, such as goethite and hematite, was also studied.     

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.     

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.     

The role of carbonate ions in pyrite oxidation in aqueous systems

The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO₃⁻, FeCO₃⁰, Fe(CO₃)(OH)⁻ and FeCO₃²⁻) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O₂, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O₂ is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.     

An experimental study of magnesite dissolution rates at neutral to alkaline conditions and 150 and 200 °C as a function of pH, total dissolved carbonate concentration, and chemical affinity

Steady-state magnesite dissolution rates were measured in mixed-flow reactors at 150 and 200 °C and 4.6 < pH < 8.4, as a function of ionic strength (0.001 M ? I ? 1 M), total dissolved carbonate concentration (10⁻⁴ M < ΣCO₂ < 0.1 M), and distance from equilibrium. Rates were found to increase with increasing ionic strength, but decrease with increasing temperature from 150 to 200 °C, pH, and aqueous CO₃²⁻ activity. Measured rates were interpreted using the surface complexation model developed by Pokrovsky et al. (1999a) in conjunction with transition state theory (Eyring, 1935). Within this formalism, magnesite dissolution rates are found to be consistent with

     

The stability of palladium(II) hydroxide and hydroxy–chloride complexes: an experimental solubility study at 25–85°C and 1 bar

Solubility methods were employed to determine conditional equilibrium constants for the formation of hydroxide and mixed hydroxy–chloride complexes of Pd(II). Measurements were made over a temperature range of 25–85°C, a pH range from 0 to 12, and ionic strengths of 0.1, 0.2, 0.5 and 1.0 molal in both KCl and NaClO₄ media. Several speciation models were fit to the data using nonlinear regression, and the model yielding the best fit with the fewest number of species was accepted for each temperature and ionic strength. The conditional equilibrium constants were then fit to a function of ionic strength and temperature (including a Debye–Hückel term) to facilitate interpolation and extrapolation to infinite dilution. The following species were found to be important in KCl solutions: PdCl₄²⁻, PdCl₃(OH)²⁻, and Pd(OH)₂⁰. The relative proportions of the species are dependent on pH and ionic strength (chloride concentration). In perchlorate media the predominant species were Pd(OH)₃⁻, Pd(OH)₂⁰, PdOH⁺ and Pd²⁺, depending on pH. Conditional stability constants determined in this study agree well with those reported in previous work for the simple chloride and hydroxide complexes, but our results suggest that mixed complexes may be more important than previously thought, and that PdCl₃(OH)²⁻ may be the dominant species in seawater, followed by Pd(OH)₂⁰.     

Surface complexation model for multisite adsorption of copper(II) onto kaolinite

We measured the adsorption of Cu(II) onto kaolinite from pH 3-7 at constant ionic strength. EXAFS spectra show that Cu(II) adsorbs as (CuO₄H_n)ⁿ⁻⁶ and binuclear (Cu₂O₆H_n)ⁿ⁻⁸ inner-sphere complexes on variable-charge ≡AlOH sites and as Cu²⁺ on ion exchangeable ≡X--H⁺ sites. Sorption isotherms and EXAFS spectra show that surface precipitates have not formed at least up to pH 6.5. Inner-sphere complexes are bound to the kaolinite surface by corner-sharing with two or three edge-sharing Al(O,OH)₆ polyhedra. Our interpretation of the EXAFS data are supported by ab initio (density functional theory) geometries of analog clusters simulating Cu complexes on the {110} and {010} crystal edges and at the ditrigonal cavity sites on the {001}. Having identified the bidentate (≡AlOH)₂Cu(OH)₂⁰, tridentate (≡Al₃O(OH)₂)Cu₂(OH)₃⁰ and ≡X--Cu²⁺ surface complexes, the experimental copper(II) adsorption data can be fit to the reactions

     

Comparative geochemistries of Pd and Pt:: Formation of mixed hydroxychloro and chlorocarbonato-complexes in seawater

Comparative observations of Pd^II and Pt^II hydrolysis in chloride solutions indicate that [PdCl₃OH²⁻]/[PdCl42−] and [PtCl₃OH²⁻]/[PtCl42−] concentration ratios in salinity 35 seawater (S = 35) are smaller than one at a typical surface ocean pH (∼8.2), and are larger than one at pH = 8.2 when S < 10. The hydrolysis behaviors of PdCl42− and PtCl42− are very similar. In 0.5 M NaCl at 25°C the hydrolysis constant for both elements, written in the form β₁ = [MCl₃OH²⁻][Cl⁻][H⁺][MCl42−]⁻¹, is logβ1*=−8.97. Between ionic strengths 0.3 M and 1.0 M for Pd^II and between 0.1 M and 1.0 M for Pt^II, log β₁ is within approximately 0.1 units of the value appropriate to 0.5 M NaCl. This small dependence of PdCl42− and PtCl42− hydrolysis constants on ionic strength is consistent with predictions based on expected activity coefficient behavior.Carbonate is observed to complex PdCl42− significantly, but to a smaller extent than OH⁻ under conditions appropriate to seawater. Complexation of PtCl42− by CO32− was observed in this work but the rate of complexation was too slow to allow equilibrium observations. The principal dissimilarity between the chemistries of Pd^II and Pt^II in our investigation was the sharp contrast in observed Pd^II and Pt^II reaction rates. Differences in reaction kinetics may cause fractionation of Pd^II and Pt^II in the environment. The speciation of Pt^II, unlike Pd^II, is likely to be based on chemical environments experienced by Pt^II over a period of days, and perhaps weeks.     

Determination of carbon fractionation factor between aqueous carbonate and CO2(g) in two-direction isotope equilibration

The experiments were conducted in the open CO₂ system to find out the equilibrium fractionation between the carbonate ion and CO₂(g). The existence of isotopic equilibrium was checked using the two-direction approach by passing the CO₂−N₂ gases with different δ¹³C compositions (− 1.5‰ and − 23‰) through the carbonate solution with δ¹³C = − 4.2‰. The Δ_{CO3T²⁻−CO2(g)} equilibrium fractionation is given as 6.03 ± 0.17‰ at 25 °C. Discussion is provided about the significance of carbonate complexing in determination of Δ_{CO3T²⁻−CO2(g)} and Δ_{HCO3T⁻−CO2(g)} fractionations. Finally, an isotope numerical model of flow and kinetics of hydration and dehydroxylation is built to predict the isotopic behaviour of the system with time.     

Kinetic isotopic fractionation during carbonate dissolution in laboratory experiments: Implications for detection of microbial CO2 signatures using δC-DIC

Laboratory experiments on reagent-grade calcium carbonate and carbonate rich glacial sediments demonstrate previously unreported kinetic fractionation of carbon isotopes during the initial hydrolysis and early stages of carbonate dissolution driven by atmospheric CO₂. There is preferential dissolution of Ca¹²CO₃ during hydrolysis, resulting in δ¹³C-DIC values that are significantly lighter isotopically than the bulk carbonate. The fractionation factor for this kinetic isotopic effect is defined as ε_carb. ε_carb is greater on average for glacial sediments (−17.4‰) than for calcium carbonate (−7.8‰) for the < 63 μm size fraction, a sediment concentration of 5 g L⁻¹ and closed system conditions at 5°C. This difference is most likely due to the preferential dissolution of highly reactive ultra-fine particles with damaged surfaces that are common in subglacial sediments. The kinetic isotopic fractionation has a greater impact on δ¹³C-DIC at higher CaCO₃:water ratios and is significant during at least the first 6 h of carbonate dissolution driven by atmospheric CO₂ at sediment concentrations of 5 g L⁻¹. Atmospheric CO₂ dissolving into solution following carbonate hydrolysis does not exhibit any significant equilibrium isotopic fractionation for at least ∼ 6 h after the start of the experiment at 5°C. This is considerably longer than previously reported in the literature. Thus, kinetic fractionation processes will likely dominate the δ¹³C-DIC signal in natural environments where rock:water contact times are short <6-24 h (e.g., glacial systems, headwaters in fluvial catchments) and there is an excess of carbonate in the sediments. It will be difficult apply conventional isotope mass balance techniques in these types of environment to identify microbial CO₂ signatures in DIC from δ¹³C-DIC data.     

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10^?5 M, (dissolved uranyl 10^?5 to 10^?8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO₃ solutions containing up to 0.01 M NaHCO₃. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10^?3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO₂OH⁺ and (UO₂)₃(OH)⁺₅, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO₂OH⁺ surface complex, and a monodentate, bidentate or tridentate (UO₂)₃(OH)⁺₅surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10^?2 and 10^?3 M.