Surface complexation of U(VI) on goethite (α-FeOOH)

Sorption of U(VI) to goethite is a fundamental control on the mobility of uranium in soil and groundwater. Here, we investigated the sorption of U on goethite using EXAFS spectroscopy, batch sorption experiments and DFT calculations of the energetics and structures of possible surface complexes. Based on EXAFS spectra, it has previously been proposed that U(VI), as the uranyl cation , sorbs to Fe oxide hydroxide phases by forming a bidentate edge-sharing (E2) surface complex, >Fe(OH)₂UO₂(H₂O)_n. Here, we argue that this complex alone cannot account for the sorption capacity of goethite (α-FeOOH). Moreover, we show that all of the EXAFS signal attributed to the E2 complex can be accounted for by multiple scattering. We propose that the dominant surface complex in CO₂-free systems is a bidentate corner-sharing (C2) complex, (>FeOH)₂UO₂(H₂O)₃ which can form on the dominant {101} surface. However, in the presence of CO₂, we find an enhancement of UO₂ sorption at low pH and attribute this to a (>FeO)CO₂UO₂ ternary complex. With increasing pH, U(VI) desorbs by the formation of aqueous carbonate and hydroxyl complexes. However, this desorption is preceded by the formation of a second ternary surface complex (>FeOH)₂UO₂CO₃. The three proposed surface complexes, (>FeOH)₂UO₂(H₂O)₃, >FeOCO₂UO₂, and (>FeOH)₂UO₂CO₃ are consistent with EXAFS spectra. Using these complexes, we developed a surface complexation model for U on goethite with a 1-pK model for surface protonation, an extended Stern model for surface electrostatics and inclusion of all known UO₂-OH-CO₃ aqueous complexes in the current thermodynamic database. The model gives an excellent fit to our sorption experiments done in both ambient and reduced CO₂ environments at surface loadings of 0.02-2.0 wt% U.     

Seasonal patterns of carbon chemistry and isotopes in tufa depositing groundwaters of southwestern Japan

Annually laminated carbonates, known as tufas, commonly develop in limestone areas and typically record seasonal patterns of oxygen- and carbon-isotope compositions. δ¹⁸O values are principally controlled by seasonal changes of water temperature, whereas δ¹³C values are the result of complex reactions among the gaseous, liquid, and solid sources of carbon in the system. We examined the processes that cause the seasonal patterns of δ¹³C in groundwater systems at three tufa-depositing sites in southwestern Japan by applying model calculations to geochemical data. Underground inorganic carbon species are exchanged with gaseous CO₂, which is mainly introduced to the underground hydrological system by natural atmospheric ventilation and by diffusion of soil air. These processes control the seasonal pattern of δ¹³C, which is low in summer and high in winter. Among the three sites we investigated, we identified two extreme cases of the degree of carbon exchange between liquid and gaseous phases. For the case with high radiocarbon composition (Δ¹⁴C) and low pCO₂, there was substantial carbon exchange because of a large contribution of atmospheric CO₂ and a small water mass. For the other extreme case, which was characterized by low Δ¹⁴C and high pCO₂, the contribution of atmospheric CO₂ was small and the water mass was relatively large. Our results suggest that at two of the three sites water residence time within the soil profile was longer than 1 year. Our results also suggested a short residence time (less than 1 year) of water in the soil profile at the site with the smallest water mass, which is consistent with large seasonal amplitude of the springwater temperature variations. The Δ¹⁴C value of tufas is closely related to the hydrological conditions in which they are deposited. If the initial Δ¹⁴C value of a tufa-depositing system is stable, ¹⁴C-chronology can be used to date paleo-tufas.     

Kinetics of Cd sorption, desorption and fixation by calcite: A long-term radiotracer study

Time-dependent sorption and desorption of Cd on calcite was studied over 210 days utilizing ¹⁰⁹Cd as a tracer to distinguish between ‘labile’ and ‘non-labile’ forms of sorbed Cd. Stabilizing the calcite suspensions for 12 months under atmospheric P_CO2 and controlled temperature was necessary to reliably follow Cd dynamics following initial sorption. Results revealed time-dependant Cd sorption and marked desorption hysteresis by calcite under environmentally relevant conditions. Data obtained were fitted to a first-order kinetic model and a concentric shell diffusion model. Both models described the progressive transfer of Cd²⁺ to a less reactive form within calcite and subsequent desorption of Cd subject to different initial contact times. The kinetic model provided a better fit to the combined sorption and desorption data (R² = 0.992). It differentiates between two ‘pools’ of sorbed Cd²⁺ on calcite, ‘labile’ and ‘non-labile’, in which labile sorbed Cd is in immediate equilibrium with the free Cd²⁺ ion activity in solution whereas non-labile Cd is kinetically restricted. For the diffusion model (R² = 0.959), the rate constants describing Cd dynamics in calcite produced a half-life for Cd desorption of ∼175 d, for release to a ‘zero-sink’ solution. Results from this study allow comment on the likely mechanisms occurring at the calcite surface following long-term Cd sorption.     

Cr-spinel/olivine and Cr-spinel/liquid nickel partition coefficients from natural samples

The Nernst partition coefficient of nickel (D^Ni) between Cr-spinel and silicate melt in natural systems has been investigated using mid-ocean ridge basalts (MORB) and other volcanic rocks. The Cr-spinel/olivine D^Ni values in volcanic rocks are between 1.2 and 0.3, indicating that the Cr-spinel/liquid D^Ni values vary from slightly higher to significantly lower than the olivine/liquid D^Ni values in natural systems. The Cr-spinel/liquid D^Ni values from the MORB samples vary between 6 and 11, slightly higher than those from the S-bearing experiments of Satari et al. [Satari P., Brenan J. M., Horn I. and McDonough W. F. (2002) Experimental constraints on the sulfide- and chromite-silicate melt partitioning behavior of rhenium and platinum-group elements. Economic Geology97, 385-398]. The results of the MORB samples and the experiments of Satari et al. (2002) indicate a negative correlation between the Cr-spinel/liquid D^Ni and the X_Cr values in Cr-spinels (Cr cation number on the basis of 3 total cations in the spinel structure). Variations of Cr-spinel/liquid D^Ni values with Cr-spinel compositions can be estimated from an empirical equation based on the results of the MORB samples and the experiments by Satari et al. (2002). The choice of Cr-spinel/liquid D^Ni = 10 for numerical modeling by Righter et al. [Righter K., Leeman W. P. and Hervig R. L. (2006) Partitioning of Ni, Co, and V between spinel-structured oxides and silicate melts: importance of spinel composition. Chemical Geology227, 1-25] is reasonable for basaltic systems. For picritic and komatiitic systems a lower value of ∼5 is more appropriate.     

Evidence for crustal degassing of CF4 and SF6 in Mojave Desert groundwaters

Dissolved tetrafluoromethane (CF₄) and sulfur hexafluoride (SF₆) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF₄ and SF₆ are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF₄ from the lithosphere. A gradual basin-wide enhancement in dissolved CF₄ and SF₆ concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF₄ and SF₆ concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF₄, but not of SF₆, is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF₄ and SF₆ into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF₄ and SF₆.     

Atomistic simulation of the mechanisms of noble gas incorporation in minerals

Atomistic simulations have been carried out to investigate the mechanisms of noble gas incorporation in minerals using both the traditional two-region approach and the “supercell” method. The traditional two-region approach has been used to calculate defect energies for Ne, Ar, Kr and Xe incorporation in MgO, CaO, diopside and forsterite in the static limit and at one atmosphere pressure. The possibilities of noble gas incorporation via both substitution and interstitial mechanisms are studied. The favored mechanism varies from mineral to mineral and from noble gas to noble gas. In all minerals studied, the variation of the solution energies of noble gas substitution with atomic radius appears approximately parabolic, analogous to those for 1⁺, 2⁺, 3⁺ and 4⁺ trace element incorporation on crystal lattice sites. Noble gas solution energies thus also fall on a curve, similar to those previously observed for cations with different charges, but with much lower curvature.The “supercell” method has been used to investigate the pressure dependence of noble gas incorporation in the same systems. Results indicate a large variation of the solubility of the larger noble gases, Kr and Xe with pressure. In addition, explicit simulation of incorporation at the (0 0 1) surface of MgO shows that the solubility of the heavier noble gases may be considerably enhanced by the presence of interfaces.     

Association of dissolved aluminum with silica: Connecting molecular structure to surface reactivity using NMR

We studied uptake mechanisms for dissolved Al on amorphous silica by combining bulk-solution chemistry experiments with solid-state Nuclear Magnetic Resonance techniques (²⁷Al magic-angle spinning (MAS) NMR, ²⁷Al{¹H} cross-polarization (CP) MAS NMR and ²⁹Si{¹H} CP-MAS NMR). We find that reaction of Al (1 mM) with amorphous silica consists of at least three reaction pathways; (1) adsorption of Al to surface silanol sites, (2) surface-enhanced precipitation of an aluminum hydroxide, and (3) bulk precipitation of an aluminosilicate phase. From the NMR speciation and water chemistry data, we calculate that 0.20 (±0.04) tetrahedral Al atoms nm⁻² sorb to the silica surface. Once the surface has sorbed roughly half of the total dissolved Al (∼8% site coverage), aluminum hydroxides and aluminosilicates precipitate from solution. These precipitation reactions are dependent upon solution pH and total dissolved silica concentration. We find that the Si:Al stoichiometry of the aluminosilicate precipitate is roughly 1:1 and suggest a chemical formula of NaAlSiO₄ in which Na⁺ acts as the charge compensating cation. For the adsorption of Al, we propose a surface-controlled reaction mechanism where Al sorbs as an inner-sphere coordination complex at the silica surface. Analogous to the hydrolysis of , we suggest that rapid deprotonation by surface hydroxyls followed by dehydration of ligated waters results in four-coordinate (>SiOH)₂Al(OH)₂ sites at the surface of amorphous silica.     

Anaerobic oxidation of methane (AOM) in marine sediments from the Skagerrak (Denmark): II. Reaction-transport modeling

A steady-state reaction-transport model is applied to sediments retrieved by gravity core from two stations (S10 and S13) in the Skagerrak to determine the main kinetic and thermodynamic controls on anaerobic oxidation of methane (AOM). The model considers an extended biomass-implicit reaction network for organic carbon degradation, which includes extracellular hydrolysis of macromolecular organic matter, fermentation, sulfate reduction, methanogenesis, AOM, acetogenesis and acetotrophy. Catabolic reaction rates are determined using a modified Monod rate expression that explicitly accounts for limitation by the in situ catabolic energy yields. The fraction of total sulfate reduction due to AOM in the sulfate-methane transition zone (SMTZ) at each site is calculated. The model provides an explanation for the methane tailing phenomenon which is observed here and in other marine sediments, whereby methane diffuses up from the SMTZ to the top of the core without being consumed. The tailing is due to bioenergetic limitation of AOM in the sulfate reduction zone, because the methane concentration is too low to engender favorable thermodynamic drive. AOM is also bioenergetically inhibited below the SMTZ at both sites because of high hydrogen concentrations (∼3-6 nM). The model results imply there is no straightforward relationship between pore water concentrations and the minimum catabolic energy needed to support life because of the highly coupled nature of the reaction network. Best model fits are obtained with a minimum energy for AOM of ∼11 kJ mol⁻¹, which is within the range reported in the literature for anaerobic processes.     

Seasonal δS variations in two high elevation snow pits measured by S-S double spike thermal ionization mass spectrometry

δ³⁴S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a ³³S-³⁶S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ³⁴S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ¹⁸O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months.The δ³⁴S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ³⁴S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ³⁴S and concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ³⁴S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.     

Upwelling, species, and depth effects on coral skeletal cadmium-to-calcium ratios (Cd/Ca)

Skeletal cadmium-to-calcium (Cd/Ca) ratios in hermatypic stony corals have been used to reconstruct changes in upwelling over time, yet there has not been a systematic evaluation of this tracer’s natural variability within and among coral species, between depths and across environmental conditions. Here, coral skeletal Cd/Ca ratios were measured in multiple colonies of Pavona clavus, Pavona gigantea and Porites lobata reared at two depths (1 and 7 m) during both upwelling and nonupwelling intervals in the Gulf of Panama (Pacific). Overall, skeletal Cd/Ca ratios were significantly higher during upwelling than during nonupwelling, in shallow than in deep corals, and in both species of Pavona than in P. lobata. P. lobata skeletal Cd/Ca ratios were uniformly low compared to those in the other species, with no significant differences between upwelling and nonupwelling values. Among colonies of the same species, skeletal Cd/Ca ratios were always higher in all shallow P. gigantea colonies during upwelling compared to nonupwelling, though the magnitude of the increase varied among colonies. For P. lobata, P. clavus and deep P. gigantea, changes in skeletal Cd/Ca ratios were not consistent among all colonies, with some colonies having lower ratios during upwelling than during nonupwelling. No statistically significant relationships were found between skeletal Cd/Ca ratios and maximum linear skeletal extension, δ¹³C or δ¹⁸O, suggesting that at seasonal resolution the Cd/Ca signal was decoupled from growth rate, coral metabolism, and ocean temperature and salinity, respectively. These results led to the following conclusions, (1) coral skeletal Cd/Ca ratios are independent of skeletal extension, coral metabolism and ambient temperature/salinity, (2) shallow P. gigantea is the most reliable species for paleoupwelling reconstruction and (3) the average Cd/Ca record of several colonies, rather than of a single coral, is needed to reliably reconstruct paleoupwelling events.