Applied Geochemistry (v.42, #C)

Carbonate dissolution in Mesozoic sand- and claystones as a response to CO2 exposure at 70 °C and 20 MPa by R. Weibel; C. Kjøller; K. Bateman; T. Laier; L.H. Nielsen; G. Purser (1-15).
The response to CO2 exposure of a variety of carbonate cemented rocks has been investigated using pressurised batch experiments conducted under simulated reservoir conditions, 70 °C and 20 MPa, and with a durations of up to14 months. Calcite, dolomite, ankerite and siderite cement were present in the unreacted reservoir rocks and caprocks. Core plugs of the reservoir rocks were used in order to investigate the alterations in situ. Crushing of the caprock samples was necessary to maximise reactions within the relatively short duration of the laboratory experiments. Synthetic brines were constructed for each batch experiment to match the specific formation water composition known from the reservoir and caprock formations in each well. Chemical matched synthetic brines proved crucial in order to avoid reactions due to non-equilibra of the fluids with the rock samples, for example observations of the dissolution of anhydrite, which were not associated with the CO2 injection, but rather caused by mismatched brines.Carbonate dissolution as a response to CO2 injection was confirmed in all batch experiments by both petrographical observations and geochemical changes in the brines. Increased Ca and Mg concentrations after 1 month reaction with CO2 and crushed caprocks are ascribed to calcite and dolomite dissolution, respectively, though not verified petrographically. Ankerite and possible siderite dissolution in the sandstone plugs are observed petrographically after 7 months reaction with CO2; and are accompanied by increased Fe and Mn contents in the reacted fluids. Clear evidence for calcite dissolution in sandstone plugs is observed petrographically after 14 months of reaction with CO2, and is associated with increased amounts of Ca (and Mg) in the reacted fluid. Dolomite in sandstones shows only minor dissolution features, which are not clearly supported by increased Mg content in the reacted fluid.Silicate dissolution cannot be demonstrated, either by chemical changes in the fluids, as Si and Al concentrations remain below the analytical detection limits, nor by petrographical changes, as partly dissolved feldspar grains and authigenic analcime are present in the sediments prior to the experiments. It is noteworthy, that authigenic K-feldspar and authigenic albite in sandstones show no signs of dissolution and consequently seem to be stable under the experimental conditions.

Reductive reactivity of iron(III) [Fe(III)] oxides in surface sediments of the East China Sea (ECS) and its variations while the sediments were subjected to repeated redox cycles were characterized by three rate parameters: m 0 (the initial amount of ascorbate-reducible Fe(III) oxides), k′ (the rate constant, s−1) and γ (a parameter describing heterogeneity of reactivity), obtained by applying a generalized dissolution rate law to time-dependent dissolution data in a buffered ascorbate solution (pH 7.5). The spatial pattern of the m 0 values is generally controlled by that of the clay fraction, but the values cannot be quantitatively corresponding to any specific reactive Fe(III) pools operationally defined by the conventional chemical extractions. Unlike the m 0, the spatial patterns of k′ and γ are more complex and may be controlled by more intricate factors relevant to Fe redox chemistry. At most sites studied, Fe(III) oxides have lower reactivity (k′) and higher heterogeneity (γ) than those of fresh ferrihydrite. Repeated redox cycles rendered m 0 and γ apparently decreased mainly during the initial 2–3 cycles due to mineral ripening and neoformation of “purified” Fe(III) oxides, respectively. Although the initial 1–2 cycles resulted in a “temporary” increase in Fe(III)-oxide reactivity, prolonged cycles eventually rendered the reactivity decreasing relative to the initial value. An a priori assumption that long-term redox cycles in natural sediments could enhance reductive reactivity of Fe(III) oxides as well as heterogeneity of the reactivity is unwarranted.

An assessment of hydrotalcite (HT) formation as a method to neutralise acidity and remove trace elements was undertaken using barren lixiviant from Heathgate Resources’ Beverley North in situ recovery (ISR) U mine in South Australia. This study demonstrated proof of concept in terms of the neutralisation of acidity and concomitant removal of a range of trace elements and U–Th series radionuclides from the barren lixiviant using MgCl2 as a supplementary Mg source to optimise Mg:Al mol ratios and NaOH as the neutralising agent. Hydrotalcite was the predominant mineral formed during neutralisation, hosting a range of elements including substantial U (∼0.2%) and rare earth elements (REE ∼0.1%). High U and REE recovery (∼99%) from barren lixiviant after HT precipitation indicates a potential to both remediate barren lixiviant and to offset remediation costs. Alternatively, HT precipitates formed during barren lixiviant neutralisation may be further stabilised via calcination, silicification or a combination thereof forming minerals potentially amenable for inclusion in a long-term waste repository at the cessation of ISR mining. Importantly, the composition of the neutralised barren lixiviant produced via HT precipitation is similar to that of existing groundwater allowing for the possibility of direct aquifer re-injection after remediation. A potential exists to apply this HT-based remediation technology to conventional or ISR U mines (or mines exploiting other commodities) and allows for the prospect of a fully integrated ISR mining, processing and lixiviant remediation strategy consistent with stringent environmental and mine closure standards.

The development of microbially mediated technologies for subsurface remediation and rock engineering is steadily increasing; however, we are lacking experimental data and models to predict bacterial movement through rock matrices. Here, breakthrough curves (BTCs) were obtained to quantify the transport of the ureolytic bacterium, Sporosarcina pasteurii, through sandstone cores, as a function of core length (1.8–7.5 cm), bacterial density (4 × 106 to 9 × 107  cells/ml) and flow rate (5.8–17.5 m/s). S. pasteurii was easily immobilised within the homogeneous sandstone matrix (>80%) in comparison to a packed sand column (<20%; under similar experimental conditions), and percentage recovery decreased almost linearly with increasing rock core length. Moreover, a decrease in bacterial density or flow rate enhanced bacterial retention. A numerical model based on 1D advection dispersion models used for unconsolidated sand was fitted to the BTC data obtained here for sandstone. Good agreement between data and model was obtained at shorter rock core lengths (<4 cm), suggesting that physicochemical filtration processes are similar in homogeneous packed sand and sandstones at these lengths. Discrepancies were, however observed at longer core lengths and with varying flow rates, indicating that the attributes of consolidated rock might impact bacterial transport progressively more with increasing core length. Implications of these results on microbial mineralisation technologies currently being developed for sealing fluid paths in subsurface environment is discussed.

The comparison of the stoichiometry of several nuclear waste glasses and basaltic glasses with their associated secondary smectites evidenced that Si/Al ratios of secondary smectites are nearly equal to the Si/Al ratios of parent glasses. This information may be very useful in constraining secondary smectites structure and stoichiometry in cases where other identification methods are difficult to apply.

Display OmittedA key component to closing the nuclear fuel cycle is the storage and disposition of nuclear waste in geologic systems. Multiphase ceramic waste forms have been studied extensively as a potential host matrix for nuclear waste. Understanding the speciation, partitioning, and release behavior of radionuclides immobilized in multiphase ceramic waste forms is a critical aspect of developing the scientific and technical basis for nuclear waste management. In this study, we evaluated a sodalite-bearing multiphase ceramic waste form (i.e., fluidized-bed steam reform sodium aluminosilicate [FBSR NAS] product) as a potential host matrix for long-lived radionuclides, such as technetium (99Tc). The FBSR NAS material consists primarily of nepheline (ideally NaAlSiO4), anion-bearing sodalites (ideally M 8[Al6Si6O24]X 2, where M refers to alkali and alkaline earth cations and X refers to monovalent anions), and nosean (ideally Na8[AlSiO4]6SO4). Bulk X-ray absorption fine structure analysis of the multiphase ceramic waste form, suggest rhenium (Re) is in the Re(VII) oxidation state and has partitioned to a Re-bearing sodalite phase (most likely a perrhenate sodalite Na8[Al6Si6O24](ReO4)2). Rhenium was added as a chemical surrogate for 99Tc during the FBSR NAS synthesis process. The weathering behavior of the FBSR NAS material was evaluated under hydraulically unsaturated conditions with deionized water at 90 °C. The steady-state Al, Na, and Si concentrations suggests the weathering mechanisms are consistent with what has been observed for other aluminosilicate minerals and include a combination of ion exchange, network hydrolysis, and the formation of an enriched-silica surface layer or phase. The steady-state S and Re concentrations are within an order of magnitude of the nosean and perrhenate sodalite solubility, respectively. The order of magnitude difference between the observed and predicted concentration for Re and S may be associated with the fact that the anion-bearing sodalites contained in the multiphase ceramic matrix are present as mixed-anion sodalite phases. These results suggest the multiphase FBSR NAS material may be a viable host matrix for long-lived, highly mobilie radionuclides which is a critical aspect in the management of nuclear waste.

Display OmittedIron and aluminium oxides are available in many climatic regions and play a vital role in many environmental processes, including the interactions of microorganisms in contaminated soils and groundwater with their ambient environment. Indigenous microorganisms in contaminated environments often have the ability to degrade or transform those contaminants, a concept that supports an in situ remediation approach and uses natural microbial populations in order to bio-remediate polluted sites. These metal oxides have a relatively high pH-dependent surface charge, which makes them good candidates for studying mineral–bacterial adhesion. Given the importance of understanding the reactions that occur at metal oxide and bacterial cell interfaces and to investigate this phenomenon further under well-characterized conditions, some of the most common iron and aluminium oxides; hematite, goethite and aluminium hydroxide, were synthesized and characterized and a coating method was developed to coat polystyrene well-plates as a surface exposable to bacterial adhesion with these minerals (non-treated polystyrene-12 well-plates which are used for cell cultures). The coating process was designed in a way that resembles naturally coated surfaces in aquifers. Hematite, Fe2O3, was synthesized from acidic FeCl3 solution, while goethite, FeOOH, and aluminium hydroxide, Al(OH)3, were prepared from an alkaline solution of Fe(NO3)3 and Al(NO3)3. They were further characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR), potentiometric titration and contact angle measurements. Characterization results show that the pure phases of hematite, goethite and aluminium hydroxides are formed with a point of zero charge (PZC) of 7.5, 8.5 and 8.9, respectively. The coating process was based on the direct deposition of mineral particles from an aqueous suspension by evaporation. Then, altered polystyrene surface properties were analyzed using X-ray photoelectron spectroscopy (XPS), attenuated total reflection-Fourier transform infrared (ATR-IR), water drop contact angle measurements and vertical scanning interferometry (VSI). The surface analysis tests prove that the coated polystyrene surface has physicochemical properties that are similar to the reference synthetic hematite, goethite and aluminium hydroxide minerals. These prepared and well-characterized mineral well-plates are similar to naturally occurring surfaces in aquifers and enable us to study the different steps of bacterial adhesion and biofilm formation on these metal oxides under laboratory-controlled conditions.

Am(III) sorption onto TiO2 samples with different crystallinity and varying pore size distributions by Nadezda N. Gracheva; Anna Yu. Romanchuk; Eugene A. Smirnov; Maria A. Meledina; Alexey V. Garshev; Eugene A. Shirshin; Victor V. Fadeev; Stepan N. Kalmykov (69-76).
Display OmittedVarious parameters influence the kinetics and the thermodynamics (surface complexation) of cation sorption onto minerals, including the pHPZC, the pHIEP, the crystallinity, the pore size distribution, and the surface roughness. In this paper, we address the effect of two of these parameters, i.e., the crystal structure and the pore size distribution on Am3+/Eu3+ sorption onto crystalline (anatase) and amorphous TiO2 microspheres. For both samples, the sorbed cation speciation was found to be the same, as determined by time-resolved laser-induced fluorescence spectroscopy (TRLIFS). As determined from sorption studies, the variation in the crystallinity of TiO2 defines its рНPZC and has a dominant effect on the cation sorption onto a mineral surface, whereas the pore size distribution has a minor effect on the Am3+ distribution. Surface complexation data were obtained from sorption data that fits well with other cation sorption onto TiO2, as determined from the linear free-energy relationship (LFER).

Geochemical and mineralogical investigation of uranium in multi-element contaminated, organic-rich subsurface sediment by Nikolla P. Qafoku; Brandy N. Gartman; Ravi K. Kukkadapu; Bruce W. Arey; Kenneth H. Williams; Paula J. Mouser; Steve M. Heald; John R. Bargar; Noémie Janot; Steve Yabusaki; Philip E. Long (77-85).
Subsurface regions of alluvial sediments characterized by an abundance of refractory or lignitic organic carbon compounds and reduced Fe and S bearing minerals, which are referred to as naturally reduced zones (NRZ), are present at the Integrated Field Research Challenge site in Rifle, CO (a former U mill site), and other contaminated subsurface sites. A study was conducted to demonstrate that the NRZ contains a variety of contaminants and unique minerals and potential contaminant hosts, investigate micron-scale spatial association of U with other co-contaminants, and determine solid phase-bounded U valence state and phase identity. The NRZ sediment had significant solid phase concentrations of U and other co-contaminants suggesting competing sorption reactions and complex temporal variations in dissolved contaminant concentrations in response to transient redox conditions, compared to single contaminant systems. The NRZ sediment had a remarkable assortment of potential contaminant hosts, such as Fe oxides, siderite, Fe(II) bearing clays, rare solids such as ZnS framboids and CuSe, and, potentially, chemically complex sulfides. Micron-scale inspections of the solid phase showed that U was spatially associated with other co-contaminants. High concentration, multi-contaminant, micron size (ca. 5–30 μm) areas of mainly U(IV) (53–100%) which occurred as biogenic UO2 (82%), or biomass – bound monomeric U(IV) (18%), were discovered within the sediment matrix confirming that biotically induced reduction and subsequent sequestration of contaminant U(VI) via natural attenuation occurred in this NRZ. A combination of assorted solid phase species and an abundance of redox-sensitive constituents may slow U(IV) oxidation rates, effectively enhancing the stability of U(IV) sequestered via natural attenuation, impeding rapid U flushing, and turning NRZs into sinks and long-term, slow-release sources of U contamination to groundwater.

Geochemical techniques have been used to estimate groundwater recharge and its spatial variability in basement terrain in a semi-arid area of southern Zimbabwe. Recharge rates estimated by chloride mass balance have been determined in the Romwe catchment, a small (4.6 km2) headwater catchment underlain by banded gneisses, with good hydrological and geological control. The results support the findings from piezometric monitoring that there are significant differences in hydrogeological properties of weathered basement derived from different primary lithologies. Annual recharge estimates for shallow weathered aquifers derived from melanocratic bedrock (dominated by pyroxene gneiss), 22 mm, and leucocratic felsic bedrock, 6.7 mm, are 3.7% and 1.1% respectively of the long-term mean annual rainfall. The significant uncertainties associated with the chloride mass balance recharge estimates are discussed. Groundwater derived from each lithology generally has a distinctive geochemistry (Na/Cl, K/Na, Mg/Ca, Na/Cl, B, Ba). The information from the Romwe catchment control area was then scaled up using information from remote sensing images (which defined areas of dark and light soils above the banded gneiss) to confirm the higher recharge rates in the melanocratic lithology in the unexplored Greater Romwe (225 km2) area. It is concluded that properly calibrated, remote sensing images could be further regionalized to site groundwater sources in basement terrain, providing a relatively inexpensive development tool.