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Natural forsterite was reacted in bulk liquid water saturated with supercritical CO2 (scCO2) and scCO2 saturated with water at 35–80°C and 90atm. The solid reaction products were analyzed with nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), and confocal Raman spectroscopy. Two carbonate phases, nesquehonite (MgCO3·3H2O) and magnesite (MgCO3), were identified with the proportions of the two phases depending on experimental conditions. In liquid water saturated with scCO2, nesquehonite was the dominant carbonate phase at 35–80°C with only a limited number of large, micron size magnesite particles forming at the highest temperature, 80°C. In contrast, in scCO2 saturated with H2O magnesite formation was identified at all three temperatures: 35, 50, and 80°C. Magnesite was the dominant carbonation reaction product at 50 and 80°C, but nesquehonite was dominant at 35°C. The magnesite particles formed under scCO2 saturated with H2O conditions exhibited an extremely uniform submicron grain-size and nearly identical rhombohedral morphologies at all temperatures. The distribution and form of the particles were not consistent with nucleation and growth on the forsterite surface. [ABSTRACT FROM AUTHOR]

Abstract: The nature of the reaction products that form on the surfaces of nanometer-sized forsterite particles during reaction with H2O-saturated supercritical CO2 (scCO2) at 35°C and 50°C were examined under in situ conditions and ex situ following reaction. The in situ analysis was conducted by X-ray diffraction (XRD). Ex situ analysis consisted of scanning electron microscopy (SEM) examination of the surface phases and chemical characterization of precipitates using a combination of confocal Raman spectroscopy, 13C and 29Si NMR spectroscopy, and energy-dispersive X-ray spectroscopy (EDS). The results show that the forsterite surface is highly reactive with the primary reaction products being a mixture of nesquehonite (MgCO3·3H2O) and magnesite (MgCO3) at short reaction times (∼3–4days) and then magnesite (MgCO3) and a highly porous amorphous silica phase at longer reaction times (14days). After 14days of reaction most of the original forsterite transformed to reaction products. Importantly, the formation of magnesite was observed at temperatures much lower (35°C) than previously thought needed to overcome its well-known sluggish precipitation kinetics. The conversion of nesquehonite to magnesite liberates H2O which can potentially facilitate further metal carbonation, as postulated by previous investigators, based upon studies at higher temperature (80°C). The observation that magnesite can form at lower temperatures implies that water recycling may also be important in determining the rate and extent of mineral carbonation in a wide range of potential CO2 storage reservoirs. [Copyright &y& Elsevier]

Part II. Reports on the development of aqueous thermodynamic models for the complexation of sodium and strontium with organic chelators. Description of the effects of ionic strength, carbonate concentration and temperature on the complexation of strontium by N-(2-Hydroxyethyl)ethylenedinitrilotriacetic acid under basic conditions. Use of pitzer ion-interaction parameters.

ab initio Thermodynamics is used to determine how free energies of magnesium carbonates and hydrates in a CO 2 -rich environment change with water concentration across a range temperature and pressure relevant to geochemistry and carbon sequestration (275 K to 375 K and pCO 2 of 1 to 210 bar). The methodology is based on first principles density-functional theory (DFT) calculations of the total energies and vibrational entropy of periclase, magnesite, brucite, nesquehonite, and hydromagnesite coupled to the experimental chemical potentials of CO 2 and H 2 O. The impact of water in supercritical CO 2 (scCO 2 ), even though less than 1% at saturation, is found to have a significant impact on the stability of hydrated carbonate minerals. Hydromagnesite and nesquehonite are found to be more thermodynamically stable than periclase and brucite in water-saturated scCO 2 and hence may be expected to result kinetically from carbonation of these minerals during CO 2 sequestration. Under dehydrating conditions nesquehonite destabilizes rapidly, whereas hydromagnesite is much more likely to persist in a CO 2 -rich environment, consistent with the observations that hydromagnesite is widespread in nature and nesquehonite is relatively rare. [ABSTRACT FROM AUTHOR]

Spectral induced polarization (SIP) responses are not well understood within the context of remediation applications at contaminated sites. Systematic SIP studies are needed to gain further insights into the complex electrical response of dynamic, biogeochemical states to enable the use of SIP for subsurface site characterization and remediation monitoring. Although SIP measurements on zero valent iron have been previously published, the SIP response for sulfur modified iron (SMI), a similar potential subsurface reductive amendment, has not yet been reported. Hence, the purpose of this laboratory-scale study was to evaluate SIP for nonintrusive monitoring of SMI under relevant subsurface conditions. SMI was separately mixed with silica sand or sediments from the Hanford Site (Washington, USA) and then packed into columns for geochemical and SIP analysis for up to 77 days under fully saturated conditions. SMI exhibited distinguishable phase peaks between 0.1 and 1.0 Hz, which changed in magnitude based on content and were detected as low as 0.3 wt%. In the initial days, the complex conductivity, phase maxima, and chargeability increased while the peak locations shifted to higher frequency (decreasing relaxation times), suggesting an initial increase in polarization and concurrent decrease in the length scales (potentially due to changes in particle size and mineralogy). Then, after 77 days, the phase maxima and chargeability decreased with a concurrent increase in relaxation times, suggesting that over longer periods, less polarizable phases are forming and particle size or connectivity of polarizable phases is increasing. These results demonstrated a unique SIP response to SMI transformations that might be applied to monitoring of SMI emplaced as a subsurface barrier or injected in the field. • Sulfur modified iron in sediments characterized for the first time by spectral induced polarization. • Sulfur modified iron is detected at low frequency with a decrease in relaxation times during oxidation. • New monitoring capability for sulfur modified iron reductant for contaminant remediation [ABSTRACT FROM AUTHOR]

The presence of mineral surfaces can affect the outcome of geochemical reactions by providing alternative nucleation pathways, but not in ways that can yet be reliably quantified. In this work, the reaction of Co(II) with calcite (CaCO 3) and magnesite (MgCO 3) powders at room temperature was used to quantify the effects of the nature of the mineral substrate and of solution chemistry on the competitive heterogeneous growth between cobalt carbonate (CoCO 3) and cobalt hydroxide (Co(OH) 2). Experiments were first performed to determine the appropriate solubility product constants of CoCO 3 and Co(OH) 2 , for which several values have been reported in the literature. X-ray photoelectron spectroscopy measurements were then performed to quantify the relative proportion of each phase in surface precipitates as a function of the nature of the substrate, initial saturation level, and pH. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were also used to characterize the morphology and composition of surface precipitates. On calcite powders, Co(OH) 2 formed predominantly despite the initial solutions being more supersaturated with respect to CoCO 3 than to Co(OH) 2 , indicating that the kinetics of heterogenous growth were faster for Co(OH) 2 than for CoCO 3. In contrast, magnesite powders were much more favorable to the growth of CoCO 3 because of the low lattice mismatch between the two phases, which allowed for heteroepitaxial growth. However, the proportion of CoCO 3 in surface precipitates decreased with increasing initial supersaturation, likely due to the resulting decrease of the free energy barrier for Co(OH) 2 nucleation and the rapid kinetics of Co(OH) 2 growth. Lowering pH increased the proportion of CoCO 3 on both substrates. These findings highlight how the interplay between lattice mismatch, heterogeneous nucleation barriers, and rates of heterogenous growth can influence competitive heterogeneous growth and dramatically affect the outcome of geochemical reactions. [ABSTRACT FROM AUTHOR]

Reactions involving variably hydrated super critical CO2 (scCO2) and a Na saturated dioctahedral smectite (Na-STX-1) were examined by in situ high-pressure X-ray diffraction at 50 °C and 90 bar, conditions that are relevant to long-term geologic storage of CO2. Both hydration and dehydration reactions were rapid with appreciable reaction occurring in minutes and near steady state occurring within an hour. Hydration occurred stepwise as a function of increasing H2O in the system; 1W, 2W-3W, and >3W clay hydration states were stable from ∼2-30%, ∼31-55 < 64%, and ≥ ∼71% H2O saturation in scCO2 respectively. Exposure of sub IW clay to anhydrous scCO2 caused interlayer expansion, not contraction as expected for dehydration, suggesting that CO2 intercalated the interlayer region of the sub 1W clay, which might provide a secondary trapping mechanism for CO2. In contrast, control experiments using pressurized N2 and similar initial conditions as in the scCO2 study, showed little to no change in the d001 spacing, or hydration states, of the clay. A salient implication for cap rock integrity is that clays can dehydrate when exposed to wet scCO2. For example, a clay in the ∼3W hydration state could collapse by ∼3 Å in the c* direction, or ∼15%, if exposed to scCO2 at less than or equal to about 64% H2O saturation. [ABSTRACT FROM AUTHOR]

The remediation of co-located contaminants in the vadose zone can be challenging due to accessibility and responses of different contaminants to remedial actions. At the Hanford Site (WA, USA), multiple radionuclides and other hazardous contaminants are present in the vadose zone and groundwater, including iodine-129 (I), technetium-99 (Tc), uranium-238 (U), chromium (Cr), and nitrate (NO 3 −). We evaluated a layered Bi oxyhydroxide material for its potential to remove individual and co-located contaminants with a series of batch experiments that investigated a range of plume conditions, followed by solid phase characterization of the reacted bismuth material. The results demonstrated successful removal of four contaminants (>98% removal of I, Tc, U, and Cr from the aqueous phase after 30 days) when tested individually. When contaminants were combined, a slight decrease in Tc removal occurred (−6%p). The addition of sediment decreased the removal for Tc and I, but U and Cr removal was unaffected. The results of these batch tests demonstrated that the bismuth based oxy-hydroxide material is a promising material for sequestering multiple contaminants in situ. • Bismuth materials were used to remove aqueous Tc, U, I, and Cr. • Promising results for removal of both individual and comingled contaminants. • Removal mechanisms are contaminant specific. • Competition with other soluble contaminants can reduce effectiveness of removal. [ABSTRACT FROM AUTHOR]

Ferrihydrite (Fh) is a major Fe(III)-(oxyhydr)oxide nanomineral distinguished by its poor crystallinity and thermodynamic metastability. While it is well known that in suboxic conditions aqueous Fe(II) rapidly catalyzes Fh transformation to more stable crystalline Fe(III) phases such as lepidocrocite (Lp) and goethite (Gt), because of the low solubility of Fe(III) the mass transfer pathways enabling these rapid transformations have remained unclear for decades. Here, using a selective extractant, we isolated and quantified a critical labile Fe(III) species, one that is more reactive than Fe(III) in Fh, formed by the oxidation of aqueous Fe(II) on the Fh surface. Experiments that compared time-dependent concentrations of solid-associated Fe(II) and this labile Fe(III) against the kinetics of phase transformation showed that its accumulation is directly related to Lp/Gt formation in a manner consistent with the classical nucleation theory. 57Fe isotope tracer experiments confirm the oxidized Fe(II) origin of labile Fe(III). The transformation pathway as well as the accelerating effect of Fe(II) can now all be explained on a unified basis of the kinetics of Fe(III) olation and oxolation reactions necessary to nucleate and sustain growth of Lp/Gt products, rates of which are greatly accelerated by labile Fe(III). [ABSTRACT FROM AUTHOR]

Mechanism of hexavalent chromium removal (Cr(VI) as CrO 4 2-) by the weak-base ion exchange (IX) resin ResinTech® SIR-700-HP (SIR-700) from simulated groundwater is assessed in the presence of radioactive contaminants iodine-129 (as IO 3 -), uranium (U as uranyl UO 2 2+), and technetium-99 (as TcO 4 -), and common environmental anions sulfate (SO 4 2-) and chloride (Cl-). Batch tests using the acid sulfate form of SIR-700 demonstrated Cr(VI) and U(VI) removal exceeded 97%, except in the presence of high SO 4 2- concentrations (536 mg/L) where Cr(VI) and U(VI) removal decreased to ≥ 80%. However, Cr(VI) removal notably improved with co-mingled U(VI) that complexes with SO 4 2- at the protonated amine sites. These U–SO 4 2- complexes are integral to U(VI) removal, as confirmed by the decrease in U(VI) removal (<40%) when the acid chloride form of SIR-700 was used instead. Solid phase characterization revealed that CrO 4 2- is removed by IX with SO 4 2- complexes and/or reduced to amorphous Cr(III)(OH) 3 at secondary alcohol sites. Tc(VII)O 4 - and I(V)O 3 - also undergo chemical reduction, following a similar removal mechanism. Oxyanion removal preference is determined by the anion reduction potential (CrO 4 2->TcO 4 ->IO 3 -), geometry, and charge density. For these reasons, 39% and 69% of TcO 4 - and 17% and 39% of IO 3 - are removed in the presence and absence of Cr(VI), respectively. [Display omitted] • SIR-700 weak-base IX resin with SO 4 2- parent anion removes ≥ 97% Cr(VI) and U(VI). • Cr(VI) is reduced to Cr(III)(OH) 3 phase at secondary alcohol sites. • Other anions also removed by IX and reduction, following the trend Cr(VI) > Tc(VII) > I(V). • U(VI) removed by complexation with SO 4 2- parent anion, Cl--form had low U(VI) uptake. [ABSTRACT FROM AUTHOR]

At the Hanford Site in southeastern Washington, discharge of radionuclide laden liquid wastes resulted in vadose zone contamination, providing a continuous source of these contaminants to groundwater. The presence of multiple contaminants (i.e., 99 Tc and 129 I) increases the complexity of finding viable remediation technologies to sequester contaminants in situ and protect groundwater. Although previous studies have shown the efficiency of zero valent iron (ZVI) and sulfur modified iron (SMI) in reducing mobile Tc(VII) to immobile Tc(IV) and iodate incorporation into calcite, the coupled effects from simultaneously using these remedial technologies have not been previously studied. In this first-of-a-kind laboratory study, we used reductants (ZVI or SMI) and calcite-forming solutions to simultaneously remove aqueous Tc(VII) and iodate via reduction and incorporation, respectively. The results confirmed that Tc(VII) was rapidly removed from the aqueous phase via reduction to Tc(IV). Most of the aqueous iodate was transformed to iodide faster than incorporation into calcite occurred, and therefore the I remained in the aqueous phase. These results suggested that this remedial pathway is not efficient in immobilizing iodate when reductants are present. Other experiments suggested that iodate removal via calcite precipitation should occur prior to adding reductants for Tc(VII) removal. When microbes were included in the tests, there was no negative impact on the microbial population but changes in the makeup of the microbial community were observed. These microbial community changes may have an impact on remediation efforts in the long-term that could not be seen in a short-term study. The results underscore the importance of identifying interactions between natural attenuation pathways and remediation technologies that only target individual contaminants. [ABSTRACT FROM AUTHOR]

Despite the numerous studies on changes within the reservoir following CO 2 injection and the effects of CO 2 release into overlying aquifers, little or no literature is available on the effect of CO 2 release on rock between the storage reservoirs and subsurface. This is important, because the interactions that occur in this zone between the CO 2 storage reservoir and the subsurface may have a significant impact on risk analysis for CO 2 storage projects. To address this knowledge gap, relevant rock materials, temperatures and pressures were used to study mineralogical and elemental changes in this intermediate zone. After rocks reacted with CO 2 -acidified 0.01 M NaCl, liquid analysis showed an increase of major elements (e.g., Ca and Mg) and variable concentrations of potential contaminants (e.g., Sr and Ba); lower aqueous concentrations of these elements were observed in N 2 control experiments, likely due to differences in pH between the CO 2 and N 2 experiments. In experiments with As/Cd and/or organic spikes, representing potential contaminants in the CO 2 plume originating in the storage reservoir, most or all of these contaminants were removed from the aqueous phase. SEM and Mössbauer spectroscopy results showed the formation of new minerals and Fe oxides in some CO 2 -reacted samples, indicating potential for contaminant removal through mineral incorporation or adsorption onto Fe oxides. These experiments show the interactions between the CO 2 -laden plume and the rock between storage reservoirs and overlying aquifers have the potential to affect the level of risk to overlying groundwater, and should be considered during site selection and risk evaluation. [ABSTRACT FROM AUTHOR]

The aim of this work was to study the separate effect of fission fragment damage on the dissolution of simulant UK advanced gas-cooled reactor nuclear fuel in water. Plain UO 2 and UO 2 samples, doped with inactive fission products to simulate 43 GWd/tU of burn-up, were fabricated. A set of these samples were then irradiated with 92 MeV 129 Xe 23+ ions to a fluence of 4.8 × 10 15 ions/cm 2 to simulate the fission damage that occurs within nuclear fuels. The primary effect of the irradiation on the UO 2 samples, observed by scanning electron microscopy, was to induce a smoothening of the surface features and formation of hollow blisters, which was attributed to multiple overlap of ion tracks. Dissolution experiments were conducted in single-pass flow-through (SPFT) mode under anoxic conditions (<0.1 O 2 ppm in Ar) to study the effect of the induced irradiation damage on the dissolution of the UO 2 matrix with data collection capturing six minute intervals for several hours. These time-resolved data showed that the irradiated samples showed a higher initial release of uranium than unirradiated samples, but that the uranium concentrations converged towards ∼10 −9 mol/l after a few hours. Apart from the initial spike in uranium concentration, attributed to irradiation induced surficial micro-structural changes, no noticeable difference in uranium chemistry as measured by X-ray electron spectroscopy or ‘effective solubility’ was observed between the irradiated, doped and undoped samples in this work. Some secondary phase formation was observed on the surface of UO 2 samples after the dissolution experiment. [ABSTRACT FROM AUTHOR]

Abstract In situ atomic force microscopy (AFM) measurements were performed to probe surface precipitates that formed on the 10 1 ¯ 4 surface of calcite (CaCO 3 ) single crystals following reaction with Mn 2+ -bearing aqueous solutions. Three-dimensional epitaxial islands were observed to precipitate and grow on the surfaces. In situ time-sequenced measurements demonstrated that the growth rates were commensurate with those obtained for epitaxial islands formed on calcite crystals reacted with Cd 2+ -bearing aqueous solutions of the same range in supersaturation with respect to the pure metal carbonate phase. This finding was unexpected as rhodochrosite (MnCO 3 ) and calcite display a 10% lattice mismatch, based on the area of their 10 1 ¯ 4 surface unit cells, whereas the lattice mismatch is only 4% for otavite (CdCO 3 ) and calcite. Coatings of varying thicknesses were therefore synthesized by reacting calcite single crystals in calcite-equilibrated aqueous solutions with up to 250 μM MnCl 2 . Ex situ X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectivity (XRR), and AFM measurements of the reacted crystals demonstrated the formation of an epitaxial (Mn,Ca)CO 3 solid solution. The epitaxial solid solution had a spatially complex composition, whereby the first few nanometers were rich in Ca and the Mn content increased with distance from the original calcite surface, culminating in a topmost region of almost pure MnCO 3 for the thickest coatings. The effective lattice mismatch was therefore much smaller than the nominal mismatch thus explaining the measured growth rates. These findings highlight the strong influence played by the substrate on the composition of surface precipitates in aqueous conditions. [ABSTRACT FROM AUTHOR]

• Fe-colloids are Si-coated ferrihydrite nanoparticles embedded in organic matter. • Colloid stability is attributed to the passivation by the si and organic matter. • Fe-colloids persist under diverse redox conditions. • Colloid persistence and mobility will facilitate element transport and cycling. Fe-rich mobile colloids play vital yet poorly understood roles in the biogeochemical cycling of Fe in groundwater by influencing organic matter (OM) preservation and fluxes of Fe, OM, and other essential (micro-)nutrients. Yet, few studies have provided molecular detail on the structures and compositions of Fe-rich mobile colloids and factors controlling their persistence in natural groundwater. Here, we provide comprehensive new information on the sizes, molecular structures, and compositions of Fe-rich mobile colloids that accounted for up to 72% of aqueous Fe in anoxic groundwater from a redox-active floodplain. The mobile colloids are multi-phase assemblages consisting of Si-coated ferrihydrite nanoparticles and Fe(II)-OM complexes. Ferrihydrite nanoparticles persisted under both oxic and anoxic conditions, which we attribute to passivation by Si and OM. These findings suggest that mobile Fe-rich colloids generated in floodplains can persist during transport through redox-variable soils and could be discharged to surface waters. These results shed new light on their potential to transport Fe, OM, and nutrients across terrestrial-aquatic interfaces. [Display omitted] [ABSTRACT FROM AUTHOR]

We report an in situ high-pressure NMR capability that permits natural abundance 17O and 25Mg NMR characterization of dissolved species in aqueous solution and in the presence of supercritical CO2 fluid (scCO2). The dissolution of Mg(OH)2 (brucite) in a multiphase water/scCO2 fluid at 90 atm pressure and 50 °C was studied in situ, with relevance to geological carbon sequestration. 17O NMR spectra allowed identification and distinction of various fluid species including dissolved CO2 in the H2O-rich phase, scCO2, aqueous H2O, and HCO3[sup -]. The widely separated spectral peaks for various species can all be observed both dynamically and quantitatively at concentrations as low as 20 mM, Measurement of the concentrations of these individual species also allows an in situ estimate of the hydrogen ion concentration, or pCH[sup +] values, of the reacting solutions. The concentration of Mg2+can be observed by natural abundance 25Mg NMR at a concentration as low as 10 mM. Quantum chemistry calculations of the NMR chemical shifts on cluster models aided in the interpretation of the experimental results. Evidence for the formation of polymeric Mg2+ clusters at high concentrations in the H2O-rich phase, a possible critical step needed for magnesium carbonate formation, was found. [ABSTRACT FROM AUTHOR]

The ability of surface precipitates to form heteroepitaxially is an important factor that controls the extent of heterogeneous growth. In this work, the growth of cadmium and cobalt carbonate phases on 10 1 ¯ 4 calcite surfaces is compared for a range of initial saturation states with respect to otavite (CdCO 3 ) and sphaerocobaltite (CoCO 3 ), two isostructural metal carbonates that exhibit different lattice misfits with respect to calcite (− 4% and − 15%, respectively, based on 10 1 ¯ 4 surface areas). Calcite single crystals were reacted in static conditions for 16 h with CdCl 2 and CoCl 2 aqueous solutions with initial concentrations 0.3 ≤ [Cd 2 + ] 0 ≤ 100 μM and 25 ≤ [Co 2 + ] 0 ≤ 200 μM. The reacted crystals were imaged in situ with atomic force microscopy (AFM) and analyzed ex situ with X-ray photoelectron spectroscopy (XPS). AFM images of Cd-reacted crystals showed the formation of large islands elongated along the 42 1 ¯ direction, clear evidence of heteroepitaxial growth, whereas surface precipitates on Co-reacted crystals were small round islands. Deformation of calcite etch pits in both cases indicated the incorporation of Cd and Co at step edges. XPS analysis pointed to the formation of a Cd-rich (Ca,Cd)CO 3 solid solution coating atop the calcite substrate. In contrast, XPS measurements of the Co-reacted crystals provided evidence for the formation of a mixed hydroxy-carbonate cobalt phase despite supersaturation with respect to CoCO 3 . The combined AFM and XPS results suggest that the lattice misfit between CoCO 3 and CaCO 3 is too large to allow for heteroepitaxial growth of a pure cobalt carbonate phase on calcite surfaces in aqueous solutions and at ambient conditions. The use of the satellite structure of the Co 2 p 3/2 photoelectron line as a tool for determining the nature of cobalt surface precipitates is also discussed. [ABSTRACT FROM AUTHOR]

• Freeze-thaw altered pore connectivity and pore throat diameters in permafrost aggregates. • Freeze-thaw resulted in more singly connected pores. • Freeze-thaw decreased volume of connected water-filled pores. • Pore network response to freeze–thaw varied with initial pore morphology. Climate change in Arctic landscapes may increase freeze–thaw frequency within the active layer as well as newly thawed permafrost. Freeze-thaw is a highly disruptive process that can deform soil pores and alter the architecture of the soil pore network with varied impacts to water transport and retention, redox conditions, and microbial activity. Our objective was to investigate how freeze–thaw cycles impacted the pore network of newly thawed permafrost aggregates to improve understanding of what type of transformations can be expected from warming Arctic landscapes. We measured the impact of freeze–thaw on pore morphology, pore throat diameter distribution, and pore connectivity with X-ray computed tomography (XCT) using six permafrost aggregates with sizes of 2.5 cm3 from a mineral soil horizon (Bw; 28–50 cm depths) in Toolik, Alaska. Freeze-thaw cycles were performed using a laboratory incubation consisting of five freeze–thaw cycles (−10 °C to 20 °C) over five weeks. Our findings indicated decreasing spatial connectivity of the pore network across all aggregates with higher frequencies of singly connected pores following freeze–thaw. Water-filled pores that were connected to the pore network decreased in volume while the overall connected pore volumetric fraction was not affected. Shifts in the pore throat diameter distribution were mostly observed in pore throats ranges of 100 µm or less with no corresponding changes to the pore shape factor of pore throats. Responses of the pore network to freeze–thaw varied by aggregate, suggesting that initial pore morphology may play a role in driving freeze–thaw response. Our research suggests that freeze–thaw alters the microenvironment of permafrost aggregates during the incipient stage of deformation following permafrost thaw, impacting soil properties and function in Arctic landscapes undergoing transition. [ABSTRACT FROM AUTHOR]

The effect of pH and contact time on uranium extractability from quartz surfaces was investigated using either acidic or carbonate (CARB) extraction solutions, time-delayed spikes of different U isotopes (238U and 233U), and liquid helium temperature time-resolved laser-induced fluorescence spectroscopy (TRLFS). Quartz powders were reacted with 238U(VI) bearing solutions equilibrated with atmospheric CO2 at pH 6, 7, and 8. After 42 days, the suspensions were spiked with 233U(VI) and reacted for an additional 7 days. Sorbed U was then extracted with either dilute nitric acid or CARB. For the CARB, but not the acid, extraction there was a systematic decrease in extraction efficiency for both isotopes from pH 6 to 8, which was mimicked by less desorption of 238U, after the 233U spike, from pH 6 to 8. The efficiency of 233U extraction was consistently greater than that of 238U, indicating a strong temporal component to the strength of U association with the surface that was accentuated with increasing pH. TRLFS revealed a strong correlation between CARB extraction efficiency and sorbed U speciation as a function of pH and time. Collectively, the observations show that aging and pH are critical factors in determining the form and strength of uranium-silica interactions. [ABSTRACT FROM AUTHOR]

[Display omitted] • CoAl and NiAl LDHs were synthesized and estimated for the iodine removal mechanism. • Iodine loading capacities of CoAl were about 1.67 (I−) and 2.16 (IO 3 −) mmol g−1. • Iodine loading capacities of NiAl were about 2.10 (I−) and 2.26 (IO 3 −) mmol g−1. • High layer charge density of LDHs affected low I− removal capacity. • Both I− and IO 3 − were efficiently removed by CoAl and NiAl through ion-exchange. The treatment of radioactive iodine released from nuclear power plants and radiological waste disposal sites is of great concern due to its high mobility and toxicity. In particular, iodide (I−) and iodate (IO 3 −) are the major iodine species of concern under various pHs and groundwater conditions. Herein, CoAl and NiAl layered double hydroxides (LDHs) were synthesized and investigated to identify the iodine removal mechanism and efficiency. Both CoAl and NiAl LDHs exhibited rapid iodine removal processes within 20 min, following the pseudo-second-order model via ion-exchange with parent NO 3 − anion in the LDHs. The CoAl LDH's maximum sorption capacities for I− and IO 3 − were about 1.67 and 2.16 mmol g−1, respectively, whereas for the NiAl LDH, these were about 2.10 and 2.26 mmol g−1, and they followed the Langmuir isotherm model. Interestingly, both the CoAl and NiAl LDHs showed a preferential ion-exchange affinity for IO 3 − over I−, which was attributed to the structural similarity of the IO 3 − and NO 3 − as well as new formation of secondary Co(IO 3) 2 ·2H 2 O or Ni(IO 3) 2 ·2H 2 O phases. In addition, a desorption study indicated that the selectivity order was SO 4 2− ≥ IO 3 − ≥ OH− > HCO 3 − > Cl− > NO 3 − ≥ I− and demonstrated the higher retention of the IO 3 − than I− anion. This study provides insights into promising iodine sorbents and the different removal mechanisms of I− and IO 3 − using CoAl and NiAl LDHs. [ABSTRACT FROM AUTHOR]

Radioactive iodine-129 (129I) and technetium-99 (99Tc) pose a risk to groundwater due to their long half-lives, toxicity, and high environmental mobility. Based on literature reviewed in Moore et al. (2019) and Pearce et al. (2019), natural and engineered materials, including iron oxides, low-solubility sulfides, tin-based materials, bismuth-based materials, organoclays, and metal organic frameworks, were tested for potential use as a deployed technology for the treatment of 129I and 99Tc to reduce environmental mobility. Materials were evaluated with metrics including capacity for IO 3 − and TcO 4 − uptake, selectivity and long-term immobilization potential. Batch testing was used to determine IO 3 − and TcO 4 − sorption under aerobic conditions for each material in synthetic groundwater at different solution to solid ratios. Material association with IO 3 − and TcO 4 − was spatially resolved using scanning electron microscopy and X-ray microprobe mapping. The potential for redox reactions was assessed using X-ray absorption near edge structure spectroscopy. Of the materials tested, bismuth oxy(hydroxide) and ferrihydrite performed the best for IO 3 −. The commercial Purolite A530E anion-exchange resin outperformed all materials in its sorption capacity for TcO 4 −. Tin-based materials had high capacity for TcO 4 −, but immobilized TcO 4 − via reductive precipitation. Bismuth-based materials had high capacity for TcO 4 −, though slightly lower than the tin-based materials, but did not immobilize TcO 4 − by a redox-drive process, mitigating potential negative re-oxidation effects over longer time periods under oxic conditions. Cationic metal organic frameworks and polymer networks had high Tc removal capacity, with TcO 4 − trapped within the framework of the sorbent material. Although organoclays did not have the highest capacity for IO 3 − and TcO 4 − removal in batch experiments, they are available commercially in large quantities, are relatively low cost and have low environmental impact, so were investigated in column experiments, demonstrating scale-up and removal of IO 3 − and TcO 4 − via sorption, and reductive immobilization with iron- and sulfur-based species. Unlabelled Image • 129I and 99Tc are a risk to groundwater due to toxicity, long half-life, and mobility. • Natural and engineered materials were tested for potential to reduce mobility. • Bismuth oxy(hydroxide) was the most promising material for iodate immobilization. • Anion-exchange resins outperformed all materials for pertechnetate sorption. • Immobilization capacity of material depends on uptake mechanism and reaction product. [ABSTRACT FROM AUTHOR]

Mineral weathering is a balanced interplay among physical, chemical, and biological processes. Fundamental knowledge gaps exist in characterizing the biogeochemical mechanisms that transform microbe-mineral interfaces at submicron scales, particularly in complex field systems. Our objective was to develop methods targeting the nanoscale by using high-resolution microscopy to assess biological and geochemical drivers of weathering in natural settings. Basalt, granite, and quartz (53–250 µm) were deployed in surface soils (10 cm) of three ecosystems (semiarid, subhumid, humid) for one year. We successfully developed a reference grid method to analyze individual grains using: (1) helium ion microscopy to capture micron to sub-nanometer imagery of mineral-organic interactions; and (2) scanning electron microscopy to quantify elemental distribution on the same surfaces via element mapping and point analyses. We detected locations of biomechanical weathering, secondary mineral precipitation, biofilm formation, and grain coatings across the three contrasting climates. To our knowledge, this is the first time these coupled microscopy techniques were applied in the earth and ecosystem sciences to assess microbe-mineral interfaces and in situ biological contributors to incipient weathering. [ABSTRACT FROM AUTHOR]

The soil region influenced by plant roots, i.e., the rhizosphere, is one of the most complex biological habitats on Earth and significantly impacts global carbon flow and transformation. Understanding the structure and function of the rhizosphere is critically important for maintaining sustainable plant ecosystem services, designing engineered ecosystems for long-term soil carbon storage, and mitigating the effects of climate change. However, studying the biological and ecological processes and interactions in the rhizosphere requires advanced integrated technologies capable of decoding such a complex system at different scales. Here, we review how emerging approaches in sensing, imaging, and computational modeling can advance our understanding of the complex rhizosphere system. Particularly, we provide our perspectives and discuss future directions in developing in situ rhizosphere sensing technologies that could potentially correlate local-scale interactions to ecosystem scale impacts. We first review integrated multimodal imaging techniques for tracking inorganic elements and organic carbon flow at nano- to microscale in the rhizosphere, followed by a discussion on the use of synthetic soil and plant habitats that bridge laboratory-to-field studies on the rhizosphere processes. We then describe applications of genetically encoded biosensors in monitoring nutrient and chemical exchanges in the rhizosphere, and the novel nanotechnology-mediated delivery approaches for introducing biosensors into the root tissues. Next, we review the recent progress and express our vision on field-deployable sensing technologies such as planar optodes for quantifying the distribution of chemical and analyte gradients in the rhizosphere under field conditions. Moreover, we provide perspectives on the challenges of linking complex rhizosphere interactions to ecosystem sensing for detecting biological traits across scales, which arguably requires using the best-available model predictions including the model-experiment and image-based modeling approaches. Experimental platforms relevant to field conditions like SMART (Sensors at Mesoscales with Advanced Remote Telemetry) soils testbed, coupled with ecosystem sensing and predictive models, can be effective tools to explore coupled ecosystem behavior and responses to environmental perturbations. Finally, we envision that with the advent of novel high-resolution imaging capabilities at nano- to macroscale, and remote biosensing technologies, combined with advanced computational models, future studies will lead to detection and upscaling of rhizosphere processes toward ecosystem and global predictions.

Importance: Fungal species are foundational members of soil ecosystems with vital contributions that support interspecies resource translocation. The minute details of these biogeochemical processes are poorly investigated. Here, we addressed this knowledge gap by probing fungal growth in a novel mineral-doped soil micromodel platform using spatially-resolved imaging methodologies. We found that fungi uptake K from K-rich minerals using organic acids exuded in a distance-dependent manner from a carbon-rich hotspot. While identification of specific mechanisms within soil remains challenging, our findings demonstrate the significance of reduced complexity platforms such as the mineral-doped micromodel in probing biogeochemical processes. These findings provide visualization into hyphal uptake and transport of mineral-derived nutrients in a resource-limited environment., Competing Interests: The authors declare no conflict of interest.

Focused ion beam (FIB) is frequently used to prepare electron- and X-ray-beam-transparent thin sections of samples, called lamellae. Typically, lamellae are prepared from only a subregion of a sample. In this paper, we present a novel approach for FIB lamella preparation of microscopic samples, wherein the entire cross-section of the whole sample can be investigated. The approach was demonstrated using spherical, porous, and often hollow microprecipitates of biologically precipitated calcium carbonate. The microprecipitate morphology made these biogenic samples more fragile and challenging than materials commonly investigated using FIB lamellae. Our method enables the appropriate orientation of the lamellae required for further electron/X-ray analyses after attachment to the transmission electron microscopy (TEM) grid post and facilitates more secure adhesion onto the grid post. We present evidence of autofluorescence in bacterially precipitated vaterite using this lamella preparation method coupled with TEM selected area diffraction. This innovative approach allows studying biomineralization at the micro to nano scales, which can provide novel insights into bacterial responses to microenvironmental conditions., Competing Interests: Conflict of Interest The authors declare that they have no competing interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of the Microscopy Society of America.)

This study investigates the sequestration and transformation of silver (Ag) and arsenic (As) ions in soil organic matter (OM) at the nanoscale using the combination of atom probe tomography (APT), transmission electron microscopy (TEM), focused ion beam (FIB), ion mill thinning and scanning electron microscopy (SEM). Silver-arsenic contaminated organic-rich soils were collected along the shore of Cobalt Lake, a former mining and milling site of the famous Ag deposits at Cobalt, Ontario, Canada. SEM examinations show that particulate organic matter (OM grains) contains mineral inclusions composed of mainly Fe, S, and Si with minor As and traces of Ag. Four OM grains with detectable concentrations of Ag (by SEM-EDS) were further characterized with either a combination of TEM and APT or TEM alone. These examinations show that As is predominantly sequestered by OM through either co-precipitation with Fe-(hydr)oxide inclusions or adsorption on Fe-(hydr)oxides and their subsequent transformation into scorodite (FeAsO 4 ·2H 2 O)/amorphous Fe-arsenate (AFA). Silver nanoparticles (NPs) with diameters in the range of ∼5-20 nm occur in the organic matrix as well as on the surface of Fe-rich inclusions (Fe-hydroxides, Fe-arsenates, Fe-sulfides), whereas Ag sulfide NPs were only observed on the surfaces of the Fe-rich inclusions. Rims of Ag-sulfides on Ag NPs (TEM data), accumulation of S atoms within and around Ag NPs (APT data), and the occurrence of dendritic as well as euhedral acanthite NPs with diameters in the range of ∼100-400 nm (TEM data) indicate that the sulfidation of the Ag NPs occurred via a mineral-replacement reaction (rims) or a complete dissolution of the Ag NPs, the subsequent precipitation of acanthite NPs and their aggregation (dendrites) and Ostwald ripening (euhedral crystals). These results show the importance of OM and, specifically the mineral inclusions in the sequestration of Ag and As to less bioavailable forms such as acanthite and scorodite, respectively.

Soil fungi facilitate the translocation of inorganic nutrients from soil minerals to other microorganisms and plants. This ability is particularly advantageous in impoverished soils because fungal mycelial networks can bridge otherwise spatially disconnected and inaccessible nutrient hot spots. However, the molecular mechanisms underlying fungal mineral weathering and transport through soil remains poorly understood primarily due to the lack of a platform for spatially resolved analysis of biotic-driven mineral weathering. Here, we addressed this knowledge gap by demonstrating a mineral-doped soil micromodel platform where mineral weathering mechanisms can be studied. We directly visualize acquisition and transport of inorganic nutrients from minerals through fungal hyphae in the micromodel using a multimodal imaging approach. We found that Fusarium sp. strain DS 682, a representative of common saprotrophic soil fungus, exhibited a mechanosensory response (thigmotropism) around obstacles and through pore spaces (~12 μm) in the presence of minerals. The fungus incorporated and translocated potassium (K) from K-rich mineral interfaces, as evidenced by visualization of mineral-derived nutrient transport and unique K chemical moieties following fungus-induced mineral weathering. Specific membrane transport proteins were expressed in the fungus in the presence of minerals, including those involved in oxidative phosphorylation pathways and the transmembrane transport of small-molecular-weight organic acids. This study establishes the significance of a spatial visualization platform for investigating microbial induced mineral weathering at microbially relevant scales. Moreover, we demonstrate the importance of fungal biology and nutrient translocation in maintaining fungal growth under water and carbon limitations in a reduced-complexity soil-like microenvironment. IMPORTANCE Fungal species are foundational members of soil microbiomes, where their contributions in accessing and transporting vital nutrients is key for community resilience. To date, the molecular mechanisms underlying fungal mineral weathering and nutrient translocation in low-nutrient environments remain poorly resolved due to the lack of a platform for spatial analysis of biotic weathering processes. Here, we addressed this knowledge gap by developing a mineral-doped soil micromodel platform. We demonstrate the function of this platform by directly probing fungal growth using spatially resolved optical and chemical imaging methodologies. We found the presence of minerals was required for fungal thigmotropism around obstacles and through soil-like pore spaces, and this was related to fungal transport of potassium (K) and corresponding K speciation from K-rich minerals. These findings provide new evidence and visualization into hyphal transport of mineral-derived nutrients under nutrient and water stresses.

The large surface areas in porous organic matter (OM) and on the surface of altered minerals control the sequestration of metal(loid)s in contaminated soils and sediments. This study explores the sequestration of Cu by OM in surficial forest soil in close proximity to the Horne smelter, Rouyn-Noranda, Quebec, Canada. The organic-rich soils have elevated concentrations of Cu (Cu = 〈0.75〉 wt%) but lack associations between organic matter (OM) and Cu-sulfides, commonly observed in organic-rich Cu-contaminated soils. This provides a unique opportunity to study the sequestration of Cu by OM in a sulfur-depleted environment using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). In two examined OM particles, Cu is predominantly sequestered as (I) nano- to micrometer-size Cu-bearing spinels, (II) as cuprite (Cu 2 O) nanoparticles or (III) finely dispersed Cu in association with clusters of magnetite (Fe 3 O 4 ) nanoparticles embedded in amorphous silica-rich pockets and (IV) in the OM matrix. The occurrence of euhedral crystals and nanoparticles in the single-digit range within the OM matrix indicate that the majority of the nanoparticles formed in situ within the OM particles. A model is developed which proposes that the sequestration of Cu in OM is promoted by (I) the partial mineralization of the OM matrix by amorphous silica; (II) the nucleation of magnetite nanoparticles on highly reactive silanol groups; (III) the diffusion of Cu within mineralized and altered areas of the OM; (IV) the availability of Cu-bearing species, which in turn is controlled by the hydrodynamic properties of the pore channels; (V) the formation of precursors and nucleation of Cu-bearing nanoparticles. This study shows that the combination of SEM, TEM and APT provides new insights into the sequestration of metal contaminants by OM at various scales ranging from the single-digit nano- to micrometer scale.

Nutrient foraging by fungi weathers rocks by mechanical and biochemical processes. Distinguishing fungal-driven transformation from abiotic mechanisms in soil remains a challenge due to complexities within natural field environments. We examined the role of fungal hyphae in the incipient weathering of granulated basalt from a three-year field experiment in a mixed hardwood-pine forest (S. Carolina) to identify alteration at the nanometer to micron scales based on microscopy-tomography analyses. Investigations of fungal-grain contacts revealed (i) a hypha-biofilm-basaltic glass interface coinciding with titanomagnetite inclusions exposed on the grain surface and embedded in the glass matrix and (ii) native dendritic and subhedral titanomagnetite inclusions in the upper 1-2 µm of the grain surface that spanned the length of the fungal-grain interface. We provide evidence of submicron basaltic glass dissolution occurring at a fungal-grain contact in a soil field setting. An example of how fungal-mediated weathering can be distinguished from abiotic mechanisms in the field was demonstrated by observing hyphal selective occupation and hydrolysis of glass-titanomagnetite surfaces. We hypothesize that the fungi were drawn to basaltic glass-titanomagnetite boundaries given that titanomagnetite exposed on or very near grain surfaces represents a source of iron to microbes. Furthermore, glass is energetically favorable to weathering in the presence of titanomagnetite. Our observations demonstrate that fungi interact with and transform basaltic substrates over a three-year time scale in field environments, which is central to understanding the rates and pathways of biogeochemical reactions related to nuclear waste disposal, geologic carbon storage, nutrient cycling, cultural artifact preservation, and soil-formation processes., (© 2022. The Author(s).)

Progress in understanding crystallization pathways depends on the ability to unravel relationships between intermediates and final crystalline products at the nanoscale, which is a particular challenge at elevated pressure and temperature. Here we exploit a high-pressure atomic force microscope to directly visualize brucite carbonation in water-bearing supercritical carbon dioxide (scCO 2 ) at 90 bar and 50 °C. On introduction of water-saturated scCO 2 , in situ visualization revealed initial dissolution followed by nanoparticle nucleation consistent with amorphous magnesium carbonate (AMC) on the surface. This is followed by growth of nesquehonite (MgCO 3 ·3H 2 O) crystallites. In situ imaging provided direct evidence that the AMC intermediate acts as a seed for crystallization of nesquehonite. In situ infrared and thermogravimetric-mass spectrometry indicate that the stoichiometry of AMC is MgCO 3 ·xH 2 O (x = 0.5-1.0), while its structure is indicated to be hydromagnesite-like according to density functional theory and X-ray pair distribution function analysis. Our findings thus provide insight for understanding the stability, lifetime and role of amorphous intermediates in natural and synthetic systems., (© 2021. The Author(s), under exclusive licence to Springer Nature Limited.)

Biodegradable plastics can reach full degradation when disposed of appropriately and thus alleviate plastic pollution caused by conventional plastics. However, additives can be released into the environment during degradation and the fate of these additives can be affected by the degradation process. Here, we characterized TiO 2 particles released from a biodegradable plastic mulch during composting and studied the transport of the mulch-released TiO 2 particles in inert sand and agricultural soil columns under unsaturated flow conditions. TiO 2 particles (238 nm major axis and 154 nm minor axis) were released from the biodegradable plastic mulch in both single-particle and cluster forms. The mulch-released TiO 2 particles were fully retained in unsaturated soil columns due to attachment onto the solid-water interface and straining. However, in unsaturated sand columns, the mulch-released TiO 2 particles were highly mobile. A comparison with the pristine TiO 2 revealed that the mobility of the mulch-released TiO 2 particles was enhanced by humic acid present in the compost residues, which blocked attachment sites and imposed steric repulsion. This study demonstrates that TiO 2 particles can be released during composting of biodegradable plastics and the transport potential of the plastic-released TiO 2 particles in the terrestrial environment can be enhanced by compost residues.

We used IR and XRD, with supporting theoretical calculations, to investigate the swelling behavior of Na + -, NH 4 + -, and Cs + -montmorillonites (SWy-2) in supercritical fluid mixtures of H 2 O, CO 2 , and CH 4 . Building on our prior work with Na-clay that demonstrated that H 2 O facilitated CO 2 intercalation at relatively low RH, here we show that increasing CO 2 /CH 4 ratios promote H 2 O intercalation and swelling of the Na-clay at progressively lower RH. In contrast to the Na-clay, CO 2 intercalated and expanded the Cs-clay even in the absence of H 2 O, while increasing fluid CO 2 /CH 4 ratios inhibited H 2 O intercalation. The NH 4 -clay displayed intermediate behavior. By comparing changes in the HOH bending vibration of H 2 O intercalated in the Cs-, NH 4 -, and Na-clays, we posit that CO 2 facilitated expansion of the Na-clay by participating in outer-sphere solvation of Na + and by disrupting the H-bond network of intercalated H 2 O. In no case did the pure CH 4 fluid induce expansion. Our experimental data can benchmark modeling studies aimed at predicting clay expansion in humidified fluids with varying ratios of CO 2 and CH 4 in real reservoir systems with implications for enhanced hydrocarbon recovery and CO 2 storage in subsurface environments.

Radioiodine ( 129 I) poses a risk to the environment due to its long half-life, toxicity, and mobility. It is found at the U.S. Department of Energy Hanford Site due to legacy releases of nuclear wastes to the subsurface where 129 I is predominantly present as iodate (IO 3 - ). To date, a cost-effective and scalable cleanup technology for 129 I has not been identified, with hydraulic containment implemented as the remedial approach. Here, novel high-performing sorbents for 129 I remediation with the capacity to reduce 129 I concentrations to or below the US Environmental Protection Agency (EPA) drinking water standard and procedures to deploy them in an ex-situ pump and treat (P&T) system are introduced. This includes implementation of hybridized polyacrylonitrile (PAN) beads for ex-situ remediation of IO 3 - -contaminated groundwater for the first time. Iron (Fe) oxyhydroxide and bismuth (Bi) oxyhydroxide sorbents were deployed on silica substrates or encapsulated in porous PAN beads. In addition, Fe-, cerium (Ce)-, and Bi-oxyhydroxides were encapsulated with anion-exchange resins. The PAN-bismuth oxyhydroxide and PAN-ferrihydrite composites along with Fe- and Ce-based hybrid anion-exchange resins performed well in batch sorption experiments with distribution coefficients for IO 3 - of >1000 mL/g and rapid removal kinetics. Of the tested materials, the Ce-based hybrid anion-exchange resin was the most efficient for removal of IO 3 - from Hanford groundwater in a column system, with 50% breakthrough occurring at 324 pore volumes. The functional amine groups on the parent resin and amount of active sorbent in the resin can be customized to improve the iodine loading capacity. These results highlight the potential for IO 3 - remediation by hybrid sorbents and represent a benchmark for the implementation of commercially available materials to meet EPA standards for cleanup of 129 I in a large-scale P&T system.

Despite widespread industrial importance, predicting metal solubilities in highly concentrated, multicomponent aqueous solutions is difficult due to poorly understood ion-ion and ion-solvent interactions. Aluminum hydroxide solid phase solubility in concentrated sodium hydroxide (NaOH) solutions is one such case, with major implications for ore refining, as well as processing of radioactive waste stored at U.S. Department of Energy legacy sites, such as the Hanford Site, Washington State. The solubility of gibbsite (α-Al(OH) 3 ) is often not well predicted because other ions affect the activity of hydroxide (OH - ) and aluminate (Al(OH) 4 - ) anions. In the present study, we systematically examined the influence of key anions, nitrite (NO 2 - ) and nitrate (NO 3 - ), as sodium salts on the solubility of α-Al(OH) 3 in NaOH solutions taking care to establish equilibrium from both under- and oversaturation. Rapid equilibration was enabled by use of a highly pure and crystalline synthetic nano-gibbsite of well-defined particle size and shape. Measured dissolved aluminum concentrations were compared with those predicted by an α-Al(OH) 3 solubility model derived for simple Al(OH) 4 - /OH - systems. Specific anion effects were expressed as an enhancement factor (Al enhc ) conveying the excess of dissolved aluminum. At 45 °C, NaNO 2 and NaNO 3 -containing systems exhibited Al enhc values of 2.70 and 1.88, respectively, indicating significant enhancement. The solutions were examined by Raman and high-field 27 Al NMR spectroscopy, indicating specific interactions including Al(OH) 4 - -Na + contact ion pairing and Al(OH) 4 - -NO 2 - /NO 3 - ion-ion interactions. Dynamic evolution of the α-Al(OH) 3 particles including growth and agglomeration was observed revealing the importance of dissolution/reprecipitation in establishing equilibrium. These studies indicate that incomplete ion hydration, as a result of the low water activity in these concentrated electrolytes, results in: (i) enhanced reactivity of the hydroxide ion with respect to α-Al(OH) 3 ; (ii) increased concentrations of Al(OH) 4 - in solution; and (iii) stronger ion-ion interactions that act to stabilize the supersaturated solutions. This information on the mechanisms by which α-Al(OH) 3 becomes supersaturated is essential for more energy-efficient aluminum processing technologies, including the treatment of millions of gallons of Al(OH) 4 - -rich high-level radioactive waste.

Olivines are divalent orthosilicates with important geologic, biological, and industrial significance and are typically comprised of mixtures of Mg 2+ and Fe 2+ ranging from forsterite (Mg 2 SiO 4 ) to fayalite (Fe 2 SiO 4 ). Investigating the role of Fe(II) in olivine reactivity requires the ability to synthesize olivines that are nanometer-sized, have different percentages of Mg 2+ and Fe 2+ , and have good bulk and surface purity. This article demonstrates a new method for synthesizing nanosized fayalite and Mg-Fe mixture olivines.First, carbonaceous precursors are generated from sucrose, PVA, colloidal silica, Mg 2+ , and Fe 3+ . Second, these precursors are calcined in air to burn carbon and create mixtures of Fe(III)-oxides, forsterite, and SiO 2 . Finally, calcination in reducing CO-CO 2 gas buffer leads to Fe(II)-rich olivines. XRD, Mössbauer, and IR analyses verify good bulk purity and composition. XPS indicates that surface iron is in its reduced Fe(II) form, and surface Si is consistent with olivine. SEM shows particle sizes predominately between 50 and 450 nm, and BET surface areas are 2.8-4.2 m 2 /g. STEM HAADF analysis demonstrates even distributions of Mg and Fe among the available M1 and M2 sites of the olivine crystals. These nanosized Fe(II)-rich olivines are suitable for laboratory studies with in situ probes that require mineral samples with high reactivity at short timescales., (Copyright © 2018 Elsevier Inc. All rights reserved.)

We report an in situ high-pressure NMR capability that permits natural abundance 17 O and 25 Mg NMR characterization of dissolved species in aqueous solution and in the presence of supercritical CO 2 fluid (scCO 2 ). The dissolution of Mg(OH) 2 (brucite) in a multiphase water/scCO 2 fluid at 90 atm pressure and 50 °C was studied in situ, with relevance to geological carbon sequestration. 17 O NMR spectra allowed identification and distinction of various fluid species including dissolved CO 2 in the H 2 O-rich phase, scCO 2 , aqueous H 2 O, and HCO 3 - . The widely separated spectral peaks for various species can all be observed both dynamically and quantitatively at concentrations as low as 20 mM. Measurement of the concentrations of these individual species also allows an in situ estimate of the hydrogen ion concentration, or pC H + values, of the reacting solutions. The concentration of Mg 2+ can be observed by natural abundance 25 Mg NMR at a concentration as low as 10 mM. Quantum chemistry calculations of the NMR chemical shifts on cluster models aided in the interpretation of the experimental results. Evidence for the formation of polymeric Mg 2+ clusters at high concentrations in the H 2 O-rich phase, a possible critical step needed for magnesium carbonate formation, was found.

Geologic storage of CO(2) requires that the caprock sealing the storage rock is highly impermeable to CO(2). Swelling clays, which are important components of caprocks, may interact with CO(2) leading to volume change and potentially impacting the seal quality. The interactions of supercritical (sc) CO(2) with Na saturated montmorillonite clay containing a subsingle layer of water in the interlayer region have been studied by sorption and neutron diffraction techniques. The excess sorption isotherms show maxima at bulk CO(2) densities of ≈ 0.15 g/cm(3), followed by an approximately linear decrease of excess sorption to zero and negative values with increasing CO(2) bulk density. Neutron diffraction experiments on the same clay sample measured interlayer spacing and composition. The results show that limited amounts of CO(2) are sorbed into the interlayer region, leading to depression of the interlayer peak intensity and an increase of the d(001) spacing by ca. 0.5 Å. The density of CO(2) in the clay pores is relatively stable over a wide range of CO(2) pressures at a given temperature, indicating the formation of a clay-CO(2) phase. At the excess sorption maximum, increasing CO(2) sorption with decreasing temperature is observed while the high-pressure sorption properties exhibit weak temperature dependence.

CO(2) injection into deep geologic formations for long-term storage will cause a decrease in aqueous pH due to CO(2) dissolution into reservoir water/brine. Current studies seeking to assess chemical changes under geological CO(2) sequestration (GCS) conditions rely largely on thermodynamic modeling due to the lack of reliable experimental methods. In this work, a spectrophotometric method utilizing bromophenol blue to measure pH in laboratory experiments under GCS-relevant conditions was developed. The method was tested in simulated reservoir fluids (CO(2)-NaCl-H(2)O) at different temperatures, pressures, and ionic strengths, and the results were compared with those from other experimental studies and geochemical models. Measured pH values were generally in agreement with the models, but inconsistencies were present between the models. In situ pH measurements for a basalt rock-CO(2)-brine system were conducted under GCS conditions. The pH increased to 3.52 during a 10-day period due to rock dissolution, compared to pH 2.95 for the CO(2)-brine system without rock. The calculated pH values from geochemical models were 0.22-0.25 units higher than the measured values (assuming all iron in the system was in the form of Fe(2+)). This work demonstrates the use of in situ spectrophotometry for pH measurement under GCS-relevant conditions.