Helen E. King

Effect of Secondary Phase Formation on the Carbonation of Olivine

Large-scale olivine carbonation has been proposed as a potential method for sequestering CO(2) emissions. For in situ carbonation techniques, understanding the relationship between the formation of carbonate and other phases is important to predict the impact of possible passivating layers on the reaction. Therefore, we have conducted reactions of olivine with carbonated saline solutions in unstirred batch reactors. Altering the reaction conditions changed the Mg-carbonate morphology. We propose that this corresponded to changes in the ability of the system to precipitate hydromagnesite or magnesite. During high-temperature reactions (200 degrees C), an amorphous silica-enriched phase was precipitated that was transformed to lizardite as the reaction progressed. Hematite was also precipitated in the initial stages of these reactions but dissolved as the reaction proceeded. Comparison of the experimental observations with reaction models indicates that the reactions are governed by the interfacial fluid composition. The presence of a new Mg-silicate phase and the formation of secondary products at the olivine surface are likely to limit the extent of olivine to carbonate conversion.

Experimental investigations into the silicification of olivine: Implications for the reaction mechanism and acid neutralization

Abstract Neutralization of acidic fluids by means of fluid-olivine interactions is important in volcanic environments and has been proposed as a practical scheme for the neutralization of acidic sulfaterich fluids. To understand the interaction of olivine with highly acidic fluids we have reacted whole olivine crystals and a dunite cube with different sulfuric acid solutions at temperatures ranging from 60-120 °C. Reaction of olivine with 2 and 3.6 M acid concentrations produced a layered amorphous silica pseudomorph of the original olivine grain. The mechanism of pseudomorphic replacement was studied by reacting olivine with an 18O-enriched acid solution and examining the products using Raman spectroscopy. Peak shifts in the Raman spectra show that 18O was incorporated into the silica rim, including the siloxane ring structures. The formation of a layered silica pseudomorph, the incorporation of 18O into the silica rim and the dependence of the replacement rim strength on the acid concentration indicate that the pseudomorphic replacement occurred by means of an interface-coupled dissolution-reprecipitation mechanism. When olivine was reacted with 1 M sulfuric acid amorphous silica was produced but no longer formed a pseudomorph of the olivine grain. Reaction with 0.1 M acid, or solutions containing Na, encouraged the formation of hematite as well as amorphous silica. From the known Fe-phase stabilities for our experimental conditions and the dependence of hematite formation on the presence of Na we propose that initially jarosite phases precipitated, which transformed into hematite during the experiment.

Control of silicate weathering by interface-coupled dissolution-precipitation processes at the mineral-solution interface

The mechanism of surface coating formation (the so-called surface altered layers [SALs] or leached layers) during weathering of silicate minerals is controversial and hinges on understanding the saturation state of the fluid at the dissolving mineral surface. Here we present in-situ data on the evolution of the interfacial fluid composition during dissolution of wollastonite (CaSiO3), obtained using interferometry and micro pH and ion-selective electrodes. Steep concentration gradients develop at the mineral interface as soon as it makes contact with the solution. This interfacial fluid becomes supersaturated with respect to amorphous silica that forms a surface coating, limiting fluid access to the mineral surface and hence affecting the dissolution rate. The thickness of the supersaturated zone and the precipitated layer depends on the relative rates of mass transport and surface reaction in the system; this effect could contribute to the discrepancy between dissolution rates measured in the field and in the laboratory. As well, our results have implications for predictions of silicate weathering rates and hence climate evolution, as different assumptions on dissolution mechanisms affect calculations on CO2 drawdown during weathering and consequent effects on estimates of global mean temperatures.

Where on Earth has our water come from

The presence of water in the Earth has long been an enigma. However, computer modelling techniques have shown that the adsorption of water onto the fractal surfaces of interplanetary dust particles, which are present in the planetary accretion disk, is sufficiently strong to provide a viable origin of terrestrial water.

Surface specific measurements of olivine dissolution by phase-shift interferometry

Abstract Natural olivine dissolution and replacement often occurs preferentially along specific crystallographic planes. Thus, olivine reactivity at specific surfaces was examined in situ using phase-shift interferometry, which has a detection limit <10-5 nm/s, by dissolving two smoothed olivine crystal faces and a third sample corresponding to a surface that was generated by preferential dissolution along structural defects. The experiments were conducted at 22 °C and ambient pressure in 0.1 M NaCl solutions that were acidified to pHs between 1 and 4 using 0.1 M HCl. These experiments show that olivine dissolution can vary from one surface to another as well as in different areas of the same surface that have similar characteristics. The fastest vertical retreat occurred at the surfaces related to defects. However, only vertical advancement was observed at pH 1 on this surface consistent with the observation of isolated islands on the surface during atomic force microscopy investigations after the experiment. Raman analysis of the precipitated phase showed that it was not one of the thermodynamically stable phases expected from PHREEQC modeling. However, the correlation between the siloxane ring peak of amorphous silica with a similar peak in the precipitate spectrum, in conjunction with previous experimental and natural observations, indicates that the precipitate was a Si-enriched amorphous phase. Therefore, precipitation can facilitate the further dissolution of olivine on this surface as long as it does not completely armor the surface. Precipitate formation on surfaces associated with outcropping defects supports the natural observations of preferential dissolution and serpentinization along these defects implying that the fast dissolution of these surfaces will play a critical role during olivine replacement. In addition, comparison with flow-through experiments indicates that outflow fluid chemistry may provide an incomplete picture of processes occurring during olivine dissolution.

Sequestration of selenium on calcite surfaces revealed by nanoscale imaging.

Calcite, a widespread natural mineral at the Earths surface, is well-known for its capacity to sequester various elements within its structure. Among these elements, selenium is important because of its high toxicity in natural systems and for human health. In the form of selenite (Se((IV))), selenium can be incorporated into calcite during growth. Our in situ atomic force microscopy observations of calcite surfaces during contact with selenium-bearing solutions demonstrate that another process of selenium trapping can occur under conditions in which calcite dissolves. Upon the injection of solutions containing selenium in two states of oxidation (either Se((IV)) or Se((VI))), precipitates were observed forming while calcite was still dissolving. In the presence of selenate (Se((VI))), the precipitates formed remained small during the observation period. When injecting selenite (Se((IV))), the precipitates grew significantly and were identified as CaSeO3·H2O, based on SEM observations, Raman spectroscopy, and thermodynamic calculations. An interpretation is proposed where the dissolution of calcite increases the calcium concentration in a thin boundary layer in contact with the surface, allowing the precipitation of a selenium phase. This process of dissolution-precipitation provides a new mechanism for selenium sequestration and extends the range of thermodynamic conditions under which such a process is efficient.

Nanoscale observations of magnesite growth in chloride- and sulfate-rich solutions.

Magnesite growth in chloride and sulfate-rich solutions has been examined at 90 °C in situ using phase-shift interferometry (PSI) and ex situ using atomic force microscopy (AFM) to evaluate the feasibility of cosequestering SO2 and CO2 in Mg-rich rocks. Although sulfate may assist desolvation at the magnesite surface, evidence for enhanced growth was only found at specific surface sites. The overall growth rates fit with those observed for chloride experiments in similarly saturated solutions. Thus, the formation of Mg-SO4 ion pairs in solution, which lowers the supersaturation with respect to magnesite, will have the dominant effect during sequestration. Lowering the activity of Mg(2+) ions in solution also inhibited the nucleation of other hydrated Mg-carbonate phases. As no evidence was found for sulfate incorporation into the growing magnesite, the presence of sulfate in solution will be detrimental to CO2 sequestration and is not expected to be cosequestered. The PSI data also emphasize the variability of reactivity over the surface and how this changes as a function of solution saturation and composition.

Novel anatomic adaptation of cortical bone to meet increased mineral demands of reproduction

The goal of this study was to investigate the effects of reproductive adaptations to mineral homeostasis on the skeleton in a mouse model of compromised mineral homeostasis compared to adaptations in control, unaffected mice. During pregnancy, maternal adaptations to high mineral demand include more than doubling intestinal calcium absorption by increasing calcitriol production. However, calcitriol biosynthesis is impaired in HYP mice, a murine model of X-linked hypophosphatemia (XLH). In addition, there is a paucity of mineralized trabecular bone, a primary target of bone resorption during pregnancy and lactation. Because the highest density of mineral is in mature cortical bone, we hypothesized that mineral demand is met by utilizing intracortical mineral reserves. Indeed, analysis of HYP mice revealed dramatic increases in intracortical porosity characterized by elevated serum PTH and type-I collagen matrix-degrading enzyme MMP-13. We discovered an increase in carbonate ion substitution in the bone mineral matrix during pregnancy and lactation of HYP mice, suggesting an alternative mechanism of bone remodeling that maintains maternal bone mass during periods of high mineral demand. This phenomenon is not restricted to XLH, as increased carbonate in the mineral matrix also occurred in wild-type mice during lactation. Taken together, these data suggest that increased intracortical perilacunar mineral turnover also contributes to maintaining phosphate levels during periods of high mineral demand. Understanding the mechanisms of skeletal contribution to mineral homeostasis is important to improving the treatment and prevention of fracture risk and bone fragility for female patients with XLH, but also provides important insight into the role and unique adaptations of the maternal skeleton to the demands of fetal development and the needs of postnatal nutrition.

Visualizing Organophosphate Precipitation at the Calcite–Water Interface by in Situ Atomic-Force Microscopy

Esters of phosphoric acid constitute a large fraction of the total organic phosphorus (OP) in the soil environment and, thus, play an important role in the global phosphorus cycle. These esters, such as glucose-6-phosphate (G6P), exhibit unusual reactivity toward various mineral particles in soils, especially those containing calcite. Many important processes of OP transformation, including adsorption, hydrolysis, and precipitation, occur primarily at mineral-fluid interfaces, which ultimately governs the fate of organophosphates in the environment. However, little is known about the kinetics of specific mineral-surface-induced adsorption and precipitation of organophosphates. Here, by using in situ atomic-force microscopy (AFM) to visualize the dissolution of calcite (1014) faces, we show that the presence of G6P results in morphology changes of etch pits from the typical rhombohedral to a fan-shaped form. This can be explained by a site-selective mechanism of G6P-calcite surface interactions that stabilize the energetically unfavorable (0001) or (0112) faces through step-specific adsorption of G6P. Continuous dissolution at calcite (1014)-water interfaces caused a boundary layer at the calcite-water interface to become supersaturated with respect to a G6P-Ca phase that then drives the nucleation and growth of a G6P-Ca precipitate. Furthermore, after the introduction of the enzyme alkaline phosphatase (AP), the precipitates were observed to contain a mixture of components associated with G6P-Ca, amorphous calcium phosphate (ACP)-hydroxyapatite (HAP) and dicalcium phosphate dihydrate (DCPD). These direct dynamic observations of the transformation of adsorption- and complexation-surface precipitation and enzyme-mediated pathways may improve the mechanistic understanding of the mineral-interface-induced organophosphate sequestration in the soil environment.

Coupled Dissolution and Precipitation at the Cerussite-Phosphate Solution Interface: Implications for Immobilization of Lead in Soils

In situ atomic force microscopy (AFM) has been used to study the interaction of phosphate-bearing solutions with cerussite, PbCO3, (010) surfaces. During the dissolution of cerussite we observed simultaneous growth of needle-shaped or spherical pyromorphite phases. This occurred at two different pH values and ionic strengths relevant to soil solution conditions. The initial dissolution processes occurring at the cerussite solid-phosphate solution interface were clearly distinguished, and heterogeneous nucleation and growth rates of pyromorphites at phosphate concentrations ranging from 0.1 μM to 10 mM were quantitatively defined. Enhanced cerussite dissolution in the presence of high salt (NaCl or NaF) concentrations leads to an increase in pyromorphite nucleation and growth rates. The newly formed pyromorphites were found to be stable upon contact with water or citrate-bearing solutions under acidic or alkaline conditions in the pH range 4-8. These in situ observations may improve the mechanistic understanding of processes resulting in lead immobilization in diverse soil systems as well as to enhance the effectiveness of phosphate-based treatments for remediation of lead-polluted soils.