CO2 sequestration via carbonation of widely available low-cost minerals, such as olivine, can permanently dispose of CO2 in an environmentally benign and a geologically stable form. We report the results of studies of the mechanisms that limit aqueous olivine carbonation reactivity under the optimum sequestration reaction conditions observed to date: 1 M NaCl + 0.64 M NaHCO3 at Te 185 degrees C and P(CO2) approximately equal to 135 bar. A reaction limiting silica-rich passivating layer (PL) forms on the feedstock grains, slowing carbonate formation and raising process cost. The morphology and composition of the passivating layers are investigated using scanning and transmission electron microscopy and atomic level modeling. Postreaction analysis of feedstock particles, recovered from stirred autoclave experiments at 1500 rpm, provides unequivocal evidence of local mechanical removal (chipping) of PL material, suggesting particle abrasion. This is corroborated by our observation that carbonation increases dramatically with solid particle concentration in stirred experiments. Multiphase hydrodynamic calculations are combined with experimentto better understand the associated slurry-flow effects. Large-scale atomic-level simulations of the reaction zone suggest that the PL possesses a "glassy" but highly defective SiO2 structure that can permit diffusion of key reactants. Mitigating passivating layer effectiveness is critical to enhancing carbonation and lowering sequestration process cost.

Lamellar dehydroxylation mechanisms are important in understanding highly reactive nanocrystal oxide formation as well as a variety of applied lamellar hydroxide reaction processes. These mechanisms have been observed for the first time via in situ nanoscale imaging for the prototype lamellar hydroxide: Mg(OH) 2 . Environmental-cell, dynamic highresolution transmission electron microscopy was combined with advanced computational modeling in discovering that lamellar nucleation and growth processes govern dehydroxylation. The host lamella can guide the formation of a solid solution series of lamellar oxyhydroxide intermediates en route to oxide formation. This investigation opens the door to a deeper, atomic-level understanding of lamellar dehydroxylation reaction processes, nanocrystal oxide formation, and the range of potential new intermediate materials and third component reaction pathways they can provide.

As compared with other candidate carbon sequestration technologies, mineral carbonation offers the unique advantage of permanent disposal via geologically stable and environmentally benign carbonates. The primary challenge is the development of an economically viable process. Enhancing feedstock carbonation reactivity is key. Heat activation dramatically enhances aqueous serpentine carbonation reactivity. Although the present process is too expensive to implement, the materials characteristics and mechanisms that enhance carbonation are of keen interest for further reducing cost. Simultaneous thermogravimetric and differential thermal analysis (TGA/DTA) of the serpentine mineral lizardite was used to isolate a series of heat-activated materials as a function of residual hydroxide content at progressively higher temperatures. Their structure and composition are evaluated via TGA/DTA, X-ray powder diffraction (including phase analysis), and infrared analysis. The meta-serpentine materials that were observed to form ranged from those with longer range ordering, consistent with diffuse stage-2 like interlamellar order, to an amorphous component that preferentially forms at higher temperatures. The aqueous carbonation reaction process was investigated for representative materials via in situ synchrotron X-ray diffraction. Magnesite was observed to form directly at 15 MPa CO2 and at temperatures ranging from 100 to 125 degrees C. Carbonation reactivity is generally correlated with the extent of meta-serpentine formation and structural disorder.

Gas-phase magnesium hydroxide carbonation processes were investigated at high CO 2 pressures to better understand the reaction mechanisms involved. Carbon and hydrogen elemental analysis, secondary ion mass spectrometry, ion beam analysis, X-ray diffraction, and thermogravimetric analysis were used to follow dehydroxylation/rehydroxylation/carbonation reaction processes. Dehydroxylation is found to generally precede carbonation as a distinct but interrelated process. Above the minimum CO 2 pressure for brucite carbonation, both carbonation and dehydroxylation reactivity decrease with increasing CO 2 pressure. Low-temperature dehydroxylation before carbonation can form porous intermediate materials with enhanced carbonation reactivity at reduced (e.g., ambient) temperature and pressure. Control of dehydroxylation/ rehydroxylation reactions before and/or during carbonation can substantially enhance carbonation reactivity.

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