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Kinetics of Olivine Carbonation for Carbon Sequestration Natalie C. Johnson 1,2 , Burt Thomas 3 , Kate Maher 2 , Dennis K. Bird 2 , Robert J. Rosenbauer 3 , Gordon E. Brown, Jr. 1,2 1. Department of Chemical Engineering, Stanford University, Stanford CA 2. Department of Geologic and Environmental Sciences, Stanford University, Stanford CA 3. United States Geologic Survey, Menlo Park, CA Ultramafic and mafic rocks Sandstone Shale caprock Motivation Dickson-type Rocker Bombs Solution Analysis 1 g/L salicylic acid no additions 0.1 g/L salicylate (buffered to pH=3.1) Solid Product Formation pH Control of Kinetics ml CO2 headspace Time of rxn (days) Other additions Extent of carbonation A 15 39 1 g/L salicylic acid 29 ± 4% B 15 39 none 7 ± 2% C 15 33 0.1 g/L salicylate buffered to pH of 3 <3% D 0 33 0.1 g/L salicylate buffered to pH of 3 0 Several ways CO 2 can be trapped underground A. Structural trapping B. Solubility trapping C. Mineral Trapping Reaction is contained inside a flexible gold bag surrounded by pressure fluid, which allows for the withdrawal of liquid samples without changing pressure/temperature conditions. Reactor is contained in a rocking furnace to maintain desired temperature and constant mixing. All reactions carried out at 60 o C, 100 bar CO 2 pressure, 20:1 water:rock ratio by mass, and 3 wt% NaCl. 0 1000 2000 3000 4000 5000 6000 7000 0 20 40 [Mg] (ppm) time (days) 0 50 100 150 200 250 0 20 40 [Fe] (ppm) time (days) 0 50 100 150 200 250 300 0 20 40 [Si] (ppm) time (days) 5 μm 1 μm 20 μm SiO 2 MgCO 3 Magnesite 100 μm 10 μm 2 μm SiO 2 Silica 2 μm SiO 2 Fe-oxide or Fe-carbonate Chromite Un-altered Mg- silicate 100 μm 10 um μm Fe-phase Three distinct phases were identified by XRD, SEM, EDS, and BSE. Magnesite, found by XRD, was confirmed with EDS to be layered nodules that grew to be several microns in diameter. An amorphous iron phase was detected with EDS and identified with BSE. Silica was detected by XRD (as a large hump centered at 2θ=27) and EDS, with it’s morphology shown to be many spheres fused together. Based on saturation calculations using the solution composition at each time point, we expect magnesite, siderite, and silica to precipitate. 2 3 2 4 2 SiO 2MgCO 2CO SiO Mg The net reaction comprises three main steps: silicate dissolution (1), carbon dioxide dissolution (2), and magnesite precipitation (3). Mineral trapping is the best long-term solution because once reacted, the CO 2 is stable in mineral form for hundreds of millions of years. No need to monitor for leaks Potential for value-added products Abundant feedstock O 2H SiO 2Mg 4H SiO Mg 2 aq 2 2 4 2 H HCO O H CO 3 2 2 H MgCO HCO Mg 3 3 2 (1) (2) (3) http://geology.csustan.edu/fieldtrips/delpuerto/ Photos courtesy of Pablo Garcia del Real and USGS archives Image by Dennis Bird and Pablo Garcia del Real Photos courtesy of Robert Rosenbauer Lemke et al. 2008 Solution compositions were measured with ICP-OES. We saw initial non-stiochiometric dissolution (Mg:Si ratio greater than 2) which indicates a preferential release of Mg from the olivine surface. The concentration of Si reaches a critical level, at which point the rate of change drops to zero. This likely indicates secondary phase precipitation rather than further dissolution. Effects of Salicylic Acid citrate oxalate water salicylate Bennett et al. 1988 Previous studies have shown that organic acids increase the dissolution rates of silicates, including quartz, although the mechanism by which this occurs is not understood. Current ideas center around a organic-metal complex at the mineral surface which weakens bonds within the mineral, reducing the energy required to break those bonds. Our results show that salicylic acid appears to increase the concentration of Mg in solution, suggesting the formation of a Mg-salicylate complex. Salicylic acid has no apparent effect on silica solubility. Quartz 4 4.5 5 5.5 6 6.5 0 20 40 pH time (days) We see a correlation between the pH of solution and the concentration of Si in solution, but it is difficult to determine the causality: Is the Si release slower because the pH is lower? Or is the pH lower because the rate of Si release is slower? Possible Explanation: The solution pH is controlled by both carbonate system and the rate of proton exchange with the rock Mg 2+ exchanges with H + to form Si-rich layer Si-rich layer behaves like SiO 2(am) or quartz Quartz and SiO 2 dissolve faster at higher pH Over longer times (days), olivine dissolves faster at near-neutral pH than very acidic pH Once Si-rich layer is formed, ion exchange occurs much slower because the process becomes diffusion-limited Bearat et al. 2006 Conclusions + Future Work SiO 2 reaches saturation very quickly Most Fe precipitates in separate phase from Mg The kinetic effect of pH likely depends on the surface composition Salicylic acid increases solubility of Mg-phases but does not affect solubility of Si-phases Initial dissolution rate does not necessarily correlate with apparent carbonate appearance rate How can we stabilize Si in solution? Accelerate dissolution of/prevent formation of Si-rich layer Improve solubility of SiO 2 to reduce volume change Rates pH Other additions Extent of carbonation Olivine dissolution rate over 24 hours Apparent MgCO 3 appearance rates (mol s -1 cm -2 ) 4.3 1 g/L salicylic acid 29 ± 4% 5.0 x 10 -13 1.1 x 10 -13 4.5 none 7 ± 2% 8.8 x 10 -13 2.6 x 10 -14 4.5 0.1 g/L salicylate buffered to pH of 3 <3% 1.4 x 10 -12 4.5 x 10 -15 Acknowledgements We wish to thank the Global Climate and Energy Project at Stanford University for financial support, and Robert Jones and Pablo Garcia Real for technical contributions. Bearat, McKelvy, Chizmeshya, Gormley, Nunez, Carpenter, Squires, & Wolf. Environ. Sci. Technol. 2006, 40:4802-4808 Bennett, Melcer, Diegel, & Hassett. Geochim. Cosmochim. Acta 1988, 52:1521-1530 Lemke, Rosenbauer, Bischoff, & Bird. Chemical Geology 2008, 252:136-144
