Possible Explanation: Mg CO H O o MgCO...All reactions carried out at 60oC, 100 bar CO 2 pressure,...
Transcript of Possible Explanation: Mg CO H O o MgCO...All reactions carried out at 60oC, 100 bar CO 2 pressure,...
Kinetics of Olivine Carbonation for Carbon SequestrationNatalie C. Johnson1,2, Burt Thomas3, Kate Maher2, Dennis K. Bird2, Robert J. Rosenbauer3, Gordon E. Brown, Jr.1,2
1. Department of Chemical Engineering, Stanford University, Stanford CA2. Department of Geologic and Environmental Sciences, Stanford University, Stanford CA
3. United States Geologic Survey, Menlo Park, CA
Ultramafic and
mafic rocksSandstone
Shale caprock
Motivation
Dickson-type Rocker Bombs
Solution Analysis1 g/L salicylic acidno additions0.1 g/L salicylate(buffered to pH=3.1)
Solid Product Formation
pH Control of Kinetics
ml CO2 headspace
Time of rxn (days)
Other additionsExtent of carbonation
A 15 39 1 g/L salicylic acid 29 ± 4%
B 15 39 none 7 ± 2%
C 15 330.1 g/L salicylate buffered
to pH of 3<3%
D 0 330.1 g/L salicylate buffered
to pH of 30
Several ways CO2 can be trapped underground
A. Structural trappingB. Solubility trappingC. 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 60oC, 100 bar CO2 pressure, 20:1 water:rock ratio by mass, and 3 wt% NaCl.
0
1000
2000
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4000
5000
6000
7000
0 20 40
[Mg]
(p
pm
)
time (days)
0
50
100
150
200
250
0 20 40
[Fe
] (p
pm
)
time (days)
0
50
100
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0 20 40
[Si]
(p
pm
)
time (days)
5 μm
1 μm
20 μm
SiO2
MgCO3
Magnesite
100 μm
10 μm
2 μm
SiO2
Silica
2 μm SiO2
Fe-oxide or Fe-carbonate
Chromite Un-altered Mg-silicate
100 μm10 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.
23242 SiO2MgCO2COSiOMg
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 CO2 is stable in mineral form for hundreds of millions of years.
No need to monitor for leaksPotential for value-added productsAbundant feedstock
O2HSiO2Mg4HSiOMg 2
aq
2
2
42
HHCOOHCO 322
HMgCOHCOMg 33
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:Siratio 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
Mg2+ exchanges with H+ to form Si-rich layer
Si-rich layer behaves like SiO2(am) or quartz
Quartz and SiO2 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 WorkSiO2 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 layerImprove solubility of SiO2 to reduce volume change
RatespH Other additions
Extent of
carbonation
Olivine
dissolution rate
over 24 hours
Apparent MgCO3
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
4.50.1 g/L salicylate
buffered to pH of 3<3% 1.4 x 10-12 4.5 x 10-15
AcknowledgementsWe 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, 40:4802-4808Bennett, Melcer, Diegel, & Hassett. Geochim. Cosmochim. Acta 1988, 52:1521-1530Lemke, Rosenbauer, Bischoff, & Bird. Chemical Geology 2008, 252:136-144