-
Joey M. Nelson1, John R. Bargar2, Gordon E. Brown, Jr.1,2,3,
Kate Maher1
1 Department of Geological and Environmental Sciences, Stanford
University 2 Department of Photon Science and Stanford Synchrotron
Radiation Lightsource, SLAC 3 Department of Chemical Engineering,
Stanford University
Pore size dependent geochemistry
Global Climate & Energy Project Distinguished Student
Lecture October 8, 2013
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
2
Motivation: CO2 sequestration and engineering subsurface
reactivity
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3
EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Data from Brantley and Mellott (Am. Min., 2000), and Mayer
(Chem. Geol., 1994)
1 nm
1 µm
m
m
m
m
m
m
m
m
m
m
Macro
po
res (> 5
0 n
m)
Micro
po
res (< 2
nm
) M
eso
po
res
(2-5
0 n
m)
IUPAC pore size ranges
Stubbs et al., GCA, 2009 Velbel and Baker, CCM, 2008 Smith et
al., PNAS, 1999
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Effects of Mesopores
Enhanced Adsorption Capacity
4
Diffusion Limited Transport
EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
(Zn adsorbed to alumina data from Wang et al., JCIS, 2002)
Equilibration Time (s)
Macroporous
Mesoporous
pH
(U adsorbed to alumina data from Sun et al., Separ. Purif.
Tech., 2011)
Ad
sorp
tio
n C
apac
ity
(ml/
g)
pH
Macroporous
Mesoporous
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Hypotheses 1. Meso-confinement phenomena result from unique
geochemical processes, not transport limitations.
2. The chemical mechanisms of sorption differ between macropores
and mesopores, which could significantly impact CO2
sequestration.
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Objectives 1. Quantify sorption rates and capacities with
respect
to pore size.
2. Determine molecular structure of sorption complexes in
macropores and mesopores.
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Zn Adsorption on Silica
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Batch Adsorption Experiments • Controlled Pore Glass
(SiO2(am))
– Macroporous (300 nm): 8.58 m2 g-1
– Mesoporous (13.9 nm): 174 m2 g-1
– Each batch contained equivalent amounts of substrate surface
area
Choice of Experimental System
• Molecular geometry of sorbed CO2 is difficult to observe with
XAFS
• Zn exists in various complexes and is easily observed with
XAFS
• Silica is a simple substrate and the dominant mineral in
saline aquifers
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Geochemical Modeling
• Diffusion-Reaction Model – Rate coefficient, k = 10-6 mol
s-1
– Diffusivity, Deff = 10-7 cm2 s-1
• Batch reaction is well-mixed (i.e., not diffusion-limited)
• Meso-confinement effects are due to unique geochemistry 7
EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Reaction vs. Transport
𝜕𝐶
𝜕𝑡= 𝐷𝑒𝑓𝑓
𝜕2𝐶
𝜕𝑥2− 𝑘𝐶
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Zn Adsorption Results
• Zn adsorption capacity is enhanced by mesopores.
• Indirect evidence that mesopores promote inner-sphere
complexation (i.e., stronger binding).
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
• Extended X-ray absorption fine structure spectroscopy at BL
11-2 at SSRL, SLAC
• Spectra unique to molecular environment of Zn in sample
(Roberts et al., JCIS, 2003)
C.N. 6
4.6
4
4
6.6
4
4
4
6
Molecular Fingerprinting with EXAFS
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
Zn EXAFS Results
• Zn is dominantly 6-fold coordinated in macropores.
• Zn is dominantly 4-fold coordinated in mesopores.
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• Pore size is a tunable parameter for environmental remediation
and chemical separations.
• More accurate reactive transport models
• Induce or enhance subsurface mineralization
Conclusions
• Zn adsorption capacity is enhanced and molecular configuration
is altered in mesoporous silica.
• Sorption of CO2 molecules in meso-confined pore volumes may
also differ from less confined spaces.
• Future study of CO2 sorption geometries with ATR-FTIR
spectroscopy and quantum calculations.
Implications
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EXPERIMENTATION CONCLUSIONS INTRODUCTION SPECTROSCOPY
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Acknowledgements
Kate Maher, Gordon E. Brown Jr., Jen Wilcox, Scott Fendorf, Mike
Massey, Adam Jew, Caroline Harris, Karrie Weaver, John Bargar,
Noemie Janot
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PORE SIZE DEPENDENT GEOCHEMISTRY GCEP RESEARCH SYMPOSIUM
2013
Thank you! Questions? Poster 59: Enhanced Sorption of Zn in
Mesoporous Silica