Surface Reactivity from the Atomic to the Global Scale

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Surface Reactivity from the Atomic to the Global
Scale
Eric H. Oelkers Experimental Geochemistry and
Biogeochemistry LMTG/Université Paul Sabatier,
Toulouse, FRANCE
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The Calcite water interface deduced from X-ray
reflectivity
After Fenter et al., 2000
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Transition State Theory
Mineral lt-gt A lt-gt B lt-gt Activated complex1 -gt
Products
r kActivated Complexes
r- k-Activated Complexes
Overall rate

• rr - r- r (1-exp(-A/nRT))

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Activated Complex Formation
1) Sorption of aqueous species
2) Breaking bridging bonds by removal of adjacent
atoms
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AFM Images of Dissolving Calcite
Corners dissolve more rapidly than edges
Jordan and Rammensee (1998) T24 C, pure H2O
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Multi-oxide Mineral Dissolution Metal-Oxygen
bonds break at very different rates
Single (Hydr)oxide dissolution rates pH2, T25 C
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Dissolution Mechanism Depends on Mineral Structure
Minerals that require breaking of more than one
bond

• Minerals that require breaking of only a single
  bond

e.g. quartz, olivine
e.g. alkali-feldspars, clay minerals
Activated complex formed by sorption reactions
Activated complex formed by exchange reactions
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Basaltic Glass Dissolution Mechanism
Si
Al
Si
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Basaltic Glass Dissolution Mechanism Step 1
Mono/divalent metal exchange
Si
Al
Si
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Basaltic Glass Dissolution Mechanism Step 2
Trivalent metal-proton exchange
Si
Al
Si
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Basaltic Glass Dissolution Mechanism Step 2
Trivalent metal-proton exchange
Si
Al
Si
Si
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Basaltic Glass Dissolution Mechanism Step 3 Si
removal
Si
Al
Si
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Basaltic Glass Dissolution Rate Equation
Aqueous Activity Ratio
Chemical Affinity of Al-Si-Fe surface layer
normalized to 1 Si
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Variation of measured Basaltic Glass Dissolution
rates with (aH/aAl3) (Oelkers and Gislason,
2001)
A single regression curve is found for all data
at both acidic and basic pH
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Computed and measured basaltic glass dissolution
rates as a function of pH (Gislason and Oelkers,
2003)
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Whole Core flow reactor
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Whole core dissolution Experiments
Fontainebleau Sandstone Keiffer et al., (1999)
Permeability Variation
Solution Chemistry Variation -gt Reactive surface
area
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Core flow reactor in situ fluid evolution
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Whole core dissolution studies in situ
analysis of rock during dissolution
Noiriel et al. (2004)
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daprès D. Bernard
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Precipitation rates What do we know?
Diffuse Growth

• r k (1-exp(-A/RT))

Spiral Growth
Germination/ 2-D growth
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Example Modeling of illite formation in Garn
formation (Oelkers et al., in prep.)
Al2Si2O5(OH)4 KAlSi3O8 KAl3Si3O10(OH)2
2SiO2 H2O
K-spar
Illite
quartz
kaolinite
Model results using published rate data
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Temperature, C
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Model results taking account of illite nucleation
2-D growth
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Mineral Reactions and Global Climate Moderation
dissolution of primary silicate minerals
coupled to CO2 consumption
2CO2 3H2O CaAl2Si2O8 -gt Ca2 2HCO3-
Al2Si2O5(OH)4
Clay formation
Ca-silicate Dissolution
CO2 consumption
Temperature dependence of dissolution far
stronger in sea water than most surface waters
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Global Suspended material Flux (Milliman and
Meade, 1981)
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• Jökulsá á Fjöllum
• Jökulsá á Dal.
• Jökulsá í Fljótsdal
• Fellsá
• Grímsá.
• Fjardará
• Lagarfljót

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The suspended flux is much more runoff dependent
than the dissolved Ca-flux (Gislason, Oelkers and
Snorrason 2005)
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Because of the runoff dependence difference
between the suspended and dissolved Ca fluxes,
the suspended flux is more climate/runoff
dependent. (Gislason, Oelkers and Snorrason
2005)
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Surface Reactivity from the Atomic to the Global
Scale
Eric H. Oelkers Experimental Geochemistry and
Biogeochemistry LMTG/Université Paul Sabatier,
Toulouse, FRANCE