Magnesium inhibition of calcite dissolution kinetics
We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO2-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 x 10-3 molal. At the same pH, magnesium concentrations of less than 0.05 x 10-3 molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch-pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch-pit formation.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 878670
- Report Number(s):
- PNNL-SA-49046; 4391; KC0303020
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 3 Vol. 70; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
AFM
ATOMIC FORCE MICROSCOPY
CALCITE
CO2
DEFECTS
DISSOLUTION
Environmental Molecular Sciences Laboratory
GEOMETRY
INTERFEROMETRY
KINETICS
MAGNESIUM
MAGNESIUM ADDITIONS
MORPHOLOGY
NUCLEATION
REACTION KINETICS
SATURATION
VELOCITY
VSI
atomic force microscopy
calcite
carbonate
dissolution
etch pit
kinetics
magnesium
surface chemistry
vertical scanning interferometry