Calcite, dolomite and magnesite dissolution kinetics in aqueous solutions at acid to circumneutral pH, 25 to 150 °C and 1 to 55 atm pCO 2: New constraints on CO 2 sequestration in sedimentary basins
Dissolution rates of calcite, dolomite and magnesite were measured at 25, 60, 100 and 150 °C at far from equilibrium conditions in 0.1 M NaCl solutions of pH from 3 to 6 as a function of bicarbonate ion concentration (10 − 5 M ≤ [NaHCO 3] ≤ 0.1 M) and CO 2 partial pressure (1 ≤ pCO 2 ≤ 55 atm). In...
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Published in | Chemical geology Vol. 265; no. 1; pp. 20 - 32 |
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Main Authors | , , , |
Format | Journal Article |
Language | English |
Published |
Elsevier B.V
2009
|
Subjects | |
Online Access | Get full text |
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Summary: | Dissolution rates of calcite, dolomite and magnesite were measured at 25, 60, 100 and 150 °C at far from equilibrium conditions in 0.1 M NaCl solutions of pH from 3 to 6 as a function of bicarbonate ion concentration (10
−
5
M
≤
[NaHCO
3]
≤
0.1 M) and CO
2 partial pressure (1
≤
pCO
2
≤
55 atm). In solutions without initially added NaHCO
3, calcite dissolution rates (
R) increase with
pCO
2 (and equilibrium H
+ activity); however, recalculation of rates normalized to a constant pH
=
4.0 yields only a weak dependence of
R on
pCO
2 at 25, 60, 100, and 150 °C. The apparent activation energy for calcite dissolution at 25–100 °C is equal to 48.2
±
4.6 kJ mol
−
1
. Dolomite dissolution rates increase with increasing
pCO
2 at 1
≤
pCO
2
≤
10 atm and stay constant when
pCO
2 is further increased to 50 atm in HCO
3-free solutions at 3.1
≤
pH
≤
4.2. The apparent activation energy for dissolution is equal to 34, 21 and 16 kJ mol
−
1
at pH
=
4 (1–50 atm
pCO
2), pH
=
4.8 (30 atm
pCO
2) and pH
=
5.5 (50 atm
pCO
2), respectively. Magnesite dissolution rates increase by a factor of 3 at 1 to 5–10 atm
pCO
2 but remain constant from 5 to 55 atm
pCO
2 in NaHCO
3-free solutions at pH
≤
4. Apparent activation energy for magnesite dissolution at
pCO
2 from 2 to 50 atm decreases from 44
±
2 kJ mol
−
1
at 3.1
≤
pH
≤
4.0 to 34 kJ mol
−
1
at pH
=
5.4.
The rates of magnesite and dolomite dissolution at 25–100 °C and far from equilibrium conditions can be rationalized using a surface complexation approach with a unique set of surface adsorption and kinetic constants. The dissolution rates increase with increase of
pCO
2 in carbonate-free acid solutions can be explained by the increase of >CO
3H° species concentration with pH decrease. In circumneutral solutions in the presence of added NaHCO
3, the dissolution rates decrease with
pCO
2 increase is due to the inhibition by dissolved (HCO
3
−/CO
3
2−), which favors the formation of >MgCO
3
− and >MgHCO
3° at the expense of the rate-controlling >MgOH
2
+ species. Finally, very weak effect of temperature on dissolution rates of all carbonate minerals between 100 and 150 °C in acidic solutions can be explained by the increase of the enthalpy of >CO
3
− protonation reaction. At the conditions of CO
2 storage, the effect of dissolved CO
2 on carbonate mineral reactivity is expected to be of second order importance compared to that of pH and dissolved carbonate/bicarbonate ions. Both high temperature and high partial CO
2 pressure decrease carbonate mineral reactivity in aqueous solution which is likely to help carbon dioxide sequestration in deep carbonate sedimentary basins. |
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ISSN: | 0009-2541 1872-6836 |
DOI: | 10.1016/j.chemgeo.2009.01.013 |