 |
| Titel |
pH-dependence of calcite growth kinetics at constant solution calcium to carbonate activity ratio and supersaturation: an in situ Atomic Force Microscopy study |
| VerfasserIn |
Encarnacion Ruiz-Agudo, Christine V. Putnis, Carlos Manuel Rodriguez-Navarro, Andrew Putnis |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250038373
|
|
|
|
|
|
| Zusammenfassung |
| Calcite-solution reactions (growth, dissolution and replacement) are critical in a range of both
engineering and natural processes. Classical crystal growth theory relates calcite growth
rates to the degree of supersaturation. The solution composition may also affect
the growth rate of carbonate minerals, via the Ca2+ to CO32- concentration ratio
(Nehrke et al., 2007; Perdikouri et al., 2009), ionic strength (Zuddas and Mucci,
1998) or the presence of organic matter (Hoch et al., 2000). Most calcite growth
studies so far have been performed at a constant pH of ca. 8 or 10, or changing the
pH together with the degree of supersaturation with respect to calcite and/or the
aCa2+ to aCO32- ratio in solution, which hinders an evaluation of the pH effect on
calcite growth kinetics. In this work, in situ Atomic Force Microscopy (AFM) was
employed to study the growth of calcite at a constant supersaturation (Ω = 6.46) and
solution stoichiometry (Ca2+-CO32- = 1) in the pH range 7.5 to 12. How pH may
influence calcite growth is relevant to improve our understanding of the effects on
carbonate-solution reactions when variations in atmospheric CO2result in changes in
the pH of the oceans and surface waters. We observed that the calcite growth rate
decreases with increasing pH in the studied range. The results can be successfully
explained by the mechanistic model for calcite growth based on surface complexation
proposed by Nilsson and Sternbeck (1999) and by solute hydration. At pH below
8.5, growth occurs mainly by CaCO30 incorporation at >CaHCO30 surface sites.
CaCO30 should be more easily incorporated than free Ca2+ ions, as water exchange
usually is faster if water molecules in the ion hydration shells are substituted for
by other ligands, as in CaCO30. However, at pH above 9, Ca2+ incorporation at
>CaHCO30 sites also contributes to calcite growth as a result of increased frequency of
water exchange in calcium hydration shells due to the presence of strongly hydrated
OH-. The decrease in calcite growth rate is a consequence of decreasing surface
concentration of growth active sites (i.e. >CaHCO30) with increasing pH in our
experimental conditions. Changes in 2D island morphology were observed at high pH (12),
possibly due to the stabilization of polar scalenohedral faces by the presence of OH-
ions.
References
Hoch, A.R.; Reddy, M.M.; Aiken, G.R. Calcite crystal growth inhibition by humic
substances with emphasis on hydrophobic acids from the Florida Everglades. Geochim.
Cosmochim. Acta 2000, 64, 61-72.
Nehrke, G.; Reichart, G. J.; Van Cappellen, P.; Meile, C.; Bijma, J. Dependence of calcite
growth rate and Sr partitioning on solution stoichiometry: Non-Kossel crystal growth.
Geochim. Cosmochim. Acta 2007, 71, 2240–2249
Nilsson O.; Sternbeck J. A mechanistic model for calcite crystal growth using surface
speciation. Geochim. Cosmochim.Acta 1999, 63, 217–225.
Perdikouri, C.; Putnis, C.V.; Kasioptas, A.; Putnis, A. An Atomic Force Microscopy
Study of the Growth of a Calcite Surface as a Function of Calcium/Total Carbonate
Concentration Ratio in Solution at Constant Supersaturation. Cryst. Growth Des. 2009, 9,
4344-4350.
Zuddas, P.; Mucci, A. Kinetics of Calcite Precipitation from Seawater: II. The
Influence of the Ionic Strength. Geochim. Cosmochim. Acta 1998, 62, 757-766. |
|
|
|
|
|
|