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| Titel |
The effect of sulphate on magnesite growth: in-situ and ex-situ nanoscale observations |
| VerfasserIn |
Helen E. King, Hisao Satoh, Katsuo Tsukamoto, Andrew Putnis |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250076664
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| Zusammenfassung |
| The composition of a solution has important implications for growth mechanisms and the
incorporation of impurities during growth. For example, the presence of sulphate
during CO2 sequestration in Mg-silicate rocks such as ophiolites is expected to
restrict magnesite (MgCO3) growth due to the formation of Mg-sulphate ion pairs in
solution (Pye et al. 1998), which lower the solution supersaturation with respect to
magnesite. Conversely, direct interactions of sulphate with the magnesite surface,
observed during dissolution (King and Putnis, 2013), could limit the negative effects
of Mg-sulphate ion pair formation in solution by aiding growth through assisted
desolvation of the Mg2+ ion (e.g., Piana et al. 2006). Furthermore, if the adsorbed
sulphate is incorporated into the magnesite structure during growth it removes the need
for expensive SO2 flue gas scrubbers as both CO2 and SO2 can be sequestered
simultaneously.
To explore the implications of sulphate in solution for CO2 sequestration we have
observed the growth of cleaved magnesite {104} surfaces in-situ using phase shift
interferometry (PSI), a technique specially designed to monitor ultra-slow growing or
dissolving mineral surfaces. In addition, we have conducted batch experiments in
Teflon-lined steel autoclaves and examined the surfaces ex-situ using atomic force
microscopy (AFM). All experiments were conducted at 90 Ë C in solutions of 0.2 M
NaHCO3 and 0.8 M NaCl or 0.4 M Na2SO4. Supersaturation of the solution was varied by
changing the concentration of either MgCl2 for Cl-rich or MgSO4 in sulphate-rich solutions.
For the PSI experiments a pressure of 1 MPa was used to prevent the formation of bubbles. In
these experiments magnesite was grown in a flowing solution (100 μL/min) for 12 hours,
whereas the batch experiments were conducted for 1-7 days under static conditions. The
in-situ observations from the PSI experiments indicate that the presence of sulphate increased
the rate of obtuse step propagation at etch pits present on the surface prior to the growth
experiment, but that it limits 2D nucleation at the magnesite terraces. This is consistent
with ex-situ observations from the AFM and the production of hydromagnesite
(Mg2(CO3)4(OH)24(H2O)) in Cl-bearing solutions during the batch reactions, identified
using Raman spectroscopy.
Thus although sulphate can aid magnesite growth and possibly be incorporated into the
obtuse step edge, it will have a negative effect during CO2 sequestration into Mg-rich rock
formations as nucleation in these scenarios is the critical step.
King, H.E. and Putnis C.V. (2013) Geochim. Cosmochim. Acta., accepted for
publication.
Piana S., Jones F. and Gale J.D. (2006) J. Am. Chem. Soc., 128, 13568-13574.
Pye C.C. and Rudolph W.W. (1998) J. Phys. Chem. A., 102, 9933-9943. |
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