『Abstract
The rate of dissolution of polycrystalline siderite planes has
been measured at 25, 60, 80 and 100℃ in 0.1 M (NaCl + HCl) solutions
and pH from 1 to 4.6 at far from equilibrium conditions as a function
of CO2 partial pressure. Experiments were
performed in batch titanium high pressure reactor under controlled
hydrodynamic conditions using the rotating disk technique with
stirring speed up to 1000 rpm. Few experiments were also carried
out at 25℃ with siderite powder. Rates R were deduced from measurement
of the amount of Fe released as a function of time using R = (d[Fe]/dt)/S,
where t designates the elapsed time, [Fe] stands for the number
of iron moles released from the solid, and S is the siderite surface
area.
Linear dependencies between logarithm of dissolution rate and
pH values (2≦pH≦4) were obtained, which allow us to determine
the values of the kinetic constant and the order of the dependence
on H+ activity for siderite dissolution rate (R = k+H+・aH+n) for different
temperatures: k+H+ =1.79・10-8 mol・cm-2・s-1,
n = 0.75 at 25℃; k+H+ =6.20・10-7 mol・cm-2・s-1,
n = 0.96 at 60℃; n = 0.94 at 80℃ and k+H+ =3.80・10-6
mol・cm-2・s-1, n = 0.90 at 100℃. Note that
our results could not permit the determination of the rate constant
at 80℃ with a reasonable accuracy.
Apparent activation energy for siderite dissolution (Eapp)
varies from 61 kJ・mol-1 at pH = 2.0 to 48 kJ・mol-1
at pH = 4.0, in good agreement with values recently determined
by Dufaud [Dufaud,F., 2006. Etude experimentale（二番目のeの頭に´）
des reactions（eの頭に´） de carbonatation minerale（最初のeの頭に´） du CO2 dans les
roches basiques et ultrabasiques. Unpublished PhD thesis, IPG,
Paris]. Finally, the effect of CO2 partial
pressure on siderite dissolution kinetics appears to be very weak,
in agreement with previous works on calcite, dolomite and magnesite
[Pokrovsky,O.S., Golubev,S.V., Schott,J., 2005. Dissolution kinetics
of calcite, dolomite and magnesite at 25℃ and 0 to 50 atm pCO2. Chem. Geol. 217, 239-255; Pokrovsky,O.S., Golubev,S.V.,
Schott,J., Castillo,A., 2009. Calcite, dolomite and magnesite
dissolution kinetics in aqueous solutions at acid to circumneutral
pH, 25 to 150℃ and 1 to 55 atm pCO2; new
constraint on CO2 sequestration in sedimentary
basins. Chem. Geol. (this issue)].
Keywords: Siderite; Dissolution; Kinetics; CO2
sequestration』
1. Introduction
2. Theoretical background
3. Experimental methods
3.1. Experimental procedure
3.2. In-situ pH measurements in high-pressure batch reactors
3.3. Analyses
4. Results and discussion
4.1. Effect of pH and temperature on dissolution rates
4.2. Effect of pCO2
5. Conclusions
Acknowledgments
References