[American Journal of Science, Vol. 301, September, 2001, P. 597– 626]
American Journal of Science
SEPTEMBER 2001
KINETICS AND MECHANISM OF DOLOMITE DISSOLUTION IN
NEUTRAL TO ALKALINE SOLUTIONS REVISITED
OLEG S. POKROVSKY AND JACQUES SCHOTT
Géochimie: Tranferts et Mécanismes, CNRS (UMR 5563)-OMP-Université
Paul-Sabatier, 38, rue des Trente-Six Ponts, 31400 Toulouse, France
ABSTRACT. Steady-state dissolution rates of dolomite were measured at 25°C in a
mixed-flow reactor as a function of pH (from 5-12), ionic strength (0.002 < I < 0.1 M),
total dissolved carbonate (10ⴚ5 < ⌺CO2 < 0.1 M), calcium (10ⴚ6-0.003 M), magnesium (3 䡠 10ⴚ7-0.005 M), and inorganic (sulfate) and organic (acetate, ascorbate,
formiate, tartrate, oxalate, citrate, and EDTA) ligands concentration. Dissolution rates
were found to be pH-independent at 6 < pH < 8 and to decrease with increasing pH at
pH > 8 and ⌺CO2 > 10ⴚ3 M. In the alkaline pH region, carbonate and bicarbonate
ions significantly inhibit dissolution rates at far from equilibrium conditions. Dissolved
Ca was found to be a strong inhibitor of dolomite dissolution at pH above 7, whereas
dissolved Mg has no effect on the dissolution rate. The surface complexation model
developed by Pokrovsky, Schott, and Thomas (1999b) was used to correlate dolomite
dissolution kinetics with its surface speciation. At the conditions of this study (5 <
pH < 12), dissolution is controlled by the hydration of Mg surface sites and formation
of >MgOH2ⴙ species. This Ca and CO3-free surface precursor complex allows us to
account for the inhibiting effect of aqueous calcium and carbonate ions on dolomite
dissolution. Based on these results and those of Pokrovsky, Schott, and Thomas
(1999b), the following rate equation, consistent with transition state theory, was used
to describe dolomite dissolution kinetics over the full range of solution composition:
R ⴝ [k CO3 䡠 {>CO3H°}2.0 ⴙ kMg 䡠 {>MgOH2ⴙ}1.9] 䡠 (1 ⴚ exp(ⴚ1.9A/RT))
or, alternatively, at pH above 6 and I ⴝ 0.1 M,
R ⴝ k *Mg 䡠
冎 冋 冉 冊册
再
K*CO3 䡠 K*Ca
K*CO3 䡠 K*Ca ⴙ K*Ca 䡠 aCO32⫺ ⴙ aCO32⫺ 䡠 aCa2ⴙ
1.9
䡠 1ⴚ
Q
0
Ksp
1.9
where {>i} stands for surface species concentration (mol/m2), A refers to the
chemical affinity of the overall reaction, kCO3, kMg, k*Mg, K*CO3, K*Ca are constants, and
0
(Q/Ksp
) stands for dolomite saturation index. This equation reflects the formation of
two different precursor complexes that contain two protonated >CO3H° and two
hydrated >MgOH2ⴙ groups in acid and in neutral and alkaline solutions, respectively.
Crystallization of dolomite was found to occur in highly supersaturated solutions as
confirmed by outlet solutions analysis and SEM observation of reacted grains. Very low
dolomite crystallization rates (that is, ⬃10ⴚ16 mol/cm2/s) are consistent with those
observed in natural conditions and predicted by the empirical model of Arvidson and
Mackenzie (1997). Dolomite dissolution rate is promoted by the addition of inorganic
and organic ligands with the following effectiveness: sulfate ⬇ formiate ⬇ tartrate <
acetate < ascorbate < oxalate < citrate Ⰶ EDTA. The effect of these ligands can be
modeled within the framework of the surface coordination theory.
597
598
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
introduction
This work represents part of concerted efforts aimed at characterizing the
reactivity of dolomite in surficial aquatic environments. Earlier studies from the same
laboratory (Gautelier, Oelkers, and Schott, 1999; Pokrovsky, Schott, and Thomas,
1999b) investigated the mechanism of dolomite dissolution in acid solutions and
presented a preliminary model for the surface speciation control of dolomite reactivity
in acid to alkaline carbonate-bearing solutions. Although numerous kinetic studies
have already been devoted to dolomite dissolution mechanisms (Lund, Fogler, and
McCune, 1973; Busenberg and Plummer, 1982; Sverdrup and Bjerle, 1982; Herman
and White, 1984; Chou, Garrels, and Wollast, 1989; Anderson, 1991; Orton and Unwin,
1993), few attempts have been made to correlate carbonate minerals dissolution
kinetics to their surface speciation in aqueous solutions. Only Van Cappellen and
others (1993) and Arakaki and Mucci (1995) used surface complexation models to
interpret calcite dissolution rates. Recently, Sternbeck (1997) and Nillson and Sternbeck (1999) applied Van Cappellen and others’ (1993) surface complexation model to
describe rhodochrosite and calcite crystal growth.
The Mg-bearing carbonates surface speciation model of Pokrovsky, Schott, and
Thomas (1999a,b) provides new tools to interpret magnesite and dolomite dissolution/
precipitation kinetics and sorption behavior. It allows us to revisit the elementary
mechanisms that govern dolomite reactivity in low-temperature aquatic systems. In this
study, the results of dolomite dissolution rates measured at 25°C over a wide range of
pH, ionic strength, and aqueous, solution composition as well as those reported earlier
by Pokrovsky, Schott, and Thomas (1999b) were depicted using the dolomite/solution
interface speciation model. This approach provides new insights into the mechanisms
that control dolomite dissolution kinetics over a wide range of chemical affinity and
solution composition.
theoretical considerations
It is generally believed that, like calcite, dolomite dissolution can be described by
three parallel reactions occurring at the solid/water interface (Plummer, Wigley, and
Parkhurst, 1978; Chou, Garrels, and Wollast, 1989; Wollast, 1990):
k1
CaMg共CO 3 兲 2 ⫹ 2H ⫹ O
¡ Mg 2⫹ ⫹ Ca 2⫹ ⫹ 2HCO 3⫺
(1)
k2
¡ Mg 2⫹ ⫹ Ca 2⫹ ⫹ 4HCO 3⫺
CaMg共CO 3 兲 2 ⫹ 2H 2 CO *3 O
(2)
k3
CaMg共CO 3 兲 2 ¢
O
¡ Mg 2⫹ ⫹ Ca 2⫹ ⫹ 2CO 32⫺
(3)
k ⫺3
Assuming dolomite crystallization is dominated by reaction 3, its overall dissolution
rate (R) can be expressed as:
R ⫽ k 1 䡠 a Hn ⫹ ⫹ k 2 䡠 a Hp 2CO3* ⫹ k 3 ⫺ k ⫺3 䡠 a Mg 2⫹ 䡠 a Ca 2⫹ 䡠 a CO 32⫺
2
(4)
where ki are the rate constants of reactions 1 to 3, and ai stands for the activity of the
subscribed aqueous species. The exponent n varies from 0.5 (Busenberg and Plummer, 1982) to 0.75 (Chou, Garrels, and Wollast, 1989) whereas p was found to be equal
to 1 (Busenberg and Plummer, 1982). Within this mechanistic scheme, the first term in
eq 4 corresponds to dolomite surface protonation, the second to its carbonatation, the
in neutral to alkaline solutions revisited
599
third to surface hydration, and the fourth term accounts for the precipitation reaction.
Note, however, that the rate dependence on the activity of H⫹ and H2CO3 does not
correspond to the stoichiometry of reaction 1 and 2, respectively, suggesting more
complex reactions occur at dolomite surface.
Carbonate dissolution and crystallization rates can also be modeled in terms of
surface complexes by considering transition state theory (TST). According to TST, the
overall rate (R) of a mineral dissolution reaction per unit surface area can be described
using (Lasaga, 1981; Oelkers, Schott, and Devidal, 1994; Schott and Oelkers, 1995;
Oelkers and Schott, 1995)
R ⫽ R ⫹ ⫺ R ⫺ ⫽ R ⫹ 䡠 关1 ⫺ exp共⫺A/␴RT兲兴
(5)
where R⫹ and R⫺ designate specific forward and backward dissolution rates, respectively, R refers to the gas constant, T designates the absolute temperature, and ␴ stands
for Temkin’s average stoichiometric number equal to the ratio of the rate of activated
or precursor complex destruction relative to the overall dissolution rate. A denotes the
chemical affinity for the overall hydrolysis reaction (that is, reaction 3 for dolomite
dissolution) given by
A ⫽ ⫺RT ln共Q/K°sp兲
(6)
where Q designates the ion activity quotient, and K°sp stands for dolomite thermodynamic solubility product.
The forward reaction rate, R⫹, is equal to the product of two factors, the
concentration of a rate-controlling surface complex, sometimes referred to as precursor complex (P#), and the rate of destruction of this precursor to form reaction
products (Wieland, Wehrli, and Stumm, 1988; Stumm and Wieland, 1990; Oelkers,
Schott, and Devidal, 1994). This concept is consistent with
R ⫹ ⫽ k P #关P # 兴
(7)
where kP# refers to a rate constant compatible with the P# precursor complex, and [P#]
stands for its surface concentration. Expressing the law of mass action for the creation
of this complex yields:
关P 兴 ⫽ K
#
#
写
i
a ni i
␥#
(8)
(where ␥# denotes the activity coefficient of the precursor complex, ni signifies the
stoichiometric coefficient of the ith species involved in the precursor complex forming
reaction, ai stands for the activity of precursor-forming species, and K# symbolizes the
equilibrium constant for the precursor complex formation reaction).
Because the overall dissolution reaction is equal to the sum of the rates of each
parallel elementary reaction, eq (5) transforms into
R⫽
冘 冉k 写 a
j
P#
j
j
n ij
i
冊
⫻ 关1 ⫺ exp共⫺A/␴jRT兲兴
(9)
where kPj # is the forward rate constant of the jth parallel reaction. Application of eq (9)
to dolomite dissolution requires knowledge of the rate limiting steps in the reaction
mechanisms and/or the precursor complexes compositions and formation reactions.
Such information can be deduced by comparing dolomite dissolution rates with its
surface speciation. Indeed, Furrer and Stumm (1986), Wieland, Wehrli, and Stumm
(1988), and Stumm and Wieland (1990) demonstrated that mineral dissolution rates
600
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Table 1
Surface complexation reactions and their intrinsic stability constants at the dolomite-solution
interface (Pokrovsky, Schott, and Thomas, 1999b)
can be related to the concentration of ligand or protonated complexes present at
oxide and silicate mineral surfaces.
The development of carbonate minerals surface speciation models (Van Cappellen and others, 1993; Pokrovsky, Schott, and Thomas, 1999a, b; Pokrovsky and
others, 2000) allows analysis of the dissolution kinetics of these minerals within the
framework of surface coordination theory. The surface complexation model used in
this study assumes the existence of two primary hydration sites, ⬎MeOH°, where Me ⫽
Mg, Ca, and ⬎CO3H°. It is based on independent measurements of dolomite surface
chemistry including surface titrations, electrokinetic measurements, and is consistent
with spectroscopic surface analyses of carbonates (Pokrovsky, Schott, and Thomas,
1999; Pokrovsky and others, 2000; Fenter and others, 2000). Equilibrium at the
dolomite/aqueous solution interface is assumed to be governed by the surface
reactions listed in table 1 together with their intrinsic stability constants. The concentration of the various surface complexes can be calculated as a function of aqueous
solution composition using this set of surface stability constants. The identity of those
species that control dolomite dissolution (that is, the precursor complex) is determined in
the present study by comparing these surface compositions with dolomite dissolution
rates obtained as a function of pH, solution composition, and chemical affinity.
experimental methods
A natural polycrystalline dolomite (Cap de Bouc, Aude, France) was used in this
study. Dolomite obtained from this locality was also used for surface titrations and
electrokinetic measurements by Pokrovsky, Schott, and Thomas (1999b). ICP-MS and
XRD analyses reveal that this dolomite contains less than 0.5 percent impurities.
Crystals ⬃1 cm in size were hand-picked, gently grinded with an agate mortar and
pestle, and sieved. The size fraction between 50 and 100 ␮m was reacted several
seconds in 1 percent HCl, ultrasonically cleaned using alcohol to remove fine particles,
rinsed repeatedly with distilled water, and dried overnight at 60°C. The specific surface
area of this cleaned powder was 1050 ⫾ 100 cm2/g as determined by krypton
absorption using the B.E.T. method.
Steady-state dissolution rates were obtained at distinct solution compositions and
saturation states using a mixed-flow reaction vessel immersed in a water bath held at
constant temperature of 25.0 ⫾ 0.2°C. The input fluid was stored in a compressible
in neutral to alkaline solutions revisited
601
Fig. 1. Concentration of Ca and Mg in outlet solutions as a function of time during experiment N° 27
(T ⫽ 25°C).
polyethylene container during the experiments. It was injected into the reactor using a
Gilson peristaltic pump that allows flow rates ranging from 0.05 to 10 mL/min.
Dolomite dissolution occurred in a 250 mL Azlon plastic beaker which was continuously stirred with a floating teflon magnetic stirrer. Stirring was controlled by a stirplate
located directly beneath the bath. The solution left the reactor through a 1 ␮m Teflon
filter. A combined pH-electrode could be fixed into the reactor cover to enable in-situ
pH measurements. The saturation state and fluid composition can be regulated by
either changing the flow rate or the composition of the inlet solution without
dismantling the reactor and/or changing the amount of mineral present during the
experiment.
Between 0.5 and 5 g of dolomite was allowed to react in fluids of prescribed input
compositions. Varying the mass of reacting dolomite by a factor of 3 did not change the
normalized dissolution rate. Steady-state dissolution rates, as indicated by constant
output Mg and Ca concentration, were obtained after 8 hrs to 5 days, depending on
flow rate values. An example of steady state attainment is shown in figure 1 where
outlet Mg and Ca concentration is plotted as a function of time. It can be seen that
within the uncertainties of measurements (⫾10 percent), outlet Mg and Ca concentrations remain constant during the course of the 30 h experiment. This demonstrates
602
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
that the times necessary to reach mechanical and chemical steady-states are indistinguishable.
Reacting fluids were comprized of deionized degassed H2O, Merck reagent grade
HCl, NaOH, NaHCO3, Na2CO3, NaCl, Na2SO4, CaCl2, MgCl2, and organic acids. In a
mixed-flow reactor, it is not possible to fix H2CO*3 activity or pCO2. This contrasts with
the pH-stat method used by most investigators where pCO2 is controlled by bubbling
continuously a gas of fixed pCO2 through the reactor. However, the pCO2 in the
reactor may be deduced from steady-state outlet solution alkalinity and pH. In most
experiments, the pCO2 was below 10⫺3 atm. Solution compositions used in the present
study are listed in app. A and B.
All input and output solutions were analyzed for magnesium ([Mg2⫹]t), calcium
([Ca2⫹]t), alkalinity ([Alk]), and pH as a function of time. Magnesium and calcium
were measured by flame atomic absorption with an uncertainty of ⫾2 percent and a
detection limit of 4 䡠 10⫺8 and 7 䡠 10⫺8 M, respectively. Alkalinity was determined
following a standard HCl titration procedure with an uncertainty of ⫾1 percent and a
detection limit of 5 䡠 10⫺5 M. The output solution pH was measured at 25°C
immediately after sampling, using a Schott (N62) combined electrode, with an
accuracy of ⫾0.002 units. Measured outlet fluid pH was always within 0.1 pH unit of
corresponding in-situ values determined using an electrode inserted into the reactor.
All outlet solutions were undersaturated with respect to all minerals other than
dolomite.
Freshly ground and reacted dolomite powders were analyzed using an ESCALAB
VG 220i-XL X-ray Photoelectron Spectrometer (XPS) to determine the Mg/Ca ratios
at the dolomite surface. This technique allows us to probe the first 80 to 100 Å of the
mineral surface. Non monochromatic twin Al K␣ X-rays (h␯ ⫽ 1486.6 eV) were used as
the excitation source at a power of 200 W. An analyzer pass energy of 150 eV with a step
size of 1 eV was used for survey scans; for the regional (narrow) scans, a 20 eV analyzer
pass energy with a step size of 0.1 eV was used. The relative abundances of elements at
the dolomite surface were obtained from measured peak areas and Scofield sensitivity
factors (Scofield, 1976) for Ca2s, Mg1s, C1s, and O1s.
Homogeneous solution equilibria as well as surface speciation and chemical
affinities were calculated for each solution composition using the MINTEQA2 code
(Allison, Brown, and Novo-Gradac, 1991) together with surface reactions stability
constants given in table 1. The activity coefficients of free aqueous ions and charged
complexes were calculated using the Davies equation. The activity coefficients of
surface species were set equal to 1. The standard state for the aqueous species is the
hypothetical 1 molal solution whose behavior is ideal. The standard state chosen for
surface species is a concentration of 1 molal for the adsorbed species and zero surface
potential (Sverjensky and Sahai, 1996). The value of dolomite solubility product used
in this study is K°sp ⫽ 10⫺17.0 (MINTEQA2 database; Langmuir, 1965, 1971; Berner,
1967; Lippman, 1973). This value is in agreement with more recent determinations
based on calorimetric (K°sp ⫽ 10⫺17.09; Robie, Hemingway, and Fisher, 1978) and
solubility (K°sp ⫽ 10⫺17.38, Konigsberger and Gamsjäger (1987) measurements. Note
the recent determination at 80°C of the solubility of the dolomite used in this study
(Gautelier, ms) yields a value of K°sp ⫽ 10⫺16.8 at 25°C.
results and discussion
Results of 130 steady-state dissolution experiments performed at ionic strengths
from 0.002 to 0.5 M are presented in app. A and B. For neutral and alkaline
carbonate-rich solutions, the data are sorted by decreasing dissolution rates. Included
in these tables are reacting solid surface area, fluid flow rates, inlet fluid compositions,
outlet fluid pH, [Alk], [Mg2⫹]tot, [Ca2⫹]tot, and chemical affinities, concentrations of
in neutral to alkaline solutions revisited
603
surface species at the dolomite-solution interface, and steady-state dolomite dissolution rates.
Steady-state dissolution rates (R, mol/cm2/s) given in app. A and B were computed from measured solution composition using
R ⫽ ⫺q 䡠 ⌬关Me2⫹兴tot/s
(10)
where q (L/sec) designates the fluid flow rate, Me ⫽ Ca, Mg, ⌬[Me2⫹]tot (mol/L)
stands to the difference between the input and output Me solution concentration, s
(cm2) refers to the total mineral surface area. The surface area used to calculate the
rates listed in app. A and B was that measured on the fresh (unreacted) dolomite
powder. The lowest measurable rates were about 10⫺16 mol/cm2/s which originated
from the analytical detection limits and lowest Mg and Ca concentration in blank
experiments.
Uncertainties on the steady-state rate constants given in app. A and B are 10 to 15
percent and are dominated by the uncertainty on BET surface area measurements
(⫾10 percent) and the standard deviation of average Mg and Ca concentration at
steady-state (⫾5 percent). Repeated runs performed in solutions of similar composition indicate that dissolution rates do not vary by more than ⬃10 percent after elapsed
time of greater than 150 hrs. Moreover, dolomite specific surface area decreased by less
than 10 percent during dissolution experiments lasting more than 150 hrs as measured
by the B.E.T. method. The uncertainties in the computed surface species concentrations are dominated by the reproducibility of outlet pH and [Alk] concentrations
leading to an average error of ⫾0.01 log units. The absolute uncertainties on these
surface species concentrations are substantially larger, however, owing to the uncertainties on the intrinsic surface stability constants (⫾0.15 log K°int units) reported by
Pokrovsky, Schott, and Thomas (1999b). The uncertainties in the computed chemical
affinities are difficult to assess due to the large number of equilibrium constants upon
which they depend (dolomite K°sp, association constants of MeCO°3, MeHCO3⫹, NaCO3⫺
ion pairs, pK1, and pK2 dissociation constants of carbonic acid, activity coefficients
calculation) but are estimated to be ⫾2 kJ/mol.
Effect of dissolved calcium and magnesium.—In the first part of this study (Pokrovsky,
Schott, and Thomas, 1999b), it was shown that dolomite dissolution rate correlates
with the total concentration of hydrated metal centers (⬎CaOH2⫹ ⫹ ⬎MgOH2⫹). In
order to check if the rate is controlled by ⬎MgOH2⫹, ⬎CaOH2⫹, or by both surface
centers, a series of experiments with different amount of aqueous Ca and Mg in
reacting solutions has been performed (app. A). The effect of dissolved Ca2 and Mg2⫹
on the dolomite dissolution rates at pH above 7 is depicted in figure 2. It can be seen
that, within the uncertainty of experiments, dissolved magnesium has no effect on
dolomite dissolution rate whereas dissolved Ca significantly inhibits dissolution at
aCa2⫹ ⬎ 10⫺4.5. Aqueous calcium inhibition implies that the surface precursor that
controls dolomite dissolution does not contain calcium, and its formation first requires
the removal of a calcium atom from the surface. This is also confirmed by a
preferential Ca release relative to Mg at the initial stage of dissolution of fresh
(unreacted) powder as illustrated in figure 3. It can be seen from this figure that the
Ca/Mg ratio in the first outlet solutions can be as high as 1.35. Such initial fast Ca
release may be understood in view of much lower hydration energy of Ca versus Mg
and thus its lower stability at the mineral/water interface. The approximated thickness
of the resulting calcium-free surface layer does not exceed 1 molecular layer thus
making its identification by modern surface spectroscopy techniques very uncertain.
Indeed, the surface Mg/Ca atomic ratio stays constant for experiments in Ca or
Mg-rich solutions at pH from 7 to 10 and 10⫺5 ⱕ ⌺CO2 ⱕ 0.02 M as measured by XPS
in this study. It is worth noting that preferential calcium release from fresh dolomite
604
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 2. The dissolution rate of dolomite as a function of aqueous calcium (open symbols) and
magnesium (closed symbols) activities. The decrease of the dissolution rate with increasing aCa2⫹ implies that
the rate controlling surface precursor does not contain Ca. The solid (pH ⫽ 9) and dashed (pH ⫽ 11) lines
are calculated using the model (eq 15) generated in this study.
surfaces was first reported by Busenberg and Plummer (1982). Moreover, the same
authors showed an inhibiting effect of Ca on dolomite dissolution rates at 45°C and pH
around 4, in contrast to the absence of any influence of Mg up to 0.05 M at the same
conditions. This finding is in complete agreement with the results of the present study,
although the inhibiting effect of Ca is much stronger at higher pH due to the
importance of surface metal hydration at these conditions. Preferential loss of Ca over
Mg in the course of dolomite dissolution was also shown by Paquette, Vali, and
Mountjoy (1994) in their transmission electron microscopy (TEM) study of dolomite
cement crystals. These results are in contrast with those of atomistic simulations of
dolomite surfaces (Titiloye, de Leew, and Parker, 1998) that show a preference of Ca
over Mg ions for the surface that should lead to the formation of a calcium-rich layer.
However, Titiloye, de Leew, and Parker (1998) did not take into account the different
hydration energies of Ca2⫹ and Mg2⫹ ions on the dolomite surface which may be
in neutral to alkaline solutions revisited
605
Fig. 3. Temporal evolution of Ca/Mg ratio in outlet solutions for experiments performed at pH ⫽ 7.3
and 9. The preferential Ca release at the beginning of the dissolution of the fresh powder indicates the
formation of a Mg-rich surface layer. Its thickness does not exceed 1 molecular layer as calculated from the
integration of measured Ca and Mg concentration dependence on time.
responsible for the different composition of dolomite cleavage surface in the vacuum
and in aqueous solution.
Dissolution rates as a function of surface speciation.—The total number of surface sites
was approximated assuming a 1:1:2 stoichiometry between calcium, magnesium, and
carbonate sites exposed at the dolomite surface. The site density was assumed to be 7
␮mol/m2 for Ca and Mg and 14 ␮mol/m2 for carbonate as inferred from surface
titration data (Pokrovsky, Schott, and Thomas, 1999b). Dolomite surface speciation
modeling shows that in solutions at pH below 4, the protonated species ⬎CO3H°
dominates surface speciation of carbonate groups. As pH increases, deprotonation of
surface carbonates occurs, and ⬎CO3⫺ becomes dominant. The speciation at metal
sites, which is generally dominated by ⬎MeOH2⫹ and ⬎MeCO3⫺ species, depends on
⫺
pH and dissolved carbonate concentration; ⬎MeOH⫹
2 species are replaced by ⬎MeCO3
at pH ⬎ 8 in solutions in equilibrium with atmospheric CO2.
It has been shown that the dissolution of carbonate minerals at acid conditions is
controlled by the protonation of surface carbonate groups as described by the first
term of eq (4) (Van Cappellen and others, 1993; Pokrovsky and Schott, 1999;
Pokrovsky, Schott, and Thomas, 1999b). At neutral to alkaline conditions, as for
oxides, it is the hydration of surface metal sites that controls dolomite dissolution as
described by the third term on the right hand side of eq (4). Dolomite surface
speciation at pH ⬎ 5 together with results presented in the previous section suggest
that ⬎MgOH2⫹ is the surface complex most likely to control dolomite dissolution at
606
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 4. Dolomite dissolution rate at 25°C, pH of 6 to 12, and ⌺CO2 of 10⫺5 to 0.1 M as a function of
{⬎MgOH2⫹}. The slope close to 2 shows that the dolomite dissolution requires the hydration of two surface
magnesium attached to a surface carbonate site.
these conditions. Indeed, a linear correlation (Rsq ⫽ 0.966) is observed between
log RH2O and log {⬎MgOH2⫹} in figure 4, consistent with:
⫹
R H 2O ⫽ k Mg
䡠 兵⬎MgOH2⫹其1.9
(11)
⫹
⫽ (1.12 ⫾ 0.1) ⫻ 105 mol⫺1 䡠 cm2 䡠 s⫺1. Eq (11) suggests that dolomite
where kMg
dissolution requires the hydration of two surface magnesium surrounding a surface
carbonate site. The second order dependence of dolomite dissolution rates on both
{⬎CO3H°} (Pokrovsky, Schott, and Thomas, 1999b) and {⬎MgOH2⫹} (this study) is not
fortuitous but suggests that metal detachment always requires protonation (hydration)
of two adjoining surface sites.
607
in neutral to alkaline solutions revisited
Construction and application of a general rate equation for dolomite dissolution/
precipitation at neutral to alkaline conditions.—A general model of dolomite dissolution/
precipitation kinetics can be obtained within the framework of TST (eqs 5 and 9) using
the empirical equations describing its H2O-promoted dissolution in neutral to alkaline
solutions (eq 11). The first step of dolomite dissolution is the preferential Ca release
with formation of Mg surface sites:
⬎Ca共Mg兲CO3⫹ ⫽ ⬎MgCO3⫺ ⫹ Ca2⫹,
K*Ca
(12)
In this reaction, all dolomite initial surface metal sites (⬎Ca⫹ ⫹ ⬎Mg⫹) are depicted
as ⬎Ca(Mg)CO3⫹. The second and rate controlling step is the hydration of exposed Mg
surface sites which is the same as that governing magnesite dissolution (Pokrovsky and
Schott, 1999):
⬎MgCO3⫺ ⫹ H2O ⫽ ⬎MgOH2⫹ ⫹ CO32⫺,
K*CO3
(13)
This equation accounts for the inhibition of forward dolomite dissolution rate by
aqueous CO32⫺ ions, which was shown earlier (Pokrovsky, Schott, and Thomas, 1999b)
and is illustrated in figure 5. Metal sites conservation requires
兵⬎CaMgCO3⫹其 ⫹ 兵⬎MgCO3⫺其 ⫹ 兵⬎MgOH2⫹其 ⫽ const ⫽ 7 䡠 10⫺6 mol/m2
(14)
Eq (14) may be combined with eqs (11, 12, and 13) to express dolomite H2Opromoted forward dissolution rate
R H 2O ⫽ k *Mg 䡠
再
K *CO 3 䡠 K *Ca
*
*
K CO 3 䡠 K Ca ⫹ K *Ca 䡠 a CO 32⫺ ⫹ a CO 32⫺ 䡠 a Ca 2⫹
冎
1.9
(15)
where K*Ca and K*CO3 stand for the equilibrium constants of reactions (12) and (13),
respectively. In this equation, the contribution of the electrostatic term on the
calculation of {⬎MgOH2⫹} is neglected. As a result, eq (15) can be used to describe the
experimental data obtained at constant ionic strength only. This equation implies that
both Ca2⫹ and CO32⫺ act as inhibitors of dolomite dissolution rate at far from
equilibrium conditions. The extent to which eq (15) can be used to predict the effect
of calcium and carbonate ions on dolomite dissolution rate is illustrated in figures 2
and 5, respectively, where the lines represent a fit of experimental data to eq (15)
assuming k*Mg ⫽ (6.3 ⫾ 1.3) 䡠 10⫺13 mol/cm2/s, K*Ca ⫽ (3.5 ⫾ 0.5) 䡠 10⫺5, and K*CO3 ⫽
(4.5 ⫾ 0.5) 䡠 10⫺5. The uncertainties attributed to these values correspond to the range
of best fits obtained by varying the values of k*Mg, K*Ca, and K*CO3. The close correspondence between the curves and most experimental data in a wide range of pH, ⌺CO2,
and activity of dissolved Ca and Mg supports the hypothesis that dolomite dissolution
rate is controlled by the hydrolysis of magnesium sites with the liberation of Ca2⫹ and
CO32⫺ from the surface. The low value of K*Ca is consistent with the formation of a thin
Ca-depleted layer at the dolomite surface. As a result, such an altered layer is not
detectable by available spectroscopic techniques.
An overall rate equation for dolomite dissolution/precipitation at neutral to
alkaline conditions can be generated within the framework of TST, if it is noted that
rate-controlling reversible dissolution/precipitation reactions occur only on Mg surface sites. Hydration of n magnesium sites leads to the precursor complex [n ⬎
MgOH2⫹]# formation according to
n ⬎ CaMgCO 3⫹ ⫹ nH 2 O ⫽ 关n ⬎ MgOH 2⫹ 兴 # ⫹ nCa 2⫹ ⫹ nCO 32⫺
(16A)
n ⬎ CaMgHCO 32⫹ ⫹ nH 2 O ⫽ 关n ⬎ MgOH 2⫹ 兴 # ⫹ nCa 2⫹ ⫹ nHCO 3⫺
(16B)
n ⬎ CaMgOH 2⫹ ⫹ nH 2 O ⫽ 关n ⬎ MgOH 2⫹ 兴 # ⫹ nCa 2⫹ ⫹ nOH ⫺
(16C)
608
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 5. Dolomite dissolution rates at 25°C as a function of CO32⫺ activity for I ⫽ 0.1 M and pH ⫽ 6 –11.5.
The symbols represent experimental data, but the solid line is generated using eq 15.
For the sake of simplicity, it is assumed that all metal sites contain either ⬎CaMgCO3⫹
or ⬎MgOH2⫹ and the percentage of ⬎ MgHCO°3 and ⬎MgOH° sites is negligible.
According to eq (7), one can write
⫺n
⫺n
⫹
⫹
#
R ⫹ ⫽ k Mg
兵⬎MgOH2⫹其n ⫽ kMg
䡠 K⫹
兵⬎CaMgCO3⫹其naCO32⫺ 䡠 aCa
⫹⫹
(17)
where K#⫹ is the equilibrium constant of reaction 16A.
Assuming that close to equilibrium (⫺2 ⱕ A ⱕ 2 kJ/mol) the surface precursor complex, [n ⬎ MgOH2⫹]#, is the same for dolomite dissolution and
precipitation, the precursor complex reaction formation for precipitation can be
expressed as:
609
in neutral to alkaline solutions revisited
n ⬎ CaMgCO 3⫹ ⫹ nMg 2⫹ ⫹ nCO 32⫺ ⫹ nH 2 O
⫽ 关n ⬎ MgOH 2⫹ 兴 # ⫹ nCaMg共CO 3 兲 2 共solid兲
(18)
which leads to the following expression for dolomite precipitation rate (R⫺):
n
⫺
⫺
#
2⫺
R ⫺ ⫽ k Mg
兵⬎MgOH2⫹其n ⫽ kMg
䡠 K⫺
兵⬎CaMgCO3⫹其naHn 2OaMg
2⫹ 䡠 a
CO3
n
(19)
where K#⫺ is the equilibrium constant of reaction 18. Combining eqs (9, 11, 17, and 19)
yields the following expression for dolomite overall reaction rate in alkaline and
carbonate-bearing solutions
冋 冉 冊册
⫹
䡠 兵⬎MgOH2⫹其n 䡠 1 ⫺
R ⫽ k Mg
Q
K0sp
n
(20)
where n ⫽ 1.9. Substituting {⬎MgOH2⫹} by its expression deduced from reactions 12
and 13 leads to the following alternative expression, consistent with TST, for dolomite
overall reaction:
R ⫽ k *Mg 䡠
再
K *CO 3 䡠 K *Ca
K *CO 3 䡠 K *Ca ⫹ K *Ca 䡠 a CO 32⫺ ⫹ a CO 32⫺ 䡠 a Ca 2⫹
冎 冋 冉 冊册
1.9
䡠 1⫺
Q
K 0sp
1.9
(21)
Eqs (20 and 21) allow description of dolomite dissolution rate in neutral to alkaline
solutions over a broad range of pH, aCa2⫹, and aCO32⫺. Their first terms describe the
H2O-promoted dissolution or hydration of surface Mg centers. They are equivalent to
the third term in eq (4). It has been advocated that the dissolution of dolomite is
controlled by proton or H2O attack on the MgCO3 component of the solid (Busenberg
and Plummer, 1982). This is consistent with the model developed in this study where
the surface precursor complex (⬎MgOH2⫹) does not contain Ca. The inhibiting effect
of HCO3⫺ and CO32⫺ at far from equilibrium conditions, earlier recognized by Busenberg and Plummer (1982), is explicitly accounted for by the second term in eq (21).
Note that the reverse precipitation reaction at close to the equilibrium conditions
described by the fourth term of eq (4) is accounted for in the present study by the [1 ⫺
0 1.9
(Q/Ksp
) ] term.
Effect of chemical affinity on dolomite dissolution and crystallization.—Dolomite overall
dissolution and crystallization rates are plotted as a function of chemical affinity at
⌺CO2 ⫽ 0.01 and I ⫽ 0.1 M in figure 6. It can be seen that the dissolution rate decrease
starts at A ⬃ 25 kJ/mol, a value much higher than that predicted by eq (5) (A ⬃ 2
kJ/mol), which suggests that this decrease is not related to the reverse reaction
(dolomite precipitation) but to the effect of aqueous carbonate species on the forward
reaction. This was shown earlier for magnesite (Pokrovsky and Schott, 1999). The
effect of chemical affinity on dolomite reaction rates at close to equilibrium conditions
in a wide range of carbonate concentrations (0.05 ⱕ ⌺CO2 ⱕ 10⫺5 M) is magnified in
figure 7. In this figure, the solid lines labeled 1 through 3 correspond to the
predictions of eq (20) for ⌺CO2 ranging from 10⫺5 to 0.05 M. The excellent
agreement between experimental dolomite dissolution rates and the model predictions at close to equilibrium conditions in the absence of added dissolved carbonate
(solid circles and line 3 in fig. 7) is noticeable. Note that TST prediction of dolomite
crystallization rates in highly supersaturated solutions 共Q/K°sp ⱖ 2.5) is not possible as
the same precursor complex is not likely to control both dissolution and precipitation.
A study of dolomite crystallization has been performed in Mg- and carbonate-rich
solutions supersaturated only with respect to dolomite or magnesite. As magnesite cannot
be precipitated at 25°C (Lippmann, 1973; Pokrovsky and Schott, 1999), dolomite rate of
precipitation was computed from the difference between the inlet and outlet Ca concentra-
610
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 6. Dolomite overall reaction rate at 25°C as a function of chemical affinity at ⌺CO2 ⫽ 0.01 M and
I ⫽ 0.1 M.
tion. SEM photomicrographs of grains reacted in solutions supersaturated with respect to
dolomite show the formation of triangular growth steps as a result of syntaxial growth on
the 1014 crystallographic planes (fig. 8D and E). These steps are not dissolution features as
they were not found in samples reacted in alkaline solutions undersaturated with respect to
dolomite (fig. 8B). Note that the formation of similar terrace steps was reported during
calcite layer growth at ambient temperature (Dove and Hochella, 1993) and syntaxial
growth of dolomite at 180°C (Arvidson and Mackenzie, 1999).
From the experimental data obtained in this study, it can be concluded that,
within the uncertainty of measurements, the rate of dolomite crystallization is practically independent of chemical affinity for 0 ⬍ A ⬍ ⫺10 kJ/mol. The very low dolomite
crystallization rates measured in this work do not allow us to account for the effect of
pH, ⌺CO2, and Mg2⫹ aqueous concentrations. Even at far from equilibrium conditions
(A ⬃ ⫺12 kJ/mol), the measured precipitation rates at 25°C fall within the range of
(1 ⫾ 0.8) ⫻ 10⫺15 mol/cm2/s which is several orders of magnitude lower than
predictions from the TST. This value, however, is in close agreement with that derived
by Arvidson and Mackenzie (1997, 1999) from a simple kinetic model using data on
high-temperature dolomite synthesis experiments.
Effect of inorganic and organic ligands on dolomite dissolution rate.—The effect of
different inorganic and organic ligands (sulfate, acetate, formiate, tartrate, ascorbate,
oxalate, citrate, and EDTA) on dolomite dissolution rate has been investigated at 25°C
in neutral to alkaline solutions revisited
611
Fig. 7. Dolomite overall reaction rate as a function of chemical affinity at close to equilibrium
conditions for I ⫽ 0.1– 0.01 M. Total carbonate concentration: F, ⬍10⫺4 M; Œ, 0.01 M; 夹, 0.05 ⫾ 0.03 M.
Lines 1, 2, and 3 are calculated using eq (20) with K °sp ⫽ 10⫺17.
in CO2-free solutions (⌺CO2 ⬍ 10⫺4 M) at far from equilibrium conditions. Results of
61 steady-state dissolution experiments performed with ligand concentrations ranging
from 10⫺5 to 0.2 M and ionic strength of 0.1 to 0.2 M are listed in app. C and illustrated
in figure 9 where dolomite dissolution rates are plotted as a function of aqueous ligand
concentration. The experiments were performed at pH above 7 so that most investigated organic acids were completely dissociated. The addition of these ligands in
solution leads to an increase in the dissolution rate following the order sulfate ⬇
formiate ⬇ tartrate ⬍ acetate ⬍ ascorbate ⱕ oxalate ⬍ citrate Ⰶ EDTA. In the
presence of organic ligands characteristic etch pits develop on dolomite surface (see in
fig. 8C an example of the formation of prismatic etch pits during EDTA-promoted
dissolution at pH 7.3). Note that similar triangular etch pits have been observed for
tartrate-affected calcite dissolution at pH ⫽ 4.6 (Barwise, Compton, and Unwin, 1990).
This ligand catalytic effect can be explained within the framework of the surface
coordination approach (Stumm, 1986, 1992) assuming the sorption of organic acids
on dolomite surface, which is well documented for calcite (Lahann and Campbell,
1980; Giannimaras and Koutsoukosm, 1988; Geffroy and others, 1999), promotes its
612
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 8. Scanning Electron Microscopic (SEM) photomicrographs of dolomite grains used in this study.
(A) Initial sample; (B) Exp N° 14-5, dissolution in 0.01 M Na2CO3 solution at pH ⫽ 11.1. (C) Dissolution at
pH ⫽ 7.3 in the presence of 5 䡠 10⫺4 M EDTA; (D) Exp N° 45-4, crystallization at pH ⫽ 9.8 (solutions were
undersaturated with respect to any minerals except magnesite and dolomite). (E) Exp N° 33, crystallization
at pH ⫽ 10.9 (solutions were undersaturated with respect to any minerals except magnesite and dolomite).
Note triangular etch pits formed in alkaline solutions (B) and strong leaching of the surface by EDTA (C).
The evidence of dolomite crystallization is seen from the development of triangular growth steps on the
{1014} face oriented parallel to crystallographic axis. Similar features have been reported for dissolution and
growth of calcite (Dove and Hochella, 1993) and syntaxial dolomite growth (Arvidson and MacKenzie,
1999).
in neutral to alkaline solutions revisited
Fig. 8 (continued)
613
614
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Fig. 8 (continued)
dissolution. Indeed, the adsorption of ligands on mineral surface brings negative
charges into the coordination sphere of surface metal species thus polarizing the
original Me-oxygen bonds of the crystal lattice and facilitating the detachment of the
metal from the surface. According to this scheme, the rate of the ligand-promoted
dissolution is proportional to the concentration of the surface metal complex ⬎MeL
which can be deduced from the stability constant of the reaction
⬎MeOH2⫹ ⫹ L⫺ ⫽ ⬎MeL° ⫹ H2O,
K *Me-L ⫽
(22)
兵⬎MeL°其
兵⬎MeOH2⫹其 䡠 关L⫺兴
(23)
where {⬎i} and [i] stand for the surface and solution concentration of the ith species,
respectively. In the presence of ligands the forward dolomite dissolution rate is thus
the sum of the H2O- and ligand-promoted dissolution according to
⫹
R ⫹ ⫽ k Mg
䡠 兵⬎MgOH2⫹其1.9 ⫹ kL 䡠 兵⬎MgL0其
(24)
Magnesium sites conservation requires
兵⬎MgOH2⫹其 ⫹ 兵⬎MgL°其 ⫽ ST ⫽ 7 䡠 10⫺6 mol/m2
Combination of eqs (23, 24, and 25) yields
冉
⫹
R ⫹ ⫽ k Mg
䡠 S T1.9 䡠 1 ⫺
K *Me-L 䡠 关L ⫺ 兴
1 ⫹ K *Me-L 䡠 关L ⫺ 兴
冊
1.9
⫹ kL 䡠 ST 䡠
K *Me-L 䡠 关L ⫺ 兴
.
1 ⫹ K *Me-L
(25)
(26)
Rigorous application of eq (26) to model the experimental dependence of dolomite
dissolution on ligand concentration requires accurate values of K*Me-L for the adsorp-
in neutral to alkaline solutions revisited
615
Fig. 9. The effect of various ligands on dolomite dissolution rates at 25°C in 0.1 M NaCl. The symbols
are the experimental data, but the solid curves were calculated using eq (26) with the parameters tabulated
in table 2. (A) ascorbate, acetate, and tartrat; (B) citrate, oxalate, and formiate; (C) EDTA and sulfate.
616
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Table 2
Parameters of eq (26), obtained for different ligands. The data for log Kaq of aqueous
complexes are taken from Martell, Smith, and Motekaitis (1997)
tion reactions on dolomite of the ligands investigated in this study. Such data are only
available for fatty acids (Zullig and Morse, 1988). Thus, only preliminary estimations of
K*Me-L were carried out in this study. As a first approximation, the values of the stability
constants for surface adsorption reactions were set similar to the corresponding values
for association reactions in homogeneous aqueous solution (Schindler and Stumm,
1987). Then, the values of rate constants kL were allowed to vary in order to fit the
experimental dependence of dissolution rate on ligand concentration using eq (26).
When fitting with only one parameter was not possible, the values of K*Me-L were
allowed to vary within one order of magnitude from the initial values (except for
citrate). The final values of constants used in eq (26) are listed in table 2. The
uncertainties attributed to the values given in this table correspond to the range of best
fits obtained by varying the kL and K*Me-L. The degree to which eq (26) can be used to
describe the effect of organic ligands on dolomite dissolution rate can be assessed in
figure 9. The solid curves depicted in this figure were computed with eq (26) together
⫹
⫽ (1.12 ⫾ 0.1) ⫻ 105 mol⫺1 䡠 cm2 䡠
with values of K*Me-L and kL listed in table 2 and kMg
⫺1
s . The close correspondence between the solid curves and experimental data
represented by symbols in figure 9 demonstrates their consistency with eq (26). It is
interesting to note that eq (26) implies that the effect of ligand on dissolution rates
⫹
depends on the relative values of kMg
, kL, and K*Me-L. The ligand will act as a catalyzer
⫹
1.9
⫹
when kL 䡠 ST ⬎ kMg 䡠 ST or as an inhibitor if kL 䡠 ST ⬍ kMg
䡠 ST1.9. Besides, the higher the
value of K*Me-L, the stronger will be the catalyzing or the inhibiting effect of the ligand.
Analyses of kL and K*Me-L values reported in table 2 provide useful correlations between
the catalytic effects on dissolution of organic ligands and their structural arrangement
on dolomite surface. The sequence of rate constants listed in table 2 shows that
carboxylic acids like acetate and formiate, which form monodendate surface complexes, are promoting dissolution to a lesser extent than those forming surface
chelates (oxalate, EDTA). Among bidendates, oxalate, which forms five-membered
rings, is more efficient than tartrate which forms seven-membered rings. This is
consistent with the observation that an increase of chelate ring size leads to a decrease
of complex stability for six coordinate metals like Mg or Ca due to steric strain as shown by
changes in enthalpies of complex formation (Martell and Hanckock, 1996). Strong effect of
in neutral to alkaline solutions revisited
617
two hydroxyl-bearing ligands, ascorbate and citrate, can be understood in view of the
marked affinity of their hydroxyl groups for surface Mg and Ca as is the case for calcite
(Geffroy and others, 1999). Finally, H2EDTA2⫺, which forms very stable five-membered
chelate rings with surface Mg/Ca ions, presents the strongest catalyzing effect on dissolution.
Note that the catalyzing effect of most organic acids on dolomite dissolution rates
differs from that for silicates. It has been argued (Oelkers and Schott, 1998) that the
effect of organic acid anion on feldspar dissolution rates is primarily due to a decrease
of the activity of aqueous free aluminum that acts as a strong inhibitor of dissolution.
Such a mechanism is unlikely to operate for dolomite dissolution as in all our
experiments conducted in the presence of organic ligands, the activity of aqueous
calcium never exceeds 10⫺5 M. At these conditions, the inhibiting effect of Ca2⫹ is very
weak (fig. 2). This strongly supports a major role of organic acid adsorption and not
aqueous complexation in the enhancement of dolomite dissolution rates.
Although there are a large number of studies devoted to the effect of organic ligands
on calcite dissolution (Morse, 1974; Thomas and Longo, 1986; Barwise, Compton, and
Urwin, 1990; Compton and Sanders, 1993; Compton and Brown, 1995; DeMaio and
Grandsaff, 1995; Teng and Dove, 1997; Fredd and Fogler, 1998), the effect of organic
ligands and sulfate on dolomite dissolution rate has not been yet investigated. The results
available for calcite indicate that there is a decrease of calcite dissolution rate in the
presence of organic ligands such as tartrate (Barwise, Compton, and Urwin, 1990),
succinate, phtalate, maleate (Fredd and Fogler, 1998), and humate (Compton and Brown,
1995), whereas the rate increases significantly in the presence of chelating agents such as
EDTA (Bodine and Fernalld, 1973; Fredd and Fogler, 1998). The results obtained for
dolomite in this study are thus consistent with most previous findings on calcite.
applications
The results of this study bring new light on dolomite behavior in aquatic systems. In
particular, the inhibiting effect of Ca on dolomite dissolution allows us to explain the high
stability of dolomite in many sedimentary environments. The conversion of dolomite to
calcite under the influence of calcium-rich solutions (often called «dedolomitization»),
CaMg共CO 3 兲 2 ⫹ Ca 2⫹ 3 2CaCO 3 ⫹ Mg 2⫹ ,
is extremely rare in nature compared to the dolomitization of limestones (Katz, 1968;
Lippmann, 1973), although the thermodynamic requirements for this reaction are often
met. The very sluggish transformation of dolomite into calcite likely results from the strong
inhibition of dolomite dissolution by Ca2⫹ at pH ⬎ 7 as demonstrated in this study.
Eq (19) implies that the rate of dolomite crystallization is proportional to the
square of the product of Mg2⫹ and CO32⫺ activities in solution. This is consistent with
results of laboratory experiments on high-magnesian calcites, protodolomite, and
dolomite synthesis at ambient and high temperatures, respectively (Katz and Mattews,
1977; Sibley, Nordeng, and Borkowski, 1994; Pokrovsky, 1996, 1998; Arvidson and
Mackenzie, 1999). It is also worth noting that the control of dolomite crystallization by
Mg and carbonate supply, as postulated in this study (reaction 18), is consistent with
the dolomite formation reaction earlier proposed by Lippman (1973):
CaCO 3 共solid兲 ⫹ Mg 2⫹ 共aq兲 ⫹ CO 32⫺ 共aq兲 ⫽ CaMg共CO 3 兲 2 共solid兲
Most modern naturally-occurring low-temperatures dolomites are poorly ordered
and exhibit large excess of calcium compared to the stoichiometry (Hardie, 1987; Last,
1990). The formation of authigenic well-ordered dolomite has been reported recently by
Kohut, Muehlenbachs, and Dudas (1995) in a saline soil of Canada where it grows at
extremely low rates from sulfate and Mg-rich solutions having carbonate concentration
from 10⫺3 to 7 䡠 10⫺3 M at pH values around 8. These conditions are favorable for Mg2⫹
618
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
adsorption onto dolomite surface and the formation of ⬎MgOH2⫹ species in accord with
eqs (18 and 19).
The limiting step for dolomite crystallization should be a transformation of
outer-sphere adsorbed Mg (⬎CO3Mg ⫻ nH2O; Pokrovsky and others, 2000) into
inner-sphere ⬎MgOH2⫹ species. This can be facilitated by any ligand favoring
dehydration of magnesium ions in solution or on the mineral surface. For example,
the presence of organic ligands is known to favor significantly the precipitation of
high-Mg calcites and protodolomite from aqueous solutions at ambient temperatures (Kitano and Kanamori, 1966; Kazanski, Kataeva, and Mandrikova, 1972). The
sulfate ion was also proposed to catalyze dolomite crystallization by dehydration of
Me surface sites (Brady, Krumhansl, and Papenguth, 1996). The modern formation
of «organogenic» dolomite in the course of the diagenetic destruction of organic
matter (Compton, 1988; Slaughter and Hill, 1991), crystallization of poorly ordered dolomite in the presence of algal mats (Peterson, Bein, and Berner, 1963),
and microbial dolomitization by nannobacteria (Vasconcelos and McKenzie, 1997)
involves the action of various organic ligands that can probably facilitate the
dehydration of adsorbed Mg and formation of ⬎MgOH2⫹ species as consistent with
the reaction mechanism proposed in this study.
conclusions
Dolomite steady-state dissolution rates have been measured at 25°C in a mixed
flow reactor in neutral to alkaline carbonate-bearing solutions, and the variation of
dissolution rate with solution composition has been correlated with dolomite surface
speciation within the framework of transition state theory. Dissolved Ca was found to
strongly inhibit dolomite dissolution at pH above 7 which is consistent with formation
of a Ca-deficient surface precursor complex comprizing hydrated Mg groups, ⬎MgOH⫹
2,
which is similar to that controlling magnesite reactivity. This is in agreement with the
suggestion by Busenberg and Plummer (1982) that dolomite dissolution is controlled
by the protonation/hydrolysis of its MgCO3 sites.
Dolomite crystallization, confirmed by solution analysis and microscopic observations, was found to be independent of chemical affinity, although, close to equilibrium,
dissolved Mg2⫹ and CO32⫺ should favor dolomite precipitation as suggested by natural
observations and dolomite laboratory synthesis at hydrothermal conditions. Different
organic ligands have been found to accelerate the rate of dolomite dissolution at pH above
7. This is due to electron transfer from ligand oxygen donor atoms to metal orbitals which
weakens the original Mg-O bonds of the crystal lattice thus facilitating the detachment of
the metal-ligand complex from the surface. This catalyzing effect can be correlated with
the structural arrangement of investigated organic ligands on dolomite surface.
Finally, it should be noted that the surface speciation model of the dolomitesolution interface used in this study to model dolomite dissolution represents only
preliminary description of surface speciation control on dolomite reactivity in aqueous
solutions. In particularly, we did not take into account the diversity of microtopographical sites exposed on real carbonate surface and their mobile nature. It would be
desirable to relate the surface speciation approach developed for flat (terrace) planes
to step and kink sites that are likely to control both the dissolution and precipitation.
In this regard, application of atomic force microscopy (AFM) for studying dissolution
of dolomite at various crystallographic planes and direct measurements of step migration
rates (see Shiraki, Rock, and Casey (2000) for calcite) should lead to more comprehensive
picture of reacting carbonate surfaces.
acknowledgments
The authors are grateful to D. Okab for assistance with SEM analyses and J.-M.
Gautier for B.E.T. surface area measurements. The authors also wish to thank J. Fein
in neutral to alkaline solutions revisited
619
and an anonymous reviewer for their helpful comments. This work was supported by a
visiting research position awarded to O.P. by the Centre National de la Recherche
Scientifique (CNRS).
APPENDIX
Table A1
Summary of dolomite dissolution experiments performed in neutral and alkaline solutions
and in carbonate-rich solutions. Ionic strength is adjusted by
NaCl/NaHCO3 /Na2CO3 addition.
[Ca2⫹]input ⫽ [Mg2⫹]input ⫽ 0. N.D. ⫽ Not Determined
620
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Table A1
(continued)
in neutral to alkaline solutions revisited
Table A1
(continued)
Note: *-with external addition of Ca2⫹; **-with external addition of Mg2⫹.
621
Table A2
Summary of dolomite dissolution/crystallization rates measured at 25°C in mixed-flow reactor
system at close-to-equilibrium conditions. Negative rates mean precipitation
622
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
Table A3
Summary of dolomite dissolution experiments performed in the presence of different ligands. 0.1 M NaCl;
[Alk] ⬍ 10⫺4 M. N.D. ⫽ non determined
in neutral to alkaline solutions revisited
623
624
O.S. Pokrovsky and J. Schott—Kinetics and mechanism of dolomite dissolution
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