Solubility investigations in the amorphous calcium magnesium carbonate system

Article abstract of DOI:10.1039/c8ce01596a

Amorphous precursors are known to occur in the early stages of carbonate mineral formation in both biotic and abiotic environments. Although the Mg content of amorphous calcium magnesium carbonate (ACMC) is a crucial factor for its temporal stabilization, to date little is known about its control on ACMC solubility. Therefore, amorphous Ca_xMg_1-xCO₃·nH₂O solids with 0 ≤ x ≤ 1 and 0.4 ≤ n ≤ 0.8 were synthesized and dispersed in MgCl₂-NaHCO₃ buffered solutions at 24.5 ± 0.5 °C. The chemical evolution of the solution and the precipitate clearly shows an instantaneous exchange of ions between ACMC and aqueous solution. The obtained ion activity product for ACMC (IAP_ACMC = "solubility product") increases as a function of its Mg content ([Mg]_ACMC = (1 - x) × 100 in mol%) according to the expression: log(IAP_ACMC) = 0.0174 (±0.0013) × [Mg]_ACMC - 6.278 (±0.046) (R² = 0.98), where the log(IAP_ACMC) shift from Ca (-6.28 ± 0.05) to Mg (-4.54 ± 0.16) ACMC endmember, can be explained by the increasing water content and changes in short-range order, as Ca is substituted by Mg in the ACMC structure. The results of this study shed light on the factors controlling ACMC solubility and its temporal stability in aqueous solutions.

Full text of DOI:10.1039/c8ce01596a

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Solubility investigations in the amorphous calcium
magnesium carbonate system†
Cite this: CrystEngComm, 2019, 21,
155
Bettina Purgstaller,
a Katja E. Goetschl,a
*
Vasileios Mavromatisab and Martin Dietzela
Amorphous precursors are known to occur in the early stages of carbonate mineral formation in both
biotic and abiotic environments. Although the Mg content of amorphous calcium magnesium carbonate
(ACMC) is a crucial factor for its temporal stabilization, to date little is known about its control on ACMC
solubility. Therefore, amorphous CaxMg1−xCO3·nH2O solids with 0 ≤ x ≤ 1 and 0.4 ≤ n ≤ 0.8 were synthe-
sized and dispersed in MgCl2–NaHCO3 buffered solutions at 24.5 0.5 °C. The chemical evolution of the
solution and the precipitate clearly shows an instantaneous exchange of ions between ACMC and aqueous
solution. The obtained ion activity product for ACMC (IAPACMC = “solubility product”) increases as a func-
tion of its Mg content ([Mg]ACMC = (1 − x) × 100 in mol%) according to the expression: logIJIAPACMC) =
0.0174 ( 0.0013) × [Mg]ACMC − 6.278 ( 0.046) (R2 = 0.98), where the logIJIAPACMC) shift from Ca (−6.28
Received 18th September 2018,
Accepted 16th November 2018
0.05) to Mg (−4.54
0.16) ACMC endmember, can be explained by the increasing water content and
DOI: 10.1039/c8ce01596a
changes in short-range order, as Ca is substituted by Mg in the ACMC structure. The results of this study
shed light on the factors controlling ACMC solubility and its temporal stability in aqueous solutions.
rsc.li/crystengcomm
calcite. These authors also documented an ion activity prod-
Introduction
uct of ACC (logIJIAPACC); “solubility” of ACC) of −6.39 0.02 at
Amorphous calcium carbonate (ACC) is a highly soluble solid
phase that commonly occurs as a precursor of crystalline
CaCO3 (e.g. calcite, aragonite) in modern biotic and abiotic
precipitation environments. Its (trans)formation has been ob-
served in calcifying organisms,1–8 microbialites from alkaline
lakes,9,10 biofilms from hot springs11 and speleothems.12 ACC
can include significant amounts of MgCO3, which is known
to play a significant role in its temporal stabilization.8,10 In
this context, the transformation of amorphous calcium mag-
nesium carbonates (ACMC) into crystalline Ca–Mg carbonate
minerals is of particular interest as it represents an energeti-
cally favorable pathway for high magnesian-calcite (Mg-cal-
cite) and disordered dolomite formation.13–17 However, the
impact of Mg on the temporal stability and transformation
behavior of ACMC is not fully elucidated and a systematic
study on the solubility of the ACMC system is still missing.
The earlier experimental work of Brečević and Nielsen18
showed that Mg-free ACC is significantly more soluble than
25 °C. More recently, Gebauer et al.19 found the solubility of
ACC to be lower and reported logIJIAPACC) values of −7.51 and
−7.42 for ACC phases with a “calcite-like” and a “vaterite-like”
short range order, respectively. To our knowledge the only
published values so far for ACMC are those obtained in our
previous study (Purgstaller et al.15), where we documented
logIJIAPACMC) values ranging from −6.14 to −7.01 for ACMC
with ≤10 mol% Mg.
In the scope of an energetic study on synthetic ACMCs,
Radha et al.20 observed that ACMC with low Mg content
transforms faster into crystalline carbonates than ACMC with
high Mg content under atmospheric conditions. The longer
stability of ACMC with elevated Mg content has been attrib-
uted to the strong bond between structural water and Mg
ions which retards the process of dehydration and subse-
quent phase transformation.20,21 However, inconsistent data
are available for the water contents of ACMC at distinct Mg
concentrations.20–22 For example, the moles of water per unit
formula CaxMg1−xCO3 are reported to vary from 0.81 to 1.75
for ACMC with 45 2 mol% Mg.20,21
a Institute of Applied Geosciences, Graz University of Technology,
Rechbauerstrasse 12, 8010 Graz, Austria. E-mail: bettina.purgstaller@tugraz.at
b Géosciences Environnement Toulouse (GET), CNRS, UMR 5563, Observatoire
Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, France
New findings indicate that the stability of Mg-free ACC un-
der atmospheric conditions is significantly controlled by the
availability of H2O in the transformation environment. In this
regard, Konrad et al.23 proposed that H2O molecules are
adsorbed by ACC promoting its transformation into calcite/
vaterite via a dissolution and reprecipitation mechanism.
† Electronic supplementary information (ESI) available: Supplementary tables
and thermogravimetric analyses (TGA), differential calorimetry results (DSC),
X-ray diffraction patterns (XRD) and scanning electron microscope images
(SEM) of synthesized ACMCs. See DOI: 10.1039/c8ce01596a
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More recently, experimental studies on ACMC (trans)forma-
tion in an aqueous solution suggested that the chemical com-
position of the reactive solution (e.g. Mg/Ca ratio) signifi-
cantly affects the temporal stability of ACMC and its
subsequent transformation into the final product (Mg-calcite,
monohydrocalcite etc.).24–27 However, the exact mechanisms
controlling ACMC stability and transformation behavior is
still under debate and further experimental work is needed
to describe the interaction between solutions and ACMC
phases.
(CaCl2·2H2O, MgCl2·6H2O and Na2CO3 from Roth) mixed
with ultrapure water.
Experimental setup for solubility study
The chemical composition of the synthesized ACMC solids
used for solubility investigations is shown in Table 1. Experi-
ments were performed at 24.5 0.5 °C in a 100 mL glass re-
actor containing 50 mL of a 100 mM NaHCO3 and 30 mM
MgCl2 solution, stirred at 350 rpm. The pH of the solution
was adjusted to 8.33
0.03 by the addition of a 500 mM
The aim of the present work is to provide an advanced un-
derstanding about the “solubility product” of ACMC as a
function of its Mg content in terms of an ion activity product
at chemical steady-state conditions. Therefore, distinct
ACMCs (CaxMg1−xCO3·nH2O; 0 ≤ x ≤ 1 and 0.4 ≤ n ≤ 0.8)
were synthesized and subsequently dispersed in MgCl2–
NaHCO3 buffered solutions in order to assess i) the elemental
exchange of ACMC with the aqueous phase and ii) the factors
controlling ACMC solubility and its temporal stability in
aqueous solutions.
NaOH solution. At the onset of the experiment, 1.5 g of the
synthetic ACMC sample was introduced into the reactor. The
temporal evolution of the mineralogy was monitored using
an in situ Raman probe immersed in the suspension. In or-
der to follow the chemical composition of the solution and
the solid phase, homogeneous sub-samples of the experimen-
tal solution/suspension (1.5 mL) were collected with a pipette
at certain reaction times (Table S1†). The solids were sepa-
rated from the solution by a 0.2 μm cellulose acetate filter
using a suction filtration unit, washed with ethanol and dried
in an oven at 40 °C.
Experimental section
Synthesis of amorphous calcium magnesium carbonates
Solid phase characterization
X-ray diffraction (XRD) patterns of the synthesized ACMCs
(Fig. S1†) and of the ACMCs separated from the experimental
solutions during the solubility study (not shown) were ac-
quired using a PANalytical X'Pert Pro diffractometer (Co-Kα
radiation) at a 2Θ range from 4 to 85° and a scan speed of
0.03° s−1. Thermogravimetric analyses (TGA) of freeze-dried
Amorphous calcium magnesium carbonates with Mg con-
tents ranging from 0 to 100 mol% Mg (hereafter referred
to as ACMCs) were synthesized by a previously described
method.23 Briefly, 80 mL of a 250 mM (Ca,Mg)Cl2 solution
was poured into a beaker containing 80 mL of a 250 mM
Na2CO3 solution. The reaction products were immediately
separated from solution by a 0.2 μm cellulose filter using
a suction filtration unit. Subsequently, the separated pre-
cipitate was washed with ultrapure water (Millipore Inte-
gral 3: 18.2 MΩ cm−1) and transferred into a freeze dryer
(Virtis Benchtop 3L). The freeze-dried ACMCs were stored
in closed vials in a desiccator with silica gel (relative hu-
midity = 3%). In total, 9 synthesis experiments were car-
ried out, where the Mg content of the (Ca,Mg)Cl2 stock
solution, [Mg]stock = {[Mg]/([Ca] + [Mg])} × 100, was system-
atically varied between 0 and 100 mol% (see Table 1). All
solutions were prepared by analytical grade chemicals
ACMCs (Fig. S2†) were realized using
a PerkinElmer
STA8000. The samples were heated from 25 °C to 800 °C at
10 °C min−1 in the presence of 99.999% N2 atmosphere. Se-
lected synthesized ACMCs were gold coated and imaged (Fig.
S3†) using a scanning electron microscope (SEM, ZEISS DSM
982 Gemini). Time-resolved in situ Raman spectroscopy of
the experimental solution/suspension was realized using a
Raman RXN2™ analyzer from Kaiser Optical Systems with a
Kaiser MR Probe head (quarter-inch immersion optic) and a
785 nm laser beam. In situ Raman spectra were collected ev-
ery 60 s in the 100–1890 cm−1 region with a resolution of 1
cm−1.
Table 1 Chemical composition of synthesized ACMCs
c
Sample
[Mg]stocka (mol%)
[Mg]ACMCb (mol%)
nH2Op.f. (mol)
Sample composition
ACC
0
0
9.0
0.44
0.49
0.51
0.53
0.56
0.59
0.66
0.71
0.79
CaCO3·0.44H2O
ACMC_9
ACMC_15
ACMC_22
ACMC_31
ACMC_39
ACMC_53
ACMC_80
AMC
20.1
30.3
40.1
50.1
60.7
72.1
89.1
100
Ca0.91Mg0.09CO3·0.49H2O
Ca0.85Mg0.15CO3·0.51H2O
Ca0.78Mg0.22CO3·0.53H2O
Ca0.69Mg0.31CO3·0.56H2O
Ca0.61Mg0.39CO3·0.59H2O
Ca0.47Mg0.53CO3·0.66H2O
Ca0.20Mg0.80CO3·0.71H2O
MgCO3·0.79H2O
14.9
21.9
30.8
39.4
53.4
80.0
100
a
b
c
Mg content of the stock solution in mol%. Mg content of the ACMC solid in mol%. Moles of water per formula unit CaxMg1−xCO3, where
(1 − x) = [Mg]ACMC/100.
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Chemical composition of the experimental solutions and
solids
During the experimental run, the pH of the experimental so-
lution was measured with a SI Analytics Silamid® gel
electrode, which was calibrated against NIST buffer standard
solutions at pH 4.01 and 7.00. The total alkalinity of the ex-
perimental solutions was measured by a Schott TitroLine al-
pha plus titrator using a 10 mM HCl solution with a preci-
sion of 2%. The aqueous Ca, Mg and Na concentrations of
the (Ca,Mg)Cl2 stock solutions, of the experimental solutions
and of the solids (digested in 6% HNO3) were determined
using inductively coupled plasma optical emission spectrom-
etry (Perkin Elmer Optima 8300 DV). The analytical error was
< 3% for Ca and Mg analyses and < 5% for Na analyses.
Fig.
1 Mg content of the stock colutions, [Mg]stock, versus Mg
Aqueous speciation and ion activity product calculation
content of the ACMC precipitates, [Mg]ACMC, and respective values
reported by Radha et al.20 The increase of [Mg]ACMC as a function of
The aqueous speciation of the experimental solutions was
calculated at 25 °C using the PHREEQC software together
with its minteq.v4 database. For the ionic strength of our ex-
periments (i.e. 0.17 0.02 M; Table S2†), the calculation of
individual ion activity coefficients for the solute species is
based on the Davis equation.28,29 The activities (a) of Ca2+,
[Mg]stock of this study can be described by the equation [Mg]ACMC
0.0009 × [Mg]stock)} (R2
0.99; solid line). Analytical uncertainties are included in the symbol
size.
=
=
−21.03243
2.1968 × {1 − exp(0.0175
Mg2+ and CO3 ions in solution and the stoichiometry of the
sized ACMCs are presented in Fig. S2 (ESI†). The DSC curve
of ACC (Ca endmember of ACMC) shows an endothermic
peak at 120 °C, a sharp exothermic peak at 340 °C and a
second endothermic peak at 731 °C (Fig. S2A†). These DSC
features have been earlier shown to be associated with the
enthalpies of dehydration, crystallization and decomposition
of ACC.20,35 In contrast to ACC, AMC (Mg endmember of
ACMC) decomposes without crystallization, as it is indicated
by the absence of the sharp exothermic peak in the DSC
curve (Fig. S2F†). The weight loss curves of ACC and AMC
show two weight loss steps (Fig. S2A and F†), where the first
step is due to the loss of structural water and the second step
due to the decomposition of CaCO3 and MgCO3 to CaO,
MgO and CO2.20,35 After ACC dehydration, the TGA curve
achieves a plateau in heating before CaCO3 decomposes to
CaO and CO2 at near 600 °C (Fig. S2A†). In contrast, AMC
shows a continuous drop of the TGA curve (without a pla-
teau) for the same temperature range, which indicates that
AMC dehydration and decomposition may overlap (Fig.
S2F†). Based on our TGA data we suggest that AMC decom-
position occurs at around 275 °C, as it is indicated by the
inflection point of the weight loss curve (Fig. S2F†). In con-
trast to the pure endmembers, the ACMCs show multistep
weight loss curves (Fig. S2B–E†). These trends are in accor-
dance with the findings of Radha et al.,20 who associated
these features with multistep carbonate decomposition
caused by (i) an initial heterogeneous amorphous material
or (ii) thermally induced phase segregation during the
heating process.
2−
digested CaxMg1−xCO3·nH2O solids (where (1 − x) = [Mg]ACMC
/
100) were used to calculate ion activity products for the amor-
phous calcium magnesium carbonates (IAPACMC) as a func-
tion of experimental time according to equation
IAPACMC = (aCa2+)x(aMg2+)1−x(aCO32−)(aH2O)n
(1)
In the present solubility experiments, the aH2O values of
the experimental solutions remained constant at 0.955
0.004. Thus, for the calculation of the IAPACMC, the aH2O was
assumed to be unity.
Results and discussion
Characterization of synthesized ACMC material
The measured Mg contents of the stock solutions, [Mg]stock
,
and of the synthesized ACMC samples, [Mg]ACMC are
,
reported in Table 1 and displayed in Fig. 1. The results reveal
a preferential enrichment of the amorphous solid in Ca and
are in good agreement with those of Radha et al.,20 who syn-
thesized a set of amorphous CaxMg1−xCO3·nH2O solids by
batch method using (Ca,Mg)Cl2 and Na2CO3 solutions
(Fig. 1). The preferential enrichment of the amorphous phase
in Ca likely stems from the strong free energy of hydration of
the Mg2+ ion compared to Ca2+.20,30 The strongly hydrated
aqueous Mg2+ is a well-known limitation in the formation of
anhydrous crystalline Ca–Mg-carbonates, such as Mg-calcite,
dolomite and magnesite, at ambient temperatures.31 Instead,
hydrous crystalline Mg-carbonates (e.g. hydromagnesite)32
and hydrous ACMC phases33,34 tend to precipitate.
The water contents of the synthetic ACMCs were calcu-
lated from the weight losses between 25 and 275 °C. Above
this temperature, mass loss might occur due to the decompo-
sition of MgCO3 to MgO, as it is suggested by the TGA curve
Representative thermogravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) curves of the synthe-
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where R2 = 0.99. Previous experimental work documented sig-
nificantly higher nH2Op.f. values for ACMCs (Fig. 2).20 These
differences probably originate from different drying methods
(freeze-dryer versus vacuum oven). In the study of Radha
et al.,20 the loosely bound physisorbed H2O on the ACMC
particles were probably not completely removed by the vac-
uum oven prior to thermogravimetric analyses. However, our
findings are in good agreement with the results of Lin
et al.,21 who reported nH2Op.f. values of 0.6, 0.8 and 0.9 for
ACMC with 17, 44 and 100 mol% Mg (Fig. 2). In the latter
case, the synthesized ACMC samples were washed with etha-
nol and lyophilized prior to TGA-analysis.
Earlier models on the structure of Mg-free ACC
suggested the presence of partially mobile water in the
amorphous phase.36,37 In this context, ACC has been de-
scribed to consist of a porous Ca-framework with inter-
connected channels formed by water molecules and car-
bonate ions, allowing water mobility to a certain extent.38
More recently, Jensen et al.39 has shown that water mole-
cules are mainly coordinated to Ca and carbonate ions
and less frequently to other water molecules, ruling out
the scenario of hydrogen-bonded networks. This is consis-
tent with the NMR-results of Lin et al.21 who concluded
that the coordination shell of Mg in ACMC contains at
least one water molecule. Indeed, in the present study,
the nH2Op.f. increases as a function of the Mg content
(Fig. 2), indicating that partially hydrated Mg is incorpo-
rated into the amorphous solid. Our obtained results how-
ever suggest that the nH2Op.f. values for ACMC are signifi-
cantly lower than previously reported.20,22
Fig. 2 Mg content of ACMC, [Mg]ACMC, versus the moles of water per
unit formula CaxMg1−xCO3, nH2Op.f., from synthesis experiments
conducted in the present study and from literature. Analytical
uncertainties are included in the symbol size.
of AMC (Fig. S2F†). The moles of water per unit formula
CaxMg1−xCO3 (nH2Op.f.) determined by thermal analyses vary
between 0.4 for ACC and 0.8 for AMC (Table 1, Fig. 2). As it
can be seen in Fig. 2, the water content determined for ACC
is in excellent agreement with those reported by Schmidt
et al.35 and Konrad et al.23 Overall, thermal analyses revealed
a linear correlation between the nH2Op.f. and the [Mg]ACMC
according to the equation
nH2Op.f. = 0.0034 0.0002[Mg]ACMC + 0.4574 0.0124 (2)
Fig. 3 3D plots of in situ Raman spectra showing the temporal evolution of the v1 band of ACMC for the experiments conducted with ACMC_9
(A) and ACMC_53 (B). The Raman bands at 1067 cm−1 and 1017 cm−1 correspond to aqueous CO32− and HCO3−, respectively.
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Table 2 Summary of in situ Raman results and calculated ion activity product values of solubility experiments
Raman v1 banda (cm−1
)
TimeACMCb (min) [Mg]ACMC_av (mol%) logIJaCO3
)
av
logIJaCa2+
)
av
logIJaMg2+
)
av
logIJIAPACMC)av
c
2−
d
d
d
e
Sample
ACC
ACMC_9
1080
1082
1
1
1
1
1
1
1
1
1
15
24
54
85
64
51
20
9
2.2 0.2
10.5 0.2
15.8 0.3
21.2 0.7
29.3 1.6
37.4 0.5
51.3 0.6
78.5 0.4
100
−3.14 0.01
−3.01 0.02
−2.91 0.01
−2.87 0.03
−2.67 0.02
−2.58 0.02
−2.51 0.02
−2.39 0.03
−2.30 0.01
−3.08 0.03
−3.22 0.03
−3.31 0.04
−3.38 0.05
−3.54 0.08
−3.68 0.06
−3.78 0.05
−4.10 0.10
—
−2.25 0.02
−2.23 0.01
−2.21 0.01
−2.20 0.03
−2.18 0.05
−2.21 0.06
−2.12 0.01
−2.14 0.01
−2.12 0.01
−6.20 0.02
−6.13 0.02
−6.04 0.03
−6.00 0.03
−5.81 0.07
−5.71 0.03
−5.43 0.02
−4.96 0.01
−4.42 0.01
ACMC_15 1083
ACMC_22 1084
ACMC_31 1085
ACMC_39 1086
ACMC_53 1088
ACMC_80 1095
AMC
1098
5
a
b
Raman v1 peak positions of ACMC solids dispersed in the MgCl2–NaHCO3 buffered solutions. Time interval in which ACMC was stable in
c
the experimental solution. Average Mg content of the ACMC solid at chemical steady state conditions (used for IAPACMC calculation).
d
Average activities of CO32−, Ca2+ and Mg2+ ions in the experimental solution at chemical steady state conditions (used for IAPACMC
e
calculation). Average ion activity product for ACMC calculated according to eqn (1). Note that the input data for speciation calculations and
selected output data are listed in Table S2.
Exchange of ions between ACMC and aqueous solution
documented a significant change in the chemical composi-
tion of the solids and experimental solutions within about 10
s after the ACMCs were dispersed into the MgCl2–NaHCO3 so-
lutions. In each experiment, chemical steady state conditions
were attained at a reaction time of about 2 min (Fig. 4A–C;
Table S1†). The fast reaction between the amorphous
solid and the solution can be explained by the nano-porous
structured ACMC material consisting of large and highly re-
active surface areas (Fig. S3†). The shifts in pH and alkalinity
of the MgCl2–NaHCO3 buffered solution increase with increas-
ing Mg content of the synthesized ACMCs (Fig. 4D). For ex-
ample, dispersion of ACMC with 9 mol% Mg (ACMC_9) yields
a slight shift in pH and alkalinity from 8.3 to 8.5 and 104 to
107 mM, respectively, whereas for ACMC with 53 mol% Mg
(ACMC_53) a significant shift in pH to 9.1 and alkalinity to
143 mM was observed (Fig. 4A and B). This indicates that for
attainment of chemical equilibrium between solid and solu-
tion, larger amounts of ACMC are required to dissolve in the
experiment with ACMC_53 (53 mol% Mg) compared to the
experiment conducted with ACMC_9 (9 mol% Mg).
After the synthesized ACMCs were dispersed in the MgCl2–
NaHCO3 buffered solutions, the collected in situ Raman spec-
tra of the suspensions revealed the presence of a broad CO3
2−
symmetric stretch (v1 band) of ACMC at 1080–1098 cm−1 (e.g.
experiments ACMC_9 and ACMC_53 in Fig. 3). The v1 band
shows systematic shifts in peak position (Table 2) and peak
broadening due to different Mg contents of the ACMCs.
These features arise from changes in the metal–oxygen bond
length due to the presence of shorter Mg–O bonds compared
to those of Ca–O in the amorphous solid.15,40 The intensities
of the v1 bands of the ACMC phases remained constant for
experimental times ranging from 5 to 85 min (referred to as
TimeACMC in Table 2). After this time interval, the v1 bands of
the ACMCs decreased in intensity, while the v1 bands of crys-
talline hydrous and/or anhydrous Ca–Mg carbonates (Mg-cal-
cite, monohydrocalcite and/or nesquehonite) evolved (not
shown here). Thus, TimeACMC denotes the time interval in
which the ACMC was stable in the experimental solution
(Table 2). Note that, the in situ Raman observations were con-
firmed by ex situ XRD analyses of the collected solid samples.
Although, TimeACMC increased as a function of the Mg
content in experiments conducted with ACMCs containing
≤22 mol% Mg, a reverse trend was obtained in experiments
with ACMCs containing >22 mol% (Table 2). These observa-
tions imply that the temporal stability of ACMC in a solution
is not strictly controlled by its primary Mg content. In con-
trast, the obtained data revealed changes in the chemical
composition of the reacting ACMC and the experimental so-
lution which might play a crucial role in the temporal stabili-
zation of the precursor phase and its subsequent transforma-
tion into distinct crystalline calcium magnesium carbonates.
The chemical composition of the experimental solutions
and solids collected during TimeACMC can be found in Table
S1.† Exemplarily, the temporal evolution of the chemical com-
position of the solutions and solids in experiments
conducted with ACMC_53 (53 mol% Mg) and with ACMC_9
(9 mol% Mg) is illustrated in Fig. 4A–C. The obtained results
The results revealed two distinctive trends of Mg and Ca ex-
change between ACMCs and the solution, depending on the Mg
concentrations of synthesized ACMCs: (i) in experiments
conducted with ACMCs containing ≤15 mol% Mg a net release
of Ca into the solution together with an uptake of Mg from the
solution into the solid is observed, which results in ACMCs with
slightly higher Mg contents compared to the synthesized ACMC
(e.g. ACMC_9 in Fig. 4B and C; Table S1†). (ii) In contrast, in ex-
periments conducted with ACMCs containing ≥20 mol% Mg a
net release of Mg into the experimental solution and lower
[Mg]ACMC values compared to the synthesized ACMCs were ob-
served (e.g. ACMC_53 in Fig. 4B and C; Table S1†). These obser-
vations suggest a dynamic exchange of Me2+ (Ca2+ and Mg2+)
ions between the amorphous solid and the experimental solu-
tion. It is likely that the total availability of Me2+ (Ca2+ and Mg2+)
2−
and CO3 ions in the prevailing solution affects the formation
of distinct crystalline calcium magnesium carbonates and the
temporal stability of the ACMC precursor in solution. Evidence
for that also comes from a recent study on ACC transformation,
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Fig. 4 Temporal evolution of (A) pH and alkalinity concentration, (B) aqueous Ca and Mg concentration of the experimental solution, [Ca]aq and
[Mg]aq, and (C) Mg content of ACMC solid, [Mg]ACMC, after synthesized ACMC solids were dispersed into the MgCl2–NaHCO3 solutions. Exemplarily
the data for ACMC_53 and ACMC_9 are shown, where the experimental solution achieves constant chemical composition after 2 min of reaction
time. Note that chemical data obtained at >2 min were used for estimating the ion activity products of the distinct ACMCs according to eqn (1)
(Table 2). (D) Average alkalinity concentration versus pH values of the experimental solution at chemical steady state conditions (>2 min, see Table
S2†). The shifts in pH and alkalinity increase with increasing [Mg]ACMC
.
showing prolonged stability of Mg-free ACC of up to ∼11 hours
in high concentrated MgCl2 solutions (up to 1000 mM).26 It was
concluded that strong aquocomplex formation of the dissolved
carbonate molecule with Mg2+ ions retarded mineral formation
and increased the stability of ACC in solution.26
atmosphere and thus CO2 is likely degassing from the experi-
mental solutions. However, in context of the present study,
samples of the suspensions were collected during a short
time interval (<45 min) in which pCO IJg), pH and alkalinity
2
values were monitored to be constant within the analytical
precision (Table S2†). Thus, the obtained activities of Ca2+,
2−
Mg2+ and CO3 ions of the solution and the ACMC stoichi-
Solubility approach
ometry at the given reaction time were used to calculate indi-
vidual IAPACMC (solubility) values according to eqn (1) (Table
S2†). The average logIJIAPACMC) values are listed in Table 2
and plotted in Fig. 5 as a function of the Mg content of the
ACMC. Note here that the average Mg content of the ACMC
solid at chemical steady state conditions (after Ca and Mg
ion exchange) was used for IAPACMC calculations (Table 2)
and was plotted in Fig. 5. The results clearly show that the
solubility of ACMC increases as a function of the Mg content
(Fig. 5). The linear fit of the logIJIAPACMC) values obtained in
the context of the present study is in good agreement with
the log(IAP) value for ACC documented by Brečević and Niel-
sen,18 while the solubility products for ACCs reported by
Gebauer et al.19 are significantly smaller. The obtained
The experimental solution in all experimental runs achieved
chemical steady state conditions after 2 min of reaction time.
These conditions remained steady during the time interval in
which solid analyses (Raman spectra and XRD pattern) docu-
mented the stability of ACMC in the aqueous phase
(TimeACMC in Table 2). Thus, speciation calculations were
performed at reaction times ≥2 min and ≤TimeACMC. The in-
put data for speciation calculations using PHREEQC model-
ing and selected output data are summarized in Table S2.†
The results showed that the experimental solutions exhibit
internal partial pressures of CO2 ranging from 10−1.8 to 10−2.9
atmospheres (see pCO IJg) in Table S2†). As such, the pCO pres-
2
2
sure of the experimental solutions is higher compared to the
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Paper
duced during the heating of the ACMC solids during
thermogravimetric analyses (thermally driven MgCO3 segrega-
tion).20 More recently, the 13C-NMR measurements on
ACMCs by Yang et al.41 showed two types of carbonate ions
in synthetic ACMC solids, whose short-range orders are iden-
tical to those of ACC and AMC. Based on these observations,
Yang et al.41 suggested that the ACMC solids comprise a
homogeneous mixture of the nano-clusters of ACC and AMC.
In order to test the validity of the Ca–Mg carbonate solid-
solution system, we calculated the IAP values of the experi-
mental solutions at chemical steady state conditions with re-
spect to the endmembers ACC and AMC according to the
equations IAPACC = (aCa2+)·(aCO32−) and IAPAMC = (aMg2+)
·(aCO32−), respectively. As the Mg content of the solid in-
creases from
2 to 78 mol% Mg, the logIJIAPAMC) and
Fig. 5 Average Mg content of ACMC, [Mg]ACMC_av, versus the average
logIJIAPACC) values change from −5.4 to −4.5 and −6.2 to −6.5,
ion activity product of ACMC, logIJIAPACMC av (resembling the solubility
)
respectively. If the ACMCs consist of an ideal two phase mix-
product of the ACMC phase), calculated according to eqn (1) at
chemical steady state conditions (see values in Table 2). The
logIJIAPACMC) values for Mg-free ACC are taken from Brečević and Niel-
sen18 and Gebauer et al.,19 while those for ACMC with ≤10 mol% Mg
are taken from Purgstaller et al.15 The analytical uncertainties are in-
cluded in the symbol size.
ture of ACC and AMC, we would expect constant logIJIAPACC
)
and logIJIAPAMC) in all experiments, irrespective of the Mg
content of the solid initially introduced into the reactor. As
such, the presence of discrete AMC and ACC phases in the
ACMC solid can be excluded. In contrast, the obtained linear
correlation between the average logIJIAPACMC) and [Mg]ACMC
values (see Fig. 5) points toward the presence of an homoge-
neous single phase in each experimental run.
logIJIAPACMC) values of −6.19 0.02 and −6.13 0.02 for ACMCs
with 2 and 10 mol% Mg, respectively, lay within the range of
It is also worth pointing out that the 13C-NMR measure-
ments on ACMCs by Yang et al.41 indicated the existence of
bicarbonate species embedded in the matrix formed by ACC
and AMC. Moreover, based on the Mg isotope composition of
Mg-ACCs, Mavromatis et al.42 suggested that during the very
fast precipitation of the amorphous phase from a highly
−6.14
0.04 for the low-Mg ACMC reported by Purgstaller
et al.15 In this respect, it has to be noted that the solids col-
lected in the present experiments were filtered through 0.2 μm
membranes. Although the separation yields aggregated spheri-
cal nano-particles (see Fig. S4†) and the solutions were visibly
clear after filtration, the presence of ACMC individuals in the
filtered solutions cannot be completely ruled out at this stage.
Partially remaining ACMC in the solution would slightly in-
crease the measured ion concentrations. However, the linear fit
of the herein obtained logIJIAPACMC) values is in good agree-
ment with the logIJIAPACC) value from Brečević and Nielsen,18
who determined the solubility of ACC by titration of water into
the suspension without physical separation of the solid phase,
indicating the effect of ACMC individuals in the filtered solu-
tions to be negligibly small. The positive correlation between
the logIJIAPACMC) and [Mg]ACMC values obtained from this study,
from Brečević and Nielsen18 and Purgstaller et al.15 can be de-
scribed by the expression:
+
0
supersaturated solution, MgHCO3 and MgCO3 species
might be directly incorporated into the precipitating solid to-
gether with MgIJH2O)62+. If this holds true, the activities of Ca
and Mg carbonate species have to be considered in the calcu-
lation of the IAPACMC values. For this purpose, however, a
more detailed characterization of the structure of ACMC is re-
quired, which should be in the focus of future studies.
Although the solubility of ACC is about two orders of mag-
nitude higher than that of calcite, the dependence of IAPACMC
on the Mg content of the solid (Fig. 5) is somehow similar to
that observed for the Mg-calcite system.43 Indeed experimen-
tal work conducted during the last 3 decades documents an
overall increase in solubility of Mg-bearing calcites with in-
creasing Mg content.43–45 Note, that significant discrepancies
exist between reported solubility values of Mg-calcites, which
were attributed to differences in mineral source and structure
(biogenic or synthetic solids) as well as different experimen-
tal setups.43–45 The change in calcite solubility as Mg is incor-
porated in its structure can likely be attributed to the distor-
tion of the octahedral site in calcite. The substitution of Ca
by Mg in the calcite structure results in a decrease of inter-
atomic metal–O distances and shortening of the c-axis.46,47
Consequently, Ca, C and O atoms exhibit large thermal mo-
tions which cause periodic stretching and weakening of the
bonds in the structure.46
log(IAPACMC) = 0.0174 0.0013 × [Mg]ACMC − 6.278 0.046 (3)
where R2 = 0.98.
Note here that the calculation of the IAPACMC values is
based on the assumption that the ACMCs are amorphous
Ca–Mg carbonate solid-solutions. However, a previous study
on synthetic ACMCs by Radha et al.20 points towards a phase
separation of ACMC solids with ≥47 mol% Mg. It has been
suggested that ACMC with ≥47 mol% are composed by a
mixture of a material nearly pure AMC with a material nearly
50 mol% Mg. However, such heterogeneity may also be in-
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In contrast to the crystalline Ca–Mg carbonate system,
Water, Minerals and Rocks. This work has been financially
supported by Austrian Science Fund (FWF) project number T
920-N29, and in part by DFG-FWF collaborative research ini-
tiative CHARON II (DFG Forschergruppe 1644; FWF I3028-
N29) and by NAWI Graz.
amorphous carbonates exhibit no order beyond 1.5 nm.40,48 In
situ Raman results from the present work and from previous
studies15,40 however showed that the substitution of Ca2+ by
Mg2+ changes the average metal–oxygen bond lengths in the
amorphous phase, which might change its short-range order
and solubility. Moreover, the linear relationship between the
IAPACMC and [Mg]ACMC values (Fig. 1) as well as of the [Mg]ACMC
and nH2Op.f. values (Fig. 2) suggests that the increase in solu-
bility of ACMC as a function of the Mg content is attributed to
the increasing water content in the structure of ACMC. A simi-
lar behavior has been observed for the hydrated crystalline Mg-
carbonate system, where the more hydrated lansfordite
(MgCO3·5H2O) exhibits a higher solubility than the less hy-
drated nesquehonite (MgCO3·3H2O) at 25 °C.49
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