Additive impact on early-stage magnesium carbonate mineralisation
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Keywords: amorphous magnesium carbonate, citrate, prenucleation clusters
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Abstract
Figures
 Figure 1 Results of titration experiments. (a) Evolution of transmittance in the presence of citrate at pH 11. Black arrows mark the point at which samples were taken for transmission electron microscopy (TEM) analysis. (b) Bar plot illustrating the effect of citrate on the different slopes of the decreasing part of the transmittance curve. (c) Time evolution of calculated (blue line) and measured (red line) conductivity. (d) Evolution of the calculated concentration of ion associates in the presence of different amounts of citrate at pH 11. For details on the calculation see Supplementary Information. |  Figure 2 Size evolution of the different (pre- and post-nucleation) species formed during titration experiments obtained by in situ dynamic light scattering (DLS): (a) size range from 0 to 40 nm, control runs; (b) size range from 0 to 6000 nm, control runs; (c) size range from 0 to 40 nm, experiments in the presence of 1 mM citrate; (d) size range from 0 to 6000 nm, experiments in the presence of 1 mM citrate. Onset of nucleation is marked with a red line. |  Figure 3 Nanostructural features of the pre- and post-nucleation species. (a, b) TEM images and SAED patterns of dried aliquots drawn from the solution immediately prior to the onset of nucleation—(a) control runs and (b) sodium citrate 1 mM experiments. (c, d) FESEM secondary electron images of the final precipitates—(c) control runs and (d) sodium citrate 1 mM experiments—separated by filtration from the reaction media at the end of the titration experiments. |  Figure 4 Scheme of the multi-step formation of AMC in control solutions (blue circles, no additives in the media) and in the presence of sodium citrate (red circles). |
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Introduction
MgCO3 minerals represent only a minor percentage of the carbonate deposits on the Earth. Consequently, fewer works have focused on magnesium carbonates compared to their calcium counterparts (Scheller et al., 2021
Scheller, E.L., Swindle, C., Grotzinger, J., Barnhart, H., Bhattacharjee, S., Ehlmann, B.L., Farley, K., Fischer, W.W., Greenberger, R., Ingalls, M., Martin, P.E., Osorio-Rodriguez, D., Smith, B. (2021) Formation of Magnesium Carbonates on Earth and Implications for Mars. Journal of Geophysical Research-Planets 126. https://doi.org/10.1029/2021JE006828
). Several anhydrous and hydrous minerals can be formed in the system, although magnesite (MgCO3) is the thermodynamically stable phase (Hanchen et al., 2008Hanchen, M., Prigiobbe, V., Baciocchi, R., Mazzotti, M. (2008) Precipitation in the Mg-carbonate system - effects of temperature and CO2 pressure. Chemical Engineering Science 63, 1012–1028. https://doi.org/10.1016/j.ces.2007.09.052
). However, for kinetic reasons it does not form under surficial P–T conditions. From an environmental point of view, carbonation processes are important since they reduce CO2 concentration in the atmosphere and regulate the Earth’s climate (e.g., Berner et al.,1983Berner, R.A., Lasaga, A.C., Garrels, R.M. (1983) The Carbonate-Silicate Geochemical Cycle and Its Effect on Atmospheric Carbon-Dioxide over the Past 100 Million Years. American Journal of Science 283, 641–683. https://doi.org/10.2475/ajs.283.7.641
). On the Earth’s surface, carbonation occurs through chemical weathering of Ca2+, Mg2+ and/or Fe2+ primary silicates. Although significant research has focused on mimicking this process for long-term CO2 storage (MacDowell et al., 2010MacDowell, N., Florin, N., Buchard, A., Hallett, J., Galindo, A., Jackson, G., Adjiman, C.S., Williams, C.K., Shah, N., Fennell, P. (2010) An overview of CO2 capture technologies. Energy & Environmental Science 3, 1645–1669. https://doi.org/10.1039/c004106h
; Bui et al., 2018Bui, M., Adjiman, C.S., Bardow, A., Anthony, E.J., Boston, A., Brown, S., Fennell, P.S., Fuss, S., Galindo, A., Hackett, L.A., Hallett, J.P., Herzog, H.J., Jackson, G., Kemper, J., Krevor, S., Maitland, G.C., Matuszewski, M., Metcalfe, I.S., Petit, C., Puxty, G., Reimer, J., Reiner, D.M., Rubin, E.S., Scott, S.A., Shah, N., Smit, B., Trusler, J.P.M., Webley, P., Wilcox, J., Mac Dowell, N. (2018) Carbon capture and storage (CCS): the way forward. Energy & Environmental Science 11, 1062–1176. https://doi.org/10.1039/C7EE02342A
), this strategy is limited in the case of Mg-minerals due to the elevated temperature needed for the direct formation of magnesite and its slow rate of precipitation from solution.Additionally, the synthesis of MgCO3 with specific morphologies and structures such as rosette spheres (Zhang et al., 2006
Zhang, Z.P., Zheng, Y.J., Ni, Y.W., Liu, Z.M., Chen, J.P., Liang, X.M. (2006) Temperature- and pH-dependent morphology and FT-IR analysis of magnesium carbonate hydrates. Journal of Physical Chemistry B 110, 12969–12973. https://doi.org/10.1021/jp061261j
), needle-like particles (Cheng, 2009Cheng, W.T., Li, Z.B., Demopoulos, G.P. (2009) Effects of Temperature on the Preparation of Magnesium Carbonate Hydrates by Reaction Of MgCl2 with Na2CO3. Chinese Journal of Chemical Engineering 17, 661–666. https://doi.org/10.1016/S1004-9541(08)60260-8
) and mesoporous, nanostructured nanomaterials (e.g., Baglioni and Giorgi, 2006Baglioni, P., Giorgi, R. (2006) Soft and hard nanomaterials for restoration and conservation of cultural heritage. Soft Matter 2, 293–303. https://doi.org/10.1039/b516442g
), has been pursued for a broad variety of applications. Moreover, the mineral phase, morphology, and microstructural evolution of magnesium carbonates, formed upon carbonation of dolomitic lime, determine the physico-mechanical characteristics and functionality of mortars and plasters (e.g., Elsen et al., 2022Elsen, J., Jackson, M.D., Ruiz-Agudo, E. (2022) Historic Concrete Science: Opus Caementicium to “Natural Cements”. Elements 18, 301–307. https://doi.org/10.2138/gselements.18.5.301
; Oriols et al., 2022Oriols, N., Salvado, N., Pradell, T., Jimenez, N., Cotte, M., Gonzalez, V., Buti, S. (2022) Carbonation of fresco mural paintings with a dolomitic mortar. Cement and Concrete Research 157. https://doi.org/10.1016/j.cemconres.2022.106828
).In all the above applications, the early stages of MgCO3 formation may significantly impact the stability, morphology and size of the final product. Research has focused on accelerating MgCO3 formation by modifying the precipitation environment using additives (Toroz et al., 2022
Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
and references therein). Additives have been also suggested to lower the barrier for Mg2+ dehydration in solution (Toroz et al., 2022Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
), which could impact the water content of the final phase formed. Recent studies have shown that MgCO3 precipitation is non-classic, and report the occurrence of amorphous magnesium carbonate (AMC) prior to the stable carbonate phase (White et al., 2014White, C.E., Henson, N.J., Daemen, L.L., Hartl, M., Page, K. (2014) Uncovering the True Atomic Structure of Disordered Materials: The Structure of a Hydrated Amorphous Magnesium Carbonate (MgCO3·3D2O). Chemistry of Materials 26, 2693–2702. https://doi.org/10.1021/cm500470g
; Tanaka et al., 2019Tanaka, J., Kawano, J., Nagai, T., Teng, H. (2019) Transformation process of amorphous magnesium carbonate in aqueous solution. Journal of Mineralogical and Petrological Sciences 114, 105–109. https://doi.org/10.2465/jmps.181119b
). The early stages of MgCO3 formation via AMC remain, however, highly unexplored as compared to CaCO3 (e.g., Politi et al., 2010Politi, Y., Batchelor, D.R., Zaslansky, P., Chmelka, B.F., Weaver, J.C., Sagi, I., Weiner, S., Addadi, L. (2010) Role of Magnesium Ion in the Stabilization of Biogenic Amorphous Calcium Carbonate: A Structure-Function Investigation. Chemistry of Materials 22, 161–166. https://doi.org/10.1021/cm902674h
; Rodriguez-Blanco et al., 2011Rodriguez-Blanco, J.D., Shaw, S., Benning, L.G. (2011) The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite. Nanoscale 3, 265–271. https://doi.org/10.1039/C0NR00589D
; Bots et al., 2012Bots, P., Benning, L.G., Rodriguez-Blanco, J.D., Roncal-Herrero, T., Shaw, S. (2012) Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC). Crystal Growth & Design 12, 3806–3814. https://doi.org/10.1021/cg300676b
; Rodriguez-Navarro et al., 2015Rodriguez-Navarro, C., Kudlacz, K., Cizer, O., Ruiz-Agudo, E. (2015) Formation of amorphous calcium carbonate and its transformation into mesostructured calcite. CrystEngComm 17, 58–72. https://doi.org/10.1039/C4CE01562B
) and only a few studies have aimed at elucidating how these early stages are impacted by the presence of additives. Here, the synthesis of AMC was carried out at constant pH using a potentiometric titration setup. We aim at investigating the influence of sodium citrate, a low molecular weight additive, on the nucleation, growth, and stability of MgCO3 phases.top
Results and Discussion
Citrate inhibits AMC formation by destabilising prenucleation-associates. Titration experiments revealed the strong nucleation inhibition exerted by citrate. Solution transmittance (Fig. 1a) remained initially constant (425–450 mV) and, subsequently, a drop was observed that marked the onset of MgCO3 formation (as AMC, see below). Citrate delayed the beginning of MgCO3 precipitation. This effect can be best quantified using a scale factor that compares the increase in the time for nucleation relative to the control (additive-free) experiment, which showed a stronger inhibition—longer times for precipitation—with increasing citrate concentration (Fig. 1b). Interestingly, the transmittance plot showed different slopes after the precipitation onset, slopes being shallower at higher citrate concentrations.
Figure 1 Results of titration experiments. (a) Evolution of transmittance in the presence of citrate at pH 11. Black arrows mark the point at which samples were taken for transmission electron microscopy (TEM) analysis. (b) Bar plot illustrating the effect of citrate on the different slopes of the decreasing part of the transmittance curve. (c) Time evolution of calculated (blue line) and measured (red line) conductivity. (d) Evolution of the calculated concentration of ion associates in the presence of different amounts of citrate at pH 11. For details on the calculation see Supplementary Information.
Additionally, MgCO3 precipitation can be tracked by conductivity measurements. Experimental data were compared to theoretical values (κcal, see Supplementary Information), showing significant deviation. Before precipitation, a continuous decrease in the conductivity of the solution was measured, while calculated values showed a steady increase due to the constant Mg2+ (and Cl−) addition (Fig. 1c). This can be related to the development of ion associates in solution (i.e. any solution specie containing Mg2+ and CO32−). While the formation of simple ion pairs in the system is certainly a possibility, previous results by Verch and co-workers showed the presence of ion associates larger than simple ion pairs during the early stages of MgCO3 formation using analytical ultracentrifugation (AUC) (interpreted as prenucleation clusters, PNCs) (Verch et al., 2012
Verch, A., Antonietti, M., Colfen, H. (2012) Mixed calcium-magnesium pre-nucleation clusters enrich calcium. Zeitschrift Fur Kristallographie-Crystalline Materials 227, 718–722. https://doi.org/10.1524/zkri.2012.1529
). The difference between κ and κcal could thus be used in the calculation of the concentration of MgCO3 associates present in the solution (Fig. 1d) (see Supplementary Information). For citrate concentrations ≤0.1 mM, Mg-binding increased with citrate concentration. However, further increase in citrate concentration led to less pronounced Mg2+ binding into ion pairs and/or bigger associates, which are thus thermodynamically destabilised. It has been proposed that ion association prior to nucleation is predominantly driven by entropy, and not by energy release associated with ionic binding (Kellermeier et al., 2016Kellermeier, M., Raiteri, P., Berg, J.K., Kempter, A., Gale, J.D., Gebauer, D. (2016) Entropy Drives Calcium Carbonate Ion Association. ChemPhysChem 17, 3535–3541. https://doi.org/10.1002/cphc.201600653
); the release of coordination water upon ion association represents the main entropic contribution, related to the increase in H2O translational and rotational degrees of freedom (Kellermeier et al., 2016Kellermeier, M., Raiteri, P., Berg, J.K., Kempter, A., Gale, J.D., Gebauer, D. (2016) Entropy Drives Calcium Carbonate Ion Association. ChemPhysChem 17, 3535–3541. https://doi.org/10.1002/cphc.201600653
). This entropic contribution is expected to be key in the case of Mg2+ due to its strongly hydrated character. However, atomistic simulations (Toroz et al., 2022Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
) have shown that citrate promotes Mg2+ dehydration; thus, in this case, the entropy gain associated with Mg-CO3 associates formation—relative to the free ions in solution—would be lower than when no additive is present, making the development of Mg-CO3 associates in solution less favourable. The pronounced nucleation inhibition of citrate highlights: (i) the importance of dehydration events for solute clustering, and (ii) the key role of MgCO3 binding in prenucleation associates for AMC nucleation.After the precipitation onset, the measured conductivity started increasing, but it was still lower than the calculated conductivity, due to ion incorporation in the solid during growth. Assuming a 1:1 Mg2+ to CO32− ratio in the solid, and a constant concentration of ion associates in equilibrium with the solid, the growth rate can be determined as the ratio of the difference between theoretical and experimental conductivity and the molar specific conductivity. Values are plotted in Figure S-1, where it is observed that citrate inhibited AMC growth at higher concentrations, while showing a slight trend to promote growth at low concentration.
In situ dynamic light scattering (DLS) measurements provided data on the size evolution of pre- and post-nucleation species formed during titration experiments (Fig. 2). In control runs, species with a hydrodynamic radius between 5–10 nm were observed during the prenucleation regime. We interpret these species as aggregates of ion associates, possibly pre-nucleation clusters. The size range was slightly higher than that reported for CaCO3 (Gebauer et al., 2014
Gebauer, D., Kellermeier, M., Gale, J.D., Bergstrom, L., Colfen, H. (2014) Pre-nucleation clusters as solute precursors in crystallisation. Chemical Society Reviews 43, 2348–2371. https://doi.org/10.1039/C3CS60451A
), in agreement with the larger sedimentation coefficients reported for MgCO3 PNCs (Verch et al., 2012Verch, A., Antonietti, M., Colfen, H. (2012) Mixed calcium-magnesium pre-nucleation clusters enrich calcium. Zeitschrift Fur Kristallographie-Crystalline Materials 227, 718–722. https://doi.org/10.1524/zkri.2012.1529
; Gebauer et al., 2014Gebauer, D., Kellermeier, M., Gale, J.D., Bergstrom, L., Colfen, H. (2014) Pre-nucleation clusters as solute precursors in crystallisation. Chemical Society Reviews 43, 2348–2371. https://doi.org/10.1039/C3CS60451A
). Subsequently, the size of the solution species remained between 2 and 20 nm. It is unlikely that such fast particle growth was exclusively due to the growth of single particles, but most probably caused by continued nanoparticle aggregation. No significant change in size was observed upon nucleation of solid MgCO3 in control runs (see dotted red line in Fig. 2c).Figure 2 Size evolution of the different (pre- and post-nucleation) species formed during titration experiments obtained by in situ dynamic light scattering (DLS): (a) size range from 0 to 40 nm, control runs; (b) size range from 0 to 6000 nm, control runs; (c) size range from 0 to 40 nm, experiments in the presence of 1 mM citrate; (d) size range from 0 to 6000 nm, experiments in the presence of 1 mM citrate. Onset of nucleation is marked with a red line.
In the case of citrate, smaller clusters (approximately 2 nm) were initially observed, which rapidly grow up to 40 nm, with a broader size distribution. These features align with the formation of a liquid precursor upon spinodal decomposition (see below). Prior to the onset of solid MgCO3 nucleation, a continuous decrease in the size of the entities in solution down to 5–10 nm was observed. This could be due to the dehydration of the dense liquid-like phase (see below) and AMC nucleation. At longer reaction times (t > 3500 s), aggregates of sizes from 1 to 5 μm were observed, not detected in the control runs. Similar trends were observed for a 5 mM citrate concentration, but the size of the different species observed was significantly larger (Fig. S-2). This size agrees with that of the individual aggregates observed by FESEM (Fig. 3). We propose that, as it has been shown in the case of Au nanoparticle stabilisation by citrate (Park and Shumaker-Parry, 2014
Park, J.-W., Shumaker-Parry, J.S. (2014) Structural study of citrate layers on gold nanoparticles: role of intermolecular interactions in stabilizing nanoparticles. Journal of the American Chemical Society. 36, 1907–1921. https://pubs.acs.org/doi/10.1021/ja4097384
), its binding to the surface of AMC nanoparticles leads to the formation of 1-D citrate chains, which assemble into layers through van der Waals interactions, leading to steric repulsion between citrate layers that prevent the initial aggregation of AMC nanoparticles. We have detected such layers in the case of citrate-stabilised amorphous calcium oxalate (Ruiz-Agudo et al., 2017Ruiz-Agudo, E., Burgos-Cara, A., Ruiz-Agudo, C., Ibanez-Velasco, A., Colfen, H., Rodriguez-Navarro, C. (2017) A non-classical view on calcium oxalate precipitation and the role of citrate. Nature Communications 8, 768. https://doi.org/10.1038/s41467-017-00756-5
). However, with increasing Mg2+ concentration, complexation between Mg2+ ions and these citrate chains may contribute to crosslinking AMC nanoparticles and promote the formation of μm-sized aggregates (Fig. 4). These observations can explain the different slopes observed in the transmittance plot (Fig. 1a) after the onset of precipitation, likely corresponding to different regimes of the growth and/or aggregation of AMC nanoparticles. The shallower slopes observed in the presence of citrate agree with the observed growth inhibition and the retardation of nanoparticle aggregation/growth determined from conductivity and particle size measurements.Figure 3 Nanostructural features of the pre- and post-nucleation species. (a, b) TEM images and SAED patterns of dried aliquots drawn from the solution immediately prior to the onset of nucleation—(a) control runs and (b) sodium citrate 1 mM experiments. (c, d) FESEM secondary electron images of the final precipitates—(c) control runs and (d) sodium citrate 1 mM experiments—separated by filtration from the reaction media at the end of the titration experiments.
Figure 4 Scheme of the multi-step formation of AMC in control solutions (blue circles, no additives in the media) and in the presence of sodium citrate (red circles).
Finally, aliquots collected shortly after the first drop in transmittance slope were quenched in ethanol and analysed using transmission electron microscopy (TEM) and selected area electron diffraction (SAED) (Fig. 3). Shapeless structures with irregular morphologies were observed when citrate was added to the solution, resembling aggregates of spherical nanoparticles with darker contrast, connected by neck-like bridges with lighter contrast, similar to those observed during CaCO3 (e.g., Rodriguez-Navarro et al., 2015
Rodriguez-Navarro, C., Kudlacz, K., Cizer, O., Ruiz-Agudo, E. (2015) Formation of amorphous calcium carbonate and its transformation into mesostructured calcite. CrystEngComm 17, 58–72. https://doi.org/10.1039/C4CE01562B
) or CaPO4 precipitation (Ruiz-Agudo et al., 2021Ruiz-Agudo, E., Ruiz-Agudo, C., Di Lorenzo, F., Alvarez-Lloret, P., Ibanez-Velasco, A., Rodriguez-Navarro, C. (2021) Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization. ACS Biomaterials Science & Engineering 7, 2346–2357. https://doi.org/10.1021/acsbiomaterials.1c00196
). SAED confirmed their amorphous nature, interpreted as being a dried residue of an emulsion (liquid-like) precursor phase formed after spinodal decomposition, as stated above, which subsequently transforms into AMC particles (for further details on this process, see Rodriguez-Navarro et al., 2015Rodriguez-Navarro, C., Kudlacz, K., Cizer, O., Ruiz-Agudo, E. (2015) Formation of amorphous calcium carbonate and its transformation into mesostructured calcite. CrystEngComm 17, 58–72. https://doi.org/10.1039/C4CE01562B
or Ruiz-Agudo et al., 2021Ruiz-Agudo, E., Ruiz-Agudo, C., Di Lorenzo, F., Alvarez-Lloret, P., Ibanez-Velasco, A., Rodriguez-Navarro, C. (2021) Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization. ACS Biomaterials Science & Engineering 7, 2346–2357. https://doi.org/10.1021/acsbiomaterials.1c00196
and references therein). Remarkably, these distinct structures were not observed in the control samples. The diffuse rings observed in the SAED pattern of both control and citrate bearing AMCs at ∼1.5Å, ∼2.0 Å and at ∼2.5 Å match the (160), (240) and (230) d-spacings of hydromagnesite, which suggests that AMC could have a hydromagnesite-like protostructure, in agreement with previous reports (Yamamoto et al., 2021Yamamoto, G., Kyono, A., Okada, S. (2021) Temperature dependence of amorphous magnesium carbonate structure studied by PDF and XAFS analyses. Scientific Reports 11. https://doi.org/10.1038/s41598-021-02261-8
).Ex-situ characterisation of MgCO3 precipitates. Powder X-ray diffraction (XRD) confirmed that the precipitates formed at the end of the titration runs (Fig. S-3), with and without citrate, are amorphous. After ageing for one week in the reaction media, AMC recrystallised into nesquehonite (MgCO3·3H2O) and dypingite (Mg5(CO3)4(OH)2·5H2O), in the absence and in the presence of citrate, respectively (Fig. S-3). Figure S-4 shows the Fourier transform infrared (FTIR) spectra of precipitates. In control precipitates, broad absorption bands corresponding to CO32− were observed at 835 cm−1, 1024 cm−1 and 1389 cm−1 (White, 1971
White, W.B. (1971) Infrared Characterization of Water and Hydroxyl Ion in Basic Magnesium Carbonate Minerals. American Mineralogist 56, 46–53.
; Zhang et al., 2006Zhang, Z.P., Zheng, Y.J., Ni, Y.W., Liu, Z.M., Chen, J.P., Liang, X.M. (2006) Temperature- and pH-dependent morphology and FT-IR analysis of magnesium carbonate hydrates. Journal of Physical Chemistry B 110, 12969–12973. https://doi.org/10.1021/jp061261j
). In addition, the O-H bending and stretching modes of water gave rise to a low intensity shoulder at 1644 cm−1 and a broad band observed in the 3000 cm-l range. The weak, broad shoulder observed at 3664 cm−1 (not seen in the presence of citrate) could be linked to OH− groups present in the control AMC. The broad features observed, in addition to the presence of the band at 1024 cm−1 and the absence of the band at ∼748 cm−1, support the amorphous nature of the precipitates, when compared to the sharper, better defined features of the FTIR spectra of crystalline MgCO3 (White, 1971White, W.B. (1971) Infrared Characterization of Water and Hydroxyl Ion in Basic Magnesium Carbonate Minerals. American Mineralogist 56, 46–53.
; Zhang et al., 2006Zhang, Z.P., Zheng, Y.J., Ni, Y.W., Liu, Z.M., Chen, J.P., Liang, X.M. (2006) Temperature- and pH-dependent morphology and FT-IR analysis of magnesium carbonate hydrates. Journal of Physical Chemistry B 110, 12969–12973. https://doi.org/10.1021/jp061261j
; Tanaka et al., 2019Tanaka, J., Kawano, J., Nagai, T., Teng, H. (2019) Transformation process of amorphous magnesium carbonate in aqueous solution. Journal of Mineralogical and Petrological Sciences 114, 105–109. https://doi.org/10.2465/jmps.181119b
). Citrate absorption bands (Mudunkotuwa and Grassian, 2010Mudunkotuwa, I.A., Grassian, V.H. (2010) Citric Acid Adsorption on TiO2 Nanoparticles in Aqueous Suspensions at Acidic and Circumneutral pH: Surface Coverage, Surface Speciation, and Its Impact on Nanoparticle-Nanoparticle Interactions. Journal of the American Chemical Society 132, 14986–14994. https://doi.org/10.1021/ja106091q
) overlapped with carbonate bands, and were not observed. However, carbonate bands were blue-shifted to 858, 1084 and 1402 cm−1 in the presence of citrate; this could be explained by citrate-Mg interactions that weaken Mg-CO3 bonding in the MgCO3 phase, thus increasing the strength of C-O bonds. Also, the subtle red-shift found in the O-H vibration bands could indicate citrate-OH bonding.Thermogravimetric analysis (TGA) of control AMC showed two main weight losses (Fig. S-5a), associated with AMC dehydration (∼280 °C) and decarbonation (280–950 °C) (Radha et al., 2012
Radha, A.V., Fernandez-Martinez, A., Hu, Y.D., Jun, Y.S., Waychunas, G.A., Navrotsky, A. (2012) Energetic and structural studies of amorphous Ca1-xMgxCO3·nH2O (0 ≤ x ≤ 1). Geochimica Et Cosmochimica Acta 90, 83–95. https://doi.org/10.1016/j.gca.2012.04.056
). In the case of citrate-bearing AMC, two dehydration steps were seen, suggesting that water exists in different environments. The second step was observed at higher temperatures, indicating that part of the water is more tightly bonded in the presence of citrate. TGA revealed that control AMC contained 1.82 ± 0.17 moles of water per mole of MgCO3, which is in the range of values previously reported (1.28–2) (e.g., Radha et al., 2012Radha, A.V., Fernandez-Martinez, A., Hu, Y.D., Jun, Y.S., Waychunas, G.A., Navrotsky, A. (2012) Energetic and structural studies of amorphous Ca1-xMgxCO3·nH2O (0 ≤ x ≤ 1). Geochimica Et Cosmochimica Acta 90, 83–95. https://doi.org/10.1016/j.gca.2012.04.056
; Tanaka et al., 2019Tanaka, J., Kawano, J., Nagai, T., Teng, H. (2019) Transformation process of amorphous magnesium carbonate in aqueous solution. Journal of Mineralogical and Petrological Sciences 114, 105–109. https://doi.org/10.2465/jmps.181119b
; (Yamamoto et al., 2021Yamamoto, G., Kyono, A., Okada, S. (2021) Temperature dependence of amorphous magnesium carbonate structure studied by PDF and XAFS analyses. Scientific Reports 11. https://doi.org/10.1038/s41598-021-02261-8
)). This phase plots close to dypingite in the ternary diagram of hydrated MgCO3 (Fig. S-6). The water content of AMC was reduced for the lowest citrate concentrations (Fig. S-6). This could be related to the fact that carboxylate anions lower the barrier for Mg2+ dehydration in solution by stabilising undercoordinated Mg2+ hydration configurations, as shown by recent molecular simulations (Toroz et al., 2022Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
). The differential scanning calorimetry (DSC) profile of AMC (Fig. S-5b) showed two endothermic events at 130 °C and 430 °C, corresponding to dehydration and decarbonation, respectively. Since no other thermal event was detected, it is clear that AMC decomposes without crystallisating. TGA and DSC results showed an increase in decarbonation temperature (ca. 100 °C) in the presence of citrate, which suggests that the dehydrated AMC is more stable. Finally, zeta potential values were negative in all AMC samples (Table S-1). The presence of OH− groups in the structure can explain such values. The less negative values of the citrate-bearing AMC could be related with its lower degree of hydration, possibly associated with its lower OH− content.top
Conclusions and Implications
We propose that MgCO3 solution species form by Mg2+ and CO32− association, driven by the entropy gain linked to the release of coordination water, and subsequent grow by aggregation prior to the beginning of AMC nucleation (Fig. 4). −COO− in citrate can lower the barrier for Mg2+ dehydration by stabilising undercoordinated Mg2+ hydration configurations, as shown by molecular simulations (Toroz et al., 2022
Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
and references therein). The entropy gain associated with water removal during Mg2+ and CO32− association will thus be lower than in the absence of citrate, making Mg2+ binding in the pre-nucleation ion associates less favourable. Formation of ion associates and subsequent aggregation are key processes for AMC nucleation, and both are hampered by citrate. This inhibits AMC nucleation, so that the system passes the spinodal limit leading to the formation of a dense liquid-like precursor. This is technologically relevant, since a small-weight, environmentally friendly carboxylic acid, such as citrate, bears similar effects as more complex polymers, and can potentially be used to precipitate particles with intricate morphologies. Moreover, low concentrations of citrate (<1 mM) accelerate AMC growth and reduce the water content of AMC, resulting in the formation of less hydrated crystalline MgCO3 phases, as is the case of dypingite, as opposed to nesquehonite formed after AMC in the absence of citrate. Citrate works similarly in other mineral systems, which could be linked to its high radial charge density that enables its interaction with ions in solution, promoting Mg2+ (or other cations or ion associates) dehydration. This could have a significant impact on mineralisation where cation dehydration is an essential step. Less hydrated Mg-carbonate phases are more stable and efficient as carbon storage medium compared to highly hydrated phases, due to the lower mass and volume per mole of CO2 stored and the greater chemical stability/lower solubility, suitable for long-term storage, ultimately having a positive effect on the effectiveness and expense of CO2 mineralisation (Swanson et al., 2014Swanson, E.J., Fricker, K.J., Sun, M., Park, A.H. (2014) Directed precipitation of hydrated and anhydrous magnesium carbonates for carbon storage. Physical Chemistry Chemical Physics 16, 23440–234450. https://doi.org/10.1039/C4CP03491K
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Acknowledgements
This research has been funded by the EC (ACT_ERA NET no. 691712, PCI2019-111931-2 and H2020 Programme, Marie Skłodowska-Curie Action ETN-ITN SUBlime, grant agreement n° 955986), the Spanish Government (grant PID2021-125305NB-I00), Junta de Andalucía (research group RNM-179 and grant P20_00675) and University of Granada (Unidad Científica de Excelencia UCE-PP2016-05).
Editor: Satish Myneni
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References
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Show in context Additionally, the synthesis of MgCO3 with specific morphologies and structures such as rosette spheres (Zhang et al., 2006), needle-like particles (Cheng, 2009) and mesoporous, nanostructured nanomaterials (e.g., Baglioni and Giorgi, 2006), has been pursued for a broad variety of applications.
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Show in context From an environmental point of view, carbonation processes are important since they reduce CO2 concentration in the atmosphere and regulate the Earth’s climate (e.g., Berner et al.,1983).
View in article
Bots, P., Benning, L.G., Rodriguez-Blanco, J.D., Roncal-Herrero, T., Shaw, S. (2012) Mechanistic Insights into the Crystallization of Amorphous Calcium Carbonate (ACC). Crystal Growth & Design 12, 3806–3814. https://doi.org/10.1021/cg300676b
Show in context The early stages of MgCO3 formation via AMC remain, however, highly unexplored as compared to CaCO3 (e.g., Politi et al., 2010; Rodriguez-Blanco et al., 2011; Bots et al., 2012; Rodriguez-Navarro et al., 2015) and only a few studies have aimed at elucidating how these early stages are impacted by the presence of additives.
View in article
Bui, M., Adjiman, C.S., Bardow, A., Anthony, E.J., Boston, A., Brown, S., Fennell, P.S., Fuss, S., Galindo, A., Hackett, L.A., Hallett, J.P., Herzog, H.J., Jackson, G., Kemper, J., Krevor, S., Maitland, G.C., Matuszewski, M., Metcalfe, I.S., Petit, C., Puxty, G., Reimer, J., Reiner, D.M., Rubin, E.S., Scott, S.A., Shah, N., Smit, B., Trusler, J.P.M., Webley, P., Wilcox, J., Mac Dowell, N. (2018) Carbon capture and storage (CCS): the way forward. Energy & Environmental Science 11, 1062–1176. https://doi.org/10.1039/C7EE02342A
Show in context Although significant research has focused on mimicking this process for long-term CO2 storage (MacDowell et al., 2010; Bui et al., 2018), this strategy is limited in the case of Mg-minerals due to the elevated temperature needed for the direct formation of magnesite and its slow rate of precipitation from solution.
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Cheng, W.T., Li, Z.B., Demopoulos, G.P. (2009) Effects of Temperature on the Preparation of Magnesium Carbonate Hydrates by Reaction Of MgCl2 with Na2CO3. Chinese Journal of Chemical Engineering 17, 661–666. https://doi.org/10.1016/S1004-9541(08)60260-8
Show in context Additionally, the synthesis of MgCO3 with specific morphologies and structures such as rosette spheres (Zhang et al., 2006), needle-like particles (Cheng, 2009) and mesoporous, nanostructured nanomaterials (e.g., Baglioni and Giorgi, 2006), has been pursued for a broad variety of applications.
View in article
Elsen, J., Jackson, M.D., Ruiz-Agudo, E. (2022) Historic Concrete Science: Opus Caementicium to “Natural Cements”. Elements 18, 301–307. https://doi.org/10.2138/gselements.18.5.301
Show in context Moreover, the mineral phase, morphology, and microstructural evolution of magnesium carbonates, formed upon carbonation of dolomitic lime, determine the physico-mechanical characteristics and functionality of mortars and plasters (e.g., Elsen et al., 2022; Oriols et al., 2022).
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Gebauer, D., Kellermeier, M., Gale, J.D., Bergstrom, L., Colfen, H. (2014) Pre-nucleation clusters as solute precursors in crystallisation. Chemical Society Reviews 43, 2348–2371. https://doi.org/10.1039/C3CS60451A
Show in context The size range was slightly higher than that reported for CaCO3 (Gebauer et al., 2014), in agreement with the larger sedimentation coefficients reported for MgCO3 PNCs (Verch et al., 2012; Gebauer et al., 2014).
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Hanchen, M., Prigiobbe, V., Baciocchi, R., Mazzotti, M. (2008) Precipitation in the Mg-carbonate system - effects of temperature and CO2 pressure. Chemical Engineering Science 63, 1012–1028. https://doi.org/10.1016/j.ces.2007.09.052
Show in context Several anhydrous and hydrous minerals can be formed in the system, although magnesite (MgCO3) is the thermodynamically stable phase (Hanchen et al., 2008).
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Kellermeier, M., Raiteri, P., Berg, J.K., Kempter, A., Gale, J.D., Gebauer, D. (2016) Entropy Drives Calcium Carbonate Ion Association. ChemPhysChem 17, 3535–3541. https://doi.org/10.1002/cphc.201600653
Show in context It has been proposed that ion association prior to nucleation is predominantly driven by entropy, and not by energy release associated with ionic binding (Kellermeier et al., 2016); the release of coordination water upon ion association represents the main entropic contribution, related to the increase in H2O translational and rotational degrees of freedom (Kellermeier et al., 2016).
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MacDowell, N., Florin, N., Buchard, A., Hallett, J., Galindo, A., Jackson, G., Adjiman, C.S., Williams, C.K., Shah, N., Fennell, P. (2010) An overview of CO2 capture technologies. Energy & Environmental Science 3, 1645–1669. https://doi.org/10.1039/c004106h
Show in context Although significant research has focused on mimicking this process for long-term CO2 storage (MacDowell et al., 2010; Bui et al., 2018), this strategy is limited in the case of Mg-minerals due to the elevated temperature needed for the direct formation of magnesite and its slow rate of precipitation from solution.
View in article
Mudunkotuwa, I.A., Grassian, V.H. (2010) Citric Acid Adsorption on TiO2 Nanoparticles in Aqueous Suspensions at Acidic and Circumneutral pH: Surface Coverage, Surface Speciation, and Its Impact on Nanoparticle-Nanoparticle Interactions. Journal of the American Chemical Society 132, 14986–14994. https://doi.org/10.1021/ja106091q
Show in context Citrate absorption bands (Mudunkotuwa and Grassian, 2010) overlapped with carbonate bands, and were not observed. However, carbonate bands were blue-shifted to 858, 1084 and 1402 cm−1 in the presence of citrate; this could be explained by citrate-Mg interactions that weaken Mg-CO3 bonding in the MgCO3 phase, thus increasing the strength of C-O bonds.
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Oriols, N., Salvado, N., Pradell, T., Jimenez, N., Cotte, M., Gonzalez, V., Buti, S. (2022) Carbonation of fresco mural paintings with a dolomitic mortar. Cement and Concrete Research 157. https://doi.org/10.1016/j.cemconres.2022.106828
Show in context Moreover, the mineral phase, morphology, and microstructural evolution of magnesium carbonates, formed upon carbonation of dolomitic lime, determine the physico-mechanical characteristics and functionality of mortars and plasters (e.g., Elsen et al., 2022; Oriols et al., 2022).
View in article
Park, J.-W., Shumaker-Parry, J.S. (2014) Structural study of citrate layers on gold nanoparticles: role of intermolecular interactions in stabilizing nanoparticles. Journal of the American Chemical Society. 36, 1907–1921. https://pubs.acs.org/doi/10.1021/ja4097384
Show in context We propose that, as it has been shown in the case of Au nanoparticle stabilisation by citrate (Park and Shumaker-Parry, 2014), its binding to the surface of AMC nanoparticles leads to the formation of 1-D citrate chains, which assemble into layers through van der Waals interactions, leading to steric repulsion between citrate layers that prevent the initial aggregation of AMC nanoparticles.
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Politi, Y., Batchelor, D.R., Zaslansky, P., Chmelka, B.F., Weaver, J.C., Sagi, I., Weiner, S., Addadi, L. (2010) Role of Magnesium Ion in the Stabilization of Biogenic Amorphous Calcium Carbonate: A Structure-Function Investigation. Chemistry of Materials 22, 161–166. https://doi.org/10.1021/cm902674h
Show in context The early stages of MgCO3 formation via AMC remain, however, highly unexplored as compared to CaCO3 (e.g., Politi et al., 2010; Rodriguez-Blanco et al., 2011; Bots et al., 2012; Rodriguez-Navarro et al., 2015) and only a few studies have aimed at elucidating how these early stages are impacted by the presence of additives.
View in article
Radha, A.V., Fernandez-Martinez, A., Hu, Y.D., Jun, Y.S., Waychunas, G.A., Navrotsky, A. (2012) Energetic and structural studies of amorphous Ca1-xMgxCO3·nH2O (0 ≤ x ≤ 1). Geochimica Et Cosmochimica Acta 90, 83–95. https://doi.org/10.1016/j.gca.2012.04.056
Show in context TGA revealed that control AMC contained 1.82 ± 0.17 moles of water per mole of MgCO3, which is in the range of values previously reported (1.28–2) (e.g., Radha et al., 2012; Tanaka et al., 2019; (Yamamoto et al., 2021)).
View in article
Rodriguez-Blanco, J.D., Shaw, S., Benning, L.G. (2011) The kinetics and mechanisms of amorphous calcium carbonate (ACC) crystallization to calcite, via vaterite. Nanoscale 3, 265–271. https://doi.org/10.1039/C0NR00589D
Show in context The early stages of MgCO3 formation via AMC remain, however, highly unexplored as compared to CaCO3 (e.g., Politi et al., 2010; Rodriguez-Blanco et al., 2011; Bots et al., 2012; Rodriguez-Navarro et al., 2015) and only a few studies have aimed at elucidating how these early stages are impacted by the presence of additives.
View in article
Rodriguez-Navarro, C., Kudlacz, K., Cizer, O., Ruiz-Agudo, E. (2015) Formation of amorphous calcium carbonate and its transformation into mesostructured calcite. CrystEngComm 17, 58–72. https://doi.org/10.1039/C4CE01562B
Show in context The early stages of MgCO3 formation via AMC remain, however, highly unexplored as compared to CaCO3 (e.g., Politi et al., 2010; Rodriguez-Blanco et al., 2011; Bots et al., 2012; Rodriguez-Navarro et al., 2015) and only a few studies have aimed at elucidating how these early stages are impacted by the presence of additives.
View in article
Shapeless structures with irregular morphologies were observed when citrate was added to the solution, resembling aggregates of spherical nanoparticles with darker contrast, connected by neck-like bridges with lighter contrast, similar to those observed during CaCO3 (e.g., Rodriguez-Navarro et al., 2015) or CaPO4 precipitation (Ruiz-Agudo et al., 2021).
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SAED confirmed their amorphous nature, interpreted as being a dried residue of an emulsion (liquid-like) precursor phase formed after spinodal decomposition, as stated above, which subsequently transforms into AMC particles (for further details on this process, see Rodriguez-Navarro et al., 2015 or Ruiz-Agudo et al., 2021 and references therein).
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Ruiz-Agudo, E., Burgos-Cara, A., Ruiz-Agudo, C., Ibanez-Velasco, A., Colfen, H., Rodriguez-Navarro, C. (2017) A non-classical view on calcium oxalate precipitation and the role of citrate. Nature Communications 8, 768. https://doi.org/10.1038/s41467-017-00756-5
Show in context We have detected such layers in the case of citrate-stabilised amorphous calcium oxalate (Ruiz-Agudo et al., 2017).
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Ruiz-Agudo, E., Ruiz-Agudo, C., Di Lorenzo, F., Alvarez-Lloret, P., Ibanez-Velasco, A., Rodriguez-Navarro, C. (2021) Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization. ACS Biomaterials Science & Engineering 7, 2346–2357. https://doi.org/10.1021/acsbiomaterials.1c00196
Show in context Shapeless structures with irregular morphologies were observed when citrate was added to the solution, resembling aggregates of spherical nanoparticles with darker contrast, connected by neck-like bridges with lighter contrast, similar to those observed during CaCO3 (e.g., Rodriguez-Navarro et al., 2015) or CaPO4 precipitation (Ruiz-Agudo et al., 2021).
View in article
SAED confirmed their amorphous nature, interpreted as being a dried residue of an emulsion (liquid-like) precursor phase formed after spinodal decomposition, as stated above, which subsequently transforms into AMC particles (for further details on this process, see Rodriguez-Navarro et al., 2015 or Ruiz-Agudo et al., 2021 and references therein).
View in article
Scheller, E.L., Swindle, C., Grotzinger, J., Barnhart, H., Bhattacharjee, S., Ehlmann, B.L., Farley, K., Fischer, W.W., Greenberger, R., Ingalls, M., Martin, P.E., Osorio-Rodriguez, D., Smith, B. (2021) Formation of Magnesium Carbonates on Earth and Implications for Mars. Journal of Geophysical Research-Planets 126. https://doi.org/10.1029/2021JE006828
Show in context MgCO3 minerals represent only a minor percentage of the carbonate deposits on the Earth. Consequently, fewer works have focused on magnesium carbonates compared to their calcium counterparts (Scheller et al., 2021).
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Swanson, E.J., Fricker, K.J., Sun, M., Park, A.H. (2014) Directed precipitation of hydrated and anhydrous magnesium carbonates for carbon storage. Physical Chemistry Chemical Physics 16, 23440–234450. https://doi.org/10.1039/C4CP03491K
Show in context Less hydrated Mg-carbonate phases are more stable and efficient as carbon storage medium compared to highly hydrated phases, due to the lower mass and volume per mole of CO2 stored and the greater chemical stability/lower solubility, suitable for long-term storage, ultimately having a positive effect on the effectiveness and expense of CO2 mineralisation (Swanson et al., 2014).
View in article
Tanaka, J., Kawano, J., Nagai, T., Teng, H. (2019) Transformation process of amorphous magnesium carbonate in aqueous solution. Journal of Mineralogical and Petrological Sciences 114, 105–109. https://doi.org/10.2465/jmps.181119b
Show in context Recent studies have shown that MgCO3 precipitation is non-classic, and report the occurrence of amorphous magnesium carbonate (AMC) prior to the stable carbonate phase (White et al., 2014; Tanaka et al., 2019).
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The broad features observed, in addition to the presence of the band at 1024 cm−1 and the absence of the band at ∼748 cm−1, support the amorphous nature of the precipitates, when compared to the sharper, better defined features of the FTIR spectra of crystalline MgCO3 (White, 1971; Zhang et al., 2006; Tanaka et al., 2019).
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TGA revealed that control AMC contained 1.82 ± 0.17 moles of water per mole of MgCO3, which is in the range of values previously reported (1.28–2) (e.g., Radha et al., 2012; Tanaka et al., 2019; (Yamamoto et al., 2021)).
View in article
Toroz, D., Song, F., Uddin, A., Chass, G.A., Di Tommaso, D. (2022) A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation. Crystal Growth & Design 22, 3080–3089. https://doi.org/10.1021/acs.cgd.1c01525
Show in context Research has focused on accelerating MgCO3 formation by modifying the precipitation environment using additives (Toroz et al., 2022 and references therein).
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Additives have been also suggested to lower the barrier for Mg2+ dehydration in solution (Toroz et al., 2022), which could impact the water content of the final phase formed.
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However, atomistic simulations (Toroz et al., 2022) have shown that citrate promotes Mg2+ dehydration; thus, in this case, the entropy gain associated with Mg-CO3 associates formation—relative to the free ions in solution—would be lower than when no additive is present, making the development of Mg-CO3 associates in solution less favourable.
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This could be related to the fact that carboxylate anions lower the barrier for Mg2+ dehydration in solution by stabilising undercoordinated Mg2+ hydration configurations, as shown by recent molecular simulations (Toroz et al., 2022).
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−COO− in citrate can lower the barrier for Mg2+ dehydration by stabilising undercoordinated Mg2+ hydration configurations, as shown by molecular simulations (Toroz et al., 2022 and references therein).
View in article
Verch, A., Antonietti, M., Colfen, H. (2012) Mixed calcium-magnesium pre-nucleation clusters enrich calcium. Zeitschrift Fur Kristallographie-Crystalline Materials 227, 718–722. https://doi.org/10.1524/zkri.2012.1529
Show in context While the formation of simple ion pairs in the system is certainly a possibility, previous results by Verch and co-workers showed the presence of ion associates larger than simple ion pairs during the early stages of MgCO3 formation using analytical ultracentrifugation (AUC) (interpreted as prenucleation clusters, PNCs) (Verch et al., 2012).
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The size range was slightly higher than that reported for CaCO3 (Gebauer et al., 2014), in agreement with the larger sedimentation coefficients reported for MgCO3 PNCs (Verch et al., 2012; Gebauer et al., 2014).
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White, C.E., Henson, N.J., Daemen, L.L., Hartl, M., Page, K. (2014) Uncovering the True Atomic Structure of Disordered Materials: The Structure of a Hydrated Amorphous Magnesium Carbonate (MgCO3·3D2O). Chemistry of Materials 26, 2693–2702. https://doi.org/10.1021/cm500470g
Show in context Recent studies have shown that MgCO3 precipitation is non-classic, and report the occurrence of amorphous magnesium carbonate (AMC) prior to the stable carbonate phase (White et al., 2014; Tanaka et al., 2019).
View in article
White, W.B. (1971) Infrared Characterization of Water and Hydroxyl Ion in Basic Magnesium Carbonate Minerals. American Mineralogist 56, 46–53.
Show in context In control precipitates, broad absorption bands corresponding to CO32− were observed at 835 cm−1, 1024 cm−1 and 1389 cm−1 (White, 1971; Zhang et al., 2006).
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The broad features observed, in addition to the presence of the band at 1024 cm−1 and the absence of the band at ∼748 cm−1, support the amorphous nature of the precipitates, when compared to the sharper, better defined features of the FTIR spectra of crystalline MgCO3 (White, 1971; Zhang et al., 2006; Tanaka et al., 2019).
View in article
Yamamoto, G., Kyono, A., Okada, S. (2021) Temperature dependence of amorphous magnesium carbonate structure studied by PDF and XAFS analyses. Scientific Reports 11. https://doi.org/10.1038/s41598-021-02261-8
Show in context The diffuse rings observed in the SAED pattern of both control and citrate bearing AMCs at ∼1.5Å, ∼2.0 Å and at ∼2.5 Å match the (160), (240) and (230) d-spacings of hydromagnesite, which suggests that AMC could have a hydromagnesite-like protostructure, in agreement with previous reports (Yamamoto et al., 2021).
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TGA revealed that control AMC contained 1.82 ± 0.17 moles of water per mole of MgCO3, which is in the range of values previously reported (1.28–2) (e.g., Radha et al., 2012; Tanaka et al., 2019; (Yamamoto et al., 2021)).
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Zhang, Z.P., Zheng, Y.J., Ni, Y.W., Liu, Z.M., Chen, J.P., Liang, X.M. (2006) Temperature- and pH-dependent morphology and FT-IR analysis of magnesium carbonate hydrates. Journal of Physical Chemistry B 110, 12969–12973. https://doi.org/10.1021/jp061261j
Show in context Additionally, the synthesis of MgCO3 with specific morphologies and structures such as rosette spheres (Zhang et al., 2006), needle-like particles (Cheng, 2009) and mesoporous, nanostructured nanomaterials (e.g., Baglioni and Giorgi, 2006), has been pursued for a broad variety of applications.
View in article
In control precipitates, broad absorption bands corresponding to CO32− were observed at 835 cm−1, 1024 cm−1 and 1389 cm−1 (White, 1971; Zhang et al., 2006).
View in article
The broad features observed, in addition to the presence of the band at 1024 cm−1 and the absence of the band at ∼748 cm−1, support the amorphous nature of the precipitates, when compared to the sharper, better defined features of the FTIR spectra of crystalline MgCO3 (White, 1971; Zhang et al., 2006; Tanaka et al., 2019).
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Supplementary Information
The Supplementary Information includes:
- Methodology
- Figures S-1 to S-6
- Table S-1
- Supplementary Information References
Download the Supplementary Information (PDF)
Figures
Figure 1 Results of titration experiments. (a) Evolution of transmittance in the presence of citrate at pH 11. Black arrows mark the point at which samples were taken for transmission electron microscopy (TEM) analysis. (b) Bar plot illustrating the effect of citrate on the different slopes of the decreasing part of the transmittance curve. (c) Time evolution of calculated (blue line) and measured (red line) conductivity. (d) Evolution of the calculated concentration of ion associates in the presence of different amounts of citrate at pH 11. For details on the calculation see Supplementary Information.
Figure 2 Size evolution of the different (pre- and post-nucleation) species formed during titration experiments obtained by in situ dynamic light scattering (DLS): (a) size range from 0 to 40 nm, control runs; (b) size range from 0 to 6000 nm, control runs; (c) size range from 0 to 40 nm, experiments in the presence of 1 mM citrate; (d) size range from 0 to 6000 nm, experiments in the presence of 1 mM citrate. Onset of nucleation is marked with a red line.
Figure 3 Nanostructural features of the pre- and post-nucleation species. (a, b) TEM images and SAED patterns of dried aliquots drawn from the solution immediately prior to the onset of nucleation—(a) control runs and (b) sodium citrate 1 mM experiments. (c, d) FESEM secondary electron images of the final precipitates—(c) control runs and (d) sodium citrate 1 mM experiments—separated by filtration from the reaction media at the end of the titration experiments.
Figure 4 Scheme of the multi-step formation of AMC in control solutions (blue circles, no additives in the media) and in the presence of sodium citrate (red circles).