Nirrupama
Kamala Ilango
a,
Hoang
Nguyen
a,
Mohammad
Alzeer‡
a,
Frank
Winnefeld
b and
Paivo
Kinnunen
*a
aFibre and Particle Engineering Research Unit, University of Oulu, Pentti Kaiteran katu 1, 90014 Oulu, Finland. E-mail: paivo.kinnunen@oulu.fi
bSwiss Federal Laboratories for Material Science and Technology (Empa), Laboratory for Concrete & Asphalt, Überlandstrasse 129, 8600 Dübendorf, Switzerland
First published on 5th December 2024
Nesquehonite (MgCO3·3H2O) is of interest as a carbon sink for mineral carbonation as its formation is kinetically favored at ambient temperatures and pressures and offers the highest CO2:
MgO ratio compared to most other hydrated magnesium carbonates (HMCs). However, the phase tends to convert to more stable HMCs depending on the environment and time leading to long-term instability. Here, we report a successful attempt to stabilize nesquehonite using a phosphate-based pH 7 buffer, while controlling the equilibrium of aqueous carbonate species did not stabilize the phase. Phosphate interacts with nesquehonite to form a Mg-phosphate phase on nesquehonite's surface. We suggest that a protective layer is formed, which prevents further transformation of nesquehonite.
Magnesite (MgCO3) is the most stable magnesium carbonate in the MgO–CO2–H2O system and is an ideal carbon sink; however, it requires elevated temperatures and pressures to precipitate. However, a range of metastable hydrated magnesium carbonates (HMCs) are formed depending on the reaction conditions.5 For instance, nesquehonite (MgCO3·3H2O) readily forms by the addition of MgCl2 solution to an aqueous solution of Na2CO3 at ambient conditions of 25 °C and 1 bar PCO2, while it takes about 5–15 hours at 120 °C and 3 bar PCO2 for magnesite precipitation with hydromagnesite (Mg5(CO3)4(OH)2·4H2O) as an intermediate.6 This makes nesquehonite an attractive product of several CCUS processes.7,8 Furthermore, it finds application in the production of low-CO2 construction materials such as gypsum-like products9 and thermal energy storage.10 Hydrated magnesium carbonates also have great potential as an alternative to conventional cement.11
The bottleneck in using nesquehonite in such applications is the stability of the phase as it transforms to thermodynamically more stable HMCs with a change in temperature, pH, CO2 concentration, water saturation, and/or aging.7,12,13 This phase transformation is accompanied by a large decrease in volume14 leading to structural instability and some loss of CO2. The transformation pathways of nesquehonite with temperature have been studied extensively. At 0 K, the free energy of formation of nesquehonite (−245.46 kJ mol−1 from oxides) is much lower than for hydromagnesite (−139.33 kJ mol−1) and magnesite (−114.14 kJ mol−1) indicating high stability; however, the phase becomes less stable with increase in temperature.15 Early studies have shown that in an aqueous medium, the phase starts decomposing at around 50 °C to hydromagnesite through various intermediates, such as amorphous magnesium carbonates and dypingite (Mg5(CO3)4(OH)2·5–8H2O).5,12,16 However, recent calculations from thermodynamic data show that the transition occurs at much lower temperatures (see Fig. S1 in the ESI†).17 To date, there are not many attempts to stabilize nesquehonite reported in the literature, and the mechanism and pathway to stabilize the phase remain unclear.18
The chemical formula of nesquehonite has been a subject of open debate in the literature, some favoring the presence of bicarbonate Mg(HCO3)(OH)·2H2O19 in contrast to carbonate MgCO3·3H2O20,21 in the structure. Here, we do not intend to contribute to this debate, but consider the possibility of the existence of either bicarbonate (HCO3−) or carbonate (CO32−) ions in the crystal. Therefore, the equation, HCO3− ↔ CO3− + H+ may play a vital role in regulating the stability of nesquehonite. The equilibrium of the solution shifts towards carbonate with the increase in temperature and pH (Fig. S2 in ESI†). Furthermore, the transformation of nesquehonite to dypingite and/or hydromagnesite, both these minerals host carbon in the form of carbonates in the structure, is accelerated with increase in temperature and is accompanied by a shift in the solution equilibrium from bicarbonate to carbonate, which is known to be a temperature-dependent relation.22 Here, we hypothesize that the stability of nesquehonite is connected to the solution equilibrium between bicarbonate and carbonate ions, which drives the conversion of nesquehonite to other HMCs via e.g., temperature increase. In that case, nesquehonite could be stabilized by regulating the equilibrium between bicarbonates and carbonates. Thus, to test this hypothesis, we experimentally validate whether nesquehonite is stabilized at elevated temperatures by shifting the solution equilibrium towards bicarbonate through three approaches:
1. By increasing the carbonate ion concentration with the addition of sodium carbonate solutions of different molar concentrations.
2. By protonation; adding hydrochloric acid as the source of H+.
3. Using a pH 7 buffer solution to maintain the pH of the solution at 7 to have mainly bicarbonate speciation.
The precipitates were collected after several days and were analyzed for phase composition using various analytical techniques (see Methods in the ESI†). In further experiments, we investigated the effect of several pH buffers with different compositions on the stability of nesquehonite to further investigate the mechanisms of nesquehonite stabilization.
The characteristic reflections of nesquehonite at 15.80° (6.50 Å), 21.01° (4.90 Å), 26.86° (3.85 Å), 32.16° (3.22 Å) 2θ CoKα are present in the sample stored in the pH 7 buffer solution (CB). A broad hump between 25–42° 2θ CoKα is also observed and could indicate the presence of amorphous phases. From the results, it is evident that in the pH 7 buffer solution (CB), nesquehonite remains stable even after 10 days at 50 °C, while it transforms to dypingite in the case of sodium carbonate or hydrochloric acid. The pH of the solutions after 10 days is measured to be 9.21, 8.30 and 9.52 in pH 7 (CB), 0.1 M HCl and 0.1 M Na2CO3, respectively. It can be observed that the phase transformation occurs at a pH of 8.30 and 9.52, but not at the intermediate pH of 9.21, suggesting that there should be other potential reason for the observed phenomenon. Thus, we postulate that the phosphate in the commercial buffer could be the reason and not the pH of the solution. To confirm the effect of the anion in stabilizing nesquehonite, further experiments were conducted with pH 7 buffers prepared with (pH-7 (CB) and phosphate) and without phosphate (pH 7-amine).
The thermal decomposition of the precipitates (Fig. 3b) as measured by thermogravimetry also confirms these findings. The mass losses at 110, 160 and 220 °C of the solids stored in pH 7 (CB) and pH 7-phosphate samples indicate the dehydration of nesquehonite. The decarbonation occurs at 440 and 485 °C, similar to results reported elsewhere.23 For the reference and pH 7-amine samples dehydration occurs around 250 °C, dehydroxylation at about 300 °C and decarbonation at 450 °C and 415 °C, respectively. The total mass losses of 58.7% and 59.7% for the reference and pH 7-amine, respectively, are close to the theoretical mass loss of dypingite (58.5%).24 The amount of nesquehonite after 28 days is quantified considering the mass loss around 350–600 °C due to the decarbonation of nesquehonite (see Methods in ESI†). About 80% and 86% of nesquehonite remained after 28 days in pH 7(CB) and pH 7-phosphate, respectively, which is similar to the values after 7 days. This indicates that the phase is stable at 50 °C up to 28 days. In conclusion, the effect of pH is not the primary factor that prevents the phase transformation, but the interaction between the phosphate-based buffer and nesquehonite resulting in the formation of a precipitate on the surface of nesquehonite (see Fig. S6 in the ESI†), potentially inhibiting further reaction.
Fig. 4a shows the EDS mapping done on precipitates after 28 days in pH 7-phosphate. The nesquehonite mineral is surrounded by phosphate and potassium rich phases. From thermodynamic calculations using GEMS, bobierrite (Mg3(PO4)2·8H2O) is predicted to be the stable magnesium phosphate that formed in this system (Fig. S7 in ESI†); this phase might be similar in bulk composition to the phase observed experimentally via SEM on the surface of nesquehonite. The broad hump in XRD between 25–42° 2θ CoKα (Fig. 3a) may indicate the presence of an amorphous magnesium potassium phosphate (MKP) phase. Experimental evidence has shown that the crystallization of MKP phases from MgO occurs from the formation of intermediate amorphous phases.25 Furthermore, the dehydration of MKP phases occurs around 70–250 °C and strongly overlaps with that of nesquehonite. However, the mass losses around 65, 110 and 150 °C, see Fig. 3b, possibly corresponding to the dehydration of Mg2KH(PO4)2·15H2O, K-struvite and bobierrite, respectively,26 tentatively indicate the presence of more than one MKP phase. Also, from the ternary diagram obtained from the EDS map (Fig. 4b), a strong intermixing between nesquehonite and the MKP phases close to the compositions of Mg2KH(PO4)2·15H2O and MgKPO4·6H2O (K-struvite) can be seen. This is a further indication that they form on the nesquehonite surface. Therefore, the results suggest that the tendency of the magnesium ions to complex with both phosphate and carbonate ions at the relevant pH of about 9.2 (as calculated by GEMS, see Fig. S8 in the ESI†) results in the formation of magnesium phosphate phases on the surface.
The findings reported here show a possible way to stabilize nesquehonite for safe use in various CCUS processes where poor stability of the phase has been a major hurdle for its utilization. For instance, considering its application as a construction material, the phase conversion from nesquehonite to dypingite would result in a 40% decrease in the solid volume as calculated from the molar volume of both phases14 and some loss in CO2. This could lead to cracks and destabilization in the long-term posing serious issues in structural applications. Here, we show that the use of phosphate effectively prevents nesquehonite conversion by about 80–85% under the investigated conditions. Further investigations are required to maximize the efficiency of the system by optimizing the Mg/P ratio, thereby preventing the loss of CO2. The effect of higher temperatures and longer curing time needs to be looked at along with alternatives to phosphate, given its application as a fertilizer.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ma00947a |
‡ Current address: Department of Chemistry, University of Helsinki A.I. Virtasenaukio 1, Helsinki 00014, Finland. |
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