3.1. Effect of glycine concentration on CaCO3 polymorphs
In this work, the influence of glycine concentration on CaCO3 precipitation via carbonation of FGDG was investigated. The XRD patterns of the carbonation products obtained with different glycine concentration are shown in Fig. 1a. It was found that a mixture a calcite and vaterite are obtained in the presence of glycine. However, the diffraction peak intensity of vaterite gradually increased and calcite diffraction peak intensity decreased. In order to quantitatively assess the effect of glycine on the polymorph of CaCO3, the content of vaterite and calcite in samples was calculated according to Eqs. 2 and 3 (Fig. 1b). As shown in Fig. 1b, the content of vaterite was about 60% in the absence of glycine, while the content of vaterite gradually increased from 60–97% with increasing the glycine concentration up to 20 wt% and then kept a constant value with adding more glycine. Correspondingly, the content of calcite decreases first and then reach a platform with ~3% calcite.
Further, all carbonation products obtained at different concentration of glycine were analyzed by SEM (Fig. S1). The as-prepared samples obtained in the absence of glycine shown sphere-like and rhombohedral particles structure (Fig. S1a), which are similar with our previous result (Wang et al. 2019). However, the sphere-like vaterite gradually increased with increasement of glycine concentration, and finally vaterite was dominated phase (Fig. S1b-f). The SEM images furtherly revealed that these microspheres were composed of the aggregation of numerous primary nanoparticles (Fig. S1e-f). Many vaterite microspheres were observed to be interconnected with each other demonstrating there were a tendency of conglutination or clustering between vaterite particles. Besides, the carbonation products were analyzed by FTIR. With the increase of the concentration of glycine, the characteristic peak of calcite at 712 cm−1 gradually weakened, and the characteristic peak of vaterite (745 cm−1) gradually increased (Fig. S2). The above results confirm that the content of vaterite in carbonation products increase with glycine concentration showing that glycine indeed promote the formation of vaterite and inhibit the growth of calcite.
Based on the above result, nearly pure vaterite CaCO3 was generated with 20 wt% glycine, which was analyzed further by TG-DSC. Fig. S3 shows the TG-DSC curve of carbonation product at the glycine concentration of 20 wt%. TG curve has three temperature ranges of weight loss at 50–200°C, 200–600°C and 600–850°C were ascribed to water evaporation, glycine decomposition and CaCO3 decomposition respectively. For DSC curve, the endothermic peaks near 100°C and 782°C are due to water evaporation and CaCO3 decomposition. The exothermic peak near 498°C is due to the phase transformation of vaterite to calcite under high temperature. The TG results further showed that there was a small amount of glycine in CaCO3 products, which could stabilize the metastable vaterite and prevent it from transforming into calcite (Lai et al. 2015; Luo et al. 2015).
3.2. Mechanism of glycine on precipitated CaCO3 polymorph
To understand the influence of glycine on CaCO3 polymorphs, the precipitation process of CaCO3 was analyzed without glycine. The FGDG carbonation reaction was divided into four main stages: gaseous, liquid, and solid phases were involved, leading to a number of chemical reactions (Lu et al. 2016; Song et al. 2012): (1) absorption of gas CO2 in the ammonia solution, (2) formation of ammonium (bi)carbonate, (3) chemical reaction between ammonium (bi)carbonate and gypsum, and (4) precipitation of CaCO3. The relevant reactions are shown as follow:
As shown in Fig. 1b, a mixture of vaterite (60%) and calcite (40%) was obtained via FGDG carbonation without glycine. However, nearly pure vaterite was generated at 20 wt% glycine (Fig. b). Hence, the assumption that a series of complex reactions might be occurred after adding the glycine was proposed. Firstly, glycine will react with NH4OH to form ammonium glycinate (NH2CH2COONH4) according to the theory of acid-base reaction (Eq. 14). Then, the resulting NH2CH2COONH4 further combined with FGDG (CaSO4·2H2O) to form Ca(NH2CH2COO)2 and ammonium sulfate ((NH4)2SO4) based on the double decomposition reaction due to the low solubility of Ca(NH2CH2COO)2 (Eq. 15). At the same time, Ca(NH2CH2COO)2 is a kind of complex which was partial ionized based on the reaction of Eq. 16.
In order to prove this assumption that the Ca(NH2CH2COO)2 was formed, the filtrate from separating FGDG suspension containing ammonia with 20 wt% glycine was obtained to analyze by ESI-MS (Fig. 2a). It can be seen clearly from Fig. 2a, the mass to charge ratio of 189.01 is Ca(NH2CH2COO)2, indicating that Ca(NH2CH2COO)2 as an intermediate was indeed generated. Furthermore, we assumed the intermediate of Ca(NH2CH2COO)2 plays critical role in promoting vaterite growth. When CO2 was injected into the system, the carbonate (CO32−) reacts with Ca(NH2CH2COO)2 to form vaterite. On the other hand, the CO32− react with the dissociative Ca2+ to form the calcite. Similar results have been reported that the vaterite was induced through the intermediate chelated by aspartic acid and Ca(OH)2 (Luo et al. 2015). Our previous work indicated that the impurity of dolomite in the FGDG selectively produce the calcite due to the hydrophilicity and negative surface charge of dolomite (Wang et al. 2019). When the glycine was added into the system, the resulting of NH2CH2COONH4 might preferentially react with Ca2+ dissolved from the FGDG to reduce the adsorption of Ca2+ on the dolomite surface. Consequently, the vaterite content enhanced in the presence of glycine. Additionally, the capability of combining Ca2+ might increase with the concentration of glycine. Therefore, the vaterite content also increased with increasing of glycine concentration (Fig. 1b).
In order to prove Ca(NH2CH2COO)2 can promote vaterite growth, the carbonation products of Ca(NH2CH2COO)2 carbonation were analyzed further (Fig. 2b-c). The results showed that the dominated vaterite and small amount calcite were obtained via carbonation of Ca(NH2CH2COO)2. Because of the dissolution equilibrium of Ca(NH2CH2COO)2, the solution includes the dissociative Ca2+ during the carbonation. The formation of vaterite was ascribed to the Ca(NH2CH2COO)2, which provided a local lower Ca2+/CO32− ratio to form vaterite (Abebe et al. 2015; Chang et al. 2016; Svenskaya et al. 2018). Meanwhile, the reason of formation of calcite might be attributed to the dissociative Ca2+. The above results indicated that Ca(NH2CH2COO)2 played an important role in the formation of vaterite.
Glycine indeed promoted the growth of vaterite in the carbonation of FGDG. To further investigate the formation mechanism of vaterite in the presence of glycine, the filtrate obtained by solid-liquid separation of FGDG suspension containing ammonia with glycine (20 wt%) was used to react with CO2 (200 mL/min). Meanwhile, the pH change of the whole process and the precipitation process were monitored. Fig. 3a depicts the pH of filtrate with CO2 bubbling time. The initial pH was about 10.5, it then quickly dropped to around 8.5, and finally kept a constant value of 7.5. The XRD patterns of the reaction products between the filtrate and CO2 at different reaction times is shown in Fig. 3b. Only calcite CaCO3 was found in the product for reaction of 3 min. However, vaterite was found after 6 min of reaction, calcite was dominated phase. As the reaction going, the diffraction peak intensity of vaterite gradually increased, while the diffraction peak intensity of calcite gradually decreased. Finally, the product was dominated by vaterite after 24 min reaction.
The morphology of the reaction products from the reaction of filtrate with CO2 at different times was further analyzed (Fig. 3c-h). The results showed that the reaction products were mainly calcite particles accumulated by cubic or hexahedral particles at 3 min (Fig. 3c). Spherical vaterite particles were observed for reaction of 6 min, however, the most of particles were still cubic calcite (Fig. 3d). As the reaction goes on, spherical vaterite particles gradually increased and dominated in products (Fig. 3e-h).
The emergence of CaCO3 polymorphs could be ascribed to the energy barrier to nucleation, which could be explained by Ostwald’s step rule (Lakshminarayanan et al. 2003; Rodriguez-Blanco et al. 2011). The metastable vaterite was generally formed first and then transformed into the thermodynamically most stable calcite. But glycine plays a role like some organic additives could protect the crystal surface of the vaterite by interfacial adsorption and stabilize the unstable phase (Abebe et al. 2015; Fu et al. 2013; Nagaraja et al. 2014). According to TG analysis, there was a weight loss of 2.25 wt % at 200−600°C due to the decomposition of glycine (Fig. 3), further revealing that the resulting CaCO3 contained a few of glycine. These results indicated that the existence of glycine had a stabilizing effect on the metastable vaterite and suppressed its transition to the calcite.
Based on the above results, the formation mechanism of spherical vaterite in the presence of glycine was proposed. Fig. 4 is the schematic diagram of formation mechanism of vaterite with glycine via FGDG carbonation. Initially, Ca(NH2CH2COO)2 was generated during carbonation (Fig. 2a). Hence, when CO2 was injected into reaction system, CO32− gradually generated at pH=10.5. The CO32− increased and combined with Ca2+ that bound to H2NCH2COO− to form vaterite CaCO3, which was due to the local higher ratio of CO32−/Ca2+ (Dang et al. 2017; Svenskaya et al. 2017). Meanwhile, a little of calcite was also generated during the carbonation. Before injecting CO2, the dissociative Ca2+ was generated through ionizing Ca(NH2CH2COO)2 based on Eq. 16 and dissolving from FGDG. Once CO2 was flowed into reaction system, CO32− was more likely to generate calcite CaCO3 with dissociative Ca2+ due to its low supersaturation and local higher ratio of Ca2+/CO32− (Chang et al. 2016; Wang et al. 2019).