Glycine-Induced Synthesis of Vaterite via Direct Aqueous Mineral Carbonation of Flue Gas Desulfurization Gypsum

Mineral carbonation of ue gas desulfurization gypsum (FGDG) can not only sequester CO 2 to mitigate the greenhouse effect, but also produce CaCO 3 to generate economic benet. A mixture of calcite and vaterite CaCO 3 was produced by FGDG carbonation in our previous study. Nevertheless, the production of uniform crystalline CaCO 3 , especially for vaterite, still maintains a big challenge via carbonation of FGDG. Herein, nearly pure vaterite was synthesized via FGDG carbonation in the presence of glycine was reported rstly. The results show that the content of vaterite increased from 60% to 97% with increasing glycine concentration and then kept a constant value, indicating that glycine can promote the formation of vaterite and inhibit the growth of calcite. Additionally, the investigation of vaterite growth mechanism in the presence of glycine demonstrated that the formation of intermediate, glycinate calcium, played an important role to stimulate the growth of vaterite. This study provides a new insight to produce a high-valued vaterite CaCO 3 during the direct mineral carbonation of FGDG.


Introduction
Flue gas desulfurization gypsum (FGDG) is a kind of industrial waste discharged from limestone-gypsum wet desulphurization process, which has leaded to serious environmental problems duo to lack of effective and environmentally friendly disposal methods. For instance, a series of problems including occupying land, polluting water, particle pollution and ecological destruction have been caused due to the  Tan et al. 2018). Furthermore, the annual production of FGDG was estimated to undergo an increasing tendency in the coming future. Although numerous stacking FGDG causes many environmental or ecological issues, FGDG is a kind of useful resource actually because it contains rich calcium and sulfur element. Consequently, the utilization and management of hazardous FGDG are indispensable way to reduce the pollution and realize the high-e ciency and clean production process.
Recently, mineral carbonation of FGDG to sequester CO 2 has received attentions expensively. This technology is a "win-win" strategy because it can not only achieve CO 2 reduction, but also realize the resource utilization of FGDG to produce toxic-free CaCO 3 and achieve the goal of treating waste with wastes. Synthesis of CaCO 3 by FGDG carbonation presents a high yield and conversion e ciency . CaCO 3 is important llers or additives in paper, plastic, food, cosmetics, medical materials and composite functional materials because of its unique characteristics of nontoxicity and harmlessness, biocompatibility, easy fabrication and controllable structure (Konopacka-Łyskawa et al. 2017; Mori et al. 2009; Wang et al. 2018). Therefore, the hazardous FGDG was transformed into eco-friendly CaCO 3 is an alternative strategy to deal with million tons of discarded FGDG thereby reducing its pollution to the natural environment and improve its high value-added. Generally, CaCO 3 has three anhydrous polymorphs (calcite, aragonite and vaterite), among which calcite shows the highest stability, while aragonite and vaterite are metastable and thermodynamically unstable phases respectively (Zhou et al. 2010 Over the past decades, many works have been focused on using FGDG to produce high-valued CaCO 3 by carbonation method. The high-purity CaCO 3 (calcite) particles were synthesized by using the solution which was extracted from the induction period of CaCO 3 during the process of FGDG carbonation (Song et al. 2012). However, the yield of pure calcite is relatively low (about 5% of FGDG) because of the short induction period of CaCO 3 crystallization. To improve the production of pure CaCO 3 (calcite), polyacrylic acid (PAA) was used to prolong the induction time of CaCO 3 crystallization during FGDG carbonation and the yield of calcite was improved to 60% (Song et al. 2016). Moreover, our previous work found that the content of vaterite increased from 60-80% in CaSO 4 -NH 3 -CO 2 -H 2 O supernatant system due to the ultrasonic induction (Cheng et al. 2016). Nevertheless, the yield of CaCO 3 is relatively low and further improvement on the production of CaCO 3 should be considered. In other work, a mixture of vaterite and calcite was also observed in FGDG suspension carbonation system below 60°C of reaction temperature Hence, the aim of this work is to synthesize vaterite via direct aqueous carbonation of FGDG. Herein, the glycine was chosen as the additives to investigate the effect of glycine on the polymorph CaCO 3 during the FGDG carbonation. Nearly pure vaterite was produced at the optimal conditions. The formation mechanism of vaterite via FGDG carbonation in the presence of glycine was also explored. The research nding might nd a new approach to manage the FGDG and enhance its added high-value.

Materials
The ue gas desulfurization gypsum (FGDG) was obtained from Taiyuan Iron Steel (group) Co., LTD.

Characterization
The phase composition of the samples was analyzed by XRD (Bruker D2 Advance with a Cu Kα source at 30 kV and 10 mA) with the scanning scope (2θ) from 5° ~ 80° under the speed of 0.02° s −1 . Jade 5.0 was used to identify the minerals components of the samples. The content of each CaCO 3 polymorph was calculated from their characteristic XRD peak intensities by using the following equations [23]. Herein

Effect of glycine concentration on CaCO 3 polymorphs
In this work, the in uence of glycine concentration on CaCO 3 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 CaCO 3 , the content of vaterite and calcite in samples was calculated according to Eqs. 2 and 3 (Fig. 1b). As shown in 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 nally 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 con rm 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 CaCO 3 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 CaCO 3 decomposition respectively. For DSC curve, the endothermic peaks near 100°C and 782°C are due to water evaporation and CaCO 3 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 CaCO 3 products, which could stabilize the metastable vaterite and prevent it from transforming into calcite (Lai et al. 2015;Luo et al. 2015).

Mechanism of glycine on precipitated CaCO 3 polymorph
To understand the in uence of glycine on CaCO 3 polymorphs, the precipitation process of CaCO 3 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  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 NH 4 OH to form ammonium glycinate (NH 2 CH 2 COONH 4 ) according to the theory of acid-base reaction (Eq. 14). Then, the resulting NH 2 CH 2 COONH 4 further combined with FGDG (CaSO 4 ·2H 2 O) to form Ca(NH 2 CH 2 COO) 2 and ammonium sulfate ((NH 4 ) 2 SO 4 ) based on the double decomposition reaction due to the low solubility of Ca(NH 2 CH 2 COO) 2 (Eq. 15). At the same time, Ca(NH 2 CH 2 COO) 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(NH 2 CH 2 COO) 2 was formed, the ltrate 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(NH 2 CH 2 COO) 2 , indicating that Ca(NH 2 CH 2 COO) 2 as an intermediate was indeed generated. Furthermore, we assumed the intermediate of Ca(NH 2 CH 2 COO) 2 plays critical role in promoting vaterite growth. When CO 2 was injected into the system, the carbonate (CO 3 2− ) reacts with Ca(NH 2 CH 2 COO) 2 to form vaterite. On the other hand, the 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 NH 2 CH 2 COONH 4 might preferentially react with Ca 2+ dissolved from the FGDG to reduce the adsorption of Ca 2+ on the dolomite surface. Consequently, the vaterite content enhanced in the presence of glycine. Additionally, the capability of combining Ca 2+ 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(NH 2 CH 2 COO) 2 can promote vaterite growth, the carbonation products of Ca(NH 2 CH 2 COO) 2 carbonation were analyzed further (Fig. 2b-c) Meanwhile, the reason of formation of calcite might be attributed to the dissociative Ca 2+ . The above results indicated that Ca(NH 2 CH 2 COO) 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 ltrate obtained by solid-liquid separation of FGDG suspension containing ammonia with glycine (20 wt%) was used to react with CO 2 (200 mL/min). Meanwhile, the pH change of the whole process and the precipitation process were monitored. Fig. 3a depicts the pH of ltrate with CO 2 bubbling time. The initial pH was about 10.5, it then quickly dropped to around 8.5, and nally kept a constant value of 7.5. The XRD patterns of the reaction products between the ltrate and CO 2 at different reaction times is shown in Fig. 3b. Only calcite CaCO 3 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 ltrate with CO 2 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). to the decomposition of glycine (Fig. 3), further revealing that the resulting CaCO 3 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(NH 2 CH 2 COO) 2 was generated during carbonation (Fig. 2a). Hence, when CO 2 was injected into reaction system, CO 3 2− gradually generated at pH=10. 5

Conclusions
In conclusion, nearly pure vaterite via direct aqueous mineral carbonation of FGDG in the presence of glycine was synthesized. Moreover, formation mechanism of vaterite was also investigated. The vaterite content increased with glycine concentration and then kept a constant value. The highest content of vaterite was about 97% under the optimal conditions. Glycine can promote the formation of vaterite and inhibit the growth of calcite. The study of formation mechanism of vaterite shows that intermediate of Ca(NH 2 CH 2 COO) 2     Schematic diagram of induction mechanism of glycine for promoting formation of vaterite by CO2 mineralization of FGDG. Note the calcite was formed through CO32-combined with dissociative Ca2+, while vaterite was generated by CO32− combined with Ca2+ that bound to H2NCH2COO− to form vaterite CaCO3.

Supplementary Files
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