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[Hiroshi SAKUMA](https://orcid.org/0000-0002-6522-0704), [Shigeru SUEHARA](https://orcid.org/0000-0001-7423-2830), [Hidenobu NAKAO](https://orcid.org/0000-0002-4014-9366), Je-Deok KIM, [Kenji TAMURA](https://orcid.org/0000-0001-6578-0923)

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[Synthesis of needle-like aragonite using carbonation method: A review](https://mdr.nims.go.jp/datasets/be9ec6bc-bbee-4ecc-897c-3134984d69ab)

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Synthesis of needle–like aragonite using carbonation method: A reviewSynthesis of needle–like aragonite using carbonation method:A reviewHiroshi SAKUMA*, Shigeru SUEHARA*, Hidenobu NAKAO**, Je–Deok KIM* and Kenji TAMURA**Environmental Circulation Composite Materials Group, National Institute for Materials Science, Tsukuba 305–0044, Japan**Hydrogen Related Materials Group, National Institute for Materials Science, Tsukuba 305–0044, JapanThe environmental problems caused by CO2 emissions endanger human lives. Under these circumstances, car-bon dioxide capture, utilization, and storage (CCUS) is likely to reduce CO2 gas emissions with economicadvantages. An application of CCUS is the reinforcement of mechanically weak bio–based composite plastics.Needle–like calcium carbonate crystals can potentially reinforce weak plastics. Moreover, the synthesis of cal-cium carbonate via carbonation can reduce CO2 emissions. Here, we review the development of synthetic meth-ods for needle–like aragonite (which is a polymorph of calcium carbonate crystals) using a gaseous CO2 forfurther development of efficient synthetic conditions and precise control of morphology. Various factors influ-encing the synthesis of aragonite have been discussed. These include the temperature; degree of supersaturationof CaCO3; pH; additives; and external stimuli such as high gravity, sound waves, and microbubbles. The esti-mated elastic moduli of aragonite were anisotropic depending on the crystal direction. This indicated the im-portance of controlling the long–axis direction of aragonite as a novel reinforcement material.Keywords: Aragonite, Aspect ratio, Elastic constants, CO2INTRODUCTIONAn increase in atmospheric carbon dioxide concentrationcan cause severe environmental transitions on Earth’s sur-face through the greenhouse effect. Carbon dioxide cap-ture and storage (CCS) has been studied to store CO2underground. However, this process does not generateeconomic advantages because the recovered CO2 is onlywaste. Carbon dioxide capture, utilization, and storage(CCUS) is likely to have an economic impact by utilizingCO2 gas (Woodall et al., 2019). Currently, CO2 gas is usedmainly to improve oil recovery. However, this processdoes not contribute to CO2 reduction. Therefore, there isa need for effective utilization methods.One of the utilization methods is the use of precipi-tated calcium carbonate (PCC) from CO2 gas as a rein-forcing agent filler in the plastic, rubber, and paper indus-tries (Fig. 1) (Hu et al., 2009; Jimoh et al., 2018; Liendo etal., 2022; Zhong et al., 2024b). Bio–based polymers arelikely to replace petroleum–based polymers in commonplastics to reduce the CO2 emissions during oil mining.A drawback of bio–based plastics is their low mechanicalstrength. Composite materials are generally used for plas-tics by adding reinforcement materials to polymers. If thePCC crystal made from CO2 gas can be used as a rein-forcement material, CO2 gas may be utilized effectively.PCC is light, non–hazardous, and can be recycled conven-iently with acids. Acids have the capacity to dissolve cal-cium carbonates, resulting in the emission of CO2 gas. Inthe recycling process, the CO2 gas can be utilized oncemore to synthesize calcium carbonate particles.PCC has been applied for reinforcing plastics andcan improve the impact strength of polypropylene (PP)(Lin et al., 2010; Moritomi et al., 2010). However, theFigure 1. Utilization of CO2 gas as a reinforcing agent for bio–base plastics and rubbers.doi:10.2465/jmps.240819H. Sakuma, SAKUMA.Hiroshi@nims.go.jp Corresponding author© 2025 Japan Association of Mineralogical SciencesJournal of Mineralogical and Petrological Sciences (2025) 120:004REVIEWflexural modulus is not improved significantly comparedwith conventional fillers such as glass fiber and talc (Mori-tomi et al., 2010). One of the reasons for the low flexuralmodulus may be the low aspect ratio of calcite (a poly-morph of calcium carbonate), although the properties ofcomposites depend on various factors such as the degreeof dispersion, crystal size, and interfacial interaction be-tween polymers and crystals. Theoretically, as shown bythe Halpin–Tsai model (Halpin, 1969; Halpin and Kardos,1976; Mallick, 2007), an increase in the aspect ratio of thefillers improves the flexural modulus of the composites.The Young’s modulus of an ideal composite estimated bythe Halpin–Tsai model increases with increases in theaspect ratio of fiber reinforcement and volume fractionof reinforcement (vr), as shown in Figure 2. The Young’smoduli of the matrix and reinforcement were assumed tobe 1 GPa and 70 GPa, respectively. In fact, an increase inthe aspect ratio of glass fibers improves the Young’s mod-ulus of epoxy in their composites (Wakashima, 1976).These studies indicated that controlling the morphologyof PCC is critical for improving the mechanical propertiesof composite materials.The morphology of the PCC varies depending on thepolymorphs of calcite, aragonite, and vaterite. Calcite isthe most stable polymorph under ambient conditions. Itsshape is mainly rhombohedral or spindle–like (scalenohe-dron). Aragonite has a denser structure than calcite and ismetastable at room temperature and 1 atm pressure. How-ever, it can be synthesized by increasing the temperatureof the starting solution and/or the presence of additives.The morphology of aragonite is characterized by needle–like and branched forms, as shown in Figure 3 (Ota et al.,1995; Ahn et al., 2004). Vaterite is metastable and exhibitsspherical and plate–like shapes. Owing to the large aspectratio of the fillers, which improves the mechanical proper-ties of composite materials, aragonite is the best candidateamong these three polymorphs. The attempt to apply co-lumnar calcium carbonates as a filler of PP has improvedthe tensile and flexural strength and these moduli (Jing etal., 2018). The mechanically tough and shock–resistancecrystal phase of PP (β phase) can be formed using arago-nite as a nucleating agent (Guan et al., 2013).Aragonite synthesis has a long history, and variousconditions have been tested to find the conditions for purearagonite synthesis. There are several ways to obtain ara-gonite, such as metathesis reaction, urea hydrolysis, andbiomineralization; however, the carbonation method di-rectly using gaseous CO2 has become the primary methodrecently due to the need for carbon reduction. This reviewprovides the synthesis by the carbonation method to con-trol its morphology and aspect ratio of aragonite. The fac-tors likely to influence the aspect ratio, such as the tem-perature, Ca and CO2 concentrations, pH, additives (Mg,NH3, etc.), and external stimuli, are discussed for the fur-ther development of aragonite synthesis.ARAGONITE SYNTHESIS USINGA CARBONATION METHODThe available reports on the synthesis of aragonite usingcarbonation methods are listed in Table 1. The synthesisof calcium carbonate from CO2 gas can be classified basedon the source of Ca. One is the use of lime (CaO) andslaked lime [Ca(OH)2], and the other is the use of Ca salts.Seawater, slag, and concrete waste should be consideredas potential sources of Ca (Ho and Iizuka, 2023; Shen etal., 2023; Zhong et al., 2024a). However, these materialscontain various ions, minerals, and organic matter; there-fore, their complicated composition is beyond the scopeof this paper when it comes to finding optimal conditionsfor needle–like aragonite synthesis. Here we focus on sim-ple Ca sources such as lime and Ca salts.Figure 2. Young’s modulus of a composite material calculated us-ing the Halpin–Tsai model. Here, the Young’s moduli of matrixand aragonite reinforcement are assumed as 1 GPa and 70 GPa,respectively. The results for three volume fractions of reinforce-ment material (vr) are plotted.Figure 3. Morphology of (a) needle–like (Ota et al., 1995) and (b)branched (Ahn et al., 2004) aragonite synthesized by the carbo-nation method. Reprinted with permission from Wiley.H. Sakuma, S. Suehara, H. Nakao, J.–D. Kim and K. Tamura2Lime and slaked lime as the Ca sourceThe carbonation of lime is an industrial method. The limebecomes slaked lime [Ca(OH)2] in water. Calcium car-bonate was produced by injecting gaseous CO2 into aCa(OH)2 suspension in water as follows:Ca2þ ðaqÞ þ 2OH� ðaqÞ þ CO2 ðaqÞ! CaCO3 ðsÞ þ H2O ðlÞ ð1Þ:This method can provide pure calcium carbonate precip-itates. However, most of the precipitated crystals are cal-cite. Moreover, it is difficult to modify the conditions forsynthesizing aragonite because of the relatively low sol-ubility of Ca(OH)2 in water (Ding et al., 2018), as listedin Table 2 (Kendall, 1912; Linke and Seidell, 1958;Duchesne and Reardon, 1995). Two methods have suc-ceeded in precipitating aragonite without additives or ex-ternal stimuli (Fig. 4).Method 1: Columnar aragonite is synthesized by pre-cisely controlling the temperature and CO2 gas feed rate(Fig. 4a: Reactor 1) (Murakami et al., 1977). First, thetemperature was maintained at a controlled range of 5–20 °C, and the CO2 feed rate was set at 7–15 mL/min for1 g Ca(OH)2 until the carbonation rate reached 2–10%.Subsequently, the CO2 feed rate was regulated between0.5 and 2 mL/min for 1 g Ca(OH)2 at temperatures be-tween 7–25 °C until the carbonation rate reached 10–60%. Thirdly, the temperature was maintained at a level ex-ceeding 45 °C, while the CO2 feed rate was regulated at arate exceeding 2 mL/min for a quantity of 1 g Ca(OH)2.The authors do not provide a clear rationale for the suc-cess of this three steps method, however, it may controlthe pH by changing the feed rate of CO2 gas. In the initialstage of the process, the elevated pH was rapidly reducedby introducing the gas with a relatively high feeding rate.This may result in an optimal pH for aragonite formation.In the second step, the feeding rate was reduced, and theTable 1. Experimental conditions of aragonite synthesis using carbonation method, arranged in chronological order of publicationMorphologyAspectratioCa sourceAdditives,special techniquesCO2 feed rate(CO2 gas,L/min/Solution, L)T (°C) ReferenceNeedle like ? Ca(OH)2 Sucrose ~ 0.39 60–90 Bennett and Gardiner, 1967Columnar ~ 10 Ca(OH)2 Three–step reactions 0.02–15 1: 5–202: 7–253: >45Murakami et al., 1977Needle like ~ 10 Ca(OH)2 MEA, HNO3 0.0008–0.04 >60 Langelin et al., 1984Needle like ~ 20 Ca(OH)2 Sucrose 1.0 >70 Ota et al., 1986Needle like ~ 20 CaCl2 Mg(OH)2 0.5 80–85 Ota et al., 1991Needle like ~ 20 Ca(OH)2 Na–aluminate 0.05 >50 Ota et al, 1991Needle like 20–80 Ca(OH)2 MgCl2 0.05 80 Ota et al., 1995Needle like ? CaO MEA, HNO3 0.33–1 60, 80 Vučak et al., 1997Needle like ? Scallop shell SrCO3 0.13 35 Sasaki et al., 1998aNeedle like ~ 10 Scallop shell MgCl2 0.03–0.06 35 Sasaki et al., 1998bNeedle like,Columnar10–20 Ca(OH)2 NaOH, KOH 2–10 L/min/kg CaO 20–80 Konno et al., 2000Needle like 4–15 Ca(OH)2 H3PO4 0.21–0.83 ? Wang et al., 2004Needle like ~ 10 Ca(OH)2 MgCl2 0.1 80 Ahn et al., 2004Branched – Ca(OH)2 MgCl2 0.1 80 Ahn et al., 2004Needle like 3–10 Ca(OH)2 MgCl2, Phthalic acid 0.025 80 Park et al., 2008Needle like 8–16 Ca(OH)2 MgCl2 0.98–1.75 70 Hu et al., 2009Needle like 4–6 Ca(OH)2 MgCl2, SrCO3 0.1 30 Kim et al., 2009? ? Ca(NO3)2 NH3, N2, NH4OH, HNO3 ? 25 Matsumoto et al., 2010Needle like ? Ca(OH)2 MgCl2, Sonication 0.24–0.36 >30 Santos et al., 2012Needle like 10–12 CaCl2 NH3 1.0 80 Ding et al., 2018Branched – CaCl2 NH3 1.0 80 Ding et al., 2018Needle like ? CaCl2 MgO, MgCl2, Amine, NaOH 0.08–5.0 >30 Rivera and Van Gerven, 2020Needle like ? Ca(OH)2 NaOH ? 40–80 Sakaguchi et al., 2021Needle like 2–10 Ca(OH)2 NaOH ? 40–80 Sakaguchi et al., 2022Needle like ? CaCl2 MEA 3.0 70 Mao et al., 2023Synthesis of needle–like aragonite using carbonation method: A review 3pH was maintained at a relatively constant level. In thethird step, an elevated temperature may be preferable foraragonite formation.Method 2: aragonite, in the form of needle–like crys-tals, including a small amount of calcite, was synthesizedby gradually adding a Ca(OH)2 aqueous slurry (at a rate ofless than 150 mL/min) to 1 L of hot water (at a temper-ature exceeding 70 °C) while introducing carbon dioxide(Fig. 4b: Reactor 2) (Ota et al., 1986). Although the rea-son for the success of this method remains unclear, thepH of the starting solution was maintained at a low leveldue to the absence of a Ca(OH)2 slurry. The addition of aCa(OH)2 slurry may result in an increase in pH, but thedegree of the change in pH can be controlled by introduc-ing CO2 gas to the solution. Consequently, the synthesisof aragonite seems to be dependent on the pH and temper-ature.These two methods reduce the degree of supersatu-ration of CaCO3. However, additives and external stimuliare commonly used for the rapid, efficient, and conven-ient synthesis of aragonite crystals.Additives, temperature, pH, and external stimuli.Various additives such as sucrose (Bennett and Gardiner,1967; Ota et al., 1986), monoethanolamine (MEA) andHNO3 (Langelin et al., 1984; Vučak et al., 1997), MgCl2(Ota et al., 1995; Ahn et al., 2004; Park et al., 2008; Hu etal., 2009), H3PO4 (Wang et al., 2004), and sodium alumi-nate (Ota et al., 1991) in Reactor 1 (Fig. 4a) and NaOH orKOH (Konno et al., 2000; Sakaguchi et al., 2021; Saka-guchi et al., 2022) in Reactor 2 (Fig. 4b) were confirmedto be effective in the synthesis of aragonite.Aragonite was precipitated when Ca(OH)2 dissolvedin a sucrose solution reacted with CO2 gas at high temper-ature (Bennett and Gardiner, 1967). The preferred syn-thetic conditions for aragonite were 20–50 wt% sucrose,0.05–0.5 M Ca(OH)2, a pH between 7 and 9, and temper-atures ranging from 60 to 90 °C. The sucrose should bepure in the absence of organic acids; otherwise, calcitewill be obtained.Monoethanolamine (MEA)–HNO3 solution has beenused to remove impurities from CaO by selective hy-droxide precipitations such as Mg(OH)2, Fe(OH)3, andAl(OH)3 (Langelin et al., 1983). This is because thesource rock of calcium in their study was dolomite,which contains Mg ions as impurities. The reaction isshown below:CaO ðsÞ þ 2Hþ ðaqÞ þ CH2OH�CH2NH2 ðlÞ! Ca2þ ðaqÞ þ CH2OH�CH2NHþ3 ðaqÞ þ OH� ðaqÞð2Þ;Ca2þ ðaqÞ þ 2OH� ðaqÞ þ CO2 ðaqÞ! CaCO3 ðsÞ þ H2O ðlÞ (1’):In this experiment, needle–like aragonite has been synthe-sized by precisely controlling the temperature and CO2feed rate (Langelin et al., 1984; Vučak et al., 1997). Inaddition to removing the impurities using MEA, variousaliphatic amines, diamines, and amino acids as additivesinfluence the polymorphs and morphology of calciumcarbonates (Chuajiw et al., 2014). Both hydrophilic func-tional groups and hydrophobic alkyl groups appear to in-fluence the polymorphs and morphology of the precipi-tated CaCO3 particles.Figure 4. Conventional carbonation methods to synthesize arago-nite using Ca(OH)2 as a Ca source at an elevated temperature.(a) Reactor 1: Control of CO2 gas feed rate for Ca(OH)2 satu-rated slurry. (b) Reactor 2: Control of Ca concentration and CO2gas feed rate.Table 2. Solubility of Ca–bearing minerals in waterCa–bearing MineralsSolubility in water at 25 °C(mol/kg)ReferenceCa(OH)2 0.0222 Duchesne and Reardon, 1995CaCl2·6H2O (Antarcticite) 7.5 Linke and Seidell, 1958Ca(NO3)2 8.4 Linke and Seidell, 1958CaCO3 (Calcite) 1.4 × 10−4 Kendall, 1912CaCO3 (Aragonite) 1.5 × 10−4 Kendall, 1912H. Sakuma, S. Suehara, H. Nakao, J.–D. Kim and K. Tamura4The addition of MgCl2 to water succeeded in synthe-sizing needle–like aragonites with aspect ratios rangingfrom 8 to 80 (Ota et al., 1995; Sasaki et al., 1998b; Ahnet al., 2004; Park et al., 2008; Kim et al., 2009; Hu et al.,2009; Santos et al., 2012). In numerous experiments, thetemperature was maintained at 70–80 °C. However, incases where specific seed crystals, additional additives,or external stimuli were present, aragonite could be syn-thesized at relatively low temperatures, above 30 °C (Ta-ble 1). The presence of Mg2+ in water enhances the pre-cipitation of aragonite rather than calcite.Aragonite was synthesized by mixing a CO2–dis-solved NaOH solution and Ca(OH)2 slurry (Konno et al.,2000; Sakaguchi et al., 2021; Sakaguchi et al., 2022). Theaddition of CO2 gas into the alkali solutions increased thedissolved Na2CO3 or K2CO3 species in water. Calciumcarbonate was precipitated via the reaction betweenCa(OH)2 and NaCO3 or K2CO3 in water:Preparation of a CO2­dissolved solution:2OH�ðaqÞ þ CO2ðaqÞ ! CO2�3 ðaqÞ þ H2OðlÞ ð3Þ:Mixing Ca­ and CO2­dissolved solutions:Ca2þðaqÞ þ CO2�3 ðaqÞ ! CaCO3 ð4Þ:Needle–like aragonite was precipitated at higher temper-atures (40–80 °C). The diameter of the needle–like arago-nite particles increased with the increase in temperature.A high–gravity environment also precipitated nee-dle–like aragonite with H3PO4 as an additive (Wang etal., 2004). The addition of H3PO4 is essential for synthe-sizing aragonite. A high–gravity environment influencesthe size distribution of the synthesized particles and reac-tion times. This is likely to be owing to the enhancementof the mixing state of the reagents (Chen et al., 2000). Thehigh gravity is of an order several hundred or thousandtimes larger than that of the gravity on the Earth’s surface.The aragonite content relative to calcite and the aspectratio of the needle–like aragonite were altered by the ro-tating speed of the reactor (Wang et al., 2004). At lowerrotating speeds, the micro–mixing was not optimal. Fur-thermore, an inhomogeneous supersaturation ratio existeddepending on the local area, resulting in increased calcitegrowth. At higher rotating speeds, the micro–mixing in-tensified, leading to a higher supersaturation ratio and cal-cite precipitation.External stimuli of sound wave (Zhou et al., 2004;Santos et al., 2012) have been tested to synthesize arago-nite. Sound waves in the range of 16–100 kHz induce theformation of small cavities or microbubbles in solution.The sonication system requires a cooling bath to lower thetemperature generated by sound waves. The collapsingmicrobubbles produce high local temperatures, pressures,and shear forces. This method has successfully producedaragonite (Santos et al., 2012). The polymorph ratios,size, and morphology of crystals were altered by severalprocess parameters, namely, the Mg/Ca ratio, ultrasoundamplitude, continuous ultrasound, sonication time appliedto the Ca(OH)2 slurry before introduction of CO2 gas (ul-trasound pre–breakage), CO2 feed rate, and concentrationof Ca(OH)2.CaCl2 and Ca(NO3)2 as the Ca sourceAs an alternative source of Ca ion, calcium chloride(CaCl2) and calcium nitrate [Ca(NO3)2] have been tested(Ota et al., 1991; Matsumoto et al., 2010; Ding et al.,2018; Rivera and Van Gerven, 2020; Mao et al., 2023).Additives, temperature, pH, and external stimuli.In this case, additives such as NH3, NaOH, and KOH arerequired to provide OH− in water. Calcium carbonate isproduced by injecting CO2 gas into a CaCl2–NH3 aque-ous solution (Ding et al., 2018) as follows:NH3 ðaqÞ þ H2O ðlÞ ! NHþ4 ðaqÞ þ OH� ðaqÞ ð5Þ;Ca2þ ðaqÞ þ 2OH� ðaqÞ þ CO2 ðaqÞ! CaCO3 ðsÞ þ H2O ðlÞ (1’):The difference with Ca(OH)2 as the Ca source is the con-centration of Ca2+ and OH− ions. Needle–like aragonitehas been synthesized in a CaCl2–NH3–CO2 aqueous sys-tem at 80 °C (Ding et al., 2018). Plate–shaped vaterite wasformed at the beginning of the reaction (10 min). A phasetransformation from vaterite to aragonite was observed ata reaction time of 30 min. Most of the vaterite transformedinto needle–like aragonite at 60 min. The formation ofvaterite at the beginning of the reaction was interpretedusing Ostwald’s rule (wherein the least stable polymorphcrystallizes first rather than stable ones). The phase trans-formation from vaterite to aragonite, rather than calcite,was interpreted as the kinetics at high temperatures.As an external stimuli, CO2/NH3/N2 microbubbleswere supplied continuously to an aqueous Ca(NO3)2 solu-tion, and aragonite was synthesized under constant pH(9.7–10.5) conditions at 298 K (Matsumoto et al., 2010).The solution pH was maintained constant by addingHNO3 and NH4OH solutions. The average bubble sizewas maintained at 40–1000 µm by controlling the rotationrate of the bubble generator and N2 flow rate. The poly-morphs of vaterite and calcite were predominantly synthe-sized at pH of 9.0 and 11.0, respectively.Synthesis of needle–like aragonite using carbonation method: A review 5ARAGONITE GROWTH ON SEED CRYSTALSThe crystal growth from the aragonite seed crystals wastested to control the particle size and morphology, as listedin Table 3. Needle–like aragonite was obtained by addingthe columnar seed crystals of aragonite to a Ca(OH)2–CO2aqueous system at >50 °C (Tanaka et al., 1988). The elon-gated aragonite crystals were obtained at the pH rangefrom eight to nine. The pH was controlled by the concen-tration of columnar aragonite seed crystals. This impliesthat, in addition to the effect of pH, the quantity of seedcrystals may also affect the polymorphs and morphologyof the precipitate. Needle–like long aragonite has alsobeen obtained by the addition of aragonite seed crystalsand H3PO4 salts (Shibata et al., 1991). This method caninhibit the coagulation of aragonite particles. Therefore,the presence of H3PO4 may prevent aragonite particle ag-gregation.PURE ARAGONITE SYNTHESISThe synthesis of pure aragonite is a desirable objective, asit serves as a reinforcing material that can help to avoid theintroduction of unwanted complications resulting fromthe presence of impurities. In this review, the criterionfor pure aragonite was defined as a composition of >90% aragonite in the polymorphs, as determined by theXRD analysis. Pure aragonite was synthesized by variouscarbonation methods (Ota et al., 1995; Sasaki et al.,1998b; Konno et al., 2000; Wang et al., 2004; Park et al,2008; Hu et al., 2009; Kim et al., 2009; Matsumoto et al.,2010; Santos et al., 2012; Rivera and van Gerven, 2020;Sakaguchi et al., 2022; Mao et al., 2023). Identifying theoptimal conditions for pure aragonite formation is a chal-lenging. However, research has indicated that pH levelsaround 9, low CO2 feed rates, the addition of amine or Mgions, and elevated temperatures are conducive to achiev-ing this goal.CONTROL OF ASPECT RATIOThe aspect ratio of aragonite influences the elastic modu-lus of a composite material as predicted by the Halpin–Tsai model (Fig. 2). The preferred aspect ratio for rein-forcement materials is between 50 and 100 as expectedby the Halpin–Tsai model, though, the aspect ratio of syn-thetic aragonite is typically around 10, as listed in Tables1 and 3. An aspect ratio greater than 20 was achievedthrough crystal growth in the presence of aragonite seedcrystals or MgCl2 as an additive. However, the factorscontrolling the aspect ratio are not yet fully understood.The aspect ratio of aragonite particles varies margin-ally depending on the concentration of the H3PO4 additiveunder high–gravity conditions (Wang et al., 2004). In thisexperiment, 7.0 g/L of H3PO4 was the best condition toobtain a high aspect ratio of 10–15 as shown in Figure 5. Apotential role of H3PO4 was the generation of needle–likehydroxylapatite (HAP) as a reaction between H3PO4 andthe Ca(OH)2 slurry. HAP functioned as a heterogeneousnucleator, and needle–like aragonite grew on it. When theH3PO4 concentration was excessively high, aragonite ag-gregated and formed irregular shapes.The Mg/Ca ratio influences the aspect ratio of arago-nite (Park et al., 2008), as shown in Figure 6. As the Mg2+ion concentration increased, the aspect ratio increased un-til a concentration of 71 mol%. This is likely to have beenowing to the transition of polymorphs from Mg–calcite toaragonite. The aspect ratio decreased with the increase inMg/Ca ratios above 75 mol%. The role of the Mg ions wasinterpreted as the promotion of growth along the majoraxis of the needle–like crystals through a selective sidepoisoning mechanism. When the Mg/Ca ratio was higherthan 75 mol%, Mg2+ was adsorbed on various crystalfaces, and the relative growth of the minor axis increased.This resulted in a decrease in the aspect ratio.The highest aspect ratio of aragonite was obtained inthe presence of MgCl2 as an additive (Ota et al., 1995) asshown in Figure 3a. The Mg/Ca ratio was 0.76 molMgCl2/0.5 mol Ca(OH)2 = 1.52 (= 60 mol%). This Mgconcentration is similar to the optimal concentrations (71–75 mol%) reported in a similar work (Park et al., 2008),although the aspect ratio is different. The difference in theCO2 gas feed rate of 0.05 and 0.025 (LCO2/min/Lsolution)may have influenced the aspect ratio. Here, LCO2 andLsolution are the volumes (Litre) of CO2 gas and aqueoussolution, respectively.Table 3. Aragonite crystal growth from aragonite seed crystalsMorphologyAspectratioCa source Seed crystalCO2 feed rate(CO2 gas,L/min/Solution, L)T (°C) ReferenceNeedle like 20–35 Ca(OH)2 Aragonite ? 50–80 Tanaka et al., 1988Needle like ~ 20 Ca(OH)2 H3PO4 0.0027–0.033 30–80 Shibata et al., 1991H. Sakuma, S. Suehara, H. Nakao, J.–D. Kim and K. Tamura6DISCUSSIONS: KEY FACTORSOF THE ARAGONITE FORMATIONThe critical factors include the additives, Ca source, CO2feed rate, pH, and temperature. Herein, these factors arediscussed to understand the mechanism of aragonite syn-thesis.AdditivesInfluence of Mg ions. The mechanism of aragonite pre-cipitation in the presence of Mg2+ ions has been interpret-ed as the inhibition of calcite crystal growth by the adsorp-tion of hydrated Mg ions on the calcite surface (De Grootand Duyvis, 1966; Reddy and Nancollas, 1976). An atom-ic–force–microscopy study (Davis et al., 2000) reportedthat the enhanced mineral solubility of Mg–incorporatedcalcite reduces calcite growth. However, this mechanismmay not be simple (Ahn et al., 2004; Ramakrishna et al.,2017; Boon et al., 2020). Density functional theory calcu-lations indicate that the presence of Mg ion on a calcitesurface alters the surface energy and influences the sub-sequent adsorption of ions and water molecules (Sakumaet al., 2014; Andersson et al., 2016). The nucleation ofaragonite was predicted by the DFTstudy to occur at high-er Mg/Ca ratios (>2) in solution, indicating that the alter-ation of the surface energy by the dissolved Mg and Caions inhibits the calcite nucleation rather than the increas-ed solubility mechanism (Sun et al., 2015). The addi-tion of MgCl2 decreases the pH by the precipitation ofMg(OH)2 at elevated pH (Hu et al., 2009). This resultsin an increase in the Ca2+ ions dissolved from Ca(OH)2.The increased solubility of Ca(OH)2 may be related to theformation of aragonite.Figure 5. Morphology of synthe-sized CaCO3 particles as a functionof H3PO4 concentrations (a) 0 g/L,(b) 3.5 g/L, (c) 7.0 g/L, and (d)10.5 g/L. The main polymorphsof CaCO3 particles are (a) calciteand (b)–(d) aragonite (Wang et al.,2004). Reprinted with permissionfrom Elsevier.Figure 6. Aspect ratio of aragonite as a function of Mg/Ca ratio(Park et al., 2008). Reprinted with permission from Elsevier.Synthesis of needle–like aragonite using carbonation method: A review 7Enhancement of aragonite nucleation. The pres-ence of seed crystals such as aragonite and SrCO3 enhan-ces the precipitation of aragonite (Tanaka et al., 1988;Shibata et al., 1991; Sasaki et al., 1998a; Kim et al., 2009).SrCO3 was selected because the difference in the latticeparameters is less than 5% of those of aragonite (as listedin Table 4) and SrCO3 crystals are more stable in waterthan metastable aragonite (Sasaki et al., 1998a). A poten-tial role of H3PO4 is the formation of needle–like HAPbefore the precipitation of calcium carbonate. Aragonitemay have nucleated on the needle–like HAP after the startof carbonation. Because the lattice parameters of HAP arenot similar to those of aragonite, as listed in Table 4, thesurface structure may be related to the nucleation of ara-gonite. The aragonite formation on the seed crystal im-plied that the barrier to aragonite formation was reducedby the presence of seed crystals.What is the role of elevated temperature?Most aragonite was synthesized at higher temperatures(>50 °C). The effect of higher temperatures can be under-stood from attempts at aragonite synthesis at lower tem-peratures. The reaction temperature can be lowered by in-cluding seed crystals of aragonite and SrCO3 in the solu-tion (Shibata et al., 1991; Sasaki et al., 1998a; Kim et al.,2009). It can thus be concluded that elevated temperaturesmay be a prerequisite for the nucleation of aragonite in theabsence of seed crystals, external stimuli, and MgCl2. Thesonication technique with the MgCl2 additive successfullysynthesized aragonite at low temperatures (<30 °C) (San-tos et al., 2012). The mechanism was explained by thelocalized high–temperature field in the solution createdby sonication, which resulted in the nucleation of arago-nite seeds. Aragonite was synthesized at 25 °C using theCO2/NH3 microbubble technique (Matsumoto et al.,2010). The reason for the successful synthesis of arago-nite at low temperature is not clear, but the microbubblemay alter the nucleation stage of aragonite by the local pHnear the gas–liquid interface, which can differ from theoverall pH in the bulk liquid. This difference may alterthe supersaturation at the gas–liquid interface and thereby,result in the selective synthesis of aragonite, calcite, andvaterite depending on the pH of the bulk liquid. Theseobservations imply that the temperature influences the nu-cleation stage of aragonite.Role of the solution pHThe chemical species of CO2 dissolved in water vary withpH. At pH < 6, H2CO3 is the major species, whereasHCO3− and CO32− are dominant at pH > 6. This was calcu-lated using the PhreeqC program (Parkhurst and Appelo,2013) (Fig. 7). To precipitate calcium carbonate, the pres-ence of ionic species HCO3− and CO32− is preferable.Therefore, a high pH (>6) is preferable for synthesizingcalcium carbonates. The pH of the solution was also usedto determine the degree of CaCO3 precipitation whenCa(OH)2 was used as the Ca source. Before feeding theCO2 gas, the pH of the solution was high (~ 12) due to thedissolution of Ca(OH)2. The hydroxide ion concentrationTable 4. Lattice parameters of calcite, aragonite, and seed crystals of strontianite (SrCO3), and hydroxylapatite (HAP)Crystals Crystal systems a (Å) b (Å) c (Å) ReferenceCalcite Trigonal systemusing hexagonal axes4.990 4.990 17.062 Graf, 1961Aragonite Orthorhombic 4.961 7.967 5.740 De Villiers, 1971Strontianite (SrCO3) Orthorhombic 5.090(+2.6%)*8.358(+4.9%)*5.997(+4.5%)*De Villiers, 1971Hydroxylapatite (HAP) Hexagonal 9.417 9.417 6.875 Hughes et al., 1989*The percentage indicates the difference from those of aragonite.Figure 7. Chemical species of CO2 (1 mmol/kg) dissolved in wa-ter at 25 °C (CO32−: solid line; HCO3−: dashed line; and dissolvedCO2: dotted line), along with the percentage of dissolved CO2[CO2(aq) and H2CO3] (long dashed short dashed line) as simu-lated by the PhreeqC program using the minteq.v4 database.Here C represents the concentration of chemical species (mol/kg). An arrow indicates that the vertical axis of the percentageof dissolved CO2 is on the right.H. Sakuma, S. Suehara, H. Nakao, J.–D. Kim and K. Tamura8decreased upon feeding of CO2 gas as shown in Reaction1. When all the Ca(OH)2 had reacted with CO2 gas, the pHbecame constant at ~ 8.The polymorph is dependent on the pH. The produc-tion yield of vaterite synthesized in pH–controlled experi-ments conducted at 30 °C demonstrated a positive corre-lation with decreasing pH, reaching a maximum at pH <8.2 (Chen et al., 1997). The maximum yield of calcite wasobserved at a pH of 8.6. In their experiments, the temper-ature was maintained at a relatively low level, and no ad-ditive were employed to facilitate aragonite synthesis.However, the presence of a small amount of aragonitewas confirmed at pH values exceeding 8.6. The synthesisemploying the microbubble technique unambiguously re-vealed that the maximum production rates of vaterite, ara-gonite, and calcite were at pH 9, 9.5, and 11, respectively(Matsumoto et al., 2010). At higher pH values, the con-centration of chemical species of CO32− increases (Fig. 7),resulting in the attainment of supersaturation conditionsthat are higher than usual. Such conditions may result ina change in the calcium carbonate polymorph. Calcite ex-hibits a preference for high supersaturation conditions,whereas aragonite and vaterite display a preference forlow supersaturation conditions.The pH of the solution is critical for the selectiveprecipitation of impurities as metal hydroxides. The chem-ical species of Ca, Fe, Al, and Mg in water were estimatedas functions of pH using the PhreeqC program (Fig. 8). Cais dissolved as Ca2+ at a low pH and as Ca(OH)+ at excep-tionally high pH. It does not precipitate as Ca(OH)2. Feprecipitates as Fe(OH)3 at pH > 2. Al precipitates asAl(OH)3 at a low pH (>4), although it redissolves asAl(OH)4− at pH > 6. Fe and Al can be conveniently re-moved as hydroxides by lowering the pH of the solution.Mg is dissolved as Mg2+ and MgOH+ species at pH < 10.However, it precipitates as Mg(OH)2 at higher pH. This isconsistent with certain experiments using Mg2+ ions as anadditive, where the precipitation of brucite [Mg(OH)2] atpH > 10 and dissolution of Mg(OH)2 at lower pH wereverified (Ahn et al., 2004).Supersaturation of CaCO3: Ca2+ and CO2 concentra-tionsBoth nucleation and crystal growth depend on the super-saturation of the solution. The solubility products of cal-cite and aragonite at 25 °C are 3.3 × 10−9 and 5.0 × 10−9,respectively (U.S. Environmental Protection Agency,1998). The calcium concentration in the majority of car-bonation methods is adequate for the precipitation of cal-cium carbonates, indicating that the feeding rate of CO2gas exerts control over the degree of supersaturation. Nee-dle–like aragonite was synthesized under low–supersatu-ration conditions, whereas calcite was precipitated underhigh–supersaturation conditions (Hu and Deng, 2003).In the process of using the MEA and HNO3 solution,a high concentration of CO2 gas in air (>99.6%) shortenedthe carbonation reaction time compared with the gas mix-ture (33% CO2) (Vučak et al., 1997). Meanwhile, the ratioof needle–like aragonite to calcite increased with 33%CO2 compared with pure CO2. This is consistent with theobservations wherein the transformation from aragonite tocalcite was reduced at a lower feed rate of CO2 gas (0.008LCO2/min/Lsolution) from that at 0.04 LCO2/min/Lsolution(Langelin et al., 1984). The content of aragonite also in-creased with the decrease in the CO2 feed rate when syn-thesized under ultrasound with MgCl2 as the additive(Santos et al., 2012) and in a high–gravity environmentwith H3PO4 as the additive (Wang et al., 2004). Theseexplained the aragonite formation owing to the low super-saturation due to relatively low concentration of CO32− inthe reaction zone. It should be noted that in their experi-ments as well, H3PO4 enhanced the formation of aragoniteby providing a seed crystal formed in water.In the majority of carbonation processes, the concen-tration of CO32− is significantly lower than that of Ca2+.Consequently, the degree of supersaturation of CaCO3can be regulated by controlling the concentration ofCO32−. The concentration of CO32− ions is significantly af-fected by the solution pH, as shown in Figure 7, and main-taining pH control is critical with regard to the polymor-phism. In the case of a liquid–liquid synthesis, such as themixing of CaCl2 and Na2CO3 solutions, a high concentra-tion of CO32− ions can be achieved, allowing for concen-tration of Ca2+ ions to be used to control the supersatura-tion degree. In this case, aragonite can be formed even atFigure 8. Chemical species of metals in water as a function ofsolution pH at 25 °C simulated by PhreeqC program usingthe minteq.v4 database. Here, A is the activity of each species(mol/kg).Synthesis of needle–like aragonite using carbonation method: A review 9high pH conditions when the concentration of Ca2+ is suf-ficient for realizing a moderate supersaturation degree.Needle–like aragonite was synthesized by gradually add-ing a Ca(OH)2 aqueous slurry to water while introducingcarbon dioxide (Fig. 4b) (Ota et al., 1986). This methodrealized low–supersaturation conditions by reducing theconcentration of Ca ions in water.External stimuliTwo effects of sonication on the aragonite formation wereconsidered. One is the local high temperature around theimploding cavities, and the other is the generation of nu-cleation sites. The addition of MgCl2 was required to syn-thesize aragonite in the carbonation method (Santos et al.,2012). The reason for the requirement of MgCl2 is notclear. However, it may be related to the supersaturationof CaCO3 in the solution.Small microbubbles have certain advantages forcrystal synthesis. These include an increase in the gas–liquid interface, which increases the reaction field andthe average residence time of the bubbles by decreasingthe buoyancy (Matsumoto et al., 2010). The local pH atthe gas–liquid interface depends on the mixture ratio ofCO2 and NH3 gas. CO2 gas decreases the local pH, where-as NH3 gas increases it. When the local pH is higher thanthe solution pH, the degree of supersaturation of CaCO3 atthe interface should increase. Therefore, calcite precipita-tion is preferable. Meanwhile, a low local pH decreasesthe supersaturation of CaCO3 at the interface. This resultsin vaterite precipitation. The optimal pH value for arago-nite is between the local pH values.DISCUSSION ON THEARAGONITE MORPHOLOGYCrystal faces of aragoniteAragonite is an orthorhombic carbonate with space groupPmcn (Speer, 1983). Its lattice parameters under ambientconditions are listed in Table 4. The experimentally ob-served morphology of single–crystal aragonite is shown inFigure 9. Three habits are commonly reported (Klein andDutrow, 2007): (1) Needle–shaped (acicular pyramidal)—planes (Fig. 9a) consisting of long {010} and {110} planesterminated by a significantly steep dipyramid and {011}.(2) Tabular (Fig. 9b)—consisting of a prominent {010}modified by {110} and a low prism {011}. (3) Pseudohex-agonal twins (not shown in Fig. 9)—consisting of three in-dividuals twinned on {110} terminated by a basal plane.The surface energy of crystal faces may determinethe morphology of crystals. These have been evaluatedby atomistic simulations (De Leeuw and Parker, 1998;Akiyama et al., 2011; Sekkal and Zaoui, 2013; Massaroet al., 2014; Kawano et al., 2015). The surface energy ofthe low–Miller–index surfaces revealed that the common-ly observed {011} faces were the most stable face amongthe calculated low–index surfaces (De Leeuw and Parker,1998; Massaro et al., 2014) in dry and wet conditions.However, the other commonly observed {110} and {010}faces were comparable to the other tested faces (DeLeeuw and Parker, 1998; Akiyama et al., 2011; Massaroet al., 2014). The estimated equilibrium morphology ofaragonite based on these surface energies and the Wulffand Gibbs theorem (Wulff, 1901; Gibbs, 1928) was incon-sistent with the experimentally observed morphology. Thediscrepancy between the experimental and modeled mor-phologies has not been adequately elucidated. One hy-pothesis is that the surface energy in the classical modelcan be modified by the presence of adsorbed ions, such asmagnesium ions. This phenomenon was not considered inthe papers. Meanwhile, the estimated growth morphologywas similar to the experimentally observed tabular mor-phology (De Leeuw and Parker, 1998).The anisotropic crystal habits of aragonite such as itsneedle–like morphology, cannot be explained only by theequilibrium and growth morphology. This also requiresthe variation in the growth rate of the crystal faces. Thegrowth rate may vary as a function of temperature, degreeof supersaturation, and selective adsorption of water andadditives. For example, the presence of a local dipole mo-ment on a particular surface of an ionic crystal enhancesthe adsorption of polar impurities including water mole-cules. At high supersaturation, the removal of adsorbedFigure 9. Morphology and crystal faces of common aragonite.Numbers indicate the Miller indices of the set of all symmetri-cally equivalent crystal faces. A tip of (a) needle shape and (b)tabular shape. The crystal shapes were created by JCrystal(Weber, 2011).H. Sakuma, S. Suehara, H. Nakao, J.–D. Kim and K. Tamura10molecules may become the rate–determining step for thegrowth of the face (Hartman, 1973). From density–func-tional–theory calculations, the adsorption energy of Mgion is different between the {001} and {110} faces of ara-gonite (Kawano et al., 2015). This difference in the ad-sorption energy of ions can alter the crystal habit of arago-nite.The anisotropic crystal habits of aragonite such as itsneedle–like morphology, cannot be explained only by theequilibrium and theoretically estimated growth morphol-ogy. This also requires the variation in the growth rate ofthe crystal faces. The growth rate may vary as a functionof temperature, degree of supersaturation, and selectiveadsorption of water and additives. For example, the pres-ence of a local dipole moment on a particular surface of anionic crystal enhances the adsorption of polar impuritiesincluding water molecules. At high supersaturation, theremoval of adsorbed molecules may become the rate–de-termining step for the growth of the face (Hartman, 1973).From density–functional–theory calculations, the adsorp-tion energy of Mg ion is different between the {001} and{110} faces of aragonite (Kawano et al., 2015). This dif-ference in the adsorption energy of ions can alter the crys-tal habit of aragonite.Elastic modulus of calcite and aragonite along certaincrystal directionsThe Halpin–Tsai model implies that the strength of thecomposite depends on the elastic modulus and aspect ra-tio of aragonite. The Young’s modulus depending on thecrystal axis can be calculated from the elastic complianceconstant of crystals (Nye, 1957). The elastic complianceconstants of various crystals were summarized in a paper(Huntington, 1958) and those of aragonite (Voigt, 1907)and calcite (Voigt, 1890; Bhimasenacrar, 1945) were usedfor estimating the Young’s modulus. The calculatedYoung’s moduli along the typical crystal axes are listedin Table 5.The calcite surface is characterized by the {104}cleavage. The Young’s modulus parallel to the [104] di-rection normal to the cleavage plane is 58.6 GPa. The longaxis of needle–like aragonite is parallel to the [001] direc-tion. The Young’s modulus 82.0 GPa is higher than that ofthe calcite perpendicular to the cleavage. Therefore, thedifference in the tensile strength of the composite origi-nating from the difference in the polymorphs of calciteand aragonite is owing to the differences in the aspect ratioand Young’s modulus based on the Halpin–Tsai model. Ifthe long axis of aragonite is parallel to the [100] direction,the Young’s modulus can be 1.7 times higher than that ofthe commonly observed needle–like aragonite elongatedparallel to the [001] direction. Such controlled crystalgrowth should be studied further for the development ofnovel aragonite fillers.CONCLUDING REMARKSTwo methods are commonly used for calcium carbonatesynthesis with gaseous CO2. The first is the injecting gas-eous CO2 into a Ca(OH)2 slurry. The second is adding aCa(OH)2 slurry to CO2 gas–injected water. Both methodscontrol the concentrations of CO2 and Ca ions in the solu-tions. In both methods, low supersaturation of CaCO3 ispreferable for synthesizing aragonite. Additives such asMgCl2, ammonia, amines, nitric acid, sucrose, H3PO4,NaOH, and KOH are critical for synthesizing aragonite.The mechanism of aragonite precipitation is not fully un-derstood. However, most additives alter the pH and super-saturation of CaCO3, and certain additives function as ara-gonite seed crystals. The pH of the solution should behigher than eight. This is related to the chemical speciesof the dissolved CO2 gas in the water. The chemical spe-cies HCO3− and CO32− are common at higher pH. WhenMgions are included in the solution as additives, the final pHshould be lower than 10 to prevent the precipitation ofbrucite. A high temperature (>50 °C) is preferable for ara-gonite synthesis, but this temperature can be reduced inthe presence of seed crystals. Therefore, temperature isrelated to the nucleation of aragonite. High–gravity exter-nal stimuli can narrow the particle size distribution andreduce the reaction time. The sound wave and microbub-ble methods can alter the local temperature and pH of thesolution, successfully synthesizing aragonite. Controllingthe aragonite morphology and crystallographic growth di-rection is critical to the elastic properties of compositematerials. The morphology of aragonite appears to varywith the quantity of seed aragonite crystals or seed hy-Table 5. Calculated Young’s moduli of calcite and aragonite along some crystal directionsTensile directionCalcite(Trigonal system using hexagonal axes)Tensile directionAragonite(Orthorhombic system)[100] 90.9 GPa [100] 143.9 GPa[104] 58.6 GPa [010] 75.8 GPa[001] 57.8 GPa [001] 82.0 GPaSynthesis of needle–like aragonite using carbonation method: A review 11droxyapatite crystals. The Mg/Ca ratio in water also influ-ences the aspect ratio of the needle–like aragonite. Thelong axes of most synthetic needle–like aragonites are par-allel to the [001] direction. The calculated elastic modu-lus, depending on the crystallographic direction of arago-nite, is anisotropic. 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