Article Simple, Reproducible Synthesis of Pure Monohydrocalcite with Low Mg Content
Takuma Kitajima 1, Keisuke Fukushi 2,* , Masahiro Yoda 3,4, Yasuo Takeichi 5 and Yoshio Takahashi 4 1 Division of Earth and Environmental Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan; [email protected] 2 Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan 3 Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, 2-12-1IE-1, Ookayama, Tokyo 152-8550, Japan; [email protected] 4 Department of Earth and Planetary Sciences, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan; [email protected] 5 Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan; [email protected] * Correspondence: fukushi@staff.kanazawa-u.ac.jp; Tel.: +81-76-264-6520
Received: 10 March 2020; Accepted: 10 April 2020; Published: 13 April 2020
Abstract: Monohydrocalcite (MHC) is a metastable hydrous calcium carbonate that requires Mg in the mother solution during formation in the laboratory. MHC prepared by previously reported methods always contains a large amount of Mg (Mg/Ca ratio up to 0.4) because of the simultaneous formation of amorphous Mg carbonate during synthesis, which has hindered detailed elucidation of the mineralogical characteristics of MHC. Here, we synthesized MHC at low temperature (5 ◦C) and found that it contained little Mg (Mg/Ca ratio < 0.01). X-ray absorption near-edge structure analysis of synthesized MHC revealed that the Mg present was structurally incorporated within the MHC, and that the chemical speciation of this Mg was similar to that of Mg in aragonite. Thus, low-temperature synthesis is an effective means of producing MHC without also producing amorphous Mg carbonate.
Keywords: monohydrocalcite; amorphous Mg carbonate; synthesis; low temperature; X-ray absorption near-edge structure
1. Introduction Monohydrocalcite (MHC, CaCO H O) is a hydrous calcium carbonate, and it is metastable with 3· 2 respect to anhydrous phases (CaCO3), such as calcite and aragonite [1,2]. MHC most frequently occurs as an authigenic mineral in alkaline saline lakes [3,4]. Recent research has suggested that the water chemistries of alkaline saline lakes, such as pH and the concentrations of Ca and Mg, are controlled by the formation of MHC in association with amorphous Mg carbonate (AMC, MgCO nH O) [5]. 3· 2 The metastable nature of MHC makes it a promising material for environmental remediation [1]. Previous studies have shown that MHC effectively sorbs contaminants, such as arsenic, lead, and phosphorous [6–8], although the presence of impurities, especially for Mg with MHC, suppresses the sorption ability of MHC for arsenic and prosperous [9,10]. Thus, a simple, reproducible method for the synthesis of MHC containing low levels of impurities would be useful. Nishiyama et al. [11] systematically examined the formation of MHC from Na2CO3–CaCl2–MgCl2 solutions and found that MHC forms when the initial Na2CO3 concentration exceeds the total Ca concentration and a large amount of Mg is present. Although methodologies based on these conditions allow for reproducible synthesis of MHC, the MHC obtained contains a large amount of Mg as an
Minerals 2020, 10, 346; doi:10.3390/min10040346 www.mdpi.com/journal/minerals Minerals 2020, 10, 346 2 of 8 impurity [12–14]. The dominant host of this Mg is AMC, which is formed together with MHC [14,15]. However, the co-existence of AMC with MHC has hindered detailed elucidation of the mineralogical and physicochemical characteristics of MHC [2,16]. Here, we report a simple, reproducible method for the synthesis of MHC with low Mg content.
2. Materials and Methods
All of the following experiments were conducted in a temperature-controlled room at 5 ◦C. A series of 50 mL solutions containing 0.05 mol/kg MgCl2 and 0.01 mol/kg CaCl2 were prepared in polypropylene bottles. Stock Na2CO3 solution was then added to the bottles to obtain initial Na2CO3 concentrations of 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, and 0.12 mol/kg. Immediately after the addition of the Na2CO3 solution, whitish precipitates began to form in the bottles. The bottles were covered with Kimwipe paper to allow interaction of the solutions with atmospheric CO2 during the reaction period [5]. After the bottles were left for 400 h to allow the reaction to proceed, the obtained suspensions were vacuum-filtered through 0.2 µm membranes. The filtrates were acidified with concentrated HNO3 to produce 0.6% HNO3 solutions, which were then examined by inductively coupled plasma optical emission spectroscopy (ICP-OES: ES-710S; Varian, Inc., Palo Alto, CA, USA). The solids on the filter papers were freeze-dried without washing and then used for mineralogical characterization. X-ray diffraction was used to determine the mineralogical compositions of the solids (Ultima IV: Cu Kα, 40 kV, 30 mA; Rigaku Corp., Tokyo, Japan). The selected MHC samples were observed using scanning electron microscopy (SEM; S-3000N; Hitachi High-Technologies Corp., Tokyo, Japan) with energy dispersed X-ray spectroscopy (EDS; EMAX500; Horiba Ltd., Kyoto, Japan) to elucidate their mineralogy. ICP-OES was used to determine the concentrations of Ca and Mg in the filtrates. For the solids identified to contain MHC, the Mg/Ca ratio was determined by wet chemical analysis by first dissolving 100 mg of solid in 40 mL of 0.6% HNO3 solution and then measuring the Ca and Mg concentrations in the solution by ICP-OES. Triplicate measurements of the obtained solutions showed that the analytical errors with the ICP-OES are always < 2%. Our previous study showed that the uncertainties of the Mg/Ca ratio associated with the procedure of the wet chemical analyses of the MHC are < 4% [15]. The significant digits given in this study (Table1) are based on the analytical errors and the uncertainties. The Mg K-edge X-ray absorption near-edge structure spectrum of the solid obtained with an initial Na2CO3 concentration of 0.10 mol/kg was measured using the BL-19B beamline at the Photon Factory, KEK, Japan. A powdered solid sample was loaded on conductive double-sided carbon tape attached to a stainless steel sample holder. The spectrum was obtained in fluorescence X-ray yield mode with a silicon drift detector under a high vacuum (approx. 1 10 6 Pa) × − and processed using the REX2000 data analysis software package (Rigaku Co. Ltd., Tokyo, Japan). Minerals 2020, 10, 346 3 of 8
Table 1. Summary of the synthesis experiments conducted in the present study and by Nishiyama et al. [11] and Fukushi et al. [15].
Reacted Solution Temperature Initial Concentrations (mol/kg) Mg/Ca in Partitioning Crystallite Size Batch# Concentrations (mol/kg) Mineralogy Reference (◦C) Solids Coefficient log D Ca Mg CO3 Ca Mg (nm) 1 5 0.047 0.0095 0.029 1.6 10 2 1.0 10 2 - - Calcite - This study × − × − 2 5 0.050 0.010 0.040 1.1 10 2 1.0 10 2 - - Calcite - This study × − × − 3 5 0.051 0.010 0.050 2.8 10 3 8.8 10 3 - - Calcite - This study × − × − 4 5 0.050 0.010 0.060 4.1 10 4 3.7 10 3 - - Mg-Calcite - This study × − × − 5 5 0.051 0.010 0.071 1.2 10 4 9.4 10 3 - - MHC, Ara - This study × − × − 6 5 0.050 0.010 0.080 1.1 10 4 9.5 10 3 0.0077 4.1 MHC 37.0 7.4 This study × − × − − ± 7 5 0.050 0.010 0.089 8.4 10 5 1.0 10 2 0.0083 4.2 MHC 36.3 4.5 This study × − × − − ± 8 5 0.050 0.010 0.10 6.7 10 5 9.8 10 3 - - MHC 38.2 2.0 This study × − × − ± 9 5 0.050 0.010 0.11 6.5 10 5 9.5 10 3 0.0101 4.2 MHC 37.2 3.1 This study × − × − − ± 10 5 0.050 0.010 0.12 6.3 10 5 9.7 10 3 0.0102 4.2 MHC 38.5 3.9 This study × − × − − ± N1 25 0.052 0.010 0.060 8.1 10 4 7.4 10 3 - - Ara, Cal - Nishiyama et al. [11] × − × − N2 25 0.052 0.010 0.077 1.6 10 4 9.1 10 3 0.02 3.5 MHC - Nishiyama et al. [11] × − × − † − N3 25 0.052 0.010 0.120 1.9 10 4 7.3 10 3 0.06 2.8 MHC - Nishiyama et al. [11] × − × − † − F1 25 0.050 0.035 0.090 2.4 10 4 1.9 10 2 0.085 0.003 3.0 MHC 28.8 1.2 Fukushi et al. [15] × − × − ± − ± F2 25 0.050 0.015 0.100 1.3 10 4 6.7 10 3 0.136 0.002 2.6 MHC 21.3 2.1 Fukushi et al. [15] × − × − ± − ± F3 25 0.160 0.040 0.200 3.5 10 4 2.7 10 2 0.281 0.005 2.4 MHC 11.7 0.9 Fukushi et al. [15] × − × − ± − ± Calculated from mass balance. † Minerals 2020, 10, 346 4 of 8
3. Results and Discussion
Figure1 shows the X-ray di ffraction patterns of the solids obtained with various initial Na2CO3 concentrations. When the initial Na2CO3 concentration was 0.06 mol/kg, the solid contained only magnesian calcite ((Ca,Mg)CO3), and when the initial Na2CO3 concentration was 0.05 mol/kg or less, the solid contained only calcite. However, when the initial Na2CO3 concentration was 0.07 mol/kg or greater, the solid contained MHC. When the initial Na2CO3 concentration was 0.07 mol/kg, small peaks attributed to aragonite were observed in the X-ray diffraction pattern, but when the initial Na2CO3 concentration was 0.08 or greater, the solid contained only MHC. These findings are consistent with those of Nishiyama et al. [11] who examined the formation of MHC using the same concentrations of CaCl2 and MgCl2 but a higher temperature (25 ◦C) than that used in the present study (5 ◦C) and found that the solids obtained when the initial Na2CO3 concentration was greater than 0.07 mol/kg contained only crystalline MHC (Table1). Thus, the formation of MHC from Na 2CO3–CaCl2–MgCl2 solutions at low temperature (5 C) was comparable with that at 25 C[5,11,15]. Minerals 2020, 10, x FOR PEER REVIEW◦ ◦ 3 of 8
C Initial Na2CO3[mol/kg] C C C C C C C C 0.03 C C 0.04 0.05 MC 0.06 MC MC MC MC MC MC MC MC M M M M M M M M M M M 0.07 AA A A M M M 0.08 0.09 0.10 0.11 0.12 10 20 30 40 50 60 °2θ(CuKα)
Figure 1. X-ray diffraction patterns of the solids obtained with different initial concentrations of Na2CO3. Figure 1. X-ray diffraction patterns of the solids obtained with different initial concentrations of A: aragonite (CaCO3), C: calcite (CaCO3), MC: magnesian calcite ((Ca,Mg)CO3), M: monohydrocalcite Na2CO3. A: aragonite (CaCO3), C: calcite (CaCO3), MC: magnesian calcite ((Ca,Mg)CO3), M: (CaCO nH O). 3monohydrocalcite· 2 (CaCO3·nH2O).
In the presentIn the present study, thestudy, initial the concentrationinitial concentratio of Mgn of was Mg 0.01 was mol 0.01/kg mol/kg for each for synthesiseach synthesis experiment. After reaction,experiment. the concentrationAfter reaction, ofthe Mg concentration in the filtrates of Mg removed in the filtrates from theremoved solids from that containedthe solids that MHC only was in thecontained range MHC 0.0095 only to 0.010was in mol the/ kg,range and 0.0095 no relationship to 0.010 mol/kg, between and no Mg relationship concentration between in Mg the filtrate concentration in the filtrate and initial Na2CO3 concentration was observed. The Mg/Ca ratio of the and initial Na2CO3 concentration was observed. The Mg/Ca ratio of the obtained MHC was 0.01 or obtained MHC was 0.01 or lower, as estimated by wet chemical analysis (Table 1). In contrast to our lower, assyntheses estimated at low by wettemperature, chemical the analysis syntheses (Table of Nishiyama1). In contrast et al. to[9] ourat 25 syntheses °C revealed at lowfiltrate temperature, Mg the synthesesconcentration of Nishiyama in the range et 0.0075 al. [9] to at 0.0092 25 ◦C and revealed Mg/Ca ratios filtrate in the Mg range concentration of 0.02 to 0.06 in from the synthesis range 0.0075 to 0.0092 andexperiments Mg/Ca conducted ratios inthe under range the comparable of 0.02 to 0.06initial from solution synthesis compositions experiments at 25 °C. conducted under the comparableIt initial is reported solution that there compositions are two types at 25of Mg◦C. speciation associated with MHC formation, where It isMg reported can be structurally that there coordinated are two types either of with Mg speciationMHC or with associated the discrete with phase MHC AMC formation, [15]. Fukushi where Mg et al. [15] suggested that when Mg is structurally coordinated with MHC, the coordination can be structurally coordinated either with MHC or with the discrete phase AMC [15]. Fukushi et al. [15] environment of Mg resembles that in aragonite. Figure 2 shows Mg K-edge spectra obtained for the suggested that when Mg is structurally coordinated with MHC, the coordination environment of Mg MHC obtained in the present study with an initial Na2CO3 concentration of 0.1 mol/kg (#9 in Table 1); resemblesAMC; that MHC in aragonite. with high (#F3 Figure in Table2 shows 1) or Mglow K-edge(#F1 in Table spectra 1) Mg obtained content synthesized for the MHC at 25 obtained °C; and in the presentMg-doped study with aragonite an initial [15]. Na The2CO shape3 concentration of the spectra offor 0.1 the mol MHCs/kg with (#9 inhigh Table and1 low); AMC; Mg content MHC can with high be attributed to contributions by the two endmembers, i.e., Mg in aragonite and AMC [15]. The shape (#F3 in Table1) or low (#F1 in Table1) Mg content synthesized at 25 ◦C; and Mg-doped aragonite [15]. The shapeof the of spectrum the spectra for forthe MHC the MHCs obtained with in the high presen andt lowstudy Mg was content well-matched can be with attributed that of Mg-doped to contributions aragonite, suggesting that AMC formation was almost negligible during MHC synthesis at low by the two endmembers, i.e., Mg in aragonite and AMC [15]. The shape of the spectrum for the MHC temperature.
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obtained in the present study was well-matched with that of Mg-doped aragonite, suggesting that AMC formation was almost negligible during MHC synthesis at low temperature.
Figure 2. Mg K-edge X-ray absorption near-edge structure spectra of the monohydrocalcite (MHC) Figure 2. Mg K-edge X-ray absorption near-edge structure spectra of the monohydrocalcite (MHC) synthesized in the present study at 5 C and an initial Na CO concentration of 0.1 mol/kg and four synthesized in the present study at◦ 5 °C and an initial Na2CO2 3 concentration3 of 0.1 mol/kg and four referencereference materials: materials: amorphous amorphous Mg Mg carbonate carbonate (AMC;(AMC; Fukushi Fukushi et etal. al.[15]), [15 MHC]), MHC with high with or high low Mg or low Mg contentcontent synthesized synthesized at 25 at◦ 25C (Fukushi°C (Fukushi et et al., al., [15[15]),]), and Mg-doped Mg-doped aragonite aragonite [15]. [The15]. spectrum The spectrum for for AMC showedAMC showed two peaks,two peaks, at 1312 at 1312 and and 1315 1315 eV eV (black(black dotted dotted lines), lines), and and that thatfor aragonite for aragonite showed showed a a sharp peaksharp at peak around at around 1313 1313 eV (redeV (red dotted dotted line). line).
The Mg partitioning behavior was also examined from the solution phase analyses. The The Mg partitioning behavior was also examined from the solution phase analyses. The partitioning partitioning behavior of Mg in calcium carbonates from aqueous solutions can be assessed with the behaviorapparent of Mg partitioning in calcium coefficient carbonates (D) defined from aqueousby the following solutions Equation can [17]: be assessed with the apparent partitioning coefficient (D) defined by the following Equation [17]: {Mg}solid /{Ca} solid D = (1) Mg{Mg} /{Ca}/ Ca solid D = { }solidsol{ } sol (1) Mg sol/ Ca sol where {Mg}sol and {Ca}sol denote the concentration{ } of Mg{ and} Ca in the solutions. {Mg}solid and {Ca}solid denote the concentration of Mg and Ca in solids. Therefore, {Mg}solid/{Ca} solid corresponds to Mg/Ca where {Mg} and {Ca} denote the concentration of Mg and Ca in the solutions. {Mg} and ratio insol Table 1. Thesol log D obtained at 25 °C [11,15] widely varies from −3.5 to −2.4. The MHCs withsolid {Ca}solidthedenote higher the Mg/Ca concentration generally exhibit of Mg the and higher Ca inlog solids. D (Table Therefore, 1). The XANES {Mg} solidspectra/{Ca} showedsolid correspondsthat the to Mg/Ca ratioAMC incontents Table 1in. Thethe solids log D withobtained MHC increased at 25 ◦C[ with11,15 the] widelyMg/Ca ratio varies of the from solids.3.5 Therefore, to 2.4. Thethe MHCs − − with thedependency higher Mg of/Ca the generallylog D on the exhibit Mg/Ca theratio higher at 25 °C log mayD come(Table from1). Thethe contribution XANES spectra of AMC showed with that the AMCMHC contents in the sample in the solidsrather than with the MHC partitioning increased in the with structure the Mg of /theCa MHC, ratio because of the solids. the increase Therefore, in the AMC simply results in the increase in {Mg}solid relative to {Ca}solid in (1). On the other hand, the log D dependency of the log D on the Mg/Ca ratio at 25 ◦C may come from the contribution of AMC with obtained from the low-temperature synthesis at 5 °C takes almost constant value at −4.2 (Table 1). MHC in the sample rather than the partitioning in the structure of the MHC, because the increase in The XANES spectrum suggests that the AMC formation is almost negligible at 5 °C. The constant AMC simplypartitioning results coefficients in the increaserepresent the in partitio {Mg}solidningrelative of Mg in the to {Ca}structuresolid ofin MHC (1). Onat 5 °C. the other hand, the log D obtained from the low-temperature synthesis at 5 C takes almost constant value at 4.2 (Table1). ◦ − The XANES spectrum suggests that the AMC formation is almost negligible at 5 ◦C. The constant Minerals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/minerals partitioning coefficients represent the partitioning of Mg in the structure of MHC at 5 ◦C. The simultaneous formation of AMC is essential for the formation of MHC, and it has been suggested that AMC likely plays a role in preventing the dehydration of MHC [11,15,18]. However, Minerals 2020, 10, 346 6 of 8
Minerals 2020, 10, x FOR PEER REVIEW 2 of 8 we found that MHC formation at low temperature produced little AMC. During MHC formation, when the initialThe Na2 COsimultaneous3 concentration formation exceeds of AMC the Cais essential concentration for the in formation the presence of MHC, of a largeand it amount has been of Mg, 2+ 2 2+ 2 after allsuggested of the Cathat AMChas reacted likely plays with a COrole3 in− ,preventi Mg inng the the solutiondehydration reacts of MHC with the[11,15,18]. remaining However, CO3 − to formwe AMC found [11 ].that In theMHC present formation study, at low the initialtemperature solution produced conditions little wereAMC. the During same MHC as those formation, used in the when the initial Na2CO3 concentration exceeds the Ca concentration in the presence of a large amount experiments of Nishiyama et al. [11] that were conducted at 25 ◦C. Therefore, AMC most likely forms 2+ 2− 2+ 2− togetherof Mg, with after MHC all of at the the Ca start has of reacted synthesis. with InCO a3 preliminary, Mg in the solution study, we reacts confirmed with the an remaining initial decrease CO3 in Mg concentrationto form AMC [11]. in the In the solution present at study, the start the initial of synthesis solution conditions (Figure A1 were in Appendixthe same asA those); however, used in the the experiments of Nishiyama et al. [11] that were conducted at 25 °C. Therefore, AMC most likely AMC that formed was gradually dissolved during the aging processes. The higher concentrations of forms together with MHC at the start of synthesis. In a preliminary study, we confirmed an initial Mg observeddecrease in Mg the concentration solutions after in synthesisthe solution in at the the present start of studysynthesis compared (Figure A1 with in thoseAppendix reported A); by Nishiyamahowever, et al. the [11 AMC] (Table that1) formed suggest was that gradually the solubility dissolved of AMC during is higher the aging at 5 ◦processes.C than it isThe at 25higher◦C. It has beenconcentrations also documented of Mg that observed the solubility in the solutions of nesquehonite after synthesis (MgCO in the3 3H present2O), astudy hydrous compared Mg carbonate, with · increasesthose with reported the decreaseby Nishiyama in temperature et al. [11] (Table from 1) suggest 25 ◦C to that 5 ◦ theC[ 19solubility]. In addition, of AMC is the higher transformation at 5 °C rate ofthan MHC it is has at 25 been °C. It shown has been to bealso significantly documented retarded that the solubility with decreasing of nesquehonite temperature (MgCO [203·3H]. Therefore,2O), a the MHChydrous formed Mg carbonate, during the increases initial with stages the ofdecrease the reaction in temperature is able tofrom persist 25 °C evento 5 °C after [19]. In the addition, dissolution of AMC.the transformation rate of MHC has been shown to be significantly retarded with decreasing temperature [20]. Therefore, the MHC formed during the initial stages of the reaction is able to persist The MHC with low Mg obtained from this study can provide the implication of the role of the Mg even after the dissolution of AMC. on the crystal growth and the morphology of MHC. The peak widths of XRD peak are related to the The MHC with low Mg obtained from this study can provide the implication of the role of the crystalliteMg on size the accordingcrystal growth to Scherrer and the morphology Equation [21 of]: MHC. The peak widths of XRD peak are related to the crystallite size according to Scherrer Equation [21]: Kλ L = (2) β cosKλθ L = (2) βθcos where L denotes the crystallite size, β denotes full width at half maximum of the selected diffraction peak,whereK represents L denotes the the Scherrer crystallite constants size, β denotes (0.94), full and widthθ denotes at half themaximum position of the ofthe selected selected diffraction diffraction peak, K represents the Scherrer constants (0.94), and θ denotes the position of the selected diffraction peak. Fukushi et al. [15] also measured the crystallite size of the MHC obtained at 25 ◦C. They showed that thepeak. crystallite Fukushi et size al. negatively[15] also measured correlates the crystallite with the size Mg /ofCa the ratio MHC (Table obtained1). The at 25 crystallite °C. They showed size of the that the crystallite size negatively correlates with the Mg/Ca ratio (Table 1). The crystallite size of the MHC obtained at 5 ◦C takes almost constant at around 37 nm regardless of the initial solution conditions MHC obtained at 5 °C takes almost constant at around 37 nm regardless of the initial solution (Table1). The size is significantly higher than any synthesized MHC at 25 C, which always exhibits conditions (Table 1). The size is significantly higher than any synthesized MHC◦ at 25 °C, which always the higherexhibits Mg the/Ca. higher This Mg/Ca. suggests This that suggests the presence that the of presence AMC with of AMC MHC with acts MHC as the acts inhibitor as the inhibitor for the MHC crystallitefor the growth, MHC crystallite as suggested growth, by as Fukushisuggested et by al. Fukushi [15]. Figure et al. [15].3 shows Figure the 3 shows SEM the image SEM ofimage the of MHC with thethe MHC EDSspectrum with the EDS obtained spectrum from obtained the initial from Na the2CO initial3 concentration Na2CO3 concentration of 0.11mol of 0.11/kg mol/kg (#9 in Table(#9 1). The MHCin Table obtained 1). The at MHC 5 ◦C wasobtained the aggregates at 5 °C was of the irregularly aggregates shaped of irregularly particles. shaped The morphology particles. The is very differentmorphology from the is MHCvery different obtained from at 25the◦ C,MHC which obtained commonly at 25 °C, exhibits which commonly the two connected exhibits the spherules two connected spherules with a diameter from several to 30 μm [15]. The crystallite sizes of MHC with a diameter from several to 30 µm [15]. The crystallite sizes of MHC obtained at 5 ◦C were not obtained at 5 °C were not significantly different from those at 25 °C. The difference of MHC between significantly different from those at 25 ◦C. The difference of MHC between at 5 and 25 ◦C was the Mg at 5 and 25 °C was the Mg contents in MHC. This suggests that the AMC may play a role in controlling contents in MHC. This suggests that the AMC may play a role in controlling the morphology. the morphology.
FigureFigure 3. Scanning 3. Scanning electron electron microscopy microscopy (SEM) (SEM) imageimage with with energy energy di dispersedspersed X-ray X-ray spectroscopy spectroscopy (EDS) (EDS) spectrumspectrum of the of MHC the MHC (batch#9 (batch#9 in Table in Table1) synthesized 1) synthesized at 5 ◦ atC( 5a )°C and (a) SEM and imageSEM image of the of MHC the MHC (batch#F1 in Table(batch#F11) at 25 in◦ TableC (Fukushi 1) at 25 et°C al. (Fukushi [13]) (b et). al. [13]) (b).
Minerals 2020, 10, 346 7 of 8 Minerals 2020, 10, x FOR PEER REVIEW 3 of 8
4. Conclusions Conclusions Here, we report that MHC with low Mg content (Mg(Mg/Ca/Ca < 0.01) can be synthesized at low temperature (5(5◦ C)°C) using using conditions conditions reported reported previously previously [9] (i.e., [9] initial (i.e., concentration initial concentration of CO3 exceeding of CO3 exceedingthe total concentration the total concentration of Ca in theof Ca presence in the presen of a largece of amounta large amount of Mg). of The Mg). X-ray The X-ray absorption absorption near nearnear-edge near-edge structure structure spectrum spectrum of the of solidthe solid obtained obtained at 5 at◦C 5 with°C with an initialan initial Na 2NaCO2CO3 concentration3 concentration of of0.10 0.10 mol mol/kg/kg showed showed that that the the Mg Mg in thein the solid solid was was structurally structurally incorporated incorporated into into the the MHC MHC and and that that the thechemical chemical speciation speciation of this of Mg this was Mg similar was similar to that to of Mgthat in of aragonite. Mg in aragonite. Thus, low-temperature Thus, low-temperature synthesis synthesiscan be used can to be produce used to MHC produce without MHC the without production the production of AMC e ffofectively. AMC effectively. Author Contributions: Conceptualization, T.K. and K.F.; Methodology; T.K., M.Y., Ya.T., and Yo.T.; Writing- Author Contributions: Conceptualization, T.K. and K.F.; Methodology; T.K., M.Y., Y.T. (Yasuo Takeichi), and OriginalY.T. (Yoshio Draft Takahashi); Preparation, Writing-Original T.K. and K.F.; Draft Writing-Review Preparation,T.K. and and Editin K.F.;g, Writing-ReviewM.Y., Ya.T., and and Yo.T; Editing, Funding M.Y., Acquisition,Y.T. (Yasuo Takeichi), K.F. M.Y. andand Y.T.Yo.T. (Yoshio Takahashi); Funding Acquisition, K.F. M.Y. and Y.T. (Yoshio Takahashi). All authors have read and agreed to the published version of the manuscript. Funding: Financial support was provided by Grants-in-Aid for Scientific Research from the Japan Society for PromotionFunding: Financial of Science support (no. 17H06458) was provided and by Grants-in-Aida cooperative research for Scientific program Research of the from Institute the Japan of Nature Society and for Promotion of Science (no. 17H06458) and by a cooperative research program of the Institute of Nature and Environmental Technology, Kanazawa University (nos. 19035 and 20061). This work was performed under the Environmental Technology, Kanazawa University (nos. 19035 and 20061). This work was performed under the approval of the Photon Factory Program AdvisoryAdvisory Committee (Proposal No.No. 2018S1-001). Acknowledgments: We wouldwould likelike to to thank thank S. S. Tan Tan for for the the experimental experimental support support for for the the X-ray X-ray absorption absorption near-edge near- edgestructure structure analysis. analysis. Discussions Discussions with Y.with Sekine Y. Sekine are also are greatly also greatly appreciated. appreciated.
ConflictsConflicts of Interest: The authors declare no conflictsconflicts of interest. Appendix A Appendix A
Figure A1. Changes in Mg concentration and Mg/Ca ratio over time during monohydrocalcite synthesis Figure A1. Changes in Mg concentration and Mg/Ca ratio over time during monohydrocalcite at 5 ◦C. The initial solutions were 0.015 mol/kg in MgCl2, 0.015 mol/kg in CaCl2, and 0.08 mol/kg in synthesis at 5 °C. The initial solutions were 0.015 mol/kg in MgCl2, 0.015 mol/kg in CaCl2, and 0.08 Na2CO3. mol/kg in Na2CO3. References References 1. Fukushi, K.; Munemoto, T.; Sakai, M.; Yagi, S. Monohydrocalcite: A Promising Remediation Material for 1. Fukushi,Hazardous K.; Anions. Munemoto,Sci. Technol.T.; Sakai, Adv. M.; Mater. Yagi, 2011S. Monohydrocalcite:, 12, 64702. [CrossRef A Promising][PubMed Remediation] Material for 2. HazardousRodriguez-Blanco, Anions. J.D.;Sci. Technol. Shaw, Adv. S.; Bots, Mater. P.; 2011 Roncal-Herrero,, 12, 064702. T.; Benning, L.G. The Role of Mg in the 2. Rodriguez-Blanco,Crystallization of Monohydrocalcite. J.D.; Shaw, S.; Bots,Geochim. P.; Roncal-Herrero, Cosmochim. Acta T.;2014 Benning,, 127, 204–220. L.G. The [CrossRef Role of] Mg in the 3. CrystallizationTaylor, G.F. The of Occurrence Monohydrocalcite. of Monohydrocalcite Geochim. Cosmochim. in Two Small Acta 2014 Lakes, 127 in, the 204–220. South-East of South Australia. 3. Taylor,Am. Mineral. G.F. The1975 Occurrence, 60, 690–697. of Monohydrocalcite in Two Small Lakes in the South-East of South Australia. 4. Am.Last, Mineral. F.M.; Last, 1975 W.M.;, 60, Halden,690–697. N.M. Carbonate Microbialites and Hardgrounds from Manito Lake, an Alkaline, 4. Last,Hypersaline F.M.; Last, Lake W.M.; in the Halden, Northern N.M. Great Carbonate Plains of Canada.MicrobialitesSediment. and Geol.Hardgrounds2010, 225 ,from 34–49. Manito [CrossRef Lake,] an Alkaline, Hypersaline Lake in the Northern Great Plains of Canada. Sediment. Geol. 2010, 225, 34–49. 5. Fukushi, K.; Matsumiya, H. Control of Water Chemistry in Alkaline Lakes: Solubility of Monohydrocalcite and Amorphous Magnesium Carbonate in CaCl2–MgCl2–Na2CO3 Solutions. ACS Earth Space Chem. 2018, 2, 735–744.
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