Fine Biocompatible Powders Synthesized from Calcium Lactate and Ammonium Sulfate

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Article Fine Biocompatible Powders Synthesized from Calcium Lactate and Ammonium Sulfate

Maksim Kaimonov 1,* , Tatiana Shatalova 1,2 , Yaroslav Filippov 1,3 and Tatiana Safronova 1,2

1 Department of Materials Science, Lomonosov Moscow State University, Building, 73, Leninskie Gory, 1, 119991 Moscow, Russia; [email protected] (T.S.); ﬁ[email protected] (Y.F.); [email protected] (T.S.) 2 Department of Chemistry, Lomonosov Moscow State University, Building, 3, Leninskie Gory, 1, 119991 Moscow, Russia 3 Research Institute of Mechanics, Lomonosov Moscow State University, Michurinsky pr., 1, 119192 Moscow, Russia * Correspondence: [email protected]; Tel.: +7-952-889-11-43

Abstract: Fine biocompatible powders with different phase compositions were obtained from a

0.5 M solution of ammonium sulfate (NH4)2SO4 and calcium lactate Ca(C3H5O3)2. The powder ◦ after synthesis and drying at 40 C included calcium sulfate dehydrate CaSO4·2H2O and calcite ◦ CaCO3. The powder after heat treatment at 350 C included β-hemihydrate calcium sulfate β- CaSO4·0.5H2O, γ-anhydrite calcium sulfate γ-CaSO4 and calcite CaCO3. The phase composition of ◦ powder heat-treated at 600 C was presented as β-anhydrate calcium sulfate β-CaSO4 and calcite ◦ CaCO3. Increasing the temperature up to 800 C leads to the sintering of a calcium sulfate powder β β consisting of -anhydrite calcium sulfate -CaSO4 main phase and a tiny amount of calcium oxide CaO. The obtained ﬁne biocompatible powders of calcium sulfate both after synthesis and after heat Citation: Kaimonov, M.; Shatalova, treatment at temperature not above 600 ◦C can be recommended as a ﬁller for producing unique T.; Filippov, Y.; Safronova, T. Fine composites with inorganic (glass, ceramic, cement) or polymer matrices. Biocompatible Powders Synthesized from Calcium Lactate and Keywords: calcium lactate; ammonium sulfate; synthesis of powder; calcium sulfate Ammonium Sulfate. Ceramics 2021, 4, 391–396. https://doi.org/10.3390/ ceramics4030028

1. Introduction Academic Editors: Gilbert Fantozzi and Anna Lukowiak Calcium sulfate has been known for over 100 years [1,2] as a relatively cheap biocompatible material widely used for orthopaedic, dental and pharmaceutical purposes [3]. Received: 15 April 2021 Implants based on calcium sulfate are resorbed almost completely at in vivo assays [4,5] Accepted: 11 June 2021 without causing inflammatory reactions. Additionally, calcium sulfate can be used in 3D Published: 4 July 2021 printing due to its binding properties [6]. Three crystalline modifications of calcium sulfate exist, differing in degree of hydra- Publisher’s Note: MDPI stays neutral tion: dihydrate calcium sulfate (gypsum) CaSO4·2H2O, hemihydrate calcium sulfate (basan- with regard to jurisdictional claims in ite) CaSO4·0.5H2O, and anhydrous calcium sulfate (anhydrite) CaSO4. Dihydrate calcium published maps and institutional affil- ◦ sulfate CaSO4·2H2O is stable up to a temperature of 60–90 C in an atmosphere of unsatu- iations. rated water vapour, and it decomposes to β-hemihydrate calcium sulfate β-CaSO4·0.5H2O upon further heating up to a temperature of 100–150 ◦C. It leads to dispersing and loosening the crystal lattice that forms fine crystals of β-CaSO4·0.5H2O. On the other hand, dihydrate calcium sulfate can decompose to α-hemihydrate calcium sulfate α-CaSO4·0.5H2O in an Copyright: © 2021 by the authors. atmosphere of saturated water vapour. In this case, the crystals of CaSO4·2H2O are re- Licensee MDPI, Basel, Switzerland. placed by densely packed prismatic crystals of α-CaSO4·0.5H2O that form large and dense This article is an open access article crystals, having a clear prismatic habit. The structure of CaSO4·0.5H2O generally can be distributed under the terms and represented as a deformed monoclinic crystal lattice of CaSO4·2H2O. Hemihydrate calcium conditions of the Creative Commons sulfate is metastable in water. When the temperature reaches 180–220 ◦C, water-soluble Attribution (CC BY) license (https:// γ-anhydrite γ-CaSO4 is formed, the polymorphic form of which exists up to temperatures creativecommons.org/licenses/by/ ◦ of 300–350 C. Further heating leads to the formation of insoluble β-anhydrite β-CaSO4, the 4.0/).

Ceramics 2021, 4, 391–396. https://doi.org/10.3390/ceramics4030028 https://www.mdpi.com/journal/ceramics Ceramics 2021, 4 392

◦ polymorphic form of which exists up to temperatures of 800–1200 C. β-CaSO4 is widely met in nature, is stable at high temperatures, and has sufﬁcient strength for use as bone ◦ material. At temperatures above 1200 C, β-CaSO4 transforms into a stable polymorphic form α-CaSO4 [2,7–9]. Chemical synthesis methods include various reactions and processes (precipitation from solutions, thermal decomposition, reduction reactions, hydrolysis and others). The rate of formation and growth of obtaining new-phase nuclei are controlled by changing the ratio of the number of reagents, the degree of supersaturation, the pH of solutions, the temperature of a process, etc. The coprecipitation method is a simple method for producing highly dispersed powders of various forms using catalysts, surfactants, or without them. Hence, the synthesis of calcium sulfate from aqueous solutions makes it possible to manage particle sizes and shapes, and soluble carboxylic acids in the synthesis of powders of inorganic substances may often play the role of surfactants. Thus, this work aimed to produce ﬁne biocompatible powders from calcium lactate and ammonium sulfate aqueous solutions.

2. Materials and Methods 2.1. Powder Preparation

A Ca(C3H5O3)2 aqueous solution (0.5 M) was obtained by adding calcium carbonate (Khimmed, GOST-4530-76) to 1 L of a 1 M solution of lactic acid C3H6O3 (RusKhim, Russia), according to the method described in previous work [10]. Then, 1 L of an aqueous solution of (NH4)2SO4 (Glavkhimreaktiv, GOST-3769-47) with a 0.5 M concentration was added at room temperature drop by drop to the aqueous solution of Ca(C3H5O3)2 for one hour with constant stirring. The obtained suspension (reaction 1) was still kept with a magnetic stirrer for one hour at room temperature.

Ca(C3H5O3)2 + (NH4)2SO4 + 2H2O → CaSO4·2H2O + 2NH4C3H5O3 (1)

The formed precipitate of calcium sulfate was separated from the mother liquid using a Buchner funnel. Then, it was dried in a thin layer at 40 ◦C in the air for 2 days. The dried powder was heat-treated at 350 ◦C, 600 ◦C, 700 ◦C and 800 ◦C for 2 h with a heating rate of 5 ◦C/min and cooled in a furnace.

2.2. Characterization The phase analysis of the synthesized and heat-treated powders was examined by X-ray diffraction (XRD) (diffractometer Rigaku D/Max-2500 (Rigaku Corporation, Tokyo, Japan) with a rotating anode, a 2θ angle range of 2–70◦ with a step of 0.02◦, and a rate of spectrum registration of 5◦/min, CuKα radiation). The morphology and phase composition of the powders was studied using LEO SUPRA 50VP scanning electron microscopy (Carl Zeiss, Jena, Germany); the imaging was performed at an accelerating voltage of 21 kV (SE2 detector). Thermal analysis was performed to determine the total mass loss of the synthesized product at heating up to 800 ◦C using NETZSCH STA 409 PC Luxx (NETZSCH, Selb, Ger- many). The gas-phase composition was monitored by the quadrupole mass spectrometer (QMS 403C Aëolos, NETZSCH, Selb, Germany). The mass spectra were registered for the ◦ following m/Z values: 18 (H2O); 44 (CO2); 46 (NO2); the heating rate was 10 C/min.

3. Results and Discussion The XRD data (Figure1a) show the phase composition of the powder synthesized and dried at 40 ◦C. Calcium sulfate powder was predominantly represented by gypsum CaSO4·2H2O and calcium carbonate CaCO3. The peaks of calcium carbonate CaCO3 on the X-ray diffraction pattern indicate an incomplete interaction process of lactic acid C3H6O3 with the original commercial calcium carbonate CaCO3 at the stage of obtaining calcium lactate Ca(C3H5O3)2. Ceramics 2021, 4 FOR PEER REVIEW 3

X-ray diffraction pattern indicate an incomplete interaction process of lactic acid C3H6O3 Ceramics 2021, 4 with the original commercial calcium carbonate CaCO3 at the stage of obtaining calcium393 lactate Ca(C3H5O3)2.

FigureFigure 1. 1. XRDXRD patterns patterns of ofthe the calcium calcium sulfate sulfate powders powders after after synthesis synthesis and drying and drying (a) and (a )after and after heatheat-treatment-treatment at at 350 350 °C◦C( (bb),), 600 600 °C◦C( (c),c), 700 700 °C◦C( (dd) )and and 800 800 °C◦C( (e).e ).

InIn a aprevious previous work work [10] [10, it], was it was pointed pointed out that out thatlactic lactic acid C acid3H6O C33 isH capable6O3 is capable of taking of parttaking in a part polycondensation in a polycondensation reaction owing reaction to owingthe presence to the presenceof hydroxyl of hydroxyl(–ОН) and (–OH) carboxyl and (–carboxylCOOH) (–COOH)groups that groups allow that the allowformation the formation of dimers of [11] dimers or polylactides [11] or polylactides with low with molec- low ularmolecular weight weight((C3H4O ((C2)n3)H [12]4O 2from)n)[12 lactic] from acid. lactic acid. Thus,Thus, the the possible possible formation formation of of dimers dimers or or polylactides polylactides with with l lowow molecular molecular weight weight duringduring the the prolonged prolonged storage storage of of lactic lactic acid acid in in turn turn led led to to the the incomplete incomplete interaction interaction of of the the initialinitial reaction reaction products products at at the the stage stage of of obtaining obtaining calcium calcium lactate lactate Ca(C Ca(C33HH5O5O3)32). 2. AfterAfter heat heat treatment treatment at at 350 350 °C◦C (Figure (Figure 1b),1b), the phase composit compositionion of the powder was representedrepresented by by ββ--hemihydratehemihydrate calcium calcium sulfate β --CaSOCaSO44··0.5H0.5H2O,2O, metastable metastable hexagonal hexagonal modificationmodiﬁcation of of γγ--anhydriteanhydrite calcium calcium sulfate sulfate γγ--CaSOCaSO44, ,and and unreacted unreacted calcium calcium carbonate carbonate CaCOCaCO3.3 .The The ββ-hemihydrate-hemihydrate calcium calcium sulfate sulfate ββ-CaSO-CaSO4·40.5H·0.5H2O2 Ois isformed formed due due to to heat heat treat- treat- ◦ mentment in in the the air air atmosphere, andand itit existsexists at at up up to to 180–220 180–220C °C according according to to literature literature data. data As. Asγ-anhydrite γ-anhydriteγ-CaSO γ-CaSO4 is4 metastableis metastable in in H2 HO2O and and air, air, it readilyit readily rehydrates rehydrates to to hemihydrate hemihydrate [7], [7],so thatso that may may explain explain the presencethe presence of β -hemihydrateof β-hemihydrate calcium calcium sulfate sulfateβ-CaSO β-CaSO4·0.5H4·20.5HO after2O ◦ afterheat heat treatment treatment at 350 at 350C in °C the in powder.the powder. ◦ TheThe phase phase composition composition of of the the powder powder after after heat heat treatment treatment at at 600 600 °CC was was represented represented β β byby the the stable stable orthorhombic orthorhombic modification modiﬁcation of ofβ-anhydrite-anhydrite calcium calcium sulfate sulfate β-CaSO-CaSO4, and4, un- and reactedunreacted calcium calcium carbonate carbonate CaCO CaCO3 was3 was also also saved saved (Figure (Figure 1c).1 c). ◦ ◦ AtAt the the temperature temperaturess of of 700 700 °CC and and 800 800 °CC,, the the phase phase composition composition of of the the powder powder was was β β representedrepresented by by β--anhydriteanhydrite calcium calcium sulfate sulfate β--CaSOCaSO44, ,calcium calcium carbonate carbonate CaCO CaCO33, ,calcium calcium oxide CaO and β-anhydrite calcium sulfate β-CaSO , and calcium oxide CaO, respectively oxide CaO and β-anhydrite calcium sulfate β-CaSO4,4 and calcium oxide CaO, respectively (Figure(Figure 1d,e).1d,e). TheThe TA TA data data ( (FigureFigure 2)2) revealed that the total mass loss of the synthesized powder heating up to 800 ◦C was 23% and proceeded in two stages, which correspond to the heating up to 800 °C ◦was 23% and proceeded◦ in two stages, which correspond to the in- tervalsintervals of 100 of 100–180–180 °C andC and 640 640–750–750 °C. C.

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The mass loss for the powder in the range of 100–180 °C corresponded to the decomposition of calcium sulfate dihydrate to crystalline hemihydrate calcium sulfate (reaction 2) and amounted to 16%.

CaSO4·2Н2О → CaSO4·0.5Н2О + 1.5Н2О (2) It is known by literature data that further heating of hemihydrate calcium sulfate leads to a complete loss of water with the formation of polymorphic forms, firstly water- Ceramics 2021, 4 394 soluble γ-anhydrite calcium sulfate γ-CaSO4, and then insoluble β-anhydrite calcium sulfate β-CaSO4, which was confirmed by XRD data (Figure 1b,c).

Figure 2. TA data forfor synthesizedsynthesized andand drieddried atat 4040 ◦°CC calcium sulfate powder.

◦ TheFurther mass heating loss for of thethe powdersynthesized in the calcium range sulfate of 100–180 powderC corresponded led to mass loss to in the the decom- range positionof 640–750 of calcium°C, which sulfate was dihydratecaused by to the crystalline decomposition hemihydrate of unreacted calcium calcium sulfate (reaction carbonate 2) andinto amountedcalcium oxide to 16%. CaO and carbon dioxide CO2 (reaction 3) and amounted to 5%. It was confirmed by XRD data (Figure 1d,e). The intensity of the calcium carbonate peak (2θ = · → · 29.4°) was much smallerCaSO at 7004 2H °C2O than CaSOat 6004 0.5H°C, as2O well + 1.5H as 2theO appearance of calcium (2) oxide CaO peaks. Increasing the temperature up to 800 °C demonstrated the complete It is known by literature data that further heating of hemihydrate calcium sulfate decomposing of unreacted calcium carbonate CaCO3, so the main phase of the powder leads to a complete loss of water with the formation of polymorphic forms, ﬁrstly water- was presented by β-anhydrite calcium sulfate β-CaSO4, and there was a tiny amount of soluble γ-anhydrite calcium sulfate γ-CaSO4, and then insoluble β-anhydrite calcium calcium oxide CaO too. It should be noted that there could be a contribution to mass loss sulfate β-CaSO4, which was conﬁrmed by XRD data (Figure1b,c). by theFurther possible heating combustion of the synthesized of the by-products calcium sulfate of the powder reaction led (ammonium to mass loss lactate/lactic in the range ofacid) 640–750 absorbed◦C, which on the was surface caused of calcium by the decomposition sulfate particles of at unreacted lower temperatures calcium carbonate too.

into calcium oxide CaO and carbonCaCO dioxide3 → CaO CO2 +(reaction CO2 3) and amounted to 5%.(3) It was conﬁrmed by XRD data (Figure1d,e). The intensity of the calcium carbonate peak (2θ =According 29.4◦) was muchto mass smaller-spectrometric at 700 ◦C thandata, at the 600 release◦C, as wellof CO as2 the, H2 appearanceO and NO2 of were calcium not oxiderecorded CaO in peaks.the range Increasings of 320– the380 temperature°C, 580–620 °C up and to 800750–◦800C demonstrated °C. The powder the after complete heat- treatment at 600 °C was light brown, caused apparently by the presence of decomposed decomposing of unreacted calcium carbonate CaCO3, so the main phase of the powder organic by-products. Figure 3 presents the photomicrographs of the calcium sulfate pow- was presented by β-anhydrite calcium sulfate β-CaSO4, and there was a tiny amount of calciumders obtained oxide CaOafter too.synthesis It should and bedrying noted at that 40 there°C (a), could and after be a contributionheat treatment to massat 350 loss °C by(b), the 600 possible °C (c) and combustion 800 °C (d). of the by-products of the reaction (ammonium lactate/lactic acid) absorbed on the surface of calcium sulfate particles at lower temperatures too.

CaCO3 → CaO + CO2 (3)

According to mass-spectrometric data, the release of CO2,H2O and NO2 were not recorded in the ranges of 320–380 ◦C, 580–620 ◦C and 750–800 ◦C. The powder after heat- treatment at 600 ◦C was light brown, caused apparently by the presence of decomposed organic by-products. Figure3 presents the photomicrographs of the calcium sulfate powders obtained after synthesis and drying at 40 ◦C (a), and after heat treatment at 350 ◦C (b), 600 ◦C (c) and 800 ◦C (d). Ceramics 2021, 4 FOR PEER REVIEW 5

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Figure 3. SEM images of the calcium sulfate powders after synthesis and drying at 40 ◦C(a); after heat-treatment at 350 ◦C( b), 600 ◦C(c) and 800 ◦C(d).

◦ FigureThe 3. SEM synthesized images of the and calcium dried sulfate 40 C powders calcium after sulfate synthesis powder and (Figuredrying at3a) 40 is °C presented (a); after heatin the-treatment form of at irregular350 °C (b), agglomerates 600 °C (c) and 800 with °C a (d size). of 1–10 µm. It is known that the form of calcium sulfate particles is acicular crystals. The by-product of the reaction probably presentThe insynthesized the powder and in dried a small 40 °C amount calcium leads sulfate to the powder sticking-together (Figure 3a) is of presented the powder in theparticles form of and irregular determines agglomerates the multilevel with structurea size of of1–10 the μm. aggregates. It is known After that heat the treatment form of calciumat 350 ◦ Csulfate (Figure particles3b), the is microstructure acicular crystals. of theThe calcined by-product powder of the inherits reaction the probably synthesized pre- sentmicrostructure. in the powder After in a small heat treatmentamount leads at 600 to the◦C stick (Figureing3-togetherc), the calcium of the powder sulfate particles powder andconsists determines of irregular the multilevel agglomerates structure with a of particle the aggregates. size distribution After heat of 1 treatment to 6 µm that at 350 consist °C (ofFigure smaller 3b), aggregates the microstructure 0.5–1 µm in of size, the whichcalcined are powder respectively inherits composed the synthesized of rhombohedral microstructure.particles 100–250After heat nm treatment in size. After at 600 heat °C ( treatmentFigure 3c), at the 800 calcium◦C (Figure sulfate3d), powder the sintering consists of ofcalcium irregular sulfate agglomerates powder is with observed. a particle The size size distribution of the grains of is 1 100–200 to 6 μm nm that that consist formed of smalleragglomerates aggregates with 0.5 irregular–1 μm forms.in size, which are respectively composed of rhombohedral particlesThus, 100 the–250 method nm in considered size. After inheat this treatment work for obtainingat 800 °C calcium(Figure 3d), sulfate the powder sintering from of calciumcalcium sulfate lactate powder and ammonium is observed. sulfate The aqueoussize of the solutions grains is makes100–200 it possiblenm that formed to produce ag- glomeratessubmicron with particles irregular of calcium forms sulfate. both after synthesis and after heat treatment. Thus, the method considered in this work for obtaining calcium sulfate powder from 4. Conclusions calcium lactate and ammonium sulfate aqueous solutions makes it possible to produce submicronThe presented particles researchof calcium has sulfate shown both the possibilityafter synthesis of obtaining and after ﬁne heat biocompatible treatment. powders from calcium lactate and ammonium sulfate with different phase compositions. The 4.powder Conclusions just after synthesis and drying included calcium sulfate dehydrate CaSO4·2H2O and calcite CaCO , and it is presented in the form of irregular agglomerates with a size of The presented3 research has shown the possibility of obtaining fine biocompatible 1–10 µm. The powder after heat treatment at 350 ◦C, which inherits the synthesized powder powders from calcium lactate and ammonium sulfate with different phase compositions. microstructure, included β-hemihydrate calcium sulfate β-CaSO ·0.5H O, γ-anhydrite The powder just after synthesis and drying included calcium4 sulfate2 dehydrate calcium sulfate γ-CaSO4, and calcite CaCO3. In addition, the phase composition of powder CaSO4·2H2O and calcite CaCO3, and it is presented in the form of irregular agglomerates heat treated at 600 ◦C, which consists of particles 100–250 nm in size linked to each other with a size of 1–10 μm. The powder after heat treatment at 350 °C, which inherits the and which formed agglomerates 1–6 µm in size, was presented by β-anhydrate calcium synthesized powder microstructure, included β-hemihydrate calcium◦ sulfate β- sulfate β-CaSO4 and calcite CaCO3. Increasing the temperature up to 800 C leads to the

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sintering of calcium sulfate powder consisting of β-anhydrite calcium sulfate β-CaSO4 main phase and a tiny amount of calcium oxide CaO. The ﬁne biocompatible powders of calcium sulfate obtained both after synthesis and after heat treatment at a temperature not above 600 ◦C can be recommended as a ﬁller for producing unique composites with inorganic (glass, ceramic, cement) or polymer matrices.

Author Contributions: Conceptualization, M.K.; Methodology, T.S. (Tatiana Safronova); Investiga- tion, M.K., T.S. (Tatiana Safronova), Y.F., T.S. (Tatiana Shatalova); Visualization, Y.F. and T.S. (Tatiana Shatalova); Writing—original draft, M.K. and T.S. (Tatiana Safronova); Writing—review & editing, M.K.; Supervision, T.S. (Tatiana Safronova); Project administration, T.S. (Tatiana Safronova). All authors have read and agreed to the published version of the manuscript. Funding: This work was carried out with financial support from the Russian Foundation for Basic Research (RFBR) (grant No. 20-03-00550 A). Acknowledgments: This work was carried out using equipment purchased at the expense of the Moscow University Development Program. Conflicts of Interest: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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