Journal of Asian Ceramic Societies · 2017. 2. 1. · 86 S.-C. Wu et al. / Journal of Asian Ceramic Societies 4 (2016) 85–90 Eggshells generated after breaking eggs represent a

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Journal of Asian Ceramic Societies 4 (2016) 85–90

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Journal of Asian Ceramic Societies

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ynthesis of hydroxyapatite from eggshell powders through ballilling and heat treatment

hih-Ching Wua, Hsueh-Chuan Hsua, Shih-Kuang Hsua, Ya-Chu Changb, Wen-Fu Hoc,∗

Department of Dental Technology and Materials Science, Central Taiwan University of Science and Technology, Taichung, Taiwan, ROCDepartment of Materials Science and Engineering, Da-Yeh University, Changhua, Taiwan, ROCDepartment of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung, Taiwan, ROC

r t i c l e i n f o

rticle history:eceived 26 September 2015eceived in revised form8 November 2015ccepted 3 December 2015vailable online 30 December 2015

a b s t r a c t

Every day, several million tons of eggshells are being generated as bio-waste across the world. This studydemonstrates the synthesis of HA powder using dicalcium phosphate dehydrate (CaHPO4·2H2O, DCPD)and eggshell powders via ball milling and subsequent heat treatment. The formation of HA phase can beinitiated by sintering the 1 h milled sample at 1000 ◦C for 1 h, while pure HA phase can be obtained uponsintering the 10 h milled sample. Additionally, the final products composed of biphasic calcium phosphate(HA + �-TCP crystals) can easily be prepared by ball milling for 5 h followed by heat treatment at 1000 ◦C

eywords:ggshellydroxyapatite-Tricalcium phosphateall milling

for 1 h. The carbonate peaks observed in the FTIR analysis of the as-prepared HA closely matched thoseof A- and B-type carbonates, which is typical of the biological apatite. The elemental composition of theas-synthesized HA showed the presence of Ca, P, Mg, and Sr.

© 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Production and hosting byElsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

eat treatment

. Introduction

Hydroxyapatite (Ca10(PO4)6(OH)2, HA) is being actively inves-igated as a potential catalyst or a catalyst support for a variety ofhemical reactions [1,2]. Besides, HA is also being analyzed as andsorbent for environmental protection, owing to its strong affin-ty toward heavy metal ions [3,4]. Furthermore, synthetic HA findspplication in various medical and dental applications as a promis-ng material for healing damaged bones and teeth, for implant andcaffold, and as a drug delivery agent, due to its biocompatibilitynd bioactivity as well as its similarity to the inorganic componentf the hard tissues in natural bones [5,6]. However, the properties ofatural apatite crystals and conventional synthetic stoichiometricA are significantly different [7]. For example, the natural apatite

n the human body contains significant amounts of carbonate andrace elements [8,9]. Natural bone is clearly a non-stoichiometricA [10]. The properties of synthetic HA are largely determined by itsarticle size, morphology, crystallinity, and composition, which in

∗ Corresponding author at: Department of Chemical and Materials Engineering,ational University of Kaohsiung, 700 Kaohsiung University Rd., Nanzih District,aohsiung 81148, Taiwan, ROC. Tel.: +886 7 591 7276; fax: +886 7 591 9277.

E-mail addresses: [email protected], [email protected] (W.-F. Ho).Peer review under responsibility of The Ceramic Society of Japan and the Korean

eramic Society.

ttp://dx.doi.org/10.1016/j.jascer.2015.12.002187-0764 © 2015 The Ceramic Society of Japan and the Korean Ceramic Society. Producti

icense (http://creativecommons.org/licenses/by-nc-nd/4.0/).

licenses/by-nc-nd/4.0/).

turn depend on the synthesis precursors and processing conditions[11].

To this end, a variety of synthesis techniques, including sol–gel[12], aqueous precipitation [13], hydrothermal technique [14–16],and solid-state reactions [17–19] have been reported for the syn-thesis of HA. Over the past years, biologically derived naturalmaterials, such as fish bone, bovine bone, corals, oyster shell, andeggshells, have been converted into useful biomaterials like HA.For instance, the study reported by Lemos et al. [20] demonstratesthe transformation of natural aragonite from cuttlefish bone intoHA via hydrothermal treatment at 200 ◦C. Ooi et al. [21] reportedthe synthesis of porous HA from bovine bone via heat treatmentin the temperature range of 400–1200 ◦C. Sivakumar et al. [22]have demonstrated the conversion of coral into monophasic HAvia a low temperature hydrothermal process. A recent study hasdemonstrated the synthesis of HA from oyster shell powders mixedwith calcium pyrophosphate (Ca2P2O7) or dicalcium phosphatedehydrate (CaHPO4·2H2O, DCPD) by ball milling and subsequentlyheat treatment [18]. More recently, Ho et al. [16] synthesizednanosized HA via hydrothermal treatment of eggshell and severalbiomolecules obtained from waste materials (pomelo, grape, andsweet potato peel extracts). These materials offer promising oppor-

tunities, given the fact that the raw materials are wastes. Besides,the use of ‘biological apatite’ containing some trace elements asbone substitutes, instead of ‘chemical apatite’, would be much ben-eficial for bone defect healing [23].

on and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND

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8 n Ceramic Societies 4 (2016) 85–90

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Fig. 1. XRD patterns of the powders produced by milling DCPD and eggshell powders

in milling time. The microstructure of the powders milled for 1 and5 h showed larger particles of irregular shape and non-uniform sizedistribution, which could be related to the unreacted precursors.

6 S.-C. Wu et al. / Journal of Asia

Eggshells generated after breaking eggs represent a significantaste because they become typically useless after the use of egg

ontents and its derivatives. Such eggshell wastes are commonlyisposed of in landfills without any pre-treatment. Such activi-ies are highly undesirable to the environment, especially fromhe viewpoint of the odor generated during the biodegradation.ggshells constitute 11% of the total weight of the egg and areainly composed of calcium carbonate (CaCO3) [24]. In the present

nvestigation, an attempt has been made to synthesize pure andone-like HA powder from eggshells. The eggshell powder, whichas used as the Ca source, was combined with DCPD powder. In the

ypical process, the powder mixture was ball-milled for 1, 5, and0 h, followed by heat treatment at 1000 ◦C for 1 h. Although therere several reports on the synthesis of HA from eggshells, most pro-esses are done by wet chemical methods. Clearly, this techniques lengthy, complicated and require pH adjustment and control. Tohe best of our knowledge, no study has reported the synthesis ofA powder using DCPD and eggshell powders via ball milling and

ubsequent heat treatment. This study shows a great potential forhe conversion of eggshell waste into highly valuable compoundssing simple yet effective processes.

. Materials and methods

In this study, DCPD (Yakuri Chemicals Co., Ltd., Japan) andggshell powders were used as starting materials for the syn-hesis of HA. Raw membrane-bound hen eggshell was collectedrom a breakfast shop in the university campus and immediatelytored in a refrigerator. The eggshells were pre-treated by strip-ing the membrane off the eggshell, followed by rinsing withater, drying, and then crushing and powdering using an agateortar. The eggshell powders thus obtained were sieved using a

25-mesh sieve. Subsequently, the hand-ground eggshell powdernd DCPD were homogeneously mixed with deionized water in

zirconia container. The ratio of eggshell powders to DCPD was:3 (mole ratio). The resulting mixture was wet-milled in a plan-tary ball-milling machine (QM-3SP4J, Nangjing, China) for 1, 5,r 10 h at a speed of 170 rpm in a zirconia bottle. After milling,he slurry was dried completely in a convection oven at 150 ◦Cor 24 h. Following that, the dried powder was heated to 1000 ◦Ct a rate of 10 ◦C/min, and subsequently held at that temperatureor 1 h.

The crystalline phases of the ball-milled powders before andfter heat treatment were analyzed by using powder X-ray diffrac-ion with Cu K� radiation (XRD; XRD-6000, Shimadzu, Japan).he phases were identified by comparing the experimental X-rayiffractograms with the standards compiled by the Joint Committeen Powder Diffraction Standards (JCPDS). The microstructure of theowders was observed using scanning electron microscope (SEM;

SM-6700F, JEOL, Japan) under secondary electron mode. Further-ore, the Fourier transform infrared (FTIR; Bio-Rad, FTS-40, USA)

pectra of the powdered samples were obtained in the wavenum-er range of 600–4000 cm−1. The composition of the synthesizedowders was analyzed by using inductively coupled plasma-atomicmission spectroscopy (ICP-AES; ICAP 9000, Jarrell-Ash Co., USA).

From the XRD data, the crystallinity (Xc) of the HA particles wasalculated according to the following equation [25]:

c = 1 − V112/300

I300

here I300 is the intensity of (300) diffraction peak and V112/300s the intensity of the hollow between (112) and (300) diffractioneaks of HA.

for various durations (a) 1, (b) 5, and (c) 10 h.

The average crystallite size of the synthesized HA particles wascalculated by using the Scherrer’s formula as follows [26]:

Xs = 0.9�

FWHM cos �

where Xs is the average crystallite size (nm); � is the wavelength ofthe X-ray used for the analysis (1.5406 A); FWHM is the full widthat half maximum for the diffraction peak under consideration (rad)and � (◦) is the Bragg’s angle. In this study, the (002) diffractionpeak was chosen for calculation of the crystallite size since it wasisolated and sharper than the other peaks.

3. Results and discussion

3.1. Characterization of ball-milled powders

Fig. 1 shows the XRD patterns of the DCPD and eggshell powdermixtures milled for 1, 5, and 10 h. As can be seen from the diffrac-tion pattern of the 1 h milled powder, the peaks mainly correspondto dicalcium phosphate anhydrous (DCPA) and CaCO3 with traceamounts of �-TCP and HA phase. The DCPD precursor completelydecomposed to DCPA because of the increased temperature andvigorous stirring during ball milling [27]. Upon milling for 5 h, theintensities of DCPA and CaCO3 decreased with the obvious appear-ance of diffraction peaks corresponding to �-TCP phase, togetherwith trace amounts of HA phase. With further increase in millingtime to 10 h, the diffraction peaks indicated the formation of HAphase with small amounts of DCPA and CaCO3 from precursors.When compared to 5 h milled sample, the XRD pattern of the 10 hmilled sample shows a decrease in �-TCP content, which indicatesthe gradual replacement of �-TCP by HA.

The microstructure of the DCPD and eggshell powders milledfor 1, 5, and 10 h showed remarkable differences in size and shape(Fig. 2). The reduction in the particle size was greater with increase

The submicron-sized particles observed in the 5 h milled samplecould possibly be related to the �-TCP and HA phase. However,with increase in milling time to 10 h, the microstructure revealedsmaller particles with relatively uniform size distribution.

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S.-C. Wu et al. / Journal of Asian Ceramic Societies 4 (2016) 85–90 87

Fig. 2. SEM images of the powders produced by milling DCPD mixed wi

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ig. 3. XRD patterns of the products synthesized by milling DCPD and eggshell pow-ers for various durations (a) 1, (b) 5, and (c) 10 h, followed by heat treatment at000 ◦C for 1 h.

.2. Characterization of the powders after heat treatment

Fig. 3 shows the X-ray diffraction patterns of the samples syn-hesized by milling DCPD and eggshell powders for 1, 5, and 10 hollowed by subsequent heat-treated at 1000 ◦C for 1 h. The XRDattern of the 1 h milled and sintered sample shown in Fig. 3a indi-ates the presence of unreacted Ca2P2O7 (pyrophosphate derivedrom DCPD after heating) and CaO (obtained from eggshell pow-ers after heating). Besides, this stage had large amounts of �-TCPhase. However, with further increase in milling time to 5 h andubsequent heat-treatment, there were no peaks correspondingo CaO or Ca2P2O7. This indicates that the unreacted precursoras been incorporated into the HA and �-TCP lattice through

igh temperature diffusion. Furthermore, the intensity ratio, �-CP(0210)/HA(211), also decreased with increase in ball-millingime, indicating that the HA phase is preferred over the �-TCPhase. With further increase in milling time to 10 h and subsequent

th eggshell powders for various durations (a) 1, (b) 5, and (c) 10 h.

heat treatment, the XRD peak corresponding to �-TCP phase dis-appeared completely and replaced by HA. The final products (10 hmilled and heat-treated sample) were composed entirely of HAcrystals. An earlier study [28] has indicated the formation of HApowder via a one-step ball milling process without any heat treat-ment. However, in the current study, it seems that the milling timeof 10 h is not sufficient to obtain a pure HA sample without sinter-ing the powder at elevated temperatures. It is evident from Fig. 3that the formation of HA phase has been initiated through sinter-ing the 1 h milled sample at 1000 ◦C for 1 h, while pure HA phase isformed only upon sintering the 10 h milled sample. The 1 and 5 hmilled samples had additional XRD peaks corresponding to �-TCPphase.

The final products composed entirely of HA crystals or biphasiccalcium phosphate (HA + �-TCP crystals) can easily be preparedby varying the ball milling time, as shown in Fig. 3b and c. Theformation of �-TCP phase under certain experimental conditionscan be considered beneficial, although preparation of phase-pureHA from eggshell is the original intention of the present work.Recently, biphasic calcium phosphate (HA/TCP) composite has beenstudied to overcome the low biodegradability of HA. Tetsuya et al.[29] demonstrated that HA ceramics containing TCP induced betterosseointegration than pure HA ceramics when implanted in sheepfemora. In addition, biphasic calcium phosphate ceramics havebeen evaluated in both osseous and non-osseous tissues [30,31].Results have shown that the biphasic ceramics are biologicallymore active than pure HA ceramics, and that the biological behaviorof the biphasic ceramics is superior in the formation of new bone.

Fig. 4 shows the SEM images of the products synthesized fromDCPD and eggshell powders milled for 1, 5, and 10 h and thenheat-treated at 1000 ◦C for 1 h. As is seen, the powders have spher-ical shaped particles of nano to submicron size range. The averageparticle size decreased from 2.21 to 1.25 �m with increase in ball-milling time. Upon subsequent heat treatment, the agglomerationof the particles became more prominent due to high temperature

sintering and high surface areas.

Table 1 summarizes the crystallinity and crystallite size of theas-prepared HA powders milled for 1, 5, and 10 h, followed by heat-treatment at 1000 ◦C for 1 h. The crystallinity of the synthesized HA

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88 S.-C. Wu et al. / Journal of Asian Ceramic Societies 4 (2016) 85–90

F ders for various durations (a) 1, (b) 5, and (c) 10 h, followed by heat treatment at 1000 ◦Cf

pwoshw1attiTi

tip1tOCat(b[iade

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ig. 4. SEM images of the products synthesized by milling DCPD and eggshell powor 1 h.

articles, as calculated from the XRD data, was found to increaseith ball-milling time, although it was slightly lower than that

f commercial HA. According to the XRD results (Fig. 3c), the as-ynthesized HA powder exhibited good crystallinity (96.4%) andigh phase-purity. The crystallite sizes of the HA particles, whichere obtained by milling DCPD and eggshell powders for 1, 5, and

0 h followed by heat-treatment at 1000 ◦C for 1 h, were 37.1, 40.8,nd 36.0 nm, respectively. According to previous reports, bone crys-als can be 30–50 nm in length, 15–30 nm in width, and 2–10 nm inhickness [32]. The nanostructure of bone-substituting materialss closely related to their good bioactivity and osteoconductivity.hat said, synthetic HA composed of nanosized crystals could offerncreased osteoblast functions [33].

Furthermore, FTIR analysis was performed to identify the func-ional groups present in the HA particles. This in turn providednformation about the constitution and phase composition of theroducts synthesized by milling DCPD and eggshell powders for0 h followed by heat treatment at 1000 ◦C for 1 h. The FTIR spec-ra of the samples shown in Fig. 5 had bands corresponding toH−, PO4

3− and H2O. Further analysis of the IR spectrum revealedO3

2− substitution, as identified by characteristic peaks of CO32−

t around 888 and 1521 cm−1, which are attributed to the vibra-ional modes of the carbonate ions substituted at the hydroxide ionA-type) [34]. Moreover, the carbonate ion band at 1465 cm−1 cane ascribed to B-type carbonate substitution on phosphate ion sites35]. In this experiment, the carbonate peaks observed for the spec-

mens closely matched those of A- and B-type carbonates. Indeed,nionic sites within the HA crystals are very susceptible to carbonioxide originated from the atmosphere, and hence the CO3

2− gen-rated from CO2 can substitute hydroxyls (A-type) or phosphates

able 1ffect of ball milling on the crystallinity, crystallite size, and particle size of as-prepared000 ◦C for 1 h.

Ball milling time (h) Crystallinity, Xc (%)

1 85.0

5 94.1

10 96.4

Fig. 5. FTIR spectrum of the HA prepared by milling DCPD and eggshell powders for10 h, followed by heat treatment at 1000 ◦C for 1 h.

(B-type) in the crystal lattice [36]. Accordingly, it can be concludedthat the prepared HA is chemically and structurally analogous toapatite of natural bone. The apatite in natural bone contains sig-nificant amounts of carbonate ions, ranging from about 4 to 6 wt%[32]. In recent years, several in vitro studies [37,38] have demon-strated the outstanding properties of carbonated HA for use as abioresorbable bone substitute, as compared to pure stoichiomet-

ric HA. Also, in vivo study indicated that the dissolution rate ofsubcutaneously implanted sintered carbonated HA ceramics wasintermediate between �-TCP and pure HA [39]. The carbonated HA

HA powders obtained by milling for 1, 5, or 10 h, followed by heat treatment at

Crystallite size, Xs (nm) Particle size (�m)

37.1 2.2140.8 1.9836.0 1.25

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S.-C. Wu et al. / Journal of Asia

as been found to be more effective with a shorter post-operativeehabilitation program [40]. Thus, among the variety of HA-basedioceramics, carbonated HA has attracted significant attention.iven this fact, the awareness of the contribution of carbonatedA to bone and dental enamel health has been greatly increasing

41].In the current study, pure HA is the main constituent of the prod-

ct obtained from DCPD and eggshell powders ball-milled for 10 hollowed by heat treatment at 1000 ◦C for 1 h (Fig. 3c). Elementalomposition of the synthesized HA, as analyzed by using ICP-AES,howed the presence of Ca (39.2 wt%), P (16.8 wt%), Mg (0.648 wt%),nd Sr (0.031 wt%), with an average Ca/P molar ratio of around 1.81.he higher Ca/P molar ratio observed in the present study, as com-ared to that of stoichiometric HA, can be attributed to the presencef carbonate ions substituting the phosphate. This type of HA is typ-cal of the mineral phase of apatite in natural bone [42], where thearbonates strongly contribute to the variation of Ca/P ratio. Othertudies have observed similar high value of Ca/P ratio in HA fromovine bones [43].

Eggshells are essentially composed of calcium carbonate (94%),alcium phosphate (1%), organic matter (4%), and magnesiumarbonate (1%) [24]. The natural biological origin of eggshells,ontaining several trace elements that remain in the crystallinetructure of as-prepared HA making its composition alike humanone, will benefit the overall physiological functioning after

mplantation [23]. Mg is known to be an important trace elementn bone and teeth. Moreover, Mg plays an important role in bone

etabolism and hence its depletion causes bone fragility and boneoss [44]. Consequently, the incorporation of Mg ions into the HAtructure is of great interest for the development of artificial bones.esides, in conventional biomimetic coatings on Ti metals, theresence of Mg greatly influences both the physical and biologi-al properties of the deposited coatings. Barrere et al. studied theole of Mg2+ ions in the deposition of apatite coating on Ti usingoncentrated SBF solution [45]. It appeared that the relatively highnterfacial concentration in Mg2+ favors heterogeneous nucleationf tiny Ca–P globules onto the Ti-6Al-4V substrate, thereby enhanc-ng physical adhesion at the early stages of the coating formation.n addition, in vitro results showed higher cell adhesion of Mg2+

patite coating than apatite coating, promoting proliferation andxpression of collagen type I with respect to bare Ti [46]. Similarly,r is reported to improve bone strength and provide benefits in thereatment of osteoporosis [47]. Sr-containing HA has also exhib-ted better mechanical properties than those of stoichiometric HA,nd has shown to enhance the proliferation and differentiation ofsteoblast cells in vitro [48]. Moreover, Xue et al. [49] demonstratedhat plasma-sprayed Sr-HA coatings on Ti-6Al-4V substrate exhib-ted good biocompatibility with human osteoblasts, favoring theurvival and proliferation of cells on the surface of the coatings.

. Conclusions

This work demonstrates the synthesis of HA from eggshellowders mixed with DCPD via ball milling and subsequent heatreatment. The following key results can be summarized based onhe results obtained in this study:

1) The XRD pattern of the 1 h milled sample indicated a smallamount of �-TCP and HA phases. With increase in milling timeup to 10 h, the �-TCP phase decreased, resulting in predomi-nantly HA phase with small amounts of DCPA and CaCO3 from

precursors.

2) The formation of HA phase can be initiated by sintering the1 h milled sample at 1000 ◦C for 1 h, while pure HA phase canbe obtained upon sintering the 10 h milled sample. Even after

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mic Societies 4 (2016) 85–90 89

sintering, the presence of �-TCP phase was observed in the XRDpatterns of the 1 and 5 h milled samples. No peaks correspond-ing to the starting materials were found in the powders milledfor 5 and 10 h followed by heat treatment at 1000 ◦C for 1 h.

(3) The final products composed entirely of pure HA crystals orbiphasic calcium phosphate (HA + �-TCP crystals) can easilybe prepared by using DCPD and eggshell powders after 5 and10 h of ball-milling, respectively, and subsequent sintering at1000 ◦C for 1 h.

(4) The FTIR analysis of 10 h milled and sintered sample showedbands corresponding to OH−, PO4

3− and H2O. The carbonatepeaks of the specimen closely matched those of A- and B-typecarbonates.

(5) Based on the XRD results, it can be concluded that the as-synthesized HA powder obtained by milling DCPD and eggshellpowders for 10 h followed by heat treatment at 1000 ◦C for 1 hexhibited good crystallinity (96.4%) and high phase-purity. Theelemental composition of the synthesized HA, as evaluated bythe ICP-AES method, showed the presence of Ca (39.2 wt%), P(16.8 wt%), Mg (0.648 wt%), and Sr (0.031 wt%).

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