Novel Sustainable Route for Synthesis of Hydroxyapatite

Research Article pubs.acs.org/journal/ascecg

Novel Sustainable Route for Synthesis of Hydroxyapatite Biomaterial from Biowastes Karly Ronan and M. Bobby Kannan* Biomaterials and Engineering Materials (BEM) Lab, College of Science and Engineering, James Cook University, 1 James Cook Drive, Townsville, Queensland 4811, Australia ABSTRACT: In this study, we demonstrated that hydroxyapatite (HAp) biomaterial can be synthesized entirely from biowastes, i.e., eggshells and urine. Eggshells were cleaned, crushed, and calcined at 900 °C and then the powder was dissolved in water to form an aqueous calcium hydroxide solution. The aqueous solution was mixed with synthetic urine (SU) in stoichiometric amounts corresponding to HAp (Ca/P ratio ∼1.67). Calcium phosphate (CaP) was potentiostatically synthesized on magnesium or stainless steel electrode at cathodic potentials (−2 to −4 V). CaP particles formed on the metal surface at −2 V. The growth of the particles increased as the potential was increased from −2 to −3 V. However, at −4 V, the particles formed on the metal surface decreased as a result of excessive hydrogen evolution on the metal surface disrupting particle adhesion. An increase in the electrolyte concentration by 3-fold enhanced the particle growth, but further increase in the concentration by 5-fold did not show any additional improvement. X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR) analysis confirmed that the powder formed on the metal sample was an amorphous CaP. Alkaline treatment at 80 °C for 2 h converted the amorphous CaP into crystalline HAp. KEYWORDS: Bioceramic, Eggshell, Urine, Bioimplant, Electrosynthesis

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due to their versatility.2,5,6,8,11 This method is relatively simple and cheap and also can be undertaken at room temperatures. Generally, for CaP synthesis, chemicals such as calcium nitrate (Ca(NO3)2) and ammonium dihydrogen phosphate (NH4H2PO4) are used as calcium and phosphate precursors, respectively.14−18 Eggshell waste is a potential source of calcium for producing calcium-based biomaterials. Eggshell consists of almost entirely (∼94%) calcium carbonate (CaCO3).19 It was estimated that over 70 million tons of eggs were produced globally in 2014; since eggshell waste makes up approximately 11% of the total egg’s weight, this equates to approximately 7.7 million tons of eggshell waste generated in 2014.20,21 Converting the eggshell waste to CaP/HAp will not only enormously reduce biowaste accumulation, but produce useful products for medical applications. In fact, some research work has been done on eggshellbased HAp synthesis using various techniques. Sasikumar et al. showed that nanocrystalline HAp powder can be synthesized using a low temperature technique.22 A microwave irradiation technique was used by Krishna et al. for HAp synthesis and reported superior hardness and density as compared to commercially available HAp.23 Gergely et al. reported nanosized homogeneous HAp synthesis using a mechanochemical activation technique.24 Recently, a flowerlike nanostructured

INTRODUCTION Over the years, the world has become increasingly focused on waste management and reduction. Transforming wastes into value-added products will tremendously enhance sustainable economic development, as well as pave the way for a more effective waste management. Currently, there is a great demand for biomaterials, e.g., orthopedic and dental implants, and drug delivery systems for musculoskeletal diseases and bone tissue regeneration. The market for such products is likely to grow steadily due to the growing and aging population.1 Hence, synthesis of biomaterials from natural wastes presents an enormous opportunity for addressing the future medical care demands in a sustainable manner. Hydroxyapatite (HAp) is a calcium phosphate-based biomaterial which is commonly used in medical applications.1−3 The ceramic biomaterial is naturally found in hard tissues and its synthesis, in powder form or as coatings on metallic materials, has been investigated extensively using various techniques2,4−6 Generally, calcium phosphate (CaP) precipitates in aqueous solutions containing calcium and phosphate ions in sufficient concentrations. The precipitation process can be accelerated by altering the pH, temperature or solution composition.7,8 The conventional methods of CaP synthesis include wet chemical methods, hydrothermal systems, RF magnetron sputtering, solid state reactions, and electrochemical techniques.2,9−13 However, electrochemical techniques involving electrodes, an electrolyte, and applied potentials are ideal for producing CaP coatings on metallic biomedical implants © 2017 American Chemical Society

Received: October 18, 2016 Revised: December 20, 2016 Published: January 20, 2017 2237

DOI: 10.1021/acssuschemeng.6b02515 ACS Sustainable Chem. Eng. 2017, 5, 2237−2245

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Sustainable Route for Synthesizing HAp

Table 1. Synthetic Urine Composition32

HAp was produced by Kumar et al. using a microwave conversion process.25 Although researchers have used eggshell waste as a calcium precursor for HAp synthesis, common phosphatebased chemical solutions have been used as a phosphate precursor, e.g., diammonium phosphate ((NH4)2HPO4) and orthophosphoric acid (H3PO4).22,23,26,27 Utilization of a phosphate-containing waste as a phosphate precursor would make the HAp synthesis process even more sustainable. Urine is a potential source of phosphate for HAp synthesis since it contains phosphate compounds. In fact, in recent years there has been growing interest in recycling nutrients recovered from urine. A significant amount of work has been done on struvite (MgNH4PO4·6H2O) synthesis from urine for fertilizer applications.28−31 In this study, an attempt for the first time was made to synthesize HAp biomaterial entirely from biowastes. An electrochemical route was used to synthesize HAp from eggshell waste and synthetic urine. Characterization techniques were used to identify the chemical nature and morphology of the synthesized powder.

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compound and chemical formula

g/L

urea (CH4N2O) creatinine (C4H7N3O) trisodium citrate dihydrate (Na3C6H5O7.2H2O) sodium chloride (NaCl) potassium chloride (KCl) ammonium chloride (NH4Cl) calcium chloride dihydrate (CaCl2·2H2O) magnesium sulfate (MgSO4·7H2O) sodium bicarbonate (NaHCO3) sodium oxalate (Na2C2O4) sodium sulfate (Na2SO4) sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) disodium hydrogen phosphate (Na2HPO4)

12.012 0.452 1.470 3.155 2.236 0.802 0.441 0.492 0.168 0.0134 1.278 0.561 0.056

EXPERIMENTAL SECTION

Materials. Eggshell waste and synthetic urine (SU) were used as precursors of calcium and phosphate, respectively, for HAp synthesis. The novel sustainable route developed to produce crystalline HAp from eggshell and SU is illustrated in Scheme 1. Eggshell powDer. Eggshells were first scrubbed and cleaned with distilled water to remove the inner membranes, then washed with methanol and dried in an oven (Model: LABEC, ODWF14) for 24 h at 100 °C to eliminate the odor and any volatile contaminants. The dried eggshells were then ground using a mortar and pestle to produce a fine powder. Particle size analysis of the fine powder was performed using a particle size analyzer (Model: Malvern Mastersizer 3000 with Hydro EV). Calcium Precursor. To eliminate any organic phases and pathogens, the crushed eggshell powder was heat treated in a furnace (Model: Carbolite Furnaces, ELF 10/14) at 450 °C (with a heating rate of 5 °C/min) for 2 h in a furnace. The furnace temperature was increased to 900 °C (with a heating rate of 1 °C/min) and held for 3 h

Figure 1. Cumulative particle size distribution curve of crushed eggshell powder. to obtain calcium oxide (CaO); this process is commonly known as calcination. The fine powder, before and after calcination, was analyzed using 2238


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Phosphate Precursor. The chemical composition of the SU used in this study is listed in Table 1.32 Three different concentrations of SU solutions were used in this study, i.e., normal, three-times, and five-times concentrated. Electrosynthesis of HAp. For the synthesis of HAp, a standard electrochemical cell was used, as shown in Scheme 1, consisting of a three-electrode system, i.e., pure magnesium or 316 stainless steel alloy as a working electrode, Ag/AgCl as a reference electrode and a platinum mesh as a counter electrode. A potentiostatic technique was used to synthesize HAp using a potentiostat (Model: VersaSTAT3, Princeton Applied Research, Oak Ridge, USA). The applied potentials ranged from −2 to −4 V for a duration of 1 h. The electrolyte was prepared by mixing the calcium precursor and the phosphate precursor solutions in stoichiometric amounts such that the Ca/P ratio was approximately equal to 1.67, which is characteristic of HAp. The resulting solutions had different concentrations, i.e., normal, three-times, and five-times; which are hereafter referred to as E1x, E3x, and E5x, respectively. The pH of the electrolyte was adjusted to 5 with 1 M hydrochloric acid (HCl). The conductivity of the electrolyte was measured using a conductivity meter (Model: Thermo Scientific Orion 5-Star Benchtop). The powder product produced using the electrochemical method was treated in 1 M sodium hydroxide (NaOH) at 80 °C for 2 h to produce crystalline HAp. Product Characterization. The morphology of the powder product was analyzed using scanning electron microscope (SEM) (Model: Jeol, JSM 5410). Fourier transform infrared spectroscopy (FTIR) (Model: Nicolet 6700 FT-IR), energy dispersive spectroscopy (EDS) analysis (Model: Jeol, JXA-8200 Superprobe) and XRD were used to analyze the chemical nature of the powder product.

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RESULTS AND DISCUSSION Eggshell XRD Analysis. The particle size distribution of the crushed eggshell powder is shown in Figure 1. The average particle size (D50) was found to be ∼8.5 μm. Figure 2a and b shows the XRD spectra of the crushed eggshell powder and calcined powder, respectively. The spectra of the crushed eggshell reveals that the major component is calcite, a stable polymorphous calcium carbonate (CaCO3).33,34 After calcination at 900 °C, the calcite transformed into calcium oxide (CaO) as shown in Figure 2b.33,35 Magnesium Electrode: Electrolyte E1x. Figure 3 shows the current density versus time curves of magnesium at various applied cathodic potentials in E1x electrolyte. At −2 V potential,

Figure 2. XRD spectra of (a) crushed eggshell powder and (b) calcined eggshell powder. X-ray diffraction technique (XRD) (Model: Bruker, D2 Phaser, second Gen). The CaO powder was dissolved in distilled water to prepare a saturated aqueous solution of calcium hydroxide (Ca(OH)2. The chemical conversion reactions from CaCO3 to Ca(OH)2 are shown below: calcination

CaCO3(s) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ CaO(s) + CO2(g) 900 ° C

CaO(s) + H 2O(l) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ca(OH)(aq) room temperature

(1) (2)

Figure 3. Current density versus time curves obtained at different applied cathodic potentials in E1x electrolyte (cathode material: magnesium metal). 2239


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was not uniform; clustering of particles was observed. At low current density, it can be assumed that the pH near the metal surface was not uniformly favorable for the precipitation to occur. However, the particles were prominent and uniformly adhered to the magnesium sample held at −3 V. Interestingly, at −4 V application, the particles formation was not as high as compared to that at −3 V. The electrochemical reactions for CaP formation is given in the below, eqs 3−6:12,36

the current density decreased with time and reached a relatively stable value (∼0.28 mA/cm2) after 1400 s. The decrease in the current density with the increase in time is expected since the formation of CaP, a nonconducting ceramic compound, would increase the cell resistance and consequently decrease the current density. Increasing the cathodic potential to −3 V increased the relatively stable current density to 2.6 mA/cm2. However, it took a longer time (∼2300 s) to achieve a relatively stable current density. Further increase in the cathodic potential to −4 V showed a serratedlike current density curve which was almost five times higher than that observed at −3 V. The macrographs of the magnesium samples after CaP electrosynthesis are shown in Figure 4. At −2 V, the particle formation

H 2PO4 − → HPO4 + H+

(3)

2H 2O + 2e− → H 2 + 2OH−

(4)

OH− + H 2PO4 − ⇄ H 2O + HPO4 2 −

(5)

Ca 2 + + HPO4 2 − + 2H 2O ⇄ CaHPO4 ·2H 2O

(6)

As can be noted in the above, eqs 3−6, the hydrogen evolution reaction (eq 4) is an important part of CaP formation. Evolution of hydrogen gas increases the pH of the electrolyte near the electrode surface, and as a result, nucleation and growth of CaP occur. As the applied cathodic voltage is increased toward the negative direction, the hydrogen evolution (cathodic reaction) also increases. In fact, visual observations of the magnesium electrode during the electrosynthesis process revealed increase in hydrogen bubble formation with the increase in the cathodic potential. Although the hydrogen evolution reaction is critical for CaP formation, a high number of hydrogen bubbles causes particles to fall from the magnesium sample, which induces a loss of particles to the solution. The growth and detachment of hydrogen bubbles from the magnesium metal surface affect the adhesion of the particles due to nonuniform formation of particles on the metal surface. This phenomenon is also demonstrated by the serrated curve observed in the current density plot for −4 V (Figure 3), i.e., the fluctuating current density caused by rapid particles forming and detaching from the metal surface. Figure 5a−d shows the SEM images of the particles formed on the magnesium sample at −3 V and the EDS analysis of the

Figure 4. Macrographs of the bare magnesium metal and the particles formed on magnesium in E1x electrolyte at various applied cathodic potentials.

Figure 5. (a−c) SEM micrographs of the particles formed on magnesium metal at −3 V in the E1x electrolyte. (d) EDS spectra of the particles. 2240


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coordinated, highly hydrated clusters.38 According to the reaction schemes shown in eqs 3−6 and the product being both CaP-based and hydrated, it can be suggested that the particles are dicalcium phosphate dihydrate (DCPD). Interestingly, the Ca/P ratio of the particles obtained from EDS analysis was found to be 1.425, which suggests that the particles are rich in calcium. The higher Ca/P ratio may be due to the incorporation of CO32− group in the CaP structure. Magnesium Electrode: Electrolytes E3x and E5x. In order to enhance the formation of CaP particles on the metal sample, the electrolyte concentration was increased. Concentrations of three and five times the SU were used and mixed with Ca(OH)2 solution to the same stoichiometric ratio previously used (Ca/P ≈ 1.67). The current density versus time curves in different electrolyte concentrations are shown in Figure 7.

particles. It can be noted that the magnesium sample was completely covered by the particles. Two types of particle morphologies were observed, i.e., a dominant cloudlike morphology (Figure 5b) and flakelike particles (Figure 5c). EDS analysis (Figure 5d) indicates that the particles are rich in calcium and phosphorus with a small amount of magnesium. FTIR analysis suggests that the particles are a CaP-based compound (Figure 6). The presence of PO43− is evident from the band observed at ∼1000 cm−1.37 The sharp band at ∼1600 cm−1 and the broad band at ∼3000−3500 cm−1 both indicate the presence of absorbed H2O. Additionally, the presence of CO32− group, at bands ∼1400 and ∼800 cm−1, suggests that some CO32− ions may have substituted PO43− ions in the CaP structure.37 It has been reported that rapid formation of CaP can cause the ions to coalesce into irregularly

Figure 6. FTIR spectra of the particles formed on magnesium metal at −3 V in E1x electrolyte.

Figure 7. Current density versus time curves at −3 V in different concentrations of the electrolyte (cathode material: magnesium). 2241


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ACS Sustainable Chemistry & Engineering In the E3x electrolyte, the current densities are almost double that in the E1x electrolyte. However, further increase in the

Figure 11. Macrographs of stainless steel and the particles formed on the alloy at −3 V in different concentrations of the electrolyte. Figure 8. Macrographs of the bare magnesium metal and the particles formed on magnesium at −3 V in different concentrations of the electrolyte.

electrolyte concentration, i.e., E5x, did not show any significant increase in the current density. Figure 8 compares the bare metal and particles formed on the magnesium sample in different electrolyte concentrations at −3 V. The concentrated electrolytes, E3x and E5x, yielded more product as compared to E1x electrolyte. However, a similar degree of particle formation was observed between E3x and E5x electrolytes. The measured current density can be related to the hydrogen evolution reaction since it is the predominant cathodic reaction i.e., high current density means high hydrogen evolution. Recently, we demonstrated that the hydrogen evolution reaction can be controlled by reducing the conductivity of the electrolyte.12 Addition of an organic solvent, ethanol, reduced the conductivity of the electrolyte and hence showed lower current density. It was found that very low current density will lead to less product formation, whereas very high current

Figure 9. Conductivity data of E1x, E3x, and E5x electrolytes at room temperature.

Figure 10. Current density versus times curves at −3 V in different concentrations of the electrolyte (cathode material: 316 stainless steel). 2242


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E3x and E5x electrolytes being more than double that observed in the E1x electrolyte. The macrographs of the bare metal and the samples after electrosynthesis of CaP are shown in Figure 11, which suggests that the formation of particles have increased in E3x and E5x electrolytes as compared to that in E1x electrolyte. Interestingly, the current densities observed for the stainless steel were consistently higher than those observed for magnesium. This can be attributed to the higher overvoltage applied for stainless steel as compared to magnesium. The open circuit potential of stainless steel was nobler than magnesium, as shown in Figure 12, i.e., stainless steel = −250 mV and magnesium = −1760 mV. It was noted that the yield of the CaP particles on stainless steel was significantly lower than that on magnesium. This could be attributed to the higher hydrogen evolution reaction on stainless steel which affected the particle adhesion. The SEM images and EDS spectra of the particles formed on the magnesium sample at −3 V in E3x electrolyte are shown in Figure 13a−d. In comparison to the particles formed in E1x

density will detach the product from the electrode due to high hydrogen bubble evolution. To examine if a similar phenomenon was responsible for the difference in the current density observed between the three electrolytes (E1x, E3x, and E5x) in this study, the conductivities of the electrolytes were measured and the results are shown in Figure 9. The conductivity increased by more than two order of magnitudes from increasing the electrolyte concentration by 3-fold. However, the difference in conductivity between E3x and E5x electrolytes was not significant. This demonstrates that the conductivity of the electrolyte did play a critical role in the formation of the particles. Stainless Steel Electrode. To understand if a similar concentration effect applies for a different metal, a stainless steel alloy (316 grade) was used as a cathode material in place of magnesium. The current density versus time curves of the stainless steel in different electrolyte concentrations are shown in Figure 10. As noted in magnesium, a similar shift in the current density was observed for the stainless steel, with the current density in

Figure 12. Open circuit potential versus time curves of magnesium and stainless steel in the E1x electrolyte.

Figure 13. (a−c) SEM micrographs of the particles formed on magnesium metal at −3 V in E3x electrolyte. (d) EDS spectra of the particles. 2243



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CONCLUSIONS The study suggests that crystalline HAp biomaterial can be synthesized entirely from biowastes using an electrochemical method. Calcined eggshell powder and synthetic urine (SU) in an aqueous solution produced an amorphous calcium phosphate (CaP) on an electrode (magnesium or stainless steel) held at cathodic potentials (−2 to −4 V). Electrosynthesis using a concentrated electrolyte at −3 V produced a greater yield of CaP powder. Alkaline treatment at 80 °C for 2 h converted the amorphous CaP to crystalline HAp.

electrolyte, in E3x electrolyte the particles were more densely formed (Figures 13a and b). A closer look at the particles revealed that they are nanosized (Figure 13c). The EDS spectra (Figure 13d) suggest that the particles are rich in calcium and phosphorus (Ca/P ratio 1.459) with a small amount of magnesium, which was similar to that observed in the E1x electrolyte. Figure 14a shows the XRD spectra of the powder particles formed on the magnesium sample at −3 V in E3x electrolyte.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +61-7-47815080. Fax: +61-7-47816788. E-mail: bobby. [email protected]. ORCID

M. Bobby Kannan: 0000-0003-0480-8587 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the staff at JCU’s Advanced Analytical Centre (AAC), Dr. Rosalie Hockings and Ms. Saira Mumtaz, for their assistance in materials characterization.

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ABBREVIATIONS CaP, calcium phosphate EDS, energy dispersive spectroscopy FTIR, Fourier transform infrared spectroscopy HAp, hydroxyapatite SEM, scanning electron microscopy SU, synthetic urine XRD, X-ray diffraction REFERENCES

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Figure 14. XRD of (a) particles formed on magnesium metal at −3 V in E3x electrolyte and (b) particles after alkaline treatment.

Interestingly, no high intensity distinct peaks were observed, which indicated that the deposited CaP is amorphous in nature. It has been reported in the literature that rapid mixing of calcium and phosphate ions in aqueous solutions typically forms amorphous CaP.38 Studies have reported that the “halo” region is associated with the broadening of apatitic diffraction peaks.39−42 In order to convert the amorphous CaP to a crystalline HAp, an alkaline treatment was performed. The XRD spectra of the alkaline-treated powder is shown in Figure 14b, which confirms the formation of crystalline HAp. In this study, we have successfully demonstrated for the first time that HAp can be synthesized exclusively from biowastes. A simple electrochemical method was employed which could facilitate a large-scale and sustainable production of HAp for biomedical applications. Converting biowastes into value-added product such as HAp will have a prodigious and sustainable economic development and also lead to a more effective waste management. 2244


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