Facile synthesis of B-type carbonated nanoapatite with tailored microstructure
Graphical abstract
Controlled synthesis of carbonated apatite at moderate temperatures using nanolime and sodiumhexametaphosphate as starting reagent.
Introduction
Hydroxylapatite is a pure end-member of the apatite group, having the composition Ca5(PO4)3OH, with two formula units in the crystallographic unit cell [1]. Atomic substitution in the cation sites as well as the anion sites of the apatite structure is readily obtained as long as geometrical restrictions and charge neutrality is preserved. Substitution by ions with different charge is accompanied by vacancies or coupled ion substitutions. For example, apatites in bone and teeth host CO32− [2]. The carbonate ions can substitute either OH− in the channel site and PO43− in the tetrahedral site, leading to the so-called “A-type” and “B-type” substitution, respectively [1]. The deficit in negative charge accompanied by incorporation of the carbonate ion in place of PO43− in B-type substituted apatites can be compensated by loss of positive charge by removal of Ca2+ from the lattice or exchange of Ca2+ for monovalent atoms such as Na+. Consequently, the Ca/P ratio of the structure becomes higher than 1.67 (stoichiometric hydroxylapatite).
Not surprisingly, the scientific interest of apatitic phosphates is greatest in the field of biomedicine [2]. In particular, synthetic nanosized apatites have been in focus due to similarities with the mineralogical counterpart found in hard tissues of mammals [3]. Apatites are also promising candidates for purification of waters contaminated with heavy metals [4], fluoride [5] and organic pollutants [6] as well as treatment of contaminated soils [7].
Many different routes for the synthesis of apatite have been developed over the years and have recently been reviewed by Sadat-Shojai et al. [3]. So-called wet methods are most promising for precise control of the size and morphology of nano-crystals although accompanied by draw-backs such as simultaneous precipitation of other CaP phases, low crystallinity and undesired incorporation of ions from solution [3]. The most popular synthesis route is simple chemical precipitation which generally involves slow addition of one reagent solution to another followed by aging [3]. Important parameters governing the product outcome is pH and temperature during mixing, the type of reagents and their concentrations as well as aging time and temperature during aging [3]. The possibility to tailor the product quality in terms of e.g. morphology and particle-size distribution by changing the synthesis conditions is accompanied by the need of precise process control which complicates industrial scale-up [3].
Many synthesis routes adopting alternative and more economically attractive reagents compared to commercial chemicals have been proposed. Some of these are phosphate minerals [8], nacreous materials [9], eggshell waste [10] and gypsum waste [11]. Nanolime obtained by calcination and consequent slaking of natural limestone followed by long-term storage under excess of water [12] would certainly fall under this category but has to the authors knowledge not yet been explored.
Determining the yield in solution-aided synthesis of nano-sized particles is not straight-forward. A common approach is to separate and purify the solid product from the mother liquor by centrifugation or vacuum filtering. The yield is then referred to the yield of the final purified product [13]. A more useful definition of yield, especially when the process efficiency is in focus, is the percentage yield obtained by dividing the obtained product with the theoretically possible amount. Even in this case, quantitative phase analyses (QPA) of the purified product is usually performed and the absolute weight of the product obtained from the synthesis is calculated based on the absolute weight of the precipitate determined gravimetrically [14]. Not questioning the accuracy of this approach, the purification step and a correct determination of the precipitate weight is both tedious and requires care of the operator. An alternative approach to determine the process yield is QPA of the solid obtained following removal of the solvent by for example freeze-drying. In this case, the powder may contain, in addition to the desired reaction product, soluble salts and other by-products. However, this type of analysis is rather complex using the classical X-ray diffractometry approach including measured integrated intensities and calibration curves e.g. [15] as many phases may be present in the mixture, including a large amount of amorphous material. In addition, the small crystallite size of the product as well as structure defects give rise to broad diffraction peaks that further complicates a correct determination of peak intensity by profile fitting without structural constraints. Recently, Reid and Hendry proposed to use X-ray Powder Diffraction (XRPD) data and the Rietveld method to determine the crystalline phase compositions of multiphase calcium phosphate mixtures [16]. However, this approach does not allow the quantification of amorphous calcium phosphates that are readily present in wet syntheses [17]. A promising approach to overcome these difficulties is full quantitative phase analysis (FQPA) using the Rietveld method and data collected from a sample mixed with a known quantity of a suitable standard [18], [19]. Although this technique has been used to determine the crystallinity of synthetic apatite [20], the method can be extended to complex multi-phase mixtures of low crystallinity. Once setting up a control file by careful calibration, Rietveld analyses are rather straightforward and time-efficient. Another important advantage using this method is that accurate unit cell parameters of the constituting phases are obtained as the sample displacement/zero shift in conventional Bragg–Brentano set-up is calibrated against the standard with certified unit cell parameters. This information is then useful to gain information regarding ion substitutions e.g. [21]. Regarding calcium phosphates, the Rietveld method is mostly applied, often in combination with spectroscopic techniques, for studies of the crystallographic structure and chemical composition [21], [22], [23], [24], [25].
In this work, the use of naturally derived nanolime and a phosphate-based Ca2+ chelating agent as calcium and phosphate source, respectively, in wet chemical synthesis of apatite was explored. Process parameters known to affect product quality were varied in a controlled manner using Design of Experiment (DOE) [26]. The Rietveld method was used as main characterization technique to evaluate the efficiency and product quality in terms of yield, chemical composition, crystallite size and structural defects. The proposed synthesis route will be shown to have high potential for sustainable industrial scale-up owing to process stability and simplicity.
Section snippets
Reagents and synthesis procedure
The apatite samples were prepared by a wet synthesis approach using lime putty as well as sodium hexametaphosphate (Carlo Erba) and double-distilled water as reagents. An analytical balance was used for all weight determinations. The lime putty is a commercial building material for repair of historical buildings, aged for ca 3 years to increase the quality in terms of practical performance, meaning a reduction in particle size and an increased specific surface area [12]. Table 1 displays the
Results and discussion
The apatite synthesis approach adopted in this work was very simple from a practical point of view, including rapid mixing of lime putty (concentrated suspension of portlandite) and a neutral sodium hexametaphophate solution followed by aging.
Although portlandite (Ca(OH)2) is frequently used as calcium source, the general procedure includes a neutralization reaction with phosphoric acid, requiring precise monitoring of solution pH to avoid the formation of calcium phosphates other than
Conclusion
In this work, nano-sized B-type carbonated apatite was successfully synthesized in batch mode using naturally derived nanolime and sodium hexametaphosphate as starting reagents. The proposed synthesis route may offers sustainable mass-production of nano-sized apatite with tailored microstructure due to its simplicity and process stability. In fact, synthesis is executed at moderate temperatures (<60 °C) for a short time (24 h) in static mode. In addition, the pH of the system is self-regulating.
Acknowledgments
P. Miselli and M. Cannio are kindly acknowledged for help in the laboratory. The authors also thank M. Zapparoli for performing the TEM analyses. A. Rattazzi and C. Polidoro, La Banca della Calce srl, Bologna, Italy, are acknowledged for providing aged lime putty used as a reagent in this work. E. Leo is kindly acknowledged for sample lyophilization.
References (63)
- et al.
Mater. Sci. Eng., C
(2005) Acta Biomater.
(2010)- et al.
Acta Biomater.
(2013) - et al.
Inorg. Chim. Acta
(2012) - et al.
J. Fluorine Chem.
(2009) - et al.
Chemosphere
(2013) - et al.
J. Eur. Ceram. Soc.
(2006) - et al.
Acta Biomater.
(2010) - et al.
J. Solid State Chem.
(2001) - et al.
Biomaterials
(2000)