Conversion of waste eggshells to mesoporous hydroxyapatite nanoparticles with high surface area Abdul-Rauf Ibrahim, Wenxin Wei, Deng Zhang, Hongtao Wang, Jun Li n Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols, Ethers and Esters, Xiamen University, Xiamen 361005, PR China article info Article history: Received 2 May 2013 Accepted 4 August 2013 Available online 13 August 2013 Keywords: Biomaterials Porous materials Hydroxyapatite Waste eggshells Surface area Nanoparticles abstract The objective of this work was to convert waste eggshells to mesoporous hydroxyapatite (HAp) with high surface area by using a simple and inexpensive protocol without complexes. The eggshells were initially dissolved in concentrated nitric acid under vigorous agitation to form Ca(NO 3 ) 2 solution, followed by the production of HAp nanoparticles through the addition of dilute phosphoric acid solution to the calcium solution at room temperature with a syringe pump. The HAp product possessed high surface area (212.4 m 2 /g), large pore size (16.8 nm) and small particle size ( o10 nm) as shown by BET and small angle XRD analyses. Moreover, only about 8% of the HAp phase was converted to whitlockite at high temperature (950 1C), indicating its stability. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Hydroxyapatite (HAp) has gained much popularity in recent years due to its potential applications in several fields such as in bioceramic industry, medical and pharmaceutical areas and in environmental pollution control [1–3]. For applications in these and other areas, control of several parameters, including particle size, surface area, pore size, purity and morphology, is important as they dictate the properties of the HAp products [4]. Consequently, many techniques aimed at producing HAp that meets these and other properties have been explored [5]. For medical, pharmaceu- tical and bioceramic applications, HAp synthesized from biological calcium sources is preferred [6]. Accordingly, HAp synthesized from waste eggshells has been highly studied [7–10]. However, issues such as impurity, low surface area, poor thermal stability and larger particle size of the synthesized products have made the use of waste eggshells to generate HAp intended for the above fields almost unattractive [11]. To meet these quality issues, surfactants and structure directing agents, templates and in some cases complex procedures have generally been used [12,13], necessitating several purification steps. Calcium species for the syntheses of HAp from waste eggshells are obtained via several routes: dissolving the eggshells in concentrated acid [7], forming calcium complex [9] and converting the eggshells to CaO at high temperatures [8,10]. However, not much has been done towards improving the surface area or pore size of such biogenic HAp, although HAp powder with higher surface area and smaller particle sizes are reported to possess enhanced sinterability kinetics leading to enhanced properties [14] for diverse applications. In this letter, we employed a simple procedure to synthesize mesoporous HAp nanoparticles directly from waste eggshells at room temperature. The objective was to convert eggshells into HAp with higher surface area ( 4200 m 2 /g) and smaller particle size ( o10 nm). 2. Experimental Waste eggshells were collected from the Furong canteen in Xiamen University, China. Phosphoric acid (purity Z85%), nitric acid (68%) and sodium hydroxide (purity Z96%) were purchased from Sinopharm Reagent Co. Ltd, China. The eggshells were washed with detergent, then hexane and then distilled water, and dried at 80 1C overnight. The synthesis of HAp was achieved by dissolving the eggshells initially in concentrated nitric acid under vigorous agitation (800–1000 rpm) to control froth formation, at ambient condition through the reaction CaCO 3 þ 2HNO 3 -CaðNO 3 Þ 2 þ CO 2 þ H 2 O ð1Þ followed by the reaction (for 15 min) of 50 ml of 0.1 M of this calcium solution with 50 ml of 0.06 M of dilute phosphoric acid solution at an additional rate of 200 ml/h using a syringe pump (TCI-IV, China) while stirring (500 rpm) at ambient conditions. Note that the reaction was performed after pH of the Ca(NO 3 ) 2 solution was controlled to 10.4 by using NaOH solution according to 5CaðNO 3 Þ 2 þ 3H 3 PO 4 þ H 2 O-Ca 5 ðPO 4 Þ 3 OH↓ þ 10HNO 3 ð2Þ The highlight of the procedure is the use of a syringe pump to control the addition of phosphoric acid, which facilitated effective Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$- see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.08.014 n Corresponding author. Tel./fax: þ86 592 2183055. E-mail address: junnyxm@xmu.edu.cn (J. Li). Materials Letters 110 (2013) 195–197