Conversion of sub-µm calcium carbonate (calcite) particles to hollow hydroxyapatite agglomerates in K2HPO4 solutions

Sun Yanyan,Satoshi Hayakawa,Akiyoshi Osaka,Li Wuhui,Wang Yaming,Wang Guangxin

doi:10.1515/ntrev-2020-0070

Sun Yanyan, Satoshi Hayakawa + Show 4 more

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https://doi.org/10.1515/ntrev-2020-0070

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Abstract

Abstract Sub-µm CaCO3 (calcite; CC) particles were converted to calcium monohydrogenphosphate dihydrate (DCPD) and hydroxyapatite (HAp) via soaking treatments in K2HPO4 solutions with varied pH (3–12) and concentrations (0.1–1.5 M) at 37°C for up to 10 days. DCPD was derived from the solutions with pH ≤ 6; while hollow HAp was yielded when pH ≥ 7 in assemblies of petal-like crystallites. Results of magic angle spinning (MAS) and cross-polarization magic angle spinning (CP-MAS) NMR studies have shown that the HAp lattice has only PO4 2− but no HPO4 2− at B (phosphate) sites. Trace amounts of CO3 2− have occupied both A (OH) and B (PO4) sites, and H2O is adsorbed on surface crystallites. The primary crystallite size of HAp derived from Scherrer equation increases quickly in a 12 h period and becomes gradually stable afterward. Samples of particles soaked within 3 h in a temperature range of 20–80°C were analyzed by X-ray diffraction. It is shown that the rate constant of 1 M solution is about an order of magnitude greater than that of 0.1 M solution and the apparent activation energy is 33 kJ/mol. In this work, the conversion of CC to HAp can be quantitatively controlled to solve the problem of slow degradation of HAp.

Highlights

Calcium carbonate (CC) is considered to be a potential bone repair material, because it can be converted to hydroxyapatite (HAp) when soaked in phosphate solutions [1,2,3,4,5,6]
According to ICDD 09-0432 and 09-0077, a set of sharp diffractions at 11.7°, 21.0°, 23.5°, and 29.35° and all other smaller peaks were assigned to DCPD for samples soaked in solutions of pH ≤ 6, and all diffractions in the top three profiles were assigned to HAp
A reaction model is proposed as presented in Figure 10, with following assumptions: (a) Spherical CC particles in contact with K2HPO4 solution, as shown in Figure 10(a). (b) Active site density remains constant on the reaction front. (c) Component species can freely migrate to the reaction front

Summary

Introduction

Calcium carbonate (CC) is considered to be a potential bone repair material, because it can be converted to hydroxyapatite (HAp) when soaked in phosphate solutions [1,2,3,4,5,6]. Ruffini et al [2] applied hydrothermal procedure to convert wood-derived CC (calcite) to HAp. Minh et al [3] soaked CC particles in H3PO4 solutions of varied concentrations at 80°C and found that the conversion reaction was slow and incomplete even after 168 h. Portela da Silva et al [6] derived HAp from stoichiometric mixtures of CC particles and NH4H2PO4 solution of pH > 7 at 25°C. They found the precipitation of calcium monohydrogenphosphate dihydrate (CaHPO4·2H2O; DCPD) in the course of the reaction, despite that Pourbaix [7] suggesting that HAp was most stable among several Ca-P phases in the range 6 < pH < 13. Sethmann et al [9] prepared macroporous apatite

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