Mineralogical-geochemical study of the anionic competition effect on the octacalcium phosphate reaction into fluorapatite

Abstract

The unstable compound octacalcium phosphate (OCP) is one of the crystalline precursors of the apatite mineral series composed by hydroxyapatite, fluorapatite and chlorapatite. The feature of OCP to react into apatite, depending on the media conditions, has been mainly exploited for biomedical applications as bone and tooth substitute material. Recently, some important applications of OCP have been documented: e.g. as electrode material for supercapacitors and as fluoride remover reagent for environmental purposes. With the aim of deepening the property of OCP to be the crystalline precursor of apatite and assessing if and how the anionic competition can influence the formation of the different apatite end-members, the OCP → apatite reaction has been here investigated placing 0.223 mmol of OCP in 50 mL aqueous solution with 0.368 mmol of dissolved fluoride, chloride, hydroxyl and carbonate anions (fluoride alone, fluoride with each of the other anions, and all the anions together) at room temperature. The post-experiment analyses of solid and liquid phases, conducted by using XRD, ESEM and ICP-OES, show that fluoride is always the main anion removed from solution during the OCP transformation reaction. The precise mineralogical characterization of solid phases formed, performed using the Rietveld algorithm, shows that fluorapatite is always the main resulting apatitic phase, followed by hydroxyapatite. Taking into account the different application fields of OCP, these results could be significant in better defining the OCP → apatite reaction in aqueous solutions where different competing anions are involved.

Keywords: Anionic competition; Fluorapatite formation; Fluoride uptake from aqueous solution; Octacalcium phosphate.

Figure 1

XRD pattern of the OCP used in the experiments. The indexed reflections written in black font are from Brown (1962), those in blue are from Frazier et al. (1991), and those in red are from the present study.

Figure 2

Two ESEM images of the synthesized OCP used for the anionic competition experiments: a) radial crystal aggregates; b) crystals with prismatic bladed habitus.

Figure 3

Plot of the fluoride empirical removal capacity (q_e) and percentage of fluoride removed for each anionic competition experiment. The No_Comp experiment is taken as a reference (100 % fluoride removed) for the other experiments.

Figure 4

Selected portion of X-ray diffraction patterns of the solid phases collected at the end of the anionic competition experiments.

Figure 5

ESEM images and EDS spectrum of the solid phase collected at the end of the anionic competition experiment Tot_Comp: a) aggregate of fluorapatite (FAp) acicular crystals, the red square is one of the areas analysed by EDS; b) magnification of FAp aggregates; c) magnification of FAp aggregates within the red square in a) with the EDS analysis spots (see Figure SD6 in the Supplemental Data for the EDS spectra); d) example of EDS spectrum with elemental identification.