Arsenate substitution in synthetic hydroxylapatite – structural characterization of the Ca5(PO4)x(AsO4)3-xOH solid solution

Abstract

CC-BY 4.0 License requiring that the original work has been properly cited.Occurences of dissolved As in surface and ground waters and observed adverse health effects have emphasized the need for better understanding of reactions that govern As mobility. Precipitation of Ca apatites is quite often used for immobilization of toxic forms P and As in the environment. Hydroxylapatite -Ca 5 (PO 4 ) 3 OH (HAP) and johnbaumite -Ca 5 (AsO 4 ) 3 OH (AsHAP) are isostructural apatite minerals naturally occurring in the oxidation zones.The main objective of this research is identification of systematic relation between structural and spectral properties and As content in the Ca 5 (PO 4 ) 3 3 OH synthesized phases were characterized using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), powder X-Ray diffraction (XRD), Infrared absorption spectroscopy (FTIR) and Raman spectroscopy.SEM-EDS analysis confirms that the products of the synthesis are homogeneous. The crystals size varies between 0.1-1.0 μm and increases with the increase of Ascontent. Semi-quantitative elemental analysis by EDS indicates that Ca, O, P and Asare the only elements detected and that the molar proportions correspond to theoretical composition of HAP, Ca 5 (PO 4 ) 2 (AsO 4 )OH, Ca 5 (PO 4 ) 1,5 (AsO 4 ) 1,5 OH, Ca 5 (PO 4 )(AsO 4 ) 2 OH and finally AsHAP.Systematic shift of peaks is apparent on the diffraction patterns. Hexagonal system and P63/m group of symmetry were used for unite cell parameters calculation. The results show systematic increase in the unit cell parameters with substitution of As for P in the series: parameter a from 9.41 to 9.43 and parameter c from 6.85 to 6.87. Crystal volumes also increase with increasing amount of As in synthesized phases. Calculated values are within the error similar to these reported by Zhu et al. (2009). In contrary to their results, a linear increase of unit cell parameters with As content is observed. This is in accordance with Lee et al. (2009).The intensity and the position of PO 4 and AsO 4 absorption bands varies systematically in the FTIR spectra being shifted towards higher wavenumber with increasing As content. The bands originating from CaO and OH (3400 cm -1 ) are practically not affected in all five spectra.

The effect of Ca-Pb and P-As substitutions on the solubulity of hydroxylapatites [abstract].

“…The system was considered in Hydroxylapatites are important biomaterials. Substitutions of Pb for Ca and As for P in hydroxylapatites are recently intensively studied due to their significance in the environmental immobilization of Pb and As (Lee et al 2009, Chlebowska et al 2015, Motyka et al 2015. The general chemical formula of minerals in the apatite group is expressed by A 5 (XO 4 ) 3 Z, where A are bivalent cations (e.g., Ca 2+ or Pb 2+ , cations are distributed on two distinct crystallographic sites), XO 4 is a trivalent oxyanion (e.g., PO 4 3-, AsO 4 3-), and Z is a monovalent anion (OH, F, Cl, or O).…”