Rietveld refinement-based structural analysis of biogenic hydroxyapatite and its PVA composite for dye removal

Abstract

The disposal of fish scales is not only an environmental issue but also a missed opportunity to recover valuable resources. In this study, fish-scale waste was transformed into hydroxyapatite (HAp), a material with possible applications in environmental remediation, particularly in removing contaminants from water. While synthetic HAp, with the stoichiometric formula Ca10(PO4)6(OH)2, has been extensively studied, the structural properties of biogenic HAp derived from natural sources, such as fish scales, remain underexplored. To bridge this gap, Rietveld refinement was used to uncover detailed crystallographic information. Using a straightforward calcination process at 800 degrees C, HAp was successfully extracted from recycled European seabass (Dicentrarchus labrax) scales. The Rietveld analysis of the X-ray powder diffraction data confirms crystallization in a hexagonal phase (space group P63/m), with refined lattice parameters comparable to synthetic HAp and a Ca/P molar ratio of 1.474(7). An impurity phase, Mg-whitlockite (15.9(1) % by weight), was also identified, offering potential benefits for bioactivity and osteoconductivity. Further analysis, including Fourier transform infrared spectroscopy, confirms the presence of PO43 - and OH- functional groups, while scanning electron microscopy provides insights into the morphology showing variations in crystallization and particle sizes, ranging from 30 to 100 nm across different regions. Additionally, HAp was incorporated into a polyvinyl alcohol (PVA) matrix, producing composite films with an adsorption capacity of 38-40 % for methylene blue dye, compared to 36-38 % for pure PVA films. This approach provides a sustainable solution for repurposing fish-scale waste into useful materials with notable potential in environmental remediation, especially in the removal of dyes from wastewater.