Deproteinization Controlled Phase Selection in Calcium Phosphate Biomaterials Derived from <i>Tenualosa ilisha</i> Fish Scale Waste: A Sustainable Valorization Approach

The sustainable management of fishery waste is a growing environmental concern, with fish scales offering promising sources for high-value biomaterials. In this study, calcium phosphate biomaterials (CPBs) were selectively synthesized by varying the chemical treatment of Tenualosa ilisha fish scales. Single-step acid (1 N HCl) or base (1 N NaOH) deproteinization followed by calcination at 1000 °C yielded the magnesium-containing β-tricalcium phosphate (β-TCP, whitlockite) phase, confirmed through X-ray diffraction (XRD) analysis. In contrast, a sequential acid–base treatment led to the formation of highly crystalline monoclinic hydroxyapatite (HAp). Detailed crystallographic analysis revealed superior crystallinity, larger crystallite size, and lower lattice strain for the HAp phase compared to those of the whitlockite phase. Crystallite sizes of 52, 48, and 118 nm were estimated using the Scherrer equation for samples prepared by the acid, base, and acid–base methods, respectively. Surface compositional analysis by X-ray photoelectron spectroscopy (XPS) further corroborated the integration of Mg into the β-TCP lattice structure. A clear distinction was evident in the Fourier transform infrared spectroscopy (FTIR) spectra between the two phases, where β-TCP contained bands for the P2O74– group caused by incomplete transformation of β-TCP from TCP and/or high temperature treatment. Differences in PO43– Raman band positions and splitting of the symmetric P–O stretching band for the whitlockite phase further highlighted structural distinctions. Morphological characterization in terms of transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) demonstrated finer, smaller, and more uniform particle formation for the HAp phase. FESEM-based energy dispersive X-ray spectroscopy (EDX) analysis revealed that all samples were Ca-deficient, with Ca/P ratios (0.92-1.38) consistently lower than theoretical values. Cell viability studies indicated excellent biocompatibility (>95% viability) in all samples. Mechanistically, the phase formation is proposed to follow pH-dependent ionic dynamics, where single-step treatment enhanced Mg incorporation and favors β-TCP formation, while sequential acid–base processing likely redistributes ions and stabilizes phosphate, promoting HAp crystallization. This phase-selective approach provides a simple, waste-derived pathway to tailor CPBs for diverse biomedical and environmental applications.