Low-cost and sustainable hydroxyapatite synthesis using technical-grade phosphoric acid for environmental applications: Structural and morphological insights

Hydroxyapatite is a versatile material with strong potential for environmental remediation, yet its large-scale use is limited by the cost and purity requirements of conventional precursors. This study introduces a low-cost and sustainable synthesis route for hydroxyapatite using technical-grade phosphoric acid, highlighting the beneficial role of industrial impurities in tuning its structural, optical, and adsorption properties, thus promoting its broader use in environmental applications. Hydroxyapatite powders were synthesized from calcium hydroxide and technical-grade phosphoric acid (Ca/P = 1.67) at 25 °C in aqueous medium, then dried and calcined (500–1100 °C). Structural, chemical, and optical analyses revealed single-phase nano-hydroxyapatite (12.6–57.3 nm) with high surface area (176.95 m²·g-1) and strong wettability. The optical band gap decreased from 4.5 ± 0.4 eV to 3.2 ± 0.4 eV after calcination, indicating defect-induced electronic modification. The poorly crystallized hydroxyapatite exhibited excellent Cd²⁺ and Pb²⁺ adsorption (99 % and 97 % removal in 100 min) but limited bisphenol A elimination (2.73 % in 180 min).