Microwave-assisted hydrothermal synthesis of hydroxyapatite from fermented fish by-product for removal of lead from contaminated water

Lead contamination in water is a critical global issue, with severe health and environmental impacts. Conventional treatment methods typically rely on chemical precipitation and adsorption using activated carbon. However, these approaches can be costly and prone to generating secondary waste. Hydroxyapatite offers a cleaner alternative for heavy metal adsorption due to its strong affinity for lead and its environmental benignity. Importantly, fish bone waste from Thailand’s fermented fish industry is an abundant, sustainable calcium-rich feedstock that can be converted into hydroxyapatite via hydrothermal process. Valorizing this by-product into an effective adsorbent not only addresses waste disposal challenges but also supports circular-economy goals. Hydroxyapatite was successfully synthesized from fermented fish bone using a hydrothermal process, optimizing conditions at 210 °C and a Ca/P ratio of 2 (sample HT9). Characterization revealed that HT9 had the high crystallinity, featuring a thin, porous sheet-like morphology with a specific surface area of 14.08 m2/g and pore volume of 0.0975 cm3/g. Key functional groups (PO43−, OH−, CO32−) critical for heavy metal adsorption were confirmed. The synthesized HT9 demonstrated excellent lead removal efficiency, achieving 99.6% removal within 60 min at pH 7 and an adsorbent dosage of 2.0 g/L. Pb adsorption on HT9 followed the Langmuir isotherm model (R2 = 0.9997), indicating monolayer adsorption with a maximum capacity of 2.20 mg/g. Kinetic analysis showed the process adhered to a pseudo-second-order model, suggesting chemisorption as the rate-limiting step, with a rate constant of 1.2012 g/mg·min. The results highlight the potential of hydroxyapatite from fermented fish bone waste as a cost-effective, eco-friendly adsorbent for efficient lead removal and promote sustainable water-treatment solutions.