Bioinspiration synthesis of hydroxyapatite nanoparticles using eggshells as a calcium source: Evaluation of Congo red dye adsorption potential

Hydroxyapatite nanoparticles (HAp NPs) were synthesized with Muntingia calabura leaf extract as solvent and eggshell waste as a calcium source. The synthesized nanoparticles were irregular rod-like, as visualized by FESEM, while EDX and XPS studies confirmed the formation of hydroxyapatite. The synthesized HAp NPs were polycrystalline and highly thermostable as per XRD and TGA studies respectively. FTIR confirmed the stable coating of plant phytochemicals onto the HAp NPs. Congo red adsorption using HAp NPs nanoadsorbent was optimized by central composite design, wherein 89.96% of 33.18 mg/L dye was adsorbed in 137 min at 180 RPM. The adsorption process was in line with pseudo-second-order model and the Freundlich isotherm indicated chemical adsorption. The adsorption process was feasible, exothermic, and spontaneous as indicated by the thermodynamic studies. The adsorption capacity of HAp NPs stood at a maximum of 217 mg/g, indicating its superiority among other adsorbents reported earlier. It is evident from the convincing results that the HAp NPs synthesized in the present investigation may have a major role in developing novel adsorbents for dye removal out of wastewater streams.