Author: N. Jamarun, A. Prasejati, N. A. Trycahyani, M. R. Novesar, Zilfa, V. Sisca, I. T. Septiana, and I. Md Isa

DOI: http://doi.org/10.31788/RJC.2026.1919440

The adaptable substance hydroxyapatite (HAp) is used extensively in environmental and biomedical applications. Inthis study, eggshell-derived calcium oxide (CaO) was dissolved using nitric acid (HNO3) and acetic acid(CH3COOH) to determine the most effective route for synthesising HAp. The results showed that dissolving CaOin1 M acetic acid (HAp-3) produced HAp with superior structural and adsorption properties. XRDconfirmedthat HAp-3 corresponded to the standard ICSD 157481 pattern, while FTIR verified the presence of characteristic –OHand PO4 3- groups. SEM-EDS revealed a needle-like shape, and the BET surface area study confirmed a mesoporousstructure with a higher specific surface area (77.442 m2 /g). The pseudo-second-order model (R 2 > 0.9996) wasfollowed by the adsorption kinetics, and the adsorption pattern suited the Langmuir isotherm model well (R2 >0.9508). Moreover, HAp-3 maintained high adsorption efficiency after three regeneration cycles, demonstratingitsreusability. These findings highlight that acetic acid-assisted synthesis of HAp from eggshell waste offers anecofriendly, cost-effective, and sustainable pathway for developing high-performance adsorbents for dye-contaminatedwastewater treatment.

Keywords: Eggshell-derived CaO, Acetic Acid, Hydroxyapatite, Adsorption, Congo Red, and Regeneration.