Synthesis of Hydroxyapatite/Chitosan–Glutamic Acid Nanocomposite for Highly Efficient Removal of Congo Red Dye from Water

Water pollution by synthetic dyes from industrial effluents poses serious environmental and health risks. This study reports the synthesis, characterization, and application of a novel hydroxyapatite/chitosan-glutamic acid composite (HAP-Cs-Glu) for Congo red (CR) dye removal from aqueous solutions. The composite was synthesized via chemical precipitation at 80°C and pH 10 with 5 hours aging. Characterization was performed using XRD, FESEM, TEM, BET, and FTIR techniques. XRD confirmed hydroxyapatite formation with low crystallinity and 12.27 nm average crystal size. FESEM and TEM revealed spherical particles (~45.77 nm) composed of needle-like nanocrystals. BET analysis showed a surface area of 58.958 m²/g, pore volume of 0.5538 cm³/g, and average pore diameter of 37.572 nm. FTIR confirmed successful functionalization with characteristic phosphate, amino, and carboxyl bands. Batch adsorption experiments investigated effects of contact time (15-75 min), adsorbent dose (0.03-0.15 g), pH (3-9), initial concentration (200-600 mg/L), and temperature (313-333 K). Optimum conditions were: 600 mg/L CR concentration, pH 3, 0.03 g adsorbent, 45 min contact time, and 60°C, achieving maximum adsorption capacity of 833.33 mg/g. Kinetic data followed pseudo-second-order model (R² = 0.9996). Langmuir isotherm provided better fit than Freundlich, indicating monolayer adsorption. Thermodynamic analysis revealed spontaneous (ΔG° < 0) and endothermic (ΔH° > 0) adsorption with positive entropy change. The HAP-Cs-Glu composite demonstrates excellent potential as an eco-friendly adsorbent for CR removal from contaminated water.