Fluoro-Hydroxyapatite/Chitosan composites as an eco-friendly adsorbent for Direct Red 23 dye removal: Optimization through Response Surface Methodology

• FHAP-CS is environmentally and economically sustainable absorbed, with the ability to be regenerated and used up to six times. • FHAPS-CS exhibited high surface properties, including porosity and specific surface area. • The adsorption process optimized using Response Surface Methodology (RSM) based on a Central Composite Design (CCD). • DR23 dye adsorption on FHAP-CS involves hydrogen bonding, chelation, n- π interactions, and electrostatic interactions. • DR23 dye adsorption onto FHAP-CS was spontaneous, endothermic, and involved physicosorption, occurring as monolayer adsorption. This study presents the development of an innovative and eco-friendly Fluoro-Hydroxyapatite/Chitosan (FHAP-CS) adsorbent for removing Direct Red 23 (DR23) dye from aqueous solutions. Synthesized via the sol-gel method by incorporating fluoride (F) and chitosan (CS) into hydroxyapatite, FHAP-CS was characterized using FTIR, XRD, SEM, TGA, and N2 adsorption/desorption to evaluate its structural, surface, and morphological properties. The results demonstrated that FHAP-CS exhibited enhanced particle definition, reduced particle size, increased surface area, and superior adsorption capacity compared to pure HAP, HAP-CS, and FHAP. The adsorption of DR23 dye onto FHAP-CS was optimized using Central Composite Design and Response Surface Methodology. The results demonstrated that the quadratic regression models were statistically significant (R² = 0.99) and can accurately predict adsorption behavior. The monolayer adsorption mechanism of DR23 dye onto FHAP-CS was confirmed by fitting the batch adsorption data to the Langmuir isotherm model, and the process followed pseudo-second-order kinetic models. Under optimal conditions: a [ D R 23 ] 0 = 350 mg/L, pH of 5.11, FHAP-CS dose of 1.60 g/L, agitation speed of 240 rpm, and contact time of 120 min the adsorption capacity reached approximately 152.36 mg g -1 . These findings suggest that FHAP-CS is an effective and cost-efficient adsorbent for removing hazardous azo dyes from wastewater.