Innovative sodium benzoate-modified hydroxyapatite for enhanced dye removal using a combined experimental and DFT approach

Abstract

This study presents the development of a novel adsorbent based on hydroxyapatite (CaHAp) grafted with sodium benzoate (SB) at varying proportions (5%, 10%, 15%) via a double decomposition method. Structural and chemical characterizations (XRD, FTIR, SEM, TGA/DTA) confirmed the successful integration of SB, with noticeable improvements in crystallinity (+ 10%), porosity, and thermal stability compared to pristine CaHAp. Adsorption experiments using methylene blue (MB⁺) as a model pollutant showed that the CaHAp-(SB)15 composite achieved an adsorption efficiency of 90%. The adsorption process followed Langmuir isotherm (R² > 0.96) and pseudo-second-order kinetics (R² > 0.99), indicating monolayer chemisorption as the dominant mechanism. Increasing SB content correlated with a significant rise in adsorption capacity from 5.41 to 9.42 mg·g⁻¹, demonstrating the role of –CO2⁻ groups in enhancing electrostatic interactions with cationic MB⁺. DFT simulations supported the experimental findings, showing favorable interactions between SB and CaHAp at both molecular and atomic levels, particularly through non-covalent forces such as van der Waals and electrostatic interactions. This combined theoretical–experimental approach highlights CaHAp-(SB)15 as a promising, low-cost, and eco-friendly adsorbent for dye-contaminated wastewater, offering high efficiency, reproducibility, and environmental compatibility.