The effect of the number of SO₃⁻ groups on the adsorption of anionic dyes by the synthesized hydroxyapatite/Mg–Al LDH nanocomposite

In this study, a new nanocomposite of hydroxyapatite (HA)/Mg–Al layered double hydroxide (LDH) was successfully formed via a facile co-precipitation method and applied to adsorb three anionic dyes of alizarin red S (ARS), Congo red (CR), and reactive red 120 (RR120) differing in the number of SO₃⁻ groups from aqueous solution. Based on a combination of characterization analysis and adsorption experiments, HA/Mg–Al LDH nanocomposite showed better adsorption performance than HA and Mg–Al LDH. Using XRD and TEM analyses, the crystallinity and the presence of nanoparticles were confirmed. According to the SEM investigation, the Mg–Al LDH layers in the nanocomposite structure were delaminated, while HA nanorods were formed at the surface of Mg–Al LDH nanoparticles. The higher BET surface area of the novel HA/Mg–Al LDH nanocomposite compared to HA and Mg–Al LDH provided its superior adsorption performance. Considering an effective amount of adsorbent dosage, pH 5 was selected as the optimum pH for each of the three dye solutions. According to the results from the study of contact time and initial concentration, the pseudo-second-order kinetic (R² = 0.9987, 0.9951, and 0.9922) and Langmuir isotherm (R² = 0.9873, 0.9956, and 0.9727) best fitted the data for ARS, CR, and RR120, respectively. Anionic dyes with different numbers of SO₃⁻ groups demonstrated distinct adsorption mechanisms for HA and Mg–Al LDH nanoparticles, indicating that the adsorption capacity is influenced by the number of SO₃⁻ groups, with HA/Mg–Al LDH nanocomposite offering superior performance toward dyes with higher numbers of SO₃⁻ groups. Furthermore, ΔH° less than 40 kJ/mol, positive ΔS°, and negative ΔG° accompanied by the mechanism clarifying show physical spontaneous adsorption without an external source of energy and increase the randomness of the process during the adsorption, respectively. Finally, the regeneration study demonstrated that the nanocomposite could be utilized for multiple adsorption–desorption cycles, proposing the HA/Mg–Al LDH as an economically and environmentally friendly adsorbent in the adsorption of anionic dyes in water treatment processes. Graphical Abstract: (Figure presented.)