Advanced equilibrium studies on the integration impact of β-cyclodextrin on the effectiveness of magnesium rich hydroxyapatite as adsorbent for tetracycline residuals: Steric and energetic studies

Hydroxyapatite nanoparticles enriched in magnesium ions (Mg.HP) were synthesized and hybridized with β-cyclodextrin (CD/HP) as potential adsorbents of tetracycline residuals (TCC). The adsorption behaviors were described on the basis of the energetic steric and energetic factors of the monolayer model of one energy. The hybridizing process with β-cyclodextrin enhanced the adsorption of TCC effectively to 265.4 mg/g in comparison to Mg.HP (106.2 mg/g). However, the steric investigation demonstrated no remarkable changes in the occupied active site density of CD/HP (Nm = 11.3–13.6 mg/g) as compared to Mg.HP (Nm = 13.5–16.16 mg/g), and the capacity of each site enhanced strongly after the hybridization step. The integration of β-cyclodextrin strongly induces the organic affinities of Mg.HP, which significantly enhances the affinities of each adsorption site to uptake more TCC molecules, prompting their aggregation behaviors during their interaction with the surface of CD/HP particles. Each site on the surface of CD/HP can adsorb up to 24 molecules of TCC (n = 17.14–23.5) as compared to 7 molecules of Mg.HP (n = 5.5–6.56). These values also contribute to the uptake of TCC molecules in vertical form by various multi-molecular mechanisms. The energetic investigations of Gaussian energy (< 8.6 KJ/mol) and adsorption energy (˂ 40 KJ/mol) validate the physical adsorption of TCC, which might involve the cooperation of dipole bonding forces, van der Waals, and hydrogen bonding. The properties and entropy values, free enthalpy, and intern energy declared the endothermic and spontaneous behaviors of the TCC adsorption.