Advanced equilibrium study on the synthesis and characterization of Mg-doped hydroxyapatite nano-fibers as a potential enhanced adsorbent of Zn (II) and malachite green dye

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Novel Mg-doped hydroxyapatite nanofibers (Mg.HAPNFs) were synthesized using inexpensive natural phosphorite and a facile method. The as-synthesized Mg.HAPNFs were characterized using several analytical methods and used as excellent adsorbent for Zn2+ ions and malachite green (MG) dye. The experimental adsorption capacities of the as-synthesized Mg.HAPNFs for Zn2+ ions and MG dye were 441.2 and 635.1 mg/g, respectively, whereas the theoretical adsorption capacities at saturation were 458.7 and 673.5 mg/g, respectively. The kinetics of the Mg.HAPNFs adsorption reactions for Zn2+ ions and MG dye were described using the pseudo-first-order kinetic model. A homogeneous monolayer mechanism for the adsorption of Zn2+ ions and MG dye molecules onto Mg.HAPNFs was proposed by fitting the experimental data with the theoretical Langmuir isotherm. The advanced equilibrium study of the Mg.HAPNF–Zn2+ and Mg.HAPNF–MG adsorption systems demonstrated that each active site of Mg.HAPNF vertically adsorbed three Zn2+ ions (n = 2.04) and two MG dye molecules (n = 1.92) via multi-ionic processes. The effective density of adsorption sites of Mg.HAPNFs for MG dye (547.78 mg/g) was higher than that for Zn2+ ions (224.56 mg/g). The Gaussian energies (4.20 kJ/mol (Zn2+ ions) and 5.37 kJ/mol (MG dye)), adsorption energies (−5.4 kJ/mol (Zn2+ ions) and − 17.85 kJ/mol (MG dye)), and experimental data for the Mg.HAPNF adsorbent indicated that adsorption involved several physical and chemical mechanisms (i.e., hydrogen bonding, dissolution–precipitation, van der Waals forces, ion exchange, and dipole–dipole interactions). Thermodynamic studies revealed that Mg.HAPNFs spontaneously and exothermically adsorbed Zn2+ ions and MG dye molecules.