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

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 multiionic 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.