Synthesis of novel nanoporous zinc phosphate/hydroxyapatite nano-rods (ZPh/HPA_NRs) core/shell for enhanced adsorption of Ni²⁺ and Co²⁺ ions: Characterization and application

Novel nanoporous zinc phosphate/hydroxyapatite nano-rods core/shell (ZPh/HPA_NRs) was synthesized and characterized as a modified form of hydroxyapatite with superior Ni²⁺ and Co²⁺ adsorption capacities. The ZPh/HPA_NRs structure exhibits saturation adsorption capacities of 515.4 mg/g (Ni²⁺) and 758 mg/g (Co²⁺). The Ni²⁺ and Co²⁺ uptake reactions by ZPh/HPA_NRs follow the theoretical behavior of Pseudo-First order and Pseudo-Second order kinetics, respectively. The assumptions of the classic Langmuir isotherm and advanced Monolayer model with one energy site were applied to illustrate the equilibrium properties of both Ni²⁺ and Co²⁺ uptake reactions. Considering the steric parameters, the numbers of adsorbed Ni²⁺ (n = 1.52–2.58) and Co²⁺ (n = 1.07–2.55) suggest their adsorption as two or three metal ions per each active site of ZPh/HPA_NRs by multi-ionic processes and in a vertical orientation. The structure show higher active site density for the Co²⁺ (Nm = 702.6 mg/g) than Ni²⁺ ions (Nm = 339.14 mg/g). The Gaussian energies (Ni²⁺ (3.13–4.49 kJ/mol) and Co²⁺ (8.63–9.44 kJ/mol)), adsorption energies Ni²⁺ (−27.3 to −28.49 kJ/mol) and Co²⁺ (−25.11 to −28.2 kJ/mol)), FT-IR analysis and EDX analysis demonstrate complex chemical (precipitation and chemical complexation), physical (hydrogen bonding), and ion exchange mechanisms. The thermodynamic functions display the spontaneous and exothermic properties of the Ni²⁺ and Co²⁺ uptake reactions.