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

Novel nanoporous zinc phosphate/hydroxyapatite nano-rods core/shell (ZPh/HPANRs) was synthesized and characterized as a modified form of hydroxyapatite with superior Ni2+ and Co2+ adsorption capacities. The ZPh/HPANRs structure exhibits saturation adsorption capacities of 515.4 mg/g (Ni2+) and 758 mg/g (Co2+). The Ni2+ and Co2+ uptake reactions by ZPh/HPANRs 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/HPANRs 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.