New direct consecutive formation of spinel phase in (Fe,Co,Ni)Al2O4 composites for enhanced Pd(II) ions removal

Spinel (Fe,Co,Ni)Al2O4 composites (FeCoNiAl) were prepared by an introduction of Ni, Co and Fe ions into a γ-Al2O3 framework via a simple electrochemical method. The physicochemical properties of the catalysts were studied by XRD, N2 adsorption-desorption, TEM, MP-AES, FTIR, XPS, ESR, and VSM analyses. The characterization data showed that an altered arrangement of the γ-Al2O3 framework was observed with a different structure, particularly with a formation of metal oxides and spinel phases as a consequence of dealumination, isomorphous substitution and surface oxygen defects that contributed to the magnetism properties. The adsorptivity toward Pd2+ was in the following order: FeCoNiAl > CoNiAl > NiAl. A strong coercive fields possessed by hercynite (FeAl2O4) in the FeCoNiAl was found to play an important role in the adsorption. The detailed adsorption equilibrium isotherms, kinetic, thermodynamics, and optimization were also studied to clarify the related results. The equilibrium data were fitted using the Langmuir model with 0.1 g L−1 FeCoNiAl giving the maximum monolayer adsorption capacity (qm) of 303 mg g−1 at 303 K and pH 5. The adsorption kinetics were best described by the pseudo-first-order model. Thermodynamic studies showed that the adsorption was exothermic and not spontaneous at high temperatures. Optimization by response surface methodology (RSM) with a central composite design (CCD) model supported this result by demonstrating that the reaction temperature played an important role in the adsorption.