The potential of Gd doping as a promising approach for enhancing the adsorption properties of nickel–cobalt ferrites.

The study shows that the addition of gadolinium ions has a significant impact on the structure, morphology, and adsorption properties of Ni-Co spinel ferrite that was synthesized by the sol–gel auto-combustion method. The research also indicates that the higher the Gd content, the greater the increase in the lattice parameter, which suggests that Gd³⁺ ions uniformly replaced the octahedral Fe³⁺ ions. The morphology and chemical composition of Gd-doped Ni-Co ferrites have been studied using SEM and EDS. Gd adding to the NiCoFe matrix increases the BET surface area by 50% (from 48 to 72 m²/g) and promotes the formation of mesopores with an average radius from 3.9 to 4.9 nm. The pH_PZC values of Gd-doped ferrites are in the range of 7.22–7.39, which means that the ferrite surface will acquire a positive charge at natural pH, so this will promote the adsorption of Congo red anionic dye through electrostatic interaction forces. Langmuir, Freundlich, and Dubinin-Radushkevich models were used to explain the mechanism of CR adsorption on the Ni_0.5Co_0.5Gd_xFe_2-xO₄ adsorbent surface. The ionic-covalent parameter has been estimated to describe the surface acid–base properties. Overall, this study highlights the potential of Gd³⁺ doping as a promising approach for enhancing the adsorption properties of nickel–cobalt ferrites. [ABSTRACT FROM AUTHOR]