Engineering of Mesoporous Gd-substituted Ni-Co Ferrites as Adsorbents for Efficient Elimination of Congo Red Dye and Oxytetracycline

The study investigates the structure, morphology, and adsorption properties of Gd-doped Ni-Co ferrites synthesized via a modified reduction method. Physicochemical analysis of the Gd-doped and undoped Ni-Co ferrites was conducted using TG-DTG-DTA, XRD, FT-IR, SEM, EDS, and BET techniques. The XRD analysis revealed an increase in the lattice parameter due to the substitution of Fe ions by Gd ions, impacting the crystallite size, which decreased from 14 to 4 nm. Additionally, the synthesized powders exhibited a well-developed mesoporous structure and a significantly increased specific surface area, reaching up to 139 m²/g. The study indicated that the presence of Gd(III) ions led to the distortion of octahedral sublattices, resulting in the formation of surface-active centers and a modification of the surface charge of the ferrites. This modification led to improved adsorption properties of Gd-substituted ferrites in solutions with natural pH. The adsorption studies demonstrated the enhanced capacity of the Ni_0.5Co_0.5Gd_0.05Fe_1.95O₄ sample of 254 mg(CR)/g and 298 mg(OTC)/g, which are three times and two times higher, respectively, compared to the undoped NCF sample. The adsorption mechanism was best described by the Langmuir model, indicating chemisorption during pollutant removal, supported by the calculated adsorption energy ranging from 14.74 to 18.90 kJ/mol. XPS and FTIR analyses showed that CR and OTC adsorption onto Gd-doped ferrite samples involves the chemisorption. The study concludes that the modified reductive coprecipitation method contributes to the formation of a mesoporous surface, while Gd substitution significantly enhances both surface charge and, as a consequence, the adsorption properties. This work sheds light on the potential of Gd doping to produce advanced adsorbents for water treatment.