Enhanced Activity of La-Substituted Nickel–Cobalt Ferrites in Congo Red Dye Removal and Hydrogen Peroxide Decomposition

La-substituted Ni-Co ferrites Ni_0.5Co_0.5La_xFe_2-xO₄ (x = 0; 0.01; 0.025; 0.05), synthesized by sol–gel auto-combustion method, were tested as adsorbents for the Congo Red dye removal and as catalysts for hydrogen peroxide decomposition. XRD, SEM, EDS, and IR spectroscopy techniques were used for structure-morphology characterization. The presence of La ions resulted in a decrease in both the lattice parameter and crystallite size. The chemical composition was confirmed to be close to the expected stoichiometry through EDS analysis. The IR spectra revealed two absorption bands within the 400–600 cm^–1 range, indicating the formation of the spinel structure and the introduction of La ions into the crystal lattice. The adsorption studies conducted on La-substituted Ni-Co ferrites revealed that the presence of La ions improved their adsorption properties for Congo Red dye removal. The maximum adsorption capacity has been observed for the sample with x(La) = 0.025. The obtained data fit well with the Langmuir model. The Dubinin-Radushkiewicz isotherm model was employed to determine the free energy of adsorption, which was found to be in the range of 22–25 kJ/mol, indicating the chemosorption of Congo Red molecules onto La-doped ferrites. The removal efficiency of Congo Red dye for the most active sample reached 98%. The samples manifested sufficient activity as Fenton-like catalysts in hydrogen peroxide decomposition. The NiCoLa-25 sample turned out to be the most active, decomposing H₂O₂ by 94%. Synthesized La-substituted Ni-Co ferrites could be both promising adsorbents and catalysts for wastewater treatment.