The increase in environmental pollution, especially water pollution, has intensified the requirement for new strategies for the treatment of water sources. Furthermore, the improved properties of nano-ferrites permit their usage in wastewater treatment. In this regard, novel Mg0.33Ni0.33Co0.33LaxFe2−xO4 nanoparticles (NPs), where 0.00≤x≤0.08, were synthesized to test their photocatalytic, antibacterial and antibiofilm activities. The structural and optical properties of the prepared NPs were investigated by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-Vis spectroscopy and photoluminescence (PL) analysis. As La content increases, the bandgap energy increases, whereas the particle size decreases. The photocatalytic activity of the prepared NPs is evaluated by the degradation of methylene blue (MB) dye under sunlight irradiation. Superior activity is exhibited by Mg0.33Ni0.33Co0.33La0.01Fe1.99O4 NPs. The influence of catalyst dosage, pH, temperature and addition of graphene (Gr) on the photodegradation reaction was studied. Increasing the pH and temperature improved the rate of the photodegradation reaction. The antibacterial and antibiofilm activities of the NPs were assessed against Escherichia coli, Leclercia adecarboxylata, Staphylococcus aureus and Enterococcus faecium. Mg0.33Ni0.33Co0.33Fe2O4 NPs inhibited bacterial growth. They had bacteriostatic activity on all isolates, with a greater effect on Gram-positive bacteria. All tested nano-ferrites had significant antibiofilm activities against some biofilms.

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This study reports the synthesis of Mg0.33Ni0.33Co0.33GdxFe2‐xO4 nanoparticles (NPs) with 0.00≤x≤0.08 by the co‐precipitation method. The structural and optical properties of NPs are investigated. As Gd content increases, the lattice parameter and the bandgap energy increase whereas the particle size decreases. The photocatalytic performance of NPs is evaluated in the degradation of methyl violet (MV) dye under sunlight irradiation. The best activity is exhibited by Mg0.33Ni0.33Co0.33GdxFe2‐xO4 where x=0.04. The rate of the photodegradation reaction is boosted by increasing the pH and temperature. 95.4 % of MV is degraded after 240 min upon the addition of 5 wt. % carbon dot. The NPs are tested against Gram‐positive (Staphylococcus aureus and Enterococcus faecium) and Gram‐negative bacteria (Escherichia coli and Leclercia adecarboxylata) isolated from wastewater. The antibacterial activity was tested by the minimum inhibitory concentration and minimum bactericidal concentration broth microdilution assay, agar well diffusion assay and time‐kill test. The results show that the NPs exhibit inhibitory activity against Gram‐positive bacteria. Furthermore, NPs show bacteriostatic activity after 3 hours of incubation. The antibiofilm activity of the NPs is tested by the inhibition of biofilm formation and the destruction of pre‐formed biofilm assays. The results show an inhibitory activity of the NPs against Gram‐positive biofilms.