The effect of Fe₂NiO₄ and Fe₄NiO₄Zn magnetic nanoparticles on anaerobic digestion activity

Two types of magnetic nanoparticles (MNPs), i.e. Ni ferrite nanoparticles (Fe₂NiO₄) and Ni Zn ferrite nanoparticles (Fe₄NiO₄Zn) containing the trace metals Ni and Fe, were added to the anaerobic digestion of synthetic municipal wastewater at concentrations between 1 and 100 mg Ni L⁻¹ in order to compare their effects on biogas (methane) production and sludge activity. Using the production of methane over time as a measure, the assays revealed that anaerobic digestion was stimulated by the addition of 100 mg Ni L⁻¹ in Fe₂NiO₄ NPs, while it was inhibited by the addition of 1–100 mg Ni L⁻¹ in Fe₄NiO₄Zn NPs. Especially at 100 mg Ni L⁻¹, Fe₄NiO₄Zn NPs resulted in a total inhibition of anaerobic digestion. The metabolic activity of the anaerobic sludge was tested using the resazurin reduction assay, and the assay clearly revealed the negative effect of Fe₄NiO₄Zn NPs and the positive effect of Fe₂NiO₄ NPs. Re-feeding fresh synthetic medium reactivated the NPs added to the anaerobic sludge, except for the experiment with 100 mg Ni L⁻¹ addition of Fe₄NiO₄Zn NPs. The findings in this present study indicate a possible new strategy for NPs design to enhance anaerobic digestion.