Magnetically recyclable spinel ferrites MFe2O4 (M = Ni, Cu, Zn) as piezocatalysts for sustained water purification

To develop a highly efficient mechanical stress-induced and conveniently recyclable piezocatalyst, in this study, spinel ferrite-based catalysts with different metal ions (Ni, Cu, Zn) were synthesized via a simple hydrothermal method for the piezocatalytic degradation of ciprofloxacin in wastewater under external mechanical vibration. NiFe₂O₄ exhibited superior catalytic performance compared to other catalysts, with the degradation efficiency of 76.1 % ciprofloxacin within 60 minutes. The kinetic rate constant reached 0.0208 min^-1, which was 4.4 and 11.6 times higher than those of ZnFe₂O₄ and CuFe₂O₄, respectively. Scavenger test, reactive radicals detection, and electrochemical analysis revealed that the piezoelectric field generated in NiFe₂O₄ boosted the charge separation and facilitated the formation of hydroxyl radicals (HO^•) and superoxide radical (O₂^•—) through simultaneous oxidation of hole (h⁺) and reduction of electron (e^-), and HO^• plays a pivotal role for ciprofloxacin oxidation. The magnetic ability of NiFe₂O₄ allows its convenient separation and excellent recyclability. This study demonstrates the potential of integrating magnetic and piezoelectric properties in a single catalyst for the degradation of organic pollutants in wastewater.