Iron oxide magnetic nanoparticles (MNPs) can effectively activate persulfate, generating free sulfate radicals (SO4-.), thereby presenting a promising technology for degradation of recalcitrant organic contaminants, such as p-nitroaniline (PNA), in wastewater. MNPs were prepared through liquid-phase co-precipitation method and characterized using transmission electron microscopy (TEM) and X-ray diffraction (XRD) for their shape and structure. The target contaminant PNA can be destroyed by sulfate radicals (SO4-.) depending on several parameters such as persulfate, Fe3O4, PNA concentrations, pH and reaction temperature. The results show that the degradation of PNA follows the pseudo-first-order kinetics. Approximately 100% removal efficiency of PNA was reached within 270 min, and the mineralization rate reached 67% after 300 min of reaction time. Moreover, the Fe3O4 MNP catalysts could be reused for four times. The degradation efficiency decreased because after the reaction with persulfate, Fe(II) on the surface of Fe3O4 was gradually converted to Fe(III), as determined through X-ray photoelectron spectroscopy (XPS). Based on the rate constants at different temperatures, the activation energy of this process was calculated as 65.6 kJ mol(-1). (C) 2015 Elsevier B.V. All rights reserved.