The oxidation of acetaminophen (ACT) was investigated in the catalytic ozonation process (COPs) an ozonation process with modified MgO (m-MgO) nanoparticles as catalyst. The effect of main operational variables including solution pH and concentration of m-MgO, ACT and ozone concentration was investigated on ACT degradation in the COP. The complete degradation and 94% mineralization of 50 mg/L ACT obtained at the solution pH around 5, MgO concentration of 2 g/L and ozone concentration of 1.8 mgO(3)/min, within a reaction time of 30 min. The cytotoxicity fresh and COP-treated ACT solution was assessed using the human embryonic kidney (HEK) cultured cells. The results indicated that treating ACT solution in the m-MgO/O-3 process under optimum experimental conditions as compared to the fresh (untreated) ACT solution considerably increased the viability of HEK cells. Therefore, the COP with m-MgO nanoparticle as the catalyst is a promising and efficient process for the oxidation and detoxification of ACT as a model of emerging contaminants in the contaminated water. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.