catalysts Article Optimization Parameters, Kinetics, and Mechanism of Naproxen Removal by Catalytic Wet Peroxide Oxidation with a Hybrid Iron-Based Magnetic Catalyst Ysabel Huaccallo-Aguilar 1,2 , Silvia Álvarez-Torrellas 1 , Marcos Larriba 1 , V. Ismael Águeda 1 , José Antonio Delgado 1 , Gabriel Ovejero 1 and Juan García 1, * 1 Catalysis and Separation Processes Group, Chemical Engineering and Materials Department, Faculty of Chemistry, Complutense University, Avda. Complutense s/n, 28040 Madrid, Spain; Ysabelhu@ucm.es (Y.H.-A.); satorrellas@ucm.es (S.Á.-T.); marcoslarriba@ucm.es (M.L.); viam@ucm.es (V.I.Á.); jadeldob@ucm.es (J.A.D.); govejero@ucm.es (G.O.) 2 Departamento de Ingeniería Química, Universidad Nacional de San Agustín, Av. Independencia s/n, 04001 Arequipa, Peru * Correspondence: jgarciar@ucm.es; Tel.: +34-913-945-207 Received: 13 February 2019; Accepted: 15 March 2019; Published: 20 March 2019   Abstract: This work presents a study of the assessment of the operating parameters of the catalytic wet peroxide oxidation (CWPO) of naproxen (NAP) using magnetite/multi-walled carbon nanotubes (Fe 3 O 4 /MWCNTs) as a catalyst. The effect of pH, temperature, and H 2 O 2 dosage on CWPO process was evaluated by using the response surface model (RSM), allowing us to obtain an optimum NAP removal of 82% at the following operating conditions: pH = 5, T = 70 ◦ C, [H 2 O 2 ] 0 = 1.5 mM, and [NAP] 0 = 10.0 mg/L. Therefore, NAP degradation kinetics were revealed to follow a pseudo-second-order kinetic model, and an activation energy value of 4.75 kJ/mol was determined. Adsorption and using only H 2 O 2 experiments, both considered as blank tests, showed no significant removal of the pollutant. Moreover, Fe 3 O 4 /MWCNTs material exhibited good recyclability along three consecutive cycles, finding an average NAP removal percentage close to 80% in each cycle of 3 h reaction time. In addition, the scavenging tests confirmed that the degradation of NAP was mainly governed by • OH radicals attack. Two reaction sequences were proposed for the degradation mechanism according to the detected byproducts. Finally, the versatility of the catalyst was evidenced in the treatment of different environmentally relevant aqueous matrices (wastewater treatment plant effluent (WWTP), surface water (SW), and a hospital wastewater (HW)) spiked with NAP, obtaining total organic carbon (TOC) removal efficiencies after 8 h in the following order: NAP-SW > NAP-HW > NAP-WWTP. Keywords: CWPO; magnetic catalyst; naproxen; response surface methodology; wastewater 1. Introduction Pharmaceutical compounds are an important part of toxic materials in wastewater that are currently growing around the world [1]. They have been frequently detected in the aqueous medium, supposing a growing environmental problem for the scientific community. The excessive drug production demanded and consumed by humans and animals usually ends up in surface water [1], sewage effluents [2], groundwater [3], and even drinking water [4] due to the discharges from municipal wastewater treatment plants (WWTPs) [1], and, consequently, these contaminants can lead to devastating effects on the environment [3,5]. Particularly, non-steroidal anti-inflammatory drugs (NSAIDs) have been found in water effluents at increasing concentrations and their removal from the environment is becoming a real challenge [6]. In this work, naproxen (NAP) has been selected due Catalysts 2019, 9, 287; doi:10.3390/catal9030287 www.mdpi.com/journal/catalysts