Heterogeneous Fenton-like oxidation of petrochemical wastewater using a magnetically separable catalyst (MNPs@C): process optimization, reaction kinetics and degradation mechanisms

Abstract

The strong tendency of magnetite nanoparticles (MNPs) to agglomerate limits their application in oxidation processes, due to the reduction of surface/volume ratio, dispersion stability and catalytic activity. To solve this problem, we successfully coated MNPs with an average size of 50 nm on an activated carbon surface in order to prepare a magnetic recoverable composite (MNPs@C). It was employed as a heterogeneous catalyst in the Fenton oxidation for petrochemical wastewater (PCW) treatment, due to its high ability in decompose H₂O₂ molecules. XRD, BET, VSM, SEM, TEM and EDX techniques were utilized to determine the catalyst's characteristics. The activity of the catalyst was assessed for the Fenton reaction using COD removal efficiency. Experiments related to the Fenton oxidation process (FOP) were carried out in order for process optimization and evaluation of the degradation kinetics and mechanism. It was observed that both oxidation Fenton and adsorption processes occurred simultaneously in the MNPs@C/H₂O₂ system. The results clearly showed that the organic compounds in the PCW have been degraded by the hydroxyl free radicals (˙OH) released from decomposition of H₂O₂ in the presence of MNPs@C. Under the optimum operating conditions (pH 3.0, 1 g L⁻¹ catalyst and 50 mM H₂O₂), the removal efficiency of COD was found to be 83.5% within 120 min reaction time. More than 65% of COD was experimentally removed after five catalytic cycles, which demonstrates the promising application of the catalyst in the oxidative degradation of organic pollutants. In addition, MNPs@C exhibited low iron leaching (<0.3 g L⁻¹) and stable catalytic activity upon five recycling cycles. The catalyst could be easily recovered and showed high potential for applications in wastewater treatment without secondary pollution. In conclusion, the MNPs@C/H₂O₂ system, as a promising technique, can provide appropriate conditions for the pretreatment of PCW prior to biological processes or as a tertiary treatment for the reuse of effluents.