Key role of La in La@nZVI/rGO for the efficient and steady PMS activation towards ofloxacin degradation via a non-radical process

Abstract

The nano zero-valent iron (nZVI)/peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs) are promising for removal of antibiotic contaminants in groundwater. However, the inherent defects of rapid passivation and inactivation limit the efficient and steady application of nZVI. Herein, the lanthanum@nZVI/rGO (La@nZVI/rGO) was synthesized in which nZVI was coated with La and distributed evenly on reduced graphene oxide (rGO) matrix. The introduction of La coating protected the nZVI from direct contact with PMS molecules thus the La@nZVI/rGO appeared efficient and steady activization of PMS to degrade ofloxacin (OFX) without rapid inactivation. The removal rate was promoted to 96.5 % compared with the uncoated nZVI/rGO (only 85.2 %). The degradation reaction represented steady and continuous activation of PMS with pseudo-first-order kinetics (0.0451 min−1). While the two-stage kinetics of nZVI/rGO/PMS with initial 0.2581 min−1 and instant 0.0024 min−1 resulted from the rapid inactivation of nZVI. The reaction of La@nZVI/rGO/PMS exhibited minimal leaching of Fe ions (less than 1.8 µg/L) while it reached 6.4 mg/L in nZVI/rGO/PMS. XRD and XPS analyses further demonstrated the excellent durability of La@nZVI/rGO and the obvious passivation of nZVI/rGO. ESR and quenching tests confirmed that La coating regulated the PMS activation and ROS generation mechanism. The degradation was dominated by 1O2-induced non-free radical process in La@nZVI/rGO/PMS system and different from radical oxidation in nZVI/rGO/PMS. Moreover, the dissolved oxygen and XPS analysis revealed the important role of La coating and oxygen vacancy in the catalytic degradation. This work could provide a novel understanding of the nZVI with protective coating in AOPs environmental remediation.