Hypothesis: Interfacially active magnetic Janus nanoparticles (M-Janus NPs) of asymmetric surface wettability are more interfacially active than biwettable nanoparticles (M-CMC-EC NPs) of uniform surface wettability as such that they could anchor more securely at an oil-water interface to separate emulsified oil droplets from oily wastewaters more effectively by magnetic separation. Experiments: Interfacial tension and interfacial pressure-area isotherms of M-Janus NPs at oil-water interface were measured and the results were correlated to coalescence time of oil droplets determined in various types of aqueous phase (wastewaters) and to the performance of oil removal or recovery from corresponding oily wastewaters by M-Janus NPs and M-CMC-EC NPs of uniform surface wettability by an external magnetic field. Findings: Despite the presence of natural/synthetic surfactants in the oily wastewaters, M-Janus NPs exhibited stronger interfacial activities and anchored more firmly at oil-water interfaces than M-CMC-EC NPs of uniform surface wettability. The application of M-Janus NPs could remove/recover >91.5% oil from oily wastewaters by an external magnetic field as compared with >84.3% achieved by M-CMC-EC NPs of uniform surface wettability for treating different oily wastewaters. The M-Janus NPs could be facilely recycled and efficiently reused in the subsequent applications to oil removal/recovery without complex regeneration. (C) 2020 Elsevier Inc. All rights reserved.