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This article reports a novel, clean and energy-efficient thermal-plasma assisted synthesis technique, for very high rate production of defective, ferromagnetic, 2D stoichiometric molybdenum-oxide, maximum up to 750 g/h, but without compromising with the sizes of the nanomaterials or their state of aggregation. A collimated, laminar beam of plasma with large cross-section and uniform profile is produced as the jet expands supersonically into a vacuum chamber that engulfs a large molybdenum target plate and heat it up in a controlled manner. The surface of the target is rapidly oxidized into a sublimating vapour, from which nanometric metal-oxides are nucleated through gas phase condensation process. Surface oxygen-defects are introduced into the product, but even without employing an additional step of defect-engineering, which endowed them with robust room-temperature ferromagnetic properties (maximum saturation magnetization = 1.84 emu/g) and easy dispersibility in aqueous solutions. It is further demonstrated that the as-synthesized, phase-pure α-MoO 3 sample with hierarchical morphology can adsorb cationic methylene blue (MB) dyes from an aqueous solution at 94.1 % in 30 min, and the corresponding maximum adsorption capacity is 1045.9 mg/g. The ferromagnetic nanoadsorbents may be separated from wastewater with a simple lab scale magnetic field, to reduce secondary pollution and for reuse of the adsorbents. • Very high rate, energy-efficient production of nanometric 2D α-MoO 3 up to 750 g/h. • Doesn't compromise on particle sizes & their aggregation even at highest production. • O vacancies introduced even without additional step of hazardous defect engineering. • Induced max saturation magnetization is 1.84 emu/g, separable with a small magnet. • Fast MB adsorption kinetics, large capacity (1045.9 mg/g), reusable nanoadsorbents. [ABSTRACT FROM AUTHOR]