This work details the synthesis and characterization of iron nanoparticles (GT-INP) from green-tea extract and its effect on biohydrogen production from CO by anaerobic biomass. Analysis of the GTINP by UV-visible spectroscopy revealed spherical or quasi-spherical shape of the iron oxide nanoparticles. X-ray diffraction analysis confirmed the presence of iron oxide (Fe3O4) and iron(III) oxidehydroxide (FeO(OH)) in the GT-INP. The appearance of strong vibrational bands in fourier transform infrared spectra indicated the presence of amide, C-N and hydroxyl groups due to flavonoids, alkaloids and polyphenols as capping agents in the GT-INP. Field emission scanning electron microscopy and field emission transmission electron microscopy analyses revealed that the size of GT-INP was within 50-90 nm range with a spherical shaped aggregated structure. In the presence of GT-INP at 1000 mg/I, more than 56 +/- 5.32% enhancement in aqueous solubility of CO was achieved along with a maximum H-2 production of 1.58 +/- 0.13 mmolil by anaerobic biomass, which is nearly 44% more than that obtained without any GT-INP addition. Continuous bioreactor study using a gas lift reactor further established the potential of GT-INP in improving biohydrogen production from CO with a maximum H-2 titre of 30.7 mmol/l due to GT-INP addition in the system. (C) 2019 Elsevier Ltd. All rights reserved.