In this study, the environmentally benign citric acid (CA) was utilized to improve the aerobic degradation of 4-chlorophenol (4-CP) over bismuth modified nanoscale zero-valent iron (Bi/Fe). The characterization results revealed the existence of bismuth covering on the Fe surface under zero-valent state. And, the Bi/Fe-0-CA+ O-2 system performed excellent reactivity in degradation of 4-CP due to the generation of reactive oxygen species (ROS), which was confirmed by electron spin resonance (ESR) spectroscopy. After 30 min of reaction, 80% of 4-CP was removed using Bi/Fe-0-CA + O-2 accompanying with high dechlorination rate. The oxidative degradation intermediates were analyzed by HPLC and LC-MS. We found that CA could promote the bismuth-iron system to produce much reactive oxygen species ROS under both aerobic and anaerobic conditions due to its ligand function, which could react with Fe3+ to form a ligand complex (Fe(III)Cit), accompanying with a considerable production of Fe2+ and H2O2. This study provides a new strategy for generating ROS on nZVI and suggests its application for the mineralization of many recalcitrant pollutants. (C) 2016 Elsevier B.V. All rights reserved.