Enhancing the reactivity of bimetallic Bi/Fe⁰ by citric acid for remediation of polluted water

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/Fe⁰). The characterization results revealed the existence of bismuth covering on the Fe⁰ surface under zero-valent state. And, the Bi/Fe⁰-CA + O₂ 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/Fe⁰-CA + O₂ 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.