Iron electrocoagulation is designed for sustainable high-efficiency and high-flexibility water purification applications. Recent advances reported that hydroxyl radicals (center dot OH)-based oxidative transformation of organic contaminants can occur in iron electrocoagulation. However, there is still a lack of mechanistic understanding the production of center dot OH in bicarbonate electrolyte, which presents a critical knowledge gap in the optimization of iron electrocoagulation technology towards practical application. Combined with contaminant degradation, radical quenching experiments, and spectroscopic techniques, we found that center dot OH was produced at rate of 16.1 mu M center dot h - 1 during 30-mA iron electrocoagulation in bicarbonate electrolyte through activation of O2 by Fe(II) under pH-neutral conditions. High yield of center dot OH occurred at pH 8.5, likely due to high adsorbed Fe(II) that can activate O2 to enhance center dot OH production. Mo center dot ssbauer and X-ray photoelectron spectroscopy measurements substantiated that Fe(II)-adsorbed lepidocrocite was the dominant solid Fe(II) species at pH 8.5. A process-based kinetic modeling was developed to describe the dynamic of center dot OH production, Fe(II) oxidation, and contaminant degradation processes in iron electrocoagulation. Findings of this study extend the functionality of electrocoagulation from phase separation to center dot OH-based advanced oxidation process, which provides a new perspective for the development of electrocoagulation-based next generation sustainable water purification technology.