Nanoscale zerovalent iron (nZVI) is efficient for removing Pb2+ and nitrate from water. However, the influence of nitrate, a common groundwater anion, on Pb2+ removal by nZVI is not well understood. In this study, we showed that under excess Fe conditions (molar ratio of Fe-0/nitrate > 4), Pb2+ ions were immobilized more quickly (<5 min) than in nitrate-free systems (similar to 15 min) due to increasing pH. With nitrate in excess (molar ratio of Fe-0/nitrate < 4), nitrate stimulated the formation of crystal PbxFe3-xO4 (ferrite), which provided additional Pb2+ removal. However, similar to 7% of immobilized Pb2+ ions were released into aqueous phase within 2 h due to ferrite deformation. Oxidation-reduction potential (ORP) values below -600 mV correlated with excess Fe-0 conditions (complete Pb2+ immobilization), while ORP values >=-475 mV characterized excess nitrate conditions (ferrite process and Pb2+ release occurrence). This study indicates that ORP monitoring is important for proper management of nZVI-based remediation in the subsurface to avoid lead remobilization in the presence of nitrate. (C) 2014 Elsevier B.V. All rights reserved.