Effects of nitrate on the treatment of lead contaminated groundwater by nanoscale zerovalent iron

Nanoscale zerovalent iron (nZVI) is efficient for removing Pb 2+ and nitrate from water. However, the influence of nitrate, a common groundwater anion, on Pb 2+ removal by nZVI is not well understood. In this study, we showed that under excess Fe 0 conditions (molar ratio of Fe 0 /nitrate > 4), Pb 2+ ions were immobilized more quickly (<5 min) than in nitrate-free systems (∼15 min) due to increasing pH. With nitrate in excess (molar ratio of Fe 0 /nitrate < 4), nitrate stimulated the formation of crystal Pb x Fe 3−x O 4 (ferrite), which provided additional Pb 2+ removal. However, ∼7% of immobilized Pb 2+ 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 Pb 2+ immobilization), while ORP values ≥−475 mV characterized excess nitrate conditions (ferrite process and Pb 2+ 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.