ENHANCEMENT OF NITRATE REDUCTION IN FEM0-PACKED COLUMNS BY SELECTED CATIONS

Although permeable reactive barriers (PRBs) using zero-valent iron (ZVI) as reactive media are receiving increasing attention in recent years, the application of ZVI barriers, despite early promise, has not been used as extensively as expected due to the clogging of the reactive media and the passivation of ZVI from the formation of a passive oxide coating. In this paper, we describe the effects of adding some selected cations (e.g., Fe2+, Fe3+, Al3+, and Cu2+) on nitrate removal in Fe0-packed column reactors with the focus on understanding the problems of media clogging and Fe0 passivation. It was found that, due to a rapid passivation of Fe0, only very poor nitrate removal could be achieved in Fe0-packed columns without adding catalytic cations. Long-term performance of the fixed-bed ZVI columns could be remarkably enhanced by adding selected cations, e.g., Fe2+, Fe3+, and AlFe3+; adding Fe3+ achieved the best performance among the four selected cations. In this study, magnetite (Fe3O4) was found to be the major iron corrosion product by nitrate reduction under anoxic conditions; cementation mainly occurred in the inlet zone and was the major cause of the hydraulic clogging. Based on X-ray diffraction analyses, the cementation was likely caused by the formation of lepidocrocite and goethite. The formation of magnetite coating would not necessarily cause clogging of the media. These findings indicate that adding some selected cations into ZVI PRBs might be a method to overcome the passivation problem of iron fillings by its corrosion products.