Enhancement of nitrate reduction in Fe⁰-packed columns by selected cations

Tests were conducted in Fe⁰-packed columns to investigate the effects of adding selected cations on nitrate removal by Fe⁰. Due to a rapid passivation of Fe⁰, only negligible nitrate was reduced in the columns without adding the selected cation. However, adding certain selected cations (Fe²⁺, Fe³⁺, or Al³⁺) in feed solution can significantly enhance nitrate reduction. Extending hydraulic retention time (HRT) increased nitrate removal by the columns, but the increase was not linearly proportional to HRT. Decreases in columns' hydraulic conductivity (K) were monitored in an 8 month operating period. A modest decrease in K was recorded in the upper and the middle section of the media bed, whereas a significant decrease in K occurred in the inlet section. X-ray diffraction analyses indicate that magnetite (Fe₃O₄) was the dominant species of the iron corrosion products in the entire height of the column media under anoxic and other test conditions. In the inlet section, however, lepidocrocite and goethite were also identified. Cementation was found to occur only in the inlet section, suggesting that lepidocrocite and goethite, rather than magnetite, might be responsible for the cementation and thereby cause the hydraulic clogging. The magnetite coating would not necessarily cause clogging of the media. Journal of Environmental Engineering