Fe(0)-Supported Autotrophic Denitrification

Proof of concept was obtained that Fe(0) can stoichiometrically reduce nitrate to ammonium and that cathodic hydrogen [produced during anaerobic Fe(0) corrosion by water] can sustain microbial denitrification to reduce nitrate to more innocuous products (i.e., N2O and N2). Autotrophic, denitrifying growth on Fe(0) was proven through the use of a dual-flask apparatus. Cathodic H2 from a flask containing Fe(0) was allowed to diffuse to another (anoxic) flask containing a pure culture of Paracoccus denitrificans, where denitrification and microbial growth were observed. Nitrate reduction and end product distribution were studied in batch reactors amended with either steel wool or Fe(0) powder. Steel wool, with a smaller specific surface area, was less reactive, and its corrosion did not significantly increase the pH of the solution. This allowed for a greater participation of denitrifiers in the nitrate removal process, which increased nitrate removal rates and transformed a greater portion of the added nitrate to innocuous gases rather than to ammonium. Combining denitrifiers with the more reactive Fe(0) powder did not increase removal rates or decrease the proportion of nitrate reduced to ammonium. This was attributed to a corrosion-induced increase in pH above the tolerance range of the bacteria (pH > 10). Nitrate removal was sustained over 4 months in flow-through columns packed with steel wool and seeded with autotrophic denitrifiers. Increasing the hydraulic retention time from 0.67 to 2.33 days increased the nitrate removal efficiency and decreased the fraction of nitrate reduced to ammonium. The finding that Fe(0) can sustain autotrophic denitrification may have practical applications to treat nitrate-contaminated waters in exsitu or in-situ reactive filters.