Effects of Oxide Coating and Selected Cations on Nitrate Reduction by Iron Metal
2003
Huang, Yong H. | Zhang, Tian C. | Shea, Patrick J. | Comfort, Steve D.
Under anoxic conditions, zerovalent iron (Fe⁰) reduces nitrate to ammonium and magnetite (Fe₃O₄) is produced at near-neutral pH. Nitrate removal was most rapid at low pH (2–4); however, the formation of a black oxide film at pH 5 to 8 temporarily halted or slowed the reaction unless the system was augmented with Fe²⁺, Cu²⁺, or Al³⁺ Bathing the corroding Fe⁰ in a Fe²⁺ solution greatly enhanced nitrate reduction at near-neutral pH and coincided with the formation of a black precipitate. X-ray diffractometry and scanning electron microscopy confirmed that both the black precipitate and black oxide coating on the iron surface were magnetite. In this system, ferrous iron was determined to be a partial contributor to nitrate removal, but nitrate reduction was not observed in the absence of Fe⁰ Nitrate removal was also enhanced by augmenting the Fe⁰–H₂O system with Fe³⁺, Cu²⁺, or Al³⁺ but not Ca²⁺, Mg²⁺, or Zn²⁺ Our research indicates that a magnetite coating is not a hindrance to nitrate reduction by Fe⁰, provided sufficient aqueous Fe²⁺ is present in the system.
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