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Chemical Reduction of Nitrate by Ferrous Iron¹

ABSTRACT

Knowledge concerning the chemical reduction of NO₃^– to gaseous products, a process of potential practical significance as an antipollution device, is sparse. The influence of pH on chemical reduction of NO₃^–-N (approximate concentration 25 ppm) by Fe²⁺ in the presence and absence of Cu²⁺ was studied over a pH range from 6 to 10. After 24-hours of controlled pH incubations under a helium atmosphere NO₃^–, NO₂^–, N₂O, NO, N₂, and NH₄⁺ were determined. The initial Fe²⁺/NO₃^– mole ratio was 8. Reduction of NO₃^– was negligible in the absence of Cu²⁺, but was pronounced above pH 7 in the presence of approximately 5 ppm Cu²⁺. Formation of NH₄⁺ increased with pH and was the dominant process at pH 9 and 10. Nitrous oxide and N₂ accumulations were greatest in the pH range from 8 to 8.5 and negligible at pH 6 and 10. Nitrite formation was small except at pH 9 and 10. Trace quantities of NO accumulated during incubation if the pH was allowed to drop below 6.

Levels of Cu²⁺ and Fe²⁺ influenced the extent and nature of NO₃^– reduction at pH 8. Maximum reduction of NO₃^– (93%) and maximum gas production (equivalent to 61% of the original NO₃^–) occurred when the Fe²⁺/NO₃^– mole ratio was 12 and the Cu²⁺ level was approximately 10 ppm. The N₂O/N₂ mole ratio in the evolved gases decreased as the Cu²⁺ level was increased from approximately 1 to 10 ppm and as the Fe²⁺/NO₃^– mole ratio was increased from 8 to 12. Nitrate was relatively stable at a Cu²⁺ content of 0.1 ppm irrespective of the Fe²⁺/NO₃^– ratio.

Key Words: denitrification • nitrous oxide • nitrogen • ammonium • nitrate removal • copper • waste water

¹ Contribution from the Dep. of Soils, North Dakota State Univ., Fargo, ND 58102. Published with the approval of the Director as Journal No. 634.

² Graduate Assistant and Professor, respectively.

Received for publication October 24, 1974.

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