Nitrate reduction by metallic iron

Chemical reduction of nitrate by metallic iron (Fe-0) was studied as a potential technology to remove nitrate from water. The effects of pH and the iron-to-nitrate ratio on both nitrate reduction rate and percent removal were investigated. Rate constants and the apparent reaction order with respect to nitrate were determined and a mass balance was obtained. Rapid nitrate reduction by iron powder was observed only at pH less than or equal to 4. pH control with sulfuric acid significantly prolonged nitrate reduction and increased the percent removal. At high nitrate loadings, both the rate and the percent removal increased with decreasing pH. An iron-to-nitrate ratio of 120 m(2) Fe-0/mol NO3 or higher was required to completely remove nitrate within an hour. An apparent reaction order of 1.7 with respect to nitrate was observed, which may be partly due to the inhibitory effect of sulfate. Ammonia was the end product of nitrate reduction and accounted for all nitrate transformed under our experimental conditions. Acidity is the principal factor which controls the rate and the extent of nitrate removal by Fe-0. The rapid reduction of nitrate at low pH was most likely due to either direct reduction by Fe-0 or indirect reduction by surface hydrogen derived from proton. Ferrous species, Fe2+ and Fe(OH)(2), were probably not involved in this reaction. (C) 1998 Elsevier Science Ltd. All rights reserved.