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 perce
nt 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 significantl
y prolonged nitrate reduction and increased the percent removal. At hi
gh nitrate loadings, both the rate and the percent removal increased w
ith decreasing pH. An iron-to-nitrate ratio of 120 m(2) Fe-0/mol NO3 o
r higher was required to completely remove nitrate within an hour. An
apparent reaction order of 1.7 with respect to nitrate was observed, w
hich may be partly due to the inhibitory effect of sulfate. Ammonia wa
s the end product of nitrate reduction and accounted for all nitrate t
ransformed under our experimental conditions. Acidity is the principal
factor which controls the rate and the extent of nitrate removal by F
e-0. The rapid reduction of nitrate at low pH was most likely due to e
ither direct reduction by Fe-0 or indirect reduction by surface hydrog
en derived from proton. Ferrous species, Fe2+ and Fe(OH)(2), were prob
ably not involved in this reaction. (C) 1998 Elsevier Science Ltd. All
rights reserved.