CHEMICAL CATALYSIS OF NITRATE REDUCTION BY IRON(II)

Experiments have been conducted to investigate the chemical reduction of nitrate under conditions relevant to the often low organic carbon e nvironment of groundwaters. At pH 8 and 20 +/- 2 degrees C, in the pre sence of Cu(II), NO3-, was chemically reduced by Fe(II) to NH4- with a n average stoichiometric liberation of 8 protons. The rate of the reac tion systematically increased with pH in the range pH 7-8.5. The half- life for nitrate reduction, t(1/2), was inversely related to the total molar copper concentration, [CUr] by the equation log t(1/2) = -1.35 log [Cu-T] - 2.616, for all measured values of t(1/2) from 23 min to 1 5 days. At the Cu(II) concentrations used of 7 x 10(-6)-10(-3) M, Cu w as present mainly as a solid phase, either adsorbed to the surfaces of precipitated iron oxides or as a saturated solid. It is this solid ph ase copper rather than Cu2+ in solution which is catalytically active. Neither magnetite, which was formed as a product of the reaction, nor freshly prepared lepidocrocite catalysed the reaction, but goethite d id. Although traces of oxygen accelerated the reaction, at higher part ial pressures (>0.01 atm) the reduction of nitrate was inhibited, prob ably due to competition between NO3- and O-2 for Fe(II). Appreciable c atalytic effects were also observed for solid phase forms of Ag(I), Cd (II), Ni(II), Hg(II), and Pb(II). Mn(II) enhanced the rate slightly, a nd there was evidence for slow abiotic reduction in the absence of any added metal catalysts. These results suggest that the chemical reduct ion of nitrate at catalytic concentrations and temperatures appropriat e to groundwater conditions is feasible on a timescale of months to ye ars. (C) 1997 Elsevier Science Ltd.