Effects of low pH on nitrate reduction by iron powder

The effect of low pH (2-4.5) on nitrate reduction in an iron/nitrate/water system was investigated through batch experiments conducted in a pH-stat. The results showed that nitrate could be rapidly reduced to ammonium at pH 2-4.5. A black coating, consisted of both Fe(II) and Fe(III), was formed on the surface of iron grains as an iron corrosion product. X-ray diffractometry indicated that the black coating was poorly crystalline, and its spectrum could not be matched with commonly known iron oxides/hydroxides/oxide hydroxides or green rust I/II. The black coating does not inhibit the reactivity of Fe0 (at least at pH < 3). The black coating was unstable and evolved with time into other oxides under certain conditions. A kinetic model incorporating the effects of pH on nitrate reduction and Langmuir adsorption of nitrate was proposed, and the parameters were estimated by nonlinear curve fitting. Based on this model, the two major effects of pH on the kinetics of nitrate reduction are that: (a) H+ ions directly participate in the redox reaction of nitrate reduction following first-order kinetics; and (b) H+ ions affect the nitrate adsorption onto reactive sites.