MECHANISTIC STUDY OF THE HETEROGENEOUS OXIDATION OF FE(II) BY NITRITE

We studied the oxidation of Fe(II) by nitrite in the presence of hydrous ferric oxide (HFO). Ferric (hydr)oxides are ubiquitous, reactive solid phases in natural and engineered systems; the Fe(II)/Fe(III) redox couple is relatively labile and the concentration, speciation, and reactivity of Fe(II) and Fe(III) affect the speciation, toxicity, and mobility of many contaminants. Tamura et al (1976), Sung and Morgan (1980), and Dempsey et al (2001) studied heterogeneous oxidation of adsorbed Fe(II) by O2 and reported rates 5-6 orders of magnitude larger than for homogeneous reactions under conditions pertinent to environmental systems. Park and Dempsey (2005) proposed an anode/cathode mechanism with O2 reduced at sorbed-Fe(II) sites and Fe(II) oxidized at electron-deficient sites without sorbed Fe(II). Nitrite is reduced by well-known one-electron transfers and oxidation of Fe(II) is spontaneous for most environmental conditions (Klausen et.al.,1995; Snoeyink and Jenkins, 1980). Increasing nitrate concentrations are found in many soils; nitrite and nitrous oxide are the products of chemo-denitrification (Sorensen and Thorling, 1989); NO2 is a greenhouse gas and contributes to stratospheric ozone depletion (EPA, 2002;Wang, et.al., 1976).

Most experiments were conducted with 2.5 mM Fe(III) as HFO and pH 6.8, using an O2-trap inside an anaerobic glove bag to ensure absolute removal of O2 (Jeon et al, 2002). Reaction was rapid in the presence of ferric oxide and was not observed in the absence of ferric oxide. The rate equation R=-k2*[Fe(II)diss]*[Fe(II)ads]*[NO2-] accurately predicted the concentrations of dissolved ferrous iron and nitrite versus time. Mössbauer spectroscopy was used to monitor the phase changes. Results were consistent with our previous work using O2 as oxidant, i.e. the rate was dependent on both dissolved and sorbed Fe(II) and sorbed Fe(II) remained constant during the initial phase of the experiment while nearly all dissolved Fe(II) was oxidized. Dynamic molecular modeling also demonstrated that anode/cathode reactions were feasible. The experimental procedures will be extended to the heterogeneous oxidation of Fe(II) with the reduction of uranium and arsenic.