Dept. of Environmental Science and Engineering
Oregon Graduate Institute of Science & Technology
The widespread use of chlorinated aliphatic and nitro aromatic compounds has made them among the most frequently encountered environmental contaminants. A passive, cost effective alternative for in situ treatment of these organic contaminants is the use of zero-valent iron (Fe0) as a reductant in permeable reactive barriers (PRBs). The resounding success of Fe0PRBs has sparked intense interest in the chemistry of the Fe0-water interface. Reactivity at the Fe0-water interface, however, is complicated by the accumulation of corrosion products due to the dissolution of Fe0 in the presence of water and other oxidants. To evaluate the role of the oxide film, three conceptual models of the oxide are presented and interpreted in terms of their implications for Fe0PRBs, including the oxide as a passive film, a semiconductor, and a coordinating surface. An oxide-free Fe0 rotating disk electrode (RDE) was used to quantify the effect of mass transport and surface reaction on the kinetics of reduction by Fe0. Experiments with the Fe0RDE show that rate of carbon tetrachloride (CCl4) reduction is dominated by reaction at the Fe0 surface, whereas the rate of nitrobenzene (ArNO2) reduction is influenced by both surface reaction and mass transport. The kinetic limitation imposed by the rate of surface reaction observed in CCl4 reduction by Fe0 appears to extend to other chlorinated aliphatic compounds. Correlation analysis with surface area normalized rate constants (k) for chlorinated aliphatic compounds reveals satisfactory correlations with a variety of measures of electron affinity. Linear free energy relationships (LFERs) are developed based on estimated one-electron reduction potentials (E1), and lowest unoccupied molecular orbital (LUMO) energies calculated from semi-empirical and ab initio methods. To determine whether surface-active substances could influence the reduction of contaminants by Fe0, a series of batch experiments with seven natural organic matter (NOM) samples and eight anthropogenic surfactants was undertaken. For the range of conditions studied, there was no observable influence on the rate of ArNO2 reduction and only a slight inhibition of rate was observed for the reduction of CCl4 in the presence of high concentrations of NOM.
Scherer, Michelle Marie, "Reduction of chlorinated aliphatic and nitro aromatic compounds at the Fe0-oxide-water interface" (1998). Scholar Archive. 298.