Reduction of nitro aromatic compounds by zero-valent iron metal

The properties of iron metal that make it useful in remediation of chlorinated solvents may also lead to reduction of other groundwater contaminants such as nitro aromatic compounds (NACs). Nitrobenzene is reduced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and azoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs more slowly (typical pseudo-first-order rate constants 3.5 x 10^-2, 3.4 x 10^-2, and 8.8 x 10^{- 3} min^-1, respectively, in the presence of 33 g/L acid-washed, 18-20 mesh Fluka iron turnings). The nitro reduction rate increased linearly with concentration of iron surface area, giving a specific reaction rate constant (3.9 ± 0.2 x 10^-2 min^-1 m^-2 L). The minimal effects of solution pH or ring substitution on nitro reduction rates, and the linear correlation between nitrobenzene reduction rate constants and the square-root of mixing rate (rpm), suggest that the observed reaction rates were controlled by mass transfer of the NAC to the metal surface. The decrease in reduction rate for nitrobenzene with increased concentration of dissolved carbonate and with extended exposure of the metal to a particular carbonate buffer indicate that the precipitation of siderite on the metal inhibits nitro reduction.