REDUCTION OF NITRO AROMATIC-COMPOUNDS BY ZERO-VALENT IRON METAL

The properties of iron metal that make it useful in remediation of chl orinated solvents may also lead to reduction of other groundwater cont aminants such as nitro aromatic compounds (NACs). Nitrobenzene is redu ced by iron under anaerobic conditions to aniline with nitrosobenzene as an intermediate product. Coupling products such as azobenzene and a zoxybenzene were not detected. First-order reduction rates are similar for nitrobenzene and nitrosobenzene, but aniline appearance occurs mo re 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 reducti on rate increased linearly with concentration of iron surface area, gi ving 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 ni tro reduction rates, and the linear correlation between nitrobenzene r eduction rare constants and the square-root of mixing rate (rpm), sugg est that the observed reaction rates were controlled by mass transfer of the NAC to the metal surf:ace. The decrease in reduction rate for n itrobenzene with increased concentration of dissolved carbonate and wi th extended exposure of the metal to a particular carbonate buffer ind icate that the precipitation of side rite on the metal inhibits nitro reduction.