Reduction of Substituted Nitrobenzenes by Fe(II) in Aqueous Mineral Suspensions

The kinetics of the reduction of 10 monosubstituted nitrobenzenes (NBs) by Fe(II) has been investigated under various experimental conditions in aqueous suspensions of minerals commonly present in soils and sediments. Aqueous solutions of Fe(II) were unreactive, In suspensions of Fe(III)- containing minerals (magnetite, goethite, and lepidocrocite), Fe(II) readily reduced the NBs to the corresponding anilines in a strongly pH-dependent reaction. Our results suggest that on other mineral surfaces (gamma-aluminum oxide, amorphous silica, titanium dioxide, and kaolinite) iron (hydr)oxide coatings are indispensable to promote the reduction of NBs by adsorbed Fe(II). Apparent pseudo-first-order rate constants, k(obs), were used to describe the initial kinetics of the NE reduction, covering several half-lives of the compounds. The distinct effect of substituents on k(obs) and the observed pronounced competition between different NBs indicate that precursor complex formation as well as the (re)generation of reactive surface sites are rate-determining steps in the overall reduction of the NBs. The results of this study demonstrate that Fe(II) adsorbed on iron (hydr)oxide surfaces or surface coatings may play an important role in the reductive transformation of organic pollutants in subsurface environments. Our findings may also contribute to a better understanding of the various redox processes involved in groundwater remediation techniques based on Fe(0) as the bulk reductant.