The effect ofpH and mineral mediation on the reactivity of ferrous iron in the reduction of nitroaromatic compounds

This study has extended the knowledge of the redox properties of ferrous iron in the natural environment. The reduction of nitroaromatic contaminants (NACs) is possible using ferrous iron as the reducing agent. This reaction works best in sediments which contain minerals with iron-oxide content or at pH values above 7.25.; A mechanistic model of this reaction has been proposed. Ferrous iron sorbed to surfaces is most reactive when it is bound to an oxygen attached to an iron atom in the crystalline lattice of the mineral. A reactive intermediate is formed by the transfer of an electron from a surface bound ferrous iron. This reactive intermediate is bound to the surface through the formation of a complex with the ferric iron formed in the transfer. This reactive intermediate is immobile and can not obtain a second electron from another surface site. The second electron must come from ferrous iron in solution. Ferrous monohydroxide (FeOH+) is capable of reducing NACs, but the concentration of FeOH+ is too low to be of importance below pH 7.25.; A kinetics model is presented. The sorbed iron is divided into two pools based on sorption data, S1 and S2. The kinetics of this reaction was modeled using three reducing agents; FeOH+, iron sorbed to surface site 1 (S1) and iron sorbed to surface site 2 (S2). This rate expression is: kobs=k *F eOH+ FeOH+* NB* OH-+ k *S1FeS1 *NB *OH- 2+k *S2FeS2 *NB *OH- 2 The intrinsic rate constant for the reaction with FeOH+ was measured directly in the absence of a mediating mineral. The intrinsic rate constants for S1 and S2 are determined by fitting the expression to the observed rate constants.; The sorption of ferrous iron to smectite clays, kaolinite and several oxide minerals is reported over the pH range of 6 to 8. Ferrous iron does not sorb very extensively to kaolinite or the oxide minerals, but does sorb very highly to the smectite clays. There is an increase in sorption with increasing pH, in the smectite clays this increase is larger than can be explained by the pH effect.