QSAR study of the reduction of nitroaromatics by Fe(II) species

The development of predictive models for the reductive transformation of nitroaromatics requires further clarification of the effect of environmentally relevant variables on reaction kinetics and the identification of readily available molecular descriptors for calculating reactivity. Toward these goals, studies were performed on the reduction of a series of monosubstituted nitrobenzenes in Fe(II)-treated goethite suspensions. The energy of the lowest unoccupied molecular orbital, ELUMO (B3LYP/6-31G*,water), of the nitrobenzenes was capable of explaining 99% of the variability in the rates. Results of experiments in which the surface area loading of ferric oxides was systematically varied indicate that (i) the reactivity of mineral-surface-associated Fe(II), Fe(II)surf, toward the reduction of p-cyanonitrobenzene (CNNB) decreased in the order hematite > goethite > lepidocrocite > ferrihydrite and (ii) the surface density of Fe(II)surf did not play a crucial role in determining the observed reactivity trend. CNNB was reduced in Fe(II)-only control experiments in a pH range of 7.28-7.97 with a pH dependency consistent with the transformation of Fe(II) to Fe(OH)3 or related oxides. The pH dependency of the reduction of CNNB in Fe(II)-treated ferric oxide suspensions (pH 6.1-7.97) could be accounted for by the oxidation of Fe(II)surf, forming an Fe(III) oxide.