Reductive Degradation of 4-Chloronitrobenzene by Fe(II) on Aluminum-Substituted Goethite Nanoparticles

Iron oxide minerals play important roles in geo- and environmental chemistry, with substantial impacts on the chemical, physical, and (micro)biological properties of soils and also the behavior, fate, and transport of pollutants. The trivalent aluminum cation is commonly found substituted in natural iron oxides as a result of its abundance in soil and its similarity in both charge and size compared to ferric ion. Here, we examine the impact of aluminum substitution on the reactivity of Fe(II) on the iron oxide mineral goethite toward 4-chloronitrobenzene (4-ClNB), which serves as the oxidant and model pollutant. The impact of aluminum substitution on both the kinetics of contaminant reduction and the concurrent oxidative mineral growth of goethite were quantified. Rate constants were determined using both the Langmuir-Hinshelwood-Haugen-Watson (LHHW) equation and a pseudo-first-order model applied to data obtained from reactors prepared using initial concentrations of 4-ClNB from 10 to 100 μM. LHHW more accurately describes the kinetics of the reduction of 4-ClNB by Fe(II) on goethite nanoparticles and provides insights into the nature of the reactivity of goethite nanocrystals. Results demonstrate that low levels of aluminum substitution (≤4 mol % Al) result in increased reactivity, as quantified by transmission electron microscopy surface-area-normalized pseudo-first-order rate constants, and that higher levels of aluminum substitution (>4 mol % Al) result in a decrease in reactivity. In addition, aluminum substitution does not impact the surface on which the oxidative mineral growth occurs, with goethite nanoparticle lengths increasing without a concurrent increase in width observed over all particle compositions.