Evolving Mineralogy and Reactivity of Hematite-Coated Sands During Reduction of 4-Chloronitrobenzene by Fe(II) in Flow-Through Reactors

Naturally-occurring iron oxide nanoparticles provide reactive surfaces for the reduction of nitroaromatic compounds, which are common groundwater pollutants, by Fe(II). In many natural aquifer systems, iron oxide minerals continuously react with groundwater pollutants and other chemical species. To closely emulate field conditions, continuous flow columns packed with hematite-coated sands were used to study the reduction of 4-chloronitrobenzene (4-ClNB) by Fe(II) associated with the iron oxide. Columns were packed with sands coated with either a high or low mass loading of hematite nanoparticles (0.19 or 0.43 mg hematite per gram of sand after flushing). Following 36 hours of reaction (200-225 pore volumes), the total mass of iron oxide present in the columns increased, resulting from the concurrent Fe(III) oxidative mineral growth. The greatest increase was observed at the bottom of the column packed with the higher hematite mass loading sand. Acicular particles were observed on the post-reaction materials of both the high and low hematite loading sands. The acicular morphology is characteristic of goethite nanoparticles, and the presence of goethite was detected by low temperature magnetometry. Similar to results obtained under batch reactor conditions, goethite crystals heterogeneously nucleated on hematite as a result of the reductive degradation of 4-ClNB by Fe(II). Results tracking the rates of reductive degradation of the 4-ClNB and evolution of mineralogy demonstrate that reactivity is determined by the accessible reactive surface area, which evolves as goethite is deposited on hematite over time.