Insights on the reactivity of environmental iron-containing nanoparticles.

Abstract

Iron oxide minerals play an important role in natural processes including redox reactions and adsorption of trace species. Ferrihydrite (Fe5HO8*4H2O) is a metastable iron oxide that forms particles in the 3-10 nm size range and is believed to be a major player in the environment. The reactivity of ferrihydrite is influenced by the available surface area which is known to increase with decreasing particle size; however, size may also influence structural and/or thermodynamic properties. The size dependence of reductive dissolution was investigated for well-characterized ferrihydrite, with average lengths of 3.4 to 5.9 nm. Characterization and kinetics results revealed there was little or no dependence on size for nanoparticle structure, electronic structure, or redox reactivity of the ferrihydrite samples. In contrast, ferrihydrite nanoparticle size has a significant affect on the kinetics of growth by oriented aggregation to produce goethite nanorods, with rates increasing significantly with decreasing precursor size. First, we demonstrate that ferrihydrite nanoparticle size can be controlled by the hydrolysis temperature during nucleation from a homogeneous solution. Next, we show that the size of goethite nanocrystals formed by aging depends directly on the size of precursor ferrihydrite nanoparticles. Inorganic arsenic species are carcinogenic and due to the affect on human health, high concentrations of arsenic in some groundwater systems have led to the study of arsenic's geochemical cycling. Arsenic has a high adsorption affinity for iron oxide and ferrihydrite materials and may influence the reactivity of such materials upon incorporation. The reductive dissolution kinetics of ferrihydrite prepared by coprecipitation with or adsorption of up to 10 wt% arsenate were quantified. Arsenic adsorbed preferentially onto the most reactive surface sites substantially lowering the reductive dissolution rate. Coprecipitation led to structural defects, but the initial rate of reductive dissolution was similar to pure ferrihydrite. Synthetic zero-valent iron (ZVI) materials have been studied for their application in remediation of contaminated sites with halogenated organic compounds that are resistant to degradation. Incorporation of up to 20 mol% copper into the ZVI materials increased the rates of carbon tetrachloride degradation but led to higher concentrations of chloroform during reaction. In addition, the solid-state products of ZVI oxidation varied substantially. In the end, copper is not an ideal ZVI additive because chloroform is not a preferred product. However, the use of metal additives is a promising method for the degradation of carbon tetrachloride if chloroform production can be minimized.