Characterization and reactivity of iron nanoparticles prepared with added Cu, Pd, and Ni

The association of a secondary metal with iron particles affects redox reactivity in engineered remediation systems. However, the structural characteristics of the metal additives and mechanism responsible for changes in reactivity have not been fully elucidated. Here, we synthesized iron nanoparticles with Cu, Pd, and Ni content ranging from 0-2 mol % via a solution deposition process (SDP), hydrogen reduction process (HRP), or hydrogen reduction of ferrihydrite coprecipitated with the metal cations (HRCO). Results from solid-state characterization show that the synthesis methods produced similar iron core/magnetite shell particles but produced substantial differences in terms of the distribution of the metal additives. In SDP, the metal additives were heterogeneously distributed on the surface of the particles. The metal additives were clearly discernible in TEM images as spherical nanoparticles (5-20 nm) on the HRP and HRCO particles. Because the metals were integral to the synthesis process, we hypothesize that the metal additive is present as solute within the iron core of the HRCO particles. Kinetic batch experiments of carbon tetrachloride (CT) degradation were performed to quantitatively compare the redox reactivity of the particles. Overall, metal additives resulted in enhanced pseudo-first-order rate constants of CT degradation (k_O,CT) compared to that of the iron nanoparticles. For the bimetallic iron nanoparticles prepared by SDP and HRP, k_O,CT increased with the concentration of metal additives. The values of chloroform yield (Y_CF) were independent of the identity and amount of metal additives. However, both k_O,CT and Y_CF of the HRCO iron particles were significantly increased. Results suggest that it is the distribution of the metal additives that most strongly impacts reactivity and product distribution. For example, for materials with ca. 0.9 mol % Ni, reactivity and Y_CF varied substantially (HRCO > SDP > HRP), and HRCO-NiFe resulted in the lowest final chloroform concentration because chloroform was rapidly dechlorinated. In addition, sequential spike experiments for long-term reactivity demonstrated that the presence of the metal additives facilitated reduction by enabling greater utilization of Fe⁰.