Reactive oxidant generation by nanoparticulate zero-valent iron: Contaminant oxidation and toxicity

The reaction of nanoparticulate zero-valent iron (nZVI) and oxygen can produce reactive oxidants that are capable of oxidizing organic and inorganic contaminants, but are potentially harmful to biological systems. An understanding of the reactive oxidants produced by the reaction of nZVI with oxygen provides insight into the applicability of the system as an advanced oxidation process and into the potential toxicity of the nanoparticles.;To gain insight into the reaction mechanism and to assess the nature of the oxidant, the oxidation of a series of probe compounds by the reaction of nanoparticulate nZVI or ferrous iron (Fe[II]) with O2 was studied. Under acidic conditions, product formation is attributable to hydroxyl radical (OH·) production through the Fenton reaction, involving hydrogen peroxide and Fe(II) produced during nZVI oxidation. At circumneutral pH, the oxidation of Fe(II) by oxygen is the dominant mechanism of the oxidant production. Product yields at circumneutral pH values are consistent with a different oxidant, such as the ferryl ion (Fe[IV]).;The oxidant yield in the nZVI system in the absence of iron-complexing ligands is too low for practical applications. The addition of ligands such as oxalate, nitrilotriacetic acid (NTA), or ethylenediaminetetracetic acid (EDTA) to oxygen-containing solutions containing nZVI significantly increases oxidant yield. These ligands improve oxidant production by limiting iron precipitation and by accelerating the rates of ferrous iron oxidation by oxygen and hydrogen peroxide. In the presence of oxalate and NTA, OH· is produced over the entire pH range. Probe compound oxidation is attributed to OH· under acidic conditions and a mixture of OH · and Fe(IV) at circumneutral pH in the presence of EDTA.;To identify the mechanism through which nZVI damages cells, a series of experiments were conducted in which nZVI was exposed to oxygen in the presence and absence of human bronchial epithelial cells. Membrane damage and internal reactive oxygen species production in cells exposed to nZVI is equivalent to the response observed when cells are exposed to the same concentration of dissolved Fe(II). The behavior of the oxidant produced by nZVI is consistent with an oxidant such as the ferryl ion, rather than hydroxyl radical.