Ligand-enhanced reactive oxidant generation by nanoparticulate zero-valent iron and oxygen

Abstract

The reaction of zero-valent iron or ferrous iron with oxygen produces reactive oxidants capable of oxidizing organic compounds. However, the oxidant yield in the absence of ligands is too low for practical applications. The addition of oxalate, nitrilotriacetic acid (NTA), or ethylenediaminetetraacetic acid (EDTA) to oxygen-containing solutions of nanoparticulate zero-valent iron (nZVI) significantly increases oxidant yield, with yields approaching their theoretical maxima near neutral pH. These ligands improve oxidant production by limiting iron precipitation and by accelerating the rates of key reactions, including ferrous iron oxidation by oxygen and hydrogen peroxide. Product yields indicate that the oxic nZVI system produces hydroxyl radical (OH*) over the entire pH range in the presence of oxalate and NTA. In the presence of EDTA, probe compound oxidation is attributed to OH under acidic conditions and a mixture of OH* and ferryl ion (Fe[IV]) at circumneutral pH.

Figure 1

HCHO yield after exposure of 150 μM nZVI to O₂ in the presence of oxalate, NTA (inset), or EDTA (inset) in 100 mM CH₃OH and 1 mM PIPES (pH 7, t = 60 min). The dashed line indicates the concentration of nZVI added.

Figure 2

Product yield after 60 min for (a) 150 μM nZVI + 10mM oxalate and (b) 150 μM Fe(II) + 10mM oxalate in 100 mM CH₃OH, 100 mM 2-propanol, and 10 mM benzoic acid.

Figure 3

Product yield after 60 min for (a) 150 μM nZVI + 1 mM NTA and (b) 150 μM Fe(II) + 1 mM NTA in 100 mM CH₃OH, 100 mM 2-propanol, and 10 mM benzoic acid.

Figure 4

Product yield after 60 min for (a) 150 μM nZVI + 200 μM EDTA and (b) 150 μM Fe(II) + 200 μM EDTA in 100 mM CH₃OH, 100 mM 2-propanol, and 10 mM benzoic acid.

Figure 5

(a) Fe(II) and (b) filterable Fe in 150 μM nZVI in 100 mM CH₃OH at 60 min. Data for no ligand, 10 mM oxalate, 1 mM NTA, and 200 μM EDTA. Initial pH as indicated.