Environ. Sci. Technol. 2011, 45, 589–594
Formation and Stabilization of Combustion-Generated Environmentally Persistent Free Radicals on an Fe(III)2O3/Silica Surface ERIC VEJERANO, SLAWOMIR LOMNICKI, AND BARRY DELLINGER* Louisiana State University, Chemistry Department, 232 Choppin Hall, Baton Rouge, Louisiana 70803, United States
Received August 18, 2010. Revised manuscript received November 17, 2010. Accepted November 22, 2010.
Previous studies have shown environmentally persistent free radicals (EPFRs) form when chlorine- and hydroxy-substituted benzenes chemisorb on Cu(II)O-containing surfaces under postcombustion conditions. This paper reports the formation of EPFRs on silica particles containing 5% Fe(III)2O3. The EPFRs are formed by the chemisorption of substituted aromatic molecular adsorbates on the metal cation center followed by electron transfer from the adsorbate to the metal ion at temperatures from 150 to 400 °C. Depending on the nature of the adsorbate and the temperature, two organic EPFRs were formed: a phenoxyl-type radical, which has a lower g-value of 2.0024-2.0040, and a second semiquinone-type radical, with a g-value of 2.0050-2.0065. Yields of EPFRs were ∼10× lower for iron than copper; however, the half-lives of EPFRs on iron ranged from 24 to 111 h, compared to the half-lives on copper of 27 to 74 min. The higher oxidation potential of Fe(III)2O3 is believed to result in greater decomposition of the adsorbate, resulting in the lower EPFR yields, but increased stabilization of the EPFR once formed, resulting in the longer half-lives.
Introduction The correlation between acute and chronic human health impacts and exposure to airborne fine particles (PM2.5) has led to a quest for the origin of their toxicity (1, 2). Production of reactive oxygen species (ROS) can produce immunological and inflammatory responses in cells and is a suspected source of the toxicity of airborne PM2.5 and other sources of particulate matter (PM) (3-5). Transition metals are known to produce ROS and have been implicated in the toxicity of PM (5-7). The bulk iron content in urban atmosphere has been reported as 5-15%, with iron oxides and hydroxides contributing to 10-70% of the bulk iron (8). Iron oxides have been proposed as a catalyst for Fenton reactions, in which soluble Fe2+ generates ROS (9-11). However, iron in PM typically exists as Fe3+ oxides, which is not soluble in water, is not particularly bioavailable, and does not catalyze Fentontype reactions (12). There have been attempts to explain this contradiction via reduction of iron oxides by biological agents to produce soluble Fe2+ ions complexed with ferritin (13). We previously demonstrated various chlorine- and hydroxy-substituted benzenes chemisorb to Cu2+ ions on * Corresponding author e-mail:
[email protected]. 10.1021/es102841s
2011 American Chemical Society
Published on Web 12/07/2010
Cu(II)O-containing particles (used as surrogates for municipal and hazardous waste incinerator fly ash) and form environmentally persistent free radicals (EPFRs) by electron transfer from the adsorbate to the metal ion, which also results in reduction of Cu2+ to Cu+ (14). This process occurred under typical postcombustion conditions at 150-400 °C (15). The electron paramagnetic resonance (EPR) spectral characteristics of the laboratory-generated EPFRs were very similar to those observed in airborne PM2.5 samples and, since combustion-generated particles are a major component of airborne PM2.5, suggested the same process may be responsible for EPFRs in airborne PM2.5 (16, 17). Studies of the biological response of these copper-EPFR complexes indicated they were potent ROS generators, resulting in pulmonary dysfunction in rat models and suggesting they are potential sources of the health impacts of airborne PM2.5 (18). Considering iron is usually the dominant transition metal in airborne PM2.5 as well as emissions from some types of combustion sources, the formation and stabilization of EPFRs involving reaction with iron might contribute more than copper to the total number of the metal-radical complexes in PM. In this manuscript, we report the results of a detailed laboratory study of the formation, structure, and persistence of EPFRs formed from various molecular adsorbates on particles consisting of 5% Fe(III)2O3 on silica. We selected simple phenolic compounds, phenol, catechol, and hydroquinone, which are ubiquitous in combustion sources and chlorinated aromatics, chlorobenzene, 1,2-dichlorobenzene, and 2-monochlorophenol, which are present in combustion sources containing chlorine, such as hazardous waste and municipal waste incinerators (19-23).
Experimental Section Particle surrogate samples (5% Fe(III)2O3 (3.50% Fe) by weight supported on silica) were prepared by impregnation of silica gel with iron(III) nitrate nonahydrate using the method of incipient wetness followed by calcination. Silica gel powder (Sigma-Aldrich, grade 923, 100-200 mesh size) was introduced into a sufficient amount of 0.840 M aqueous solution of the iron(III) nitrate nonahydrate (Sigma-Aldrich, iron(III) nitrate nonahydrate, 99+%) for incipient wetness to occur. The resulting paste was allowed to equilibrate for 24 h at room temperature and dried at 120 °C for 12 h before calcination in air for 5 h at 450 °C. We use surrogate particle samples to create a simple, reproducible, model system where it is possible to study interactions among specific, welldefined components of the fly ash. Combustion-generated fly ash is far more complex, and other components of the fly ash may be involved in EPFR formation and stabilization. The adsorbate chemicals, hydroquinone (HQ, Sigma, 99+%), catechol (CT, Aldrich, 99+%), phenol (PH, Aldrich, 99+%), 2-monochlorophenol (2-MCP, Aldrich, 99+%), monochlorobenzene (MCBz, Aldrich, 99.8% anhydrous), and 1,2-dichlorobenzene (1,2-DCBz, Sigma-Aldrich, 99% HPLC grade), were used as received without further purification. The particulate samples were exposed to the vapors of the adsorbates using a custom-made vacuum exposure system (15). The system consisted of a vacuum gauge, dosing vial port, equilibration chamber, and 2 reactors. Prior to adsorption, each sample was oxidized in air in situ at 450 °C. Vapors of the molecular adsorbates were introduced into the equilibration chamber at the desired pressure (∼10 Torr), and the particles were exposed to the adsorbate vapors at the desired temperature for 5 min at the selected temperature, VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
589
FIGURE 1. Temperature dependence of the first-derivative EPR spectra of 5% Fe(III)2O3/silica particles dosed with various adsorbates. ranging from 150 to 400 °C. Once the exposure was completed, the sample was allowed to cool, and the dosing tube was evacuated for 1 h at 150 °C at 10-2 Torr. The reactor was then sealed under vacuum, and the sample was cooled to room temperature prior to EPR measurements. Each adsorption experiment was performed in at least triplicate, followed by EPR measurement. The results were reproducible, with deviation of radical concentration of
