Environmentally Persistent Free Radicals: Insights on a New Class of Pollutants

Abstract

Environmentally persistent free radicals, EPFRs, exist in significant concentration in atmospheric particulate matter (PM). EPFRs are primarily emitted from combustion and thermal processing of organic materials, in which the organic combustion byproducts interact with transition metal-containing particles to form a free radical-particle pollutant. While the existence of persistent free radicals in combustion has been known for over half-a-century, only recently that their presence in environmental matrices and health effects have started significant research, but still in its infancy. Most of the experimental studies conducted to understand the origin and nature of EPFRs have focused primarily on nanoparticles that are supported on a larger micrometer-sized particle that mimics incidental nanoparticles formed during combustion. Less is known on the extent by which EPFRs may form on engineered nanomaterials (ENMs) during combustion or thermal treatment. In this critical and timely review, we summarize important findings on EPFRs and discuss their potential to form on pristine ENMs as a new research direction. ENMs may form EPFRs that may differ in type and concentration compared to nanoparticles that are supported on larger particles. The lack of basic data and fundamental knowledge about the interaction of combustion byproducts with ENMs under high-temperature and oxidative conditions present an unknown environmental and health burden. Studying the extent of ENMs on catalyzing EPFRs is important to address the hazards of atmospheric PM fully from these emerging environmental contaminants.

Figure 1.

Zone theory of combustion. Recombination of atoms or small molecules in the flame zone forms hydrocarbons (HCs), chlorinated hydrocarbons (CHCs), and brominated hydrocarbons (BHCs) in the post flame zone as the temperature decreases. Surface-mediated reactions catalyzed by transition metals at temperature lower than 600 °C form polychlorinated dibenzofurans (PCDFs), polychlorinated dibenzo-p-dioxins (PCDDs), and polycyclic aromatic hydrocarbons (nitro-PAHs and oxy-PAHs). Adapted from Cormier et al., 2006.

Figure 2.

Conceptual mechanism of EPFR formation on a Cu(II)O surface.

Figure 3.

(A) Concentration of EPFRs formed on various transition metal oxide surfaces. EPFR concentration in airborne PM is in the order of 10¹⁶ spins/g (1 spin equals 1 free radical). EPFR concentration for Al(III)₂O₃ is unavailable. (B) Relaxation times of EPFRs formed on different metal oxide surfaces range from ~0.4 h to 2.5 months. We define relaxation time as the time required for ~37% of the EPFR concentration to decrease. Values used in the graphs were obtained from refs , , , , and .

Figure 4.

Type of free radicals responsible for the long lifetimes in PM. Phenoxyl radicals (I and II) are more persistent than o- and p-semiquinone radicals (III and IV). Adapted from Lomnicki et al., 2008 with permission from the American Chemical Society. Copyright 2008.

Figure 5.

EPFRs are converted to molecular species by several reaction pathways, including, chlorination, hydrogen abstraction that converts them to their initial structure, and recombination reactions that produce dibenzofurans, dibenzo-p-dioxins, and biphenyl ethers, or biphenyls. For clarity, we omitted surface representation for structures I, II, and V. These free radicals are formed on surfaces as labeled. Organic structures that are formed on surfaces are labeled with “surfaces”, otherwise, the species are formed in solutions. Adapted from Truong et al., 2010 with permission from the American Chemical Society. Copyright 2010.

Figure 6.

A semiquinone-type radical (C) reduces molecular oxygen to form a superoxide anion radical by catalytic cycling. Superoxide anion radical can dismutate to hydrogen peroxide and produce hydroxyl radical via exogeneous Fenton/Fenton-type reactions. Adapted from Khachatryan et al., 2011 with permission from the American Chemical Society. Copyright 2011.

Figure 7.

A surface-bound chlorophenoxyl EPFR reduces molecular oxygen to superoxide, which dismutates to hydrogen peroxide and produce hydroxyl radical via exogeneous Fenton/Fenton-type reactions.

Figure 8.

Band bending of a donor molecule for small and large particles. CB and VB are the conduction band and the valence band, respectively. V_BBS and V_BBL refer to the magnitude of band bending for small and large particles, respectively. A small particle has a lower magnitude of V_BB whereas a larger particle has a higher magnitude V_BB. Adapted from Albery, 1984 and Zhang and Yates, 2012 with permission from the American Chemical Society. Copyright 1984 and 2012.

Figure 9.

Hypothesized mechanism of the formation of dibenzofuran from recombination of a surface-bound free radical on a silver oxide surface. Adapted from Vejerano et al., 2013 with permission from the American Chemical Society. Copyright 2013.