Assessing the Prevalence of Environmentally Persistent Free Radicals in Soil

“…EPFRs are not confined to combustion-generated particulate matter and are more widely present in the environment than originally suspected”

Environmentally persistent free radicals (EPFRs) represent a human health hazard since they may cause lung damage through oxidative stress. They are produced as a result of interactions between aromatic hydrocarbons and can persist for several decades. Recently, it has been shown that they occur at high concentration in combustion-generated airborne particulate matter¹.

As soil contains the three key components for the formation of EPFRs (the redox-active transition metal, the molecular precursor-organic pollutant, and the silica/clay matrix support), it was hypothesized that soil and sediment could also pose an EPFR risk. In addition, soil is known to accumulate organic contaminants, which can themselves form hazardous free radicals. This is of particular concern since inhaled soil organic matter is known to reduce antioxidant levels in fluid lining the lungs², and so the human risk from EPFRs is heightened.

In the United States, the Environmental Protection Agency initiated the Superfund Program to clean up some of the nation’s most contaminated land. The project aims to remove contaminants from affected sites in order to protect both public health and the environment. With increasing attention on EPFRs and knowledge of their longevity, concerns arose that these so-called Superfund sites may still be a source of EPFRs despite having been cleaned. Increased effort has thus recently been targeted towards evaluating the extent of the EPFR risk at Superfund sites.

Analysis of soil samples collected from a former wood treatment facility that had been contaminated with pentachlorophenol for over 25 years revealed the presence of EPFRs³. It was postulated that these EPFRs must have been formed through interactions between organic compounds trapped in the soil matrix and other inorganic and biological soil components and electron transfer to a soil substrate. It therefore appears that cleaned soils with a history of contamination with hazardous wastes do indeed pose a risk of EPFRs.

A recent study has investigated the extent of EPFRs formation at three additional Superfund sites where the soil had been contaminated with hazardous compounds, such as pentachlorophenol (PCP), polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and polybrominated biphenyl ethers (PBDEs) up to 100 years ago⁴. Soil samples from each site were analysed using a Bruker EMX – 10/2.7 electron paramagnetic resonance (EPR) spectrometer.

EPFR concentrations in samples with a history of contamination were found to be up to thirty times higher than in uncontaminated soil samples⁴. The highest concentration of EPFRs was detected at middle depth soil (between 10 and 20cm) and the bottom layer soil (more than 20–30cm) contained fewest EPFRs. It was also revealed that the contaminated soil samples had a higher carbon content than the uncontaminated soil samples, indicating that additional organic matter (pollutants) was present. Furthermore, there was a strong linear correlation between EPFR concentration and total carbon content. This indicates that soil organic matter may therefore actually encourage the production of reactive oxygen species rather than scavenge them.

The presence of EPFRs in the soil at sites that were contaminated between a decade and a century ago suggests that EPFRs are continually being formed from molecular contaminants in the soil. These findings confirm that elevated EPFR concentrations are not restricted to combustion-generated particulate matter but also occur in contaminated soil and are thus more prevalent in the environment than was originally suspected.

References

1. Yang L, et al Highly Elevated Levels and Particle-Size Distributions of Environmentally Persistent Free Radicals in Haze-Associated Atmosphere. Environ. Sci. Technol. 2017;51(14):7936–7944.
2. Qi S, et al. Damage to lung epithelial cells and lining fluid antioxidant defense by humic acid. Environ Toxic Pharm. 2008;26:96–101.
3. dela Cruz ALN, et al. Detection of environmentally persistent free radicals at a superfund wood treating site. Environ Sci Technol. 2011;45(15):6356–6365.
4. dela Cruz ALN, et al. Assessment of Environmentally Persistent Free Radicals in Soils and Sediments from Three Superfund Sites. Environ Sci Process Impacts 2014;16(1):44–52.