A critical review of environmentally persistent free radical (EPFR) solvent extraction methodology and retrieval efficiency
Graphical abstract
Introduction
Environmentally persistent free radicals (EPFR) are long-lived, resonance-stabilized radicals formed in the cool zone of a combustion process (Dellinger et al., 2007). The concept of persistent free radicals was first introduced in the 1950s (Ingram et al., 1954), and the signal of these radicals was detected by Pryor and co-workers in the 1970s (Pryor, 1970; Pryor et al., 1976). Dellinger et al. (2007) proposed a detailed mechanism of EPFR formation in which the substituted aromatic compounds act as molecular precursors to react with transition metal oxides. The process starts with the physisorption of precursors onto the surface of transition metal oxides. Later, chemisorption takes place through elimination of water and/or hydrogen halide molecules, and electrons are then transferred to form surface-associated EPFR (Fig. 1).
Different precursors interact with transition metal oxides to form various EPFR. The structure of the absorbate, together with the properties of the absorption site under certain combustion conditions, determine the structure of the newly-formed EPFR and their different reactivities (Dellinger et al., 2007; Lomnicki et al., 2008). Delocalization of the unpaired electron could lead to production of either a carbon-centered EPFR or an oxygen-centered EPFR.
Some recent studies investigated other mechanisms for EPFR formation on different molecular precursors and under varied reaction conditions. Studies provided evidence for EPFR formation in the presence of metal oxides other than those involving transition metals (Wu et al., 2020a; Thibodeaux et al., 2015; Assaf et al., 2016). Vejerano et al. (2018a) discussed EPFR formation mechanisms on metal oxides and engineered nanomaterials (ENMs) during combustion and found that the molecular precursors and the type of metal oxide present affected concentration and lifetime of EPFR. D'Arienzo et al. (2017) revealed the formation of EPFR with unsubstituted benzene as the precursor. Borrowman et al. (2016a) explored EPFR formation through heterogeneous oxidation of ozone with polycyclic aromatic compounds. Zhu et al. (2019) examined EPFR formation on microplastics under light irradiation. These studies provide insight into different EPFR formation pathways and their environmental impacts.
A growing body of research has examined persistence of EPFR. The lifetimes of EPFR range from hours to years (Gehling and Dellinger, 2013; Chen et al., 2018a, 2019; Yang et al., 2017) and theoretically may be infinite under a vacuum (Lomnicki et al., 2008). In contrast, traditional free radicals such as hydroxyl radicals (•OH) have half-lives on the order of 10−9 s (Pryor, 1986).
The toxicity of EPFR stems from their persistence in the environment coupled with their ability to generate •OH, which may lead to the downstream generation of other reactive oxygen species (ROS) (Kelley et al., 2013) including peroxyl (RO2•) and alkoxyl (RO•) radicals. These ROS could induce oxidative stress in biological systems (Khachatryan et al., 2011; Dellinger et al., 2001; Ayres et al., 2008; Nel et al., 2006). In vitro and in vivo studies of inhalation exposure to EPFR containing particulate matter (PM) identified cardiac (Burn and Varner, 2015; Lord et al., 2011; Mahne et al., 2012; Chuang et al., 2017) and pulmonary (Fahmy et al., 2010; Balakrishna et al., 2011; Filep et al., 2016; Thevenot et al., 2013; Wang et al., 2011) dysfunction and effects on the central nervous system (CNS) (Wang et al., 2017; Allen et al., 2017; Costa et al., 2015; Solaimani et al., 2017; Ljubimova et al., 2018). Saravia et al. (2013) also studied adverse health effects of PM-associated EPFR on infants.
Electron Paramagnetic Resonance (EPR) is a spectroscopic technique used to study samples containing radicals. The g-factor obtained from an EPR spectrum is characteristic of a specific molecular structure (Rich and Wesley, 1972) and thus provides information about the radical. A higher g-factor indicates that the unpaired electron is closer to an oxygen atom. For carbon-centered radicals, such as phenyl radicals, g-factors are typically less than 2.0030 while for oxygen-centered radicals, such as semiquinone radicals, g-factors are typically greater than 2.0040 (Dellinger et al., 2007; Lomnicki et al., 2008; Gehling and Dellinger, 2013; Borrowman et al., 2016b; Dela Cruz et al., 2011). The g-factor of EPFR from an environmental sample is usually in the range of 2.0030–2.0040 (Xu et al., 2019).
There are two main approaches for using EPR to measure the radical signal in a sample potentially containing EPFR. The first method is the direct measurement of solid samples (D'Arienzo et al., 2017; Zhu et al., 2019; Oyana et al., 2017; Wang et al., 2020a) or of filter samples (Gehling and Dellinger, 2013; Xu et al., 2020a, 2020b; Runberg et al., 2020; Guo et al., 2020; Arangio et al., 2016) within a quartz tube inserted into the EPR cavity. For filters, a process of folding or rolling to fit them within the EPR quartz tube may be required. Chen et al. (2018b) proposed a method to clamp a piece of a quartz sheet filter into a flat cell within the EPR cavity to avoid possible changes in sample uniformity caused by folding or rolling. The other approach for EPFR detection by EPR is to extract EPFR from the collection matrix using solvents and then measure radical signals in the extracts or residues (Chen et al., 2018a, 2018b; Yang et al., 2017; Dela Cruz et al., 2011; Wang et al., 2018; Zhao et al., 2019, 2020; Liu et al., 2020; Kiruri et al., 2013; Truong et al., 2010).
EPFR have been detected in PM (Chen et al., 2018a, 2018b; Yang et al., 2017; Wang et al., 2018), ENMs (Vejerano et al., 2018a), soil (Dela Cruz et al., 2011; Liu et al., 2020; Cruz et al., 2012; Jia et al., 2019), soot (Wang et al., 2018; Jia et al., 2020), fly ash (Zhao et al., 2019; Feld-Cook et al., 2017), and biomass (Wu et al., 2020b; Wang et al., 2020b; Shi et al., 2020; Sun et al., 2019; Mosonik et al., 2018). Despite many original studies (Chen et al., 2018a, 2018b; Yang et al., 2017; Dela Cruz et al., 2011; Wang et al., 2018; Zhao et al., 2019, 2020; Liu et al., 2020; Kiruri et al., 2013; Truong et al., 2010) and review articles (Vejerano et al., 2018a; Saravia et al., 2013; Xu et al., 2019; Odinga et al., 2020; Ndirangu et al., 2019; Ruan et al., 2019; Pan et al., 2019; Dugas et al., 2016) focused on EPFR, there has been no systematic method for reporting EPFR extraction methodologies and their impact on retrieval efficiency and radical characteristics. A critical review comparing EPFR solvent extraction methods and their retrieval efficiencies is urgently needed to promote reporting consistency in this emerging field.
In this review, we evaluated studies concerning the possible loss of EPFR signals and changes to the radical characteristics during the extraction processes. We present extraction methodology and retrieval data from reviewed studies. By comparing methods used and extraction recoveries among EPFR studies, we hope to provide some information to assist with selection of EPFR extraction methods. We aim to guide reporting in future EPFR studies with our review.
Section snippets
Methods
The literature search and screening processes were conducted based on the guidelines and the checklist of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) (Moher et al., 2009) (Fig. 2). The broad literature search, modeled after the search conducted by the U.S. Environmental Protection Agency in the Integrated Science Assessment for Particulate Matter (US EPA, 2019), began with an advanced search using the “topic” tag:
((TS = "particulate") OR (TS = "particulates") OR
EPFR analysis
The number of studies regarding EPFR sampling has increased in the past decade. Among the 120 references identified since 2007, over half of the papers were published between 2016 and 2021. Among the 94 papers meeting one of our criteria of involving field or laboratory studies, 29 studied EPFR measured in environmental samples, and 65 examined surrogate samples synthesized or contaminated in a laboratory. The details of these studies were summarized in Table S1.
EPFR solvent extraction
Ten papers met our criteria for
Synthesis of findings
The g-factor change after solvent extraction provides information about the influence of extraction on the radical structure for different types of samples. Among the ten studies (Chen et al., 2018a, 2018b; Yang et al., 2017; Dela Cruz et al., 2011; Wang et al., 2018; Zhao et al., 2019, 2020; Liu et al., 2020; Kiruri et al., 2013; Truong et al., 2010), four (Chen et al., 2018a, 2018b; Zhao et al., 2019; Kiruri et al., 2013) of them reported the g-factor of samples before and after the
Conclusions and perspectives
EPFR are an emerging contaminant that has received increased research attention in recent years. EPFR can be generated during combustion processes, and they are believed to be related to certain adverse respiratory and cardiovascular effects due to their ability to generate reactive oxygen species.
The solvent extractability of EPFR depends on the chemical composition of the sample and the solvent type. We have discovered two main findings in this review. First, the polar solvent seems to have a
Funding source
This work was supported by the NIEHS Superfund Research Program (P42 ES013648).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References (73)
- et al.
Rapid determination of environmentally persistent free radicals (EPFRs) in atmospheric particles with a quartz sheet-based approach using electron paramagnetic resonance (EPR) spectroscopy
Atmos. Environ.
(2018) - et al.
Characteristics of environmentally persistent free radicals in PM2.5: concentrations, species and sources in Xi’an, northwestern China
Environ. Pollut.
(2019) - et al.
Formation and stabilization of persistent free radicals
Proc. Combust. Inst.
(2007) - et al.
Formation of environmentally persistent free radicals during the transformation of anthracene in different soils: roles of soil characteristics and ambient conditions
J. Hazard Mater.
(2019) - et al.
Occurrence, formation, environmental fate and risks of environmentally persistent free radicals in biochars
Environ. Int.
(2020) - et al.
Environmentally persistent free radicals: occurrence, formation mechanisms and implications
Environ. Pollut.
(2019) - et al.
Formation, characteristics, and applications of environmentally persistent free radicals in biochars: a review
Bioresour. Technol.
(2019) - et al.
Stability of environmentally persistent free radicals (EPFR) in atmospheric particulate matter and combustion particles
Atmos. Environ.
(2020) - et al.
Evolution and stabilization of environmental persistent free radicals during the decomposition of lignin by laccase
Chemosphere
(2020) - et al.
Probing environmentally significant surface radicals: crystallographic and temperature dependent adsorption of phenol on ZnO
Chem. Phys. Lett.
(2015)
Levels, spatial distribution, and source identification of airborne environmentally persistent free radicals from tree leaves
Environ. Pollut.
Adsorption behavior of Cr(VI) by magnetically modified enteromorpha prolifera based biochar and the toxicity analysis
J. Hazard Mater.
A density functional theory calculation for revealing environmentally persistent free radicals generated on PbO particulate
Chemosphere
Mediation of rhodamine B photodegradation by biochar
Chemosphere
Assessment of personal exposure to environmentally persistent free radicals in airborne particulate matter
J. Hazard Mater.
Risk evaluation of environmentally persistent free radicals in airborne particulate matter and influence of atmospheric factors
Ecotoxicol. Environ. Saf.
Developmental neurotoxicity of inhaled ambient ultrafine particle air pollution: parallels with neuropathological and behavioral features of autism and other neurodevelopmental disorders
Neurotoxicology
Quantification of environmentally persistent free radicals and reactive oxygen species in atmospheric aerosol particles
Atmos. Chem. Phys.
Formation of environmentally persistent free radicals on α-Al2O3
Environ. Sci. Technol.
Evaluating the toxicity of airborne particulate matter and nanoparticles by measuring oxidative stress potential - a workshop report and consensus statement
Inhal. Toxicol.
Environmentally persistent free radicals induce airway hyperresponsiveness in neonatal rat lungs
Part. Fibre Toxicol.
Formation of environmentally persistent free radicals from the heterogeneous reaction of ozone and polycyclic aromatic compounds
Phys. Chem. Chem. Phys.
Formation of environmentally persistent free radicals from the heterogeneous reaction of ozone and polycyclic aromatic compounds
Phys. Chem. Chem. Phys.
Environmentally persistent free radicals compromise left ventricular function during ischemia/reperfusion injury
Am. J. Physiol. Heart Circ. Physiol.
Dominant fraction of EPFRs from nonsolvent-extractable organic matter in fine particulates over Xi’an, China
Environ. Sci. Technol.
Dominant fraction of EPFRs from nonsolvent-extractable organic matter in fine particulates over Xi’an, China
Environ. Sci. Technol.
Environmentally persistent free radicals cause apoptosis in HL-1 cardiomyocytes
Cardiovasc. Toxicol.
Neurotoxicity of traffic-related air pollution
Neurotox. traffic-related air Pollut
Effect of low temperature thermal treatment on soils contaminated with pentachlorophenol and environmentally persistent free radicals
Environ. Sci. Technol.
Detection of environmentally persistent free radicals at a Superfund wood treating site
Environ. Sci. Technol.
Role of free radicals in the toxicity of airborne fine particulate matter
Chem. Res. Toxicol.
Addressing emerging risks: scientific and regulatory challenges associated with environmentally persistent free radicals
Int. J. Environ. Res. Publ. Health
Experimental and theoretical investigation on the catalytic generation of environmentally persistent free radicals from benzene
J. Phys. Chem. C
In vitro and in vivo assessment of pulmonary risk associated with exposure to combustion generated fine particles
Environ. Toxicol. Pharmacol.
Effect of particulate matter mineral composition on environmentally persistent free radical (EPFR) formation
Environ. Sci. Technol.
Exposure to urban PM1 in rats: development of bronchial inflammation and airway hyperresponsiveness
Respir. Res.
Cited by (12)
Scientific and regulatory challenges of environmentally persistent free radicals: From formation theory to risk prevention strategies
2023, Journal of Hazardous MaterialsProgress of catalytic oxidation of typical chlorined volatile organic compounds (CVOCs): A review
2023, Science of the Total EnvironmentMolecular characteristics of microalgal extracellular polymeric substances were different among phyla and correlated with the extracellular persistent free radicals
2023, Science of the Total EnvironmentCitation Excerpt :Given the high intensity of PFRs produced by Cyanophyta, the level of PFRs in eutrophic lakes and reservoirs predominated by Cyanophyta may be considerably high. Other organisms in the water column, such as bacteria and zooplankton are bound to be stressed by elevated level of PFRs (Guo and Richmond-Bryant, 2021). Although PFRs can break down some persistent organic pollutants, the presence of substantial volumes of natural DOM in water bodies (Li and Hur, 2017) can stymie the decomposition of persistent organic pollutants.
Unexpected catalytic influence of atmospheric pollutants on the formation of environmentally persistent free radicals
2022, ChemosphereCitation Excerpt :In addition to metals, inorganic, and molecular organic components, relatively newly identified pollutants, environmentally persistent free radicals (EPFRs) (Dellinger et al., 2007), also exist in significant mass concentration in PM (Chen et al., 2019; Sun et al., 2019; Wang et al., 2019, 2020a). Based on the g-factor, which is derived from electron paramagnetic resonance (EPR) measurement (Guo and Richmond-Bryant, 2021), EPFRs are generally divided into three types: semiquinone, phenoxy, and cyclopentadienyl radicals (Xu et al., 2019). Of these EPFR types, a huge amount of phenoxy-type EPFRs produced by phenolic precursors have been found abundant in haze events (Pan et al., 2019b).