Environmentally persistent free radicals in PM2.5: a review

Environmental Persistent Free Radicals in highly polluted soils and the association with polycyclic aromatic compounds

Environmental persistent free radicals (EPFRs), as emerging contaminants in environment, can induce oxidative stress causing severe adverse health outcomes. The formation of EPFRs is thought to be associated with the transformation of aromatic compounds like polycyclic aromatic hydrocarbons (PAHs). Herein this study firstly evaluated EPFRs in industrial soils being highly polluted by PAHs, and explored its associated with PAHs, with the modification of soil organic matter content. Soil EPFRs from two industrial plants were 4.1 × 1016 and 4.5 × 1016 spins/g, respectively, that were significantly higher than the levels in the surrounding areas. Carbon-centered EPFRs account for approximately 80% inside the plant, but outside the plants, nearly 50-70% of EPFRs were carbon-centered with adjacent heteroatoms. As one important precursor of EPFRs, PAHs exhibited a significantly positive correlation with EPFRs in industrial soils (p < 0.05), explaining 40%-60% of the variation in EPFRs concentration in the present study. The relationship between soil organic matter and EPFRs concentration normalized by PAHs forms an inverted V-shape, suggesting an inhibition effect of soil organic matter on the EPFR formation potentials from PAHs, that is worthy to be further examed in future laboratory and field experiments.

Tracking Long-Lived Free Radicals in Dandelion Caused by Air Pollution Using Electron Paramagnetic Resonance Spectroscopy

Studies on particulate air pollution indicate that a new type of pollutant should be considered from mainly fossil fuel combustion and automobile exhaust emissions, i.e., environmentally persistent free radicals. These radicals, ubiquitous in the environment, have a long life span and are capable of producing harmful reactive oxygen species. Samples of dandelion were collected in 2020 and 2021 in spring and late summer. Roots, leaves, flower stalks, and inflorescences of Taraxacum sp. were collected from six sites with three plants each, along with monitoring of particulate matter air pollution. Four sites were located at streets with heavy traffic and two were control sites in the rural part of the city. The free radical content in each part of the plant was measured by electron paramagnetic resonance. The leaf was selected as the most appropriate part of the plant for the measurement of carbon-derived free radicals. The geff value and the total number of spins were calculated. Relationships were found between location, season, and measurements. The electron paramagnetic resonance spectrum consists of at least two components, which can be attributed to C-type radicals and mixed C + O radicals. Their increase in numbers in the fall seasons, compared to the spring seasons, is also noticeable. It has also been observed that leaves collected in autumn have a higher geff value, which is probably related to the higher amount of oxygen- and carbon-derived free radicals.

Residential Wood Burning and Vehicle Emissions as Major Sources of Environmentally Persistent Free Radicals in Fairbanks, Alaska

Environmentally persistent free radicals (EPFRs) play an important role in aerosol effects on air quality and public health, but their atmospheric abundance and sources are poorly understood. We measured EPFRs contained in PM2.5 collected in Fairbanks, Alaska, in winter 2022. We find that EPFR concentrations were enhanced during surface-based inversion and correlate strongly with incomplete combustion markers, including carbon monoxide and elemental carbon (R2 > 0.75). EPFRs exhibit moderately good correlations with PAHs, biomass burning organic aerosols, and potassium (R2 > 0.4). We also observe strong correlations of EPFRs with hydrocarbon-like organic aerosols, Fe and Ti (R2 > 0.6), and single-particle mass spectrometry measurements reveal internal mixing of PAHs, with potassium and iron. These results suggest that residential wood burning and vehicle tailpipes are major sources of EPFRs and nontailpipe emissions, such as brake wear and road dust, may contribute to the stabilization of EPFRs. Exposure to the observed EPFR concentrations (18 ± 12 pmol m-3) would be equivalent to smoking ∼0.4-1 cigarette daily. Very strong correlations (R2 > 0.8) of EPFR with hydroxyl radical formation in surrogate lung fluid indicate that exposure to EPFRs may induce oxidative stress in the human respiratory tract.

The Effect of α-Fe2O3(0001) Surface Containing Hydroxyl Radicals and Ozone on the Formation Mechanism of Environmentally Persistent Free Radicals

The formation of environmentally persistent free radicals (EPFRs) is mediated by the particulate matter's surface, especially transition metal oxide surfaces. In the context of current atmospheric complex pollution, various atmospheric components, such as key atmospheric oxidants ·OH and O3, are often absorbed on particulate matter surfaces, forming particulate matter surfaces containing ·OH and O3. This, in turn, influences EPFRs formation. Here, density functional theory (DFT) calculations were used to explore the formation mechanism of EPFRs by C6H5OH on α-Fe2O3(0001) surface containing the ·OH and O3, and compare it with that on clean surface. The results show that, compared to EPFRs formation with an energy barrier on a clean surface, EPFRs can be rapidly formed through a barrierless process on these surfaces. Moreover, during the hydrogen abstraction mechanism leading to EPFRs formation, the hydrogen acceptor shifts from a surface O atom on a clean surface to an O atom of ·OH or O₃ on these surfaces. However, the detailed hydrogen abstraction process differs on surfaces containing oxidants: on surfaces containing ·OH, it occurs directly through a one-step mechanism, while, on surfaces containing O3, it occurs through a two-step mechanism. But, in both types of surfaces, the essence of this promotional effect mainly lies in increasing the electron transfer amounts during the reaction process. This research provides new insights into EPFRs formation on particle surfaces within the context of atmospheric composite pollution.

Ecotoxicity of three typical tire wear particles to periphytic biofilms: The potentiating role after natural water-incubation-aging

Tire wear particles (TWPs), abundant in the aquatic environment, pose potential ecological risks, yet their implications have not been extensively studied. Rolling friction TWPs, sliding friction TWPs (S-TWPs) and cryogenically milled tire treads were used as research objects to study the ecotoxicity and difference of the above materials before and after aging in natural water (AS-TWPs) to the periphytic biofilm. The results showed that there were significant differences in the microstructure, surface elements, size, functional groups and environmentally persistent free radicals (EPFRs) of the three TWPs. After aging in natural water, the properties of the three TWPs mentioned above showed homogenization, but the EPFRs and reactive oxygen species (ROS) yield were different. After exposure to TWPs (10 mg L-1), total organic carbon and adenosine triphosphate decreased significantly (p < 0.05), and the production of extracellular polymeric substances (EPS) in the periphytic biofilm increased, in which the content of humic-like substance and proteins (tryptophan protein and humic acid-like substances) increased obviously. The increment of TB-EPS was higher than that of LB-EPS, and S-TWPs and AS-TWPs had the strongest promoting effect on EPS secretion. In addition, 10 mg L-1 TWPs caused massive cell death in the periphytic biofilm, which was more obvious in the S-TWPs and AS-TWPs exposure group. The toxic mechanism of TWPs promotes intracellular ROS accumulation and leads to the release of lactate dehydrogenase, which was attributed to the formation of EPFRs on the surface of TWPs and an increase in EPFRs intensity after aging in natural water. TWPs at environmentally relevant concentrations (0.1 mg L-1) had no biological toxicity to periphytic biofilms. This study fills the gap in the study of the surface structure characteristics of TWPs on the toxicity of periphytic biofilms, and is of great significance to the study of the aquatic toxicity mechanism of TWPs.

New Features of Laboratory-Generated EPFRs from 1,2-Dichlorobenzene (DCB) and 2-Monochlorophenol (MCP)

The present research is primarily focused on investigating the characteristics of environmentally persistent free radicals (EPFRs) generated from commonly recognized aromatic precursors, namely, 1,2-dichlorobenzene (DCB) and 2-monochlorophenol (MCP), within controlled laboratory conditions at a temperature of 230 °C, termed as DCB230 and MCP230 EPFRs, respectively. An intriguing observation has emerged during the creation of EPFRs from MCP and DCB utilizing a catalyst 5% CuO/SiO2, which was prepared through various methods. A previously proposed mechanism, advanced by Dellinger and colleagues (a conventional model), postulated a positive correlation between the degree of hydroxylation on the catalyst's surface (higher hydroxylated, HH and less hydroxylated, LH) and the anticipated EPFR yields. In the present study, this correlation was specifically confirmed for the DCB precursor. Particularly, it was observed that increasing the degree of hydroxylation at the catalyst's surface resulted in a greater yield of EPFRs for DCB230. The unexpected finding was the indifferent behavior of MCP230 EPFRs to the surface morphology of the catalyst, i.e., no matter whether copper oxide nanoparticles are distributed densely, sparsely, or completely agglomerated. The yields of MCP230 EPFRs remained consistent regardless of the catalyst type or preparation protocol. Although current experimental results confirm the early model for the generation of DCB EPFRs (i.e., the higher the hydroxylation is, the higher the yield of EPFRs), it is of utmost importance to closely explore the heterogeneous alternative mechanism(s) responsible for generating MCP230 EPFRs, which may run parallel to the conventional model. In this study, detailed spectral analysis was conducted using the EPR technique to examine the nature of DCB230 EPFRs and the aging phenomenon of DCB230 EPFRs while they exist as surface-bound o-semiquinone radicals (o-SQ) on copper sites. Various aspects concerning bound radicals were explored, including the hydrogen-bonding tendencies of o-semiquinone (o-SQ) radicals, the potential reversibility of hydroxylation processes occurring on the catalyst's surface, and the analysis of selected EPR spectra using EasySpin MATLAB. Furthermore, alternative routes for EPFR generation were thoroughly discussed and compared with the conventional model.

Formation Mechanism of Environmentally Persistent Free Radicals on Alkaline Earth Oxide Surfaces

The formation of environmentally persistent free radicals (EPFRs) is usually related to transition-metal oxides in particulate matter (PM). However, recent studies suggest that alkaline-earth-metal oxides (AEMOs) in PM also influence EPFRs formation, but the exact mechanism remains unclear. Here, density functional theory calculations were performed to investigate the formation mechanism of EPFRs by C6H5OH on AEMO (MgO, CaO, and BaO) surfaces and compare it with that on transition-metal oxide (ZnO and CuO) surfaces. Results indicate that EPFRs can be rapidly formed on AEMOs by dissociative adsorption of C6H5OH, accompanied by electrons transfer. As the alkalinity of AEMOs increases, both adsorption energy and the number of electron transfers gradually increase. Also, the stability of the formed EPFRs is mainly attributed to the electrostatic and van der Waals interactions between the phenoxy radical and surfaces. Notably, the formation mechanism of EPFRs on AEMOs is similar to that on ZnO but differs from that on CuO, as suggested through geometric structure and charge distribution analyses. This study not only elucidates the formation mechanisms of EPFRs on AEMOs but also provides theoretical insights into addressing EPFRs pollution.

Oxidative potential, environmentally persistent free radicals and reactive oxygen species of size-resolved ambient particles near highways

Particulate matter (PM) is a group of atmospheric pollutants with an uncertain toxicity, particularly when collected near highways. This study examined the oxidative potential (OP) of, as well as the environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS) present in PM samples collected near highways in Xiamen, China. Our findings revealed that PM had a relatively high OP, ranging from 3.8 to 18.5 nmol/min/μg, surpassing values reported in previous research. The oxidative potential of the water-insoluble fraction (OPWIS), which accounted for 68% of the total oxidative potential (OPTotal), demonstrated rapid toxicity, whereas the oxidative potential of the water-soluble fraction (OPWS) displayed a steadier toxicity release pattern. The primary free radicals detected in PM were oxygen-centered. The measured concentration of EPFRs was 6.073 × 1014 spins/m3, which is lower than that reported in previous studies, possibly because of the high relative humidity of the road environment in Xiamen. We also investigated the interaction between PM and water near highways and observed the generation of R and OH radicals. Additionally, we analysed the sample composition and evaluated the contributions of the different components to OPTotal. Transition metals (Fe, Cu, and Zn) were identified as the major contributors, accounting for 33.2% of the OPTotal. The positive correlation observed between EPFRs and ROS suggests that EPFRs may be involved in ROS generation. The correlation analysis indicated that the oxidative potential measured using the DTT method (OPDTT) could serve as an indicator of ROS generation. Finally, based on the relationship between OPDTT, EPFRs, and ROS, we propose that reducing the emission of transition metals, particularly Fe, represents an effective control measure for mitigating PM toxicity near highways.

A short review on environmental distribution and toxicity of the environmentally persistent free radicals

Environmentally Persistent Free Radicals (EPFRs) are usually generated by the electron transfer of a certain radical precursor on the surface of a carrier. They are characterized with high activity, wide migration range, and relatively long half-life period. In this review, we summarized the literature on EPFRs published since 2010, including their environmental occurrence and potential cytotoxicity and biotoxicity. The EPFRs in the atmosphere are the most abundant in the environment, mainly generated from the combustion of raw materials or biochar, and the C-center types (quinones, semiquinones radicals, etc.) may exist for a relatively long time. These EPFRs can transform into other substances (such as reactive oxygen species, ROS) under the influence of environmental factors, and partly enter soil and water by wet and dry deposition of particulate matter, which may promote the generation of EPFRs in those media. The wide distribution of EPFRs in the environment may lead to their exposure to biota including humans, resulting in cytotoxicity and biotoxicity. The EPFRs can influence the normal redox process of the biota, and generate a large number of free radicals like ROS. Exposure to EPFRs may change the expression of gene and activity of metabolic enzymes, and damage the cells, as well as some organs such as the lung, trachea, and heart. However, due to the difficulty in sample extraction, identification, and quantification of the specific EPFR individuals, the toxicity and exposure evaluation of biota are still limited which merits study in the future.

Size-resolved environmentally persistent free radicals in urban road dust and association with transition metals

Environmental persistent free radicals (EPFRs) are receiving growing concerns owing to their potentially adverse impacts on human health. Road dust is one important source of air pollution in most cities and may pose significant health risks. Characteristics of EPFRs in urban road dusts and its formation mechanism(s) are still rarely studied. Here, we evaluated occurrence and size distributions of EPFRs in road dusts from different functional areas of an urban city, and assessed relationship between EPFRs and some transition metals. Strong electron paramagnetic resonance signals of 6.01 × 1016 - 1.3 × 1019 spins/g with the mean g value of 2.0029 ± 0.0019 were observed, indicating that EPFRs consisted of a mixture of C-centered radicals, and C-centered radicals with an adjacent oxygen atom in the urban road dust. Much more EPFRs enriched in finer dust particles. EPFRs significantly correlated with the total Fe, but not water-soluble Fe, suggesting different impacts of water-soluble and insoluble metals in the formation of EFPRs. Health risk assessment results indicated high risk potentials via the ingestion and dermal exposure to EPFRs in road dusts. Future studies are calling to look into formation mechanisms of EPFRs in urban road dusts and to quantitatively evaluate its potential risks on human health.

Leaves are a Source of Biogenic Persistent Free Radicals

Nonsenescent and senescent leaves of selected coniferous and broadleaf plants contained substantial levels of naturally occurring persistent free radicals (PFRs). These biogenic PFRs (BPFRs) were stable and persistent despite multiple wetting and drying cycles, implying that BPFRs can leach and sorb on soil particles. Results suggest that endogenous chemicals in plants and their transformation byproducts can stabilize unpaired electrons in leaves under ambient conditions. Thus, the vast amount and perpetual supply of leaf litter is an unaccounted natural source of BPFRs. If toxic, inhaling and accidentally ingesting fine soil dust and powder from degraded leaf litter may increase our environmental and health burdens to PFRs. We expect that this finding will generate more studies on natural sources of PFRs, establish their properties, and distinguish them from those formed from combustion and thermal processes.

Tire wear particles in different water environments: occurrence, behavior, and biological effects-a review and perspectives

As an important source of microplastics, the water ecological risk of tire wear particles (TWPs) has attracted widespread attention worldwide. However, the occurrence and behavior of TWPs and their biological effects in water environments have not been clearly analyzed. For example, most contemporary studies have focused on the evaluation of the aquatic toxicity of TWPs leachate, and little attention has been paid to the behavior process and potential risks of its surface properties in water environments. In addition, most studies rely on preparing TWPs under laboratory conditions or purchasing commercial TWPs for studying their water environmental behavior or exposure. These obviously cannot meet the requirements of accurate assessment of water ecological risks of TWPs. As thus, in addition to describing the occurrence, distribution, and (aging) transformation of TWPs in different water environments, we further tried to explain the potential water environment behavior process and multiple pathways leading to potential adverse impacts of TWPs on aquatic organisms from the perspectives of particle self-toxicity and release toxicity, as well as synergistic effects of TWPs and other substances are also discussed. The existing data, such as studies on the self-characteristics of TWPs, environmental factors, and subjects, are insufficient to comprehensively evaluate the recent changes in essential water ecosystem services and multifunctions caused by TWPs, implying that the impact of TWPs on water environmental health needs to be further evaluated, and the corresponding countermeasures should be recommended. In this context, the current review provides an outlook on future research on TWPs in aquatic environments.

Comparative 6+studies of environmentally persistent free radicals on nano-sized coal dusts

Coal dust is the major hazardous pollutant in the coal mining environment. Recently environmentally persistent free radicals (EPFRs) were identified as one of the key characteristics which could impart toxicity to the particulates released into the environment. The present study used Electron Paramagnetic Resonance (EPR) spectroscopy to analyze the characteristics of EPFRs present in different types of nano-size coal dust. Further, it analyzed the stability of the free radicals in the respirable nano-size coal dust and compared their characteristics in terms of EPR parameters (spin counts and g-values). It was found that free radicals in coal are remarkably stable (can remain intact for several months). Also, Most of the EPFRs in the coal dust particles are either oxygenated carbon centered or a mixture of carbon and oxygen-centered free radicals. EPFRs concentration in the coal dust was found to be proportional to the carbon content of coal. The characteristic g-values were found to be inversely related to the carbon content of coal dust. The spin concentrations in the lignite coal dust were between 3.819 and 7.089 μmol/g, whereas the g-values ranged from 2.00352 to 2.00363. The spin concentrations in the bituminous coal dust were between 11.614 and 25.562 μmol/g, whereas the g-values ranged from 2.00295 to 2.00319. The characteristics of EPFRs present in coal dust identified by this study are similar to the EPFRs, which were found in other environmental pollutants such as combustion-generated particulates, PM_2.5, indoor dust, wildfires, biochar, haze etc., in some of the previous studies. Considering the toxicity analysis of environmental particulates containing EPFRs similar to those identified in the present study, it can be confidently hypothesized that the EPFRs in the coal dust might play a major role in modulating the coal dust toxicity. Hence, it is recommended that future studies should analyze the role of EPFR-loaded coal dust in mediating the inhalation toxicity of coal dust.

The generation of environmentally persistent free radicals on photoaged microbeads from cosmetics enhances the toxicity via oxidative stress

Microbeads used in personal care products have been one of the important sources of microplastics (MPs), and little has been reported on their environmental behaviors and health risks. The characteristics of environmentally persistent free radicals (EPFRs) and the toxicity assessment of MPs (environmentally relevant concentrations) from cosmetics during photoaging remains largely unknown. In this study, the formation of EPFRs on polyethylene (PE) microbeads from facial scrubs under light irradiation and their toxicity were investigated using C. elegans as a model organism. The results suggested that light irradiation induced the generation of EPFRs, which accelerates the aging process and alters the physicochemical properties of PE microbeads. Acute exposure to PE (1 mg/L) at photoaged times of 45-60 d significantly decreased the physiological indicators (e.g., head thrashes, body bends, and brood size). The oxidative stress response and stress-related gene expression were also enhanced in nematodes. The addition of N-acetyl-l-cysteine induced significant inhibition of toxicity and oxidative stress in nematodes exposed to 45-60 d of photoaged PE. The Pearson correlation results showed that the concentration of EPFRs was significantly correlated with physiological indicators, oxidative stress, and related-genes expression in nematodes. The data confirmed that the generation of EPFRs combined with heavy metals and organics contributed to toxicity induced by photoaged PE, and oxidative stress might be involved in regulating adverse effects in C. elegans. The study provides new insight into the potential risks of microbeads released into the environment during photoaging. The findings also highlight the necessity for considering the role of EPFRs formation in evaluating the impacts of microbeads.

Effects of tire wear particles with and without photoaging on anaerobic biofilm sulfide production in sewers and related mechanisms

Tire wear particles (TWPs) are considered to be one of the major sources of microplastics (MPs) in sewers; however, little has been reported on the surface properties and photochemical behavior of TWPs, especially in terms of their environmental persistent radicals, leachate type, and response after photoaging. It is also unknown how TWPs influence the production of common pollutants (e.g., sulfides) in anaerobic biofilms in sewers. In our study, the effects of cryogenically milled tire treads (C-TWPs) and their corresponding photoaging products (photoaging-TWPs, A-TWPs) on anaerobic biofilm sulfide production in sewers and related mechanisms were studied. The results showed that the two TWPs at a low concentration (0.1 mg L^-1) exerted no significant (p > 0.05) effects on sulfide yield, whereas exposure to a high concentration of TWPs (100 mg L^-1) inversely affected sulfide yield, with A-TWPs exerting a significant inhibitory effect on sulfide yield in the sewers (p < 0.01). The main reason was that A-TWPs carried higher concentrations of reactive environmental persistent radicals on their surfaces after photoaging than C-TWPs, which could induce the formation of oxygen radicals. In addition, A-TWPs were more uniformly distributed in the wastewater system and could penetrate the biofilm to damage bacterial cells, and their ability to leach polycyclic aromatic hydrocarbons and heavy metals such as zinc additives enhanced their toxic effects. In contrast, C-TWPs contributed significantly to sulfide production (p < 0.01), primarily because of their low biotoxicity, ability to leach a considerable amount of sulfide, and stimulatory effect on anaerobic biofilm surface sulfate-reducing bacteria. Our study complements the toxicity studies of the TWPs particles themselves and provides insight on a new influencing factor for determining the changes in sulfide generation and control measures in sewers.

Unexpected catalytic influence of atmospheric pollutants on the formation of environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) have been recognized as harmful and persistent environmental pollutants. In polluted regions, many acidic and basic atmospheric pollutants, which are present at high concentrations, may influence the extent of the formation of EPFRs. In the present paper, density functional theory (DFT) and ab-initio molecular dynamics (AIMD) calculations were performed to investigate the formation mechanisms of EPFRs with the influence of the acidic pollutants sulfuric acid (SA), nitric acid (NA), organic acid (OA), and the basic pollutants, ammonia (A), dimethylamine (DMA) on α-Al₂O₃ (0001) surface. Results indicate that both acidic and basic pollutants can enhance the formation of EPFRs by acting as "bridge" or "semi-bridge" roles by proceeding via a barrierless process. Acidic pollutants enhance the formation of EPFRs by first transferring its hydrogen atom to the α-Al₂O₃ surface and subsequently reacting with phenol to form an EPFR. In contrast, basic pollutants enhance the formation of EPFRs by first abstracting a hydrogen atom from phenol to form a phenoxy EPFR and eventually interacting with the α-Al₂O₃ surface. These new mechanistic insights will inform in understanding the abundant EPFRs in polluted regions with high mass concentrations of acidic and basic pollutants.

Metal-Free Biomass-Derived Environmentally Persistent Free Radicals (Bio-EPFRs) from Lignin Pyrolysis

To assess contribution of the radicals formed from biomass burning, our recent findings toward the formation of resonantly stabilized persistent radicals from hydrolytic lignin pyrolysis in a metal-free environment are presented in detail. Such radicals have particularly been identified during fast pyrolysis of lignin dispersed into the gas phase in a flow reactor. The trapped radicals were analyzed by X-band electron paramagnetic resonance (EPR) and high-frequency (HF) EPR spectroscopy. To conceptualize available data, the metal-free biogenic bulky stable radicals with extended conjugated backbones are suggested to categorize as a new type of metal-free environmentally persistent free radicals (EPFRs) (bio-EPFRs). They can be originated not only from lignin/biomass pyrolysis but also during various thermal processes in combustion reactors and media, including tobacco smoke, anthropogenic sources and wildfires (forest/bushfires), and so on. The persistency of bio-EPFRs from lignin gas-phase pyrolysis was outlined with the evaluated lifetime of two groups of radicals being 33 and 143 h, respectively. The experimental results from pyrolysis of coniferyl alcohol as a model compound of lignin in the same fast flow reactor, along with our detailed potential energy surface analyses using high-level DFT and ab initio methods toward decomposition of a few other model compounds reported earlier, provide a mechanistic view on the formation of C- and O-centered radicals during lignin gas-phase pyrolysis. The preliminary measurements using HF-EPR spectroscopy also support the existence of O-centered radicals in the radical mixtures from pyrolysis of lignin possessing a high g value (2.0048).

Real-World Emission Characteristics of Environmentally Persistent Free Radicals in PM2.5 from Residential Solid Fuel Combustion

Environmentally persistent free radicals (EPFRs) can induce reactive oxygen species, causing adverse health impacts, and residential fuel (biomass and coal) combustion is believed to be an important emission source for EPFRs; however, the residential emission characteristics of EPFRs are rarely studied in the real world. Here, we conducted a field campaign evaluating the presence and characteristics of EPFRs generated from residential biomass and coal burning in rural China. The emission factors (EFs) of EPFRs (with units of 10²⁰ spins·kg^-1) in PM_2.5 from the combustion of crop residues (3.97 ± 0.47) were significantly higher than those from firewood (2.06 ± 0.19) and coal (2.13 ± 0.33) (p < 0.05). The EPFRs from residential solid fuel combustion were carbon-centered free radicals adjacent to oxygen atoms. The fuel type was a primary factor controlling EPFR discharge, explaining 68% of the variation in EPFR EFs. The emissions from biomass burning had higher EPFRs per particle than those from coal combustion. EPFRs had stronger relationships with carbonaceous components than with other incomplete combustion products. The EPFRs from biomass burning were mostly generated during the pyrolysis of fuels, while the EPFRs generated from coal combustion were mainly associated with refractory organic compounds. This study provides valuable information for evaluating the fates of EPFRs, promoting a better understanding of the health impacts of air pollution.

Spatial distribution, pollution characterization, and risk assessment of environmentally persistent free radicals in urban road dust from central China

Environmentally persistent free radicals (EPFRs) have aroused widespread concern due to their potential adverse health effects. Research on EPFRs in road dust is still very limited. In this study, 86 road dust samples were collected using vacuum sampling in a rapidly developing city in central China. The pollution characterization and health risk of EPFRs in the urban road dust were then systematically analyzed. The results showed the average concentrations of EPFRs in urban road dust and fraction of particle with aerodynamic diameters lower than 10 μm (PM₁₀) were 2.24 × 10¹⁷ to 3.72 × 10¹⁹ spins·g^-1 and 6.02 × 10¹⁷ to 1.41 × 10²⁰ spins g^-1, respectively. The concentrations of EPFRs in dust from expressways, arterial roads, and secondary trunk roads were significantly higher than those found in the remaining road types. The g-factors of 2.0032-2.0039 indicated that the EPFRs have consisted of oxygen-centered and carbon-centered radicals or carbon-centered radicals with nearby oxygen or halogen atoms. Moreover, three decay patterns of EPFRs were observed: a fast decay followed by a slow decay, a single slow decay, and the slowest decay. In addition, a comparative evaluation was made for probabilistic risk assessments of exposure to the EPFRs in road dust and the PM₁₀ fraction. Compared with road dust, the probability of the number of equivalent cigarettes to exceed the 100 and 200 cigarettes for inhaling EPFRs in the PM₁₀ fraction increased by 27.0% and 25.0%, respectively. The simulation results showed the PM₁₀ fraction were primarily deposited in the upper respiratory tract regions (57.1%) and pulmonary regions (28.8%). The findings of this study suggest a potential risk of EPFRs in inhalable particles and provide a new insight for further exploration of the EPFRs in fine particles of road dust.

Electrospun Nanofiber Membranes for Air Filtration: A Review

Nanomaterials for air filtration have been studied by researchers for decades. Owing to the advantages of high porosity, small pore size, and good connectivity, nanofiber membranes prepared by electrospinning technology have been considered as an outstanding air-filter candidate. To satisfy the requirements of material functionalization, electrospinning can provide a simple and efficient one-step process to fabricate the complex structures of functional nanofibers such as core-sheath structures, Janus structures, and other multilayered structures. Additionally, as a nanoparticle carrier, electrospun nanofibers can easily achieve antibacterial properties, flame-retardant properties, and the adsorption properties of volatile gases, etc. These simple and effective approaches have benefited from the significate development of electrospun nanofibers for air-filtration applications. In this review, the research progress on electrospun nanofibers as air filters in recent years is summarized. The fabrication methods, filtration performances, advantages, and disadvantages of single-polymer nanofibers, multipolymer composite nanofibers, and nanoparticle-doped hybrid nanofibers are investigated. Finally, the basic principles of air filtration are concluded upon and prospects for the application of complex-structured nanofibers in the field of air filtration are proposed.

Effect of long-term low concentrations of TiO2 nanoparticles on dewaterability of activated sludge and the relevant mechanism: the role of nanoparticle aging

Nanoparticles can undergo aging phenomena in sewage treatment systems, which alter their physical and chemical properties. However, the effect of aged nanoparticles on the dewatering performance of activated sludge under long-term low concentrations is yet to be reported in sewage treatment systems. Here, we compared the chronic effects of pristine and aged TiO₂ nanoparticles on the sludge dewatering index, which includes specific resistance to filtration (SRF) and bound water (BW) in a sequencing batch reactor (SBR) at μg/L concentration levels, and analyzed the relevant mechanisms. The results indicated that aging in the sludge supernatant altered the photosensitivity and water stability of nanoparticles, which was mainly due to the changes in the zeta potential and energy band of the particle and was ultimately attributed to the combined effect of particle surface inclusions such as organic matter and inorganic salt. At 10 μg/L, nanoparticles reduced the sludge dewaterability, which observed an improvement at 100 μg/L. This is because 10 μg/L promoted the secretion of extracellular polymeric substances (EPS), which regulated the structure of sludge flora and increased the abundance of secreted quorum sensing-acyl-homoserine lactones (QS-AHL) and EPS genera, while the corresponding exposure results for 100 μg/L were the opposite, owing to the damage and necrosis effects caused by exposure under long-term light, which reduced EPS production and increased sludge density. Interestingly, aging could alleviate the effects of two exposure concentrations on sludge dewatering, mainly because of the decrease in the photoactivity of the nanoparticles. The results of this study show that environmental aging could delay, but not reverse the results of exposure to specific concentrations of nanoparticles. However, the significantly different ecological effects of photosensitive nanoparticles with two environmentally relevant concentration should be refined and confirmed again in freshwater environments to provide a basis for subsequent scientific management and control of photosensitive nanoparticles.

A critical review of environmentally persistent free radical (EPFR) solvent extraction methodology and retrieval efficiency

Long-lived environmentally persistent free radical (EPFR) exposures have been shown in toxicology studies to lead to respiratory and cardiovascular effects, which were thought to be due to the persistence of EPFR and their ability to produce reactive oxygen species. To characterize EPFR exposure and resulting health impacts, it is necessary to identify and systematize analysis protocols. Both direct measurement and solvent extraction methods have been applied to analyze environmental samples containing EPFR. The use of different protocols and solvents in EPFR analyses makes it difficult to compare results among studies. In this work, we reviewed EPFR studies that involved solvent extraction and carefully reported the details of the extraction methodology and retrieval recovery. EPFR recovery depends on the structure of the radical species and the solvent. For the limited number of studies available for review, the polar solvents had superior recovery in more studies. Radicals appeared to be more oxygen-centered following extraction for fly ash and particulate matter (PM) samples. Different solvent extraction methods to retrieve EPFR may produce molecular products during the extraction, thus potentially changing the sample toxicity. The number of studies reporting detailed methodologies is limited, and data in these studies were not consistently reported. Thus, inference about the solvent and protocol that leads to the highest EPFR extraction efficiency for certain types of radicals is not currently possible. Based on our review, we proposed reporting criteria to be included for future EPFR studies.