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Article

Study on Non-Metal-Induced EPFRs in PM2.5 Generated from Flue Gas of Cellulose Combustion

by
Lixin Zhang
*,
Boru An
,
Jingmin Chen
,
Yuwei Zhang
and
Guojiao Yu
School of Environmental Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(1), 301; https://doi.org/10.3390/su17010301
Submission received: 9 December 2024 / Revised: 31 December 2024 / Accepted: 1 January 2025 / Published: 3 January 2025
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Abstract

:
Environmental persistent free radicals (EPFRs) are a type of environmental risk substances existing in atmospheric particulate matter, which pose a challenge to human survival and sustainable development. The current understanding is that the formation mechanism of EPFRs is generally related to metallic materials. However, this study analyzed the PM2.5 generated from cellulose combustion and found that EPFRs could be generated even without the metallic materials. Therefore, this paper explores the emission characteristics of non-metal-induced EPFRs, aiming to reveal the influencing factors, distribution, and decay characteristics of non-metal-induced EPFRs generated from cellulose combustion. The results show that combustion conditions such as combustion temperature and oxygen concentration have a significant impact on the emission concentration of non-metal-induced EPFRs in PM2.5 from cellulose combustion. The emission concentrations of non-metal-induced EPFRs in PM2.5 are at the order of magnitude of 1014 spins/m3 and over 50% is distributed in the inextricable substances. Their g-factor are in the range from 2.0015 to 2.0022, indicating that these EPFRs are carbon-centered radicals. Furthermore, non-metal-induced EPFRs in PM2.5 from cellulose combustion have a half-life of several years or even longer, which exhibit distinct characteristics different from metal-induced EPFRs.

1. Introduction

Haze is one of the main environmental problems in the process of modernization development in China. Atmospheric particulate matter (PM2.5) is one of the important factors contributing to the formation of haze, and the health problems it causes are a highly concerning hot topic. In recent years, research has found that the health risks caused by a type of environmental risk substances in atmospheric particulate matter-environmental persistent free radicals (EPFRs) cannot be ignored [1,2,3]. They are widely present in the atmosphere [4,5,6], soil [7,8,9], and even water environments [10]. Compared with the traditional OH, O2− and other short-lived free radicals (with a lifetime of milliseconds), EPFRs have a longer life, which can be minutes, days, and even months [11]. This means that EPFRs can migrate with atmospheric particulate matter over long distances, and can cause long-term lasting damage to human health after entering the human body through respiration. Studies have shown that EPFRs can induce the production of active free radicals such as OH, O2−, which can aggravate the oxidative stress reaction of the biological system and cause cell and DNA damage [12,13]. Compared with pure particulate matter or original organic compounds, EPFRs are more likely to cause lung and cardiovascular diseases and are one of the causes of cancer [2,3,14]. Therefore, EPFRs have become one of the new hotspots in the study of atmospheric pollution in recent years.
Rapidly in recent years, many scientific problems need to be studied in depth and systematically. One of the key scientific problems to be solved is the chemical nature and formation mechanism of EPFRs in atmospheric particulate matter. The study of this problem is the basis for the relevant research of EPFRs in atmospheric particulate matter, and it is of great significance for a comprehensive understanding of the environmental chemical behavior and toxicological characteristics of EPFRs in atmospheric particulate matter.
Fuel combustion is one of the main sources of atmospheric particulate matter. Studies have found that EPFRs exist in the particles generated by tobacco, biomass, coal and gas/diesel combustion [15,16], particles and residues generated by various plastic combustion [17], and fly ash from garbage incineration [18]. Therefore, EPFRs in atmospheric particulate matter are closely related to fuel combustion. The widely recognized EPFRs’ formation mechanism in the combustion process is that EPFRs are formed by electron transfer between aromatic compounds generated by thermal decomposition of fuel and transition metal oxides in particulate matter [19,20,21,22].
Specifically, in the low-temperature region of combustion (<600 °C), at first the precursor molecules containing aromatic compounds are adsorbed on the surface of particles containing transition metals through physical adsorption; then, chemical adsorption occurs between precursor molecules and transition metals contained in the particles, strong chemical bonds are formed by removing H2O or HCl molecules between them [21]; finally, through electron transfer, the electrons of precursor molecules are transferred to transition metals, thus forming EPFRs.
In this study, the formation of EPFRs based on the mechanism described above is defined as metal-induced EPFRs. The formation mechanism determines that the formation of metal-induced EPFRs is strongly correlated with transition metal elements. Our recent study on EPFRs in actual atmospheric particles in Xi’an found that EPFRs generated based on the metal-induced mechanism cannot completely explain EPFRs in actual atmospheric particles in Xi’an [23,24]. On the contrary, 88 ± 10% of EPFRs in actual atmospheric particles in Xi’an are contributed by OC3 and OC4 substances. Their g-factor (a characteristic parameter for judging the type of EPFRs) and decay characteristics (affected by the type of EPFRs) are significantly different from metal-induced EPFRs. Furthermore, correlation analysis shows that the formation of such EPFRs is weakly correlated with metal elements [23]. Therefore, it can be speculated that there should be other EPFR formation mechanisms in addition to metal-induced ones in the combustion process.
Preliminary, experiments were carried out, the PM2.5 generated by the combustion of cellulose (the main component of biomass, without metal elements) was analyzed, and an expected existence of EPFRs was observed. The experimental results directly proved that without the transition metal element, the EPFRs could still be generated in the combustion process, in other words, there were other mechanisms for the generation of EPFRs in addition to metal induction in the combustion process. Here, we defined the EPFRs generated by such a non-metal inducted mechanism as non-metal-induced EPFRs. Based on the study of Chen et al. on the actual atmospheric particulate matter PM2.5 in Xi’an [23], we deduce that the main contributor of EPFRs in actual atmospheric particulate matter is very likely formed through the non-metal-inducted mechanism (accounting for 88 ± 10% of the total EPFRs in atmospheric particulate matter). Therefore, the systematic study of the chemical nature and generation mechanism of non-metal-induced EPFRs is crucial to the comprehensive understanding of the environmental chemical behavior of EPFRs in atmospheric particulate matter and the evaluation of its toxicological characteristics.
At present, there are few related studies on non-metal-induced EPFRs. Therefore, in this study, the emission characteristics of non-metal-induced EPFRs in PM2.5 generated by cellulose combustion under different experimental conditions were investigated to reveal its main influencing factors and the distribution characteristics. Furthermore, the decay characteristics of non-metal-induced EPFRs were studied to provide theoretical support for the comprehensive understanding of the environmental chemical behavior of EPFRs.

2. Materials and Methods

2.1. Instruments and Reagents

The instruments and reagents used in this study are shown in Table 1.

2.2. Sample Preparation and Collection Methods

The experimental system used for sampling PM2.5 from the combustion gas of the cellulose is shown in Figure 1. It consists of a gas supply system, a reaction system, a dilution system for the exhaust gas, and a sampling system. The gas supply system consists of an air compressor, a venting valve, a tertiary filter, and two flow meters. The reaction system comprises a quartz tube reactor with a diameter of 50 mm placed in a tubular furnace and quartz boats placed inside the quartz tube. Specific information about the apparatus can be found in the Supplementary Materials.
A brief description of the experimental procedure is as follows: to simulate different combustion conditions, the flow rates of the air to the gas supply system and the dilution system for the exhaust gas were adjusted. When the tube furnace reached the preset temperature, 5.00 g of cellulose was weighed into a quartz boat. The sample was burned for 30 min at the designated temperature, followed by an additional 30 min of sampling after the combustion time ended, totaling 60 min of sampling, and the sampling flow rate was set at 16.7 L/min.
In this study, the specific setting of combustion temperature is referred to the study of Dellinger et al. [19]. The details of the sampling conditions can be found in Table 2, where “Yab”, a is used to represent the composition of the air, and b is used to represent the combustion temperature. When the value of a is 1, the condition is pyrolysis. When the value of a is 0.5, the condition is combustion.

2.3. Sample Solvent Extraction Treatment Method

In this study, the collected PM2.5 samples were extracted. The extraction object was the whole PM2.5 filter membrane. Firstly, ultrapure water was used for extraction, 10 mL each time, and after three times extraction, a total of 30 mL of water-soluble substance (WSS) was collected; then, organic solvents (methanol, dichloromethane and n-hexane) were extracted in sequence, respectively, with each solvent being extracted three times, 10 mL each time, and about 90 mL of water-insoluble substance (WISS) being collected; the rest of the substances remaining on the quartz membrane due were insoluble (IS).

2.4. OC/EC Analysis

Using the steel punch provided with the OCEC instrument, quartz membranes attached with PM2.5 were cut into 8 mm circular pieces. After pre-treatment, the samples were analyzed using the OC/EC analyzer, employing the thermal-optical transmission method for quantitative analysis of carbonaceous components.

2.5. Environmental Persistent Free Radical Detection

The EPR (Electron Paramagnetic Resonance) parameters were set as follows: magnetic field strength of 3360 Gs, detection time of 25 s, modulation amplitude of 1 Gs, and one detection cycle, the microwave power was set at 2.0 mW. The obtained EPR signal spectrum was processed and analyzed using Origin software (https://www.originlab.com/). The signal was first smoothed to remove noise, followed by baseline correction and Gaussian function fitting to obtain characteristic values such as g-factor (behavior of electrons in a magnetic field) and ΔHP-P (Magnetic field intensity difference of signal in EPR spectrum). Simultaneously, the EPR software Xenon (https://www.bruker.com/en/products-and-solutions/mr/epr-instruments/epr-software/xenon.html, accessed on 31 December 2024) was used for quantitative analysis of the standard substance DPPH. The calculation formula for the concentration of EPFRs (spins/m3) in the sample is as follows:
C EPFRs =   C dpph × S EPFRs S dpph
In the formula:
Cdpph—Standard DPPH total spin quantity, which is 2.12 × 1014 (spins);
SEPFRs—Integral area of the sample EPFRs absorption curve (A.U);
Sdpph—Integral area of the standard DPPH’s EPR absorption curve.

2.6. Evaluation of the Decay Period of Environmentally Persistent Free Radicals

In order to investigate the decay characteristics of non-metal-induced EPFRs under natural conditions, the EPR tests were performed on collected PM2.5 samples on days 0, 1, 3, 5, 7, 20, 40, and 70 after the initial collection. The half-life of EPFRs in the samples was calculated according to formulas, and the decay period was evaluated.
l n C / C 0 =   − kt
t 1 / e = 1 / kt
In the formula:
C0—Initial concentration of EPFRs (t = 0);
C—EPFRs concentration after decay;
t—Decay time;
t1/e—Half-life;
k—Decay rate constant, obtained from the slope of the logarithm of the ratio of EPFRs concentration (C/C0) to time.

3. Results

3.1. Emission Characteristics of Non-Metal-Induced EPFRs in PM2.5 Generated from Cellulose Combustion

The EPR spectrum of the PM2.5 samples generated from cellulose combustion is shown in Figure 2. It can be seen from Figure 2 that there are certain differences in the signal intensities of PM2.5 samples under different experimental conditions. In addition, the total concentration of non-metal-induced EPFRs is on the order of 1014 (2.89 × 1014 spins/m3–4.8 × 1014 spins/m3) under the current conditions in this study, which is similar to the concentration of EPFRs in the atmosphere of Xi’an from 2018 to 2021 (4.33 × 1014 spins/m3–5.28 × 1014 spins/m3) [25]. The g-factor of non-metal-induced EPFRs generated from cellulose combustion is in the range from 2.0015 to 2.0022, which is less than 2.0030. These values are typical for carbon-centered radicals. In contrast, many studies have found that metal-induced EPFRs, which are formed through electron transfer between aromatic compounds generated by the thermal decomposition of fuel and transition metal oxides in particulate matter and which typically have g-factors ranging from 2.0029 to 2.0067 [21,22,26,27], represent oxygen-centered radicals and radicals influenced by heteroatoms near the carbon center. Therefore, the type of radicals for non-metal-induced EPFRs in PM2.5 generated from cellulose combustion is quite different from that of metal-induced EPFRs.
Based on Yen’s study on EPFRs in coal/tar, some free radicals or unpaired electrons can be stabilized in the large condensed aromatic structures of coal/tar under spatial hindrance effects, leading to the formation of EPFRs [28]. Additionally, Senesi et al. found that aromatic ring systems in humic substances can form conjugated systems to stabilize free radicals [29]. Further studies by Watanabe revealed that the higher the carbon content in the aromatic ring system, the more developed the conjugated system, which is beneficial for stabilizing free radicals [30]. Therefore, we speculate that similar spatial hindrance effects or conjugated systems exist in PM2.5 from cellulose combustion, stabilizing some free radicals generated during combustion and forming EPFRs. This leads to the formation of non-metal-induced EPFRs. The probable formation mechanism of non-metal-induced EPFRs is shown in Figure 3.
Further analysis was conducted on the carbonaceous components of PM2.5 samples in this study. The results are shown in Figure 4: The proportion of organic carbon (OC) exceeds 90%. Among them, the proportion of OC1 ranks first, and the rest are OC2, OC3, EC1, OC4, EC2, and EC3 in sequence. In addition, it can be found that the proportion of the OC1 component in PM2.5 samples generated by cellulose combustion under the conditions of 500 °C and 700 °C is less than that in PM2.5 samples under the condition of 300 °C. These components are generally organic components that are physically adsorbed on the surface of particles or bound by weaker chemical bonds. As for the OC2 component, the proportion of this component in PM2.5 samples generated under the conditions of 500 °C and 700 °C is greater than that of the OC1 component in PM2.5 samples under the condition of 300 °C. Such substances may cover medium-molecular-weight organic polymers, long-chain hydrocarbons, and so on. For the OC3 component, it exhibits a proportion rule similar to that of the OC2 component. This component includes aromatic compounds, polycyclic aromatic hydrocarbons, etc., and it is also similar to the rule of non-metallic-induced environmentally persistent free radicals (EPFRs) generated at different temperatures mentioned in Section 3.2 below. The distribution rule of the OC3 component further corroborates the potential mechanism of the generation of non-metallic-induced EPFRs proposed above.

3.2. Influencing Factors of Non-Metal-Induced EPFRs in PM2.5 Produced by Cellulose Combustion

The emission characteristics of non-metal-induced EPFRs in PM2.5 from cellulose combustion are influenced by combustion conditions. As shown in Figure 5, the total concentration of non-metal-induced EPFRs in PM2.5 samples prepared under different combustion temperatures shows a trend of first increasing and then decreasing with increasing combustion temperature. The reason for this may be that, at first, due to the low combustion temperature (300 °C), cellulose combustion is incomplete. Then, as the combustion temperature increases to a moderate temperature (500 °C), it is conducive to generating more large condensed aromatic structures through pyrolysis and partial oxidation reactions during the combustion of cellulose, thus creating spatial hindrance effects or conjugated systems to form non-metal-induced EPFRs. Finally, when the combustion temperature increases to a higher temperature (700 °C), cellulose is fully burned, and the large condensed aromatic compounds are completely oxidized and decomposed, leading to a decrease in EPFRs’ concentration with increasing combustion temperature.
Yang et al. found that the EPR signal of biochar prepared by pyrolysis of pine wood at 500 °C was higher than that of biochar prepared at 300 °C, while when the pyrolysis temperature increased to 700 °C, the EPR signal was significantly weakened [31]. In addition, Guo et al. investigated the effect of temperature on the generation of EPFRs in the CT-SiO2 system and obtained similar results: in the low-temperature range (<400 °C), the concentration of EPFRs increased with the combustion temperature, but with further increase in temperature, the concentration of EPFRs showed a decreasing trend [32]. These findings indicate that the concentration of non-metal-induced EPFRs generated under different combustion temperatures in this study follows the same variation trend as that in metal and metal/non-metal coexisting systems.
The gas atmosphere is another important factor that influences the combustion of cellulose. In this study, under various combustion temperature conditions, the concentration of EPFRs in PM2.5 samples generated from cellulose combustion with an air ratio of 1 was significantly higher than under the condition of an air ratio of 0.5. This means that the increase in oxygen content during cellulose combustion favors the formation of non-metal-induced EPFRs, which is quite different from the study by Maskos and Dellinger [33]. In their study of the generation of EPFRs in the pyrolysis process of tobacco, when in the atmosphere of oxygen, the carbon-centered radicals will combine with oxygen to form oxygen-centered radicals. However, this study did not detect oxygen-centered radicals. The specific reasons for this difference remain to be further explored.
The g-factor is commonly used to characterize the types of free radicals. As shown in Table 3, the g-factor range of EPFRs in PM2.5 generated from cellulose combustion under different experimental conditions is between 2.0015 and 2.0022, which is less than 2.0030, indicating carbon-centered radicals such as aromatic radicals. This suggests that under the current experimental conditions of this study, variations in combustion conditions such as combustion temperature and air ratio do not significantly affect the types of non-metal-induced EPFRs.

3.3. Distribution Characteristics of Non-Metal-Induced EPFRs in Different Extracted Fractions of PM2.5 Samples

According to Figure 6, the pattern of distribution of non-metal-induced EPFRs in different extracted fractions of PM2.5 samples collected under different combustion conditions is very similar. The overall distribution presents as follows: Inextricable substance (63.67 ± 1.17%) > Water-insoluble substance (28.57 ± 1.67%) > Water-soluble substances (7.737 ± 1.43%). There are significant differences in the concentrations of non-metal-induced EPFRs in different extracted fractions of PM2.5 samples.
The concentrations of non-metal-induced EPFRs in inextricable substances account for more than half of that in the original sample. These inextricable substances may be produced by incomplete combustion of cellulose, forming low-volatility and high-molecular-weight polycyclic aromatic hydrocarbons, asphaltenes, and graphite-like substances. These substances are resistant to solvents and cannot be extracted by solvents [34,35]. As mentioned above, the presence of these substances is conducive to the formation of non-metal-induced EPFRs, providing further evidence for the formation mechanism of non-metal-induced EPFRs.
According to Figure 7, the g-factors of EPFRs in the original PM2.5 samples and their solvent-extracted fractions collected under different combustion conditions show no significant difference, ranging from 2.0015 to 2.0023, which is less than 2.0030, indicating carbon-centered radicals. This result contrasts significantly with the g-factor ranges from 2.0028 to 2.0033 and 2.0032 to 2.0038 for EPFRs in PM2.5 samples from the Xi’an and Wanzhou regions as reported [23,36]. It is speculated that non-metal-induced EPFRs are not primary contributors to that actual atmospheric particulate matter. Although EPFRs exhibit relatively persistent stability, their resonance structure and lifespan are still influenced by atmospheric environmental factors. Dellinger et al. have found that carbon-centered radicals are generally more reactive than oxygen-centered radicals, and the presence of oxygen in the atmosphere may preferentially react with carbon-centered EPFRs, leading to an increase in the g-factor [19].

3.4. Decay Characteristics of Non-Metal-Induced EPFRs Generated from Cellulose Combustion

Figure 8 shows the decay characteristics of non-metal-induced EPFRs in PM2.5 generated from cellulose combustion under different combustion conditions within 70 days. For the original PM2.5 samples, two decay modes were observed: one with rapid decay followed by slow decay (33% of samples), and the other with rapid decay followed by slow decay with no further decay (67% of samples). The corresponding t1/e for rapid decay, slow decay, and no decay were 18 ± 6 days, 291 ± 89 days, and 1800 ± 189 days. Yang et al. [26] studied the effects of metal oxides on the types and life of EPFR and found that the half-lives of surface free radicals on Al2O3, ZnO, CuO, and NiO were 108 days, 68 days, 81 days, and 86 days. In conclusion, the decay mode of non-metal-induced EPFRs is significantly different from that of EPFRs formed by metal oxides. Metal-induced EPFRs exhibit a very high decay rate, with reported half-lives ranging from hours to months [22,26,27].
During the first 7 days, all samples exhibited rapid decay, with an average decay of 34 ± 8.0% of the original EPFRs content. From day 7 to day 40, the samples experienced a slow decay, with an average decay of 7.5 ± 7.41% of the original EPFR content. After 40 days, the decay could be considered negligible. It was observed that the decay during the first 7 days was more signific. A comparison of the decay of EPFRs in PM2.5 samples over the first 7 days revealed that the EPFRs’ concentrations in PM2.5 samples from combustion at different temperatures decreased by 40.4% (700 °C), 34.1% (500 °C), and 26% (300 °C) when the air ratio was 1. A similar trend was observed when the air ratio was 0.5. Therefore, the higher combustion temperatures resulted in a more pronounced decay of non-metal-induced EPFRs in PM2.5 samples. Furthermore, the EPFRs’ concentrations in the Y1 series samples decayed on an average of 33.5%, while those in the Y0.5 series samples decayed on an average of 34.2%, indicating that the air ratio did not have a significant impact on the decay of EPFRs.
The decay analysis of EPFRs in solvent-extracted components of PM2.5 samples revealed five decay modes: only rapid decay (33% of samples); rapid decay followed by slow decay or no decay (11% of samples); rapid decay followed by slow decay (17% of samples); slow decay followed by no decay (11% of samples); and only slow decay (28% of samples). The corresponding values t1/e for rapid decay, slow decay, and no decay were 17 ± 18 days, 213 ± 174 days, and 1716 ± 607 days.
The decay of WSS in the samples follows a rapid decay mode, and the signal intensity detected on the 7th day is extremely weak, with concentrations close to zero, indicating that the decay is essentially complete. The decay modes of WISS follow three modes and are relatively complex, differing significantly from the decay modes of their respective original samples. For the IS portion, the decay modes in samples generated under different conditions are divided into two categories: slow decay and rapid decay followed by slow decay.
As shown in Figure 9, in this study the g-factor of different types of non-metal-induced EPFRs in the original and extracted samples ranges from 2.0014 to 2.0024, all lower than 2.0030, consistently indicating carbon-centered radicals. This suggests that the types of EPFRs remain relatively stable during the decay process, showing no significant variations.

4. Conclusions

In this study, the PM2.5 have collected emissions from cellulose combustion under different combustion conditions (temperature, air ratio), and the concentration and g-factor of EPFRs in PM2.5 were analyzed to investigate the influence of combustion conditions on the emission characteristics of non-metal-induced EPFRs in PM2.5 from cellulose combustion, and finally, the decay characteristics of non-metal-induced EPFRs were examined. The specific conclusions are as follows:
  • In this study, under current experimental conditions, temperature and the air ratio have a certain influence on the concentration of non-metal-induced EPFRs in PM2.5 generated from the combustion of cellulose. Among them, the concentration of non-metal-induced EPFRs in the sample is the highest at 500 °C and when there is an air ratio of 1. In addition, temperature and the air ratio have no significant effect on the types of non-metal-induced EPFRs, which are all carbon-centered radicals.
  • A large quantity of non-metal-induced EPFRs are generated in PM2.5 emissions from cellulose combustion, with concentrations at the level of 1014 spins/m3, similar to the EPFRs concentration in PM2.5 from the atmosphere of Xi’an during the period from 2018 to 2021. The non-metal-induced EPFRs in this study are important contributors to EPFRs in the actual atmosphere. In addition, extraction of PM2.5 samples generated from cellulose combustion reveals that non-metal-induced EPFRs are mainly concentrated in inextricable substances. Their g-factors are in the range of 2.0015~2.0023, which differs significantly from the g-factor range from 2.0028 to 2.0033 for EPFRs in actual atmospheric particles before and after extraction. It is speculated that non-metal-induced EPFRs are not primary contributors to actual atmospheric particulate matters.
  • Unlike the half-life of metal-induced EPFRs ranging from hours to months, most non-metal-induced EPFRs in PM2.5 samples from cellulose combustion have long lifetimes, with half-lives of several years or longer, and their types remain relatively stable during the decay process.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17010301/s1, Details of the experimental system for sampling PM2.5 in cellulose combustion gas are in Support information.

Author Contributions

Conceptualization, L.Z.; methodology, L.Z. and B.A.; formal analysis, B.A. and G.Y.; investigation, B.A. and G.Y.; resources, L.Z.; data curation, B.A. and G.Y.; writing—original draft preparation, L.Z., B.A. and G.Y.; writing—review and editing, J.C. and Y.Z.; supervision, J.C. and Y.Z.; project administration, L.Z. and B.A.; funding acquisition, L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (Program No. 22076110), Technology Innovation Leading Program of Shaanxi (Program No. 2022QFY06-04), China and Technology Innovation Leading Program of Shaanxi (Program No. 2023-YD-CGZH-27), Key Research and Development Projects of Shaanxi Province (Program No. 2022SF-216).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Acknowledgments

We thank the Ministry of Science, Technology and Innovation for the financial support granted for the development of the proposal.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Yang, Y.; Sun, Z. Research progress on a new class of environmentally harmful substances, environmentally persistent free radicals (EPFRs). Bull. Mineral. Petrol. Geochem. 2012, 31, 287–290. [Google Scholar]
  2. Saravia, J.; Lee, G.I.; Lomnicki, S.; Dellinger, B.; Cormier, S.A. Particulate matter containing environmentally persistent free radicals and adverse infant respiratory health effects: A review. J. Biochem. Mol. Toxicol. 2013, 27, 56–68. [Google Scholar] [CrossRef]
  3. Dugas, T.R.; Lomnicki, S.; Cormier, S.A.; Dellinger, B.; Reams, M. Addressing emerging risks: Scientific and regulatory challenges associated with environmentally persistent free radicals. Int. J. Environ. Res. Public. Health 2016, 13, 573. [Google Scholar] [CrossRef] [PubMed]
  4. Yang, L.; Liu, G.; Zheng, M.; Jin, R.; Zhu, Q.; Zhao, Y.; Wu, X.; Xu, Y. Highly elevated levels and particle-size distributions of environmentally persistent free radicals in haze-associated atmosphere. Environ. Sci. Technol. 2017, 51, 7936–7944. [Google Scholar] [CrossRef]
  5. Wang, P.; Pan, B.; Li, H.; Huang, Y.; Dong, X.; Ai, F.; Liu, L.; Wu, M.; Xing, B. The overlooked occurrence of environmentally persistent free radicals in an area with low-rank coal burning, Xuanwei, China. Environ. Sci. Technol. 2018, 52, 1054–1061. [Google Scholar] [CrossRef]
  6. Yang, L.; Zheng, M.; Xu, Y. Pollution Characteristics and Generation Mechanism of Environmental Persistent Free Radicals. Sci. China: Chem. 2018, 48, 1226–1235. [Google Scholar]
  7. Fang, G.; Liu, C.; Gao, J.; Dionysiou, D.D.; Zhou, D. Manipulation of persistent free radicals in biochar to activate persulfate for contaminant degradation. Environ. Sci. Technol. 2015, 49, 5645–5653. [Google Scholar] [CrossRef] [PubMed]
  8. Jia, H.; Zhao, S.; Nulaji, G.; Tao, K.; Wang, F.; Sharma, V.K.; Wang, C. Environmentally persistent free radicals in soils of past coking sites: Distribution and stabilization. Environ. Sci. Technol. 2017, 51, 6000–6008. [Google Scholar] [CrossRef]
  9. Jia, H.; Zhao, S.; Shi, Y.; Fan, X.; Wang, T. 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, 362, 214–223. [Google Scholar] [CrossRef]
  10. Li, X.; Zhao, H.; Qu, B.; Tian, Y. Photoformation of environmentally persistent free radicals on particulate organic matter in aqueous solution: Role of anthracene and formation mechanism. Chemosphere 2022, 291, 132815. [Google Scholar] [CrossRef]
  11. Han, L.; Chen, B. Mechanism of environmental persistent free radical production and environmental chemical behavior. Prog. Chem. 2017, 29, 1008–1020. [Google Scholar]
  12. Ziech, D.; Franco, R.; Georgakilas, A.G.; Georgakila, S.; Malamou-Mitsi, V.; Schoneveld, O.; Pappa, A.; Panayiotidis, M.I. The role of reactive oxygen species and oxidative stress in environmental carcinogenesis and biomarker development. Chem.-Biol. Interact. 2010, 188, 334–339. [Google Scholar] [CrossRef]
  13. Burn, B.R.; Varner, K.J. Environmentally persistent free radicals (EPFRs) compromise left ventricular function during ischemia/reperfusion injury. Am. J. Physiol. Heart Circ. Physiol. 2015, 308, H998–H1006. [Google Scholar] [CrossRef]
  14. Balakrishna, S.; Lomnicki, S.; McAvey, K.M.; Cole, R.B.; Dellinger, B.; Cormier, S.A. Environmentally persistent free radicals amplify ultrafine particle mediated cellular oxidative stress and cytotoxicity. Part. Fibre Toxicol. 2009, 6, 11. [Google Scholar] [CrossRef] [PubMed]
  15. Liao, S.; Pan, B.; Li, H.; Zhang, D.; Xing, B. Detecting Free Radicals in Biochars and Determining Their Ability to Inhibit the Germination and Growth of Corn, Wheat and Rice Seedlings. Environ. Sci. Technol. 2014, 48, 8581–8587. [Google Scholar] [CrossRef]
  16. Singer, L.S. Free radicals in coals and synthetic fuels: L. Petrakis and D. W. Grandy Elsevier, Amsterdam, 1983. Fuel 1985, 64, 1334–1335. [Google Scholar] [CrossRef]
  17. Valavanidis, A.; Iliopoulos, N.; Gotsis, G.; Fiotakis, K. Persistent free radicals, heavy metals and PAHs generated in particulate soot emissions and residue ash from controlled combustion of common types of plastic. J. Hazard. Mater. 2008, 156, 277–284. [Google Scholar] [CrossRef] [PubMed]
  18. Wang, T.J.; Chen, T.; Zhan, M.X.; Guo, Y.; Xiao-Dong, L.I. Correlation of Persistent Free Radicals, PCDD/Fs and Metals in Waste Incineration Fly Ash. Environ. Sci. 2016, 37, 1163–1170. [Google Scholar]
  19. Dellinger, B.; Lomnicki, S.; Khachatryan, L.; Maskos, Z.; Hall, R.W.; Adounkpe, J.; McFerrin, C.; Truong, H. Formation and stabilization of persistent free radicals. Proc. Combust. Inst. Int. 2007, 31, 521–528. [Google Scholar] [CrossRef]
  20. Nwosu, U.G.; Khachatryan, L.; Youm, S.G.; Roy, A.; Aln, D.C.; Nesterov, E.E.; Dellinger, B.; Cook, R.L. Model System Study of Environmentally Persistent Free Radicals Formation in a Semiconducting Polymer Modified Copper Clay System at Ambient Temperature. RSC Adv. 2016, 6, 43453–43462. [Google Scholar] [CrossRef]
  21. Kiruri, L.W.; Lavrent, K.; Barry, D.; Slawo, L. Effect of copper oxide concentration on the formation and persistency of environmentally persistent free radicals (EPFRs) in particulates. Environ. Sci. Technol. 2014, 48, 2212–2217. [Google Scholar] [CrossRef]
  22. Vejerano, E.; Lomnicki, S.; Dellinger, B. Formation and stabilization of combustion-generated environmentally persistent free radicals on an Fe(III)2O3/silica surface. Environ. Sci. Technol. 2011, 45, 589–594. [Google Scholar] [CrossRef] [PubMed]
  23. Chen, Q.; Sun, H.; Wang, M.; Mu, Z.; Wang, Y.; Li, Y.; Wang, Y.; Zhang, L.; Zhang, Z. Dominant fraction of EPFRs from nonsolvent-extractable organic matter in fine particulates over Xi’an, China. Environ. Sci. Technol. 2018, 52, 9646–9655. [Google Scholar] [CrossRef] [PubMed]
  24. Sun, H.; Chen, Q.; Mou, Z.; Wang, Y.; Shen, Z.; Zhang, L.; Li, Y. Properties and sources of Environmental persistent free radicals (EPFRs) in PM2.5 in Xi ’an. J. Environ. Sci. 2019, 39, 197–203. [Google Scholar] [CrossRef]
  25. Ding, J.; Chen, Q.; Zhang, Z.; Han, Y. Impact of the “Three-year Action Plan” on the Pollution Characteristics and Sources of Atmospheric Persistent Free Radicals. China Environ. Sci. 2023, 43, 5041–5051. [Google Scholar]
  26. Yang, L.; Liu, G.; Zheng, M.; Jin, R.; Zhao, Y.; Wu, X.; Xu, Y. Pivotal roles of metal oxides in the formation of environmentally persistent free radicals. Environ. Sci. Technol. 2017, 51, 12329–12336. [Google Scholar] [CrossRef] [PubMed]
  27. Vejerano, E.; Lomnicki, S.; Dellinger, B. Lifetime of combustion-generated environmentally persistent free radicals on Zn (II) O and other transition metal oxides. J. Environ. Monit. 2012, 14, 2803–2806. [Google Scholar] [CrossRef] [PubMed]
  28. Yen, T.F.; Erdman, J.; Saraceno, A.J. Investigation of the Nature of Free Radicals in Petroleum Asphaltenes and Related Substances by Electron Spin Resonance. Anal. Chem. 1962, 34, 694–700. [Google Scholar] [CrossRef]
  29. Senesi, N. Application of electron spin resonance (ESR) spectroscopy in soil chemistry. In Advances in Soil Science; Springer: New York, NY, USA, 1990; Volume 14, pp. 77–130. [Google Scholar]
  30. Watanabe, A.; McPhail, D.B.; Maie, N.; Kawasaki, S.; Anderson, H.A.; Cheshire, M.V. Electron spin resonance characteristics of humic acids from a wide range of soil types. Org. Geochem. 2005, 36, 981–990. [Google Scholar] [CrossRef]
  31. Yang, J.; Pan, B.; Li, H.; Liao, S.; Zhang, D.; Wu, M.; Xing, B. Degradation of p-nitrophenol on biochars: Role of persistent free radicals. Environ. Sci. Technol. 2016, 50, 694–700. [Google Scholar] [CrossRef]
  32. Guo, C.; Hasan, F.; Lay, D.; Dela Cruz, A.L.N.; Ghimire, A.; Lomnicki, S.M. Phytosampling—A supplementary tool for particulate matter (PM) speciation characterization. Environ. Sci. Pollut. Res. 2021, 28, 39310–39321. [Google Scholar] [CrossRef]
  33. Maskos, Z.; Khachatryan, L.; Dellinger, B. Precursors of radicals in tobacco smoke and the role of particulate matter in forming and stabilizing radicals. Energy Fuels 2005, 19, 2466–2473. [Google Scholar] [CrossRef]
  34. Apicella, B.; Carpentieri, A.; Alfè, M.; Barbella, R.; Tregrossi, A.; Pucci, P.; Ciajolo, A. Mass spectrometric analysis of large PAH in a fuel-rich ethylene flame. Proc. Combust. Inst. 2007, 31, 547–553. [Google Scholar] [CrossRef]
  35. Rüger, C.P.; Sklorz, M.; Schwemer, T.; Zimmermann, R. Characterisation of ship diesel primary particulate matter at the molecular level by means of ultra-high-resolution mass spectrometry coupled to laser desorption ionisation—Comparison of feed fuel, filter extracts and direct particle measurements. Anal. Bioanal. Chem. 2015, 407, 5923–5937. [Google Scholar] [CrossRef] [PubMed]
  36. Qian, R.; Zhang, S.; Peng, C.; Zhang, L.; Yang, F.; Tian, M.; Huang, R.; Wang, Q.; Chen, Q.; Yao, X. Characteristics and potential exposure risks of environmentally persistent free radicals in PM2.5 in the three gorges reservoir area, Southwestern China. Chemosphere 2020, 252, 126425. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. Schematic diagram of fuel combustion and sample collection device.
Figure 1. Schematic diagram of fuel combustion and sample collection device.
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Figure 2. EPR spectrum of PM2.5 samples collected under different combustion conditions.
Figure 2. EPR spectrum of PM2.5 samples collected under different combustion conditions.
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Figure 3. Schematic diagram of the potential formation mechanism of non-metal-induced EPFRs.
Figure 3. Schematic diagram of the potential formation mechanism of non-metal-induced EPFRs.
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Figure 4. The proportion of different types of OC/EC in PM2.5 samples relative to TC.
Figure 4. The proportion of different types of OC/EC in PM2.5 samples relative to TC.
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Figure 5. The concentration of EPFRs in PM2.5 samples collected under different combustion conditions.
Figure 5. The concentration of EPFRs in PM2.5 samples collected under different combustion conditions.
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Figure 6. Concentration chart of EPFRs in PM2.5 original samples and their solvent-extracted fractions under different combustion conditions ((a) for air proportion of 0.5; (b) for air proportion of 1).
Figure 6. Concentration chart of EPFRs in PM2.5 original samples and their solvent-extracted fractions under different combustion conditions ((a) for air proportion of 0.5; (b) for air proportion of 1).
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Figure 7. g-factor chart of PM2.5 original samples and their solvent-extracted fractions collected under different combustion conditions.
Figure 7. g-factor chart of PM2.5 original samples and their solvent-extracted fractions collected under different combustion conditions.
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Figure 8. The relative contents of EPFRs in different kinds of samples after extraction and original samples changed over time (for conditions (a–c), the air ratio is 0.5 and the temperatures are 300 °C, 500 °C, and 700 °C respectively. For conditions (d–f), the air ratio is 1 and the temperatures are 300 °C, 500 °C, and 700 °C respectively).
Figure 8. The relative contents of EPFRs in different kinds of samples after extraction and original samples changed over time (for conditions (a–c), the air ratio is 0.5 and the temperatures are 300 °C, 500 °C, and 700 °C respectively. For conditions (d–f), the air ratio is 1 and the temperatures are 300 °C, 500 °C, and 700 °C respectively).
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Figure 9. The g-factor of EPFRs of different kinds of samples after extraction and the original sample changes over time.
Figure 9. The g-factor of EPFRs of different kinds of samples after extraction and the original sample changes over time.
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Table 1. Instruments and reagents.
Table 1. Instruments and reagents.
CategoryNameDetails
ReagentsMethanolchromatographic grade, Macklin, Van Alstyne, TX, USA, ≥99.5%
ReagentsDichloromethanechromatographic grade, Chengdu Cologne Chemical Co., Ltd., Chengdu, China ≥99.5%
Reagentsn-Hexanechromatographic grade, Chengdu Cologne Chemical Co., Ltd., Chengdu, China ≥99.5%
ReagentsEthanolanalytical grade, Tianjin KeMiOu Chemical Reagent Co., Ltd., Tianjin, China ≥99.5%
MaterialsQuartz filter membraneTISSUQUARTZ2500QAT-UP, Pall Life Science, Pensacola, FL, USA
MaterialsCelluloseD50, 180–280 um, Shanghai Macroscopic Biochemical Technology Co., Ltd., Shanghai, China
InstrumentsTube furnaceOTF-1200X, Hefei Kejing Material Technology Co., Ltd., Hefei, China
InstrumentsOC/EC analyzerDRI 2001A, Atmoslytic, Calabasas, CA, USA
InstrumentsElectron paramagnetic resonance spectrometerEMX Plus, Bruker, Leipzig, Germany
Table 2. Numbers and combustion conditions of collected PM2.5 samples in flue gas.
Table 2. Numbers and combustion conditions of collected PM2.5 samples in flue gas.
Sample NumberSample Weight (g)Combustion Temperature (°C)Gas Supply Flow Rate (L/min)Gas Dilution Flow Rate (m3/h)Sampling Flow Rate (L/min)
Y13005.00300Air 0.65216.7
Y15005.00500Air 0.65216.7
Y17005.00700Air 0.65216.7
Y0.53005.00300Air 0.325
Nitrogen 0.325		216.7
Y0.55005.00500Air 0.325
Nitrogen 0.325		216.7
Y0.57005.00700Air 0.325
Nitrogen 0.325		216.7
Table 3. Characteristic information of EPFRs in PM2.5 samples collected under different combustion conditions.
Table 3. Characteristic information of EPFRs in PM2.5 samples collected under different combustion conditions.
Sample NumberThe Concentration of EPFRs (spins/m3)g-FactorΔHP-P
Y13003.29 × 1014 ± 1.102 × 10132.00154.7948
Y15004.80 × 1014 ± 1.343 × 10132.00194.9094
Y17004.56 × 1014 ± 1.868 × 10132.00195.2605
Y0.53002.89 × 1014 ± 2.104 × 10132.00204.8367
Y0.55003.96 × 1014 ± 1.202 × 10132.00204.9961
Y0.57003.76 × 1014 ± 1.059 × 10132.00224.7163
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Zhang, L.; An, B.; Chen, J.; Zhang, Y.; Yu, G. Study on Non-Metal-Induced EPFRs in PM2.5 Generated from Flue Gas of Cellulose Combustion. Sustainability 2025, 17, 301. https://doi.org/10.3390/su17010301

AMA Style

Zhang L, An B, Chen J, Zhang Y, Yu G. Study on Non-Metal-Induced EPFRs in PM2.5 Generated from Flue Gas of Cellulose Combustion. Sustainability. 2025; 17(1):301. https://doi.org/10.3390/su17010301

Chicago/Turabian Style

Zhang, Lixin, Boru An, Jingmin Chen, Yuwei Zhang, and Guojiao Yu. 2025. "Study on Non-Metal-Induced EPFRs in PM2.5 Generated from Flue Gas of Cellulose Combustion" Sustainability 17, no. 1: 301. https://doi.org/10.3390/su17010301

APA Style

Zhang, L., An, B., Chen, J., Zhang, Y., & Yu, G. (2025). Study on Non-Metal-Induced EPFRs in PM2.5 Generated from Flue Gas of Cellulose Combustion. Sustainability, 17(1), 301. https://doi.org/10.3390/su17010301

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