Role of the Filters in the Formation and Stabilization of Semiquinone Radicals Collected from Cigarette Smoke

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.

Efficient activation of peracetic acid by mixed sludge derived biochar: Critical role of persistent free radicals

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) were increasingly identified as the alternative scheme in wastewater treatment. Cost-effective and easily available catalyst for activation of PAA was in urgent demand for promoting engineering application process. In this study, a new type of biochar catalyst derived from pyrolysis of mixture of primary sludge (PSD) and secondary sludge (SSD) was prepared and showed effective PAA activation ability. The degradation of p-chlorophenol (4-CP) improved with PAA activation by mixed sludge derived biochar (PS-SDBC) than secondary sludge derived biochar (S-SDBC) and primary sludge derived biochar (P-SDBC), and the highest removal efficiency achieved by PS-SDBC with the PSD/SSD ratio of 5/5 (k_obs=0.057 1/(M·min), pH 9). Correlation analysis firstly indicated that persistent free radicals (PFRs) rather than chemical composition and material structure dominated PAA activation and organic radicals (RO•) was proved to be the major reactive species through electron paramagnetic resonance (EPR) detection. The mixture of PSD and SSD caused the synergy of inorganic metals and organic matters through pyrolysis processes, resulting in larger specific surface area (SSA) (110.71 m²/g), more abundant electron-donating groups (e.g., C = O, -OH) and massive defects (I_D/I_G = 1.519) of PS-SDBC than P-SDBC and S-SDBC, which eventually promoted PFRs formation. A fascinating phenomenon was observed in PS-SDBC/PAA system that the active sites of PFRs could be regenerated by RO• attacking onto PS-SDBC, which contributed to the wide pH applicability and continuous stability of PS-SDBC/PAA system in practical wastewater treatment. This study not only significantly deepened the understanding of the reaction mechanism between PAA and biochar, but also provided a potential PAA-based AOPs for micropollutants removal in wastewater.

Generation of Environmentally Persistent Free Radicals on Metal-Organic Frameworks

Environmentally persistent free radicals (EPFRs) have been recognized as one of the important emerging contaminants with biological toxicity, environmental persistence, and global mobility. Previous studies have identified the catalytic role of surface metal oxides in EPFRs formation and illustrated the metal-dependence of EPFRs by studying on various metal oxide nanoparticles and single crystals. However, there is still lack of an understanding on the formation of EPFRs from the point of view of metal sites. Various factors (e.g., crystalline phases and surface species) of metal oxides are regarded to contribute to the generation of EPFRs, which present profound difficulties for scientists to tease apart the impact of metal type. Herein, a laboratory investigation, in terms of the acidity and oxidation strength of metal cations, was conducted by selecting metal-variable isostructural metal-organic frameworks as material platforms. Specifically, we evaluated EPFRs generation on MIL-100(M) (M = Al, Cr, Fe) from chlorine-substituted phenol vapor and catechol under thermal conditions. It is found that high Lewis acidity of metal sites is crucial for capturing the above two phenolic precursors, activating the O-H bond and promoting EPFRs formation. Radical species with half-life as long as 70 days were generated on MIL-100 rich in 5-fold coordinated Al³⁺ sites. The unpaired electron spin density donation was further confirmed by using ²⁷Al solid-state nuclear magnetic resonance spectroscopy. Despite their higher oxidation power than Al³⁺, the exposed Cr³⁺ and Fe³⁺ sites show undetectable catalytic activity for the formation of EPFRs, because of their insufficient Lewis acidity. Our results suggest that the surface species rather than Lewis acid sites may be a major contributor to the formation of EPFRs on metal oxides like Fe₂O₃.

Environmentally persistent free radicals in PM2.5: a review

Environmentally persistent free radicals (EPFRs) are a new class of pollutants that are long-lived in fine particles (PM_2.5), i.e., their 1/e lifetime ranges from days to months (or even infinite). They are capable of producing harmful reactive oxygen species such as hydroxyl radicals. The redox cycling of EPFRs is considered as an important pathway for PM_2.5 to induce oxidative stress inside the humans, causing adverse health effects such as respiratory and cardiovascular diseases. Consequently, research regarding their toxicity, formation and environmental occurrences in PM_2.5 has attracted increasing attentions globally during the past two decades. However, literature data in this field remain quite limited and discrete. Hence, an extensive review is urgently needed to summarize the current understanding of this topic. In this work, we systematically reviewed the analytical methods and environmental occurrences, e.g., types, concentrations, and decay behaviors, as well as possible sources of EPFRs in PM_2.5. The types of pretreatment methods, g-values of common EPFRs and categories of decay processes were discussed in detail. Moreover, great efforts were made to revisit the original data of the published works of EPFRs in airborne particulate matter and provided additional useful information for comparison where possible, e.g., their mean and standard deviation of g-values, line widths (ΔH _p-p), and concentrations. Finally, possible research opportunities were highlighted to further advance our knowledge of this emerging issue.

Efficient removal of several estrogens in water by Fe-hydrochar composite and related interactive effect mechanism of H2O2 and iron with persistent free radicals from hydrochar of pinewood

Recently, hydrochar (HC) with existed persistent free radicals (PFRs) has attracted researches' attention for the potential application in heterogeneous Fenton-like reactions, but studies on the interactive effects of H₂O₂, iron, and HC in removal of organic pollutants are still limited. In this paper, magnetic iron (hydr)oxides immobilized hydrochar composite (Fe/HC) derived from hydrothermal carbon (HTC) of pinewood were synthesized and characterized. The interactive effects of H₂O₂, iron, and HC in the removal of several estrogens were systematically investigated to understand the removal performance and related mechanism, especially at a pH range close to natural water environment. Batch experiments results showed that estrogens could be efficiently removed over Fe/HC material under a wide pH range of 4-9. Based on the analysis of electron spin resonance, X-ray photoelectron spectroscopy, Mössbauer spectroscopy, and electrochemical impedance spectroscopy, mechanism study indicated that the carbon-centered PFRs on the surface of hydrochar can act as electron donors, and transfer the electrons on adsorbed O₂ to generate O₂^- rapidly, while the addition of H₂O₂ enhanced the transmission ability of electron to produce OH_(ads) on the material surface. The iron and hydrochar components contributed to the desirable removal of estrogens via the synergistic effect between catalysis and adsorption. This study provides a promising application for the use of Fe/HC materials on remediation of pollution with trace estrogens in water environment.