Formation Of Persistent Free Radicals Research Articles

Biochar-Derived Persistent Free Radicals: A Plethora of Environmental Applications in a Light and Shadows Scenario.

Biochar (BC) is a carbonaceous material obtained by pyrolysis at 200-1000 °C in the limited presence of O2 from different vegetable and animal biomass feedstocks. BC has demonstrated great potential, mainly in environmental applications, due to its high sorption ability and persistent free radicals (PFRs) content. These characteristics enable BC to carry out the direct and PFRs-mediated removal/degradation of environmental organic and inorganic contaminants. The types of PFRs that are possibly present in BC depend mainly on the pyrolysis temperature and the kind of pristine biomass. Since they can also cause ecological and human damage, a systematic evaluation of the environmental behavior, risks, or management techniques of BC-derived PFRs is urgent. PFRs generally consist of a mixture of carbon- and oxygen-centered radicals and of oxygenated carbon-centered radicals, depending on the pyrolytic conditions. Here, to promote the more productive and beneficial use of BC and the related PFRs and to stimulate further studies to make them environmentally safer and less hazardous to humans, we have first reviewed the most common methods used to produce BC, its main environmental applications, and the primary mechanisms by which BC remove xenobiotics, as well as the reported mechanisms for PFR formation in BC. Secondly, we have discussed the environmental migration and transformation of PFRs; we have reported the main PFR-mediated application of BC to degrade inorganic and organic pollutants, the potential correlated environmental risks, and the possible strategies to limit them.

Photodegradation of poly(ether-b-amide) catalyzed by bismuth radiopaque additive

Bismuth compounds (e.g., bismuth subcarbonate and bismuth oxychloride) are often used as radiopaque additives in poly(ether-b-amide) (PEBA) for medical catheter applications to enhance the visibility of the catheter tips during fluoroscopy-guided minimally invasive surgeries. However, there is an increasing number of reports on bismuth-based photocatalysts in other fields; yet the impact of bismuth additives on the shelf life of PEBA has never been studied/reported. In this work, the stability of PEBA containing bismuth subcarbonate was tested against PEBA containing other radiopaque additives, barium sulfate and tungsten, in several real-life-relevant lighting conditions. It was found bismuth-containing PEBA degraded significantly faster than these counterpart materials under light exposure conditions including indoor fluorescent light, daylight-simulating xenon light and high-flux visible blue light. The extents of photodegradation of bismuth-containing PEBA were characterized by optical imaging, tensile test, FTIR analysis, Raman imaging and ESR radical analysis. The results showed material surface cracking, material hardening, surface oxidation and radical concentration increase over the course of light exposure. Various environmental factors were studied; it was found PEBA containing bismuth not only degraded under lights containing UV light but also degraded under high intensity visible blue light. Larger doses of light exposure and higher environmental temperatures led to more material degradation. Additional shelf aging studies on light-exposed samples were performed, which showed latent aging with continued degradation shortening the shelf life after the initial light exposure, due to the formation of persistent free radicals. Finally, several design and use considerations were discussed in the context of catheter material photodegradation.

Formation and evolution mechanism of persistent free radicals in biochar during biomass pyrolysis: Insights from biochar’s element composition and chemical structure

The persistent free radicals (PFRs) in biochar can play a crucial role during biochar’s utilization, but the formation and evolution mechanism of PFRs in biochar are poorly understood because of biochars’ element specificity and chemical structure complexity. In order to further understand the formation and evolution mechanism of PFRs, the PFRs characteristics, chemical structure and element composition of six types of biomasses were investigated. The results revealed that the changing trend of the PFRs types, concentration and line width with pyrolysis temperature is similar for all six feedstocks. The difference in the feedstocks’ properties just affects the absolute value of PFRs’ characteristic parameters. The arrangement and combination of elements and carbon skeleton structure in biochar determine the evolution of PFRs in the pyrolysis process. Additionally, the (H + O)/C atomic ratio of the biomass feedstock and the content of substitutional groups on aromatic rings were found to influence the types of PFRs in biochar. When biochar has a similar O/C atom ratio, a higher H content can reduce the carbon-centered PFRs in biochar. Besides, the reduction in O element content contributes to a decrease in line width due to the reduction in the effect of heteroatoms. The good correlation between O/C atomic ratio, H/C atomic ratio, chemical structure and PFRs characteristics can be used to predict the PFRs characteristics. Combining the changing characteristics in PFRs concentration and carbon skeleton structure of biochar, a three-stage model of PFRs formation and evolution was constructed.

Iron phthalocyanine doped carbon-based as a bifunctional material for peroxymonosulfate activation toward Reactive Red 24 degradation: Consolidated adsorption and multiple oxidation

The industrial wastewater containing some refractory macromolecule active dyes is usually hard to be treated effectively through the traditional physio-chemical and biological methods. In this study, an innovative non-homogeneous catalyst system with efficacious oxidation capacity, namely iron phthalocyanine (FePc)-doped millet biochar (mBC) with the function of activating peroxymonosulfate (PMS), was constructed for the degradation of reactive red 24 (RR24). The obtained FePc-mBC exhibited an efficient adsorption with a catalysis performance of a 93 %-removal efficiency for RR24 in 120 min, and meanwhile this novel catalyst performed remarkable adaptability at the wide pH range from 3 to 10. The FePc-introduced significantly weakened the boundary layer thickness of the material to absorb RR24 uniformly under acidic conditions, which creating a favorable condition for subsequent efficient oxidation. Both free radicals and non-free radicals were involved in the oxidation reaction of which, the constructed N(C)-O- acted as the axial ligand to bridge with Fe in FePc and induced -O-O- heterolysis to produce Fe (IV) = O, which was identified as the effective precursor product of 1O2, the main player for the RR24 oxidation reaction. Besides, the FePc prompted the formation of persistent free radicals (PFRs) on the mBC, as a single-electron body, which induced the redox cycling of Fe (III) and Fe (II) and maintained the “continuity” capacity of the system. Overall, this work provides a new insight into the development of bifunctional metal complex/biochar composites and creates a reasonable system containing diversified and multi-functional active species for the oxidation of refractory organic pollutants.

Effects of Heterogeneous Metals on the Generation of Persistent Free Radicals as Critical Redox Sites in Iron-Containing Biochar for Persulfate Activation

Metal components have critical influences on the generation of persistent free radicals (PFRs) in biochar. However, little is known about the effect on the formation of PFRs brought about by interactions among metal components when pyrolyzing biomass containing multiple metal components (e.g., sewage sludges and hyperaccumulators) for biochar. Herein, iron-modified biochar (FeO/BC) and iron-containing bimetallic oxide-loaded biochar (ZnFeO/BC, CoFeO/BC, and NiFeO/BC) were prepared. Due to the synergistic effect, the coupling of Fe with Co and Ni further enhances the abundance of PFRs in iron-containing biochars (IBCs) compared with FeO/BC catalysts, while Zn inhibits the generation of PFRs via blocking the electron transfer process during biomass pyrolysis. Furthermore, persulfate activation experiments on IBCs were conducted, and a positive correlation coefficient of 0.980 (or 0.974) was found between kobs values and PFR intensities (or the consumption rate of PFRs), indicating that PFRs are doubtless the critical redox sites. Finally, NiFeO/BC as a more efficient catalyst for PS activation was confirmed due to the higher adsorption energy and more transferred electrons based on experimental results and theoretical calculations. This study provides new methods for regulating PFRs in biochar and updates the understanding about heterogeneous metal-containing biomass pyrolysis processes.

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 (kobs=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 (ID/IG = 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 Mechanism of Persistent Free Radicals in Lignocellulose-Derived Biochar: Roles of Reducible Carbonyls.

Persistent free radicals (PFRs) in biochar can influence biochar reactivity, promoting organic contaminant degradation or even causing certain toxic impacts. However, the PFR generation mechanism is not still well understood. An investigation of the relationship between PFR formation and the chemical structure of biochar is essential for understanding the PFR formation mechanism. Our in situ measurement results showed that PFR intensities increased from 0-509.5 to 146-5678 a.u. after being pyrolyzed at 300 °C for 60 min. The significant positive correlation between PFR intensities and the peak areas of C═O and aromatic C═C groups indicated that the generation of PFRs was highly dependent on the C═O and aromatic C═C structures. The reduction of biochars by KBH4 resulted in a 32.2 ± 2.49% decrease in the C═O content and a relative increase in the C-O content, while other physicochemical properties did not change. Thus, the observed 49.3% decrease in PFR signals after this reduction suggested that the reducible C═O groups, possibly in aldehydes, aromatic ketones, and quinones, were closely associated with PFRs in biochars. This study provides an in situ insight into the PFR generation mechanism and guides the corresponding biochar design and property manipulation.

Formation of persistent free radicals and reactive chlorine species during photochemical processes of Polychlorophenols: Effect of temperature, humidity and particles

Persistent free radicals (PFRs) generated by polychlorophenol pollutions (CPs) have attracted widespread attention, while the hazards of reactive chlorine species (RCS) generated during the oxidation process of PFRs to environment and human health have been largely ignored. In this study, the PFRs and RCS generation pathways in gas and on Si3O10H8 cluster from 2,4,6-trichlorophenol (2,4,6-TCPs) and pentachlorophenol (PCPs) initiated by OH radical are investigated by quantum chemistry method. In the gas phase, 2,4,6-TCPs mainly undergoes H-abstraction reaction to generate PFRs, while on Si3O10H8 cluster, the OH-addition reaction at the phenolic hydroxyl site becomes dominant and the addition product is dehydrated into PFRs under the synergisticeffectof H2O and Si3O10H8 cluster. Conversely, PCPs mainly undergoes H-abstraction reaction both in the gas phase and on Si3O10H8 clusters to form PFRs. This is because the PCPs form strong hydrogen bonds with Si3O10H8 clusters and the (highest occupied molecular orbital) HOMO of PCPs moves to higher energy levels, which reduces the electron binding ability and promotes the electrophilic reaction of OH radical. Introducing a carbonyl group ortho to the carbonyl group of phenolate-type PFRs leads to spontaneous ring-opening and subsequent decomposition into a series of RCS that can react with Cl, NO2 and O2 to form phosgene, chloroformyl peroxynitrate and peroxychloroformyl radical, respectively. These generated chlorine-containing compounds are highly irritating to human eyes and skin and have potential environmental toxicity.

Collaborative optimization for the performance of ZnO/biochar composites on persulfate activation through plant enrichment-pyrolysis method

A novel method of plant enrichment for ZnO nanoparticles (ZnO NPs) and pyrolysis was employed to gain the ZnO/biochar (ZnO/BC) composites. Comparing to pristine biochar and two zinc oxides/biochar composites obtained by conventional metal salts impregnation method, here the ZnO/BC exhibited distinct advantages in persulfate (PS) activation for tetracycline hydrochloride (TC) degradation. The activation mechanism analysis revealed that PS accepted electrons from persistent free radicals (PFRs) in ZnO/BC to directly or indirectly generate sulfate radicals (SO4∙-) and hydroxyl radicals (HO∙). Coincidentally, ZnO NPs promoted the secretion of total phenols and flavonoids, as precursors of PFRs, which accelerated the formation of more oxygen-centered PFRs with stronger catalytic ability and longer half-life. Moreover, owing to the tight combination of ZnO NPs with internal structures of the plant, higher porosity and large specific surface area in the biochar composites were favorable for reducing the electrochemical impedance of ZnO/BC, which could effectively facilitate the electron transfer process between PFRs and PS. Meanwhile, due to the injection of electrons during PFRs formation, slight narrow in the bandgap brought about the mild enhancement of photocatalytic properties for ZnO NPs. Overall, the method in this research not only provided a new strategy to acquire metal oxide/biochar composites with catalytic ability, but also opened up a novel way to collaboratively optimize the performance of biochar matrix and metal nanoparticles.

Identifying the Persistent Free Radicals (PFRs) Formed as Crucial Metastable Intermediates during Peroxymonosulfate (PMS) Activation by N-Doped Carbonaceous Materials.

A nonradical mechanism involved in peroxymonosulfate (PMS) activation in carbonaceous materials (CMs) is still controversial. In this study, we prepared N-doped CMs, including hollow carbon spheres (NHCSs) and carbon nanotubes (N-CNTs), to probe the crucial intermediates during PMS activation. The results suggested that the higher efficiency and lower activation energy (13.72 kJ mol-1) toward phenol (PN) degradation in an NHCS/PMS system than PMS alone (∼24.07 kJ mol-1) depended on a typical nonradical reaction. Persistent free radicals (PFRs) with a g factor of 2.0033-2.0045, formed as crucial metastable intermediates on NHCS or N-CNT in the presence of PMS, contribute largely to the organic degradation (∼73.4%). Solid evidence suggested that the formation of PFRs relied on the attack of surface-bonded •OH and SO4•- or peroxides in PMS, among which surface-bonded SO4•- was most thermodynamically favorable based on theoretical calculations. Electron holes within PFRs on NHCSs shifted the Fermi level to the positive energy with the valance band increasing from 1.18 to 1.98 eV, promoting the reactivity toward nucleophilic substances. The degradation intermediates of aromatic compounds (e.g., PN) and electron rearrangement triggered the evolution of PFRs from oxygen-centered to carbon-centered radicals. Moreover, due to the specific electron configuration, graphitic N on NHCS was critical for stabilizing the PFRs. This study provides insightful understanding of the fate of organic contaminants and the structure-activity relationship of reactivity of CMs toward PMS activation.

Formation of persistent free radicals in sludge biochar by hydrothermal carbonization

The uses of biochar as soil fertilizer can offset global warming and reduce dependence on limited mineral resources in the future circular economy, yet biochar may contain contaminants that can ultimately enter the food chain. In particular, persistent free radicals are emerging contaminants previously detected in biochar but underlying mechanisms of radical formation are not yet established. Here we studied radical generation during hydrothermal carbonization of waste sludge at 160–220 ºC for 0.5–2 h with solid weight ratios of 10%w–40%w using electron paramagnetic resonance and Fourier transform infrared spectrometry. Results reveal that radical concentration increases with temperature, reaction time, and weight ratio in sludge biochars, reaching a content of 47.2 × 1015 spins/g for 220 ºC, 2 h heating, and 40%w solid ratio. Moreover, low temperature of about 160 ºC favors the production of oxygen-centered radicals, whereas higher temperature of 220 ºC produces carbon-centered radicals. Our findings imply that biochar ecotoxicity should be assessed prior applications to prevent adverse health effects.

Photoinduced formation of persistent free radicals, hydrogen radicals, and hydroxyl radicals from catechol on atmospheric particulate matter

SummaryCatechol is speculated to be a potential precursor of environmentally persistent free radicals (EPFRs) in the atmosphere. EPFRs absorbed on PM2.5 have attracted public attention because their toxicity is similar to cigarette smoke. In this study, we found that catechol could produce EPFRs, which were oxygen-centered phenoxy and semiquinone radicals. These free radical species had half-lives of up to 382 days. CaO, CuO, and Fe2O3 markedly promoted EPFR formation from catechol. The valence states of Cu and Fe changed during the photochemical reactions of catechol but no valence state changed for Ca. Alkaline nature of CaO is possibly the key for promoting the free radical formations through acid-base reactions with catechol. In addition to hydroxyl free radicals, hydrogen free radicals and superoxide anions formed from the photochemical reactions of catechol were first discovered. This is of concern because of the adverse effects of these free radicals on human health.

Plant and algal toxicity of persistent free radicals and reactive oxygen species generated by heating anthracene-contaminated soils from 100 to 600 °C

Persistent free radicals (PFRs) are emerging contaminants of increasing concern, yet their formation, fate, toxicity and health risk are poorly known. Thermal treatment, a common remediation technique to clean industrial soils, induces the formation of PFRs, which could paradoxically increase soil toxicity, contrary to the original objective of remediation. Actually, there is little knowledge on the formation and toxicity of PFRs in soils contaminated by polycyclic aromatic hydrocarbons (PAHs). Here we studied the generation of PFRs of soils spiked with anthracene and heated 1 h from 100 to 600 °C, using electron paramagnetic resonance. We also investigated the formation of reactive oxygen species (ROS), e.g. superoxide radical (O2·−), hydrogen peroxide (H2O2) and hydroxyl radical (·OH), in the aqueous phase of thermal-treated soil, and the impact of heating on soil oxidative potential, wheat growth and green algae activity. Results showed that PFRs, ROS, soil oxidative potential, plant toxicity and algal toxicity show a similar trend with an increase from 100 to 300 °C, followed by a decrease to 600 °C. Scavenger trapping tests reveal that algal toxicity is mainly due to the generation of O2·−, ·OH and H2O2 induced by anthracene-PFRs and that anthracene and anthracene-PFRs have negligible direct algal toxicity. Overall, our findings reveal the unintended formation of toxic compounds peaking at 300 °C during the thermal remediation of PAH-contaminated soils. These results should help to assess the environmental risk of thermally treated PAH-contaminated soils.

Sunlight-driven degradation of diethyl phthalate via magnetically modified biochar catalysts in water: Internal electron transfer mechanism

This study presents a one-step synthetic approach for magnetic biochar (MBC) photo-degradation of diethyl phthalate (DEP). The results showed that MBC exhibited better catalytic property for DEP degradation than BC, and its catalytic performance was influenced by the amount of Fe doping. Electron paramagnetic resonance (EPR), quenching experiments, and chemical probe studies confirmed the presence of persistent free radicals (PFRs), hydroxyl radicals (·OH), and superoxide anion radical (·O2−) in both of BC and MBC. Solar light promoted the formation of PFRs in BC system, which transferred electrons to oxygen to form ·O2−, thus yielding ·OH. On the other hand, electron transfer occurred between PFRs and Fe3+ for MBC, Fe2+ played an important role in activation of O2 and ·O2− production. Subsequently, photo-Fenton reaction was primarily responsible for ·OH formation. This work compared the different generation pathways for ROS between BC and MBC and provides new insight into the possible mediatory roles of BC in O2 activation under solar light by transition metals.

Persistent free radicals on N-doped hydrochar for degradation of endocrine disrupting compounds

Metal-free carbocatalysts are raising increased interest in advanced oxidation processes (AOPs) for green degradation of aqueous organic pollutants. In the present study, a series of N-doped hydrochars (NHCs) was prepared from pinewood biomass and used in carbocatalysis toward catalytic peroxymonosulfate (PMS) activation to degrade refractory endocrine disrupting compounds. Nitrogen-doping was found to regulate the concentrations and types of persistent free radicals (PFRs) on hydrochar, promoting the electron transfer and oxidative degradation of the contaminants. Mechanistic studies revealed that graphitic N not only shows a strong electron transfer to facilitate PMS activation in a nonradical pathway, but also induces the formation of oxygen-centered PFRs from the structure defects for a radical oxidation pathway. More importantly, the PMS/NHC system demonstrates efficient degradation and satisfactory stability for real water treatment. This study has gained in-depth understanding on the roles of N-dopants in radical and nonradical pathways of biomass-based carbocatalysis, and will provide a promising and environmental-friendly route for efficient degradation of refractory endocrine disrupting compounds for green remediation.

Photodegradation Elevated the Toxicity of Polystyrene Microplastics to Grouper (Epinephelus moara) through Disrupting Hepatic Lipid Homeostasis

Microplastics (MPs) have caused increasing global concerns due to their detrimental effects on marine ecosystems. However, the role of photodegradation in altering toxicity of MPs to marine organisms is poorly understood. We therefore investigated the photolytic transformation of pristine polystyrene fragments (P-PS) by 60-day ultraviolet (UV) irradiation, and compared the toxicity of P-PS, photodegraded PS (PD-PS), and commercially available polystyrene microbeads (C-PS) to juvenile grouper (Epinephelus moara). Photodegradation reduced the size from ∼55.9 μm of P-PS to ∼38.6 μm of PD-PS, even produced nanoparticles (∼75 nm) with a yield of 7.03 ± 0.37% (w/w), and induced surface oxidation and formation of persistent free radicals (e.g., CO•, COO•). Also, endogenous pollutants (chemical additives and polymer fragments) were leached out. Thus, PD-PS had the highest growth inhibition and lipidosis-driven hepatic lesions of grouper, followed by P-PS and C-PS, which was mainly explained by increased hepatic bioaccumulation of MPs/NPs and released endogenous toxicants. Furthermore, oxidative stress-triggered mitochondrial depolarization, suppression of fatty acid oxidation and transport, and promotion of inflammation were identified as the key mechanisms for the enhanced hepatotoxicity after photodegradation. This work provides new insight into the potential hazard and harm of MPs in marine environments after photodegradation.

Formation of persistent free radicals in biochar derived from rice straw based on a detailed analysis of pyrolysis kinetics

The presence of persistent free radicals (PFR) in biochars may greatly broaden the application of biochars in pollution control, but may also cause negative impacts to the environment. Understanding the structural basis and the formation mechanisms of PFR is essential for a targeted biochar production and application. This study used rice straw (RS), a ubiquitous agricultural waste, to investigate the generation processes of PFR in relation to RS pyrolysis kinetics. Based on a detailed thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis, the activation energy was calculated by Kissinger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO) methods. This work combined pyrolysis kinetics analysis and solid particle characterization. Our results showed that lignin started to pyrolyze at a lower temperature than cellulose and hemicellulose. Lignin was the main factor for PFR generation. Chemical bond breaking contributed only slightly to PFR formation. The reconfiguration of the carbonaceous structures may be a more important contributor to PFR formation, while the cross-linking between different compositions and the interactions between the chemical compositions and inorganic minerals may play a significant role for PFR generation and stabilization in RS. This study provides useful theoretical basis to understand the thermal pyrolysis process of RS and the manipulation of biochar properties.

Photoformation of persistent free radicals on a montmorillonite-humic acid complex simulated as particulate organic matter in an aqueous solution.

This study investigates the formation of persistent free radicals (PFRs) on particulate organic matter (POM) under irradiation in water. A montmorillonite-humic acid complex (Mnt-HA complex) was prepared to simulate POM, and the generated PFRs were detected by the electron paramagnetic resonance (EPR) technique. EPR signals with the trend of an initial increase and then a decrease were observed under irradiation for 8 days, and the g factors were in the range of 2.0034-2.0039, which indicated the generation of carbon-centered radicals with electrophilic moieties. Different concentrations and types of halophenols and transition-metal ions were respectively adsorbed on the Mnt-HA complex to probe their influence on the formation of PFRs. The amount of PFRs generated in the Mnt-HA complexes was in the order: 2-bromophenol (2-BP) > 2,4-dibromophenol (2,4-DBP) > 2,4-dichlorophenol (2,4-DCP), which implied that halogen substitution and the number of substituents in the halophenols could affect the generation of PFRs. The effects of transition-metal ions that resulted in the reduction of PFRs when adsorbed on the Mnt-HA complex were as follows: Fe3+ > Zn2+ > Cu2+ > Mn2+, and this is in agreement with their redox capacity. Analyzing the induced generation of reactive oxygen species (ROS) and electrons on POM, it is found that halophenols and transition metal ions also affected this process under irradiation. These findings indicate that the photoformation of PFRs on POM could be a source of PFRs in aqueous environments and requires further attention.

Decontamination of Cr(VI) facilitated formation of persistent free radicals on rice husk derived biochar

This study investigated the facilitation of Cr(VI) decontamination to the formation of persistent free radicals (PFRs) on rice husk derived biochar. It was found that Cr(VI) remediation by biochar facilitated the production of PFRs, which increased with the concentration of treated Cr(VI). However, excessive Cr(VI) would induce their decay. Biochar with high pyrolysis temperature possessed great performance to Cr(VI) removal, which was mainly originated from its reduction by biochar from Inductively Coupled Plasma Optical Emission Spectroscopy and X-ray Photoelectron Spectroscopy. And the corresponding generation of PFRs on biochar was primarily ascribed to the oxidization of phenolic hydroxyl groups by Cr(VI) from Fourier Transform Infrared Spectroscopy analysis, which was further verified by the H2O2 treatment experiments. The findings of this study will help to illustrate the transformation of reactive functional groups on biochar and provide a new insight into the role of biochar in environmental remediation.

Nonnegligible role of biomass types and its compositions on the formation of persistent free radicals in biochar: Insight into the influences on Fenton-like process

Biochar, a green material that possesses high catalytic potential due to its persistent free radicals (PFRs), has attracted increasing attention in the removal of refractory pollutants from water. The influences of biomass types (bamboo, corn stalk, and pig manure) and its compositions (metals and phenolic compounds) on the formation of PFRs in biochar were investigated by electron paramagnetic resonance (EPR). It was found that the amounts of PFRs in biochar would decrease sharply with the decrease of the initial metals and phenolic compounds existed in biomass, and the effect of metals contents on PFRs formation was much greater than that of phenolic compounds contents. This finding was supported by the results obtained from elemental analysis and molar H/C analysis of three types of biochar, which suggested that pig-manure biochar (PM500) had the highest concentrations of PFRs of 14.13 × 1018 spins·g−1 due to the high content of metals and phenolic compounds in pig manure. The EPR trapping experiment and Linear sweep voltammetry (LSV) measurements in biochar/H2O2 systems showed that OH was the dominant reactive radical and electrons transfer pathways might be responsible for the activation of H2O2 by biochar. Possible degradation pathways of the tetracycline in biochar/H2O2 systems were also proposed. Besides, high degradation efficiency and good stability were observed in real wastewater application of the PM500/H2O2 system. These findings would provide a clearer insight into the formation mechanisms of PFRs in biochar, and also present a better strategy for biochar preparation and application in Fenton-like system.