A Comparative Study on the Formation of Environmentally Persistent Free Radicals (EPFRs) on Hematite and Goethite: Contribution of Various Catechol Degradation Byproducts

Emerging contaminants: A One Health perspective

Environmental pollution is escalating due to rapid global development that often prioritizes human needs over planetary health. Despite global efforts to mitigate legacy pollutants, the continuous introduction of new substances remains a major threat to both people and the planet. In response, global initiatives are focusing on risk assessment and regulation of emerging contaminants, as demonstrated by the ongoing efforts to establish the UN's Intergovernmental Science-Policy Panel on Chemicals, Waste, and Pollution Prevention. This review identifies the sources and impacts of emerging contaminants on planetary health, emphasizing the importance of adopting a One Health approach. Strategies for monitoring and addressing these pollutants are discussed, underscoring the need for robust and socially equitable environmental policies at both regional and international levels. Urgent actions are needed to transition toward sustainable pollution management practices to safeguard our planet for future generations.

Formation and biotoxicity of environmentally persistent free radicals in steelworks soil under thermal treatment

Thermal treatment are commonly used to address organic contaminated soils. In particular, the pyrolysis of organic substances can result in the creation of environmentally persistent free radicals (EPFRs). We investigated a steelworks site in Chongqing (China) to observe changes in EPFRs before and after thermal treatment. Our findings revealed that the EPFRs were carbon-centered radicals with a g-factor < 2.0030 and a spin density ranging from n.d.-5.23 × 1015 spins/mg. The formation of EPFRs was driving by polycyclic aromatic hydrocarbons (PAHs), Mn, Cu, and total organic carbon (TOC). Following the thermal treatment, the spin densities of EPFRs increased by a factor of 0.25 to 1.81, with maximum levels reached at 300 °C. High molecular weight PAHs exhibited high heat capacity, enabling the generation of more EPFRs. The thermal decay of EPFRs occurred in two stages, with the shortest 1/e lifetime lasting up to 16.8 h. Raising the temperature or prolonging time can significantly reduce EPFRs levels. Thermal treatment increased the generation of EPFRs, hydroxyl radicals (•OH) and superoxide radical (•O2-), leading to a decrease in bacterial luminescence. Specifically, •OH contributed to approximately 73% of the B. brilliantus inhibition. Our results highlight that the thermal treatment significantly enhance EPFRs concentrations, and the treated soil remained ecologically risky. The knowledge of the formation of EPFRs and their biotoxicity is shedding new light on the thermal treatment risk management.

A cryptand-like Ti-coordination compound with visible-light photocatalytic activity in CO2 storage

A cryptand-like Ti-coordination compound, namely Ti12Cs, comprising two Ti6-salicylate cages and hosting two Cs+ ions, was synthesized by the solvothermal method. It exhibits strong visible-light absorption with an absorption band edge of 652 nm, attributed to the electron transition from salicylate ligands to Ti ions. Electrochemical impedance, visible-light transient photocurrent response, and photoluminescence spectra confirm that Ti12Cs has excellent visible-light response and charge-separation properties. Ti12Cs can be used as a heterogeneous and recyclable photocatalyst for CO2/epoxide cycloaddition, with high utilization efficiency of visible-light under mild conditions. The mechanism investigation points to a synergistic effect of photocatalysis and Lewis acid catalysis.

Roles of Natural Phenolic Compounds in Polycyclic Aromatic Hydrocarbons Abiotic Attenuation at Soil-Air Interfaces through Oxidative Coupling Reactions

Little information is available on the roles of natural phenolic compounds in polycyclic aromatic hydrocarbons (PAHs) attenuation at dry soil-air interfaces. The purpose of this study was to determine the roles of model phenolic constituents of soil organic matter (SOM) on the abiotic attenuation of PAHs. The phenolic compounds can significantly change the attenuation rates of PAHs, among which hydroquinone was the most effective in promoting anthracene and benzo[a]anthracene attenuation. Product identification and sequential extraction experiments revealed hydroquinone enhanced the formation of oxidative coupling products and promoted the incorporation of PAHs into humic analogues, thereby reducing potential risks to humans and ecosystems. Electron paramagnetic resonance spectroscopy analyses showed both PAHs and phenolic compounds could donate electrons to Lewis acid sites of soil minerals, resulting in the generation of persistent free radicals (PFRs). PFRs could promote the generation of ·OH to enhance PAH oxidation and could cross-couple with PAHs, resulting in high-molecular-weight oxidative coupling products. This study revealed for the first time the reaction mechanism between PAHs and phenolic components of SOM under relatively dry conditions and provided new insights into promoting PAHs detoxification in soils but also a potential strategy to increase the organic carbon sequestration.

Enhanced Cr(VI) bioreduction by biochar: Insight into the persistent free radicals mediated extracellular electron transfer

Biochar can act as a shuttle to accelerate the extracellular electron transfer (EET) by exoelectrogens. However, it is poorly understood how the persistent free radicals (PFRs) in biochar affected EET and the redox reaction. Herein, the effects of the biochar and chitosan modified biochar (CBC) on the Cr(VI) bioreduction by Shewanella oneidensis MR-1 (MR-1) was investigated. Kinetic study indicated that the Cr(VI) bioreduction rate constant by MR-1 was increased by 1.8-33.7 folds in the presence of biochar, and by 2.7-60.2 folds in the presence of CBC, respectively. Moreover, Cr(VI) bioreduction rates increased with the decreasing pH. Results suggested that the electrostatic attraction between Cr(VI) and redox-active particles could accelerate the EET by c-cytochrome due to the promotion of the Cr(VI) migration from aqueous phase to biochar or CBC. Electron paramagnetic resonance analysis suggested that the PFRs affected the electron transfer from the ·O₂^- generated by MR-1 to Cr(VI) and accelerate the Cr(VI) bioreduction. Remarkably, in the presence of PFRs, this electron shuttling process was dependent on the non-metal-reducing respiratory pathway. Our results offer new insights that free radicals may be widely involved in the EET and strongly impact on the redox reaction in the environment.

The fate of diclofenac in anaerobic fermentation of waste activated sludge

Diclofenac (DCF), a nonsteroidal anti-inflammatory drug, is one of the most commonly detected pharmaceuticals in wastewater treatment plants. However, the fate of DCF in waste activated sludge (WAS) anaerobic fermentation has not been well-understood so far. This work therefore aims to comprehensively reveal whether and how DCF is transformed in WAS mesophilic anaerobic fermentation through both experimental investigation and density functional theory (DFT) calculation. Experimental results showed that ∼28.8% and 45.8% of DCF were respectively degraded during the batch and long-term fermentation processes. Based on the detected intermediates and DFT-predicted active sites, three metabolic pathways, i.e., chlorination, hydroxylation, and dichlorination, responsible for DCF transformation were proposed. DFT calculation also showed that the Gibbs free energy (ΔG) of the three transformation pathways was respectively 19.0, -4.3, and -19.3 kcal/mol, suggesting that the latter two reactions (i.e., hydroxylation and dichlorination) were thermodynamically favorable. Illumina MiSeq sequencing analyses revealed that DCF improved the populations of complex organic degradation microbes such as Proteiniclasticum and Tissierellales, which was in accord with the chemical analyses above. This work updates the fundamental understanding of the degradation of DCF in WAS anaerobic fermentation process and enlightens engineers to apply theoretical calculation to the field of sludge treatment or other complex microbial ecosystems.

Medium-Low Temperature Conditions Induce the Formation of Environmentally Persistent Free Radicals in Microplastics with Conjugated Aromatic-Ring Structures during Sewage Sludge Pyrolysis

Medium-low temperature pyrolysis is an effective method of retaining active components in sludge char. However, we found that incomplete cracking reactions resulted in residues of microplastics (MPs) remaining in the char; moreover, high levels of environmentally persistent free radicals (EPFRs) were detected in these MPs. Here, we investigated the temperature-dependent variations in the char-volatile products derived from sludge and MPs under different pyrolysis scenarios using multiple in situ probe coupling techniques and electron paramagnetic resonance spectroscopy, thereby identifying the sources of EPFRs and elucidating the corresponding formation-conversion mechanisms. The temperature was the key factor in the formation of EPFRs; in particular, in the 350-450 °C range, the abundance of EPFRs increased exponentially. Reactive EPFR readily formed in MPs with conjugated aromatic-ring structures (polyethylene terephthalate and polystyrene) at a temperature above 350 °C; EPFR concentrations were 5-17 times higher than those found in other types of polymers, and these radicals exhibited half-lives of more than 90 days. The EPFR formation mechanism could be summarized as solid-solid/solid-gas interfacial interactions between the polymers and the intermediate products from sludge pyrolysis (at 160-350 °C) and the homolytic cleavage-proton transfer occurring in the polymers themselves under the dual action of thermal induction and acid sites (at 350-450 °C). Based on the understanding of the evolution of EPFRs, temperature regulation and sludge components conditioning may be effective approaches to inhibit the formation of EPFRs in MPs, constituting reliable strategies to diminish the environmental risk associated with the byproducts of sludge pyrolysis.

Direct toxicity of environmentally persistent free radicals to nematode Caenorhabditis elegans after excluding the concomitant chemicals

Environmentally persistent free radicals (EPFRs) have attracted extensive attention due to their potential toxicity. However, EPFRs-containing particles always coexist with their parent organic contaminants and intermediate degradation products (IM), which may have hindered the toxicity assessment of EPFRs. In this study, the toxicity of EFFRs was specifically verified after comparing the systems without EPFRs, such as the immediate mixture of catechol (CT) and particles, solutions of CT only, IM extracted from the particles, as well as particles after EPFRs quenching. Caenorhabditis elegans (C. elegans) were used as model organisms. Our results showed that EPFRs-containing particles (Si-Al-CT) exhibited significant toxicity to C. elegans, but not for the parent chemical CT and IM on the particles. Higher levels of reactive oxygen species (ROS) and malondialdehyde (MDA) in the Si-Al-CT system were attributed to the mediated generation of ·O₂^- and ·OH via EPFRs. EPFRs could increase gene expressions related not only to oxidative stress and biotransformation in C. elegans, but also to indications of disturbances in energy homeostasis, survival, proliferation, cell and embryonic development. Overall, these results confirmed the direct toxicity of EPFRs and highlighted the key role of EPFRs which may be neglected in assessing the environmental risks of organic contaminants.

Catalyzed H2O2 decomposition over iron oxides and oxyhydroxides: Insights from oxygen production and organic degradation

Iron minerals, such as iron oxides and iron oxyhydroxides, are the main influential soil components in catalyzed hydrogen peroxide propagation (CHP). Due to their dual effects on H₂O₂ activation to produce reactive oxygen species (ROS) and invalid consumption to produce oxygen, the intrinsic reactivity of iron minerals toward H₂O₂ decomposition requires comprehensive investigations. Herein, six iron minerals (hematite, magnetite, maghemite, goethite, feroxyhyte, and ferrihydrite) for H₂O₂ decomposition were investigated by a combination of normalized kinetic rate constants of H₂O₂ decomposition (Nk_H2O2), O₂ production (Nk_O2), benzoic acid degradation (Nk_BA), and hexachloroethane degradation (Nk_HCA) over the surface area of each mineral. The results indicate H₂O₂ decomposition over iron minerals is a surface-related heterogeneous process. Hematite and goethite are the most promising minerals for environmental cleanup in terms of ROS production, because their H₂O₂ utilization efficiency for benzoic acid (BA) degradation (0.138 and 0.024 mol BA/mol H₂O₂ for hematite and goethite, respectively) are highest among the six iron minerals. Magnetite and maghemite are highly active for both H₂O₂ decomposition and O₂ production at neutral and basic pHs. The presence of organic compounds suppresses O₂ production by more than 60%, which favors H₂O₂ utilization. Ferrihydrite and feroxyhyte are considered as the problematic mineral for CHP due to that the two minerals acquire a high O₂ production and negligible ROS generation at all pHs. The results of this study provide new insights to increase the understandings of H₂O₂-iron mineral systems and guide the application of iron minerals in chemical oxidation technologies.

Photoformation of environmentally persistent free radicals on particulate organic matter in aqueous solution: Role of anthracene and formation mechanism

Environmentally persistent free radicals (EPFRs) generated under irradiation have been widely detected in soil particles, atmospheric particles and microplastic particles, but the formation of EPFRs in water is not well understood. This study investigated the formation of EPFRs on particulate organic matter (POM) in water contaminated by anthracene (Ant) under irradiation. The photoformation and decay progress of EPFRs was represented with the help of electron paramagnetic resonance (EPR) technique on both actual POM and Fe(III)-montmorillonite simulated samples. EPR signals at the range of 10¹⁶ to 10¹⁷ spin/g were detected and the half-life time of EPFRs stored in water was at around 16.62 h and 60.80 h, much shorter than those in the air. The g factors were all larger than 2.0040, which indicated the generation of oxygen centered EPFRs. The primary intermediates were identified by gas chromatography-mass spectrometer (GC-MS) and a possible EPFR formation pathway during Ant degradation was proposed. The interaction between Ant and POM, and the hydroxylation and carbonylation of the intermediates made contributions to the generation of EPFRs. Meanwhile, the indirect photodegradation of bisphenol A (BPA) has been demonstrated by analyzing the reactive oxygen species (ROS) and photogenerated electrons in the solution with POM containing EPFRs. It is found that hydroxyl radicals (•OH) and singlet oxygen (¹O₂) were induced and might promote the photodegration. Overall, our present study provided useful information to understand the photoformation of EPFRs on POM and their fate in aqueous environments.

Formation of environmentally persistent free radicals from photodegradation of triclosan by metal oxides/silica suspensions and particles

Metal oxides play an essential role in the photocatalysis of contaminants and substantially increase in the environment by the engineering production. However, whether emerging contaminants will be produced during photocatalysis of contaminants remains unclear. Here, triclosan (TCS) photodegradation in metal oxides/silica suspensions and particles, simulated as the states of metal oxides in water and soil environments, were studied. The photodegradation results confirmed that metal oxides exhibited a double-effect. They promoted TCS photodegradation by generating reactive oxidizing species (ROS) in metal oxides/silica suspensions and inhibited the photodegradation by competing with TCS for irradiation in metal oxides/silica particles. In this study, the critical discovery was the formation of emerging contaminants, environmentally persistent free radicals (EPFRs), and EPFRs yields were promoted by metal oxides (Al₂O₃, ZnO, TiO₂). They were more stable in metal oxides than silica, and the half-lives ranged from 6.7 h to 90.9 d. Although CuO did not increase EPFRs yields compared to silica, the half-lives of EPFRs were also longer. In addition, this study found that EPFRs yields were dependent on the metal oxides concentrations. Our results provided a new insight into the negative environmental impacts of metal oxides and improved our understanding of the formation and fate of EPFRs by metal oxides in soil and aquatic environments.

Humidity induces the formation of radicals and enhances photodegradation of chlorinated-PAHs on Fe(III)-montmorillonite

Chlorinated-PAHs (ClPAHs) are widely detected in the soil surface and atmospheric particles. However, the underlying mechanisms of their photodegradation are not well understood. In the present study, the formation of radicals on ClPAHs-contaminated clay minerals was quantitatively monitored via electron paramagnetic resonance (EPR) spectroscopy, and the impact of relative humidity (RH) was systematically explored. ClPAHs removal (> 75%) was attributed to electron transfer and •OH attack. The degradation easiness of ClPAHs follows: 2-ClNAP >2-ClANT >9-ClPHE >1-ClPYR. Light irradiation significantly improved the generation of reactive oxygen species (ROS, such as •OH and •O₂^-), and further generate a series of hydroxylated products of ClPAHs. Persistent free radicals (PFRs) were only detected on clay minerals contaminated with 2-ClANT and 1-ClPYR. RH 10-80%, the concentration of •OH and •O₂^- increased by 1.07 and 62.79 times respectively, which facilitated transformation of PFRs and ClPAHs degradation. The results of quantum chemical calculations indicate that the initial reaction of ClPAHs photodegradation is mediated by the substitution of •OH for chlorine groups. The present work implies that higher humidity may decrease the generation of PFRs on clay minerals and help mitigate the threats of PFRs and ClPAHs to human health.

Environmental persistent free radicals in diesel engine exhaust particles at different altitudes and engine speeds

The occurrence of environmental persistent free radicals (EPFRs) in the environment has attracted a great deal of research attention. Although the major sources of EPFRs in the environment is diesel engine exhaust, the study on the emission characteristics of EPFRs at different working conditions is still very limited. An integrated engine system was adopted to simulate different working conditions of various altitudes and engine speeds, and to examine the emission process of a diesel engine. The results suggested that low engine speed and high altitude are generally associated with high PM₁₀ emission with more stable and ordered structures. Based on the analysis of PAHs on solid and gas phases, PM₁₀ generated from diesel engine at altitude higher than 2000 m may contain substantial amounts of PAHs embedded inside particles, but not adsorbed on the surface. EPFRs signal up to 1.66 × 10²⁰ spins/g were detected in PM₁₀ of the diesel exhaust. Higher engine speed and lower altitude were associated with stronger EPR signals on PM₁₀. However, the accumulated EPR signal intensities after consuming 1 L of diesel were higher at lower engine speed and higher altitude, suggesting higher overall risks. A positive correlation between R value (signal strength ratio of D and G peaks on the Raman spectra) and EPFRs intensity indicated that the EPR signals were associated with the defects of carbon structure. EPFRs intensity in particles showed no significant change in dark, and over 70% of the EPR signals survived under UV light in a one-month aging simulation. The strong persistence of these EPFRs suggested their potential long lasting and widespread risks, which should be investigated extensively.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles

Microbial-mediated transformation of anthropogenic Cd²⁺ controls its distribution, bioavailability, and potential risks. However, the processes readily form CdS nanoparticles (CdS-NPs), which exhibit dissolution behavior different from that of larger sized particles. Here, we investigated the effects of morphologies and facets of CdS-NPs on their photoinduced dissolution. Three CdS-NPs, CdS-sphere, CdS-rod, and CdS-sheet, and one nanosized biogenic CdS (Bio-CdS) were synthesized with different dominant facets of {101}, {100}, {001}, and {111} and thus distinct surface chemistry. As explored by HRTEM, EPR, and DFT calculations, photogenerated e^-/h⁺ pairs were more likely to generate on CdS-sheet surfaces due to higher surface energies and a narrower band gap, facilitating the formation of •OH and thereby faster dissolution (k_obs = 6.126-6.261 × 10^-2 h^-1). The wider band gaps of CdS-sphere and CdS-rod caused less formation of O₂^•- and •OH, leading to slower oxidative dissolutions (k_obs = 0.090-0.123 and 2.174-3.038 × 10^-2 h^-1, respectively). Given the similar surface energy as that of CdS-sheet, the dissolution rate of Bio-CdS was close to that of CdS-rod and CdS-sheet, which was 1.6-3.5 times faster than that of larger sized CdS, posing higher environmental risks than thought. Altogether, this work revealed the facet effects on the dissolution of CdS-NPs, manifesting a deeper understanding of metal sulfides' environmental behaviors.

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.

CuO and TiO2 particles generated more stable and stronger EPFRs in dark than under UV-irradiation

Environmentally persistent free radicals (EFPRs) have recently attracted a great deal of research attention because of their significant toxicity and ubiquitous occurrence in the environment. The information is still very limited on how to estimate the intensity of EPFRs under ambient circumstances. This study is designed to specifically compare EPFRs generation during catechol degradation in dark and UV light irradiation. CuO and TiO₂ were selected as model metal oxides to coat on silica at 1% CuO has a large electron exchange capacity, which may mediate catechol degradation in dark, while TiO₂ possesses strong photocatalytic property and could accelerate catechol degradation under UV light. Under UV light irradiation, EPFRs were generated very quickly and reached the maximum value in 4 d, which was related to the photocatalytic property of the particle. However, these EPFRs dissipated quickly in 14 d. On the contrary, the intensities of EPFRs generated in dark were 2 times higher, and stabled for over 2 months. Therefore, the environmental impacts of EPFRs in dark may be widespread and long-lasting, which should be monitored more carefully. It should be noted that for CuO-coated silica, a significant amount of EPFRs (20% of the maximum) survived the UV-light irradiation and stabled during the experimental period (45 d). Stronger EPFRs were associated with more abundant dimer structures, suggesting the dimer structures were related to EPFRs formation during catechol degradation. Monitoring the generation of dimer structures in the degradation of organic chemicals may provide useful information to estimate EPFRs generation and risks.

Emission factors of environmentally persistent free radicals in PM2.5 from rural residential solid fuels combusted in a traditional stove

Emission factors (EFs) are crucial for establishing emission inventory and subsequent health risk assessment of pollutants. However, the EFs of environmentally persistent free radicals (EPFRs) in PM_2.5 have not been well investigated. We measured EPFRs in PM_2.5 from burning of different solid fuels in a traditional stove widely used in rural China and calculated the EFs of EPFRs (EF_EPFRs). The characteristics of EPFRs varied greatly with PM_2.5 depending on the feedstock, and the EF_EPFRs of crop residue, firewood and bitumite was 2.13 ± 1.04, 1.40 ± 0.76 and 1.08 ± 0.39 (10²⁰ spins·kg^-1), respectively. The estimated results of EPFRs emission associated with PM_2.5 showed that the crop residue was the main contributor to the top four provinces with high EPFRs emissions in China in 2010. A wide range (0.03-4.89 cig·person^-1·day^-1) of equivalent cigarette number converted by inhaling EPFRs in PM_2.5 was observed. Provinces with higher equivalent cigarette number were mainly agricultural provinces, because the rural residents tend to use readily available fuels. Additionally, EPFRs in collected PM_2.5 during 2 - month photoaging were more stable in particles with higher organic carbon contents. Our findings provided a new insight into the risk assessment of PM_2.5 from different sources by taking EPFRs into consideration.

Sorption mechanisms of antibiotic sulfamethazine (SMT) on magnetite-coated biochar: pH-dependence and redox transformation

Sorption of sulfonamides (SAs) on magnetite-coated biochar (MBC) is a promising approach for the remediation of antibiotic contaminants, due to its extended adsorption capacity and irreversibility. However, the actual sorption mechanisms of SAs on MBC remain unclear and the gap in knowledge hinders understanding of the fate of SAs in soils or sediments. In this study, various MBCs were prepared under different pyrolysis temperatures, with batch sorption experiments conducted using SMT as the model pollutant. Results of a two-compartment kinetic model demonstrated that aromatic components of MBCs dominated slow-sorption mechanisms, whereas the embedded magnetite further accelerated fast-sorption due to H-bonding. Modification of BC with magnetite improved the distribution coefficient (K_d) and isotherm linearity of SMT. Multi-parameter model results indicated that the pH-dependence of SMT sorption on BCs and MBCs occurred via a dominant mechanism of π-bond assisted H-bonding. Compared to pristine BCs, the change in pH-dependent sorption characteristics of SMT on MBC results from the regulation of π-bonding and proton configuration. Simultaneous transformation of SMT to sulfate ions on BCs or MBCs was also observed. The degradation of SMT occurred because of persistent free radicals (PFRs) on BCs or the inherent redox of iron minerals on MBCs. However, the small fraction of SMT transformed on BCs or MBCs was not found to result in overestimation of SMT sorption. This study presents the critical mechanisms of SMT sorption on pyrochars and provides novel understanding of the fate of SMT on carbonaceous materials during practical remediation applications.

Persistent Free Radicals from Low-Molecular-Weight Organic Compounds Enhance Cross-Coupling Reactions and Toxicity of Anthracene on Amorphous Silica Surfaces under Light

Polycyclic aromatic hydrocarbon (PAH) contamination has raised great environmental concerns, while the effects of low-molecular-weight organic compounds (LMWOCs) on PAH photodegradation at amorphous silica (AS)/air interfaces have been largely ignored. In this study, the phototransformation of anthracene (ANT) at amorphous silica (AS)/air interfaces was investigated with the addition of LMWOCs. ANT removal was attributed to ^•OH attacking and the energy transfer process via ³ANT*. Light irradiation induced the fractured ≡SiO^• or ≡Si^• generation on AS surfaces, which could react with absorbed H₂O and O₂ to generate ^•OH and further yield a series of hydroxylated products of ANT. The presence of citric acid and oxalic acid improved ^•OH generation and enhanced ANT removal by 1.0- and 2.2-fold, respectively. For comparison, the presence of catechol and hydroquinone significantly decreased ANT removal and produced coupling products. The results of density functional theory calculations suggest that persistent free radicals (PFRs) on AS surfaces from catechol or hydroquinone after ^•OH attacking prefer to cross-couple with ANT via C-C bonding rather than self-couple. Dianthrone and cross-coupling products might possess higher ecotoxicity, while hydroxylated products were less ecotoxic than their parent compounds based on Ecological Structure Activity Relationships (ECOSAR) estimation. The results of this study revealed the potential ecotoxicity of PAH-adsorbed particulates coexisting with LMWOCs and also provided a new insight into PAH transformation through PFR pathways.

The exposed hematite surface and the generation of environmentally persistent free radicals during catechol degradation

Environmentally persistent free radicals (EPFRs) have drawn increasing attention. It is reported that EPFR formation is dependent on the presence of transition metals; however the size of the metal particles is ignored. In this study, we hypothesized that transition metals in smaller particle sizes could more efficiently promote the generation of EPFRs and thus have higher risks. Nanosized hematite (nanoHMT) and microsized hematite (microHMT) were studied and compared. We monitored the degradation of catechol and the generation of EPFRs under both dark and ultraviolet light conditions. Catechol degradation was inhibited in the presence of hematite in the dark, with more significant inhibition by nanoHMT. However, under ultraviolet light, catechol degradation was promoted by hematite, with more significant promotion by nanoHMT. The yield of free radicals in the nanoHMT system was always higher than that in the microHMT system. More dimers were detected in the nanoHMT system, which may have played an important role in stabilizing free radicals. More trivalent Fe was converted to divalent Fe in the nanoHMT system than in the microHMT system. The relatively more active sites for the catechol interaction promoted EPFR generation. These results highlighted that size-dependent reactions should be well considered when predicting the environmental behavior and risks of organic contaminants.

Modification on biochars for applications: A research update

Biochars are the solid product of biomass under pyrolysis or gasification treatment, whose wholesale prices are lower than commercial activated carbons and other fine materials now in use. The employment of biochars as a renewable resource for field applications, if feasible, would gain apparent economic niche. Modification using physical or chemical protocol to revise the surface properties of biochar for reaching enhanced performances of target application has attracted great research interests. This article provided an overview of biochar application, particularly with the respect to the use of modified biochar as preferred soil amendment, adsorbent, electrochemical material, anaerobic digestion promotor, and catalyst. Based on literature works the current research trends and the prospects and research needs were outlined.

The key role of persistent free radicals on the surface of hydrochar and pyrocarbon in the removal of heavy metal-organic combined pollutants

We show that persistent free radicals (PFRs) on the surface of biochar can produce hydroxyl radicals (•OH) by catalyzing H₂O₂ to facilitate the removal of the combined pollutant BPA-Cr(VI). Microstructure characterization showed that the structures of pyrocarbon and hydrochar were significantly different when prepared at different temperatures. As the preparation temperature and preparation time for biochar increased, the concentration of PFRs first increased and then decreased. When biochar, PFRs, and H₂O₂ were present in the same solution, the single pollutants BPA and Cr(VI) as well as the combined pollutant BPA-Cr(VI) could be removed effectively, with removal rates greater than 90%. However, when PFRs, BPA, H₂O₂, and Cr(VI) were present in the same solution, Cr(VI) competed with H₂O₂ for electrons and promoted the removal of BPA. The results of this study could be applied to sludge recycling and be used to develop approaches to catalytically degrade combined pollutants.