Effect of straw decomposition on hexavalent chromium removal by straw: Significant roles of surface potential and dissolved organic matter.

Mobility and bioavailability of hexavalent chromium (Cr(VI)) in agricultural soils are affected by interactions between Cr(VI) and returned crop straws. However, the effect of straw decomposition on Cr(VI) removal and underlying mechanisms remain unclear. In this study, Cr(VI) removal by pristine and decomposed rice/rape straws was investigated by batch experiments and a series of spectroscopies. The results showed that straw decomposition inhibited Cr(VI) removal, regardless of straw types. However, the potential mechanisms of the inhibition were distinct for the two straws. For the rice straw, a lower zeta potential after decomposition suppressed Cr(VI) sorption and subsequent reduction. In addition, less Cr(VI) was reduced by the decomposed rice straw-derived dissolved organic matter (DOM) than the pristine one. In contrast, for the rape straw, due to the increased zeta potential after decomposition, the decreased Cr(VI) removal was mainly ascribed to less Cr(VI) reduction by the rape straw-derived DOM. These results emphasized the significant roles of straw surface potential and DOM in Cr(VI) removal, depending on straw types and decomposition, which facilitate the fundamental understanding of Cr(VI) removal by straws and are helpful for predicting the environmental risk of Cr and rational straw return in Cr(VI)-contaminated fields.

Overlooked Roles and Transformation of Carbon-Centered Radicals Produced from Natural Organic Matter in a Thermally Activated Persulfate System.

The presence and induced secondary reactions of natural organic matter (NOM) significantly affect the remediation efficacy of in situ chemical oxidation (ISCO) systems. However, it remains unclear how this process relates to organic radicals generated from reactions between the NOM and oxidants. The study, for the first time, reported the vital roles and transformation pathways of carbon-centered radicals (CCR•) derived from NOM in activated persulfate (PS) systems. Results showed that both typical terrestrial/aquatic NOM isolates and collected NOM samples produced CCR• by scavenging activated PS and greatly enhanced the dehalogenation performance under anoxic conditions. Under oxic conditions, newly formed CCR• could be oxidized by O2 and generate organic peroxide intermediates (ROO•) to catalytically yield additional •OH without the involvement of PS. Nuclear magnetic resonance (NMR) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) results indicated that CCR• predominantly formed from carboxyl and aliphatic structures instead of aromatics within NOM through hydrogen abstraction and decarboxylation reactions by SO4•- or •OH. Specific anoxic reactions (i.e., dehalogenation and intramolecular cross-coupling reactions) further promoted the transformation of CCR• to more unsaturated and polymerized/condensed compounds. In contrast, oxic propagation of ROO• enhanced bond breakage/ring cleavage and degradation of CCR• due to the presence of additional •OH and self-decomposition. This study provides novel insights into the role of NOM and O2 in ISCO and the development of engineered strategies for creating organic radicals capable of enhancing the remediation of specific contaminants and recovering organic carbon.

Unlocking the roles of wheat root exudates in regulating laccase-catalyzed estrogen humification.

While laccase humification has an efficient capacity to convert estrogenic pollutants, the roles of wheat (Triticum aestivum L.) root exudates (W-REs) in the enzymatic humification remain poorly understood. Herein, we presented the research into the effects of W-REs on 17ß-estradiol (E2) and bisphenol A (BPA) conversion in vitro laccase humification. W-REs inhibited E2 removal but promoted BPA conversion in the enzymatic humification, and the first-order kinetic constants for E2 and BPA were 0.27-0.69 and 0.28-0.55 h-1, respectively. Specialized small phenols and amino acids in W-REs were susceptible to laccase humification, resulting in increased copolymerization of estrogen and W-REs. In greenhouse hydroponics, the accumulated amounts of E2 (BPA) in the roots and shoots were estimated to be 0.87 (2.15) and 0.43 (0.51) nmol·plant-1 at day 3, respectively. By forming low- and eventually non-toxic copolymeric precipitates between estrogen and W-REs, laccase humification lowered the phytotoxicity and bioavailability of estrogen in the rhizosphere solution, consequently relieving its uptake, accumulation, and distribution in the wheat cells. This work sheds light on the roles of W-REs in regulating laccase-catalyzed estrogen humification, and gives an insight into the path of addressing organic contamination in the rhizosphere and ensuring food safety.

Long-term Application of Agricultural Amendments Regulate Hydroxyl Radicals Production during Oxygenation of Paddy Soils.

Hydroxyl radicals (•OH) play a significant role in contaminant transformation and element cycling during redox fluctuations in paddy soil. However, these important processes might be affected by widely used agricultural amendments, such as urea, pig manure, and biochar, which have rarely been explored, especially regarding their impact on soil aggregates and associated biogeochemical processes. Herein, based on five years of fertilization experiments in the field, we found that agricultural amendments, especially coapplication of fertilizers and biochar, significantly increased soil organic carbon contents and the abundances of iron (Fe)-reducing bacteria. They also substantially altered the fraction of soil aggregates, which consequently enhanced the electron-donating capacity and the formation of active Fe(II) species (i.e., 0.5 M HCl-Fe(II)) in soil aggregates (0-2 mm), especially in small aggregates (0-3 µm). The highest contents of active Fe(II) species in small aggregates were mainly responsible for the highest •OH production (increased by 1.7-2.4-fold) and naphthalene attenuation in paddy soil with coapplication of fertilizers and biochar. Overall, this study offers new insights into the effects of agricultural amendments on regulating •OH formation in paddy soil and proposes feasible strategies for soil remediation in agricultural fields, especially in soils with frequent occurrences of redox fluctuations.

Organic radicals driving polycyclic aromatic hydrocarbon polymerization with peracetic acid activation in soil.

The use of peracetic acid (PAA) in advanced oxidation processes has gained significant attention recently, but the knowledge of activating PAA to degrade polycyclic aromatic hydrocarbons (PAHs) is limited due to the variety and selectivity of reactive substances in PAA oxidation system. This paper presented the first systemically study on the degradation of PAHs by PAA activation in soil. It was found that heat-activated peracetic acid (heat/PAA) was capable of degrading phenanthrene (PHE) efficiently with degradation efficiency > 90 % within 30 min. Experimental results demonstrated that a series of reactive oxygen species (ROS) including organic radicals (RO•), hydroxyl radicals (HO•) and singlet oxygen (1O2) were generated, while acetylperoxyl (CH3C(O)OO•) and acetyloxyl (CH3C(O)O•) radicals were primarily responsible for PHE degradation in soil. Further analysis shows that polymerization products such as diphenic acid, 2'-formyl-2-biphenylcarboxylic acid and other macromolecules were dominant products of PHE degradation, suggesting polymerization driving PHE degradation instead of the conventional mineralization process. Toxicity analysis shows that most of the polymerization products had less toxicity than that of PHE. These results indicate that PAA activation was a highly effective remediation method for PAHs contaminated soil, which also provided a novel mechanism for pollutant degradation with the PAA activation process for environmental remediation.

Airborne environmentally persistent free radicals (EPFRs) in PM2.5 from combustion sources: Abundance, cytotoxicity and potential exposure risks.

As an emerging atmospheric pollutant, airborne environmentally persistent free radicals (EPFRs) are formed during many combustion processes and pose various adverse health effects. In health-oriented air pollution control, it is vital to evaluate the health effects of atmospheric fine particulate matter (PM2.5) from different emission sources. In this study, various types of combustion-derived PM2.5 were collected on filters in a partial-flow dilution tunnel sampling system from three typical emission sources: coal combustion, biomass burning, and automobile exhaust. Substantial concentrations of EPFRs were determined in PM2.5 samples and associated with significant potential exposure risks. Results from in vitro cytotoxicity and oxidative potential assays suggest that EPFRs may cause substantial generation of reactive oxygen species (ROS) upon inhalation exposure to PM2.5 from anthropogenic combustion sources, especially from automobile exhaust. This study provides important evidence for the source- and concentration-dependent health effects of EPFRs in PM2.5 and motivates further assessments to advance public health-oriented PM2.5 emission control.

Laccase-induced decontamination and humification mechanisms of estrogen in water-crop matrices.

Enzymatic humification plays a crucial biogeochemical role in eliminating steroidal estrogens and expanding organic carbon stocks. Estrogenic contaminants in agroecosystems can be taken up and acropetally translocated by crops, but the roles of laccase-triggered rhizospheric humification (L-TRH) in pollutant dissipation and plant uptake remain poorly understood. In this study, the laccase-induced decontamination and humification mechanisms of 17ß-estradiol (E2) in water-crop media were investigated by performing greenhouse pot experiments with maize seedlings (Zea mays L.). The results demonstrated that L-TRH effectively dissipated E2 in the rhizosphere solution and achieved the kinetic constants of E2 dissipation at 10 and 50 µM by 8.05 and 2.75 times as much as the treatments without laccase addition, respectively. The copolymerization of E2 and root exudates (i.e. phenols and amino acids) consolidated by L-TRH produced a larger amount of humified precipitates with the richly functional carbon architectures. The growth parameters and photosynthetic pigment levels of maize seedlings were greatly impeded after a 120-h exposure to 50 µM E2, but L-TRH motivated the detoxication process and thus mitigated the phytotoxicity and bioavailability of E2. The tested E2 contents in the maize tissues initially increased sharply with the cultivation time but decreased steadily. Compared with the treatment without laccase addition, the uptake and accumulation of E2 in the maize tissues were obviously diminished by L-TRH. E2 oligomers such as dimer, trimer, and tetramer recognized in the rhizosphere solution were also detected in the root tissues but not in the shoots, demonstrating that the acropetal translocation of E2 oligomers was interrupted. These results highlight a promising strategy for decontaminating estrogenic pollutants, boosting rhizospheric humification, and realizing low-carbon emissions, which would be beneficial for agroenvironmental bioremediation and sustainability.

Strong Substance Exchange at Paddy Soil-Water Interface Promotes Nonphotochemical Formation of Reactive Oxygen Species in Overlying Water.

Photochemically generated reactive oxygen species (ROS) are widespread on the earth's surface under sunlight irradiation. However, the nonphotochemical ROS generation in surface water (e.g., paddy overlying water) has been largely neglected. This work elucidated the drivers of nonphotochemical ROS generation and its spatial distribution in undisturbed paddy overlying water, by combining ROS imaging technology with in situ ROS monitoring. It was found that H2O2 concentrations formed in three paddy overlying waters could reach 0.03-16.9 µM, and the ROS profiles exhibited spatial heterogeneity. The O2 planar-optode indicated that redox interfaces were not always generated at the soil-water interface but also possibly in the water layer, depending on the soil properties. The formed redox interface facilitated a rapid turnover of reducing and oxidizing substances, creating an ideal environment for the generation of ROS. Additionally, the electron-donating capacities of water at soil-water interfaces increased by 4.5-8.4 times compared to that of the top water layers. Importantly, field investigation results confirmed that sustainable •OH generation through nonphotochemical pathways constituted of a significant proportion of total daily production (>50%), suggesting a comparable or even greater role than photochemical ROS generation. In summary, the nonphotochemical ROS generation process reported in this study greatly enhances the understanding of natural ROS production processes in paddy soils.

Formation of reactive intermediates in paddy water from different temperature zones for the promotion of abiotic ammonification.

The paddy field is a hot area of biogeochemical process. The paddy water has a large capacity in photo-generation of reactive intermediates (RIs) due to abundant photosensitive dissolved organic matter (DOM), which is influenced by the spatial heterogeneity of paddy soils but rarely been explored. Our work presents the first investigation of the role of soil properties on photochemistry in paddy water. Soil organic matter (SOM), determined by the temperature, was the dominant factor for the photo-generation of RIs in paddy water of main rice producing areas. The RI concentrations generated with abundant SOM from cool regions are 0.05-8.71 times higher than those for the warm regions in China. The humic-like substance and aromatic-like compounds of DOM plays an essential role in RIs generation, which is abundant in paddy soils rich in SOM from Chinese cool regions. In addition, RIs can efficiently accelerate the photo-ammonification of urea and free amino acids by 15.2 %-164 %, leading to 0.13-0.17 mmol/L/d photo-produced ammonium after fertilization, which is preferentially absorbed by rice. The findings of this study will extend our knowledge of the geochemistry of global paddy field ecosystem. The potential role of RIs in nitrogen cycle should be highlighted in the agroecosystem.

Matched Kinetics Process Over Fe2O3-Co/NiO Heterostructure Enables Highly Efficient Nitrate Electroreduction to Ammonia.

Tandem nitrate electroreduction reaction (NO3 -RR) is a promising method for green ammonia (NH3) synthesis. However, the mismatched kinetics processes between NO3 --to-NO2 - and NO2 --to-NH3 results in poor selectivity for NH3 and excess NO2 - evolution in electrolyte solution. Herein, a Ni2+ substitution strategy for developing oxide heterostructure in Co/Fe layered double oxides (LDOs) was designed and employed as tandem electrocataltysts for NO3 -RR. (Co0.83Ni0.16)2Fe exhibited a high NH3 yield rate of 50.4 mg ⋅ cm-2 ⋅ h-1 with a Faradaic efficiency of 97.8 % at -0.42 V vs. reversible hydrogen electrode (RHE) in a pulsed electrolysis test. By combining with in situ/operando characterization technologies and theoretical calculations, we observed the strong selectivity of NH3 evolution over (Co0.83Ni0.16)2Fe, with Ni playing a dual role in NO3 -RR by i) modifying the electronic behavior of Co, and ii) serving as complementary site for active hydrogen (*H) supply. Therefore, the adsorption capacity of *NO2 and its subsequent hydrogenation on the Co sites became more thermodynamically feasible. This study shows that Ni substitution promotes the kinetics of the NO3 -RR and provides insights into the design of tandem electrocatalysts for NH3 evolution.

Mechanism of sulfite enhanced As(III) oxidation in the As(III)-Fe minerals under ambient conditions.

Iron (Fe) minerals are known to be effective adsorbents for arsenic (As). However, the effects of sulfur species formed from the reductive dissolution of Fe minerals on the transformation of As(III) during the redox fluctuations processes under ambient conditions were poorly understood. Herein, we synthesized the As(III)-Fe minerals using sodium arsenite and ferric nitrate to investigate the effects of sulfur species on As(III) transformation in the As(III)-Fe minerals. Experimental results showed that sulfite rather than elemental sulfur and thiosulfate significantly accelerated As(III) oxidation. The oxidation rate of As(III) increased markedly from 0.0050 to 0.0168 min-1 with the increase of sulfite concentration from 0.5 to 2.0 mM. Sulfate radicals (SO4•-) and hydroxyl radicals (•OH) were identified as the dominant reactive species for As(III) oxidation. Besides, the underlying mechanism of Fe(II)/Fe(III) cycling for enhancing As(III) oxidation was further explored in the homogeneous Fe(II)/sulfite systems. Finally, interactions between sulfite and soil components induced radical formation, leading to As(III) oxidation in the soil environments. This study gives new insights into As(III) transformation co-existed with Fe minerals and sulfur species, which shed light on developing remediation strategies for regulating As contamination in temporarily flooded soils. ENVIRONMENTAL IMPLICATION: "New Insights into the Mechanism of Sulfur Species Induced As(III) Oxidation in the As-Fe Minerals" This study systematically explored the coupled effects between sulfur species and Fe minerals on As(III) transformation in the As-Fe-minerals under oxic conditions, which showed that sulfite significantly accelerated As(III) oxidation to As(V) via the enhanced formation reactive oxygen species (e.g., SO4•- and •OH). This study shed light on the development of remediation strategies in the contaminated soils with toxic pollutants via introducing sulfur species. We strongly believe this study is of great interest to environmental scientists and chemical engineers, especially those who works on the remediation of contaminated sites and wish to explore the high-efficiency strategies for the control of toxic pollutants like As.

Recovery of Chlorosilane Residual Liquid to Prepare Nano-Silica via the Reverse Micro-Emulsion Process.

In this paper, nano-silica particles were prepared from chlorosilane residue liquid using an inverse micro-emulsions system formed from octylphenyl polyoxyethylene ether (TX-100)/n-hexanol/cyclohexane/ammonia. The influence of different reaction conditions on the morphology, particle size, and dispersion of nano-silica particles was investigated via single-factor analysis. When the concentration of chlorosilane residue liquid (0.08 mol/L), hydrophile-lipophilic-balance (HLB) values (10.50), and the concentration of ammonia (0.58 mol/L) were under suitable conditions, the nano-silica particles had a more uniform morphology, smaller particle size, and better dispersion, while the size of the nano-silica particles gradually increased with the increase in the molar ratio of water to surfactant (ω). The prepared nano-silica was characterized through XRD, FT-IR, N2 adsorption/desorption experiments, and TG-DSC analysis. The results showed that the prepared nano-silica was amorphous mesoporous silica, and that the BET specific surface area was 850.5 m2/g. It also had good thermal stability. When the temperature exceeded 1140 °C, the nano-silica underwent a phase transition from an amorphous form to crystalline. This method not only promoted the sustainable development of the polysilicon industry, it also provided new ideas for the protection of the ecological environment, the preparation of environmental functional materials, and the recycling of resources and energy.

Dynamic Production of Hydroxyl Radicals during the Flooding-Drainage Process of Paddy Soil: An In Situ Column Study.

Frequent cycles of flooding and drainage in paddy soils lead to the reductive dissolution of iron (Fe) minerals and the reoxidation of Fe(II) species, all while generating a robust and consistent output of reactive oxygen species (ROS). In this study, we present a comprehensive assessment of the temporal and spatial variations in Fe species and ROS during the flooding-drainage process in a representative paddy soil. Our laboratory column experiments showed that a decrease in dissolved O2 concentration led to rapid Fe reduction below the water-soil interface, and aqueous Fe(II) was transformed into solid Fe(II) phases over an extended flooding time. As a result, the •OH production capacity of liquid phases was reduced while that of solid phases improved. The •OH production capacity of solid phases increased from 227-271 µmol kg-1 (within 1-11 cm depth) to 500-577 to 499-902 µmol kg-1 after 50 day, 3 month, and 1 year incubation, respectively. During drainage, dynamic •OH production was triggered by O2 consumption and Fe(II) oxidation. ROS-trapping film and in situ capture revealed that the soil surface was the active zone for intense H2O2 and •OH production, while limited ROS production was observed in the deeper soil layers (>5 cm) due to the limited oxygen penetration. These findings provide more insights into the complex interplay between dynamic Fe cycling and ROS production in the redox transition zones of paddy fields.

Alcohols radicals can efficiently reduce recalcitrant perfluorooctanoic acid.

Alcohols are commonly used as eluents for the regeneration of per/poly-fluoroalkyl substances (PFASs) adsorbents, but their potential effects on the subsequent treatment of these eluates have not been fully explored. This work investigated the effect of alcohols on perfluorooctanoic acid (PFOA) degradation by persulfate (PS) based advanced oxidation processes. The results showed that ethanol significantly promoted PFOA degradation in thermal/PS system. Under anoxic conditions, 25.5±1.4% or 91.2±1.6% of PFOA was degraded within 48 h in the absence or presence of ethanol. Electron paramagnetic resonance (EPR) detection, free radical quenching experiments, and chemical probe studies clearly demonstrated that the sulfate radicals (SO4•-) generated from PS activation would react with ethanol to form alcohol radicals, which could efficiently degrade PFOA. The transformation pathways of PFOA were proposed based on degradation products analysis and density function theory (DFT) calculation. The reaction between SO4•- and other alcohols could also induce the formation of alcohol radicals and facilitate to the degradation of PFOA. This work represents the positive roles of alcohols in the degradation of PFASs, providing new insights into developing simple and efficient treatments for PFASs eluate or PFAS-contaminated water.

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.

Oxygen vacancies-dominated reactive species generation from peroxymonosulfate activated by MoO3-x for pollutant degradation.

Interface oxygen vacancies (OVs) are commonly used to improve the catalytic performance of activators in persulfate-based advanced oxidation processes, but the underlying mechanism was not fully explored. This work reports a facile heat treatment method to regulate OVs in MoO3-x to elucidate the mechanism of peroxymonosulfate (PMS) activated by OVs to degrade 2,4,4-Trichlorobiphenyl (PCB28). Electron spin resonance, free radical quenching, X-ray photoelectron spectroscopy, and Raman spectroscopy confirmed that both reducing Mo species and OVs of MoO3-x surface were responsible for PMS activation. Further experiments and Density Function Theory (DFT) calculation suggest that OVs in MoO3-x induced the formation of superoxide radical (O2•-), and then O2•- was transformed into singlet oxygen (1O2) or mediated PMS activation to generate radicals, which contritbued to 70.2% of PCB28 degradation. The steady-state concentrations of free radical calculated with the kinetics model show that OVs were more favorable to mediate PMS to generate hydroxyl radicals (•OH) under oxic conditions, while reducing Mo species would like to induce PMS to produce sulfate radicals (SO4•-). Overall, this study is dedicated to a new insight into the in-depth mechanism of PMS activation by OVs-rich catalysts and provides a novel strategy for reactive species regulation in PMS based oxidation process.

Amendment of organic acids significantly enhanced hydroxyl radical production during oxygenation of paddy soils.

Recently, hydroxyl radical (•OH) production during soil redox fluctuations has been increasingly reported, but the low efficiency of contaminant degradation is the barrier for engineering remediation. The widely distributed low-molecular-weight organic acids (LMWOAs) might greatly enhance •OH production due to their strong interactions with Fe(II) species, but it was less investigated. Herein, we found that LMWOAs amendment (i.e., oxalic acid (OA) and citric acid (CA)) significantly enhanced •OH production by 1.2 -19.5 times during oxygenation of anoxic paddy slurries. Compared with OA and acetic acid (AA) (78.4 -110.3 µM), 0.5 mM CA showed the highest •OH accumulation (140.2 µM) due to the elevated electron utilization efficiency derived from its strongest capacity for complexation. Besides, increasing CA concentrations (within 6.25 mM) dramatically enhanced the •OH production and imidacloprid (IMI) degradation (increased by 48.6%), and further decreased due to the extensive competition from excess CA. Compared to 0.5 mM CA, the synergistic effects of acidification and complexation induced by 6.25 mM CA rendered more formation of exchangeable Fe(II) that easily coordinated with CA, and thus significantly enhanced its oxygenation. This study proposed promising strategies for regulating natural attenuation of contaminants using LMWOAs in agricultural fields, especially soils with frequent occurrence of redox fluctuations.

Phosphorus doping significantly enhanced the catalytic performance of cobalt-single-atom catalyst for peroxymonosulfate activation and contaminants degradation.

Increasing studies have been conducted to explore strategies for enhancing the catalytic performance of metal-doped C-N-based materials (e.g., cobalt (Co)-doped C3N5) via heteroatomic doping. However, such materials have been rarely doped by phosphorus (P) with the higher electronegativity and coordination capacity. In current study, a novel P and Co co-doped C3N5 (Co-xP-C3N5) was developed for peroxymonosulfate (PMS) activation and 2,4,4'-trichlorobiphenyl (PCB28) degradation. The PCB28 degradation rate increased by 8.16-19.16 times with Co-xP-C3N5 compared to conventional activators under similar reaction conditions (e.g., PMS concentration). The state-of-the-art techniques, including X-ray absorption spectroscopy and electron paramagnetic resonance etc., were applied to explore the mechanism of P doping for enhancing Co-xP-C3N5 activation. Results showed that P doping induced the formation of Co-P and Co-N-P species, which increased the contents of coordinated Co and improved Co-xP-C3N5 catalytic performance. The Co mainly coordinated with the first shell layer of Co1-N4, with successful P doping occurring in the second shell layer of Co1-N4. The P doping favored electron transfer from the C to N atom near Co sites and thus strengthened PMS activation owing to its higher electronegativity. These findings provide new strategy for enhancing the performance of single atom-based catalysts for oxidant activation and environmental remediation.

Mechanism of nitro-byproducts formation during persulfate-based electrokinetic in situ oxidation for remediation of anthracene contaminated soil.

Persulfate-based electrokinetic (EK) chemical oxidation appears to be a novel and viable strategy for the in situ remediation of polycyclic aromatic hydrocarbons (PAHs) polluted soil; however, the possible toxic byproducts of PAHs have been overlooked. In this study, we systematically investigated the formation mechanism of the nitro-byproducts of anthracene (ANT) during the EK process. Electrochemical experiments revealed that NH4+ and NO2- originating from nitrate electrolyte or soil substrates were oxidized to NO2• and NO• in the presence of SO4•-. Liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS/MS) analysis with 15N labeling revealed the formation of nitro-byproducts (14 kinds), including 1-hydroxy-4-nitro-anthraquinone and its similar derivatives, 4-nitrophenol, and 2,4-dinitrophenol. The nitration pathways of ANT have been proposed and described, mainly including the formation of hydroxyl-anthraquinone-oxygen and phenoxy radicals and the subsequent addition of NO2• and NO•. ANT-based formation of nitro-byproducts during EK, which is usually underestimated, should be further investigated due to their enhanced acute toxicity, mutagenic effects, and potential threat to the ecosystem.

New insights into persulfate decomposition by soil minerals: radical and non-radical pathways.

Persulfate (PS)-based in situ chemical oxidation (ISCO) has been widely used for pollutant remediation in soil and groundwater. However, the underlying mechanism of interactions between mineral and PS was not fully explored. In this study, several soil model minerals including goethite, hematite, magnetite, pyrolusite, kaolin, montmorillonite, and nontronite were selected to investigate their potential effects on PS decomposition and free radical evolution. It was found the decomposition efficiency of PS by these minerals varied significantly, and both the radical and non-radical decomposition processes were included. Pyrolusite has the highest reactivity for PS decomposition. However, PS decomposition is prone to form SO42- through non-radical pathway, and thus, the amounts of free radicals (e.g., •OH and SO4•-) produced are relatively limited. However, PS mainly decomposed to produce free radicals in the presence of goethite and hematite. In the presence of magnetite, kaolin, montmorillonite, and nontronite, PS both decomposed to produce SO42- and free radicals. Furthermore, the radical process exhibited the high degradation performance for model pollutant such as phenol with relatively high utilization efficiency of PS, while non-radical decomposition has limited contribution to phenol degradation with extremely low utilization efficiency of PS. This study deepened the understanding of interactions between PS and minerals during the PS-based ISCO in soil remediation.