Sulfur-Containing Persistent Free Radicals and Reactive Species on Photoaged Microplastics: Identification and the Formation Mechanism.

The elemental composition may affect the persistent free radical (PFR) and reactive species (RS) formation associated with photoaging microplastics; however, a relevant study is still lacking. This study systematically investigated the formation, evolution, and types of PFRs and RS on sulfur-containing microplastics (S-MPs) under simulated sunlight. Electron paramagnetic resonance detection and power saturation curve analysis isolated three different PFRs on each photoaging poly(phenylene sulfide) (PPS) and polysulfone (PSF). Combining the results of characterization and density functional theory calculation, these observed PFRs on the irradiated S-MPs were classified as oxygen-centered radicals with an adjacent S atom (namely, thio-oxygen radicals), oxygen-centered and sulfur-centered radicals, where the thio-oxygen radicals on PPS were benzenethiol-like radicals, and oxygen-centered radicals and sulfur-centered radicals on PSF that were identified as benzenesulfonic-like radicals and phenyl sulfonyl-like radicals, respectively. Moreover, potential precursor molecule fragments of PFRs on the photoaging S-MPs, including p-toluenesulfinic acid and benzenesulfonic acid, were detected by pyrolysis-gas chromatography/mass spectrometry and liquid chromatography-mass spectrometry. Interestingly, reactive sulfur species (SO3•-) was also observed on irradiated S-MPs in addition to reactive oxygen species, which was mainly derived from the reaction of •OH and sulfonyl radicals. These results have implications for assessing the potential risks of atmospheric S-MPs.

Soil pH determines arsenic-related functional gene and bacterial diversity in natural forests on the Taibai Mountain.

Arsenic-related functional genes are ubiquitous in microbes, and their distribution and abundance are influenced by edaphic factors. In arsenic-contaminated soils, soil arsenic content and pH determine the distribution of arsenic metabolizing microorganisms. In the uncontaminated natural ecosystems, however, it remains understudied for the key variable factor in determining the variation of bacterial assembly and mediating the arsenic biogeographical cycles. Here, we selected natural forest soils from southern and northern slopes along the altitudinal gradient of Taibai Mountain, China. The arsenic-related functional genes and soil bacterial community was examined using GeoChip 5.0 and high-throughput sequencing of 16S rRNA genes, respectively. It was found that arsenic-related functional genes were ubiquitous in tested forest soils. The gene arsB has the highest relative abundance, followed by arsC, aoxB, arrA, arsM, and arxA. The arsenic-related functional genes distribution on two slopes were decoupled from their corresponding bacterial community. Though there are higher abundance of bacterial communities on the northern slope than that on the southern slope, for arsenic-related functional genes, the abundance has the contrary trend which showing the more arsenic-related functional genes on the southern slope. In the top ten phyla, Proteobacteria and Actinobacteria were dominant phyla which affected the abundance of arsenic-related functional genes. Redundancy analysis and variance partitioning analysis indicated that soil pH, organic matter and altitude jointly determined the arsenic-related functional genes diversity in the two slopes of Taibai Mountain, and soil pH was a key factor. This indicates that the lower pH may shape more microbes with arsenic metabolic capacity. These findings suggested that soil pH plays a significant role in regulating the distribution of arsenic-related functional microorganisms, even for a forest ecosystem with an altitudinal gradient, and remind us the importance of pH in microbe mediated arsenic transformation.

Reliable model established depending on soil properties to assess arsenic uptake by Brassica chinensis.

Generally, prediction of arsenic (As) bioavailability, mobility and its transfer from soil to plant is very important with respect to management of environment and food safety. In this study, pakchoi (Brassica chinensis) was sown in a greenhouse to evaluate the As transfer characteristics from different soils to plant system, and to investigate the possible prediction equations and key factors involved in As bioavailability. The results showed that As uptake of plant and soil As concentration was significantly and positively correlated (R2 = 0.778; P < 0.01). A log-transformed data provided a better correlation (R2 = 0.901; P < 0.01). Results obtained from stepwise multiple linear regression (SMLR) showed that soil pH and total As were important variables involved in the contribution of As transfer to plant. The As accumulation in plant exhibited a positive correlation with soil As content and pH. Various prediction equations were obtained from different As sources, whereas the most favourable equation was screened by root mean square error (RMSE) between the measured and predicted Log [plant As] content. The prediction model (Log [plant As] =1.34 Log [soil As] +0.18pH-1.25) showed the greatest accuracy of R2 = 0.978 and RMSE = 0.11, by combining the data of three As treatments (45 observed data points). These current findings are quite useful and could be used for predicting the As transfer from soil to plant system.

Understanding the variation of microbial community in heavy metals contaminated soil using high throughput sequencing.

To improve the understanding of bacterial community in heavy metals contaminated soils, we studied the effects of environmental factors on the bacterial community structure in contaminated fields located in Shaanxi Province of China. Our results showed that microbial community structure varied among sites, and it was significantly affected by soil environmental factors such as pH, soil organic matter (SOM), Cd, Pb and Zn. In addition, Spearman's rank-order correlation indicated heavy metal sensitive (Ralstonia, Gemmatimona, Rhodanobacter and Mizugakiibacter) and tolerant (unidentified-Nitrospiraceae, Blastocatella and unidentified-Acidobacteria) microbial groups. Our findings are crucial to understanding microbial diversity in heavy metal polluted soils of China and can be used to evaluate microbial communities for scientific applications such as bioremediation projects.

Major controlling factors and prediction models for arsenic uptake from soil to wheat plants.

The application of current Chinese agriculture soil quality standards fails to evaluate the land utilization functions appropriately due to the diversity of soil properties and plant species. Therefore, the standards should be amended. A greenhouse experiment was conducted to investigate arsenic (As) enrichment in various soils from 18 Chinese provinces in parallel with As transfer to 8 wheat varieties. The goal of the study was to build and calibrate soil-wheat threshold models to forecast the As threshold of wheat soils. In Shaanxi soils, Wanmai and Jimai were the most sensitive and insensitive wheat varieties, respectively; and in Jiangxi soils, Zhengmai and Xumai were the most sensitive and insensitive wheat varieties, respectively. Relationships between soil properties and the bioconcentration factor (BCF) were built based on stepwise multiple linear regressions. Soil pH was the best predictor of BCF, and after normalizing the regression equation (Log BCF=0.2054 pH- 3.2055, R(2)=0.8474, n=14, p<0.001), we obtained a calibrated model. Using the calibrated model, a continuous soil-wheat threshold equation (HC5=10((-0.2054 pH+2.9935))+9.2) was obtained for the species-sensitive distribution curve, which was built on Chinese food safety standards. The threshold equation is a helpful tool that can be applied to estimate As uptake from soil to wheat.

Major factors influencing cadmium uptake from the soil into wheat plants.

At present, soil quality standards for agriculture have not been improved for many years and are applied uniformly for a diverse variety of crops and different soil types, not fully considering the effects of soil properties on cadmium (Cd) uptake via soil-plant transfer. In this study, the characteristics of Cd transfer from soil to eight wheat varieties were investigated, and the results showed that Xiaoyan 22 was moderately sensitive to Cd. Upon growing Xiaoyan 22 in 18 different Chinese soils, we studied the major controlling factors of Cd transfer and constructed a bioaccumulation prediction model from the soil properties. The results showed that pH was the most important factor contributing to Cd uptake. After calibration for the eight wheat varieties, a continuous soil threshold model for wheat was derived for the species sensitive distribution based on food safety standards.

[Safe Utilization Effect of Passivator on Mild to Moderate Cadmium Contaminated Farmland].

In order to study the safe utilization of acid cadmium （Cd） contaminated soil， light and moderate Cd-contaminated farmland in Shangluo， Shaanxi Province was taken as the research object， and lime， biochar， and calcium magnesium phosphate fertilizer were applied. Through the wheat-maize rotation experiment， the safe utilization effect of different amounts of passivator on Cd-contaminated soil was explored， and the best ratio of passivator was selected. The results showed that： ① the soil quality could be improved to varying degrees by applying the passivator. ② After the application of amendments， the grain yield of wheat and maize increased to different degrees. ③ The lime 2 340 kg·hm-2 （C3） treatment had the best effect， which increased the soil pH of wheat and corn by 1.453 and 1.717 units， respectively， and reduced the available Cd content by 34.38% and 30.20%， respectively. ④ The application of biochar 1 800 kg·hm-2 （B2） treatment had the best effect on reducing the Cd contents in wheat roots， straws， and grains， which were significantly reduced by 53.60%， 38.86%， and 52.96%， respectively， compared with that in CK. The Cd content in wheat grains was reduced to 0.09 mg·kg-1， which was lower than the limit value of wheat Cd （0.1 mg·kg-1） specified in the "National food safety standard food pollutant limit" （GB 2762-2017）. The application of the biochar 1 260 kg·hm-2 （B1） treatment had the best comprehensive effect on reducing the Cd contents of maize roots， straws， and grains， which were significantly reduced by 43.74%， 53.20%， and 94.57%， respectively， compared with that in CK. The Cd content of maize grains was reduced to 0.001 9 mg·kg-1， which was far lower than the limit value of maize Cd （0.1 mg·kg-1） specified in the "National food safety standard food pollutant limit" （GB 2762-2017）. Therefore， under the conditions of the field experiment， considering the influence of various indexes， biochar had the best effect on farmland soil in the wheat-maize rotation area with mild to moderate Cd pollution.

Arsenite adsorption and oxidation affected by soil humin: The significant role of persistent free radicals and reactive oxygen species.

Humin (HM), as the main component of soil organic matter, carries various reactive groups and plays a crucial regulatory role in the transformation of arsenic (As). However, current research on the redox pathway of As and its interactions with HM is relatively limited. This study aimed to explore the impact of different HM samples on the redox characteristics of As. The results showed that HM can not only adsorb arsenite [As(III)] but also oxidize As(III) into arsenate [As(V)]. However, once As(III) is adsorbed on the HM, it cannot undergo further oxidation. HMNM (extracted from peat soil) exhibited the highest adsorption capacity of As(III), with a maximum amount of 1.95 mg/kg. The functional groups of HM involved in As complexation were primarily phenolic hydroxyl and carboxyl groups. The adsorption capacity of HM samples for As(III) was consistent with their carboxyl group contents. The oxygen-containing functional groups and environmentally persistent free radicals (EPFRs) on HM can directly oxidize As(Ⅲ) through electron transfer, or indirectly induce the production of reactive oxygen species (ROS), such as hydroxyl radicals, to further oxidize As(Ⅲ). This study provides new insight into the transport and transformation process of As mediated by soil HM, and establishes a theoretical basis for As remediation.

Respecting catalytic efficiency of soil arylsulfatase as soil Sb contamination bio-indicator by enzyme kinetic strategy.

Antimony (Sb), a toxic metalloid, is ubiquitous in the environment and threatens human and ecological health. Soil arylsulfatase (ARS) activity indicates heavy metal pollution. However, the enzyme's substrate concentration can affect the toxicity evaluation of heavy metals using enzyme activity. Enzyme kinetic parameters directly reflect the potency of heavy metals, and the magnitude of these parameters does not change with the substrate concentration of soil enzyme. In this work, seventeen soils were exposed to Sb contamination to investigate the change of kinetic parameters of soil arylsulfatase under Sb stress. Results showed that Sb inhibited soil arylsulfatase activity. The maximum reaction rate (Vmax) of soil arylsulfatase was reduced by 11.58-46.72% in 16 tested soils and unchanged in S15 when exposed to Sb. The Michaelis constant (Km) presented three trends: unchanged, increased by 28.46-41.27%, and decreased by 19.71-29.91% under Sb stress. The catalytic efficiency (Ka as the ratio of Vmax to Km) decreased by 12.56-55.17% in all soils except for S12 and S16. Antimony acted as a non-competitive and linear mixed inhibitor by decreasing ARS activity in S1-S12, S14, and S17-S18 soils, as an uncompetitive inhibitor in S13 and S16 soils and as a competitive inhibitor in S15. The competitive and uncompetitive inhibition constants (Kic and Kiu) were 0.058-0.142 mM and 0.075-0.503 mM. The ecological dose values of Sb to catalytic efficiency (Ka) of ARS (ED10-Ka) ranged from 50 to 1315 mg kg-1. Soil pH and total phosphorus (TP) contents were the dominant factors responsible for Sb toxicity on Ka by affecting the interaction of inhibitor (Sb) with enzyme-substrate (ES) complex. The findings of this study advance the current knowledge on Sb toxicity to soil enzymes and have significant implications for the risk assessment of Sb in soils.

Soil phosphatase assay to evaluate arsenic toxicity should be performed at the soil's actual pH.

Soil phosphatase is considered an indicator to assess soil arsenic (As) pollution. In the phosphatase activity determination, a fixed buffer value (pH 5-10) is commonly used for all soils, ignoring the soil's actual pH. Here, we determined the soil phosphatase activity of 20 soils under As stress at the soils' pH, and the As inhibition mechanism was also explored by the enzyme kinetics. Our results show that soil phosphatase activity was significantly inhibited under As stress. The inhibition rate in acid soils (39.2 %) was considerably higher than in alkaline soils (25.4 %) when As concentration was 600 mg kg-1. For alkaline soils, As inhibited phosphatase by competitive inhibition or linear mixed inhibition, while for acid soils, it was more complex, including linear mixed inhibition, non-competitive inhibition, and anti-competitive inhibition. Simultaneously, our results showed that the ecological dose (ED10) described by the partial inhibition model was far below than the complete inhibition model. According to the partial inhibition model, the ED10 of As ranged from 2.66 to 164.07 mg kg-1 for alkaline soils and 0.11 to 89.95 mg kg-1 for acid soils. Moreover, Vmax/Km of phosphatase is a more sensitive index for evaluating As contamination than Vmax in partial inhibition models. The ED10 obtained based on the relationship between Vmax/Km and As concentration was 0.64-34.75 mg kg-1 for acid soils and 8.48 to 20.16 mg kg-1 for alkaline soils. This also confirms Vmax/Km as a sensitive and ideal index for assessing As pollution under soils' actual pH. Furthermore, soil pH and cation exchange capacity are dominant factors affecting As inhibition on soil phosphatase. The above kinetic studies indicate that performing the assay by adjusting the buffer pH to the soil pH is essential for more accurately evaluating arsenic toxicity.

Effects of atrazine on microbial metabolic limitations in black soils: Evidence from enzyme stoichiometry.

Long-term input of agricultural chemicals such as pesticides into the soil can increase soil pollution, thereby affecting the productivity and quality of black soil. Triazine herbicide atrazine has been shown to have long-lasting residual effects in black soil. The atrazine residues affected soil biochemical properties, further leading to microbial metabolism restriction. It is necessary to explore the strategies to mitigate the limitations on microbial metabolism in atrazine-contaminated soils. Here, we evaluated the effect of the atrazine on microbial nutrient acquisition strategies as indicated by extracellular enzyme stoichiometry (EES) in four black soils. Atrazine degradation in soil followed the first-order kinetics model across various concentrations ranging from 10 to 100 mg kg-1. We found that the atrazine was negatively correlated with the EES for C-, N-, and P-acquisition. Vector lengths and angles decreased and increased significantly with an increase of atrazine concentration in tested black soils except for Lishu soils. Moreover, the vector angles were >45° for tested four black soils, indicating that atrazine residue had the greatest P-limitation on soil microorganisms. Interestingly, microbial C- and P-limitations with different atrazine concentrations showed a strong linear relationship, especially in Qiqihar and Nongan soils. Atrazine treatment significantly negatively affected microbial metabolic limitation. Soil properties and EES interaction explained up to 88.2% for microbial C-/P-limitation. In conclusion, this study confirms the EES as a useful method in evaluating the effects of pesticides on microbial metabolic limitations.

Inorganic anions influenced the photoaging kinetics and mechanism of polystyrene microplastic under the simulated sunlight: Role of reactive radical species.

The photo-transformation of microplastic (MP) in natural water may involve interactions with various ingredients, but the photoaging kinetics and underlying mechanism are not well understood. This work systematically explored the photoaging process of polystyrene microplastic (PS-MP) in the presence of commonly-found inorganic anions, including NO3-, HCO3-, Br- and Cl-. The addition of these ions led to more obvious changes in the morphology, functional groups and molecular weight of photoaging PS-MP. The evolution of carbonyl index value for the photoaged samples conformed to pseudo-first-order kinetic model, and the photoaging rate constant (k) in the presence of inorganic anions at their environmentally relevant concentrations of 0.6 mM, 1.2 mM, 0.1 M and 0.1 mM was calculated to be kHCO3- = 0.0074 d-1, kNO3- = 0.01001 d-1, kCl- = 0.00783 d-1, and kBr- = 0.00888 d-1, which was higher than that in ultrapure water (k=0.00705 d-1). Electron paramagnetic resonance technique and quenching experiments demonstrated that photo-transformation of PS-MP was mainly mediated by indirect photolysis, i.e., the formation of reactive radical species. The photosensitivity of NO3- promoted more •OH production, thereby accelerated the indirect photoaging of PS-MP. Meanwhile, the presence of halide ions promoted the generation of reactive halogen species, which were also involved in the indirect photoaging of PS-MP. Interestingly, as •OH scavenger, HCO3- had no inhibitory effect on PS-MP photoaging, attributing to the oxidation of CO3•-. This study provides valuable insights into the understanding of photo-transformation of MPs in natural aquatic environments.

Short-term effects of wildfire on soil arthropods in a semi-arid grassland on the Loess Plateau.

Fires lead to dramatic shifts in ecosystems and have a large impact on the biota. Soil organisms, especially soil fauna, are often used as indicators of environmental change. At present, minimal attention has been paid to using soil fauna as an indicator of environmental change after a fire. Here, a field survey of burnt herbaceous vegetation in semi-arid areas was conducted to determine the response of soil arthropods to fire and their short-term recovery after fire. Overall, the abundance and biomass of soil arthropods was more sensitive to fire than the number of groups. The number of soil arthropod groups, especially the dominant groups (mites and springtails), was not significantly affected by wildfires. At the unburned site, soil arthropod abundance showed significant seasonal shifts that may be related to the vegetation properties, temperature, and precipitation caused by seasonal changes. In contrast, soil arthropods at the burnt sites showed a delayed recovery and had only reached 56%-82%, 17%-54%, and 91%-190% of the biomass in the unburnt forest at the 3, 6, and 9 months after the burning event. Our findings of soil arthropod abundance changes in the present study suggest that fire-induced changes in soil and vegetation properties (e.g., AN, LT, and VC) were crucial factors for the changes in soil arthropod abundance in this semi-arid grassland. We conclude that fire disturbance reduces the seasonal sensitivity of soil arthropods by altering their habitat. This study furthers our understanding of wildfire impact recovery by documenting the short-term temporal dynamics of soil arthropods.

Understanding the sorption behaviors of heavy metal ions in the interlayer and nanopore of montmorillonite: A molecular dynamics study.

The molecular-scale adsorption mechanism of heavy metal ions in the interlayer and nanopore regions of montmorillonite (MMT) were investigated by molecular dynamics simulations. Three typical heavy metals (zinc, cadmium, and lead) were selected as the model ions, and two types of MMT (Arizona and Wyoming) were considered. The results showed that Cd2+ and Pb2+ can form both inner- and outer-sphere complexes on Wyoming MMT, while Zn2+ only formed outer-sphere complex due to the stronger hydration interaction of Zn2+ than Cd2+ and Pb2+. For Arizona MMT, all of the three ions only formed outer-sphere complexes on its interlayer and external basal surface in which the cations remained a fully hydrated state. The calculated diffusion coefficients of three cations in interlayer and nanopore indicated that their diffusion abilities were significantly impaired, implying that MMT adsorbents have a strong ability to fix and retard heavy metal ions. The derived results and mechanisms are instrumental to a profound understanding of the transport and retention of heavy metal elements in subsurface environments, and provide guidance for the management of heavy metal pollution.

Soil organic carbon transformation and dynamics of microorganisms under different organic amendments.

Organic amendments (OAs) application is a practical strategy to improve soil organic carbon (SOC) in agriculture. The present study evaluated the impact of different OAs on the transformation of carbon and the dynamics of microorganisms in a 77-day incubation experiment. The OA treatments applied included wheat straw (U + WS), pig manure (U + PM), compost (U + CP), and improved compost (U + IC), and the no amendment group was the CK. After incubation, the SOC increased significantly in the U + WS group, but the other OA treatments had no significant effect relative to the CK. Among the OA treatments, U + CP and U + IC had lower CO2-C cumulative mineralization and the highest humification of dissolved organic carbon (DOC). U + PM had the lowest SOC content and the lowest aromatization of DOC. Redundancy analyses (RDA) showed that the CO2-C cumulative mineralization directly influenced the DOC, extracted organic carbon (EOC) and microbial biomass carbon (MBC) in all treatments. Proteobacteria positively correlated with SOC and MBC, Bacteroidetes were significantly related to DOC, and Gemmatimonadetes had a significant negative relationship with CO2-C cumulative mineralization. These results showed that U + CP and U + IC were more conducive to carbon sequestration, and U + PM was the most unfavourable during the incubation. Wheat straw played an important role in the steady improvement of the SOC.

How important is abiotic dissipation in natural attenuation of polycyclic aromatic hydrocarbons in soil?

Natural attenuation capacity, as one of the most important ecosystem functions in soil, plays a vital role in the detoxification of organic pollutants such as polycyclic aromatic hydrocarbons (PAHs). However, despite the role of biodegradation is established, the contribution of abiotic dissipation to natural attenuation has long been overlooked. Herein, the abiotic dissipations of 16 types of PAHs in a past coking site and of anthracene (ANT) in various cultivated soils were studied. Results showed that the contributions of abiotic dissipation to the total attenuation were in a wide range from 11.8 to 99.7% depending on the types of PAHs. Specifically, abiotic dissipation is higher for heavy PAHs (68.3-99.7%) than for light PAHs (11.8-71.5%), with the exception of ANT (80.7%). Similarly, the contribution of abiotic dissipation to ANT attenuation ranged from 30.7 to 68.6% in eight soils. The abiotic dissipation rate of ANT followed the order of lateritic-red earth > gray-desery soil > coastal solonchaks > cumulated-irrigated soil > cinnamon soil > fluvo-aquic soil > purplish soil ~ yellow-brown earth, which was positively correlated with transition metal contents in soils. These findings demonstrated that the abiotic dissipation of PAHs is determined by both molecule properties and soil types. Overall, this work provided valuable insights into clarifying the roles of abiotic dissipation in PAH attenuation in soil.

Interfacial reaction between organic acids and iron-containing clay minerals: Hydroxyl radical generation and phenolic compounds degradation.

Reactive oxygen species, especially hydroxyl radicals (OH), exert a distinguished role in the transformation of contaminants, and their in-situ generation attracts wide attentions in environmental and geochemical areas. The present work explored the potential formation of OH during the interactions between iron-containing clay minerals and environmentally prevalent organic acids in dark environments. The results demonstrated that the accumulative OH concentrations were related to the solution pH, the types of clay minerals, and the nature of organic acid species. At pH 5.5, 1.2- 15.2 times of OH were generated from the reduction of Na-nontronite-2 (Na-NAu-2) compared with other clay minerals in the presence of ascorbic acid (AA) at 144 h. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) analyses indicated that Fe(III) was reduced to Fe(II) by AA during OH formation. Meanwhile, chemical probe tests coupled with quenching experiments confirmed the generation of H2O2 and superoxide radical (O2-), which participated in the formation of OH. The produced OH/O2- can transform 68.4%, 86.4%, and 50.1% of phenol, p-nitrophenol, and 2,4-dichlorophenol within 168 h in AA-Na-NAu-2 suspension, respectively. This work provides valuable insights into OH production in the mutual interaction between organic acids and iron-bearing clays, which is helpful for the development of a new method for removing organic pollutants from contaminated water and soil environments.

Deciphering the transformation mechanism of substituted polycyclic aromatic hydrocarbons on Al(III)-montmorillonite: An experimental and density functional theory study.

The researches on transformation of polycyclic aromatic hydrocarbons (PAHs) on clay minerals modified by metal ions have received increasing attention. However, the transformation of PAHs with electron-withdrawing or electron-donating substitutional groups on clay minerals is not well understood currently. In this study, the degradation of anthracene (ANT) with different substituents (including -CH3, -CHO, -Br, -OMe, and -NO2) on Al(III)-montmorillonite (MMT) was investigated in the dark. The results showed that aromatic compounds were degraded with the rate constants (kobs) of 0.004-0.141 d-1. Moreover, ANT with electron-donating substituents (e.g., -CH3, -OMe) had a higher transformation rate than that with electron-withdrawing substituents (e.g., -Br, -NO2). The reactive oxygen species (ROS) quenching experiments indicated that ROS played a significant role in the transformation of ANT and ANT derivatives. Density functional theory (DFT) calculations revealed that the reactivity of single substituted PAHs was highly correlated with their ionization potential (IP), the energy of highest occupied molecular orbital (EHOMO), the energy of lowest unoccupied molecular orbital (ELUMO), and electronegativity (ζ), while independent of hardness (η). This study provides novel insights into predicting the reactivity of PAHs derivatives, and lays a fundamental basis for better understanding the fate of substituted PAHs in soils.

Greatly enhanced oxidative activity of δ-MnO2 to degrade organic pollutants driven by dominantly exposed {-111} facets.

The reactivity of oxidizing materials is highly related to the exposed crystal facets. Herein, δ-MnO2 with different exposure facets were synthesized and the oxidative activities of the as-prepared materials were evaluated by degrading phenol in water without light. The degradation rate of phenol by δ-MnO2-{-111} was significantly higher than that by δ-MnO2-{001}. δ-MnO2-{-111} also displayed high degradation efficiency to a variety of other organic pollutants, such as ciprofloxacin, bisphenol A, 3-chlorophenol and sulfadiazine. Comprehensive characterization and theoretical calculation verified that the {-111} facet had high density of Mn3+, thus displaying enhanced direct oxidative capacity to degrade organic pollutants. In addition, the dominant {-111} facet promoted adsorption/activation of O2, thus favored the generation of superoxide radical (O2•-), which actively participated in the degradation of pollutants. The phenol degradation kinetics could be divided into two distinct phases: the rapid phase (k1obs = 0.468 min-1) induced by Mn3+ and the slower phase (k2obs = 0.048 min-1) dominated by O2•-. The synergistically promoted non-radical and radical based reactions resulted in greatly enhanced the oxidative activity of the δ-MnO2-{-111}. These findings deepen the understanding of facet-dependent oxidative performance of materials and provided valuable insights into the possible practical application of δ-MnO2 for water purification.

Evolution and stabilization of environmental persistent free radicals during the decomposition of lignin by laccase.

Soil microbial enzymes may induce lignin decomposition, accompanied by generation of free radicals. The evolution of environmentally persistent free radicals (EPFRs) and reactive oxygen species (ROS) during laccase-catalyzed lignin decomposition remains unclear. Characterization by electron paramagnetic resonance spectroscopy revealed gradually increased concentration of EPFRs, with maximum levels within 6 h that remained constant, accompanied by the increase in g-factor from 2.0037 to 2.0041. The results suggested the generation of oxygen-centered radicals on lignin. The EPFRs produced on solid samples slowly decreased by 17.2% over 17 d. ROS were also detected to have a similar trend as that of the evolution of EPFRs. Scanning electron microscopy, attenuated total reflectance-Fourier transform infrared spectroscopy, gel permeation chromatography and nuclear magnetic resonance analyses suggested the demethylation and oxidation of lignin. We clarify the biogeochemical transformation of lignin and potential contributions to the generation of EPFRs and ROS in soil.