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.

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.

Biotic Process Dominated the Uptake and Transformation of Ag+ by Shewanella oneidensis MR-1.

Silver ions (Ag+) directly emitted from industrial sources or released from manufactured Ag nanoparticles (AgNPs) in biosolid-amended soils have raised concern about the risk to ecosystems. However, our knowledge of Ag+ toxicity, internalization, and transformation mechanisms to bacteria is still insufficient. Here, we combine the advanced technologies of hyperspectral imaging (HSI) and single-particle inductively coupled plasma mass spectrometry to visualize the potential formed AgNPs inside the bacteria and evaluate the contributions of biological and non-biological processes in the uptake and transformation of Ag+ by Shewanella oneidensis MR-1. The results showed a dose-dependent toxicity of Ag+ to S. oneidensis MR-1 in the ferrihydrite bioreduction process, which was primarily induced by the actively internalized Ag. Moreover, both HSI and cross-section high-resolution transmission electron microscopy results confirmed that Ag inside the bacteria existed in the form of particulate. The Ag mass distribution in and around live and inactivated cells demonstrated that the uptake and transformation of Ag+ by S. oneidensis MR-1 were mainly via biological process. The bioaccumulation of Ag+ may be lethal to bacteria. A better understanding of the uptake and transformation of Ag+ in bacteria is central to predict and monitor the key factors that control Ag partitioning dynamics at the biointerface, which is critical to develop practical risk assessment and mitigation strategies.

Pyrogenic Carbon Initiated the Generation of Hydroxyl Radicals from the Oxidation of Sulfide.

Sulfide is one of the most abundant reductants in the subsurface environment, while pyrogenic carbon is a redox medium that widely exists in sulfide environment. Previous studies have found pyrogenic carbon can mediate the reductive degradation of organic pollutants under anoxic sulfide conditions; however, the scenario under oxic sulfide conditions has rarely been reported. In this study, we found that pyrogenic carbon can mediate hydroxyl radicals (•OH) generation from sulfide oxidation under dark oxic conditions. The accumulated •OH ranged from 2.07 to 101.90 µM in the presence of 5 mM Na2S and 100 mg L-1 pyrogenic carbon at pH 7.0 within 240 min. The Raman spectra and electrochemical cell experiments revealed that the carbon defects were the possible chemisorption sites for oxygen, while the graphite crystallites were responsible for the electron transfer from sulfide to O2 to generate H2O2 and •OH. Quenching experiments and degradation product identification showed that As(III) and sulfanilamide can be oxidized by the generated •OH. This research provides a new insight into the important role of pyrogenic carbon in redox reactions and dark •OH production.

Effect of Straw Return on Hydroxyl Radical Formation in Paddy Soil.

Straw return, as an important agricultural management measure, is receiving growing attention. Hydroxyl radical (•OH) can be produced when subsurface soil interacts with oxygen, but the effects of straw incorporation on •OH formation have rarely been evaluated. In this study, we found that straw return had a significant effect on soil properties. Soil pH and redox potential (Eh) decreased while electronic conductivity (EC) showed an increment. Dissolved organic carbon content of soil initially increased and then decreased to the same level as the control by the end of the experiment of 120 days. Moreover, Fe(II) formation was promoted by straw return under anaerobic conditions. •OH was produced in the flooded paddy soil when exposed to oxygen, which correlated well with Fe(II) content. The effect of rape (Brassica campestris L.) straw on •OH formation rate was more evident as compared to wheat (Triticum aestivum L.) straw, suggesting a potentially more profound influence of rape straw return on pollutant transformation in paddy soils.

Effect of Polyvinyl Chloride Microplastics on Bacterial Community and Nutrient Status in Two Agricultural Soils.

Knowledge of the influence of microplastics on soil microbiome and nutrients is important for understanding the ecological consequences of microplastic pollution in terrestrial ecosystems. In this study, we investigated whether polyvinyl chloride (PVC) microplastic pollution at environmentally relevant concentrations would affect soil bacterial community and available nitrogen/phosphorus content. The results showed that although PVC microplastics at 0.1% and 1% levels did not have a significant effect on overall bacterial community diversity and composition in soil over the course of 35 days, a number of bacterial genera were significantly reduced or enriched by the presence of microplastics. Potentially due to their effect on certain functional groups, microplastics caused a significant change in soil available P content. It is noteworthy that, depending on soil type, pollution level and plasticizer presence, contrasting effects of microplastics may be observed. Further research is definitely warranted to gain a clearer picture of the threats posed by microplastic pollution in soil environments.

Cotransformation of Carbon Dots and Contaminant under Light in Aqueous Solutions: A Mechanistic Study.

In this study, the photochemistry of carbon dots (CDs) and their effects on pollutant transformation were systematically examined. Diethyl phthalate (DEP) degradation was strongly enhanced by CDs under UV light, with the observed reaction rate constant ( kobs) increased by 2.4-15.1-fold by CDs at a concentration of 0.5-10 mg/L. Electron paramagnetic resonance (EPR) spectrometry combined with free radical quenching experiments with various chemical probes indicated the production of reactive oxygen species (ROS), including hydroxyl radicals (•OH), singlet oxygen (1O2), and superoxide radical anions (O2•-), and these contributed to the enhanced DEP degradation. Meanwhile, CDs were also degraded to low-molecular-weight species and partially mineralized to CO2 by ROS, as evidenced by Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) and total organic carbon (TOC) analysis, and transformation of CDs was accelerated by DEP. Furthermore, CDs were degraded rapidly under natural sunlight, accompanied by the formation of •OH and 1O2. Anions such as CO32-, NO3-, and Cl- had limited effects on transformation of CDs, while humic substances greatly inhibited this process. Our results indicate that photoreactions of CDs play an important role in influencing the transformation of pollutants and CDs themselves in the natural aquatic environment. The findings provide invaluable information for evaluating risks associated with the release of CDs into the natural environment.

Occurrence and Ecological Impacts of Microplastics in Soil Systems: A Review.

Microplastics, as a group of emerging contaminants, are receiving growing attention. During the last decade, their occurrence and toxicity in aquatic ecosystems have been intensively studied and reviewed, but less attention has been paid on soil ecosystems. Given the importance of soil ecosystems and the call for increasing research on soil from scientific communities, it is predicted that relevant studies will boom in the following years. The present review intends to provide a comprehensive overview of current knowledge on microplastic pollution in soil environments. We critically summarize the source, contamination level and fate of microplastics in (industrial and arable) soils. Then, we thoroughly describe what effects have been observed on soil microbes, animals and plants, and analyze what insights we can get from available information. Finally, we identify knowledge gaps that need to be filled and give suggestions for future research.

Reductive Hexachloroethane Degradation by S2O8•- with Thermal Activation of Persulfate under Anaerobic Conditions.

Despite that persulfate radical (S2O8•-) is an important radical species formed from the persulfate (PS) activation process, its reactivity toward contaminant degradation has rarely been explored. In this study, we found that S2O8•- efficiently degrades the contaminant hexachloroethane (HCA) under anaerobic conditions, whereas HCA degradation is negligible in the presence of oxygen. We observed dechlorination products such as pentachloroethane, tetrachloroethylene, and Cl- during HCA degradation, which suggest that HCA degradation is mainly a reductive process under anaerobic conditions. Using free radical quenching and electron paramagnetic resonance (EPR) experiments, we confirmed that S2O8•- forms from the reaction between sulfate radical (SO4•-) and S2O82-, which are the dominant reactive species in HCA degradation. Density functional theory (DFT) calculations were used to elucidate the pathways of HCA degradation and S2O8•- radical decomposition. Further investigation showed that S2O8•- can efficiently degrade HCA and DDTs in soil via reduction during the thermal activation of PS under anaerobic conditions. The finding of this study provide a novel strategy for the reductive degradation of contaminant when PS-based in situ chemical oxidation used in the remediation of soil and groundwater, particularly those contaminated with highly halogenated compounds.

Mechanisms of Interaction between Persulfate and Soil Constituents: Activation, Free Radical Formation, Conversion, and Identification.

Persulfate-based in situ chemical oxidation (ISCO) for soil remediation has received great attention in recent years. However, the mechanisms of interaction between persulfate (PS) and soil constituents are not fully understood. In this study, PS decomposition, activation, free radical formation and conversion processes in 10 different soils were examined. The results showed that soil organic matter (SOM) was the dominant factor affecting PS decomposition in soil, but Fe/Mn-oxides were mainly responsible for PS decomposition when SOM was removed. Electron paramagnetic resonance (EPR) spectroscopy analysis showed that sulfate radicals (SO4•-) and hydroxyl radicals (•OH) generated from PS decomposition subsequently react with SOM to produce alkyl-like radicals (R•), and this process is dependent on SOM content. R• and SO4•-/•OH radicals predominated in soil with high and low SOM, respectively, and all three radicals coexist in soil with medium SOM. Chemical probe analysis further identified the types of radicals, and R• can reductively degrade hexachloroethane in high SOM soil, while SO4•- and •OH oxidatively degrade phenol in low SOM soil. These findings provide valuable information for PS-ISCO, and new insight into the role of SOM in the remediation of contaminated soil.

Critical roles of low-molecular-weight organic acid in enhancing hydroxyl radical production by ferrous oxidation on γ-Al2O3 mineral surface.

Recognizing the pervasive presence of alumina minerals and low-molecular-weight organic acids (LMWOAs) in the environment, this study addressed the gap in the interaction mechanisms within the ternary system involving these two components and Fe(II). Specifically, the impacts of LMWOAs on hydroxyl radicals (•OH) production and iron species transformation during Fe(II) oxidation on γ-Al2O3 mineral surface were examined. Results demonstrated that adding 0.5 mM oxalate (OA) or citrate (CA) to the γ-Al2O3/Fe(II) system (28.1 µM) significantly enhanced •OH production by 1.9-fold (51.9 µM) and 1.3-fold (36.2 µM), respectively, whereas succinate (SA) exhibited limited effect (30.7 µM). Raising OA concentration to 5 mM further promoted •OH yield to 125.0 µM after 24 h. Deeper analysis revealed that CA facilitated the dissolution of adsorbed Fe(II) and its subsequent oxygenation by O2 through both one- and two-electron transfer mechanisms, whereas OA enhanced the adsorption of dissolved Fe(II) and more efficient two-electron transfer for H2O2 production. Additionally, LMWOAs presence favored the formation of iron minerals with poor crystallinity like ferrihydrite and lepidocrocite rather than well-crystallized forms such as goethite. The distinct impacts of various LMWOAs on Fe(II) oxidation and •OH generation underscore their unique roles in the redox processes at mineral surface, consequently modulating the environmental fate of prototypical pollutants like phenol.

Key role of hydrogen atoms in the preparation of sulfidated zero valent iron.

Sulfidated zero valent iron (ZVI) is a popular material for the reductive degradation of halogenated organic pollutants. Simple and economic synthesis of this material is highly demanded. In this study, sulfidated micro/nanostructured ZVI (MNZVI) particles were prepared by simply heating MNZVI particles and sulfur elements (S0) in pure water (50℃). The iron oxides on the surface of MNZVI particles were conducive to sulfidation reaction, indicating the formation of iron-sulphide minerals (FeSx) on the surface of MNZVI particles might not be from the direct reaction of Fe0 with S0 (Fe0 and S0 acted as reductant and oxidant, respectively). As an important reductant, hydrogen atom (H•) can be generated from the reduction of H+ by MNZVI particles and participate in the formation of FeSx. Quenching experiment and cyclic voltammetry analysis proved the existence of H• on the surface of MNZVI particles. DFT calculation found that the potential barrier of H•/S0 and Fe0/S0 were 1.91 and 7.24 eV, respectively, indicating that S0 would preferentially react with H• instead of Fe0. The formed H• can quickly react with S0 to generate hydrogen sulfide (H2S), which can further react with iron oxides such as α-Fe2O3 on the surface of MNZVI particles to form FeSx. In addition, the H2 partial pressure in water significantly affected the amount of H• generated, thereby affecting the sulfidation efficiency. For TCE degradation, as the sulfur loading of sulfidated MNZVI particles increased, the contribution of H• significantly decreased while the contribution of direct electron transfer increased. This study provided new insights into the synthesis mechanism of sulfidated ZVI in water.

Nonmicrobial mechanisms dominate the release of CO2 and the decomposition of organic matter during the short-term redox process in paddy soil slurry.

Both biotic and abiotic mechanisms play a role in soil CO2 emission processes. However, abiotically mediated CO2 emission and the role of reactive oxygen species are still poorly understood in paddy soil. This study revealed that •OH promoted CO2 emission in paddy soil slurries during short-term oxidation (4 h). •OH generation was highly hinged on active Fe(II) content, and the •OH contribution to CO2 efflux was 10%-33% in topsoil and 40%-77% in deep-soil slurries. Net CO2 efflux was higher in topsoil slurries, which contained more dissolved organic carbon (DOC). CO2 efflux correlated well with DOC contents, suggesting the critical role of DOC. Microbial mechanisms contributed 9%-45% to CO2 production, as estimated by γ-ray sterilization experiments in the short-term reoxidation process. Solid-aqueous separation experiments showed a significant reduction in net CO2 efflux across all soil slurries after the removal of the original aqueous phase, indicating that the water phase was the main source of CO2 emission (>50%). Besides, C emission was greatly affected by pH fluctuation in acidic soil but not in neutral/alkaline soils. Fourier transform ion cyclotron resonance mass spectrometry and excitation-emission matrix results indicated that recalcitrant and macromolecular dissolved organic matter (DOM) components were more easily removed or attacked by •OH. The decrease in DOM content during the short-term reoxidation was the combined result of •OH oxidation, co-precipitation, and soil organic matter release. This study emphasizes the significance of the generally overlooked nonmicrobial mechanisms in promoting CO2 emission in the global C cycle, and the critical influence of the aqueous phase on C loss in paddy environments.

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.

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.

Anti-passivation ability of sulfidated microscale zero valent iron and its application for 1,1,2,2-tetrachloroethane degradation.

The performance of sulfidated zero valent iron (ZVI) for the degradation of chlorinated hydrocarbons under aerobic conditions remains unclear. In this study, sulfidated microscale ZVI (S-mZVI) was prepared for 1,1,2,2-tetrachloroethane (TeCA) degradation under aerobic conditions. Compared with mZVI, S-mZVI showed excellent passivation resistance during the degradation of TeCA and its hydrolysis/reduction products. This was probably because the existence of FeSx shell (FeS/FeS2/FeSn) protected the internal ZVI core from passivation. Though the outer layer of FeSx shell could be oxidized to FeSn and Fe2(SO4)3 as the reaction proceeded, the inner layer remained stable, which maintained the fast electron transfer capability of S-mZVI. The high temperature could enhance the degradation of TeCA, without compromising the anti-passivation and reusability of S-mZVI. Even after the fifth cycle, S-mZVI could still efficiently degrade 90% of TeCA within 24 h. Furthermore, it was found that the degradation of TeCA and its reduction products (e.g., dichloroethylene (DCE)) by S-mZVI relied on direct electron transfer and hydrogen radical (H•), respectively, which might explain the lower levels of toxic DCE in the S-mZVI system. This study provides valuable information for the practical application of S-mZVI in the treatment of wastewater containing halogenated hydrocarbons under ambient conditions.

Preparation of Fe/Cu bimetals by ball milling iron powder and copper sulfate for trichloroethylene degradation: Combined effect of FeSx and Fe/Cu alloy.

The addition of a secondary metal (such as Cu, Co, Ni and Pd) to form iron-based bimetallic particles could enhance the reactivity of zero valent iron (ZVI). This study proposed a new synthesis method for preparing Cu-Fe bimetals (Cu-Febm (CuSO4)) by ball milling mZVI and CuSO4. During ball-milling process, 40% of Cu2+ can be reduced to Cu0, which formed galvanic couple with Fe0 in a way of Fe/Cu alloy structure. Part Cu2+ was only reduced to Cu+ (corresponding to Cu2O), while 29% of SO42- was reduced to Sx2- (corresponding to FeSx). The appearance of Cu2O was not conducive to the activity of Cu-Febm (CuSO4) particles, the formation of Fe0/FeSx structure compensated for the partial loss of Fe/Cu alloy. H•abs was identified as the main active species for TCE degradation by Cu-Febm (CuSO4) bimetals. The Cu-Febm (CuSO4) bimetals has great potential for the removal of chlorinated hydrocarbons in water.

Linking pyrogenic carbon redox property to arsenite oxidation: Impact of N-doping and pyrolysis temperature.

Pyrogenic carbon-mediated arsenite (As(III)) oxidation shows great potential as a prerequisite for the efficient removal of arsenic in groundwater. Herein, the critical role of N-containing functional groups in low and high-temperature prepared pyrogenic carbons for mediating As(III) oxidation was systemically explored from an electrochemistry perspective. The pyrogenic carbon electron donating capacity and area-normalized specific capacitance were the key parameters explained the As(III) oxidation kinetics mediated by low electrical conductive 500 °C biomass-derived pyrogenic carbons (N contents of 0.36-7.72 wt%, R2 = 0.87, p < 0.001) and high electrical conductive 800 °C pyrogenic carbons (N contents of 1.00-8.00 wt%, R2 = 0.99, p < 0.001), respectively. The production of H2O2 from the reaction between electron donating phenol groups or semiquinone radicals and oxygen, and the direct electron transfer between semiquinone radicals and As(III) contributed to these pyrogenic carbons mediated As(III) oxidation. While the electron accepting quinone, pyridinic-N, and pyrrolic-N groups did not significantly contribute to the 500 °C pyrogenic carbons mediated As(III) oxidation, the direct electron conduction by these functional groups was responsible for the facilitated As(III) oxidation by the 800 °C pyrogenic carbons. Furthermore, the pyridinic-N and pyrrolic-N groups showed higher electron conduction efficiency than that of the quinone groups. The findings help to develop robust pyrogenic carbons for As(III) contaminated groundwater treatment.

Extensive production of hydroxyl radicals during oxygenation of anoxic paddy soils: Implications to imidacloprid degradation.

Hydroxyl radical (•OH) plays a critical role in driving organic pollutants degradation during redox fluctuations. Such processes have been frequently investigated in sedimentary environments, but rarely referred to the agricultural fields, such as paddy soils with frequent occurrence of redox fluctuations. Our findings demonstrated that extensive •OH (40.3-1061.4 µmol kg-1) was produced during oxygenation of anoxic paddy slurries under circumstance conditions. Wet chemical sequential extractions, Mössbauer spectra, and X-ray photoelectron spectroscopy characterizations collectively corroborated that 0.5 M HCl-extracted Fe(II) (i.e., surface-bound Fe and Fe in low-crystalline minerals) contributed to more •OH production than aqueous Fe2+. The produced •OH can efficiently induce the oxidative transformation of organic carbon and the degradation of imidacloprid (IMP), which in turn produced the by-products, such as IMP-urea, IMP-olefin, and 6-chloronicontinic acid, via •OH-attacking mechanisms. Quenching experiments showed that hydrogen peroxide (H2O2) was the important intermediate for •OH formation via Haber-Weiss mechanisms during oxygenation processes. These findings indicate that abundant •OH can be produced during the redox fluctuations of paddy soil, which might be of great significance to predict the removal of organic contaminants and the mineralization of organic carbon in paddy fields.