Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils

Tidal fluctuations induce accumulation and transformation of seawater Cr(Ⅵ) in coastal sediments

Tidal fluctuations play a critical role in regulating the transport and fate of contaminants in coastal environments. This study explored the dynamic redistribution of chromium (Cr) from seawater to sediment under tidal influence, as well as the accumulation and transformation of Cr in sediment through laboratory experiments and numerical simulations. After 35 tidal cycles, Cr concentrations in seawater declined rapidly and stabilized at approximately 27 % of the initial level. Notably, Cr migrated into sediment, ultimately accumulating in the bottom layer. Colloidal particles (350-800 nm) composed of clay minerals served as the primary transport vectors for Cr within sediment. During tidal fluctuations, 94.5 %-98.2 % of Cr(VI) in sediment was reduced to Cr(III), predominantly mediated by Fe(II) in the top sediment and by sulfur-reducing bacteria in the bottom layers. Consistent with experimental findings, numerical reactive transport modeling demonstrated that Cr(III) initially peaked in the middle sediment layer before stabilizing in the bottom layer, whereas Cr(VI) remained confined to the top layer. These findings elucidate tide-induced mobilization and natural reduction mechanisms governing Cr-contaminated seawater infiltration into sediment, offering novel insights into the fate of Cr discharged from coastal wastewater sources within seawater-sediment systems.

Carbon Isotope Fractionation of Dissolved Organic Matter Due to •OH-Based Oxidation

•OH-based oxidation plays a crucial role in dissolved organic matter (DOM) transformation and carbon flux, whereas quantifying the contribution of this pathway remains challenging. Here we combined the concentration with the carbon isotope analysis of DOM and its generated CO2 to quantify the contribution of •OH-based oxidation. Results showed that the 13C enrichment factors (ε values) were -8.1‰ to -8.9‰ for benzene ring oxidation in aromatic compounds, -4.2‰ to -28.9‰ for lower-molecular-weight organic acids, and -13.0‰ for DOM from sediment. The fractionation of sediment DOM reflects the average ε value of humic substances and organic acids. These ε values were more negative than those of the photochemical and microbial processes, enabling the identification of DOM transformation mechanisms. Using an end-member mix model, we found that the proportion of •OH-based mineralization in total CO2 emission ranged from 20.9% to 39.8% for 100 g/L sediment oxidation by 5-20 mM H2O2 under pH-neutral condition within 2 h and was only 2% for oxidation by air under the same conditions. We also found that inorganic carbon degassing contributed greatly to CO2 emission during sediment oxidation. This study presents a new isotope-based tool to quantitatively assess the contribution of •OH-based oxidation to the emission of CO2 from DOM.

Predicting trichloroethene attenuation in aquifers with reduced iron minerals under oxygen perturbation: From kinetic model to reactive transport model

Pollutant attenuation in aquifers due to the reactivity of in situ reduced iron minerals (RIM) under dynamic redox conditions has gained popularity because of its value in designing green and sustainable strategies for soil and groundwater remediation. In this study, a novel approach that integrates RIM-based kinetic modeling in reactive transport modeling was initiated to predict trichloroethene (TCE) attenuation in RIM-containing aquifers. The kinetic model was optimized and simplified based on previous efforts and verified with data from batch experiments (R2 > 0.85). Using the refined kinetic model as a reaction module, a multi-scale and multi-process reactive transport model (RTM) was developed, which comprehensively linked geochemical reactions, dispersion, and advection of TCE in the aquifers with RIM and accurately simulated the variations of TCE concentrations in experiments (R2 >0.95). Notably, the importance of RIM oxygenation in TCE attenuation during O2 perturbation was demonstrated by the model; the ratio of contributions from RIM-based anaerobic dichlorination, hydrodynamics, and RIM-based aerobic degradation to TCE removal in low-permeability zones after 10 d was approximately 1:7:21. Besides, the RTM helped to elucidate the effects of variations in key elements (i.e., Fe(II) species, O2) on the TCE attenuation system. This study can facilitate the evaluation and prediction of the TCE fate in aquifers with RIM under O2 perturbation, so that remediation strategies can be formulated. Meanwhile, the limitations of the study motivate future work to determine the effects of more elements (e.g., organic matters, microbes), environmental changes, and scale differences to develop more sophisticated models.

Microbial advanced oxidation aroused by bacteria-algae symbiosis induced abiotic methane production in anaerobic digestion

The slow decomposition of recalcitrant substrate limits the conversion efficiency of anaerobic digestion. Microbial advanced oxidation, capable of in-situ generating reactive oxygen species (ROS) with the microbial aerobic/anaerobic respiration, provided a potential way to strengthen the substrate-methane conversion in anaerobic digestion. In this study, microalgae were inoculated in anaerobic system and formed redox oscillation under the intermittent illumination, which ultimately increased the methane production by 27.4 %. With the redox oscillation, •OH, the typical ROS, showed a 6.27-fold increase in production (72.95 ± 9.06 μM vs. 10.03 ± 1.49 μM), facilitating the decomposition of lignocelluloses. Notably, abiotic methanation was observed in anaerobic digestion with the occurrence of microbial advanced oxidation. ROS quenching experiments revealed that abiotic methanation roughly accounted for 17.5 % of the total methane production. Microbial advanced oxidation formed by redox oscillation showed the potential to strengthen anaerobic digestion. Notably, for the first time, it was confirmed that abiotic methanation could be established in anaerobic digestion with the ROS generated by microbial advanced oxidation, which offered a new perspective to understand and improve the performances of natural and engineered ecosystems.

Role of reactive oxygen species (ROS) on biochar enhanced chromium phytoremediation in the soil-plant system: Exploration on detoxification mechanism

Biochar, as an amendment to enhance phytoremediation of heavy metal contamination, can mediate reactive oxygen species (ROS) generation. However, the role of biochar-mediated ROS (BMR) during soil-plant phytoremediation remains inadequately understood. In this study, a combination of pot experiments, chemical extraction, and partial least squares path modeling (PLS-PM) was employed to investigate BMR dynamics and their influence on chromium (Cr) accumulation and detoxification in plants. Biochar addition promoted Cr removal efficiency and decreased ROS concentrations in soil, notably reaching the largest removal efficiency of 80.60 % and the lowest ROS concentration of 37.53 μmol/kg in BC-3 group at 90d. Decreased ROS concentrations in soil facilitated the plant absorbing water-soluble Cr (VI), adsorbed Cr (VI), and chromate-precipitated Cr (VI) in soil, and enhanced Cr accumulation in metabolically inactive compartments (cell walls and vacuoles). When biochar was added at concentrations of 2 % and 3 % (w/w), ROS concentrations in plant tissues decreased to signaling molecule thresholds. This reduction further stimulated antioxidant enzyme activity, promoted the reduction of Cr (VI) within subcellular organelles, and enhanced Cr cell wall fixation and vacuolar compartmentation, ultimately achieving their synergistic integration with Cr detoxification with accumulation. This study provides an in-depth understanding of BMR-related mechanisms during phytoremediation and valuable insights into strategies for enhancing mitigation of variable valence heavy metals in soils.

Photic versus aphotic production of organohalogens from native versus invasive wetland plants-derived dissolved organic matter

The aphotic formation of natural organohalogens (NOHs) remains inadequately understood, in contrast to the well-documented photo-halogenation process of dissolved organic matter (DOM), despite the significant biogeochemical implications associated with NOHs. This study investigates the differences in the formation of chlorinated and brominated compounds from the photochemical and aphotic reactions of native Phragmites australis (PA-DOM) and invasive Spartina alterniflora (SA-DOM). The findings indicate that SA-DOM exhibits a greater potential for photochemical halogenation, attributed to its higher aromatic content and enhanced photostability. Utilizing advanced mass spectrometry, the study identifies nitrogen-containing and free saturated compounds as primary precursors for both types of DOM during photochemical halogenation. Notably, significant disparities in the halogenation processes of lignin/CRAM, nitrogen-containing/free saturated compounds, and amino sugars between SA-DOM and PA-DOM are observed, leading to a higher production of NOHs in PA-DOM during aphotic reactions compared to photic reactions, even in artificial seawater. Furthermore, the study emphasizes the critical role of dissolved oxygen in the formation of NOHs from PA-DOM under aphotic conditions. Given the rapid fluctuations in oxygen levels, salinity, and solar intensity, alongside tidal and diurnal cycles, the significance of both photic and aphotic pathways for NOHs formation should not be overlooked.

Developing Methylmercury-Targeted Strategies to Safeguard Rice Consumers

Mitigating mercury (Hg) risk in the rice-paddy system is crucial for safeguarding food safety and human health, as rice is a main source of human exposure to neurotoxic methylmercury (MeHg). Current mitigation strategies predominantly focus on reducing the availability of inorganic Hg (IHg) for Hg methylation, achieved primarily through Hg emission control and in situ Hg immobilization. While these IHg-targeted approaches have effectively reduced MeHg bioaccumulation and subsequent human exposure, their efficacy is largely undermined by Hg transformations and fluctuating environmental conditions due to the complex and protracted pathway linking IHg from environmental sources to MeHg at the point of human exposure. In light of recent advancements in MeHg-related transformations, we emphasize the development of MeHg-targeted strategies to improve the overall efficiency of Hg risk management in rice-paddy systems. MeHg-targeted strategies include microbial regulation to diminish net MeHg production, facilitating MeHg demethylation in soils, and promoting the in vivo MeHg degradation within rice plants. Although these approaches are still in their nascent stages, they hold significant promise due to their potential high mitigation efficacy and reduced uncertainties, owing to the shorter pathway between MeHg production and human exposure. Integrating IHg- and MeHg-targeted strategies offers a comprehensive and synergistic approach, paving the way for more effective mitigation of human exposure to MeHg in rice-paddy systems.

A Novel Perspective on the Role of Hydroxyl Radicals in Soil Organic Carbon Mineralization within the Detritusphere: Stimulating C-Degrading Enzyme Activities

Detritusphere is a hotspot of carbon cycling in terrestrial ecosystems, yet the mineralization of soil organic carbon (SOC) within this microregion associated with reactive oxygen species (ROS) remains unclear. Herein, we investigated ROS production and distribution in the detritusphere of six representative soils and evaluated their contributions to SOC mineralization. We found that ROS production was significantly correlated with several soil chemical and biological factors, including pH, water-soluble phenols, water-extractable organic carbon, phenol oxidase activity, surface-bound or complexed Fe(II) and Fe(II) in low-crystalline minerals, highly crystalline Fe(II)-bearing minerals, and SOC. These factors collectively contributed to 99.6% of the variation in ROS production, as revealed by redundancy analyses. Among ROS, hydroxyl radicals (•OH) were key contributors to SOC mineralization, responsible for 10.4%-38.7% of CO2 emissions in ROS quenching experiments. Inhibiting •OH production decreased C-degrading enzyme activities, indicating that •OH stimulates CO2 emissions by increasing enzyme activity. Structural equation modeling further demonstrated that •OH promotes C-degrading enzyme activities by degrading water-soluble phenols to unlock the "enzyme latch" and by increasing SOC availability to upregulate C-degrading gene expression. These pathways contributed equally to SOC mineralization and exceeded its direct effect. These findings provide detailed insight into the mechanistic pathways of •OH-mediated carbon dynamics within the detritusphere.

Electron transfer tuning for persulfate activation via the radical and non-radical pathways with biochar mediator

Electron mediator-based in-situ chemical oxidation (ISCO) offers a novel strategy for groundwater remediation due to diverse reaction pathways. However, distinguishing and further tuning the reaction pathway remains challenging. Herein, biochar as an electron mediator targeted active peroxysulphate (PDS) via the radical or non-radical pathway. Exemplified by the triazin pesticides removal, the complex radical (•OH and SO4•-) and non-radical active species (electron transfer oxidation) were generated and identified in different biochar/PDS systems. The electron transfer process between biochar and PDS was significantly distinguished via an innovatively in-situ visualization of radical pathway, and the electron transfer oxidation non-radical pathway is directly unveiled via a galvanic cell experiment combined with LC-MS analyses. The electron transfer mechanism was revealed via establishing the quantitative structure-activity relationships between biochar and ln kobs. The redox capacity of biochar was assessed as a key for tuning the atrazine degradation by non-radical pathway, and the surface carbon-centered persistent free radicals (PFRs) were identified as key electron donors for triggering the radical pathway. This study gives new insights into the electron transfer mechanism during tuning radical and non-radical activation pathways and the enhanced utilization of oxidants in ISCO technology.

Photodegradation of typical psychotropic drugs in the aquatic environment: a critical review

Continuous consumption combined with incomplete removal during wastewater treatment means residues of psychotropic drugs (PDs), including antidepressants, antipsychotics, antiepileptics and illicit drugs, are continuously entering the aquatic environment, where they have the potential to affect non-target organisms. Photochemical transformation is an important aspect to consider when evaluating the environmental persistence of PDs, particularly for those present in sunlit surface waters. This review summarizes the latest research on the photodegradation of typical PDs under environmentally relevant conditions. According to the analysis results, four classes of PDs discussed in this paper are influenced by direct and indirect photolysis. Indirect photodegradation has been more extensively studied for antidepressants and antiepileptics compared to antipsychotics and illicit drugs. Particularly, the photosensitization process of dissolved organic materials (DOM) in natural waters has received significant research attention due to its ubiquity and specificity. The direct photolysis pathway plays a less significant role, but it is still relevant for most PDs discussed in this paper. The photodegradation rates and pathways of PDs are influenced by various water constituents and parameters such as DOM, nitrate and pH value. The contradictory results reported in some studies can be attributed to differences in experimental conditions. Based on this analysis of the existing literature, the review also identifies several key aspects that warrant further research on PD photodegradation. These results and recommendations contribute to a better understanding of the environmental role of water matrixes and provide important new insights into the photochemical fate of PDs in aquatic environments.

Microscale Spatiotemporal Variation of Reactive Oxygen Species in the Charosphere: Underlying Formation Mechanism and Their Role in CO2 Emission

Charosphere, a highly active zone between biochar and surrounding soil, is widely present in agricultural and wildfire-affected soils, yet whether reactive oxygen species (ROS) are produced within the charosphere remains unclear. Herein, the production and spatiotemporal evolution of charosphere ROS were explored. In situ ROS capture visualized a gradual decrease in ROS production with increasing distance from the biochar/soil interface. Temporally, O2•- and H2O2 contents initially increased and then declined with increasing incubation time, peaking at 3.04 and 5.40 μmol kg-1, respectively, while •OH content decreased continuously. High-throughput sequencing revealed that dissolved biochar (DBC) facilitated ROS production by promoting the growth of bacteria with electron-releasing capacity, such as Bacteroidetes, Acidobacteria, Actinobacteria, and Chloroflexi. Additionally, adding electron transfer-weakened DBC significantly decreased ROS contents (ANOVA, P < 0.05), demonstrating that DBC also served as the electron shuttle and electron-storing materials to promote ROS production by accelerating electron transfer. This was further confirmed via fluorescence imaging, which visually showed stronger electron transfer ability near the soil/biochar surface. Inhibition and isotope experiments revealed the critical role of charosphere ROS in CO2 emissions, primarily from soil organic carbon. This study highlights the charosphere as a prevalent yet overlooked ROS hotspot, advancing our understanding of organic carbon turnover in soils.

Visualizing Hydrogen Peroxide Diffusion in Soils with Precipitation-Based Fluorescent Probe

Hydrogen peroxide (H2O2)-based advanced oxidation technology has emerged as a cost-effective and green solution for tackling soil pollution. Given the highly heterogeneous nature of soil, the effectiveness of H2O2 remediation is significantly influenced by its diffusion distance in soils. However, the dynamics of H2O2 diffusion and its effective range remain largely unexplored, primarily due to the lack of analytical methods for mapping H2O2 in soils. This study introduces a precipitation-based fluorescent probe (PFP) method for in situ, high-resolution (micrometer scale) mapping of H2O2 diffusion in soils. Using the PFP method, we visualized real-time H2O2 diffusion in various types of soils, revealing distinct diffusion patterns with rates ranging from 0.011 to >0.56 mm min-1. The observed differences in diffusion rates are associated with soil permeability. Additionally, soils exhibited a wide range of diffusion distances, from 0.22 to >11 mm in 20 min. Soil's reactivity for H2O2 decomposition, a previously overlooked factor, is critical in determining the diffusion distance of H2O2. We further demonstrate that the efficacy of H2O2 diffusion in soils is a pivotal factor in controlling pollutant degradation and soil remediation efficiency. These findings enhance our understanding of reagent diffusion processes in soil remediation, informing the optimization of more efficient soil remediation strategies.

Production of Reactive Oxygen Species during Redox Manipulation and Its Potential Impacts on Activated Sludge Wastewater Treatment Processes

Reactive oxygen species (ROS) are ubiquitous in redox-fluctuating environments, exerting profound impacts on biogeochemical cycles. However, whether ROS can be generated during redox manipulation in activated sludge wastewater treatment processes (AS-WTPs) and the underlying impacts remain largely unknown. This study demonstrates that ROS production is ubiquitous in AS-WTPs due to redox manipulation and that the frequency and capacity of ROS production depend on the operating modes. The anaerobic/oxic continuous-flow reactor showed persistent ROS generation (0.8-2.1 μM of instantaneous H2O2), whereas the oxic/anoxic sequencing batch reactor (0.21-0.28 mM of H2O2 per cycle) and the anaerobic/anoxic digestion reactor (0.27-0.29 mM of H2O2 per cycle) exhibited periodic ROS production. Our results illustrated that ROS generated during redox manipulation can contribute to the removal of organic micropollutants. Due to their high activity, ROS can directly accelerate the abiotic oxidation of organic phenolics and Fe(II) minerals in sludges. ROS could also affect biotic nitrification by changing the microbial community composition and regulating the relative expression of functional genes, such as amoA, nrxA, and nrxB. This research demonstrates the ubiquitous production of ROS during redox manipulation in AS-WTPs, which provides new insights into pollutant removal and the abiotic and biotic elemental transformation in AS-WTPs.

The successive reduction of iodate to iodide driven by iron redox cycling

Ferrous iron (Fe(II)) produced by microbial Fe(III) reduction and reactive oxygen species (ROS) generated from aerobic Fe(II) oxidation can mediate iodate (IO3-) reduction and iodide (I-) oxidation, respectively. Nevertheless, how Fe redox cycling under redox fluctuating conditions drives transformation of iodine species remain unclear. In this study, Shewanella oneidensis MR-1 wildtype (WT) and its mutant △dmsEFAB, which lost the ability to enzymatically reduce IO3-, were chosen to conduct ferrihydrite/goethite/nontronite culture experiments under consecutive cycles of anoxic reduction of Fe(III) and re-oxidation of Fe(II) by O2 to reveal the role of Fe redox cycling in the transformation of iodine species. The results showed that both surface-adsorbed and mineral structural Fe(II) chemically reduced IO3-. Chemical IO3- reduction by biogenic Fe(II) was slower than enzymatic IO3- reduction by WT. Compared to △dmsEFAB cultures, WT cultures all showed higher Fe(II) concentrations under anoxic conditions but lower cumulative •OH under oxic conditions, which imply the chemical reaction between I- and ROS. I- oxidation by ROS, however, did not lead to a significant production of IO3- compared with I- formed under anoxic conditions. Consequently, Fe redox cycling successively reduced IO3- to I-, which highlights vital roles of Fe(III)-reducing bacteria in I- formation and mobilization in environments.

Redox Oscillation-Driven Production of Reactive Oxygen Species from Black Carbon

Wildfire and stubble burning release substantial amounts of black carbon (BC) into natural environments that experience frequent redox oscillations, such as wetlands and farmlands. Here, we report that redox oscillations can effectively drive ROS production from BC. Following sequential microbial reduction and air exposure, 6.5 ± 0.2 μM/gC hydrogen peroxide (H2O2) and 285.3 ± 9.5 nM/gC hydroxyl radical (•OH) were produced from BC. Moreover, BC derived from various biomass sources, temperatures, and particle sizes exhibits 111.5-fold variations in ROS production. Electrochemical analyses revealed that both the electron transfer capacity and the ROS production selectivity are critical determinants of ROS generation under redox oscillations. The variation in electron transfer capacity (0.3-5.7 mmol e-/gC) is primarily governed by the abundance of electron-storing moieties such as quinones, while the ROS generation selectivity (26.2-72.0%) is influenced by the presence of competitive sites for oxygen reduction reactions, such as carbon defects. These findings provide insights into ROS production from BC under fluctuating redox conditions, with potential implications for elemental cycles and pollutant dynamics in regions prone to wildfire and stubble burning events and substantial BC deposition (e.g., wetlands and rice paddies).

Enhancing the production of reactive oxygen species in the rhizosphere to promote contaminants degradation in sediments by electrically strengthening microbial extracellular electron transfer

The production of reactive oxygen species (ROS) in the rhizosphere is limited by the low extracellular electron transfer capacity of indigenous microorganisms. In the present study, electrical stimulation was used to promote the generation of rhizospheric ROS by accelerating extracellular electron transfer. The result showed that •OH concentrations in the electrically stimulated group (ES group) exceeded the control group by 15.76 %. Accordingly, the removal rate of the target pollutant (i.e., 2,4-dichlorophenol, and sulfamethoxazole) was 20.01 %-24.80 % higher in the ES group than in the control group. The sediment of the ES group had a higher capacity (30.55 %) and a lower electrical resistance (29.15 %) compared to the control group, which subsequently promoted the dissimilatory iron reduction to produce Fe(II) for triggering a Fenton-like process. The increased extracellular respiratory capacity under electrical stimulation could be attributed to the polarization of C-N and CO bonds, which provided more electron storage sites and thus participated in proton-coupled electron transfer. In addition, the concentration of ATP and co-enzymes (NADH/NAD+ and Complex I/Complex III), reflecting electron exchange within respiratory chains, increased distinctly under electrical stimulation. Applying electrical stimulation seemed feasible to increase ROS production and contaminant degradation in the rhizosphere, deepening the understanding of electrical stimulation to promote the production of ROS in the natural system.

The generation and transformation mechanisms of reactive oxygen species in the environment and their implications for pollution control processes: A review

Reactive oxygen species (ROS), substances with strong activity generated by oxygen during electron transfer, play a significant role in the decomposition of organic matter in various environmental settings, including soil, water and atmosphere. Although ROS has a short lifespan (ranging from a few nanoseconds to a few days), it continuously generated during the interaction between microorganisms and their environment, especially in environments characterized by strong ultraviolet radiation, fluctuating oxygen concentration or redox conditions, and the abundance of metal minerals. A comprehensive understanding of the fate of ROS in nature can provide new ideas for pollutant degradation and is of great significance for the development of green degradation technologies for organic pollutants. At present, the review of ROS generally revolves around various advanced oxidation processes, but lacks a description and summary of the fate of ROS in nature, this article starts with the definition of reactive oxidants species and reviews the production, migration, and transformation mechanisms of ROS in soil, water and atmospheric environments, focusing on recent developments. In addition, the stimulating effects of ROS on organisms were reviewed. Conclusively, the article summarizes the classic processes, possible improvements, and future directions for ROS-mediated degradation of pollutants. This review offers suggestions for future research directions in this field and provides the possible ROS technology application in pollutants treatment.

Geobatteries in environmental biogeochemistry: Electron transfer and utilization

The efficiency of direct electron flow from electron donors to electron acceptors in redox reactions is significantly influenced by the spatial separation of these components. Geobatteries, a class of redox-active substances naturally present in soil-water systems, act as electron reservoirs, reversibly donating, storing, and accepting electrons. This capability allows the temporal and spatial decoupling of redox half-reactions, providing a flexible electron transfer mechanism. In this review, we systematically examine the critical role of geobatteries in influencing electron transfer and utilization in environmental biogeochemical processes. Typical redox-active centers within geobatteries, such as quinone-like moieties, nitrogen- and sulfur-containing groups, and variable-valent metals, possess the potential to repeatedly charge and discharge. Various characterization techniques, ranging from qualitative methods like elemental analysis, imaging, and spectroscopy, to quantitative techniques such as chemical, spectroscopic, and electrochemical methods, have been developed to evaluate this reversible electron transfer capacity. Additionally, current research on the ecological and environmental significance of geobatteries extends beyond natural soil-water systems (e.g., soil carbon cycle) to engineered systems such as water treatment (e.g., nitrogen removal) and waste management (e.g., anaerobic digestion). Despite these advancements, challenges such as the complexity of environmental systems, difficulties in accurately quantifying electron exchange capacity, and scaling-up issues must be addressed to fully unlock their potential. This review underscores both the promise and challenges associated with geobatteries in responding to environmental issues, such as climate change and pollutant transformation.

The effect of redox fluctuation on carbon mineralization in riparian soil: An analysis of the hotspot zone of reactive oxygen species production

Riparian zones are important depositional environments at the catchment scale and provide environmental services such as carbon sequestration. This zone is a highly dynamic interface for oxygen and electron exchange, which confers the basis for reactive oxygen species (ROS) production. However, the differences in soil ROS production and their impact on carbon turnover across various redox locations within the riparian zone remain to be fully elucidated. In this study, we investigated the distribution characteristics and generation mechanism of ROS in riparian soil based on soil samples collected in a three-month field monitoring experiment, with additional incubation experiments conducted to examine the effect of hydroxyl radical (•OH) on soil organic carbon (SOC) mineralization. The obtained results demonstrated that the riverine wetland was the hotspot zone for •OH production, with the production flux of 13.05 μmol kg-1 d-1, which was significantly higher than that in floodplain (7.29 μmol kg-1 d-1) and riverbank soils (8.61 μmol kg-1 d-1). Moreover, •OH levels displayed distinct rhythmic fluctuations, with significantly higher concentrations at low water levels compared to those at high water levels, and remained essentially flat over three cycles. The statistic analysis revealed that the ROS production was highly dependent on reduced species and microbial community structure, which function as biogeochemical batteries and electron shuttles under redox fluctuations. Furthermore, the generated •OH involved in the abiotic mineralization of SOC, contributing to 13.1‒21.8 % of total CO2 efflux. Compared to particulate organic carbon (POC), mineral-associated organic carbon (MAOC) fractions of SOC were more susceptible to •OH attacks. The findings provide a novel insight to comprehensively assess the redox process on riparian carbon turnover.

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Adsorption of oxidized humic acid onto redox-inert mineral surfaces induces formation of hydroxyl radicals and carbon dynamics

The dark formation of hydroxyl radicals (·OH) from O2 activation by reduced humic substances at oxic-anoxic interfaces has been extensively documented. However, their generation in oxic subsurface environments is typically overlooked due to the scarcity of electron donors, especially in the presence of minerals. In this study, the formation of ·OH during the adsorption of oxidized humic acids (HA) onto redox-inert minerals was investigated under oxic and pH-neutral conditions. Batch experiment results demonstrated that the adsorption of oxidized HA onto aluminum (hydr)oxide and Fe-free clay minerals induces the formation of ·OH (e.g., 16/28 μmol/g C) without the addition of exogenous electron donors. In contrast, the interaction of oxidized HA alone with O2 did not result in measurable ·OH production. The enhanced electron-donating capacity (EDC) and humification of the whole HA (mainly in adsorbed state) were measured after adsorption. The surface-catalyzed polymerization of oxidizable polyphenols in HA is proposed as the plausible mechanism for the observed EDC enhancement, which in turn triggers O2 activation for ·OH production. Furthermore, substantial chemical alterations of lignins and condensed aromatics within HA were observed, producing more compounds exhibiting higher molecular weight, aromaticity, O/C ratio, and nominal oxidation state of carbon. It is indicated that the contribution of oxidative polymerization outweighs ·OH oxidation in the molecular transformation of adsorbed HA. Overall, our findings extend the understanding of HA-induced ·OH production from oxic-anoxic interfaces to the oxic zone and present a novel pathway for the abiotic transformation of recalcitrant organic matter in subsurface environments with extensive surface water-groundwater interactions.

Spatiotemporal dynamics of reactive oxygen species and its effect on beta-blockers' degradation in aquatic plants' rhizosphere

The pathway for pollutant degradation involving reactive oxygen species (ROS) in the rhizosphere is poorly understood. Herein, a rootchip system was developed to pinpoint the ROS hotspot along the root tip of Iris tectorum. Through mass balance analysis and quenching experiment, we revealed that ROS contributed significantly to rhizodegradation for beta-blockers, ranging from 22.18 % for betaxolol to 83.83 % for atenolol. The identification of degradation products implicated ROS as an important agent to degrade atenolol into less toxic transformation products during phytoremediation. Moreover, an active production of ROS in rhizosphere was identified by mesocosm experiment. Across three root-associated regions aquatic plants inhabiting the rhizosphere accumulated the highest •OH of ∼1200 nM after 3 consecutive days, followed by rhizoplane (∼230 nM) and bulk environment (∼60 nM). ROS production patterns were driven by rhizosphere chemistry (Fe and humic substances) and microbiome variations in different rhizocompartments. These findings not only deepen understanding of ROS production in aquatic plants rhizosphere but also shed light on advancing phytoremediation strategies.

Iron coupled with hydroxylamine turns on the "switch" for free radical degradation of organic pollutants under high pH conditions

Reducing iron by hydroxylamine (HA) can promote the generation of reactive oxygen species (ROS) in the Fenton reaction and play a crucial role in the degradation of organic pollutants. However, the performance of this system at wider environmental thresholds is still not sufficiently understood, especially in the highly alkaline environments resulting from human activities. Here, we assessed the impact of solution pH on organic pollutant degradation by goethite with the addition of HA and H2O2. The solid phase variation and ROS generation were analyzed using Mössbauer spectroscopy, X-ray absorption near edge structure spectroscopy, and electron paramagnetic resonance analysis. This study found that under alkaline conditions, the system can continuously scavenge organic pollutants through oxygen-mediated generation of free radicals. At lower pH levels, organic pollutant decomposition, exemplified by the breakdown of bisphenol A (BPA), is primarily driven by the Fenton reaction facilitated by iron. As pH increases, hydroxyl radical (•OH) production decreases, accompanied by decreased BPA removal efficiency. However, the removal efficiency of BPA increased significantly at pH > 9. At pH 12, the removal of BPA exceeded that of the acidic condition after one hour, which is consistent with observations in soil system studies. Unlike the Fenton reaction, which is not sensitive to oxygen content, the removal of BPA under alkaline conditions occurs only under aerobic conditions. H2O2 is hardly involved in the reaction, and the depletion of HA becomes a critical factor in the decomposition of BPA. Importantly, in contrast to acidic conditions, where the dramatic decomposition of BPA occurs mainly in the first 10 min, the decomposition of BPA under alkaline conditions continued to occur over the 2 h of observation until complete removal. For natural systems, the remediation of pollutants depends more on the active time of ROS than on their reactivity. Therefore, this idea can reference pollution remediation strategies in anthropogenically disturbed environments.

Hybrid mechanism of microplastics degradation via biological and chemical process during composting

Little is known about the synergistic effects of abiotic aging and biodegradation on microplastics (MPs) transformation in the environment. Herein, a hybrid process of MPs degradation was proposed by analyzing the effect of microorganisms and abiotic aging on aging MPs and non-aging MPs degradation during composting. The results showed that composting facilitated the oxidation and depolymerization of aging MPs, and its degradation efficiency was about three times that of non-aging MPs. Further investigation revealed that aging MPs contained higher abundance of plastic-degrading bacteria and enzyme activity than non-aging MPs. In addition, free radicals also influenced the degradation of MPs. However, path model and shielding experiments confirmed that free radicals mainly facilitated the non-aging MPs degradation (contribution was 68.8 %), while aging MPs was easily degraded by microorganisms (contribution was 72.6 %). This study provides promising strategies for scaling up plastic treatment in bioreactors through a hybrid collaboration of biological and abiotic processes.

Radial Oxygen Loss Triggers Diel Fluctuation of Cadmium Dissolution in the Rhizosphere of Rice

Cadmium (Cd) contamination poses a significant global threat to human health, primarily through dietary intake, with rice serving as a major source. While Cd predominantly resides in bound states in soil, the physiological processes by which rice facilitates Cd absorption in the rhizosphere remain largely elusive. This study delves into the mechanisms governing Cd uptake by rice plants in the rhizosphere, emphasizing the impact of daytime and nighttime fluctuations in microenvironmental conditions. Employing a microfluidic chip setup, the research reveals that radial oxygen loss from rice roots triggers dissolution of Cd in the rhizosphere. Notably, Cd mobility exhibits distinct diurnal fluctuations, peaking at 44.0 ± 4.1 nM during the daytime and dropping to 8.3 ± 1.3 nM during the nighttime. Further investigations reveal that variations in dissolved oxygen and hydroxyl radical concentrations influence Cd release, while pH changes and microbial reduction reactions play crucial roles in Cd immobilization. These findings provide insights into the intricate processes governing Cd mobilization in the rice rhizosphere, highlighting the importance of regulating these processes for effective Cd adsorption control in rice crops and safeguarding public health.

Reduction capacity in the transmissive zones fueled by the embedded low-permeability lenses: Implications for contaminant transformation in heterogeneous aquifers

Redox conditions play a decisive role in regulating contaminant and nutrient transformation in groundwater. Here we quantitatively described and interpreted the temporal and spatial variations of aquifer reduction capacity formation in lens-embedded heterogeneous aquifers in 1-D columns. Experimental results indicated that the aquifer reduction capacity exported from the low-permeability lens permeated into the downstream sandy zones, where it subsequently accumulated and extended. Reactive transport modeling suggested that reduction capacity within the lens preferentially diffused to the transmissive zones around the lens-sand interface, and was then transported via convection to downstream transmissive zones. A low-permeability lens of the same volume, but more elongated in the flow direction, led to less concentrated reduction capacity but extended further downgradient from the lens. The increased flow velocity attenuated the maintenance of aquifer reduction capacity by enhancing mixing and diluting processes in the transmissive zones. The reduction zones formed downstream from the low-permeability lens were hotpots for resisting the oxidative perturbation by O2. This study highlights the important role of low-permeability lenses as large and long-term electron pools for the transmissive zones, and thus providing aquifer reduction capacity for contaminant transformation and remediation in heterogeneous aquifers.

Enhancing the microbial advanced oxidation of P-nitrophenol in sediment through accelerating extracellular respiration with electrical stimulation

Microbial advanced oxidation, a fundamental process for pollutant degradation in nature, is limited in efficiency by the weak respiration of indigenous microorganisms. In this study, an electric field was employed to enhance microbial respiration and facilitate the microbial advanced oxidation of p-nitrophenol (PNP) in simulated wetlands with alternation of anaerobic and aerobic conditions. With intermittent air aeration, an electric field of 0.8 V promoted extracellular electron transfer to increase Fe2+ generation through dissimilatory iron reduction and the production of hydroxyl radicals (•OH) through Fenton-like reactions. As a result, the PNP removal rate of the electrically-stimulated group was higher than that of the control (72.15 % vs 46.88 %). Multiple lines of evidence demonstrated that the electrically-induced polarization of respiratory enzymes expedited proton-coupled electron transfer within the respiratory chain to accelerate microbial advanced oxidation of PNP. The polarization of respiratory enzymes with the electric field hastened proton outflow to increase cell membrane potential for adenosine triphosphate (ATP) generation, which enhanced intracellular electron transportation to benefit reactive oxygen species generation. This study provided a new method to enhance microelectrochemical remediation of the contaminant in wetlands via the combination of intermittent air aeration.

Microbially Driven Iron Cycling Facilitates Organic Carbon Accrual in Decadal Biochar-Amended Soil

Soil organic carbon (SOC) is pivotal for both agricultural activities and climate change mitigation, and biochar stands as a promising tool for bolstering SOC and curtailing soil carbon dioxide (CO2) emissions. However, the involvement of biochar in SOC dynamics and the underlying interactions among biochar, soil microbes, iron minerals, and fresh organic matter (FOM, such as plant debris) remain largely unknown, especially in agricultural soils after long-term biochar amendment. We therefore introduced FOM to soils with and without a decade-long history of biochar amendment, performed soil microcosm incubations, and evaluated carbon and iron dynamics as well as microbial properties. Biochar amendment resulted in 2-fold SOC accrual over a decade and attenuated FOM-induced CO2 emissions by approximately 11% during a 56-day incubation through diverse pathways. Notably, biochar facilitated microbially driven iron reduction and subsequent Fenton-like reactions, potentially having enhanced microbial extracellular electron transfer and the carbon use efficiency in the long run. Throughout iron cycling processes, physical protection by minerals could contribute to both microbial carbon accumulation and plant debris preservation, alongside direct adsorption and occlusion of SOC by biochar particles. Furthermore, soil slurry experiments, with sterilization and ferrous iron stimulation controls, confirmed the role of microbes in hydroxyl radical generation and biotic carbon sequestration in biochar-amended soils. Overall, our study sheds light on the intricate biotic and abiotic mechanisms governing carbon dynamics in long-term biochar-amended upland soils.

Superoxide Photoproduction from Wetland Plant-Derived Dissolved Organic Matter: Implications for Biogeochemical Impacts of Plant Invasion

Although the impacts of exotic wetland plant invasions on native biodiversity, landscape features, and carbon-nitrogen cycles are well appreciated, biogeochemical consequences posed by ecological competition, such as the heterogeneity of dissolved organic matter (DOM) from plant detritus and its impact on the formation of reactive oxygen species, are poorly understood. Thus, this study delves into O2•- photogeneration potential of DOM derived from three different parts (stem, leaf, and panicle) of invasive Spartina alterniflora (SA) and native Phragmites australis (PA). It is found that DOM from the leaves of SA and the panicles of PA has a superior ability to produce O2•-. With more stable aromatic structures and a higher proportion of sulfur-containing organic compounds, SA-derived DOM generally yields more O2•- than that derived from PA. UVA exposure enhances the leaching of diverse DOM molecules from plant detritus. Based on the reported monitoring data and our findings, the invasion of SA is estimated to approximately double the concentration of O2•- in the surrounding water bodies. This study can help to predict the underlying biogeochemical impacts from the perspective of aquatic photochemistry in future scenarios of plant invasion, seawater intrusion, wetland degradation, and elevated solar UV radiation.

Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

In subsurface environments, Fe(II)-bearing clay minerals can serve as crucial electron sources for O2 activation, leading to the sequential production of O2•-, H2O2, and •OH. However, the observed •OH yields are notably low, and the underlying mechanism remains unclear. In this study, we investigated the production of oxidants from oxygenation of reduced Fe-rich nontronite NAu-2 and Fe-poor montmorillonite SWy-3. Our results indicated that the •OH yields are dependent on mineral Fe(II) species, with edge-surface Fe(II) exhibiting significantly lower •OH yields compared to those of interior Fe(II). Evidence from in situ Raman and Mössbauer spectra and chemical probe experiments substantiated the formation of structural Fe(IV). Modeling results elucidate that the pathways of Fe(IV) and •OH formation respectively consume 85.9-97.0 and 14.1-3.0% of electrons for H2O2 decomposition during oxygenation, with the Fe(II)edge/Fe(II)total ratio varying from 10 to 90%. Consequently, these findings provide novel insights into the low •OH yields of different Fe(II)-bearing clay minerals. Since Fe(IV) can selectively degrade contaminants (e.g., phenol), the generation of mineral Fe(IV) and •OH should be taken into consideration carefully when assessing the natural attenuation of contaminants in redox-fluctuating environments.

Abiotic Methane Production Driven by Ubiquitous Non-Fenton-Type Reactive Oxygen Species

Abiotic CH4 production driven by Fenton-type reactive oxygen species (ROS) has been confirmed to be an indispensable component of the atmospheric CH4 budget. While the chemical reactions independent of Fenton chemistry to ROS are ubiquitous in nature, it remains unknown whether the produced ROS can drive abiotic CH4 production. Here, we first demonstrated the abiotic CH4 production at the soil-water interface under illumination. Leveraging this finding, polymeric carbon nitrides (CNx) as a typical analogue of natural geobattery material and dimethyl sulfoxide (DMSO) as a natural methyl donor were used to unravel the underlying mechanisms. We revealed that the ROS, photocatalytically produced by CNx, can oxidize DMSO into CH4 with a high selectivity of 91.5 %. Such an abiotic CH4 production process was further expanded to various non-Fenton-type reaction systems, such as electrocatalysis, pyrocatalysis and sonocatalysis. This work provides insights into the geochemical cycle of abiotic CH4, and offers a new route to CH4 production via integrated energy development.

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.

Seasonal and Spatial Fluctuations of Reactive Oxygen Species in Riparian Soils and Their Contributions on Organic Carbon Mineralization

Reactive oxygen species (ROS) are ubiquitous in the natural environment and play a pivotal role in biogeochemical processes. However, the spatiotemporal distribution and production mechanisms of ROS in riparian soil remain unknown. Herein, we performed uninterrupted monitoring to investigate the variation of ROS at different soil sites of the Weihe River riparian zone throughout the year. Fluorescence imaging and quantitative analysis clearly showed the production and spatiotemporal variation of ROS in riparian soils. The concentration of superoxide (O2•-) was 300% higher in summer and autumn compared to that in other seasons, while the highest concentrations of 539.7 and 20.12 μmol kg-1 were observed in winter for hydrogen peroxide (H2O2) and hydroxyl radicals (•OH), respectively. Spatially, ROS production in riparian soils gradually decreased along with the stream. The results of the structural equation and random forest model indicated that meteorological conditions and soil physicochemical properties were primary drivers mediating the seasonal and spatial variations in ROS production, respectively. The generated •OH significantly induced the abiotic mineralization of organic carbon, contributing to 17.5-26.4% of CO2 efflux. The obtained information highlighted riparian zones as pervasive yet previously underestimated hotspots for ROS production, which may have non-negligible implications for carbon turnover and other elemental cycles in riparian soils.

Iron Promotes the Retention of Terrigenous Dissolved Organic Matter in Subtidal Permeable Sediments

Marine permeable sediments are important sites for organic matter turnover in the coastal ocean. However, little is known about their role in trapping dissolved organic matter (DOM). Here, we examined DOM abundance and molecular compositions (9804 formulas identified) in subtidal permeable sediments along a near- to offshore gradient in the German North Sea. With the salinity increasing from 30.1 to 34.6 PSU, the DOM composition in bottom water shifts from relatively higher abundances of aromatic compounds to more highly unsaturated compounds. In the bulk sediment, DOM leached by ultrapure water (UPW) from the solid phase is 54 ± 20 times more abundant than DOM in porewater, with higher H/C ratios and a more terrigenous signature. With 0.5 M HCl, the amount of leached DOM (enriched in aromatic and oxygen-rich compounds) is doubled compared to UPW, mainly due to the dissolution of poorly crystalline Fe phases (e.g., ferrihydrite and Fe monosulfides). This suggests that poorly crystalline Fe phases promote DOM retention in permeable sediments, preferentially terrigenous, and aromatic fractions. Given the intense filtration of seawater through the permeable sediments, we posit that Fe can serve as an important intermediate storage for terrigenous organic matter and potentially accelerate organic matter burial in the coastal ocean.

Investigation on the role of ·OH for BPA removal in coastal sediments: The important mediation of low reactivity Fe(II)

Bisphenol A (BPA) in seawater tends to be deposited in coastal sediments. However, its degradation under tidal oscillations has not been explored comprehensively. Hydroxyl radicals (·OH) can be generated through Fe cycling under redox oscillations, which have a strong oxidizing capacity. This study focused on the contribution of Fe-mediated production of ·OH in BPA degradation under darkness. The removal of BPA was investigated by reoxygenating six natural coastal sediments, and three redox cycles were applied to prove the sustainability of the process. The importance of low reactivity Fe(II) in the production of ·OH was investigated, specifically, Fe(II) with carbonate and Fe(II) within goethite, hematite and magnetite. The degradation efficiency of BPA during reoxygenation of sediments was 76.78-94.82%, and the contribution of ·OH ranged from 36.74% to 74.51%. The path coefficient of ·OH on BPA degradation reached 0.6985 and the indirect effect of low reactivity Fe(II) on BPA degradation by mediating ·OH production reached 0.5240 obtained via partial least squares path modeling (PLS-PM). This study emphasizes the importance of low reactivity Fe(II) in ·OH production and provides a new perspective for the role of tidal-induced ·OH on the fate of refractory organic pollutants under darkness.

Enzyme-induced reactive oxygen species trigger oxidative degradation of sulfamethoxazole within a methanotrophic biofilm

Although microorganisms carrying copper-containing membrane-bound monooxygenase (CuMMOs), such as particulate methane monooxygenase (pMMO) and ammonia monooxygenase (AMO), have been extensively documented for their capability to degrade organic micropollutants (OMPs), the underlying reactive mechanism remains elusive. In this study, we for the first time demonstrate biogenic reactive oxygen species (ROS) play important roles in the degradation of sulfamethoxazole (SMX), a representative OMP, within a methane-fed biofilm. Highly-efficient and consistent SMX biodegradation was achieved in a CH4-based membrane biofilm reactor (MBfR), manifesting a remarkable SMX removal rate of 1210.6 ± 39.0 μg·L-1·d-1. Enzyme inhibition and ROS clearance experiments confirmed the significant contribution of ROS, which were generated through the catalytic reaction of pMMO and AMO enzymes, in facilitating SMX degradation. Through a combination of density functional theory (DFT) calculations, electron paramagnetic resonance (EPR) analysis, and transformation product detection, we elucidated that the ROS primarily targeted the aniline group in the SMX molecule, inducing the formation of aromatic radicals and its progressive mineralization. In contrast, the isoxazole-ring was not susceptible to electrophilic ROS attacks, leading to accumulation of 3-amino-5-methylisoxazole (3A5MI). Furthermore, microbiological analysis suggested Methylosarcina (a methanotroph) and Candidatus Nitrosotenuis (an ammonia-oxidizing archaea) collaborated as the SMX degraders, who carried highly conserved and expressed CuMMOs (pMMO and AMO) for ROS generation, thereby triggering the oxidative degradation of SMX. This study deciphers SMX biodegradation through a fresh perspective of free radical chemistry, and concurrently providing a theoretical framework for the advancement of environmental biotechnologies aimed at OMP removal.

Soil carbon and nitrogen cycles driven by iron redox: A review

Soil carbon and nitrogen cycles affect agricultural production, environmental quality, and global climate. Iron (Fe), regarded as the most abundant redox-active metal element in the Earth's crust, is involved in a biogeochemical cycle that includes Fe(III) reduction and Fe(II) oxidation. The redox reactions of Fe can be linked to the carbon and nitrogen cycles in soil in various ways. Investigating the transformation processes and mechanisms of soil carbon and nitrogen species driven by Fe redox can provide theoretical guidance for improving soil fertility, and addressing global environmental pollution as well as climate change. Although the widespread occurrence of these coupling processes in soils has been revealed, explorations of the effects of Fe redox on soil carbon and nitrogen cycles remain in the early stages, particularly when considering the broader context of global climate and environmental changes. The key functional microorganisms, mechanisms, and contributions of these coupling processes to soil carbon and nitrogen cycles have not been fully elucidated. Here, we present a systematic review of the research progress on soil carbon and nitrogen cycles mediated by Fe redox, including the underlying reaction processes, the key microorganisms involved, the influencing factors, and their environmental significance. Finally, some unresolved issues and future perspectives are addressed. This knowledge expands our understanding of the interconnected cycles of Fe, carbon and nitrogen in soils.

Natural Disinfection-like Process Unveiled in Soil Microenvironments by Enzyme-Catalyzed Chlorination

Substantial natural chlorination processes are a growing concern in diverse terrestrial ecosystems, occurring through abiotic redox reactions or biological enzymatic reactions. Among these, exoenzymatically mediated chlorination is suggested to be an important pathway for producing organochlorines and converting chloride ions (Cl-) to reactive chlorine species (RCS) in the presence of reactive oxygen species like hydrogen peroxide (H2O2). However, the role of natural enzymatic chlorination in antibacterial activity occurring in soil microenvironments remains unexplored. Here, we conceptualized that heme-containing chloroperoxidase (CPO)-catalyzed chlorination functions as a naturally occurring disinfection process in soils. Combining antimicrobial experiments and microfluidic chip-based fluorescence imaging, we showed that the enzymatic chlorination process exhibited significantly enhanced antibacterial activity against Escherichia coli and Bacillus subtilis compared to H2O2. This enhancement was primarily attributed to in situ-formed RCS. Based on semiquantitative imaging of RCS distribution using a fluorescence probe, the effective distance of this antibacterial effect was estimated to be approximately 2 mm. Ultrahigh-resolution mass spectrometry analysis showed over 97% similarity between chlorine-containing formulas from CPO-catalyzed chlorination and abiotic chlorination (by sodium hypochlorite) of model dissolved organic matter, indicating a natural source of disinfection byproduct analogues. Our findings unveil a novel natural disinfection process in soils mediated by indigenous enzymes, which effectively links chlorine-carbon interactions and reactive species dynamics.

Field Quantification of Hydroxyl Radicals by Flow-Injection Chemiluminescence Analysis with a Portable Device

Hydroxyl radical (•OH) is a powerful oxidant abundantly found in nature and plays a central role in numerous environmental processes. On-site detection of •OH is highly desirable for real-time assessments of •OH-centered processes and yet is restrained by a lack of an analysis system suitable for field applications. Here, we report the development of a flow-injection chemiluminescence analysis (FIA-CL) system for the continuous field detection of •OH. The system is based on the reaction of •OH with phthalhydrazide to generate 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence (CL) when oxidatively activated by H2O2 and Cu3+. The FIA-CL system was successfully validated using the Fenton reaction as a standard •OH source. Unlike traditional absorbance- or fluorescence-based methods, CL detection could minimize interference from an environmental medium (e.g., organic matter), therefore attaining highly sensitive •OH detection (limits of detection and quantification = 0.035 and 0.12 nM, respectively). The broad applications of FIA-CL were illustrated for on-site 24 h detection of •OH produced from photochemical processes in lake water and air, where the temporal variations on •OH productions (1.0-12.2 nM in water and 1.5-37.1 × 107 cm-3 in air) agreed well with sunlight photon flux. Further, the FIA-CL system enabled field 24 h field analysis of •OH productions from the oxidation of reduced substances triggered by tidal fluctuations in coastal soils. The superior analytical capability of the FIA-CL system opens new opportunities for monitoring •OH dynamics under field conditions.

Stability and Biotransformation of 6:2 Fluorotelomer Sulfonic Acid, Sulfonamide Amine Oxide, and Sulfonamide Alkylbetaine in Aerobic Sludge

The 6:2 fluorotelomer sulfonamide (6:2 FTSAm)-based compounds signify a prominent group of per- and polyfluoroalkyl substances (PFAS) widely used in contemporary aqueous film-forming foam (AFFF) formulations. Despite their widespread presence, the biotransformation behavior of these compounds in wastewater treatment plants remains uncertain. This study investigated the biotransformation of 6:2 FTSAm-based amine oxide (6:2 FTNO), alkylbetaine (6:2 FTAB), and 6:2 fluorotelomer sulfonic acid (6:2 FTSA) in aerobic sludge over a 100-day incubation period. The biotransformation of 6:2 fluorotelomer sulfonamide alkylamine (6:2 FTAA), a primary intermediate product of 6:2 FTNO, was indirectly assessed. Their stability was ranked based on the estimated half-lives (t1/2): 6:2 FTAB (no obvious products were detected) ≫ 6:2 FTSA (t1/2 ≈28.8 days) > 6:2 FTAA (t1/2 ≈11.5 days) > 6:2 FTNO (t1/2 ≈1.2 days). Seven transformation products of 6:2 FTSA and 15 products of 6:2 FTNO were identified through nontarget and suspect screening using high-resolution mass spectrometry. The transformation pathways of 6:2 FTNO and 6:2 FTSA in aerobic sludge were proposed. Interestingly, 6:2 FTSAm was hardly hydrolyzed to 6:2 FTSA and further biotransformed to perfluoroalkyl carboxylic acids (PFCAs). Furthermore, the novel pathways for the generation of perfluoroheptanoic acid (PFHpA) from 6:2 FTSA were revealed.

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.

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.

Impact of redox fluctuations on microbe-mediated elemental sulfur disproportionation and coupled redox cycling of iron

Elemental sulfur (S0) plays a vital role in the coupled cycling of sulfur and iron, which in turn affects the transformation of carbon and various pollutants. These processes have been well characterized under static anoxic or oxic conditions, however, how the natural redox fluctuations affect the bio-mediated sulfur cycling and coupled iron cycling remain enigmatic. The present work examined S0 disproportionation as driven by natural microbial communities under fluctuating redox conditions and the contribution of S0 disproportionation to ferrihydrite transformation. Samples were incubated at either neutral or alkaline pH values, applying sequential anaerobic, aerobic and anaerobic conditions over 60 days. Under anaerobic conditions, S0 was found to undergo disproportionation to sulfate and sulfide, which subsequently reduced ferrihydrite at both pH 7.4 and 9.5. Ferrihydrite promoted S0 disproportionation by scavenging biogenic sulfide and maintaining a suitable degree of sulfate formation. After an oxic period, during the subsequent anoxic incubation, bioreduction of sulfate occurred and the biogenic sulfide reduced iron (hydr)oxides at a rate approximately 25 % lower than that observed during the former anoxic period. A 16S rDNA-based microbial community analysis revealed changes in the microbial community in response to the redox fluctuations, implying an intimate association with the coupled cycling of sulfur and iron. Microscopic and spectroscopic analyses confirmed the S0-mediated transformation of ferrihydrite to crystalline iron (hydr)oxide minerals such as lepidocrocite and magnetite and the formation of iron sulfides precipitated under fluctuating redox conditions. Finally, a reaction mechanism based on mass balance was proposed, demonstrating that bio-mediated sulfur transformation maintained a sustainable redox reaction with iron (hydr)oxides under fluctuating anaerobic-aerobic-anaerobic conditions tested in this study. Altogether, the finding of our study is critical for obtaining a more complete understanding of the dynamics of iron redox reactions and pollutant transformation in sulfur-rich aquatic environments.

Electromotive force induced by dynamic magnetic field electrically polarized sediment to aggravate methane emission

As a primary driving force of global methane production, methanogens like other living organisms are exposed to an environment filled with dynamic electromagnetic waves, which might induce electromotive force (EMF) to potentially influence the metabolism of methanogens. However, no reports have been found on the effects of the induced electromotive force on methane production. In this study, we found that exposure to a dynamic magnetic field enhanced bio-methanogenesis via the induced electromotive force. When exposed to a dynamic magnetic field with 0.20 to 0.40 mT of intensity, the methane emission of the sediments increased by 41.71%. The respiration of methanogens and bacteria was accelerated by the EMF, as the ratios of F420H2/F420 and NAD+/NADH of the sediment increased by 44.12% and 55.56%, respectively. The respiratory enzymes in respiration chains might be polarized with the EMF to accelerate the proton-coupled electron transfer to enhance microbial metabolism. Together with the enriched exoelectrogens and electrotrophic methanogens, as well as the increased sediment electro-activities, this study indicated that the EMF could enhance the electron exchange among extracellular respiratory microorganisms to increase the methane emission from sediments.

Dry-to-wet fluctuation of moisture contents enhanced the mineralization of chloramphenicol antibiotic

Due to livestock wastewater irrigation, soil is becoming one of the major sinks of antibiotics in the environment. Recently, it is getting recognized that a variety of minerals under low moisture conditions can induce strong catalytic hydrolysis to antibiotics. However, the relative importance and implication of soil water content (WC) for natural attenuation of soil residual antibiotics has not been well recognized. In order to explore the optimal moisture levels and the key soil properties dominating for the high catalytic hydrolysis activities of soils, this study collected 16 representative soil samples across China, and assessed their performances to degrade chloramphenicol (CAP) under different moisture levels. The results showed that the soils with low organic matter contents (< 20 g/kg) and high amounts of crystalline Fe/Al were particularly effective in catalyzing CAP hydrolysis when exposed to low WC (< 6%, wt/wt), leading to CAP hydrolysis half-lives of <40 d Higher WC greatly suppressed the catalytic activity of the soil. By utilizing this process, it is possible to integrate abiotic and biotic degradation to enhance the mineralization of CAP, attributing to that the hydrolytic products are more available for soil microorganisms. As expected, the soils experienced periodic dry-to-wet moisture conditions (i.e., the WC shifting from 1 to 5% to 20-35%, wt/wt) exhibited higher degradation and mineralization of ¹⁴C-CAP, in comparison with the constant wet treatment. Meanwhile, the bacterial community composition and the specific genera showed that the dry-to-wet fluctuation of soil WC relieved the antimicrobial stress to bacterial community. Our study verifies the critical role of soil WC in mediating the natural attenuation of antibiotics, and guides to remove antibiotics from both wastewater and soil.

Neglected but Efficient Electron Utilization Driven by Biochar-Coactivated Phenols and Peroxydisulfate: Polyphenol Accumulation Rather than Mineralization

We report an unrecognized but efficient nonradical mechanism in biochar-activated peroxydisulfate (PDS) systems. Combining a newly developed fluorescence trapper of reactive oxygen species with steady-state concentration calculations, we showed that raising pyrolysis temperatures of biochar (BC) from 400 to 800 °C remarkably enhanced trichlorophenol degradation but inhibited the catalytic production of radicals (SO₄^•- and ^•OH) in water and soil, thereby switching a radical-based activation into an electron-transfer-dominated nonradical pathway (contribution increased from 12.9 to 76.9%). Distinct from previously reported PDS* complex-determined oxidation, in situ Raman and electrochemical results of this study demonstrated that the simultaneous activation of phenols and PDS on the biochar surface triggers the potential difference-driven electron transfer. The formed phenoxy radicals subsequently undergo coupling and polymerization reactions to generate dimeric and oligomeric intermediates, which are eventually accumulated on the biochar surface and removed. Such a unique nonmineralizing oxidation achieved an ultrahigh electron utilization efficiency (e_phenols/e_PDS) of 182%. Through biochar molecular modeling and theoretical calculations, we highlighted the critical role of graphitic domains rather than redox-active moieties in lowering band-gap energy to facilitate electron transfer. Our work provides insights into outstanding contradictions and controversies related to nonradical oxidation and inspiration for more oxidant-saving remediation technologies.

Organic ligands activate the dark formation of hydroxyl radicals (HO•) in surface soil/sediment: Yields, mechanisms, and applications

Soil is an important sink for various pollutants. Recent findings suggest that soil and sediment would spontaneously form HO^• through Fenton or Fenton-like reactions under natural conditions. In this study, the effects and mechanisms of organic ligands (OLs) on the occurrence of HO^• in surface soil/sediment were experimentally and computationally examined. Results confirmed that HO^• generation was ND-12.92 nmol/g in surface soil/sediment, and the addition of EDTA-2Na would significantly enhance the yields of HO^• by 1.4-352 times. Moisture was the decisive factor of soil HO^• generation. The release of Fe(II) from solid into the aqueous phase was essential for the stimulation of HO^• in EDTA-2Na suspensions. Furthermore, complexation reactions between Fe(II) and OLs would enhance single electron transfer (SET) reactions and the formation of O₂^•-. Interestingly, for specific OLs, their stimulations on SET and formation of O₂^•- would depress HO^• generation. Provoking HO^• generation by OLs could be efficiently used to degrade sulfamethoxazole in rice field sediment. The study provided new knowledge on how commonly synthetic OLs affect the HO^• generation in surface soil/sediment, and it additionally shed light on the engineered stimulation of in-situ Fenton reactions in natural soil/sediment.

Reactive oxygen species affect the potential for mineralization processes in permeable intertidal flats

Intertidal permeable sediments are crucial sites of organic matter remineralization. These sediments likely have a large capacity to produce reactive oxygen species (ROS) because of shifting oxic-anoxic interfaces and intense iron-sulfur cycling. Here, we show that high concentrations of the ROS hydrogen peroxide are present in intertidal sediments using microsensors, and chemiluminescent analysis on extracted porewater. We furthermore investigate the effect of ROS on potential rates of microbial degradation processes in intertidal surface sediments after transient oxygenation, using slurries that transitioned from oxic to anoxic conditions. Enzymatic removal of ROS strongly increases rates of aerobic respiration, sulfate reduction and hydrogen accumulation. We conclude that ROS are formed in sediments, and subsequently moderate microbial mineralization process rates. Although sulfate reduction is completely inhibited in the oxic period, it resumes immediately upon anoxia. This study demonstrates the strong effects of ROS and transient oxygenation on the biogeochemistry of intertidal sediments.

Reactive Oxygen Species Promote Nitrous Oxide (N2O) Emissions from Soil/Sediment during the Anoxic-Oxic Transition

Reactive oxygen species (ROS)-induced element/pollutant geochemical processes in fluctuating anoxic-oxic areas have received increasing attention in recent years. Nitrous oxide (N₂O) is a strong greenhouse gas; however, the relationship between ROS and N₂O emissions in these areas has not been established. This work revealed the essential role of ROS in promoting N₂O emissions in soil/sediment during the anoxic-oxic transition. ROS decreased the rate of nitrate reduction by 26-31% and increased N₂O emissions by 8.8-31.3% (at 48 h). ROS-induced N₂O emission was via inhibiting the step of N₂O reduction. During the anoxic-oxic transition, the contribution of ROS to inhibit the step of N₂O reduction was higher than 52.6%, demonstrating the important role of ROS. The downregulated relative transcription of the NosZ gene demonstrated inhibition at the gene level. Hydrogen peroxide was the dominant ROS species inhibiting N₂O reduction, while the role of hydroxyl radicals was negligible, suggesting a different behavior of N₂O emission with common pollutant conversion induced by ROS during the anoxic-oxic transition. This study demonstrated an overlooked factor in promoting N₂O emission in the soil/sediment and appealed to a re-examination of the mechanism of N₂O emissions in fluctuating anoxic-oxic areas.

Reductive Stress Boosts the Horizontal Transfer of Plasmid-Borne Antibiotic Resistance Genes: The Neglected Side of the Intracellular Redox Spectrum

The dissemination of plasmid-borne antibiotic resistance genes (ARGs) among bacteria is becoming a global challenge to the "One Health" concept. During conjugation, the donor/recipient usually encounter diverse stresses induced by the surrounding environment. Previous studies mainly focused on the effects of oxidative stress on plasmid conjugation, but ignored the potential contribution of reductive stress (RS), the other side of the intracellular redox spectrum. Herein, we demonstrated for the first time that RS induced by dithiothreitol could significantly boost the horizontal transfer of plasmid RP4 from Escherichia coli K12 to different recipients (E. coli HB101, Salmonella Typhimurium, and Pseudomonas putida KT2440). Phenotypic and genotypic tests confirmed that RS upregulated genes encoding the transfer apparatus of plasmid RP4, which was attributed to the promoted consumption of intracellular glutamine in the donor rather than the widely reported SOS response. Moreover, RS was verified to benefit ATP supply by activating glycolysis (e.g., GAPDH) and the respiratory chain (e.g., appBC), triggering the deficiency of intracellular free Mg²⁺ by promoting its binding, and reducing membrane permeability by stimulating cardiolipin biosynthesis, all of which were beneficial to the functioning of transfer apparatus. Overall, our findings uncovered the neglected risks of RS in ARG spreading and updated the regulatory mechanism of plasmid conjugation.