Active iron species driven hydroxyl radicals formation in oxygenation of different paddy soils: Implications to polycyclic aromatic hydrocarbons degradation

Root Exudates Mediate the Production of Reactive Oxygen Species in Rhizosphere Soil: Formation Mechanisms and Ecological Effects

Reactive oxygen species (ROS), as redox messengers, play an important role in regulating plant growth, sensing biotic and abiotic stresses, and integrating different environmental signals. As the microenvironment of the interaction between root, soil and microorganism, the rhizosphere is the hotspot of ROS production and action. Root exudates are an important medium for communication between roots and the soil environment, and they have a significant regulatory effect on the production of ROS in the rhizosphere. At the same time, the formation of rhizosphere ROS is determined by the coupling of various biotic and abiotic factors, and it is also affected by environmental stresses such as temperature, humidity, and disease. This review summarizes how root exudates affect plant growth and induce plant defense mechanisms by regulating the generation and distribution of ROS. It also discusses the role of ROS in promoting the decomposition of soil organic matter, nutrient cycling, and pollutant degradation and transformation. In-depth study of the regulation mechanism of root exudates on ROS not only helps to reveal the molecular mechanism of plant adaptation to environmental stress but also provides theoretical support and practical guidance for sustainable agricultural development and ecological environment protection.

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.

Interactions between iron mineral and low-molecular-weight organic acids accelerated nitrogen conversion and release in lake sediments

Endogenous nitrogen (N) release from lake sediments is one of main causes affecting water quality, which can be affected by the presence of iron (Fe) minerals and organic matter, especially low-molecular-weight organic acids (LMWOAs). Although these substances always coexist in sediments, their interaction effect on N fate is not yet clear. In this study, the role and mechanisms of the coexistence of iron mineral (ferrihydrite, Fh) and LMWOAs, i.e. citric acid (CA) and galacturonic acid (GA) on the release and transformation of N in lake sediments were systematically evaluated via microcosm cultivation for 45 d Results showed that the addition of Fh+LMWOAs significantly accelerated N mineralization and conversion in lake sediments, accompanied by increasing ferrous iron content and decreasing redox potential. Biotic pathways played more critical roles than abiotic oxidation pathways during this process, and Fh+LMWOAs strengthened cooperation among microbial species by forming complex topologies and higher positive correlations. Correspondingly, cellular functions, iron respiration, and N metabolism modules were increased. CA with high carboxyl content showed greater total nitrogen removal and metabolic abundance. The present findings facilitate understanding the synergies of iron minerals and organic matter on N fate and N biogeochemical cycling in lake sediments.

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.

Agricultural amendments enhanced the redox cycling of iron species and hydroxyl radical formation during redox fluctuation of paddy soil

Hydroxyl radical (•OH) plays a critical role in accelerating organic contaminant attenuation during water-table decline in paddy soil, but the impacts of widely applied agricultural amendments (e.g., organic manure, rice straw, and biochar) on these processes have been rarely explored. Hence, the effects of agricultural amendments on •OH formation and pollutant degradation were examined based on field experiments. Compared with control, organic fertilizer (supplying more organic carbon (OC) and bioavailable elements that promoted Fe(II) formation by microorganisms) enhanced •OH production by 0.8-1.3 times, while straw returning and biochar have negligible effects, probably due to the decreased pH and inhibition of microorganisms. The increased oxidation of active Fe(II) species (e.g., exchangeable Fe(II) and Fe(II) in lower-crystallinity minerals) mainly contributed to •OH production. Further analyses showed that organic fertilizers significantly enhanced the redox cycling of Fe species mainly through increasing the contents of soil organic carbon and relative abundances of Fe(III)-reducing microorganisms. In addition, the increased •OH formation markedly enhanced imidacloprid degradation by 24.3-42.4 %, with the toxicity of intermediates increased versus the parent compound. This study systematically examined the effects of typical agricultural amendments on the •OH formation and organic contaminant attenuation in paddy soil, which probably provides promising strategies for regulating contaminant remediation in agricultural fields.

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.

Effects of Low Concentrations of Hypochloric Acid on the Agricultural Soil Properties

The oxidant (e.g., hypochlorous acid, HClO) has been widely used to remediate contaminated sites. However, the effects of HClO on soil health in agricultural fields have been rarely explored. Based on the laboratory and field experiments, we found that low concentrations of HClO changed soil bioavailable elements, enzyme activities, and the structure of microbial communities. Specifically, the application of 5‰ HClO increased DOC content by 13.9-19.9% compared with CK. The bioavailable Fe and Mn contents also increased by 15.0% and 48.7%-61.8%, respectively. Correspondingly, in field experiments, 5‰ HClO enhanced the DOC contents by 12.0%. Besides, the application of HClO also elevated enzyme activities and improved the structure of the microbial community of soil. These findings might provide new strategies for improving the fertility and quality of agricultural soil.

Humic acid inhibits hydroxyl radical generation during oxygenation of Fe(II) on goethite surface

The dark formation of hydroxyl radicals (•OH) by the oxidation of Fe(II) has been increasingly recognized at anoxic-oxic interface. Minerals play significant roles on oxidation of Fe(II) owing to the adsorption changed the reactivity of Fe(II). However, the impact of dissolved organic matter (DOM) on the oxidation of mineral adsorbed Fe(II) and •OH generation remains unknown. Herein, we examined the impact of humic acid (HA) on •OH accumulation during oxygenation of goethite surface-adsorbed Fe(II). We found the addition of 100-500 mg C•L-1 HA inhibited 3.7 % - 24.8 % •OH accumulation, compared to •OH generation facilitated by goethite-adsorbed Fe(II), and the electron utilization efficiency for •OH generation reduced from 15.4 % to 11.8 %. The adsorption experiment under anoxic condition showed that HA compete absorption with Fe(II) on goethite surface. XPS spectroscopy revealed that HA reduces the surface Fe-OH functional group by 4.8 %, thereby reducing the active sites on goethite. Voltammetric cycling curves demonstrated that HA decreased the reducing capacity and inhibited the electrical conductivity of the goethite-adsorbed Fe(II). This study elucidates the effects and mechanisms of HA adsorption on •OH generation during the oxygenation of Fe(II) on goethite surface.

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.

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.

Mechanistic insights into the pH-driven radical transformation of the Fe(II)/nCP in groundwater remediation

Calcium peroxide nanoparticles (nCP) as a versatile and safe solid H2O2 source, have attracted significant research interst for their application potential in groundwater remediation. Compared to the traditional Fenton system, the nCP-based Fenton-like system has a wider pH-working window for contaminants degradation. This results from the dominant radical transformation under different pH. Unlike the traditional Fenton system which is only effective in acid conditions with hydroxyl radical (•OH) as the main active species, the release of H2O2 and O2 from nCP provides multiple contaminants degradation pathways. In acidic environments, •OH and Fe(IV) predominate as the active species, facilitated by substantial H2O2 production which activates the Fenton reaction. In neutral or alkaline conditions, the production of H2O2 was dramatically decreased. While the O2 released from nCP can be catalyzed by Fe(II) to form superoxide radical (•O2-), which subsequently generate singlet oxygen (1O2). The formation pathway of •O2- was tracked by O18 isotope labeling experiment. The impact of the water matrix on radical generation in the Fe(II)/nCP Fenton-like system was also studied. This research deepens the understanding of the radical formation mechanisms in nCP-based Fenton-like system, offering insights to support their application in remediating contaminated groundwater.

DNA-stable isotope probing and metagenomics reveal Fe(II) oxidation by core microflora in microoxic rhizospheric habitats to mitigate the accumulation of cadmium and phenanthrene in rice

Rice rhizosphere soil-porewater microdomains exist within an iron (Fe)-rich microoxic habitat during paddy soil flooding. However, the response mechanisms of core microflora in this habitat to Fe(II)-oxidation-mediated cadmium (Cd) and phenanthrene (Phen) remain unclear. Using gel-stabilized gradient systems to replicate the microoxic conditions in the rice rhizosphere porewater, we found that microaerophilic rhizobacteria drove Fe(II) oxidation to yield iron oxides, thereby reducing the Cd and Phen contents in the rhizosphere porewater and rice (Cd and Phen decreased by 15.9%-78.0% and 10.1%-37.4%, respectively). However, co-exposure to Cd and Phen resulted in a greater reduction in the Cd uptake and a greater increase in the Phen uptake in rice as compared to those in the Cd or Phen treatments, possibly attributing to the cation-π interactions between Cd and Phen, as well as competition between the adsorption sites on the roots. The elevation of Cd-tolerant genes and Phen-degradation genes in biogenic cell-mineral aggregates unveiled the survival strategies of rhizobacteria with respect to Cd and Phen in the microoxic habitat. Potential Cd-tolerant rhizobacteria (e.g., Pandoraea and Comamonas) and Phen-degradation rhizobacteria (e.g., Pseudoxanthobacter) were identified through the DNA-SIP and 16S rRNA gene amplicon sequencing. Metagenomic analysis further confirmed that these core microbes harbor Cd-tolerant, Phen-degradation, and Fe(II) oxidation genes, supporting their metabolic potential for Cd and/or Phen in the microoxic habitat of the rice rhizosphere. These findings suggest the potential mechanism and ecological significance of core rhizospheric microbial-driven Fe(II) oxidation in mitigating the bioavailability of Cd and Phen in paddy soil during flooding.

Evaluation of the immobilized enzymes function in soil remediation following polycyclic aromatic hydrocarbon contamination

The bioremediation of polycyclic aromatic hydrocarbon (PAHs) from soil utilizing microorganisms, enzymes, microbial consortiums, strains, etc. has attracted a lot of interest due to the environmentally friendly, and cost-effective features. Enzymes can efficiently break down PAHs in soil by hydroxylating the benzene ring, breaking the C-C bond, and catalyze the hydroxylation of a variety of benzene ring compounds via single-electron transfer oxidation. However, the practical application is limited by its instability and ease to loss function under harsh environmental conditions such as pH, temperature, and edaphic stress etc. Therefore, this paper focused on the techniques used to immobilize enzymes and remediate PAHs in soil. Moreover, previous research has not adequately covered this topic, despite the employment of several immobilized enzymes in aqueous solution cultures to remediate other types of organic pollutants. Bibliometric analysis further highlighted the research trends from 2000 to 2023 on this field of growing interest and identified important challenges regarding enzyme stability and interaction with soil matrices. The findings indicated that immobilized enzymes may catalyzed PAHs via oxidation of OH groups in benzene rings, and generate benzyl radicals (i.e., •OH and •O2) that undergo further reaction and release water. As a result, the intermediate products of PAHs further catalyze by enzyme and enzyme induced microbes producing carbon dioxide and water. Meanwhile efficiency, activity, lifetime, resilience, and sustainability of immobilized enzyme need to be further improved for the large-scale and field-scale clean-up of PAHs polluted soils. This could be possible by integrating enzyme-based with microbial and plant-based remediation strategies. It can be coupled with another line of research focused on using a new set of support materials that can be derived from natural resources.

Accelerated Indirect Photodegradation of Organic Pollutants at the Soil-Water Interface

Indirect photolysis driven by photochemically produced reactive intermediates (PPRIs) is pivotal for the transformations and fates of pollutants in nature. While well-studied in bulk water, indirect photolysis processes at environmental interfaces remain largely unexplored. This study reveals a significant acceleration of indirect photodegradation of organic pollutants at the soil-water interface of wetlands. Organic pollutants experienced ubiquitously enhanced indirect photodegradation at the soil-water interfaces, with rates 1.41 ± 0.01 to 4.27 ± 0.03-fold higher than those in bulk water. This enhancement was observed across various natural and artificial wetlands, including coastal wetlands and rice paddies. In situ mapping indicated that soil-water interfaces act as hotspots, concentrating both organic pollutants and PPRIs by 9.30- and 4.27-folds, respectively. This synchronized colocation is the primary cause of the accelerated pollutant photolysis. Additionally, the contribution of each PPRI species to pollutant photolysis and a coupled transformation pathway at the soil-water interface significantly differed from those in bulk water. For instance, the contribution of singlet oxygen to metoxuron photolysis increased from 10.1% in bulk water to 44.4% at the soil-water interface. Our study highlights the rapid indirect photolysis of organic pollutants at the soil-water interfaces, offering new insights into the natural purification processes in wetlands as "Earth's kidneys."

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.

Contribution of complexed Fe(Ⅱ) oxygenation to norfloxacin humification and stabilization: Producing and trapping of more humified products

Organic pollutants polymerization in advanced oxidation processes or environmental matrices has attracted increasing attention, but little is known about stabilization of the polymerization products. The results in this work revealed the contribution of Fe(Ⅱ) oxygenation to stabilization of the products from norfloxacin (NOR) humification. It was found that upon oxygenation of Fe(Ⅱ) complexed by catechol (CT), NOR polymerized into the products with larger molecular weight through nucleophilic addition. Around 83.9-89.7 % organic carbon (OC) can be retained in the reaction solution and the precipitates at different Fe(II)/CT molar ratio. In this system with humification potential, the produced hydroxyl radical (HO•) dominantly modified, instead of decomposed, the structure of transformation products (TPs). TPs with diversified side chains were formed through hydroxylation and ring-opening, leading to the more humified products. In the subsequent Fe(Ⅱ) oxidative precipitation, Fe-TPs composites were formed as spherical particle clusters, which could steadily incorporate OC species with molecular fractionation. Specifically, lignin-like, tannins-like, condensed aromatic and high-molecular-weight TPs were preferentially preserved in the precipitates, while the recalcitrant aliphatic products mainly retained in the solution. These findings shed light on the role of Fe(Ⅱ) oxygenation in stabilizing the products from pollutants humification, which could strengthen both decontamination and organics sequestration.

Biotic/abiotic transformation mechanisms of phenanthrene in iron-rich constructed wetland under redox fluctuation

Iron-rich constructed wetlands (CWs) could promote phenanthrene bioremediation efficiently through biotic and abiotic pathways, which have gained increasing attention. However, the biotic/abiotic transformation mechanisms of trace organic contaminants in iron-rich CW are still ambiguous. Herein, three CWs (i.e., CW-A: Control; CW-B: Iron-rich CW, CW-C: Iron-rich CW + tidal flow) were constructed to investigate the transformation mechanisms of phenanthrene through Mössbauer spectroscopy and metagenomics. Results demonstrated CW-C achieved the highest phenanthrene removal (94.0 %) and bacterial toxicity reduction (92.1 %) due to the optimized degradation pathway, and subsequently achieved the safe transformation of phenanthrene. Surface-bound/low-crystalline iron regulated hydroxyl radical (·OH) production predominantly, and its utilization was promoted in CW-C, which also improved electron transfer capacity. The enhanced electron transfer capacity led to the enrichment of PAH-degrading microorganisms (e.g., Thauera) and keystone species (Sphingobacteriales bacterium 46-32) in CW-C. Additionally, the abundances of phenanthrene transformation (e.g., EC:1.14.12.-) and tricarboxylic-acid-cycle (e.g., EC:2.3.3.1) enzyme were up-regulated in CW-C. Further analysis indicated that the safe transformation of phenanthrene was mainly attributed to the combined effect of abiotic (·OH and surface-bound/low-crystalline iron) and biotic (microbial community and diversity) mechanisms in CW-C, which contributed similarly. Our study revealed the essential role of active iron in the safe transformation of phenanthrene, and was beneficial for enhanced performance of iron-rich CW.

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.

Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic-Oxic Coupling Mechanism

Natural attenuation of organic contaminants can occur under anoxic or oxic conditions. However, the effect of the coupling anoxic-oxic process, which often happens in subsurface soil, on contaminant transformation remains poorly understood. Here, we investigated 2,4-dichlorophenol (2,4-DCP) transformation in Fe-rich soil under anoxic-oxic alternation. The anoxic and oxic periods in the alternating system showed faster 2,4-DCP transformation than the corresponding control single anoxic and oxic systems; therefore, a higher transformation rate (63.4%) was obtained in the alternating system relative to control systems (27.9-42.4%). Compared to stable pH in the alternating system, the control systems presented clear OH- accumulation, caused by more Fe(II) regeneration in the control anoxic system and longer oxygenation in the control oxic system. Since 2,4-DCP was transformed by ion exchangeable Fe(II) in soil via direct reduction in the anoxic process and induced ·OH oxidation in the oxic process, OH- accumulation was unbeneficial because it competed for proton with direct reduction and inhibited •OH generation via complexing with Fe(II). However, the alternating system exhibited OH--buffering capacity via anoxic-oxic coupling processes because the subsequent oxic periods intercepted Fe(II) regeneration in anoxic periods, while shorter exposure to O2 in oxic periods avoided excessive OH- generation. These findings highlight the significant role of anoxic-oxic alternation in contaminant attenuation persistently.

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

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

Novel Insights into Sb(III) Oxidation and Immobilization during Ferrous Iron Oxygenation: The Overlooked Roles of Singlet Oxygen and Fe (oxyhydr)oxides Formation

Reactive oxygen species (ROS) produced from the oxygenation of reactive Fe(II) species significantly affect the transformation of metalloids such as Sb at anoxic-oxic redox interfaces. However, the main ROS involved in Sb(III) oxidation and Fe (oxyhydr)oxides formation during co-oxidation of Sb(III) and Fe(II) are still poorly understood. Herein, this study comprehensively investigated the Sb(III) oxidation and immobilization process and mechanism during Fe(II) oxygenation. The results indicated that Sb(III) was oxidized to Sb(V) by the ROS produced in the aqueous and solid phases and then immobilized by formed Fe (oxyhydr)oxides via adsorption and coprecipitation. In addition, chemical analysis and extended X-ray absorption fine structure (EXAFS) characterization demonstrated that Sb(V) could be incorporated into the lattice structure of Fe (oxyhydr)oxides via isomorphous substitution, which greatly inhibited the formation of lepidocrocite (γ-FeOOH) and decreased its crystallinity. Notably, goethite (α-FeOOH) formation was favored at pH 6 due to the greater amount of incorporated Sb(V). Moreover, singlet oxygen (1O2) was identified as the dominant ROS responsible for Sb(III) oxidation, followed by surface-adsorbed ·OHads, ·OH, and Fe(IV). Our findings highlight the overlooked roles of 1O2 and Fe (oxyhydr)oxide formation in Sb(III) oxidation and immobilization during Fe(II) oxygenation and shed light on understanding the geochemical cycling of Sb coupled with Fe in redox-fluctuating environments.

Simultaneously removal of PAHs from contaminated soil and effluent by integrating soil washing and advanced oxidation processes in a continuous system: Water saving, optimization and scale up modeling

Every year a large amount of clean water turns into contaminated effluent by soil washing (SW) process. The release of this effluent has become a growing environmental threat. In this study, a sustainable approach was developed for effective removal of PAHs from contaminated soil and the effluent by integrating SW and advanced oxidation processes (AOPs) in a continuous system. In the constructed continuous system, first small amount of clean water passed through the contaminated soil to remove PAHs. Then, the polluted effluent was treated by a quick AOPs and recycled for SW processes again and again until a complete removal of PHE be achieved. The performance of the continuous system was optimized and compared with batch system (no circulation) at lab scale. In addition, a scale up modeling was developed to predict the performance of continuous system at large scale. According to the results, under the optimum conditions: Tween 80 (TW80) = 6 g/L, ultrasonic = 160 kW, UV = 30 W, O3 = 5 g/h and TiO2 = 2 g/m2, the final PHE degradation efficiency of 98 % and 94 % were achieved by the continuous and batch systems after 130 and 185 min, respectively. The continuous system used 5 times less water volume than the batch system but resulted in better PAHs degradation. The scale up modeling revealed at large scale (100 kg soil), the continuous system could decrease the energy consumption and the required washing solution (water + TW80) up to 50 % and 80 %, respectively in comparison to the batch system. This work suggests a promising and practical approach for contaminated soil remediation without producing polluted water.

Reducing the accumulation of cadmium and phenanthrene in rice by optimizing planting spacing: Role of low-abundance but core rhizobacterial communities

Optimizing planting spacing is a common agricultural practice for enhancing rice growth. However, its effect on the accumulation of cadmium (Cd) and phenanthrene (Phen) in soil-rice systems and the response mechanisms of rhizobacteria to co-contaminants remain unclear. This study found that reducing rice planting spacing to 5 cm and 10 cm significantly decreased the bioavailability of Cd (by 7.9 %-29.5 %) and Phen (by 12.9 %-47.6 %) in the rhizosphere soil by converting them into insoluble forms. The increased accumulation of Cd and Phen in roots and iron plaques (IPs) ultimately led to decreased Cd (by 32.2 %-39.9 %) and Phen (by 4.2 %-17.3 %) levels in brown rice, and also significantly affected the composition of rhizobacteria. Specifically, reducing rice planting spacing increased the abundance of low-abundance but core rhizobacteria in the rhizosphere soil and IPs, including Bacillus, Clostridium, Sphingomonas, Paenibacillus, and Leifsonia. These low-abundance but core rhizobacteria exhibited enhanced metabolic capacities for Cd and Phen, accompanied by increased abundances of Cd-resistance genes (e.g., czcC and czcB) and Phen-degradation genes (e.g., pahE4 and pahE1) within the rhizosphere soil and IPs. Reduced planting spacing had no noticeable impact on rice biomass. These findings provide new insights into the role of low-abundance but core rhizobacterial communities in Cd and Phen uptake by rice, highlighting the potential of reduced planting spacing as an eco-friendly strategy for ensuring the safety of rice production on contaminated paddy soils.

Humic substance mitigated the microplastic-induced inhibition of hydroxyl radical production in riparian sediment

Hydroxyl radicals (·OH) generated during the flooding-drought transformation process play a vital role in affecting nutrient cycles at riparian zone. However, information on the processes and mechanisms for ·OH formation under the influence of microplastics (MPs) remains unclear. In this study, the effects of MPs on ·OH production from riparian sediments with different biomass [e.g., vegetation lush (VL) and vegetation barren (VB)] were studied. The results showed that presence of MPs inhibited the production of ·OH by 27 % and 7.5 % for VB and VL sediments, respectively. The inhibition was mainly resulted from the MP-induced reduction of the biotic and abiotic mediated Fe redox processes. Spectral analysis revealed that VL sediments contained more high-molecular-weight humic-like substances. Presence of MPs increased the abundances and activities of Proteobacteria, Acidobacteria and Actinobacteria, which were conducive to the changes in humification and polar properties of organic matters. The reduced humic- and fulvic-like substances were accumulated in the flooding period and substantially oxidized during flooding/drought transformation due to the enhanced MP-mediated electron transfer abilities, thus mitigated the MP-induced inhibition effects. Therefore, in order to better understanding the biogeochemical cycling of contaminants as influenced by ·OH and MPs in river ecosystems, humic substances should be considered systematically.

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.

Contaminant transformation during sediment oxygenation: Temporal variation of oxidation mechanisms mediated by hydroxyl radicals and aerobic microbes

Sediment oxidation by oxygen is ubiquitous, whereas the mechanisms of concurrent contaminant oxidation, particularly the temporal variation of chemical and biological oxidation, remain inadequately understood. This study investigated the oxidation of two contaminants (phenol and trichloroethylene) with different responses during the oxygenation of four natural sediments with different redox properties. Results showed that contaminant oxidation was initially dominated by hydroxyl radicals (•OH) (first stage), stabilized for different time for different sediments (second stage), and was re-started by microbial mechanism (third stage). In the first short stage, the contribution of chemical oxidation by •OH was mainly determined by the variation of sediment electron-donating capacity (EDC). In the second long stage, the stabilization time was dependent on sediment redox properties, that is, the abundance and growth of aerobic microbes capable of degrading the target contaminants. A more reduced sediment resulted in a higher extent of oxidation by •OH and a longer stabilization time. When the third stage of aerobic microbial oxidation was started, the contaminants like phenol that can be utilized by microbes can be oxidized quickly and completely, and those refractory contaminants like trichloroethylene remained unchanged. The study differentiates chemical and biological mechanisms for contaminant oxidation during sediment oxygenation.

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.

Effect of interaction between dissolved organic matter and iron/manganese (hydrogen) oxides on the degradation of organic pollutants by in-situ advanced oxidation techniques

Iron and manganese (hydrogen) oxides (IMHOs) exhibit excellent redox capabilities for environmental pollutants and are commonly used in situ chemical oxidation (ISCO) technologies for the degradation of organic pollutants. However, the coexisting dissolved organic matter (DOMs) in surface environments would influence the degradation behavior and fate of organic pollutants in IMHOs-based ISCO. This review has summarized the interactions and mechanisms between DOMs and IMHOs, as well as the properties of DOM-IMHOs complexes. Importantly, the promotion or inhibition impact of DOM was discussed from three perspectives. First, the presence of DOMs may hinder the accessibility of active sites on IMHOs, thus reducing their efficiency in degrading organic pollutants. The formation of compounds between DOMs and IMHOs alters their stability and activity in the degradation process. Second, the presence of DOMs may also affect the generation and transport of active species, thereby influencing the oxidative degradation process of organic pollutants. Third, specific components within DOMs also participate and affect the degradation pathways and rates. A comprehensive understanding of the interaction between DOMs and IMHOs helps to better understand and predict the degradation process of organic pollutants mediated by IMHOs in real environmental conditions and contributes to the further development and application of IMHO-mediated ISCO technology.

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.

Biosurfactants-based mixed polycyclic aromatic hydrocarbon degradation: From microbial community structure toward non-targeted metabolomic profile determination

Biosurfactants-based bioremediation is considered an efficient technology to eliminate environmental pollutants including polycyclic aromatic hydrocarbons (PAHs). However, the precise role of rhamnolipids or lipopeptide-biosurfactants in mixed PAH dissipation, shaping microbial community structure, and influencing metabolomic profile remained unclear. In this study, results showed that the maximum PAH degradation was achieved in lipopeptide-assisted treatment (SPS), where the pyrene and phenanthrene were substantially degraded up to 74.28 % and 63.05 % respectively, as compared to rhamnolipids (SPR) and un-aided biosurfactants (SP). Furthermore, the high throughput sequencing analysis revealed a significant change in the PAH-degrading microbial community, with Proteobacteria being the predominant phylum (>98 %) followed by Bacteroidota and Firmicutes in all the treatments. Moreover, Pseudomonas and Pannonibacter were found as highly potent bacterial genera for mixed PAH degradation in SPR, SPS, and SP treatments, nevertheless, the abundance of the genus Pseudomonas was significantly enhanced (>97 %) in SPR treatment groups. On the other hand, the non-targeted metabolomic profile through UHPLC-MS/MS exhibited a remarkable change in the metabolites of amino acids, carbohydrates, and lipid metabolisms by the input of rhamnolipids or lipopeptide-biosurfactants whereas, the maximum intensities of metabolites (more than two-fold) were observed in SPR treatment. The findings of this study suggested that the aforementioned biosurfactants can play an indispensable role in mixed PAH degradation as well as seek to offer new insights into shifts in PAH-degrading microbial communities and their metabolic function, which can guide the development of more efficient and targeted strategies for complete removal of organic pollutants such as PAH from the contaminated environment.

Biodegradation of pyrene by bacterial consortia: Impact of natural surfactants and iron oxide nanoparticles

Polycyclic aromatic hydrocarbons (PAHs) are potentially hazardous compounds that could cause a severe impact on many ecosystems. They are very challenging to remove using conventional methods due to their hydrophobic nature. However, this issue can be resolved by utilizing surface-active molecules to increase their bioavailability. In this study, pyrene was chosen as the PAH compound to explore its degradability by the effect of individual bacterial strains (Pseudomonas stutzeri NA3 and Acinetobacter baumannii MN3) and mixed consortia (MC) along with natural surfactant derived from Sapindus mukorossi and iron oxide nanoparticles (NPs). Additionally, fatty acids esters, dipeptides, and sugar derivative groups were identified as potent bioactive components of natural surfactants. Various techniques, such as XRD, VSM, TEM, and FE-SEM with EDX, were utilized to characterize the pristine and Fenton-treated iron oxide NPs. The analytical results confirmed that the Fe3O4 crystal phase and spherical-shaped NPs exhibited excellent magnetic properties. The impact of natural surfactants and iron oxide NPs has significantly contributed to the biodegradation process, resulting in a prominent decrease in chemical oxygen demand (COD) levels. Gas chromatography-mass spectrometry (GC-MS) analysis showed that biodegradation systems produced primary hydrocarbon intermediates, which underwent oxidative degradation through Fenton treatment. Interestingly, synthesized iron oxide NPs effectively produced hydroxyl radical (•OH) during the Fenton reaction, which was confirmed by electron paramagnetic resonance (EPR) spectra, and the pristine iron oxide NPs underwent a material transformation observed. The study demonstrated an integrated approach for biodegradation and the Fenton reaction process to enhance the pyrene degradation efficiency (90%) compared to other systems. Using natural surfactants and iron oxide NPs in aquatic environments serves as a crucial platform at the interface of microorganisms and contaminated oil products. This interaction offers a promising solution for PAHs bioremediation.

Facet-Dependent Productions of Reactive Oxygen Species from Pyrite Oxidation

Reactive oxygen species (ROS) are widespread in nature and play central roles in numerous biogeochemical processes and pollutant dynamics. Recent studies have revealed ROS productions triggered by electron transfer from naturally abundant reduced iron minerals to oxygen. Here, we report that ROS productions from pyrite oxidation exhibit a high facet dependence. Pyrites with various facet compositions displayed distinct efficiencies in producing superoxide (O2• -), hydrogen peroxide (H2O2), and hydroxyl radical (•OH). The 48 h •OH production rates varied by 3.1-fold from 11.7 ± 0.4 to 36.2 ± 0.6 nM h-1, showing a strong correlation with the ratio of the {210} facet. Such facet dependence in ROS productions primarily stems from the different surface electron-donating capacities (2.2-8.6 mmol e- g-1) and kinetics (from 1.2 × 10-4 to 5.8 × 10-4 s-1) of various faceted pyrites. Further, the Fenton-like activity also displayed 10.1-fold variations among faceted pyrites, contributing to the facet depedence of •OH productions. The facet dependence of ROS production can greatly affect ROS-driven pollutant transformations. As a paradigm, the degradation rates of carbamazepine, phenol, and bisphenol A varied by 3.5-5.3-fold from oxidation of pyrites with different facet compositions, where the kinetics were in good agreement with the pyrite {210} facet ratio. These findings highlight the crucial role of facet composition in determining ROS production and subsequent ROS-driven reactions during iron mineral oxidation.

Fluctuating redox conditions accelerate the electron storage and transfer in magnetite and production of dark hydroxyl radicals

Magnetite (Fe3O4), known as a geo-battery that can store and transfer electrons, often co-occurs with sulfide in subsurface environments with fluctuating redox conditions. However, little is known about how fluctuating redox conditions (e.g., sulfidation-oxidation) affect the electron storage and transfer in Fe3O4 that was associated with the production of dark hydroxyl radicals (⋅OH) and the oxidation of dissolved organic matter (DOM). This study revealed that Fe3O4 sulfidated by sulfide (S-Fe3O4) at neutral pH exhibited higher ⋅OH production upon oxygenation than Fe3O4, in which the cumulative ⋅OH concentration increased with increasing initial S/Fe ratio (≤ 0.50), sulfidation duration and number of sulfidation-oxidation cycle. X-ray photoelectron spectroscopy and wet-chemical analyses of Fe and S species of S-Fe3O4 showed that sulfidation enables electron storage in Fe3O4 by increasing both structural and surface Fe(II). Sulfide was converted into S0, acid volatile sulfur (AVS), and chromium-reducible sulfur (CRS) during Fe3O4 sulfidation. S-Fe3O4 with lower AVS/CRS ratio exhibited higher reactivity to produce ⋅OH, indicating the important role of CRS in transferring electrons from Fe(II) to O2. Based on quenching experiments and electron paramagnetic resonance analysis, a one-step two-electron transfer mechanism was proposed for O2 reduction during S-Fe3O4 oxygenation, and surface-bound rather than free ⋅OH were identified as the primary reactive oxygen species. The ⋅OH from S-Fe3O4 oxygenation was shown to be efficient in degradation of DOM. Overall, these results suggested that sulfidation-oxidation can accelerate the electron storage and transfer in Fe3O4 for dark ⋅OH production, having an important impact on the carbon cycling in subsurface environments.

Using compound-specific isotope analysis to identify the mechanism of acetochlor degradation during oxygenation of hyporheic zone sediment

Biodegradation is recognized as the main pathway for acetochlor attenuation in aquatic environments. However, the potential abiotic degradation of acetochlor by hydroxyl radicals (•OH) generated during oxygenation of hyporheic zone sediments has not been investigated. This study aims to examine the production of •OH during oxygenation of hyporheic zone sediments and its effects on acetochlor attenuation. A significant decrease of acetochlor, ranging from 77.9% to 100%, was observed in the water-sediment systems with extensive •OH production. The primary sources of •OH production were found to be the oxidation of Fe(II) and reduced humic acids. Furthermore, a •OH quenching experiment suggests that •OH driven oxidation is the dominant pathway for acetochlor attenuation. Carbon isotope fractionation of acetochlor degradation during oxygenation of sediments (εbulk,C ranged from -1.5‰ to -0.5 ± 0.3‰) was close to that during acetochlor degradation by •OH in a H2O2-Fe3O4 Fenton system (εbulk,C = -0.5 ± 0.1‰), but significantly smaller than that during acetochlor biodegradation (εbulk,C = -5.8 ± 0.9‰). Compound-specific isotope analysis (CSIA) further suggests that •OH produced by sediment oxygenation plays a critical role in acetochlor attenuation in aquatic environments. Results of calculated apparent kinetic isotope effect of carbon (AKIEC) and transformation products indicate that SN1 and SN2-type nucleophilic substitution are the first steps in acetochlor attenuation through •OH driven oxidation (AKIEC = 1.007 ± 0.001) and aerobic biodegradation (AKIEC = 1.088 ± 0.013), respectively. Our findings highlight the potential of CSIA to assess the acetochlor degradation in water-sediment system, which can help to elucidate the fate of herbicide in aquatic environments.

Distribution, sources, ecological risk and microbial response of polycyclic aromatic hydrocarbons in Qingdao bays, China

Bay ecosystem has garnered significant attention due to the severe threat posed by organic pollutants, particularly polycyclic aromatic hydrocarbons (PAHs). However, there is a dearth of information regarding the extent of PAHs pollutant risk and its impact on microbial communities and metabolism within this environment. In this study, the distribution, sources, ecological risk, and microbial community and metabolic response of PAHs in Jiaozhou Bay, Aoshan Bay, and Lingshan Bay in Qingdao, China were investigated. The results showed that the average concentration of ∑PAHs ranged from 120 to 614 ng/L across three bays, with Jiaozhou and Aoshan Bay exhibiting a higher risk than Lingshan Bay due to an increased concentration of high-molecular-weight PAHs. Further analysis revealed a negative correlation between dissolved organic carbon concentration and ∑PAHs concentration in water. Metagenomic analysis demonstrated that higher levels of PAHs can lead to decreased microbial diversity, while the abundance of PAHs-degrading bacteria is enhanced. Additionally, the Erythrobacter, Jannaschia and Ruegeria genera were found to have a significant correlation with low-molecular-weight PAH concentrations. In terms of microbial metabolism, higher PAH concentrations were beneficial for carbohydrate metabolic pathway but unfavorable for amino acid metabolic pathways and membrane transport pathways in natural bay environments. These findings provide a foundation for controlling PAHs pollution and offer insights into the impact of PAHs on bacterial communities and metabolism in natural bay environments.

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.

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.

Kinetic models for hydroxyl radical production and contaminant removal during soil/sediment oxygenation

Hydroxyl radical (•OH) oxidation has been identified as a significant pathway for element cycling and contaminant removal in redox fluctuating environments. Fe(II) has been found to be the main electron contributor for •OH production. Despite the recognition of the mechanisms of •OH production from the oxidation of Fe(II) in soils/sediments by O2, the kinetic model about Fe(II) oxidation, •OH production and contaminant removal is not yet clear. To address this knowledge gap, we conducted a series of experiments to explore the variation of different Fe(II) species, •OH and trichloroethylene (TCE, a representative contaminant) during sediment oxygenation, followed by the development of a kinetic model. In this model, Fe(II) species in sediments was divided into three categories based on the sequential chemical extraction method: ion exchangeable Fe(II), surface-adsorbed Fe(II) and mineral structural Fe(II),. Results showed that the kinetic model accurately fitted the concentration time trajectories of different Fe(II) species, •OH and TCE in this study as well as in previous studies. Model analysis indicated that the relative contribution of surface-adsorbed Fe(II) and reactive mineral structural Fe(II) in •OH production was 16.4%-33.9% and 66.1%-83.6%, respectively. However, ion-exchangeable Fe(II) not only fails to contribute to •OH production but also reduces the •OH yield relative to H2O2 decomposition. Poorly reactive mineral structural Fe(II) can serve as an electron pool to regenerate these reactive Fe(II) and facilitate •OH production. Regarding TCE degradation, Fe(II) species plays a dual role in contributing to •OH production while competing with TCE for •OH consumption, with the quenching efficiency being related to their content and reactivity toward •OH. This kinetic model offers a practical approach to describing and predicting •OH production and associated environmental impacts at the oxic-anoxic interface.

Chlorinated organic pollutants in global flooded soil and sediments: Pollution status and potential risk

Chlorinated organic pollutants (COPs) were widely detected in anaerobic environments while there is limited understanding of their pollution status and potential environmental risks. Here, we applied meta-analysis to identify the occurrence status, pollution sources, and environmental risk of COPs from 246 peer-published literature, including 25 kinds of COPs from 977 sites. The results showed that the median concentrations of COPs were at the ng g^-1 level. By the combination of principal component analysis (PCA) and positive matrix factorization (PMF), we established 7 pollution sources for COPs. Environmental risk assessment found 73.3% of selected sites were at a security level but the rest were not, especially for the wetlands. The environmental risk of COPs was usually underestimated by the existing evaluation methods, such as without the consideration of the non-extractable residues (NER) and the multi-process coupling effect. Especially, the synergetic coupling associations between dechlorination and methanogenesis might increase the risk of methane emission that has barely been previously considered in previous risk assessment approaches. Our results expanded the knowledge for the pollution control and remediation of COPs in anaerobic environments.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

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.

A Mechanistic Study of Goethite-Based Fenton-Like Reactions for Imidacloprid Degradation

Biotic transformation of imidacloprid (IMD) has been widely investigated in the environments. However, little was known about IMD degradation via abiotic pathways, such as reactive oxygen species (ROS)-based oxidation processes. Here we systematically investigated the mechanism of hydroxyl radical (^•OH) production and the associated IMD degradation in the goethite (α-FeOOH)-based Fenton-like systems. Results showed that IMD can be efficiently degraded in the α-FeOOH/H₂O₂ systems, with degradation rate exceeded 80% within 48 h. Based on the examination of electron paramagnetic resonance (EPR) and chemical probes, ^•OH was identified as the key ROS that responsible for IMD degradation. IMD can be decomposed via hydroxylation or removal of -N-NO₂ to produce hydroxylated IMD, cyclic urea and 6-chloronicotinic acid, with the associated toxicities also evaluated. In addition, the increasing H₂O₂ concentration and decreasing solution pH both significantly increased IMD degradation. This study provides theoretical understanding for the implications of soil mineral-based Fenton-like reactions in the abiotic transformation of pesticide pollutants.

Insights into the degradation process of phenol during in-situ thermal desorption: The overlooked oxidation of hydroxyl radicals from oxygenation of reduced Fe-bearing clay minerals

In-situ thermal desorption (ISTD) has attracted increasing attention owing to the efficient removal of organic contaminants from contaminated sites. However, it is poorly understood that whether and to what extent contamination degradation occurs upon oxygenation of reduced Fe-bearing clay minerals (RFC) in the subsurface during ISTD. In this study, we evaluated the mechanism of contaminant degradation upon oxygenation of reduced clay minerals during the ISTD. Reduced nontronite (rNAu-2) and montmorillonite (rSWy-3) were selected as RFC models. Results showed that thermal treatment during ISTD could significantly enhance phenol degradation, which increased from 25.8 % at 10 °C to 74.4 % at 70 °C in rNAu-2 and from 17.7 % at 10 °C to 49.8 % at 70 °C in rSWy-3. Correspondingly, the cumulative •OH at steady-state ([•OH]_ss) increased by 3.7 and 1.5 times, respectively. The acceleration of Fe(II) oxidation with increasing temperature could be mainly responsible for [•OH]_ss generation, which degrades phenol. Moreover, thermal treatment improved the fast oxidation of trioctahedral entities Fe(II)Fe(II)Fe(II) (TOF) and the slow oxidation of dioctahedral entities Fe(II)Fe(II) (DTF₁), AlFe(II) (DAF₁), and Fe(II)Fe(III) (DTF₂). Our study suggests that the overlooked degradation progress of phenol by oxygenation of RFC during ISTD, and it could be favorable for contaminant degradation during remediation.

Insight into the Photodegradation of Microplastics Boosted by Iron (Hydr)oxides

Iron (hydr)oxides as a kind of natural mineral actively participate in the transformation of organic pollutants, but there is a large knowledge gap in their impacts on photochemical processes of microplastics (MPs). This study is the first to examine the degradation of two ordinary plastic materials, polyethylene (PE) and polypropylene (PP), mediated by iron (hydr)oxides (goethite and hematite) under simulated solar light irradiation. Both iron (hydr)oxides significantly promoted the degradation of MPs (particularly PP) with a greater effect by goethite than hematite, related to hydroxyl radical (^•OH) produced by iron (hydr)oxides. Under light irradiation, the surface Fe(II) phase catalyzed the production of H₂O₂ and promoted the release of Fe²⁺, leading to the subsequent light-driven Fenton reaction which produced a large amount of ^•OH. As the iron (hydr)oxides were modified with NaF at various concentrations, the activity of the surface Fe(II) as well as the release of Fe²⁺ were greatly reduced, and thus the ^•OH formation and MP degradation were depressed remarkably. It is worth noting that the surface hydroxyl groups (especially ≡FeOH) affected the reaction kinetics of ^•OH by regulating the activity of Fe species. These findings unveil the distinct impacts and intrinsic mechanisms of iron (hydr)oxides in influencing the photodegradation of MPs.

Quantification of the Influence of Citrate/Fe(II) Molar Ratio on Hydroxyl Radical Production and Pollutant Degradation during Fe(II)-Catalyzed O2 and H2O2 Oxidation Processes

Ligand-enhanced hydroxyl radical (•OH) production is an important strategy for Fe(II)-catalyzed O2 and H2O2 oxidation processes. However, the influence of the molar ratio of ligands to Fe(II) on •OH production remains elusive. This study employed citrate and inorganic dissolved Fe(II) (Fe(II)dis) as the representative ligand and Fe(II) species, respectively, to quantify this relationship. Results showed that •OH production was highly dependent on the citrate/Fe(II) molar ratio. For instance, for the oxygenation of Fe(II)dis, the •OH accumulations were 2.0-8.5, 3.4-28.5 and 8.1-42.3 μM at low (0.25-0.5), moderate (0.5-1), and high (1-2) citrate/Fe(II) molar ratios, respectively. At low citrate/Fe(II) molar ratio (<0.5), inorganic Fe(II)dis mainly contributed to •OH production, with the increase in the citrate/Fe(II) molar ratio to a high level (1-2), Fe(II)-citrate complex turned to the electron source for •OH production. The change in Fe(II) speciation with the increase of citrate/Fe(II) molar ratio elevated •OH production. For pollutant degradation, 1 mg/L phenol was degraded by 53.6% within 40 min during oxygenation of Fe(II)-citrate system (1:1) at pH 7. Our results suggest that a moderate molar ratio of ligand/Fe(II) (0.5-1) may be optimal for Fe(II)-catalyzed O2 and H2O2 oxidation processes.

Insights into phenanthrene attenuation by hydroxyl radicals from reduced iron-bearing mineral oxygenation

The oxygenation of Fe(II)-bearing minerals for hydroxyl radicals (HO^•) formation and contaminant attenuation receive increasing attention, while the mechanisms for specific Fe(II) species in manipulating HO^• formation and contaminant attenuation are unclear. Herein, a total of four Fe(III)-bearing minerals were applied in the reduction-oxygenation processes to produce HO^•. Results showed that the total HO^• generated from the Fe-(oxyhydr)oxides were significantly higher than those from the Fe-silicates, with the order of goethite and hematite (~1500 μmol kg^-1) > Fe-montmorillonite (~550 μmol kg^-1) > chlorite (~120 μmol kg^-1). The HO^• formation was largely hinged on the reactive Fe(II) species, i.e., the surface-adsorbed/low-crystalline Fe(II) in the Fe-bearing minerals. For the co-incubation of minerals and phenanthrene, the concentrations of phenanthrene decreased from the initial 3.0 mg L^-1 to 0.7 mg L^-1 and 1.9 mg L^-1 for Fe-montmorillonite and goethite, respectively, suggesting the HO^• mediated by the Fe-montmorillonite was more conducive for phenanthrene attenuation. The goethite tended to promote the formation of free HO^•, while the Fe-montmorillonite with interlayer structure can provide attachment sites for the surface-adsorbed/low-crystalline Fe(II), resulting in high potential for surface-bound HO^• formation and phenanthrene attenuation. This study highlights the importance of Fe-bearing minerals in manipulating HO^• formation, providing new insight into the removal of contaminants in ecosystems.

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation

Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe(II) is considered as the main electron donor for O₂ activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HA_red) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HA_red suspension. Solid HA_red transferred electrons rapidly to O₂ via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HA_red to O₂ for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

Hydroxyl radical formation during oxygen-mediated oxidation of ferrous iron on mineral surface: Dependence on mineral identity

Many studies have examined the redox behavior of ferrous ions (Fe(II)) sorbed to mineral surfaces. However, the associated hydroxyl radical (^•OH) formation during Fe(II) oxidation by O₂ was rarely investigated at circumneutral pH. Therefore, we examined ^•OH formation during oxygenation of adsorbed Fe(II) (Fe(II)_sorbed) on common minerals. Results showed that 16.7 ± 0.4-25.6 ± 0.3 μM of ^•OH was produced in Fe(II) and α/γ-Al₂O₃ systems after oxidation of 24 h, much more than in systems with dissolved Fe(II) (Fe²⁺_aq) alone (10.3 ± 0.1 μM). However, ^•OH production in Fe(II) and α-FeOOH/α-Fe₂O₃ systems (6.9 ± 0.1-8.3 ± 0.1 μM) slightly decreased compared to Fe²⁺_aq only. Further analyses showed that enhanced oxidation of Fe(II)_sorbed was responsible for the increased ^•OH production in the Fe(II)/Al₂O₃ systems. In comparison, less Fe(II) was oxidized in the α-FeOOH/α-Fe₂O₃ systems, which was probably ascribed to the quick electron-transfer between Fe(II)_sorbed and Fe(III) lattice due to their semiconductor properties and induced formation of high-crystalline Fe(II) phases that hindered Fe(II) oxidation and ^•OH formation. The types of minerals and solution pH strongly affected Fe(II) oxidation and ^•OH production, which consequently impacted phenol degradation. This study highlights that the properties of minerals exert great impacts on surface-Fe(II) oxidation and ^•OH production during water/soil redox fluctuations.