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

Water Vapor Condensation Triggers Simultaneous Oxidation and Hydrolysis of Organic Pollutants on Iron Mineral Surfaces

Increasing worldwide contamination with organic chemical compounds is a paramount environmental challenge facing humanity. Once they enter nature, pollutants undergo transformative processes that critically shape their environmental impacts and associated risks. This research unveils previously overlooked yet widespread pathways for the transformations of organic pollutants triggered by water vapor condensation, leading to spontaneous oxidation and hydrolysis of organic pollutants. These transformations exhibit variability through either sequential or parallel hydrolysis and oxidation, contingent upon the functional groups within the organic pollutants. For instance, acetylsalicylic acid on the goethite surface underwent sequential hydrolysis and oxidation that first hydrolyzed to salicylic acid followed by hydroxylation oxidation of the benzene moiety driven by the hydroxyl radical (•OH). In contrast, chloramphenicol underwent parallel oxidation and hydrolysis, forming hydroxylated chloramphenicol and 2-amino-1-(4-nitrophenyl)-1,3-propanediol, respectively. The spontaneous oxidation and hydrolysis occurred consistently on three naturally abundant iron minerals with the key factors being •OH production capacity and surface binding strength. Given the widespread presence of iron minerals on Earth's surface, these spontaneous transformation paths could play a role in the fate and risks of organic pollutants of health concerns.

Round-the-Clock Adsorption-Degradation of Tetracycline Hydrochloride by Ag/Ni-TiO2

The synergy of adsorption and photocatalysis is a good method to remove organic pollutants in wastewater. In recent decades, persistent photocatalysis has gained considerable interest for its ability to sustain the catalytic degradation of organic pollutants in the dark. Herein, we report three different TiO2 nanomaterials to remove tetracycline hydrochloride (TCH) in solution. We found that the removal ability of TiO2, Ni-TiO2, and Ag/Ni-TiO2 is 8.8 mg/g, 13.9 mg/g and 23.4 mg/g, respectively, when the initial concentration of TCH is 50 mg/L. Chemical adsorption could be the rate-determining step in the TCH adsorption process. Moreover, Ag nanoparticles dispersed on Ni doped TiO2 surface act as traps to capture photo-generated electrons upon illumination with indoor light. The holes in Ag/Ni-TiO2 serve as critical oxidative species in TCH degradation under dark conditions. This work provides new insights into the design of persistent photocatalysts that can be activated by weak illumination and degrade organic pollutants in wastewater after sunset.

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.

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.

Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.

Rice straw returning enhances cadmium activation by accelerating iron cycling thus hydroxyl radical production in paddy soils during drainage

Straw returning is widely found elevating the bioavailability of cadmium (Cd) in paddy soils with unclear biogeochemical mechanisms. Here, a series of microcosm incubation experiments were conducted and spectroscopic and microscopic analyses were employed. The results showed that returning rice straw (RS) efficiently increased amorphous Fe and low crystalline Fe (II) to promote the production of hydroxyl radicals (OH) thus Cd availability in paddy soils during drainage. On the whole, RS increased OH and extractable Cd by 0.2-1.4 and 0.1-3.3 times, respectively. While the addition of RS effectively improved the oxidation rate of structural Fe (II) mineral (i.e., FeS) to enhance soil Cd activation (up to 38.5 %) induced by the increased OH (up to 69.2 %). Additionally, the existence of CO32- significantly increased the efficiency level on OH production and Cd activation, which was attributed to the improved reactivity of Fe (II) by CO32- in paddy soils. Conclusively, this study emphasizes risks of activating soil Cd induced by RS returning-derived OH, providing a new insight into evaluating the safety of straw recycling.

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.

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.

Contaminant degradation by •OH during sediment oxygenation: Effect of abundant solid matrix in aquifer

Aquifer oxygenation for hydroxyl radical (•OH) production has been recently proposed as a promising strategy for in-situ remediation. However, the high performance of this process was justified at low solid-to-liquid ratios (SLRs) of suspension systems. It remains unclear whether and how the performance is affected by abundant solid matrixes. Here we assessed the influence of SLR on •OH production and contaminant degradation during sediment oxygenation. Cumulative •OH increased from 21.8 to 165.2 μM when the SLR increased from 200 to 1600 g/L, while phenol degradation increased with the increase in SRL at the values lower than 1200 g/L and decreased at higher SLRs. As the main sediment component, silica exhibited a negligible effect on •OH production and phenol degradation because of the weak adsorption towards aqueous Fe(II). Whereas, the other component, alumina, significantly inhibited •OH production and phenol degradation because it strongly adsorbed Fe(II). •OH scavenging by solid reactive matrixes was mainly responsible for the inhibition at high SLRs. The scavenging effect could be mitigated by mediating the main reactive Fe(II) species from solid-adsorbed to dissolved phase with ligand addition. Our findings are important for understanding the side reactions and optimizing the remediation performance during aquifer oxygenation.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Siderite's green revolution: From tailings to an eco-friendly material for the green economy

Siderite, extensively mined as a natural iron mineral, is often discarded as tailings due to the low grade of the ore and due to the high cost of current sorting technologies. Yet, this mineral has demonstrated significant potential in several pivotal areas of the environmental remediation. Siderite not only possesses exceptional adsorption, catalytic, and microbial carrier capabilities but also offers an eco-friendly and cost-effective solution for the environmental pollution management. This article consolidates research advancements and achievements over the past few decades concerning siderite's role in pollution control, delving deeply into its various remediation pathways. Initially, the paper contrasts the performance differences between natural and synthetic siderite, followed by a comprehensive overview of siderite's adsorption mechanisms for various inorganic pollutants. Furthermore, this paper analyzes the unique physicochemical attributes of siderite as both, a reductant and the catalyst, with a special emphasis on its use in the preparation of SCR catalysts and in the catalytic advanced oxidation processes for organic pollutants' degradation. This paper also enumerates and discusses the myriad advantages of siderite as a microbial carrier, thereby enhancing our understanding of biogeochemical cycles and pollutant transformations. In essence, this review systematically elucidates the mechanisms and intrinsic physicochemical properties of siderite in pollution control, paving the way for novel strategies to augment siderite's environmental remediation performance.

Significant contribution of different sources of particulate organic matter to the photoaging of microplastics

Particulate organic matter (POM), as an important component of organic matter, can act as a redox mediator and thus intervene in the environmental behavior of microplastics (MPs). However, quantitative information on the role of POM in the photoaging of MPs under ultraviolet (UV) light is still lacking. To raise the knowledge gap, through environmental simulation experiments and qualitative/quantitative experiments of active substances, we found that POM from peat soil has stronger oxidation capacity than POM from sediment, and the involvement of POM at high water content makes the aging of MPs more obvious. This is because the persistent radicals and electron-absorbing groups on the surface of POM indirectly generate reactive oxygen species (ROS) by promoting electron transfer, and the dissolved organic matter (DOM) released from POM under UV light (POM-DOM) is further excited to generate triplet-state photochemistry of DOM (3DOM*) to promote the aging of MPs. Theoretical calculations revealed that the benzene ring, mainly C = C, and C = O in the main chain in the plastic macromolecule structure are more susceptible to ROS attack, and the differences in the vulnerable sites contained in different plastic structures as well as the differences in the energy band gaps lead to differences in their aging processes. This study firstly elucidates the key role and intrinsic mechanism of POM in the photoaging of MPs, providing a theoretical basis for a comprehensive assessment of the effect of POM on MPs in the environment.

Biogeochemical cycling in paddy soils controls antimony transformation: Roles of iron (oxyhydr)oxides, organic matter and sulfate

In paddy fields, periodic flooding and drainage phases can significantly affect the availability of antimony (Sb), but the underlying mechanisms remain unclear. In this study, Sb-contaminated paddy soil was incubated under anaerobic (40 day) and subsequently aerobic (40-55 day) conditions. The Sb fractions was investigated and a kinetic model was established to quantitatively evaluate the main processes controlling Sb transformation. Under anaerobic conditions, the reductive dissolution of iron (Fe) (oxyhydr)oxides, the release of soil colloids, and dissolved organic carbon (DOC) could facilitate the release of Sb(V), while newly released Sb(V) were synchronously reduced to Sb(III) that could be incorporated into the solid phase (34.1%, 40 day) or precipitated as Sb2S3 (9.7%, 40 day). After soil aeration, a significant increase in dissolved and extracted Sb(V) (34.7%, 45 day) was observed due to the Sb(III) oxidization by the reactive oxygen species (ROS) generated from Fe(II) oxidization. The dissolved and extracted Sb(V) were simultaneously incorporated into the solid phase as the re-aggregation of soil colloids and DOC, and only contributed to 17.1% of the total Sb content at the end of aerobic phase (55 day). Our results elucidated the mechanisms about how biogeochemical Fe/S/C cycling jointly controlled Sb transformation in paddy systems.

Differential interactions between natural clay minerals and dissolved organic matter affect reactive oxygen species formation

Naturally occurring reactive oxygen species (ROS) are widely involved in many environmental processes. Here we investigated the ROS generation associated with the interaction between complexed natural clay minerals (CMs) and dissolved organic matter (DOM). Our results showed that among the nine chemical-reduced CMs (CR-CMs), the light brown CR-CM (CR-CM 7) generated the highest ROS via oxygenation, relying on the reactive structural Fe(II) (Fe species that can transfer electrons to oxygen) instead of total structural Fe(II) as previously reported. Moreover, DOM affected the oxygenation of CR-CMs differently. The tight interaction between DOM and CR-CM 7 formed DOM-complexed Fe, while the weak interaction between DOM and the dark gold CR-CM (CR-CM 1) and the black CR-CM (CR-CM 5) exhibited decreased efficiencies. Mechanism studies revealed that ROS were generated through three pathways but all followed a similar one-electron transfer process in the presence of DOM. We further developed a three-layer geobattery model system and demonstrated that long electron transfer driven by CR-CMs/DOM could extend ROS generation to several centimetres across the oxic-anoxic interface, even without redox switching. These findings offer new insights into CMs-involved ROS generation and associated organic matter transformation in natural environments.

The effect of the synergistic thermal treatment and stabilization on the transformation and transportation of arsenic, chromium, and cadmium in soil

Thermal treatments commonly used to remedy organic-contaminated soils can inadvertently impact the behavior of non-targeted pollutants, notably heavy metal(loid)s in soil. This study introduces an integrated calcination-stabilization remediation strategy employing steel slag as a stabilizing agent, with a focus on elucidating the transformations and remobilization tendencies of As, Cr, and Cd. Thermal treatment alters the mobility of these elements by modifying soil properties, with pH and redox conditions playing pivotal roles. After anaerobic calcination, the leaching concentrations of As reached 163 μg L-1, far surpassed 7.57 μg L-1 after the aerobic calcination. Although Cr and As share oxygen-containing anion forms, they display opposing leaching tendencies after thermal treatment. At 400 °C, Cr leaching from aerobically treated soil reaches 64.5 mg L-1, dropping to 6.63 mg L-1 after anaerobic heating due to pH-induced Cr(OH)3 formation. Thermal treatment significantly amplifies the leaching of Cd cations. In contrast to the leaching concentration of 122 μg L-1 in the untreated soil, aerobic and anaerobic heating (400 °C) resulted in leaching concentrations of 896 μg L-1 and 132 μg L-1, respectively. Noteworthy, the integrated treatment (400 °C anaerobically) decreases leached As and Cd concentrations to 68.3 μg L-1 and 15.4 μg L-1, attributed to stabilizer alkalinity and porosity. Column leaching shows initial rapid release followed by continuous behavior for As and Cd, with the average leaching concentrations of the remediated soil decreasing to 60.5 μg L-1 and 1.32 μg L-1, ensuring safe backfilling. In conclusion, this study contributes to the understanding of the mobility and stabilization of heavy metal(loid)s subsequent to the integrated calcination-stabilization process.

Roles of extracellular polymeric substances in the adsorption and removal of norfloxacin during hydrothermal treatment of sewage sludge

Hydrothermal treatment (HT) is promising to remove antimicrobials from sewage sludge (SS); however, the mechanism of antimicrobial degradation during the HT of SS is not fully understood. In this study, the roles of extracellular polymeric substances (EPS) in the removal and transformation of norfloxacin (NOR) during the HT of SS at temperatures of 100 and 160 °C were investigated. The results indicated that the degradation of NOR increased with increasing HT temperature, with maximum NOR removal (52%) achieved at 160 °C. Furthermore, the NOR in sludge showed higher degradation efficiencies than the control as HT temperature was higher than 120 °C. Evident promotion effects of bound-EPS (B-EPS) in sludge were observed on the NOR degradation as HT temperature was higher than 120 °C, leading to the mineralization and deamination of protein-like components in EPS during HT. Beside, the adsorption capacity of NOR during the HT of SS decreased at temperatures higher than 120 °C. The evolution of the spatial structure of B-EPS was predominantly responsible for the adsorption of antimicrobials, a spontaneous process driven mainly by hydrophilic interactions. With the hydrothermal conversion of B-EPS, the electron transfer, and reactive species (3EPS* and ·OH) derived from B-EPS could facilitate the degradation of NOR. In particular, hydrogen bonds between B-EPS and NOR increased the apparent yield of ·OH and accelerated the decarboxylation of NOR during HT at temperatures higher than 120 °C. A toxicity evaluation suggested that HT for NOR degradation could attenuate toxicity, whereas deep oxidation or mineralization would be needed to promote ecosystem safety. These findings provide new insights into the hydrothermal activation of EPS and the interrelated hydrothermal fate of antimicrobials and other toxic pollutants in sludge.

Ozone Micronano-bubble-Enhanced Selective Degradation of Oxytetracycline from Production Wastewater: The Overlooked Singlet Oxygen Oxidation

The efficient and selective removal of refractory antibiotics from high-strength antibiotic production wastewater is crucial but remains a substantial challenge. In this study, a novel ozone micronano-bubble (MNB)-enhanced treatment system was constructed for antibiotic production wastewater treatment. Compared with conventional ozone, ozone MNBs exhibit excellent treatment efficiency for oxytetracycline (OTC) degradation and toxicity decrease. Notably, this study identifies the overlooked singlet oxygen (1O2) for the first time as a crucial active species in the ozone MNB system through probe and electron paramagnetic resonance methods. Subsequently, the oxidation mechanisms of OTC by ozone MNBs are systematically investigated. Owing to the high reactivity of OTC toward 1O2, ozone MNBs enhance the selective and anti-interference performance of OTC degradation in raw OTC production wastewater with complex matrixes. This study provides insights into the mechanism of ozone MNB-enhanced pollutant degradation and a new perspective for the efficient treatment of high-concentration industrial wastewater using ozone MNBs. In addition, this study presents a promising technology with scientific guidance for the treatment of antibiotic production wastewater.

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.

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

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

An overlooked influence of reactive oxygen species on ammonia-oxidizing microbial communities in redox-fluctuating aquifers

Reactive oxygen species (ROS) are ubiquitous in O₂-perturbed aquifers, but their role in shaping ammonia-oxidizing microbial communities is not clear. This study examined the dynamic responses of ammonia-oxidizing microorganisms (AOMs) in redox-fluctuating aquifers to ROS via field investigation and in-lab verification using transcriptomes/ metatranscriptome and RT-qPCR. Ammonia-oxidizing archaea (AOA) dominated recharge aquifers with lower ROS levels, whereas ammonia-oxidizing bacteria (AOB) and heterotrophic nitrifying aerobic bacteria (HNB) predominated in discharge areas with higher ROS levels. Similar succession in AOM enrichments was found in that the dominant AOMs changed from AOA Nitrosopumilus to AOB Nitrosomonas with increasing ROS. Ammonia oxidation and antioxidant capacity differed significantly among three AOM isolates exposed to ROS. ROS decreased the amoA gene expression of AOA strain Nitrososphaera viennensis PLX03, accompanied by inhibited ammonia oxidation capacity. By contrast, the catalase and superoxide dismutase activities of the AOB strain Nitrosomonas oligotropha PLL12 and HNB strain Pseudomonas aeruginosa PLL01 increased, and the antioxidant genes katG, sodA, ahpC, and ahpF were significantly upregulated. These results demonstrate that ROS exert an important influence on AOMs in redox-fluctuating aquifers. This study improves our understanding of the ecological niches of AOMs in surface/subsurface 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.

The overlooked toxicity of environmentally persistent free radicals (EPFRs) induced by anthracene transformation to earthworms (Eisenia fetida)

Environmentally persistent free radicals (EPFRs) as intermediate products exist widely in the PAHs-contaminated soils, but toxicity assessment associated with EPFRs for terrestrial invertebrates remains unclear. Using the model organism Eisenia fetida, we compared the adverse effects among anthracene (ANT), anthraquinone (ANQ), and EPFRs induced by ANT transformation on clay surfaces. Our results showed that EPFRs-exposed earthworms experienced histopathological damage, which was more severe than ANT and ANQ-exposed earthworms. The source of EPFRs damage was associated with the obvious dysbiosis of reactive oxygen species in earthworms. Specifically, EPFRs trigged more severe antioxidant responses and oxidative damages (e.g., membrane lipid and DNA injury) in comparison with ANT and ANQ exposure, as evidenced by the values of integrated biomarker response (IBR) following the order of EPFRs (14.5) > ANT (12.8) > ANQ (10.9). Moreover, high-throughput sequencing found that EPFRs induced dramatic changes in the composition and structure of earthworm gut microbiota, which may involve immune and metabolism dysfunction, in turn aggravated EPFRs toxicity. Overall, the obtained information highlights the more severe injury of EPFRs to terrestrial organisms, deserving more attentions for the assessment of potential risks associated with radical intermediates in PAHs-contaminated soils.

Microscale Spatiotemporal Variation and Generation Mechanisms of Reactive Oxygen Species in the Rhizosphere of Ryegrass: Coupled Biotic-Abiotic Processes

Reactive oxygen species (ROS) play key roles in soil biogeochemical processes, yet the occurrence and accumulation of ROS in the rhizosphere are poorly documented. Herein, we first developed a ROS-trapping membrane to in situ determine ROS in the ryegrass rhizosphere and then quantified the temporal and spatial variations of representative ROS (i.e., O₂^•─, H₂O₂, and ^•OH). Fluorescence imaging clearly visualized the production of ROS in the rhizosphere. Both O₂^•─ and H₂O₂ content increased first and then declined throughout the life cycle of ryegrass, while ^•OH concentration decreased continuously. Spatially, ROS contents remained at a relatively high level at 0-5 mm and then descended with increasing distance. The concentrations of ROS in different soils followed the order of black soil > latosol soil > yellow-brown soil > tier soil ∼ red soil. Analysis of soil properties suggested that both biotic factors (microbial community) and abiotic factors (Fe(II) and water-soluble phenols) played critical roles in ROS production. The combined processes, including Fe(II) and water-soluble phenol-mediated electron transfer, microbial community-driven extracellular O₂^•─ release, and Fe(II)/Fe(III) cycling, may be responsible for ROS production. These findings provide insights into ROS-associated rhizosphere effects and inspiration for the phytoremediation of pollutants and element cycling.

Effect of C/Fe Molar Ratio on H2O2 and •OH Production during Oxygenation of Fe(II)-Humic Acid Coexisting Systems

Hydrogen peroxide (H₂O₂) and hydroxyl radical (^•OH) production during oxygenation of reduced iron (Fe(II)) and natural organic matter (NOM) in the subsurface has been increasingly discovered, whereas the effect of the C/Fe molar ratio in Fe(II) and NOM coexisting systems remains poorly understood. In this study, aqueous Fe(II) and reduced humic acid (HA_red) mixture at different C/Fe molar ratios (0-20) were oxygenated. Results show that both H₂O₂ and ^•OH accumulation increased almost linearly with the increase in the C/Fe ratio, with a more prominent increase in ^•OH accumulation at high C/Fe molar ratios. At low C/Fe molar ratios (C/Fe ≤ 1.6), electrons mainly transferred from dissolved inorganic Fe(II), surface-adsorbed Fe(II), and a low proportion of HA-complexed Fe(II) to O₂; with the increase in the C/Fe ratio to a high level (C/Fe ≥ 5), the main electron source turned to HA-complexed Fe(II) and free HA_red. The changes in the electron source and electron transfer pathway with the increase in the C/Fe ratio increased the yield of ^•OH relative to H₂O₂. This study highlights the important role of the C/Fe ratio in controlling H₂O₂ and ^•OH production and therefore in accurately evaluating the associated environmental impacts.