Contaminant Degradation by •OH during Sediment Oxygenation: Dependence on Fe(II) Species

Effects of biochar on tire wear particle-derived 6PPD, 6PPD-Q, and antimony levels and microbial community in soil

Although several studies have documented tire wear particles (TWP)-contaminated soil could increase N-(1,3-dimethylbutyl)-N-phenyl-p-phenylenediamine (6PPD), 6PPD-quinone (6PPD-Q), and antimony (Sb) levels, despite this, effective strategies to address the problem are still lacking. This study focused on mitigating environmental risks from TWPs, a significant but overlooked contaminant. We examined the impact of biochar (BC) on TWP contamination at different soil moisture levels. 6PPD levels in TWP-amended soil peaked at 4.239 µg/g by day 60 in flooded conditions. BC amendments reduced 6PPD and 6PPD-Q concentrations by 85-90 % in both conditions. BC also reduced Sb(III) and Sb(V) levels by 80-83 %, while boosting dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) levels by up to 75 %, improving soil fertility. Our results showed that 6PPD and 6PPD-Q exposure altered bacterial composition, with Desulfobacterota and Planctomycetota thriving in flooded conditions, while Gemmamonadota and Verrucomicrobiota declined in 50 % water holding capacity (WHC). Key results indicated a strong reduction in alpha diversity under 50 % WHC, while treatments with MBc400 maintain higher biodiversity, as indicated by the Shannon index, and higher species richness, shown by the Chao index, especially in 50 % WHC. These results implied that higher-temperature BC effectively reduced 6PPD, 6PPD-Q, and Sb bioavailability while mitigating TWP contamination by enhancing microbial diversity, especially under 50 % WHC.

New insights into soil active substances enhance the biochar/periodate process for remediation of sulfadiazine: The changes of soil properties and toxicity

In recent years, periodate (SPI)-based advanced oxidation processes have been successfully applied in wastewater treatment. However, their application in soil pollution remediation remains limited. To our knowledge, this study represents the first attempt to utilize SPI catalyzed by the Eichhornia crassipes biochar (EBC) system for the remediation of sulfadiazine (SD)-contaminated soil. In the EBC/SPI system, the degradation performance of SD-spiked soils was significantly improved, achieving complete degradation within 60 min, which indicates a clear synergistic effect between SPI and EBC. Notably, our findings highlighted that active soil constituents play crucial roles in SPI activation. Specifically, free Fe-oxides in soil were essential for SPI activation to form reactive species (RS) compared to amorphous Fe-oxides and dissolved Fe, leading to superior SD degradation. Soil organic matter (SOM) also contributed to RS formation and conversion. Adding Fe3+, Cl-, and humic acid accelerated SD elimination, whereas Mn2+ and HCO3- inhibited it. Quenching experiments and electron paramagnetic resonance spectroscopy confirmed the formation of singlet oxygen, superoxide radicals, and iodate radicals, which actively degraded SD. Analysis of soil properties, including SOM content, total phosphorus, functional groups, crystal structure, and pH value, showed negligible changes after EBC/SPI treatment. Additionally, potential decomposition pathways of SD were proposed based on identified SD intermediates. Ecotoxicity analyses and phytotoxicity tests indicated a marked reduction in the toxicity of these intermediates compared to SD. These findings provide an efficient strategy for soil remediation and offer new insights into the role of inherent substances in the field of contaminated soil remediation.

Organic acid-enhanced production of hydroxyl radicals during H2O2-based chemical oxidation for the remediation of contaminated soil

Hydrogen peroxide (H2O2)-based in situ chemical oxidation (ISCO) is widely used for remediating contaminated groundwater and soil. However, its effectiveness can be limited by a low efficiency of H2O2 utilization, leading to increased costs. In this study, we showed that ascorbic acid (AA), citric acid, and hydroxylamine hydrochloride (used for comparison) significantly increased •OH production (by 2.3-108.0-fold) and chlorobenzene degradation (by 6.4-30.5-fold) in H2O2/site soil systems. Further analysis revealed that AA significantly enhanced the formation and oxidation of active Fe(II) species (e.g., 0.5 M HCl-, 5 M HCl-, and HF-Fe(II)) via the mechanisms of acid dissolution, complexation, and reduction. As a result, these processes inhibited the transformation of low-crystallinity Fe phases into high-crystallinity forms, thereby preserving the activity of the Fe phases. The different capacities of these ligands for acidification and complexation or reduction are significantly influenced by their characteristics, such as the presence of specific functional groups, as well as their concentration. This variation, in turn, affects •OH production and the degradation of contaminants in treatment systems. This study provides valuable insights into how low-molecular-weight organic acids enhance the formation of •OH and contaminant degradation during H2O2-based ISCO. These findings also contribute to the development of efficient, environmentally friendly, and cost-effective remediation technologies for the treatment of contaminated groundwater and soil.

Superoxide anion radicals mediated degradation of tetrachloropicolinic acid in biochars-FexP@Fe-FexC/O2 system with excellent reactivity durability

Activation of oxygen by zero-valent iron (ZVI) to in-situ produce reactive oxidant species (ROS) provides a promising low-carbon and "green" technology for water purification. However, poor ROS yields and easy inactivation limit its engineering application for organic pollutants elimination. Herein, we fabricated a novel Fe-based catalyst with Fe(II)-regenerative surface derived from phosphatized sewage sludge and iron salts. The achieved materials were composed of sludge biochars, FexP, Fe, and FexC (SL-FexP@Fe-FexC) and possessed core/shell structure. SL-FexP@Fe-FexC showed high efficiency in degrading recalcitrant organic pollutants 3,4,5,6-tetrachloropicolinic acid (TCPA) from water at pH 3-10 or in different salts solution without the need of exogenous H2O2. When sludge was pretreated with 1.0 M H3PO4 and then soaked in 50 mM FeCl3 solution before carbonization, the obtained SL1.0M-FexP@Fe-FexC50mM could degrade TCPA with almost 100 % efficiency in ten consecutive recycle runs. This material demonstrates better activity persistence than most of the reported Fe-based catalysts. The EPR and quenching tests indicated that O2•- radicals generated from Fe(II)/O2 reaction were the main active species for TCPA degradation. The electrochemical experiments revealed that strong affinity of O2 and fast electron transfer from inner Fe/FexC to SL-FexP shell improved the yields of O2•- and regeneration of Fe(II) species.

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

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

Enhanced in-situ sulfide removal via goethite-fulvic acid bio-reduction and iron-based catalysis in activated sludge recycling odor control system

H2S poses serious challenges in wastewater treatment plants, including unpleasant odors issues, toxicity conditions and infrastructure corrosion. In this study, we propose a novel in-situ H2S odor control process, which introduced goethite/goethite-fulvic acid (FA) bio-reduction and Fe-based catalysis into activated sludge recycling. This novel process reduced the activated sludge recycling rate from 40 % to 5 %, while increasing the sulfide removal efficiency from 52.97 % to 87.61 %. The sulfide removal capacities were 99.78 mgS/g Fe for goethite and 247.38 mgS/g Fe for goethite-FA. The bio-reduction of recycled sludge further enhanced the sulfide removal capacity to 103.43 mgS/g Fe in goethite and 337.74 mgS/g Fe for goethite-FA. Fulvic acid disrupted crystal structure, reduced electron transfer resistance and increased surface area of goethite, thereby enhancing bio-reduction efficiency and sulfide removal capacity. Moreover, aeration of inlet works further increased the sulfide removal efficiency from 12.67 % to 65.50 % in goethite sludge and from 23.73 % to 87.61 % in goethite-FA sludge. This enhancement was due to the catalytic effect of dissolved and ion-exchangeable Fe, which generated through complexation and electronegativity of recycled Fe-activated sludge. Overall, the novel H2S control process can achieve high sulfide removal efficiency while maintaining low recycling rate and operation costs.

Demethylation by Reactive Oxygen Species Lowers Methylmercury Accumulation in Rice

It is a paradox that the accumulation of neurotoxic methylmercury (MeHg) in rice is generally low despite efficient MeHg production in paddies and absorption by rice plants. Because rice-paddy systems are conducive to reactive oxygen species (ROS) generation and ROS have recently been revealed to efficiently demethylate MeHg absorbed in autotrophs, we propose that ROS, generated in soils and plants, are significant yet overlooked drivers that lower MeHg accumulation in rice. ROS-mediated demethylation in both soils and rice plants is estimated to have reduced rice MeHg by 84%, highlighting the importance of these overlooked drivers in safeguarding global rice consumers.

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.

Enhancing emerging pollutant removal mediated by root iron plaques: Integrated abiotic and biotic effects

As a contact interface among plants, microbes, and the liquid phase, root iron plaque (IP) occupies a crucial ecological niche on the root surface in constructed wetlands. However, research on the integrated work mechanisms of the various processes mediated by root IP in removing emerging pollutants is limited. This study analyzed four IP-mediated pathways in plant hydroponic systems, categorizing them into abiotic (adsorption and ·OH oxidation) and biotic (plant uptake and microbial degradation) effects. Employing sulfamethoxazole (SMX) as a target emerging pollutant, the results revealed that root IP improved pollutant removal efficiency by 2.44 times, which could be attributed to the enhanced abiotic effects. Root IP increased the amount of adsorbed SMX from 0.135 to 1.328 µg/g FW. Fe2 + in root IP could activate O2 to generate adsorbed hydroxyl radicals (·OH), which subsequently facilitate the oxidative removal of adsorbed SMX. The contribution of ·OH oxidation to SMX removal reached 27.35 %. However, from the biotic perspective, the stable adsorbed SMX was difficult to transport into plant root tissues, and root IP served as a "barrier" that hindered the plant uptake. In addition, IP weakened biotic effects by altering root-associated microbial community. The total relative abundance of SMX degradation-related genera decreased by 12.4 % with root IP, and the amounts of microbial degradation decreased from 8.33 to 3.49 µg/L.

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.

Immobilization or mobilization of heavy metal(loid)s in lake sediment-water interface: Roles of coupled transformation between iron (oxyhydr)oxides and natural organic matter

Iron (Fe) (oxyhydr)oxides and natural organic matter (NOM) are active substances ubiquitously found in sediments. Their coupled transformation plays a crucial role in the fate and release risk of heavy metal(loid)s (HMs) in lake sediments. Therefore, it is essential to systematically obtain relevant knowledge to elucidate their potential mechanism, and whether HMs provide immobilization or mobilization effect in this ternary system. In this review, we summarized (1) the bidirectional effect between Fe (oxyhydr)oxides and NOM, including preservation, decomposition, electron transfer, adsorption, reactive oxygen species production, and crystal transformation; (2) the potential roles of coupled transformation between Fe and NOM in the environmental behavior of HMs from kinetic and thermodynamic processes; (3) the primary factors affecting the remediation of sediments HMs; (4) the challenges and future development of sediment HM control based on the coupled effect between Fe and NOM from theoretical and practical perspectives. Overall, this review focused on the biogeochemical coupling cycle of Fe, NOM, and HMs, with the goal of providing guidance for HMs contamination and risk control in lake sediment.

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.

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

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

Freeze-Thaw Cycle Events Enable the Deep Disintegration of Biochar: Release of Dissolved Black Carbon and Its Structural-Dependent Carbon Sequestration Capacity

Biochar is widely regarded as a recalcitrant carbon pool. However, the impact of freeze-thaw cycle events on its storage capacity, particularly on the release of dissolved black carbon (DBC), has remained poorly investigated. This study investigated the release behavior of DBC from biochar pyrolyzed at 300-700 °C during freeze-thaw cycles and their retention capacity in soil. Freeze-thaw cycles dramatically promoted DBC release (33.08-230.74 mg C L-1), exhibiting an order of magnitude higher than those without freeze-thaw process. The release kinetics of freeze-thaw-induced DBC varied depending on the pyrolysis temperature of biochar due to the different disintegration mechanisms. Interestingly, the retention capacity of freeze-thaw-induced DBC in soil showed a reduction ranging from 7.7 to 29.5% compared to DBC without the freeze-thaw process. This reduction can be attributed to numerous hydrophilic low-molecular-weight compounds (16.97-75.31%) in freeze-thaw-induced DBC, as evidenced by the results of size exclusion chromatography, fluorescence excitation/emission matrix, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. These compounds tend to concentrate in the aqueous phase rather than being retained in the soil, potentially exacerbating the outflow of dissolved organic carbon. These findings clarify the release behavior of DBC during freeze-thaw cycles and reveal their contribution to the attenuation of carbon pools in cold regions.

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."

Coupled effects of redox-active substances and microbial communities on reactive oxygen species in rhizosphere sediments of submerged macrophytes

Reactive oxygen species (ROS) play crucial roles in element cycling and pollutant dynamics, but their variations and mechanisms in the rhizosphere of submerged macrophytes are poorly investigated. This study investigated the light-dark cycle fluctuations and periodic variations in ROS, redox-active substances, and microbial communities in the rhizosphere of Vallisneria natans. The results showed sustained production and significant diurnal fluctuations in the O2•- and •OH from 27.6 ± 3.7 to 61.7 ± 3.0 μmol/kg FW and 131.0 ± 6.8 to 195.4 ± 8.7 μmol/kg FW, respectively, which simultaneously fluctuated with the redox-active substances. The ROS contents in the rhizosphere were higher than those observed in non-rhizosphere sediments over the V. natans growth period, exhibiting increasing-decreasing trends. According to the redundancy analysis results, water-soluble phenols, fungi, and bacteria were the main factors influencing ROS production in the rhizosphere, showing contribution rates of 74.0, 17.3, and 4.4 %, respectively. The results of partial least squares path modeling highlighted the coupled effects of redox-active substances and microbial metabolism. Our findings also demonstrated the degradation effect of ROS in rhizosphere sediments of submerged macrophytes. This study provides experimental evidence of ROS-related rhizosphere effects and further insights into submerged macrophytes-based ecological restoration.

Phosphate ligand-mediated production of reactive oxygen species during oxygenation of Fe(II)-phosphate complexes

Reactive oxygen species (ROS) production upon the oxygenation of reduced iron minerals is of critical importance to redox cycles of Fe and the fate of refractory organic contaminants. The environmental impact factors during this process, however, have been underappreciated. In this study, prominently enhanced production of hydroxyl radicals (•OH) was observed by oxygenation of Fe(II) with 5-50 mM phosphate. The results of spin trap electron spin resonance (ESR) experiment showed that Fe(II)-phosphate complexes facilitated the generation of •OH. The degradation experiment of p-nitrophenol (PNP) confirmed that •OH formation was dominated by a consecutive one-electron O2 reduction (90.2-96.9 %), and the quantification of PNP degradation products revealed that Fe(II)/phosphate molar ratios regulated the O2 activation pathways for O2•- or •OH production. The further experimental and theoretical investigation demonstrated that the coordination of phosphate with Fe(II) plays a dual role in ROS generation that facilitated O2•- formation by lowering the energy barrier for Fe(II) oxidation and altered the reaction pathway of •OH formation due to its occupation of sites for electron transfer. The present work highlights an important role of natural oxyanions in O2 activation by Fe(II) and raises the possibility of in situ degradation of contaminants in subsurface environment.

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

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

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

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

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.

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

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

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.

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

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

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.

Iron minerals enhance Fe(II)-mediated abiotic As(III) oxidation

The abiotic oxidation of As(III) is simultaneously mediated by the oxidation of Fe(II) in microaerobic environment, but the role of Fe minerals in the Fe(II)-mediated As(III) oxidation have been neglected. This work mimicked the microaerobic environment and examined the mechanisms of Fe(II) mediated the As(III) oxidation in the presence of Fe minerals using a variety of iron minerals (lepidocrocite, goethite, etc.). The results indicated the Fe(II) and As(III) oxidation rate were improved with Fe minerals, while As(III) oxidation efficiency increased by 1.3-1.8 times in comparison to that without minerals. Fe(II) mediated the As(III) oxidation happened on Fe minerals surface in the presence of Fe minerals. The As(III) oxidation efficiency increased with increasing Fe mineral concentrations (from 0.5 to 2 g L-1) but decreased with increasing pH values. Reactive oxygen species (ROS) that play a crucial role in As(III) oxidation were Fe(IV) and ·O2-, accounting for 42.7%-47.9% and 24.1%-29.8%, respectively. The Fe minerals facilitated the oxidation of As(III) by ROS and stimulated the release of ROS through the adsorbed-Fe(II) oxidation, both of which favored As(III) oxidation. This work highlighted the potential mechanisms of Fe minerals in promoting Fe(II) mediated the As(III) oxidation in microaerobic environment, especially in terms of As(III) oxidation efficiency, shedding a valuable insight on optimization of arsenic contaminated wastewater treatment processes.

The metal release and transformation mechanisms of V-Ti magnetite tailings: Role of the alternate flooding and drying cycles and organic acids

V-Ti magnetite tailings (VTMTs) contain various heavy metals, such as Fe, Mn, V, Co, and Ni. The groundwater pollution caused by the tailing metal release has become a local environmental concern. Although studies have demonstrated the influence of alternate flooding and drying cycles (FDCs) on metal form and mobility in minerals, little was known about whether FDCs affect the metal release of VTMTs and the transformation of released metals. This study investigated the metal release kinetics of VTMTs and the metal transformation under FDCs in the absence and presence of acid rain (sulfuric and nitric acids) and bio-secreted organic acids (acetic, oxalic, and citric acids). The results showed that FDCs promoted metal release whether or not acids were present. The maximum released concentrations of V, Mn, Fe, Co, and Ni were as high as 78.63 mg L-1,1.47 mg L-1, 67.96 μg L-1, 1.34 mg L-1, and 0.80 mg L-1, respectively, under FDCs and citric acids. FDCs enhanced the tailing metal release by increasing the metal labile fraction proportion. However, the concentrations of released Fe, Mn, V, Co, and Ni all gradually decreased due to their (co-)precipitation. These precipitates conversely inhibited the subsequent mineral dissolution by covering the tailing surface. FDCs also enhanced the tailings' porosities by 2.94%-9.94%. The mineral dissolution, expansion and shrinkage, and changes in tension destroyed the tailing microstructure during FDCs. This study demonstrated the low metal pollution risk of VTMTs under FDCs, either in acid rain or bio-secreted organic acids. However, the increase in tailing porosity should be seriously considered as it would affect the tailing pond safety.

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.

Exploring the promoting effect of nitrilotriacetic acid on hydroxyl radical and humification during magnetite-amended composting of sewage sludge

The OH production by adding magnetite (MGT) alone has been reported in composting. However, the potential of nitrilotriacetic acid (NTA) addition for magnetite-amended sludge composting remained unclear. Three treatments with different addition [control check (CK); T1: 5 % MGT; T2: 5 % MGT + 5 % NTA] were investigated to characterize hydroxyl radical, humification and bacterial community response. The NTA addition manifested the best performance, with the peak OH content increase by 52 % through facilitating the cycle of Fe(Ⅱ)/Fe(Ⅲ). It led to the highest organic matters degradation (22.3 %) and humic acids content (36.1 g/kg). Furthermore, NTA addition altered bacterial community response, promoting relative abundances of iron-redox related genera, and amino acid metabolism but decreasing carbohydrate metabolism. Structural equation model indicated that temperature and Streptomyces were the primary factors affecting OH content. The study suggests that utilizing chelators is a promising strategy to strengthen humification in sewage sludge composting with adding iron-containing minerals.

Insights into the Crystallinity-Dependent Photochemical Productions of Reactive Oxygen Species from Iron Minerals

Iron minerals are widespread in earth's surface water and soil. Recent studies have revealed that under sunlight irradiation, iron minerals are photoactive on producing reactive oxygen species (ROS), a group of key species in regulating elemental cycling, microbe inactivation, and pollutant degradation. In nature, iron minerals exhibit varying crystallinity under different hydrogeological conditions. While crystallinity is a known key parameter determining the overall activity of iron minerals, the impact of iron mineral crystallinity on photochemical ROS production remains unknown. Here, we assessed the photochemical ROS production from ferrihydrites with different degrees of crystallinity. All examined ferrihydrites demonstrated photoactivity under irradiation, resulting in the generation of hydrogen peroxide (H2O2) and hydroxyl radical (•OH). The photochemical ROS production from ferrihydrites increased with decreasing ferrihydrite crystallinity. The crystallinity-dependent photochemical •OH production was primarily attributed to conduction band reduction reactions, with the reduction of O2 by conduction band electrons being the rate-limiting key process. Conversely, the crystallinity of iron minerals had a negligible influence on photon-to-electron conversion efficiency or surface Fenton-like activity. The difference in ROS productions led to a discrepant degradation efficiency of organic pollutants on iron mineral surfaces. Our study provides valuable insights into the crystallinity-dependent ROS productions from iron minerals in natural systems, emphasizing the significance of iron mineral photochemistry in natural sites with abundant lower-crystallinity iron minerals such as wetland water and surface soils.

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.

Degradation of methylmercury into Hg(0) by the oxidation of iron(II) minerals

Mercury contamination is a global concern, and the degradation and detoxification of methylmercury have gained significant attention due to its neurotoxicity and biomagnification within the food chain. However, the currently known pathways of abiotic demethylation are limited to light-induced photodegradation process and little is known about light-independent abiotic demethylation of methylmercury. In this study, we reported a novel abiotic pathway for the degradation of methylmercury through the oxidation of both mineral structural iron(II) and surface-adsorbed iron(II) in the absence of light. Our findings reveal that methylmercury can be oxidatively degraded by reactive oxygen species, specifically hydroxyl and superoxide radicals, which are generated from the oxidation of iron(II) minerals under dark conditions. Surprisingly, Hg(0) trapping experiments demonstrated that inorganic Hg(II) resulting from the oxidative degradation of methylmercury was rapidly reduced to gaseous Hg(0) by iron(II) minerals. The demethylation of methylmercury, coupled with the generation of Hg(0), suggests a potential natural attenuation process for methylmercury. Our results highlight the underappreciated roles of iron(II) minerals in the abiotic degradation of methylmercury and the release of gaseous Hg(0) into the atmosphere.

Abiotic natural attenuation of 1,2,3-trichloropropane by natural magnetite under O2 perturbation

1,2,3-Trichloropropane (TCP) is an emerging groundwater pollutant, but there is a lack of reported studies on the abiotic natural attenuation of TCP by iron minerals. Furthermore, perturbation by O2 is common in the shallow subsurface by both natural and artificial processes. In this study, natural magnetite was selected as the reactive iron mineral to investigate its role in the degradation of TCP under O2 perturbation. The results indicated that the mineral structural Fe(II) on magnetite reacted with dissolved oxygen to generate O2-· and HO·. Both O2-· and HO· contributed to TCP degradation, with O2-· playing a more important role. After 56 days of reaction, 66.7% of TCP was completely dechlorinated. This study revealed that higher magnetite concentrations, smaller magnetite particle sizes, and lower initial TCP concentrations favored TCP degradation. The presence of <10 mg/L natural organic matter (NOM) did not affect TCP degradation. These findings significantly advance our understanding of the abiotic natural attenuation mechanisms facilitated by iron minerals under O2 perturbation, providing crucial insights for the study of natural attenuation.

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.

Zeolite facilitates sequestration of heavy metals via lagged Fe(II) oxidation during sediment aeration

Aeration of sediments could induce the release of endogenous heavy metals (HMs) into overlying water. In this study, experiments involving FeS oxygenation and contaminated sediment aeration were conducted to explore the sequestering role of zeolite in the released HMs during sediment aeration. The results reveal that the dynamic processes of Fe(II) oxidation play a crucial role in regulating HMs migration during both FeS oxygenation and sediment aeration in the absence of zeolite. Based on the release of HMs, Fe(II) oxidation can be delineated into two stages: stage I, where HMs (Mn2+, Zn2+, Cd2+, Ni2+, Cu2+) are released from minerals or sediments into suspension, and stage II, released HMs are partially re-sequestered back to mineral phases or sediments due to the generation of Fe-(oxyhydr) oxide. In contrast, the addition of zeolite inhibits the increase of HMs concentration in suspension during stage I. Subsequently, the redistribution of HMs between zeolite and the newly formed Fe-(oxyhydr) oxide occurs during stage II. This redistribution of HMs generates new sorption sites in zeolite, making them available for resorbing a new load of HMs. The outcomes of this study provide potential solutions for sequestering HMs during the sediment aeration.

Production and prediction of hydroxyl radicals in distinct redox-fluctuation zones of the Yellow River Estuary

Hydroxyl radicals (·OH) produced in subsurface sediments play an important role in biogeochemical cycles. One of the major sources of·OH in sediments is associated with reduced compounds (e.g., iron and organic matter) oxygenation. Moreover, the properties of iron forms and dissolved organic matter (DOM) components varied significantly across redox-fluctuation zones of estuaries. However, the influence of these variations on mechanisms of·OH production in estuaries remains unexplored. Herein, sediments from riparian zones, wetlands, and rice fields in the Yellow River Estuary were collected to systematically explore the diverse mechanisms of·OH generation. Rhythmic continuous·OH production (82-730 μmol/kg) occurred throughout the estuary, demonstrating notable spatial heterogeneity. The amorphous iron form and humic-like DOM components were the key contributors to·OH accumulation in estuary wetlands and freshwater restoration wetlands, respectively. The crystalline iron form and protein-like DOM components influenced the capabilities of iron reduction and continuous·OH production. Moreover, the orthogonal partial least squares models outperformed various multivariate models in screening crucial factors and predicting the spatiotemporal production of·OH. This study provides novel insights into varied mechanisms of·OH generation within distinct redox-fluctuation zones in estuaries and further elucidates elemental behavior and contaminant fate in estuarine environments. ENVIRONMENTAL IMPLICATION: Given that estuaries serve as sinks for anthropogenic pollutants, various organic pollutants (e.g., emerging contaminants such as antibiotics) have been widely detected in estuarine environments. The production of·OH in sediments has been proven to affect the fate of contaminants. Therefore, the varied mechanisms of·OH in estuarine environments, dominated by diverse iron forms and DOM components, were explored in this study. MLR and OPLS models exhibited good performance in screening crucial factors and predicting·OH production. Our work highlights that in estuarine subsurface environments, the presence of·OH potentially leads to a natural degradation of pollutants.

Dissolved organic matter accelerates microbial degradation of 17 alpha-ethinylestradiol in the presence of iron mineral

Dissolved organic matter (DOM) and iron minerals widely existing in the natural aquatic environment can mediate the migration and transformation of organic pollutants. However, the mechanism of interaction between DOM and iron minerals in the microbial degradation of pollutants deserves further investigation. In this study, the mechanism of 17 alpha-ethinylestradiol (EE2) biodegradation mediated by humic acid (HA) and three kinds of iron minerals (goethite, magnetite, and pyrite) was investigated. The results found that HA and iron minerals significantly accelerated the biodegradation process of EE2, and the highest degradation efficiency of EE2 (48%) was observed in the HA-mediated microbial system with pyrite under aerobic conditions. Furthermore, it had been demonstrated that hydroxyl radicals (HO•) was the main active substance responsible for the microbial degradation of EE2. HO• is primarily generated through the reaction between hydrogen peroxide secreted by microorganisms and Fe(II), with aerobic conditions being more conducive. The presence of iron minerals and HA could change the microbial communities in the EE2 biodegradation system. These findings provide new information for exploring the migration and transformation of pollutants by microorganisms in iron-rich environments.

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.

Effects of Fe3O4 NMs based Fenton-like reactions on biodegradable plastic bags in compost: New insight into plastisphere community succession, co-composting efficiency and free radical in situ aging theory

Biodegradable plastic bags (BPBs), meant for eco-friendly, often inadequately degrade in compost, leading to microplastic pollution. In this study, the effect of Fenton-like reaction with Fe3O4 nanoparticles (NMs) on the plastisphere microorganisms' evolution and the BPBs' aging mechanism was revealed by co-composting of food waste with BPBs for 40 days. The establishment of the Fenton-like reaction was confirmed, with the addition of Fenton-like reagent treatments resulting in an increase of 57.67% and 37.75% in H2O2 levels during the composting, compared to the control group. Moreover, the structural characterization reveals that increasing oxygen content continuously generates reactive free radicals on the surface, leading to the formation of oxidative cavities. This process results in random chain-breaking, significantly reducing molecular weights by 39.27% and 38.81%, thus showcasing a deep-seated transformation in the plastic's molecular structure. Furthermore, the microbial network suggested that the Fenton-like reaction enriched plastisphere keystone species, thus accelerating the BPBs' aging. Additionally, the Fenton-like reaction improved compost maturity and reduced greenhouse gas emissions. These results reveal the bio-chemical mechanisms of BPBs aging and random chain-breaking by the Fenton-like reaction, under alternating oxidative/anoxic conditions of composting and provide a new insight to resolve the BPBs' pollutions.

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.

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

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

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.

Occurrence and cycle of dissolved iron mediated by humic acids resulting in continuous natural photodegradation of 17α-ethinylestradiol

17α-ethinylestradiol (EE2), a synthetic endocrine-disrupting chemical, can degrade in natural waters where humic acids (HA) and dissolved iron (DFe) are present. The iron is mostly bound in Fe(III)-HA complexes, the formation process of Fe(III)-HA complexes and their effect on EE2 degradation were explored in laboratory experiments. The mechanism of ferrihydrite facilitated by HA was explored with results indicating that HA facilitated the dissolution of ferrihydrite and the generation of Fe(III)-HA complexes with the stable chemical bonds such as C-O, CO in neutral, alkaline media with a suitable Fe/C ratio. 1O2, •OH, and 3HA* were all found to be important in the photodegradation of EE2 mediated by Fe(III)-HA complexes. Fe(III)-HA complexes could produce Fe(II) and hydrogen peroxide (H2O2) to create conditions suitable for photo-Fenton reactions at neutral pH. HA helped to maintain higher dissolved iron concentrations and alter the Fe(III)/Fe(II) cycling. The natural EE2 photodegradation pathway elucidated here provides a theoretical foundation for investigating the natural transformation of other trace organic contaminants in aquatic environments.

A novel iron sulfide phase with remarkable hydroxyl radical generation capability for contaminants degradation

The hydroxyl radical (·OH) stands as one of the most potent oxidizing agents, capable of engaging in non-selective and instantaneous reactions with contaminants in water. Herein, we present a novel iron sulfide phase (S-FeS) characterized by an unprecedented structure, accompanied by its remarkable hydroxyl radical generation capability and contaminant degradation efficiency surpassing that of the conventional metastable iron sulfide phase, namely, the Mackinawite (FeS). In comparison to FeS, S-FeS exhibits enhanced degradation kinetics and higher efficacy in the removal of methylene blue, ciprofloxacin, and trivalent arsenic. Utilizing density functional theory (DFT) calculations, we postulate the mechanism for the exceptional contaminant degradation performance of S-FeS to be attributed to the increased exposure of the highly reactive (110) crystal facets. This research uncovers a new metastable phase that expands the polymorphisms within the iron sulfide family and showcases its capability for driving the oxygen reduction reaction.

Synergistic effects of Fe-based nanomaterial catalyst on humic substances formation and microplastics mitigation during sewage sludge composting

In this study, a novel Fe-based nanomaterial catalyst (Fe0/FeS) was synthesized via a self-heating process and employed to explore its impact on the formation of humic substances and the mitigation of microplastics. The results reveal that Fe0/FeS exhibited a significant increase in humic acid content (71.01 mg kg-1). Similarly, the formation of humic substances resulted in a higher humification index (4.91). Moreover, the addition of Fe0/FeS accelerated the degradation of microplastics (MPs), resulting in a lower concentration of MPs (9487 particles/kg) compared to the control experiments (22792 particles/kg). Fe0/FeS significantly increased the abundance of medium-sized MPs (50-200 μm) and reduced the abundance of small-sized (10-50 μm) and large-sized MPs (>1000 μm). These results can be attributed to the Fe0/FeS regulating the ▪OH production and specific microorganisms to promote humic substance formation and the degradation of MPs. This study proposes a feasible strategy to improve composting characteristics and reduce contaminants.

Dependence of Biotic and Abiotic H2O2 and •OH Production on the Redox Conditions and Compositions of Sediment during Oxygenation

Reactive oxygen species (ROS) production in O2-perturbed subsurface environments has been increasingly documented in recent years. However, the constraining conditions under which abiotic and/or biotic mechanisms predominate for ROS production remain ambiguous. Here, we demonstrate that the ROS production mechanism, biotic and abiotic, is determined by sediment redox properties and sediment compositions. Upon the oxygenation of 10 field sediments, the cumulative H2O2 concentrations reached up to 554 μmol/kg within 2 h. The autoclaving sterilization experiments showed that H2O2 could be produced by both biotic and abiotic processes depending on the redox conditions. However, only the abiotic process could produce significant levels of •OH, and the production yield was closely related to the sediment components, particularly sediment Fe(II) and organic matter. Fe(II) bound with organic matter led to high yields of H2O2 and •OH production. Sediment oxygenation contributed to the appearance of H2O2 in groundwater, with the abiotic mechanism producing higher instantaneous H2O2 concentrations than the biotic mechanism. These findings reveal that the redox conditions, compositions, and texture of sediments collectively control abiotic and biotic mechanisms for ROS production, which assists the identification of ROS production hotspots and the understanding of ROS distribution and utilization in the subsurface.

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

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