Interfacial reaction between organic acids and iron-containing clay minerals: Hydroxyl radical generation and phenolic compounds degradation

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.

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

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

Mechanistic insight into Cu(II)-mediated transformation of oxytetracycline on Fe-bearing smectite: Evidence from kinetic modeling

Heavy metals like Cu(II) and oxytetracycline (OTC) frequently coexist in livestock manure and wastewater due to their widespread use as feed additives, forming Cu(II)-OTC complexes that influence OTC transport and transformation in groundwater. This study investigates Cu(II)-mediated OTC transformation on Fe-bearing smectite under dark, anoxic conditions. Results show that OTC adsorption on Fe-bearing smectite undergoes further transformation. Cu(II)-OTC complexes significantly enhance OTC adsorption, accounting for 23.6%, 99.4%, and 92.9% of total OTC at pH 3, 5, and 8.5, respectively, but inhibit OTC transformation. FTIR, XPS, and UPLC-QTOF analyses reveal redox reactions between Cu(II)-OTC complexes and Fe-bearing smectite, reducing Fe(III) to Fe(II). Transformation primarily involves the tricarbonylamide group at pH 3 and 5, extending to the dimethylamino group at pH 8.5. A kinetic model of OTC transformation identifies two key parameters: the transformation rate (k') and the reactive site quantity (Srxn). The k' varies with pH as follows: pH 3 < pH 8.5 < pH 5, while Fe concentration exhibits an inverse relationship with k', suggesting that mineral dissolution reduces the redox activity of Fe-bearing smectite. Furthermore, a linear correlation between the fraction of redox-active Cu(II)-OTC complexes and Srxn indicates that Cu(II) complexation occupies reactive sites on Fe-bearing smectite, inhibiting transformation. These findings highlight the importance of considering heavy metal-antibiotic interactions when assessing groundwater risks.

Catalytic detoxification of mitoxantrone by graphitic carbon nitride (g-C3N4) supported Fe/Pd bimetallic nanoparticles

The overuse of antibiotics and antitumor drugs has resulted in more and more extensive pollution of water bodies with organic drugs, causing detrimental ecological effects, which have attracted attention towards effective and sustainable methods for antibiotics and antitumor drug degradation. Here, the hybrid nanomaterial (g-C3N4@Fe/Pd) was synthesized and used to remove a kind of both an antibiotic and antitumor drug named mitoxantrone (MTX) with 92.0% removal efficiency, and the MTX removal capacity is 450 mg/g. After exposing to the hybrid material the MTX aqueous solution changed color from dark blue to lighter progressively, and LC-UV results of residual solutions show that a new peak at 3.0 min (MTX: 13.2 min) after removal by g-C3N4@Fe/Pd appears, with the simultaneous detection of intermediate products indicating that g-C3N4@Fe/Pd indeed degrades MTX. Detailed mass spectrometric analysis suggests that the nuclear mass ratio decreased from 445.2 (M+1H) to 126.0 (M+1H), 169.1 (M+1H), 239.2 (M+1H), 267.3 (M+1H), 285.2 (M+1H), 371.4 (M+1H) and 415.2 (M+1H), and the maximum proportion (5.63%) substance of all degradation products (126.0 (M+1H)) is 40-100 times less toxic than MTX. A mechanism for the removal and degradation of mitoxantrone was proposed. Besides, actual water experiments confirmed that the maximum removal capacity of MTX by g-C3N4@Fe/Pd is up to 492.4 mg/g (0.02 g/L, 10 ppm).

Phytoremediation of trichloroethylene in the soil/groundwater environment: Progress, problems, and potential

Trichloroethylene (TCE) poses a significant environmental threat in groundwater and soil, necessitating effective remediation strategies. Phytoremediation offers a cost-effective and environmentally friendly approach to remediation. However, the mechanisms governing plant uptake, volatilisation, and degradation of TCE remain poorly understood. This review explores the mechanisms of TCE phytoremediation, metabolic pathways, and influencing factors, emphasizing future research directions to improve the understanding of TCE phytoremediation. The results showed that although the proportion of TCE phytovolatilisation is limited, it is important at sites chronically contaminated with TCE. The rhizosphere is a key microzone for pollutant redox reactions that significantly enhance its effectiveness when its characteristics are fully utilised and manipulated through reinforcement. Future research should focus on manipulating microbial communities through methods such as the application of endophytic bacteria and genetic modification. However, practical applications are in their infancy and further investigation is needed. Furthermore, many findings are based on non-uniform parameters or unstandardised methods, making them difficult to compare. Therefore, future studies should provide more standardised experimental parameters and employ accurate and standardised methods to develop suitable prediction models, enhancing data comparability and deepening our understanding of plant detoxification mechanisms.

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.

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

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

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.

The mechanisms of ·OH formation in MnO2 and oxalate system: Implication for ATZ removal

Manganese oxides (MnO2) are commonly prevalent in groundwater, sediment and soil. In this study, we found that oxalate (H2C2O4) dissolved MnO2, leading to the formation of Mn(II)/(III), CO2(aq) and reactive oxygen species (·CO2-/O2·-/H2O2/·OH). Notably, CO2(aq) played a crucial role in ·OH formation, contributing to the degradation of atrazine (ATZ). To elucidate underneath mechanisms, a series of reactions with different gas-liquid ratios (GLR) were conducted. At the GLR of 0.3, 3.76, and + ∞ 79.4 %, 5.32 %, and 5.28 % of ATZ were eliminated, in which the cumulative ·OH concentration was 39.6 μM, 8.11 μM, and 7.39 μM and the cumulative CO2(aq) concentration was 11.2 mM, 4.7 mM, and 2.8 mM, respectively. The proposed reaction pathway was that CO2(aq) participated in the formation of a ternary complex [C2O4-Mn(II)-HCO4·3 H2O]-, which converted to a transition state (TS) as [C2O4-Mn(II)-CO3-OH·3 H2O]-, then decomposed to a complex radical [C2O4-Mn(II)-CO3·3 H2O]·- and ·OH after electron transfer within TS. It was novel to discover the role of CO2(aq) for ·OH yielding during MnO2 dissolution by H2C2O4. This finding helps revealing the overlooked processes that CO2(aq) influenced the fate of ATZ or other organic compounds in environment and providing us ideas for new technique development in contaminant remediation. ENVIRONMENTAL IMPLICATION: Manganese oxides and oxalate are common in soil, sediment and water. Their interactions could induce the formation of Mn(II)/(III), CO2(aq) and ·CO2-/O2·-/H2O2. This study found that atrazine could be effectively removed due to ·OH radicals under condition of high CO2(aq) concentration. The concentrations of Mn (0.0002-8.34 mg·L-1) and CO2(aq) (15-40 mg·L-1) were high in groundwater, and the surface water or rainfall seeps into groundwater and bring organic acids, which might promote the ·OH formation. The results might explain the missing steps of herbicides transformation in these environments and be helpful in developing new techniques in remediation in future.

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

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

Robust reactive oxygen species production in interfacial reaction between organic acids and biochar: The combined effect of electron acceptance and electron conduction

Interfacial electron transport and reactive oxygen species (ROS) generation in the redox action between biochar (BC) and low-molecular-weight organic acids (LMWOAs) have been overlooked during the utilization of BC in soil amelioration/remediation. Herein, BC and N-doped BC (NBC) with various physicochemical properties were prepared at pyrolysis temperatures of 350, 550 and 750 °C (namely BC/NBC350, 550 and 750) and their interactions with LMWOAs were systematically investigated. Results of ROS quenching and electrochemical cell experiments revealed that BC/NBC could strongly interact with LMWOAs, especially ascorbic acid (AA), and acted as an electron shuttle to mediate one electron transfer from AA to oxygen to generate O2•-, followed by the robust generation of H2O2 and •OH. Interestingly, the generation rates of •OH and H2O2 in the NBC-AA system were significantly higher than that in the BC-AA system, of which the NBC550-AA system exhibited the best performance, enabling its superior ability in bisphenol A degradation. Different from conventional wisdom, the electron shuttle capacity depended on a combined aromatization degree and electron-accepting capacity of BC/NBC. These findings complement the interfacial electron transfer mechanism during the natural BC-LMWOAs interaction and provide new inspiration for the development of green and efficient organic pollutant removal technologies.

Activation of persulfate for the degradation of ethyl-parathion in soil: Combined effects of microwave with biochar

Sulfate radical (SO4•-), formed by persulfate (PS) activation during advanced oxidation process (AOPs), can be used for the remediation of organic contaminated soil. However, the role of biochar and microwave (MW) in the activation of PS is not fully understood, especially the corresponding mechanism. Herein, biochar combined with MW was used to activate PS for the remediation of ethyl-parathion (PTH)-polluted soil. The dynamic evolutions of PTH under different conditions, such as biochar content, particle size, reaction temperature, and the degradation mechanisms of PTH were also systematically investigated. Significant enhancement performance on PTH removal was observed after adding biochar, which was 88.78% within 80 min. Meanwhile, activating temperature exhibited remarkable abilities to activate PS for PTH removal. The higher content of adsorption sites in nano-biochar facilitated the removal of PTH. Furthermore, chemical probe tests coupled with quenching experiments confirmed that the decomposition of PS into active species, such as SO4•-, •OH, O2•- and 1O2, contributed to the removal of PTH in biochar combined with MW system, which could oxidize PTH into oxidative products, including paraoxon, 4-ethylphenol, and hydroquinone. The results of this study provide valuable insights into the synergistic effects of biochar and MW in the PS activation, which is helpful for the potential application of biochar materials combined with MW-activated PS in the remediation of pesticide-polluted soils.

Unrecognized Role of Organic Acid in Natural Attenuation of Pollutants by Mackinawite (FeS): The Significance of Carbon-Center Free Radicals

Organic acid is prevalent in underground environments and, against the backdrop of biogeochemical cycles on Earth, holds significant importance in the degradation of contaminants by redox-active minerals. While earlier studies on the role of organic acid in the generation of reactive oxygen species (ROS) primarily concentrated on electron shuttle or ligand effects, this study delves into the combined impacts of organic acid decomposition and Mackinawite (FeS) oxidation in contaminant transformation under dark aerobic conditions. Using bisphenol A (BPA) as a model, our findings showed that oxalic acid (OA) notably outperforms other acids in enhancing BPA removal, attaining a rate constant of 0.69 h-1. Mass spectrometry characterizations, coupled with anaerobic treatments, advocate for molecule-O2 activation as the principal mechanism behind pollutant transformation. Comprehensive results unveiled that carbon center radicals, initiated by hydroxyl radical (•OH) attack, serve as the primary agents in pollutant oxidation, accounting for at least 93.6% of the total •OH generation. This dynamic, driven by the decomposition of organic acids and the concurrent formation of carbon-centered radicals, ensures a steady supply of electrons for ROS generation. The obtained information highlights the importance of OA decomposition in the natural attenuation of pollutants and offers innovative strategies for FeS and organic acid-coupled decontamination.

Mechanism of polycyclic aromatic hydrocarbons degradation in the rhizosphere of Phragmites australis: Organic acid co-metabolism, iron-driven, and microbial response

Microbial co-metabolism is crucial for the efficient biodegradation of polycyclic aromatic hydrocarbons (PAHs); however, their intrinsic mechanisms remain unclear. To explore the co-metabolic degradation of PAHs, root organic acids (ROAs) (phenolic ROAs: caffeic acid [CA] and ferulic acid [FA]; non-phenolic ROAs: oxalic acid [OA]) were exogenously added as co-metabolic substrates under high (HFe) and low (LFe) iron levels in this study. The results demonstrated that more than 90% of PAHs were eliminated from the rhizosphere of Phragmites australis. OA can promote the enrichment of unrelated degrading bacteria and non-specific dioxygenases. FA with a monohydroxy structure can activate hydroxylase; however, it relies on phytosiderophores released by plants (such as OA) to adapt to stress. Therefore, non-specific co-metabolism occurred in these units. The best performance for PAH removal was observed in the HFe-CA unit because: (a) HFe concentrations enriched the Fe-reducing and denitrifying bacteria and promoted the rate-limiting degradation for PAHs as the enzyme cofactor; (b) CA with a dihydroxyl structure enriched the related degrading bacteria, stimulated specific dioxygenase, and activated Fe to concentrate around the rhizosphere simultaneously to perform the specific co-metabolism. Understanding the co-metabolic degradation of PAHs will help improve the efficacy of rhizosphere-mediated remediation.

Synergistic effect and mechanism of Cd(II) and As(III) adsorption by biochar supported sulfide nanoscale zero-valent iron

Biochar derived from bamboo was used to support sulfide nanoscale zero-valent iron (S-nZVI@BC) for simultaneous removal of Cd(II) and As (III) from aqueous media. Scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) characterization confirmed the successful synthesis of the S-nZVI@BC. Adsorption kinetics and isotherms indicated that co-adsorption of Cd(II) and As(III) onto S-nZVI@BC was well represented by pseudo-second-order model (R²_Cd(II) = 0.990, R²_As(III) = 0.995) and Langmuir model (R²_Cd(II) = 0.954, R²_As(III) = 0.936). The maximum adsorption was 162.365 and 276.133 mg/g for Cd(II) and As(III), respectively, in a co-adsorption system, which was significantly higher than that in a single adsorption system (103.195 and 223.736 mg/g, respectively). Batch experiments showed that the Cd(II)-to-As(III) concentration ratio significantly affected the co-adsorption with the optimal ratio of 1:2. Ca²⁺ and Mg²⁺ significantly inhibited Cd(II) removal. In contrast, phosphate and humic acid significantly inhibited As(III) removal. Electrochemical analysis indicated S-nZVI@BC had a lower corrosion potential and resistance than nZVI@BC, making it more conducive to electron transfer and chemical reaction. Electrostatic adsorption, complexation, co-precipitation, and redox were the primary mechanisms for Cd(II) and As(III) removal. Overall, the present study provides new insights into the synergistic removal of Cd(II) and As(III) by S-nZVI@BC, which is a very promising adsorbent for the effective removal of Cd(II) and As(III) from contaminated wastewater.

On-site generation of reactive oxidative radicals from dithionite treated oxic soil slurry

Whereas dithionite has been extensively used as a reducing agent in soil and sediment remediation, here, we demonstrate that it can be used as a potential source of oxidizing radical in oxic soils with potential application in organic pollutant remediation. Benzoic acid was used as a probe compound and the generation of its oxidative product para-hydroxybenzoic acid (p-HBA) was detected to quantify the production of oxidative radicals (ROS). By increasing the dithionite concentration from 2.5-10 Mm, the accumulated P-HBA concentration in 120 min increased from 15.0-27 µM. Whereas, above 10 mM, the p-HBA concentration decreased due to radical scavenging. Increasing soil dosage from 2.5-15 g/100 mL the accumulated p-HBA amount increased from 22.8-33.7 µM. Temperature 25-35 ^oC and pH 6.2-7.5 were favoured for p-HBA generation. Furthermore, we investigated the roles of different active intermediates in the reaction system and proposed the mechanism behind the ROS genearation. This study suggested that dithionite can be used as an active reagent for advanced oxidation remediation in oxic soil medium.

Iron redox cycling in layered clay minerals and its impact on contaminant dynamics: A review

A majority of clay minerals contain Fe, and the redox cycling of Fe(III)/Fe(II) in clay minerals has been extensively studied as it may fuel the biogeochemical cycles of nutrients and govern the mobility, toxicity and bioavailability of a number of environmental contaminants. There are three types of Fe in clay minerals, including structural Fe sandwiched in the lattice of clays, Fe species in interlayer space and adsorbed on the external surface of clays. They exhibit distinct reactivity towards contaminants due to their differences in redox properties and accessibility to contaminant species. In natural environments, microbially driven Fe(III)/Fe(II) redox cycling in clay minerals is thought to be important, whereas reductants (e.g., dithionite and Fe(II)) or oxidants (e.g., peroxygens) are capable of enhancing the rates and extents of redox dynamics in engineered systems. Fe(III)-containing clay minerals can directly react with oxidizable pollutants (e.g., phenols and polycyclic aromatic hydrocarbons (PAHs)), whereas structural Fe(II) is able to react with reducible pollutants, such as nitrate, nitroaromatic compounds, chlorinated aliphatic compounds. Also structural Fe(II) can transfer electrons to oxygen (O₂), peroxymonosulfate (PMS), or hydrogen peroxide (H₂O₂), yielding reactive radicals that can promote the oxidative transformation of contaminants. This review summarizes the recent discoveries on redox reactivity of Fe in clay minerals and its links to fates of environmental contaminants. The biological and chemical reduction mechanisms of Fe(III)-clay minerals, as well as the interaction mechanism between Fe(III) or Fe(II)-containing clay minerals and contaminants are elaborated. Some knowledge gaps are identified for better understanding and modelling of clay-associated contaminant behavior and effective design of remediation solutions.

Adsorption Characteristics of Iron on Different Layered Loess Soils

In view of the problem of Fe³⁺ pollution in an iron sulfur mine, different layers of loess soil in the Bijie area were used for adsorption to alleviate the mine wastewater pollution by natural treatment. The effects of the initial concentration of Fe³⁺, adsorption time and pH value on the adsorption performance of top, core and subsoil layers of loess soils were studied by the oscillatory equilibrium method, and the adsorption mechanism of these three soils was analyzed through a kinetic adsorption experiment and infrared spectroscopy. The results showed that the adsorption capacity of Fe³⁺ was improved by increasing the initial concentration and reaction time, but the adsorption rate of the adsorption capacity of Fe³⁺ was reduced. The adsorption rate of Fe³⁺ in the subsoil layer was faster than that in the other two layers. The higher the pH, the higher the adsorption capacity. After the pH was higher than 3.06, it had little effect on the adsorption capacity, but the adsorption rate increased. The first-order kinetic equation, second-order kinetic equation and Elovich equation were suitable for iron adsorption kinetics of three soils. The fitting correlation coefficient of the second-order kinetic equation was close to one, indicating the main role of chemical adsorption. The adsorption rate constant of the subsoil layer was about two times and three times that of the core soil layer and the topsoil layer. The Langmuir model can better fit the isothermal adsorption process. The results of infrared spectroscopy of soil showed that the content of soil organic matter played an important role in the adsorption capacity of Fe³⁺. The subsoil layer had a higher concentration of organic matter and more abundant functional groups, so the adsorption capacity of Fe³⁺ was the highest. The results could provide a theoretical basis for the removal of iron in acid mine wastewater.

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.