Highly sensitive and selective demethylase FTO detection using a DNAzyme-mediated CRISPR/Cas12a signal cascade amplification electrochemiluminescence biosensor with C-CN/PCNV heterojunction as emitter

Fat mass and obesity-associated protein (FTO) has gained attention as the first RNA N6-methyladenosine (m6A) modification eraser due to its overexpression being associated with various cancers. In this study, an electrochemiluminescence (ECL) biosensor for the detection of demethylase FTO was developed based on DNAzyme-mediated CRISPR/Cas12a signal cascade amplification system and carboxylated carbon nitride nanosheets/phosphorus-doped nitrogen-vacancy modified carbon nitride nanosheets (C-CN/PCNV) heterojunction as the emitter. The biosensor was constructed by modifying the C-CN/PCNV heterojunction and a ferrocene-tagged probe (ssDNA-Fc) on a glassy carbon electrode. The presence of FTO removes the m6A modification on the catalytic core of DNAzyme, restoring its cleavage activity and generating activator DNA. This activator DNA further activates the trans-cleavage ability of Cas12a, leading to the cleavage of the ssDNA-Fc and the recovery of the ECL signal. The C-CN/PCNV heterojunction prevents electrode passivation and improves the electron-hole recombination, resulting in significantly enhanced ECL signal. The biosensor demonstrates high sensitivity with a low detection limit of 0.63 pM in the range from 1.0 pM to 100 nM. Furthermore, the biosensor was successfully applied to detect FTO in cancer cell lysate and screen FTO inhibitors, showing great potential in early clinical diagnosis and drug discovery.

Cr(VI) Reduction and Sequestration by FeS Nanoparticles Formed in situ as Aquifer Material Coating to Create a Regenerable Reactive Zone

Remediation of large and dilute plumes of groundwater contaminated by oxidized pollutants such as chromate is a common and difficult challenge. Herein, we show that in situ formation of FeS nanoparticles (using dissolved Fe(II), S(-II), and natural organic matter as a nucleating template) results in uniform coating of aquifer material to create a regenerable reactive zone that mitigates Cr(VI) migration. Flow-through columns packed with quartz sand are amended first with an Fe2+ solution and then with a HS- solution to form a nano-FeS coating on the sand, which does not hinder permeability. This nano-FeS coating effectively reduces and immobilizes Cr(VI), forming Fe(III)-Cr(III) coprecipitates with negligible detachment from the sand grains. Preconditioning the sand with humic or fulvic acid (used as model natural organic matter (NOM)) further enhances Cr(VI) sequestration, as NOM provides additional binding sites of Fe2+ and mediates both nucleation and growth of FeS nanoparticles, as verified with spectroscopic and microscopic evidence. Reactivity can be easily replenished by repeating the procedures used to form the reactive coating. These findings demonstrate that such enhancement of attenuation capacity can be an effective option to mitigate Cr(VI) plume migration and exposure, particularly when tackling contaminant rebound post source remediation.

Eco-friendly natural mineral biotite as a cesium adsorbent: Utilizing low-concentration acid and hydrogen peroxide

Biotite, a phyllosilicate mineral, possesses significant potential for cesium (Cs) adsorption owing to its negative surface charge, specific surface area (SSA), and frayed edge sites (FES). Notably, FES are known to play an important role in the adsorption of Cs. The objectives of this study were to investigate the Cs adsorption capacity and behavior of artificially weathered biotite and identify mineralogical characteristics for the development of an eco-friendly geologically-based Cs adsorbent. Through various analyses, it was confirmed that the FES of biotite was mainly formed by mineral structural distortion during artificial weathering. The Cs adsorption capacity is improved by approximately 39% (from 20.53 to 28.63 mg g-1) when FES are formed in biotite through artificial weathering using a low-concentration acidic solution mixed with hydrogen peroxide (H2O2). Especially, the Cs selectivity in Cs-containing seawater, including high concentrations of cations and organic matter, was significantly enhanced from 203.2 to 1707.6 mL g-1, an increase in removal efficiency from 49.5 to 89.2%. These results indicate that FES of artificially weathered biotite play an essential role in Cs adsorption. Therefore, this simple and economical weathering method, which uses a low-concentration acidic solution mixed with H2O2, can be applied to natural minerals for use as Cs adsorbents.

Simultaneous removal of antibiotic resistance genes and improved dewatering ability of waste activated sludge by Fe(II)-activated persulfate oxidation

Waste activated sludge properties vary widely with different regions due to the difference in living standards and geographical distribution, making a big challenge to developing a universally effective sludge dewatering technique. The Fe(II)-activated persulfate (S2O82-) oxidation process shows excellent ability to disrupt sludge cells and extracellular polymeric substances (EPS), and release bound water from sludge flocs. In this study, the discrepancies in the physicochemical characteristics of sludge samples from seven representative cities in China (e.g., dewaterability, EPS composition, surface charge, microbial community, relative abundance of antibiotic resistance genes (ARGs), etc.) were investigated, and the role of Fe(II)-S2O82- oxidation in enhancing removal of antibiotic resistance genes and dewatering ability were explored. The results showed significant differences between the EPS distribution and chemical composition of sludge samples due to different treatment processes, effluent sources, and regions. The Fe(II)-S2O82- oxidation pretreatment had a good enhancement of sludge dewatering capacity (up to 76 %). Microbial analysis showed that the microbial community in each sludge varied significantly depending on the types of wastewater, the wastewater treatment processes, and the regions, but Fe(II)-S2O82- oxidation was able to attack and rupture the sludge zoogloea indiscriminately. Genetic analysis further showed that a considerable number of ARGs were detected in all of these sludge samples and that Fe(II)-S2O82- oxidation was effective in removing ARGs by higher than 90 %. The highly active radicals (e.g., SO4-·, ·OH) produced in this process caused drastic damage to sludge microbial cells and DNA stability while liberating the EPS/cell-bound water. Co-occurrence network analysis highlighted a positive correlation between population distribution and ARGs abundance, while variations in microbial communities were linked to regional differences in living standards and level of economic development. Despite these variations, the Fe(II)-S2O82- oxidation consistently achieved excellent performance in both ARGs removal and sludge dewatering. The significant modularity of associations between different microbial communities also confirms its ability to reduce horizontal gene transfer (HGT) by scavenging microbes.

A redox switch allows binding of Fe(II) and Fe(III) ions in the cyanobacterial iron-binding protein FutA from Prochlorococcus

The marine cyanobacterium Prochlorococcus is a main contributor to global photosynthesis, whilst being limited by iron availability. Cyanobacterial genomes generally encode two different types of FutA iron-binding proteins: periplasmic FutA2 ABC transporter subunits bind Fe(III), while cytosolic FutA1 binds Fe(II). Owing to their small size and their economized genome Prochlorococcus ecotypes typically possess a single futA gene. How the encoded FutA protein might bind different Fe oxidation states was previously unknown. Here, we use structural biology techniques at room temperature to probe the dynamic behavior of FutA. Neutron diffraction confirmed four negatively charged tyrosinates, that together with a neutral water molecule coordinate iron in trigonal bipyramidal geometry. Positioning of the positively charged Arg103 side chain in the second coordination shell yields an overall charge-neutral Fe(III) binding state in structures determined by neutron diffraction and serial femtosecond crystallography. Conventional rotation X-ray crystallography using a home source revealed X-ray-induced photoreduction of the iron center with observation of the Fe(II) binding state; here, an additional positioning of the Arg203 side chain in the second coordination shell maintained an overall charge neutral Fe(II) binding site. Dose series using serial synchrotron crystallography and an XFEL X-ray pump-probe approach capture the transition between Fe(III) and Fe(II) states, revealing how Arg203 operates as a switch to accommodate the different iron oxidation states. This switching ability of the Prochlorococcus FutA protein may reflect ecological adaptation by genome streamlining and loss of specialized FutA proteins.

Tracking the formation potential of vivianite within the treatment train of full-scale wastewater treatment plants

Phosphorus recovery is a vital element for the circular economy. Wastewater, especially sewage sludge, shows great potential for recovering phosphate in the form of vivianite. This work focuses on studying the iron, phosphorus, and sulfur interactions at full-scale wastewater treatment plants (Viikinmäki, Finland and Seine Aval, France) with the goal of identifying unit processes with a potential for vivianite formation. Concentrations of iron(III) and iron(II), phosphorus, and sulfur were used to evaluate the reduction of iron and the formation potential of vivianite. Mössbauer spectroscopy and X-ray diffraction (XRD) analysis were used to confirm the presence of vivianite in various locations on sludge lines. The results show that the vivianite formation potential increases as the molar Fe:P ratio increases, the anaerobic sludge retention time increases, and the sulfate concentration decreases. The digester is a prominent location for vivianite recovery, but not the only one. This work gives valuable insights into the dynamic interrelations of iron, phosphorus, and sulfur in full-scale conditions. These results will support the understanding of vivianite formation and pave the way for an alternative solution for vivianite recovery for example in plants that do not have an anaerobic digester.

Structural Insights into the Reaction between Hydrogen Peroxide and Di-iron Complexes at the Ferroxidase Center of Ferritin

The Fe(II) oxidation mechanism in the ferroxidase center of heavy chain ferritin has been studied extensively. However, the actual production of H2O2 was found to be substantially lower than expected at low flux of Fe(II) to ferritin subunits. Here, we demonstrated that H2O2 could interact with the di-iron nuclear center, leading to the production of hydroxyl radicals and oxygen. Two reaction intermediates were captured in the ferroxidase center by using the time-lapse crystallographic techniques in a shellfish ferritin. The crystal structures revealed the binding of H2O2 as a μ -1,2-peroxo-diferric species and the binding of O2 to the diferric structure. This investigation sheds light on the reaction between the di-iron nuclear center and H2O2 and provides insights for the exploitation of metalloenzymes.

Co-precipitation of iron and silicon: Reaction kinetics, elemental ratios and the influence of phosphorus

A sufficient supply of dissolved silicon (DSi) relative to dissolved phosphorus (DP) may decrease the likelihood of harmful algal blooms in eutrophic waters. Oxidative precipitation of Fe(II) at oxic-anoxic interfaces may contribute to the immobilization of DSi, thereby exerting control over the DSi availability in the overlying water. Nevertheless, the efficacy of DSi immobilization in this context remains to be precisely determined. To investigate the behavior of DSi during Fe(II) oxidation, anoxic solutions containing mixtures of aqueous Fe(II), DSi, and dissolved phosphorus (DP) were exposed to dissolved oxygen (DO) in the batch system. The experimental data, combined with kinetic reaction modeling, indicate that DSi removal during Fe(II) oxidation occurs via two pathways. At the beginning of the experiments, the oxidation of Fe(II)-DSi complexes induces the fast removal of DSi. Upon complete oxidation of Fe(II), further DSi removal is due to adsorption to surface sites of the Fe(III) oxyhydroxides. The presence of DP effectively competes with DSi via both of these pathways during the initial and later stages of the experiments, with as a result more limited removal of DSi during Fe(II) oxidation. Overall, we conclude that at near neutral pH the oxidation of Fe(II) has considerable capacity to immobilize DSi, where the rapid homogeneous oxidation of Fe(II)-DSi results in greater DSi removal compared to surface adsorption. Elevated DP concentration, however, effectively outcompetes DSi in co-precipitation interactions, potentially contributing to enhanced DSi availability within aquatic systems.

Enhanced dewaterability of sludge by Fe(II)-sludge biochar activate persulfate

Sludge biochar supported Fe(II) (Fe(II)-SBC) was successfully prepared using waste activated sludge as peroxydisulfate (PDS) activator to condition sludge for deep dewatering. The experimental results showed that Fe(II)-SBC with FeO on it could effectively active PDS to produce SO 4 - ⋅ and HO ⋅ . The radicals could destroy the structure of sludge cells and extracellular polymeric substance (EPS), transformed the hydrophilic and tightly bound EPS into soluble-EPS, degrade partial proteins and polysaccharides and released bound water. The negatively charged groups on sludge floc were dripped off by SO 4 - ⋅ /HO ⋅ or neutralized with Fe2+, Fe3+, H+, or Fe(II)-SBC, leading to an increase in zeta potential to -2.24 mV and sludge destabilization. The residual Fe(II)-SBC served as a skeleton builder that decreased the compression coefficient of the sludge cake to 0.75. Under the combined functions, the CST and SRF were reduced by 70% and 82.7%, respectively, and Wc was reduced to 72.4%. The byproducts of Fe3+ and SO42- finally remained in sludge cake in the form of NaFeSi2O6 and CaSO4.

Deciphering the Diversified Metabolic Behavior of Hydroxyalkyl Ferrocidiphenols as Anticancer Complexes

Ferrocidiphenols possessing appropriate substituents in the aliphatic chain have very promising anticancer properties, but a systematic approach to deciphering their diversified metabolic behavior has so far been lacking. Herein, we show that a series of novel ferrocidiphenols bearing different hydroxyalkyl substituents exhibit strong anticancer activity as revealed in a range of in vitro and in vivo experiments. Moreover, they display diversified oxidative transformation profiles very distinct from those of previous complexes, shown by the use of chemical and enzymatic methods and in cellulo and in vivo metabolism studies. In view of this phenomenon, unprecedented chemo-evolutionary sequences that connect all the ferrocidiphenol-related intermediates and analogues have been established. In addition, a comprehensive density functional theory (DFT) study has been performed to decipher the metabolic diversification profiles of these complexes and demonstrate the delicate modulation of carbenium ions by the ferrocenyl moiety, via either α- or β-positional participation.

Critical Functions of Soil Components for In Situ Persulfate Oxidation of Sulfamethoxazole: Inherent Fe(II) Minerals-Coordinated Nonradical Pathway

Naturally occurring iron (Fe) minerals have been proved to activate persulfate (PS) to generate reactive species, but the role of soil-inherent Fe minerals in activating PS as well as the underlying mechanisms remains poorly understood. Here, we investigated sulfamethoxazole (SMX) degradation by PS in two Fe-rich soils and one Fe-poor soil. Unlike with the radical-dominant oxidation processes in Fe-poor soil, PS was effectively activated through nonradical pathways (i.e., surface electron-transfer) in Fe-rich soils, accounting for 68.4%-85.5% of SMX degradation. The nonradical mechanism was evidenced by multiple methods, including electrochemical, in situ Raman, and competition kinetics tests. Inherent Fe-based minerals, especially those containing Fe(II) were the crucial activators of PS in Fe-rich soils. Compared to Fe(III) minerals, Fe(II) minerals (e.g., ilmenite) were more liable to form Fe(II) mineral-PS* complexes to initiate the nonradical pathways, oxidizing adjacent SMX via electron transfer. Furthermore, mineral structural Fe(II) was the dominant component to coordinate such a direct oxidation process. After PS oxidation, low-crystalline Fe minerals in soils were transformed into high-crystalline Fe phases. Collectively, our study shows that soil-inherent Fe minerals can effectively activate PS in Fe-rich soils, so the addition of exogenous iron might not be required for PS-based in situ chemical oxidation. Outcomes also provide new insights into the activation mechanisms when persulfate is used for the remediation of contaminated soils.

Deciphering Deoxynybomycin Biosynthesis Reveals Fe(II)/α-Ketoglutarate-Dependent Dioxygenase-Catalyzed Oxazoline Ring Formation and Decomposition

The antibacterial agents deoxynybomycin (DNM) and nybomycin (NM) have a unique tetracyclic structure featuring an angularly fused 4-oxazoline ring. Here, we report the identification of key enzymes responsible for forming the 4-oxazoline ring in Embleya hyalina NBRC 13850 by comparative bioinformatics analysis of the biosynthetic gene clusters encoding structurally similar natural products DNM, deoxynyboquinone (DNQ), and diazaquinomycins (DAQs). The N-methyltransferase DnmS plays a crucial role in catalyzing the N-dimethylation of a tricyclic precursor prenybomycin to generate NM D; subsequently, the Fe(II)/α-ketoglutarate-dependent dioxygenase (Fe/αKGD) DnmT catalyzes the formation of a 4-oxazoline ring from NM D to produce DNM; finally, a second Fe/αKGD DnmU catalyzes the C-12 hydroxylation of DNM to yield NM. Strikingly, DnmT is shown to display unexpected functions to also catalyze the decomposition of the 4-oxazoline ring and the N-demethylation, thereby converting DNM back to prenybomycin, to putatively serve as a manner to control the intracellular yield of DNM. Structure modeling, site-directed mutagenesis, and quantum mechanics calculations provide mechanistic insights into the DnmT-catalyzed reactions. This work expands our understanding of the functional diversity of Fe/αKGDs in natural product biosynthesis.

Revealing the mechanism of quartz sand seeding in accelerating phosphorus recovery from anaerobic fermentation supernatant through vivianite crystallization

The recovery of phosphorus (P) through vivianite crystallization offers a promising approach for resource utilization in wastewater treatment plants. However, this process encounters challenges in terms of small product size and low purity. The study aimed to assess the feasibility of using quartz sand as a seed material to enhance P recovery and vivianite crystal characteristics from anaerobic fermentation supernatant. Various factors, including seed dosage, seed size, Fe/P ratio, and pH, were systematically tested in batch experiments to assess their influence. Results demonstrated that the effect of seed enhancement on vivianite crystallization was more pronounced under higher seed dosages, smaller seed sizes, and lower pH or Fe/P ratio. The addition of seeds increased P recovery by 4.43% in the actual anaerobic fermentation supernatant and also augmented the average particle size of the recovered product from 19.57 to 39.28 μm. Moreover, introducing quartz sand as a seed material effectively reduced co-precipitation, leading to a notable 12.5% increase in the purity of the recovered vivianite compared to the non-seeded process. The formation of an ion adsorption layer on the surface of quartz sand facilitated crystal attachment and growth, significantly accelerating the vivianite crystallization rate and enhancing P recovery. The economic analysis focused on chemical costs further affirmed the economic viability of using quartz sand as a seed material for P recovery through vivianite crystallization, which provides valuable insights for future research and engineering applications.

Degradation of Nitrobenzene by Mackinawite through a Sequential Two-Step Reduction and Oxidation Process

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO2) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO2 to aniline (ArNH2), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO2. In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals (•OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing •OH production for ArNH2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO2.

Abiotic Reduction of Organic and Inorganic Compounds by Fe(II)-Associated Reductants: Comprehensive Data Sets and Machine Learning Modeling

Iron-associated reductants play a crucial role in providing electrons for various reductive transformations. However, developing reliable predictive tools for estimating abiotic reduction rate constants (logk) in such systems has been impeded by the intricate nature of these systems. Our recent study developed a machine learning (ML) model based on 60 organic compounds toward one soluble Fe(II)-reductant. In this study, we built a comprehensive kinetic data set covering the reactivity of 117 organic and 10 inorganic compounds toward four major types of Fe(II)-associated reductants. Separate ML models were developed for organic and inorganic compounds, and the feature importance analysis demonstrated the significance of resonance structures, reducible functional groups, reductant descriptors, and pH in logk prediction. Mechanistic interpretation validated that the models accurately learned the impact of various factors such as aromatic substituents, complexation, bond dissociation energy, reduction potential, LUMO energy, and dominant reductant species. Finally, we found that 38% of the 850,000 compounds in the Distributed Structure-Searchable Toxicity (DSSTox) database contain at least one reducible functional group, and the logk of 285,184 compounds could be reasonably predicted using our model. Overall, the study is a significant step toward reliable predictive tools for anticipating abiotic reduction rate constants in iron-associated reductant systems.

Dioxygen Reduction and Bioinspired Oxidations by Non-heme Iron(II)-α-Hydroxy Acid Complexes

ConspectusAerobic organisms involve dioxygen-activating iron enzymes to perform various metabolically relevant chemical transformations. Among these enzymes, mononuclear non-heme iron enzymes reductively activate dioxygen to catalyze diverse biological oxidations, including oxygenation of C-H and C═C bonds and C-C bond cleavage with amazing selectivity. Several non-heme enzymes utilize organic cofactors as electron sources for dioxygen reduction, leading to the generation of iron-oxygen intermediates that act as active oxidants in the catalytic cycle. These unique enzymatic reactions influence the design of small molecule synthetic compounds to emulate enzyme functions and to develop bioinspired catalysts for performing selective oxidation of organic substrates with dioxygen. Selective electron transfer during dioxygen reduction on iron centers of synthetic models by a sacrificial reductant requires appropriate design strategies. Taking lessons from the role of enzyme-cofactor complexes in the selective electron transfer process, our group utilized ternary iron(II)-α-hydroxy acid complexes supported by polydentate ligands for dioxygen reduction and bioinspired oxidations. This Account focuses on the role of coordinated sacrificial reductants in the selective electron transfer for dioxygen reduction by iron complexes and highlights the versatility of iron(II)-α-hydroxy acid complexes in affecting dioxygen-dependent oxidation/oxygenation reactions. The iron(II)-coordinated α-hydroxy acid anions undergo two-electron oxidative decarboxylation concomitant with the generation of reactive iron-oxygen oxidants. A nucleophilic iron(II)-hydroperoxo species was intercepted in the decarboxylation pathway. In the presence of a Lewis acid, the O-O bond of the nucleophilic oxidant is heterolytically cleaved to generate an electrophilic iron(IV)-oxo-hydroxo oxidant. Most importantly, the oxidants generated with or without Lewis acid can carry out cis-dihydroxylation of alkenes. Furthermore, the electrophilic iron-oxygen oxidant selectively hydroxylates strong C-H bonds. Another electrophilic iron(IV)-oxo oxidant, generated from the iron(II)-α-hydroxy acid complexes in the presence of a protic acid, carries out C-H bond halogenation by using a halide anion.Thus, different metal-oxygen intermediates could be generated from dioxygen using a single reductant, and the reactivity of the ternary complexes can be tuned using external additives (Lewis/protic acid). The catalytic potential of the iron(II)-α-hydroxy complexes in performing O2-dependent oxygenations has been demonstrated. Different factors that govern the reactivity of iron-oxygen oxidants from ternary iron(II) complexes are presented. The versatile reactivity of the oxidants provides useful insights into developing catalytic methods for the selective incorporation of oxidized functionalities under environmentally benign conditions using aerial oxygen as the terminal oxidant.

Simultaneous stabilization of arsenic and antimony co-contaminated mining soil by Fe(Ⅱ) activated-Fenton sludge: Behavior and mechanisms

Fenton sludge (FS) with high iron contents that discharged from the Fenton process was rarely studied for soil remediation. Herein, a novel Fe(Ⅱ) activated-Fenton sludge (FS-FeSO4) was proposed to stabilize arsenic (As) and antimony (Sb) co-contaminated soil meanwhile disposing FS. Multiple characteristic analyses revealed that the porous structures and rich functional groups of FS-FeSO4 involved in As and Sb adsorption. Meanwhile, Fe (hydro)oxides played a key role in As and Sb stabilization. Under the optimal application parameters (stabilizers dosage: 5%, incubation time: 60 days), the available As and Sb content decreased by 88.6% and 83.3%, respectively, and the leachability of As and Sb was reduced by 100% and 72.6% for FS-FeSO4 stabilized soil. Moreover, the mobile As and Sb fractions (F1 and F2) were transformed into the most stable fraction (F5). The adsorption of As and Sb on FS-FeSO4 was well fitted by pseudo-second-order kinetic and Langmuir models, while FS-FeSO4 exhibited a better affinity for As than Sb under competition conditions. Poorly crystalline α-FeOOH and amorphous Fe (hydro)oxides provided sufficient active sites for As and Sb, and the generation of Fe-As/Sb and Ca-Sb chemical bonds promoted the stability of As and Sb. This study demonstrated that FS-FeSO4 was a potentially effective stabilizer for As and Sb co-contaminated soil remediation.

Rates and Mechanism of Vivianite Dissolution under Anoxic Conditions

The iron phosphate mineral vivianite Fe(II)3(PO4)2·8H2O has emerged as a potential renewable P source. Although the importance of vivianite as a potential P sink in the global P cycle had previously been recognized, a mechanistic understanding of vivianite dissolution at the molecular level, critical to its potential application, is still elusive. The potential of vivianite as a P sink or source in natural or engineered systems is directly dependent on its dissolution kinetics under environmentally relevant conditions. To understand the thermodynamic and kinetic controls on bioavailability, the oxidation and dissolution processes of vivianite must be disentangled. In this study, we conducted controlled batch and flow-through experiments to quantitatively determine the dissolution rates and mechanisms of vivianite under anoxic conditions as a function of pH and temperature. Our results demonstrate that vivianite solubility and dissolution rates strongly decreased with increasing solution pH. Dissolution was nonstoichiometric at alkaline pH (>7). The rapid initial dissolution rate of vivianite is related to the solution saturation state, indicating a thermodynamic rather than a kinetic control. A defect-driven dissolution mechanism is proposed. Dissolution kinetics over pH 5-9 could be described with a rate law with a single rate constant and a reaction order of 0.61 with respect to {H+}: R exp = 36.0 · e - 1.41 · pH · [ 1 - e ( 0.2 · Δ G / RT ) ] 4.7 The activation energy of vivianite dissolution proved low (Ea = 20.3 kJ mol-1), suggesting hydrogen bridge dissociation as the rate-determining step.

Effects of Fe(II) concentration on the biosynthesis of schwertmannite by Acidithiobacillus ferrooxidans and the As(III) removal capacity of schwertmannite

The effect of Fe(II) concentrations on schwertmannite bio-synthesis and the As(III) removal capacity of schwertmannite were investigated in this study. Acidithiobalillus ferrooxidans (A. ferrooxidans) were inoculated into five FeSO4 systems with initial concentrations of 50, 100, 200, 300, and 400 mmol/L, respectively, to bio-synthesize schwertmannite. The Fe(II) of the systems were almost completely oxidised at 48, 72, 120, 168, and 192 h, respectively, and the bio-schwertmannite yield was 1.99, 3.81, 9.36, 12.42, and 21.60 g/L. The results of this study indicate that all minerals harvested from the different systems are schwertmannite. As the initial Fe(II) concentration increases, the effect of the minerals removing As(III) decreases; moreover, the structure and extracellular polymeric substance (EPS) of schwertmannite may regulate the As(III) removal process. The EPS generated by the A. ferrooxidans can absorb As(III). The outcomes of this study provide fresh insights into the bio-synthetic regulation of schwertmannite and play a significant role in treating As-containing groundwater.

The Renaissance of Ferrocene-Based Electrocatalysts: Properties, Synthesis Strategies, and Applications

The fascinating electrochemical properties of the redox-active compound ferrocene have inspired researchers across the globe to develop ferrocene-based electrocatalysts for a wide variety of applications. Advantages including excellent chemical and thermal stability, solubility in organic solvents, a pair of stable redox states, rapid electron transfer, and nontoxic nature improve its utility in various electrochemical applications. The use of ferrocene-based electrocatalysts enables control over the intrinsic properties and electroactive sites at the surface of the electrode to achieve specific electrochemical activities. Ferrocene and its derivatives can function as a potential redox medium that promotes electron transfer rates, thereby enhancing the reaction kinetics and electrochemical responses of the device. The outstanding electrocatalytic activity of ferrocene-based compounds at lower operating potentials enhances the specificity and sensitivity of reactions and also amplifies the response signals. Owing to their versatile redox chemistry and catalytic activities, ferrocene-based electrocatalysts are widely employed in various energy-related systems, molecular machines, and agricultural, biological, medicinal, and sensing applications. This review highlights the importance of ferrocene-based electrocatalysts, with emphasis on their properties, synthesis strategies for obtaining different ferrocene-based compounds, and their electrochemical applications.

Nitric oxide binding to ferrous nitrobindins: A computer simulation investigation

Nitrobindins (Nbs) represent an evolutionary conserved all-β-barrel heme-proteins displaying a highly solvent-exposed heme-Fe(III) atom, coordinated by a proximal His residue. Interestingly, even if the distal side is exposed to the solvent, the value of the second order rate constants for ligand binding to the ferrous derivative is almost one order of magnitude lower than those reported for myoglobins (Mbs). Noteworthy, nitric oxide binding to the sixth coordination position of the heme-Fe(II)-atom causes the cleavage or the severe weakening of the proximal His-Fe(II) bond. Here, we provide a computer simulation investigation to shed light on the molecular basis of ligand binding kinetics, by dissecting the ligand binding process into the ligand migration and the bond formation steps. Classical molecular dynamics simulations were performed employing a steered molecular dynamics approach and the Jarzinski equality to obtain ligand migration free energy profiles. The formation of the heme-Fe(II)-NO bond took into consideration the iron atom displacement from the heme plane. The ligand migration is almost unhindered, and the low rate constant for NO binding is due to the large displacement of the Fe(II) atom with respect to the heme plane responsible for the barrier for the Fe(II)-NO bond formation. In addition, we investigated the weakening and breaking of the proximal His-Fe(II) bond, observed experimentally upon NO binding, by means of a combination of classical molecular dynamics simulations and quantum-classical (QM-MM) optimizations. In both human and M. tuberculosis Nbs, a stable alternative conformation of the proximal His residue interacting with a network of water molecules was observed.

Ferrous nitrosylated cytochrome c: The unusual strength of the proximal His18-Fe bond

NO binding to horse heart cytochrome c (hhcyt c) has been investigated as a function of pH by both optical absorption and EPR spectroscopies. Lowering pH from 3.5 to 1.5 induces: (i) a blue-shift of the maximum of the optical absorption spectrum in the Soret region from 415 to about 404 nm, and (ii) the appearance of a strong three hyperfine splitting in the gz region of the EPR spectrum. Both spectroscopic features indicate the cleavage of the proximal His18-Fe(II)-NO bond giving rise to the five-coordinated Fe(II)-NO species. By quantification of the relative weight for the six- and the five-coordinated component in the EPR spectra, the pKa value was determined. The apparent pKa of the proximal His Nε atom (1.8 ± 0.1) is unusually low for a ferrous nitrosylated form since in all investigated ferrous NO-bound heme-proteins the pKa value for the cleavage of the proximal His-Fe(II) bond ranges between 3.7 and 5.8. The pKa value of ferrous nitrosylated hhcyt c indicates that the strength of the proximal His18-Fe(II) bond (= 27.9 kJ/mol) is about 10-22 kJ/mol higher than that observed in all investigated heme-proteins. The strong coordination of the heme-Fe atom by His18 is extremely important to maintain the redox efficiency of cyt c and to keep apoptosis under control. This is a crucial point in tissues, such as retina, where apoptosis might trigger macular degenerative processes.

Synthesis and characterization of green rust-deposited MoS2 composites for adsorptive removal of EDTA-chelated Ni(II) in wastewater

Ethylenediamminetetraacetatonickel(II) (EDTA-Ni(II)) has emerged as a significant soil and groundwater contaminant due to the increasing agricultural and industrial activities, posing environmental challenges. This study focuses on addressing the reactivity of green rust (GR), which can be hindered by oxidation with oxygen, limiting its effectiveness in remediation processes. To overcome this limitation and enhance the adsorptive capacities, the combination of sulfate green rust (SO4-GR) with various Fe(II)/Fe(III) ratios with a high-surface-area adsorbent, MoS2, resulting in the formation of binary composites of green rust-deposited MoS2 (MSGs) were explored. The aim was to improve the removal efficiency of EDTA-Ni(II) from contaminated wastewater. To characterize the MSGs, a comprehensive analysis using XRD, SEM, TEM, FTIR, and X-ray absorption spectroscopy was performed. The surface areas of the MSGs were smaller than that of MoS2 but larger than that of the SO4-GRs, indicating a promising composite material. XANES spectra analysis revealed that both MSGs and SO4-GRs exhibited a mixture of ferrous and ferric ions, as evident from their spectral positioning between FeO and Fe2O3. The optimal pH for efficient removal of EDTA-Ni(II) was 3, which resulted in removal efficiencies of 45.6%, 47.3%, 46.0%, and 46.2% for MSG 1, MSG 2, MSG 3, and MSG 4 after 24 h, respectively. Reducing the initial concentration of EDTA-Ni(II) to 50 mg Ni(II)/L effectively doubled the removal efficiency. Notably, as EDTA-Ni(II) was removed, an increased leaching of iron was observed, leading to a total iron concentration exceeding 40 mg/L for the composites with higher Fe(II)/Fe(III) ratios. These findings underscore the potential of MSG as a promising material for degrading EDTA-Ni(II) in contaminated wastewater, offering a viable solution to mitigate the environmental impact of this emerging contaminant. This study contributes to the understanding of green rust reactivity and provides valuable insights for developing effective strategies to address the challenges associated with EDTA-Ni(II) contamination.

Facet-dependent U(VI) removal of hematite with confined ferrous ions

The presence of ferrous minerals has been demonstrated to have a significant impact on the destiny, migration, and availability of uranyl (U(VI)) in natural surroundings. The iron oxide/Fe(II) system is a multifaceted iron reduction system anchored to surfaces, encompassing various forms of iron and ferrous ions. Several studies have investigated the effectiveness of adsorbed ferrous iron on iron-based minerals to facilitate the reduction of heavy metal ions and radioactive nuclides. A range of techniques for characterization, including X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy, were employed to explore the process of U(VI) adsorption and deposition, focusing on the limited region containing ferrous iron on the exposed crystalline surface of hematite. In this specific investigation, two kinds of hematite nanocrystals primarily exposing {001} and {012} crystal facets, referred to as HNPs and HNCs, were synthesized. Their ability to remove U(VI) was examined. Ferrous ions (Fe(II)) adsorbed onto the surface of hematite nanocrystals significantly enhanced the efficiency of U(VI) remediation. Furthermore, the HNCs/Fe(II) system showed better U(VI) reduction ability than the HNPs/Fe(II) system. Remarkably, HNCs produced and consumed more electrons and hydroxyl radicals, indicating a more intense response. This finding serves to highlight the significance of their role in interfacial effects and in predicting the spatial distribution of U(VI) in aqueous systems.

Nitric oxide delivery and heme-assisted S-nitrosation by the bedbug nitrophorin

Nitrophorins are heme proteins used by blood feeding insects to deliver nitric oxide (NO) to a victim, leading to vasodilation and antiplatelet activity. Cimex lectularius (bedbug) nitrophorin (cNP) accomplishes this with a cysteine ligated ferric (Fe(III)) heme. In the acidic environment of the insect's salivary glands, NO binds tightly to cNP. During a blood meal, cNP-NO is delivered to the feeding site where dilution and increased pH lead to NO release. In a previous study, cNP was shown to not only bind heme, but to also nitrosate the proximal cysteine, leading to Cys-NO (SNO) formation. SNO formation requires oxidation of the proximal cysteine, which was proposed to be metal-assisted through accompanying reduction of ferric heme and formation of Fe(II)-NO. Here, we report the 1.6 Å crystal structure of cNP first chemically reduced and then exposed to NO, and show that Fe(II)-NO is formed but SNO is not, supporting a metal-assisted SNO formation mechanism. Crystallographic and spectroscopic studies of mutated cNP show that steric crowding of the proximal site inhibits SNO formation while a sterically relaxed proximal site enhances SNO formation, providing insight into specificity for this poorly understood modification. Experiments examining the pH dependence for NO implicate direct protonation of the proximal cysteine as the underlying mechanism. At lower pH, thiol heme ligation predominates, leading to a smaller trans effect and 60-fold enhanced NO affinity (Kd = 70 nM). Unexpectedly, we find that thiol formation interferes with SNO formation, suggesting cNP-SNO is unlikely to form in the insect salivary glands.

Efficient removal of chloroform from groundwater using activated percarbonate by cellulose nanofiber-supported Fe/Cu nanocomposites

Chloroform (CF) is a recalcitrant halogenated methane (HM) that has received widespread attention due to its frequent detection in groundwater and its potential carcinogenic risk. In this study, TEMPO-oxidized cellulose nanofiber-supported iron/copper bimetallic nanoparticles (TOCNF-Fe/Cu), a novel composite catalyst, was synthesized to activate sodium percarbonate (SPC) for the removal of CF from groundwater. The results showed that over 96.3% of CF could be removed in a neutral reaction medium (pH 6.5-9) within 180 min using 0.66 g L-1 of TOCNF (0.32)-Fe/Cu (1) and 1 mM of SPC, which outperforms typical advanced oxidation processes. The reaction mechanism of the TOCNF-Fe/Cu-SPC system for the CF removal was elucidated. As demonstrated through electron paramagnetic resonance and quenching experiments, the TOCNF-Fe/Cu-SPC system was found to include •OH and O2•-, where the latter played a dominant role in the CF removal. DFT calculations indicated that TOCNF improved the electron transport capability of Fe/Cu and reduced the transition state energy. The Fe species on the surface of TOCNF-Fe/Cu were identified as the primary active sites for SPC activation, whereas the Cu species were beneficial to the regeneration of the Fe species. Additionally, TOCNF-Fe/Cu was found to have good recyclability and stability. The feasibility of the TOCNF-Fe/Cu-SPC system was further confirmed by applying it for the efficient removal of composite HMs from actually contaminated groundwater. Overall, the TOCNF-Fe/Cu-SPC system is an attractive candidate for the treatment of HM-contaminated groundwater.

Behavior of organic components and the migration of heavy metals during sludge dewatering by different advanced oxidation processes via optical spectroscopy and molecular fingerprint analysis

A comparative study of the different advanced oxidation processes (Fe(II)-Oxone, Fe(II)-H2O2, and Fe(II)-NaClO) was carried out herein to analyze the characteristics of organic components and the migration of heavy metals in waste activated sludge. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, sludge dewaterability was significantly improved, however, sludge dewaterability was deteriorated by the Fe(II)-NaClO treatment. The enhanced sludge dewaterability by the Fe(II)-Oxone and Fe(II)-H2O2 treatments was strongly correlated with the shifted organic components, particularly proteins, in soluble extracellular polymeric substances (S-EPS), while the deteriorated sludge dewaterability by the Fe(II)-NaClO treatment was strongly correlated with the over release of organic components from bound EPS (B-EPS) to S-EPS. For both the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the radicals preferentially attacked humic acid-like organic components over the protein-like organic components in S-EPS, while for the Fe(II)-NaClO treatment, interestingly, the radicals preferentially attacked the protein-like organic components in both S-EPS and B-EPS. The hydrophilic functional groups like phenolic OH and CO of polysaccharides may be more preferentially migrated to S-EPS of sludge by the Fe(II)-NaClO treatment compared to the other two treatments. With the Fe(II)-Oxone and Fe(II)-H2O2 treatments, the proportion of aliphatic compounds as well as the much oxygenated organic components with a low desaturation and a low molecular weight increased. While with the Fe(II)-NaClO treatment, the proportion of low oxygenated organic components with a high desaturation and a high molecular weight increased. The concentration of total organic carbon, particularly the concentration of proteins, may be the key factor determining the shift of Zn and Cu from sludge solid to liquid phase, along with the high oxidation extent of organic components and close binding to CHOS and CHON compounds as indicated by density functional theory (DFT) calculation. This study systematically revealed the simultaneous sludge dewatering and migration of heavy metals when the role of organic components was factored into herein.

Atomic-Level Structural Differences between Fe(III) Coprecipitates Generated by the Addition of Fe(III) Coagulants and by the Oxidation of Fe(II) Coagulants Determine Their Coagulation Behavior in Phosphate and DOM Removal

In situ Fe(III) coprecipitation from Fe2+ oxidation is a widespread phenomenon in natural environments and water treatment processes. Studies have shown the superiority of in situ Fe(III) (formed by in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (using a Fe(III) coagulant directly) in coagulation, but the reasons remain unclear due to the uncertain nature of amorphous structures. Here, we utilized an in situ Fe(III) coagulation process, oxidizing the Fe(II) coagulant by potassium permanganate (KMnO4), to treat phosphate-containing surface water and analyzed differences between in situ and ex situ Fe(III) coagulation in phosphate removal, dissolved organic matter (DOM) removal, and floc growth. Compared to ex situ Fe(III), flocs formed by the natural oxidizing Fe2+ coagulant exhibited more effective phosphate removal. Furthermore, in situ Fe(III) formed through accelerated oxidation by KMnO4 demonstrated improved flocculation behavior and enhanced removal of specific types of DOM by forming a more stable structure while still maintaining effective phosphate removal. Fe K-edge extended X-ray absorption fine structure spectra (EXAFS) of the flocs explained their differences. A short-range ordered strengite-like structure (corner-linked PO4 tetrahedra to FeO6 octahedra) was the key to more effective phosphorus removal of in situ Fe(III) than ex situ Fe(III) and was well preserved when KMnO4 accelerated in situ Fe(III) formation. Conversely, KMnO4 significantly inhibited the edge and corner coordination between FeO6 octahedra and altered the floc-chain-forming behavior by accelerating hydrolysis, resulting in a more dispersed monomeric structure than ex situ Fe(III). This research provides an explanation for the superiority of in situ Fe(III) in phosphorus removal and highlights the importance of atomic-level structural differences between ex situ and in situ Fe(III) coprecipitates in water treatment.

Goethite Formed in the Periplasmic Space of Pseudomonas sp. JM-7 during Fe Cycling Enhances Its Denitrification in Water

Denitrification-driven Fe(II) oxidation is an important microbial metabolism that connects iron and nitrogen cycling in the environment. The formation of Fe(III) minerals in the periplasmic space has a significant effect on microbial metabolism and electron transfer, but direct evidence of iron ions entering the periplasm and resulting in periplasmic mineral precipitation and electron conduction properties has yet to be conclusively determined. Here, we investigated the pathways and amounts of iron, with different valence states and morphologies, entering the periplasmic space of the denitrifier Pseudomonas sp. JM-7 (P. JM-7), and the possible effects on the electron transfer and the denitrifying ability. When consistently provided with Fe(II) ions (from siderite (FeCO3)), the dissolved Fe(II) ions entered the periplasmic space and were oxidized to Fe(III), leading to the formation of a 25 nm thick crystalline goethite crust, which functioned as a semiconductor, accelerating the transfer of electrons from the intracellular to the extracellular matrix. This consequently doubled the denitrification rate and increased the electron transport capacity by 4-30 times (0.015-0.04 μA). However, as the Fe(II) concentration further increased to above 4 mM, the Fe(II) ions tended to preferentially nucleate, oxidize, and crystallize on the outer surface of P. JM-7, leading to the formation of a densely crystallized goethite layer, which significantly slowed down the metabolism of P. JM-7. In contrast to the Fe(II) conditions, regardless of the initial concentration of Fe(III), it was challenging for Fe(III) ions to form goethite in the periplasmic space. This work has shed light on the likely effects of iron on environmental microorganisms, improved our understanding of globally significant iron and nitrogen geochemical cycles in water, and expanded our ability to study and control these important processes.

Directed Evolution of an Iron(II)- and α-Ketoglutarate-Dependent Dioxygenase for Site-Selective Azidation of Unactivated Aliphatic C-H Bonds

FeII - and α-ketoglutarate-dependent halogenases and oxygenases can catalyze site-selective functionalization of C-H bonds via a variety of C-X bond forming reactions, but achieving high chemoselectivity for functionalization using non-native functional groups remains rare. The current study shows that directed evolution can be used to engineer variants of the dioxygenase SadX that address this challenge. Site-selective azidation of succinylated amino acids and a succinylated amine was achieved as a result of mutations throughout the SadX structure. The installed azide group was reduced to a primary amine, and the succinyl group required for azidation was enzymatically cleaved to provide the corresponding amine. These results provide a promising starting point for evolving additional SadX variants with activity on structurally distinct substrates and for enabling enzymatic C-H functionalization with other non-native functional groups.

Heavy-metal resistant bio-hybrid with biogenic ferrous sulfide nanoparticles: pH-regulated self-assembly and wastewater treatment application

Self-assembled bio-hybrids with biogenic ferrous sulfide nanoparticles (bio-FeS) on the cell surface are attractive for reduction of toxic heavy metals due to higher activity than bare bacteria, but they still suffer from slow synthesis and regeneration of bio-FeS and bacterial activity decay for removal of high-concentration heavy metals. A further optimization of the bio-FeS synthesis process and properties is of vital importance to address this challenge. Herein, we present a simple pH-regulation strategy to enhance bio-FeS synthesis and elucidated the underlying regulatory mechanisms. Slightly raising the pH from 7.4 to 8.3 led to 1.5-fold higher sulfide generation rate due to upregulated expression of thiosulfate reduction-related genes, and triggered the formation of fine-sized bio-FeS (29.4 ± 6.1 nm). The resulting bio-hybrid exhibited significantly improved extracellular reduction activity and was successfully used for treatment of high-concentration chromium -containing wastewater (Cr(VI), 80 mg/L) at satisfactory efficiency and stability. Its feasibility for bio-augmented treatment of real Cr(VI)-rich electroplating wastewater was also demonstrated, showing no obvious activity decline during 7-day operation. Overall, our work provides new insights into the environmental-responses of bio-hybrid self-assembly process, and may have important implications for optimized application of bio-hybrid for wastewater treatment and environmental remediation.

Comparison of peracetic acid and sodium hypochlorite enhanced Fe(Ⅱ) coagulation on algae-laden water treatment

In this study, Fe(Ⅱ)/peracetic acid (PAA) and Fe(Ⅱ)/sodium hypochlorite (NaClO) systems were applied as the combined preoxidation and coagulation process to enhance algae removal. A high removal rate of algae and turbidity could be achieved, with most algal cells keeping intact when adding reasonable concentrations of PAA and NaClO to enhance Fe(Ⅱ) coagulation. The variations of chlorophyll a, malondialdehyde, and intracellular reactive oxygen species suggested that moderate oxidation with only destroying surface-adsorbed organic matter rather than cell integrity was realized. The generated organic radicals, Fe(Ⅳ), and hydroxy radical played the major roles in the Fe(Ⅱ)/PAA system for the moderate oxidation of algal cells, but direct oxidation by NaClO rather than producing reactive species in the Fe(Ⅱ)/NaClO process contributed to the preoxidation. Concurrently, the in-situ formed Fe(Ⅲ) greatly promoted the agglomerating and settling of algae. The analysis of cell integrity, biochemical compositions, and fluorescence excitation-emission matrices spectra demonstrated that excess NaClO but not PAA would seriously damage the algal cells. This might be because that NaClO would directly oxidize the cell wall/membrane, while PAA mainly permeates into the cell to inactivate algae. These results suggest that Fe(Ⅱ)/PAA is an efficient strategy for algae-laden water treatment without serious algae lysis.

Inhibition of Extracellular Enzyme Activity by Reactive Oxygen Species upon Oxygenation of Reduced Iron-Bearing Minerals

The dual roles of minerals in inhibiting and prolonging extracellular enzyme activity in soils and sediments are governed by enzyme adsorption to mineral surfaces. Oxygenation of mineral-bound Fe(II) generates reactive oxygen species (ROS), yet it is unknown whether and how this process alters the activity and functional lifespan of extracellular enzymes. Here, the effect of mineral-bound Fe(II) oxidation on the hydrolytic activity of a cellulose-degrading enzyme β-glucosidase (BG) was studied using two pre-reduced Fe-bearing clay minerals (nontronite and montmorillonite) and one pre-reduced iron oxide (magnetite) at pH 5 and 7. Under anoxic conditions, BG adsorption to mineral surfaces decreased its activity but prolonged its lifespan. Under oxic conditions, ROS was produced, with the amount of •OH, the most abundant ROS, being positively correlated with the extent of structural Fe(II) oxidation in reduced minerals. •OH decreased BG activity and shortened its lifespan via conformational change and structural decomposition of BG. These results suggest that under oxic conditions, the ROS-induced inhibitory role of Fe(II)-bearing minerals outweighed their adsorption-induced protective role in controlling enzyme activity. These results disclose a previously unknown mechanism of extracellular enzyme inactivation, which have pivotal implications for predicting the active enzyme pool in redox-oscillating environments.

Facet Dependence of Biosynthesis of Vivianite from Iron Oxides by Geobacter sulfurreducens

Vivianite plays an important role in alleviating the phosphorus crisis and phosphorus pollution. The dissimilatory iron reduction has been found to trigger the biosynthesis of vivianite in soil environments, but the mechanism behind this remains largely unexplored. Herein, by regulating the crystal surfaces of iron oxides, we explored the influence of different crystal surface structures on the synthesis of vivianite driven by microbial dissimilatory iron reduction. The results showed that different crystal faces significantly affect the reduction and dissolution of iron oxides by microorganisms and the subsequent formation of vivianite. In general, goethite is more easily reduced by Geobacter sulfurreducens than hematite. Compared with Hem_{100} and Goe_L{110}, Hem_{001} and Goe_H{110} have higher initial reduction rates (approximately 2.25 and 1.5 times, respectively) and final Fe(II) content (approximately 1.56 and 1.20 times, respectively). In addition, in the presence of sufficient PO₄^3-, Fe(II) combined to produce phosphorus crystal products. The final phosphorus recoveries of Hem_{001} and Goe_H{110} systems were about 5.2 and 13.6%, which were 1.3 and 1.6 times of those of Hem_{100} and Goe_L{110}, respectively. Material characterization analyses indicated that these phosphorous crystal products are vivianite and that different iron oxide crystal surfaces significantly affected the size of the vivianite crystals. This study demonstrates that different crystal faces can affect the biological reduction dissolution of iron oxides and the secondary biological mineralization process driven by dissimilatory iron reduction.

Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs

Vivianite (Fe₃(PO₄)₂·8H₂O) crystallization has attracted increasing attention as a promising approach for removing and recovering P from wastewaters. However, Fe^II is susceptible to oxygen with its oxidation inevitably influencing the crystallization of vivianite. In this study, the profile of vivianite crystallization in the presence of dissolved oxygen (DO) was investigated at pHs 5-7 in a continuous stirred-tank reactor. It is found that the influence of DO on vivianite crystallization was highly pH-related. At pH 5, the low rate of Fe^II oxidation at all of the investigated DO of 0-5 mg/L and the low degree of vivianite supersaturation resulted in slow crystallization with the product being highly crystalline vivianite, but the P removal efficiency was only 30-40%. The removal of P from the solution was substantially more effective (to >90%) in the DO-removed reactors at pH 6 and 7, whereas the efficiencies of P removal and especially recovery decreased by 10-20% when Fe^II oxidation became more severe at DO concentrations >2.5 mg/L (except at pH 6 with 2.5 mg/L DO). The elevated degree of vivianite supersaturation and enhanced rate and extent of Fe^II oxidation at the higher pHs led to decreases in the size and homogeneity of the products. At the same pH, amorphous ferric oxyhydroxide (AFO)─the product of Fe^II oxidation and Fe^III hydrolysis─interferes with vivianite crystallization with the induction of aggregation of crystal fines by AFO, leading to increases in the size of the obtained solids.

Iron Vacancy Accelerates Fe(II)-Induced Anoxic As(III) Oxidation Coupled to Iron Reduction

Chemical oxidation of As(III) by iron (Fe) oxyhydroxides has been proposed to occur under anoxic conditions and may play an important role in stabilization and detoxification of As in subsurface environments. However, this reaction remains controversial due to lack of direct evidence and poorly understood mechanisms. In this study, we show that As(III) oxidation can be facilitated by Fe oxyhydroxides (i.e., goethite) under anoxic conditions coupled with the reduction of structural Fe(III). An excellent electron balance between As(V) production and Fe(III) reduction is obtained. The formation of an active metastable Fe(III) phase at the defective surface of goethite due to atom exchange is responsible for the oxidation of As(III). Furthermore, the presence of defects (i.e., Fe vacancies) in goethite can noticeably enhance the electron transfer (ET) and atom exchange between the surface-bound Fe(II) and the structural Fe(III) resulting in a two time increase in As(III) oxidation. Atom exchange-induced regeneration of active goethite sites is likely to facilitate As(III) coordination and ET with structural Fe(III) based on electrochemical analysis and theoretical calculations showing that this reaction pathway is thermodynamically and kinetically favorable. Our findings highlight the synergetic effects of defects in the Fe crystal structure and Fe(II)-induced catalytic processes on anoxic As(III) oxidation, shedding a new light on As risk management in soils and subsurface environments.

Incorporation of Cu into Goethite Stimulates Oxygen Activation by Surface-Bound Fe(II) for Enhanced As(III) Oxidative Transformation

The dark production of reactive oxygen species (ROS) coupled to biogeochemical cycling of iron (Fe) plays a pivotal role in controlling arsenic transformation and detoxification. However, the effect of secondary atom incorporation into Fe(III) oxyhydroxides on this process is poorly understood. Here, we show that the presence of oxygen vacancy (OV) as a result of Cu incorporation in goethite substantially enhances the As(III) oxidation by Fe(II) under oxic conditions. Electrochemical and density functional theory (DFT) evidence reveals that the electron transfer (ET) rate constant is enhanced from 0.023 to 0.197 s^-1, improving the electron efficiency of the surface-bound Fe(II) on OV defective surfaces. The cascade charge transfer from the surface-bound Fe(II) to O₂ mediated by Fe(III) oxyhydroxides leads to the O-O bond of O₂ stretching to 1.46-1.48 Å equivalent to that of superoxide (^•O₂^-), and ^•O₂^- is the predominant ROS responsible for As(III) oxidation. Our findings highlight the significant role of atom incorporation in changing the ET process on Fe(III) oxyhydroxides for ROS production. Thus, such an effect must be considered when evaluating Fe mineral reactivity toward changing their surface chemistry, such as those noted here for Cu incorporation, which likely determines the fates of arsenic and other redox sensitive pollutants in the environments with oscillating redox conditions.

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.

Ferrocene Derivatives as New Generation of Antimalarial Agents: Opportunity or Illusion?

Despite significant scientific progress over the last two decades, malaria remains a global burden that causes thousands of deaths every year. In the absence of effective and practical preventive measures, the only current option for reducing the mortality and morbidity of malaria is chemotherapy. However, due to the minimal stock of active antiparasitic analogs, issues of toxicity, and the repeated appearance of drug resistance, scientists must broaden the arsenal of existing therapies beyond conventional medicinal chemistry. To curb this menace, a series of potential metal-based hybrids have been synthesized and screened. Ferrocene is one of the potent organometallic candidates and the hybridization of ferrocene with other pharmacophores results in compounds with enhanced biological activities. Many researchers have reported the ferrocene compounds as potent pharmacophores and useful as anticancer and antimalarial agents when hybridized with other pharmaceutical hybrids. Drug, such as Ferroquine (FQ, SSR97193), is currently the most advanced organometallic compound developed from the hybridization of ferrocene and chloroquine and has demonstrated great potency in clinical trials against both drug-sensitive and drug-resistant malaria. Not only ferroquine but its derivatives have shown significant activity as antimalarial agents. The present review focuses on the discovery of FQ, the hypothesis of its mode of action, and recent clinical trials of ferrocene compounds as a new class of antimalarial agents. The structure-activity relationship (SAR) of ferrocene derivatives is also discussed to provide insight into the rational design of more effective antimalarial candidates. Finally, efforts have been made to discuss the future expectations for ferrocene-based antimalarial drugs.

Simultaneous elimination and detoxification of arsenite in the presence of micromolar hydrogen peroxide and ferrous and its environmental implications

Experiments for simultaneous elimination and detoxification of microgram level of As(Ⅲ) in the presence of micromolar H2O2 and Fe(Ⅱ) which are frequently encountered in natural water were conducted. The results showed that the molar ratio of oxidant to As(III) plays important role in As(III) oxidation under the experimental conditions. The extent of As(Ⅲ) oxidation with single H2O2 or Fe(Ⅱ) ranged from 7.9 % to 60.3 % and 22.2-46.6 %, respectively. Treatments with H2O2/As(Ⅲ) molar ratios in the range 150: 1-750: 1 or Fe(Ⅱ)/As(Ⅲ) molar ratios in the range 37.5: 1-375: 1 were more favor for As(Ⅲ) oxidation respectively, and increasing oxidant concentration did not result in complete As(Ⅲ) oxidation. As(Ⅲ) was completely oxidized and eliminated following the precipitation of ferric hydroxides in 5 reaction minutes when H2O2 and Fe(Ⅱ) coexisted in the reaction system. The interface characterization for the reacted precipitates after the experiment were conducted by using a high-resolution field emission scanning electron microscopy (SEM) coupled with an EX-350 energy dispersive X-ray spectrometer (EDX) and X-ray photoelectron spectroscopy (XPS), respectively. The results showed that As(Ⅴ) was the merely arsenic species and As oxide primary situated in the subsurface layer of the reacted precipitates, whereas Fe was more concentrated in the outermost surface layer. Our research showed that H2O2 and Fe(Ⅱ) at natural level may exert significant influence on arsenic mobilization in natural water. Considering the much more toxic of As(Ⅲ) than that of As(Ⅴ), the research also provide us an environmental friendly choice in the elimination and detoxification of microgram As(Ⅲ) in drinking water.

Speciation analysis and formation mechanism of iron-phosphorus compounds during chemical phosphorus removal process

Iron (Fe) salt was applied extensively to remove phosphorus (P) in wastewater treatment plants (WWTPs). Exploring the formation mechanism of iron-phosphorus compounds (FePs) during the chemical P removal (CPR) process is beneficial to P recovery. In this study, the performance of P removal, FePs speciation analysis and the kinetics of P removal under different conditions (pH, Fe/P molar ratio (Fe/P_mol), type of Fe salt, dissolved organic matters) were comprehensively investigated. More than 95% of P was removed under the optimal conditions with pH = 4.7, Fe/P_mol = 2, FeCl₃ or polymeric ferric sulfate (PFS) as the coagulant. The FePs formation mechanism was considerably influenced by reaction conditions. Iron-phosphate compounds were the dominant FePs species (>76%) at pH < 6.2, while more iron oxides were formed at pH ≥ 6.2 with decreased P removal efficiency. When the initial Fe/P_mol was 2, iron-phosphate compound was the only product that was formed by the reaction between PO₄^3- and Fe(III) or Fe(II) ions directly. More iron oxides were generated when the initial Fe/P_mol was 1 or 3. At Fe/P_mol = 1, the Fe(III) was hydrolyzed to form iron oxides and trapped PO₄^3-, while at Fe/P_mol = 3, iron-phosphate compounds were produced firstly and the remaining Fe(III) was hydrolyzed to form iron oxides. The pseudo-second-order kinetic model simulated the chemical P removal process well. The reaction rate of P with Fe(II) was slower than that with Fe(III), but complete removal was still achieved when the reaction time was more than 30 min. Poly-Fe salt exhibited a fast P removal rate, while the removal efficiency depended on its iron content. Organic matters in wastewater with large molecular weight and multiple functional groups (such as humic acids) inhibited P removal rate but hardly affect the removal amount. This study provides an insight into CPR by Fe salts and is beneficial for P recovery in WWTPs.

Influence of aluminate and silicate on selenate immobilization using alkaline-earth metal oxides and ferrous salt

Effects of aluminate and silicate species on the SeO₄^2- immobilization using alkali-earth metal oxides and ferrous species have not been clearly elucidated. In the present study, Al and Si species were separately added into MgO/Fe(II) and CaO/Fe(II) reactions containing SeO₄^2-, studied by toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), X-ray absorption fine structure (XAFS), and PHREEQC simulation. Approximately 42 % of SeO₄^2- was reduced to SeO₃^2- for MgO/Fe(II) reaction in the presence of Al species, being consistent with the case without Al species. The Al species only showed slight inhibition of Se leaching for the MgO/Fe(II) reaction. Most of Se oxyanions were adsorbed onto Mg(OH)₂ through outer-sphere complexation. For CaO/Fe(II) reaction, all of SeO₄^2- was reduced to SeO₃^2- with or without Al species. However, the Se leaching amount (3 %) of sample added with Al species (CE3) is much lower than that (12 %) of sample without Al species (CE2). This is mainly because SeO₃^2- can be sorbed onto the iron-based minerals through binuclear bidentate corner-sharing (²C) complexation instead of monodentate mononuclear corner-sharing (¹V) complexation of the case without Al species. On the other hand, SeO₄^2- was not reduced to SeO₃^2- in the presence of silicate, and almost all of Se was leached out for silicate-contained samples except CaO/Fe(II) reaction with the addition of Al species. This is due to the polymerization of Al and Si species under a high-alkalinity environment, thereby stabilizing SeO₄^2- in the amorphous silicon-aluminum structure and contributing to the decrease of Se leaching.

Mechanism of the improved Fe(III)/persulfate reaction by gallic acid for ibuprofen degradation

Gallic acid (GA), a natural plant polyphenol, was applied as amendment of Fe(III)/persulfate (PS) system for ibuprofen (IBP) degradation in this study. The impacts of all agentia (GA, Fe(III), PS) concentration and initial pH values on IBP removal efficiency were investigated, and their corresponding observed pseudo-first-order rate constants (k_obs) were calculated. The addition of GA has significantly improved elimination efficiency of IBP due to the enhanced Fe(III)/Fe(II) cycle. Electron paramagnetic resonance (EPR) results confirmed that SO₄^•-, HO^• and O₂^•- were involved in GA/Fe(III)/PS system. However, quenching experiments further affirmed the impact of SO₄^•- and HO^• towards IBP decomposition instead of O₂^•-, with contribution ratio to IBP removal was 69.12% and 30.88%, respectively. SO₄^•- was the main radicals formed by directly activation of PS with Fe(II), while HO^• was the transformation product of SO₄^•-. Based on instrumental analysis (stopped-flow UV-vis spectrum and MS) and theoretical calculation, the potential reaction mechanism between GA and Fe(III) in the presence of PS was further proposed. GA complexed with Fe(III) firstly and the Fe(III)-GA complex was then converted into quinone substance, accompanied by the generation of Fe(II). Furthermore, the application of GA extended the optimal pH range to neutral as well, which made it a promising treatment in practical application.

The nature of metal atoms incorporated in hematite determines oxygen activation by surface-bound Fe(II) for As(III) oxidation

The incorporation of secondary metal atoms into iron oxyhydroxides may regulate the surface chemistry of mediating electron transfer (ET) and, therefore, the biogeochemical pollutant processes such as arsenic (As) in the subsurface and soils. The influence of incorporating two typical metals (Cu and Zn) into a specific {001} hematite facet on O₂ activation by surface-bound Fe(II) was addressed. The results showed that Cu-incorporated hematite enhances As(III) oxidation in the presence of Fe(II) under oxic conditions and increases with increasing Cu content. Conversely, Zn incorporation leads to the opposite trend. The As(III) oxidation induced by surface-bound Fe(II) is positively related to the Fe(II) content and is favorable under acidic conditions. Reactive oxygen species (ROS), such as superoxide (·O^2-) and H₂O₂, predominantly contribute to As(III) oxidation as a result of 1-electron transfer from bound Fe(II) to surface O₂ on hematite and radical propagation. Electrochemical analysis demonstrates that Cu incorporation significantly lower the oxidation potential of Fe(II) on hematite, whereas Zn led to a higher reaction potential for Fe(II) oxidation. Subsequently, distinct surface reactivities of hematite for the activation of O₂ to form ROS by surface-bound Fe(II) are evidenced by metal incorporation. Our study provides a new understanding of the changes in the surface chemistry of iron oxyhydroxides because of incorporating metals (Zn and Cu), and therefore impact the biogeochemical processes of pollutants in soils and subsurface environments.

Molecular Hybridization Strategy on the Design, Synthesis, and Structural Characterization of Ferrocene-N-acyl Hydrazones as Immunomodulatory Agents

Immunomodulatory agents are widely used for the treatment of immune-mediated diseases, but the range of side effects of the available drugs makes necessary the search for new immunomodulatory drugs. Here, we investigated the immunomodulatory activity of new ferrocenyl-N-acyl hydrazones derivatives (SintMed(141−156). The evaluated N-acyl hydrazones did not show cytotoxicity at the tested concentrations, presenting CC50 values greater than 50 µM. In addition, all ferrocenyl-N-acyl hydrazones modulated nitrite production in immortalized macrophages, showing inhibition values between 14.4% and 74.2%. By presenting a better activity profile, the ferrocenyl-N-acyl hydrazones SintMed149 and SintMed150 also had their cytotoxicity and anti-inflammatory effect evaluated in cultures of peritoneal macrophages. The molecules were not cytotoxic at any of the concentrations tested in peritoneal macrophages and were able to significantly reduce (p < 0.05) the production of nitrite, TNF-α, and IL-1β. Interestingly, both molecules significantly reduced the production of IL-2 and IFN-γ in cultured splenocytes activated with concanavalin A. Moreover, SintMed150 did not show signs of acute toxicity in animals treated with 50 or 100 mg/kg. Finally, we observed that ferrocenyl-N-acyl hydrazone SintMed150 at 100 mg/kg reduced the migration of neutrophils (44.6%) in an acute peritonitis model and increased animal survival by 20% in an LPS-induced endotoxic shock model. These findings suggest that such compounds have therapeutic potential to be used to treat diseases of inflammatory origin.

Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

The new nonheme iron complexes FeII(BNPAPh2O)(N3) (1), FeIII(BNPAPh2O)(OH)(N3) (2), FeII(BNPAPh2O)(OH) (3), FeIII(BNPAPh2O)(OH)(NCS) (4), FeII(BNPAPh2O)(NCS) (5), FeIII(BNPAPh2O)(NCS)2 (6), and FeIII(BNPAPh2O)(N3)2 (7) (BNPAPh2O = 2-(bis((6-(neopentylamino)pyridin-2-yl) methyl)amino)-1,1-diphenylethanolate) were synthesized and characterized by single crystal X-ray diffraction (XRD), as well as by 1H NMR, 57Fe Mössbauer, and ATR-IR spectroscopies. Complex 2 was reacted with a series of carbon radicals, ArX3C· (ArX = p-X-C6H4), analogous to the proposed radical rebound step for nonheme iron hydroxylases and halogenases. The results show that for ArX3C· (X = Cl, H, tBu), only OH· transfer occurs to give ArX3COH. However, when X = OMe, a mixture of alcohol (ArX3COH) (30%) and azide (ArX3CN3) (40%) products was obtained. These data indicate that the rebound selectivity is influenced by the electron-rich nature of the carbon radicals for the azide complex. Reaction of 2 with Ph3C· in the presence of Sc3+ or H+ reverses the selectivity, giving only the azide product. In contrast to the mixed selectivity seen for 2, the reactivity of cis-FeIII(OH)(NCS) with the X = OMe radical derivative leads only to hydroxylation. Catalytic azidation was achieved with 1 as catalyst, λ3-azidoiodane as oxidant and azide source, and Ph3CH as test substrate, giving Ph3CN3 in 84% (TON = 8). These studies show that hydroxylation is favored over azidation for nonheme iron(III) complexes, but the nature of the carbon radical can alter this selectivity. If an OH· transfer pathway can be avoided, the FeIII(N3) complexes are capable of mediating both stoichiometric and catalytic azidation.

Evaluation of iron-loaded granular activated carbon used as heterogeneous fenton catalyst for degradation of tetracycline

To optimize the efficiency of general adsorption-Fenton oxidation treatment, iron-loaded granular activated carbon (Fe-GAC) was prepared, characterized, and used as a catalyst in the heterogeneous Fenton oxidation of tetracycline (TC). Characterization revealed that the Fe(II) was successfully introduced onto the original granular activated carbon (GAC) and diversified the materials' surface morphology and elemental compounds. Under an initial pH of 3.0, the Fe-GAC/Fenton system obtained a maximum removal rate of 92.6%, with hydrogen peroxide (H₂O₂) dosages of 9 mmol g^-1. And the GAC/Fenton without iron supplementation was 89.5%, with H₂O₂ dosages of 8 mmol g^-1. Additionally, the Fe-GAC/Fenton system consumed a lower Fe(II) dosage than GAC/Fenton, with Fe(II)/H₂O₂ molar ratios of 0.007:1 and 0.04:1, respectively. Analysis of total organic carbon demonstrated higher mineralization efficiency in the Fe-GAC/Fenton system (67.2%), which was approximately 1.3 times of GAC/Fenton. Desorption experiments showed that the adsorption and degradation accounted for 19.22% and 80.78% of the total TC removal by GAC/Fenton, and 10.58% and 89.42% in the Fe-GAC/Fenton system, respectively. Electron paramagnetic resonance (EPR) technique and quenching experiments demonstrated that the dominant reactive oxygen species (ROS) in synergistic treatments were hydroxyl (•OH) and hydroxy peroxyl (HO₂•) radicals. In addition, three potential degradation pathways for TC were proposed according to the detected fourteen intermediates. Catalyst regeneration treatments were evaluated over six cycles, and the regeneration was 6.5% higher with the iron-supplemented carbon granules. Overall, the Fe-GAC can be used as an efficient catalyst in practical water treatment, and this study demonstrated a promising method to develop adsorption-Fenton technology.

Improved Dewaterability of Waste Activated Sludge by Fe(II)-Activated Potassium Periodate Oxidation

Fe(II)-activated potassium periodate (KIO₄) oxidation was used to improve the dewaterability of waste-activated sludge for the first time. Compared with those of raw sludge, the capillary suction time (CST), specific resistance filtration (SRF), and water content of filter cake (W_C) of sludge treated using the Fe(II)/KIO₄ process under the optimal conditions (i.e., the initial pH = 6.8, KIO₄ dose = 1.4 mmol/g volatile suspended solids, Fe(II)/KIO₄ molar ratio = 1.2) decreased by 64.34%, 84.13%, and 6.69%, respectively. For conditioned sludge flocs, the Zeta potential and particle size were increased, and hydrophilic proteins in extracellular polymeric substances (EPS) were partly degraded, accompanied by the transformation of tightly bound EPS into soluble EPS and the conversion of dense sludge flocs into loose and porous ones. During Fe(II)/KIO₄ oxidation, Fe(IV) and the accompanying ^•OH were determined as the predominant reactive species and the underlying mechanism of sludge EPS degradation was proposed. This work provides a prospective method for conditioning the sludge dewaterability.

Production of Hydroxyl Radicals from Oxygenation of Simulated Acid Mine Drainage in Presence of Extracellular Polymers Substances

The reaction mechanisms Fe(II) abiotic oxidation produce ·OH by CaCO₃-induced in acid mine drainage (AMD) are well-documented, but little is known about the influence of extracellular polymeric substances (EPS) secreted by microorganisms on Fe(II) oxidation in AMD. In this study, ·OH production was experimently measured from oxygenation of simulated AMD in the presence of EPS. The cumulative ·OH increased from 56.75 to 158.70 µM within 24 h at pH 3 with the increase in EPS concentration from 0 to 12 mg/L. An appropriate pH (about 6) and EPS (6 mg/L) concentration were required for the moderate rate of Fe(II) oxidation. Besides, the yield of ·OH increased remarkably with the addition of Fe³⁺. In the presence of EPS, ·OH production is attributed mainly the complexation of Fe(II) with EPS, of which is rich of carboxyl and hydroxyl groups. The findings provide fundamental supplement of ·OH production from Fe(II) oxidation by microorganisms in natural AMD.

The role of Fe dissolution in olivine-hydroxylamine-induced Fenton reaction for enhanced oxidative degradation of organic pollutant

In this study, a dye pollutant (methyl orange, MO) was effectively oxidized in a hydroxylamine (HA)-assisted Fenton system using various Al/Si/Fe- and Fe-containing minerals. The fastest degradation kinetics of MO were observed in the olivine-HA Fenton system, whereas other Al/Si/Fe and Fe-rich minerals (magnetite and lepidocrocite) demonstrated much slower degradation kinetics. The degradation rate constants were proportional to dissolved Fe(II) quantities in mineral suspensions (R² = 0.98), indicating the crucial role of dissolved Fe(II) quantity in HA-assisted Fenton reactions. Radical scavenging and electron spin resonance results revealed that MO was dominantly oxidized by ·HO produced in the olivine-HA Fenton system. The continuous production of aqueous Fe(II) via direct Fe(II) dissolution at a pH of 3 and further Fe dissolution from the reductive dissolution of surface Fe(III) by HA was the main driving force for efficient MO degradation. Furthermore, lowering the pH by the addition of hydroxylamine hydrochloride resulted in the effective removal of MO under various pH conditions (3-9), indicating the additional advantage of HA use in Fenton reactions. Liquid chromatography-mass spectroscopy analysis revealed that the cleavage of C-N and C-C bonds, demethylation, hydroxylation, and dehydroxylation were the main processes for MO oxidation in the olivine-HA Fenton system.