Mechanistic insights into surface catalytic oxidation of fluoroquinolone antibiotics on sediment mackinawite

Unveiling the reaction pathways in the degradation mechanism of enrofloxacin by hydroxyl radicals: A DFT and experiment study

Fluoroquinolone antibiotics, widely used in daily life, contribute to environmental pollution due to their persistence in natural ecosystems. However, the degradation mechanism of fluoroquinolones remains elusive, which not only hinders the understanding of their environmental behavior but also restricts the development of effective remediations. This study investigates the degradation mechanism of enrofloxacin (ENR) through hydroxyl radicals (•OH), integrating density functional theory (DFT) calculations and experimental validations. The degradation process involves key steps such as bond activation (C-F, C-H, C-C) and decarboxylation, with the C-F bond and decarboxylation identified as rate-limiting steps. Experimental results confirm the theoretical predictions of degradation pathways and major by-products. Toxicity analysis shows that most degradation products exhibit significantly reduced toxicity compared to ENR. This work provides valuable insights into the degradation behavior of fluoroquinolones and lays the groundwork for designing advanced environmental remediation strategies.

Manipulating Charge Dynamics in Carbon Nitride by Carbon Dot Doping for Efficient Photocatalysis

Graphitic carbon nitride (g-C3N4), a prominent metal-free semiconductor photocatalyst, faces limitations due to its high exciton binding energy. While significant efforts have been focused on optimizing charge-carrier processes, the interplay of exciton and free carrier in this system have received less attention. Herein, this density-functional theory (DFT) and time-dependent DFT calculations demonstrate that carbon dot-functionalized g-C3N4 (g-C3N4/CD), synthesized via a facile thermal polymerization, shifts the excited state from localized to charge transfer characteristics. The g-C3N4/CD exhibits reduced exciton binding energy from 41.0 to 24.6 meV, as shown by temperature-dependent photoluminescence spectroscopy. Particularly, g-C3N4/CD-10 (10 wt.% CD solution in precursors) achieves a 3-fold increase in the photodegradation rate (k = 0.020 min⁻¹) of an emerging environmental pollutant, levofloxacin (LEV), under 10 W LED light. Enhanced photocatalytic performances correlate with optimized band structure and efficient charge transport, as confirmed by photophysical and photoelectrochemical analyses. Although the excited state lifetime in g-C3N4/CD is slightly reduced compared to pristine g-C3N4, photocatalytic activity remains unaffected, underscoring the critical role of charge excited state in enhancing photocatalytic efficiency. This work offers insights onto the potential of manipulating charge transfer excited state dynamics for improved g-C3N4-based photocatalysis in environmental applications.

Surface catalytic degradation of ciprofloxacin by ferrihydrite sulfidation under ambient conditions

Dissolved sulfide (S(-II)) is abundant in sediments and capable of initiating the sulfidation reactions of iron-bearing minerals, in which the reaction mechanisms have been well documented. However, the impact of the S(-II)/Fe concentration ratio on reactive oxygen species (ROS) formation and the fate of co-existing contaminants upon iron-bearing minerals sulfidation under ambient conditions remains inadequately explored. Herein, the transformation of ciprofloxacin (CIP) by ferrihydrite sulfidation under ambient conditions was systematically investigated. Our findings indicated that the rate of CIP degradation initially increased with rising S(-II)/Fe concentration ratios, but subsequently decreased as the ratio continued to elevate. Evidence from electron paramagnetic resonance, molecular probing and quenching experiments revealed that the superoxide anion radical (O2•-), primarily produced from the reaction between O2 and surface-bound Fe(II), was the dominant contributor to the accelerated transformation of CIP. Upon being attacked by ROS, CIP underwent degradation via carboxyl substitution, defluorination, hydroxylation, piperazine ring ketonization and piperazine ring cleavage. Additionally, common water quality factors, i.e., pH, natural organic matter (NOM) and inorganic anions could also significantly influence the degradation processes of CIP during ferrihydrite sulfidation under ambient conditions. This research offers valuable insights into the significant function of mineral sulfidation in facilitating the formation and reactivity of ROS, which sheds light on enhanced elimination of organic contaminants in sediments.

Simultaneous removal of hexavalent chromium and tetracycline from simulated wastewater by FexSy impregnated hydrogel

Hexavalent chromium (Cr(VI)) and tetracycline (TC) have become widely distributed pollutants, and their combined pollution has attracted widespread attention from researchers. In this study, FexSy particles impregnated hydrogels were prepared to simultaneously remove Cr(VI) and TC from simulated wastewater. The effects of the initial concentration of pollutants, pH value, and coexisting dissolved organic matter on the removal rate of pollutants in single pollutant systems and dual pollutant systems were investigated in detail. In single pollutant systems, Cr(VI) was mainly reduced to Cr(III) by Fe(II) and S(-II), and then separated from the solution by complexation or precipitation. And TC firstly formed the Fe (II)-TC complex with Fe (II), which was subsequently oxidized to form Fe(III)-TC complex and generated O2⋅-, leading to the degradation of TC. S(-II) mainly played a role in reducing Fe (III), promoting the circulation between Fe(II) and Fe(III). The co-removal mechanism of Cr(VI) and TC was further understood in dual pollutant systems. The degradation pathway of TC was proposed through HPLC-MS analysis. Overall, our work provided economical and effective materials for simultaneously removing Cr(VI) and TC from simulated wastewater.

Fluoroquinolone antibiotics in the aquatic environment: environmental distribution, the research status and eco-toxicity

The increasing presence of fluoroquinolone (FQ) antibiotics in aquatic environments is a growing concern due to their widespread use, negatively impacting aquatic organisms. This paper provides an overview of the environmental distribution, sources, fate, and both single and mixed toxicity of FQ antibiotics in aquatic environments. It also examines the accumulation of FQ antibiotics in aquatic organisms and their transfer into the human body through the food chain. The study identifies critical factors such as metabolism characteristics, physiochemical characteristics, light, temperature, dissolved oxygen, and environmental compatibility that influence the presence of FQ antibiotics in aquatic environments. Mixed pollutants of FQ antibiotics pose significant risks to the ecological environment. Additionally, the paper critically discusses advanced treatment technologies designed to remove FQ antibiotics from wastewater, focusing on advanced oxidation processes (AOPs) and electrochemical advanced oxidation processes (EAOPs). The discussion also includes the benefits and limitations of these technologies in degrading FQ antibiotics in wastewater treatment plants. The paper concludes by proposing new approaches for regulating and controlling FQ antibiotics to aid in the development of ecological protection measures.

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

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

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

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

Fungal biomineralization of toxic metals accelerates organic pollutant removal

Fungal biomineralization plays an important role in the biogeochemical cycling of metals in the environment and has been extensively explored for bioremediation and element biorecovery. However, the cellular and metabolic responses of fungi in the presence of toxic metals during biomineralization and their impact on organic matter transformations are unclear. This is an important question because co-contamination by toxic metals and organic pollutants is a common phenomenon in the natural environment. In this research, the biomineralization process and oxidative stress response of the geoactive soil fungus Aspergillus niger were investigated in the presence of toxic metals (Co, Cu, Mn, and Fe) and the azo dye orange II (AO II). We have found that the co-existence of toxic metals and AO II not only enhanced the fungal biomineralization of toxic metals but also accelerated the removal of AO II. We hypothesize that the fungus and in situ mycogenic biominerals (toxic metal oxalates) constituted a quasi-bioreactor, where the biominerals removed organic pollutants by catalyzing reactive oxygen species (ROS) generation resulting from oxidative stress. We have therefore demonstrated that a fungal/biomineral system can successfully achieve the goal of toxic metal immobilization and organic pollutant decomposition. Such findings inform the potential development of fungal-biomineral hybrid systems for mixed pollutant bioremediation as well as provide further understanding of fungal organic-inorganic pollutant transformations in the environment and their importance in biogeochemical cycles.

Physiological responses and removal mechanisms of ciprofloxacin in freshwater microalgae

Antibiotics, such as ciprofloxacin (CIP), are frequently detected in various environmental compartments, posing significant risks to ecosystems and human health. In this study, the physiological responses and elimination mechanisms of CIP in Chlorella sorokiniana and Scenedesmus dimorphus were determined. The exposure CIP had a minimal impact on the growth of microalgae, with maximum inhibit efficiency (IR) of 5.14% and 22.74 for C. sorokiniana and S. dimorphus, respectively. Notably, the photorespiration in S. dimorphus were enhanced. Both microalgae exhibited efficient CIP removal, predominantly through bioaccumulation and biodegradation processes. Intermediates involved in photolysis and biodegradation were analyzed through Liquid Chromatography High Resolution Mass Spectrometer (HPLC-MS/MS), providing insights into degradation pathways of CIP. Upregulation of key enzymes, such as dioxygenase, oxygenase and cytochrome P450, indicated their involvement in the biodegradation of CIP. These findings enhance our understanding of the physiological responses, removal mechanisms, and pathways of CIP in microalgae, facilitating the advancement of microalgae-based wastewater treatment approaches, particularly in antibiotic-contaminated environments.

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.

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.

Dramatically enhanced phenol degradation upon FeS oxygenation by low-molecular-weight organic acids

Oxidizing potential of FeS for organic contaminants degradation due to hydroxyl radicals (•OH) production has been recently documented, but the oxidizing efficiency was limited. Here, we revealed that low-molecular-weight organic acids (LMWOAs) can immensely enhance phenol degradation during FeS oxygenation due to increased utilization efficiency of FeS electron for •OH production. Upon oxygenation of 0.5 g/L FeS, phenol degradation boosted from 7.1% without LMWOAs to 91.5%, 84.6% and 95.0% with the addition of 1 mM oxalate, citrate and EDTA, respectively. Electron utilization efficiency of Fe(II) for •OH production dramatically rose from 0.3% with FeS alone to respective 2.0%, 2.5% and 2.7% in the LMWOAs systems. An increase in oxalate concentrations benefited •OH formation and phenol degradation. Coexisting oxalate led to an additional •OH production pathway from Fe(II)-oxalate oxidation, which expanded the O2 reduction to H2O2 from a two- to one-electron transfer process. Meanwhile, electron transfer from FeS to dissolved Fe(III)-oxalate promoted the redox cycling of Fe(III)/Fe(II), thus supplying the Fe(II) oxidation for •OH production. Moreover, the presence of oxalate decreased the crystallinity and particles size of lepidocrocite generated from FeS oxidation. Consequently, this study shed lights on the LMWOAs-enhanced contaminant degradation in either natural or engineered FeS oxidation systems.