Hydroxyl Radical-Involving p-Nitrophenol Oxidation during Its Reduction by Nanoscale Sulfidated Zerovalent Iron under Anaerobic Conditions

Depassivation of Zero-Valent Iron through Continuous Surface Renewal with Mechanochemical Processing

Zero-valent iron (ZVI) is a solid reductant that can react with a wide array of contaminants; however, its effectiveness is significantly reduced by the surface passivation layer formed on its surface. This study introduces mechanochemical depassivation to revitalize passivated ZVI and restore its reactivity. The method is exemplified through mechanical milling to depassivate ZVI with different passivation layers formed in solutions containing Cr(VI), Ni(II), and/or trichloroethylene, as well as in a complex water matrix. The results demonstrate rapid restoration of ZVI reactivity within 10 min of milling, achieving a 7-fold increase in contaminant removal capacity after eight cycles. The method universally outperforms acid pickling and ultrasonication across all of the tested passivation scenarios. Mechanistic analysis attributes its efficacy to the ductile-brittle disparity between the metallic iron and the passivation layers: brittle passivation layers fragment under mechanical stress, while ductile iron resists structural damage. Discrete element method simulations provide further mechanical insights into the milling dynamics and identify the contact frequency as the key factor determining depassivation efficiency. The study provides a versatile, environmentally benign, and effective depassivation method for ZVI reactions that can be integrated into ZVI reuse processes for water treatment applications.

Insights into the Restoration of Micropolluted Groundwater by Pyrite: The Contribution of Fe(IV) and Outer-Sphere Electron Transfer

Pyrite has been investigated for its potential to modulate the redox microenvironment of groundwater porous media through self-activation. However, the self-purification process of the contaminants by pyrite after their migration from surface water to groundwater has been neglected. This process is accompanied by a decrease in pollutant concentration with a transition from aerobic to anaerobic environments. Here, we selected sulfamethoxazole (SMX), a micropollutant frequently detected in groundwater, as a modeled micropollutant for the investigation. The findings indicate that pyrite could degrade micropollutants SMX (20 μg/L) by self-activation with nearly 100% degradation efficiency under anaerobic conditions. It was also found that •OH was not the primary reactive oxygen species (ROS), but rather the longer-lived and more stable Fe(IV) generated by •OHad-mediated oxidation of structural Fe(III). Additionally, SMX can be degraded by outer-sphere electron transfer with dissolved Fe3+ in the system. Whereas, the reduction product Fe2+ facilitates the supply of electrons to pyrite and promotes the production of Fe (IV). The contributions of Fe(IV) and outer-sphere electron transfer to SMX degradation were 67.5% and 32.5%, respectively. Furthermore, pyrite self-activation exhibited selective oxidation of electron-rich pollutants under anaerobic conditions. This finding provides a new insight into the self-purification of micropollutants in groundwater environments.

Controlled Synthesis of the FeB Nanometallic Glasses with Stronger Electron Donating Capability to Activate Molecular Oxygen for the Enhanced Ferroptosis Therapy

Considering the strong electron-donating ability and the superior biocompatibility, the integration of zero-valent iron nanostructure Fe0 (electron-reservoir) and zero-valent boron nanostructure B0 offers great promise for fabricating novel ferroptosis nanoagents. Nevertheless, the controlled and facile synthesis of alloyed Fe0 and B0 nanostructure-FeB nanometallic glasses (NMGs) has remained a long-standing challenge. Herein, a complexion-reduction strategy is proposed for the controlled synthesis of FeB NMGs with greater electron donating capacity to activate the molecular oxygen for improved ferroptosis therapy. In-depth mechanism reveales that the complexion-reduction strategy effectively prevent the long-range diffusion of Fe0, resulting in the amorphous alloyed Fe0 and B0 nanostructure-FeB nanoparticles (FeB NPs). The FeB NPs display stronger electron donating capability and electron transfer rate 9.4 times higher than that of the Fe0 NPs, which effectively activate the molecular oxygen to produce ∙O2 -, H2O2 and ∙OH. The in vitro cellular experiments confirm the FeB-ss-SiO₂ NPs (encapsulation with SiO2 outlayer containing -S-S- bonds) demonstrates the enhanced ferroptosis. The tumor-bearing mice models shows that FeB-ss-SiO₂ NPs exhibited superior biocompatibility and tumor inhibition effect (inhibition rate of 73%), which improve the overall survival rate for 30 days post-treatment. This study will provide an innovative way to design therapeutic nanoagents for cancer treatments.

An enhanced Bi/nZVI activated molecular oxygen process for the degradation of sulfonamide antibiotics in a citrate buffering system

Citric acid (CA) and sodium citrate (NaCA) have been effectively employed to synergize with bismuth-doped nanoscale zero-valent iron (Bi/nZVI) to degrade sulfonamide antibiotics (SAs) without the need for additional H2O2. In the integrated Bi/nZVI-CA/NaCA system, excellent oxidation activity of sulfamethazine (SM2), sulfadiazine (SD) and sulfamethoxazole (SMX) in the mixed solution was obtained. The bimetallic enhancement alongside ligand complexation significantly promoted Bi/nZVI to catalyze molecular oxygen and was conducive to the spontaneous generation of H2O2. Fe(II)[Cit]- was formed in the CA/NaCA system, and then underwent a Fenton-like reaction with spontaneously produced H2O2 to achieve the oxidation of SAs. Long service life was confirmed by the results of characterization, electrochemical analysis, utilization rate (UR), electronic efficiency (EE) and cycling degradation experiments. In the Bi/nZVI-CA/NaCA system, two comparable degradation pathways (hydroxylation and SO2 extrusion) for SM2, SMX and SD were obtained, while another degradation pathway for SMX was reflected in the opening of the N-O bond on the benzene ring. Additionally, post-reactive solution toxicity was assessed to ensure environmental safety. Overall, our findings provide a theoretical research basis for the effective elimination of SAs from contaminated environments.

Novel crystalline/amorphous heterophase Fe-Mn core-shell chains on-site generate hydrogen peroxide in aqueous solution

Hydrogen peroxide (H2O2) is a crucial eco-friendly oxidizer with increasing demand due to its wide range of applications. Activating O2 with catalysts to generate H2O2 on-site offers a promising alternative to traditional production methods. Here, we design unique crystalline/amorphous heterophase Fe-Mn core-shell chains (ZVI-Mn) for efficient on-site generation of H2O2 and manipulation of subsequent H2O2 activation. The yield of H2O2 on-site produced by ZVI-Mn in water within 5 min was 103.7 mg·L-1, which was much greater than that of zero-valent iron (ZVI) and amorphous Mn (A-Mn) (0 and 42.5 mg·L-1). Raman and density functional theory (DFT) calculations confirmed that *OOH is the key species involved in the on-site generation of H2O2. Electrochemical tests confirmed the excellent electron-transferring ability, while electron paramagnetic resonance (EPR) revealed oxygen vacancy defects in the catalysts, which proved to be conducive to improving the catalytic activity of ZVI-Mn. Additionally, by regulating the pH of aqueous solution, ZVI-Mn can simultaneously achieve efficient on-site generation of H2O2 and in-situ removal of enrofloxacin from aqueous solution.

Sulfidation of Nanoscale Zero-Valent Iron by Sulfide: The Dynamic Process, Mechanism, and Role of Ferrous Iron

Sulfidation of nanoscale zerovalent iron (nZVI) can enhance particle performance. However, the underlying mechanisms of nZVI sulfidation are poorly known. We studied the effects of Fe2+ on 24-h dynamics of nZVI sulfidation by HS- using a dosed S to Fe molar ratio of 0.2. This shows that in the absence of Fe2+, HS- rapidly adsorbed onto nZVI particles and reacted with surface iron oxide to form mackinawite and greigite (<0.5 h). As nZVI corrosion progressed, amorphous FeSx in solution deposited on nZVI, forming S-nZVI (0.5-24 h). However, in the initial presence of Fe2+, the rapid reaction between HS- and Fe2+ produced amorphous FeSx, which deposited on the nZVI and corroded the surface iron oxide layer (<0.25 h). This was followed by redeposition of colloidal iron (hydr)oxide on the particle surface (0.25-8 h) and deposition of residual FeSx (8-24 h) on S-nZVI. S loading on S-nZVI was 1 order of magnitude higher when Fe2+ was present. Surface characterization of the sulfidated particles by TEM-SAED, XPS, and XAFS verified the solution dynamics and demonstrated that S2- and S22-/Sn2- were the principal reduced S species on S-nZVI. This study provides a methodology to tune sulfur loading and S speciation on S-nZVI to suit remediation needs.

Enhanced hydrodechlorination of 4-chlorophenol through carboxymethylcellulose-modified Pd/Fe nanosuspension synthesized by one-step methods

Palladized iron (Pd/Fe) represents one of the most common modification strategies for nanoscale zero-valent iron (nZVI). Most studies prepared Pd/Fe by reducing iron salts and depositing Pd species on the surface of pre-synthesized nZVI, which can be called the two-step method. In this study, we proposed a one-step method to obtain Pd/Fe by the concurrent formation of Fe0 and Pd0 and investigated the effects of these two methods on 4-chlorophenol (4-CP) removal, with carboxymethylcellulose (CMC) coated as a surface modifier. Results indicated that the one-step method, not only streamlined the synthesis process, but also Pd/Fe-CMCone-step, synthesized by it, exhibited a higher 4-CP removal rate (97.9%) compared to the two-step method material Pd/Fe-CMCtwo-step (82.4%). Electrochemical analyses revealed that the enhanced activity of Pd/Fe-CMCone-step was attributed to its higher electron transfer efficiency and more available reactive species, active adsorbed hydrogen species (Hads*). Detection of intermediate products demonstrated that, under the influence of Pd/Fe-CMCone-step, the main route of 4-CP was through hydrodechlorination (HDC) to form phenol and H* was the main active specie, supported by EPR tests, quenching experiments and product analysis. Additionally, the effects of initial 4-CP concentration, initial pH, O2 concentration, anions such as Cl-, SO42-, HCO3-, and humic acid (HA) were also investigated. In conclusion, the results of this study suggest that Pd/Fe-CMCone-step, synthesized through the one-step method, is a convenient and efficient nZVI-modifying material suitable for the HDC of chlorinated organic compounds.

Self-acclimation mechanism of pyrite to sulfamethoxazole concentration in terms of degradation behavior and toxicity effects caused by reactive oxygen species

Pyrite has been extensively tested for oxidizing contaminants via the activation of water molecule or dissolved oxygen, while the changing of oxidation species induced by contaminant's concentration has been largely underestimated. In this study, we revealed a self-acclimation mechanism of pyrite in terms of •OH conversion to 1O2 during the sulfamethoxazole (SMX) degradation process under oxic conditions. Two reaction stages of SMX degradation by pyrite were observed. The SMX concentration decreased by 70% rapidly in the first 12 h after the reaction was initiated, then, the removal rate began to decrease as the SMX concentration decreased. Importantly, •OH and O2•- were the dominant oxidizing species in stage one, while 1O2 was responsible for the further degradation of SMX in stage two. The self-acclimated mechanism of pyrite was proven to be caused by the conversion of oxidative species at the surface of pyrite. This process can overcome the shortages of •OH such as ultrashort lifetime and limited effective diffusion in the decontamination of micropollutant. Moreover, different reactive oxygen species will lead to different degradation pathways and environmental toxicity while degrading pollutants. This finding of oxidizing species' self-acclimation mechanism should be of concern when using pyrite for water treatment.

Sulfide-modified nanoscale zero-valent iron as a novel therapeutic remedy for septic myocardial injury

INTRODUCTION

Myocardial injury is a serious complication in sepsis with high mortality. Zero-valent iron nanoparticles (nanoFe) displayed novel roles in cecal ligation and puncture (CLP)-induced septic mouse model. Nonetheless, its high reactivity makes it difficult for long-term storage.

OBJECTIVES

To overcome the obstacle and improve therapeutic efficiency, a surface passivation of nanoFe was designed using sodium sulfide.

METHODS

We prepared iron sulfide nanoclusters and constructed CLP mouse models. Then the effect of sulfide-modified nanoscale zero-valent iron (S-nanoFe) on the survival rate, blood routine parameters, blood biochemical parameters, cardiac function, and pathological indicators of myocardium was observed. RNA-seq was used to further explore the comprehensive protective mechanisms of S-nanoFe. Finally, the stability of S-nanoFe-1d and S-nanoFe-30 d, together with the therapeutic efficacy of sepsis between S-nanoFe and nanoFe was compared.

RESULTS

The results revealed that S-nanoFe significantly inhibited the growth of bacteria and exerted a protective role against septic myocardial injury. S-nanoFe treatment activated AMPK signaling and ameliorated several CLP-induced pathological processes including myocardial inflammation, oxidative stress, mitochondrial dysfunction. RNA-seq analysis further clarified the comprehensive myocardial protective mechanisms of S-nanoFe against septic injury. Importantly, S-nanoFe had a good stability and a comparable protective efficacy to nanoFe.

CONCLUSIONS

The surface vulcanization strategy for nanoFe has a significant protective role against sepsis and septic myocardial injury. This study provides an alternative strategy for overcoming sepsis and septic myocardial injury and opens up possibilities for the development of nanoparticle in infectious diseases.

Particle-Scale Understanding of Arsenic Interactions with Sulfidized Nanoscale Zerovalent Iron and Their Impacts on Dehalogenation Reactivity

Co-occurrence of organic contaminants and arsenic oxoanions occurs often at polluted groundwater sites, but the effect of arsenite on the reactivity of sulfidized nanoscale zerovalent iron (SNZVI) used to remediate groundwater has not been evaluated. Here, we study the interaction of arsenite [As(III)] with SNZVI at the individual-particle scale to better understand the impacts on the SNZVI properties and reactivity. Surface and intraparticle accumulation of As was observed on hydrophilic FeS-Fe0 and hydrophobic FeS2-Fe0 particles, respectively. X-ray absorption spectroscopy indicated the presence of realgar-like As-S and elemental As0 species at low and high As/Fe concentration ratios, respectively. Single-particle inductively coupled plasma time-of-flight mass spectrometry analysis identified As-containing particles both with and without Fe. The probability of finding As-containing particles without Fe increased with the S-induced hydrophobicity of SNZVI. The interactions of SNZVI materials with coexisting arsenite inhibited their reactivity with water (∼5.8-230.7-fold), trichloroethylene (∼3.6-67.5-fold), and florfenicol (∼1.1-5.9-fold). However, the overall selectivity toward trichloroethylene and florfenicol relative to water was improved (up to 9.0-fold) because the surface-associated As increased the SNZVI hydrophobicity. These results indicate that reactions of SNZVI with arsenite can remove As from groundwater and improve the properties of SNZVI for dehalogenation selectivity.

Immobilization of cadmium in river sediments by different modified nanoscale zero-valent iron: performance, mechanisms, and Fe dissolution

Modified nanoscale zero-valent iron (NZVI) exhibited great potential for the remediation of heavy metal contaminated river sediments, but its mechanisms and environmental risks are still unclear. This study systematically discussed the performance and the mechanisms of modified NZVI materials, i.e., sodium alginate-coated NZVI (SNZVI), rhamnolipid-coated NZVI (RNZVI), and graphene oxide-loaded NZVI (GNZVI), for the stabilization of Cd in sediment, with the exploration of their stability to Cd at various pH values and Fe dissolution rate. Compared with the control, the toxicity characteristic leaching procedure (TCLP) leachable Cd decreased by 52.66-96.28%, and the physiologically based extraction test (PBET) extractable Cd decreased by 44.68-70.21% after 56 days of incubation with the immobilization efficiency varying according to GNZVI > RNZVI > SNZVI > NZVI. Besides, the adsorption behavior of Cd on materials was fitted with the Freundlich model and classified as an endothermic, spontaneous, and chemical adsorption process. SEM-EDX, XRD, and FTIR results verified that the stabilization mechanisms of Cd were principally based on the adsorption, complexation of Cd2+ with secondary Fe minerals (including Fe2O3, γ-Fe2O3, and γ-FeOOH) and precipitation (Cd(OH)2). From the risk assessment results, it was observed that the materials were favorable for Cd stabilization at a pH range from 7 to 11, meanwhile, the leaching concentration of Fe in the overlying water was detected below the limit value. These findings pave the way to developing an effective strategy to remediate Cd contaminated river sediments.

Three-dimensional rGO/CNT/g-C3N4 macro discs as an efficient peroxymonosulfate activator for catalytic degradation of sulfamethoxazole

Over the past few years, advanced oxidation processes (AOPs) have shown promising efficiencies for wastewater remediation. Carbocatalysis, in particular, has been exploited widely thanks to its sustainable and economical properties but has an issue of recovery and reusability of the catalysts. To address this, three-dimensional (3D) binary and ternary graphene-based composites in the form of macro discs were created to activate peroxymonosulfate (PMS) for catalytic oxidation of sulfamethoxazole (SMX). Graphene oxide served as the base, while graphitic carbon nitride (g-C3N4) and/or single-walled carbon nanotubes (SWCNTs) were added. Among the various discs synthesized, rGNTCN discs (ternary composite) were proven to be the most efficient by completely degrading SMX in 60 min owing to their large surface area and nitrogen loading. The catalytic system was further optimized by varying the reaction parameters, and selective radical quenching and electron paramagnetic resonance tests were performed to identify the active radical, revealing the synergistic role of both radical and non-radical pathways. This led to the development of possible SMX degradation pathways. This research not only provides insights into ternary carbocatalysis but also gives a novel breakthrough in catalyst recovery and reusability by transforming nanocatalysts into macro catalysts.

MgxCu-biochar activated peroxydisulfate triggers reductive species for the reduction and enhanced electron-transfer degradation of electron-deficient aromatic pollutants

In wastewater treatment by persulfate-based advanced oxidation processes (PS-AOPs), electron-deficient aromatic pollutants (EDAPs) are refractory to nonradical pathway. To explore an efficient degradation pathway for EDAPs, Mg_xCu-biochar (BC) (x = 0.5, 1, 1.5) activated peroxydisulfate (PDS) was developed, which could trigger reductive species (•H) to reduce EDAPs first, and subsequently facilitate electron-transfer degradation of reduced intermediates. The roles of Mg-doping in Mg_xCu-BC to promote PDS activation and 2,4-dibromophenol (DBP) degradation were investigated. The mechanisms were then explored via electron paramagnetic resonance (EPR), chemical probes and Density Functional Theory (DFT) calculations. The results showed that Mg-doping improved metal-support interactions (MSIs) of Mg_xCu-BC, inducing •H formation via electron transfer from Cu atoms during PDS activation, which was thermodynamically favorable. The degradation rate of DBP (k_obs, 0.0494 min^-1) and Br^- release (5.35 mg L^-1) in Mg₁Cu-BC systems were more 31 and 33 times than that in Cu-BC/PDS system, respectively. The degradation mechanism of •H-enhanced electron transfer processes was that •H attacked one Br group of DBP, and then debrominated intermediates were mineralized by electron transfer processes in the Mg₁Cu-BC/PDS system. Overall, this study reports a novel pathway in PS-AOPs for selective degradation of EDAPs in wastewaters.

Colloidal iron species driven enhanced H2O2 decomposition into hydroxyl radicals for efficient removal of methylene blue from water

Colloids are wide-spread in natural waters and colloid-facilitated transport via adsorption was established as the most important mechanism for the mobilization of aqueous contaminants. This study reports another possible, but reasonable, role of colloids for the contaminants driven by redox reactions. Under the same conditions (pH 6.0, 0.3 ml 30% H₂O₂, and 25 °C), the degradation efficiencies of methylene blue (MB) at 240 min over Fe colloid, Fe ion, Fe oxide and Fe(OH)₃ were 95.38%, 42.66%, 4.42% and 9.40%. We suggested that, Fe colloid can promote the H₂O₂ based in-situ chemical oxidation process (ISCO) compared with other iron species such as Fe(Ⅲ) ion, Fe oxide and Fe(OH)₃ in natural water. Furthermore, the MB removal via adsorption by Fe colloid was only 1.74% at 240 min. Hence, the occurrence, behavior and fate of MB in Fe colloid containing natural water system mainly depends on the reduction-oxidation rather than adsorption-desorption process. Based on the mass balance of colloidal iron species and characterization of iron configurations distribution, Fe oligomers were the active and dominant components for Fe colloid-driven enhanced H₂O₂ activation among three types of Fe species. The quick and steady conversion of Fe(III) to Fe(II) was proven to be reason why Fe colloid can efficiently react with H₂O₂ to produce hydroxyl radicals.

A facile approach to synthesize CdS-attapulgite as a photocatalyst for reduction reactions in water

At room temperature, a facile approach has been utilized for preparing novel CdS–attapulgite (CdS–ATP) composites and the composites were applied in photocatalytic reduction of p-nitrophenol and Cr(vi). The effect of ATP on the photocatalytic activity of the CdS–ATP composites were studied by controlling the mass ratio of attapulgite. The results showed that the CdS–20%ATP composite has an excellent photocatalytic activity. In order to figure out the key to improve the photocatalytic efficiency, the prepared composites were characterized by Brunauer–Emmett–Teller (BET) specific surface area, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). The superior photocatalytic performance of the CdS–20%ATP composite can be ascribed to the existence of the ATP which can fix the CdS and prevent agglomeration. The interaction between ATP and CdS in the composites facilitates the electron transfer and also promoted their photocatalytic performance. This work provides us with some significant guidance in the development of CdS–ATP composite photocatalysts.

The application of CdS–attapulgite composites in photocatalytic reduction of p-nitrophenol and Cr(vi) demonstrated that the attapulgite could overcome the limitations of CdS.

Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

2021 marks 10 years since controlled abiotic synthesis of sulfidated nanoscale zerovalent iron (S-nZVI) for use in site remediation and water treatment emerged as an area of active research. It was then expanded to sulfidated microscale ZVI (S-mZVI) and together with S-nZVI, they are collectively referred to as S-(n)ZVI. Heightened interest in S-(n)ZVI stemmed from its significantly higher reactivity to chlorinated solvents and heavy metals. The extremely promising research outcomes during the initial period (2011-2017) led to renewed interest in (n)ZVI-based technologies for water treatment, with an explosion in new research in the last four years (2018-2021) that is building an understanding of the novel and complex role of iron sulfides in enhancing reactivity of (n)ZVI. Numerous studies have focused on exploring different S-(n)ZVI synthesis approaches, and its colloidal, surface, and reactivity (electrochemistry, contaminant selectivity, and corrosion) properties. This review provides a critical overview of the recent milestones in S-(n)ZVI technology development: (i) clear insights into the role of iron sulfides in contaminant transformation and long-term aging, (ii) impact of sulfidation methods and particle characteristics on reactivity, (iii) broader range of treatable contaminants, (iv) synthesis for complete decontamination, (v) ecotoxicity, and (vi) field implementation. In addition, this review discusses major knowledge gaps and future avenues for research opportunities.