Facet-Dependent Activation of Oxalic Acid over Magnetic Recyclable Fe3S4 for Efficient Pollutant Removal under Visible Light Irradiation: Enhanced Catalytic Activity, DFT Calculations, and Mechanism Insight

Modulating d-orbital electronic configuration of magnetic iron sulfide nanocrystals for maximized treatment efficiency of chromium-contaminated water

Hexavalent chromium (Cr(VI)) is one of the most rigorously regulated contaminants frequently detected in surface and groundwater. Magnetic iron sulfides are naturally abundant, environmentally friendly materials ideal for the removal of Cr(VI) from contaminated water, but the high-spin states of Fe(III) ions limits their adsorption affinity. Herein, we develop a heteroatom-doping approach to boost the Cr(VI) removal efficacy of greigite. Compared to pristine greigite, cobalt doping significantly enhances the capability of greigite to adsorption Cr(VI) by decreasing electron occupancy in the eg orbitals and reducing the spin state of Fe ions. With a combination of electrochemical characterizations and theoretical calculations, we confirm that cobalt doping significantly enhances its reduction capacity toward Cr(VI) by elevating the d-band center and increasing electron transfer rate. We corroborate the finding by showing that Cu-doping, which has the opposite effects on d-orbital electron configures, compromises the efficacy of greigite. Of note, the magnetic properties of greigite remain largely unaffected upon heteroatom doping, allowing easy separation and recovery of the materials from the aqueous solutions. This work provides valuable mechanistic insights for nanomaterial design in contaminant removal and may inform the development of nanotechnology for green remediation of contaminated sites.

Hydrothermal reduction and phase transformation of Fe(III) minerals induced by rice straw to improve the heterogeneous Fenton degradation of metolachlor

Heterogeneous Fenton technology is effective in degrading residual pesticides in soil, but the reduction of Fe(III) in the mineral structure presents a bottleneck. This study combined rice straw with Schwertmannite (Sch), ferrihydrite (Fh), and magnetite (Mag) via a hydrothermal process to obtain iron oxides-hydrothermal carbon composites (Sch@HTC, Fh@HTC, and Mag@HTC). Poor-crystallized Sch and Fh, which were more capable of accepting electrons compared to well-crystallized Mag, exhibited obvious phase transformation to highly active Fe(II)-mineral (humboldtine) via the combination of oxalic acid, an intermediate product, with reduced Fe(II), while Mag was hard to achieve. After hydrothermal treatment, all composites showed enhanced catalytic activity, which increased with the degree of phase transformation. Especially, Sch@HTC demonstrated the highest catalytic activity, degrading 85 % of metolachlor in soil within 24 hours, 2-10 times faster than the others. Surprisingly, the solid-phase Fe(II) in soil increased slightly after the Fenton reaction. Moreover, the in-situ fluorescence intensity of HO• in soil was continuously enhanced, and the effective utilization of H2O2 to HO• was improved. These results confirmed that HTC could provide electrons to Fe(III) during the hydrothermal process, facilitating the Fe(III)/Fe(II) redox cycle and sustaining reactive Fe(II), thus overcoming key challenges in heterogeneous Fenton catalysis.

Bioengineered iron-based heterojunction orientation in optimizing activation pathways for superoxide radical-mediated photoreduction of Cr(VI) from water

Photocatalytic technology has been widely employed for Cr(VI) remediation. However, the inadequate generation of reactive oxygen species associated with the Cr(VI) reduction, caused by the uncontrollable photo-Fenton reaction, significantly restricts the reduction efficiency. Herein, a bioengineered iron-based heterojunction (Bio-Fe2O3/Fe2(WO4)3) was fabricated via a two-step process of biomineralization and calcination, where tungstate was doped into the precursor during iron metabolism in acidophilic bacteria to optimize the heterojunction structure. Bio-Fe2O3/Fe2(WO4)3 exhibited a short-range ordered structure and superior photocatalytic performance, achieving 100 % reduction of 20 mg/L Cr(VI) within 60 min by photocatalytic oxalic acid (OA) under simulated light conditions. The system provided robust operation in complex environments, notably, operating effectively under mild solar radiation as an alternative to the simulated light. The heterojunction structure intensified the H2O2 activation and selectively boosted the yield of superoxide radical (O2·-), the primary Cr(VI)-reducing species, from 48.02 % to 72.96 %. The high oxidation state of Fe in Bio-Fe2O3/Fe2(WO4)3 contributed to stronger adsorption performance towards OA and H2O2, accompanied with the tendency to take the O2·--generated activation pathway. This work provides a broader perspective on the rational design of photocatalysts to modulate the OA photocatalysis and the H2O2 activation pathway, selectively elevating the yield of O2·- for Cr(VI) reduction.

Modification of iron-containing silicate tailings with oxalic acid to develop a long-efficacy utilization peroxymonosulfate-based system for the efficient decomplexation and removal of Cr(III)-ethylenediamine tetraacetic acid

Ferrous oxalate (FeC2O4)-based composite has been recognized as an eminent catalyst for Cr(III)-ethylenediamine tetraacetic acid (Cr(III)-EDTA) decomplexation. However, their practical application has been limited by low cycling capacity and an ambiguous mechanism. In this research, a composite catalyst consisting of biotite loaded with nano FeC2O4 (CFS90) was prepared directly from iron-containing silicate tailing. The removal efficiency (91.3 %, kobs = 0.0185 min-1) of Cr(III)-EDTA by CFS90/peroxymonosulfate (PMS) system was remarkably higher than that of other typical systems. The Si site in biotite lost electrons while the electron cloud density around the Fe atom in FeC2O4 increased, which facilitates the activation of PMS and the generation of reactive oxygen species (ROS). In this system, abundant singlet oxygen (1O2) was primarily produced via interactions between carbon-centered radicals (CO2·-) and dissolved oxygen (DO), rather than through oxygen vacancies (Ovs) in CFS90. Both CO2·- and Fe(II) provided reducing conditions, preventing the released Cr(III) from being re-oxidized. Notably, the released Cr(III) was effectively precipitated by elevating the solution pH with NaOH, therefore endowing superior stability and deactivation capacity of CFS90 to enable its removal rate of Cr(III)-EDTA to remain above 84.1 % for 18 h in a fix-bed reactor. These findings provide an in-depth analysis of the enhanced Cr(III)-EDTA removal mechanism and highlight the environmental remediation potential of iron-containing silicate tailings.

Sodium citrate-modification enhanced Fe3S4 for Cr(Ⅵ) removal from aqueous solution and soil

Fe3S4 has been widely employed to remove Cr(Ⅵ) from wastewater, however, its practical effectiveness is often limited by agglomeration and passivation. This study introduces sodium citrate (SC) as a ligand to synthesize an Fe3S4-SC magnetic micro-crystal for Cr(Ⅵ) removal from aqueous solutions and contaminated soils. Experimental results show that Fe3S4-SC exhibits superior Cr(Ⅵ) removal efficiency, especially in acidic environments, with a maximum adsorption capacity of 449.12 mg/g. When Fe3S4-SC was used to remediate Cr(Ⅵ)-contaminated soil with a Cr(Ⅵ) content of 664.98 mg/kg and a TCLP-Cr(Ⅵ) concentration of 26.57 mg/L, the removal efficiencies of Cr(Ⅵ) and TCLP-Cr(Ⅵ) were 99.29% and 98.52% after 60 days. Cr speciation shifted from exchangeable fraction and weak acid-soluble fraction to more stable species bound to Fe-Mn oxides and residual fraction. Cr(Ⅵ) removal was primarily facilitated by surface Fe(Ⅱ), dissolved Fe(Ⅱ), and surface S(-Ⅱ). Surface S(-Ⅱ) provided electrons to Fe(Ⅲ), facilitating Fe(Ⅱ) regeneration for the continuous reduction of Cr(Ⅵ). The SC ligand enhanced material dispersion and stability, promoted Fe(Ⅱ) dissolution, reduced passivation layer formation, and improved electron transfer efficiency, thus increasing the efficacy of Fe3S4-SC in Cr(Ⅵ) removal. These findings provide a valuable reference for effectively remediating Cr(Ⅵ) contamination in wastewater and soil.

Modulation of Charge-Ordered Carriers Within 3D Fe3S4 Polyurethane Foam (Fe3S4-PUF) for Efficient Iron Redox Cycling and Continuous-Flow Photocatalytic Antibiotics Degradation

Photocatalytic antibiotic degradation is an energy-efficient and environmentally friendly approach with the potential for large-scale application but is severely constrained by the lack of efficient and stable catalysts to produce reactive oxygen species (ROS). This research introduces a charge-ordered 3D Fe3S4-PUF composite integrated into a custom-built photocatalytic tandem continuous-flow cylinder reactor (TCCR) for antibiotic degradation. The system consistently achieves 100% tetracycline (TC) degradation efficiency with Fe3S4-PUF during 130 h of continuous operation, benefiting from the charge-ordered 3D Fe3S4-PUF framework and the TCCR design. Mechanism investigations reveal that the abundant Lewis basic ≡SH site and light-induced sustainable Fe2+/Fe3+ redox cycling within Fe3S4 facilitates the production of H2O2 and ROS. Density functional theory (DFT) calculations indicate that Fe2+ acts as an active site for capturing and activating O2, leading to either one-electron (O2→O2 •-→H2O2→•OH) or two-electron transfer (O2→H2O2) pathways. Meanwhile, photogenerated electron and the oxygen atoms in H2O2 provide electrons to Fe3+, facilitating the reduction of Fe3+ to Fe2+, thus elucidating the Fe2+/Fe3+ redox cycling mechanism. Moreover, the 3D PUF structure enhances the mass transfer and pollutant-ROS interactions. The continuous-flow photocatalytic reaction validate the efficient antibiotic degradation of Fe3S4-PUF composite, suggesting its potential for implementation in large-scale antibiotic wastewater treatment systems.

Controlling the amount of MoSe2 loaded SrTiO3 to activate peroxymonosulfate for efficient elimination of organic pollutants

It is critical to effectively eliminate recalcitrant organic pollutants from wastewater. In this paper, the MoSe2/SrTiO3 (MST) catalysts were synthesized through simply controlling the amount of MoSe2 in the hydrothermal method to activate peroxymonosulfate (PMS) for the degradation of pollutants. The results demonstrated that sulfamethoxazole and tetracycline were almost eliminated by PMS/MST-3 (MoSe2/SrTiO3 mass ratio 0.3: 1) activation system. The effect of inorganic anions (Cl -, H2PO4 -, HCO3 -) and metal ions (Cu2+, Ni2+, Zn2+) commonly found in actual water bodies on catalytic reaction was explored. Moreover, SO4• -, •OH and 1O2 were identified by EPR tests and scavenger experiments, where the SO4• - and •OH were the dominant reactive species. The XPS analysis indicated that the oxygen vacancies and charge transfer on the catalyst surface were the keys of PMS activation. The effect of active sites in SMX and TC on the catalytic degradation activity was explored by density functional theory, and it was obtained that the central nitrogen site of SMX was more vulnerable in the catalytic system, while the edge oxygen site of TC was more susceptible to attack.

Efficient photocatalytic degradation of microplastics by constructing a novel Z-scheme Fe-doped BiO2-x/BiOI heterojunction with full-spectrum response: Mechanistic insights and theory calculations

Recently, microplastics (MPs) have garnered significant attention as a challenging emerging pollutant to address. Here, a full-spectrum light-driven Fe-doping BiO2-x/BiOI (FBI) Z-scheme heterojunction was constructed for efficiently degrading MPs in waters. Compared with BiO2-x, Fe doping BiO2-x, and BiOI, the optimal photocatalyst (40-FBI) can cause deep cracks in the polyethylene terephthalate (PET) within 10 h under the irradiation of full-spectrum light. Meanwhile, FT-IR characterization revealed that the absorption peak intensities of the C-O group, CO group, -CH stretching vibration, and -OH group on the MPs surface gradually increased with degradation time. A series of experiments and theory calculations revealed that the introduction of Fe creates impurity levels, accelerating the separation of photo-generated carriers and reducing the work function of BiO2-x, thereby enhancing the transport of photo-generated carriers between Z-scheme heterojunctions. This study offers a valuable idea for designing an efficient photocatalyst by simultaneously introducing ion doping and constructing heterojunctions for enhancing MPs degradation.

Optimization of band structure by facet engineering to enhance photocatalytic/photothermal synergistic therapy of cobalt sulfide nanosheets

With the rapid development of optics and nanotechnology, optical therapy is recognized as a potential strategy to combat bacterial drug resistance. Unfortunately, the limitation of band structure on the therapeutic effect of photocatalytic nano-antimicrobial materials has become a major challenge in the development of optical therapy. Here, Co9S8 and Co9S8-T200 were prepared by facet engineering design with major exposed facets (311) and (222), respectively. The experimental results demonstrate that the antimicrobial ability of Co9S8-T200 was significantly higher than that of Co9S8 under the excitation of near-infrared light. Theoretical calculations show that the (222) facets can modulate the d-band center of Co9S8-T200 closer to the Fermi energy level. This promotes the binding of the intermediate to the active site. The strong adsorption capacity for O2 and H2O and the high efficiency of electron-hole pair separation facilitate the production of reactive oxygen species dominated by singlet oxygen, and the photocatalytic therapy is effectively enhanced. In addition, Co9S8-T200 has a narrow bandgap to promote photothermal conversion efficiency and light absorption capacity and exhibits excellent photothermal therapy. This work expands new ideas for the development of superior optical antimicrobial agents with synergistic photothermal and photocatalytic therapy.

Disclosing the influence mechanism of facet-dependent pyrite photo-activation and photo-dissolution processes on the reduction of Cr(VI)

The photo-activation and photo-dissolution processes of pyrite (FeS2) can affect the environmental behavior of the co-existing hexavalent chromium (Cr(VI)). But the photochemical performance of FeS2 is intimately dependent on its exposed facets. Herein, FeS2 nanosheets (FeS2 NS) and FeS2 nanocubes (FeS2 NC) with the dominant exposed facets of (001) and (210)/(100) respectively are prepared. The more Fe3+, Fe2+, and SO42- are released in the FeS2 NS system than the other system due to its more excellent generation ability of photogenerated electrons and reactive oxygen species. The higher surface energy on (001) facet leads to the faster dissolution rate of FeS2 NS. Due to the optimal production ability of photogenerated electrons and Fe2+ of (001) facet, the much higher Cr(VI) elimination efficiency in the FeS2 NS system is observed than that in the FeS2 NC (72.8%) system within 120 min. This work could help to unveil the influence of FeS2 on the fate of Cr(VI) in surface environment, and offer a theoretical support to clarify the influence of heavy metal ions on the iron sulfide minerals.

2D TiO2 Nanosheets Decorated Via Sphere-Like BiVO4: A Promising Non-Toxic Material for Liquid Phase Photocatalysis and Bacterial Eradication

An in-depth investigation was conducted on a promising composite material (BiVO4/TiO2), focusing on its potential toxicity, photoinduced catalytic properties, as well as its antibiofilm and antimicrobial functionalities. The preparation process involved the synthesis of 2D TiO2 using the lyophilization method, which was subsequently functionalized with sphere-like BiVO4 through wet impregnation. Finally, we developed BiVO4/TiO2 S-scheme heterojunctions which can greatly promote the separation of electron-hole pairs to achieve high photocatalytic performance. The evaluation of concentration- and time-dependent viability inhibition was performed on human lung carcinoma epithelial A549 cells. This assessment included the estimation of glutathione levels and mitochondrial dehydrogenase activity. Significantly, the BiVO4/TiO2 composite demonstrated minimal toxicity towards A549 cells. Impressively, the BiVO4/TiO2 composite exhibited notable photocatalytic performance in the degradation of rhodamine B (k=0.135 min-1) and phenol (k=0.016 min-1). In terms of photoinduced antimicrobial performance, the composite effectively inactivated both gram-negative E. coli and gram-positive E. faecalis bacteria upon 60 minutes of UV-A light exposure, resulting in a significant log 6 (log 10 CFU/mL) reduction in bacterial count. In addition, a 49 % reduction of E. faecalis biofilm was observed. These promising results can be attributed to the unique 2D morphology of TiO2 modified by sphere-like BiVO4, leading to an increased generation of (intracellular) hydroxyl radicals, which plays a crucial role in the treatments of both organic pollutants and bacteria. This research has significant potential for various applications, particularly in addressing environmental contamination and microbial infections.

The Surface Confinement of FeO Assists in the Generation of Singlet Oxygen and High-Valent Metal-Oxo Species for Enhanced Fenton-Like Catalysis

Transition metal compounds (TMCs) have long been potential candidate catalysts in persulfate-based advanced oxidation process (PS-AOPs) due to their Fenton-like catalyze ability for radical generation. However, the mechanism involved in TMCs-catalyzed nonradical PS-AOPs remains obscure. Herein, the growth of FeO on the Fe3O4/carbon precursor is regulated by restricted pyrolysis of MIL-88A template to activate peroxymonosulfate (PMS) for tetracycline (TC) removal. The higher FeO incorporation conferred a 2.6 times higher degradation performance than that catalyzed by Fe3O4 and also a higher interference resistance to anions or natural organic matter. Unexpectedly, the quenching experiment, probe method, and electron paramagnetic resonance quantitatively revealed that the FeO reassigned high nonradical species (1O2 and FeIV═O) generation to replace original radical system created by Fe3O4. Density functional theory calculation interpreted that PMS molecular on strongly-adsorbed (200) and (220) facets of FeO enjoyed unique polarized electronic reception for surface confinement effect, thus the retained peroxide bond energetically supported the production of 1O2 and FeIV═O. This work promotes the mechanism understanding of TMCs-induced surface-catalyzed persulfate activation and enables them better perform catalytic properties in wastewater treatment.

Nitrogen-doped titanium dioxide/schwertmannite nanocomposites as heterogeneous photo-Fenton catalysts with enhanced efficiency for the degradation of bisphenol A

Potential health risks related to environmental endocrine disruptors (EEDs) have aroused research hotspots at the forefront of water treatment technologies. Herein, nitrogen-doped titanium dioxide/schwertmannite nanocomposites (N-TiO2/SCH) have been successfully developed as heterogeneous catalysts for the degradation of typical EEDs via photo-Fenton processes. Due to the sustainable Fe(III)/Fe(II) conversion induced by photoelectrons, as-prepared N-TiO2/SCH nanocomposites exhibit much enhanced efficiency for the degradation of bisphenol A (BPA; ca. 100% within 60 min under visible irradiation) in a wide pH range of 3.0-7.8, which is significantly higher than that of the pristine schwertmannite (ca. 74.5%) or N-TiO2 (ca. 10.8%). In this photo-Fenton system, the efficient degradation of BPA is mainly attributed to the oxidation by hydroxyl radical (•OH) and singlet oxygen (1O2). Moreover, the possible catalytic mechanisms and reaction pathway of BPA degradation are systematically investigated based on analytical and photoelectrochemical analyses. This work not only provides a feasible means for the development of novel heterogeneous photo-Fenton catalysts, but also lays a theoretical foundation for the potential application of mineral-based materials in wastewater treatment.

Facet-dependent activation of oxalic acid over hematite nanocrystals under the irradiation of visible light for efficient degradation of pollutants

Naturally occurring hematite has been widely studied in the Fenton-like system for water pollutant remediation due to its abundance and non-toxicity. However, its inadequate catalytic activity results in difficulty in effectively degrading pollutants in the catalytic degradation system that it constitutes. Thus, we constructed a photochemical system composed of hematite with {001} facet of high activity facet and low-cost and non-toxic oxalic acid (OA) for the removal of various types of pollutants. The removal rate for the degradation of metronidazole, tetracycline hydrochloride, Rhodamine B, and hexavalent chromium by hematite nanoplate with the exposed {001} facet activating OA under visible light irradiation was 4.75, 2.25, 2.33, and 2.74 times than that by the exposed {110} facet, respectively. Density functional theory (DFT) calculation proved that the OA molecule was more easily adsorbed on the {001} facet of hematite than that on the {110} facet, which would favor the formation of the more Fe(III)-OA complex and reactive species. In addition, the reactive site of metronidazole for the attraction of radicals was identified on the basis of the DFT calculation on the molecular occupied orbitals, and the possible degradation pathway for metronidazole included carbon chain fracture, hydroxyethyl-cleavage, denitrogenation, and hydroxylation. Thus, this finding may offer a valuable direction in designing an efficient iron-based catalyst based on facet engineering for the improved activity of Fenton-like systems such as OA activation.

Z-scheme heterojunction composed of Fe-doped g-C3N4 and Bi2MoO6 for photo-fenton degradation of antibiotics over a wide pH range: Activity and toxicity assessment

In photo-Fenton technology, the narrower pH range limits its practical application for antibiotic wastewater remediation. Therefore, in this study, a Z-scheme heterojunction photo-Fenton catalyst was constructed by Fe-doped graphite-phase carbon nitride in combination with bismuth molybdate for the degradation of typical antibiotics. Fe doping can shorten the band gap and increase visible-light absorption. Simultaneously, the constructed Z-scheme heterojunction provides a better charge transfer pathway for the photo-Fenton reaction. Within 30 min, Fe3CN/BMO-3 removed 95.54% of tetracycline hydrochloride (TC), and its remarkable performance was the higher Fe3+/Fe2+ conversion efficiency through the decomposition of H2O2. The Fe3CN/BMO-3 catalyst showed remarkable photo-Fenton degradation performance in a wide pH range (3.0-11.0), and it also had good stability in the treatment of TC wastewater. Furthermore, the order of action of the active species was h+ > ·O2- > 1O2 > ·OH, and the toxicity assessment suggested that Fe3CN/BMO-3 was effective in reducing the biotoxicity of TC. The catalyst proved to be an economically feasible and applicable material for antibiotic photo-Fenton degradation, and this study provides another perspective on the application of elemental doping and constructed heterojunction photo-Fenton technology for antibiotic water environmental remediation.

Insights into sunlight-driven transformation of tetracycline by iron (hydr)oxides: The dominating role of self-generated hydrogen peroxide

Iron (hydr)oxides are abundant in surface environment, and actively participate in the transformation of organic pollutants due to their large specific surface areas and redox activity. This work investigated the transformation of tetracycline (TC) in the presence of three common iron (hydr)oxides, hematite (Hem), goethite (Goe), and ferrihydrite (Fh), under simulated sunlight irradiation. These iron (hydr)oxides exhibited photoactivity and facilitated the transformation of TC with the initial phototransformation rates decreasing in the order of: Hem > Fh > Goe. The linear correlation between TC removal efficiency and the yield of HO• suggests that HO• dominated TC transformation. The HO• was produced by UV-induced decomposition of self-generated H2O2 and surface Fe2+-triggered photo-Fenton reaction. The experimental results indicate that the generation of HO• was controlled by H2O2, while surface Fe2+ was in excess. Sunlight-driven H2O2 production in the presence of the highly crystalline Hem and Goe occurred through a one-step two-electron reduction pathway, while the process was contributed by both O2-induced Fe2+ oxidation and direct reduction of O2 by electrons on the conduction band in the presence of the poorly crystalline Fh. These findings demonstrate that sunlight may significantly accelerate the degradation of organic pollutants in the presence of iron (hydr)oxides.

Research on carbon black and cerium co-doped Ti4O7-CB-Ce electrocatalytic oxidation of tetracycline-based antibiotics

Elemental doping is a promising way for enhancing the electrocatalytic activity of metal oxides. Herein, we fabricate Ti/ Ti4O7-CB-Ce anode materials by the modification means of carbon black and cerium co-doped Ti4O7, and this shift effectively improves the interfacial charge transfer rate of Ti4O7 and •OH yield in the electrocatalytic process. Remarkably, the Ti4O7-CB-Ce anode exhibits excellent efficiency of minocycline (MNC) wastewater treatment (100% removal within 20 min), and the removal rate reduces from 100 to 98.5% after five cycles, which is comparable to BDD electrode. •OH and 1O2 are identified as the active species in the reaction. Meanwhile, it is discovered that Ti/ Ti4O7-CB-Ce anodes can effectively improve the biochemical properties of the non-biodegradable pharmaceutical wastewater (B/C values from 0.25 to 0.44) and significantly reduce the toxicity of the wastewater (luminescent bacteria inhibition rate from 100 to 26.6%). This work paves an effective strategy for designing superior metal oxides electrocatalysts.

Elucidating Facet-Dependent Photocatalytic Activities of Metastable CdS and Au@CdS Core-Shell Nanocrystals

Controlling the crystal facets of semiconductor nanocrystals (NCs) has been proven as an effective approach to tune their physicochemical properties. However, the study on facet-engineering of metastable zinc blende CdS (zb-CdS) and its heterostructures is still not fully explored. In this study, the zb-CdS and Au@zb-CdS core-shell NCs with tunable terminating facets are controllably synthesized, and their photocatalytic performance for water splitting are evaluated. It is found that the {111} facets of the zb-CdS NCs display higher intrinsic activity than the {100} counterparts, which originates from these surfaces being much more efficient, facilitating electron transition to enhance the adsorption ability and the dissociation of the adsorbed water, as revealed by theoretical calculations. Moreover, the Au@zb-CdS core-shell NCs exhibit better photocatalytic performance than the zb-CdS NCs terminated with the same facets under visible light irradiation (≥400 nm), which is mainly ascribed to the accelerated electron separation at the interface, as demonstrated by femtosecond transient absorption (fs-TA) spectroscopy. Importantly, the quantum yield of plasmon-induced hot electron transfer quantified by fs-TA in the Au@zb-CdS core-shell octahedrons can be reached as high as 1.2% under 615 nm excitation, which is higher than that of the Au@zb-CdS core-shell cubes. This work unravels the face-dependent photocatalytic performance of the metastable semiconductor NCs via a combination of experiments and theoretical calculations, providing the understanding of the underlying mechanism of these photocatalysts.

Pyrite-activated persulfate to degrade 3,5,6-trichloro-2-pyridyl in water: Degradation and Fe release mechanism

TCP (3,5,6-trichloro-2-pyridinol), the main recalcitrant degradation product of chlorpyrifos, poses a high risk to human health and ecological systems. This study provided a comprehensive exploration of the pyrite-activated persulfate (PS) system for the removal of TCP in water and placed particular emphasis on the pyrite oxidation process that releases Fe. The results showed that the pyrite-activated PS system can completely degrade TCP within 300 min at 5.0 mmol/L PS and 1000 mg/L pyrite at 25 °C, wherein small amounts of PS (1 mmol/L) can effectively facilitate TCP removal and the oxidation of pyrite elements, while excessive PS (>20 mmol/L) can lead to competitive inhibitory effects, especially in the Fe release process. Aimed at the dual effects, the evident positive correlation (R2 > 0.90) between TCP degradation (kTCP) and Fe element release (kFe), but the value of k (0.00237) in the pyrite addition variable experiment was less than that in the PS experiment (k = 0.00729), further indicating that the inhibition effect of excessive addition consists of PS but not notably pyrite. Moreover, the predominant free radicals and non-free radicals produced in the pyrite/PS system were tested, with the order of significance being •OH < Fe (Ⅳ) < SO4•- < •O2- < 1O2, wherein 1O2 emerged as the principal player in both TCP degradation and Fe release from the pyrite oxidation process. Additionally, CO32- can finitely activate PS but generally slows TCP degradation and inhibit pyrite oxidation releasing Fe process. This study provides a theoretical basis for the degradation of TCP using pyrite-activated PS.

Synthesis of 3D Sb2O3-based heterojunction reinforced by SPR effect and photo-Fenton mechanism for upgraded oxidation of metronidazole in water environments

The traditional homogenous and heterogenous Fenton reactions have frequently been restrained by the lower production of Fe2+ ions, which significantly obstructs the generation of hydroxyl radicals from the decomposition of H2O2. Thus, we introduce novel photo-Fenton-assisted plasmonic heterojunctions by immobilizing Fe3O4 and Bi nanoparticles onto 3D Sb2O3 via co-precipitation and solvothermal approaches. The ternary Sb2O3/Fe3O4/Bi composites offered boosted photo-Fenton behavior with a metronidazole (MNZ) oxidation efficiency of 92% within 60 min. Among all composites, the Sb2O3/Fe3O4/Bi-5% hybrid exhibited an optimum photo-Fenton MNZ reaction constant of 0.03682 min- 1, which is 5.03 and 2.39 times higher than pure Sb2O3 and Sb2O3/Fe3O4, respectively. The upgraded oxidation activity was connected to the complementary outcomes between the photo-Fenton behavior of Sb2O3/Fe3O4 and the plasmonic effect of Bi NPs. The regular assembly of Fe3O4 and Bi NPs enhances the surface area and stability of Sb2O3/Fe3O4/Bi. Moreover, the limited absorption spectra of Sb2O3 were extended into solar radiation by the Fe3+ defect of Fe3O4 NPs and the surface plasmon resonance (SPR) effect of Bi NPs. The photo-Fenton mechanism suggests that the co-existence of Fe3O4/Bi NPs acts as electron acceptor/donor, respectively, which reduces recombination losses, prolongs the lifetime of photocarriers, and produces more reactive species, stimulating the overall photo-Fenton reactions. On the other hand, the photo-Fenton activity of MNZ antibiotics was optimized under different experimental conditions, including catalyst loading, solution pH, initial MNZ concentrations, anions, and real water environments. Besides, the trapping outcomes verified the vital participation of •OH, h+, and •O2- in the MNZ destruction over Sb2O3/Fe3O4/Bi-5%. In summary, this work excites novel perspectives in developing boosted photosystems through integrating the photocatalysis power with both Fenton reactions and the SPR effects of plasmonic materials.

Degradation of microplastic in water by advanced oxidation processes

Plastic products have gained global popularity due to their lightweight, excellent ductility, high durability, and portability. However, out of the 8.3 billion tons of plastic waste generated by human activities, 80% of plastic waste is discarded due to improper disposal, and then transformed into microplastic pollution under the combined influence of environmental factors and microorganisms. In this comprehensive study, we present a thorough review of recent advancements in research on the source, distribution, and effect of microplastics. More importantly, we conducted deep research on the catalytic degradation technologies of microplastics in water, including advanced oxidation and photocatalytic technologies, and elaborated on the mechanisms of microplastics degradation in water. Besides, various strategies for mitigating microplastic pollution in aquatic ecosystems are discussed, ranging from policy interventions, the initiative for plastic recycling, the development of efficient catalytic materials, and the integration of multiple technological approaches. This review serves as a valuable resource for addressing the challenge of removing microplastic contaminants from water bodies, offering insights into effective and sustainable solutions.

Preparation of novel Fe-containing zeolite-A for KN-R decolorization by Fenton-like reaction

Herein, novel catalysts of Fe-containing zeolite-A (Fe/zeolite-A) were synthesized by exchanging iron ions into zeolite-A framework, and short-chain organic acids (SCOAs) were employed as chelating agents. Reactive Brilliant Blue KN-R (KN-R) was used as a model pollutant to evaluate the performance of these catalysts based on the heterogeneous Fenton reaction. The results showed that Fe-OA/3A, which applied zeolite-3A as the supporter and oxalic as the chelating agent, presented the most prominent KN-R decolorization efficiency. Under the initial pH of 2.5, 0.4 mM KN-R could be totally decolorized within 20 min. However, the mineralization efficiency of KN-R was only 58.2%. Therefore, anthraquinone dyes were introduced to modify zeolite-3A. As a result, the mineralization efficiency of KN-R was elevated to 92.7% when using Alizarin Violet (AV) as the modifier. Moreover, the modified catalysts exhibited excellent stability, the KN-R decolorization efficiency could be maintained above 95.0% within 20 min after operating for nine cycles. The mechanism revealed that the Fe(II)/Fe(III) cycle was accelerated by AV-modified catalyst thus prompting the KN-R decolorization in Fenton-like system. These findings provide new insights for preparing catalysts with excellent activity and stability for dye wastewater treatment.

Nitrogen-doped metal-free granular activated carbons as economical and easily separable catalysts for peroxymonosulfate and hydrogen peroxide activation to degrade bisphenol A

The fabrication of low-cost, highly efficient, environmentally friendly, and easily separable metal-free heterogeneous catalysts for environmental remediation remains a challenge. In this study, granular nitrogen-doped highly developed porous carbons with a particle size of 0.25-0.30 mm were prepared by preoxidation and subsequent NH3 modification of a commercially available coconut-based activated carbon, and used to activate peroxymonosulphate (KHSO5) or hydrogen peroxide (H2O2) to degrade bisphenol A (BPA). The nitrogen-doped carbon (ACON-950) prepared by NH3 modification at 950 °C, with the addition of only 0.15 g/L could remove 100% of 50 mg/L BPA in 150 min, and more than 90% of the removed BPA was due to degradation. The removal rates of total organic carbon of ACON-950/KHSO5 and ACON-950/H2O2 systems reached 60.4% and 66.2% respectively, indicating the excellent catalytic activity of ACON-950. The reaction rate constant was significantly positively correlated with the absolute content of pyridinic N (N-6) and graphitic N (N-Q) and negatively and weakly positively correlated with pyrrolic N (N-5) and defects. Quenching experiments combined with electron paramagnetic resonance demonstrated that singlet oxygen was the dominant reactive oxidative species for BPA degradation. ACON-950 was characterized before and after the degradation reaction using N2 adsorption-desorption analyzer, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results confirmed the prominent contribution of both the N-6 and N-Q to the catalytic performance of nitrogen-doped carbons. The reusability of ACON-950 and its application in actual water bodies further demonstrated its remarkable potential for the remediation of organic pollutants in wastewater.

Reaction Thermodynamics of Zerovalent Iron and Hexavalent Chromium at the Solid-Liquid Interface and in Solution

Zerovalent iron (Fe0) is a promising candidate for remediating hexavalent chromium (Cr(VI)) via adsorption and (or) reduction. Herein, the reaction between Fe0 and Cr(VI) at the solid-liquid interface and in solution under varying pHs was inspected using the methodology of equilibrium thermodynamics. First, species distribution functions of aqueous Cr(VI), Cr(III), Fe(III), and Fe(II) are deduced to illuminate the quantitative distribution of aqueous metal species. Second, the plausible reaction at pH = 0-14 either at the solid-liquid interface or in solution is determined according to the species distribution function. Third, the spontaneity of each reaction is evaluated via a thermodynamic calculation based on the van't Hoff equation. The results present the following. (1) At the solid-liquid interface, the redox reaction 2Cr(VI) + 3Fe0 → 2Cr(III) + 3Fe(II) is spontaneous, inducing complete Cr(VI) → Cr(III) reduction at pH = 0-14. Especially, the high spontaneity of the redox reaction is mainly ascribed to Fe0 oxidation, which serves as a highly spontaneous subreaction. (2) In solution, the redox reaction Cr(VI) + 3Fe(II) → Cr(III) + 3Fe(III) is nonspontaneous at pH = 6 and 7, whereas it is spontaneous at pH = 6-7, 0-5, and 8-14. Accordingly, no Cr(VI) → Cr(III) reduction at pH = 6-7 and complete Cr(VI) → Cr(III) reduction at pH = 0-5 and 8-14 are expected. Particularly, the nonspontaneity of the Cr(VI) reduction at pH = 6-7 is majorly attributed to water ionization, which is involved as a highly nonspontaneous subreaction. On the contrary, the spontaneity of the Cr(VI) reduction at pH = 0-5 and 8-14 is mainly owing to acid-base neutralization, which is involved as a highly spontaneous subreaction. This work may deepen our knowledge about the chemistry involved in hexavalent chromium remediation by the zerovalent iron.

Synthesis of X@DRHC (X=Co, Ni, Mn) catalyst from comprehensive utilization of waste rice husk and spent lithium-ion batteries for efficient peroxymonosulfate (PMS) activation

Highly efficient resource recycling and comprehensive utilization play a crucial role in achieving the goal of reducing resource wasting, environmental protection, and achieving goal of sustainable development. In this work, the two kinds waste resources of agricultural rice husk and metal ions (Co, Ni, and Mn) from spent lithium-ion batteries have been skillfully utilized to synthesize novel Fenton-like catalysts. Desiliconized rice husk carbon (DRHC) with rich pore structure and large specific surface area from rice husk has been prepared and used as scalable carrier, and dandelion-like nanoparticles cluster could be grown in situ on the surface of the carrier by using metal ions contained waste water. The designed catalysts (X@DRHC) as well as their preparation process were characterized in detail by SEM, TEM, BET, XRD and XPS, respectively. Meanwhile, their catalytic abilities were also studied by activating potassium peroxomonosulfate (PMS) to remove methylene blue (MB). The results indicate X@DRHC displays excellent degradation efficiency on MB with wide pH range and stable reusability, which is suitable for the degradation of various dyes. This work has realized the recycling and high-value utilization of waste resources from biomass and spent lithium-ion batteries, which not only creates an efficient way to dispose waste resources, but also shows high economic benefits in large-scale water treatment.

A novel visible-light-driven Z-scheme C3N5/BiVO4 heterostructure with enhanced photocatalytic degradation performance

As a visible-light response semiconductor materials, bismuth vanadate (BiVO4) is extensively applied in photodegradation organic dye field. In this study, we synthesized C3N5 nanosheets and coupled with decahedral BiVO4 to construct a Z-scheme C3N5/BiVO4 heterostructure with close interface contact. By introducing C3N5 into BiVO4, the built Z-scheme transfer pathway provides silky channel for charge carrier migration between different moieties and enables photoexcited electrons and holes accumulated on the surface of BiVO4 and C3N5. The accelerated separation of charge carriers ensures C3N5/BiVO4 heterostructures with a powerful oxidation capacity compared with pure BiVO4. Due to the synergistic effect in Z-scheme heterostructure, the C3N5/BiVO4 demonstrated an improved photodegradation ability of rhodamine B (RhB) and methylene blue (MB) that of bare BiVO4.

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.

Novel heterogeneous Fenton catalysts for promoting carbon iron electron transfer by one-step hydrothermal synthesization

Carbon materials play a crucial role in promoting the Fe(III)/Fe(II) redox cycle in heterogeneous Fenton reactions. However, the electron transfer efficiency between carbon and iron is typically low. In this study, we prepared a novel heterogeneous Fenton catalyst, humboldtine/hydrothermal carbon (Hum/HTC), using a one-step hydrothermal method and achieved about 100 % reduction in Fe(III) during synthesis. Moreover, the HTC continuously provided electrons to promote Fe(II) regeneration during the Fenton reaction. Electron paramagnetic resonance (EPR) and quenching experiments showed that Hum/HTC completely oxidized As(III) to As(V) via free radical and non-free radical pathways. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) and two-dimensional correlation spectroscopy (2D-COS) analyses revealed that monodentate mononuclear (MM) and bidentate binuclear (BB) structures were the dominant bonding methods for As(V) immobilization. 40 %Hum/HTC exhibited a maximum As(III) adsorption capacity of 167 mg/g, which was higher than that of most reported adsorbents. This study provides a novel strategy for the efficient reduction of Fe(III) during catalyst synthesis and demonstrates that HTC can continuously accelerate Fe(II) regeneration in heterogeneous Fenton reactions.

Fe/sponge structure peanut shell carbon composite preparation for efficient Fenton oxidation crystal violet

In order to obtain super synergy effect between adsorption and Fenton oxidation for crystal violet (CV) removement from water, in this study, Fe modified on a sponge structure peanut shell carbon (Fe/SPSC) nanocomposite was successfully synthesized by a wet impregnation method. In the Fe/SPSC sample, the prepared peanut shell carbon had a sponge-like structure, (002) crystal plane of graphite crystallite, and Fe/SPSC composite coexisted Fe2O3 and Fe3O4 crystalline, which could adsorb and enrich crystal violet molecule, decrease the concentration of CV solution rapidly. And also SPSC could do better for electrons transfer and further promote CV oxidation degradation. The removal efficiency results showed that the 7% Fe/SPSC (500 °C, 2 h) had the best CV removal activity. The composite prepared under the optimum conditions is 2.0 g/L, 0.1 mL 30% H2O2, pH = 7.0, 300 mg/L crystal violet water solution, and the CV degradation rate can reach 95.5%, and the CV degradation amount for Fe/SPSC was 143.25 mg/g. It was confirmed that hydroxyl radicals (•OH) is the active center of Fenton oxidation degradation reaction. XPS results showed that Fe, O, and C elements coexist in the 7% Fe/SPSC composite, and N element content increases after the reaction. Remarkable synergies between adsorption and Fenton oxidation, which could make Fe/SPSC, have quick CV abatement ability. The possible systematic effect mechanism of adsorption and Fenton-oxidation CV was also supplied. The present system has advantages on high CV dye degradation performance, no other Fe sludge formation, short reaction time, and better catalyst reusability.

Spin state-dependent in-situ photo-Fenton-like transformation from oxygen molecule towards singlet oxygen for selective water decontamination

The development of 1O2-dominanted selective decontamination for water purification was hampered by extra H2O2 consumption and poor 1O2 generation. Herein, we proposed the reconstruction of Fe spin state using near-range N atom and long-range N vacancies to enable efficient generation of H2O2 and sequential activation of H2O2 into 1O2 after visible-light irradiation. Theoretical and experimental results revealed that medium-spin Fe(III) strengthened O2 adsorption, penetrated eg electrons to antibonding p-orbital of oxygen, and lowered the free energy of O2 activation, enabling the oxygen protonation for H2O2 generation. Thereafter, the electrons of H2O2 could be extracted by low-spin Fe(III) and rapidly converted into 1O2 in a nonradical path. The developed 1O2-dominated in-situ photo-Fenton-like system had an excellent pH universality and anti-interference to inorganic ions, dissolved organic matter, and even real water matrixes (e.g., tap water and secondary effluent). This work provided a novel insight for sustainable and efficient 1O2 generation, which motivated the development of new-generation selective water treatment technology.

Boosting Fenton's Oxidation Reaction by a Food Waste-Derived Catalyst for Oxidizing Organic Dyes: Synergistic Effect of Complex Iron Oxides and the Layer Carbon Structure

The constant increase in the human population drives the demand for food supply and thereby increasing the food wastage dramatically all over the world. Especially, around 60% of banana biomass has been generated as inedible domestic waste. Herein, we successfully employed banana waste as a catalyst for Fenton's oxidation reaction. The biomass-derived catalysts were subjected to various characterization techniques such as XRD, ATR-FTIR, confocal Raman spectroscopy, and XPS, XRF, BET, SEM, and TEM analyses. The XRD results revealed that, after carbonization of the dried banana bract material, a perloffite-like metal oxide phase was formed due to the aerial oxidation reaction. Characterization results of Raman and ATR-FTIR confirm that the carbonized catalyst possesses a layer-like structure with different types of functional groups. The calcium, magnesium, potassium, sodium, and iron are the dominating metal species in the resultant material, which was evident from the XRF and EDAX analyses. The carbonized banana bract catalyst is successfully utilized for the Fenton's oxidation reaction at neutral pH. The experimental results showed that the degradation efficiency of the fresh catalyst was 95% in 4 h of reaction time, and the stability of the catalyst was retained up to nine consecutive cycles. The high activity of MB, methylene blue, is mainly attributed to the strong interaction between oxy functional groups of the catalyst and MB molecule as compared to RhB. Further, the calculated efficiency of the hydrogen peroxide was found to be 99% and the self-decomposition of hydrogen peroxide by the formed metal oxides was highly limited.

Photocatalytic Self-Fenton System of g-C3N4-Based for Degradation of Emerging Contaminants: A Review of Advances and Prospects

The discharge of emerging pollutants in the industrial process poses a severe threat to the ecological environment and human health. Photocatalytic self-Fenton technology combines the advantages of photocatalysis and Fenton oxidation technology through the in situ generation of hydrogen peroxide (H2O2) and interaction with iron (Fe) ions to generate a large number of strong reactive oxygen species (ROS) to effectively degrade pollutants in the environment. Graphite carbon nitride (g-C3N4) is considered as the most potential photocatalytic oxygen reduction reaction (ORR) photocatalyst for H2O2 production due to its excellent chemical/thermal stability, unique electronic structure, easy manufacturing, and moderate band gap (2.70 eV). Hence, in this review, we briefly introduce the advantages of the photocatalytic self-Fenton and its degradation mechanisms. In addition, the modification strategy of the g-C3N4-based photocatalytic self-Fenton system and related applications in environmental remediation are fully discussed and summarized in detail. Finally, the prospects and challenges of the g-C3N4-based photocatalytic self-Fenton system are discussed. We believe that this review can promote the construction of novel and efficient photocatalytic self-Fenton systems as well as further application in environmental remediation and other research fields.

Deep dewatering of refinery oily sludge by Fenton oxidation and its potential influence on the upgrading of oil phase

Highly efficient dewatering is essential to the reduction and reclamation disposal of oily sludge, which is a waste from the extraction, transportation, and refining of crude oil. How to effectively break the water/oil emulsion is a paramount challenge for dewatering of oily sludge. In this work, a Fenton oxidation approach was adopted for the dewatering of oily sludge. The results show that the oxidizing free radicals originated from Fenton agent effectively tailored the native petroleum hydrocarbon compounds into smaller organic molecules, hence destructing the colloidal structure of oily sludge and decreasing the viscosity as well. Meanwhile, the zeta potential of oily sludge was increased, implying the decrease of repulsive electrostatic force to realize easy coalescence of water droplets. Thus, the steric and electrostatic barriers which restrained the coalescence of dispersed water droplets in water/oil emulsion were removed. With these advantages, the Fenton oxidation approach derived the significant decrease of water content, in which 0.294 kg water was removed from per kilogram oily sludge under the optimal operation condition (i.e., pH value of 3, solid-liquid ratio of 1:10, Fe²⁺ concentration of 0.4 g/L and H₂O₂/Fe²⁺ ratio of 10:1, and reaction temperature of 50 °C). In addition, the quality of oil phase was upgraded after Fenton oxidation treatment accompanying with the degradation of native organic substances in oily sludge, and the heating value of oily sludge was increased from 8680 to 9260 kJ·kg^-1, which would facilitate to the subsequent thermal conversion like pyrolysis or incineration. Such results demonstrate that the Fenton oxidation approach is efficient for the dewatering as well as the upgrading of oily sludge.

Spontaneous Formation of Low Valence Copper on Red Phosphorus to Effectively Activate Molecular Oxygen for Advanced Oxidation Process

Efficient spontaneous molecular oxygen (O₂) activation is an important technology in advanced oxidation processes. Its activation under ambient conditions without using solar energy or electricity is a very interesting topic. Low valence copper (LVC) exhibits theoretical ultrahigh activity toward O₂. However, LVC is difficult to prepare and suffers from poor stability. Here, we first report a novel method for the fabrication of LVC material (P-Cu) via the spontaneous reaction of red phosphorus (P) and Cu²⁺. Red P, a material with excellent electron donating ability and can directly reduce Cu²⁺ in solution to LVC via forming Cu-P bonds. With the aid of the Cu-P bond, LVC maintains an electron-rich state and can rapidly activate O₂ to produce ·OH. By using air, the ·OH yield reaches a high value of 423 μmol g^-1 h^-1, which is higher than traditional photocatalytic and Fenton-like systems. Moreover, the property of P-Cu is superior to that of classical nano-zero-valent copper. This work first reports the concept of spontaneous formation of LVC and develops a novel avenue for efficient O₂ activation under ambient conditions.