Novel insights into Feammox coupled with the NDFO: A critical review.

Ammonium oxidation coupled with Fe(III) reduction, known as Feammox, and nitrate-dependent ferrous oxidation (NDFO) are two processes that can be synergistically achieved through the Fe(III)/Fe(II) cycle. This integrated approach enables the simultaneous removal of ammonia nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) from wastewater, representing a novel method for complete nitrogen removal. This study presents a systematic and exhaustive examination of the Feammox-NDFO coupled process. An initial thorough exploration of the underlying mechanisms behind the coupling process is conducted, highlighting how the Fe(III)/Fe(II) cycle enables the concurrent occurrence of these reactions. Further, the functional microorganisms associated with and playing a crucial role in the Feammox-NDFO process are summarized. Next, the key influencing factors that govern the efficiency of the Feammox-NDFO process are explored. These include parameters such as pH, temperature, carbon source, iron source, nitrogen source, and various electron shuttles that may mediate electron transfer. Understanding the impact of these factors is essential for optimizing the process. The most recent trends and endeavors on the Feammox-NDFO coupling technology in wastewater treatment applications are also examined. This includes examining both laboratory-scale studies and field trials, highlighting their successes and challenges. Finally, an outlook is presented regarding the future advancement of the Feammox-NDFO technology. Areas of improvement and novel strategies that could further enhance the efficiency of simultaneous nitrogen removal from the iron cycle are discussed. In summary, this study aspires to offer a thorough comprehension of the Feammox-NDFO coupled process, with a focus on its mechanisms, influencing factors, applications, and prospects. It is anticipated to yield invaluable insights for the advancement of process optimization, thus sparking fresh ideas and strategies aimed at accomplishing the thorough elimination of nitrogen from wastewater via the iron cycle.

FePO4/WB as an efficient heterogeneous Fenton-like catalyst for rapid removal of neonicotinoid insecticides: ROS quantification, mechanistic insights and degradation pathways.

Iron-based catalysts for peroxymonosulfate (PMS) activation hold considerable potential in water treatment. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, an iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of neonicotinoid insecticides (NEOs). Based on electron paramagnetic resonance (EPR) characterization, scavenging experiments, chemical probe approaches, and quantitative tests, both radicals (HO• and SO4⋅-) and non-radicals (1O2 and Fe(IV)) were produced in the FePO4/WB-PMS system, with relative contributions of 3.02 %, 3.58 %, 6.24 %, and 87.16 % to the degradation of imidacloprid (IMI), respectively. Mechanistic studies revealed that tungsten boride (WB) promoted the reduction of FePO4, and the generated Fe(II) dominantly activated PMS through a two-electron transfer to form Fe(IV), while a minority of Fe(II) engaged in a one-electron transfer with PMS to produce SO4⋅-, HO•, and 1O2. In addition, four degradation pathways of NEOs were proposed by analyzing the byproducts using UPLC-Q-TOF-MS/MS. Besides, seed germination experiments revealed the biotoxicity of NEOs was significantly reduced after degradation via the FePO4/WB-PMS system. Meanwhile, the recycling experiments and continuous flow reactor experiments showed that FePO4/WB exhibited high stability. Overall, this study provided a new perspective on water remediation by Fenton-like reaction. ENVIRONMENTAL IMPLICATION: Neonicotinoids (NEOs) are a type of insecticide used widely around the world. They've been found in many aquatic environments, raising concerns about their possible negative effects on the environment and health. Iron-based catalysts for peroxymonosulfate (PMS) activation hold great promise for water purification. However, the slow conversion of Fe(III) to Fe(II) restricts its large-scale application. Herein, iron phosphate tungsten boride composite (FePO4/WB) was synthesized by a simple hydrothermal method to facilitate the Fe(III)/Fe(II) redox cycle and realize the efficient degradation of NEOs. The excellent stability and reusability provided a great prospect for water remediation.

Reduced sulfur accelerates Fe(III)/Fe(II) recycling in FeS2 surface for enhanced electro-Fenton reaction.

FeS2 is well-known for its role in redox reactions. However, the mechanism within heterogeneous electron-Fenton (Hetero-EF) systems remains unclear. In this study, a novel FeS2 based three-dimensional system (GF/Cu-FeS2) with self-generation of H2O2 was investigated for Hetero-EF degradation of sulfamethazine (SMZ). The results revealed that SMZ could be completely removed in 1.5 h, accompanying with the mineralization efficiency of 96% within 4 h. This system performed excellent stability, evidenced by consistently eliminated 100% of SMZ within 2 h over 4 cycles. The generated Reactive Oxygen Species (ROS) of •OH and •O2- in every degradation cycle were quantitatively measured to confirm the stability of the GF/Cu-FeS2 system. Additionally, the redox reaction mechanism on the surface of FeS2 was thoroughly analyzed in detail. The accelerated reduction of Fe(III) to Fe(II), triggered by S22- on the surface of FeS2, promoted the iron cycling, thereby quickening the Fenton process. Density Functional Theory (DFT) results illustrated the process of S22- to be oxidized to in detail. Therefore, this work provides deeper insight into the mechanistic role of S22- in FeS2 for environmental remediation.

Facile synthesis of pyrite FeS2 on carbon spheres for high-efficiency Fenton-like reaction.

Designing iron-based catalysts for Fenton-like reactions with peroxymonosulfate (PMS) as oxidants have attracted growing attentions. Herein, pyrite FeS2 supported on carbon spheres (FeS2@C) is synthesized by a facile low-temperature method. The FeS2@C/PMS system can degrade carbamazepine (CBZ) effectively in a wide pH range. Sulfate radicals (SO4·-), hydroxyl radicals (·OH), superoxide radical (O2·-), and singlet oxygen (1O2) are the responsible reactive oxygen species (ROSs) for CBZ degradation. Moreover, in the simulated fixed-bed reactor, the FeS2@C/PMS system can maintain a high CBZ removal ratio of >95% for than 8 h, exhibiting its excellent stability. The outstanding performance of FeS2@C/PMS system is attributed to the presence of carbon spheres and lattice S2-, which together promote the Fe(III)/Fe(II) redox cycle. The FeS2@C is a promising catalyst due to its facile synthesis, low cost, high efficiency, and excellent stability to activate PMS for organics degradation.

Boosting the denitrification efficiency of iron-based constructed wetlands in-situ via plant biomass-derived biochar: Intensified iron redox cycle and microbial responses.

Considering the unsatisfied denitrification performance of carbon-limited wastewater in iron-based constructed wetlands (ICWs) caused by low electron transfer efficiency of iron substrates, utilization of plant-based conductive materials in-situ for improving the long-term reactivity of iron substrates was proposed to boost the Fe (III)/Fe (II) redox cycle thus enhance the nitrogen elimination. Here, we investigated the effects of withered Iris Pseudacorus biomass and its derived biochar on nitrogen removal for 165 days in ICWs. Results revealed that accumulate TN removal capacity in biochar-added ICW (BC-ICW) increased by 14.7 % compared to biomass-added ICW (BM-ICW), which was mainly attributed to the synergistic strengthening of iron scraps and biochar. The denitrification efficiency of BM-ICW improved by 11.6 % compared to ICWs, while its removal capacity declined with biomass consumption. Autotrophic and heterotrophic denitrifiers were enriched in BM-ICW and BC-ICW, especially biochar increased the abundance of electroactive species (Geobacter and Shewanella, etc.). An active iron cycle exhibited in BC-ICW, which can be confirmed by the presence of more liable iron minerals on iron scraps surface, the lowest Fe (III)/Fe (II) ratio (0.51), and the improved proportions of iron cycling genes (feoABC, korB, fhuF, TC.FEV.OM, etc.). The nitrate removal efficiency was positively correlated with the nitrogen, iron metabolism functional genes and the electron transfer capacity (ETC) of carbon materials (P < 0.05), indicating that redox-active carbon materials addition improved the iron scraps bioavailability by promoting electron transfer, thus enhancing the autotrophic nitrogen removal. Our findings provided a green perspective to better understand the redox properties of plant-based carbon materials in ICWs for deep bioremediation in-situ.

Ascorbic acid promoted sulfadimidine degradation in the magnetite-activated persulfate system by facilitating the Fe(III)/Fe(II) cycle.

Persulfate (PS) activation technologies were of significant importance to the organic contaminant treatment. In this study, ascorbic acid (AA) was introduced to the traditional PS-activated process by using magnetite (Fe3O4) as the activator; herein, the degradation efficiency of sulfadimidine (SM2) was improved from 30 to 93% within 3 h, and the observed removal rate was about 8.0 times higher than that of the Fe3O4/PS system. These improvements were found to be induced by the added AA because it could reduce the surface Fe(III) to Fe(II) on Fe3O4 and thus facilitate the Fe(III)/Fe(II) cycle, which was conducive to producing reactive oxygen species (ROSs) in the oxidation process during PS activation. Meanwhile, AA could also promote the Fe(III)/Fe(II) cycle in the homogeneous solution, further advancing the PS decomposition for SM2 degradation. The ROS trapping experiments indicated that SM2 removal in the Fe3O4/PS/AA system was attributed to •OH and •SO4-, and •SO4- was the dominant ROS. Moreover, the reusability test experiment revealed that magnetite retained good activity after five cycles in the Fe3O4/AA/PS system. This study provides a promising PS activation technology for efficient organics contaminant treatment.

Enhanced degradation of chloramphenicol via heterogeneous activation of peroxymonosulfate by Fe3O4 and gallic acid.

In this study, we demonstrated the effective degradation of wide-spectrum antibiotic chloramphenicol (CAP) by Fe3O4/peroxymonosulfate (PMS) system modified by gallic acid (GA). GA/Fe3O4/PMS showed a substantially higher degradation rate (77.6%) than Fe3O4/PMS (8.3%). The active components were detected by electron spin-resonance spectroscopy (ESR) and the quenching experiments. The results showed that the hydroxyl radical (HO•) was the main reason for the degradation of CAP. In the GA/Fe3O4/PMS system, the trace amount of dissolved iron ion were not the main species that activated PMS. Surface characterization and theoretical simulations showed that Fe atoms on Fe3O4 were responsible for PMS activation rather than a homogenous reaction. Five probable CAP degradation pathways were identified by density functional theory (DFT) calculations and liquid-phase mass spectrometry. Finally, the reusability of Fe3O4 was measured, and the GA/Fe3O4/PMS system maintained high efficiency after 5 times applications. The total organic carbon (TOC) removal rate reached 46.5% after reacting for 12 h. The gallic acid effectively promotes the circulation of Fe(II)/Fe(III) on solid surfaces and enhanced the degradation capacity of the original system. The research proposed a new way of directly employing plant polyphenols to boost the degradation ability of contaminants in heterogeneous systems.

A novel flow-through dual-system electro-Fenton for boosting PAEs removal efficiency in natural waters.

In a conventional electro-Fenton system with a single cathode, it is difficult to attain both high H2O2 generation by oxygen reduction reaction (ORR) and efficient iron reduction reaction (FRR). For this study, a flow-through dual-system electro-Fenton (FT-DEF) reactor was designed to overcome this shortcoming and promote mass transfer to effectively remove dimethyl phthalate (DMP) from water. By comparing the ORR and FRR performances of four different commercial carbon electrodes, the graphite felt with the highest amount of H2O2 generation was selected as the cathode of the ORR system, and the activated carbon fiber with the best Fe (III) reduction effect was selected as another cathode of the FRR system. The ORR system and FRR system operate simultaneously to form the DEF system. The FT-DEF system displayed many advantages compared with the conventional electro-Fenton (CI-ORR), presenting an improved efficiency and low energy consumption in phthalates removal. Under optimal reaction conditions, the FT-DEF system is capable to degrade 100% DMP in 20 min, which is 25% higher than the CI-ORR, while the reaction rate constant (0.271 min-1) is 16 times that of CI-ORR system (0.017min-1). In addition, the TOC removal of FT-DEF achieving 72.3% within 2 h with energy consumption of 2.35 kW h·m-3 is much better than CI-ORR that only achieves 18.3% TOC removal within 2 h with energy consumption of 8.13 kW h·m-3. Furthermore, control parameters and mechanism of FT-DEF were investigated in detail. The main intermediate products of DMP were analyzed by UPLC-ESI-HRMS, and the possible degradation path of DMP was speculated. In addition, application of FT-DEF in three types of natural water demonstrated its universal applicability of the system.

Insight into mechanism of peroxydisulfate activation by natural pyrite: Participation of Fe(IV) and regulation of Fe(III)/Fe(II) cycle by sulfur species.

In this study, natural pyrite (NP) was used to activate peroxydisulfate (PDS) for imidacloprid (IMD) degradation. NP was characterized by X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Effects of key reaction parameters (NP dosage, PDS concentration and initial pH) and co-existing ions on IMD degradation in the NP/PDS system were investigated. Quenching experiments and electron spin resonance (ESR) tests identified the existence of sulfate radical (SO4•-), hydroxyl radical (•OH), singlet oxygen (1O2) and superoxide radical (O2•-). The cumulative concentration of SO4•- and •OH were quantified by the formation of benzoquinone (BQ) and p-hydroxybenzoic acid (HBA), respectively. Meanwhile, more than 60% of methylphenyl sulfoxide (PMSO) was selectively converted to methylphenyl sulfone (PMSO2), revealing that Fe(IV) was dominant in the NP/PDS system. The order of contribution of the three reactive species in the NP/PDS system was Fe(IV) > â€¢OH > SO4•- (contributions of 1O2 and O2•- were negligible). Fe(II) released from NP played a crucial role in PDS activation, and sulfur species in NP could also boost Fe(III)/Fe(II) cycle and contribute to the generation of reactive species. Further, the possible degradation pathways of IMD have been proposed based on the detected intermediates using high-performance liquid chromatography-mass spectrometry (HPLC-MS), and the toxicity (including acute toxicity, developmental toxicity and mutagenicity) of these intermediates have been predicted using Toxicity Estimation Software Tool (T.E.S.T). Moreover, NP/PDS system was applied in four natural water bodies and IMD degradation efficiency reached more than 97% after adjusting the pH to 3. The fluorescence excitation-emission matrix (EEM) spectra showed that in addition to IMD, NP/PDS system could also remove other impurities.

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

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

Accelerating the Fe(III)/Fe(II) cycle via enhanced electronic effect in NH2-MIL-88B(Fe)/TPB-DMTP-COF composite for boosting photo-Fenton degradation of sulfamerazine.

In the photo-Fenton reactions, fast recombination of photoinduced electrons and holes in Fe-based metal-organic frameworks (Fe-MOFs) slows Fe(III)/Fe(II) cycle, which remains big challenge that significantly retards the overall process. Herein, NH2-MIL-88B(Fe) (NM88) was modified with 3,5-diaminobenzoic acid (DB) and TPB-DMTP-COF (COF-OMe) to in situ construct NM88(DB)0.85/COF-OMe composite that could strongly harvest the visible light for photo-Fenton degradation of sulfamerazine (SMR). With the addition of DB, electron-donating effect of NM88 was strengthened, which then promoted amino groups to react with aldehyde groups (Schiff-base), and thus highly facilitated the interfacial contact between NM88 and COF-OMe. Such modifications increased the degradation rate constants for NM88(DB)0.85/COF-OMe to 15.1 and 17.3 times that of NM88 and COF-OMe respectively with good reusability. Moreover, the catalyst exhibited 32-170 times higher degradation kinetics in comparison to other reported catalysts. Results showed that due to the Schiff-base reaction between NM88(DB) and COF-OMe, electron density on Fe(III) was decreased; and the photogenerated electrons of COF-OMe moved to NM88(DB) to reduce Fe(III), thus resulting in the generation of highly active Fe(II) and ·OH species. Furthermore, the main reactive species were determined to be ·OH and ·O2- by trapping experiments, and a possible mechanism of the degradation system followed Z-scheme charge transfer.

Enhanced degradation of tetracycline over FeS-based Fenton-like process: Autocatalytic decomposition of H2O2 and reduction of Fe(III).

This study constructed a FeS-based Fenton-like process to explore the degradation of TTC in the presence of copper ions. The acidic condition of pH 3 was more favorable to the H2O2 decomposition and TTC degradation, and it was slightly enhanced by Cu(II). The production of •OH from H2O2 was revealed through radical scavenging and benzoic acid probe experiments, and the ratio of H2O2 decomposition to •OH production was about 1-1.5, which is comparatively consistent with the theoretical ratio. FeS-based Fenton process was proved to be a homogenous system, the slow release of Fe(II) source and the autocatalytic cycle of Fe(III) to Fe(II) resulting from the reductive species of TTC and dissolved S(-II) improved the production of •OH and the degradation of TTC, which was proved by comparing TTC degradation, TOC removal, H2O2 decomposition and Fe(II) concentration with different iron sources (FeS, Fe(II) and Fe(III)) and external addition of dissolved S(-II). The possible degradation pathways of TTC were subsequently inferred according to the detected products by LC-MS. Understanding these autocatalytic processes is essential to reveal the transformation of redox-active substances in environments and may have potential significance in applying FeS-based Fenton-like process for the treatment of wastewater containing reductive organic matters.

Reducing agents enhanced Fenton-like oxidation (Fe(III)/Peroxydisulfate): Substrate specific reactivity of reactive oxygen species.

Reduction of Fe(III) is the rate-limiting step of iron induced Fenton-like systems, such as the iron/peroxydisulfate system, reducing agents (RAs) were frequently employed as electron donors to directly reduce Fe(III) to further promote the formation of reactive oxygen species (ROS), mainly including hydroxyl radical (•OH), sulfate radical (SO4•-), and ferryl ion (Fe(IV)). However, the intrinsic distinctions among these ROS cause the substrate specific reactivity towards oxidation of diverse organic contaminants. In this study, various RAs (representative solid amorphous boron (A-Boron) and dissolved hydroxylamine (HA)) were added to enhance the Fe(III)/PDS system for investigating the substrate specific reactivity of ROS. It is demonstrated that RAs remarkably boost the Fe(III)/Fe(II) cycles to produce •OH, SO4•-, and Fe(IV) in the RAs/Fe(III)/PDS systems, based on the results of EPR analysis, quenching tests, and chemical probe analysis. Furthermore, the different yields of methyl phenyl sulfone (PMSO2) indicate that the distribution of multiple oxidizing species changed with various factors (i.e., type and dosage of RAs added, solution pH, Fe(III) and PDS dosage). This work provides the possibility for the adjustment of oxidation selectivity of RAs/Fe(III)/PDS systems by regulating contribution of radicals and non-radical for oxidizing organic contaminants due to the substrate specific reactivity of •OH, SO4•-, and Fe(IV), moreover, the comparison of homogeneous and heterogeneous RAs provides assistance in the application of RAs for environmental remediation.

Sulfide enhances the Fe(II)/Fe(III) cycle in Fe(III)-peroxymonosulfate system for rapid removal of organic contaminants: Treatment efficiency, kinetics and mechanism.

The activation of peroxymonosulfate (PMS) by Fe(II) or Fe(III) for environmental decontamination is severely limited by the low conversion rate from Fe(III) to Fe(II). Here, we found that this puzzling problem could virtually be solved by introducing trace amounts of S2-. With the addition of 0.2 mM S2-, the bisphenol A (BPA) degradation efficiency and total organic carbon (TOC) removal in PMS/Fe(III) system were improved by 3.8 and 6.0 times, respectively. Meanwhile, the kobs and PMS utilization efficiency also markedly increased by 650% and 160%, respectively. The constructed PMS/Fe(III)/S2- system exhibited a good applicability to a wide pH range (3.2 ~ 9.5) and high resistance to humic acid, Cl- and NO3-. The main reactive oxidant species in PMS/Fe(III)/S2- system were identified by scavenging experiments, electron paramagnetic resonance measurement, chemical probe approach, and 18O isotope-labeling technique. The identification results revealed that FeIVO2+ was the primary reactive oxidant species, while •OH, SO4•-, O2•- and 1O2 were also involved in the degradation of BPA. Finally, the generalizability of PMS/Fe(III)/S2- system was evaluated by varying the target pollutants, oxidants, and reducing S species. The construction of PMS/Fe(III)/S2- system provides some insights into the treatment of organic wastewaters containing S2-, e.g., from refineries and tanneries.

Activation of peroxymonosulfate by iron oxychloride with hydroxylamine for ciprofloxacin degradation and bacterial disinfection.

Iron oxychloride (FeOCl) is a known effective iron-based catalyst and has been used in advanced oxidation processes (AOPs). This study intends to achieve more facile free radicals generation from peroxymonosulfate (PMS) activation by exploring the Fe(III)/Fe(II) cycle of FeOCl in the presence of hydroxylamine (HA). With 0.2 g/L FeOCl, 1.5 mM PMS, and 1 mM HA, the PMS/FeOCl/HA system could effectively achieve 98.88% of the oxidative degradation of 5 mg/L ciprofloxacin (CIP) in 15 min and quickly inactivate 99.99% of E. coli (108 CFU/mL) in 5 min at near-neutral pH. HA played an important role in promoting the Fe(III)/Fe(II) cycle, thereby greatly improving the oxidation activity of the system. The reactive oxygen species (ROS) such as HO, SO4- and O2- were identified as the dominated free radicals produced in the system. The intermediate products of CIP detected by liquid chromatograph-mass spectrometer (LC-MS) and three possible degradation pathways of CIP were proposed. The presence of common anions in the PMS/FeOCl/HA system, including HCO3-, Cl-, SO42-, and NO3-, enhanced the degradation efficiency of CIP to varying degrees at the concentrations of 10 mM. Moreover, FeOCl maintained a high degradation capability for CIP after several recycles. This work offers a new promising means of catalyzing the PMS-based AOPs in the degradation of refractory organics.

Enhanced activation of persulfate by Fe(III) and catechin without light: Reaction kinetics, parameters and mechanism.

An environmental-friendly plant polyphenol, catechin (CAT), was applied in Fe(III) activated persulfate (PS) system for naproxen (NPX) degradation in this research. Reaction kinetics, parameters, NPX degradation products and reaction mechanism were investigated. Combining the results of quenching experiments as well as Electron Spin Resonance (ESR), it was observed that SO4•- was critical in NPX degradation, and the contribution of HO• was minor in the Fe(III)/CAT/PS system. O2•- was generated during the reaction but did not contribute to NPX degradation. SO4•- and HO• were produced from the PS activation by Fe(II), which was formed from the transient complexing and reduction process between Fe(III) and CAT. The effects of Fe(III), CAT, PS concentration and pH value on NPX degradation were evaluated. Moreover, the mineralization rate was 20.2%, and the toxicity of the treated solution were lower than the initial solution. Nine possible intermediates were determined when using LC-QTOF-MS to analyze, and three degradation pathways were put ward. The results proved that CAT could accelerate the redox cycle of Fe(III)/Fe(II), consequently to strengthen PS activation without light. It was a promising oxidation technology as it offered an energy-saving and hypo-toxic way for refractory organic pollutants treatment, and it was applicable at a comparatively wide pH range.

Structure activity relationship study of N-doped ligand modified Fe(III)/H2O2 for degrading organic pollutants.

The performance of Fe(III)/H2O2 was extremely enhanced by a novel N-doped ligand dipicolinamide (Dpa) for removing various organic pollutants. This dramatic enhancement of contaminants degradation in Fe(III)-Dpa/H2O2 system under pH≥ 7 was ascribed to the coordinating capacity of Dpa to form the dissolved Fe(III)-Dpa/Fe(II)-Dpa, and the reductive capacity of Dpa to maintain the concentration of Fe(II), which made Dpa improve the catalytic performance of Fe(III) nearly twice as much as Fe(II). Dpa has a strong complexing ability than Cit, NTA, and EDTA to maintain the catalytic activity of Fe(III) without light. The single crystal of Fe-Dpa was obtained to reveal its structure activity relationship. Fe-Dpa was composed of four bonds of Fe-N and two bonds of Fe-Cl. The Fe-Cl bonds were labile sites, which was easily experienced ligand exchange with H2O2, resulting Fe-H2O2 bonds to initiate degradation reaction. The remaining Fe-N bonds were effectively planar, which had a large delocalized π electrons flow domain, enhancing the production of multiple reactive species, including iron(IV/V)-oxo species, HO· and O2-·. An empirical kinetic model of Fe(III)-Dpa/H2O2 system was established. In addition, the evaluation results of the toxicity of Fe-Dpa to larval zebrafish and chinese cabbage displayed that Fe-Dpa possesses low toxicity.

Catalytic degradation of micropollutant by peroxymonosulfate activation through Fe(III)/Fe(II) cycle confined in the nanoscale interlayer of Fe(III)-saturated montmorillonite.

Low cost, green, regenerable catalyst for persulfate activation is the popularly concerned topic for the degradation of persistent organic micropollutants in drinking water. In this work, natural montmorillonite (MMT) saturated with Fe(III) ions was used to activate peroxymonosulfate (PMS) for the degradation of atrazine in raw drinking water. Results showed that the adsorption of atrazine was quickly completed within 1 min and the percentage degradation was finally increased up to 94.1% in 60 min. The d001-spacing of MMT was enlarged to 2.91 nm at the most by Fe(III) saturation. Atrazine was adsorbed into the nanoscale interlayer of Fe(III)-saturated montmorillonite (Fe-MMT), where the Fe(III)/Fe(II) cycle was sustainably realized through the accelerated transformation of electrons between Fe(III) and PMS. Meanwhile, the in-situ generated Fe(II) accelerated the decomposition of PMS to further proceed the degradation of atrazine through the oxidation of HO• and SO4•- radicals. This nanoconfined effect of PMS activation by Fe(III) was further confirmed through the degradation of various micropollutants in the backgrounds of river water. The selective catalytic oxidation of micropollutants through PMS activation was attributed to the 2D mesoporous structure of Fe-MMT, inhibiting the interlayer adsorption of larger molecular backgrounds (humic acids etc.). Fe(III)-saturated montmorillonite (Fe-MMT) provided a feasible and scalable method of PMS activation by Fe(III) for the degradation of micropollutants in drinking water.

Enhancement of bisphenol A degradation by accelerating the Fe(III)/Fe(II) cycle in graphene oxide modified Fe(III)/peroxymonosulfate system under visible light irradiation.

The photocatalytic behavior of the graphene oxide (GO) modified Fe(III)/peroxymonosulfate (Fe/PMS) system for bisphenol A (BPA) degradation was investigated. With the addition of GO, a dramatic enhancement of BPA degradation was obtained at pH 3.0 under visible light irradiation. According to ESR analysis and quenching tests, both SO4- and OH are responsible for BPA degradation. The characterization analysis demonstrates that Fe(III) can chelate with the oxygenic functional groups on the surface of GO forming a stable GO-Fe(III) complex. The detections of different kinds of Fe species reveal that Fe(III) can be reduced to Fe(II) by GO via intramolecular electron transfer in the GO-Fe(III) complex, and visible light could enhance this process. The Fe(III)/Fe(II) cycle not only occurs on the surface of GO, but also in aqueous solution via homogeneous reactions. In addition, the degradation pathway of BPA was proposed based on the identification of the intermediates using GC-MS and LC-MS techniques.

Simulated solar light driven Fe(III)/Fe(II) redox cycle for roxarsone degradation and simultaneous arsenate immobilization.

Organoarsenicals remediation requires degrading organoarsenicals and simultaneously immobilizing the resulted inorganic arsenic, and is thus a great challenge. In this study, a simulated solar light driven Fe(III)/Fe(II) cycle strategy was developed to degrade roxarsone and immobilize the generated inorganic arsenic via tuning the degree of Fe(III) hydrolysis. At pH values of 2.0 and 3.0, the hydrolysis of Fe(III) in the solution was suppressed to produce photoreactive Fe(III)-hydroxyl complexes, which could be excited by simulated solar light to generate OH for 85.3 % of roxarsone degradation into arsenate within 60 min. Density functional theory calculations suggested that Fe(OH)(H2O)52+ with lower energy separation gap was the most photoactive Fe(III)-hydroxyl complex for OH generation. With further increasing pH value to 6.0, the hydrolysis of Fe(III) was promoted to precipitate the arsenate for its immobilization, accompanying with the decrease of final iron ions and arsenate concentrations to 0.012 mmol L-1 and 58 µg L-1, respectively. Meanwhile, the undegraded roxarsone was also adsorbed by the precipitate, increasing the overall roxarsone removal efficiency to 99.0 %. This study offers a promising strategy for the efficient organoarsenicals treatment, and also sheds light on the dual effects of iron based materials in organic pollutants degradation and heavy metal ions immobilization.