UVA-LED-Assisted Activation of the Ferrate(VI) Process for Enhanced Micropollutant Degradation: Important Role of Ferrate(IV) and Ferrate(V)

Roles of iron (V) and iron (IV) species in ferrate-triggered oxidation of phenolic pollutants and their transformation induced by phenoxyl radical

Ferrate is a promising oxidizing agent for water treatment. Understanding the reaction characteristics and transformation mechanism of high-valent intermediate irons [Fe(V) and Fe(IV)] remains challenging. Here, we systematically investigated the roles of Fe(VI), Fe(V), and Fe(IV) species for acetaminophen oxidation using reaction kinetics, products, and stoichiometries. Acetaminophen reacts with Fe(VI) via one-electron transfer mechanism, to initiate a sequential conversion process of "Fe(VI)-Fe(V)-Fe(IV)-Fe(III)", with a stoichiometry [Δacetaminophen/Δferrate] up to 2.20:1. The stoichiometry decreased to 1.23:1 after adding pyrophosphate to sequester Fe(V) oxidation, higher than the Fe(VI)-contributed stoichiometry of 0.58:1, indicating the involvement of Fe(IV) species, not inhibited by pyrophosphate. Dimer yields and theoretical calculations demonstrated that the generated phenoxyl radical could reduce Fe(V) into Fe(IV) even in the presence of pyrophosphate, to achieve the sequential one-electron transfer process. For other phenols containing electron-donating substituents, their phenoxyl radicals could also induce the transformation of Fe(V) into Fe(IV). This organic radical-induced conversion could occur in the reaction of ferrate with natural organic matter, and enhance the effective removal of pollutants. This study highlights the interaction of phenoxyl radical with high-valent iron species, and offers new insights to guide future identification of high-valent iron species in ferrate oxidation.

Dynamic in-situ reconstruction of active site circulators for photo-Fenton-like reactions

Developing efficient and stable heterogeneous catalysts for the continuous activation of oxidants is crucial to mitigating the global water resource crisis. Guided by computational predictions, this research achieved this goal through the synthesis of a modified graphitic carbon nitride with enhanced catalytic activity and stability. Its intrinsic activity was further amplified by dynamic in-situ reconstruction using the I-/I3- redox mediator system during photoreactions. Impressively, this reconstructed catalyst demonstrated the capability for at least 30 regeneration cycles while maintaining high purification efficacy. The mechanism underlying the in-situ reconstruction of active sites for periodate functionalization was elucidated through theoretical calculations, coupled with semi-in-situ X-ray photoelectron spectroscopy (XPS) and electrochemical analyses. The system's capacity to detoxify recalcitrant pollutants was demonstrated through successful Escherichia coli cultivation and Zebrafish embryo experiments. The economic feasibility and environmental impacts are quantitatively assessed by the Electrical Energy per Order (EE/O) metric and Life Cycle Assessment (LCA), confirming the system's scalability and applicability in real-world scenarios. This dual-site constrained interlayer insertion, and controllable in-situ catalyst reconstruction achieve durable robustness of the photocatalyst, paving the way for the development of sustainable catalytic water purification technologies.

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

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

Degradation of carbamazepine by the UVA-LED365/ClO2/NaClO process: Kinetics, mechanisms and DBPs yield

A mixed oxidant of chlorine dioxide (ClO2) and NaClO was often used in water treatment. A novel UVA-LED (365 nm)-activated mixed ClO2/NaClO process was proposed for the degradation of micropollutants in this study. Carbamazepine (CBZ) was selected as the target pollutant. Compared with the UVA365/ClO2 process, the UVA365/ClO2/NaClO process can improve the degradation of CBZ, with the rate constant increasing from 2.11×10-4 sec-1 to 2.74×10-4 sec-1. In addition, the consumption of oxidants in the UVA365/ClO2/NaClO process (73.67%) can also be lower than that of UVA365/NaClO (86.42%). When the NaClO ratio increased, both the degradation efficiency of CBZ and the consumption of oxidants can increase in the UVA365/ClO2/NaClO process. The solution pH can affect the contribution of NaClO in the total oxidant ratio. When the pH range of 6.0-8.0, the combination process can generate more active species to promote the degradation of CBZ. The change of active species with oxidant molar ratio was investigated in the UVA365/ClO2/NaClO process. When ClO2 acted as the main oxidant, HO• and Cl• were the main active species, while when NaClO was the main oxidant, ClO• played a role in the system. Both chloride ion (Cl-), bicarbonate ion (HCO3-), and nitrate ion (NO3-) can promote the reaction system. As the concentration of NaClO in the reaction solution increased, the generation of chlorates will decrease. The UVA365/ClO2/NaClO process can effectively control the formation of volatile disinfection by-products (DBPs), and with the increase of ClO2 dosage, the formation of DBPs can also decrease.

Novel catalyst-free activation of chlorine by visible light for micropollutant abatement

This study systematically investigated the direct activation of chlorine by visible light emitting diode (Vis-LED). Vis-LED could effectively activate chlorine to degrade micropollutants with degradation efficiency and pseudo-first-order degradation rate constant range of 64.3-100 % and 0.0340-0.195 min-1, respectively. Quenching experiments and modeling results suggested that reactive chlorine species (RCS, including ClO•, Cl2•-, and Cl•) and hydroxyl radical (•OH) were involved in the degradation of atenolol (ATL). The contribution ratio of ClO•, free available chlorine, Cl•, Cl2•-, and •OH to ATL degradation were 58.7 %, 17.4 %, 15.6 %, 1.8 %, and 5.9 %, respectively, in Vis-LED448/chlorine process. Moreover, the innate quantum yields of HClO and ClO- decreased from 0.229 and 0.0206 to 0.0489 and 0.0109 mol·Einstein-1, respectively, as the wavelength increased from 448 to 513 nm, leading to a decrease in ATL degradation, which was consistent with the model results. Experimental and modeling results have confirmed that ATL degradation decreased when pH increased from 4.0 to 9.0. Cl- had little effect on the degradation of ATL, while HA and HCO3- affected ATL degradation by scavenging reactive species and/or shielding effect. The concentration of disinfection by-products decreased with the increase of wavelength and pH. In summary, Vis-LED/chlorine is an efficient water treatment process even without a catalyst.

VUV Activated Fe(VI) by Promoting the Generation of Intermediate Valent Iron and Hydroxyl Radicals

In this study, vacuum ultraviolet (VUV) was first proposed to activate ferrate (Fe(VI)) for degrading micropollutants (e.g., carbamazepine (CBZ)). Results indicated that VUV/Fe(VI) could significantly facilitate the CBZ degradation, and the removal efficiencies of VUV/Fe(VI) were 30.9-83.4% higher than those of Fe(VI) at pH = 7.0-9.0. Correspondingly, the degradation rate constants of VUV/Fe(VI) were 2.3-36.0-fold faster than those of Fe(VI). Free radical quenching and probe experiments revealed that the dominant active species of VUV/Fe(VI) were •OH and Fe(V)/Fe(IV), whose contribution ratios were 43.3 to 48.6% and 48.2 to 46.6%, respectively, at pH = 7.0-9.0. VUV combined with Fe(VI) not only effectively mitigated the weak oxidizing ability of Fe(VI) under alkaline conditions (especially pH = 9.0) but also attenuated the deteriorating effect of background constituents on Fe(VI). In different real waters (tap water, river water, WWTPs effluent), VUV/Fe(VI) retained a remarkably enhanced effect on CBZ degradation compared to Fe(VI). Moreover, VUV/Fe(VI) exhibited outstanding performance in the debasement of CBZ and sulfamethoxazole (SMX), as well as six other micropollutants, displaying broad-spectrum capability in degrading micropollutants. Overall, this study developed a novel oxidation process that was efficient and energy-saving for the rapid removal of micropollutants.

A new perspective on contaminants as "activators": Aromatic amine groups promoted degradation of tetracycline by ferrate(VI)

Occasionally, our group found that the degradation of tetracycline by ferrate(VI) could be promoted by four co-exist contaminants, containing aromatic amines (ofloxacin, diatrizoic acid, sulfadiazine and alachlor). This study investigated the promotion of aromatic amine groups on tetracycline degradation by ferrate(VI) by using aniline as a model compound. The results implied that the presence of aniline increased the degradation rate of tetracycline by 2.76 times, and the enhancement was weakened gradually with the decrease of pH from 10 to 7.5. The generation of Fe(IV) and·OH by the reaction between ferrate(VI) and aniline was proposed to enhance the degradation of tetracycline, supported by quenching experiments, electron paramagnetic resonance (EPR) and theoretical calculations. A positive correlation was found between the rate constant of tetracycline degradation and the electron-donating ability of the substituted amines (quantified by the Hammett substituent constants). In addition, the degradation of tetracycline was remarkably inhibited by HA and some inorganic ions such as NO3-, SO42-, Cl-, Ca2+, and Mg2+, and the inhibition also happened in the Songhua River water and the secondary effluent. The present study provided an insight into the complex oxidation process for the degradation of micropollutants containing aromatic amine by ferrate in water treatment.

Activation of peroxymonosulfate by novel magnetically recyclable CoFe2O4/MXene quantum dots composites for rapid degradation of tetracycline: Synergistic performance and mechanisms

Tetracycline (TC), a commonly used antibiotic in wastewater, poses environmental and health risks, thus demanding advanced catalysts for its effective removal. In this work, for the first time, we integrated cobalt ferrite (CoFe2O4) and MXene quantum dots (MQDs) to form magnetic heterojunctions for rapid degradation of TC in the presence of peroxymonosulfate (PMS). Anchoring MQDs on the CoFe2O4 nanoparticles remarkably promoted the overall degradation rate of TC to 98.2% within 20 min via both radical and non-radical pathways. The first-order kinetic constant was 0.170 min-1, 3.5 and 15.5 times higher than that of CoFe2O4 and MQDs alone, respectively. Quenching experiments revealed that the addition of p-benzoquinone (p-BQ) and furfuryl alcohol (FFA) reduced the degradation of TC within 20 min to 56.2% and 28.4%, respectively, indicating that the primary reactive oxygen species for TC degradation in the CoFe2O4/MQDs + PMS system are •O2- and 1O2. CoFe2O4/MQDs also exhibited superparamagnetic property, which enabled their effective recovery by external magnetic field. Their reusability was verified by retaining 81.4% of catalytic efficacy in the consecutive 8th cycle. The CoFe2O4/MQDs + PMS system also exhibited excellent practicability in natural water samples as the degradation rates in both tap water and lake water environments exceeded 90%. Three potential pathways for TC degradation were proposed based on the liquid chromatography-mass spectrometry (LC-MS) characterizations and TC progressively transformed into 13 intermediates. This work may contribute to the ongoing efforts to develop advanced catalysts and strategies for mitigating the environmental impact of antibiotic pollution, offering a pathway toward sustainable and efficient water treatment technologies.

Ferrate (VI) oxidation of sulfamethoxazole enhanced by magnetized sludge-based biochar: Active sites regulation and degradation mechanism analysis

Developing non radical systems for antibiotic degradation is crucial for addressing the inefficiency of conventional radical systems. In this study, novel magnetic-modified sludge biochar (MASBC) was synthesized to significantly enhance the oxidative degradation of sulfamethoxazole (SMX) by ferrate (Fe (VI)). In the Fe (VI)/MASBC system, 90.46% of SMX at a concentration of 10 μM and 49.34% of the total organic carbon (TOC) could be removed under optimal conditions of 100 μM of Fe (VI) and 0.40 g/L of MASBC within 10 min. Furthermore, the Fe (VI)/MASBC system was demonstrated with broad-spectrum removal capability towards sulfonamides in single or mixture. Quenching experiments, EPR analyses, and electrochemical experiments revealed that direct electron transfer (DET) and •O2- were mainly responsible for the removal of SMX, with functional groups (e.g., -OH, C=O) and Fe-O (redox of Fe (III)/Fe (II)) acting as the active sites, while the probe experiments showed that Fe (IV)/Fe (V) made a minor contribution to the degradation of SMX. Benefiting from the DET, the Fe (VI)/MASBC system exhibited a wide pH adaptation range (e.g., from 5.0 to 10.0) and strong anti-interference ability. The N atoms and their neighboring atoms in SMX were the prior degradation sites, with the cleavage of bond and ring opening. The degradation products showed low or non-toxicity according to ECOSAR program assessment. The removal of SMX remained within a reasonable range of 71.33%-90.46% over five consecutive cycles. Also, the Fe (VI)/MASBC system was demonstrated to be effectively applied for successful SMX removal in various water matrices, including ultrapure water, tap water, lake water, Yangtze River water, and wastewater. Therefore, this study offered new insights into the mechanism of Fe (VI) oxidation and would contribute to the efficient treatment of organic pollutants.

Deciphering the Novel Picolinate-Mn(II)/peroxymonosulfate System for Sustainable Fenton-like Oxidation: Dominance of the Picolinate-Mn(IV)-peroxymonosulfate Complex

A highly efficient and sustainable water treatment system was developed herein by combining Mn(II), peroxymonosulfate (PMS), and biodegradable picolinic acid (PICA). The micropollutant elimination process underwent two phases: an initial slow degradation phase (0-10 min) followed by a rapid phase (10-20 min). Multiple evidence demonstrated that a PICA-Mn(IV) complex (PICA-Mn(IV)*) was generated, acting as a conductive bridge facilitating the electron transfer between PMS and micropollutants. Quantum chemical calculations revealed that PMS readily oxidized the PICA-Mn(II)* to PICA-Mn(IV)*. This intermediate then complexed with PMS to produce PICA-Mn(IV)-PMS*, elongating the O-O bond of PMS and increasing its oxidation capacity. The primary transformation mechanisms of typical micropollutants mediated by PICA-Mn(IV)-PMS* include oxidation, ring-opening, bond cleavage, and epoxidation reactions. The toxicity assessment results showed that most products were less toxic than the parent compounds. Moreover, the Mn(II)/PICA/PMS system showed resilience to water matrices and high efficiency in real water environments. Notably, PICA-Mn(IV)* exhibited greater stability and a longer lifespan than traditional reactive oxygen species, enabling repeated utilization. Overall, this study developed an innovative, sustainable, and selective oxidation system, i.e., Mn(II)/PICA/PMS, for rapid water decontamination, highlighting the critical role of in situ generated Mn(IV).

Ferrate(VI)-based oxidation for ultrafiltration membrane fouling mitigation in shale gas produced water pretreatment: Role of high-valent iron intermediates and hydroxyl radicals

Ultrafiltration (UF) is increasingly used in the pretreatment of shale gas produced water (SGPW), whereas severe membrane fouling hampers its actual operation. In this work, ferrate(VI)-based oxidation was proposed for membrane fouling alleviation in SGPW pretreatment, and the activation strategies of calcium peroxide (CaO2) and ultraviolet (UV) were selected for comparison. The findings indicated that UV/Fe(VI) was more effective in removing fluorescent components, and the concentration of dissolved organic carbon was reduced by 24.1 %. With pretreatments of CaO2/Fe(VI) and UV/Fe(VI), the terminal specific membrane flux was elevated from 0.196 to 0.385 and 0.512, and the total fouling resistance diminished by 52.7 % and 76.2 %, respectively. Interfacial free energy analysis indicated that the repulsive interactions between pollutants and membrane were notably enhanced by Fe(VI)-based oxidation, thereby delaying the deposition of cake layers on the membrane surface. Quenching and probe experiments revealed that high-valent iron intermediates (Fe(IV)/Fe(V)) played significant roles in both CaO2/Fe(VI) and UV/Fe(VI) processes. Besides, hydroxyl radicals (•OH) were also important reactive species in the UV/Fe(VI) treatment, and the synergistic effect of Fe(IV)/Fe(V) and •OH showed a positive influence on SGPW fouling mitigation. In general, these findings establish a theoretical underpinning for the application of Fe(VI)-based oxidation for UF membrane fouling mitigation in SGPW pretreatment.

A novel visible light-driven oxygen doped C3N4/Bi12O17Cl2/ferrate(VI) system for Bisphenol A degradation: Radical and nonradical pathways

In this study, visible light-activated photocatalyst oxygen-doped C3N4@Bi12O17Cl2 (OCN@BOC) and Fe(VI) coupling system was proposed for the efficient degradation of bisphenol A (BPA). The comprehensive characterization of the OCN@BOC photocatalyst revealed its excellent photogenerated carrier separation rate in heterogeneous structures. The OCN@BOC/Fe(VI)/Vis system exhibited a remarkable BPA removal efficiency of over 84% within 5 min. Comparatively, only 37% and 59% of BPA were degraded by single OCN@BOC and Fe(VI) in 5 min, respectively. Reactive species scavenging experiments, phenyl sulfoxide transformation experiments, and electron paramagnetic resonance experiments confirmed the involvement of superoxide radicals (⋅O2-), singlet oxygen (1O2), as well as iron(V)/iron(IV) (Fe(V)/Fe(IV)) species in the degradation process of BPA. Furthermore, density functional theoretical calculations and identification of intermediates provided insights into the potential degradation mechanism of BPA during these reactions. Additionally, simulation evaluations using an ecological structure activity relationship model demonstrated that the toxicity of BPA to the ecological environment was mitigated during its degradation process. This study presented a novel strategy for removing BPA utilizing visible light photocatalysts, highlighting promising applications for practical water environment remediation with the OCN@BOC/Fe(VI)/Vis system.

Selective oxidation decontamination in cobalt molybdate activated Fenton-like oxidation via synergic effect of cobalt and molybdenum

In this study, cobalt molybdate (CoMoO4) activated peracetic acid (PAA) was developed for water purification. CoMoO4/PAA system could remove 95% SMX with pseudo-first-order reaction rate constant of 0.15410 min-1, which was much higher than CoFe2O4/PAA, FeMoO4/PAA, and CoMoO4/persulfate systems. CoMoO4/PAA system follows a non-radical species pathway dominated by the high-valent cobalt (Co(IV)), and CH3C(O)OO• shows a minor contribution to decontamination. Density functional theory (DFT) calculation indicates that the generation of Co(IV) is thermodynamically more favorable than CH3C(O)OO• generation. The abundant Co(IV) generation was attributed to the special structure of CoMoO4 and effect of molybdenum on redox cycle of Co(II)/Co(III). DFT calculation showed that the atoms of SMX with higher ƒ0 and ƒ- values are the main attack sites, which are in accordance with the results of degradation byproducts. CoMoO4/PAA system can effectively reduce biological toxicity after the reaction. Benefiting from the selective of Co(IV) and CH3C(O)OO•, the established CoMoO4/PAA system exhibits excellent anti-interference capacity and satisfactory decontamination performance under actual water conditions. Furthermore, the system was capable of good potential practical application for efficient removal of various organics and favorable reuse. Overall, this study provides a new strategy by CoMoO4 activated PAA for decontamination with high efficiency, high selectivity and favorable anti-interference.

Direct Phototransformation of Sulfamethoxazole Characterized by Four-Dimensional Element Compound Specific Isotope Analysis

The antibiotic sulfamethoxazole (SMX) undergoes direct phototransformation by sunlight, constituting a notable dissipation process in the environment. SMX exists in both neutral and anionic forms, depending on the pH conditions. To discern the direct photodegradation of SMX at various pH levels and differentiate it from other transformation processes, we conducted phototransformation of SMX under simulated sunlight at pH 7 and 3, employing both transformation product (TP) and compound-specific stable isotope analyses. At pH 7, the primary TPs were sulfanilic acid and 3A5MI, followed by sulfanilamide and (5-methylisoxazol-3-yl)-sulfamate, whereas at pH 3, a photoisomer was the dominant product, followed by sulfanilic acid and 3A5MI. Isotope fractionation patterns revealed normal 13C, 34S, and inverse 15N isotope fractionation, which exhibited significant differences between pH 7 and 3. This indicates a pH-dependent transformation process in SMX direct phototransformation. The hydrogen isotopic composition of SMX remained stable during direct phototransformation at both pH levels. Moreover, there was no variation observed in 33S between the two pH levels, indicating that the 33S mass-independent process remains unaffected by changes in pH. The analysis of main TPs and single-element isotopic fractionation suggests varying combinations of bond cleavages at different pH values, resulting in distinct patterns of isotopic fractionation. Conversely, dual-element isotope values at different pH levels did not significantly differ, indicating cleavage of several bonds in parallel. Hence, prudent interpretation of dual-element isotope analysis in these systems is warranted. These findings highlight the potential of multielement compound-specific isotope analysis in characterizing pH-dependent direct phototransformation of SMX, thereby facilitating the evaluation of its natural attenuation through sunlight photolysis in the environment.

Ferrate(VI)/percarbonate for the oxidation of micropollutants: Interactive activation and release of low-concentration hydrogen peroxide for efficient electron utilization

A novel "ferrate/percarbonate (Fe(VI)/SPC) co-oxidation process" was used to treat ciprofloxacin (CIP) and various micropollutants (MPs), which owned better performance than mixture of Fe(VI), Na2CO3 and H2O2. The mechanism investigation found that the low-concentration H2O2 (1-2 µM) released by SPC can promote the high-valent iron intermediates (Fe(IV)/Fe(V)) of Fe(VI) to the MP oxidation, and Fe(VI) products can also activate SPC to produce hydroxyl radical (·OH). The interactive activation of Fe(VI) and SPC was realized, which retained the high selectivity of Fe(VI) to electron-rich pollutants, and also made up the oxidation of electron-deficient pollutants through •OH, improving the degradation effect of various MPs by 20-30%, and the rate constant was increased by 1 to 3 times. Moreover, non-purgeable organic carbon (NPOC) determination confirmed that •OH participation reduced the NPOC value of CIP from 5.43 mg/L to 4.37 mg/L. The transformation pathway of CIP showed that Fe(VI)/SPC resulted in more hydroxylation intermediates of CIP than Fe(VI) alone. Acute toxicity assays found that the photoinhibition rate of CIP treated with Fe(VI) alone was 14.5%, while the sample treated with Fe(VI)/SPC showed no significant photoinhibition effect, which proved that the new process had good detoxification properties for CIP.

Role of bipyridyl in enhancing ferrate oxidation toward micropollutants

Enhancing Fe(VI) oxidation ability by generating high-valent iron-oxo species (Fe(IV)/Fe(V)) has attracted continuous interest. This work for the first time reports the efficient activation of Fe(VI) by a well-known aza-aromatic chelating agent 2,2'-bipyridyl (BPY) for micropollutant degradation. The presence of BPY increased the degradation constants of six model compounds (i.e., sulfamethoxazole (SMX), diclofenac (DCF), atenolol (ATL), flumequine (FLU), 4-chlorophenol (4-CP), carbamazepine (CBZ)) with Fe(VI) by 2 - 6 folds compared to those by Fe(VI) alone at pH 8.0. Lines of evidence indicated the dominant role of Fe(IV)/Fe(V) intermediates. Density functional theory calculations suggested that the binding of Fe(III) to one or two BPY molecules initiated the oxidation of Fe(III) to Fe(IV) by Fe(VI), while Fe(VI) was reduced to Fe(V). The increased exposures of Fe(IV)/Fe(V) were experimentally verified by the pre-generated Fe(III) complex with BPY and using methyl phenyl sulfoxide as the probe compound. The presence of chloride and bicarbonate slightly affected model compound degradation by Fe(VI) in the presence of BPY, while a negative effect of humic acid was obtained under the same conditions. This work demonstrates the potential of N-donor heterocyclic ligand to activate Fe(VI) for micropollutant degradation, which is instructive for the Fe(VI)-based oxidation processes.

Mechanical energy triggered piezo-catalyzation of Bi2WO6 nanoplates on ferrate (Fe(VI)) oxidation in alkaline media: Performance and mechanism

Piezo-electricity, as a unique physical phenomenon, demonstrates high effectiveness in capturing the environmental mechanical energy into polarization charges, offering the possibility to activate the advanced oxidation processes via the electron pathway. However, information regarding the intensification of Fe(VI) through piezo-catalysis is limited. Therefore, our study is the first to apply Bi2WO6 nanoplates for piezo-catalyzation of Fe(VI) to enhance bisphenol A (BPA) degradation. Compared to Fe(VI) alone, the Fe(VI)/piezo/Bi2WO6 system exhibited excellent BPA removal ability, with the degradation rate increased by 32.6% at pH 9.0. Based on the experimental and theoretical results, Fe(VI), Fe(V), Fe(IV) and •OH were confirmed as reaction active species in the reaction, and the increased BPA removal mainly resulted from the enhanced formation of Fe(IV)/Fe(V) species. Additionally, effects of coexisting anions (e.g., Cl-, NO3-, SO42- and HCO3-), humic acid and different water matrixes (e.g., deionized water, tap water and lake water) on BPA degradation were studied. Results showed the Fe(VI)/piezo/Bi2WO6 system still maintained satisfactory BPA degradation efficiencies under these conditions, guaranteeing future practical applications in surface water treatment. Furthermore, the results of intermediates identification, ECOSAR calculation and cytotoxicity demonstrated that BPA degradation by Fe(VI)/piezo/Bi2WO6 posed a diminishing ecological risk. Overall, these findings provide a novel mechanical energy-driven piezo-catalytic approach for Fe(VI) activation, enabling highly efficient pollutant removal under alkaline condition.

How Should We Activate Ferrate(VI)? Fe(IV) and Fe(V) Tell Different Stories about Fluoroquinolone Transformation and Toxicity Changes

Many studies have investigated activation of ferrate (Fe(VI)) to produce reactive high-valent iron intermediates to enhance the oxidation of micropollutants. However, the differences in the risk of pollutant transformation caused by Fe(IV) and Fe(V) have not been taken seriously. In this study, Fe(VI)-alone, Fe3+/Fe(VI), and NaHCO3/Fe(VI) processes were used to oxidize fluoroquinolone antibiotics to explore the different effects of Fe(IV) and Fe(V) on product accumulation and toxicity changes. The contribution of Fe(IV) to levofloxacin degradation was 99.9% in the Fe3+/Fe(VI) process, and that of Fe(V) was 89.4% in the NaHCO3/Fe(VI) process. The cytotoxicity equivalents of levofloxacin decreased by 1.9 mg phenol/L in the Fe(IV)-dominant process while they significantly (p < 0.05) increased by 4.7 mg phenol/L in the Fe(V)-dominant process. The acute toxicity toward luminescent bacteria and the results for other fluoroquinolone antibiotics also showed that Fe(IV) reduced the toxicity and Fe(V) increased the toxicity. Density functional theory calculations showed that Fe(V) induced quinolone ring opening, which would increase the toxicity. Fe(IV) tended to oxidize the piperazine group, which reduced the toxicity. These results show the different-pollutant transformation caused by Fe(IV) and Fe(V). In future, the different risk outcomes during Fe(VI) activation should be taken seriously.

Oxidation of emerging contaminants by S(IV) activated ferrate: Identification of reactive species

Studies on the Fe(VI)/S(IV) process have focused on improving the efficiency of emerging contaminants (ECs) degradation under alkaline conditions. However, the performance and mechanisms under varying pH levels remain insufficiently investigated. This tudy delved into the efficiency and mechanism of Fe(VI)/S(IV) process using sulfamethoxazole (SMX) and ibuprofen (IBU) as model contaminants. We found that pH was crucial in governing the generation of reactive species, and both Fe(V/IV) and SO4•- were identified in the reaction system. Specifically, an increase in pH favored the formation of SO4•-, while the formation of Fe(VI) to Fe(V/IV) became more significant at lower pH. At pH 3.2, Fe(III) resulting from the Fe(VI) self-decay reactedwith HSO3-to produce SO4•-and •OH. Under near-neutral conditions, the coexistance of Fe(V/IV) and SO4•- in abundance contributed to the optimal oxidation of both pollutants in the Fe(VI)/S(IV) process, with the removal exceeding 74% in 5 min. Competitive quenching experiments showed that the contributions of Fe(V/IV) to SMX and IBU destruction dimished, while the contributions of radicals increased with an increase in pH. However, this evolution was slower during SMX degradation compared to IBU degradation. A comprehensive understnding of pH as the key factor is essential for the optimization of the sulfite-activated Fe(VI) oxidation process in water treatment.

Potassium permanganate oxidation enhanced by infrared light and its application to natural water

Photocoupled permanganate (PM) is an effective way to enhance the oxidation efficiency of PM, however, the activation of PM by infrared has received little attention. This study aimed to investigate the ability of infrared light to activate PM. When coupled with infrared, the degradation rate of 4-chlorophenol (4-CP) is increased to 3.54 times of PM oxidation alone. The accelerated reaction was due to the formation of vibrationally excited PM by absorbing 3.1 kJ mol-1 infrared energy, which also leads to the primary reactive intermediates Mn(V/IV) in the reaction system. The infrared coupled PM system also showed 1.14-2.34 times promotion effect on other organic pollutants. Furthermore, solar composed of 45% infrared, coupled PM system showed excellent degradation performance, where the degradation of 4-CP in 10 L of tap water and river water was 68 and 23 times faster than in ultrapure water, respectively. The faster-increased degradation rate in natural waters is mainly due to the abundant inorganic ions, which can stabilize the manganese species, and then has a positive effect on 4-CP degradation. In summary, this work develops a energy-efficient photoactivated PM technology that utilizes infrared and provides new insights into the design of novel sunlight-powered oxidation processes for water treatment.

Efficient treatment of actual glyphosate wastewater via non-radical Fenton-like oxidation

Compared to radical oxidative pathway, recent research revealed that non-radical oxidative pathway has higher selectivity, higher adaptability and lower oxidant requirement. In this work, we have designed and synthesized Cu2O/Cu nanowires (CuNWs), by pyrolysis of copper chloride and urea, to selectively generate high-valent copper (CuIII) upon H2O2 activation for the efficient treatment of actual glyphosate wastewater. The detailed characterizations confirmed that CuNWs nanocomposite was comprised of Cu0 and Cu2O, which possessed a nanowire-shaped structure. The electron paramagnetic resonance (EPR) analysis, in situ Raman spectra, chronoamperometry and liner sweep voltammetry (LSV) verified CuIII, which mainly contributed to glyphosate degradation, was selectively generated from CuNWs/H2O2 system. In particular, CuI is mainly oxidized by H2O2 into CuIIIvia dual-electron transfer, rather than simultaneously releasing OH• via single electron transfer. More importantly, CuNWs/H2O2 system exhibited the excellent potential in the efficient treatment of actual glyphosate wastewater, with 96.6% degradation efficiency and chemical oxygen demand (COD) dropped by 30%. This novel knowledge gained in the work helps to apply CuNWs into heterogeneous Fenton-like reaction for environmental remediation and gives new insights into non-radical pathway in H2O2 activation.

A two-stage Fe(VI) oxidation process enhances the removal of bisphenol A for potential application

Ferrate (Fe(VI)) has been extensively studied as a green oxidant to treat wastewater. But Fe(VI) oxidation still faces several challenges for application, such as the sensitivity of Fe(VI) to pH and the restrictions on the Fe(VI) utilization efficiency for pollutant elimination at low concentration levels. This study proposed a two-stage Fe(VI) oxidation process to enhance the bisphenol A (BPA) removal for potential applicability, consisting of the adsorption by CNTs of stage I and the degradation by Fe(VI) of stage II. The Fe(VI) utilization efficiency in the two-stage process (0.848) was higher than that in one-stage processes (0.727) and Fe(VI) alone system (0.504) at pH 9. In stage I, the adsorption process had good compliance with the Langmuir isotherm model and pseudo-second-order kinetic model. In stage II, the effective utilization of low-concentration Fe(VI) was 2.45 times more than Fe(VI) alone, and the reduction of reaction volume was beneficial to further enhance utilization. The probe experiments (sulfoxide) and the degradation experiments of other electron-donating/withdrawing pollutants (e.g., atrazine, benzoic acid) demonstrated that Fe(IV) and Fe(V) were major oxidizing species in the two-stage process. The regeneration experiments showed that CNTs still had acceptable adsorption and catalytic capabilities after five cycles. Finally, the intermediate products in the two-stage process were detected and four possible degradation pathways of BPA were proposed. These findings were meaningful for the practical application of Fe(VI) oxidation to overcome the conditional limitation and improve the utilization.

Coupling pretreatment of ultraviolet/ferrate (UV/Fe(vi)) for improving the ultrafiltration of natural surface water

Ultrafiltration (UF) is a high-potential technology for purifying natural surface water; however, the problem of membrane fouling has limited its widespread application. Herein, ultraviolet (UV)-activated ferrate (Fe(vi)) was used to purify natural surface water and improve the performance of the UF membrane. The combination of UV and Fe(vi) could generate active species (Fe(v), Fe(iv), ˙OH and O2˙-) to degrade pollutants, while the in situ produced Fe(iii) had the effect of coagulation. With the above action, pollutants were removed, and the pollution load of natural surface water was reduced. After treatment with the UV/Fe(vi) system, dissolved organic carbon was reduced by 49.38%, while UV254 was reduced by 45.00%. The removal rate was further increased to 54.88% and 51.67% after UF treatment. In addition, the fluorescent organics were reduced by 44.22%, and the molecular weight of the organics became smaller. In the stage of UF, the terminal J/J0 was increased from 0.61 to 0.92, and the membrane fouling resistance was decreased by 85.94%. The analysis of the membrane fouling mechanism indicates that the role of cake filtration was weakened among all the mechanisms. Fourier transform infrared spectroscopy showed that less pollutants were accumulated on the membrane surface, and scanning electron microscopy revealed that the membrane pore blockage was relieved. In summary, the UV/Fe(vi) co-treatment process proposed in this study can significantly improve the purification efficiency of the UF systems in natural surface water treatment.

pH-Driven Efficacy of the Ferrate(VI)-Peracetic Acid System in Swift Sulfonamide Antibiotic Degradation: A Deep Dive into Active Species Evolution and Mechanistic Insights

In the realm of wastewater treatment, the power of ferrate (Fe(VI)) and peracetic acid (PAA) as oxidants stands out. But their combined might is where the enhancement truly lies. Their collaborative effect intensifies, but the underlying mechanics, especially across varying pH levels and pollutant types, still lurks in obscurity. Our study delved into the sophisticated oxidation interplay among Fe(VI)-PAA, Fe(VI)-H2O2, and standalone Fe(VI) systems. Notably, at a pH of 9.0, boasting a kinetic constant of ∼0.127 M-1·s-1, the Fe(VI)-PAA system annihilated the pollutant sulfamethoxazole, outpacing its counterparts by a staggering 48.73-fold when compared to the Fe(VI)-H2O2 system and 105.58-fold when using Fe(VI) individually. The behavior of active species─such as the dynamic •OH radicals and high-valent iron species (Fe(IV)/Fe(V))─shifted with pH variations, leading to distinct degradation pathways. Our detailed exploration pinpoints the behaviors of certain species across pH levels from 3.0 to 9.0. In more acidic environments, the •OH species proved indispensable for the system's reactivity. Conversely, as the pH inclined, degradation was increasingly steered by high-valent iron species. This intensive probe demystifies Fe(VI) interactions, deepening our understanding of the capabilities of the Fe(VI)-centered system and guiding us toward cleaner water solutions. Importantly, pH value, often underappreciated, holds the reins in organic wastewater decontamination. Embracing this key player is vital as we strategize for more expansive systems in upcoming ventures.

Novel Synthesis Pathways for Highly Oxidative Iron Species: Generation, Stability, and Treatment Applications of Ferrate(IV/V/VI)

Difficulties arise related to the economy-of-scale and practicability in applying conventional water treatment technologies to small and remote systems. A promising oxidation technology better suited for these applications is that of electro-oxidation (EO), whereby contaminants are degraded via direct, advanced, and/or electrosynthesized oxidant-mediated reactions. One species of oxidants of particular interest includes ferrates (Fe(VI)/(V)/(IV)), where only recently has their circumneutral synthesis been demonstrated, using high oxygen overpotential (HOP) electrodes, namely boron-doped diamond (BDD). In this study, the generation of ferrates using various HOP electrodes (BDD, NAT/Ni-Sb-SnO2, and AT/Sb-SnO2) was investigated. Ferrate synthesis was pursued in a current density range of 5-15 mA cm-2 and initial Fe3+ concentrations of 10-15 mM. Faradaic efficiencies ranged from 11-23%, depending on operating conditions, with BDD and NAT significantly outperforming AT electrodes. Speciation tests revealed that NAT synthesizes both ferrate(IV/V) and ferrate(VI), while the BDD and AT electrodes synthesized only ferrate(IV/V) species. A number of organic scavenger probes were used to test the relative reactivity, including nitrobenzene, carbamazepine, and fluconazole, whereby ferrate(IV/V) was significantly more oxidative than ferrate(VI). Finally, the ferrate(VI) synthesis mechanism by NAT electrolysis was elucidated, where coproduction of ozone was found to be a key phenomenon for Fe3+ oxidation to ferrate(VI).

Ozone- and Hydroxyl Radical-Induced Degradation of Micropollutants in a Novel UVA-LED-Activated Periodate Advanced Oxidation Process

In this study, novel light emitting diode (LED)-activated periodate (PI) advanced oxidation process (AOP) at an irradiation wavelength in the ultraviolet A range (UVA, UVA-LED/PI AOP) was developed and investigated using naproxen (NPX) as a model micropollutant. The UVA-LED/PI AOP remarkably enhanced the degradation of NPX and seven other selected micropollutants with the observed pseudo-first-order rate constants ranging from 0.069 ± 0.001 to 4.50 ± 0.145 min-1 at pH 7.0, demonstrating a broad-spectrum micropollutant degradation ability. Lines of evidence from experimental analysis and kinetic modeling confirmed that hydroxyl radical (•OH) and ozone (O3) were the dominant species generated in UVA-LED/PI AOP, and they contributed evenly to NPX degradation. Increasing the pH and irradiation wavelength negatively affected NPX degradation, and this could be well explained by the decreased quantum yield (ΦPI) of PI. The degradation kinetics of NPX by the UVA-LED/PI AOP in the presence of water matrices (i.e., chloride, bicarbonate, and humic acid) and in real waters were examined, and the underlying mechanisms were illustrated. A total of nine transformation products were identified from NPX oxidation by the UVA-LED/PI AOP, mainly via hydroxylation, dealkylation, and oxidation pathways. The UVA-LED/PI AOP proposed might be a promising technology for the treatment of micropollutants in aqueous solutions. The pivotal role of ΦPI during light photolysis of PI may guide the future design of light-assisted PI AOPs.

Enhancing Ferrate Oxidation of Micropollutants via Inducing Fe(V)/Fe(IV) Formation Needs Caution: Increased Conversion of Bromide to Bromate

This study explores the formation of bromate (BrO3-) in the copresence of Fe(VI) and bromide (Br-). It challenges previous beliefs about the role of Fe(VI) as a green oxidant and highlights the crucial role of intermediates Fe(V) and Fe(IV) in the conversion of Br- to BrO3-. The results show that the maximum concentration of BrO3- of 48.3 μg/L was obtained at 16 mg/L Br- and that the contribution of Fe(V)/Fe(IV) to the conversion was positively related to pH. The study suggests that a single-electron transfer from Br- to Fe(V)/Fe(IV) along with the generation of reactive bromine radicals is the first step of Br- conversion, followed by the formation of OBr- which was then oxidized to BrO3- by Fe(VI) and Fe(V)/Fe(IV). Some common background water constituents (e.g., DOM, HCO3-, and Cl-) significantly inhibited BrO3- formation by consuming Fe(V)/Fe(IV) and/or scavenging the reactive bromine species. While investigations proposing to promote Fe(V)/Fe(IV) formation in Fe(VI)-based oxidation to enhance its oxidation capacity have been rapidly accumulated recently, this work called attention to the considerable formation of BrO3- in this process.

Occurrence and removal of antibiotics from aquaculture wastewater by solar-driven Fe(VI)/oxone process

In this study, the occurrence and removal of ten selected antibiotics from aquaculture wastewater by the process solar + Fe(VI)+oxone were investigated. The detection levels of the antibiotics in the aquaculture wastewater samples were at ng/L. The degradation of the selected antibiotics under the process solar + Fe(VI)+oxone followed pseudo-first-order kinetics. As the most abundant antibiotic in the studied aquaculture wastewater, norfloxacin (NFX) was used as the model compound to study the reaction mechanism and detoxification ability of the treatment system, as well as the effects of reaction parameters and environmental factors. The active species including O2•-, O21, and Fe(V)/Fe(IV) contributed to NFX degradation in the process solar + Fe(VI)+oxone. Decarboxylation, the piprazine ring opening, defluorination of the benzene ring, oxygen addition and the cleavage of the quinolone/benzene ring were main degradation pathways of NFX. Around 20% mineralization was reached and the inhibition rate of the bacteria (Escherichia Coli) growth was reduced from 95.5% to 47.1% after the NFX degradation for 60 min. Despite the suppression of NFX degradation by NO2-, PO43- and humic acid, the NFX degradation in three aquaculture wastewater samples was faster than that in ultrapure water due to the positive effect of Br-and other factors. The above results demonstrate the treatment process solar-driven Fe(VI)/oxone has a good potential in antibiotics removal from the aquaculture wastewater.

Sequential anodic oxidation and cathodic electro-Fenton in the Janus electrified membrane for reagent-free degradation of pollutants

Electrified membrane technologies have recently demonstrated high potential in tackling water pollution, yet their practical applications are challenged by relying on large precursor doses. Here, we developed a Janus porous membrane (JPEM) with synergic direct oxidation by Magnéli phase Ti4O7 anode and electro-Fenton reactions by CuFe2O4 cathode. Organic pollutants were first directly oxidized on the Ti4O7 anode, where the extracted electrons from pollutants were transported to the cathode for electro-Fenton production of hydroxyl radical (·OH). The cathodic ·OH further enhanced the mineralization of organic pollutant degradation intermediates. With the sequential anodic and cathodic oxidation processes, the reagent-free JPEM showed competitive performance in rapid degradation (removal rate of 0.417 mg L-1 s-1) and mineralization (68.7 % decrease in TOC) of sulfamethoxazole. The JPEM system displayed general performance to remove phenol, carbamazepine, and perfluorooctanoic acid. The JPEM runs solely on electricity and oxygen that is comparable to that of PEM relies on large precursor doses and, therefore, operation friendly and environmental sustainability. The high pollutant removal and mineralization achieved by rational design of the reaction processes sheds light on a new approach for constructing an efficient electrified membrane.

Visible light activation of ferrate(VI) by oxygen doped ZnIn2S4/black phosphorus nanolayered heterostructure: Accelerated oxidation of trimethoprim

The increasing consumption of antibiotics and their subsequent release to wastewater or groundwater and ultimately to the water supply (or drinking water) has great concerns. This paper presents a visible light (VL) activated ferrate(VI) (FeVIO42-, Fe(VI)) system to degrade the selected antibiotic, trimethoprim (TMP), efficiently. An oxygen doped ZnIn2S4 nanosheet (O-ZIS) coupled with a black phosphorus (BP) heterostructure (O-ZIS/BP), is fabricated by a simple electrostatic self-assembly method. The O-ZIS/BP photocatalyst is comprehensively characterized by surface and analytical techniques, which show superior separation efficiency of the photoinduced charge carriers in the heterostructure. A VL-O-ZIS/BP-Fe(VI) system achieves more than 80% removal in 1.0 min and complete removal of TMP in 3.0 min. Comparatively, only ⁓7% and ⁓24% of TMP are degraded by O-ZIS/BP and Fe(VI) in 1.0 min, respectively. The degradation experiments using probe molecules of reactive species and electron paramagnetic resonance (EPR) measurements reveal involvement of superoxide (O2-•), hydroxyl radical (•OH), and iron(V)/iron (IV) (FeV/FeIV) species in the mechanism of TMP degradation. Oxidized products of TMP are identified and reaction pathways are given. Theoretical calculations predict the initial attack on the TMP molecule by the reactive species in the VL-O-ZIS/BP-Fe(VI) system. The activation of Fe(VI) by VL-heterostructure photocatalysts accelerates the degradation of antibiotics, demonstrating its potential for water depollution.

Enhanced ferrate(VI) oxidation of organic pollutants through direct electron transfer

Fe(VI) is a versatile agent for water purification, and various strategies have been developed to improve its pollutant removal efficiency. Herein, it was found that in addition to intermediate iron species [Fe(IV)/Fe(V)], direct electron transfer (DET) played a significant role in the abatement of organic pollutants in Fe(VI)/carbon quantum dots (CQDs) system. Around 86, 83, 73, 64, 52, 45 and 17% of BPA, DCF, SMX, 4-CP, phenol, p-HBA, and IBP (6 μM) could be oxidized by 30 μM of Fe(VI), whereas with the addition of CQDs (4 mg/L), the oxidation ratio of these pollutants increased to 98, 99, 80, 88, 87, 66 and 57%, respectively. The negative impact induced by solution pH and background constituents on Fe(VI) abatement of pollutants could be alleviated by CQDs, and CQDs acted as catalysts for mediating DET from organic pollutants to Fe(VI). Theoretical calculation revealed that iron species [Fe(VI)/Fe(V)/Fe(IV)] was responsible for the oxidation of 36% of phenol, while DET contributed to the oxidation of 64% of phenol in the Fe(VI)/CQDs system. Compared with iron species oxidation, the CQDs mediated DET from pollutants to Fe(VI) was more efficient for utilizing the oxidation capacity of Fe(VI). The DET mechanism presented in the study provides a prospective strategy for improving the pollution control potential of Fe(VI).

Enhanced hydroxyl radical generation for micropollutant degradation in the In2O3/Vis-LED process through the addition of periodate

Periodate (PI) as an oxidant has been extensively studied for organic foulants removal in advanced oxidation processes. Here PI was introduced into In2O3/Vis-LED process to enhance the formation of ·OH for promoting the degradation of organic foulants. Results showed that the addition of PI would significantly promote the removal of sulfamethoxazole (SMX) in the In2O3/Vis-LED process (from 9.26% to 100%), and ·OH was proved to be the dominant species in the system. Besides, the process exhibited non-selectivity in the removal of different organic foulants. Comparatively, various oxidants (e.g., peroxymonosulfate, peroxydisulfate, and hydrogen peroxide) did not markedly promote the removal of SMX in the In2O3/Vis-LED process. Electrochemical analyses demonstrated that PI could effectively receive photoelectrons, thus inhibiting the recombination of photogenerated electron-hole (e-/h+) pairs. The holes then oxidized the adsorbed H2O to generate ·OH, and the PI converted to iodate at the same time. Additionally, the removal rate of SMX reduced from 100% to 17.2% as Vis-LED wavelengths increased from 440 to 560 nm, because of the low energy of photons produced at longer wavelengths. Notably, the species of PI do not affect its ability to accept electrons, resulting in the degradation efficiency of SMX irrespective of pH (4.0-10.0). The coexistence of inorganic cations and anions (such as Cl-, CO32-/HCO3-, SO42-, Ca2+, and Mg2+) also had an insignificant effect on SMX degradation. Furthermore, the process also showed excellent degradation potential in real water. The proposed strategy provides a new insight for visible light-catalyzed activation of PI and guidance to explore green catalytic processes for high-efficiency removal of various organic foulants.

The overlooked role of Cr(VI) in micropollutant degradation under solar light irradiation

Hexavalent chromium (Cr(VI)) is ubiquitous in natural environments, whereas its role in the transformation of coexisting contaminants may have been overlooked. In this work, it was reported for the first time that the irradiation of Cr(VI) by solar light (solar light/Cr(VI) system) could effectively degrade various micropollutants with different structures. The removal efficiency of selected micropollutants was increased by 13.3-64.8% by the solar light/Cr(VI) system compared to that by direct solar photolysis. Meanwhile, the oxidation rates were enhanced by 2.2-21.5 folds, while they were negligible by Cr(VI) oxidation alone. Experiments by specific scavengers, probe compounds, fluorescence absorbance, and electron spin resonance analysis demonstrated that hydroxyl radical (•OH) was the major reactive species in the solar light/Cr(VI) system. Further experiments showed that the generation of •OH was closely related to the intermediate Cr(V) generated from Cr(VI) reduction, and Cr(V) could be re-oxidized back to Cr(VI). Increasing solution pH negatively affected model micropollutant (carbamazepine (CBZ)) degradation by the solar light/Cr(VI) system, mainly due to the decreased quantum yield of •OH at higher pH. Coexisting sulfate ions showed negligible effect on CBZ degradation in the solar light/Cr(VI) system, while the presence of bicarbonate, chloride, and humic acid inhibited CBZ degradation to varying degrees, owing to their diverse scavenging effects on •OH. Furthermore, moderate CBZ degradation was also achieved by natural solar light photolysis of Cr(VI). This study demonstrated the pivotal role of Cr(VI) in the transformation of micropollutants under solar irradiation, which advances the understanding of the fate of micropollutants in natural environments.

Dissolved organic matter promotes photocatalytic degradation of refractory organic pollutants in water by forming hydrogen bonding with photocatalyst

Removing refractory organic pollutants in real water using photocatalysis is a great challenge because coexisting dissolved organic matter (DOM) can quench photogenerated holes and thus prevent generation of reactive oxygen species (ROS). Herein, for the first time, we develop a hydrogen bonding strategy to avoid the scavenging of photoexcited holes, by which DOM even promotes photocatalytic degradation of refractory organic pollutants. Theoretical calculations combined with experimental studies reveal the formation of hydrogen bonding between DOM and a hydroxylated S-scheme heterojunction photocatalyst (Mo-Se/OHNT) consisting of hydroxylated nitrogen doped TiO2 (OHNT) and molybdenum doped selenium (Mo-Se). The hydrogen bonding is demonstrated to change the interaction between DOM and Mo-Se/OHNT from DOM-Ti (IV) to a hydrogen bonded complexation through the hydroxyl/amine groups of DOM and the OHNT in Mo-Se/OHNT. The formed hydrogen network can stabilize excited-state of DOM and inject its electron to the conduction band rather than the valence band of the OHNT upon light irradiation, realizing the key to preventing hole quenching. The electron-hole separation in Mo-Se/OHNT is consequently improved for generating more ROS to be involved in removing refractory organic pollutants. Moreover, this hydrogen bonding strategy is generalized to nitrogen doped zinc oxide and graphitic carbon nitride and applies to real water. Our findings provide a new insight into handling the DOM problem for photocatalytic technology towards water and wastewater treatment.

Efficient activation of ferrate by Ru(III): Insights into the major reactive species and the multiple roles of Ru(III)

Ferrate (Fe(VI)) has aroused great research interest in recent years due to its environmental benignancy and lower potential in disinfection by-product generation. However, the inevitable self-decomposition and lower reactivity under alkaline conditions severely restrict the utilization and decontamination efficiency of Fe(VI). Here, we discovered that Ru(III), a representative transition metal, could effectively activate Fe(VI) to degrade organic micropollutants, and its performance on Fe(VI) activation exceeded that of previously reported metal activators. The high-valent metal species (i.e., Fe(IV)/Fe(V) and high-valent Ru species) made a major contribution to SMX removal by Fe(VI)-Ru(III). Density functional theory calculations indicated the function of Ru(III) as a two-electron reductant, leading to the production of Ru(V) and Fe(IV) as the predominant active species. The characterization analyses proved that Ru species was deposited on ferric (hydr)oxides as Ru(III), indicating the possibility of Ru(III) as an electron shuttle with the rapid valence circulation between Ru(V) and Ru(III). This study not only develops an efficient way to activate Fe(VI) but also offers a thorough understanding of Fe(VI) activation induced by transition metals.

UV absorbance and electron donating capacity as surrogate parameters to indicate the abatement of micropollutants during the oxidation of Fe(II)/PMS and Mn(II)/NTA/PMS

In this study, the relative residual UV absorbance (UV₂₅₄) and/or electron donating capacity (EDC) was investigated as a surrogate parameter to evaluate the abatement of micropollutants during the Fe(II)/PMS and Mn(II)/NTA/PMS processes. In the Fe(II)/PMS process, due to the generation of SO₄^•- and •OH at acidic pH, UV₂₅₄ and EDC abatement was greater at pH 5. In the Mn(II)/NTA/PMS process, UV₂₅₄ abatement was greater at pH 7 and 9, while EDC abatement was greater at pH 5 and 7. This was attributed to the fact that MnO₂ was formed at alkaline pH to remove UV₂₅₄ by coagulation, and manganese intermediates (Mn(V)) were formed at acidic pH to remove EDC via electron transfer. Due to the strong oxidation capacity of SO₄^•-, •OH and Mn(V), the abatement of micropollutants increased with increasing dosages of oxidant in different waters in both processes. In the Fe(II)/PMS and Mn(II)/NTA/PMS processes, except for nitrobenzene (∼23% and 40%, respectively), the removal of other micropollutants was greater than 70% when the oxidant dosages were greater in different waters. The linear relationship between the relative residual UV₂₅₄, EDC and the removal of micropollutants was established in different waters, showing a one-phase or two-phase linear relationship. The differences of the slopes for one-phase linear correlation in the Fe(II)/PMS process (micropollutant-UV₂₅₄: 0.36-2.89, micropollutant-EDC: 0.26-1.75) were less than that in the Mn(II)/NTA/PMS process (micropollutant-UV₂₅₄: 0.40-13.16, micropollutant-EDC: 0.51-8.39). Overall, these results suggest that the relative residual UV₂₅₄ and EDC could truly reflect the removal of micropollutants during the Fe(II)/PMS and Mn(II)/NTA/PMS processes.

Efficient removal of p-arsanilic acid and arsenite by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes

The widespread occurrence of p-arsanilic acid (p-ASA) in natural environments poses big threats to the biosphere due to the generation of toxic inorganic arsenic (i.e., As(III) and As(V), especially As(III) with higher toxicity and mobility). Oxidation of p-ASA or As(III) to As(V) followed by precipitation of total arsenic using Fe-based advanced oxidation processes demonstrated to be a promising approach for the treatment of arsenic contamination. This study for the first time investigated the efficiency and inherent mechanism of p-ASA and As(III) oxidation by Fe(II)/peracetic acid (Fe(II)/PAA) and PAA processes. p-ASA was rapidly degraded by the Fe(II)/PAA process within 20 s at neutral to acidic pHs under different conditions, while it was insignificantly degraded by PAA oxidation alone. Lines of evidence suggested that hydroxyl radicals and organic radicals generated from the homolytic OO bond cleavage of PAA contributed to the degradation of p-ASA in the Fe(II)/PAA process. p-ASA was mainly oxidized to As (V), NH₄⁺, and p-aminophenol by the Fe(II)/PAA process, wherein the aniline group and its para position were the most vulnerable sites. As(III) of concern was likely generated as an intermediate during p-ASA oxidation and it could be readily oxidized to As(V) by the Fe(II)/PAA process as well as PAA alone. The in-depth investigation demonstrated that PAA alone was effective in the oxidation of As(III) under varied conditions with a stoichiometric molar ratio of 1:1. Efficient removal (> 80%) of total arsenic during p-ASA oxidation by Fe(II)/PAA process or during As(III) oxidation by PAA process with additional Fe(III) in synthetic or real waters were observed, mainly due to the adsorptive interactions of amorphous ferric (oxy)hydroxide precipitates. This study systematically investigates the oxidation of p-ASA and As(III) by the Fe(II)/PAA and PAA processes, which is instructive for the future development of arsenic remediation technology.

Ferrate(VI)/Periodate System: Synergistic and Rapid Oxidation of Micropollutants via Periodate/Iodate-Modulated Fe(IV)/Fe(V) Intermediates

The presence of organic micropollutants in water sources worldwide has created a need for the development of effective and selective oxidation methods in complex water matrices. This study is the first report of the combination of ferrate(VI) (Fe(VI)) and periodate (PI) for synergistic, rapid, and selective elimination of multiple micropollutants. This combined system was found to outperform other Fe(VI)/oxidant systems (e.g., H₂O₂, peroxydisulfate, and peroxymonosulfate) in rapid water decontamination. Scavenging, probing, and electron spin resonance experiments showed that high-valent Fe(IV)/Fe(V) intermediates, rather than hydroxyl radicals, superoxide radicals, singlet oxygen, and iodyl radicals, played a dominant role in the process. Further, the generation of Fe(IV)/Fe(V) was evidenced directly by the ⁵⁷Fe Mössbauer spectroscopic test. Surprisingly, the reactivity of PI toward Fe(VI) is rather low (0.8223 M^-1 s^-1) at pH 8.0, implying that PI was not acting as an activator. Besides, as the only iodine sink of PI, iodate also played an enhanced role in micropollutant abatement by Fe(VI) oxidation. Further experiments proved that PI and/or iodate might function as the Fe(IV)/Fe(V) ligands, causing the utilization efficiency of Fe(IV)/Fe(V) intermediates for pollutant oxidation to outcompete their auto-decomposition. Finally, the oxidized products and plausible transformation pathways of three different micropollutants by single Fe(VI) and Fe(VI)/PI oxidation were characterized and elucidated. Overall, this study proposed a novel selective oxidation strategy (i.e., Fe(VI)/PI system) that could efficiently eliminate water micropollutants and clarified the unexpected interactions between PI/iodate and Fe(VI) for accelerated oxidation.

Photolysis of chlorite by solar light: An overlooked mitigation pathway for chlorite and micropollutants

Chlorite (ClO₂^-) is an undesirable toxic byproduct commonly produced in the chlorine dioxide and ultraviolet/chlorine dioxide oxidation processes. Various methods have been developed to remove ClO₂^- but require additional chemicals or energy input. In this study, an overlooked mitigation pathway of ClO₂^- by solar light photolysis with a bonus for simultaneous removal of micropollutant co-present was reported. ClO₂^- could be efficiently decomposed to chloride (Cl^-) and chlorate by simulated solar light (SSL) at water-relevant pHs with Cl^- yield up to 65% at neutral pH. Multiple reactive species including hydroxyl radical (•OH), ozone (O₃), chloride radical (Cl•), and chlorine oxide radical (ClO•) were generated in the SSL/ClO₂^- system with the steady-state concentrations following the order of O₃ (≈ 0.8 μΜ) > ClO• (≈ 4.4 × 10^-6 μΜ)> •OH (≈ 1.1 × 10^-7 μΜ)> Cl• (≈ 6.8 × 10^-8 μΜ) at neutral pH under investigated condition. Bezafibrate (BZF) as well as the selected six other micropollutants was efficiently degraded by the SSL/ClO₂^- system with pseudofirst-order rate constants ranging from 0.057 to 0.21 min^-1 at pH 7.0, while most of them were negligibly degraded by SSL or ClO₂^- treatment alone. Kinetic modeling of BZF degradation by SSL/ClO₂^- at pHs 6.0 - 8.0 suggested that •OH contributed the most, followed by Cl•, O₃, and ClO•. The presence of water background components (i.e., humic acid, bicarbonate, and chloride) exhibited negative effects on BZF degradation by the SSL/ClO₂^- system, mainly due to their competitive scavenging of reactive species therein. The mitigation of ClO₂^- and BZF under photolysis by natural solar light or in realistic waters was also confirmed. This study discovered an overlooked natural mitigation pathway for ClO₂^- and micropollutants, which has significant implications for understanding their fate in natural environments.

Oxidation of Pharmaceuticals by Ferrate(VI)-Amino Acid Systems: Enhancement by Proline

The occurrence of micropollutants in water threatens public health and ecology. Removal of micropollutants such as pharmaceuticals by a green oxidant, ferrate(VI) (FeVIO42-, Fe(VI)) can be accomplished. However, electron-deficient pharmaceuticals, such as carbamazepine (CBZ) showed a low removal rate by Fe(VI). This work investigates the activation of Fe(VI) by adding nine amino acids (AA) of different functionalities to accelerate the removal of CBZ in water under mild alkaline conditions. Among the studied amino acids, proline, a cyclic AA, had the highest removal of CBZ. The accelerated effect of proline was ascribed by demonstrating the involvement of highly reactive intermediate Fe(V) species, generated by one-electron transfer by the reaction of Fe(VI) with proline (i.e., Fe(VI) + proline → Fe(V) + proline•). The degradation kinetics of CBZ by a Fe(VI)-proline system was interpreted by kinetic modeling of the reactions involved that estimated the rate of the reaction of Fe(V) with CBZ as (1.03 ± 0.21) × 106 M-1 s-1, which was several orders of magnitude greater than that of Fe(VI) of 2.25 M-1 s-1. Overall, natural compounds such as amino acids may be applied to increase the removal efficiency of recalcitrant micropollutants by Fe(VI).

Oxidation towards enrofloxacin degradation over nanoscale zero-valent copper: mechanism and products

Enrofloxacin (ENR) is a widely used veterinary fluoroquinolone antibiotic and is frequently detected in water environments. The degradation of ENR was examined utilizing molecular oxygen mediation using nanometer zero-valent copper (nZVC) as the catalyst in this work. The dosage of nZVC, initial pH, and reaction temperature were investigated as contributing factors to ENR degradation. The effects of Cl^-, NO^3-, SO4^2-, and humic acid on the degradation of ENR were investigated. The actual effects were evaluated using natural water. The reactive oxygen species (ROS) that participated in the reaction were identified, their generation mechanisms were elucidated, and the effects on ENR degradation were assessed. More emphasis was given to exploring ENR degradation and transformation pathways via analyses of HPLC-TOF-MS. Data showed that at 35 ℃, with an initial pH of 3 and exposed to air, an nZVC dose of 0.5 g·L^-1 degraded ENR by 99.51% dramatically. HO^• radicals were identified as the dominant ROS, and conversions among Cu⁰, Cu⁺, and Cu²⁺ played crucial roles in the generation of ROS. The destruction mechanism of ENR was speculated based on analyses of HPLC-TOF-MS results as the transformation of the piperazine ring into an oxidized state with a -COOH substitution with HO^•, which caused ENR to be mineralized and converted into CO₂, H₂O, and [Formula: see text]. The ECOSAR program has been used to evaluate the toxicity of ENR and its degradation products, and oxidative degradation of nZVC significantly reduced its toxicity and increased its biodegradability. This research proposes a capable and practical method for removing ENR from water.

Enhanced Permanganate Activation under UVA-LED Irradiation: Unraveled Mechanism Involving Manganese Species and Hydroxyl Radical

Permanganate [Mn(VII)] has gained broad attention in water treatment. However, its limited reactivity toward some refractory micropollutants hinders its application for micropollutant degradation. Herein, we introduced UVA-LED photolysis of Mn(VII) (UVA-LED/Mn(VII)) to degrade micropollutants (diclofenac (DCF), 4-chlorophenol (4-CP), atrazine, and nitrobenzene) by selecting DCF and 4-CP as target micropollutants. The effects of operating conditions (e.g., light intensity, radiation wavelengths, pH, and water constituents) on DCF and 4-CP degradation as well as the underlying mechanisms were systematically studied. The degradation rates of DCF and 4-CP linearly decreased with increasing radiation wavelengths (from 365 to 405 nm), likely due to the decreased molar absorption coefficients and quantum yields of Mn(VII). Reactive manganese species (RMnS), including Mn(V), Mn(III), and HO•, were generated in the UVA-LED/Mn(VII) process. Mn(V) and HO• were responsible for DCF degradation, while Mn(III), HO•, and likely Mn(V) accounted for 4-CP degradation. Competitive kinetic results revealed that contributions of RMnS and HO• decreased with increasing radiation wavelengths, wherein RMnS played the dominant role. Increasing pH displayed opposite effects on DCF and 4-CP degradation with higher degradation efficiency obtained at acidic pH for the former one but alkaline pH for the latter one. The presence of water background ions (e.g., Cl^-, HCO₃^-, and Ca²⁺) barely influenced DCF and 4-CP degradation. Finally, in comparison with Mn(VII) alone, enhanced degradation of DCF and 4-CP by UVA-LED/Mn(VII) was observed in real waters. This work advances the understanding of the photochemistry of manganese species in micropollutant degradation and facilitates Mn(VII) oxidation in practical application.

Photochemical and Photophysical Dynamics of the Aqueous Ferrate(VI) Ion

Ferrate(VI) has the potential to play a key role in future water supplies. Its salts have been suggested as "green" alternatives to current advanced oxidation and disinfection methods in water treatment, especially when combined with ultraviolet light to stimulate generation of highly oxidizing Fe(V) and Fe(IV) species. However, the nature of these intermediates, the mechanisms by which they form, and their roles in downstream oxidation reactions remain unclear. Here, we use a combination of optical and X-ray transient absorption spectroscopies to study the formation, interconversion, and relaxation of several excited-state and metastable high-valent iron species following excitation of aqueous potassium ferrate(VI) by ultraviolet and visible light. Branching from the initially populated ligand-to-metal charge transfer state into independent photophysical and photochemical pathways occurs within tens of picoseconds, with the quantum yield for the generation of reactive Fe(V) species determined by relative rates of the competing intersystem crossing and reverse electron transfer processes. Relaxation of the metal-centered states then occurs within 4 ns, while the formation of metastable Fe(V) species occurs in several steps with time constants of 250 ps and 300 ns. Results here improve the mechanistic understanding of the formation and fate of Fe(V) and Fe(IV), which will accelerate the development of novel advanced oxidation processes for water treatment applications.

Oxidation of micropollutants by visible light active graphitic carbon nitride and ferrate(VI): Delineating the role of surface delocalized electrons

The treatment of recalcitrant micropollutants in water remains challenging. Ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) has emerged as a green oxidant to oxidize organic molecules, however, its reactivity with recalcitrant micropollutants are sluggish. Our results demonstrate enhanced oxidation of carbamazepine (CBZ) by three types of visible light-responsive graphitic carbon nitride (g-C₃N₄) photocatalyst in absence and presence of ferrate(VI) (Fe^VIO₄^2-, Fe(VI)) under mild alkaline conditions. The g-C₃N₄ photocatalysts were prepared by thermal process using urea, thiourea, and melamine and were named as CN-U, CN-T, and CN-M, respectively. The degradation efficiency of CBZ, in both visible light-g-C₃N₄ and visible light-g-C₃N₄-Fe^VIO₄^2- systems followed the order of CN-U > CN-T > CN-M. The mechanisms for this trend was elucidated by measuring physiochemical properties of the microstructures with various surface and analytical techniques. Results suggest the dominating role of specific surface area and surface delocalized electrons of microstructures in degrading CBZ. Crystallinity, morphology, and surface functional groups may not directly associate with CBZ degradation. The CN-U has higher specific surface area and surface delocalized electrons than CN-T and CN-M and therefore the highest degradation efficiency of CBZ. The surface electrons likely generated O₂^●- and ¹O₂ in the visible light-g-C₃N₄ system. The additional oxidants, Fe^V and Fe^IV in the visible light-g-C₃N₄- Fe^VIO₄^2- system led to higher degradation efficiency than the visible light-g-C₃N₄ system. Results suggest that the surfaces of g-C₃N₄ may be prepared preferentially with high levels of delocalized electrons at the surface of microstructures to enhance degradation of micropollutants.

Novel solar-driven ferrate(VI) activation system for micropollutant degradation: Elucidating the role of Fe(IV) and Fe(V)

This paper presents a novel process of solar-ferrate(VI) [Fe(VI)] for micropollutant degradation. The solar-Fe(VI) process promoted micropollutant degradation compared with Fe(VI) alone and solar. The radical scavenging and probing experiment results suggested that Fe(V) and Fe(IV) but not reactive oxygen species were most likely involved in the solar-Fe(VI) process. Through building a kinetic model, Fe(IV) and Fe(V) were observed to play an equally significant role in the solar-Fe(VI) process. Afterward, the reaction mechanism of the photochemistry of Fe(VI) was elaborated. Fe(IV) formed from Fe(VI) photolysis and then decomposed to Fe(II) which reacted with Fe(VI) to form Fe(V). Furthermore, the effect of pH on carbamazepine (CBZ) degradation was studied and the quantum yields of Fe(VI) were determined, with (1.98 ± 0.16)× 10^-3 mol∙einstein^-1, (5.90 ± 0.27)× 10^-4 mol∙einstein^-1, and (1.66 ± 0.14)× 10^-4 mol∙einstein^-1 at pH 7.0, 8.0, and 9.0, respectively. Inorganic ions, including Cl^-, HCO₃^-, and Br^- displayed negligible influence on the CBZ degradation, whereas humic acid inhibited the CBZ degradation. Finally, the solar-Fe(VI) process exhibited good applicability in authentic waters and under different irradiation (natural sunlight, ultraviolet light, and visible light from solar cut-off emission). Overall, this study provides a new routine for efficient micropollutant elimination and reveals the photochemistry of Fe(VI).

Preferential Destruction of Micropollutants in Water through a Self-Purification Process with Dissolved Organic Carbon Polar Complexation

Removing micropollutants in real water is a scientific challenge due to primary dissolved organic carbon (DOC) and high energy consumption of current technologies. Herein, we develop a self-purification process for the preferential destruction of various micropollutants in municipal wastewater, raw drinking water, and ultrapure water with humic acid (HA) driven by the surface microelectronic field of Fe⁰-Fe_yC_z/Fe_x-GZIF-8-rGO without any additional input. It was verified that a strongly polar complex consisting of an electron-rich HA/DOC area and an electron-poor micropollutant area was formed between HA/DOC and micropollutants, promoting more electrons of micropollutants in the adsorbed complex to delocalizing to electron-rich Fe species area and be trapped by O₂, which resulted in their surface cleavage and hydrolyzation preferentially. The higher micropollutant degradation efficiency observed in real wastewaters was due to the greater complex polarity of DOC. Moreover, the electron transfer process ensured the stability of the surface microelectronic field and continuous water purification. Our findings provide a new insight into low-energy combined-micropollution water treatment.

Degradation of Organic Contaminants by Reactive Iron/Manganese Species: Progress and Challenges

Many iron(II, III, VI)- and manganese(II, IV, VII)-based oxidation processes can generate reactive iron/manganese species (RFeS/RMnS, i.e., Fe(IV)/Fe(V) and Mn(III)/Mn(V)/Mn(VI)), which have mild and selective reactivity toward a wide range of organic contaminants, and thus have drawn significant attention. The reaction mechanisms of these processes are rather complicated due to the simultaneous involvement of multiple radical and/or nonradical species. As a result, the ambiguity in the occurrence of RFeS/RMnS and divergence in the degradation mechanisms of trace organic contaminants in the presence of RFeS/RMnS exist in literature. In order to improve the critical understanding of the RFeS/RMnS-mediated oxidation processes, the detection methods of RFeS/RMnS and their roles in the destruction of trace organic contaminants are reviewed with special attention to some specific problems related to the scavenger and probe selection and experimental results analysis potentially resulting in some questionable conclusions. Moreover, the influence of background constituents, such as organic matter and halides, on oxidation efficiency of RFeS/RMnS-mediated oxidation processes and formation of byproducts are discussed through their comparison with those in free radicals-dominated oxidation processes. Finally, the prospects of the RFeS/RMnS-mediated oxidation processes and the challenges for future applications are presented.

Graphite (GP) induced activation of ferrate(VI) for degradation of micropollutants: The crucial reduction role of carbonyl groups on GP surface

The sluggish oxidation kinetics of ferrate (Fe(VI)) at neutral and slightly alkaline pH impedes its rapid abatement of micropollutants in practical application. This work discovers that graphite (GP), a metal-free carbonaceous material, can be a promising material to improve the reactivity of Fe(VI) in the pH range of 7.0 - 9.0. The performance of the GP/Fe(VI) process for sulfamethoxazole (SMX) removal was further evaluated via altering the dosages of Fe(VI), GP, and SMX. Probe analysis and quenching experiments identified Fe(IV) and Fe(V) as the primary active species responsible for the removal of organic compounds in the GP/Fe(VI) system. The detailed activation mechanism of GP is discussed via analyzing the surface chemical changes of GP exposed to Fe(VI). It is found that the carbonyl groups on GP surface execute a critical role in Fe(VI) activation. The GP/Fe(VI) system shows powerful anti-interference ability to environmental background substances. Therefore, the new oxidation process proposed in this work holds a great application prospect for contamination remediation. Finally, we discuss the underlying degradation pathways of SMX by the GP/Fe(VI) system. This study not only develops a promising system for the removal of micropollutants but also provides an in-depth insight into the activation mechanism of metal-free carbonaceous material in Fe(VI) oxidation process.

Efficient activation of ferrate(VI) by colloid manganese dioxide: Comprehensive elucidation of the surface-promoted mechanism

Current research focuses on introducing additional energy or reducing agents to directly accelerate the formation of Fe(IV) and Fe(V) from ferrate (Fe(VI)), thereby ameliorating the oxidation activity of Fe(VI). Interestingly, this study discovers that colloid manganese dioxide (cMnO₂) can remarkably promote Fe(VI) to remove various contaminants via a novel surface-promoted pathway. Many lines of evidence suggest that high-valent Fe species are the primary active oxidants in the cMnO₂-Fe(VI) system, however, the underlying activation mechanism for the direct reduction of Fe(VI) by cMnO₂ to generate Fe(IV)/Fe(V) is eliminated. Further analysis found that Fe(VI) can combine with the vacancies in cMnO₂ to form precursor complex (cMnO₂-Fe(VI)*), which possesses a higher oxidation potential than Fe(VI). This makes cMnO₂-Fe(VI)* is more vigorous to oxidize pollutants with electron-rich moieties through the electron transfer step than alone Fe(VI), resulting in producing Fe(V) and Fe(IV). The products of Fe(VI) decay (i.e., Fe(II), Fe(III), and H₂O₂) are revealed to play vital roles in further boosting the formation of Fe(IV) and Fe(V). Most importantly, the catalytic stability of cMnO₂ in complicated waters is superior to popular reductants, suggesting its outstanding application potential. Taken together, this work provides a full-scale insight into the surface-promoted mechanism in Fe(VI) oxidation process, thus providing an efficient and green strategy for Fe(VI) activation.