Activation of persulfate by homogeneous and heterogeneous iron catalyst to degrade chlortetracycline in aqueous solution

Fluorescence visualization of CO2-responsive phase transfer materials targeting at the heterogeneous interfacial reactions in advanced oxidation of naphthenic acid in wastewater

Treatment of naphthenic acids (NAs) in wastewater is necessary due to its high toxicity and difficult degradation. In the heterogeneous Fenton-like advanced oxidation of organic pollutant system, the insufficient accessibility of oxidizing agent and NAs greatly hamper the reaction efficiency. CO2-responsive phase transfer materials derived from polyethylene glycol (PEG)-based deep eutectic solvents were specific targeted at the immiscible-binary phase system. The NAs oxidative degradation process was optimized including the kinds of catalyst (Molecular weight of PEG, constitute of DESs, and dosage.), temperature, flow rate of CO2, et al. With the help of fluorescence properties of catalyst, the hydrophilic-hydrophobic interaction was visual-monitored and further studied. The amphipathic property of PEG-200/Sodium persulfate/Polyether amine 230 (PEA230) greatly reduced the aqueous/organic phase transfer barrier between sodium persulfate and NAs (up to 84 %), thus accreting oxidation rate. The surface tension decreased from 35.364 mN/m to 28.595 mN/m. To control the reaction rate, the CO2 respond structure of amido played an important role. In addition, the interfacial transfer intermediates and oxidation pathways were also explored by nuclear magnetic resonance, flourier transform infrared spectroscopy, surface tension, and radical inhibition experiments. The mechanism of advanced oxidation of NAs catalyzed by CO2-responsive phase transfer catalyst was proposed, which would made up for the deficiency of the system theory of heterogeneous chemical oxidation of organic pollutants.

Spatial Confinement of Co Nanoparticles in N-Doped Carbon Nanorods for Wastewater Purification via CaSO3 Activation

Spatial confinement of organic pollutants and reactive oxygen species (e.g., SO4•- and •OH) with ultrashort lifetime inside the scale of chemical theoretical diffusion could provide a greatly promising strategy to overcome the limitation of mass transfer in the heterogeneous Fenton-like oxidation process. Herein, we first reported spatial confinement of cobalt nanoparticles in N-doped carbon nanorods (Co-NCNRs), by encapsulating Co nanoparticles into N-doped carbon nanorods, in activating CaSO3 for antibiotic degradation. Compared to Na2SO3 and NaHSO3, CaSO3 could slowly and persistently discharge SO32- due to its low solubility, thus avoiding the depletion of the generated SO3•- and •OH under the high concentration of sulfite ions. Fully physical characterizations confirmed that the 3D hydrogel was mostly transformed into the nanorod structure of Co-NCNRs at 550 °C. Co atoms were successfully nanoconfined into N-doped carbon nanorods, which contributes to mass transfer and prevents the agglomeration of Co nanoparticles, thus enhancing its catalytic activity and stability in activating CaSO3 for water decontamination. The catalytic performance, kinetic research, influences of inorganic anions, pH, and degradation mechanism of chlortetracycline degradation catalyzed by the Co-NCNRs/CaSO3 system have been studied in detail. This work not only proposed a facile method for synthesis of nanoconfined catalyst but also provided an excellent Co-NCNRs/CaSO3 system for wastewater treatment.

Catalytic activation of persulfate by nanoscale zero-valent iron-derived supported boron-doped porous carbon for bisphenol A degradation

In this study, boron-doped porous carbon materials (BCs) with high surface areas were synthesized employing coffee grounds as carbon source and sodium bicarbonate and boric acid as precursors; afterward, nanoscale zero-valent iron (nZVI) and BCs composites (denoted as nZVI@BCs) were further prepared through reduction of FeSO4 by NaBH4 along with stirring. The performance of the nZVI@BCs for activating persulfate (PS) was evaluated for the degradation of bisphenol A (BPA). In comparison with nZVI@Cs/PS, nZVI@BCs/PS could greatly promote the degradation and mineralization of BPA via both radical and non-radical pathways. On the one hand, electron spin resonance and radical quenching studies represented that •OH, SO4•-, and O2•- were mainly produced in the nZVI@BCs/PS system for BPA degradation. On the other hand, the open circuit voltages of nZVI@BCs and nZVI@Cs in different systems indicated that non-radical pathway still existed in our system. PS could grab the unstable unpaired electron on nZVI@BCs to form a carbon material surface-confined complex ([nZVI@BCs]*) with a high redox potential, then accelerate BPA removal efficiency via direct electron transfer. Furthermore, the performances and mechanisms for BPA degradation were examined by PS activation with nZVI@BC composites at various conditions including dosages of nZVI@BCs, BPA and PS, initially pH value, temperature, common anions, and humid acid. Therefore, this study provides a novel insight for development of high-performance carbon catalysts toward environmental remediation.

Single-atom Mn-embedded carbon nitride as highly efficient peroxymonosulfate catalyst for the harmful algal blooms control

In recent years, water quality deterioration caused by harmful algal blooms (HABs) has become one of the global drinking water safety issues, and sulfate radical driven heterogeneous advanced oxidation technology has been widely used for algae removal. However, the shortages of low active site exposure, metal leaching, and secondary contamination limit its further application. Therefore, the single-atom Mn anchored on inorganic carbon nitride was constructed to enhance the oxidation and coagulation of algal cells while maintaining cell integrity in this study. The removal efficiency of Microcystis aeruginosa was as high as 100 % within 30 min under the optimal conditions of 400 mg/L single-atom Mn-embedded g-C3N4 (SA-MCN) and 0.32 mM peroxymonosulfate (PMS). Importantly, the K+ release, malondialdehyde concentration, floccules morphology and variation of algal organic matters further showed that the algal cells still maintained high integrity without severe rupture during the catalytic reaction. Furthermore, the catalytic mechanisms of algae removal by moderate oxidation and simultaneous coagulation in this system were explored by quenching experiments, EPR analysis, theoretical calculation, and Zeta potential. In brief, this study highlighted the single-atom heterogeneous catalyst with high-efficiency and environmental-friendliness in harmful algal blooms control.

A magnetically reusable Ce-MOF/GO/Fe3O4 composite for effective photocatalytic degradation of chlortetracycline

Herein, we report a novel 1/GO/Fe3O4 photocatalyst, comprising Ce(BTB)(H2O) (MOF-1, H3BTB = 1,3,5-benzenetrisbenzoic acid), graphene oxide (GO), and iron oxide (Fe3O4) for photocatalytic degradation of chlortetracycline (CTC). This design enables the effective transfer of electrons from the MOF to GO, thereby reducing the photoelectron-hole recombination rate. Therefore, the optimized 1/GO/Fe3O4 photocatalyst with H2O2 shows the highest photocatalytic activity toward CTC. The kinetic constant is 5.4 times that in the system of MOF-1 and hydrogen peroxide, which usually acted as efficient electron acceptors to improve the photocatalytic performance of MOFs. More importantly, light absorption is extended from the ultraviolet to the visible region. Furthermore, 1/GO/Fe3O4 can be quickly recycled under an applied magnetic field and displays outstanding stability and reusability. According to the radical trapping experiments and electron paramagnetic resonance results, hydroxyl radicals, superoxide radicals, and holes all contribute to excellent photocatalytic activity. The possible catalytic mechanism of 1/GO/Fe3O4 is tentatively proposed. This work aims to explore the synergistic effect between metal-organic frameworks (MOFs) and GO, and provide a theoretical basis for MOF-based composites to remove antibiotic contaminants in the environment.

Sulfite activation by Jahn-Teller-driven oxygen vacancies Cu-Mn composite oxide for chlortetracycline degradation

Copper-manganese composite metal oxides (CuMnOy) were prepared by hydrolysis-driven oxidation-reduction method and used to activate sulfite to degrade chlortetracycline hydrochloride (CTC) for the first time. The Jahn-Teller ions Mn3+ and Cu2+ exist in CuMnOy, which form a solid electric charge transport redox system and ensure the continuous generation of reactive oxygen species (ROS). Through the systematic study of the experimental parameters such as sulfite concentration, catalyst metal molar ratio, catalyst amounts and initial pH, the optimal degradation rate of CTC could reach 91.74% within 10 min and 94.46% after 30 min. The major reactive radicals were determined by radical quenching experiments and electron paramagnetic resonance (EPR) trapping techniques, and it was confirmed that SO4•- and •O2- played a nonnegligible role in the process of degrading CTC. Density functional theory (DFT) calculations show that higher Fukui indices (f- and f0) of CTC sites are more vulnerable to free radical attack. CuMnOy has low CTC degradation intermediate toxicity, high catalytic performance, good anti-interference ability, reusability and stability, and possesses decent application potential in the actual water treatment field.

Epsilon-MnO2 simply prepared by redox precipitation as an efficient catalyst for ciprofloxacin degradation by activating peroxymonosulfate

Four kinds of manganese oxides were successfully prepared by hydrothermal and redox precipitation methods, and the obtained oxides were used for CIP removal from water by activating PMS. The microstructure and surface properties of four oxides were systematically characterized. The results showed that ε-MnO2 prepared by the redox precipitation method had large surface area, low crystallinity, high surface Mn(III)/Mn(Ⅳ) ratio and the highest activation efficiency for PMS, that is, when the concentration of PMS was 0.6 g/L, 0.2 g/L ε-MnO2 could degrade 93% of CIP within 30 min. Multiple active oxygen species, such as sulfate radical, hydroxyl radical and singlet oxygen, were found in CIP degradation, among which sulfate radical was the most important one. The degradation reaction mainly occurred on the surface of the catalyst, and the surface hydroxyl group played an important role in the degradation. The catalyst could be regenerated in situ through the redox reaction between Mn4+ and Mn3+. The ε-MnO2 had the advantages of simple preparation, good stability and excellent performance, which provided the potential for developing new green antibiotic removal technology.

Chalcopyrite as an oxidants activator for organic pollutant remediation: A review of mechanisms, parameters, and future perspectives

Advanced oxidation processes (AOPs) based on oxidants have attracted attention for the degradation of organic pollutants. The combination of chalcopyrite with oxidants such as persulfate, peroxide, percarbonate, and others shows promise as a system due to its ability to activate through various pathways, leading to the formation of numerous radical and non-radical species. In this review, the generation of sulfate radical (SR) and hydroxyl radical (HR) in AOPs were summarized. The significance of chalcopyrite in various approaches including Fenton, photo-Fenton, and photo/Fenton-like methods, as well as its involvement in electrochemical Fenton-based processes was discussed. The stability and reusability, toxicity, catalyst mechanism, and effects of operational parameters (pH, catalyst dosage, and oxidant concentration) are evaluated in detail. The review also discusses the role of Fe2+/3+, Cu1+/2+, S2- and Sn2- present in CuFeS2 in the generation of free radicals. Finally, guidelines for future research are presented in terms of future perspectives.

Efficient Photocatalytic Degradation of Tetracycline on the MnFe2O4/BGA Composite under Visible Light

In this work, the MnFe₂O₄/BGA (boron-doped graphene aerogel) composite prepared via the solvothermal method is applied as a photocatalyst to the degradation of tetracycline in the presence of peroxymonosulfate. The composite's phase composition, morphology, valence state of elements, defect and pore structure were analyzed by XRD, SEM/TEM, XPS, Raman scattering and N₂ adsorption-desorption isotherms, respectively. Under the radiation of visible light, the experimental parameters, including the ratio of BGA to MnFe₂O₄, the dosages of MnFe₂O₄/BGA and PMS, and the initial pH and tetracycline concentration were optimized in line with the degradation of tetracycline. Under the optimized conditions, the degradation rate of tetracycline reached 92.15% within 60 min, whereas the degradation rate constant on MnFe₂O₄/BGA remained 4.1 × 10^-2 min^-1, which was 1.93 and 1.56 times of those on BGA and MnFe₂O₄, respectively. The largely enhanced photocatalytic activity of the MnFe₂O₄/BGA composite over MnFe₂O₄ and BGA could be ascribed to the formation of type I heterojunction on the interfaces of BGA and MnFe₂O₄, which leads to the efficient transfer and separation of photogenerated charge carriers. Transient photocurrent response and electrochemical impedance spectroscopy tests offered solid support to this assumption. In line with the active species trapping experiments, SO₄^•- and O₂^•- radicals are confirmed to play crucial roles in the rapid and efficient degradation of tetracycline, and accordingly, a photodegradation mechanism for the degradation of tetracycline on MnFe₂O₄/BGA is proposed.

Degradation of tetracyclines by peracetic acid and UV/peracetic acid: Reactive species and theoretical computations

As an environment-friendly oxidant and disinfectant, peracetic acid (PAA) and PAA based-advanced oxidation processes (AOPs) for the treatment of emerging micropollutants have raised increasing interest, owing to their ease of activation and less generation of harmful disinfection byproducts. Tetracyclines (TCs) antibiotics as a group of wide-spectrum antibiotics are frequently detected in sewage effluents, while the knowledge of PAA-based advanced oxidation reactions to remove the substrates is quite limited. In this work, we systematically investigated the kinetics and underlying transformation mechanisms of three TCs including tetracycline (TTC), oxytetracycline (OTC), and chlortetracycline (CTC) in the UV-activated PAA oxidation process. The results indicated that three TCs can be efficiently decayed by UV/PAA. The pseudo-first-order reaction rate constants (k_obs) of TCs followed the order: k_CTC (0.453 min^-1) ≫ k_TTC (0.164 min^-1) > k_OTC (0.158 min^-1). Quenching experiments showed that the removal of CTC was mainly ascribed to the direct oxidation of PAA, while TTC and OTC were more susceptible to free radicals. The k_obs values of the three TCs by PAA oxidation presented a fairly well correlation to the global nucleophilicity and the activation energies of the TC molecules, highlighting the structure-specific reactions of TCs to PAA. Based on product identification and theoretical calculation, N-demethylation and hydroxylation were proposed as the main pathways for TCs degradation by PAA non-radical oxidation. The combination of PAA and UV irradiation can further improve the degradation efficiency of TCs and contribute to reducing the diffusion and transmission of resistance genes in the environment.

Enhanced removal of tetracycline via advanced oxidation of sodium persulfate and biochar adsorption

Advanced oxidation of antibiotic tetracycline (TC) is becoming an accessible and efficient technology. The removal of TC from the complex wastewater needs to be lucubrated. In this study, a TC removal system involving degradation and adsorption was established. TC degradation was accomplished by enhanced advanced oxidation via the addition of sodium persulfate (SP) and biochar into simulated wastewater containing Mn²⁺ and TC wastewater. The adsorption of TC and its derivatives was removed by biochar. The results indicate that the optimized reaction parameters were 3.0 g/L of biochar prepared at 600 °C (B600) and 400 mg/L of SP under acidic condition, and the removal percentage of TC was 87.48%, including 74.23% of degradation and 13.28% of adsorption; the anions Cl^-, NO₃^-, and H₂PO₄^- had negligible effects on the removal of TC in this Mn²⁺/B600/SP system. The system also functioned well with an aqueous solution with a high chemical oxygen demand (COD) concentration. Electron paramagnetic resonance (EPR) analysis indicated that ·OH and SO₄^- free radicals were present in the Mn²⁺/B600/SP system. Based on the testing and analysis results, a removal mechanism and potential TC degradation pathway for this system were proposed. TC can be degraded by ·OH and SO₄^- via three degradation pathways. Mn²⁺ can be precipitated as MnO₂, and a part of the TC and its derivatives can be adsorbed on the biochar surface. The Mn²⁺/B600/SP system also performed satisfactorily for a complex aqueous solution with various cations and antibiotics.

Mn2O3/Mn3O4-Cu1.5Mn1.5O4 spinel as an efficient Fenton-like catalyst activating persulfate for the degradation of bisphenol A: Superoxide radicals dominate the reaction

In this work, a Mn₂O₃/Mn₃O₄-Cu_1.5Mn_1.5O₄ spinel was fabricated and utilised as a catalyst to activate peroxydisulfate (PDS) leading to degradation of bisphenol A (BPA). The results showed that the system exhibited an excellent turnover frequency (TOF) of 2.7 × 10^-3 s^-1 and high stability. The amount of ion leaching was small and the degree of mineralisation was up to 66.2%. Superoxide radicals (O₂^-) were determined to be the dominant active species in the system. ≡Mn(II) and oxygen vacancies (V_o) were found to be the main active sites at the catalyst surface. The activation of PDS by the spinel catalyst and the reduction of dissolved oxygen both contributed to the production of O₂^- species. The synergistic effect of ≡Cu(I)/≡Cu(II) and ≡Mn(II)/≡Mn(III) redox pairs enabled the reaction to occur continuously. These results suggest the promise of this novel spinel catalyst in the removal of refractory organic compounds due to its excellent performance and stability. The catalyst may thus have great utility for environmental remediation.

Activated persulfate by iron-carbon micro electrolysis used for refractory organics degradation in wastewater: a review

With the rapid economic development, the discharge of industrial wastewater and municipal wastewater containing many refractory organic pollutants is increasing, so there is an urgent need for processes that can treat refractory organics in wastewater. Iron-carbon micro electrolysis and advanced oxidation based on persulfate radicals (SO₄^-·) have received much attention in the field of organic wastewater treatment. Iron-carbon micro electrolysis activated persulfate (Fe-C/PS) treatment of wastewater is characterized by high oxidation efficiency and no secondary pollution. This paper reviews the mechanism and process of Fe-C/PS, degradation of organics in different wastewater, and the influencing factors. In addition, the degradation efficiency and optimal reaction conditions (oxidant concentration, catalyst concentration, iron-carbon material, and pH) of Fe-C/PS in the treatment of refractory organics in wastewater are summarized. Moreover, the important factors affecting the degradation of organics by Fe-C/PS are presented. Finally, we analyzed the challenges and the prospects for the future of Fe-C/PS in application, and concluded that the main future directions are to improve the degradation efficiency and cost by synthesizing stable and efficient catalysts, optimizing process parameters, and expanding the application scope.

Synthetic Fe-rich nontronite as a novel activator of bisulfite for the efficient removal of tetracycline

In this work, the iron-containing smectite nontronite (NNT) was artificially prepared by hydrothermal process and used as a heterogeneous catalyst to activate bisulfite (BS) for degradation of tetracycline (TC). Two NNT samples with different iron content (NNT1 and NNT2) were characterized by XRD, FTIR, XPS and SEM-EDS analysis. Under dark condition, the TC removal rates of NNT1/BS and NNT2/BS reached about 91.7% and 95.5% respectively at 60 min. Due to the heterogeneous catalysis of structural Fe(III), the NNT catalysts showed great catalytic activity and low iron leaching at the pH range 3.0-7.5. In addition, NNT particles were also stable and reusable in activating BS for TC removal. According to the EPR and radical quenching experiments, it could be proved that the precursor radical ^•SO₃- was first generated in NNT/BS system, then ^•SO₄- and ^•OH were the active species that played a role in TC degradation. The synthetic NNT clay is a promising Fe-based catalyst for treatment of TC wastewater thanks to its high activity, good stability and effective reusability.

An efficient and low-cost magnetic heterogenous Fenton-like catalyst for degrading antibiotics in wastewater: Mechanism, pathway and stability

Metal-doped MgFe₂O₄ spinel ferrite synthesized from saprolite laterite nickel ore was verified as an efficient heterogeneous Fenton-like catalyst for degrading antibiotics including tetracycline (TC) and metronidazole (MNZ) in a "catalyst/oxalic acid (H₂C₂O₄)/visible light (vis)" system. The degradation efficiencies reached over 95% and total organic carbon (TOC) removal efficiencies were nearly 50% of the two antibiotics within 210 min, under the optimal conditions, especially 90% catalytic activity of the fresh catalyst was maintained after five cycles, suggesting the ferrite possessed excellent degrading performance, cycling stability and applicability. Moreover, the degradation mechanism and pathway of TC were elucidated in detail. Results revealed that the [≡Fe(C₂O₄)₃]^3- complex ions formed by octahedral Fe³⁺ in spinel ferrite with oxalate ions on the surface of MgFe₂O₄, played the key role in production of ·OH radicals which decomposed antibiotic TC into small molecules even mineralized in three pathways. Cost-effective preparation, high catalytic performance and long cycle life may accelerate the practical application of the heterogeneous Fenton-like catalyst.

Removal of sulfamethoxazole by ferrous iron activation of persulfate: Optimization of dosing strategy and degradation mechanism

In this paper, the degradation of sulfamethoxazole (SMX) was investigated using the ferrous iron (Fe²⁺) activation of persulfate (PS) (the Fe²⁺/PS process). The influence of the initial concentration of both PS and Fe²⁺ was investigated. It was found that increasing the PS concentration resulted in a higher SMX degradation efficiency. The influence of inhibiting reactions was found to increase with increasing Fe²⁺ concentration. In order to minimize these inhibiting reactions, different dosing strategies were applied. It was found that the SMX degradation efficiency could be enhanced significantly when changing from direct dosing (total amount of Fe²⁺ dosed at the start) to sequential dosing (dosing that same total amount but divided over specific time intervals) and even more when using continuous dosing (dosing the same total amount but continuously over 30 min reaction time). The contribution of different reactive species in this process was also investigated. It was found that hydroxyl radicals (•OH) were mainly responsible for the degradation of SMX during direct dosing, while using continuous dosing of Fe²⁺, the contribution of Fe(IV) and sulfate radicals (•SO₄^-) became more important (reduction of •OH contribution from 89 to 71%). Some degradation products formed during the SMX degradation process were identified and the difference in reaction mechanism between •OH on the one hand and Fe(IV) and •SO₄^- on the other hand was elucidated. At last, a comparison of different sulfate radical based advanced oxidation processes (SR-AOP) is performed by comparing the difference in SMX degradation efficiency, reactive species contribution and the formed degradation products. In most investigated processes, similar degradation products have been found, however, the large •OH contribution in the Fe²⁺/PS process resulted in distinct degradation products.

Hydrothermal synthesis of magnetic nano-CoFe2O4 catalyst and its enhanced degradation of amoxicillin by activated permonosulfate

Advanced oxidation process (AOP) has attracted widespread attention because it can effectively remove antibiotics in water, but its practical engineering application is limited by the problems of the low efficiency and difficult recovery of the catalyst. In the study, nano-spinel CoFe₂O₄ was prepared by hydrothermal method and served as the peroxymonosulfate (PMS) catalyst to degrade antibiotic amoxicillin (AMX). The reaction parameters such as CoFe₂O₄ dosage, AMX concentration, and initial pH value were also optimized. The reaction mechanism was proposed through free radical capture experiment and possible degradation pathway analysis. In addition, the magnetic recovery performance and stability of the catalyst were evaluated. Results showed that 85.5% of AMX could be removed within 90 min at optimal conditions. Sulfate radicals and hydroxyl radicals were the active species for AMX degradation. Moreover, the catalyst showed excellent magnetism and stability in the cycle experiment, which has great potential in the AOP treatment of antibiotic polluted wastewater.

Sulfidated zero valent iron as a persulfate activator for oxidizing organophosphorus pesticides (OPPs) in aqueous solution and aged contaminated soil columns

Sulfidation treatment is an effective method of improving the catalytic performance of zero-valent iron (ZVI). Here, we prepared sulfidated, micro-sized ZVI (S-mZVI) using ball milling technology to activate persulfate (PS) with the goal of oxidizing organophosphorus pesticides (OPPs) in aqueous solution and aged OPP-contaminated soil columns. Energy dispersive spectroscopy (EDS), X-ray powder diffraction (XRD) and X-ray photoelectron spectrometry (XPS) analyses uncovered the formation of Fe₂O₃, FeOOH, FeS and FeS₂ in the S-mZVI prepared by ball milling with different proportions of elemental S powder to make micro-sized ZVI particles. The presence of sulfur can regulate the morphology of S-mZVI with a dispersed and spherical shape, and it can improve the activation performance of PS. In aqueous solution, 11.2 mg of S-mZVI activated 2.5 mM PS (S-mZVI-PS) with an S/Fe molar ratio of 0.100, and it was the best at activating PS, leading to oxidation-rate constants of 0.030 s^-1 for 10 mg/L phorate and 0.026 s^-1 for 10 mg/L terbufos, which were much greater than those of the other S-mZVI and mZVI. The results of the soil column experiment showed that the PS, which had a low consumption for the total dosage, achieved higher degradation percentages among the three OPPs in the S-mZVI-PS treatment than those in the mZVI-PS treatment over 120 h, with the best performance achieved by oxidizing 69.7% phorate, 48.0% terbufos and 60.6% aminoparathion. The effluent concentrations of the three OPPs in the S-mZVI-PS treatment were significantly lower than those in the mZVI-PS treatment, while dissolved total iron and Fe(II) displayed the opposite results. These results indicate that S-mZVI prepared by ball milling can effectively activate PS and be applied to remediate OPP-contaminated soil.

Treatment of industrial contaminants with zero-valent iron- and zero-valent aluminium-activated persulfate: a case study with 3,5-dichlorophenol and 2,4-dichloroaniline

Zero-valent iron (ZVI)- and zero-valent aluminium (ZVA)-activated persulfate (PS) oxidation procedure was applied to remove the industrial pollutants 3,5-dichlorophenol (3,5-DCP; 12.27 µM) and 2,4-dichloroaniline (2,4-DCA; 12.34 µM) from aqueous solutions. The effects of PS concentration and pH were investigated to optimize heterogeneous treatment systems. Negligible removals were obtained for both pollutants by individual applications of nanoparticles (1 g/L) and PS (1.00 mM). PS activation with ZVI resulted in 59% (1.00 mM PS; 1 g/L ZVI; pH 5.0; 120 min) and 100% (0.75 mM PS; 1 g/L ZVI; pH 5.0; 80 min) 3,5-DCP and 2,4-DCA removals, respectively. The ZVA/PS treatment system gave rise to only 31% 3,5-DCP (1.00 mM PS; 1 g/L ZVA; pH 3.0; 120 min) and 47% 2,4-DCA (0.25 mM PS; 1 g/L ZVA; pH 3.0; 120 min) removals. The pH decreases from 5.0 to 3.0 and from 3.0 to 1.5 enhanced contaminant removals for ZVI/PS and ZVA/PS treatments, respectively. Pollutant removal rates were in correlation with the consumption rates of the oxidants. Metal ion (Al, Fe) release increased in the presence of PS and with decreasing pH.

Aminated N-doped graphene hydrogel for long-term catalytic oxidation in strong acidic environment

Metal-based catalysts in advanced oxidation processes (AOPs) are not stable under strong acidic condition due to the remarkable leaching, which will also lead to a secondary pollution. In this study, an aminated N-doped graphene hydrogel (ANGH) is synthesized from graphene oxide and ethylenediamine (EDA) via an in-situ hydrothermal process. The ANGH shows a free-standing structure and has high catalytic activity especially in phenol degradation under strong-acidic condition because of a non-radical dominated mechanism determined in this process. On the large scale, a longer lifetime of ∼1700 min for ANGH is obtained under strong-acidic condition on a dynamic amplifying device, 2.9 times longer than that at neutral condition. It is proposed that amine N can be protected by hydrogen ions from being oxidized, thus leading to the better stability. Meanwhile, the active sites of ANGH can transform from N containing groups into oxygenous groups, and the deactivated material can be reutilized 10 times for rhodamine B degradation on a large scale. The ANGH synthesized facilely and could be recycled repeatedly, which is also very stable in the strong acidic environment, thus should have great potential in wastewater remediation.

The sequestration of Cr(VI) by zero valent iron under a non-uniform magnetic field: An interfacial dynamic reaction

A non-uniform magnetic field was applied to sequester Cr(VI) with microscale zero-valent iron (ZVI). When the non-uniform magnetic field was applied, the average removal rate of Cr(VI) was increased and the lag phase was shortened with the increasing of magnetic field intensity. The instantaneous rate was fast at the beginning and about 40% of the Cr(VI) was sequestered rapidly when ZVI was added into the magnetic field system. Later, the sequestration rate of Cr(VI) was reduced and remained stable with time until Cr(VI) was removed completely. The instantaneous removal rate was positively correlated with ZVI dosage and the rate per unit mass of ZVI was 0.455 mg/(L·min·gZVI). The constant rate stage was not affected by the initial and the residual concentration of Cr(VI). In the case where no magnetic field was applied, the removal of Cr(VI) is a process in which ZVI is depassivated and its reactivity is restored continuously. The promotion of a magnetic field on the removal of Cr(VI) is mainly due to increasing the role of adsorbed reducing species of Fe²⁺ or Fe⁰ on the ZVI surface. Aging of ZVI under a magnetic field could enhance the release rate of Fe²⁺ in the initial 5 min though the remanence of this kind of ZVI had little effect on the enhancement of the sequestration of Cr(VI).

Efficient removal of acid orange 7 using a porous adsorbent-supported zero-valent iron as a synergistic catalyst in advanced oxidation process

This study focuses on the synthesis of granular red mud reinforced by zero-valent iron (Fe@GRM) and its application for the removal acid orange 7 (AO7) from aqueous solution. Then ZVI is employed as a catalyst for the activation of persulfate (PS) to produce sulfate radicals (SO₄^•-) that are produced at 900 °C in an anoxic atmosphere using the direct reduction of iron oxide in the red mud with maize straw as the reductant. Furthermore, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are used to illustrate the morphology and porous structure of the Fe@GRM. The X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) revealed that Fe@GRM was loaded with zero-valent iron. This characterization confirmed that the Fe@GRM was a porous structure material that contained zero-valent iron. The influence of conditions for AO7 elimination, including initial pH, Fe@GRM dosage, initial AO7 concentrations, and temperature, is also investigated. The removal efficiency of AO7 was 90.78% using Fe@GRM/PS, while only 18.15% was removed when Fe@GRM was used alone. The degradation kinetics were well fitted to a pseudo-first-order kinetic model, and the rate of removal increased with temperature, demonstrating an endothermic elimination process. The Arrhenius activation energy of the process was 20.77 kJ/mol, which indicated that the reduction of AO7 was a diffusion-mediated reaction. Fe@GRM is a low-cost material that demonstrated outstanding performance with great potential for wastewater treatment.

Activated persulfate by iron-based materials used for refractory organics degradation: a review

Recently, the advanced oxidation processes (AOPs) based on sulfate radicals (SRs) for organics degradation have become the focus of water treatment research as the oxidation ability of SRs are higher than that of hydroxyl radicals (HRs). Since the AOP-SRs can effectively mineralize organics into carbon dioxide and water under the optimized operating conditions, they are used in the degradation of refractory organics such as dyes, pesticides, pharmaceuticals, and industrial additives. SRs can be produced by activating persulfate (PS) with ultraviolet, heat, ultrasound, microwave, transition metals, and carbon. The activation of PS in iron-based transition metals is widely studied because iron is an environmentally friendly and inexpensive material. This article reviews the mechanism and application of several iron-based materials, including ferrous iron (Fe²⁺), ferric iron (Fe³⁺), zero-valent iron (Fe⁰), nano-sized zero-valent iron (nFe⁰), materials-supported nFe⁰, and iron-containing compounds for PS activation to degrade refractory organics. In addition, the current challenges and perspectives of the practical application of PS activated by iron-based systems in wastewater treatment are analyzed and prospected.

MnCeOx/diatomite catalyst for persulfate activation to degrade organic pollutants

Persulfate (PS)-based oxidation technologies are attracting increasing attentions in water treatment due to their high efficiency and stability. In this study, a novel diatomite supported MnCeO_x composite (MnCeO_x/diatomite) was prepared and characterized for activation of PS to degrade organic pollutants. Results indicated that diatomite not only dispersed MnCeO_x and increased the specific surface area of catalyst, but also improved the low-valence metal site (Mn²⁺ and Ce³⁺) and reactive oxygen species site (-OH) of MnCeO_x, thus enhancing the activities of MnCeO_x. MnCeO_x/diatomite/PS showed high efficiency for multiple dyes and pharmaceutical pollutants. Constant rate (k) of MnCeO_x/diatomite (k_{MnCeOx/diatomite}) was three times higher than the sum of constant rate of MnCeO_x (k_MnCeOx) and constant rate of diatomite (k_diatomite). In addition, MnCeO_x/diatomite showed wide pH application (5-9). Cl^- and NO₃^2- had no effect while SO₄^2- and humid acid had slightly negative effects on MnCeO_x/diatomite/PS system. Moreover, MnCeO_x/diatomite showed good reusability and stability. Mechanism analyses indicated that electron transfer of Mn and Ce attributed to the activation of PS and oxygen to produce free radicals. SO₄^-, OH and O₂^- on the surface of catalyst were the main active free radicals to attack pollutants.

An efficient CuO-γFe2O3 composite activates persulfate for organic pollutants removal: Performance, advantages and mechanism

CuO-γFe₂O₃ was fabricated as a novel and effective persulfate (PS) catalyst to remove bio-refractory organic pollutants. Characterization results showed that CuO-γFe₂O₃ possessed a relatively large surface area among transition metal oxides which provided favorable adsorption and activation sites for PS to degrade pollutants. There was an obvious synergy between CuO and γFe₂O₃ in the composite, which played 84.7% role in Acid orange 7 (AO7) removal. Under the optimal conditions (CuO-γFe₂O₃ dosage = 0.6 g L^-1, PS dosage = 0.8 g L^-1, unadjusted solution pH), almost complete AO7 was rapidly eliminated in 5 min. Moreover, the wide workable pH range (2-13), good stability (0.82 mg L^-1 Cu leached, almost no Fe leached) and reusability (4 times) were the significant virtues of CuO-γFe₂O₃ for wastewater treatment. Besides, the reaction mechanism mainly based on the interaction among Cu(II/III) and Fe(II/III) species for sulfate radical (SO₄^-) generation was emphatically elucidated by the analyses of radicals, PS utilization, TOC removal and metal chemical states. Finally, CuO-γFe₂O₃+PS system displayed desirable removal of multiple organic pollutants with different molecular structures. In light of the prominent advantages of CuO-γFe₂O₃+PS, this work extended activated PS process in treating refractory organic wastewater.

Effects of green synthesis, magnetization, and regeneration on ciprofloxacin removal by bimetallic nZVI/Cu composites and insights of degradation mechanism

In this study, nanoscale zerovalent iron (nZVI) with copper (Cu) bimetallic particles, whichare applied for degradation of Ciprofloxacin (CIP) under weak magnetic field (WMF), were synthesized using green tea extracts (GT-nZVI/Cu). The surface morphology and physicochemical properties of the novel catalytic materials were characterized. It was found that GT-nZVI was more stable and performed better in oxidation resistance than the nZVI synthesized by traditional chemical methods. Besides, the catalytic reactivity of GT-nZVI/Cu was measured with and without WMF, it is obvious from the experimental results the performance of GT-nZVI/Cu system was enhanced significantly with WMF. Moreover, WMF still had a certain effect even after being removed, which is called remanence effect. The mass spectrometry (MS) was utilized to analyze the degradation products of CIP, and the contribution of adsorption and Fenton/Fenton-like oxidation of GT-nZVI/Cu during CIP removal process was further evaluated. It was found that as the removal process progressed, the contribution ratio of Fenton/Fenton-like oxidation rose rapidly and exceeded adsorption after 20 min. Eventually, attempts have been made to regenerate GT-nZVI/Cu, in which physical recovery (ultrasonic) was the main route, and the CIP removal rate decreased as the regeneration times increased. This research provides new insights into the green synthesis and regeneration of nZVI and is expected to realize the practical application of nZVI.

Degradation of ronidazole by electrochemically simultaneously generated persulfate and ferrous ions

Nitroimidazoles are found in pharmaceuticals and personal care products (PPCPs) and, when discharged into the environment, have adverse effects on human health and survival. Advanced oxidation technologies (AOTs) based on persulfate (PS) can rapidly and efficiently degrade organic pollutants via strong oxidizing radicals under activation conditions. This study investigated the degradation of ronidazole (RNZ) by indirect electrolytic generation of PS and its activator, ferrous ion (Fe²⁺). An electrochemical system was developed, with a high concentration of PS generated at the anode while the activator Fe²⁺ was produced at the cathode. It showed that ammonium polyphosphate (APP) could effectively promote the electrolysis of PS. A high current efficiency (88%) at the anode could be obtained after 180 min at a high current density (300 mA cm^-2). However, Fe²⁺ was inhibited at the cathode due to material control. The degradation of RNZ in the Fe²⁺/PS system generated from the electrochemical system was also explored. Increasing PS concentration and Fe²⁺/PS ratio were beneficial to the RNZ degradation. In homogeneous reactions, the degradation efficiency of RNZ could be improved by decreasing the Fe²⁺ addition rate through a peristaltic pump. Five intermediates were also detected and the degradation pathways were proposed. These findings provide a new method and mechanism for rapid and efficient degradation of RNZ.

Persulfate activation by natural zeolite supported nanoscale zero-valent iron for trichloroethylene degradation in groundwater

In the advanced oxidation processes, using persulfate (PS) as a radical precursor for pollutant degradation in groundwater has received increasing attention. In this study, zeolite supported nZVI composites (Z/nZVI) were synthesized through an ion exchange and borohydride reduction method to investigate their ability to activate PS for the TCE degradation. Based on preliminary screening of the PS activation by the Z/nZVI (PS-Z/nZVI) system in terms of TCE degradation, Z/nZVI composite with a zeolite to nZVI mass ratio of 1:1 (Z/nZVI (1)) was optimized as the best composition and chosen for further characterization and examination. Especially, for this PS-Z/nZVI system, PS concentration, solution matrix effects (i.e., solution pH, coexisting anions and natural organic matter) were studied. Characterization results revealed that the aggregation of nZVI particles was alleviated and they were good dispersed on the zeolite sheet with a large SSA (159.49 m²/g) compared to the unsupported nZVI (8.77 m²/g). The synthesized Z/nZVI (1) composite exhibited excellent activated ability towards PS (1.5 mM) and effectively degraded 98.8% of TCE at pH 7 within 120 min. The PS-Z/nZVI system was observed to operate effectively over a wide range of pH (i.e., 4-7) for TCE degradation. Moreover, the presence of nitrates (1 mM) and bicarbonates (10 mM) decreased the TCE degradation efficiency to 91.5% and 59.6%, respectively. Scavenger tests demonstrated that both sulfate and hydroxyl radicals participated in the TCE degradation. The ion chromatography analysis suggested the formation of oxalic acid and formic acid as the reaction intermediates during the TCE degradation process in the PS-Z/nZVI system.

Sulfate Radicals-Based Technology as a Promising Strategy for Wastewater

This study was focused on the generation of sulfate radicals and their applicability as powerful oxidants for degrading complex organic compounds with the final objective of operating in flow systems. To this end, the removal of two compounds from the pharmaceutical industry was assessed, lissamine green and prednisolone. Initially, sulfate radicals were generated by the activation of persulfate with iron as homogenous catalyst, and the key parameters involved in the process, as catalyst concentration and oxidant dosage, were evaluated. Furthermore, with the aim of preventing the secondary contamination due to metal leaching and to be operate in a continuous mode, a heterogeneous catalyst was developed. For it, the iron was fixed on a cationic resin as Amberlite IR120 Na+ form. It was demonstrated that the removal of both pollutants increases with greater catalyst dosages, achieving a decay of 85% within 25 min with 30 g·L−1 of catalyst. Moreover, the reuse capability of the catalyst was tested, illustrating that it is rough enough for its reuse. Conversely, in order to develop a continuous treatment in flow system, a fixed bed reactor was constructed and its feasibility was proven. Different experiments with residence times from 10 min to 60 min were performed, obtaining a removal level of ≈95% and 90% for prednisolone and lissamine green, respectively, at residence time of 60 min. In conclusion, the potential of sulfate radicals-based technology for degrading organic contaminants has been demonstrated.

Activation of peroxymonosulfate system by copper-based catalyst for degradation of naproxen: Mechanisms and pathways

Organic degradation by zero-valent metal (ZVM)-activated peroxymonosulfate (PMS) systems has drawn great attention in water treatment. Among various types of ZVM, zero-valent copper (ZVC) showed greatest activating capacity. However, the disadvantages of the released Cu²⁺ limit the practical utilization of ZVC. In this study, the activation capacity of four normal-sized copper catalysts, namely, copper sheet, graphene-copper sheet, copper foam, and graphene-copper foam, for PMS was investigated using Naproxen (NPX) as the probe compound. Results showed that the degradation efficiency of NPX increased by 10%, while the release of Cu²⁺ decreased by 30% by coating the copper with graphene. Stability tests showed that all of the four catalysts exhibited considerable stability in PMS activation. Furthermore, we found for the first time that the hydroxyl radical was the dominant species in the degradation of NPX rather than the sulfate radical, which was proved by ESR and radical scavenging experiments. Finally, six intermediates were identified by HPLC-MS/MS, and the degradation pathways were proposed. This study confirmed the feasibility of graphene coating on metals to achieve the enhancement of PMS activation.

A facile heterogeneous system for persulfate activation by CuFe2O4 under LED light irradiation

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated. The effect of vital experimental factors, including initial solution pH, CuFe₂O₄ dosage, PS concentration, co-existing anion and initial RB concentration on the removal of RB was systematically studied. The removal of RB was in accordance with the pseudo first-order reaction kinetics. Over 96% of 20 mg L⁻¹ RB was removed in 60 min using 0.5 g L⁻¹ CuFe₂O₄ catalyst and 0.2 mM PS at neutral pH. In addition, free radical quenching experiments and electron spin resonance (EPR) experiments were performed, which demonstrated the dominant role of sulfate radical, photogenerated holes and superoxide radical in the CuFe₂O₄/PS/LED system. The morphology and physicochemical properties of the catalyst were characterized by XRD, SEM-EDS, TEM, N₂ adsorption–desorption isotherm, UV-vis DRS, and XPS measurements. Moreover, 18.23% and 38.79% total organic carbon (TOC) removal efficiency was reached in 30 min and 60 min, respectively. The catalyst revealed good performance during the reusability experiments with limited iron and copper leaching. Eventually, the major intermediates in the reaction were detected by GC/MS, and the possible photocatalytic pathway for the degradation of RB in the CuFe₂O₄/PS/LED system was proposed. The results suggest that the CuFe₂O₄/PS/LED system has good application for further wastewater treatment.

In this study, the removal performance for rhodamine B (RB) by persulfate (PS) activated by the CuFe₂O₄ catalyst in a heterogeneous catalytic system under LED light irradiation was investigated.