Unraveling the interaction of hydroxylamine and Fe(III) in Fe(II)/Persulfate system: A kinetic and simulating study

Chloride-mediated enhancement in Cu(II)-catalyzed Fenton-like reaction: The overlooked reactive chlorine species

The practical application of Cu(II)-catalyzed Fenton-like reaction (Cu(II)/H2O2) exhibits a low efficiency in the degradation of refractory compounds of wastewater. The impact of chloride ions (Cl-) on Fenton-like reactions have been investigated, but the influence mechanism is still unclear. Herein, the presence of Cl- (5 mM) significantly accelerated the degradation of benzoic acid (BA) under neutral conditions. The degradation of BA follows pseudo-first-order kinetics, with a degradation rate 7.3 times higher than the Cu(II)/H2O2 system. Multiple evidences strongly demonstrated that this reaction enables the production of reactive chlorine species (RCS) rather than HO• and high-valent copper (Cu(III)). The kinetic model revealed that Cl- could shift reactive species from the key intermediate (Cu(III)-chloro complexes) to RCS. Dichlorine radicals (Cl2•-) was discovered to play a crucial role in BA degradation, which was largely overlooked in previous reports. Although the reaction rate of Cl2•- with BA (k = 2.0 × 106 M-1 s-1) is lower than that of other species, its concentration is 10 orders of magnitude higher than that of Cu(III) and HO•. Furthermore, the exceptional efficacy of the Cu(II)/H2O2 system in BA degradation was observed in saline aquatic environments. This work sheds light on the previously unrecognized role of the metal-chloro complexes in production the RCS and water purification.

Iron (hydr)oxides-induced activation of sulfite for contaminants degradation: The critical role of structural Fe(III)

Iron-based sulfite (S(IV)) activation has emerged as a novel strategy to generate sulfate radicals (SO4•-) for contaminants degradation. However, numerous studies focused on dissolved iron-induced homogeneous activation processes while the potential of structural Fe(III) remains unclear. In this study, five iron (hydr)oxide soil minerals (FeOx) including ferrihydrite, schwertmannite, lepidocrocite, goethite and hematite, were successfully employed as sources of structural Fe(III) for S(IV) activation. Results showed that the catalytical ability of structural Fe(III) primarily depended on the crystallinity of FeOx instead of their specific surface area and particle size, with ferrihydrite and schwertmannite being the most active. Furthermore, in-situ ATR-FTIR spectroscopy and 2D-COS analysis revealed that HSO3- was initially adsorbed on FeO6 octahedrons of FeOx via monodentate inner-sphere complexation, ultimately oxidized into SO42- which was then re-adsorbed via outer-sphere complexation. During this process, strong oxidizing SO4•- and •OH were formed for pollutants degradation, confirmed by radical quenching experiments and electron spin resonance. Moreover, FeOx/S(IV) system exhibited superior applicability with respect to recycling test, real waters and twenty-six pollutants degradation. Eventually, plausible degradation pathways of three typical pollutants were proposed. This study highlights the feasibility of structural Fe(III)-containing soil minerals for S(IV) activation in wastewater treatment.

Recovering palladium and gold by peroxydisulfate-based advanced oxidation process

Palladium (Pd) and gold (Au) are the most often used precious metals (PMs) in industrial catalysis and electronics. Green recycling of Pd and Au is crucial and difficult. Here, we report a peroxydisulfate (PDS)-based advanced oxidation process (AOPs) for selectively recovering Pd and Au from spent catalysts. The PDS/NaCl photochemical system achieves complete dissolution of Pd and Au. By introducing Fe(II), the PDS/FeCl2·4H2O solution functioned as Fenton-like system, enhancing the leaching efficiency without xenon (Xe) lamp irradiation. Electron paramagnetic resonance (EPR), 18O isotope tracing experiments, and density functional theory calculations revealed that the reactive oxidation species of SO4·-, ·OH, and Fe(IV)═O were responsible for the oxidative dissolution process. Lixiviant leaching and one-step electrodeposition recovered high-purity Pd and Au. Strong acids, poisonous cyanide, and volatile organic solvents were not used during the whole recovery, which enables an efficient and sustainable precious metal recovery approach and encourage AOP technology for secondary resource recycling.

Mechanisms on the removal of gram-negative/positive antibiotic resistant bacteria and inhibition of horizontal gene transfer by ferrate coupled with peroxydisulfate or peroxymonosulfate

The existence of antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) has been a global public environment and health issue. Due to the different cell structures, gram-positive/negative ARB exhibit various inactivation mechanisms in water disinfection. In this study, a gram-negative ARB Escherichia coli DH5α (E. coli DH5α) was used as a horizontal gene transfer (HGT) donor, while a gram-positive ARB Bacillus as a recipient. To develop an efficient and engineering applicable method in water disinfection, ARB and ARGs removal efficiency of Fe(VI) coupled peroxydisulfate (PDS) or peroxymonosulfate (PMS) was compared, wherein hydroxylamine (HA) was added as a reducing agent. The results indicated that Fe(VI)/PMS/HA showed higher disinfection efficiency than Fe(VI)/PDS/HA. When the concentration of each Fe(VI), PMS, HA was 0.48 mM, 5.15 log E. coli DH5α and 3.57 log Bacillus lost cultivability, while the proportion of recovered cells was 0.0017 % and 0.0566 %, respectively, and HGT was blocked. Intracellular tetA was reduced by 2.49 log. Fe(IV) and/or Fe(V) were proved to be the decisive reactive species. Due to the superiority of low cost as well as high efficiency and practicality, Fe(VI)/PMS/HA has significant application potential in ARB, ARGs removal and HGT inhibition, offering a new insight for wastewater treatment.

Redox property of coordinated iron ion enables activation of O2 via in-situ generated H2O2 and additionally added H2O2 in EDTA-chelated Fenton reaction

The Fenton system was a generation system of reactive oxygen species via the chain reactions, which employed H2O2 and O2 as radical precursors and Fe2+/Fe3+ as electron-donor/acceptor for triggering or terminating the generation of radicals. Recent work mainly emphasized the Fe2+- activated H2O2 and the application of in-situ generated •OH, while neglecting other side-reactions. In this work, EDTA (Ethylene diamine tetraacetic acid) was employed as a chelating agent of iron ions, which simultaneously changed the redox property of coordinated iron. The Fe2+-EDTA complexes in the presence of dissolved oxygen enabled the two-electron transfer from Fe2+ to O2 and the in-situ production of H2O2, which further activate H2O2 for yielding •OH. Meanwhile, coordinated Fe3+ exhibited non-negligible reactivity toward H2O2, which was higher than that of free Fe3+ in the traditional Fenton system. The complexation of EDTA with Fe3+ could enhance the Fe2+ generation reaction by the H2O2, accompanied by the O2•- formation. The enhancement of O2•- formation and Fe2+-EDTA regeneration induced the subsequent H2O2 activation by Fe2+-EDTA, thus accelerating the Fe3+-EDTA/Fe2+-EDTA cycle for simultaneously producing O2•- and •OH. To sum up, the EDTA-chelated Fenton system extended the applicable pH range to circumneutral/alkaline level and tuned the redox property of coordinated iron for diversifying the •OH production routes. The research reinterpreted the chain reactions in the Fenton system, revealing another way to enhance the radical production or other property of the Fenton/Fenton-like system.

Factors hindering the degradation of pharmaceuticals from human urine in an iron-activated persulfate system

This study investigated the degradation of clofibric acid (CFA), bezafibrate (BZF), and sulfamethoxazole (SMX) in synthetic human urine using a novel mesoporous iron powder-activated persulfate system (mFe-PS system), and identified the factors limiting their degradation in synthetic human urine. A kinetic model was established to expose the radical production in various reaction conditions, and experiments were conducted to verify the modeling results. In the phosphate-containing mFe-PS system, the 120 min removal efficiency of CFA decreased from 95.1% to 76.6% as the phosphate concentration increased from 0.32 to 6.45 mmol/L, but recovered to 90.5% when phosphate concentration increased to 16.10 mmol/L. Meanwhile, the increased concentration of phosphate from 0.32 to 16.10 mmol/L reduced the BZF degradation efficacy from 91.5% to 79.0%, whereas SMX removal improved from 37.3% to 62.9%. The mFe-PS system containing (bi)carbonate, from 4.20 to 166.70 mmol/L, reduced CFA and BZF removal efficiencies from 100% to 76.8% and 80.4%, respectively, and SMX from 83.5% to 56.7% within a 120-min reaction time. In addition, alkaline conditions (pH ≥ 8.0) inhibited CFA and BZF degradations, while nonacidic pH (pH ≥ 7.0) remarkably inhibited SMX degradation. Results of the kinetic model indicated the formation of phosphate (H2PO4·/HPO4·-) and/or carbonate radicals (CO3·-) could limit pharmaceutical removal. The transformation products (TPs) of the pharmaceuticals revealed more incompletely oxidized TPs occurred in the phosphate- and (bi)carbonate-containing mFe-PS systems, and indicated that H2PO4·/HPO4·- mainly degraded pharmaceuticals via a benzene ring-opening reaction while CO3·- preferentially oxidized pharmaceuticals via a hydroxylation reaction.

Efficient degradation and detoxification of antibiotic Fosfomycin by UV irradiation in the presence of persulfate

Fosfomycin (FOS) as a widely used antibiotic has been found in abundance throughout the environment, but little effort has been devoted to its treatment. In this study, we systemically looked into the degradation of FOS by ultraviolet-activated persulfate (UV/PS) in aqueous solutions. Our findings demonstrated that FOS can be degraded efficiently under the UV/PS, e.g., >90 % of FOS was degraded with 19,200 mJ cm-2 of UV irradiance and 20 μM of PS. HO was the dominant radical responsible for FOS degradation. FOS degradation increased as PS dosage increased, and higher degradation efficiency was observed at neutral pH. Natural water constitutes either promoted (e.g., Cu2+, Fe3+, and SO42-) or inhibited (e.g., humic acid, HCO3-, and CO32-) FOS degradation to varying degrees. Hydroxyl substitution, CP bond cleavage, and coupling reactions were the major degradation pathways for FOS degradation. Finally, the toxicity evaluation revealed that FOS was toxic to E. coli and S. aureus, but the toxicity of the intermediate products of FOS to E. coli and S. aureus rapidly decreased over time after UV/PS treatment. Therefore, these findings provided a fundamental understanding of the transformation process of FOS and supplied useful information for the environmental elimination of FOS contamination and its toxicity.

A Novel Spectrophotometric Method for Determination of Percarbonate by Using N, N-Diethyl-P-Phenylenediamine as an Indicator and Its Application in Activated Percarbonate Degradation of Ibuprofen

Sodium percarbonate (SPC) concentration can be determined spectrophotometrically by using N, N-diethyl-p-phenylenediamine (DPD) as an indicator for the first time. The ultraviolet-visible spectrophotometry absorbance of DPD•+ measured at 551 nm was used to indicate SPC concentration. The method had good linearity (R2 = 0.9995) under the optimized experimental conditions (pH value = 3.50, DPD = 4 mM, Fe2+ = 0.5 mM, and t = 4 min) when the concentration of SPC was in the range of 0-50 μM. The blank spiked recovery of SPC was 95-105%. The detection limit and quantitative limit were 0.7-1.0 μM and 2.5-3.3 μM, respectively. The absorbance values of DPD•+ remained stable within 4-20 min. The method was tolerant to natural water matrix and low concentration of hydroxylamine (<0.8 mM). The reaction stoichiometric efficiency of SPC-based advanced oxidation processes in the degradation of ibuprofen was assessed by the utilization rate of SPC. The DPD and the wastewater from the reaction were non-toxic to Escherichia coli. Therefore, the novel Fe2+/SPC-DPD spectrophotometry proposed in this work can be used for accurate and safe measurement of SPC in water.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Dual promoted ciprofloxacin degradation by Fe0/PS system with ascorbic acid and pre-magnetization

As a widely used and hard-to-degrade pharmaceuticals and personal care product (PPCP), ciprofloxacin (CIP) was frequently found in water environment and the detected concentration was gradually increased. Although zero-valent iron (ZVI) has been shown to be effective in destroying refractory organic pollutants, the practical application and sustained catalytic performance is not satisfactory. Herein, introduction of ascorbic acid (AA) and employment of pre-magnetized Fe⁰ was achieved to maintain a high-concentration of Fe²⁺ during persulfate (PS) activation. Pre-Fe⁰/PS/AA system presented the best performance for CIP degradation, achieving almost complete elimination of 5 mg/L CIP within 40 min in the reaction conditions of 0.2 g/L pre-Fe⁰，0.05 mM AA and 0.2 mM PS. The CIP degradation retarded as excess pre-Fe⁰ and AA were added, therefore, the optimum dosages of pre-Fe⁰ and AA were determined to be 0.2 g/L and 0.05 mM, respectively. The CIP degradation gradually decreased as the initial pH increased from 3.05 to 11.03. The presence of Cl^-, HCO₃^-, Al³⁺, Cu²⁺ and humic acid significantly influenced the performance of CIP removal, while Zn²⁺, Mg²⁺, Mn²⁺, and NO₃^- slightly affected the CIP degradation. Combined with the results of HPLC analysis and previous literature, several possible degradation pathways of CIP were proposed.

Activation of the combined hydrogen peroxide and peroxymonosulphate by lepidocrocite for chloramphenicol removal: kinetics and mechanisms

The main compositions of pipe deposits from water distribution networks are potential iron resources, which can be used as catalysts to activate the combined hydrogen peroxide (HP) and peroxymonosulphate (PMS) system to produce reactive oxidative species (ROSs) to degrade pollutants. As a result, the degradation efficiency of chloramphenicol (CAP) in the HP/PMS dual-oxidant system could reach as high as 75.21% within 100 min with hydroxylamine (HA) assistance, and the dual-oxidant method had a wide pH applied range. To explore the mechanism of the dual-oxidant system in detail, several main affecting factors were investigated. In addition, the hydroxyl radical(•OH) was identified as the predominant radicals by Electron paramagnetic resonance (EPR) and the Radical scavenger test (RST). According to the competition kinetics experiment, the reaction rate of CAP with •OH was 1.933(± 0.052) × 10¹⁰ M^-1s^-1 in the HP/PMS dual-oxidant system, which was higher than the HP single oxidant system (6.10(± 0.036) × 10⁹ M^-1s^-1). And the role of HA was explored , including reduction and competition. Six degradation products were detected by the liquid chromatography-mass spectrometry (LC-MS) and their toxicity was analyzed by the ecological structure-activity relationship (ECOSAR) predictive model. These findings further provide a theoretical basis for the practical application of pipe deposits and advance the development of in-situ removal of pollutants in water distribution networks in the future promisingly.

Vivianite-induced peroxymonosulfate activation for containment removal under dark conditions: Performance, mechanism and regeneration

The performance and intrinsic mechanism of vivianite, a natural mineral containing structural Fe(II), for peroxymonosulfate (PMS) activation and pollutant degradation under dark conditions were comprehensively explored in this study. It was found that vivianite was able to efficiently activate PMS to degrade various pharmaceutical pollutants under dark conditions, in which the corresponding reaction rate constant of ciprofloxacin (CIP) degradation was 47- and 32-fold higher than that of magnetite and siderite, respectively. SO₄^·-, ^·OH, Fe(IV) and electron-transfer processes were found in the vivianite-PMS system, while SO₄^·- was the main contributor to CIP degradation. Moreover, mechanistic explorations revealed that the Fe site on the surface of vivianite could bind PMS in the form of a bridge position, and thus vivianite could rapidly activate absorbed PMS due to its strong electron-donating ability. Additionally, it was illustrated that the used vivianite could be efficiently regenerated by either chemical or biological reduction. This study may provide an alternative application of vivianite in addition to phosphorus recovery from wastewater.

Thiosulfate enhanced Cu(II)-catalyzed Fenton-like reaction at neutral condition: Critical role of sulfidation in copper cycle and Cu(III) production

Thiosulfate (S₂O₃^2-) has been proven to be an effective promoter of Fenton-like reactions by accelerating the metal ions cycle. However, up to now, little is known about the role of sulfur transformation and intermediate sulfur in the regulation of metal chemical cycle and reactive species production. Herein, free Cu(II) was selected as catalyst for the activation of H₂O₂. The introduction of S₂O₃^2- significantly enhanced the degradation of benzoic acid, and the degradation rate (k_obs) was 5.8 times that of Cu(II)/H₂O₂ system. The kinetic model revealed the transformation of sulfur species and demonstrated that sulfides (i.e., HS^-/S^2-, S₂O₃^2-) and S⁰ were the dominant electron donor for the reduction of Cu(II) into Cu(I). Consequently, the reduction and complexation roles of S₂O₃^2- significantly resolve the rate-limiting step and broaden the pH range of in Fenton-like reactions. Evidence for the critical role of high-valent copper (Cu(III)) and HO^• on BA degradation was obtained by scavengers experiments, electron paramagnetic resonance and fluorescent probes. Meanwhile, the Cu(II)/H₂O₂/S₂O₃^2- system also exhibited satisfactory anti-interference ability of the various matrix. Overall, this study offers mechanistic insight into sulfidation in Cu chemical cycle and Cu(III) generation, and highlights the potential of S₂O₃^2- for Fenton-like reactions to control pollutants.

Highly Efficient Utilization of Ferrate(VI) Oxidation Capacity Initiated by Mn(II) for Contaminant Oxidation: Role of Manganese Species

Manganese ion [Mn(II)] is a background constituent existing in natural waters. Herein, it was found that only 59% of bisphenol A (BPA), 47% of bisphenol F (BPF), 65% of acetaminophen (AAP), and 49% of 4-tert-butylphenol (4-tBP) were oxidized by 20 μM of Fe(VI), while 97% of BPA, 95% of BPF, 96% of AAP, and 94% of 4-tBP could be oxidized by the Fe(VI)/Mn(II) system [20 μM Fe(VI)/20 μM Mn(II)] at pH 7.0. Further investigations showed that bisphenol S (BPS) was highly reactive with reactive iron species (RFeS) but was sluggish with reactive manganese species (RMnS). By using BPS and methyl phenyl sulfoxide (PMSO) as the probe compounds, it was found that reactive iron species contributed primarily for BPA oxidation at low Mn(II)/Fe(VI) molar ratios (below 0.1), while reactive manganese species [Mn(VII)/Mn(III)] contributed increasingly for BPA oxidation with the elevation of the Mn(II)/Fe(VI) molar ratio (from 0.1 to 3.0). In the interaction of Mn(II) and Fe(VI), the transfer of oxidation capacity from Fe(VI) to Mn(III), including the formation of Mn(VII) and the inhibition of Fe(VI) self-decay, improved the amount of electron equivalents per Fe(VI) for BPA oxidation. UV-vis spectra and dominant transformation product analysis further revealed the evolution of iron and manganese species at different Mn(II)/Fe(VI) molar ratios.

New insights into the degradation of micro-pollutants in the hydroxylamine enhanced Fe(II)/peracetic acid process: Contribution of reactive species and effects of pH

The hydroxylamine-enhanced Fe(II)/peracetic acid (PAA) process is a promising advanced oxidation process (AOP) with the generation of reactive species (RS) including RO•, •OH and Fe(IV). Nevertheless, it is still challenging to identify which RS is the major intermediate oxidant, and the reasons why the optimal condition is pH 4.5 rather than 3.0 are also unclear. Herein, the generation of RS and their contribution to the degradation of three micro-pollutants were explored. The quenching experiments and pseudo first-order kinetic model demonstrated that RO• rather than the other two RS were predominant. Then the overall generation and evolution pathways of RS were depicted. The elevation of pH (3.0-4.5) would accelerate the Fe(II)/Fe(III) redox cycle through the enhanced reduction of Fe(III) by hydroxylamine and induce the conversion of Fe(IV) to RO•, which benefited naproxen degradation. While the adverse Fe(III) precipitation would dominate the reduced degradation performance with the solution pH higher than 4.5. The elevation of PAA and Fe(II) dosages sped up the PAA activation, while excess hydroxylamine could consume the formed RS and exhibited an inhibitory effect. This study helps further understand the role of HA and differentiate the contribution of RS in the emerging PAA-based AOPs.

WS2 significantly enhances the degradation of sulfachloropyridazine by Fe(III)/persulfate

The use of antibiotics has become an indispensable part of the production and life of human society. Among them, sulfonamide antibiotics widely used in humans and animals are considered to be one of the most crucial antibiotics. However, antibiotics are difficult to degrade naturally, leading to an accumulation in the environment and a potential hazard to human health. In this paper, WS₂ as a co-catalyst could reduce trace Fe(III) to Fe(II) which exhibited a great activating ability to PS through the exposed W(IV) active sites, and formed the Fe(III)/Fe(II) cycle to degrade sulfachloropyridazine (SCP) continuously. This paper systematically discussed the degradation of SCP under different conditions in the PS/WS₂/Fe(III) system, including the amount of WS₂, Fe(III) concentration, PS concentration, initial pH, natural organic matter (NOM) and common anions (NO₃^-, Cl^-, HCO₃^-, HPO₄^2- and H₂PO₄^-). The experimental results showed that PS/WS₂/Fe(III) system possessed a strong degradation ability for SCP in a wide pH range. NO₃^- and Cl^- could promote the degradation of SCP a little. HCO₃^-, HPO₄^2- and H₂PO₄^- could significantly inhibit the degradation of SCP. The main types of free radicals that degraded SCP were explored. In addition, the stability and reusability of WS₂ were examined, and two possible degradation pathways of SCP were proposed.

Model simulation and mechanism of Fe(0/II/III) cycle activated persulfate degradation of methylparaben based on hydroxylamine enhanced nano-zero-valent iron

The mechanism of Fe²⁺-activated peroxodisulfate (PDS) by hydroxylamine (HA) has been investigated, however, nano zero-valent iron-activated persulfate (nZVI/PDS) has a more optimal effect and needs further investigation. This study investigated the addition of HA to nZVI/PDS to improve Fe²⁺ regeneration and accelerate methylparaben (MP) degradation by Fe (0/II/III) cycle. After 60 min of reaction, the HA-enhanced nZVI/PDS (HA/nZVI/PDS) system afforded a 21% increase in MP degradation, reaching 93.26% (1 mM HA, 1 mM nZVI, and 2 mM PDS). nZVI/PDS system was a second-order reaction, but after adding HA, the reaction was more suitable for the first-order reaction. The addition of HA effectively promoted the reduction of Fe³⁺ to Fe²⁺ to improve the effect and reaction rate of PDS degradation of MP (k increased from 0.0127 min^-1 to 0.0198 min^-1) and broadened the reaction pH range. The results of various characterizations of nZVI before and after the reaction revealed that nZVI changed from a spherical structure to a bundle structure and was slightly oxidized. Changes in the Fe²⁺ and Fe³⁺ concentrations as well as in the pH of the reaction systems were monitored and the possible reactions of the HA/nZVI/PDS system were derived for the first time (k_nZVI/PDS<3.7 × 10⁶ M^-1 s^-1, k_Fe3+/NH2O· >4.2 min^-1). 12 potential compounds were investigated and MP breakdown pathways were speculated; hydroxylation was determined to be the most important pathway of degradation. And the HA/nZVI/PDS system had universal applicability.

Occurrence of Iodophenols in Aquatic Environments and the Deiodination of Organic Iodine with Ferrate(VI)

Toxic and odorous iodophenols are commonly identified as disinfection by-products (DBPs) in drinking water. Herein, ng/L levels of iodophenols were identified in river water, wastewater treatment plant effluent, and medical wastewater, with the simultaneous identification of μg/L to mg/L levels of iodide (I^-) and total organic iodine (TOI). Oxidation experiment suggested that the I^-, TOI, and iodophenols could be oxidized by ferrate [Fe(VI)], and more than 97% of TOI had been transformed into stable and nontoxic IO₃^-. Fe(VI) initially cleaved the C-I bond of iodophenols and led to the deiodination of iodophenols. The resulted I^- was swiftly oxidized into HOI and IO₃^-, with the intermediate phenolic products be further oxidized into lower molecular weight products. The Gibbs free energy change (ΔG) of the overall reaction was negative, indicating that the deiodination of iodophenols by Fe(VI) was spontaneous. In the disinfection of iodine-containing river water, ng/L levels of iodophenols and chloro-iodophenols formed in the reaction with NaClO/NH₂Cl, while Fe(VI) preoxidation was effective for inhibiting the formation of iodinated DBPs. Fe(VI) exhibited multiple functions for oxidizing organic iodine, abating their acute toxicity/cytotoxicity and controlling the formation of iodinated DBPs for the treatment of iodide/organic iodine-containing waters.

Kinetics of zero-valent iron-activated persulfate for methylparaben degradation and the promotion of Cl. JOURNAL OF ENVIRONMENTAL MANAGEMENT 2022;321:115973. [PMID: 36104884 DOI: 10.1016/j.jenvman.2022.115973]

Methylparaben (MP) is an emerging pollutant, and the optimal conditions and kinetics of MP degradation using nano-zero-valent iron-activated persulfate (nZVI/PDS) need to be further investigated. This paper firstly investigated the response surface methodology (RSM) analysis of MP degradation by the heterogeneous system nZVI/PDS and concluded that the initial pH had the most significant effect on MP degradation. The optimal experimental conditions predicted by the RSM were as follows: initial pH 2.75, [nZVI]₀ = 2.87 mM, [PDS]₀ = 2.18 mM (MP degradation level of 95.30%). First- and second-order kinetic fits were performed for different initial pH levels and different concentrations of MP, nZVI, and PDS. It was determined that k = 0.0365 min^-1 (R² = 0.984) when the initial pH was 3, [PDS]₀ = 2 mM, [MP]₀ = 20 mg L^-1, and [nZVI]₀ = 3 mM (MP degradation level of 94.25%). The rest of the conditions were more closely fitted to the second-order reactions. The effects of different concentrations of anions and humic acid (HA) on the MP degradation level and k were examined, and it was found that Cl^- could promote MP degradation to 97.69% (increased by 3.65%) and increase the k in accordance with the first-order reaction kinetics (0.0780 min^-1, R² = 0.991). Finally, the analysis of intermediates revealed 5 reaction pathways and 7 reaction intermediates, which inferred a possible reaction mechanism with the recycling performance of nZVI. In this paper, the superiority of nZVI/PDS for the purposes of activating MP degradation was affirmed. The presence of Cl^- can enhance the level of MP degradation was confirmed, which provides a new direction for future practical engineering applications.

Accelerated Removal of Acid Orange 7 by Natural Iron Ore Activated Peroxymonosulfate System with Hydroxylamine for Promoting Fe(III)/Fe(II) Cycle

In this study, peroxymonosulfate (PMS) was activated by cheap and readily available natural iron ore to remove Acid Orange 7 (AO7) in water with the assistance of hydroxylamine (HA). Results show that the presence of HA could accelerate the Fe(II)/Fe(III) cycle on the ore surface, promoting the activation of PMS to generate reactive oxidative species. The effects of ore dosage, PMS dosage, HA dosage and initial pH on the degradation of AO7 were investigated in the HA/Ore/PMS system. Under the optimal conditions, the removal of AO7 could reach 93.1% during 30 min, which was 41.4% higher than the ore/PMS system. The AO7 removal increased with the increase of HA, PMS and ore dosage, but was unaffected by the initial solution pH. Based on radical scavenging experiments and EPR tests, the dominant reactive species in the HA/Ore/PMS system were revealed to be the sulfate radical (SO4•−), singlet oxygen (1O2), superoxide radical (O2•−) and hydroxyl radical (•OH), which were responsible for the AO7 degradation. Furthermore, the possible reaction mechanism of PMS activation was proposed. This study provides an efficient technique for the removal of azo dye organic contaminant in water, which has great practical significance.

Mechanistic Insights into the Markedly Decreased Oxidation Capacity of the Fe(II)/S2O82- Process with Increasing pH

The poor oxidation capacity of the Fe(II)/S₂O₈^2- [Fe(II)/PDS] system at pH > 3.0 has limited its wide application in water treatment. To unravel the underlying mechanism, this study systematically evaluated the possible influencing factors over the pH range of 1.0-8.0 and developed a mathematical model to quantify these effects. Results showed that ∼82% of the generated Fe(IV) could be used for pollutant degradation at pH 1.0, whereas negligible Fe(IV) contribution was observed at pH 7.5. This dramatic decline of Fe(IV) contribution with increasing pH dominantly accounted for the pH-dependent performance of the Fe(II)/PDS process. Unexpectedly, Fe(II) could consume ∼80% of the generated SO₄^•- non-productively under both acidic and near-neutral conditions, while the larger formation of Fe(III) precipitates at high pH inhibited the SO₄^•- contribution mildly. Moreover, the strong Fe(II) scavenging effect was difficult to be compensated for by slowing down the Fe(II) dosing rate. The competition of dissolved oxygen with PDS for Fe(II) was insignificant at pH ≤ 7.5, where the second-order rate constants for reactions of Fe(II) with oxygen were much lower than or comparable to that between Fe(II) and PDS. These findings could advance our understanding of the chemistry and application of the Fe(II)/PDS process.

Upgrading the peroxi-coagulation treatment of complex water matrices using a magnetically assembled mZVI/DSA anode: Insights into the importance of ClO radical

The electrochemical technologies for water treatment have flourished over the last decades. However, it is still challenging to treat the actual complex water effluents by a single electrochemical process, often requiring coupling of technologies. In this study, an upgraded peroxi-coagulation (PC) process with a magnetically assembled mZVI/DSA anode has been devised for the first time. COD, NH₃-N and total phosphorous were simultaneously and effectively removed from livestock wastewater. The advantages, influence of key parameters and evolution of electrogenerated species were systematically investigated to fully understand this novel PC process. The fluorescent substances in livestock wastewater could also be almost removed under optimal conditions (300 mA, 0.2 g ZVI particles and pH 6.8). The interaction between OH and active chlorine yielded ClO with a high steady-state concentration of 6.85 × 10^-13 M, which did not cause COD removal but accelerated the oxidation of NH₃-N. The Mulliken population suggested that OH and NH₃-N had similar electron-donor behavior, whereas ClO acted as an electron-withdrawing species. Besides, although the energy barrier for the reaction between OH and NH₃-N (17.0 kcal/mol) was lower than that with ClO (18.8 kcal/mol), considering the tunneling in the H abstraction reaction, the Skodje-Truhlar method adopted for calculations evidenced a 17-fold faster NH₃-N oxidation rate with ClO. In summary, this work describes an advantageous single electrochemical process for the effective treatment of a complex water matrix.

Sorption Preconcentration and Analytical Determination of Cu, Zr and Hf in Waste Samarium–Cobalt Magnet Samples

We developed a method of sorption determination via the atomic emission of Cu, Zr and Hf metals in the waste of samarium–cobalt magnets. This method was based on the preconcentration of impurities using S- and N-containing heterochain sorbents, with further determination of the analytes via inductively coupled plasma atomic emission spectrometry (ICP-OES). Different sorbents such as PED (polyethelendiamine), TDA (polythiodimethanamine), PhED (N-phenylpolyethediamine) and PTE (polythioether) were tested for Ti, Cu, Zr, Nb and Hf extraction. The PTE sorbent ensured the maximum extraction of the analytes (recovery 60% for Ti, 80% for Nb, 95–100% for Cu, Zr and Hf) and thus was selected for further research. Additionally, various acidities of chloride solution (0.01–1 M HCl) were investigated for metal sorption. Under the optimised sorption conditions, trace impurities of Cu, Zr and Hf were determined using ICP-OES with a relative standard deviation of less than 5%. The obtained results were confirmed by the added–found method and cross-method experiments. The detection limits (DLs) were 1.5, 2, 0.15, 2 and 0.75 µg/L for Ti, Cu, Zr, Nb and Hf, respectively. The proposed method can be successfully used for the determination of various microelements in other waste REE-magnetic materials.

Hydroxylamine-enhanced Fe(II)-peroxymonosulfate activation for efficient degradation of organic pollutants: optimization by response surface methodology

In this work, the response surface methodology (RSM) was used to model and optimize the hydroxylamine (HA) enhanced Fe(II)/peroxymonosulfate (PMS) process. The enhanced effect of HA on the degradation efficiency of Orange II (AO7) in the Fe(II)/PMS system was quantitatively analyzed. Pareto analysis showed that the individual and interactive effects of HA, Fe(II) and PMS were of the following order: HA > Fe(II) > PMS and Fe(II)/PMS > HA/PMS > Fe(II)/HA. The optimal conditions of HA/Fe(II)/PMS system were as follows: Fe(II) concentration was 34.0 μM, HA concentration was 0.4 mM, and PMS concentration was 0.9 mM. When the initial pH was 4.0-6.0, the degradation efficiency of AO7 in the HA/Fe(II)/PMS system was significantly higher than that in the Fe(II)/PMS system (P < 0.05). Hydroxylamine enhances the degradation of AO7 in the Fe(II)/PMS system by reducing Fe(III) to Fe(II). The results of quenching experiment showed that SO₄^•- was the dominating reactive oxygen species (ROS) in the HA/Fe(II)/PMS system. In the HA/Fe(II)/PMS system, CO₃^2- and humic acid inhibited the degradation efficiency of AO7. This work provides a novel mathematical model for the degradation of AO7 in the HA/Fe(II)/PMS system, which can be popularized and applied in similar experiments.

Magnetically separable Fe-base deposited on different carbon sources for ultrasound/persulfate-like heterogeneous activation: Optimized synthesis and field driving process

The eco-friendly composite materials, micro-nano Fe-base/glucose-derived hydro-chars (Fe_b/HCs), were prepared, whose magnetic separation can be achieved in both preparation and water treatment stages. The performances of Fe_b/HCs after N₂ heat-treatment to activate persulfate, chlorite, hydrogen peroxide, etc., in ultrasound field, obtained great improvement by extracting "O". For comparison, other different sized and magnetic iron-carbon based composites based on different carbon sources of activated carbons, cornstalk-derived bio-chars, and glucose-derived pyrolytic carbons were prepared and applied to systematically compare the performance of activation. The Fe_b/HCs with optimizing preparation were utilized as activators to well treat different structures (triphenylmethane-, azo-, and xanthene-) contaminants. The detected p-Benzoquinone and 2-chloro-p-Benzoquinone could be considered as transitional and characteristic intermediates from carbocyclic compounds to chain compounds. The degradation mechanisms were evoked by pH and absorption to trigger via free/non-free radicals processes: high valence iron-oxo species, sulfate radicals, hydroxyl radicals, Cl-based substances, etc. The findings contrastively provided the potential applications of magnetically separable iron-carbon based composites for heterogeneous activation in environmental remediation.

Comparative Experimental and Computational Studies of Hydroxyl and Sulfate Radical-Mediated Degradation of Simple and Complex Organic Substrates

Persulfate (PS)-based advanced oxidation processes (AOPs) have been promoted as alternatives to H₂O₂-based AOPs. To gauge the potential of this technology, the PS/Fe(II) and Fenton (H₂O₂/Fe(II)) processes were comparatively evaluated using formate as a simple target compound and nanofiltration concentrate from a municipal wastewater treatment plant as a complex suite of contaminants with the aid of kinetic modeling. In terms of the short-term rate and extent of mineralization of formate and the nanofiltration concentrate, PS/Fe(II) is less effective due to slow Fe(II)/Fe(III) cycling attributable to the scavenging of superoxide by PS. However, in the concentrate treatment, PS/Fe(II) provided a sustained removal of total organic carbon (TOC), with ∼81% removed after 7 days with SO₄^•- consistently produced via homolysis of the long-life PS. In comparison, H₂O₂/Fe(II) exhibited limited TOC removal over ∼57% after 10 h due to the futile consumption of H₂O₂ by HO^•. PS/Fe(II) also offers better performance at transforming humic-like moieties to more biodegradable compounds as a result of chlorine radicals formed by the reaction of SO₄^•- with the matrix constituents present in the concentrate. The application of PS/Fe(II) is, however, subject to the limitations of slow oxidation of organic contaminants, release of sulfate, and formation of chlorinated byproducts.

Deep investigation on different effects of Cl- in transformation of reactive species in Fe(II)/NH2OH/PDS and Fe(II)/NH2OH/H2O2 systems

Hydroxylamine (NH₂OH) has been verified to efficiently strengthen pollutants oxidation in Fe(II)/peroxydisulfate (PDS) and Fe(II)/H₂O₂ systems. However, the different effects of hydroxylamine salts types were rarely recognized. Herein, the effects of two commonly used hydroxylamine salts (i.e. NH₂OH·HCl and (NH₂OH)₂·H₂SO₄) on oxidation kinetics and reactive species composition were compared in Fe(II)/PDS and Fe(II)/H₂O₂ systems for the first time. Pseudo first order kinetics could only describe benzoic acid (BA) oxidation well in Fe(II)/NH₂OH/H₂O₂ system, which was related to the different concentration changes of Fe(III) determined by [Formula: see text] . Hydroxylamine salts types influenced not kinetic rules, but reaction rates of target compounds. The empirical reaction rate constant of BA in Fe(II)/NH₂OH·HCl/PDS system was 141.5% of that in Fe(II)/(NH₂OH)₂·H₂SO₄/PDS system under the same concentration of NH₂OH (1.4 mM), while the apparent reaction rate constant in Fe(II)/NH₂OH·HCl/H₂O₂ system was 68% of that in Fe(II)/(NH₂OH)₂·H₂SO₄/H₂O₂ system. This opposite effect resulted from the differences in primary reactive species compositions and their interactions with Cl^-. Reactive species identification indicated that Cl^- would decrease the contribution of ferryl ion (Fe(IV)) and transform sulfate radical (SO₄^·-) to hydroxyl radical (·OH) in Fe(II)/NH₂OH/PDS system, while it competitively consumed the only reactive species ·OH in Fe(II)/NH₂OH/H₂O₂ system. This study highlights the importance of reductants types on strengthening Fenton oxidation and offers a reference for reasonable construction of the relevant systems.

Coupling electrode aeration and hydroxylamine for the enhanced Electro-Fenton degradation of organic contaminant: Improving H2O2 generation, Fe3+/Fe2+ cycle and N2 selectivity

To improve H2O2 generation and Fe3+/Fe2+ cycle simultaneously for enhancing Electro-Fenton performance, the electrode aeration (EA) and hydroxylamine sulfate (HA) were coupled. With dimethyl phthalate (DMP) as main target contaminant, combination of HA and EA greatly accelerated the degradation of DMP and exhibited a synergy in the pH of 2.0-6.9 through promoting the key reactions, including electrochemical two-electron reduction of O2 into H2O2 and redox cycles of Fe3+/Fe2+, which then improved the generation of hydroxyl radicals (·OH). The coupling EA and HA reduced the use of HA and converted most of HA into environment-friendly N2 (60.1-62.1% of HA products), while HA/solution aeration(SA) system consumed HA rapidly and the generated N2 only accounted for 5.8-6.7% of HA products. Furthermore, compared with HA/SA and EA Electro-Fenton systems, enhancement degree of DMP degradation in HA/EA Electro-Fenton process was higher in actual waterbody than in ultrapure water. The coupling EA and HA in the Electro-Fenton process could solve the low Fe3+/Fe2+ cycle efficiency and low H2O2 production simultaneously, and improve the N2 selectivity of HA transformation, which advanced its application in practical environmental remediation.

g-C3N4 induced acceleration of Fe3+/Fe2+ cycles for enhancing metronidazole degradation in Fe3+/peroxymonosulfate process under visible light

In this study, a Fe³⁺/g-C₃N₄ hybrid catalyst system was proposed to activate peroxymonosulfate (PMS) for the metronidazole (MNZ) photocatalytic degradation. The two catalysts, Fe³⁺ and g-C₃N₄, exhibited an obvious synergistic effect in the photocatalytic degradation process. When 1 mM PMS, 0.04 mM Fe³⁺ and 0.05 g L^-1 g-C₃N₄ were applied, the rate constant of the Fe³⁺/g-C₃N₄/PMS/LED process at 0.07288 min^-1 is around 3.6 to 6.8 times faster than that of Fe³⁺/PMS/LED and g-C₃N₄/PMS/LED processes at 0.0198 and 0.01076 min^-1, respectively. Under visible light, electron transfer from photo-activated g-C₃N₄ to Fe³⁺, resulting in the continuous regeneration of Fe²⁺ in the system, which ensures non-stopping production of radicals for MNZ degradation. UV-visible spectra were used to confirm the regeneration of Fe²⁺. In addition, EPR tests were used to identify the reactive oxygen species involved in the reaction system. Typically, the effects of various operation parameters, including the catalyst dosage, PMS dosage, initial concentration of MNZ and initial pH were examined. This work provided a new idea of promoting pollutant degradation by accelerating Fe³⁺/Fe²⁺ redox through semiconductor, which could help to use the catalyst more effectively for wastewater treatment and/or chemical industries.

Peroxymonosulfate (PMS) activation by mackinawite for the degradation of organic pollutants: Underappreciated role of dissolved sulfur derivatives

The internal Fe²⁺/Fe³⁺ cycle is important for peroxymonosulfate (PMS) activation by iron-based materials to produce the reactive oxidative species (ROS) for the breakdown of organic contaminants. Previous studies have focused on the contribution of heterogeneous sulfur species to the Fe²⁺/Fe³⁺ cycle such as lattice S(-II) and surface SO₃^2- of iron sulfides. In this study, we found that the dissolved S(-II) from mackinawite (FeS) had a substantial contribution to the Fe²⁺/Fe³⁺ cycle. Furthermore, the oxidation intermediates of the dissolved S(-II) such as S₂O₃^2- and SO₃^2- ions could convert Fe³⁺ to Fe²⁺ in solution. The elimination of target organic pollutant bisphenol A (BPA) derived from PMS activation triggered by the dissolved Fe²⁺ might be enhanced by the equivalent dissolved S(-II) in the FeS/PMS system. These results revealed that previous studies underestimated the significance of PMS activation by dissolved Fe²⁺ of iron sulfides to organic pollutant degradation. Moreover, SO₄^•- and •OH were more likely to be the main ROS for BPA degradation in the FeS/PMS system compared with FeO²⁺. Considering that the metal sulfides have been widely used to activate PMS, H₂O₂ and peroxydisulfate, this study offers a new perspective on the function of sulfur in these advanced oxidation processes.

Heterogeneous activation of H2O2/Na2S2O8 with iron ore from water distribution networks for pollutant removal

In this study, we investigated using the main composition of pipe deposits from water distribution networks as catalyst to activate dual-oxidant H₂O₂/Na₂S₂O₈ system to produce radicals for perchloroethylene and chloramphenicol removal. According to the results, the degradation efficiency of perchloroethylene by H₂O₂/Na₂S₂O₈ system was 92.05% within 8 h. Due to the slow conversion between ≡Fe³⁺ and ≡Fe²⁺, the hydroxylamine was introduced to reduce reaction time. As for the results, the degradation efficiency of chloramphenicol in the H₂O₂/Na₂S₂O₈ system with hydroxylamine assistance was 73.31% within 100 min. Meanwhile, several key affecting factors and the kinetic models were investigated. The primary radicals were identified by electron paramagnetic resonance and radical scavenging tests. Eleven degradation products were confirmed by high-resolution liquid chromatography-mass spectrometry. The result of this study provided the theoretical basis for resource utilization of pipe deposits in water treatment in case of emerging contamination events.

Ascorbic acid enhanced ferrous/persulfate system for degradation of tetracycline contaminated groundwater

Persulfate (PS) activated by Fe(ii) has been widely investigated for degradation of contaminants.

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

Iron oxychloride (FeOCl) is a known effective iron-based catalyst and has been used in advanced oxidation processes (AOPs). This study intends to achieve more facile free radicals generation from peroxymonosulfate (PMS) activation by exploring the Fe(III)/Fe(II) cycle of FeOCl in the presence of hydroxylamine (HA). With 0.2 g/L FeOCl, 1.5 mM PMS, and 1 mM HA, the PMS/FeOCl/HA system could effectively achieve 98.88% of the oxidative degradation of 5 mg/L ciprofloxacin (CIP) in 15 min and quickly inactivate 99.99% of E. coli (10⁸ CFU/mL) in 5 min at near-neutral pH. HA played an important role in promoting the Fe(III)/Fe(II) cycle, thereby greatly improving the oxidation activity of the system. The reactive oxygen species (ROS) such as HO, SO₄^- and O₂^- were identified as the dominated free radicals produced in the system. The intermediate products of CIP detected by liquid chromatograph-mass spectrometer (LC-MS) and three possible degradation pathways of CIP were proposed. The presence of common anions in the PMS/FeOCl/HA system, including HCO₃^-, Cl^-, SO₄^2-, and NO₃^-, enhanced the degradation efficiency of CIP to varying degrees at the concentrations of 10 mM. Moreover, FeOCl maintained a high degradation capability for CIP after several recycles. This work offers a new promising means of catalyzing the PMS-based AOPs in the degradation of refractory organics.

Hydroxylamine enhanced Fe(II)-activated peracetic acid process for diclofenac degradation: Efficiency, mechanism and effects of various parameters

In this study, a commonly used reducing agent, hydroxylamine (HA), was introduced into Fe(II)/PAA process to improve its oxidation capacity. The HA/Fe(II)/PAA process possessed high oxidation performance for diclofenac degradation even with trace Fe(II) dosage (i.e., 1 μM) at pH of 3.0 to 6.0. Based on electron paramagnetic resonance technology, methyl phenyl sulfoxide (PMSO)-based probe experiments and alcohol quenching experiments, Fe^IVO²⁺ and carbon-centered radicals (R-O^•) were considered as the primary reactive species responsible for diclofenac elimination. HA accelerated the redox cycle of Fe(III)/Fe(II) and itself was gradually decomposed to N₂, N₂O, NO₂^- and NO₃^-, and the environmentally friendly gas of N₂ was considered as the major decomposition product of HA. Four possible degradation pathways of diclofenac were proposed based on seven detected intermediate products. Both elevated dosages of Fe(II) and PAA promoted diclofenac removal. Cl^-, HCO₃^- and SO₄^2- had negligible impacts on diclofenac degradation, while humic acid exhibited an inhibitory effect. The oxidation capacity of HA/Fe(II)/PAA process in natural water matrices and its application to degrade various micropollutants were also investigated. This study proposed a promising strategy for improving the Fe(II)/PAA process and highlighted its potential application in water treatment.

Activation of peroxymonosulfate by molybdenum disulfide-mediated traces of Fe(III) for sulfadiazine degradation

The activation of persulfate by ferrous iron (Fe(II)) is of great interest to the environmental remediation community, but the reduction of ferric iron (Fe(III)) to Fe(II) is slow and the accumulation of iron sludge resulted from the precipitation of Fe(III) is a great concern. Here, molybdenum disulfide (MoS₂) was studied as a co-catalyst to improve the activation of peroxymonosulfate (PMS) by Fe(III) for sulfadiazine (SDZ) degradation and different characterization technologies were used to reveal the reactive species. The results showed that a strong synergy existed between MoS₂ and Fe(III); approximately 94.3% of the SDZ was removed by MoS₂-Fe(III)-PMS after reaction for 30 min, while only 8.5% and 56.4% of the SDZ was removed by Fe(III)-PMS and MoS₂-PMS, respectively. Both hydroxyl radicals and sulfate radicals were generated and the latter was the primary species. In addition to the radicals, singlet oxygen was found to be generated and contributed to the degradation of SDZ. The chemical probe reaction with methyl phenyl sulfoxide showed that the generation of high-valent iron-oxo species was not obvious by MoS₂-Fe(III)-PMS under both acidic and neutral conditions. MoS₂ had good stability. No noticeable deactivation was observed during the 1st to 5th run and no obvious oxidation of surface Mo(IV) occurred. Based on the characterization of catalyst and oxidizing species, a mechanism for the activation of PMS by MoS₂-Fe(III) was proposed. The results from this study are expected to clarify the reactive species and deepen the understanding of MoS₂-promoted persulfate activation by Fe(II)/Fe(III).

Enhancing iron redox cycling for promoting heterogeneous Fenton performance: A review

Conventional water treatment methods are difficult to remove stubborn pollutants emerging from surface water. Advanced oxidation processes (AOPs) can achieve a higher level of mineralization of stubborn pollutants. In recent years, the Fenton process for the degradation of pollutants as one of the most efficient ways has received more and more attention. While homogeneous catalysis is easy to produce sludge and the catalyst cannot be cycled. In contrast, heterogeneous Fenton-like reaction can get over these drawbacks and be used in a wider range. However, the reduction of Fe (III) to Fe(II) by hydrogen peroxide (H₂O₂) is still the speed limit step when generating reactive oxygen species (ROS) in heterogeneous Fenton system, which restricts the efficiency of the catalyst to degrade pollutants. Based on previous research, this article reviews the strategies to improve the iron redox cycle in heterogeneous Fenton system catalyzed by iron materials. Including introducing semiconductor, the modification with other elements, the application of carbon materials as carriers, the introduction of metal sulfides as co-catalysts, and the direct reduction with reducing substances. In addition, we also pay special attention to the influence of the inherent properties of iron materials on accelerating the iron redox cycle. We look forward that the strategy outlined in this article can provide readers with inspiration for constructing an efficient heterogeneous Fenton system.

Heterogeneous Fenton Degradation of Organic Pollutants in Water Enhanced by Combining Iron-type Layered Double Hydroxide and Sulfate

Water pollution derived from organic pollutants is one of the global environmental problems. The Fenton reaction using Fe²⁺ as a homogeneous catalyst has been known as one of clean methods for oxidative degradation of organic pollutants. Here, a layered double hydroxide (Fe²⁺ Al³⁺ -LDH) containing Fe²⁺ and Al³⁺ in the structure was used to develop a "heterogeneous" Fenton catalyst capable of mineralizing organic pollutants. We found that sulfate ion (SO₄ ^2- ) immobilized on the Fe²⁺ Al³⁺ -LDH significantly facilitated oxidative degradation (mineralization) of phenol as a model compound of water pollutants to carbon dioxide (CO₂ ) in a heterogeneous Fenton process. The phenol conversion and mineralization efficiency to CO₂ reached >99% and ca. 50%, respectively, even with a reaction time of only 60 min.

Iodometric spectrophotometric determination of peroxydisulfate in hydroxylamine-involved AOPs: 15 min or 15 s for oxidative coloration?

In this study, iodometric spectrophotometry, the most-used method for detecting peroxydisulfate (PDS), was modified by increasing the concentration of potassium iodide (KI) for realizing the immediate PDS determination and avoiding the interference of hydroxylamine. Kinetic studies showed that the reaction between PDS and I^- to generate the yellow-colored I₃^- followed the kinetic equation as [Formula: see text] . Detection time of the iodometric spectrophotometry was shortened from 15 min to 15 s when KI concentration was increased from 0.6 M to 4.8 M. Different with the previous iodometric spectrophotometry, the modified method using 4.8 M KI as the indicator was well tolerable to the interference of hydroxylamine at acidic pH conditions. The calibration curve of the modified method showed a well linear relationship (R² = 0.999) between the absorbance of I₃^- at 352 nm and PDS concentration in the range of 0-80 μM. The modified method was highly sensitive with the absorptivity of 2.5 × 10⁴ M^-1 cm^-1 and the limit of detection of 0.11 μM. Moreover, the modified method was successfully applied for monitoring the change of PDS concentration during the degradation of diclofenac with four different PDS-based AOPs, the calculated reaction stoichiometric efficiency (RSE(%)=DiclofenacdegradedPDSconsumed×100%) followed the order as heat/PDS system > hydroxylamine/Fe²⁺/PDS system > hydroxylamine/Cu²⁺/PDS system > Fe²⁺/PDS system.

Overlooked enhancement of chloride ion on the transformation of reactive species in peroxymonosulfate/Fe(II)/NH2OH system

Though hydroxylamine (NH₂OH) is effective for accelerating pollutants degradation in Fenton and Fenton-like systems, the effect of anions simultaneously introduced by the hydroxylamine salts have always been ignored. Herein, effect of two commonly used hydroxylamine salts, hydroxylamine hydrochloride (NH₂OH·HCl) and hydroxylamine sulfate [(NH₂OH)₂·H₂SO₄], for the degradation of dimethyl phthalate (DMP) in peroxymonosulfate (PMS)/Fe(II) system was comparatively investigated. Degradation efficiency of DMP with NH₂OH·HCl was 1.6 times of that with same dosages of (NH₂OH)₂·H₂SO₄. SO₄^·-, Fe(IV) and ·OH formed in the PMS/Fe(II)/NH₂OH system, but ·OH was the major species for DMP degradation. Addition of Cl^- significantly improved the production of ·OH and Cl·, and the exposure dose of ·OH (CT_·OH) was more than 10 times that of CT_Cl· as the concentration of Cl^- increased to 1 mM. Calculations based on branching ratios of Cl· and ·OH indicated that the reactions of Cl^- with SO₄^·- and Cl· with H₂O were not the only production sources of ·OH in the system. Further experiments with methyl phenyl sulfoxide (PMSO) as the probe indicated that Cl^- would facilitate the shift of reactive species from Fe(IV) to radicals (SO₄^·- or ·OH) in the system. Both hydroxylation and nitration intermediate products were detected in the oxidation of DMP. Cl^- promoted the formation of hydroxylation intermediates and reduced the formation of nitration intermediates. This study revealed for the first time that Cl^- could shift reactive species from Fe(IV) to radicals in PMS/Fe(II) system, raising attention to the influence of the coexisting anions (especially Cl^-) for pollutants oxidation in iron-related oxidation processes.

Activation of peroxymonosulfate by biochar and biochar-based materials for degrading refractory organics in water: a review

Water pollution caused by refractory organics has attracted widespread concern in recent years. At this time peroxymonofulfate (PMS) has been widely used to generate sulfate radicals with high reactivity and potential. The direct reaction rate between PMS and organics is very low. However, the activated PMS has a strong oxidizing ability on organics due to its conversion into sulfate radicals. Recently, the free radicals generated by oxidant PMS and catalyst biochar have proven to be an effective species in dealing with refractory organics. In order to enable researchers to better understand the current research status of PMS/biochar, and to promote the development and application of PMS/biochar system, we have written this review. This review in detail described the mechanism of PMS activated by biochar materials, and summarized the influencing factors of refractory organics degradation in the PMS/biochar system. In addition, the active sites of PMS/biochar, the degradation mechanism of refractory organics, and the reusability of biochar catalysts were also discussed. Finally, the concluding remarks and perspectives were made for future research on the PMS/biochar system in the degradation of refractory organics.

Selective degradation of acetaminophen from hydrolyzed urine by peroxymonosulfate alone: performances and mechanisms

Owing to the high concentration of pharmaceuticals in urine, the degradation of these organic pollutants before their environmental release is highly desired. Peroxymonosulfate (PMS) is a desirable oxidant that can be applied to environmental remediation; however, the performance and mechanism of PMS for the degradation of pharmaceuticals in the urine matrix have not been investigated. Herein, PMS was first discovered to efficiently degrade typical pharmaceuticals in hydrolyzed urine (HU) by selecting acetaminophen (ACE) as a target compound. Quenching experiments revealed that singlet oxygen (¹O₂) and hydroxyl radicals (HO˙) were observed in the HU/PMS system, but the principal reactive species (RS) responsible for ACE removal was ¹O₂. The major constituents of HU, including SO₄²⁻ and organics (creatine, creatinine and hippuric acid), hardly affected the elimination of ACE, whereas Cl⁻, H₂PO₄⁻ and NH₄⁺ would accelerate ACE degradation. Besides, HCO₃⁻ slightly inhibited this process. The ACE degradation efficiency was enhanced using photo-irradiation, including sunlight and visible light, although increasing the reaction temperature could, interestingly, hardly accelerate the degradation rate of ACE. Three-dimensional excitation–emission matrices (3D-EEMs) have indicated that other intermediates that have a higher fluorescence intensity might be generated in the HU/PMS system. Finally, nine intermediate products were determined and the degradation pathways of ACE were proposed. Overall, the results of this study illustrated that PMS is an efficient oxidant for the degradation of ACE in HU.

Owing to the high concentration of pharmaceuticals in urine, the degradation of these organic pollutants before their environmental release is highly desired.

Metal-free black-red phosphorus as an efficient heterogeneous reductant to boost Fe3+/Fe2+ cycle for peroxymonosulfate activation

In this work, a novel metal-free black-red phosphorus (BRP) was prepared from red phosphorus (RP) and applied in Fe²⁺/peroxymonosulfate (PMS) process. Compared with that of RP, the contaminant degradation performance of BRP was significantly elevated due to the enhanced electron transfer from BRP to Fe³⁺. This enhancement was mainly induced by size decrease effect, the removal of oxidation layer and the partial phase conversion. Moreoevr, BRP avoided the radical quenching reaction caused by reductant itself, whereas it was inevitable using homogeneous reductant like hydroxylamine. More importantly, the system had a superior recyclability and strong resistance to natural water. Though concurrent side-reaction between PMS and BRP occured, multiple PMS dosage could remarkedly alleviated the side-reaction, thus elevating PMS utilization efficiency. The dominant BRP oxidation products included phosphite and phosphate. Interestingly, moderate increase of Fe³⁺ concentration could efficiently reduce the by-product formation via the prompt PMS activation by regenerated Fe²⁺. Our work clarified the acceleration mechanism of Fe³⁺/Fe²⁺ cycle by BRP and proposed the control strategy of by-prodoct formation.

Hydroxylamine driven advanced oxidation processes for water treatment: A review

Hydroxylamine (HA) driven advanced oxidation processes (HAOPs) for water treatment have attracted extensive attention due to the acceleration of reactive intermediates generation and the improvement on the elimination effectiveness of target contaminants. In this review, HAOPs were categorized into three parts: (1) direct reaction of HA with oxidants (e.g., hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS), ozone (O₃), ferrate (Fe(VI)), periodate (IO₄^-)); (2) HA driven homogeneous Fenton/Fenton-like system (Fe(II)/peroxide/HA system, Cu(II)/O₂/HA system, Cu(II)/peroxide/HA system, Ce(IV)/H₂O₂/HA system); (3) HA driven heterogeneous Fe/Cu-Fenton/Fenton-like system (iron-bearing material/peroxide/HA system, copper-bearing material/peroxide/HA system, bimetallic composite/peroxide/HA system). Degradation efficiency of the target pollutant, reactive intermediates, and effective pH range of various HAOPs were summarized. Further, corresponding reaction mechanism was elaborated. For the direct reaction of HA with oxidants, improvement of pollutants degradation was achieved through the generation of secondary reactive intermediates which had higher reactivity compared with the parent oxidant. For HA driven homogeneous and heterogeneous Fe/Cu-Fenton/Fenton-like system, improvement of pollutants degradation was achieved mainly via the acceleration of redox cycle of Fe(III)/Fe(II) or Cu(II)/Cu(I) and subsequent generation of reactive intermediates, which avoided the drawbacks of classical Fenton/Fenton-like system. In addition, HA driven homogeneous Fe/Cu-Fenton/Fenton-like system with heterogeneous counterpart were compared. Further, formation of oxidation products from HA in various HAOPs was summarized. Finally, the challenges and prospects in this field were discussed.

Heterogeneously activation of H2O2 and persulfate with goethite for bisphenol A degradation: A mechanistic study

Advanced oxidation processes (AOPs) based on the activation of hydrogen peroxide (H₂O₂) and persulfate (PS) by minerals have received increasing interest for environmental remediation. Herein, H₂O₂ and PS activation systems employing goethite as a catalyst were discovered for the rapid degradation of BPA with the generation of reactive oxidation species (ROS) and for the reduction of total organic carbon (TOC) in aqueous solutions. The morphology of goethite were characterized by XRD, SEM, BET, TEM, etc. As a result, the oxidant efficiency of the goethite/H₂O₂ system (75.9%) was higher than that of the goethite/PS system (61.4%) after 240 min due to the restricted radical scavenging. According to the results of electron paramagnetic resonance (EPR) and radical quenching experiments, the main active ROS during the BPA degradation process were OH and SO₄^-. The two reaction systems were all pH-dependent that BPA can be effectively degraded in the goethite/PS system under acidic, neutral and weakly alkaline conditions, while the most inefficient degradation under alkaline conditions in the goethite/H₂O₂ system. Moreover, goethite showed good structural stability in the two systems. Several reaction products were detected using LC-MS, and the mechanisms for three systems were proposed. Density functional theory (DFT) was employed to study the conceivable degradation pathways of BPA in the two processes. This work reveals novel mechanistic insights regarding H₂O₂ and PS activation over goethite and implies the great potential application of the PS/mineral process in water and wastewater treatment.

Regeneration of Fe(II) from Fenton-derived ferric sludge using a novel biocathode

Fenton reactions are widely applied when degrading recalcitrant pollutants, but reusing the resulting ferric sludge remains a challenge. A novel concept for regenerating Fe(II) solution at pH 6 based on ferric sludge from neutral Fenton was herein proposed. The microbial fuel cell (MFC) with biocathode and citric acid was used for the first time to promote the regenerated rate of Fe(II) from ferric sludge. The concentration of dissolved Fe(II) reached 120 mg/L in biocathode, which was much higher than that obtained in abiotic cathode (<1 mg/L). The main chemical cost of regenerating Fe(II) was only 3.3% of the commercial Fe(II). Subsequently, the regenerated Fe(II) solution was used to activate H₂O₂, to remove pharmaceuticals from the municipal wastewater effluent. A wide range of pharmaceuticals was successfully removed at neutral pH in 60 min, and the efficiency of the treatment was similar to when the same dosage of commercial Fe(II) was applied.

Comparison of UV/H2O2, UV/PMS, and UV/PDS in Destruction of Different Reactivity Compounds and Formation of Bromate and Chlorate

In this study, we compared the decontamination kinetics of various target compounds and the oxidation by-products (bromate and chlorate) of PMS, PDS, and H₂O₂ under UV irradiation (UV/PMS, UV/PDS, UV/H₂O₂). Probes of different reactivity with hydroxyl and sulfate radicals, such as benzoic acid (BA), nitrobenzene (NB), and trichloromethane (TCM), were selected to compare the decontamination efficiency of the three oxidation systems. Experiments were performed under acidic, neutral, and alkaline pH conditions to obtain a full-scale comparison of UV/peroxides. Furthermore, the decontamination efficiency was also compared in the presence of common radical scavengers in water bodies [bicarbonate, carbonate, and natural organic matter (NOM)]. Finally, the formation of oxidation by-products, bromate, and chlorate, was also monitored in comparison in pure water and tap water. Results showed that UV/H₂O₂ showed higher decontamination efficiency than UV/PDS and UV/PMS for BA degradation while UV/H₂O₂ and UV/PMS showed better decontamination performance than UV/PDS for NB degradation under acidic and neutral conditions. UV/PMS was the most efficient among the three processes for BA and NB degradation under alkaline conditions, while UV/PDS was the most efficient for TCM degradation under all pH conditions. In pure water, both bromate and chlorate were formed in UV/PDS, small amounts of bromate and rare chlorate were observed in UV/PMS, and no detectable bromate and chlorate were formed in UV/H₂O_2. In tap water, no bromate and chlorate were detectable for all three systems.