Ferrate(VI) oxidation of polychlorinated diphenyl sulfides: Kinetics, degradation, and oxidized products

Oxidation of selected fluoroquinolones by ferrate(VI) in water: Kinetics, mechanism, effects of constituents, and reaction pathways

In this work, the oxidation of gatifloxacin (GAT), fleroxacin (FLE) and enoxacin (ENO) in aqueous solution by ferrate (Fe(VI)) was systemically investigated. Weak alkaline and high oxidant doses were favorable for the reaction. The pseudosecond-order rate constants were 0.18055, 0.29162, and 0.05476 L/(mg·min), and the activation energies were 25.13, 15.25, and 11.30 kJ/mol at pH = 8.00 and n(Fe(VI)):n(GAT) = 30:1, n(Fe(VI)):n(FLE) = 20:1, n(Fe(VI)):n(ENO) = 40:1 and a temperature of 25 °C. The maximum degradation rates of the GAT, FLE and ENO were 96.72%, 98.48% and 94.12%, respectively, well simulated by Response Surface Methodology. During the oxidation, the contribution of hydroxyl radicals (HO•) varied with time, whereas the final contribution was approximately 20% at 30 min. The removal efficiency was inhibited by anions by less than 10%, and cations by less than 25%, and significantly inhibited by high concentrations of humic acid. Moreover, two or three dominant reaction pathways were predicted, and the ring cleavages of quinolone and piperazine were mainly achieved through decarboxylation, demethlation and hydroxylation, and some pathways ended up with monocyclic chemicals, which were harmless to aquatic animals and plants. Theoretical calculations further proved that the reactions between FeO4- and neutral fluoroquinolone antibiotics were the major reactions. This work illustrates that Fe(VI) can efficiently remove fluoroquinolone antibiotics (FQs) in aqueous environments, and the results may contribute to the treatment of wastewater containing trace antibiotics and Fe(VI) chemistry.

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

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

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

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

Degradation efficiency and mechanism of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) by thermally activated persulfate system

Polybrominated diphenyl ethers (PBDEs) as a typical brominated flame retardant (BFR) have attracted worldwide attention due to the high environmental risk and resistance to conventional remediation processes. In this study, thermally activated persulfate (TAP) process was applied to degrade 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), which is the most toxic and representative PBDEs in e-waste dismantling sites. Impact factors such as PDS dosage, heating temperature, and initial pH were evaluated. Results showed that BDE-47 can be 100% degraded within 180 min under the condition of PDS:BDE-47 = 1000:1, 60 °C, and pH = 7. Quenching experiments combined with EPR analysis further proved the important role of SO₄^·- in oxidating BDE-47. According to high-resolution mass spectrometry (HRMS) analysis, only one oxidation product of low toxicity was detected during the oxidation process. Theoretical calculations further revealed that the oxidation process mainly involved radical attack at C-Br bond, cleavage of C-Br bond, and fission of ether bond, and HSO₄· may also play an important role in BDE-47 degradation in TAP system. In addition, TAP system exhibited universality as all selected PBDE congeners can be degraded, and the degradation rate of PBDEs was greatly affected by the number of substituted Br atoms in a negative trend. Overall, these findings indicate that TAP can be applied as an effective method for removal of PBDEs, and we provide a new insight for the practical application of TAP technology in BDE-47 degradation from experimental and theoretical aspects.

Insights into manganese(VII) enhanced oxidation of benzophenone-8 by ferrate(VI): Mechanism and transformation products

Benzophenones (BPs) are commonly used as UV filters in cosmetics and plastics products and are potentially toxic to the environment. This paper presents kinetics and products of BPs oxidation by ferrate(VI) (FeO₄^2-, Fe(VI)) promoted by permanganate (Mn(VII)) . Degradation of 10.0 µM 2,2'-dihydroxy-4-methoxybenzophenone (BP-8)were determined under different experimental conditions ([Mn(VII)] = 0.5-1.5 µM, [Fe(VI)] = 50-150 µM, and pH = 7.0-10.0). The addition of Mn(VII) traces to Fe(VI)-BP-8 solution enhanced kinetics and efficiency of the removal. Similar enhanced removals were also seen for other BPs (BP-1, BP-3, and BP-4) under optimized conditions. The second-order rate constants (k, M^-1s^-1) of the degradation of BPs showed positive relationship with the energy of the highest occupied orbital (E_HOMO). The possible interaction between Mn(VII) and BP-8 and the enhanced generation of Fe(V)/Fe(IV) and •OH was proposed to facilitate the oxidation of the target benzophenone, supported by in-situ electrochemical measurements, theoretical calculations and reactive species quenching experiments. Thirteen oxidation products of BP-8 suggested hydroxylation, bond breaking, polymerization and carboxylation steps in the oxidation. Toxicity assessments by ECOSAR program showed that the oxidized intermediate products posed a tapering ecological risk during the degradation process. Overall, the addition of Mn(VII) could improve the oxidation efficiency of Fe(VI).

Quantitative structure-activity relationship models for predicting apparent rate constants of organic compounds with ferrate (VI)

Ferrate (VI) (Fe (VI)) is a promising, environmentally friendly multifunctional oxidant widely applied in organic compound degradation. Oxidative kinetics of the apparent second-order rate constants (k_app) of Fe (VI) with organic compounds are critical for modeling oxidation processes. Herein, a quantitative structure-activity relationship (QSAR) model was developed using particle swarm optimization and an extreme learning machine to better understand the laws of the k_app values of organic compounds, including 33 aliphatic and aromatic hydrocarbon derivatives, during degradation by Fe (VI). Seven components-electronic hardness (H), electronic softness (S), ratio of oxygen to carbon atoms (O_n/C_n), energy of the highest occupied molecular orbital (E_HOMO), vertical ionization potential (VIP), maximum nucleophilic reaction index (f(+)_x), and minimum relative electrophilicity index (RE_n) constitute the critical molecular parameters. The developed QSAR model was verified on the basis of the coefficient of determination (R²) and the root mean square error (RMSE): for the training set, R² = 0.924 and RMSE = 1.186, whereas for the test set, R² = 0.996, and RMSE = 0.352. The applicability, reliability, and predictability of the model were verified by estimating the applicability domain (AD) of the model. Furthermore, QSAR models constructed using different methods were compared, and the main impact descriptors and conclusions obtained from previous studies were theoretically analyzed. Results indicate that constructing the QSAR model facilitates k_app prediction for Fe (VI) in the degradation of various organic compounds, improves the understanding of the degradation mechanism, and reduces the pressure on human and material resources caused by experiments.

Machine learning approaches to predict the apparent rate constants for aqueous organic compounds by ferrate

The apparent second-order rate constant with hexavalent ferrate (Fe(VI)) (k_Fe(VI)) is a key indicator to evaluate the removal efficiency of a molecule by Fe(VI) oxidation. k_Fe(VI) is often determined by experiment, but such measurements can hardly catch up with the rapid growth of organic compounds (OCs). To address this issue, in this study, a total of 437 experimental second-order k_Fe(VI) rate constants at a range of conditions (pH and temperature) were used to train four machine learning (ML) algorithms (lasso regression (LR), ridge regression (RR), extreme gradient boosting (XGBoost), and the light gradient boosting machine (LightGBM)). Using the Morgan fingerprint (MF)) of a range of organic compounds (OCs) as the input, the performance of the four algorithms was comprehensively compared with respect to the coefficient of determination (R²) and root-mean-square error (RMSE). It is shown that the RR, XGBoost, and LightGBM models displayed generally acceptable performance k_Fe(VI) (R²_test > 0.7). In addition, the shapely additive explanation (SHAP) and feature importance methods were employed to interpret the XGBoost/LightGBM and RR models, respectively. The results showed that the XGBoost/LightGBM and RR models suggestd pH as the most important predictor and the tree-based models elucidate how electron-donating and electron-withdrawing groups influence the reactivity of the Fe(VI) species. In addition, the RR model share eight common features, including pH, with the two tree-based models. This work provides a fast and acceptable method for predicting k_Fe(VI) values and can help researchers better understand the degradation behavior of OCs by Fe(VI) oxidation from the perspective of molecular structure.

The efficient degradation of diclofenac by ferrate and peroxymonosulfate: performances, mechanisms, and toxicity assessment

In this study, the degradation efficiency and reaction mechanisms of diclofenac (DCF), a nonsteroidal anti-inflammatory drug, by the combination of ferrate (Fe(VI) and peroxymonosulfate (PMS) (Fe(VI)/PMS) were systematically investigated. The higher degradation efficiency of DCF in Fe(VI)/PMS system can be obtained than that in alone persulfate (PS), Fe(VI), PMS, or the Fe(VI)/PS process at pH 6.0. DCF was efficiently removed in Fe(VI)/PMS process within a wide range of pH values from 4.0 to 8.0, with higher degradation efficiency in acidic conditions. The increasing reaction temperature (10 to 30 ℃), Fe(VI) dose (6.25 to 100 µM), or PMS concentration (50 to 1000 µM) significantly enhanced the DCF degradation. The existences of HCO₃¯, Cl¯, and humic acid (HA) obviously inhibited the DCF removal. Electron paramagnetic resonance (EPR), free radical quenching, and probing experiments confirmed the existence of sulfate radicals (SO₄^•¯), hydroxyl radicals (^•OH), and Fe(V)/ Fe(IV), which are responsible for DCF degradation in Fe(VI)/PMS system. The variations of TOC removal ratio reveal that the adsorption of organics with ferric particles, formed in the reduction of Fe(VI), also were functioned in the removal process. Sixteen DCF transformation byproducts were identified by UPLC-QTOF/MS, and the toxicity variation was evaluated. Consequently, eight reaction pathways for DCF degradation were proposed. This study provides theoretical basis for the utilization of Fe(VI)/PMS process.

Tetracycline photolysis revisited: Overlooked day-night succession of the parent compound and metabolites in natural surface waters and associated ecotoxicity

Despite the extensive study of tetracycline photolysis in aquatic environments, the phototransformation of tetracycline and its metabolites under natural day-night succession has not been examined. In this study, we investigated tetracycline photolysis and associated ecotoxicity in two natural surface waters and one artificial ultrapure water under simulated day/night cycling over two days. Previously unrecognized and highly pH- and temperature-dependent dark interconversions of tetracycline metabolites were observed. The liquid chromatography-mass spectrometry/mass spectrometry analysis identified a range of isomerized, hydroxylated, demethylated, deaminated, and open-ring photoproducts. The hydrolysis of tetracycline, isotetracycline, and several intermediate products was proposed as the major mechanism for the observed dark transformations. Exposure studies employing Escherichia coli indicated that although the tetracycline degradation products had lower bacterial toxicities than the parent compound, increasing toxicity with irradiation time after the near-complete degradation of the parent compound in natural waters implied that product mixtures retain ecotoxicity. The dark transformations also affected the bacterial toxicity and fluorescence properties of irradiated tetracycline solutions. Overall, this study provides new insights into the photochemical behavior of tetracycline and its associated ecological risk in aquatic environments.

A Comparative Study on the Oxidation Mechanisms of Substituted Phenolic Pollutants by Ferrate(VI) through Experiments and Density Functional Theory Calculations

In this work, the oxidation of five phenolic contaminants by ferrate(VI) was comparatively investigated to explore the possible reaction mechanisms by combined experimental results and theoretical calculations. The second-order rate constants were positively correlated with the energy of the highest occupied molecular orbital. Considering electronic effects of different substituents, the easy oxidation of phenols by ferrate(VI) could be ranked as the electron-donating group (-R) > weak electron-withdrawing group (-X) > strong electron-withdrawing group (-(C═O)-). The contributions of reactive species (Fe(VI), Fe(V)/(IV), and •OH) were determined, and Fe(VI) was found to dominate the reaction process. Four main reaction mechanisms including single-oxygen transfer (SOT), double-oxygen transfer (DOT), •OH attack, and electron-transfer-mediated coupling reaction were proposed for the ferrate(VI) oxidation process. According to density functional theory calculation results, the presence of -(C═O)- was more conducive for the occurrence of DOT and •OH attack reactions than -R and -X, while the tendency of SOT for different substituents was -R > -(C═O)- > -X and that of e^--transfer reaction was -R > -X > -(C═O)-. Moreover, the DOT pathway was found in the oxidation of all four substituted phenols, indicating that it may be a common reaction mechanism during the ferrate(VI) oxidation of phenolic compounds.

Enhanced ferrate oxidation of organic pollutants in the presence of Cu(II) Ion

In this study, we found that the introduction of Cu(II) (several μM, close to the concentration level of some real water/wastewater) in ferrate (Fe(VI)) oxidation can remarkably accelerate the abatement of various organic pollutants under slightly alkaline conditions. The results show that 5 μM sulfamethoxazole (SMX) can be completely degraded by Fe(VI) (50 μM) in the presence of 20 μM Cu(II) within 10 min at pH 8.0, which was 1.65 times higher than that by Fe(VI) alone. High-valent iron intermediates (i.e. Fe(V), Fe(IV)) and Cu(III) were generated as reactive species in the Cu(II)/Fe(VI) system, all of which contributed to the enhanced oxidation of SMX. Common water components, except for HCO₃^- and humic acid, exhibited no influence on SMX removal. Additionally, the enhanced removal of SMX by Cu(II)/Fe(VI) was also observed in real water with the benefit of total removal of Cu(II) by the ferrate resultant particles. Due to the presence of highly reactive and selective oxidant, the Cu(II)/Fe(VI) system could react readily with organic pollutants containing electron-rich moieties, such as phenol, olefin or amino groups. This study provided a simple, selective, and practical strategy for the abatement of organic pollutants and a simultaneous removal of Cu(II).

Oxidative degradation of sulfamethoxazole from secondary treated effluent by ferrate(VI): kinetics, by-products, degradation pathway and toxicity assessment

Sulfamethoxazole (SMX) is a typical antibiotic in the world, which is frequently detected in the aquatic environment. The current study was aimed to investigate the SMX degradation in secondary treated wastewater using potassium Ferrate [Fe(VI)]. The effects of various experimental conditions, EDTA and phosphate as chelating agents, and toxicity assessment were also considered. Secondary treated effluent was spiked with predefined SMX concentrations, and after desired reaction time with Fe(VI), residual SMX was measured using HPLC. Results indicated that SMX degradation by Fe(VI) was favored under acidic condition, where 90% of SMX degradation was achieved after 120 min. Fe(VI) and SMX reaction obeyed first-order kinetic; meantime, the SMX degradation rate under pH 3 was 7.6 times higher than pH 7. The presence of phosphate (Na₂HPO₄) and EDTA declined SMX degradation, while Fe (III) effect was contradictory. In addition to promising demolition, 10% TOC removal was achieved. Eighteen major intermediates were identified using LC-MS/MS and the degradation pathways were suggested. Transformation products (TPs) were formed due to hydroxylation, bond cleavage, transformation after bond cleavage, and oxidation reactions. The ECOSAR analysis showed that some of the SMX oxidation products were toxic to aquatic organisms (fish, daphnia and green algae).

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s40201-021-00769-9.

Reinvestigation of the oxidation of organic contaminants by Fe(VI): Kinetics and effects of water matrix constituents

Since previous studies mostly ignored the contributions of Fe(IV) and Fe(V) during the determination of reaction rate constants of ferrate (Fe(VI)) with trace organic contaminants (TrOCs), the intrinsic oxidation ability of Fe(VI) was overestimated. For the first time, this study systemically evaluated the reactivity of Fe(VI) towards four kinds of TrOCs by blocking Fe(IV)/Fe(V) over the TrOCs degradation, and evaluated the effects of coexisting water matrix constituents. Results revealed that Fe(VI) exhibited superior reactivity towards phenolic compounds. Different from other tested TrOCs, phenolic compounds were mainly degraded by Fe(VI) rather than Fe(IV)/Fe(V). Taking bisphenol A (BPA) as the target TrOC, we found that the coexisting constituents can not only affect the reactivity of different ferrate species (i.e., Fe(IV), Fe(V), and Fe(VI)), but also alter the concentrations of ferrates. HPO₄^2- inhibited the reaction between Fe(VI) and H₂O₂, while Ca²⁺, Mg²⁺, and NH₄⁺ promoted the generation of Fe(IV)/Fe(V) from Fe(VI). Besides, humic acid could increase the contribution of Fe(IV)/Fe(V) to the oxidation of BPA. These findings were validated in real water samples. Taken together, this study provides a new perspective regarding the intrinsic oxidation reactivity of Fe(VI), thereby urging reconsideration of the proper strategies for utilization of high-valent Fe species in practices.

Design of effective self-powered SnS2/halide perovskite photo-detection system based on triboelectric nanogenerator by regarding circuit impedance

Self-powered detectors based on triboelectric nanogenerators (TENG) have been considered because of their capability to convert ambient mechanical energy to electrical out-put signal, instead of conventional usage of electrochemical batteries as power sources. In this regard, the self-powered photodetectors have been designed through totally two lay out called passive and active circuit. in former model, impedance matching between the TENG and the resistance of the circuit’s elements is crucial, which is not investigated systematically till now. In this paper, a cost effective novel planar photodetector (PD) based on heterojunction of SnS₂ sheets and Cs_0.05(FA_0.83 MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ three cationic lead iodide based perovskite (PVK) layer fabricated which powered by graphene oxide (GO) paper and Kapton based contact-separated TENG (CS-TENG). To achieve the high performance of this device, the proper range of the load resistances in the circuit regards to TENG’s characterization has been studied. In the next steps, the integrated self-powered photo-detection system was designed by applying Kapton/FTO and hand/FTO TENG, separately, in the proposed impedance matching circuit. The calculated D* of integrated self-powered SnS₂/PVK supplied by tapping the Kapton and hand on FTO is 2.83 × 10¹⁰ and 1.10 × 10¹³ Jones under the 10 mW/cm² of white light intensity, the investigations determine that for designing significate performance of self-powered PD supplied by TENG, the existence of the load resistance with the well match amount to the utilized TENG is crucial. Our results which can be generalized to other types of passive self-powered sensors, are substantial to both academia and industry concepts.

Experimental and theoretical study on the degradation of Benzophenone-1 by Ferrate(VI): New insights into the oxidation mechanism

The oxidation of Benzophenone-1 (BP-1) by ferrate (Fe(VI)) was systemically investigated in this study. Neutral pH and high oxidant dose were favorable for the reaction, and the second order rate constant was 1.03 × 10³ M^-1·s^-1 at pH = 7.0 and [Fe(VI)]₀:[BP-1]₀ = 10:1. The removal efficiency of BP-1 was enhanced by cations (K⁺, Ca²⁺, Mg²⁺, Cu²⁺, and Fe³⁺), while inhibited by high concentrations of anions (Cl^- and HCO₃^-) and low concentrations of humic acid. Moreover, intermediates were identified by LC-MS, and five dominating reaction pathways were predicted, involving single hydroxylation, dioxygen transfer, bond breaking, polymerization and carboxylation. Theoretical calculations showed the dioxygen transfer could occur by Fe(VI) attacking the CC double-bond in benzene ring of BP-1 to form a five-membered ring intermediate, which was hydrolyzed twice followed by H-abstraction to generate the dihydroxy-added product directly from the parent compound. Dissolved CO₂ or HCO₃^- might be fixed to produce carboxylated products, and Cl^- led to the formation of two chlorinated products. In addition, the toxicity assessments showed the reaction reduced the environmental risk of BP-1. This work illustrates Fe(VI) could remove BP-1 in water environments efficiently, and the newly proposed dioxygen transfer mechanism herein may contribute to the development of Fe(VI) chemistry.

Effectiveness of UV/SO32- advanced reduction process for degradation and mineralization of trichlorfon pesticide in water: identification of intermediates and toxicity assessment

This study aimed to investigate the degradability, mineralization, proposed decomposition pathway, intermediate products, and toxicity of effluent from trichlorfon (TCF) degradation in water by UV/sulfite-advanced reduction process (UV/S-ARP). This study was experimentally performed in a photochemical reactor as a batch operation. The source of light was a UV lamp. Sulfite ion was used as the reducing agent. After the treatment, the residual concentration of TCF was measured by liquid chromatography equipped with tandem mass spectrometry (LC-MS/MS). UV/S-ARP had the highest performance at an initial pH of 7, a sulfite ion concentration of 120 mg/L, a contact time of 60 min, and a TCF concentration of 10 mg/L. Under such conditions, the degradation efficiency of TCF was 96.0%, and the amount of mineralization based on the removal of TOC and COD was 74.6% and 79.5%, respectively. The results of the degradation mechanism showed that eaq- and SO3•- have played the greatest role in dechlorination and transformation of TCF. Based on the identified intermediates, more complex compounds are transformed into compounds with simpler structures by UV/S-ARP. Evaluating the toxicity of TCF by-products via ECOSAR bioassay showed that as-generated intermediates do not have acute and chronic adverse effects on fish. The results of our study indicated that the advanced reduction process could be an effective process for the purification of TCF-contaminated water.

Aquatic photolysis of ketoprofen generates products with photosensitizing activity and toxicity

Ketoprofen (KTF) is a nonsteroidal anti-inflammatory drug frequently detected in natural and engineering waters. Because KTF is particularly photolabile (half-life ∼4 min), knowledge of the fate and ecological risks of KTF photoproducts in the aquatic environment is especially essential. Herein, we systematically investigated the photophysics, photochemistry, and photosensitization of KTF photoproducts in aqueous solution under 365 nm irradiation (UV₃₆₅). Results show that KTF photolyzed rapidly and formed 3-ethyl-α-hydroxylbenzophenone (EtOH-BP), 3-ethyl-α-hydroperoxylbenzophenone (EtOOH-BP), 3-acetylbenzophenone (AcBP), and 3-ethylbenzophenone (EtBP), as identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high-resolution mass spectrometry (HRMS). The presence of O₂ significantly affected the evolution of photoproducts during KTF photolysis. The photophysical properties of EtBP and AcBP were characterized by spectroscopic approaches. In particular, transient absorption spectra obtained by nanosecond laser flash photolysis (LFP) indicated that EtBP and AcBP were excited to triplet states with lifetimes of 28 and 2.4 µs, respectively. EtBP underwent further photodegradation, giving rise to EtOH-BP, EtOOH-BP, and AcBP upon UV₃₆₅ irradiation. The reaction is proposed to proceed through an excimer precursor (³[EtBP···EtBP]*) followed by intramolecular H-abstraction. In contrast, AcBP was relatively photostable, particularly under aerated condition. Both EtBP and AcBP have strong photosensitizing activity, as evidenced by the triplet probe 4-(N,N-dimethylamino)benzonitrile (DMABN). ECOSAR program suggested that the photoproducts are more ecotoxic and bioaccumulative than the parent KTF. Results of this study underscore the need to scrutinize the formation and fate of KTF photoproducts in sunlit surface waters.

Unveiling the mechanism of imidacloprid removal by ferrate(VI): Kinetics, role of oxidation and adsorption, reaction pathway and toxicity assessment

Imidacloprid (IMI), an emerging pollutant, has high toxicity to non-target organisms. This paper presents the kinetics of IMI removal by ferrate(VI) at different pH (6.0-9.0), molar ratios ([ferrate(VI)]:[IMI]) and added Fe(III) ions. The apparent second-order rate constant (k_app) decreased with increase in pH from pH 6.0 to 9.0 (i.e., (1.2 ± 0.1) × 10² M^-1 s^-1 to (8.3 ± 0.3) M^-1 s^-1). The species-specific rate constants were obtained as k (HFeO₄^-) = 1.3 × 10² M^-1 s^-1 and k (FeO₄^2-) = 6.9 M^-1 s^-1. The decreases in the concentration of HFeO₄^- with increase in pH caused the observed pH dependence in k_app. At pH 7.0, the removal of IMI increased with the molar ratio from 1.0 to 10.0 with complete removal at the highest ratio. The variation in pH from 6.0 to 9.0 had no obvious effect on removal of IMI. Experiments indicate that IMI removal is mainly by ferrate(VI) oxidation and to a lesser extent by Fe(III) adsorption. Mineralization of IMI was also observed (20-26%). The addition of Fe(III) ions to ferrate(VI)-IMI at pH 7.0 and 8.0 resulted in enhanced removal of IMI, but the presence of Ca²⁺, SO₄^2-, HCO₃^-, and humic acid (HA) has negative effects. The presence of coexisting substances in river water slightly decreased IMI removal by ferrate(VI) by less than 10%. Identification of products and frontier electron density (FED) calculations demonstrated involvement of opening of the five-membered heterocyclic moiety of IMI by ferrate(VI). Toxicity assessment with NIH 3T3 fibroblasts and ECOSAR analysis indicated lower toxicity of oxidized products than parent IMI.

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

This paper investigated ultraviolet A light-emitting diode (UVA-LED) irradiation to activate Fe(VI) for the degradation of micropollutants (e.g., sulfamethoxazole (SMX), enrofloxacin, and trimethoprim). UVA-LED/Fe(VI) could significantly promote the degradation of micropollutants, with rates that were 2.6-7.2-fold faster than for Fe(VI) alone. Comparatively, UVA-LED alone hardly degraded selected micropollutants. The degradation performance was further evaluated in SMX degradation via different wavelengths (365-405 nm), light intensity, and pH. Increased wavelengths led to linearly decreased SMX degradation rates because Fe(VI) has a lower molar absorption coefficient at higher wavelengths. Higher light intensity caused faster SMX degradation, owing to the enhanced level of reactive species by stronger photolysis of Fe(VI). Significantly, SMX degradation was gradually suppressed from pH 7.0 to 9.0 due to the changing speciation of Fe(VI). Scavenging and probing experiments for identifying oxidative species indicated that high-valent iron species (Fe(V)/Fe(IV)) were responsible for the enhanced degradation. A kinetic model involving target compound (TC) degradation by Fe(VI), Fe(V), and Fe(IV) was employed to fit the TC degradation kinetics by UVA-LED/Fe(VI). The fitted results revealed that Fe(IV) and Fe(V) primarily contributed to TC degradation in this system. In addition, transformation products of SMX degradation by Fe(VI) and UVA-LED/Fe(VI) were identified and the possible pathways included hydroxylation, self-coupling, bond cleavage, and oxidation reactions. Removal of SMX in real water also showed remarkable promotion by UVA-LED/Fe(VI). Overall, these findings could shed light on the understanding and application of UVA-LED/Fe(VI) for eliminating micropollutants in water treatments.

Ferrate (VI) Oxidation Is an Effective and Safe Way to Degrade Residual Colistin - a Last Resort Antibiotic - in Wastewater

The rise of novel mcr mobile resistance genes seriously threatens the use of colistin as a last resort antibiotic for treatment of multidrug-resistant Gram-negative bacterial infections in humans. Large quantities of colistin are released annually into the environment through animal feces. This leads to environmental toxicity and promotes horizontal transmission of the mcr gene in aqueous environments. We examined colistin degradation catalyzed by the presence of strong oxidant Fe (VI). We found almost complete colistin degradation (>95%) by Fe (VI) at initial colistin levels of 30 μM at a molar ratio of Fe (VI): colistin of 30 using an initial pH 7.0 at 25°C for 60 min. The presence of humic acid did not alter the degradation rate and had no significant impact on the removal of colistin by Fe (VI). Quantitative microbiological assays of Fe (VI)-treated colistin solutions using Escherichia coli, Staphylococcus aureus, and Bacillus subtilis indicated that the residual antibacterial activity was effectively eliminated by Fe (VI) oxidation. Luminescent bacteria toxicity tests using Vibrio fischeri indicated that both colistin and its degradation products in water were of low toxicity and the products showed decreased toxicity compared to the parent drug. Therefore, Fe (VI) oxidation is a highly effective and environment-friendly strategy to degrade colistin in water.

Reactive High-Valent Iron Intermediates in Enhancing Treatment of Water by Ferrate

Efforts are being made to tune the reactivity of the tetraoxy anion of iron in the +6 oxidation state (Fe^VIO₄^2-), commonly called ferrate, to further enhance its applications in various environmental fields. This review critically examines the strategies to generate highly reactive high-valent iron intermediates, Fe^VO₄^3- (Fe^V) and Fe^IVO₄^4- or Fe^IVO₃^2- (Fe^IV) species, from Fe^VIO₄^2-, for the treatment of polluted water with greater efficiency. Approaches to produce Fe^V and Fe^IV species from Fe^VIO₄^2- include additions of acid (e.g., HCl), metal ions (e.g., Fe(III)), and reductants (R). Details on applying various inorganic reductants (R) to generate Fe^V and Fe^IV from Fe^VIO₄^2- via initial single electron-transfer (SET) and oxygen-atom transfer (OAT) to oxidize recalcitrant pollutants are presented. The common constituents of urine (e.g., carbonate, ammonia, and creatinine) and different solids (e.g., silica and hydrochar) were found to accelerate the oxidation of pharmaceuticals by Fe^VIO₄^2-, with potential mechanisms provided. The challenges of providing direct evidence of the formation of Fe^V/Fe^IV species are discussed. Kinetic modeling and density functional theory (DFT) calculations provide opportunities to distinguish between the two intermediates (i.e., Fe^IV and Fe^V) in order to enhance oxidation reactions utilizing Fe^VIO₄^2-. Further mechanistic elucidation of activated ferrate systems is vital to achieve high efficiency in oxidizing emerging pollutants in various aqueous streams.

Influence of anions on ozonation of bisphenol AF: Kinetics, reaction pathways, and toxicity assessment

In this article, the degradation of 4, 4'-(hexafluoroisopropylidene) diphenol (bisphenol AF, BPAF) by ozone was studied and toxicity of the degradation products was evaluated. Kinetic studies showed that acidic conditions were more conducive to the ozone degradation of BPAF than alkaline conditions. In the presence of common anions, Br^- and SO₄^2- promoted the degradation of BPAF, whereas NO₂^-, NO₃^-, HSO₃^- inhibited the degradation, and the other anions and cations had no significant effect. The degradation products were analyzed by mass spectrometry, and were mainly manifested in hydroxylation, carboxylation and cleavage of benzene ring. The addition of NO₂^-, HSO₃^- and Br^-produced the corresponding free radicals, resulting in the parent compound being attacked and affecting the degradation efficiency and pathways. The theoretical calculated results showed that the ortho-site of the BPAF phenolic hydroxyl group was more active than the meta-position, and it's more likely for free radicals to attack ortho-sites and initiate substitution reactions. Toxicity assessment of the products in the process of ozone degradation showed that toxicity of the products was reduced by benzene ring cleavage and a reduction in the F atomic number. However, the toxicity of nitro and brominated products of BPAF was increased. These findings provide some new insights into the role of common ions in ozonation process and product formation, and supplement the existing conclusions. The results of this study remind future researchers to concern that inorganic ions in real water may be converted into corresponding free radicals that affect the formation of ozone oxidation products.

Sustainable ferrate oxidation: Reaction chemistry, mechanisms and removal of pollutants in wastewater

This review is intended to evaluate the use of ferrate (Fe(VI)), being a green coagulant, sustainable and reactive oxidant, to remove micro pollutants especially pharmaceutical pollutants in contaminated water. After a brief description of advanced oxidation processes, fundamental dimensions regarding the nature, reactivity, and chemistry of this oxidant are summarized. The degradation of contaminants by Fe(VI) involves several mechanisms and reactive agents which are critically evaluated. The efficiency and chemistry of Fe(VI) oxidation differs according to the reaction conditions and activation agent, such as soluble Fe(VI) processes, which involve Fe(VI), UV light, and electro-Fe(VI) oxidation. Fe(VI) application methods (including single dose, multiple doses, chitosan coating etc), and Fe(VI) with activating agents (including sulfite, thiosulfate, and UV) are also described to degrade the micro pollutants. Besides, application of Fe(VI) to remove pharmaceuticals in wastewater are intensely studied. Electrochemical prepared Fe(VI) has more wide application than wet oxidation method. Meanwhile, we elaborated Fe(VI) performance, limitations, and proposed innovative aspects to improve its stability, such as the generation of Fe(III), synergetic effects, nanopores entrapment, and nanopores capsules. This study provides conclusive direction for synergetic oxidative technique to degrade the micro pollutants.

Enhancing degradation of atrazine by Fe-phenol modified biochar/ferrate(VI) under alkaline conditions: Analysis of the mechanism and intermediate products

In this study, Fe-phenol modified biochar was prepared to enhance atrazine (AT) degradation by ferrate (Fe(VI)) under alkaline conditions, and the properties, mechanism and transformation pathways were extensively investigated. Degradation experiments showed that Fe-phenol modified biochar was more beneficial for improving the oxidation capacity of Fe(VI) than unmodified biochar, and the biochar with a molar ratio of Fe³⁺ to phenol of 0.1:5 (BC-2) showed the best promoting effect, and more than 94% of AT was removed at pH = 8 within 30 min. Moreover, the rate of oxidation (k_app) of AT by Fe(VI) increased 1.86 to 4.11 times by the addition of BC-2 in the studied pH range. Fe(Ⅴ)/Fe(Ⅳ) and ·OH were the main active oxidizing species for AT degradation in the Fe(VI)/BC-2 group and contributed to 70% and 24%, respectively, of degradation. The formation of ·OH and Fe(Ⅴ)/Fe(Ⅳ) was mainly due to the persistent free radicals and reducing groups on the surface of BC-2. AT was oxidized to 12 intermediate products in the Fe(VI)/BC-2 group through 5 pathways: alkyl hydroxylation, dealkylation, dichlorination, hydroxylation, alkyl dehydrogenation and dichlorination. Compared with those of the initial solution, the total organic carbon content and toxicity after the reaction decreased by 32.8% and 19.02%, respectively. Therefore, the combination of Fe-phenol modified biochar and Fe(VI) could be a promising method for AT removal.

Insight into the role of binding interaction in the transformation of tetracycline and toxicity distribution

The transformation of free state organic micro-pollutants (MPs) has been widely studied; however, few studies have focused on mixed and bound states MPs, even though numerous ionizable organic MPs process a strong tendency to combine with dissolved organic matters in aquatic environments. This study systemically investigated the distribution and toxicity assessment of tetracycline (TET) transformation products in free, mixed and bound states during UV, UV/H₂O₂, UV/PS and CNTs/PS processes. A total of 33 major transformation products were identified by UPLC-Q-TOF-MSMS analysis, combining the double bond equivalence and aromaticity index calculations. The binding interaction would weaken the attack on the dimethylamino (-N(CH₃)₂) group and induce the direct destruction of rings A and B of TET through the analysis of 2D Kernel Density changes and density functional theory (DFT) calculations. Toxicity assessment and statistics revealed that the intermediate products with medium molecular weight (230≤ m/z ≤ 380) exhibited higher toxicity, which was closely related to the number of the rings in molecular structures (followed as 2»3 > 1≈4). A predicted toxicity accumulation model (PTAM) was established to evaluate the overall toxicity changes during various oxidation processes. This finding provides new insight into the fate of bound MPs during various oxidation processes in the natural water matrix.

Theoretical Calculation on the Reaction Mechanisms, Kinetics and Toxicity of Acetaminophen Degradation Initiated by Hydroxyl and Sulfate Radicals in the Aqueous Phase

The •OH and SO₄^•− play a vital role on degrading pharmaceutical contaminants in water. In this paper, theoretical calculations have been used to discuss the degradation mechanisms, kinetics and ecotoxicity of acetaminophen (AAP) initiated by •OH and SO₄^•−. Two significant reaction mechanisms of radical adduct formation (RAF) and formal hydrogen atom transfer (FHAT) were investigated deeply. The results showed that the RAF takes precedence over FHAT in both •OH and SO₄^•− with AAP reactions. The whole and branched rate constants were calculated in a suitable temperature range of 198–338 K and 1 atm by using the KiSThelP program. At 298 K and 1 atm, the total rate constants of •OH and SO₄^•− with AAP were 3.23 × 10⁹ M⁻¹ s⁻¹ and 4.60 × 10¹⁰ M⁻¹ s⁻¹, respectively, considering the diffusion-limited effect. The chronic toxicity showed that the main degradation intermediates were harmless to three aquatic organism, namely, fish, daphnia, and green algae. From point of view of the acute toxicity, some degradation intermediates were still at harmful or toxic level. These results provide theoretical guidance on the practical degradation of AAP in the water.

New Findings of Ferrate(VI) Oxidation Mechanism from Its Degradation of Alkene Imidazole Ionic Liquids

Chemical reactivity, kinetics, degradation pathways and mechanisms, and ecotoxicity of the oxidation of 1-vinyl-3-ethylimidazolium bromide ([VEIm]Br), the most common alternative to organic solvents, by Fe(VI) (HFeO₄^-) were studied by lab experiments and theoretical calculations. Results show that Fe(VI) can efficiently remove VEIm through the dioxygen transfer-hydrolysis mechanism, which has not been reported yet. The reactivity of VEIm toward Fe(VI) mainly depends on the double bonds in the side chain of VEIm. The second-order rate constant for VEIm was 629.45 M^-1 s^-1 at pH 7.0 and 25 °C. Typical water constituents, except for SO₃^2-, Cl^-, and Cu²⁺, had no obvious effects on the oxidation. The oxidation products were determined by high-performance liquid chromatography hybrid quadrupole time-of-flight mass spectrometry, which proves that there were interactions between the oxidation intermediates of the anion and cation parts of [VEIm]Br during the degradation process. The structures of related products and oxidation mechanisms were further rationalized by theoretical calculations. The ecotoxicity of products from the three oxidation pathways all showed a trend of increase after the initial decrease. We hope that the findings of this work can give researchers some new inspirations on Fe(VI) degradation of other alkene-containing contaminants.

Three Kinetic Patterns for the Oxidation of Emerging Organic Contaminants by Fe(VI): The Critical Roles of Fe(V) and Fe(IV)

For the first time, this study showed that the apparent second-order rate constants (kapp) of six selected emerging organic contaminants (EOCs) oxidation by Fe(VI) increased, remained constant, or declined with time, depending on [EOC]0/[Fe(VI)]0, pH, and EOCs species. Employing excess caffeine as the quenching reagent for Fe(V) and Fe(IV), it was found that Fe(V)/Fe(IV) contributed to 20-30% of phenol and bisphenol F degradation by Fe(VI), and the contributions of Fe(V)/Fe(IV) remained nearly constant with time under all the tested conditions. However, the contributions of Fe(V)/Fe(IV) accounted for over 50% during the oxidation of sulfamethoxazole, bisphenol S, and iohexol by Fe(VI), and the variation trends of kapp of their degradation by Fe(VI) with time displayed three different patterns, which coincided with those of the contributions of Fe(V)/Fe(IV) to their decomposition with time. Results of the quenching experiments were validated by simulating the oxidation kinetic data of methyl phenyl sulfoxide by Fe(VI), which revealed that the variation trends of kapp with time were significantly determined by the change in the molar ratio of Fe(V) to Fe(VI) with time, highlighting the key role of Fe(V) in the oxidative process. This study provides comprehensive and insightful information on the roles of Fe(V)/Fe(IV) during EOC oxidation by Fe(VI).

Kinetics and mechanisms of phenolic compounds by Ferrate(VI) assisted with density functional theory

In this paper, Phenol, 4-Chlorophenol (4-CP), 2,4-Dichlorophenol (2,4-DCP) and 2,4,6-Trichlorophenol (2,4,6-TCP) were selected as model pollutants to explore the oxidant mechanism by ferrate (Fe(VI)). The reactions between ferrate (1000 μM) and four phenolic compounds (100 μM) were conformed to the second-order reaction kinetics at pH 9.2, and the order of k_obs followed as: k_4-CP (129 M^-1 s^-1) > k_2,4-DCP (96 M^-1 s^-1) > k_2,4,6-TCP (44 M^-1 s^-1) > k_Phenol (12 M^-1 s^-1). Meanwhile, the degradation rates of all four compounds by Fe(VI) increased with increased pH (3.1-9.2). A total of 14 degradation products were identified by Liquid chromatography-Time-of-Flight-Mass Spectrometry (LC-TOF-MS), and two pathways including hydroxylation of benzene ring and substitution of chlorine atom were proposed. Hydroxyl radicals, played a vital role during the degradation of phenolic compounds. Moreover, density functional theory calculations were used to explore the degradation mechanisms. The results showed that the hydroxyl radical was more favorable to substitute chlorine atom than hydrogen atom, and the substitution on ortho-position was more favorable than para-position for all four compounds. The findings of this study could greatly improve our understanding on the degradation mechanism of chlorophenol-like compounds by Fe(VI) for environmental remediation.

Ferrate self-decomposition in water is also a self-activation process: Role of Fe(V) species and enhancement with Fe(III) in methyl phenyl sulfoxide oxidation by excess ferrate

To reveal the role of ferrate self-decomposition and the fates of intermediate iron species [Fe(V)/Fe(IV) species] during ferrate oxidation, the reaction between ferrate and methyl phenyl sulfoxide (PMSO) at pH 7.0 was investigated as a model system in this study. Interestingly, the apparent second-order rate constants (k_app) between ferrate and PMSO was found to increase with ferrate dosage in the condition of excess ferrate in borate buffer. This ferrate dosage effect was diminished greatly in the condition of excess PMSO where ferrate self-decomposition was lessened largely, or counterbalanced by adding a strong complexing ligand (e.g. pyrophosphate) to sequester Fe(V) oxidation, demonstrating that the Fe(V) species derived from ferrate self-decomposition plays an important role in PMSO oxidation. A mechanistic kinetics model involving the ferrate self-decomposition and PMSO oxidation by Fe(VI), Fe(V) and Fe(IV) species was then developed and validated. The modeling results show that up to 99% of the PMSO oxidation was contributed by the ferrate self-decomposition resultant Fe(V) species in borate buffer, revealing that ferrate self-decomposition is also a self-activation process. The direct Fe(VI) oxidation of PMSO was impervious to presence of phosphate or Fe(III), while the Fe(V) oxidation pathway was strongly inhibited by phosphate complexation or enhanced with Fe(III). Similar ferrate dosage effect and its counterbalance by pyrophosphate as well as the Fe(III) enhancement were also observed in ferrate oxidation of micropollutants like carbamazepine, diclofenac and sulfamethoxazole, implying the general role of Fe(V) and promising Fe(III) enhancement during ferrate oxidation of micropollutants.

Products distribution and contribution of (de)chlorination, hydroxylation and coupling reactions to 2,4-dichlorophenol removal in seven oxidation systems

We systemically investigated the transformation behavior of 2,4-dichlorophenol (24-DCP) in seven different reaction systems including KMnO₄, heat/PS, O₃, UV, Fenton, NaClO and K₂FeO₄ treatment. The results revealed that complete removal of 24-DCP could be reached in minutes, especially for Fe(VI), KMnO₄, NaClO, Fenton and O₃ system. A total of 41 products were identified by LC-MS, and 10 of them were validated using commercial and self-synthesized standards. Hydroxyl substitution and coupling reactions were commonly observed in the studied systems. Meanwhile, extra routes such as sulfate substitution, (de)chlorination and direct oxidation were also involved for certain oxidation methods. Comparisons showed that a high degree of chlorination (>90%) occurred for NaClO system, while coupling products accounted for ~45% of the removed 24-DCP under PS oxidation. Moreover, low mineralization degree together with high aquatic toxicity was attributed to the occurrence of coupling reaction, which was possibly related to the redox potential of the main oxidative species. Considering the low abundance of coupling products and the gentle reaction condition, UV irradiation is a better option for 24-DCP removal in water and wastewaters. These findings can deepen our understanding on the transformation process of 24-DCP and provide some useful information for the environmental elimination of substituted phenols.

Revelation of Fe(V)/Fe(IV) Involvement in the Fe(VI)-ABTS System: Kinetic Modeling and Product Analysis

To quantitatively probe iron intermediate species [Fe(V)/Fe(IV)] in Fe(VI) oxidation, this study systematically investigated the reaction kinetics of Fe(VI) oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic)acid (ABTS) at different ratios of [ABTS]₀/[Fe(VI)]₀ (i.e., >1.0, =1.0, and <1.0) in pH 7.0 phosphate (10 mM)-buffered solution. Compared to the literature, a more comprehensive and robust kinetic model for the Fe(VI)-ABTS system including interactions between high-valent iron species [Fe(VI), Fe(V), and Fe(IV)], ABTS, and the ABTS^•+ radical was proposed and validated. The oxidation of ABTS by Fe(VI) (k = (5.96 ± 0.9%) × 10⁵ M^-1 s^-1), Fe(V) (k = (2.04 ± 0.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (4.64 ± 13.0%) × 10⁵ M^-1 s^-1) proceeds via one-electron transfer to generate ABTS^•+, which is subsequently oxidized by Fe(VI) (k = (8.5 ± 0.0%) × 10² M^-1 s^-1), Fe(V) (k = (1.0 ± 40.0%) × 10⁵ M^-1 s^-1), or Fe(IV) (k = (1.9 ± 17.0%) × 10³ M^-1 s^-1), respectively, via two-electron (oxygen atom) transfer to generate colorless ABTS_ox. At [ABTS]₀/[Fe(VI)]₀ > 1.0, experimental data and model simulation both indicated that the reaction stoichiometric ratio of Fe(VI)/ABTS^•+ increased from 1.0:1.0 to 1.0:1.2 as [ABTS]₀ was increased. Furthermore, the Fe(VI)-ABTS-substrate model was developed to successfully determine reactivity of Fe(V) to different substrates (k = (0.7-1.42) × 10⁶ M^-1 s^-1). Overall, the improved Fe(VI)-ABTS kinetic model provides a useful tool to quantitatively probe Fe(V)/Fe(IV) behaviors in Fe(VI) oxidation and gains new fundamental insights.

Effect of Metal Ions on Oxidation of Micropollutants by Ferrate(VI): Enhancing Role of FeIV Species

This paper investigated the oxidation of recalcitrant micropollutants [i.e., atenolol (ATL), flumequine, aspartame, and diatrizoic acid] by combining ferrate(VI) (Fe^VIO₄^2-, Fe^VI) with a series of metal ions [i.e., Fe(III), Ca(II), Al(III), Sc(III), Co(II), and Ni(II)]. An addition of Fe(III) to Fe^VI enhanced the oxidation of micropollutants compared solely to Fe^VI. The enhanced oxidation of studied micropollutants increased with increasing [Fe(III)]/[Fe^VI] to 2.0. The complete conversion of phenyl methyl sulfoxide (PMSO), as a probe agent, to phenyl methyl sulfone (PMSO₂) by the Fe^VI-Fe(III) system suggested that the highly reactive intermediate Fe^IV/Fe^V species causes the increased oxidation of all four micropollutants. A kinetic modeling of the oxidation of ATL demonstrated that the major species causing the increase in ATL removal was Fe^IV, which had an estimated rate constant as (6.3 ± 0.2) × 10⁴ M^-1 s^-1, much higher than that of Fe^VI [(5.0 ± 0.4) × 10^-1 M^-1 s^-1]. Mechanisms of the formed oxidation products of ATL by Fe^IV, which included aromatic and/or benzylic oxidation, are delineated. The presence of natural organic matter significantly inhibited the removal of four pollutants by the Fe^VI-Fe(III) system. The enhanced effect of the Fe^VI-Fe(III) system was also seen in the oxidation of the micropollutants in river water and lake water.

Kinetics and reaction pathways for the transformation of 4-tert-butylphenol by ferrate(VI)

4-tert-butylphenol (4-tBP) is a phenolic endocrine disrupting chemical that has attracted great attention due to its wide occurrence, environmental persistence, and possible toxic effects. In this study, we systematically investigated the transformation of 4-tBP in ferrate (VI) oxidation process. The second-order reaction rate constant (k_app) of Fe(VI) with 4-tBP decreases with solution pH, and the k_app value was determined as 295 M^-1·s^-1 at pH 8.0. The removal efficiency of 4-tBP was slightly decreased by Mg²⁺ and HCO₃^-, while accelerated at varying degrees by the presence of Cu²⁺ and humic acid. Product analysis revealed that 4-tBP was mainly transformed into hydroxylation products, benzene-ring cleavage products, dimers and higher polymerization products via oxygen atom transfer, ring-opening of the benzene ring and radical coupling reaction. Furthermore, initial reactions of 4-tBP were rationalized by theoretical analysis of atom partial charges, frontier electron densities, and spin densities. Nearly complete removal of 4-tBP (20 μM) was achieved after 5 min of reaction in both ultrapure water and natural waters, demonstrating the feasibility of this Fe(VI) oxidation method in treating phenols-contaminated waters.

Three-dimensional porous graphene-like biochar derived from Enteromorpha as a persulfate activator for sulfamethoxazole degradation: Role of graphitic N and radicals transformation

Three-dimensional graphene-like biochar derived from Enteromorpha (EGB) was prepared as a persulfate (PS) activator for sulfamethoxazole (SMX) degradation. The graphitic N in the EGB samples not only endowed the superior binding energy towards SMX adsorption, but also promote the PS binding with the EGB, which was crucial to the catalytic degradation of SMX in EGB/PS system. Different from the radical-based oxidation in biochar prepared at 400 °C via the persistent free radicals (PFRs), both ¹O₂ and surface electron transfer served as non-radical pathways in the EGB samples prepared above 500 °C, acting together with free radicals (O₂∙^-) on SMX degradation. Oxidation of SMX and its substructural analogues indicated that the selective oxidizing reaction occurred in the EGB/PS system and the isoxazole ring in SMX molecule was insensitive to be attacked ¹O₂. In addition, toxicity predication indicated that the overall biotoxicity of the intermediates during SMX degradation was decreased.

Effects of common inorganic anions on the ozonation of polychlorinated diphenyl sulfides on silica gel: Kinetics, mechanisms, and theoretical calculations

In this work, the ozonation properties of 2,2',3',4,5-pentachlorodiphenyl sulfide (PeCDPS) was systematically studied, with special emphasis on the underlying mechanism for the effects of inorganic ions. Kinetic experiments show that common ions can significantly reduce the oxidative properties of ozone, except for SO₃^2- and Cu²⁺. The inhibition effect of anions has been explained through the scavenging effect of free radicals and the generation of other free radicals with weaker oxidation potentials, but no research has reported on the effect of free radicals generated by anions on the degradation pathway. However, SO₃^2- and Cu²⁺ exerted a promoting effect through enhanced formation of ·OH via the hydrolysis effect and the catalyzed decomposition of O₃, respectively. According to the intermediate products identified by high performance liquid chromatography-mass spectrometry/mass spectrometry (HPLC-MS/MS) analysis, direct oxidation of S atom, substitution of Cl atom with -OH group, and hydroxylation of the benzene ring were commonly observed. The addition of NO₂^- and SO₃^2- produced new free radicals like ·NO₂, ·SO₃ and ·SO₄^-, which would attack the parent compound or its primary product, thus influencing the degradation efficiency and pathways. The radicals initiated reactions and the structures of the corresponding products were further rationalized by density functional theory (DFT) calculations. These findings provide new insights into the effects of common anions on ozone oxidation of organic compounds.

Theoretical Design of Biodegradable Phthalic Acid Ester Derivatives in Marine and Freshwater Environments

The biodegradability of phtalic acid esters in marine and freshwater environments was characterized by their binding free energy with corresponding degrading enzymes. According to comprehensive biodegradation effects weights, the binding free energy values were converted into dimensionless efficacy coefficient using ratio normalization method. Then, considering comprehensive dual biodegradation effects value and the structural parameters of PAEs in both marine and freshwater environments, a 3D‐QSAR pharmacophore model was constructed, five PAE derivatives (DBP−COOH, DBP−CHO, DBP−OH, DINP−NH₂, and DINP−NO₂) were screened out based on their environmental friendliness, functionality and stability. The prediction of biodegradation effects on five PAE derivatives by biodegradation models in marine and freshwater environment increased by 15.90 %, 15.84 %, 27.21 %, 12.33 %, and 8.32 %, and 21.57 %, 15.21 %, 20.99 %, 15.10 %, and 9.74 %, respectively. By simulating the photodegradation path of the PAE derivative molecular, it was found that DBP−OH can generate ^.OH and provides free radicals for the photodegradation of microplastics in the environment.

Alumina-mediated photocatalytic degradation of hexachlorobenzene in aqueous system: Kinetics and mechanism

Five kinds of Al₂O₃ were characterized by SEM, TEM, FT-IR and BET surface area, and then used as carriers to investigate the photochemical removal of hexachlorobenzene (HCB) in aqueous system. The results showed that HCB coated on the surfaces of all Al₂O₃ could be photodegraded rapidly, and Neutral-Al₂O₃ presented the best performance. Meanwhile, the efficient removal of HCB in real water matrices, including tap water, river water and secondary clarifier effluent showed the potential practical application of Al₂O₃. EPR and theoretical calculation revealed the generation of hydroxyl radicals on Al₂O₃ surface under 500 W Xe lamp irradiation. Nine intermediates and a small amount of Cl^- were identified by GC/MS, LC/MS and IC analysis, which was further verified by transition state calculations. These results can provide a new technique for HCB removal in water and wastewaters, and give more insights into the environmental ecological risk assessment of this pollutant.

Evaluation of enhanced coagulation combined with densadeg-ultrafiltration process in treating secondary effluent: Organic micro-pollutants removal, genotoxicity reduction, and membrane fouling alleviation

Conventional coagulation is widely used as an ultrafiltration membrane pretreatment process in wastewater reclamation, however it shows little ability to reduce organic micro-pollutants (OMPs) and genotoxicity. In this research, powdered activated carbon (PAC) and potassium ferrate were used respectively with polyaluminum chloride (PACl) to enhance coagulation. Filtration experiments of coagulation (CUF), coagulation-adsorption (CAUF) and coagulation-oxidation (COUF) pretreatment combined with densadeg-ultrafiltration processes were conducted under their optimum doses. The effluent water quality of CAUF and COUF could meet the water reuse quality standard for scenic environment use, while total phosphorus in the conventional CUF discharge was higher than the standard. The average removal efficiency of the selected fourteen OMPs was significantly improved by 1.8 times through the CAUF process compared to the CUF process (31.2%), whereas the COUF process (38.4%) showed limited improvement. Prominent reduction of genotoxicity was observed in the CAUF and COUF processes, and the effluent of the CAUF process had the least genotoxicity of 1.0 ± 0.3 μg 4-Nitroquinoline-N-oxide (4-NQO)/L. Moreover, the average transmembrane pressure increasing rate followed the order of CUF (1.5 kPa/d) > COUF (1.1 kPa/d) > CAUF (0.6 kPa/d), indicated that the enhanced coagulation process could relieve membrane fouling effectively.

Efficient removal of triclosan via peroxymonosulfate activated by a ppb level dosage of Co(II) in water: Reaction kinetics, mechanisms and detoxification

Triclosan (TCS), an extensively used broad-spectrum antimicrobial agent, has raised significant environmental concerns regarding its widespread occurrence in waters. In this study, the removal of TCS in aqueous solution via peroxymonosulfate (PMS) activated by an extremely low-level Co²⁺ (0.02 μM) was systematically investigated. During preliminary test, TCS (10 μM) was totally degraded in 30 min by using 0.1 μM Co²⁺ and 40 μM PMS at pH 7.0 with a degradation rate constant of 0.1219 min^-1. A first-order apparent degradation rate of TCS was found with respect to the PMS concentrations. At extremely low dosage of Co²⁺ (0.02 μM), the presence of NO₃^-, HCO₃^-, PLFA, and SRHA within test concentrations significantly inhibited TCS removal, while a dual effect of Cl^- on the degradation rate of TCS was observed. The quenching experiments verified that SO₄^- was the dominant reactive oxygen species (ROS) rather than OH. Six major intermediates were identified using TOF-LC-MS, based on which we proposed three associated reaction pathways including hydroxylation, ether bond breakage, and dechlorination. Toxicity predictions by ECOSAR software exhibited aquatic toxicity reduction of TCS after Co²⁺/PMS treatment. We outlook these findings to advance the feasibility of organic contaminants removal via Co²⁺/PMS system with Co²⁺ at extremely low levels.

Ferrate(VI) pretreatment before disinfection: An effective approach to controlling unsaturated and aromatic halo-disinfection byproducts in chlorinated and chloraminated drinking waters

Disinfection is an essential process of drinking water treatment to eliminate harmful pathogens, but it generates potentially toxic disinfection byproducts (DBPs). Ferrate (FeO₄^2-, Fe(VI)) was used to pre-oxidize natural organic matter (NOM, the precursor of DBPs) in source water to control DBP formation in subsequent chlorine or chloramine disinfection. Currently, it is unclear how Fe(VI) changes the structure of NOM, and no information details the effect of Fe(VI) pretreatment on the aromatic DBPs or the speciation of overall DBPs generated in subsequent disinfection of drinking water. In the present paper, Fe(VI) was applied to pretreat simulated source water samples at a Fe(VI) to dissolved organic carbon mole ratio of 1:1 at pH 8.0. ¹³C nuclear magnetic resonance spectroscopy was newly employed to characterize NOM in simulated source waters with and without Fe(VI) treatment, and it was demonstrated that Fe(VI) converted unsaturated aromatic C functional groups in NOM to saturated aliphatic ones. High-resolution mass spectrometry (HRMS) and high performance liquid chromatography/triple quadrupole MS were applied to analyze the DBPs generated in chlorination and chloramination of the source waters with and without Fe(VI) pretreatment. It was confirmed that Fe(VI) pretreatment followed by chlorination (or chloramination), generated DBPs containing less unsaturated, halogenated, and aromatic moieties than chlorination (or chloramination) without pretreatment by Fe(VI). Finally, the cytotoxicity of disinfected drinking water samples were assessed with the human epithelial colorectal adenocarcinoma Caco-2 cell line (a model of the intestinal barrier for ingested toxicants), and the results show that Fe(VI) pretreatment detoxified the chlorinated and chloraminated drinking waters.

A combined experimental and computational study on the oxidative degradation of bromophenols by Fe(VI) and the formation of self-coupling products

In this study, the degradation of eight bromophenols (BPs), including monobromophenols (2-BP, 3-BP, and 4-BP), dibromophenols (2,4-DBP, 2,6-DBP, and 3,5-DBP), a tribromophenol (2,4,6-TBP) and a pentabromophenol (PBP), by a Fe(VI) reaction process at a pH of 8.0 was systematically studied. It was concluded that their degradation rates increased with increasing Fe(VI) concentrations in solution. The removal of 2,4,6-TBP, 2-BP, and 2,6-DBP was faster than that of the other five BPs, which could be attributed to the position of the substituting Br atom. Moreover, the direct oxidation and coupling reactions greatly influenced the reactivity of the bromophenols with Fe(VI). The electron paramagnetic resonance (EPR) analysis confirmed the presence of hydroxyl radicals in present system. The oxidation reaction products of PBP and 2-BP were recognized by an electrospray time-of-flight mass spectrometer; hydroxylation, hydroxyl substitution, the cleavage of the C-C bond, direct oxidation and polymerization via an end linking mechanism were noticeably found in the reaction process, resulting in the formation of polymerization products and causing hydroxylation to occur. Theoretical calculations further determined the possible oxidation sites of 2-BP and PBP. This study may provide comprehensive and important information on the remediation of BPs by Fe(VI).

Rapid and strong biocidal effect of ferrate on sulfidogenic and methanogenic sewer biofilms

For the control of sulfide and methane in sewers, it is favorable to reduce their production rather than to remove them after generation. In this study, we revealed rapid and strong biocidal effect of ferrate (Fe(VI)) on sulfidogenic and methanogenic sewer biofilms, leading to control of sulfide and methane production in sewer. The inactivation of the microorganisms in sewer biofilms by Fe(VI) could be accomplished within 15 min for a single dosing event and the biocidal effect could be enhanced by applying pulse dosing strategy. The microbiological analysis showed that the key functional genes involved in sulfide and methane production, i.e. dsrA and mcrA, in the viable cells after Fe(VI) dosing were decreased substantially by 84.2% and 86.6%, respectively. Significant drops were also observed in the relative abundances of viable sulfide reducing bacteria (SRB) and methanogenic archaea (MA). The direct dosing of Fe(VI) into a sewer reactor led to instant and complete suppression of sulfidogenic and methanogenic activities, and the recovery of the activities resembled the regrowth of residual SRB and MA. The results of this study suggested the feasibility for developing an efficient and cost-effective sulfide and methane control strategy using Fe(VI).

Degradation of tetrabromobisphenol A by a ferrate(vi)-ozone combination process: advantages, optimization, and mechanistic analysis

This study systematically investigated the ferrate(vi)-ozone combination process for TBBPA degradation. Firstly, the advantages of a ferrate(vi)-ozone combination process were assessed as compared with a sole ozone and ferrate(vi) oxidation process. Then, the performance of the ferrate(vi)-ozone combination process was investigated under different experimental conditions, including the dosing orders of oxidants, dosing concentrations of oxidants, and the initial solution pH. At the same time, toxicity control (including the acute and chronic toxicity) and mineralization were analyzed after optimization. Finally, a mechanism was proposed about the synergetic effects of the ferrate(vi)-ozone combination process for decontamination. The ferrate(vi)-ozone combination process proved to be an efficient and promising technology for removing TBBPA from water. After being pre-oxidized by ferrate(vi) for 3 min and then co-oxidized by the two oxidants, TBBPA of 1.84 μmol L^-1 could be completely degraded by dosing only 0.51 μmol L^-1 of ferrate(vi) and 10.42 μmol L^-1 of ozone within 10 min in wide ranges of pH (5.0-11.0). Up to 91.3% of debromination rate and 80.5% of mineralization rate were obtained, respectively. In addition, no bromate was detected and the acute and chronic toxicity were effectively controlled. The analysis of the proposed mechanism showed that there might exist a superposition effect of the oxidation pathways. In addition, the interactions between the two oxidants were beneficial for the oxidation efficiency of ferrate(vi) and ozone, including the catalytic effect of ferrate(vi) intermediates on ozone and the oxidation of low-valent iron compounds by ozone and the generated ·OH radical.

Strategy of combining radiation with ferrate oxidation for enhancing the degradation and mineralization of carbamazepine

In this study, the strategy of combining radiation with ferrate oxidation was proposed to decrease the adsorbed dosse and enhance the mineralization of carbamazepine in aqueous solution. Compared to single radiation (800 Gy), the combined process of ferrate pretreatment and radiation required lower dose (600 Gy) for totally removing carbamazepine. During the combined process, the removal efficiency of total organic carbon (TOC) reached 22.2%. However, the removal efficiencies of carbamazepine and TOC decreased when ferrate and radiation were used simultaneously, indicating that the addition of ferrate during the radiation process had negative effect on the removal of carbamazepine. In contrast, the radiation followed by ferrate oxidation presented the best performance in decreasing the absorbed dose and enhancing the mineralization of carbamazepine. Carbamazepine could be completely removed under all conditions. TOC removal efficiency reached 18.3%, 31.3%, 52.9% and 60.6%, respectively, at the adsorbed dose of 100, 300, 600 and 800 Gy when 0.4 mM ferrate was adopted. The enhanced TOC removal could be due to the enhanced oxidation capacity of ferrate caused by the pH decrease at the end of radiation and the further oxidation of intermediate products formed during the radiation process by ferrate. Seven degradation products were identified in total, and thus the degradation pathway of carbamazepine was proposed. This study provides a possible way to decrease the adsorbed dose and enhance the mineralization of carbamazepine by radiation.

Formation of hydroxylated derivatives and coupling products from the photochemical transformation of polyfluorinated dibenzo-p-dioxins (PFDDs) on silica surfaces

Polyfluorinated dibenzo-p-dioxins (PFDDs) are dioxin compounds that have been detected in industrial fluoroaromatic chemicals and can cause adverse effects to organisms. In this work, the photochemical behaviors of PFDDs congeners on silica was systematically investigated. The pseudo-first-order rate constants (k, h^-1) of surface photolysis changed with the substitution number and position of fluorine atoms, and the tetra-fluorinated PFDDs tended to degrade more efficiently. Octafluorinated dibenzo-p-dioxin (OFDD) was selected as a representative to explore the reaction mechanisms. Product analysis showed that OFDD was decomposed into hydroxylated PFDDs (OH-PFDDs) and hydroxylated polyfluorinated diphenyl ethers (OH-PFDEs) via hydroxyl substitution and (OH radical mediated or direct) C-O bond cleavage. Coupling elimination reaction was also observed, resulting in the formation of three-membered and four-membered ring compounds. According to the extracted peak areas in mass spectra and the energy barrier in potential energy surface, direct homolysis of C-O bond occurs as the dominant reaction pathway. This work could provide some new insights into the environmental fate of dioxin compounds.

Oxidative degradation of chlorpyrifos using ferrate(VI): Kinetics and reaction mechanism

In this study, we investigated the degradation kinetics of chlorpyrifos, an organophosphorus (OP) compound, using ferrate(VI), and investigated the potential of this iron-based chemical oxidant on chlorpyrifos removal from water and wastewater treatments. A series of kinetic experiments were conducted to evaluate the influence of various environmental factors, such as pH, oxidant dosages, as well as the presence of anions, cations, humic acid (HA), and different water matrices. Chlorpyrifos was completely removed within 300 s under the following optimum conditions: [chlorpyrifos]₀ = 1 μM, [Fe(VI)]₀:[chlorpyrifos]₀ = 100:1, T = 25 °C, and pH = 7.0. Anions such as Cl^-, SO₄^2-, NO₃^-, and HCO₃^- and cations such as Fe³⁺, Cu²⁺, and NH₄⁺ did not appear to influence the removal of chlorpyrifos. However, the presence of Ca²⁺, Mg²⁺, and HA in water inhibited the degradation of chlorpyrifos. Experiments on removing chlorpyrifos from tap water, river water, and synthetic wastewater were performed to demonstrate the practical applications of Fe(VI). Ten oxidation products of chlorpyrifos were identified using liquid chromatography-quadrupole-time-of flight-mass spectrometry (LC-Q-TOF-MS), and their structures were further elucidated using MS/MS spectra. Then, two degradation pathways were preliminarily proposed including the oxidation of the P = S bond, cleavage of C-O bond, and hydroxyl substitution reaction. In general, Fe(VI) could be used as an efficient technology for chlorpyrifos removal from water and wastewater treatments.

Comparative study on ferrate oxidation of BPS and BPAF: Kinetics, reaction mechanism, and the improvement on their biodegradability

Bisphenol S (BPS) and bisphenol AF (BPAF) were increasingly consumed and these compounds are resistant to environmental degradation. Herein, ferrate oxidation of BPS and BPAF was investigated, and biodegradability of the oxidation products was examined. The second-order reaction rate constants of ferrate with BPS and BPAF were 1.3 × 10³ M^-1s^-1 and 3 × 10² M^-1s^-1, respectively, at pH 7.0, 25 °C. In the oxidation process, some BPS molecules dimerized, while other BPS molecules were oxidized through oxygen-transfer process, leading to the formation of hydroxylation products and benzene-ring cleavage products. The dominant reaction of BPAF with ferrate was oxygen-transfer process, and BPAF was degraded into lower molecular weight products. The variation of assimilable organic carbon (AOC) suggested that the biodegradability of BPAF and BPS was largely improved after ferrate oxidation. Compared with the BPS oxidation products, the BPAF oxidation products were easier to be bio-consumed. Pure culture test showed that BPAF inhibited the growth of Escherichia coli, while ferrate oxidation completely eliminated this toxic effect. Co-existing humic acid (HA, 1 mg C/L to 5 mg C/L) decreased the removal of BPS and BPAF with ferrate. Compared with BPAF, more oxidation intermediates formed in the ferrate oxidation of BPS may be reduced by HA to the parent molecular. Thus, the inhibition effect of HA on the ferrate oxidation of BPS was more obvious than that on BPAF.