Catalytic Ozonation System Research Articles

Synergistic mechanism of supported Mn–Ce oxide in catalytic ozonation of nitrofurazone wastewater

In this study, the catalytic materials of MnOx/γ-Al2O3, CeO2/γ-Al2O3, and MnxCe1-xO2/γ-Al2O3 for catalytic ozonation were synthesized. The catalysts were used in heterogeneous catalytic ozonation of the wastewater containing ntrofurazone (NFZ). The effects of the catalytic ozonation operational factors were systematically evaluated in terms of ozone dosing, catalyst dosing, initial NFZ concentration, and pH. The results showed that the catalytic activity of the MnxCe1-xO2/γ-Al2O3 was higher than that of the MnOx/γ-Al2O3 and CeO2/γ-Al2O3. The kinetics analysis revealed that bimetallic loading has a synergistic effect and the mechanism of this effect was investigated in the catalytic ozonation system. The catalysts were characterized by FESEM, EDS, XRD, XPS, IR, and BET. The characteristics of the catalysts revealed that Mn could alter the oxide species on the metal surface and interfere with the formation of CeO2 crystals, which led to smaller grains, enhanced adsorption oxygen, and greater specific surface area. The MnxCe1-xO2/γ-Al2O3 crystals could form a solid solution, which helps higher catalytic activity. This study adds to the understanding of the synergistic mechanism of the loaded Ce–Mn oxide catalysts in the heterogeneous catalytic ozonation system and provides a feasible method for degrading pharmaceutical wastewater.

Catalytic ozonation of toluene over amorphous Cu–Mn bimetallic oxide: Influencing factors, degradation mechanism and pathways

Herein, amorphous catalysts were employed to investigate the catalytic ozonation system, revealing the degradation mechanism and influencing factors (O3 concentration, temperature, and humidity) for toluene catalytic ozonation. Cu0.2MnOx exhibited the highest toluene oxidized and excellent stability (∼85% at 60 h) based on the suitable value of Oads/Olat and potent synergy between Cu with Mn. To explore the effect of factors, the change of fresh and post-reaction samples was compared as revealed in the relevant characterization results (SEM, XRD, BET, XPS, TGA), DRIFTS and GC-MS identified the intermediates and byproducts. The results show that appropriate temperature (100 °C) and O3 concentration (2100 ppm) can effectively enhance the number of reactive oxygen species. Although H2O can increase the production of ·OH to promote degradation, it is easier to quench the active sites on the surface of amorphous catalysts. During the reaction, the main role of Cu in Cu–Mn bimetallic oxides is adsorption of toluene and O3, formation of benzoic acid, and oxidation of short-chain products. As for the adjacent Mn, it works on the cleavage of O–O in O3 and the ring-opening of benzene. Then, the mainly catalytic ozonation pathway of toluene was proposed and followed the order: toluene, benzoic acid, benzene, maleic anhydride, short-chain carbon species, CO2, and H2O.

Interfacial mechanism of the synergy of biochar adsorption and catalytic ozone micro-nano-bubbles for the removal of 2,4-dichlorophenoxyacetic acid in water

A high-efficiency catalytic ozonation system consisting of NaOH-pretreated rice straw biochar (OHBC) and ozone micro-nano-bubbles (O3-MNBs) was developed to remove 2,4-dichlorophenoxyacetic acid (2,4-D) in water, and the interfacial mechanism was further studied in detail. The removal rate of 2,4-D was improved by 47.0% through the synergy of adsorption and catalytic ozonation in the O3-MNBs/OHBC system (89.0%) compared with the O3-MNBs alone system (42.0%), and the ozone utilization rate was increased from 0.43 mg/mg to 0.66 mg/mg. High performance for 2,4-D removal from water with diverse matrix factors could also be obtained. The many hydroxyl groups (OH) (1.25 mmol/g) on the surface of the OHBC were more powerful than the carboxyl and lactone groups (COOH and COOR) at catalyzing ozone to produce OH according to XPS and Boehm titration results. The theoretical calculations based on density functional theory (DFT) showed that ozone molecules were more readily adsorbed on the surface OH groups due to the relatively high negative adsorption energy (–0.31 eV), and they further extended the two OO bond length from 1.277 Å to 1.303 Å and 1.311 Å, thus promoting its decomposition. The degradation mechanism of 2,4-D included dechlorination, electrophilic substitution, and OH substitution, and the degraded water showed relatively low toxicity.

Catalytic ozonation of phenol by ZnFe2O4/ZnNCN: performance and mechanism.

A novel magnetic catalyst was synthesized and applied in heterogeneous catalytic ozonation process. The ZnFe2O4/ZnNCN material was synthesized by hydrothermal and high-temperature calcination method and characterized by XPS, XRD, FTIR, VSM, and SEM techniques. In the system of O3/ZnFe2O4/ZnNCN, the removal rates of phenol and chemical oxygen demand (COD) reached 93% and 43% at 60 min. Further analysis shows that ZnFe2O4/ZnNCN has a significant catalytic effect on O3, which is demonstrated by the first-order kinetic constant being 1.93 times than O3 alone. The catalyst exhibits excellent cycling stability during repeated catalytic ozonation process and can be fully recycled under an applied magnetic field. The role of hydrogen peroxide (H2O2) and surface hydroxyl groups was investigated, and a mechanism for catalytic ozonation was proposed. This work not only builds an efficient catalytic ozonation system, but also provides a potential modification strategy for spinel oxides.

Preparation of SnOx-MnOx@Al2O3 for Catalytic Ozonation of Phenol in Hypersaline Wastewater

ABSTRACT The catalytic ozonation process (COP) is an effective and advanced oxidation technology for the treatment of organic wastewater. However, high salinity has a negative impact on catalytic ozonation performance. In this work, tin oxide (SnOx) and manganese oxide (MnOx) doped γ-Al2O3 catalysts (SnOx-MnOx@Al2O3) were prepared by the incipient wetness impregnation method and characterized by SEM, XRD, BET, XRT, XPS and FT-IR techniques. The SnOx-MnOx@Al2O3 catalyst was applied to the catalytic ozonation of phenol in hypersaline wastewater, and the catalytic performance was evaluated by COD removal efficiency. When the mass ratio of MnOx to SnOx was 1:3, the pH was 7, the catalyst dosage was 40 g/L, the ozone dosage was 6 mg/(L·min), the NaCl concentration was 15 g/L and the COD removal efficiency of hypersaline phenol wastewater reached 93.8% after 240 min of catalytic ozonation treatment. Compared with the single ozonation process (SOP), the introduction of SnOx-MnOx@Al2O3 catalyst improved the COD removal efficiency by 32.3%. Cl− may quench surface •OH species and ozone to form Cl• and Cl2•− with low activity and high selectivity, resulting in the reduction of catalytic ozonation efficiency. The catalytic activity of the SnOx-MnOx@Al2O3 catalyst remained high after eight cycles. In conclusion, the SnOx-MnOx@Al2O3 catalytic ozonation system exhibits efficient and stable mineralization performance and is a promising strategy for the treatment of hypersaline organic wastewater.

Highly efficient catalytic ozonation for oxalic acid mineralization with Ag2CO3 modified g-C3N4: Performance and mechanism

A series of Ag2CO3 doping g-C3N4 composites labeled as AgCNx-T (x represented the weight content of g-C3N4 and T referred to the hydrothermal temperature) were synthesized by a simple precipitation method. Various techniques such as BET, XRD, FTIR, SEM, and XPS were employed to explore the morphology structure and physicochemical properties of catalysts ascribed to the decoration of Ag2CO3 onto g-C3N4 and the results revealed that the hydrothermal temperature played an important role in the size dimension and crystallinity of Ag2CO3. It was worthy noted, the decorating of Ag2CO3 would provide more active sites on the catalyst surface, strengthen the regeneration and transmission of electrons, improve the utilization of O3, and promote the generation of reactive oxygen species, thus improving the catalytic ozonation performance. Amongst, the AgCN0.4-100 composite had the optimal performance with 99.99% of OA degradation efficiency and 93.19% of OA mineralization. Moreover, the AgCN0.4-100 exhibited satisfactory reusability for multiple consecutive cycles (≥5) with low Ag ion release (<0.3 mg L−1). The reactive species (O2•− and 1O2) were verified to take predominant roles in the reaction through the radical scavenger experiments and ESR spectra. Accordingly, an empirical kinetic model was established to predict OA concentration with the given operational parameters. Finally, the synergistic mechanism of OA degradation in catalytic ozonation system was also proposed, which possessed promising prospect in practical water treatment for environmental applications.

Adaptive modeling of nonnegative environmental systems based on projectional Differential Neural Networks observer

A new design of a non-parametric adaptive approximate model based on Differential Neural Networks (DNNs) applied for a class of non-negative environmental systems with an uncertain mathematical model is the primary outcome of this study. The approximate model uses an extended state formulation that gathers the dynamics of the DNN and a state projector (pDNN). Implementing a non-differentiable projection operator ensures the positiveness of the identifier states. The extended form allows producing continuous dynamics for the projected model. The design of the learning laws for the weight adjustment of the continuous projected DNN considered the application of a controlled Lyapunov-like function. The stability analysis based on the proposed Lyapunov-like function leads to the characterization of the ultimate boundedness property for the identification error. Applying the Attractive Ellipsoid Method (AEM) yields to analyze the convergence quality of the designed approximate model. The solution to the specific optimization problem using the AEM with matrix inequalities constraints allows us to find the parameters of the considered DNN that minimizes the ultimate bound. The evaluation of two numerical examples confirmed the ability of the proposed pDNN to approximate the positive model in the presence of bounded noises and perturbations in the measured data. The first example corresponds to a catalytic ozonation system that can be used to decompose toxic and recalcitrant contaminants. The second one describes the bacteria growth in aerobic batch regime biodegrading simple organic matter mixture.

Can the commonly used quenching method really evaluate the role of reactive oxygen species in pollutant abatement during catalytic ozonation?

Reactive oxygen species (ROS) such as hydroxyl radicals (•OH), superoxide radicals (O2•−), and singlet oxygen (1O2) have often been suggested to play a role in ozone-resistant pollutant abatement during catalytic ozonation. However, there are significant controversies regarding their relative importance in literature. Currently, the role of ROS in pollutant abatement is commonly evaluated by the quenching method based on the assumption that the added ROS quenchers (e.g., tert-butanol (TBA) and para-benzoquinone (pBQ)) quench only the target ROS, but do not considerably influence other reaction mechanisms of catalytic ozonation. However, we hypothesized that this assumption is possibly unrealistic and a main cause for the controversies reported in literature. To test this hypothesis, this study evaluated the effects of six commonly used ROS quenchers (TBA, pBQ, methanol (MeOH), 4-chloro-7-nitrobenzo-2-oxa-1,3-dizole (NBD-Cl), furfuryl alcohol (FFA), and sodium azide (NaN3)) on the mechanism of catalytic ozonation with manganese dioxide. The results show that rather than only quenching their target ROS, these quenchers can profoundly change the catalytic ozonation system through various mechanisms, e.g., interrupting the radical chain reaction of O3 decomposition, blocking the active sites of catalysts, and consuming O3 in the system. Due to the significant confounding effects of ROS quenchers on the reaction mechanism, the quenching method actually cannot reveal the role of ROS in pollutant abatement and often misinterpreted the catalytic ozonation mechanism. The results indicate that the commonly used quenching method is probably not an appropriate way to investigate the role of ROS in pollutant abatement during catalytic ozonation, and many previously reported mechanisms obtained with the quenching method may need a revisit.

Catalytic Ozonation of the Secondary Effluents from the Largest Chinese Petrochemical Wastewater Treatment Plant—A Stability Assessment

Effluents discharged from petrochemical facilities are complex and composed of various types of highly toxic contaminants, which necessitates the development of sustainable treatment technologies. Stability is among the most important sustainability criteria of the wastewater treatment processes. In the present manuscript, the standard-reaching rate (η) index was used to evaluate the stability of the catalytic ozonation process for treating the secondary effluent from the petrochemical industry. A pilot-scale device was designed and implemented for catalytic ozonation. The effluents were taken from the secondary sedimentation tank of a petrochemical wastewater treatment plant in China. A commercially available γ-Al2O3 was used as the catalyst after a pre-treatment heating step. The catalyst was characterized using scanning electron microscopy. Three mathematical statistics indexes, discrete coefficient (Vσ), skewness coefficient (Cso), and range coefficient (VR), were used to analyze the results achieved from the catalytic ozonation process. Continuous operation of the pilot-scale device was monitored for 9 months under an ozone concentration of 36 mg/L and the contact oxidation time of 1 h. The results demonstrated that the stability evaluation grades of chemical oxygen demand (COD) and suspended solids (SS) in the effluent of the catalytic ozonation system were both 3 and A, indicating that the process was relatively stable over a long period of application. The effluent COD compliance grade was also calculated as B, indicating that the effluent COD does not meet the standard and the process parameters need to be further optimized. When the reflux ratio is 150%, the removal rate of COD is the highest (38.2%) and the COD of effluent is 49.34 mg/L. Meanwhile, to enhance the efficiency and stability of the system, the ozone concentration and the two-stage aeration ratio are 40 mg/L and 4:1, respectively. Moreover, the presence of SS in the water of the catalytic ozonation system will result in the waste of ozone and reduce the utilization rate of ozone.

Insights into mechanism of catalytic ozonation of cinnamyl alcohol over core–shell Fe3O4@SiO2@La2O3 catalyst

In this study, a magnetic Fe3O4@SiO2@La2O3 nano-catalyst with a core–shell structure was synthesized to improve the efficiency of catalytic ozonation of cinnamyl alcohol. The structure, morphology, and elemental composition of the catalyst were analyzed with the help of catalyst characterization methods. The mechanism of strengthened degradation efficiency and mineralization of cinnamyl alcohol in catalytic ozonation system in the existence of the catalyst were further discussed according to the analysis results. The results showed that the Fe3O4@SiO2@La2O3 catalyst can not only greatly improve the degradation of cinnamyl alcohol, but also has good reusability and stability. And its economic efficiency is relatively good. The degradation rate of cinnamyl alcohol could reach up to 99.8% at 30 min, and the COD could be removed by 28.5% at 60 min with the help of the catalyst. The degradation efficiency of cinnamyl alcohol by ozone oxidation alone was only 52.1% at 30 min, and the COD removal rate was only 24.2% at 60 min in the same experimental conditions. Besides, the possible degradation pathway and degradation mechanism of cinnamyl alcohol were proposed according to the intermediate products detected by gas chromatography-mass spectrometry.

Magnetically separable NiFe2O4/sepiolite catalyst for enhanced ozonation treatment of quinoline and bio-treated coking wastewater in a catalytic ozonation system

In a quest to eliminate the bio-refractory organic pollutants in industrial wastewater, the NiFe2O4/SEP composites were synthesized via a sol-gel method and investigated by XRD, SEM, XPS, FTIR, VSM. The TOC removal increased from 16.82% to 69.35% in NiFe2O4/SEP+O3 system using quinoline as a target contaminant after 30 min, which is about 4.12 times as that in O3 system. The evolution of acetic acid, oxalic acid and formic acid in quinoline degradation was analyzed, saturated carboxylic acids were inertia to ozone molecules. However, 93.54% of oxalic acid was removed after 10 min, and 77.66% of acetic acid was degraded after 30 min in catalytic ozonation. The generation of small molecular organic acids could promote the Fe3+/Fe2+ redox cycle by complexing with Fe3+ on NiFe2O4/SEP, and then accelerated the formation of reactive oxygen species. In the presence of NiFe2O4/SEP, the high oxidation potential of •OH and •O2- contributed to an excellent TOC removal rate. Meanwhile, NiFe2O4/SEP was proved to be active for bio-treated coking wastewater in catalytic ozonation. The naturally fluorescent constituents in bio-treated coking wastewater were almost entirely removed by NiFe2O4/SEP+O3 system in 60 min. These findings indicated the good potential of NiFe2O4/SEP for practical applications in industrial wastewater treatment.

Enhanced degradation of dimethyl phthalate in wastewater via heterogeneous catalytic ozonation process: performances and mechanisms.

Ozonation process is a promising yet challenging method for the removal of refractory organic matter due to the sluggish reaction for generating hydroxyl radical (˙OH) at a neutral pH condition. Herein, an efficient heterogeneous catalytic ozonation system using CeO2/Al2O3 catalyst was developed to remove dimethyl phthalate (DMP) from wastewater. Under a neutral condition of pH = 6, this system achieved almost 100% DMP removal within 15 min at an optimized catalyst dosage of 30 g L-1 and the ozone flow rate of 22.5 mg min-1. Moreover, the catalytic ozonation system exhibited a stable degradation performance of DMP in a wider pH range (pH = 5-10). The results of electron paramagnetic resonance (EPR) and quantitative tests confirmed the ultrafast conversion of O3 to ˙OH (0.774 μM min-1) on the surface of CeO2 based ceramic catalyst. The quenching experiments further supported the predominant role of ˙OH in the mineralization of DMP. These results highlight the potential of using the heterogeneous catalytic ozonation system for the efficient removal of refractory organic matter from wastewater.

Enhanced Ozone Oxidation by a Novel Fe/Mn@γ−Al2O3 Nanocatalyst: The Role of Hydroxyl Radical and Singlet Oxygen

Catalytic ozonation is a potential alternative to address the dye wastewater effluent, and developing an effective catalyst for catalyzing ozone is desired. In this study, a novel Fe/Mn@γ−Al2O3 nanomaterial was prepared and successfully utilized for catalytic ozonation toward dye wastewater effluent components (dimethyl phthalate and 1−naphthol). The synthesized Fe/Mn@γ−Al2O3 exhibited superior activity in catalytic ozonation of dimethyl phthalate and 1−naphthol in contrast to Fe@γ−Al2O3 and Mn@γ−Al2O3. Quench and probe tests indicated that HO° contributed to almost all removal of dimethyl phthalate, whereas O3, HO°, and singlet oxygen participated in the degradation of 1−naphthol in the Fe/Mn@γ−Al2O3/O3 system. The results of XPS, FT−IR, and EPR suggested that HO° and singlet oxygen were generated from the valence variations of Fe(II/III)and Mn(III/IV). Moreover, the Fe/Mn@γ−Al2O3/O3 system could also have excellent efficacy in actual water samples, including dye wastewater effluent. This study presents an efficient ozone catalyst to purify dye wastewater effluent and deepens the comprehension of the role and formation of reactive species involved in the catalytic ozonation system.

Fischer–Tropsch synthetic wastewater treatment with Fe/Mn@CH: Catalytic ozonation and process evaluation

• Fe-Mn@CH with high catalytic activity was prepared. • The organic matter is transformed into protein-like and tryptophan-like substances. • Ozone catalytic oxidation process is closer to the secondary reaction kinetics. • A multi-level-gray relational evaluation model is established. In this study, the honeycomb ceramic catalyst Fe/Mn@CH with high catalytic activity was prepared. The effect of preparation conditions on the catalytic performance of Fe/Mn@CH was systematically studied. Scanning electron microscope; X-ray energy spectrum analysis; X-ray diffraction; specific surface area, pore distribution and adsorption performance analysis; X-ray fluorescence spectroscopy; and X-ray photoelectron spectroscopy were performed to analyze the structural characteristics of the catalyst. The optimal working conditions of the catalytic ozonation system for the degradation of Fischer–Tropsch synthesis wastewater were also studied. The metal elements Fe and Mn were successfully loaded on the surface of the honeycomb ceramic carrier in the form of α-MnO 2 and α-Fe 2 O 3 oxides. The optimal COD removal rate could reach 64.37% at pH of 9, ozone dosage of 6 g/h, Fe/Mn@CH dosage of 50 g/300 mL, and reaction time of 60 min. The specific mechanism of catalytic ozonation in the presence of tert -butanol was explored through ultraviolet absorption spectroscopy and three-dimensional fluorescence spectroscopy. The COD removal efficiency for Fischer–Tropsch synthesis wastewater was analyzed on the basis of catalytic oxidation reaction kinetics. Finally, a multilevel gray correlation evaluation model was established for the quantitative evaluation of the ozonation catalytic oxidation of Fischer–Tropsch wastewater.

Unraveling the multiple roles of Ag species incorporation into OMS-2 for efficient catalytic ozonation: Structural properties and mechanism investigation

Silver incorporated manganese oxide octahedral molecular sieves (Ag-OMSX-T) were successfully synthesized via a hydrothermal-calcination method. Due to the multiple roles of Ag species doping, the optimal Ag-OMS0.2-180 nanocatalyst with the Ag/Mn molar ratio of 0.2 had the highest TOF value (2.94 × 10−5 mol g−1 h−1), which could completely degrade 50 mg L−1 OA at pH 5.0 in 30 min much higher than those of OMS-2 (22.3%) in catalytic ozonation system. The results of systematic characterization (such as XRD, XPS, Py-IR, and O2-TPD) and catalytic performance tests of Ag-OMS0.2-180 clearly demonstrated that the proper amount of Ag incorporation not only facilitated the electron transfer but also induced more formation of Lewis acid sites in Ag-OMS0.2-180 as well as metals (Ag and Mn) as activation sites to collectively favor the catalytic ozonation degradation of OA. In addition, the higher mineralization rate has been achieved (79.6%) and the Ag-OMS0.2-180 exhibited the satisfactory stability and reusability with low metal release during multiple consecutive cycles (≥ 5). Meanwhile, the electron spin resonance spectra and radical scavenger tests confirmed that superoxide radical and singlet oxygen were the main reactive oxygen species in catalytic process. The mechanism of solid-liquid interface reaction and the pathway of cycling of oxygen vacancies were also elaborated, which present a new insight for water decontamination.

V2O5-(NH4)2V6O16·1.5H2O composite catalysts as novel platforms for high-efficiency catalytic ozonation of NO under low temperature

In this study, the catalytic ozonation of NO by V-based catalysts was investigated with the aim of reducing O3 consumption and pay attention to the role of V-based species in the catalytic ozonation system. Vanadium oxide treated with varying ammonium fluoride were synthesized and the catalytic ozonation of NO were systematically studied. The VF1 catalyst displayed the highest catalytic ozonation of NO efficiency, with which NO oxidation efficiency of 71% and O3 leakage of less than 1 ppm at the molar ratio of O3/NO = 0.5. The phase of catalysts revealed that VFx (x = 0, 0.25, 0.5, 1, 1.5) was transformed from V2O5 to (NH4)2V6O16·1.5H2O species, and the surface of catalyst became rougher as the amount of ammonium fluoride treatment gradually increased. Besides, chemical properties analysis of the catalyst revealed that (NH4)2V6O16·1.5H2O promoted the content of surface –OH and the absorption of H2O, which was conducive to the generation of ·OH and ·O2− radicals with O3 introduced. Furthermore, the effect of SO2 and reaction temperature was also investigated in this system, which exhibited that SO2 was favorable for NO2 removal and temperature had little effect on NO oxidation efficiency.

Ternary catalyst Mn-Fe-Ce/Al2O3 for the ozonation of phenol pollutant: performance and mechanism.

In this study, a novel ternary catalyst Mn-Fe-Ce/Al2O3 was synthesized by co-impregnation method, and was characterized by XRD, SEM, XPS, and FTIR. The catalytic performance of this ternary catalyst was evaluated in the heterogeneous catalytic ozonation of phenol pollutants and it improved the removal rate and mineralization degree of phenol pollutants. The changes of dissolved ozone in water and the TBA experiment proved that the ternary catalyst could accelerate the decomposition of ozone into hydroxyl radicals, thus accelerating the oxidation of phenol. Phosphate experiments and surface hydroxyl density measurements proved that surface hydroxyl was the active site of the catalyst. XPS analysis showed that the ternary catalysts accelerated electron transfer through the redox cycles of Mn2+-Mn3+-Mn4+, Fe2+-Fe3+, and Ce3+-Ce4+, which also contributed to the high catalytic activity. Moreover, the catalyst maintained high catalytic activity after five cycles of use. Therefore, the ternary catalyst was considered an efficient and promising catalyst for catalytic ozonation system.

MnFe2O4 decorated graphene as a heterogeneous catalyst for efficient degradation of di-n-butyl phthalate using catalytic ozonation in water

Magnetic, stable graphene-MnFe2O4 hybrids were synthesized by co-precipitation method for efficient degradation of di-n-butyl phthalate (DBP) using catalytic ozonation in water. The various synthesized catalysts were systematically characterized, and the catalytic activities in catalytic ozonation system were investigated. Moreover, a detailed mechanism of catalytic ozonation by synergistic function of graphene oxide (GO) and magnetic MnFe2O4 was proposed. Results showed that catalytic ozonation by 5-rGO-MFO-200 (MnFe2O4 calcined at 200 °C after 5 wt% GO doped) could significant boost the DBP degradation, which illustrated by the apparent rate constant (k = 0.0058 min−1) being nearly 4.2 times than that in ozone alone (k = 0.0241 min−1). The increased activities of 5-rGO-MFO-200 for the catalytic ozonation was attributed to its rich concentration of surface hydroxyl sites (SHSC), better charge electron transfer ability, favorable lattice structure and nanoparticles diffusion in contrast to bare MnFe2O4. All catalysts presented good recycling and stability in the repeated batch experiment, and the removal efficiency of DBP only decreased 3% after 5 times. The radical quenching tests verified that hydroxyl radicals (OH) was the dominant reactive oxygen species responsible for DBP degradation. Electrons cycling between Mn2+ and Mn3+ led to the decomposition of ozone to produce OH, which attacked DBP molecules to degrade into small molecular species and then further mineralized into CO2 and H2O. The synergistic function between magnetic MnFe2O4 and GO induced efficient ozone decomposition and more OH production.

Efficient catalytic activity and bromate minimization over lattice oxygen-rich MnOOH nanorods in catalytic ozonation of bromide-containing organic pollutants: Lattice oxygen-directed redox cycle and bromate reduction

The inhibition of bromate formation is a challenge for the application of ozonation in water treatment due to the carcinogenicity and nephrotoxicity of bromate. In this study, the high-mobility lattice oxygen-rich MnOOH nanorods were synthesized successfully and applied for the bromate inhibition during catalytic ozonation in bromide and organic pollutants-containing wastewater treatment. The catalytic ozonation system using lattice oxygen-rich MnOOH nanorods exhibited an excellent performance in bromate control with an inhibition efficiency of 54.1% compared with the sole ozonation process. Furthermore, with the coexistence of 4-nitrophenol, the catalytic ozonation process using lattice oxygen-rich MnOOH nanorods could inhibit the bromate formation and boost the degradation of 4-nitrophenol simultaneously. Based on the experiments of ozone decomposition, surface manganese inactivation and reactive oxygen species detection, the inhibition of bromate could be attributed to the effective decomposition of ozone with generating more ·O2- and the reduction of bromate into bromide by lattice oxygen-rich MnOOH. The existed surface Mn(IV) on lattice oxygen-rich MnOOH can accept electrons from lattice oxygen and ·O2- to generate surface transient Mn(II)/Mn(III), in which Mn(II)/Mn(III) can promote the reduction of bromate into bromide during catalytic ozonation. This study provides a promising strategy for the development of bromate-controlling technologies in water treatment.

Efficient degradation of organic pollutants by catalytic ozonation and photocatalysis synergy system using double-functional MgO/g-C3N4 catalyst

The catalytic ozonation and photocatalysis system, which was constructed by introducing ozone and using MgO/g-C3N4 as the catalyst, exhibited great degradation performance for phenol solution. The degradation efficiency was nearly 100% within 2 min, which was 18 and 1.5 times higher than individual photocatalytic and catalytic ozonation activity, and has not been reported in previous literature. In the catalytic ozonation and photocatalysis synergy system, MgO played a dual role: MgO could greatly promote the separation of photoinduced charges in photocatalysis due to the formation of C-O and Mg-N coordination bonds between g-C3N4 and MgO, as seen from the photocurrent intensity results. Simultaneously, MgO actively improved the utilization efficiency of ozone and accelerated the conversion of ozone into hydroxyl radicals (OH), which enhanced the catalytic ozonation activity. Thus, MgO was used as a bridge to realize the (O3/MgO/g-C3N4/Vis) synergy between catalytic ozonation and photocatalysis. The EPR results further confirmed this synergy, since the intensity of OH produced by catalytic ozonation and photocatalysis was greatly higher than the photocatalysis and catalytic ozonation progress. Meanwhile, the effects of the amount of ozone and catalyst, initial pH and concentration of the phenol solution on the synergy degradation performance were investigated. Furthermore, the mineralization capabilities of the synergy system for bisphenol A and 2–4 dichlorophenol and the cyclic stable performance were examined. Finally, the degradation mechanism of the synergy system was proposed. This work provided a new idea to design double-acting catalytic materials and developed a coupled oxidation technology to degrade organic pollutants.