Advanced and green ozonation process for removal of clofibric acid in water system: Preparation and mechanism analysis of efficient copper-substituted MCM-48

Performance of alkali and Cu-modified ZSM-5 during catalytic ozonation of polyvinyl alcohol in aqueous solution

A novel hierarchical Cu/ZSM-5 was prepared over alkaline treatment and incipient wet impregnation method for the catalytic ozonation of polyvinyl alcohol (PVA). Under the optimum preparation conditions, hierarchical Cu/ZSM-5 exhibited an excellent mineralization performance during the PVA degradation process, and the removal rate of TOC after 60 min of reaction was 47.86%, much higher than that of ozonation alone (5.40%). Its high catalytic activity could attribute to the large pore volume (0.27 cm³/g) and pore size (6.51 nm) which are beneficial for the distribution of loaded copper and adsorption performance for PVA. Compared to ·OH, ¹O₂ (2.66 times in 10 min) contributed more to the removal of PVA. The degradation of PVA was a combined process of direct ozone oxidation, catalytic ozonation and adsorption. With its high catalytic performance and stability, hierarchical Cu/ZSM-5 has a very broad application prospect in the process of catalytic ozonation of refractory pollutants.

Intensification of the photodegradation efficiency of an emergent water pollutant through process conditions optimization by means of response surface methodology

Heterogeneous photocatalysis has been increasingly investigated during the past years and has been recognized as a promising technique for clean and safe water purification. The current study exploits the advantage of this technique demonstrating that the removal of a biorefractory water pollutant named clofibric acid can be really improved by photocatalysis through a parametric comprehensive investigation and optimization study based on response surface methodology. Its novelty comes from the approach used to enhance the efficiency of the photocatalytic degradation of clofibric acid. A custom central composite design consisting of 49 trials was applied for process modeling and a quadratic robust model was derived based on the analysis of variance for the optimization of the process parameters. The effective removal of the target molecule with about 70% carbon mineralization was achieved under optimal photocatalytic conditions: 1.5 mg/L as the initial concentration of pollutant, 0.61 g/L catalyst, and an irradiation time of 190 min. Further, it was provided that nitrates play a positive role in the removal of this pollutant, while hydrogenocarbonates slow down its elimination. The ecotoxicity evaluation at different trophic levels confirmed the low toxicity of photodegradation by-products. Data analysis demonstrated that response surface methodology is a reliable approach for the optimization of the interactive effects of photocatalytic process parameters and is able to enhance their performance for the complete elimination of this hardly removed water pollutant.

A comprehensive review on metal based active sites and their interaction with O3 during heterogeneous catalytic ozonation process: Types, regulation and authentication

Heterogeneous catalytic ozonation (HCO) was a promising water purification technology. Designing novel metal-based catalysts and exploring their structural-activity relationship continued to be a hot topic in HCO. Herein, we reviewed the recent development of metal-based catalysts (including monometallic and polymetallic catalysts) in HCO. Regulation of metal based active sites (surface hydroxyl groups, Lewis acid sites, metal redox cycle and surface defect) and their key roles in activating O₃ were explored. Advantage and disadvantage of conventional characterization techniques on monitoring metal active sites were claimed. In situ electrochemical characterization and DFT simulation were recommended as supplement to reveal the metal active species. Though the ambiguous interfacial behaviors of O₃ at these active sites, the existence of interfacial electron migration was beyond doubt. The reported metal-based catalysts mainly served as electron donator for O₃, which resulted in the accumulation of oxidized metal and reduced their activity. Design of polymetallic catalysts could accelerate the interfacial electron migration, but they still faced with the dilemma of sluggish Me^(n+m)+/Meⁿ⁺ redox cycle. Alternative strategies like coupling active metal species with mesoporous silicon materials, regulating surface hydrophobic/hydrophilic properties, polaring surface electron distribution, coupling HCO process with photocatalysis and H₂O₂ were proposed for future research.

Efficient catalytic ozonation via Mn-loaded C-SiO2 Framework for advanced wastewater treatment: Reactive oxygen species evolution and catalytic mechanism

Heterogeneous catalytic ozonation (HCO) is attractive for water decontamination and catalyst is a core element. However, it is difficult to maintain high efficiency and stability of catalysts under stern conditions. In this study, we proposed Mn-loaded C-SiO₂-Framework (Mn-CSF) which contained stable silica core and robust carbon shell for efficient catalytic ozonation. The pseudo-first-order kinetic rate constant for oxalic acid removal of Mn-CSF catalytic ozonation was 160 % and 875 % higher than those of Mn-SiO₂ and pristine CSF, respectively. Mn-CSF was also proven effective in gasification wastewater treatment, where the COD was decreased to 46 mg·L^-1, 37 % lower than that of Mn-SiO₂. These results indicated that the graphitization carbon layer and Mn significantly enhanced the activity of the catalyst. Furthermore, a fulvic-like component and a protein-like component were recognized through 3D-EEM in coal gasification wastewater. It was proven that Mn-CSF catalytic ozonation exhibited higher fulvic-like component and protein-like component removal compared with ozonation. Moreover, O₂^- and ¹O₂ were identified to be responsible for organic degradation in this research. Sufficient external specific surface area and porous structure were important for complex wastewater treatment. Specifically, external specific surface area could enhance the degradation of macromolecular organics while porous structures were vital for smaller molecular pollutant removal. The results highlighted that Mn-CSF was a promising HCO catalyst for advanced wastewater treatment, and this study provided evidence of relationship between structure of catalysts and HCO efficiency.

ZnO-Zn2TiO4 heterostructure for highly efficient photocatalytic degradation of pharmaceuticals

In this study, ZnO-Zn₂TiO₄ (ZTM) material was prepared through a novel synthesis method based on a ultrasound-assisted polyol-mediated process followed by calcination at a different temperature. Physical features of the samples were studied by using various analysis techniques including XRD, FT-IR, SEM/EDX, pH_PZC, and UV-Vis DRS. Subsequently, the materials were employed as catalysts for the photocatalytic degradation of clofibric acid as a model pharmaceutical contaminant. The photocatalytic performance was evaluated under different conditions of calcination temperature, catalyst dosage, starting concentration, and initial pH of clofibric acid solution. The finding results revealed that hexagonal-tetragonal phases of ZnO-Zn₂TiO₄ calcined at 600 °C (ZTM-600) with an average crystallite size of 97.8 Å exhibited the best degradation efficiency (99%). The primary bands characteristic of ZnO and Zn₂TiO₄ were displayed by FT-IR analysis and the UV-visible DRS confirms the larger absorption capacity in UV-visible regions. The photogenerated electrons are the powerful reactive species involved in clofibric acid photodegradation process. This study shows a promising photocatalyst and provides new sight to rational design the facets of photocatalysis process for enhanced photocatalytic performances and effective wastewater treatment.

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.

Removal of phenol from wastewater by high-gravity intensified heterogeneous catalytic ozonation with activated carbon

In this study, the high-gravity technique is used to intensify the heterogeneous catalytic ozonation with activated carbon (AC) as the catalyst for removal of phenol from wastewater in a rotating packed bed (RPB), and the effects of high-gravity factor, inlet O₃ concentration, liquid-gas ratio, and initial pH on the degradation and mineralization of phenol at room temperature are investigated. It is revealed that the degradation rate of phenol reaches 100% at 10 min and the removal rate of total organic carbon (TOC) reaches 91% at 40 min under the conditions of high-gravity factor β = 40, inlet O₃ concentration = 90 mg·L^-1, liquid flow rate = 80 L·h^-1, and initial pH = 11. Compared with the bubbling reactor (BR)/O₃/AC and RPB/O₃ systems, the mineralization rate of phenol by the RPB/O₃/AC system is increased by 24.78% and 34.77%, respectively. Free radical quenching experiments are performed using tertiary butanol (TBA) and benzoquinone (BQ) as scavengers of ·OH and O₂^-, respectively. It is shown that the degradation and mineralization of phenol are attributed to the direct ozonation and the indirect oxidation by ·OH generated from the decomposition of O₃ adsorbed on AC surface, respectively. ·OH and O₂^·- are also detected by electron paramagnetic resonance (EPR). Thus, it is concluded that AC-catalyzed ozonation and high-gravity technique have a synergistic effect on ·OH initiation, which in turn can significantly improve the degradation and mineralization of organic wastewater.

Enhancing catalytic ozonation activity of MCM-41 via one-step incorporating fluorine and iron: The interfacial reaction induced by hydrophobic sites and Lewis acid sites

Fe-MCM-41 had been widely used as ozonation catalyst, however, the existence of large amount of hydrophilic silanol hindered its interfacial reaction with O₃ and pollutants. To solve this problem, F-Fe-MCM-41 was synthesized by co-doping F and Fe into the framework of MCM-41 to replace silanol with Si-F groups through a one-step hydrothermal method. F introduced hydrophobic sites which contributed to more ibuprofen (IBP) chemisorption on the surface of F-Fe-MCM-41. Moreover, doping F also enhanced the acidity, which accelerated O₃ decomposition into •OH. F-Fe-MCM-41/O₃ exhibited notably activity with 96.6% IBP removal efficiency within 120 min, while only 78.5% and 80.9% in O₃ alone and Fe-MCM-41/O₃, respectively. Surface Lewis acid sites and metal hydroxyl groups were considered as important factors for O₃ activation and •OH generation. F-Fe-MCM-41 exhibited excellent catalytic performance under acidic and alkaline conditions. Comparative experiments revealed that F doping improved the interfacial reaction, especially the interfacial electron transfer, which resulted in the high catalytic activity of F-Fe-MCM-41. F-Fe-MCM-41 possessed good stability and reusability, with only 5.7% decline for IBP removal in five successive cycles. Furthermore, the possible degradation path of IBP was proposed according to DFT calculation and GC-MS analysis.

Efficient catalytic ozonation of bisphenol A by three-dimensional mesoporous CeOx-loaded SBA-16

Herein, we demonstrated the construction of three-dimensional (3D) cerium oxide (CeO_x)/SBA-16 nanocomposites for efficient removal of bisphenol A (BPA) via a catalytic ozonation, with a high BPA mineralization up to 60.9% in 90 min. On one hand, the CeO_x/SBA-16 mesoporous structured materials presented large surface area and uniform pore distribution, which was conducive to the adsorption of transformation by-products (TBPs) and then, the mass transfer. On the other hand, CeO_x/SBA-16 could enhance the ozone utilization efficiency and meanwhile facilitate the formation of OH, the main reactive oxygen species. Through the exploration of dissoluble organic matters and the identification of the reaction intermediates, two BPA degradation pathways were proposed. This approach reported here will benefit the design and construction of mesoporous structured materials for catalytic elimination of hazards to remediate the environment.

Synthesis of Silanol-Rich MCM-48 with Mixed Surfactants and Their Application in Acetone Adsorption: Equilibrium, Kinetic, and Thermodynamic Studies

Mesoporous silica MCM-48 with rich silanol was prepared using polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) as mixed templates, and the dynamic adsorption performance of acetone was evaluated by testing breakthrough curves. The mixed micelle formed by CTAB and PVP, as well as the hydrogen bond between the carbonyl group of PVP and silanol group affected the condensation process of Si-OH group during the formation of mesoporous structure, resulting in the increase of Si-OH group number on the surface of MCM-48. Compared with MCM-48 synthesized by single template (CTAB), the acetone adsorption capacity of MCM-48 (1:3) synthesized by mixed templates (PVP:CTAB = 1:3) improved by 23.86%, which was attributed to the increase of silanol group amount and the decrease of pore size. In addition, Bangham model had the highest goodness of fit to describe the adsorption process among four kinetic models for the adsorbents, conforming to the mechanism of pore diffusion. The Langmuir and Freundlich models were used to fit the adsorption isotherm data, and the Freundlich model could better describe the adsorption of acetone. Freundlich model fitting results showed that MCM-48 with rich silanol had a strong affinity for acetone, and the adsorption of acetone on MCM-48 belonged to multilayer adsorption. The thermodynamic results showed that the adsorption of MCM-48 for acetone was physical adsorption, and the adsorption behavior was exothermic. This work provided insight into how the inherent properties of an adsorbent and environmental factors (including initial concentration and adsorption temperature) affected the adsorption performance of ketones, thus more ideas could be provided for the accurate design of adsorbents. Furthermore, silanol-rich MCM-48 synthesized by mixed templates is expected to be a promising adsorbent for acetone removal.

Catalytic ozonation of bio-treated coking wastewater in continuous pilot- and full-scale system: Efficiency, catalyst deactivation and in-situ regeneration

In this study, the performance of catalytic ozonation in the treatment of bio-treated coking wastewater (BCW) using pilot- and full-scale systems was investigated. Additionally, the removal efficiency of organic pollutants from BCW, the deactivation mechanism of Mn_xCe_1-xO₂/γ-Al₂O₃, and backflushing optimization for in-situ catalyst regeneration, which have not been previously investigated, were analysed. Results of the 12-month pilot scale experiments showed that catalytic ozonation resulted in the effective removal of organic pollutants when backflushing was applied as an in-situ catalyst regeneration strategy. The effluent chemical oxygen demand (COD) content decreased from 150 to 78 mg L^-1, and remained below a discharge limitation of 80 mg L^-1, and the stable COD removal efficiencies (from 56.0% to 47.9%) indicated that catalyst deactivation, which primarily resulted from the deposition of inorganic salts on the surface of the catalyst that limited interaction between ozone and active sites and/or prevented electrons transfer, was primarily inhibited by backflushing. The catalyst regeneration via in-situ air- and water-backflushing was attributed to the scrubbing, collision, and/or the loosing effect. Additionally, in the full-scale experiment, the catalytic ozonation process with in-situ alternative backflushing exhibited a stable COD removal efficiency (above 45.6%) for 885 days when water- and air-flushing strengths of 10 L m^-2 s^-1 and 15 L m^-2 s^-1, respectively, were applied with a 7-day regeneration interval. Therefore, the results of this study provide new insights into catalytic ozonation and support its engineering application in BCW treatment.

Treatment of Bio-Treated Coking Wastewater by Catalytic Ozonation with Semi-Batch and Continuous Flow Reactors

In this work, the treatment of bio-treated coking wastewater (BCW) by catalytic ozonation was conducted in semi-batch and continuous flow reactors. The kinetics of chemical oxygen demand (COD) removal were analyzed using BCWs from five coking plants. An integral reactor with catalytic ozonation stacked by ozone absorption (IR) was developed, and its efficiency was studied. The catalyst of MnxCe1-xO2/γ-Al2O3 was efficient in the catalytic ozonation process for the treatment of various BCWs. The chemical oxygen demand (COD) removal efficiencies after 120 min reaction were 64–74%. The overall apparent reaction rate constants were 0.0101–0.0117 min−1, which has no obvious relationship with the initial COD of BCW and pre-treatment biological process. The IR demonstrated the highest efficiency due to the enhancement of mass transfer and the utilization efficiency of ozone. Bypass internal circulation can further improve the reactor efficiency. The optimal results were obtained with the ozone absorption section accounting for 19% of the valid water depth in the reactor and 250% of circulation flow ratio. The long-term and full-scale application of the novel reactor in a continuous mode indicated stable removal of COD and polycyclic aromatic hydrocarbons (PAHs). The results showed that the system of IR is a promising reactor type for tertiary treatment of coking wastewater by catalytic ozonation.

Mechanism of Synergistic Effect on Electron Transfer over Co-Ce/MCM-48 during Ozonation of Pharmaceuticals in Water

The same amount of metal was deposited on the surface of three-dimensional mesoporous MCM-48 by a facile impregnation-calcination method for catalytic ozonation of pharmaceutical and personal-care products in the liquid phase. At 120 min reaction time, Co/MCM-48 and Ce/MCM-48 showed 46.6 and 63.8% mineralization for clofibric acid (CA) degradation, respectively. Less than 33% mineralization was achieved with Co/MCM-48 and Ce/MCM-48 during sulfamethazine (SMZ) ozonation. In the presence of monometallic oxides modified MCM-48 catalysts, total organic carbon (TOC) removal of diclofenac sodium (DCF) was around 80%. The composite Co-Ce/MCM-48 catalyst exhibited significantly higher activity in terms of TOC removal of CA (83.6%), SMZ (51.7%) and DCF (86.8%). Co-Ce/MCM-48 inhibited efficiently the accumulation of small molecular carboxyl acids during ozonation. A detailed research was conducted to detect the nature of material structure and mechanism of catalytic ozonation by using a series of characterizations. The main reaction pathway of CA was determined by the analysis of liquid chromatography-mass spectrometry, in line with the results of frontier electron density calculations that reactive oxygen species (ROSs) were easy to attack negative regions of pharmaceuticals. The Si-O-Si, Co···HO-Si-O-Si-OH···Ce, and O₃···Co-HO-Si-O-Si-OH···Ce-OH···O₃ basic units in catalysts were constructed to detect the orbit-energy-level difference. The results revealed that a synergistic effect existed at the interface between cobalt and cerium oxides over MCM-48, which facilitated the ROSs sequence in solution with ozone. Therefore, the multivalence redox coupling of Ce⁴⁺/Ce³⁺ and Co³⁺/Co²⁺ along with electron transfer played an important role in catalytic ozonation process.