Visible-light-driven MIL-53(Fe)/BiOCl composite assisted by persulfate: Photocatalytic performance and mechanism

Fabrication of OVs enriched BiOCl microflowers doped with Fe3+ for effective destruction of two typical contaminants

In this study, a one-step solvothermal method was used to fabricate Fe3+ doped BiOCl microflowers with abundant oxygen vacancies (OVs) in the presence of glacial acetic acid. Various analytical techniques were employed to characterize the structural, morphological, and optical properties of the prepared samples. The presence of OVs was confirmed by low temperature electron paramagnetic resonance (EPR) analysis. The photocatalytic results show that Fe3+ doped BiOCl photocatalysts have higher activity than the bare BiOCl, and 10% Fe3+/BiOCl exhibits the highest photocatalytic performance, the photocatalytic efficiency of this sample is 2.3 and 1.1 times higher than that of the blank BiOCl toward photocatalytic degradation of perfluorooctanoic acid (PFOA) and rhodamine B (RhB), respectively. Furthermore, Fe3+ doped BiOCl demonstrates excellent reusability. Based on the experimental observations, an enhancement mechanism for the photocatalytic activity of Fe3+ doped BiOCl was reasonably elucidated.

Characterization of an Iron-Copper Bimetallic Metal-Organic Framework for Adsorption of Methyl Orange in Aqueous Solution

Iron-based organic frame material MIL-53 (Fe) was synthesized by the hydrothermal method with Cu2+ incorporated to obtain bimetallic composite MIL-53 (Fe, Cu). The structure and morphology of the material were characterized by SEM, XRD, BET, FTIR, XPS, and zeta potential. The adsorption performance of MIL-53 (Fe, Cu) on methyl orange was tested under a variety of conditions, including the effects of pH and material dosage, by the static adsorption test. The results show that under the condition of pH = 7, a temperature of 30°C, and an adsorbent dosage of 20 mg, the removal rate of MIL-53 (Fe, Cu) for methyl orange can reach more than 96% within 4 h, and the maximum adsorption capacity after fitting by the thermodynamic model can reach 294.43 mg/g, showing the excellent adsorption performance of MIL-53 (Fe, Cu) on methyl orange. In addition, by exploring the adsorption mechanism of MIL-53 (Fe, Cu) on methyl orange, it is found that the adsorption of MIL-53 (Fe, Cu) on methyl orange depends on chemical adsorption, as evidenced by combining Fe3+ and Cu2+ in the material with methyl orange molecules to form complexes to achieve adsorption. While the specific surface area of the material had no obvious effect on adsorption, the effects of pH, temperature, and concentration were explored. At a pH of 6.5, greater adsorption occurred at higher temperatures, as determined by thermodynamic model fitting, as well as with higher initial methyl orange molecule concentration.

Degradation of ciprofloxacin by persulfate activated by Fe(III)-doped BiOCl composite photocatalyst

Fe-BOC-X photocatalyst was successfully prepared by solvothermal method. The photocatalytic activity of Fe-BOC-X was determined by ciprofloxacin (CIP), a typical fluoroquinolone antibiotic. Under sunlight irradiation, all Fe-BOC-X showed better CIP removal performance than original BiOCl. In comparison, the photocatalyst with iron content of 50 wt% (Fe-BOC-3) has excellent structural stability and the best adsorption photodegradation efficiency. The removal rate of CIP (10 mg/L) by Fe-BOC-3 (0.6 g/L) reached 81.4% within 90 min. At the same time, the effects of photocatalyst dosage, pH, persulfate, persulfate concentration, and combinations of different systems (PS, Fe-BOC-3, Vis/PS, Vis/Fe-BOC-3, Fe-BOC-3/PS, and Vis/Fe-BOC-3/PS) on the reaction were systematically discussed. In reactive species trapping experiments, electron spin resonance (ESR) signals revealed that the photogenerated holes (h+), hydroxyl radical (•OH), sulfate radical (•SO4-), and superoxide radical (•O2-) played an important role in CIP degradation; hydroxyl radicals (•OH) and sulfate radicals (•SO4-) play a major role. Various characterization methods have demonstrated that Fe-BOC-X has larger specific surface area and pore volume than original BiOCl. UV-vis DRS indicate that Fe-BOC-X has wider visible light absorption and faster photocarrier transfer and provides abundant surface oxygen absorption sites for effective molecular oxygen activation. Accordingly, a large number of active species were produced and participated in the photocatalytic process, thus effectively promoting the degradation of ciprofloxacin. Based on HPLC-MS analysis, two possible decomposition pathways of CIP were finally proposed. The main degradation pathways of CIP are mainly due to the high electron density of piperazine ring in CIP molecule, which is mainly attacked by various free radicals. The main reactions include piperazine ring opening, decarbonylation, decarboxylation, and fluorine substitution. This study can better open up a new way for the design of visible light driven photocatalyst and provide more ideas for the removal of CIP in water environment.

Construction of SrTiO3-BiOCl composite catalyst via facial microwave hydrothermal for highly efficient photocatalytic activity towards organic compounds degradation

This work mainly focuses on the preparation and performance study of SrTiO₃-BiOCl composite photocatalysis. The SrTiO₃-BiOCl photocatalysts are prepared via the facial microwave hydrothermal method. XRD, UV-vis DRS, SEM, TEM, XPS, N₂ adsorption and desorption isothermal experiment, FT-IR, and PL are applied to characterize the prepared samples. The spherical particles of SrTiO₃ grow on the flaky BiOCl, and the crystal size is uniform and evenly disperses on the BiOCl. The catalytic performance of the samples was evaluated over the degradation rates of methylene blue(MB). Typically, the clearance rates of MB reached to 99.65% over SrTiO₃-BiOCl-50% under visible light, which was much higher than that of SrTiO₃ and BiOCl (55.86%, 79.79%, respectively). The active species capturing experiments and ESR showed that the holes (h⁺) and ·OH are playing the main roles in the degradation process.

A hybrid nanocomposite based on CuFe layered double hydroxide coated graphene oxide for photocatalytic degradation of trimethoprim

Photocatalytic activation of persulfate (PS) has recently been considered an effective and environmentally friendly approach for antibiotic decomposition due to its high treatment efficiency, low energy consumption, and high reliability. The development of safe and high-performance catalysts is important for PS-based advanced oxidation processes. In this study, a CuFe-layered double hydroxide (LDH) coated graphene oxide (CuFe-LDH/GO) composite was constructed as a photocatalyst for trimethoprim (TMP) decomposition. The CuFe-LDH/GO catalyst was prepared via the co-precipitation method and characterized through Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and X-ray electron microscopy (XPS) techniques. Characterization results revealed that GO was entirely covered by LDH platelets which also kept its hydrotalcite structure in the as-prepared nanocomposite. The average crystallite size of CuFe-LDH/GO was 28.22 nm. The results confirmed that CuFe-LDH/GO exhibited excellent performance for the PS activation with a TMP removal efficiency of 90.8% under UV-light irradiation. Compared with pristine CuFe-LDH, the rate constant of TMP degradation of CuFe-LDH/GO was doubled. The results also indicated that acidic and alkaline conditions were not favorable for TMP degradation, and the catalytic activity of the used photocatalyst has not decreased significantly after 720 h of continuous recycling. Overall, CuFe-LDH/GO could be a promising photocatalyst for the treatment of wastewater containing antibiotics.

Solvothermal In-Situ Synthesis of MIL-53(Fe)@Carbon Felt Photocatalytic Membrane for Rhodamine B Degradation

In this study, MIL-53(Fe) was innovatively incorporated into carbon felt (CF) by growing in-situ using the solvothermal method. MIL-53(Fe)@carbon felt (MIL-53(Fe)@CF) was prepared and used for the degradation of rhodamine B (RhB). As a new photocatalytic membrane, MIL-53(Fe)@CF photocatalytic membrane has the characteristics of high degradation efficiency and recyclability. Influence of various parameters including MIL-53(Fe)@CF loading, light, electron trapper type, and starting pH on RhB degradation were investigated. The morphology, structure, and degradation properties of MIL-53(Fe)@CF photocatalytic membrane were characterized. Corresponding reaction mechanisms were explored. The results indicated that pH at 4.5 and 1 mmol/L H₂O₂, 150 mg MIL-53(Fe)@CF could photocatalytically degrade 1 mg/L RhB by 98.8% within 120 min, and the reaction rate constant (k) could reach 0.03635 min^-1. The clearance rate of RhB decreased by only 2.8% after three operations. MIL-53(Fe)@CF photocatalytic membrane was found to be stable.

Anion-Directed Self-Assembly of Calix[4]arene-Based Silver(I) Coordination Polymers and Photocatalytic Degradation of Organic Pollutants

Coordination polymers (CPs) have recently emerged as promising candidates for heterogeneous photocatalysis due to their structural designability and tunable properties. Herein, we developed two novel Ag(I)-calix[4]arene coordination polymers with the formula {[Ag₂(μ-NO₃)L¹]}_n (CP 1) and {[AgL¹]·PF₆}_n (CP 2) (L¹ = 2-mercapto-5-methyl-1,3,4-thiadiazole resorcinol calix[4]arene). Crystallography revealed that anion coordination and self-inclusion behavior induced the cavitand and silver ions to self-assemble into well-defined CPs 1 and 2 with different topological coordination frameworks, respectively. Furthermore, CPs 1 and 2 display high photocatalytic activity for the photodegradation of rhodamine B (RhB) and methyl orange (MO) in an aqueous solution under mild conditions (WLED and UV irradiation). The comparison results demonstrate that CP 1 exhibited better photocatalytic performance than CP 2, which correlated well with the differences in their molecular structure and HOMO-LUMO energy gaps. The photocatalysis products and possible intermediates were successfully monitored and determined using mass spectrum, gas chromatography, and electron paramagnetic resonance measurements. The rational photocatalysis mechanism was further investigated and proposed.

A dual purpose aluminum-based metal organic framework for the removal of chloramphenicol from wastewater

The presence of antibiotics in the aquatic environment can cause significant environmental and human health problems even at trace concentrations. Conventional treatment systems alone are ineffective in removing these resistant antibiotics. To address this problem, oxidation and adsorption techniques were used to explore the removal of recalcitrant antibiotic chloramphenicol (CAP). An aluminum-based metal-organic framework (Al-MIL) with high surface area and extended porosity, was prepared and used both as adsorbent and catalyst for the oxidation of CAP. Characterization of the Al-MIL revealed a large surface area of 1137 m² g^-1, a homogeneous microporous structure, good crystallinity, and particle size in the range of 200-400 nm. Adsorption of CAP on Al-MIL achieved equilibrium after 1 h, reaching a maximum adsorption capacity of 96.1 mg g^-1 at the optimum pH value of 5.3. The combination of adsorption and oxidation did not improve the % TOC reduction considerably, indicating an antagonistic rather than synergistic effect between the two processes. Oxidation alone in the presence of persulfate, achieved a % TOC reduction of 71% after 2 h, compared to 56% achieved by adsorption alone at the same duration. The optimum persulfate concentration was determined as 2.5 mM. The Al-MIL structure did not demonstrate any substantial deterioriation after six repeated runs, according to the reusability experiments.

Synthesis, modifications and applications of MILs Metal-organic frameworks for environmental remediation: The cutting-edge review

Ever-increasing anthropogenic activities are radically deteriorating the environment by causing severe pollution. Thus, curtailing the environmental pollution and promotion of sustainable development, are the hot issues confronted by scientists in this modern era. Metal-organic frameworks (MOFs) have been highly recognized as emerging promising materials for environmental remediation due to their versatile structure and extraordinary properties. Among them, MILs (MIL = Matérial Institute of Lavoisier) are the series of MOFs mostly known for their incredible stability, unique tailorable pore structures, and astounding versatile environmental applications. Their exclusive physiochemical properties and multifunctionality make them proficient for a wide range of pollutants removal in the exposure of versatile harsh environments, compared to other MOFs. This piece of research summarizes the state-of-the-art of development of MILs on the broad spectrum, highlighting their specificities, such as synthesis techniques, modifications and applications for environmental remediation. However, MILs wonderful properties and extraordinary applications in multiple fields, their deployment on practical and commercial-scale pollutants remediation is hindered by insufficient scientific research on underlying mechanisms and relationships. Henceforth, this review not only signifies the emerging importance of MILs for environmental applications but also indicates the urgency to maximize the scientific research for exploitation of MOFs on a practical level and promotion of green technologies for environmental remediation.

In situ synthesis of novel type Ⅱ BiOCl/CAU-17 2D/2D heterostructures with enhanced photocatalytic activity

A novel type Ⅱ BiOCl/CAU-17 2D/2D heterostructures photocatalyst was synthesized by in-situ growth of ultrathin BiOCl on the surface of CAU-17 nanorods through a solvothermal process. The 2D/2D heterostructures endow...

Highly efficient heterogeneous photo-Fenton BiOCl/MIL-100(Fe) nanoscaled hybrid catalysts prepared by green one-step coprecipitation for degradation of organic contaminants

An excellent heterojunction structure is vital for the improvement of photocatalytic performance. In this study, BiOCl/MIL-100(Fe) hybrid composites were prepared via a one-pot coprecipitation method for the first time. The prepared materials were characterized and then used as a photo-Fenton catalyst for the removal of organic pollutants in wastewater. The BiOCl/MIL-100(Fe) hybrid exhibited better photo-Fenton activity than MIL-100(Fe) and BiOCl for RhB degradation; in particular, the hybrid with 50% Bi molar concentration showed the highest efficiency. The excellent performance can be ascribed to the presence of coordinatively unsaturated iron centers, abundant Lewis acid sites, fast H₂O₂ activation, and efficient carrier separation on BiOCl nanosheets due to the high charge carrier mobility of the nanosheets. The photo-Fenton mechanism was studied, and the results indicated that ˙OH and h⁺ were the main active species for organic pollutant degradation. The coprecipitation-based hybridization approach presented in this paper opens up an avenue for the sustainable fabrication of photo-Fenton catalysts with abundant coordinatively unsaturated metal centers and efficient electron–hole separation capacity.

An excellent heterojunction structure is vital for the improvement of photocatalytic performance.

Preparation of Mn2O3/MIL-100(Fe) composite and its mechanism for enhancing the photocatalytic removal of rhodamine B in water

In this study, Mn₂O₃/MIL-100(Fe) composite was successfully synthesized by the hydrothermal method and applied for photocatalytic removal of rhodamine B (RhB) in water. The physical and chemical properties of the synthesized materials were characterized by XRD, FTIR, SEM, UV-visible, and BET analyses. Experimental results showed a great enhancement in the photocatalytic ability of the Mn₂O₃/MIL-100(Fe) composite as compared to individual Mn₂O₃ or MIL-100(Fe) under visible light and persulfate activation. The affecting factors such as pH, photocatalyst dose, RhB concentration, and Na₂S₂O₈ concentration were investigated to find out the best conditions for efficient photocatalysis. By conducting a radical quenching test, all radicals of HO˙, SO₄˙^-, ¹O₂, and O₂˙^- were found to be important in photocatalytic decomposition. The mechanism was proposed for the enhancement of photocatalytic RhB removal via band potential calculation, charge separation, surface redox reaction, and key reactive oxidation species. With its durability, reusability, and high efficiency, the Mn₂O₃/MIL-100(Fe) composite emerges as a potential photocatalyst working under visible light for application in wastewater treatment.

Uncertainty and misinterpretation over identification, quantification and transformation of reactive species generated in catalytic oxidation processes: A review

The identification of reactive radical species using quenching and electron paramagnetic resonance (EPR) tests has attracted extensive attention, but some mistakes or misinterpretations are often present in recent literature. This review aims to clarify the corresponding issues through surveying literature, including the uncertainty about the identity of radicals in the bulk solution or adsorbed on the catalyst surface in quenching tests, selection of proper scavengers, data explanation for incomplete inhibition, the inconsistent results between quenching and EPR tests (e.g., SO₄^•- is predominant in quenching test while the signal of ^•OH predominates in EPR test), and the incorrect identification of EPR signals (e.g., SO₄^•- is identified by indiscernible or incorrect signals). In addition, this review outlines the transformation of radicals for better tracing the origin of radicals. It is anticipated that this review can help in avoiding mistakes while investigating catalytic oxidative mechanism with quenching and EPR tests.

Insights into the Stability and Activity of MIL-53(Fe) in Solar Photocatalytic Oxidation Processes in Water

MIL-53(Fe) is a metal organic framework that has been recently considered a heterogeneous photocatalyst candidate for the degradation of water pollutants under visible or solar radiation, though stability studies are rather scarce in the literature. In this work, MIL-53(Fe) was successfully synthesized by a solvothermal method and fully characterized by X-ray diffraction (XRD), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), N2 adsorption–desorption isotherm, Thermogravimetric analysis coupled with mass spectrometry (TGA-MS), UV-visible diffuse reflectance spectroscopy (DRS), elemental analysis and wavelength dispersive X-ray fluorescence (WDXRF). The effects of pH, temperature, solar radiation and the presence of oxidants (i.e., electron acceptors) such as ozone, persulfate and hydrogen peroxide on the stability of MIL-53(Fe) in water were investigated. The as-synthetized MIL-53(Fe) exhibited relatively good stability in water at pH 4 but suffered fast hydrolysis at alkaline conditions. At pH 4–5, temperature, radiation (solar and visible radiation) and oxidants exerted negative effect on the stability of the metal–organic framework (MOF) in water, resulting in non-negligible amounts of metal (iron) and linker (terephthalic acid, H2BDC) leached out from MIL-53(Fe). The photocatalytic activity of MIL-53(Fe) under simulated solar radiation was studied using phenol and metoprolol as target pollutants. MIL-53(Fe) on its own removed less than 10% of the pollutants after 3 h of irradiation, while in the presence of ozone, persulfate or hydrogen peroxide, complete elimination of pollutants was achieved within 2 h of exposure to radiation. However, the presence of oxidants and the formation of some reaction intermediates (e.g., short-chain carboxylic acids) accelerated MIL-53(Fe) decarboxylation. The findings of this work suggest that MIL-53(Fe) should not be recommended as a heterogeneous photocatalyst for water treatment before carrying out a careful evaluation of its stability under actual reaction conditions.

Bifunctional Bi12O17Cl2/MIL-100(Fe) composites toward photocatalytic Cr(VI) sequestration and activation of persulfate for bisphenol A degradation

Bifunctional Bi₁₂O₁₇Cl₂/MIL-100(Fe) composite (BMx) was firstly constructed via facile ball-milling method. The optimal BM200 was highly efficient for Cr(VI) sequestration and activation of persulfate (PS) for bisphenol A (BPA) decomposition under white light illumination, which was much more remarkable than the pristine MIL-100(Fe) and Bi₁₂O₁₇Cl₂, respectively. Furthermore, the photocatalytic reduction efficiency can be significantly improved via the addition of some green small organic acids (SOAs). As well, the BPA degradation can be achieved over an extensive initial pH range of 3.0-11.0. When the PS concentration increased to more than 2.0 mM, the BPA degradation efficiency decreased due to the SO₄^-• self-scavenging effect. It was also found that the co-existence of inorganic anions like H₂PO₄^-, HCO₃^-, SO₄^2-, Cl^- and NO₃^- could decelerate the BPA degradation. The excellent photocatalytic Cr(VI) reduction and persulfate activation performances originated from both MIL-100(Fe) with excellent PS activation ability and Bi₁₂O₁₇Cl₂ with a favorable band position, which not only enabled the efficient separation of charges but also accelerated the formation of SO₄^-• radicals. The BM200 displayed prominent stability and recyclability. More importantly, the credible degradation pathway was proposed based on UHPLC-MS analysis and DFT calculation. This research revealed that the Fe-based MOFs/bismuth-rich bismuth oxyhalides (Bi_xO_yX_z, X = Cl, Br and I) composites possessed great potential in wastewater remediation.

Improved photocatalytic degradation of ketoprofen by Pt/MIL-125(Ti)/Ag with synergetic effect of Pt-MOF and MOF-Ag double interfaces: Mechanism and degradation pathway

It is a central issue to improve the separation efficiency of photogenerated charge carriers and the utilization of visible light in the field of photocatalysis. Herein, taking MIL-125(Ti) as a host material, the Pt/MIL-125(Ti) was first prepared by solvothermal method to build the interface of Schottky junction. Ag was then introduced onto the surface of Pt/MIL-125(Ti) to form the interface with the surface plasmon resonance effect. These double interfaces in the composite play a synergistic role on the photodagradation. The morphology, crystallinity and photochemical properties of the material were tested. By comparison, Pt/MIL-125(Ti)/Ag (4 wt% Ag) exhibited the best performance in the photodegradation of ketoprofen (KP, 10 mg/L) and the degradation process conformed to the pseudo-first-order kinetics. The photodegradation rate is 0.0253 min^-1, which was higher than MIL-125(Ti) (0.0009 min^-1). The TOC removal efficiency of KP reached approximately 51.5%. The electron paramagnetic resonance (EPR) and free radical capture tests verified that h⁺ and ·OH played the prominent roles during the reaction system. The degradation process, possible pathways and reaction mechanism were proposed. The design of the double interfaces between semiconductor and noble metals is a novel strategy to enhance the photocatalytic performance.

Enhanced Activation of Persulfate by Co-Doped Bismuth Ferrite Nanocomposites for Degradation of Levofloxacin Under Visible Light Irradiation

In this study, magnetic visible light driven photocatalysts (bismuth ferrite, Bi₂Fe₄O₉, BFO and Co-doped bismuth ferrite, Co-BFO) were successfully prepared by the facile hydrothermal method. The catalyst was used in the application of heterogeneous persulfate (PS) system under visible LED light irradiation for the degradation of levofloxacin (LFX), proving to be an excellent photocatalyst when evaluated by various characterization methods. The effect of Co-doping in the BFO structure was investigated that the decrease of band gap width and the generated photoelectrons and holes would effectively reduce the recombination of photogenerated electron-hole pairs, leading to the enhancement photocatalytic activity. The results demonstrated that Co-BFO catalyst had a high photodegradation efficiency over a wide pH range of 3.0-9.0 and the Co-BFO-2 composite displayed the optimal catalytic performance. It was found that the degradation rate of LFX by Co-BFO-2 catalyst was 3.52 times higher than that of pure BFO catalyst under visible light condition. The free radical trapping experiments and EPR tests demonstrated that superoxide, photogenerated holes and sulfate radicals were the main active species in the photocatalytic degradation of LFX. And a possible photocatalytic degradation mechanism of LFX was proposed in the Vis/Co-BFO/PS process. These findings provided new insight of the mechanism of heterogeneous activation of persulfate by Co-BFO under visible light irradiation.