Ni–Ti Layered Double Hydroxide@Graphitic Carbon Nitride Nanosheet: A Novel Nanocomposite with High and Ultrafast Sonophotocatalytic Performance for Degradation of Antibiotics

Copper-Based Bio-MOF/GO with Lewis Basic Sites for CO2 Fixation into Cyclic Carbonates and C-C Bond-Forming Reactions

Several measures, including crude oil recovery improvement and carbon dioxide (CO2) conversion into valuable chemicals, have been considered to decrease the greenhouse effect and ensure a sustainable low-carbon future. The Knoevenagel condensation and CO2 fixation have been introduced as two principal solutions to these challenges. In the present study for the first time, bio-metal-organic frameworks (MOF)(Cu)/graphene oxide (GO) nanocomposites have been used as catalytic agents for these two reactions. In view of the attendance of amine groups, biological MOFs with NH2 functional groups as Lewis base sites protruding on the channels' internal surface were used. The bio-MOF(Cu)/20%GO performs efficaciously in CO2 fixation, leading to more than 99.9% conversion with TON = 525 via a solvent-free reaction under a 1 bar CO2 atmosphere. It has been shown that these frameworks are highly catalytic due to the Lewis basic sites, i.e., NH2, pyrimidine, and C═O groups. Besides, the Lewis base active sites exert synergistic effects and render bio-MOF(Cu)/10%GO nanostructures as highly efficient catalysts, significantly accelerating Knoevenagel condensation reactions of aldehydes and malononitrile as substrates, thanks to the high TOF (1327 h-1) and acceptable reusability. Bio-MOFs can be stabilized in reactions using GO with oxygen-containing functional groups that contribute as efficient substitutes, leading to an expeditious reaction speed and facilitating substrate absorption.

Synergistic effect of ZnO/Ag2O@g-C3N4 based nanocomposites embedded in carrageenan matrix for dye degradation in water

This research achieved success by synthesizing innovative nanocomposite composed of zinc oxide (ZnO), graphitic carbon nitride (g-C3N4) and silver oxide (Ag2O) nanomaterials incorporated into a carrageenan matrix, thus creating an environmentally friendly and stable support structure. The synthesis process involved hydrothermal and chemical precipitation methods to create photocatalytic g-C3N4, ZnO, and Ag2O nanocomposites. The success is evident through the characterization results, which unveiled distinctive peaks corresponding to Zn-O (590-404 cm-1) and Ag-O (2072 cm-1) stretching in the Fourier transform infrared (FTIR) and X-ray diffraction (XRD) analyses, conclusively confirming the successful synthesis of g-C3N4, ZnO, Ag2O, and their respective nanocomposites. Further validation through a scanning electron microscope coupled with an energy dispersive spectrometer (SEM-EDX) and elemental mapping affirmed the presence of Zn, O, Ag, C, and N. Additionally, transmission electron microscope (TEM) imaging unveiled the nanosheet morphology of g-C3N4, the nanorod structure of ZnO, and the spherical form of Ag2O nanomaterials. ZnO and Ag2O nanomaterials demonstrated a consistent 10-20 nm size range. To underscore their ability to harness visible light, the nanomaterials were excited at 380 nm, emitting visible light emission within the 400-450 nm range. The synthesized nanocomposites showcased outstanding adsorption and photocatalytic properties, achieving efficiency ranging from 80 % to 98 %, attributed to the synergistic interactions between the various components. These findings culminate in a confirmation of the research's success, validating the exceptional potential of these nanocomposites for various applications.

Synthesis of magnetic layered double hydroxide (Fe3O4@CuCr-LDH) decorated with ZIF-8 for efficient sonocatalytic degradation of tetracycline

A series of Fe3O4@CuCr-LDH hybrids decorated with different amount of ZIF-8 (FLZ, 10-40 wt%) was prepared using simple methods and characterized with different techniques. The activity of the synthesized nanocomposites was investigated in the sonocatalytic degradation of tetracycline (TC) antibiotic from wastewater. When the content of ZIF-8 in the nanocomposite structure was 20 wt%, the FLZ-20 sonocatalyst exhibited the high performance in the sonocatalytic removal of TC. At optimum conditions (0.7 g/L catalyst dosage, pH of 7, 50 mg/L initial concentration of antibiotic, and 15 min sonication time) of the sonocatalytic removal of TC approached to 91.4% under ultrasonic irradiation (USI) using FLZ-20. This efficiency was much higher than those of obtained results by Fe3O4@CuCr-LDH and pristine ZIF-8. The formed ●OH and ●O2- exhibited the major roles in the sonocatalytic TC degradation process. The excellent performance of FLZ-20 can be attributed to the heterojunctions created between composite components, which could improve the electron transfer ability and effectively separate e-/h+ pairs. In addition, FLZ-20 showed the superior reusability and stability during three successive recycling. Moreover, the facile magnetically separation of the sonocatalyst from the aqueous solution was another outstanding feature, which prevents the formation of secondary pollutants. It can be concluded that the fabrication of heterojunctions is an efficient procedure to promote the sonocatalytic acting of the catalyst.

Advances in ultrasound-assisted synthesis of photocatalysts and sonophotocatalytic processes: A review

Water pollution and the global energy crisis are two significant challenges that the world is facing today. Ultrasound-assisted synthesis offers a simple, versatile, and green synthetic tool for nanostructured materials that are often unavailable by traditional synthesis. Furthermore, the integration of ultrasound and photocatalysis has recently received considerable interest due to its potential for environmental remediation as a low-cost, efficient, and environmentally friendly technique. The underlying principles and mechanisms of sonophotocatalysis, including enhanced mass transfer, improved catalyst-pollutant interaction, and reactive species production have been discussed. Various organic pollutants as dyes, pharmaceuticals, pesticides, and emerging organic pollutants are targeted based on their improved sonophotocatalytic degradation efficiency. Additionally, the important factors affecting sonophotocatalytic processes and the advantages and challenges associated with these processes are discussed. Overall, this review provides a comprehensive understanding of sono-assisted synthesis and photocatalytic degradation of organic pollutants and prospects for progress in this field.

Facile fabrication of ternary NiTiFe-LDH ultrathin nanosheets for efficient conversion of amines into imines under visible light

Ternary NiTiFe-LDH with an ultrathin nanosheet morphology was successfully fabricated via a facile co-precipitation method, followed by refluxing, and was used as a catalyst for oxidative coupling of amines to produce imines under visible light. The obvious superior activity observed in NiTiFe-LDH ultrathin nanosheets compared with binary NiTi-LDH and bulk NiTiFe-LDH can be ascribed to an enhanced light absorption capability caused by the introduction of Fe3+ ions as well as the ultrathin nanosheets which can minimize the recombination of the photogenerated charge carriers and provide more catalytically active sites for the reaction. As a result, more catalytically active O2˙- radicals are generated over NiTiFe-LDH ultrathin nanosheets, which leads to their superior activity. This study not only shows the possibility of using LDHs in photocatalytic organic transformations but also demonstrates an effective strategy to promote the activity of LDH-based photocatalysts via simultaneous composition and morphology modulation of LDHs.

Recent progress in mineralization of emerging contaminants by advanced oxidation process: A review

Emerging contaminants are chemicals generated due to the usage of pesticide, endocrine disrupting compounds, pharmaceuticals, and personal care products and are liberated into the environment in trace quantities. The emerging contaminants eventually become a greater menace to living beings owing to their wide range and inhibitory action. To diminish these emerging contaminants from the environment, an Advanced Oxidation Process was considered as an efficient option. The Advanced Oxidation Process is an efficient method for mineralizing fractional or generous contaminants due to the generation of reactive species. The primary aim of this review paper is to provide a thorough knowledge on different Advanced Oxidation Process methods and to assess their mineralization efficacy of emerging contaminants. This study indicates the need for an integrated process for enhancing the treatment efficiency and overcoming the drawbacks of the individual Advanced Oxidation Process. Further, its application concerning technical and economic aspects is reviewed. Until now, most of the studies have been based on lab or pilot scale and do not represent the actual scenario of the emerging contaminant mineralization. Thus, the scaling up of the process was discussed, and the major challenges in large scale implementation were pointed out.

Recent Advances in LDH/g-C3N4 Heterojunction Photocatalysts for Organic Pollutant Removal

Environmental pollution has been decreased by using photocatalytic technology in conjunction with solar energy. An efficient method to obtain highly efficient photocatalysts is to build heterojunction photocatalysts by combining graphitic carbon nitride (g-C3N4) with layered double hydroxides (LDHs). In this review, recent developments in LDH/g-C3N4 heterojunctions and their applications for organic pollutant removal are systematically exhibited. The advantages of LDH/g-C3N4 heterojunction are first summarized to provide some overall understanding of them. Then, a variety of approaches to successfully assembling LDH and g-C3N4 are simply illustrated. Last but not least, certain unmet research needs for the LDH/g-C3N4 heterojunction are suggested. This review can provide some new insights for the development of high-performance LDH/g-C3N4 heterojunction photocatalysts. It is indisputable that the LDH/g-C3N4 heterojunctions can serve as high-performance photocatalysts to make new progress in organic pollutant removal.

Bifunctional quaternary magnetic composite as efficient heterojunctions photocatalyst for simultaneous photocatalytic visible light degradation of dye and herbicide pollutants from water and bacterial disinfection

In the present study, the magnetic Fe3O4/Ag2C2O4/Ag3PO4/Ag nanocomposite were prepared through a simple co-precipitation method by using calendula officinalis seed extract as a stabilizer. The fabricated quaternary photocatalyst was applied for to degrade food dye Brilliant Blue FCF (BB) and herbicide Paraquat (PQ) as contaminants at binary mixture in a batch and continuous flow-loop photoreactor under visible light irradiation and also the antibacterial properties was investigated. The fabricated nanocomposite was determined by XRD, FESEM, EDX, BET&BJH, UV-DRS, FT-IR and VSM methods to gain insight about structure, morphology, purity, surface area, optical, functional group and magnetic properties. The photoelectrochemical experiments, PL and DRS indicate the successful coupling of the active semiconductors. The degradation efficiency of BB and PQ was announced to be 88.9% and 92.72% under optimal conditions with a high reaction rate constant value (0.03 and 0.0326 min-1), respectively. The quaternary photocatalyst exhibited superior photocatalytic performance compared with Ag3PO4/Ag2C2O4 and Ag2C2O4. Various scavengers were used to explore the mechanism of photocatalytic performance and supports that [Formula: see text] and OH. is main active species in the degradation process of BB and PQ, respectively. Furthermore, the Fe3O4/Ag2C2O4/Ag3PO4/Ag also demonstrated bactericidal activity against Staphylococcus aureus (S. aureus) as gram-positive bacteria and Escherichia coli (E. coli) as gram-negative bacteria.

Highly efficient ultrasound-driven Cu-MOF/ZnWO4 heterostructure: An efficient visible-light photocatalyst with robust stability for complete degradation of tetracycline

Metal-organic frameworks (MOFs) are a significant class of porous, crystalline materials composed of metal ions (clusters) and organic ligands. The potential use of copper MOF (Cu-BTC) for the sonophotocatalytic degradation of Tetracycline (TC) antibiotic was investigated in this study. To enhance its catalytic efficiency, S-scheme heterojunction was created by combining Cu-BTC with Zinc tungstate (ZnWO4), employing an ultrasound-assisted hydrothermal method. The results demonstrated that the Cu-BTC/ZnWO4 heterojunction exhibited complete removal of TC within 60 min under simultaneous irradiation of visible light and ultrasound. Interestingly, the sonophotocatalytic degradation of TC using the Cu-BTC/ZnWO4 heterojunction showed superior efficiency (with a synergy index of ∼0.70) compared to individual sonocatalytic and photocatalytic degradation processes using the same heterojunction. This enhancement in sonophotocatalytic activity can be attributed to the formation of an S-scheme heterojunction between Cu-BTC and ZnWO4. Within this heterojunction, electrons migrated from Cu-BTC to ZnWO4, facilitated by the interface between the two materials. Under visible light irradiation, the built-in electric field, band edge bending, and coulomb interaction synergistically inhibited the recombination of electron-hole pairs. Consequently, the accumulated electrons in Cu-BTC and holes in ZnWO4 actively participated in the redox reactions, generating free radicals that effectively attacked the TC molecules. This study offers valuable perspectives on the application of a newly developed S-scheme heterojunction photocatalyst, demonstrating its effectiveness in efficiently eliminating diverse recalcitrant pollutants via sonophotocatalytic degradation.

A co-precipitation route for the preparation of eco-friendly Cu-Al-layered double hydroxides with efficient tetracycline degradation

The construction of novel efficient catalysts for the treatment of organic pollutants in the aqueous environment is essential. The lamellar-like Cu-Al layered double hydroxides (CuAl-LDHs) with various mole ratios were synthesized by a simple route of co-precipitation, and the corresponding degradation characteristic was tested for the removal of tetracycline (TC) using PMS activation. The degradation efficiency of TC over CuAl-LDHs increased up to 93% within 10 min for the Cu/Al mole ratio of 3:1 and almost not changed at a higher mole ratio. For further calcining the optimal catalyst at 300 ℃, the degradation efficiency of TC was found to be increased to 96%. Sulfuric radicals and singlet oxygen were analyzed to be the main reason for the change in degradation characteristics, which was proved by radical quenching experiments and electron paramagnetic resonance technique. The parameters including PMS concentration, catalyst dosage, and reaction temperature on the TC degradation as well as the degradation mechanism for PMS activation were elaborated. The best proportion of CuAl-LDHs owned splendid stability and catalytic activity after reusing, which showed enormous potential in practical application.

Photocatalytic transfer of aqueous nitrogen into ammonia using nickel-titanium-layered double hydroxide

The development of solar-driven transfer of atmospheric nitrogen into ammonia is one of the green and sustainable strategies in industrial ammonia production. Nickel-titanium-layered double hydroxide (NiTi-LDH) was synthesised using the soft-chemical process for atmospheric nitrogen fixation application under photocatalysis in an aqueous system. NiTi-LDH was investigated using advanced characterisation techniques and confirmed the potential oxygen vacancies and/or surface defects owing to better photocatalytic activity under the solar spectrum. It also exhibited a bandgap of 2.8 eV that revealed its promising visible-light catalytic activities. A maximum of 33.52 µmol L-1 aqueous NH3 was obtained by continuous nitrogen (99.9% purity) supply into the photoreactor under an LED light source. Atmospheric nitrogen supply (≈78%) yielded 14.67 µmol L-1 aqueous NH3 within 60 min but gradually reduced to 3.6 µmol L-1 at 330 min. Interestingly, in weak acidic pH, 20.90 µmol L-1 NH3 was produced compared to 11.51 µmol L-1 NH3 in basic pH. The application of NiTi-LDH for visible-light harvesting capability and photoreduction of atmospheric N2 into NH3 thereby opens a new horizon of eco-friendly NH3 production using natural sunlight as alternative driving energy.

An overview on ZnO-based sonophotocatalytic mitigation of aqueous phase pollutants

Over the past several decades, the increase in industrialization provoked the discharge of harmful pollutants into the environment, affecting human beings and ecosystems. ZnO-based photocatalysts seem to be the most promising photocatalysts for treating harmful pollutants. However, fast charge carrier recombination, photo corrosion, and long reaction time are the significant factors that reduce the photoactivity of ZnO-based photocatalysts. In order to enhance the photoactivity of such photocatalysts, a combined process i.e., sonocatalysis + photocatalysis = sonophotocatalysis was used. Sonophotocatalysis is one of several different AOP methods that have recently drawn considerable interest, as it produces high reactive oxygen species (ROS) which helps in the oxidation of pollutants by acoustic cavitation. This combined technique enhanced the overall efficiency of the individual method by overcoming its limiting factors. The current review aims to present the theoretical and fundamental aspects of sonocatalysis and photocatalysis along with a detailed discussion on the benefits that can be obtained by the combined process i.e., US + UV (sonophotocatalysis). Also, we have provided a comparison of the excellent performance of ZnO to that of the other metal oxides. The purpose of this study is to discuss the literature concerning the potential applications of ZnO-based sonophotocatalysts for the degradation of pollutants i.e., dyes, antibiotics, pesticides, phenols, etc. That are carried out for future developments. The role of the produced ROS under light and ultrasound stimulation and the degradation mechanisms that are based on published literature are also discussed. In the end, future perspectives are suggested, that are helpful in the development of the sonophotocatalysis process for the remediation of wastewater containing various pollutants.

Indulgent of the physiochemical features of MgCr-LDH nanosheets towards photodegradation process of methylene blue

In the present work, we report the preparatory strategy of MgCr-layered double hydroxide (LDH) nanosheets with 90% degree of delamination by employing a formamide-assisted co-precipitation and mild hydrothermal route for the degradation of methylene blue (MB) under solar light exposure. The as-synthesized MgCr-LDH nanosheets were characterized by assorted characterization techniques such as powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Raman, thermogravimetric analysis (TGA), N₂ adsorption-desorption measurement, X-ray photoelectron spectroscopy (XPS) and UV-Visible diffused reflectance spectroscopy (UV-DRS). The XRD pattern of MgCr-LDH nanosheets quantified the strain (ε) and dislocation density (δ) of 1.371 lines^-2 m^-4 and 0.5723 lines m^-2 related to the (110) plane with d-spacing value of 1.6169 Ȧ. With a minimum band gap of ∼2.63 eV, the as-synthesized MgCr-LDH nanosheets displayed 90.6% MB photodegradation under the experimental protocols such as catalyst dosage of 30 mg/L, initial MB concentrations of 20 ppm, pH of 7 and time duration of 2 h under solar light exposure. Further, the recyclability test of the photocatalyst signifies material stability up to four successive cycles with 90% retention of MB degradation under sunlight exposure. The superior catalytic performances of the MgCr-LDH nanosheets could be ascertained to the suppression of excitonic recombination and effective light harvestation properties, synergistically contributed by the porous structural aspects via association of uni/multi-lamellar nanosheets, surface defect sites and photoactive Cr³⁺ cations. Additionally, the surface -OH groups of LDH contributed towards the generation of ^•OH radicals for triggering the catalytic performances. This type of work advances the novel ideas for establishing highly potent photocatalysts via synergizing structural and surface properties, paving towards effective wastewater treatment.

Degradation of tetracycline antibiotic utilizing light driven-activated oxone in the presence of g-C3N4/ZnFe LDH binary heterojunction nanocomposite

In the present study, a binary heterojunction nanocomposite composed of graphitic carbon nitride (g-C₃N₄) and Zn/Fe-contained layered double hydroxide (ZnFe LDH) was employed as heterogeneous catalyst for the decomposition of tetracycline (TC) antibiotic utilizing Oxone and UV light irradiation. The sole use of g-C₃N₄/ZnFe LDH as adsorbent led to the negligible elimination of TC. In addition, the sole use of Oxone or UV (photolysis) and even their combination were not effective enough to degrade the target pollutant, while the combined process of g-C₃N₄/ZnFe LDH/Oxone/photolysis revealed significantly enhanced (synergistic) degradation of TC (92.4% within 30 min). Indirect detection tests for the identification of free radical species indicated the major role of both hydroxyl (^•OH) and sulfate (SO₄^•-) radicals in the degradation of TC by the g-C₃N₄/ZnFe LDH/Oxone/photolysis system. The elimination of TC followed a pseudo-first order kinetic model. The complete degradation of TC (degradation efficiency of 100%) was achieved within the reaction time of 25 min when ultrasound (US) was applied as enhancing agent. Furthermore, the results of total organic carbon (TOC) analysis were used to exhibit progress in the mineralization of the pollutant. The bioassay results indicated the decreased toxicity of the process effluent toward microbial population of Escherichia coli after the treatment.

Preparation of P-doped CdS nanorods as an efficient photocatalyst for the degradation of the emerging pollutant tetracycline antibiotic under blue LED light irradiation

Excessive drug usage and sewage discharges containing antibiotics have caused water contamination due to the rapid growth of pharmaceutical industries. Tetracycline (TC) is one of the most frequently applied antibiotics having a significant impact on the aquatic environment, water quality and human health and thus effective approaches for TC removal from water are urgently needed. Here, we have fabricated P-doped CdS (CdS-P_0.8) nanorods (NRs) by one-step thermal phosphorization treatment for TC degradation through photocatalytic reaction in the presence of blue and white LED light irradiation. Synthesized photocatalysts were characterized to authenticate the incorporation of P atoms on the CdS NR surface using XPS, XRD, ICP-OES and EDX mapping analyses. CdS-P_0.8 NRs have greater photocatalytic activity for tetracycline degradation under blue LED light irradiation. TC degradation on CdS-P_0.8 NRs followed pseudo-first order kinetics for both LED light sources. In the presence of blue LED light at an intensity of 10 mW cm^-2, TC degradation efficiency and pseudo-first order rate constants of CdS-P_0.8 NRs for the photocatalytic degradation reaction reached 95.4% and 0.13396 min^-1 in 20 minutes without any supplemental oxygen sources. Scavenging experiments demonstrate that reactive oxygen species are produced during the photocatalytic degradation of tetracycline. As a result, due to the extensive utilization of photogenerated oxidative species such as h⁺, O₂˙^- and OH˙, CdS-P_0.8 NRs demonstrated high photocatalytic tetracycline degradation efficiency in 20 minutes. Our findings shed more light on nonmetal P doping on CdS materials and other semiconductors, exploring new possibilities for photocatalytic degradation to efficiently reduce the amount and toxicity of TC antibiotics in wastewater.

Complete removal of Tetracycline by sonophotocatalysis using ultrasound-assisted hierarchical graphitic carbon nitride nanorods with carbon vacancies

Tuning a graphitic carbon nitride (CN) structure is an effective strategy to advance its physicochemical and electronic properties. Herein, hierarchical CN nanorods with carbon vacancy were synthesized via ultrasound-assisted thermal polycondensation method wherein melamine-HONH₂·HCl complex acts as a template. The hierarchical CN nanorods can facilitate multiple light scattering, provide large specific surface area with extensive reactive sites and endow abundant mass-transport channels for charge migration. The existence of carbon vacancies can serve as shallow charge trapping sites and prompt charge separation. Consequently, hierarchical CN nanorod possessed excellent sonophotodegradation efficiency of ∼100% towards Tetracycline (TC) antibiotic within 60 min under ultrasonic irradiation and visible light illumination. Moreover, the sonophotocatalytic degradation was higher than the sum of sonocatalytic and photocatalytic TC degradation using hierarchical CN nanorods due to its synergistic performance. A plausible sonophotocatalytic mechanism and TC degradation pathway using hierarchical CN nanorod were proposed. Lastly, hierarchical CN nanorod is durable and stable which can withstand the sonophotocatalytic condition even after the fifth run. This work offers an insight into hierarchical CN nanorod to advance sonophotocatalytic degradation performance for highly efficient removal of various recalcitrant pollutants.

Effect of cobalt doping on photocatalytic water splitting activity of NiTi-layered double hydroxide

Cobalt-doping a NiTi layered double hydroxide affords CoNiTi-LDH with smaller plate sizes and a higher degree of order, which allows the band gap to shrink from 2.7 eV to 2.4 eV. CoNiTi-LDH is active for both water oxidation and reduction.

Zinc-chromium layered double hydroxides anchored on carbon nanotube and biochar for ultrasound-assisted photocatalysis of rifampicin

In this study, ZnCr layered double hydroxide (LDH), ZnCr LDH/carbon nanotube (CNT), and ZnCr LDH/Biochar (BC) were synthesized and characterized by various analyses. The successful synthesis and the great crystallinity of the samples were consented by XRD analysis. SEM and TEM were applied to study the morphology of the synthesized samples. The simultaneous presence of C, Zn, and Cr elements was well confirmed by EDX and dot mapping analyses demonstrating the successful preparation of nanocomposites. According to the BET analysis, ZnCr LDH nanocomposites with BC and CNT had more specific surface area compared to ZnCr LDH alone. The catalytic performances of the samples were determined for the degradation of rifampicin (RF). The degradation efficiency of the sonophotocatalytic process in the presence of 0.6 g L^-1 of ZnCr LDH/BC toward 15 mg L^-1 of RF under 150 W ultrasound and visible light irradiation was found to be about 100% within 40 min. The influence of the reactive species on the sonophotocatalytic process was assessed via the addition of different scavengers (para-benzoquinone (p-BQ), formic acid (FA), isopropyl alcohol (IPA)), and enhancers (hydrogen peroxide and potassium persulfate). The GC-MS analysis was carried out and eleven by-products during the RF decomposition were detected.

Hollow microspherical Bi2MoO6/Zn–Ti layered double hydroxide heterojunction for efficient visible-light photocatalytic degradation of organic contaminants

The construction of a Bi2MoO6/Zn–Ti layered double hydroxide heterojunction can effectively enhance the photocatalytic activity owing to efficient charge separation.

Recent Progress and Prospects in Catalytic Water Treatment

Presently, conventional technologies in water treatment are not efficient enough to completely mineralize refractory water contaminants. In this context, the implementation of catalytic processes could be an alternative. Despite the advantages provided in terms of kinetics of transformation, selectivity, and energy saving, numerous attempts have not yet led to implementation at an industrial scale. This review examines investigations at different scales for which controversies and limitations must be solved to bridge the gap between fundamentals and practical developments. Particular attention has been paid to the development of solar-driven catalytic technologies and some other emerging processes, such as microwave assisted catalysis, plasma-catalytic processes, or biocatalytic remediation, taking into account their specific advantages and the drawbacks. Challenges for which a better understanding related to the complexity of the systems and the coexistence of various solid-liquid-gas interfaces have been identified.

Utilization of layered double hydroxides (LDHs) and their derivatives as photocatalysts for degradation of organic pollutants

Direct or indirect discharge of wastes containing organic pollutants have contributed to the environmental pollution globally. Decontamination of highly polluted natural resources such as water using an effective treatment is a great challenge for public health and environmental protection. Photodegradation of organic pollutants using efficient photocatalyst has attracted extensive interest due to their stability, effectiveness towards degradation efficiency, energy, and cost efficiency. Among various photocatalysts, layered double hydroxides (LDHs) and their derivatives have shown great potential towards photodegradation of organic pollutants. Herein, we review the mechanism, key factors, and performance of LDHs and their derivatives for the photodegradation of organic pollutants. LDH-based photocatalysts are classified into three different categories namely unmodified LDHs, modified LDHs, and calcined LDHs. Each LDH category is reviewed separately in terms of their photodegradation efficiency and kinetics of degradation. In addition, the effect of photocatalyst dose, pH, and initial concentration of pollutant as well as photocatalytic mechanisms are also summarized. Lastly, the stability and reusability of different photocatalysts are discussed. Challenges related to modeling the LDHs and its derivatives are addressed in order to improve their functional capacity.

Construction of a polymeric cobalt phthalocyanine@mesoporous graphitic carbon nitride composite for efficient photocatalytic CO2 reduction

Herein, we prepare a type of organic/inorganic hybrid photocatalyst by in situ polymerization of cobalt phthalocyanine on mesoporous g-C3N4 (mpg-C3N4) for photocatalytic CO2 reduction. Photocatalytic results indicate that this photocatalyst can effectively reduce CO2 to CO with high activity and remarkable long-term stability in organic solvents under visible-light irradiation. Our work provides a versatile strategy to synthesize a highly active organic/inorganic composite for photocatalysis.

Instantaneous Sonophotocatalytic Degradation of Tetracycline over NU-1000@ZnIn2S4 Core-Shell Nanorods as a Robust and Eco-friendly Catalyst

The universal pollution of diverse water bodies and declined water quality represent very important environmental problems. The development of new and efficient photocatalytic water treatment systems based on the Z-scheme mechanisms can contribute to tackling such problems. This study reports the preparation, full characterization, and detailed sonophotocatalytic activity of a new series of hybrid NU@ZIS nanocomposites, which comprise a p-n heterojunction of 3D Zr(IV) metal-organic framework nanorods (NU-1000) and photoactive ZnIn₂S₄ (ZIS) nanostars. Among the obtained materials with varying content of ZIS (5, 10, 20, and 30%) on the surface of NU-1000, the NU@ZIS20 nanocomposite revealed an ultrahigh catalytic performance and recyclability in a quick visible-light-induced degradation of the tetracycline antibiotic in water under sonophotocatalytic conditions. Moreover, increased activity of NU@ZIS20 can be ascribed to the formation of a p-n heterojunction between NU-1000 and ZIS, and a synergistic effect of these components, leading to a high level of radical production, facilitating a Z-scheme charge carrier transfer and reducing the recombination of charge carriers. The radical trapping tests revealed that ^•OH, ^•O₂^-, and h⁺ are the major active species in the sonophotocatalytic degradation of tetracycline. Possible mechanism and mineralization pathways were introduced. Cytotoxicity of NU@ZIS20 and aquatic toxicity of water samples after tetracycline degradation were also assessed, showing good biocompatibility of the catalyst and efficacy of sonophotocatalytic protocols to produce water that does not affect the growth of bacteria. Finally, the obtained nanocomposites and developed photocatalytic processes can represent an interesting approach toward diverse environmental applications in water remediation and the elimination of other types of organic pollutants.

Application of advanced materials in sonophotocatalytic processes for the remediation of environmental pollutants

Significant advances in various industrial processes have resulted in the discharge of toxic pollutants into the environment. Consequently, it is essential to develop efficient wastewater treatment processes to reduce water contamination and increase recycling/reuse. Photocatalytic degradation is considered as an efficient method for the degradation of toxic pollutants in industrial wastewater. However, the use of photocatalytic approaches is associated with numerous limitations, such as lengthy procedures and the necessity for large amounts of catalysts. Hence, it has been proposed that photocatalysis could be combined with other techniques, including sonolysis, electrochemical, photothermal, microwave, ultrafiltration, and biological reactor. The integration of photocatalysis with sonolysis could be remarkably beneficial for environmental remediation. The combination of these processes has the advantages of using uniformly dispersed catalysts, regeneration of the catalyst surface, improved mass transfer, enhanced surface area due to smaller catalyst particles, and production of more active radicals for the degradation of organic pollutants. In this review, an overview on employing sonophotocatalysis for the removal of toxic organic contaminants from aqueous environments is provided. Additionally, the limitations of photocatalysis alone and the fundamental sonophotocatalytic mechanistic pathways are discussed. The importance of utilizing advanced two-dimensional (2D) semiconductor materials in sonophotocatalysis and the common synthetic approaches for the preparation of 2D materials are also highlighted. Lastly, the review provides comprehensive insights into different materials based on metal oxides, chalcogenides, graphene, and metal organic frameworks (MOFs), which are involved in sonophotocatalytic processes employed for the remediation of environmental pollutants.

Layered double hydroxides and related hybrid materials for removal of pharmaceutical pollutants from water

Pharmaceuticals and their by-products are recalcitrant contaminants in water. Moreover, the high consumption of these drugs has many detrimental effects on body waters and ecosystems. In this timely review, the advances in molecular engineering of layered double hydroxides (LDH) that have been used for the removal of pharmaceutical pollutants are discussed. The approach starts from the strategies to obtain homogeneous synthesis of LDH that allow the doping and/or surface functionalization of different metals and oxides, producing heterojunction systems as well as composites with carbon and silica-based materials with high surface area. Adsorption is considered as a traditional removal of pharmaceutical pollutants, so the kinetic and mechanism of this phenomenon are analyzed based on pH, temperature, ionic strength, in order to obtain new insights for the formation of multifunctional LDH. Advanced oxidation methodologies, mainly heterogeneous photocatalysis and Fenton-like processes, stand out as the more efficient even to obtain the mineralization of the drugs. The LDH have the advantage of structural memory that favors regeneration processes. The reconstruction of calcined LDH can be used to improve drug removal, through a combination of adsorption capacity/catalytic activity. A meticulous analysis of the persistence, toxicity and bioaccumulation of the most common pharmaceuticals has allowed us to highlight the ability of the LDH to remove recalcitrant drugs at relatively low concentrations (ppm, ppb), in contrast to other mixed oxide nanostructures and homogeneous oxidation processes. In this sense, the mechanism of drug removal by LDH is discussed based on the importance of the use of composites, scavenger agents, Fenton and electro-Fenton processes, membranes, thin films and coatings, among others. In addition, the ecotoxicity of LDH is also reviewed to indicate that these layered structures can exhibit biocompatibility or high toxicity depending on the adsorbed drug and ions/metals that compose them. Undoubtedly, the LDH have a unique flexible structure with adsorption capacity and catalytic activity, facts that explain the important reasons for their extensive use in the environmental remediation of pharmaceutical pollutants from water.

Sonochemical processes for the degradation of antibiotics in aqueous solutions: A review

Antibiotic residues in water are general health and environmental risks due to the antibiotic-resistance phenomenon. Sonication has been included among the advanced oxidation processes (AOPs) used to remove recalcitrant contaminants in aquatic environments. Sonochemical processes have shown substantial advantages, including cleanliness, safety, energy savings and either negligible or no secondary pollution. This review provides a wide overview of the different protocols and degradation mechanisms for antibiotics that either use sonication alone or in hybrid processes, such as sonication with catalysts, Fenton and Fenton-like processes, photolysis, ozonation, etc.

Controllable One-Step Synthesis of Mixed-Phase TiO2 Nanocrystals with Equivalent Anatase/Rutile Ratio for Enhanced Photocatalytic Performance

In this study, the novel mixed-phase TiO2 nanocrystals (s-TiO2) with nearly equivalent anatase/rutile ratio were fabricated in the reagent of sec-butanol at the relatively low temperature of 80 °C by using a facile one-step condensing reflux method. The photocatalytic water splitting hydrogen production performance of s-TiO2 nanocrystals is close to that of commercial TiO2 (P25), and its photocatalytic degradation performance is about four times that of P25. The energy-level staggered interfaces and surface bridged hydroxyl groups significantly increased due to the anatase/rutile mixed-phase crystal structure and high specific surface area, which might generate the synergistic effect for the improvement of photocatalytic degradation.

Review on the hazardous applications and photodegradation mechanisms of chlorophenols over different photocatalysts

Chlorophenols are very important environmental pollutants, which have created huge problems for both aquatic and terrestrial lives. Therefore, their removal needs urgent, effective, and advanced technologies to safeguard our environment for future generation. This review encompasses a comprehensive study of the applications of chlorophenols, their hazardous effects and photocatalytic degradation under light illumination. The effect of various factors such as pH and presence of different anions on the photocatalytic oxidation of chlorophenols have been elaborated comprehensively. The production of different oxidizing agents taking part in the photodegradation of chlorophenols are given a bird eye view. The photocatalytic degradation mechanism of different chlorophenols over various photocatalyts has been discussed in more detail and elaborated that how different photocatalysts degrade the same chlorophenols with the aid of different oxidizing agents produced during photocatalysis. Finally, a future perspective has been given to deal with the effective removal of these hazardous pollutants from the environment.

Emerging Hybrid Nanocomposite Photocatalysts for the Degradation of Antibiotics: Insights into Their Designs and Mechanisms

The raising occurrence of antibiotics in the global water bodies has received the emerging concern due to their potential threats of generating the antibiotic-resistive and genotoxic effects into humans and aquatic species. In this direction, the solar energy assisted photocatalytic technique offers a promising solution to address such emerging concern and paves ways for the complete degradation of antibiotics with the generation of less or non-toxic by-products. Particularly, the designing of hybrid photocatalyticcomposite materials has been found to show higher antibiotics degradation efficiencies. As the hybrid photocatalysts are found as the systems with ideal characteristic properties such as superior structural, surface and interfacial properties, they offer enhanced photoabsorbance, charge-separation, -transfer, redox properties, photostability and easy recovery. In this context, this review study presents an overview on the recent developments in the designing of various hybrid photocatalytic systems and their efficiency towards the degradation of various emerging antibiotic pharmaceutical contaminants in water environments.

Amoxicillin photodegradation under visible light catalyzed by metal-free carbon nitride: An investigation of the influence of the structural defects

Herein, the structural defects of metal-free polymeric carbon nitrides were controlled by making use of different precursors in their syntheses, i.e. melamine (CN-M) and thiourea (CN-T), as well as a 1:1 mixture of them (CN-1M:1 T). By controlling the structural defects, the electronic, morphological and chemical properties were modified. Additionally, the activities of synthesized PCNs were evaluated for amoxicillin photodegradation under visible light irradiation (16 mW cm^-2). The results of photocatalytic tests showed that CN-T material has better efficiency (100 % removal within 48 h), which is directly related to the greater number of defects present in its structure with consequent improvement of electron-hole pairs separation efficiency. The CN-T material showed excellent stability with only 13 % decrease in its photocatalytic activity after the third cycle. A mechanism for amoxicillin degradation by CN-T was proposed based on the ESI-MS and the in situ EPR allied with spin trapping method investigations.

Sonophotocatalytic activities of FeCuMg and CrCuMg LDHs: Influencing factors, antibacterial effects, and intermediate determination

Herein, FeCuMg and CrCuMg layered double hydroxides (LDHs) were synthesized and their sonophotocatalytic activities toward Acid blue 113 (AB113) were compared. Sonolysis alone (only ultrasound) led to the decolorization efficiency of 13.0 %. A similar result was obtained in the case of the utilization of photolysis alone using a 10-W LED lamp (13.5 %). The adsorption process of AB113 onto both compounds was not efficient to significantly remove the target contaminant. The bandgap energy of 2.54 eV and 2.41 eV was calculated for FeCuMg and CrCuMg LDHs, respectively, indicating relatively higher photocatalytic activity of Cr-incorporated LDH than FeCuMg LDH. The sonophotocatalysis of AB113 (50 mg L^-1) over CrCuMg LDH (81.1 %) was more efficient than that of FeCuMg LDH (57.3 %) within the reaction time of 60 min. Intermediate byproducts of the sonophotocatalytic decomposition of organic dye over the as-synthesized tri-metal layered sonophotocatalysts were also identified. Furthermore, the antibacterial activity of both LDHs was evaluated by the CFU technique and the MBC and MIC values were determined. The antibacterial assessment confirmed the higher antibacterial activity of CrCuMg LDH than that of FeCuMg LDH against Staphylococcus aureus (S. aureus).

Synthesis of a Magnetic 2D Co@NC-600 Material by Designing a MOF Precursor for Efficient Catalytic Reduction of Water Pollutants

2D metal-organic framework (MOFs) can be ideal sacrificial templates for fabricating nanomaterials because of active sites exposed on the surface rather than in pores and channels, often exhibiting improved performance in catalysis applications. In this study, the novel 2D layered cobalt-based MOF [Co(TPT)(fma)(H₂O)₂]·3H₂O (Co-MOF) has been constructed by the selection of high N atom content ligands. On this basis, a 2D nitrogen-doped carbon-coated cobalt nanoparticle composite (Co@NC) was prepared by using this MOF as a precursor. Magnetic Co@NC has excellent catalytic activity and recycling features regarding the reaction of 4-nitrophenol (4-NP) reducing to 4-aminophenol (4-AP) in the presence of NaBH₄ at ambient temperature. 2D Co@NC-600 can reach nearly 100% conversion within 120 s and its stability remains almost unchanged after five reaction cycles. Moreover, this Co@NC catalyst also is highly active for catalytic reduction of dyes such as Rhodamine B (RhB) and Methylene blue (MB).

Current Trends in the Application of Nanomaterials for the Removal of Emerging Micropollutants and Pathogens from Water

Water resources contamination has a worldwide impact and is a cause of global concern. The need for provision of clean water is becoming more and more demanding. Nanotechnology may support effective strategies for the treatment, use and reuse of water and the development of next-generation water supply systems. The excellent properties and effectiveness of nanomaterials make them particularly suitable for water/wastewater treatment. This review provides a comprehensive overview of the main categories of nanomaterials used in catalytic processes (carbon nanotubes/graphitic carbon nitride (CNT/g-C₃N₄) composites/graphene-based composites, metal oxides and composites, metal–organic framework and commercially available nanomaterials). These materials have found application in the removal of different categories of pollutants, including pharmaceutically active compounds, personal care products, organic micropollutants, as well as for the disinfection of bacterial, viral and protozoa microbial targets, in water and wastewater matrices. Apart from reviewing the characteristics and efficacy of the aforementioned nanoengineered materials for the removal of different pollutants, we have also recorded performance limitations issues (e.g., toxicity, operating conditions and reuse) for their practical application in water and wastewater treatment on large scale. Research efforts and continuous production are expected to support the development of eco-friendly, economic and efficient nanomaterials for real life applications in the near future.

Recent Progress in Two-Dimensional Layered Double Hydroxides and Their Derivatives for Supercapacitors

High-performance supercapacitors have attracted great attention due to their high power, fast charging/discharging, long lifetime, and high safety. However, the generally low energy density and overall device performance of supercapacitors limit their applications. In recent years, the design of rational electrode materials has proven to be an effective pathway to improve the capacitive performances of supercapacitors. Layered double hydroxides (LDHs), have shown great potential in new-generation supercapacitors, due to their unique two-dimensional layered structures with a high surface area and tunable composition of the host layers and intercalation species. Herein, recent progress in LDH-based, LDH-derived, and composite-type electrode materials targeted for applications in supercapacitors, by tuning the chemical/metal composition, growth morphology, architectures, and device integration, is reviewed. The complicated relationships between the composition, morphology, structure, and capacitive performance are presented. A brief projection is given for the challenges and perspectives of LDHs for energy research.

Ultrasonically Induced Sulfur-Doped Carbon Nitride/Cobalt Ferrite Nanocomposite for Efficient Sonocatalytic Removal of Organic Dyes

The sulfur-doped carbon nitride/cobalt ferrite nanocomposite (SCN/CoFe2O4) was prepared via ultrasonication and studied for the sonocatalytic degradation of wastewater organic dye pollutants including methylene blue, rhodamine B, and Congo red. The X-ray photoelectron spectroscopy confirmed the presence and atomic ratios of S, C, N, Co, Fe, and O elements and their corresponding bonds with Co2+ and Fe3+ cations. The nanocomposite was found to have aggregated nanoparticles on a sheet-like structure. The bandgap energy was estimated to be 1.85 eV. For the sonocatalytic degradation of 25-ppm methylene blue at 20 kHz, 1 W and 50% amplitude, the best operating condition was determined to be 1 g/L of catalyst dosage and 4 vol % of hydrogen peroxide loading. Under this condition, the sonocatalytic removal efficiency was the highest at 96% within a reaction period of 20 min. SCN/CoFe2O4 outperformed SCN and CoFe2O4 by 2.2 and 6.8 times, respectively. The SCN/CoFe2O4 nanocomposite was also found to have good reusability with a drop of only 7% after the fifth cycle. However, the degradation efficiencies were low when tested with rhodamine B and Congo red due to difference in dye sizes, structural compositions, and electric charges.

An advanced composite with ultrafast photocatalytic performance for the degradation of antibiotics by natural sunlight without oxidizing the source over TMU-5@Ni–Ti LDH: mechanistic insight and toxicity assessment

Pharmaceuticals are considered as emerging organic contaminants that have become a serious environmental problem, which endanger human health and environmental bio-diversity.

Advanced Oxidation Processes for the Removal of Antibiotics from Water. An Overview

In this work, the application of advanced oxidation processes (AOPs) for the removal of antibiotics from water has been reviewed. The present concern about water has been exposed, and the main problems derived from the presence of emerging pollutants have been analyzed. Photolysis processes, ozone-based AOPs including ozonation, O3/UV, O3/H2O2, and O3/H2O2/UV, hydrogen peroxide-based methods (i.e., H2O2/UV, Fenton, Fenton-like, hetero-Fenton, and photo-Fenton), heterogeneous photocatalysis (TiO2/UV and TiO2/H2O2/UV systems), and sonochemical and electrooxidative AOPs have been reviewed. The main challenges and prospects of AOPs, as well as some recommendations for the improvement of AOPs aimed at the removal of antibiotics from wastewaters, are pointed out.

Solvent-Mediated and Mechanochemical Methods for Anion Exchange of Carbonate from Layered Double Hydroxides Using Ammonium Salts

Deintercalation of carbonate from layered double hydroxides (LDH) followed by intercalation of another anion (decarbonative intercalation) is a good method for the synthesis of crystalline LDH with different intercalated anions. We have carried out decarbonative intercalation of halides, nitrate, acetate, and sulfate by refluxing the carbonate-LDH with the corresponding ammonium salt in 1-butanol to obtain ordered LDH incorporating the desired anion. The crystallinity of the precursor LDH is retained in the anion-exchanged products, making this reaction a useful tool to prepare ordered LDH containing various anions. In addition, the morphology of the LDH is also retained after the exchange, making the reaction morphotactic. As the reaction is facilitated by the weak acidity of the ammonium salt, just grinding the carbonate-LDH with the ammonium salt of the desired anion also results in anion exchange. However, while the crystallinity of the LDH is retained, the morphology changes possibly due to breaking up of the crystals during the reaction.

Powerful combination of g-C3N4 and LDHs for enhanced photocatalytic performance: A review of strategy, synthesis, and applications

The utilization of solar energy with photocatalytic technology has been considered a good solution to alleviate environmental pollution and energy shortage. Constructing 2D/2D heterostructure photocatalysts with layered double hydroxide (LDH) and graphitic carbon nitride (g-C₃N₄) is an effective approach to attain high performance in solar photocatalysis. This paper provides a review of recent studies about 2D/2D LDH/g-C₃N₄ heterostructure photocatalysts. Main strategies for constructing the desired 2D/2D heterojunction are summarized. The planar structure of LDH and g-C₃N₄ offers a shorter transfer distance for charge carriers and reduces electron-hole recombination in the bulk phase. The face-to-face contact between the two materials can promote the charge transfer across the heterostructure interface, thus improving the electron-hole separation efficiency. The performance and mechanisms of LDH/g-C₃N₄ photocatalysts in hydrogen production, CO₂ reduction, and organic pollutant degradation are analyzed and discussed. Incorporating reduced graphene oxide or Ag nanoparticles into LDH/g-C₃N₄ heterojunction and fabricating calcined LDH/g-C₃N₄ composites are effective strategies to further facilitate charge transfer at the interface of LDH and g-C₃N₄ and improve the absorption capacity for visible light. This review is expected to provide basic insights into the design of 2D/2D LDH/g-C₃N₄ heterojunctions and their applications in solar photocatalysis.