Removal of malachite green from water: Comparison of adsorption in a residue-derived AC versus photocatalytic oxidation with TiO2 and study of the adsorption-photocatalysis synergy

The literature rarely compiles studies devoted to the removal of pollutants in aqueous media comparing adsorption and photocatalytic degradation, and does not pay enough attention to the analysis of combined adsorption-photocatalytic oxidation processes. In the present manuscript, the removal of malachite green (MG) from aqueous solutions has been investigated in three different sustainable scenarios: i) adsorption on activated carbon (AC) derived from a residue, luffa cylindrica, ii) photocatalytic oxidation under simulated solar light using titanium dioxide (TP) and iii) combined adsorption-photocatalytic oxidation using TP-AC (70/30 wt./wt.) under simulated solar light. The study has revealed that in the three scenarios and studied conditions, the total removal of this endocrine-disrupting dye from the solution takes place in the assayed time, 2 h, in some cases just in a few minutes. MG adsorption in the AC is a very fast and efficient removal method. MG photocatalytic oxidation with TP also occurs efficiently, although the oxidized MG is not totally mineralized. MG removal using the TP-AC composite under simulated solar light occurs only slightly faster to the MG adsorption in the AC, being adsorption the dominating MG removal mechanism for TP-AC. Thus, more than 90% of the removed MG with TP-AC under simulated solar light is adsorbed in this carbon-containing composite. The obtained results highlight the interest in adsorption, being the selection of the most suitable removal method dependent on several factors (i.e., the cost of the AC regeneration, for adsorption, or the toxicity of the intermediate oxidation species, for photooxidation). Paying attention to MG photooxidation with TiO2, comparison of two working photodegradation schemes shows that the direct photodegradation of MG from solution, avoiding any initial dark equilibrium period, is more efficient from a time perspective. The use of scavengers has proved that MG photodegradation occurs via an oxidation mechanism dominated by superoxide anion radicals.

Catalytic activation of peracetic acid for pelargonic acid vanillylamide degradation by Co3O4 nanoparticles in-situ anchored carbon-coated MXene nanosheets: Performance and mechanism insight

Pelargonic acid vanillylamide (PAVA), a capsaicin-type dacryagogue agent utilized for counter-terrorism and riot control, possesses a low stimulus threshold. This characteristic can lead to environmental contamination following its application and may easily result in secondary stimulation to personnel. Cobalt-doped Ti3C2-MXene nanosheets (Co3O4/Ti3C2@C) were synthesized for the purpose of activating peracetic acid (PAA) and degrading PAVA. A carbon layer was coated on the surface of Ti3C2-MXene nanosheets to address the challenge of poor oxygen resistance in MXenes, thus preventing a significant decline in surface reactivity. The BET surface area of Co3O4/Ti3C2@C was expanded to 149.6 m2/g, significantly exceeding that of Ti3C2 (13.0 m2/g) and Co3O4 (56.4 m2/g). With 0.5 mg/mL of Co3O4/Ti3C2@C and 0.35 mM of PAA, 100 mg/L of PAVA was completely degraded within 60 min. The augmented BET surface area and the presence of more active sites confer remarkable PAA activation and catalytic degradation properties toward PAVA. Parameters such as initial pH, PAVA concentration, catalyst dosage, and PAA concentration on PAVA degradation were systematically assessed. Furthermore, the reusability and stability of the nanocomposite were substantiated through recycling tests. Radical quenching experiments and electron paramagnetic resonance analysis demonstrated the acetylperoxy radical (CH3CO3) as the primary species responsible for PAVA degradation. This research serves as an illustration of the utilization of MXene and transition metal activated PAA in wastewater treatment.

Enhanced Redox Cycle of Rod-Shaped MIL-88A/SnFe2O4@MXene Sheets for Fenton-like Degradation of Congo Red: Optimization and Mechanism

This study intended to fabricate a novel Fenton-like catalyst by supporting the rod-like MIL-88A and the magnetic tin ferrite nanoparticles (SnFe2O4) on the MXene sheets (MIL-88A/SnFe2O4@MXene). The well fabrication and determination of the MIL-88A/SnFe2O4@MXene properties were investigated using SEM, XPS, VSM, Zeta potential, XRD, and FTIR tools. The Fenton-like degradation reaction of CR by MIL-88A/SnFe2O4@MXene was thoroughly studied to identify the optimal proportions of the catalyst components, the impact of CR and H2O2 concentrations, as well as the effect of raising the temperature and the pH medium of the catalytic system and the catalyst dosage. Kinetics studies were executed to analyze the decomposition of CR and H2O2 using First-order and Second-order models. Furthermore, the degradation mechanism was proposed based on the scavenging test that proceeded in the presence of chloroform and t-butanol, in addition to the XPS analysis that clarified the participation of the containing metal species: Fe, Sn, and Ti, and the formation of a continual redox cycle. The obtained intermediates during the CR degradation were defined by GC-MS. A recyclability test was performed on MIL-88A/SnFe2O4@MXene during five runs of the Fenton-like degradation of CR molecules. Finally, the novel MIL-88A/SnFe2O4@MXene Fenton-like catalyst could be recommended as a propitious heterogeneous catalyst with a continuous redox cycle and a recyclability merit.

Recent developments and perspectives of MXene-Based heterostructures in photocatalysis

Energy crises and environmental degradation are serious in recent years. Inexhaustible solar energy can be used for photocatalytic hydrogen production or CO2 reduction to reduce CO2 emissions. At present, the development of efficient photocatalysts is imminent. MXene as new two-dimensional (2D) layered material, has been used in various fields in recent years. Based on its high conductivity, adjustable band gap structure and sizable specific surface area, the MXene is beneficial to hasten the separation and reduce the combination of photoelectron-hole pairs in photocatalysis. Nevertheless, the re-stacking of layers because of the strong van der Waals force and hydrogen bonding interactions seriously hinder the development of MXene material as photocatalysts. By contrast, the MXene-based heterostructures composed of MXene nanosheets and other materials not only effectively suppress the re-stacking of layers, but also show the superior synergistic effects in photocatalysis. Herein, the recent progress of the MXene-based heterostructures as photocatalysts in energy and environment fields is summarized in this review. Particularly, new synthetic strategies, morphologies, structures, and mechanisms of MXene-based heterostructures are highlighted in hydrogen production, CO2 reduction, and pollutant degradation. In addition, the structure-activity relationship between the synthesis strategy, components, morphology and structure of MXene-based heterostructures, and their photocatalytic properties are elaborated in detail. Finally, a summary and the perspectives on improving the application study of the heterostructures in photocatalysis are presented.

Spontaneous formation of MXene-oxidized sono/chemo-dynamic sonosensitizer/nanocatalyst for antibacteria and bone-tissue regeneration

Prolonged and incurable bacterial infections in soft tissue and bone are currently causing large challenges in the clinic. Two-dimensional (2D) materials have been designed to address these issues, but materials with satisfying therapeutic effects are still needed. Herein, CaO₂-loaded 2D titanium carbide nanosheets (CaO₂-TiO_x@Ti₃C₂, C-T@Ti₃C₂) were developed. Surprisingly, this nanosheet exhibited sonodynamic ability, in which CaO₂ caused the in situ oxidation of Ti₃C₂ MXene to produce acoustic sensitiser TiO₂ on its surface. In addition, this nanosheet displayed chemodynamic features, which promoted a Fenton reaction triggered by self-supplied H₂O₂. We detected that C-T@Ti₃C₂ nanosheets increased reactive oxygen species (ROS) production in response to sonodynamic therapy, which displayed an ideal antibacterial effect. Furthermore, these nanoreactors facilitated the deposition of Ca²⁺, which promoted osteogenic transformation and enhanced bone quality in osteomyelitis models. Herein, a wound healing model and prosthetic joint infection (PJI) model were established, and the C-T@Ti₃C₂ nanosheets played a protective role in these models. Taken together, the results indicated that the C-T@Ti₃C₂ nanosheets function as a multifunctional instrument with sonodynamic features, which might reveal information regarding the treatment of bacterial infections during wound healing.

DNA controllable peroxidase-like activity of Ti3C2 nanosheets for colorimetric detection of microcystin-LR

The peroxidase-like activity of Ti₃C₂ nanosheets (Ti₃C₂ NSs) was evaluated by catalytic oxidation of colorless o-phenylenediamine (OPD) into orange-yellow 2,3-diaminophenazine (DAP) with the aid of H₂O₂. The catalytic behavior followed the typical Michaelis-Menten kinetics. Systematic studies about the catalytic activity of Ti₃C₂ NSs including cytochrome C (Cyt C) electron transfer experiments, radical capture experiments, and fluorescence analysis were conducted, revealing that the catalytic mechanism of Ti₃C₂ NSs was attributed to nanozyme-accelerated electron transfer between substrates and nanozyme-promoted generation of active species (superoxide anion free radical (·O₂^-) and holes (h⁺)). Single-stranded DNA (ssDNA) inhibited the peroxidase-like activity of Ti₃C₂ NSs, and the reduced catalytic activity was ascribed to DNA-hindered substrate accessibility to nanozyme surface. Based on the DNA controllable peroxidase-mimicking activity of Ti₃C₂ NSs, taking microcystin-LR (MC-LR) aptamer as an example, a label-free colorimetric aptasensor was proposed for the sensitive detection of MC-LR. The colorimetric aptasensor showed a wide linear range (0.01-60 ng mL^-1), low limit of detection (6.5 pg mL^-1), and high selectivity. The practicality of the colorimetric aptasensor was demonstrated by detecting different levels of MC-LR in spiked real water samples; satisfactory recoveries (97.2-102.1%) and low relative standard deviations (1.16-3.72%) were obtained.

Methyl orange as a novel colorimetric iodide indicator with in situ generation of H2O2 by etching uncoated Ag-Ti3C2 nanohybrids

Iodine intake remains a major public health concern, as both iodine excess and deficiency are related to adverse effects on health. Therefore, developing simple and economical methods to detect I^- is still in great demand. Herein, we constructed a visual I^- sensing platform based on the uncoated Ag-Ti₃C₂ nanohybrids using methyl orange (MO) as a colorimetric indicator. Plasmonic nanostructures are frequently employed in colorimetric analysis, but uncoated Ag nanoparticles (NPs) are unstable because their surface energies are usually high. Considering that Ag NPs can be etched by I^- via forming Ag-I bond, we introduce Ag-Ti₃C₂ nanohybrids because uncoated Ag NPs with immaculate surfaces are more conducive to binding with I^- and being etched. Dissolved O₂ molecules adsorbed on Ti³⁺ of Ti₃C₂ MXenes enable the in situ generation of H₂O₂ by iodine-etching of uncoated Ag-Ti₃C₂ nanohybrids. ∙OH radicals promote the degradation of MO through a self-driven Fenton-like process, exhibiting the color variation from orange to transparent. Under optimal conditions, the absorbance of MO at 465 nm decreases linearly with the concentration of I^- in the range of 0.5-300 μM, with a limit of detection as low as 0.31 μM. This work opens the feasibility of iodine-etching of Ag in developing novel probes for facile colorimetric determination of I^-.

Magnetic chitosan/TiO2 composite for vanadium(v) adsorption simultaneously being transformed to an enhanced natural photocatalyst for the degradation of rhodamine B

A magnetic chitosan/TiO₂ composite material (MCT) was developed. MCT was successfully synthesized by a one-pot method using chitosan, TiO₂, and Fe₃O₄. The absorption equilibrium time of MCT was 40 min in absorbing vanadium(v), the optimal adsorption pH was 4, and the maximum adsorption capacity of vanadium(v) was 117.1 mg g^-1. The spent MCT was applied to photocatalytic reactions for reutilization. The decolorization rates for the degradation of rhodamine B (RhB) by new and spent MCT were 86.4% and 94.3%, respectively. The new and spent MCT exhibited absorption bands at 397 and 455 nm, respectively, which showed that the spent MCT was red-shifted to the cyan light region. These results indicated that the forbidden band widths of the new and spent MCT were about 3.12 and 2.72 eV, respectively. The mechanism of the degradation reaction showed that the hydroxyl radicals as oxidants in the spent MCT mediated the photocatalytic degradation of RhB. In addition, the superoxide anion radical formation of hydroxyl radicals was the main reaction, and the hole generation of hydroxyl radicals was the subordinate reaction. The N-de-ethylated intermediates and organic acids were monitored by MS and HPLC.

Application of MXene-based materials in Fenton-like systems for organic wastewater treatment: A review

Recently, Fenton-like systems have been widely explored and applied for the removal of organic matter from wastewater. Two-dimensional (2D) MXene-based materials exhibit excellent adsorption and catalysis capacity for organic pollutants removal, which has been reported widely. However, there is no summary on the application of MXene-based materials in Fenton-like systems for organic matter removal. In this review, four types of MXene-based materials were introduced, including 2D MXene, MXene/Metal complex, MXene/Metal oxide complex, and MXene/3D carbon material complex. In addition, the Fenton-like system usually consists of adsorption and degradation processes. The oxidation process might contain hydrogen peroxide (H₂O₂) or persulfate (PS) oxidants. This review summarizes the performance and mechanisms of organic pollutants adsorption and oxidants activation by MXene-based materials systematically. Finally, the existing problems and future research directions of MXene-based materials are proposed in Fenton-like wastewater treatment systems.

Effective Removal of Sulfonamides Using Recyclable MXene-Decorated Bismuth Ferrite Nanocomposites Prepared via Hydrothermal Method

Developing a simple and efficient method for removing organic micropollutants from aqueous systems is crucial. The present study describes the preparation and application, for the first time, of novel MXene-decorated bismuth ferrite nanocomposites (BiFeO₃/MXene) for the removal of six sulfonamides including sulfadiazine (SDZ), sulfathiazole (STZ), sulfamerazine (SMZ), sulfamethazine (SMTZ), sulfamethoxazole (SMXZ) and sulfisoxazole (SXZ). The properties of BiFeO₃/MXene are enhanced by the presence of BiFeO₃ nanoparticles, which provide a large surface area to facilitate the removal of sulfonamides. More importantly, BiFeO₃/MXene composites demonstrated remarkable sulfonamide adsorption capabilities compared to pristine MXene, which is due to the synergistic effect between BiFeO₃ and MXene. The kinetics and isotherm models of sulfonamide adsorption on BiFeO₃/MXene are consistent with a pseudo-second-order kinetics and Langmuir model. BiFeO₃/MXene had appreciable reusability after five adsorption-desorption cycles. Furthermore, BiFeO₃/MXene is stable and retains its original properties upon desorption. The present work provides an effective method for eliminating sulfonamides from water by exploiting the excellent texture properties of BiFeO₃/MXene.

Emergence of MXene and MXene-Polymer Hybrid Membranes as Future- Environmental Remediation Strategies

The continuous deterioration of the environment due to extensive industrialization and urbanization has raised the requirement to devise high-performance environmental remediation technologies. Membrane technologies, primarily based on conventional polymers, are the most commercialized air, water, solid, and radiation-based environmental remediation strategies. Low stability at high temperatures, swelling in organic contaminants, and poor selectivity are the fundamental issues associated with polymeric membranes restricting their scalable viability. Polymer-metal-carbides and nitrides (MXenes) hybrid membranes possess remarkable physicochemical attributes, including strong mechanical endurance, high mechanical flexibility, superior adsorptive behavior, and selective permeability, due to multi-interactions between polymers and MXene's surface functionalities. This review articulates the state-of-the-art MXene-polymer hybrid membranes, emphasizing its fabrication routes, enhanced physicochemical properties, and improved adsorptive behavior. It comprehensively summarizes the utilization of MXene-polymer hybrid membranes for environmental remediation applications, including water purification, desalination, ion-separation, gas separation and detection, containment adsorption, and electromagnetic and nuclear radiation shielding. Furthermore, the review highlights the associated bottlenecks of MXene-Polymer hybrid-membranes and its possible alternate solutions to meet industrial requirements. Discussed are opportunities and prospects related to MXene-polymer membrane to devise intelligent and next-generation environmental remediation strategies with the integration of modern age technologies of internet-of-things, artificial intelligence, machine-learning, 5G-communication and cloud-computing are elucidated.

Self-Assembly 2D Ti3C2/g-C3N4 MXene Heterojunction for Highly Efficient Photocatalytic Degradation of Tetracycline in Visible Wavelength Range

An ultrathin 2D Ti₃C₂/g-C₃N₄ MXene (2D-TC/CN) heterojunction was synthesized, using a facile self-assembly method; the perfect microscopic-morphology and the lattice structure presented in the sample with a 2 wt% content of Ti₃C₂ were observed by the field-emission scanning electron microscopy (SEM) and transmission electron microscope (TEM). The optimized sample (2-TC/CN) exhibited excellent performance in degrading the tetracycline (TC), and the degradation rate reached 93.93% in the conditions of 20 mg/L, 50 mL of tetracycline within 60 min. Except for the increased specific-surface area, investigated by UV-vis diffuse reflectance spectra (UV-vis DRS) and X-ray photoelectron microscopy (XPS) valence spectra, the significantly enhanced photocatalytic activity of the 2-TC/CN could also be ascribed to the formation of Ti-N bonds between Ti₃C₂ and g-C₃N₄ nanosheets, which reduced the width of the band gap through adjusting the position of the valence band, thus resulting in the broadened light-absorption. Furthermore, the facilitated electron transmission was also proved by time-resolved photoluminescence (TRPL) and electrochemical impedance spectroscopy (EIS), which is effective in improving the quantum efficiency of photo-generated electrons. In addition, the resulting radical-capture experiment suggested that superoxide radicals have the greatest influence on photodegradation performance, with the photodegradation rate of TC reducing from 93.16% to 32.08% after the capture of superoxide radicals, which can be attributed to the production of superoxide radicals only, by the 2-TC/CN composites with a high conduction-band value (-0.62 eV). These facilely designed 2D Ti₃C₂/g-C₃N₄ composites possess great application potential for the photodegradation of tetracycline and other antibiotics.

Nanoparticles as Next-Generation Tooth-Whitening Agents: Progress and Perspectives

Whitening agents, such as hydrogen peroxide and carbamide peroxide, are currently used in clinical applications for dental esthetic and dental care. However, the free radicals generated by whitening agents cause pathological damage; therefore, their safety issues remain controversial. Furthermore, whitening agents are known to be unstable and short-lived. Since 2001, nanoparticles (NPs) have been researched for use in tooth whitening. Importantly, nanoparticles not only function as abrasives but also release reactive oxygen species and help remineralization. This review outlines the historical development of several NPs based on their whitening effects and side effects. NPs can be categorized into metals or metal oxides, ceramic particles, graphene oxide, and piezoelectric particles. Moreover, the status quo and future prospects are discussed, and recent progress in the development of NPs and their applications in various fields requiring tooth whitening is examined. This review promotes the research and development of next-generation NPs for use in tooth whitening.

Silver-decorated MXene nanosheets as a radical initiator for polymerization and multifunctional hydrogels

Silver nanoparticle-decorated multilayered titanium carbide MXene (Ag/Ti₃C₂T_x) itself is capable of initiating the polymerization of a variety of acrylic monomers, due to it being able to generate hydroxyl radicals via the pseudo-Fenton reaction. Furthermore, double-network hydrogel Ag/Ti₃C₂T_x@gelatin/PAAm is synthesized by a one-pot procedure and displays a good near-infrared light-triggered shape-memory performance.

N-Rich Doped Anatase TiO2 with Smart Defect Engineering as Efficient Photocatalysts for Acetaldehyde Degradation

Nitrogen (N) doping is an effective strategy for improving the solar-driven photocatalytic performance of anatase TiO2, but controllable methods for nitrogen-rich doping and associated defect engineering are highly desired. In this work, N-rich doped anatase TiO2 nanoparticles (4.2 at%) were successfully prepared via high-temperature nitridation based on thermally stable H3PO4-modified TiO2. Subsequently, the associated deep-energy-level defects such as oxygen vacancies and Ti3+ were successfully healed by smart photo-Fenton oxidation treatment. Under visible-light irradiation, the healed N-doped TiO2 exhibited a ~2-times higher activity of gas-phase acetaldehyde degradation than the non-treated one and even better than standard P25 TiO2 under UV-visible-light irradiation. The exceptional performance is attributed to the extended spectral response range from N-rich doping, the enhanced charge separation from hole capturing by N-doped species, and the healed defect levels with the proper thermodynamic ability for facilitating O2 reduction, depending on the results of ∙O2− radicals and defect measurement by electron spin resonance, X-ray photoelectron spectroscopy, atmosphere-controlled surface photovoltage spectra, etc. This work provides an easy and efficient strategy for the preparation of high-performance solar-driven TiO2 photocatalysts.

Contribution of photocatalytic and Fenton-based processes in nanotwin structured anodic TiO2 nanotube layers modified by Ce and V

In the present work, nanotwin structured TiO₂ nanotube (TNT) layers are prepared by the electrochemical anodization technique to form the anatase phase and by surface modification via spin-coating of Ce and V precursors to form Ce-TNT and V-TNT, respectively. The surface and cross-sectional images by SEM revealed that the nanotubes have an average diameter of ∼130 nm and a length of ∼14 μm. In addition, the TEM images revealed the nanotwin structures of the nanotubes, especially the anatase (001) and (112) twin surfaces, that increase the transport of photogenerated charges. The photoinduced degradation of caffeine (CAF) by TNT, Ce-TNT, and V-TNT led to a degradation extent of 16%, 26% and 33%, respectively, whereas it increased to 26%, 38%, and 46% in the presence of H₂O₂, owing to the involvement of Fenton-based processes (in addition to photocatalysis). The effect of the Fenton-based processes accounts for about 10% of the total degradation extent of CAF. Finally, the mechanism of the photoinduced degradation of CAF was investigated. The main oxidative species were the hydroxyl radicals, and the better efficiency of V-TNT over Ce-TNT and TNT was ascribed to its negative surface, thus improving the interactions with CAF.

Recent Advances in Oxidation Stable Chemistry of 2D MXenes

As an emerging star of 2D nanomaterials, 2D transition metal carbides and nitrides, named MXenes, present a large potential in various research areas owing to their intrinsic multilayer structure and intriguing physico-chemical properties. However, the fabrication and application of functional MXene-based devices still remain challenging as they are prone to oxidative degradation under ambient environment. Within this review, the preparation methods of MXenes focusing on the recent investigations on their thermal structure-stability relationships in inert, oxidizing, and aqueous environments are systematically introduced. Moreover, the key factors that affect the oxidation of MXenes, such as, atmosphere, temperature, composition, microstructure, and aqueous environment, are reviewed. Based on different scenarios, strategies for avoiding or delaying the oxidation of MXenes are proposed to encourage the utilization of MXenes in complicated environments, especially at high temperature. Furthermore, the chemistry of MXene-derived oxides is analyzed, which can offer perspectives on the further design and fabrication of novel 2D composites with the unique structures of MXenes being preserved.

Two-Dimensional Nanostructures in the World of Advanced Oxidation Processes

Two-dimensional compounds with nanostructural features are attracting attention from researchers worldwide. Their multitude of applications in various fields and vast potential for future technology advancements are successively increasing the research progress. Wastewater treatment and preventing dangerous substances from entering the environment have become important aspects due to the increasing environmental awareness, and increasing consumer demands have resulted in the appearance of new, often nonbiodegradable compounds. In this review, we focus on using the most promising 2D materials, such as MXenes, Bi2WO6, and MOFs, as catalysts in the modification of the Fenton process to degrade nonbiodegradable compounds. We analyze the efficiency of the process, its toxicity, previous environmental applications, and the stability and reusability of the catalyst. We also discuss the catalyst’s mechanisms of action. Collectively, this work provides insight into the possibility of implementing 2D material-based catalysts for industrial and urban wastewater treatment.

Bibliometric analysis and recent trends on MXene research - A comprehensive review

MXene, identified as a high performance material with superior properties, has gained significant importance in the field of applications including energy storage, photo catalysis, sensing of components and environmental pollution control. This review article is a comprehensive study on scientometric review on the research studies involving MXene and its derivatives for various applications. The aim of this study is to identify the areas of priority focused during the study period (2012-2020) and evaluate the impact of the studies in terms of different parameters. Using the suitable key words, a total of 3332 documents are identified and screened with respect to yearly count of literature, type of literature, language of publication, authors, Web of science (WoS) categories, most cited literature, author contribution, name of the affiliated institution, country of author affiliation, journals and key words. In addition, collaboration behavior and citation network are reviewed using the mapping tool. The total local citation score (TLCS) and total global citation score (TGCS) are evaluated. Based on the review data, the developments in the field of MXene applications are presented with more focus on sensing applications and photocatalysis. The top two contributing countries in the chosen field of MXene research are China and USA. Based on the number of documents published, ACS Applied Materials & Interfaces and Journal of Materials Chemistry "A" are identified as the best two journals.

In situ synthesis of novel peroxo-functionalized Ti3C2Tx adsorbent for aqueous pollutants removal: Role of oxygen-containing terminal groups

A novel peroxo-functionalized Ti₃C₂T_x adsorbent with abundant surface termination groups was facilely prepared in situ to remove aqueous anionic and cationic dyes. The adsorption behavior of methylene blue on peroxo-functionalized Ti₃C₂T_x was systematically investigated by adsorption kinetics, isotherms, and thermodynamics. Compared with Ti₃C₂T_x, the adsorption capacities of peroxo-functionalized Ti₃C₂T_x for cationic dyes methylene blue (558.0 mg g^-1), rhodamine B (524.6 mg g^-1) and anionic dyes methyl orange (292.6 mg g^-1), congo red (258.2 mg g^-1) were increased at room temperature without adjustment of pH, background ions and humic acid, etc of the contaminant solution by 7.9, 5.3, 5.9 and 4.6 times, respectively. In addition, peroxo-functionalized Ti₃C₂T_x could well tolerate the effects of pH, ionic strength, and humic acid. As revealed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, the adsorption mechanism of peroxo-functionalized Ti₃C₂T_x for anionic and cationic dyes was mainly attributed to the electrostatic interaction, hydrogen bonding interaction, and noncovalent surface-π attraction interaction. This study demonstrates a facile modification strategy for Ti₃C₂T_x adsorbent materials and aims to provide insights for the development of excellent Ti₃C₂T_x-based adsorbent materials.

Ingeniously designed Silica nanostructures as an exceptional support: Opportunities, potential challenges and future prospects for viable degradation of pesticides

Despite significant advancements in modern agricultural practices, efficient handling of pesticides is a must as they are continuously defiling our terrestrial as well as aquatic life. During the last couple of decades, substantial efforts by various research groups have been devoted to find innovative solutions to remove pesticides from our environment in a greener way. In this regard, functionalized silica nanoparticles (NPs) have gained considerable attention of scientific community due to their notable properties such as amenable design, large surface area as well as fine-tunable and uniform pore structures which make them an ideal material for pesticides removal. The present review aims to proffer current scientific progress attained by silica-based nanostructures as an excellent material for effective removal of noxious agrochemicals. Further, a brief discussion on the synthetic strategies as well as intrinsic benefits associated with different morphologies of silica have also been highlighted in this article. It also summarizes the recent reports on silica assisted degradation of pesticides via enzymatic, chemical as well as advanced oxidation protocols. Additionally, it presents a critical analysis of different support materials for decontamination of our ecosystem. The review concludes with potential challenges, their possible solutions along with key knowledge gaps and future research directions for successful deployment of silica supported materials in degradation of pesticides at commercial scale.

Advancements in Solar Desalination of Seawater by Various Ti3C2 MXene Based Morphologies for Freshwater Generation: A Review

For a few years, we have been witnessing ubiquitous fresh and drinking water scarcity in various countries. To mitigate these problematic situations, many countries relied on non-conventional freshwater generation technologies through solar desalination of seawater. In this manner, we excel the ability of new class 2D Ti3C2 MXenes as a photothermal material (solar absorber) for freshwater generation via the solar desalination technique. In this review, the air–water interfacial interaction is highlighted for improving the evaporation efficiency. To provide the dependence of the desalination efficiency on the microstructure of the solar absorbers, we summarized various forms of 2D Ti3C2 MXenes (aerosol, films, foam, hydrogel, membrane, monolith and porous structure) and their characteristics. These microstructures prevailed ultrahigh photoconversion efficiency. In this aspect, we further explained key features such as light absorption, reflection, multiple internal reflection, hydrophilicity, lower thermal conduction, light-to-heat generation, and salt rejection for achieving efficient desalination output throughout the visible and broadband region. Specifically, we targeted to explore the self-floating and salt rejection nature of various state-of-the-art 2D Ti3C2 MXene structures. Further, we highlighted the long-term stability. Among the above morphologies, Ti3C2 MXene in the form of a membrane is believed to be a promising morphology which effectively desalinates seawater into freshwater. Finally, we highlighted the challenges and future perspectives, which can pave a potential path for advancing the sustainable solar desalination of seawater into freshwater.

Review of MXene-based nanocomposites for photocatalysis

Since multilayered MXenes (Ti₃C₂T_x, a new family of two-dimensional materials) were initially introduced by researchers at Drexel University in 2011, various MXene-based nanocomposites have received increased attention as photocatalysts owing to their exceptional properties (e.g., rich surface chemistry, adjustable bandgap structures, high electrical conductivity, hydrophilicity, thermal stability, and large specific surface area). Therefore, we present a comprehensive review of recent studies on fabrication methods for MXene-based photocatalysts and photocatalytic performance for contaminant degradation, CO₂ reduction, H₂ evolution, and N₂ fixation with various MXene-based nanocomposites. In addition, this review briefly discusses the stability of MXene-based nanophotocatalysts, current limitations, and future research needs, along with the various corresponding challenges, in an effort to reveal the unique properties of MXene-based nanocomposites.

Photocatalytic inactivation of airborne bacteria in a polyurethane foam reactor loaded with a hybrid of MXene and anatase TiO2 exposing {0 0 1} facets

A hybrid of TiO2 exposing {0 0 1} facets and monolayer Ti3C2Tx nanosheet (MXene) was synthesized, characterized and used as a photocatalyst in this study. The introduction of MXene (3.4 wt%) helped to reduce the recombination of photo-induced electrons and holes, and thus enhanced the photocatalytic activity by 30%. A continuous flow-through reactor loaded with the as-prepared photocatalyst coated onto polyurethane foam was developed to inactivate airborne bacteria. The photocatalytic inactivation efficiency of airborne Escherichia coli (E. coli) achieved 3.4 lg order under ultraviolet (UV) irradiation at 254 (UV254), which was superior to that using UV254-only treatment with 2.5 lg order under the same operating condition (95% relative humidity and retention time of 4.27 s). The effect of humidity and bacteria species on inactivation performance was also investigated. The thick cell membrane could protect bacteria from photocatalytic oxidation while high humidity increased the photocatalytic inactivation efficiency by generating more reactive oxygen species. The phenomena of photo reactivation and dark repair of airborne E. coli using UV254-only treatment was observed. However, no reactivation occurred after UV photocatalytic inactivation, and even a continuous decline under visible light. These results suggested a different inactivation mechanism between UV irradiation and UV photocatalysis that the former inactivated bacteria by damaging their DNA, whereas photocatalysis physically damaged their cell structure.

Magnetic recyclable α-Fe2O3–Fe3O4/Co3O4–CoO nanocomposite with a dual Z-scheme charge transfer pathway for quick photo-Fenton degradation of organic pollutants

A novel α-Fe₂O₃–Fe₃O₄/Co₃O₄–CoO nanocomposite was developed, integrating multiple degradation pathways. The Z-scheme configuration and oxygen vacancies contributes to in situ H₂O₂ formation and simultaneous reactivation showing excellent performance.

Z-scheme TiO2@Ti3C2/Cd0.5Zn0.5S nanocomposites with efficient photocatalytic performance via one-step hydrothermal route

Photocatalytic degradation of pollutants has been proved to be an effective strategy for wastewater treatment. Herein, TiO₂ nanoparticles were synthesized on a Ti₃C₂ matrix by in situ growth, forming Z-scheme TiO₂@Ti₃C₂/Cd_0.5Zn_0.5S (TO/CZS) multilevel structured nanocomposites via one-step hydrothermal route. The effects of hydrothermal temperature and Cd_0.5Zn_0.5S content on microstructure and properties of composites were assessed. TO/CZS nanocomposites were probed into phase composition, morphological and optical properties with x-ray diffractometer, infrared radiation, scanning electron microscope and UV-vis reflective spectra. Following the hydrothermal reaction at 160 °C for 12 h, TiO₂ nanoparticles of 30 nm in diameter were generated in situ on Ti₃C₂ lamina and Cd_0.5Zn_0.5S particles were evenly distributed on the Ti₃C₂ matrix. The photocatalytic activity of TO/CZS composites were evaluated, which found that degradation rate constant (k = 0.028 min^-1) of TO/CZS-40 on Rhodamine B was 5.19 times that of pure TiO₂ and 4.48 times that of Cd_0.5Zn_0.5S. Through anchoring Ti₃C₂ as an electron transition mediator and combination with TiO₂ and Cd_0.5Zn_0.5S, the new Z-scheme between TiO₂ oxidized by Ti₃C₂ and Cd_0.5Zn_0.5S establishes a multilevel structure of separating electron-hole pairs. This work demonstrates a valid way to control electrons and hole transfer directions efficiently through designing multilevel semiconductor structural designs.

Metal-free generation of hydroxyl radicals by benzoate-mediated decomposition of peroxides

A type of metal-free organic photo-Fenton-like reaction was developed, which was realized by a combination of UV with benzoates and peroxides. The findings here not only provide a novel ˙OH production pathway, but also shed new light on the accurate quantification, better use or prevention of ˙OH in many scenarios.

Recent progress and advances in the environmental applications of MXene related materials

MXenes are a new type of two-dimensional (2D) transition metal carbide or carbonitride material with a 2D structure similar to graphene. The general formula of MXenes is M_n+1X_nT_x, in which M is an early transition metal element, X represents carbon, nitrogen and boron, and T is a surface oxygen-containing or fluorine-containing group. These novel 2D materials possess a unique 2D layered structure, large specific surface area, good conductivity, stability, and mechanical properties. Benefitting from these properties, MXenes have received increasing attention and emerged as new substrate materials for exploration of various applications including, energy storage and conversion, photothermal treatment, drug delivery, environmental adsorption and catalytic degradation. The progress on various applications of MXene-based materials has been reviewed; while only a few of them covered environmental remediation, surface modification of MXenes has never been highlighted. In this review, we highlight recent advances and achievements in surface modification and environmental applications (such as environmental adsorption and catalytic degradation) of MXene-based materials. The current studies on the biocompatibility and toxicity of MXenes and related materials are summarized in the following sections. The challenges and future directions of the environmental applications of MXene-based materials are also discussed and highlighted.

A Fenton-Like Nanocatalyst Based on Easily Separated Magnetic Nanorings for Oxidation and Degradation of Dye Pollutant

In this study, uniform Fe₃O₄ magnetic nanorings (Fe₃O₄-MNRs) were prepared through a simple hydrothermal method. The morphology, magnetic properties, and structure of the product were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM), vibrating sample magnetometer (VSM), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), respectively. The Fe₃O₄-MNRs were used as Fenton-like catalysts in the presence of hydrogen peroxide (H₂O₂) and showed excellent Fenton-catalytic activity for degradation of organic dyes such as Methylene blue (MB), Rhodamine B (RhB), and Bromophenol blue (BPB). Furthermore, the obtained Fe₃O₄-MNRs could be recycled after used for several times and still remained in a relative high activity and could rapidly be separated from the reaction medium using a magnet without considerable loss. All results reveal that Fe₃O₄-MNRs have potential for the treatment of dyes pollutants.

TiO2 Modified with Organic Acids for the Decomposition of Chlorfenvinphos under the Influence of Visible Light: Activity, Performance, Adsorption, and Kinetics

Photocatalytic decomposition of chlorfenvinphos (CFVP) in the presence of titanium dioxide (TiO₂) modified with organic acids: pyruvic (PA) and succinic (SA) under the visible light radiation has been studied. The following tests were examined: dose of photocatalysts, adsorption time, pH of the model solution, deactivation of catalysts, the role of oxygen, identification of free radicals for the CFVP decomposition, Langmuir-Hinshelwood kinetics. The synthesized materials were characterized by Scanning Electron Microscopy (SEM) and UV-Vis. At 10 wt.% of acid (90:10) decomposition of chlorfenvinphos was the most effective in the following conditions: dose of catalyst 50.0 mg/L, time of adsorption = 20 min, pH of model solution = 3.0. Under these conditions the order of photocatalyst efficiency has been proposed: TiO₂/PA/90:10 > TiO₂/SA/90:10 > TiO₂ with the removal degree of 85, 72 and 48%. The mathematically calculated half-life at this conditions was 27.0 min and 39.0 min for TiO₂/PA/90:10 and TiO₂/SA/90:10 respectively, compared to 98 min for pure TiO₂. It has been determined that the O₂^- radicals and holes (h⁺) are the main reactive species involved in the photodegradation of chlorfenvinphos. The results of this study showed that method may be an interesting alternative for the treatment of chlorfenvinphos contaminated wastewater.

2D MXenes as Co-catalysts in Photocatalysis: Synthetic Methods

Since their seminal discovery in 2011, two-dimensional (2D) transition metal carbides/nitrides known as MXenes, that constitute a large family of 2D materials, have been targeted toward various applications due to their outstanding electronic properties. MXenes functioning as co-catalyst in combination with certain photocatalysts have been applied in photocatalytic systems to enhance photogenerated charge separation, suppress rapid charge recombination, and convert solar energy into chemical energy or use it in the degradation of organic compounds. The photocatalytic performance greatly depends on the composition and morphology of the photocatalyst, which, in turn, are determined by the method of preparation used. Here, we review the four different synthesis methods (mechanical mixing, self-assembly, in situ decoration, and oxidation) reported for MXenes in view of their application as co-catalyst in photocatalysis. In addition, the working mechanism for MXenes application in photocatalysis is discussed and an outlook for future research is also provided.

Ti3C2 MXene quantum dot-encapsulated liposomes for photothermal immunoassays using a portable near-infrared imaging camera on a smartphone

Methods based on the photothermal effect (a common phenomenon in nature) have been widely applied in different fields; however, their application in bioanalysis has lagged behind. Herein, we designed a near-infrared (NIR) photothermal immunoassay for the qualitative or quantitative detection of prostate-specific antigen (PSA) using titanium carbide (Ti₃C₂) MXene quantum dot (QD)-encapsulated liposomes with high photothermal efficiency. This system involves a sandwich-type immunoreaction and photothermal measurements. Ti₃C₂ MXene QDs were utilized as innovative photothermal signal beacons and were encapsulated in liposomes for the labeling of the secondary antibody. The assay was carried out by coupling a low-cost microplate with a homemade 3D printed device. Under NIR-laser irradiation, the Ti₃C₂ MXene QDs converted the light energy into heat, and a shift in temperature corresponding with the analyte concentration was obtained on a handheld thermometer. Under optimal conditions, the Ti₃C₂ MXene QD-based photothermal immunoassay exhibited a dynamic linear range from 1.0 ng mL^-1 to 50 ng mL^-1 with a limit of detection of 0.4 ng mL^-1 for PSA detection. Also, we constructed portable equipment using a portable near-infrared imaging camera to collect visual thermal data for the semi-quantitative analysis of the target PSA within 3 min. The specificity, reproducibility and accuracy of the photothermal immunoassay were acceptable. Importantly, our strategy opens new opportunities for protein point-of-care (POC) testing and biosecurity diagnostics.

A Co–Fe Prussian blue analogue for efficient Fenton-like catalysis: the effect of high-spin cobalt

We, for the first time, demonstrated using experiments and density functional theory (DFT) calculations that high-spin Co^II exhibits excellent Fenton-like activity.