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

Recent progression in MXene-based catalysts for emerging photocatalytic applications of CO2 reduction and H2 production: A review

The development of advanced materials for efficient photocatalytic H2 production and CO2 reduction is highly recommended for addressing environmental issues and producing clean energy sources. Specifically, MXenes have emerged as two-dimensional (2D) materials extensively used as high-performance cocatalysts in photocatalyst systems owing to their outstanding features of structure and properties such as high conductivity, large specific surface area, and abundant active sites. Nevertheless, there is a lack of deep and systematic studies concerning the application of these emerging materials for CO2 reduction reaction (CRR) and H2 production (HER). This review first outlines the essential features of MXenes, encompassing the synthesis methods, composition, surface terminations, and electronic properties, which make them highly active as cocatalysts. It then examines the recent progress in MXene-based photocatalysts, emphasizing the synergy achieved by coupling MXenes as co-catalysts with semiconductors, utilizing MXenes as a support for the consistent growth of photocatalysts, leading to finely dispersed nanoparticles, and exploiting MXene as exceptional precursors for creating MXene/metal oxide photocomposite. The roles of engineering surface terminations of MXene cocatalysts, MXene quantum dots (QDs), and distinctive morphologies in MXenes-based photocatalyst systems to enhance photocatalytic activity for both HER and CRR have been explored both experimentally and theoretically using DFT calculations. Challenges and prospects for MXene-based photocatalysts are also addressed. Finally, suggestions for further research and development of effective and economical MXenes/semiconductors strategies are proposed. This comprehensive review article serves as a valuable reference for researchers for applying MXenes in photocatalysis.

MXene-based composite aerogels with bifunctional ferrous ions for the efficient degradation of phenol from wastewater

Herein, MXene-based composite aerogel (MXene-Fe2+ aerogel) are constructed by a one-step freeze-drying method, using Ti3C2Tx MXene layers as substrate material and ferrous ion (Fe2+) as crosslinking agent. With the aid of the Fe2+ induced Fenton reaction, the synthesized aerogels are used as the particle electrodes to remove phenol from wastewater with three-dimensional electrode technology. Combined with the dual roles of Fe2+ and the highly conductive MXene, the obtained particle electrode possesses extremely effective phenol degradation. The effects of experiment parameters such as Fe2+ to MXene ratio, particle electrode dosage, applied voltage, and initial pH of solution on the removal of phenol are discussed. At pH = 2.5, phenol with 50 mg/L of initial concentration can be completely removed within 50 min at 10 V with the particle electrode dosage of 0.56 g/L. Finally, the mechanism of degradation is explored. This work provides an effective way for phenol degradation by MXene-based aerogel, which has great potential for the degradation of other organic pollutants in wastewater.

MXenes as emerging adsorbents for removal of environmental pollutants

MXenes are a recently emerging class of two-dimensional nanomaterials that have gained considerable interest in the field of environmental protection. Owing to their high surface area, abundant terminal groups, and unique two-dimensional layered structures, MXenes have demonstrated high efficacy as adsorbents for various pollutants. Here we focused on the latest developments in the field of MXene-based adsorbents, including the structure and properties of MXenes, their synthesis and modification methods, and their adsorption performance and mechanisms for various pollutants. Among the pollutants that have been reported to be adsorbed by MXenes are radionuclides (U(VI), Sr(II), Cs(I), Eu(III), Ba(II), Th(IV), and Tc(VII)/Re(VII)), heavy metals (Hg(II), Cu(II), Cr(VI), and Pb(II)), dyes, per- and polyfluoroalkyl substances (PFAS), antibiotics (tetracycline, ciprofloxacin, and sulfonamides), antibiotic resistance genes (ARGs), and other contaminates. Moreover, future directions in MXene research are also suggested in this review.

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.

Catalytic degradation of crystal violet and methyl orange in heterogeneous Fenton-like processes

Metals-loaded (Fe3+, Cu2+ and Zn2+) activated carbons (M@AC) with different loading ratios (0.1%, 0.5%, 1%, 5% and 10%) were prepared and employed for catalytic degradation of dye model compounds (crystal violet (CV) and methyl orange (MO)) in wastewater by heterogeneous Fenton-like technique. Compared with Cu@AC and Zn@AC, 0.5% Fe3+ loaded AC (0.5Fe@AC) had better catalytic activity for dyes degradation. The effects of dyes initial concentration, catalyst dosage, pH and hydrogen peroxide (H2O2) volume on the catalytic degradation process were investigated. Cyclic performance, stability of 0.5Fe@AC and iron leaching were explored. Degradation kinetics were well fitted to the pseudo-second-order model (Langmuir-Hinshelwood). Almost complete decolorization (99.7%) of 400 mg L-1 CV was achieved after 30 min reaction under the conditions of CV volume (30 mL), catalyst dosage (0.05 g), H2O2 volume (1 mL) and pH (7.7). Decolorization of MO reached 98.2% under the same conditions. The abilities of pyrolysis char (PC) of dyeing sludge (DS) and metal loaded carbon to remove dye pollutants were compared. The intermediate products were analyzed and the possible degradation pathway was proposed. This study provided an insight into catalytic degradation of triphenylmethane- and aromatic azo-based substances, and utilization of sludge char.

Chalcopyrite as an oxidants activator for organic pollutant remediation: A review of mechanisms, parameters, and future perspectives

Advanced oxidation processes (AOPs) based on oxidants have attracted attention for the degradation of organic pollutants. The combination of chalcopyrite with oxidants such as persulfate, peroxide, percarbonate, and others shows promise as a system due to its ability to activate through various pathways, leading to the formation of numerous radical and non-radical species. In this review, the generation of sulfate radical (SR) and hydroxyl radical (HR) in AOPs were summarized. The significance of chalcopyrite in various approaches including Fenton, photo-Fenton, and photo/Fenton-like methods, as well as its involvement in electrochemical Fenton-based processes was discussed. The stability and reusability, toxicity, catalyst mechanism, and effects of operational parameters (pH, catalyst dosage, and oxidant concentration) are evaluated in detail. The review also discusses the role of Fe2+/3+, Cu1+/2+, S2- and Sn2- present in CuFeS2 in the generation of free radicals. Finally, guidelines for future research are presented in terms of future perspectives.

MXenes and MXene-based materials for removal of pharmaceutical compounds from wastewater: Critical review

The rapid increase in the global population and its ever-rising standards of living are imposing a huge burden on global resources. Apart from the rising energy needs, the demand for freshwater is correspondingly increasing. A population of around 3.8 billion people will face water scarcity by 2030, as per the reports of the World Water Council. This may be due to global climate change and the deficiency in the treatment of wastewater. Conventional wastewater treatment technologies fail to completely remove several emerging contaminants, especially those containing pharmaceutical compounds. Hence, leading to an increase in the concentration of harmful chemicals in the human food chain and the proliferation of several diseases. MXenes are transition metal carbide/nitride ceramics that primarily structure the leading 2D material group. MXenes act as novel nanomaterials for wastewater treatment due to their high surface area, excellent adsorption properties, and unique physicochemical properties, such as high electrical conductivity and hydrophilicity. MXenes are highly hydrophilic and covered with active functional groups (i.e., hydroxyl, oxygen, fluorine, etc.), which makes them efficient adsorbents for a wide range of species and promising candidates for environmental remediation and water treatment. This work concludes that the scaling up process of MXene-based materials for water treatment is currently of high cost. The up-to-date applications are still limited because MXenes are currently produced mainly in the laboratory with limited yield. It is recommended to direct research efforts towards lower synthesis cost procedures coupled with the use of more environmentally friendly materials to avoid secondary contamination.

Fundamental Perspectives on the Electrochemical Water Applications of Metal-Organic Frameworks

HIGHLIGHTS

The recent development and implementation of metal-organic frameworks (MOFs) and MOF-based materials in electrochemical water applications are reviewed. The critical factors that affect the performances of MOFs in the electrochemical reactions, sensing, and separations are highlighted. Advanced tools, such as pair distribution function analysis, are playing critical roles in unraveling the functioning mechanisms, including local structures and nanoconfined interactions. Metal-organic frameworks (MOFs), a family of highly porous materials possessing huge surface areas and feasible chemical tunability, are emerging as critical functional materials to solve the growing challenges associated with energy-water systems, such as water scarcity issues. In this contribution, the roles of MOFs are highlighted in electrochemical-based water applications (i.e., reactions, sensing, and separations), where MOF-based functional materials exhibit outstanding performances in detecting/removing pollutants, recovering resources, and harvesting energies from different water sources. Compared with the pristine MOFs, the efficiency and/or selectivity can be further enhanced via rational structural modulation of MOFs (e.g., partial metal substitution) or integration of MOFs with other functional materials (e.g., metal clusters and reduced graphene oxide). Several key factors/properties that affect the performances of MOF-based materials are also reviewed, including electronic structures, nanoconfined effects, stability, conductivity, and atomic structures. The advancement in the fundamental understanding of these key factors is expected to shed light on the functioning mechanisms of MOFs (e.g., charge transfer pathways and guest-host interactions), which will subsequently accelerate the integration of precisely designed MOFs into electrochemical architectures to achieve highly effective water remediation with optimized selectivity and long-term stability.