MXsorption of mercury: Exceptional reductive behavior of titanium carbide/carbonitride MXenes

Nitrogen-doped 2D MXene-based catalysts: Synthesis, properties and applications for electrochemical hydrogen production

Designing advanced materials with a trade-off between overall electrocatalytic efficiency and economic cost for electrochemical hydrogen production is crucial to overcoming the current energy crisis and environmental issues. On the more 10-year journey since the discovery, transition-metal carbides/nitrides nanosheets (MXenes) have increasingly attracted attention as potential materials toward hydrogen/oxygen evolution reactions (HER/OER) because of their unique physical and chemical characteristics, but the layered restacking and low intrinsic electrochemical activity are dragging them out water-splitting technology. Doping MXenes with nitrogen atoms has recently been introduced as a facile but efficient strategy to accelerate the HER/OER efficiency by the optimization of electronic structure, surface terminations, and adsorption/desorption energies of intermediates on pristine MXenes. However, a comprehensive evaluation of the doping mechanism and content-structure-performance relationship of N-doped 2D MXene-related catalysts is still lacking. Thus, we herein systematically summarize synthetic strategies, theoretical calculations, properties, and applications of nitrogen-doped 2D MXenes for the HER and OER to give more fundamental insights into physicochemical characteristics of nitrogen-doped 2D MXenes to further design next-generation catalysts for the electrochemical hydrogen production and other applications.

Photoresponse of Ti3C2Tx MXene promotes its adsorptive-reductive removal of Cr(VI) from water

MXenes, such as Ti3C2Tx, demonstrate tremendous potential as heavy metal adsorbents due to their abundant reaction sites, high hydrophilicity, controllable interlayer spacing, and inherent reduction ability. However, their structural dependent pollutant removal performances and the related mechanisms are far less studied. Therefore, the removing abilities of Cr(VI) from water on Ti3C2Tx MXenes with different structures (multilayer (ML-) and delaminated (DL-) Ti3C2Tx) synthesized via several etching techniques were evaluated. Focusing on the most effective ML- and DL-Ti3C2Tx obtained by acid/fluoride salt etching, the impacts of structural variations on the Cr(VI) removal performances were explored. Both ML- and DL-Ti3C2Tx demonstrate outstanding Cr(VI) adsorption and reduction capabilities, achieving equilibrium within 500 min with capacities of 92.7 and 205 mg/g, respectively. The differences in removal mechanisms stemed from the varying adsorption and reduction capacities of two MXenes. ML-Ti3C2Tx, with lower surface area and porosity, had low adsorption capacity but superior reduction ability, efficiently converting most Cr(VI) to Cr(III) (66.8%). Conversely, DL-Ti3C2Tx exhibited better removal efficiency but a lower capacity for reduction (45.7%). Notably, although the partial reduction of DL-Ti3C2Tx to TiO2 results in its limited chemical reduction capacity, Ti3C2Tx might serve as a co-catalyst for TiO2, boosting the photoresponsiveness of DL-Ti3C2Tx or TiO2 through Ti3C2Tx/TiO2 heterojunctions, thereby facilitating photocatalysis to realize the reduction of Cr(VI). Both Ti3C2Tx exhibited both excellent Cr(VI) removal capacity and detoxification capacity, demonstrating their high potential in treating heavy metal pollutants in wastewater.

Electrochemical biosensor based on RNA aptamer and ferrocenecarboxylic acid signal probe for C-reactive protein detection

Monitoring health-related biomarkers using fast and facile detection techniques provides key physicochemical information for disease diagnosis or reflects body health status. Among them, electrochemical detection of various bio-macromolecules, e.g., the C-reactive protein (CRP), is of great interest in offering potential diagnosis for acute inflammation caused by infections, heart diseases, etc. Herein, a novel electrochemical aptamer biosensor was constructed from Ti3C2Tx MXene and in-situ reduced Au NPs for thiolated-RNA aptamer immobilization and CRP protein detection using Fc(COOH) as the signal probe. The sensory performances for CRP detection were optimized based on working conditions, including the incubation times and the pH. The large surface area offered by Ti3C2Tx MXene and high electrical conductivity originating from Au NPs endowed the as-fabricated aptamer biosensor with a decent sensitivity for CRP in a wide linear range of 0.05-80.0 ng/mL, good selectivity over interfering substances, and a low detection limit of 0.026 ng/mL. Such aptamer biosensors also detected CRP in serum samples using the spike & recovery method with reasonable recovery rates. The results demonstrated the potential of the as-fabricated electrochemical aptamer biosensor for fast and facile CRP detection in practical applications.

Research Progress on Metal Ion Recovery Based on Membrane Technology and Adsorption Synergy

The development of modern industry will generate more and more waste containing metal ions. It is necessary to take appropriate measures to recover these ions, whether from the perspective of environmental protection or improving economic benefits. So far, scientists have studied many methods for recovering metal ions. Among these methods, adsorption and membrane separation have received widespread attention due to their own characteristics. Combining adsorption and membrane separation methods can better leverage their respective advantages to improve the ability of recovering metal ions. This review, therefore, focuses on the synergistic recovery of metal ions by adsorption and membrane separation methods. This article first briefly explains the theoretical principles of membrane separation and adsorption synergy, and then focuses on several technologies that have received attention in different chapters. In these chapters, membrane technology is briefly introduced, followed by the situation and progress of synergistic application with adsorption technology. Then, the article compares and elaborates on the advantages and disadvantages of the above technologies, and finally summarizes and looks forward to these technologies being used to solve the difficulties and challenges in industrial application.

Recent strategies, progress, and prospects of two-dimensional metal carbides (MXenes) materials in wastewater purification: A review

With the rapid development of industrialization, water pollution directly leads to the serious shortage of fresh water. As reported by the World Water Council, nearly 3.8 billion people will face water scarcity by 2030. Therefore, developing advanced nanomaterials to realize wastewater purification is a major challenge. Two-dimensional (2D) transition metal carbides (MXenes), as the emerging 2D layered nanomaterials, have been investigated for the applications of water purification treatment since first reported in 2011. Over 40 different MXenes have been developed for environmental remediation, and dozens more structures and properties are theoretically predicted. Here, we review the advances from the aspects of synthesis strategies for MXenes, purification mechanism, and their applications in wastewater treatment processes. The major points are 1) the synthesis and modification approaches for MXenes such as multi-layered stacked MXenes and delaminated MXenes 2) a discussion of current water remediation over MXene-based materials, 3) a brief introduction for removal behaviors and deep interaction mechanisms, 4) optimization strategies and key points for boosting the remediation performance of MXenes.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

MXenes and MXene-supported nanocomposites: a novel materials for aqueous environmental remediation

Water contamination has become a significant issue on a global scale. Adsorption is a cost-effective way to treat water and wastewater compared to other techniques such as the Advanced Oxidation Processes (AOPs), photocatalytic degradation, membrane filtration etc. Numerous research experts are continuously developing inexpensive substances for the adsorptive removal of organic contaminants from wastewater. A fresh and intriguing area of inquiry has emerged as a result of the development of MXenes. This article aims to provide a preliminary understanding of MXenes from synthesis, structure, and characterization to the scope of further research. The applications of MXenes as a new generation adsorbent for remediation of various kinds of organic pollutants and heavy metals from wastewater are also summarized. MXenes with altered surfaces may make effective adsorbents for wastewater treatment. Lastly, the mechanism of adsorption of organic contaminants and heavy metals on MXenes is also discussed for a better understanding of the readers.

Efficient mercury removal from aqueous solutions using carboxylated Ti3C2Tx MXene

Water supplies contaminated with heavy metals are a worldwide concern. MXenes have properties that make them attractive for the removal of metal ions from water. This work presents a simple one-step method of Ti₃C₂T_x carboxylation that involves the use of a chelating agent with a linear structure, providing strong carboxylic acid groups with high mobility. The carboxylation decreases the zeta-potential of Ti₃C₂T_x by ~16 to ~18 mV over a pH range of 2.0-8.5 and improves Ti₃C₂T_x stability in the presence of molecular oxygen. pH in the range of 2-6 has a negligible effect on the adsorption capacity of Ti₃C₂T_x and COOH-Ti₃C₂T_x. Compared to Ti₃C₂T_x, COOH-Ti₃C₂T_x has a slightly higher and much faster mercury uptake, and the concentration of mercury ions leached out from COOH-Ti₃C₂T_x is lower. For both Ti₃C₂T_x and COOH-Ti₃C₂T_x, the leached mercury ion concentration is far below the U.S.-EPA maximum level. At an initial Hg²⁺ concentration of 50 ppm and pH of 6, COOH-Ti₃C₂T_x has the equilibrium adsorption capacity of 499.7 mg/g and removes 95% of Hg²⁺ in less than 1 min. Moreover, it has an equilibrium time of 5 min, which is significantly shorter than that of Ti₃C₂T_x (~ 60 min). Finally, its mercury-ion uptake capacity is higher than commercially available adsorbents reported in the literature. Its mercury removal is mainly via chemisorption and monolayer adsorption.

Prospects of titanium carbide-based MXene in heavy metal ion and radionuclide adsorption for wastewater remediation: A review

Contamination of water sources with various organic and inorganic non-biodegradable pollutants is becoming a growing concern due to industrialization, urbanization, and the inefficiency of traditional wastewater treatment processes. Transition Metal Carbides/Nitrides (MXenes) are emerging as advanced nanomaterials of choice for treating contaminated water owing to their excellent conductivity, mechanical flexibility, high specific surface area, scalable production, rich surface functionalities, and layered morphology. MXenes have demonstrated enhanced ability to adsorb various organic and inorganic contaminants depending upon their surface terminal groups (-OH, -F, and -O) and interlayer spacing. Titanium carbide (Ti₃C₂T_x) is most researched to date due to its ease of processing and stability. Ti₃C₂T_x has shown excellent performance in absorbing heavy metal ions and radioactive heavy metals. This review summarizes state-of-the-art Ti₃C₂T_x synthesis, including selective etching techniques, optimization of the desired adsorption features (controlling surface functional groups, intercalation, sonication, and functionalization), and regeneration and adsorption mechanism to remove contaminants. Furthermore, the review also compares the adsorption performance of Ti₃C₂T_x with other commercial adsorbents (including chitosan, cellulose, biomass, and zeolites). Ti₃C₂T_x has been found to have an adsorption efficiency of more than 90% in most studies due to its layered structure, which makes the functional groups easily accessible, unique and novel compared to other conventional nanomaterials and adsorbents. The challenges, potential solutions, and prospects associated with the commercial development of Ti₃C₂T_x as adsorbents are also discussed. The review establishes a framework for future wastewater treatment research using MXenes to address the global problem of water scarcity.