Efficient phosphate sequestration for water purification by unique sandwich-like MXene/magnetic iron oxide nanocomposites

Highly Efficient Catalytic Performances of Nitro Compounds and Morin via Self-Assembled MXene-Pd Nanocomposites Synthesized through Self-Reduction Strategy

With development of the society, the problem of environmental pollution is becoming more and more serious. There is the urgent need to develop a new type of sustainable green material for degradable pollutants. However, the conventional preparation method is limited by conditions such as cumbersome operation, high energy consumption, and high pollution. Here, a simple method named self-reduction has been proposed, to synthesize highly efficient catalytic nitro compounds and morin self-assembled MXene-Pd nanocomposites. Palladium nanoparticles were grown in situ on MXene nanosheets to form MXene@PdNPs. MXene@PdNPs composites with different reaction times were prepared by adjusting the reduction reaction time. In particular, MXene@PdNPs20 exhibited a high catalytic effect on 4-NP and 2-NA, and the first-order rate constants of the catalysis were 0.180 s-1 and 0.089 s-1, respectively. It should be noted that after eight consecutive catalytic cycles, the conversion to catalyze 4-NP was still greater than 94%, and the conversion to catalyze 2-NA was still greater than 91.8%. Therefore, the research of self-assembled MXene@PdNPs nanocomposites has important potential value for environmental management and sustainable development of human health, and provides new clues for the future research of MXene-based new catalyst materials.

Novel Two-Dimensional Magnetic Titanium Carbide for Methylene Blue Removal over a Wide pH Range: Insight into Removal Performance and Mechanism

Two-dimensional (2D) layer-structured titanium carbide MXenes (e.g., 2D Ti₃C₂ MXene) have received tremendous attention owing to their excellent properties and unique 2D planar topology. Nevertheless, there are still several challenges to be addressed for well dispersibility and easy separation from a heterogeneous system, hindering the practical applications. Herein, 2D Ti₃C₂ MXene, as the most typical member of 2D MXenes, is functionalized with magnetic Fe₃O₄ nanoparticles via an in situ growth approach (designated as MXene@Fe₃O₄), which exhibits the intriguing phenomenon on methylene blue (MB) adsorption in the environmental remediation realm. The maximum adsorption capacity of the MXene@Fe₃O₄ composites for MB is calculated to be 11.68 mg·g^-1 by a Langmuir isotherm model. A thermodynamic study of the adsorption demonstrates that the reaction process is exothermic and entropy-driven. Attractively, the removal process is a pH-independent process, and the optimal MB adsorption capacity is achieved at pH = 3 or 11, which is ascribed to electrostatic interactions and the hydrogen bond effect. X-ray diffraction, Fourier transform spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculation results reveal that the adsorption process is based on a combination of Ti-OH···N bonding, electrostatic attraction, and reductivity. Furthermore, multiple cycle runs demonstrate an excellent stability and reusability of MXene@Fe₃O₄ composites. This study provides a promising approach for the alternative removal of MB and broadens the potential application of 2D MXene for the treatment of practical acidic or alkaline wastewater.

Mesoporous zirconia nanostructures (MZN) for adsorption of As(III) and As(V) from aqueous solutions

Mesoporous zirconia nanostructures (MZN) were synthesized by hydrothermal method to efficiently remove highly mobile and toxic arsenite (As(III)) and arsenate (As(V)) from aqueous solutions. The as-synthesized MZN were characterized by Brunauer-Emmett-Teller (BET), X-Ray diffraction (XRD), scanning electron microscopy (SEM), high resolution transmission electron microscope (HRTEM), and Energy-dispersive X-ray spectroscopy (EDX) techniques. The batch adsorption experimental results showed that the As(III) and As(V) removal capacities of the MZN were 105.03 and 110.29 mg/g, respectively, under neutral pH conditions, which were better than many recently reported adsorbents. The adsorption behavior of As(III) and As(V) on the MZN could be well described by pseudo-second-order and Langmuir isotherms models. Moreover, As(III) and As(V) adsorption on the MZN was spontaneous and endothermic. Some of the common co-existing ions had slightly affected the arsenic removal proficiency of MZN. Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the adsorption mechanism of As(III) and As(V) on the as-synthesized MZN. The as-synthesized MZN demonstrated quite fast and good treatment of simulated real arsenic (As(III,V)) contaminated water. This study suggested that the as-synthesized MZN are potential candidate for practical applications of As(III) and As(V) removal from the aqueous solutions.

Synthesis of a novel magnetic Caragana korshinskii biochar/Mg-Al layered double hydroxide composite and its strong adsorption of phosphate in aqueous solutions

Phosphate pollution of aquatic ecosystems is of great concern and requires the development of high-performance materials for effective pollution treatment. To realize efficient phosphate removal from aqueous solution, an easily separable magnetic (Fe3O4) Caragana korshinskii biochar/Mg-Al layered double hydroxide composite (denoted as FCB/MAC) was synthesized via two-step electro-assisted modification for the first time. Subsequently, the physical and chemical properties of FCB/MAC were characterized. Furthermore, the sorption mechanism for phosphate removal was investigated in detail. The results indicated that Fe3O4 and the Mg-Al layered double hydroxide were successfully embedded in the biochar matrix. Moreover, FCB/MAC exhibited a high phosphate adsorption capacity and excellent magnetic properties for easy recovery. The maximum phosphate sorption capacity of FCB/MAC was 252.88 mg g-1, which is much higher than the capacities of most magnetic phosphate adsorbents. In addition, the adsorption kinetics and isotherms indicated that phosphate adsorption by FCB/MAC was controlled by the pseudo-second-order kinetic model and the Langmuir-Freundlich isotherm model. The phosphate adsorption mechanism involves anion exchange, electrostatic attraction, and ligand exchange. After five adsorption-desorption cycles, the phosphate adsorption capacity of FCB/MAC was 25.71 mg g-1 with 51.43% removal efficiency and high recyclability. Thus, the composite prepared in this study is a promising adsorbent for phosphate removal from aqueous solution, and this work provides an excellent reference for constructing novel biochar-based phosphate adsorbents.

Insights into the thermal and chemical stability of multilayered V2CTx MXene

We report on the thermal stability of multilayered V2CTx MXenes under different atmospheres by combining in situ Raman spectroscopy with ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) in order to elucidate and monitor the molecular, electronic, and structural changes of both the surface and bulk of the V2CTx MXene which has recently received much attention. The MXene samples were heated up to 600 °C in inert (N2), oxidative (CO2, air), and reductive (H2) environments under similar conditions. In situ Raman showed that the V[double bond, length as m-dash]O vibration for two-dimensional vanadia is preserved up to 600 °C under N2, while its intensity reduces under H2. When heated above 300 °C under either CO2 or air, V2CTx slightly oxidizes or transforms into V2O5, respectively. Furthermore, SEM revealed the presence of an accordion-like layered structure for the MXene under N2 and H2, while under CO2 and air the layered structure collapses and forms VO2 (V4+) and V2O5 (V5+) crystals, respectively. XPS reveals that, regardless of the gas, surface V species oxidize above 300 °C during the dehydration process. Finally, we demonstrated that the partial dehydration of V2CTx results in the partial oxidation of the material, and the total dehydration is achieved once 700 °C is reached. We believe that our methodology is a unique alternative to tune the dehydration, oxidation, and properties of V2CTx, which allows for the expansion of applications of MXenes.

MXene Boosted CoNi-ZIF-67 as Highly Efficient Electrocatalysts for Oxygen Evolution

Oxygen evolution reaction (OER) is a pivotal step for many sustainable energy technologies, and exploring inexpensive and highly efficient electrocatalysts is one of the most crucial but challenging issues to overcome the sluggish kinetics and high overpotentials during OER. Among the numerous electrocatalysts, metal-organic frameworks (MOFs) have emerged as promising due to their high specific surface area, tunable porosity, and diversity of metal centers and functional groups. It is believed that combining MOFs with conductive nanostructures could significantly improve their catalytic activities. In this study, an MXene supported CoNi-ZIF-67 hybrid (CoNi-ZIF-67@Ti₃C₂T_x) was synthesized through the in-situ growth of bimetallic CoNi-ZIF-67 rhombic dodecahedrons on the Ti₃C₂T_x matrix via a coprecipitation reaction. It is revealed that the inclusion of the MXene matrix not only produces smaller CoNi-ZIF-67 particles, but also increases the average oxidation of Co/Ni elements, endowing the CoNi-ZIF-67@Ti₃C₂T_x as an excellent OER electrocatalyst. The effective synergy of the electrochemically active CoNi-ZIF-67 phase and highly conductive MXene support prompts the hybrid to process a superior OER catalytic activity with a low onset potential (275 mV vs. a reversible hydrogen electrode, RHE) and Tafel slope (65.1 mV∙dec⁻¹), much better than the IrO₂ catalysts and the pure CoNi-ZIF-67. This work may pave a new way for developing efficient non-precious metal catalyst materials.

Oxygen-Functionalized Ultrathin Ti3 C2 Tx MXene for Enhanced Electrocatalytic Hydrogen Evolution

Two-dimensional (2D) transition-metal carbides (MXenes) are widely adopted as potential electrocatalysts for the hydrogen evolution reaction (HER) owing to their metallic conductivity, rich tunable surface chemistry, and atomic thickness with highly exposed active sites. Previously published theoretical results indicate that MXenes functionalized entirely with oxygen have lower ΔG_H* for HER. However, MXenes contain many terminal F groups on the basal plane, which is detrimental to the HER. Herein, the development of an ultrathin Ti₃ C₂ MXene nanosheet fully functionalized with oxygen is reported for the HER. The obtained oxygen-functionalized Ti₃ C₂ (Ti₃ C₂ O_x ) exhibits a much higher HER activity (190 mV at 10 mA cm^-2 ) than that of Ti₃ C₂ T_x (T=F, OH, and O). The improved HER performance is attributed to the highly active O sites on the basal plane of Ti₃ C₂ T_x MXenes. This study paves way for electrocatalytic applications of MXene materials by tuning their surface functional groups.

Ti3C2T x MXene decorated with Sb nanoparticles as anodes material for sodium-ion batteries

A large family of two-dimensional (2D) transition metal carbides, MXene, has demonstrated potential applications for electrochemical energy storage. 2D MXene sheets may buffer large volume changes and form a 3D conductive network to facilitate the electronic transfer of working electrodes. However, multilayer Ti₃C₂T _x material could only deliver a moderate capacity in sodium ion battery cells, below the requirement of commercial applications. Herein, we decorated multilayer MXene (Ti₃C₂T _x ) with Sb nanoparticles (NPs) via a facile solution-phase method, in which Sb NPs with a diameter of about 5-10 nm are absorbed on the surface of MXene layers by the electrostatic attraction action. The hybrid material Ti₃C₂T _x @Sb-0.5 delivers a higher reversible capacity of 200 mA h g^-1 at 0.1 A g^-1 than that of pure Ti₃C₂T _x (90 mA h g^-1), and shows a much better capacity retention of nearly 98% after 500 cycles compared with Sb NPs. Also, it achieves superior rate performance (remaining a capacity of 127 mA h g^-1 at 2 A g^-1) and excellent long-term stability (a capacity retention of almost 92.3% after 8000 cycles). These results indicate that Ti₃C₂T _x @Sb-0.5 possess a potential for high-performance sodium ion batteries anodes.

Fabrication of Cobaltosic Oxide Nanoparticle-Doped 3 D MXene/Graphene Hybrid Porous Aerogels for All-Solid-State Supercapacitors

MXenes are a new family of 2 D transition metal carbides and nitrides, which have attracted enormous attention in electrochemical energy storage, sensing technology, and catalysis owing to their good conductivity, high specific surface area, and excellent electrochemical properties. In this work, a series of Co₃ O₄ -doped 3 D MXene/RGO hybrid porous aerogels is designed and prepared through a facile in situ reduction and thermal annealing process, in which the reduced graphene oxide (RGO) conductive network can electrically link the separated Co₃ O₄ -MXene composite nanosheets, leading to enhanced electronic conductivity. It is found that upon using the Co₃ O₄ -MXene/RGO hybrid porous aerogel prepared with a mass ratio of Co₃ O₄ -MXene/RGO of 3:1 (CMR31) as an electrode for a supercapacitor, a superior specific capacitance of 345 F g^-1 at the current density of 1 A g^-1 is achieved, which is significantly higher than those of Ti₃ C₂ T_x MXene, RGO, and MXene/RGO electrodes. In addition, a high capacitance retention (85 % of the initial capacitance after 10 000 cycles at a high current density of 3 A g^-1 ) and a low internal resistance R_s (0.44 Ω) can be achieved. An all-solid-state asymmetric supercapacitor (ASC) device is assembled using CMR31, and it has the ability to light up a blue LED indicator for 5 min if four ASCs are connected in series. Therefore, these novel Co₃ O₄ -MXene/RGO hybrid porous aerogels have potential practical applications in high-energy storage devices.

Enhanced interaction in TiO2/BiVO4 heterostructures via MXene Ti3C2-derived 2D-carbon for highly efficient visible-light photocatalysis

Heterostructured photocatalysts play a significant role in the removal of contaminants by decreasing the recombination of the photo-induced charges. Herein, we presented novel TiO₂/C/BiVO₄ ternary hybrids employing a 2D layered Ti₃C₂ MXene precursor, overcoming the lattice mismatching of TiO₂/BiVO₄ binary heterostructures simultaneously. Raman and XPS analyses proved the strong coupling effects of TiO₂, carbon and BiVO₄ components, and the heterostructures were identified from high-resolution transmission electron microscopy results. Moreover, the ternary TiO₂/C/BiVO₄ composites exhibit excellent photocatalytic performance of Rhodamine B degradation, which is about four times higher than pure BiVO₄ and twice that of binary TiO₂/BiVO₄ heterostructures, reaching a reaction constant of 13.7 × 10^-3 min^-1 under visible-light irradiation (λ > 420 nm). In addition, for the possible mechanism for dye elimination it was proposed that RhB molecule be directly oxidized by photo-induced holes (h⁺) on the BiVO₄ components and superoxide radical ([Formula: see text]) generated from conduction band electrons of the heterostructures. This work will provide possibilities for developing visible-light responsive nanomaterials for efficient solar utilization.

Hydrated lanthanum oxide-modified diatomite as highly efficient adsorbent for low-concentration phosphate removal from secondary effluents

The requirement to the phosphorus (P) emission from wastewater treatment plants (WWTPs) is becoming increasingly strict, which makes an advanced treatment for the low-concentration phosphate removal from secondary effluents indispensable. In present work, hydrated lanthanum (La) oxide-modified diatomite composites (La-diatomite) were fabricated by a facile method and employed as the highly efficient adsorbent for the low-concentration phosphate removal from simulating secondary effluents. Comparative experiments indicated that the La-diatomite treated by 0.1 mol/L LaCl₃ exhibited the highest La availability (P/La molar ratio of 2.30) and performed good selectivity to phosphate adsorption even with the coexistence of competing anions and humic acid. The maximum P adsorption capacity reached to 58.7 mg P/g and the 96% P was removed quickly within 30 min at initial phosphate concentration 2 mg P/L. Insignificant La leaching was observed during the process due to the La stabilization by macroporous diatomite. Eight cycles of adsorption-desorption experiments revealed that the excellent repeated use property of La-diatomite. At the column test, La-diatomite showed superior treatment capacities of 3455 kg water/kg La-diatomite for simulated secondary effluents. The La-diatomite maintained high and stable adsorption effectiveness in wide pH range, which should be attributed to the synergistic effect of electrostatic interactions, ligand exchange and Lewis acid-based interaction. This work might provide a candidate for low-concentration phosphate removal from secondary effluent to alleviate the eutrophication.

Facile Preparation of Hierarchical AgNP-Loaded MXene/Fe3O4/Polymer Nanocomposites by Electrospinning with Enhanced Catalytic Performance for Wastewater Treatment

MXene as a kind of two-dimensional nanomaterial has aroused people's strong research interest because of its excellent properties. In the present study, we introduced a new poly(vinyl alcohol)/poly(acrylic acid)/Fe3O4/MXene@Ag nanoparticle composite film fabricated by electrospinning and heat treatment as well as self-reduction reaction process. The obtained composite films showed high self-reduction ability because of the incorporation of MXene flakes. The intercalated MXene flakes in the composite nanofibers were evenly distributed, which not only solved the aggregation problem from MXene dispersion but also could self-reduce Ag nanoparticles in situ in composite materials. In addition, the composite nanofiber films exhibited good fiber structure, thermal stability, and magnetic properties. Moreover, the composite nanofiber films demonstrated excellent catalytic ability and cycle stability to 4-nitrophenol and 2-nitroaniline.

Synthesis of mesoporous lanthanum hydroxide with enhanced adsorption performance for phosphate removal

Phosphate is a ubiquitous pollutant in aquatic systems, and increasingly stringent post-treatment phosphate effluent standards necessitate increasingly efficient removal techniques. In this study, mesoporous lanthanum hydroxide (MLHO) was synthesized by a hard-template method using ordered mesoporous silica, and its potential as an adsorbent for high-efficiency phosphate removal in aqueous solutions was tested. The porosity characteristics of MLHOs were controlled by adjusting the template structure and synthesis conditions. MLHO adsorbents showed great potential for phosphate removal from solutions containing both high and low initial phosphate concentrations. The phosphate adsorption capacity of MLHO strongly depended on its surface area as this process was governed by monolayer adsorption. Moreover, the phosphate removal performance of MLHO was affected by its structural properties. MLHO showed a high adsorption capacity of 109.41 mg P g⁻¹ at 28 °C (q_m by the Langmuir isotherm model). Further, it showed ultrafast adsorption in a solution with low initial concentration of 2 mg P/L; within the first 10 min, 99.8% of phosphate was removed, and the phosphorus concentration remaining in the solution dramatically reduced to 4 μg P/L. These findings suggest that MLHO adsorbent is a good candidate for rapid and efficient low-concentration phosphate removal to meet the increasingly stringent discharge standards for wastewater treatment plants.

MLHO exhibited outstanding phosphate removal performance with a high adsorption capacity as well as efficient removal in low P concentrations.

A flexible Ti3C2Tx (MXene)/paper membrane for efficient oil/water separation

The scalable fabrication of flexible membranes for efficient oil/water separation is in high demand but still significantly underdeveloped. Here, we present a flexible membrane using Ti₃C₂T_x (MXene) as the functional layer on conventional print paper as the substrate. With a simple coating process using MXene ink, we developed a highly hydrophilic and oleophobic membrane with an underwater oil contact angle of 137°. Such a simple membrane shows outstanding flexibility and robustness, and demonstrates a facile approach for membrane scale-up using MXene ink on low-cost print paper. The membrane shows high separation efficiency for oil/water emulsions, of over 99%, and a high water permeation flux of over 450 L per m² per h per bar. We demonstrate the excellent anti-fouling property of this membrane by cleaning the membranes without chemicals. These low-cost, highly efficient, anti-fouling membranes can provide new opportunities for industrial oil/water separation applications.

A highly hydrophilic and oleophobic membrane based on Ti₃C₂T_x (MXene) coated paper demonstrated high separation efficiency for oil/water emulsions with excellent antifouling properties.

FeNC/MXene hybrid nanosheet as an efficient electrocatalyst for oxygen reduction reaction

The iron–nitrogen–carbon (FeNC) catalyst, as a highly active and stable non-precious metal catalyst, has emerged as one of the most promising alternatives to replace the platinum catalyst for oxygen reduction reaction (ORR). Herein, a novel FeNC/MXene hybrid nanosheet was, for the first time, explored via pyrolysis of an iron–ligand complex and MXene nanosheets. The structure and morphology characterizations reveal that a thin and rugged FeNC coating was closely attached on the surface of MXene, forming a hybrid nanosheet structure with an excellent conductive substrate and many electrocatalytic active sites on the substrate. The electrochemical measurements disclose that the FeNC/MXene hybrid nanosheet exhibited a remarkable electrocatalytic performance, with a 25 mV higher half-wave potential (0.814 V versus RHE) than the Pt/C counterpart. More importantly, this hybrid presented a superb durability, with only 2.6% decay after a 20 000 s continuous test, much better than the 15.8% degradation for Pt/C. This work not only demonstrates the promising performance of the FeNC/MXene hybrid nanosheet for ORR, but more importantly provides new insight into the rational design of non-noble-metal catalysts using an MXene support.

A novel FeNC/MXene hybrid nanosheet with a rugged FeNC coating closely attached on the MXene surface was explored, which exhibited remarkable electrocatalytic activity with a superb durability (only 2.6% decay after a 20 000 s continuous test).

Recent Progress on Adsorption Materials for Phosphate Removal

BACKGROUND

High concentration of phosphate has been threatening human health and the ecosystem. Adsorption is one of high-efficiency and low-cost techniques to reduce the concentration of phosphate. This mini review aims to summarize the recent development of adsorption materials for phosphate removal.

METHOD

We conducted a detailed search of "adsorption of phosphate" in the published papers and the public patents on the adsorbents for phosphate based on Web of Science database in the period from January 1 2012 to December 31 2017. The corresponding literature was carefully evaluated and analyzed.

RESULTS

One hundred and forty one papers and twenty two recent patents were included in this review. An increased trend in scientific contributions was observed in the development of adsorption materials for phosphate removal. Three kinds of promising adsorbents: layered double hydroxides, natural materials, and metal oxides were paid special attention including removal mechanism, performance as well as the relationship between adsorption performance and structure. Both the chemical composition and the morphology play a key role in the removal capacity and rate.

CONCLUSION

The findings of this review confirm the importance of phosphate removal, show the development trend of high-performance and low-cost adsorption materials for phosphate removal, and provide a helpful guide to design and fabricate high-efficiency adsorbents.

A Facile Method to Construct MXene/CuO Nanocomposite with Enhanced Catalytic Activity of CuO on Thermal Decomposition of Ammonium Perchlorate

In this work, a mixing-calcination method was developed to facilely construct MXene/CuO nanocomposite. CuO and MXene were first dispersed in ethanol with sufficient mixing. After solvent evaporation, the dried mixture was calcinated under argon to produce a MXene/CuO nanocomposite. As characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and X-ray photoelectron spectra (XPS), CuO nanoparticles (60⁻100 nm) were uniformly distributed on the surface and edge of MXene nanosheets. Furthermore, as evaluated by differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA), the high-temperature decomposition (HTD) temperature decrease of ammonium perchlorate (AP) upon addition of 1 wt% CuO (hybridized with 1 wt% MXene) was comparable with that of 2 wt% CuO alone, suggesting an enhanced catalytic activity of CuO on thermal decomposition of AP upon hybridization with MXene nanosheets. This strategy could be further applied to construct other MXene/transition metal oxide (MXene/TMO) composites with improved performance for various applications.

Chemical Stability of Ti₃C₂ MXene with Al in the Temperature Range 500⁻700 °C

Ti₃C₂T_x MXene, a new 2D nanosheet material, is expected to be an attractive reinforcement of metal matrix composites because its surfaces are terminated with Ti and/or functional groups of –OH, –O, and –F which improve its wettability with metals. Thus, new Ti₃C₂T_x/Al composites with strong interfaces and novel properties are desired. To prepare such composites, the chemical stability of Ti₃C₂T_x with Al at high temperatures should be investigated. This work first reports on the chemical stability of Ti₃C₂T_x MXene with Al in the temperature range 500–700 °C. Ti₃C₂T_x is thermally stable with Al at temperatures below 700 °C, but it reacts with Al to form Al₃Ti and TiC at temperatures above 700 °C. The chemical stability and microstructure of the Ti₃C₂T_x/Al samples were investigated by differential scanning calorimeter, X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy.

Functional MXene Materials: Progress of Their Applications

Nowadays, two-dimensional materials have many applications in materials science. As a novel two-dimensional layered material, MXene possesses distinct structural, electronic, and chemical properties; thus, it has potential applications in many fields, including battery electrodes, energy storage materials, sensors, and catalysts. Up to now, more than 70 MAX phases have been reported. However, in contrast to the variety of MAX phases, the existing MXene family merely includes Ti₂ C, Ti₃ C₂ , (Ti_1/2 , Nb_1/2 )₂ C, (V_1/2 , Cr_1/2 )₃ C₂ , Nb₂ C, Ti₃ CN, Ta₄ C₃ , V₂ C, and Nb₄ C₃ . Among these materials, the Ti₃ C₂ T_x MXene exhibits prominently high volumetric capacitance, and the rate at which it transports electron is suitable for electrode materials in batteries and supercapacitors. Hence, Ti₃ C₂ T_x is commonly utilized as an electrode material in ion batteries such as Li⁺ , Na⁺ , K⁺ , Mg²⁺ , Ca²⁺ , and Al³⁺ batteries. What is more, Ti₂ C has the biggest specific surface area among all of these potential MXene phases, and therefore, Ti₂ C has remarkably high gravimetric hydrogen storage capacities. In addition, Ti₂ CO₂ materials display extremely high activity for CO oxidation, which makes it possible to design catalysts for CO oxidation at low temperatures. Furthermore, Ti₃ C₂ T_x with O, OH, and/or F terminations can be used for water purification owing to excellent water permeance, favorable filtration ability, and long-time operation ability. This review supplies a relatively comprehensive summary of various applications of MXenes over the past few years.

Environment-Sensitive Photoresponse of Spontaneously Partially Oxidized Ti3C2 MXene Thin Films

A large family of two-dimensional transition metal carbides and nitrides (MXenes) has increasingly raised interest for electronic and optoelectronic applications due to their high electrical conductivity, potentially tunable electronic structure, nonlinear optical properties, and ability to be manufactured in the thin film state. During delamination and storage in ambient air environment, spontaneous oxidation of MXene flakes leads to formation of titanium oxide, a process that, as we demonstrate here, can be harnessed for manufacturing MXene-titania composites for optoelectronics, sensing, and other applications. We show that partially oxidized MXene thin films containing the in situ formed phase of titanium oxide have a significant photoresponse in the UV region of the spectrum. The relaxation process of photoexcited charge carriers takes a long time (∼24 h) but can be accelerated in the presence of oxygen and water vapor in the atmosphere. These properties of spontaneously formed MXene-titania thin films make them attractive materials for photoresistors with memory effect and sensitivity to the environment, as well as many other photo- and environment-sensing applications.

Self-Assembly of Transition Metal Oxide Nanostructures on MXene Nanosheets for Fast and Stable Lithium Storage

Recently, a new class of 2D materials, i.e., transition metal carbides, nitrides, and carbonitrides known as MXenes, is unveiled with more than 20 types reported one after another. Since they are flexible and conductive, MXenes are expected to compete with graphene and other 2D materials in many applications. Here, a general route is reported to simple self-assembly of transition metal oxide (TMO) nanostructures, including TiO₂ nanorods and SnO₂ nanowires, on MXene (Ti₃ C₂ ) nanosheets through van der Waals interactions. The MXene nanosheets, acting as the underlying substrate, not only enable reversible electron and ion transport at the interface but also prevent the TMO nanostructures from aggregation during lithiation/delithiation. The TMO nanostructures, in turn, serve as the spacer to prevent the MXene nanosheets from restacking, thus preserving the active areas from being lost. More importantly, they can contribute extraordinary electrochemical properties, offering short lithium diffusion pathways and additional active sites. The resulting TiO₂ /MXene and SnO₂ /MXene heterostructures exhibit superior high-rate performance, making them promising high-power and high-energy anode materials for lithium-ion batteries.

Adsorptive environmental applications of MXene nanomaterials: a review

Since titanium carbide Ti3C2 nanosheets were first produced in 2011, an increasing number of members of this new family of two-dimensional transition metal carbides/nitride (MXene) materials have been successfully synthesized. Due to their large specific surface area, hydrophilic nature and abundant highly active surface sites, MXenes have been demonstrated to adsorb a variety of environmental pollutants, including heavy metal ions, organic dyes, radionuclides, and gas molecules, and thus can be used for the removal of pollutants and even sensing. In this review, we summarize the recent research progress on MXene materials in the adsorptive remediation of environmental pollutants and highlight the main challenges in the future to understand the full potential of MXene materials in environmental systems.

Estimation of oxygen effective diffusion coefficient in a non-steady-state biofilm based on response time

In wastewater treatment, oxygen effective diffusion coefficient (D _eff ) is a key parameter in the study of oxygen diffusion-reaction process and mechanism in biofilms. Almost all the reported methods for estimating the D _eff rely on other biokinetic parameters, such as substrate consumption rate and reaction rate constant. Then, the estimation was complex. In this study, a method independent of other biokinetic parameters was proposed for estimating the dissolved oxygen (DO) D _eff in biofilms. It was based on the dynamic DO microdistribution in a non-steady-state inactive biofilm, which was measured by the oxygen transfer modeling device (OTMD) combining with an oxygen microelectrode system. A pure DO diffusion model was employed, and the expression of the DO D _eff was obtained by applying the analytical solution of the model to a selected critical DO concentration. DO D _eff in the biofilm from the bioreactor was calculated as (1.054 ± 0.041) × 10^-9 m²/s, and it was in the same order of magnitude with the reported results. Therefore, the method proposed in this study was effective and feasible. Without measurement of any other biokinetic parameters, this method was convenient and will benefit the study of oxygen transport-reaction process in biofilms and other biofouling deposits. Graphical abstract ᅟ.

Rheological Characteristics of 2D Titanium Carbide (MXene) Dispersions: A Guide for Processing MXenes

Understanding the rheological properties of two-dimensional (2D) materials in suspension is critical for the development of various solution processing and manufacturing techniques. 2D carbides and nitrides (MXenes) constitute one of the largest families of 2D materials with >20 synthesized compositions and applications already ranging from energy storage to medicine to optoelectronics. However, in spite of a report on clay-like behavior, not much is known about their rheological response. In this study, rheological behavior of single- and multilayer Ti₃C₂T _x in aqueous dispersions was investigated. Viscous and viscoelastic properties of MXene dispersions were studied over a variety of concentrations from colloidal dispersions to high loading slurries, showing that a multilayer MXene suspension with up to 70 wt % can exhibit flowability. Processing guidelines for the fabrication of MXene films, coatings, and fibers have been established based on the rheological properties. Surprisingly, high viscosity was observed at very low concentrations for solutions of single-layer MXene flakes. Single-layer colloidal solutions were found to exhibit partial elasticity even at the lowest tested concentrations (<0.20 mg/mL) due to the presence of strong surface charge and excellent hydrophilicity of MXene, making them amenable to fabrication at dilute concentrations. Overall, the findings of this study provide fundamental insights into the rheological response of this quickly growing 2D family of materials in aqueous environments as well as offer guidelines for processing of MXenes.

Clay-Inspired MXene-Based Electrochemical Devices and Photo-Electrocatalyst: State-of-the-Art Progresses and Challenges

MXene, an important and increasingly popular category of postgraphene 2D nanomaterials, has been rigorously investigated since early 2011 because of advantages including flexible tunability in element composition, hydrophobicity, metallic nature, unique in-plane anisotropic structure, high charge-carrier mobility, tunable band gap, and favorable optical and mechanical properties. To fully exploit these potentials and further expand beyond the existing boundaries, novel functional nanostructures spanning monolayer, multilayer, nanoparticles, and composites have been developed by means of intercalation, delamination, functionalization, hybridization, among others. Undeniably, the cutting-edge developments and applications of clay-inspired 2D MXene platform as electrochemical electrode or photo-electrocatalyst have conferred superior performance and have made significant impact in the field of energy and advanced catalysis. This review provides an overview of the fundamental properties and synthesis routes of pure MXene, functionalized MXene and their hybrids, highlights the state-of-the-art progresses of MXene-based applications with respect to supercapacitors, batteries, electrocatalysis and photocatalysis, and presents the challenges and prospects in the burgeoning field.

First-Principle Study of Li-Ion Storage of Functionalized Ti2C Monolayer with Vacancies

Two-dimensional transition metal carbides are notable as promising anode materials for Li-ion batteries (LIBs). Using first-principle calculations, we investigate the effect of vacancies on the Li adsorption and diffusion on Ti₂C and Ti₂CT₂ (where T denotes surface terminations, F or OH) monolayers. Interestingly, we find that the carbon vacancies (V_C) tend to enhance the adsorption of Li in Ti₂C monolayer, whereas the titanium vacancies (V_Ti) play a similar role in Ti₂CT₂ when functional groups present. The presence of vacancies further leads to a change in the diffusion behavior of Li atoms. In this context, we propose an idea to mitigate the adverse effects on Li diffusion performance by regulating the functional groups. In the presence of V_C, the surface of Ti₂C monolayer is suggested to be modified with OH- groups due to its relatively low diffusion barrier in the range of 0.025-0.037 eV when Li diffuses around V_C, whereas in the presence of V_Ti, the surface is suggested to remove the functional groups, resulting in a decrease of energy barrier by about 1 eV when Li atom diffuses around V_Ti. The present study may provide a guideline to improve the Li-ion storage performance of Ti₂C monolayers as electrode materials in LIBs, with atomic vacancies being taken into consideration.

Mercuric ion capturing by recoverable titanium carbide magnetic nanocomposite

Two-dimensional metal carbides and nitrides (MXenes) have attracted increasing attention for application in water/wastewater treatment. The functionalization of MXenes to increase their stability while demonstrating high pollutant removal can facilitate sustainable water/wastewater treatment processes. In this study, the highly stable magnetic titanium carbide (Ti₃C₂T_x) MXene nanocomposite (MGMX nanocomposite) was successfully synthesized through a facile hydrothermal approach and was tested for aqueous-phase adsorptive removal of mercuric ions. The synthesized MGMX nanocomposite was studied using characteristic analyses, showing high stability as revealed by zeta-potential analysis and dynamic light-scattering technique. The MGMX nanocomposite presented excellent Hg(II) removal in a wide range of pH conditions, and an exceptional maximum experimental Hg(II) uptake capacity of 1128.41mgg^-1 was observed. The adsorption behavior was investigated using the Redlich-Peterson adsorption isotherm, pseudo second-order kinetics, and thermodynamics models. In the adsorption/desorption investigation, the MGMX nanocomposite was reusable for up to five cycles of adsorption/desorption. The stability, hydrophilic nature, available adsorptive surfaces, and easy separation after reaction make the MGMX nanocomposite an efficient sorbent for the removal of toxic Hg(II) for water purification.

Distinguished Cr(VI) capture with rapid and superior capability using polydopamine microsphere: Behavior and mechanism

Toxic heavy metal containing Cr(VI) species is a serious threat for ecological environment and human beings. In this work, a new mussel-inspired polydopamine microsphere (PDA-sphere) is prepared through in situ oxidative polymerization at air condition with controllable sizes. The adsorption of Cr(VI) ions onto PDA-sphere is highly pH dependent with the optimal pH ranging from 2.5 to 3.8. A rapid Cr(VI) removal can approach in 8min for equilibrium. More importantly, the prepared materials exhibit a remarkable sorption selectivity, coexisting SO₄^2-, NO₃^- and Cl^- ions at high levels; The applicability model further proves its effective performances with treated capacity of 42,000kg/kg sorbent, and the effluent can be reduced from 2000ppb to below 50ppb, which meets the drinking water criterions recommended by WHO. 1kg sorbent can also purify approximately 100t Cr(VI) contaminated wastewaters basing on the wastewater discharges of China. Such capacity for application ranks the top level for Cr(VI) removal. Additionally, the exhausted materials can be well regenerated by binary alkaline and salts mixtures. Such efficient adsorption can be ascribed to the well-dispersed morphology as well as the strong affinity between Cr(VI) and catechol or amine groups by XPS investigation. All the results suggest that polydopamine microspheres may be ideal materials for Cr(VI) treatment in waters.

Novel Fe3O4 nanoparticles-based DGT device for dissolved reactive phosphate measurement

The use of Fe₃O₄ nanoparticles as a liquid binding phase in DGT is demonstrated for the sampling measurement of dissolved reactive phosphate in natural water.

Highly efficient and selective phosphate removal from wastewater by magnetically recoverable La(OH)3/Fe3O4 nanocomposites

The use of lanthanum (La)-based materials for phosphate removal from water and wastewater has received increasing attention. However, challenges remain to enhance phosphate sorption capacities and recover La-based sorbents. In this study, magnetic La(OH)₃/Fe₃O₄ nanocomposites with varied La-to-Fe mass ratios were synthesized through a precipitation and hydrothermal method. Based upon preliminary screening of synthesized La(OH)₃/Fe₃O₄ nanocomposites in terms of phosphate sorption capacity and La content, La(OH)₃/Fe₃O₄ nanocomposite with a La-to-Fe mass ratio of 4:1 was chosen for further characterization and evaluation. Specifically, for these materials, magnetic separation efficiency, phosphate sorption kinetics and isotherm behavior, and solution matrix effects (e.g., coexisting ions, solution pH, and ionic strength) are reported. The developed La(OH)₃/Fe₃O₄ (4:1) nanocomposite has an excellent magnetic separation efficiency of >98%, fast sorption kinetics of 30 min, high sorption capacity of 83.5 mg P/g, and strong selectivity for phosphate in presence of competing ions. Phosphate uptake by La(OH)₃/Fe₃O₄ (4:1) was pH-dependent with the highest sorption capacities observed over a pH range of 4-6. The ionic strength of the solution had little interference with phosphate sorption. Sorption-desorption cyclic experiments demonstrated the good reusability of the La(OH)₃/Fe₃O₄ (4:1) nanocomposite. In a real treated wastewater effluent with phosphate concentration of 1.1 mg P/L, 0.1 g/L of La(OH)₃/Fe₃O₄ (4:1) efficiently reduced the phosphate concentration to below 0.05 mg P/L. Electrostatic attraction and inner-sphere complexation between La(OH)₃ and P via ligand exchange were identified as the sorption mechanisms of phosphate by La(OH)₃/Fe₃O₄ (4:1).

Size Modulation of Zirconium-Based Metal Organic Frameworks for Highly Efficient Phosphate Remediation

Eutrophication of water bodies caused by the excessive phosphate discharge has constituted a serious threat on a global scale. It is imperative to exploit new advanced materials featuring abundant binding sites and high affinity to achieve highly efficient and specific capture of phosphate from polluted waters. Herein, water stable Zr-based metal organic frameworks (MOFs, UiO-66) with rational structural design and size modulation have been successfully synthesized based on a simple solvothermal method for effective phosphate remediation. Impressively, the size of the resulting UiO-66 particles can be effectively adjusted by simply altering reaction time and the amount of acetic acid with the purpose of understanding the crucial effect of structural design on the phosphate capture performance. Representatively, UiO-66 particles with small size demonstrates 415 mg/g of phosphate uptake capacity, outperforming most of the previously reported phosphate adsorbents. Meanwhile, the developed absorbents can rapidly reduce highly concentrated phosphate to below the permitted level in drinking water within a few minutes. More significantly, the current absorbents display remarkable phosphate sorption selectivity against the common interfering ions, which can be attributed to strong affinity between Zr-OH groups in UiO-66 and phosphate species. Furthermore, the spent UiO-66 particles can be readily regenerated and reused for multiple sorption-desorption cycles without obvious decrease in removal performance, rendering them promising sustainable materials. Hence, the developed UiO-66 adsorbents hold significant prospects for phosphate sequestration to mitigate the increasingly eutrophic problems.

Highly Efficient Water Decontamination by Using Sub-10 nm FeOOH Confined within Millimeter-Sized Mesoporous Polystyrene Beads

Millimeter-sized polymer-based FeOOH nanoparticles (NPs) provide a promising option to overcome the bottlenecks of direct use of NPs in scaled-up water purification, and decreasing the NP size below 10 nm is expected to improve the decontamination efficiency of the polymeric nanocomposites due to the size and surface effect. However, it is still challenging to control the dwelled FeOOH NP sizes to sub-10 nm, mainly due to the wide pore size distribution of the currently available polymeric hosts. Herein, we synthesized mesoporous polystyrene beads (MesoPS) via flash freezing to assemble FeOOH NPs. The embedded NPs feature with α-crystal form, tunable size ranging from 7.3 to 2.0 nm and narrow size distribution. Adsorption of As(III/V) by the resultant nanocomposites was greatly enhanced over the α-FeOOH NPs of 18 × 60 nm, with the iron mass normalized capacity of As(V) increasing to 10.3 to 14.8 fold over the bulky NPs. Higher density of the surface hydroxyl groups of the embedded NPs as well as their stronger affinity toward As(V) was proved to contribute to such favorable effect. Additionally, the as-obtained nanocomposites could be efficiently regenerated for cyclic runs. We believe this study will shed new light on how to fabricate highly efficient nanocomposites for water decontamination.

Simultaneous Oxidation and Sequestration of As(III) from Water by Using Redox Polymer-Based Fe(III) Oxide Nanocomposite

Water decontamination from As(III) is an urgent but still challenging task. Herein, we fabricated a bifunctional nanocomposite HFO@PS-Cl for highly efficient removal of As(III), with active chlorine covalently binding spherical polystyrene host for in situ oxidation of As(III) to As(V), and Fe(III) hydroxide (HFO) nanoparticles (NPs) embedded inside for specific As(V) removal. HFO@PS-Cl could work effectively in a wide pH range (5-9), and other substances like sulfate, chloride, bicarbonate, silicate, and humic acid exert insignificant effect on As(III) removal. As(III) sequestration is realized via two pathways, that is, oxidation to As(V) by the active chlorine followed by specific As(V) adsorption onto HFO NPs, and As(III) adsorption onto HFO NPs followed by oxidation to As(V). The exhausted HFO@PS-Cl could be refreshed for cyclic runs with insignificant capacity loss by the combined regeneration strategy, that is, alkaline solution to rinse the adsorbed As(V) and NaClO solution to renew the host oxidation capability. In addition, fixed-bed experiments demonstrated that the HFO@PS-Cl column could generate >1760 bed volume (BV) effluent from a synthetic As(III)-containing groundwater to meet the drinking water standard (<10 μg As/L), whereas other two HFO nanocomposites, HFO@PS-N and HFO@D201 could only generate 450 and 600 BV effluents under otherwise identical conditions.

Response surface methodology as a powerful tool to optimize the synthesis of polymer-based graphene oxide nanocomposites for simultaneous removal of cationic and anionic heavy metal contaminants

Nanocomposites containing graphene oxide (GO), polyethyleneimine (PEI), and chitosan (CS) were synthesized for chromium(vi) and copper(ii) removal from water.

Hematite nanoparticle clusters with remarkably high magnetization synthesized from water-treatment waste by one-step “sharp high-temperature dehydration”

Hematite (α-Fe₂O₃) nanoparticle clusters with an exceptionally high magnetization of 51 emu g⁻¹ were synthesized for the first time. This material was prepared from water-treatment waste by a new “sharp high-temperature dehydration” process.

Effects of organic acids of different molecular size on phosphate removal by HZO-201 nanocomposite

Various organic acids in wastewater effluent could significantly influence the performance of phosphate adsorbent. This study focused on the effects of organic acids of different-molecular-size on phosphate adsorption by a novel nanocomposite HZO-201. Three organic acids (gallic acid (GA), tannic acid (TA) and humic acid (HA)) with distinct molecular size (HA > TA > GA) were chosen for this purpose. Both isotherm and kinetic tests of phosphate adsorption were conducted in the single-phosphate and binary system, and a series of microscopic techniques (i.e., XPS, FT-IR and SEM-EDX) and N₂ adsorption-desorption test were employed to explore the underlying mechanism. It was found that GA could greatly weaken phosphate adsorption capability of HZO-201 by directly competing for ammonium group on the nanocomposite, TA exhibited significant inhibition on phosphate adsorption rate mainly through pore constriction/blockage, while HA posed negligible impact on phosphate adsorption because of the size exclusion effect. It was also observed that although GA, TA and HA showed substantial influence on bulky HZO due to complexation, their impact on the nano-HZO loaded inside HZO-201 was little. The covalently bounded ammonium group and the networking pore structure of HZO-201 may play important roles in it.

Rational control of the interlayer space inside two-dimensional titanium carbides for highly efficient uranium removal and imprisonment

Rational control of the interlayer space of multilayered Ti₃C₂T_xenables the MXene to exhibit an excellent U(vi) sorption capacity and an exceptional radionuclide encapsulation performance.

Glyphosate contaminated soil remediation by atmospheric pressure dielectric barrier discharge plasma and its residual toxicity evaluation

Glyphosate was one of the most widely used herbicides in the world. Remediation of glyphosate-contaminated soil was conducted using atmospheric pressure dielectric barrier discharge (DBD) plasma. The feasibility of glyphosate degradation in soil was explored, and the soil leachate toxicity after remediation was assessed via a seed germination test. The experimental results showed that approximately 93.9% of glyphosate was degraded within 45min of DBD plasma treatment with an energy yield of 0.47gkWh-1, and the degradation process fitted the first-order kinetic model. Increasing the discharge voltage and decreasing the organic matter content of the soil were both found to facilitate glyphosate degradation. There existed appropriate soil moisture to realize high glyphosate degradation efficiency. Glyphosate mineralization was confirmed by changes of total organic carbon (TOC), chemical oxygen demand (COD), PO43- and NO3-. The degradation intermediates including glycine, aminomethylphosphonic acid, acetic acid, formic acid, PO43- and NO3-, CO2 and CO were observed. A possible pathway for glyphosate degradation in the soil using this system was proposed. Based on the soil leachate toxicity test using wheat seed germination, the soil did not exhibit any hazardous effects following high-efficiency glyphosate degradation.

Novel PVA/MOF Nanofibres: Fabrication, Evaluation and Adsorption of Lead Ions from Aqueous Solution

Plain polyvinyl alcohol (PVA) nanofibres and novel polyvinyl alcohol benzene tetracarboxylate nanofibres incorporated with strontium, lanthanum and antimony ((PVA/Sr-TBC), (PVA/La-TBC) and (PVA/Sb-TBC)), respectively, where TBC is benzene 1,2,4,5-tetracarboxylate adsorbents, were fabricated by electrospinning. The as-prepared electrospun nanofibres were characterized by scanning electron microscope (SEM), Fourier transform infrared (FTIR) and thermogravimetric analysis (TGA). Only plain PVA nanofibres followed the Freundlich isotherm with a correlation coefficient of 0.9814, while novel nanofibres (PVA/Sb-TBC, PVA/Sr-TBC and PVA/La-TBC) followed the Langmuir isotherm with correlation coefficients of 0.9999, 0.9994 and 0.9947, respectively. The sorption process of all nanofibres followed a pseudo second-order kinetic model. Adsorption capacity of novel nanofibres was twofold and more compared to that of plain PVA nanofibres. The thermodynamic studies: apparent enthalpy (ΔH°) and entropy (ΔS°), showed that the adsorption of Pb(II) onto nanofibres was spontaneous and exothermic. The novel nanofibres exhibited higher potential removal of Pb(II) ions than plain PVA nanofibres. Ubiquitous cations adsorption test was also investigated and studied.