Surface Functionalized MXenes for Wastewater Treatment-A Comprehensive Review

Abstract

Over 80% of wastewater worldwide is released into the environment without proper treatment. Whilst environmental pollution continues to intensify due to the increase in the number of polluting industries, conventional techniques employed to clean the environment are poorly effective and are expensive. MXenes are a new class of 2D materials that have received a lot of attention for an extensive range of applications due to their tuneable interlayer spacing and tailorable surface chemistry. Several MXene-based nanomaterials with remarkable properties have been proposed, synthesized, and used in environmental remediation applications. In this work, a comprehensive review of the state-of-the-art research progress on the promising potential of surface functionalized MXenes as photocatalysts, adsorbents, and membranes for wastewater treatment is presented. The sources, composition, and effects of wastewater on human health and the environment are displayed. Furthermore, the synthesis, surface functionalization, and characterization techniques of merit used in the study of MXenes are discussed, detailing the effects of a range of factors (e.g., PH, temperature, precursor, etc.) on the synthesis, surface functionalization, and performance of the resulting MXenes. Finally, the limits of MXenes and MXene-based materials as well as their potential future research directions, especially for wastewater treatment applications are highlighted.

Keywords: 2D materials; MXene; functionalization; wastewater treatment.

Figure 1

MXene Applications: actuators (Reproduced with permission.^{[ 20 ]} Copyright 2019, Springer Nature), EMI shielding (Reproduced with permission.^{[ 21 ]} Copyright 2019, Elsevier), supercapacitor (Reproduced with permission.^{[ 22 ]} Copyright 2017, Springer Nature), sensors (Reproduced with permission.^{[ 23 ]} Copyright 2017, RSC), water purification (Reproduced with permission.^{[ 17 ]} Copyright 2020, Wiley‐VCH), photothermal therapy (Reproduced with permission.^{[ 24 ]} Copyright 2013, American Ceramic Society) and field effect transistor (Reproduced with permission.^{[ 25 ]} Copyright 2019, Elsevier).

Figure 2

Domestic, industrial, and natural sources of wastewater.

Figure 3

Classification and processes involved in conventional wastewater treatment.^{[ 49} , ⁵³ , ^{59 ]}

Figure 4

a) MXene composition: M is an early transition metal (green), A = group IIIA or IVA elements (purple), X is carbon/nitrogen or both (red). b) Structure of MAX phases and c) the corresponding MXenes.

Figure 5

XRD patterns of a) Ti₃C₂ MXene, amino functionalized MXene (MXene‐NH₂), and MXene/Polyethylenimine/Sodium alginate (labeled as MPA) showing disappearance of the (0002) peak and reduction in the intensity of the (006) and (008) peak. Reproduced with permission.^{[ 117 ]} Copyright 2021, Elsevier. b)Ti₃C₂T _x , NH₂‐Ti₃C₂T _x ‐0.2, NH₂‐Ti₃C₂T _x ‐0.5 and NH₂‐Ti₃C₂T _x ‐1.0 showing shift in (002) from 9.02° (Ti₃C₂T _x ) to 9.20° (NH₂‐Ti₃C₂T _x ‐0.2), 9.32° (NH₂‐Ti₃C₂T _x ‐0.5) and 9.48°(NH₂‐Ti₃C₂T _x ‐1.0). Reproduced with permission.^{[ 118 ]} Copyright 2021, Elsevier. c) XRD of a series of carboxyl modified MXene with different reaction ratios of diazonium salt. Reproduced with permission.^{[ 139 ]} Copyright 2020, Elsevier. XPS wide scan spectrum of d) pristine Ti₃C₂ and Ti₃C₂–SO₃H and narrow scan spectra of Ti₃C₂–SO₃H showing the C 1s, N 1s, O 1s, S 2p, Ti 2p peaks. Reproduced with permission.^{[ 127 ]} Copyright 2019, Elsevier. Multilayer Ti₃C₂T _x e) and NH₂‐Ti₃C₂T _x f) rising of a new peak of C1s which occurs at 286.3 eV which was assigned to C—N. Reproduced with permission.^{[ 118 ]} Copyright 2021, Elsevier. g) SEM image of MXene@Fe₃O₄ demonstrating the 2D lamellar structure with spherical shape Fe₃O₄ nanoparticles (inset), h) High‐resolution TEM (HRTEM) and i) elemental mapping analyses (Ti: purple, C: green, Fe: yellow, and O: red) of MXene@Fe₃O₄. Reproduced with permission.^{[ 137 ]} Copyright 2019, American Chemical Society. SEM images of j) pristine Ti₃C₂ MXene and k) MX‐MNPs, l) HRTEM of MX‐MNPs. Reproduced with permission.^{[ 137 ]} Copyright 2020, Elsevier.

Figure 6

a) N₂ adsorption‐desorption isotherms of MXene/PEI modified sodium alginate aerogel (MPA) exhibiting a typical Type‐IV curve with H3 hysteresis (inset is the corresponding pore size distribution of MPA). Reproduced with permission.^{[ 117 ]} Copyright 2021, Elsevier. b) N₂ adsorption‐desorption isotherms of bulk Ti₃C₂T _X  and the Ti₃C₂T _X nanosheets obtained by the expansion method. Reproduced with permission.^{[ 102 ]} Copyight 2021, Elsevier. c) Specific surface area of Ti₃C₂T _X and NH₂‐Ti₃C₂T _x with different contents of APTES (inset shows the mesopores size distribution). Reproduced with permission.^{[ 118 ]} Copyright 2021, Elsevier. d) FT‐IR spectra of MXene, MXene‐NH₂, MPA, MPA‐Cr(VI) and MPA‐CR. Reproduced with permission.^{[ 117 ]} Copyright 2021, Elsevier. e) FT‐IR spectra of Ti₃C₂T _X and NH₂‐ Ti₃C₂T _X ‐0.5 nanosheets. Reproduced with permission.^{[ 118 ]} Copyright 2021, Elsevier. f) FT‐IR of Ti₃C₂T _X MXene, SA20, MX‐SA_4:20, and Hg²⁺@MX SA_4:20. Reproduced with permission.^{[ 147 ]} Copyright 2019, Elsevier. g) FT‐IR spectra of MXene and MXenes with PDOPA. Reproduced with permission.^{[ 123 ]} Copyright 2019, Elsevier. h) TGA curves of MXene, MXene‐PDA (polydopamine), and MXene‐PDA‐IL (from room temperature to 800 °C under N2 ambient). Reproduced with permission.^{[ 101 ]} Copyright 2021, Elsevier. i) Thermogravimetric analysis of CA (cellulose acetate), CCA (cross‐linked CA), 10%MXene/CA, and 10%MXene@CA membranes.^{[ 112 ]} j) TGA of MXene‐carbon nanotube membranes with two stages of weight loss from 30 to 200 °C. Reproduced with permission.^{[ 116 ]} Copyright 2021, Elsevier. k) Raman spectra showing the characteristic signals of the HCS (hydrothermal carbon sphere), CPCM, and MXene/chitosan‐based porous carbon microspheres (CPCM@MXene) at 1350 cm⁻¹ (D band) and 1590 cm⁻¹ (G band). Reproduced with permission.^{[ 85 ]} Copyright 2021, Elsevier. l) Raman spectra of Ti₃C₂T _x , PmPD (poly(m‐phenylenediamine), and Ti₃C₂T _x /PmPD. Reproduced with permission.^{[ 111 ]} Copyright 2019, Environmental Research and Public Health.

Figure 7

a) Diagram of procedure of hydrothermal synthesis employed in functionalizing MoS₂ on Ti₃C₂ MXene sheets. Reproduced with permission.^{[ 136 ]} Copyright 2019, Elsevier. b) Schematic diagram of the solvothermal process employed in the preparation process of Ti₃C₂‐Bi/BiOCl composite. Reproduced with permission.^{[ 172 ]} Copyright 2020, Elsevier. c) Schematic formation of Bi₃TaO₇/Ti₃C₂ MXene sample by hydrothermal method. Reproduced with permission.^{[ 113 ]} Copyright 2020, Elsevier. d) Schematic diagram showing a novel one‐step strategy for the preparation of Fe₃O₄ nanoparticles on Ti₃C₂ MXene. Reproduced with permission.^{[ 108 ]} Copyright 2020, Elsevier. e) Schematic diagram of synthesis process involving the use of electrochemical anodization in producing g‐C₃N₄/Ti₃C₂ MXene/TNTAs. Reproduced with permission.^{[ 172 ]} Copyright 2020, Elsevier.

Figure 8

Various applications of surface functionalized MXenes used as adsorbent materials for wastewater treatment.

Figure 9

Surface modified MXene used as adsorbent a) delaminated Ti₃C₂T _x  MXene (DL‐ Ti₃C₂T _x ) showing better adsorptive properties as compared to multilayer Ti₃C₂T _x . Reproduced with permission.^{[ 179 ]} Copyright 2017, American Chemical Society. b) Digital photograph on adsorption of different metal ions (1 mg L⁻¹) on histidine His@TiO₂@d‐ Ti₃C₂T _x colloidal suspension (50 mg L⁻¹) after 5min of contact and c) removal efficiency of Cu²⁺ ions versus different concentrations of His@TiO₂ @ d‐ Ti₃C₂T _x which shows ≈75% of Cu²⁺ ions was removed by the 1gL⁻¹ His @TiO₂@ Ti₃C₂T _x  MXene. Reproduced with permission.^{[ 86 ]} Copyright 2020, John Wiley and Sons. d) High adsorption efficiency of Cr(VI) with surface functionalized Ti₃C₂T _x  MXene with poly(m‐phenylenediamine). Reproduced with permission.^{[ 206 ]} Copyright 2019, Elsevier. e) Methylene blue removal rate of alkali treated MXenes (LiOH‐Ti₃C₂T _x  MXene, NaOH‐Ti₃C₂T _x , and KOH‐ Ti₃C₂T _x ). Reproduced with permission.^{[ 178 ]} Copyright 2017, Elsevier. f) Comparison of Cu²⁺ adsorption between Ti₃C₂T _x  MXene and Ti₃C₂T _x  MXene surface functionalized with levodopa (amino acid). Reproduced with permission.^{[ 123 ]} Copyright 2019, Elsevier. g) Plot of adsorption performances of Ti₂CT _x  MXene chitosan functionalized on Ti₂CT _x  MXene (Ti₂CT _x ‐CS), lignosulfonate functionalized on Ti₂CT _x (Ti₂CT _x ‐LS), and enzymatic hydrolysis lignin functionalized on Ti₂CT _x  MXene (Ti₂CT _x ‐EHL). Reproduced with permission.^{[ 205 ]} Copyright 2019, Elsevier. h) Simultaneous adsorption for 8 toxic metal ions in a batch system (conditions: 50 mg adsorbent dose added to 30 mL aqueous solution containing 3 ppm of each metal ion agitated. Reproduced with permission.^{[ 147 ]} Copyright 2019, Elsevier.

Figure 10

Various applications of MXenes used as photocatalyst materials for wastewater treatment.^{[ 102} , ¹⁰⁸ , ¹¹³ , ^{163 ]}

Figure 11

a) Photocatalytic degradation of ciprofloxacin over surface functionalized Ti₃C₂ MXene with Bi/BiOCl. Reproduced with permission.^{[ 163 ]} Copyright 2020, Elsevier. b) Photocatalytic degradation efficiencies of Fe₃0₄‐Ti₃C₂ MXene on four organic dyes (rhodamine B, methyl orange, congo red, methylene blue) and c) degradation efficiencies of Ti₃C₂‐MNPs with different mass ratios and degradation efficiencies with only Ti₃C₂‐MNPs. Reproduced with permission.^{[ 108 ]} Copyright 2020, Elsevier. d) adsorption kinetic curves of as‐prepared Ti₃C₂ MXene—Co₃O₄ nanocomposite on i,ii) rhodamine B and iii,iv) methylene blue. Reproduced with permission.^{[ 152 ]} Copyright 2019, ACS. e) i,iii) UV–vis spectra of 4‐nitrophenol (4‐NP) and 2‐nitroamiline (2‐NA) before and after adding NaBH₄ aqueous solution, ii, iv) UV– vis spectra for the photocatalytic reduction of 4‐NP and 2‐NA with polyvinyl/polyacrylic/Fe₃O₄/Ti₃C₂T _x MXene@AgNP20 composite nanofibers. Reproduced with permission.^{[ 224 ]} Copyright 2019, ACS.

Figure 12

Novel Ti₃C₂ MXene on cellulose membrane.

Figure 13

a) Schematic demonstration of the concept design for the MXene/cellulose anti‐biofouling membrane steam generator; b) Picture of MXene/cellulose membrane having 15 cm diameter and 0.2 mm thickness (inset shows the image of the flower based on the MXene/cellulose membrane); c) Water treatment shows the wetted part from bottom area to top part of MXene/cellulose membranes that spreads with time; d) EDS mapping and e) TEM cross‐section of the MXene/cellulose membrane. The magnified picture in the inset of e) shows the layered MXene sheets structure; f) IR thermal pictures of bulk water, and the surface of the membranes of MXene/cellulose, and rGO/cellulose for 0 and 600s; g) SEM image of the MXene/cellulose membrane surface with E. coli and S. aureus; h) Densities of S. aureus and E. coli on MXene/cellulose, cellulose and rGO/cellulose membranes based on the SEM images; i) Temperatures as a function of irradiation time of the surface of MXene/cellulose, and rGO/cellulose membranes and bulk water under 1 sun solar illumination. Antibacterial efficiencies as a function of contact time for j) E. coli and k) S. aureus on MXene/cellulose and rGO/cellulose membranes; l) Pictures of agar plates with the bacterias S. aureus and E. coli cells growth exposed to and MXene/cellulose rGO/cellulose, and cellulose, membranes for 24 h contact. Reproduced with permission.^{[ 237 ]} Copyright 2019, American Chemical Society.

Figure 14

Novel Ti₃C₂ MXene for radioactive purposes.^{[ 242} , ²⁴³ , ^{244 ]}

Figure 15

a) U(VI) sorption from aqueous solution onto multi‐layered V2CT _x  MXene as a function of i) Ph, ii) contact time, iii) initial U(VI) concentration, and iv) competing metal cations. Reproduced with permission.^{[ 242 ]} Copyright 2016, American Chemical Society. b) Removal of Anionic Re(VII) from aqueous solution by Ti₂CT _x  MXene and poly(dially/dimethylammonium chloride–PDDA) nanocomposite and c) effect of competitive anion species on Re(VIII) removal by Ti₂CT _x  MXene nanocomposite/PDDA composite. Reproduced with permission.^{[ 244 ]} Copyright 2019, American Chemical Society.