Silane-Grafted MXene (Ti3C2T X ) Membranes for Enhanced Water Purification Performance.

The current communication describes the modifications of MXene (Ti3C2T x ) with silane grafting reaction for membrane preparation for enhanced water purification. The MXene was successfully grafted with n-octadecyltrichlorosilane (MODCS), n-octyltrichlorosilane (MNOCS), and triphenylchlorosilane (MTPCS) in order to make a hydrophobic MXene that could be able to bind with the organic matrix/polymers. The modified MXenes were transformed into thin membranes by forming an MXene/polyvinyl alcohol (PVA) composite over a filter paper support, that is, MCE (mixed cellulose ester filter paper). MXene membranes were also formed without the MCE support by using PVA and glutaraldehyde (PVA/GA) where GA was used as a cross-linker to stabilize PVA and make it water-resistant. The conditions of membrane formation were optimized to investigate optimum compatible conditions with the modified materials. The resulting membranes were tested for the removal of various organic pollutants that included mesitylene (or trimethylbenzene); polyaromatic hydrocarbons (chrysene, as a model); biphenyl; bisphenol A; benzene, toluene, ethylbenzene, and styrene; methylene blue; and Sudan II dyes. The MTPCS PVA/GA cross-linked membrane showed the best results for a pollutant removal efficiency up to 98%. Overall, all six types of membranes showed the removal efficiency in the range of 52-98%. It was observed that the membrane exhibits reusability up to five cycles.

Introduction

The adequate and potable water in the modern world of rapid urbanization is a requisite.[1] A global environmental challenge is posed by the pollutants like pharmaceuticals, heavy metals, aromatic compounds, dyes, and salts that are present in water and wastewater streams; a majority of them are toxic and threatening for living organisms.[2] Organic micropollutants (OMPs) are typically present (ng L–1 to μg L–1 orders) in relatively low concentrations. They are present in reservoirs of fresh water, natural bodies of water, effluents for water treatment, and soil sediments and have shown the potential for significant health threats to humans and marine life.[3] Membrane filtration is very versatile and has become increasingly popular for water treatment.[4] Especially, for the removal of micropollutants that pose a challenge in the traditional water treatment techniques and remain unaffected, membrane treatment offers a satisfactory alternative as in the case of membrane filtration, there is a mechanism of surface retention of contaminants.[5,6] Membrane filtration is considered a robust treatment technology because of the large range of selectivity or preservation of pollutants in water. There are few mechanisms for the removal of OMPs by membrane filtration, for example, (i) size exclusion[7] adsorption and interaction of hydrophobic OMP to hydrophobic membrane surfaces, (ii) electrostatic repulsion between the surface of the charged membrane and charged molecules of OMP,[8] and (iii) hydrophobic surface interactions and hydrogen bonding between OMPs and the membrane. Generally, the size exclusion mechanism is followed in membrane filtration.[5] Size exclusion membrane filtration is a widely used simple technique. In size exclusion, pollutants are sieved out based on their sizes and OMPs with a larger size than the membrane pore are retained. The mechanism of size exclusion is well understood, especially in the application for the removal of particulate matters and suspended solids, which are large in size. However, the size of OMPs should not be exclusively based on the molecular weight; the length of the molecule or shape (width) should also be considered.[5]

In 2011, Drexel University scientists discovered a new family of two-dimensional (2D) materials, reporting the 2D layered materials resulting from the exfoliation of three-dimensional (3D) transition-metal carbides through the selective etching of “A” elements from the MAX phases. The members of the MXene family are transition-metal carbides, nitrides, and carbonitrides.[9] The suffix “ene” in MXene indicates the similarity to graphene.[10] The MAX phases are precursors of the MXenes. These are 3D carbides or nitrides of ternary metals, represented as MAX (MAX), where M is a transition metal (Mo, Cr, Zr, Ti, etc.); A represents elements in the periodic table from group 13 to 16 such as Al, Si, Ge, Ga, and so forth; X may be either nitrogen or carbon, or their combination; and n varies between 1 and 3.[11] MXenes have distinct properties from their MAX phase precursors and are generally described as MXT, while T represents the terminated functional groups (−O, −F, −OH) resulting from the acid interaction in the etching step. The first synthesized MXene was Ti3C2T,[12] and up to now, MXenes (especially Ti3C2T) have drawn much attention. Owing to unique characteristics, such as high electronic conductivity and hydrophilicity and adsorptive, reductive, and antibacterial characteristics, MXenes are an excellent fit for environmental applications.[13] By reason of element abundance and nontoxic decomposition, Ti3C2T has found the most applications in water treatment.[14]

MXene membranes can alter interlayer distances and respond by intercalating ions (molecular/ion sieving mechanism). They are also used for desalination because of hydrophilic behavior.[15] The three most common fabrication techniques include[16] (i) preparation of lamellar-structure membranes by employing MXenes as skeleton materials, (ii) mixed-matrix membranes with MXenes prepared by using additives or other nanomaterials such as TiO2 and graphene oxide and so forth, and (iii) coating materials on membrane supports including polyvinylidene fluoride, anodic aluminum oxide, and so forth. The increased efficiency of MXene composite membranes can be attributed to (i) more water transport pathways provided by MXene nanosheets/nanofragments/combination with other materials than pristine membranes, (ii) increased interlayer spacing and production of abundant nanochannels due to separation of pores resulting from intercalated nanoparticles, and (iii) functionalization of MXene-based membranes with different functional groups (e.g., −COOH, −NH2, C6H6), which enhances transport of solvents like toluene, n-heptane, isopropanol, and so forth.[16] MXene polymeric and nanoparticle composites in the membrane form have also been used.[2]

Hydrophobic MXene membranes for solar steam generation have been prepared for seawater desalination, wastewater remediation, and sunlight harvesting. The author reported the use of Ti3C2 nanosheets layered with trimethoxy(1H,1H,2H,2H-perfluorodecyl)silane for solar desalination.[17] Similarly, 1H,1H,2H,2H-perfluorooctyltriethoxysilane was used for the surface modification of MXenes[18] to develop a superhydrophobic matrix for solar desalination systems. In another study, MXene membranes (Ti3C2) were fabricated using an aqueous suspension of an MXene which was filtrated using a commercial PVDF membrane, causing a deposition of Ti3C2 thin layers stacked on the upper surface of the PVDF substrate.[19] Grafted sulfonated polyelectrolyte brushes on the surface of the MXene (Ti3C2T) earlier functionalized with [3-(methacryloxy)propyl]trimethoxysilane] using a surface-initiated precipitation polymerization technique has been reported[20] to fabricate proton-conducting membranes. 3-Aminopropyltriethoxysilane was used in another study to functionalize the surface of Ti3C2T in a polyacrylonitrile or polydimethylsiloxane matrix to develop thin-film membranes that showed selectivity for alcohol-based solvents and strong enhancement in flux.[21] Functionalizing the MXene surface with silane coupling agents with various organofunctional groups will enable a wide variety of applications.[22]

In the current study, organo-MXene composite filtration membranes were synthesized using silane grafting to produce a hydrophobic MXene that could be able to bind with the organic matrix/polymers. The modified MXenes were transformed into thin membranes by forming a MXene/polyvinyl alcohol (PVA) composite over a filter paper support, that is, MCE (mixed cellulose ester filter paper). MXene membranes were also formed without the MCE support by using PVA/glutaraldehyde (GA),[23] where GA was used as a cross-linker to stabilize PVA and make it water-resistant. The conditions of membrane formation were optimized to investigate optimum compatible conditions with the modified materials. The resulting membranes were tested for the removal of various organic pollutants that included mesitylene (or trimethylbenzene); polyaromatic hydrocarbons (PAHs) (chrysene, as a model); biphenyl; bisphenol A; benzene, toluene, ethylbenzene, and xylene (BTEX); methylene blue; and Sudan II dye.

Experimental Section

Materials

The MAX phase, that is, Ti3AlC2 99% (Nanoshel, UK), hydrofluoric acid 48%, sodium hydroxide, PVA M wt 72,000, Sudan II dye, methylene blue, BTEX, chrysene, mesitylene, biphenyl and bisphenol A (Sigma-Aldrich), n-octadecyltrichlorosilane (95%), n-octyltrichlorosilane (95%), and triphenylchlorosilane (95%) were provided by Alfa Aesar Co., Germany. GA and MCE membrane filters having a pore size of 0.22 μm were provided by Merck, Germany.

Silane Grafting Procedure

The synthesis of the MXene was carried out using the reported procedure[9] as follows: 1.0 g of the MAX phase was taken in a Teflon beaker; 10 mL of 48% hydrofluoric acid was added slowly, and it was kept under stirring for 24 h at 60 °C to ensure complete etching of the aluminum layer. The resulting suspension was washed with DI water, and centrifugation was done for 10 min at 5000 rpm to obtain the settled material. The process of washing was repeated three to four times until the pH reached around 7. The MXene was finally obtained after oven drying overnight at 60 °C. The MXene was dispersed in water and then ultrasonicated for 1 h to obtain a few/single-layer flakes as the result of delamination. The resulting MXene further went through alkaline treatment with 2 g of NaOH for 2 h with constant stirring. The obtained Ti3C2(OH)2 was separated through centrifugation and dried in oven overnight at 60 °C. Silane grafting resulted when 1:1 MXene, that is, Ti3C2(OH)2, and silanes (1 g) were placed under reflux for 48 h in 30 mL of toluene and HCl gas (as a byproduct) was allowed to escape time to time. After washing with toluene, methanol, and water several times, followed by filtration and oven drying at 60 °C for 4 h, the grafted hydrophobic materials were obtained and were labeled as MODCS, MNOCS, and MTPCS, indicating MXene grafting with n-octadecyltrichlorosilane, n-octyltrichlorosilane, and triphenylchlorosilane, respectively.

Fabrication of MCE Filter Paper-Supported Silane-Grafted MXene Membranes

PVA was used as a binder to prepare MXene polymer membranes.[24] In order to prepare 1% solution of PVA, 1 g of PVA was added in 100 mL of water and left for stirring for 3 h at 90 °C. Homogeneous dispersions were obtained after 2 h stirring by adding 0.1 g each of the MXene, MODCS, MNOCS, and MTPCS separately in 100 mL of 1% PVA solution. The dispersions were then passed through an MCE membrane filter (0.22 μm as a support) through vacuum filtration assembly. The uniform layers of unmodified MXene and modified materials (i.e., silane-grafted MXenes) were deposited over the MCE filter paper.

Fabrication of silane-grafted MXene membranes with PVA/GA cross-linked (without the filter paper support).

Using PVA as a binder and glutaraldehyde (GA) as a cross-linker, silane-grafted MXene membranes were fabricated. Casting solutions were prepared by adding 0.5 mL of GA in 4 mL of 1% PVA and then dispersing 0.1 g each of the MXene, MODCS, MNOCS, and MTPCS separately in the solutions. They were then poured into Teflon Petri dishes (70 mm diameter) and placed for 3 h in an oven at 40 °C.

Procedure for Pollutant Removal by Silane-Grafted Membranes

The prepared membranes were tested for pollutant removal, which included mesitylene, PAHs (chrysene, as a model), biphenyl, bisphenol A, BTEX, methylene blue dye, and Sudan dye. Methylene blue was the only water-miscible analyte, whereas the others had low solubility in water. Therefore, 10% methanolic solution in water with the respective concentrations of pollutants was prepared (except for biphenyl, which was prepared in 50% methanolic solution in water). Thereafter, 50 ppm (methanolic water solutions) each of Sudan II, methylene blue, PAHs, bisphenol A, biphenyl, mesitylene, and BTEX was used for the removal by prepared membranes. The membranes (both MCE and PVA cross-linked with GA) were fitted in vacuum-filtration assembly and individually tested for the removal of every pollutant by using UV–visible (UV–vis) spectrophotometry (Shimadzu double beam 1800).

Results and Discussion

Characterization

FT-IR Analysis of the Silane-Grafted MXene

FT-IR spectral analysis was performed in the range of 4000–400 cm–1 for the unmodified MXene and the modified silane grafted MXene, that is, to validate the functionality. No functional group peaks were observed in case of MXenes, whereas new peaks appeared in the range of 2950–2800 cm −1and 1470–1350 cm–1, corresponding to C–H stretching and bending, respectively, showing the successful silane grafting on the MXene (Figure ). The formation of the Si–O–Ti bond in the modified material was also indicated by the peaks observed in the range of 950–700 cm–1.[25] All these peaks confirmed the modification of the MXene and successful silane grafting.

Figure 1

FT-IR spectra of (a) unmodified MXene, (b) MTPCS modified, (c) MNOCS modified, and (d) MODCS modified. FT-IR analysis of MCE filter paper-supported MXene membranes with the PVA binder.

The major peaks corresponding to wide OH stretching, CH2 stretching, C=O carbonyl stretching, and C–O stretching were observed at 3280, 2920, 1690, and 1050 cm–1, respectively, suggesting the presence of PVA.[26] The MXene and silane-grafted MXene were transformed into a membrane using PVA as a binder; also, the previously described peaks for modified materials can be seen in Figure .

Figure 2

FT-IR spectra of MCE filter paper-supported membranes with the PVA binder: (a) MXene, (b) MTPCS modified, (c) MNOCS modified, and (d) MODCS modified. FT-IR analysis of silane-grafted MXene membranes with PVA/GA cross-linked.

The characteristic peaks appearing at 3350, 2860, 1723 cm−1 and 1087 cm-1 were assigned to broad −OH stretching, stretching (symmetric and asymmetric) vibrations of the −CH2 group, C=O stretching, and −C–O–C– linkage,[26] respectively, as illustrated in Figure .

Figure 3

FT-IR spectra of the PVA/GA cross-linked modified membrane (without the filter paper support): (a) MXene, (b) MTPCS, (c) MNOCS, and (d) MODCS.

Powder X-ray Diffraction Analysis

The obtained XRD was found in agreement with the reported literature.[27]Figure a represents MAX phase XRD. In the MXene, good Al etching and exfoliation of the MAX phase were indicated by the vanishing and shifting of the main diffraction peak at 2θ ≈ 9.6°, corresponding to the (002) basal plane of the MAX phase (Ti3AlC2). In the MXene, the broad peak indicated increased interlayer spacing (Figure b). In the MAX phase (Ti3AlC2), the most intense peak is at 2θ ≈ 39°, which corresponds to the (104) suppressed in the MXene, indicating the etching of Al from the structure. The decrease in peak intensity showed the loss of crystallinity after the removal of aluminum from the MAX phase.[28]

Figure 4

pXRD spectrum of (a) MXene (Ti3C2T) and (b) MAX phase (Ti3AlC2) and (c) inset showing the peak shift.

SEM Analysis

It is evident from SEM images of the MAX phase (Ti3AlC2) (Figure ) that it had a compact structure where no distinct layers could be seen. However, after etching, multi-layered MXene structures are formed (Figure ), thus validating successful etching of Al from the MAX phase. After grafting, the MXene surface was found to be entirely covered with the silane materials (Figure ). SEM images indicated the MXene surface modification and successful silane grafting.

Figure 5

SEM morphological images of the MAX phase (Ti3AlC2) precursor at (a) 100 nm and (b) 300 nm scales.

Figure 6

SEM morphological images of the MXene (Ti3C2T) after etching at (a) 1 μm, (b) 2 μm, (c)3 μm, and (d) 500 nm scales.

Figure 7

SEM images of silane-grafted MXenes: (a,b) MNOCS, (c,d) MODCS, and (e,f) MTPCS.

Flexibility of Membranes

MXene-incorporated PVA membranes were found to be less flexible compared to silane-grafted MXenes. It was observed that the grafting resulted in more flexible attachments of MXenes with added PVA, which enhance the use in water purification applications. Figure shows the MXene membranes with and without silane modifications.

Figure 8

(a) MXene-incorporated and (b) silane-grafted MXene-incorporated PVA/GA membranes.

Pollutant Removal Studies

UV–vis spectra were recorded for the samples prior to filtration. Afterward, the filtrate was subjected to record the UV–vis spectrum in order to figure out the remaining concentration of the pollutant. The removal performance was evaluated out of reduced concentrations from the calibration curve. A range of quantity of materials was tested in order to achieve optimum pollutant removal. The removal efficiency data with varying amounts of the MXene, MNOCS, MODCS, and MTPCS incorporated in PVA/GA membranes are represented in Figure . It was observed that 0.1 g of the grafted MXene was suitable for membrane fabrication after optimization.

Figure 9

Optimization of the modified MXene PVA/GA material for membrane fabrication.

UV–visible spectra of all studied pollutants are represented in Figures S1 and S2 (Supporting Information). In each case, the highest spectral curve corresponds to the initial value of absorbance by the respective pollutant in the solution. Subsequent spectral curves indicated a decreased concentration of pollutants and hence the absorbance after being passed through MNOCS, MODCS, and MTPCS membranes. The lowest spectral curves are for pollutant removal by MTPCS membranes, which indicate maximum removal by MTPCS membranes as compared to MNOCS and MODCS membranes.

Calculating Pollutant Removal Performance

To measure the membrane pollutant removal performance, 50 ppm (methanolic water solutions) each of Sudan II, methylene blue, PAHs, bisphenol A, biphenyl, mesitylene, and BTEX was used for their removal by prepared membranes. The removal efficiency (R %) for pollutants is expressed as follows

whereas Cf is the feed concentration and Cp is the permeated concentration of pollutants in mg/L or ppm[29] evaluated from the Beer–Lambert law using corresponding absorbance and molar absorptivity in each case.

Pollutant Removal

Figures and 11 show that both types of membranes, MCE filter paper-supported membranes and membranes without the support, are excellent in removal of Sudan II, methylene blue, PAHs, bisphenol A, biphenyl, mesitylene, and BTEX. Due to the hydrophobic nature of these membranes, the pollutants with less water solubility and organic moiety in their structure (i.e., Sudan II dye) were removed efficiently. Out of all these membranes, MTPCS membranes showed the maximum removal. Furthermore, MNOCS and MODCS membranes were also very efficient in pollutant removal next to MTPCS membranes. Unmodified MXene membranes were also found to be effective; however, they were found to be least effective compared with modified membranes.

Figure 10

Graphical representation of % removal of the pollutants by MCE filter paper-supported MXene membranes.

Figure 11

Graphical representation of % removal of the pollutants by PVA with GA cross-linked MXene membranes.

Generally, the efficiency for pollutant removal increased in the case of PVA/GA cross-linked membranes as compared to their complementary MCE filter paper-supported membranes incorporated with similar grafted materials. The reason was attributed to the fact that the cross-linking of PVA by GA had resulted in the formation of a hydrophobic network where the silane-grafted MXene is attached and surrounded by multiple PVA/GA bonds. Thus, hydrophobic silane-grafted MXenes incorporated in hydrophobic PVA/GA membranes exhibited the best removals for hydrophobic pollutants by entrapping and adsorbing them onto their surface. Exceptionally, MXene-incorporated MCE filter paper-supported membranes showed better removal for MB as compared to PVA and GA membranes because of the hydrophilic nature of both the MXene and MCE membrane. Table compares the removal performances of various pollutants by newly synthesized membranes.

Table 1

Comparison of Removal of Pollutants for Different Membrane Materialsa

		membrane materials
		MXenea		MNOCS		MODCS		MTPCS
	Pollutants	MCE filter paper-supported (%)	PVA/GA cross-linked (%)	MCE filter paper-supported (%)	PVA/GA cross-linked (%)	MCE filter paper-support-ed (%)	PVA/GA cross-linked (%)	MCE filter paper-support-ed (%)	PVA/GA cross-linked (%)
removal percentages	MB	50	42	52	66	55	68	68	73
	mesitylene	45	57	60	67	63	76	75	85
	BTEX	42	62	63	70	65	73	66	89
	bisphenol A	35	65	66	75	68	78	70	90
	biphenyl	33	60	68	80	70	83	72	92
	PAHs	31	53	70	80	72	84	83	94
	Sudan II	28	70	88	93	90	96	95	98

*Without silane grafting.

Comparison with Previously Reported Studies (MXene-Based Membranes)

Lamellar membranes based on 2D MXene nanosheets supported on the aluminum oxide substate with a favorable removal rate (over 90%) for Rhodamine B and Evans blue as model pollutants were reported earlier.[30] Similarly, the hydrophilic MXene supported on the polyethersulfone ultrafiltration membrane was demonstrated for the removal of Congo red dye (92.3% at 0.1 MPa), having permselectivity in the separation of dyes from salts shown earlier.[31] The removal of dyes such as methyl red, methylene blue, rose Bengal, and brilliant blue carried out in thin (90 nm), laminated porous Ti3C2T–graphene oxide membranes was demonstrated by Kang et al..[32] The author shows the removal performance (under 5 bar pressure) for hydrated radii above 5 Å with removal performance of 68–100% for these dyes. Similarly, the removal rate for four different dyes, that is, Rhodamine B, methyl blue, crystal violet, and neutral red, was shown to be >97%[33] by another study in which the MXene nanosheets were used to produce TiO2 nanoparticles via in situ oxidation which intercalated with graphene oxide (GO) to produce GO-based nanofiltration membranes. In another study, the composite membrane was developed using GO and MXene. The author demonstrated that the heterogeneous structure of the GO/MXene membrane produced a synergistic effect in terms of substrate removal and permeability. The removal performance of common organic dyes (chrysoidine G, neutral red, methylene blue, crystal violet, brilliants blue) was found to >99.5%.[34]Table shows the comparison of various membranes in terms of support materials and removal performance with the reported grafted membranes.

Table 2

Comparison with Previously Reported Studies for the Removal of Selected Pollutant Species by MXene-Based Membranes

MXene	support layer	pollutants	key removal (%)	ref.
Ti3C2Tx	anodic aluminum oxide	Rhodamine B, Evans blue	85, 90	(30)
	polyethersulfone	Congo red, Gentian violet	92, 80	(31)
Ti3C2Tx–Ag	polyvinylidene difluoride	Rhodamine B, methyl green	79.9, 92.3	(35)
Ti3C2Tx–graphene oxide	polycarbonate and nylon	brilliant blue, rose Bengal	95.4, 94.6, 40	(32)
		methylene blue
	mixed cellulose ester (200 nm)	Rhodamine B, methyl blue crystal violet, neutral red	>97 for all dyes	(33)
	mixed cellulose ester (450 nm)	Chrysoidine G, neutral red, methylene blue, crystal violet, brilliant blue	>99 for all dyes	(34)
	porous nylon membrane	methyl orange, methylene blue, acid yellow 14, indigo carmine, eosin	>95 for all dyes	(36)
Ti3C2Tx	mixed cellulose ester (220 nm)	methylene blue	50%	Current studies
		mesitylene	45%
		BTEX	42%
		bisphenol A	35%
		biphenyl	33%
		PAHs	31%
		Sudan II	28%
	polyvinyl acetate and GA	methylene blue	42%
		mesitylene	57%
		BTEX	62%
		bisphenol A	65%
		biphenyl	60%
		PAHs	53%
		Sudan II	70%
Ti3C2Tx–n-octadecyltrichlorosilane (MODCS)	mixed cellulose ester (220 nm)	methylene blue I	52%	Current studies
		mesitylene	60%
		BTEX	63%
		bisphenol A	66%
		biphenyl	68%
		PAHs	70%
		Sudan II	88%
	polyvinyl acetate and GA	methylene blue	66%
		mesitylene	67%
		BTEX	70%
		bisphenol A	75%
		biphenyl	80%
		PAHs	80%
		Sudan II	93%
Ti3C2Tx–n-octyltrichlorosilane (MNOCS)	mixed cellulose ester (220 nm)	methylene blue	55%	Current studies
		mesitylene	63%
		BTEX	65%
		bisphenol A	68%
		biphenyl	70%
		PAHs	72%
		Sudan II	90%
	polyvinyl acetate and GA	methylene blue	68%
		mesitylene	76%
		BTEX	73%
		bisphenol A	78%
		biphenyl	83%
		PAHs	84%
		Sudan II	96%
Ti3C2Tx–triphenylchlorosilane (MPTCS)	mixed cellulose ester (220 nm)	methylene blue	68%	Current studies
		mesitylene	75%
		BTEX	66%
		bisphenol A	70%
		biphenyl	72%
		PAHs	83%
		Sudan II	95%
	polyvinyl acetate and GA	methylene blue	73%
		mesitylene	85%
		BTEX	89%
		bisphenol A	90%
		biphenyl	92%
		PAHs	94%
		Sudan II	98%

Membrane Reusability and Disposal

The reusability of membrane was tested using Ti3C2T–triphenylchlorosilane (MPTCS) PVA/GA cross-linked membranes for removal of various pollutants (Figure ) after washing with ethanol (50 mL × 5). It was observed that the membranes exhibit reusability up to five cycles. The removal efficiency dropped in the range of 8–15% of the original values after five cycles. The used membranes can be disposed off as usual as the material for membrane fabrication is environmentally friendly and the MXene and PVA matrix do not show any environmental problems.

Figure 12

Reusability of the MXene membrane [Ti3C2Tx–triphenylchlorosilane (MPTCS) PVA/GA cross-linked] for the removal of various pollutants.

Conclusions

The commercially available MAX phase (Ti3AlC2) was successfully etched with HF and exfoliated and delaminated using ultrasonication as confirmed by powder X-ray diffraction (pXRD) analysis. The modifications of the MXene (Ti3C2T) with silane grafting reaction for membrane preparation for enhances water purification were investigated. The MXene was successfully grafted with n-octadecyltrichlorosilane (MODCS), n-octyltrichlorosilane (MNOCS), and triphenylchlorosilane (MTPCS) in order to make a hydrophobic MXene that could be able to bind with the organic matrix/polymers. The grafting was confirmed with FT-IR analysis. The modified MXenes were transformed into thin membranes by forming an MXene/PVA composite over a filter paper support, that is, MCE filter paper. MXene membranes were also formed without the MCE support by using PVA/GA, where GA was used as a cross-linker to stabilize PVA and make it water-resistant. The conditions of membrane formation were optimized to investigate optimum compatible conditions with the modified materials. 0.1 g of the MXene was sufficient to develop the membrane with a high efficiency for removal. The resulting membranes were tested for removal for various organic pollutants that included mesitylene (or trimethylbenzene), PAHs (chrysene, as a model), biphenyl, bisphenol A, BTEX, methylene blue, and Sudan II dyes. MTPCS PVA/GA cross-linked membranes showed the best results for pollutant removal up to 98% efficiency. Overall, all six types of membranes showed a removal efficiency in the range of 52–98%, which is better than that of the bare MXene membrane (28–50% removal efficiency). It was observed that the membranes exhibit reusability up to five cycles.