Yuanyuan Wanga,
Hang Xu*a,
Mingmei Dinga,
Lei Zhangb,
Gang Chenc,
Jiawei Fua,
Ao Wanga,
Jiapei Chena,
Bonan Liua and
Wen Yanga
aKey Laboratory of Integrated Regulation and Resource Development on Shallow Lakes, Ministry of Education, College of Environment, Hohai University, Nanjing 210098, China. E-mail: xuhang810826@163.com
bCollege of Civil and Architecture Engineering, Chuzhou University, Chuzhou 239000, China
cState Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, College of Hydrology and Water Resources, Hohai University, Nanjing 210098, China
First published on 1st April 2022
Removing salt from dye/salt mixtures using nanofiltration (NF) membranes needs to be improved to ensure high permeability, high selectivity, and antifouling performance. In this study, we used an interfacial polymerization (IP) technique to create a novel thin-film nanocomposite NF membrane by introducing two-dimensional MXene Ti3C2Tx into the polyamide (PA) layer. Enhanced IP reaction rate facilitated the overflow of residual solvent from the fresh PA layer's edge due to the MXene-mediated IP strategy, resulting considerable bubble-like nodules on the membrane surface. The unique nanostructure of PA effective layer could be tuned by controlling the MXene concentration in aqueous phase solution, which finally promoted the obtained membranes with superb permselectivity. In this way, the water permeability was elevated to a maximum value of 45.12 L m−1 h−1, nearly 1.58-fold compared to the PA-pristine membrane. Moreover, the Ti3C2Tx/NF membrane exhibited a superior dye/monovalent salt separation coefficient of 820, outperforming the pristine PA membrane and other NF membranes in the literature. Additionally, the MXene-assisted IP strategy designed an effective dye anti-fouling hydration layer, which played a crucial role in fouling resistance. This work illustrates a novel use of Ti3C2Tx to successfully regulate high-performance TFN PA membranes for potential application in dye/salt separation.
Commercial NF membranes are thin-film composite (TFC) membranes with polyamide (PA) as the functional layer. The ultrathin selective layer allows TFC membranes to have a higher dye/monovalent salt ion separation efficiency. However, the trade-off effect between water permeability and salt rejection is still a challenge for the wide application of TFC membranes.14 Previous research has revealed that the PA layer's properties are critical for TFC-NF membrane's permeability and selectivity.12,15 With the rapid development of nanotechnology, 2D nanomaterials such as graphene oxide,16 molybdenum disulfide,17 carbon nitride18 and boron nitride nanosheets,19 were gradually applied for the regulation of PA layer due to their atomic-level thickness, mechanical properties, multifunctional groups, and to provide high-speed water channels. After incorporating with 2D nanomaterials, the physicochemical properties of PA layer including surface roughness, hydrophilic properties, surface charge, and cross-link density could be elaborately regulated, facilitating the separation performance.20 Moreover, the vulnerability of PA membranes to fouling is mainly related to the hydrophobicity of the PA active layer. The incorporation of nanomaterials tends to facilitate the formation of a stable and continuous hydration layer, which is of great significance for increasing anti-fouling property of PA layer.21,22
MXene (transition metal carbonitride), as a new type of 2D nanomaterial with good hydrophilicity, tunable electronegativity, and mechanical properties, exhibited great potential for desalination and dye separation.23–26 The unique structure of MXene nanosheets ensures sufficient strength during the separation process. The low concentration and good dispersion of MXene supernatant exhibit good affinity and interface compatibility with the PA matrix. In addition, the negatively charged hydrophilic groups on the surface of MXene can facilitate the rejection ability of anions and the resistance to contamination.27–29 Wang et al.30 doped MXene nanosheets into a PA layer via interfacial polymerization (IP), and the resulting reverse osmosis (RO) membrane showed excellent water flux and chlorine resistance. Xue et al.31 used MXene supernatant containing several layers as an aqueous phase solvent for interfacial polymerization. The prepared MXene composite NF membranes exhibited excellent low-carbon and persistent desalination performance. However, the regulation mechanism of MXene embedding on PA layer has not been well investigated. Besides, little optimization work has been conducted on introducing MXene into the PA layer for dye/salt separation, and further research is required.
In this study, we proposed a facile MXene-assisted IP strategy of designing TFN membranes for enhanced dye/salt separation and antifouling performance. The presence of MXene nanosheets significantly increased the PIP adsorption with enhanced IP reaction rate, and residual solvent flows spilled out from the edges of fresh PA layer, resulting in the formation of bubble-like nodules on the membrane surface. This special nano-structure endowed the PA effective layer with an increased effective permeable area, while the introduction of MXene nanosheets reduced the formation of non-selective pores, therefore the corresponding nodule-type NF membrane exhibited higher water permeability and ultrahigh dye/monovalent salt selectivity. Besides, the introduction of MXene facilitated the combination with water molecules through hydrogen bond; a novel dye antifouling hydration layer was constructed on the membrane surface, thereby promoting higher surface hydrophilicity and anti-fouling properties. MXene nanocomposite membranes will pave the way for dye/salt separation NF membranes in the future.
Titanium aluminum carbide powder (Ti3AlC2, 99%, 300 mesh) was acquired from Nanjing Mission New Materials Co., Ltd (Nanjing, China). A polysulfone (PSF) ultrafiltration membrane (DelStar Technologies Co., Ltd, Suzhou, China) was used as support. Throughout the experiments, deionized water was used.
The morphology of the Ti3C2Tx/NF membranes was observed using SEM (Hitachi S4800, Japan). Elemental distribution of samples was determined using an EDS (Octane Plus, USA) equipped on SEM. The roughness of the membranes was measured by AFM (Bruker Icon, Germany). The chemical compositions of the produced membranes were determined using X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250Xi, USA), and their functional chemical structures were investigated using attenuated total reflectance-Fourier transform infrared spectroscopy (FTIR; PerkinElmer Spectrum 100, USA). To analyze the surface wetting nature of the membranes, contact angle goniometry (Dataphysics OCA15EC, Germany) was utilized to monitor water contact angle. The zeta potential of the Ti3C2Tx/NF membranes was determined using a laser particle size analyzer (Malvern Mastersizer 3000, UK).
(1) |
(2) |
rs = 16.73 × 10−3 × M0.557 | (3) |
The rejection mechanism of NF membranes is also impacted by electrostatic interactions; thus, four different dyes (Methyl Orange, MO; Methylene Blue, MB; Congo Red, CR; and Crystal Violet, CV) were tested at neutral pH in the experiment. The dye rejection (R) was calculated as eqn (2). A dye rejection test using NaCl/Congo Red (CR) and NaCl/Crystal Violet (CV) mixture solutions was performed to examine the NaCl/dye selectivity of the prepared membranes. The NaCl and dye concentrations were 2000 ppm and 100 ppm, respectively. The following equation was used to calculate the selectivity of NaCl for dyes:
(4) |
Fig. 1 Scanning electron microscopy (SEM) (a) of Ti3AlC2; transmission electron microscopy (TEM) (b), X-ray diffraction (XRD) (c), and atomic force microscopy (AFM) (d and e) of Ti3C2Tx. |
After ultrasound-assisted exfoliation, the precursor Ti3AlC2 was delaminated to form several layers of MXene nanosheets. The TEM image of Ti3C2Tx nanosheets is shown in Fig. 1(b). The TEM image shows that 3–4 layers of nanosheets are stacked together, thereby presenting a 2D thin layer structure with a lateral dimension of approximately 700–800 nm.
To determine whether the Ti3C2Tx nanosheet had a single layer or a few layers, AFM was used to characterize the thickness of the nanosheet. Fig. 1(d and e) shows the AFM image of Ti3C2Tx. The theoretical thickness of the monolayer Ti3C2Tx nanosheets was 1.51 nm.35 According to the AFM image, the height difference between the nanosheets was 1.44 nm, which was roughly consistent with that in a previous report.36 Monolayer Ti3C2Tx nanosheets were successfully prepared.
Scheme 1 shows a schematic representation of the Ti3C2Tx/NF membrane synthesis. The PA separation layer, Ti3C2Tx, and PIP were bonded by van der Waals forces and hydrogen bonding to form thin-film nanocomposite membranes, and their separation characteristics were explored.
AFM scanning was used to examine the influence of MXene doping on surface roughness. The AFM images and surface characteristics of the original and produced membranes are shown in Fig. 3 and Table 1. Compared with the naked PA-NF membrane, the surface of the thin-film nanocomposite membrane was coarser when the doping content of Ti3C2Tx increased to 0.002 wt%. The PA layer's remarkable sensitivity to the presence of nanomaterials could explain the variance in its surface morphology. The inducement of MXene creates a bubble-like structure of bumps on the PA layer, thereby increasing the surface roughness of the TFN membrane. The higher surface roughness could contribute to greater increased water contact area, which would be beneficial for the improvement of water permeability. When the mass percentage of Ti3C2Tx increased to 0.005 wt%, the membrane surface became smooth owing to the increased hydrophilicity caused by Ti3C2Tx and the effect on the diffusion rate. Due to the diffusion of the aqueous monomer into the organic phase is the rate-limiting step in the IP reaction.45 Higher nanomaterial loading might have hindered the diffusion of aqueous phase monomers, resulting in a smoother membrane surface, consistent with previous studies.46
Ti3C2Tx content (wt%) | Ra | Rq |
---|---|---|
0 | 6.72 | 8.95 |
0.001 | 10.2 | 23.7 |
0.002 | 11.8 | 16.3 |
0.005 | 15.6 | 29.1 |
0.010 | 10.4 | 13.8 |
The surface functional groups on the membranes were detected by FTIR, as illustrated in Fig. 4(a). FTIR has a penetration depth of a few hundred nanometers to a few microns, making it helpful for detecting PA top layers and PSF support layers.47 Weak aromatic amide peaks were seen in all membranes at 1629 cm−1, which is the CO stretching vibration of amide I, originates from poly (piperazine amide). When Ti3C2Tx was added, the 3406 cm−1 O–H stretching vibration increased, which may be accounted for the hydroxyl (–OH) functional groups on the Ti3C2Tx surface. Additionally, the peak strength of 1489 cm−1 for O–H bending from carboxyl and 1508 cm−1 for N–H in-plane bending reduced, owing to the influence of nanomaterial doping. It might be explained for Ti3C2Tx addition helps storage aqueous monomer in IP process, accelerating the IP reaction rare, leading to unreacted TMC hydrolysis to –COOH decreased the associated peak. Moreover, the hydroxyl of MXene nanosheets forms hydrogen bonds with the piperazine group, limiting the N–H stretching vibration and thus decreasing the peak intensity at 1508 cm−1.
Fig. 4 Fourier transform infrared spectroscopy (FTIR) spectra of M0, M1, M2, M3, and M4 (a) and X-ray photoelectron spectroscopy (XPS) spectra of M0 and M2 (b). |
Benefiting from the high detection accuracy of XPS, to verify whether Ti3C2Tx nanosheets were successfully doped into the PA cortex, a series of thin-film NF membranes were prepared according to the concentration in Table 1, and XPS analysis was performed on the M0 and M2 membranes. The wide-scan XPS spectra of M0 and M2 are shown in Fig. 4(b). In the M2 spectrum, there were almost no peaks corresponding to Ti and F, which might be due to the ultra-low load of MXene nanosheets and the bulk of MXene being covered by PA. The results of the XPS content analysis for the M0 and M2 membranes are shown in Table 2. The surface of the M0 membrane contained no Ti and F elements, whereas the atomic mass fractions of Ti and F on the surface of the M2 membrane were 0.20% and 0.29%, respectively, thereby confirming the successful doping of MXene. Compared with the original membrane, the element composition of the modified membrane was slightly different. The modified membrane had a lower O/N ratio, corresponding to a higher degree of cross-linking.39,47 It was thought that the hydrogen bonds between piperazine monomers and MXene nanosheet could control the release of piperazine and then affect the IP process, promoting the formation of a tight PA barrier layer.
C 1s (%) | O 1s (%) | N 1s (%) | Ti 2p (%) | F 1s (%) | O/N ratio | D (%) | |
---|---|---|---|---|---|---|---|
M0 | 72.96 | 15.11 | 11.93 | 0 | 0 | 1.27 | 64.72 |
M2 | 72.57 | 14.77 | 12.20 | 0.19 | 0.28 | 1.21 | 71.41 |
To investigate the improvement of hydrophilic properties of MXene doping on the membrane surface, the water contact angle of the M0 and M2 membranes was measured, and the results are shown in Fig. 5(a–e). The hydrophilicity of the membrane surface increased as the water contact angle decreased when the surface shape of the membranes was similar.48 The water contact angle of the membrane reduced as the Ti3C2Tx nanosheet doping increased, indicating that Ti3C2Tx doping in the PA layer could effectively improve the membrane's hydrophilicity.49,50 The improvement in hydrophilicity is related to the hydrophilic functional groups (–OH and –O) on the surface of Ti3C2Tx. The powerful hydrophilic groups can attract water molecules and form a thin hydration layer on the membrane surface via hydrogen bond. In addition, the oxygen functional groups have a strong water affinity. This, in turn, demonstrates that Ti3C2Tx was successfully embedded in the PA layer. It is implied that the hydrophilic MXene-assisted NF membranes would exhibit outstanding permeability and anti-fouling performance.51
The charge characteristics of the membrane surface were evaluated using the zeta potential, as shown in Fig. 6(a). The isoelectric point of the original membrane without Ti3C2Tx doping and the TFN membrane doped with a Ti3C2Tx mass fraction of 0.002 wt% was 3.2. At pH > 6, the surfaces of both the initial and modified membranes doped with Ti3C2Tx were negatively charged.50 The zeta potential of the M2 membrane decreased and became more negative compared with that of the original membrane. Owing to the abundance of negative groups such as –OH and –F in the doped Ti3C2Tx, the electronegativity of the membrane surface was improved. MXene-mediated PA layer would strengthen the anionic dye repulsion by electrostatic interaction, therefore enhancing both the selectivity and fouling resistance of the modified membrane.
The membranes' MWCO was determined using PEG of different molecular weights. Fig. 6(b) shows the PEG retention rates of the M0 and M2 membranes. The MWCO of the pristine membrane is 371, whereas the MWCO of the M2 membrane modified by MXene is 360. The high degree of cross-linking corresponds to narrower pore sizes, which is consistent with the chemical characteristics shown by XPS. It is further demonstrated that the doping of MXene helps form a dense and defect-free PA layer, reducing the formation of non-selective pores, which would positively affect the separation efficiency.
The increase in water permeability may be closely related to the doping of Ti3C2Tx nanosheets and the resulting change in the surface morphology of the PA layer. For one thing, the introduction of Ti3C2Tx nanosheets leads to bubble-like bulges on the surface of the PA layer, increasing the effective water contact area. For another, the introduction of Ti3C2Tx nanosheets disrupted the polymer chain packing, increasing the system's free volume.22 Meanwhile, the hydrophilic functional groups such as OF and –OH introduced during the preparation of Ti3C2Tx nanosheets can attract water molecules in the membrane matrix and facilitate their passage through the membrane.31,50 In addition, the layered structure of Ti3C2Tx may provide additional nanoscale channels for the rapid passage of water molecules through the membrane.52,53 When the introduction of Ti3C2Tx is in a high state (>0.005 wt%), the higher nanomaterial loading results in a smoother membrane surface, reducing the effective water contact area, thereby resulting in a decrease in permeability.4,54
The influence of the introduction of Ti3C2Tx nanosheets on the efficiency of TFN membranes in retaining salt ions was further investigated, with the results shown in Fig. 7 (b). The retention of salt ions by all membranes was ranked as Na2SO4 > MgSO4 > NaCl, which could be attributed to the difference in the selectivity of the charged NF membranes for different ions. It is generally considered a result of the combined size sieving effect and the Donnan effect. The Ti3C2Tx/NF membranes were electronegative at pH > 6. Owing to the Donnan effect, the Ti3C2Tx/NF membrane repels high-valence ions (SO42−) more strongly than low-valence ions (Cl−), which is manifested by the high retention rate of SO42−.55 The electrostatic gravitational force of Mg2+ is stronger than that of Na+; the Donnan effect is weakened to a certain extent; thus, the retention rate of the Ti3C2Tx/NF membrane for Na2SO4 was slightly higher than that for MgSO4. Significantly, the rejection of both salt ions improved under a lower Ti3C2Tx loading. First of all, it could be attributed to the abundant –OH groups and –F above the Ti3C2Tx nanosheet.23 The negatively charged groups made the zeta potential on the membrane surface more negative, thereby amplifying the electrostatic repulsion between the solute and the membrane surface. The enhanced Donnan exclusion effect led to higher repulsion rates.56 Besides, the low concentration of MXene doping enhances the cross-linking degree of PA layers; the smaller pore size increases the repulsion of the membranes to salt ions through the size exclusion effect. In the end, the layered structure of MXene, allowing better distribution and diffusion of monomers during the IP reaction and reduces the formation of non-selective pores.57 In contrast, the salt rejection rate decreased when a higher Ti3C2Tx load was reached. When excessive Ti3C2Tx was added, it reduced the cross-linking degree of the PA layer, which could cause looseness of the structure and influence the integrity of the active layer.30 The size exclusion effect of the PA layer was eventually weakened by the negative effects mentioned above, therefore reducing the salt rejection rate. The M2 membrane showed great performance for both water permeation and salt rejection, and due to this reason, which was chosen to explore the dye separation performance further.
Four kinds of organic dyes with different types of charges were chosen to evaluate the dye removal ability of the Ti3C2Tx/NF membrane, as shown in Fig. 8(a). The M2 membrane showed high retention (>97.5%) for all four dyes. Meanwhile, the flux of the M2 membrane with the four dye solutions with different charge types was similar (CR: 41.83 L m−2 h−1, MO: 43.52 L m−2 h−1, CV: 42.77 L m−2 h−1, and MB: 45.10 L m−2 h−1). The two dyes with higher molecular weights (CR and CV) exhibited a higher rejection rate (>99.4%), which might have been attributed to the size exclusion effect.58 Although the molecular weights of MO and MB were similar, there were some variations in rejection. This phenomenon can be explained by the electrostatic repulsion effect.59 The retention of the anionic dye MO (98.71%) was higher than that of the cationic dye MB (97.27%) due to the enhanced electrostatic repulsion by the higher negative zeta potential on the membrane surface, owing to the introduction of MXene nanosheets. Additionally, the effective pore size of M2 shows a slight shrinking of 5.8% to 0.52 nm, which is smaller than the hydrated ionic radius of most multivalent ions and tested dyes, thereby promoting the sieving performance. The molecular structure of the dye is shown in Fig. S3.†
Effective separation of dye molecules and inorganic salts is the key to the purification process of crude dyes.59–62 Owing to the high NaCl permeability and dye retention performance of the Ti3C2Tx/NF membranes, the prepared membrane should have excellent selectivity for NaCl and various dye molecules. Fig. 8(b and c) shows the Ti3C2Tx/NF membrane's selective separation effect for the dye/salt mixture solution. The feed solution consisted of 2000 ppm NaCl and 100 ppm dye (CR and CV). For the two dye molecules with different charges, the M2 membrane showed nearly 100% rejection while maintaining a high permeability for NaCl. The separation factors of NaCl/CR and NaCl/CV were computed using eqn (4) and were calculated as 820.0 and 500.5, respectively. In addition, the water flux and dye/NaCl separation performance of the Ti3C2Tx/NF membrane were compared with those of other reported membranes.22,59–65 The Ti3C2Tx/NF membrane exhibited superior permeability and dye/NaCl separation performance compared with those of other NF membranes, making it an attractive candidate for dye/salt separation membrane in the future.
Fig. 9 Antifouling test (a) and long-term filtration (b–d) of M2 (normalized water flux (b); CR rejection (c); Na2SO4 rejection (d)). |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d2ra00335j |
This journal is © The Royal Society of Chemistry 2022 |