Journal of Membrane Science 641 (2022) 119931
Available online 1 October 2021
0376-7388/© 2021 Elsevier B.V. All rights reserved.
Simultaneously enhanced permeability and anti-fouling performance of polyethersulfone ultrafiltration membranes by structural control and mixed carbon quantum dots
Bingjie Zhang, Wenyi Wang
*, Liuyong Zhu, Ning Li, Xingyu Chen, Jiawei Tian, Xuewen Zhang
State Key Laboratory of Separation Membranes and Membrane Processes, School of Material Science and Engineering, Tiangong University, Tianjin, 300387, China
A R T I C L E I N F O Keywords:
Polyethersulfone Porous thin surface Electric field Carbon quantum dots Antifouling
A B S T R A C T
Assisted with a direct current (DC) electric field membrane forming device, the Polyethersulfone (PES) ultra- filtration (UF) membrane was fabricated by non solvent induced phase inversion method. The membrane morphology and performance were simultaneously changed because the PES polymer chains were polarized by the DC electric field. As a result, the M-0-DC membrane showed a porous and thinner separation layer structure with a water permeability of 280.1 L m−2 h−1⋅bar−1, which is ~55% higher than that of M-0. Further, a novel antifouling PES membrane containing carbon quantum dots (CQDs) with a concentration of 0.5 was prepared combined with the electric field. The water permeability (~164% higher than M − 0) of M-0.5-DC was sub- stantially improved and the rejection rate for BSA was 98.4%. The M-0.5-DC also exhibited excellent antifouling property, that is, its BSA flux recovery rate reached as high as 95.3%. Apart from, the Congo red self-cleaning ability investigation of M-0.5-DC showed that the membrane became clean again after the irradiation of ultra- violet light for 2 h. The simultaneously enhanced water permeability and improved anti-fouling property by DC electric field and CQDs are promising advances that may promote the applications of UF membranes in water treatment.
1. Introduction
With the acceleration of industrial development and urbanization, the shortage of clean water resources has become a global problem restricting human progress. In this context, various water purification technologies have attracted more and more attention, including physical adsorption [1], chemical oxidation [2], biological treatment [3] and membrane separation [4]. Due to the advantages of high separation efficiency, simple operation and easy adjustment of its scale and treat- ment capacity, membrane separation technology is developing rapidly.
Among them, organic polymer membranes occupied an important po- sition because of its low cost and easy modification such as poly- vinylidene fluoride (PVDF), polyacrylonitrile (PAN), polysulfone (PSF) and polyethersulfone (PES).
Due to the excellent mechanical strength, chemical resistance and thermal stability, PES was widely used in membrane preparation for water treatment [5–7]. However, the inherent hydrophobicity and porous features of organic membranes are easy to adsorb and accumu- late organic pollutants, which always result in serious membrane fouling
[8]. In order to alleviate the membrane pollution and achieve the pur- pose of long-term use, researchers have done a great deal of attempt to modify the surface or structure of polymer membranes. For example, the common modification methods include surface coating [9], surface grafting [10,11], ultraviolet (UV) irradiation and blending modification [12–14]. Blending carbon nanomaterials with hydrophilic groups such as carbon nanotubes (CNTs), graphene oxide (GO), carbon quantum dots (CQDs) into the membrane is considered being an excellent modification method to improve surface hydrophilicity, which could enhance the antifouling ability of ultrafiltration (UF) membranes [15]. Wang et al.
modified the PES membrane with CNTs functionalized by sodium lignosulfonate and found the antifouling performances of the f-CNT/PES membrane were dramatically increased [16]. Kong et al. prepared the PES UF membrane containing cysteine-functionalized graphene oxide (CGO) and found that the membrane had high water flux and excellent antifouling property [17]. Koulivand et al. fabricated a carbon dot-modified PES membranes for Salt and dyes removal from water, and found a significant increase in antifouling capacity of the membrane [18].
* Corresponding author.
E-mail address: wenyi-wang@hotmail.com (W. Wang).
Contents lists available at ScienceDirect
Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
https://doi.org/10.1016/j.memsci.2021.119931
Received 5 August 2021; Received in revised form 15 September 2021; Accepted 28 September 2021
In recent years, researchers began to use the external field (magnetic field, electric field, etc.) to control the arrangement of nanomaterials in the membrane, so as to maximize the separation and permeation effi- ciency of the prepared membrane. Liu et al. introduced the orientation of two-dimensional magnetic Fe3O4 nanosheets to the PAN surface with the help of a magnetic field, and prepared M-SA-Fe3O4 (0.4)/PAN with large surface roughness and efficient ethanol dehydration performance [19]. Wang et al. applied a direct current (DC) electric field to make the graphene oxide more migrate to the surface of the PES membrane, and the membrane they prepared had increased pure water permeability and antifouling ability [20]. Liu et al. used the DC electric field to make the aligned carbon nanotubes (CNTs) penetrate the surface of the PES membrane, using this as a substrate to prepare a nanofiltration mem- brane with good pure water permeability and chlorine resistance [21].
Unfortunately, most of the researchers focus on the deflection and migration of the additives in the external field, but the mechanism of the external field on the polymer itself in the membrane forming process is rarely concerned.
The performance of ultrafiltration membrane prepared by non sol- vent induced phase inversion method depends on many factors, such as membrane forming method, additive type and geometry [22–24].
Among them, the membrane forming method is one of the factors that need to be considered to have a significant impact on the performance of the membranes. It is generally recognized that polymers have unique structural characteristics completely different from small molecules. In other words, there are many rotatable single bonds in the polymer chain, and the internal rotation of single bond forms many conformations [25].
It is this property that makes it possible to regulate membrane structure in the process of membrane formation. The pore of PES ultrafiltration membrane is essentially the space between PES molecules and the conformation adjustment of molecular chain will inevitably lead to the change of membrane structure. Therefore, it is of great significance to study the effect of electric field on the microstructure of polymer membrane.
In this work, because of the possibility of conformational rear- rangement of PES molecular chain in casting solution, an DC electric field assisted membrane forming device was designed, as shown in Fig. 1. The PES casting solution salivated on the glass plate was placed in the electric field for 1 min to give enough time for molecular chain rearrangement, and then the membrane was immersed in water for 1 min by removing the “Movable block” to obtain the structure modified PES membrane. Furthermore, in order to investigate the influence of DC electric field on the microstructure of the membrane while meeting the requirements of pollution resistance, CQDs was introduced into the PES membrane. A new type of membrane having high water flux and
excellent antifouling ability was successfully prepared by this method and a highly controllable hybrid substrate platform was constructed by combining the assistance of DC electric field with doping CQDS. This study provided a new insight for the mechanism of the effect of DC electric field on the structure of PES UF membrane, and had a guiding significance for the structure design and optimization of membranes in the future.
2. Experiment section 2.1. Materials
Polyethersulfone (PES, Ultrason E6020P, Mw =5.8 ×103 Da) was purchased from BASF, Germany. Carbon quantum dots (CQDs, XF253, Granularity 2–5 nm) were provided by Jiangsu XFNANO Materials Technology Co., Ltd. N,N-dimethylacetamide (DMAc, ≥99.5%), bovine serum albumin (BSA) and Congo red (CR) were all obtained from Shanghai Aladdin Chemical Reagent Co., Ltd. Polyvinylpyrrolidone (PVP) was obtained from Tianjin Guangfu Fine Chemical Research Institute. All the chemicals used in the experiment were used directly without further purification.
2.2. Preparation of PES ultrafiltration
All membranes were prepared by non solvent induced phase inver- sion method [26,27]. Firstly, the homogeneous casting solution with a 17% concentration was prepared by dissolving dried PES power in DMAc and stirring at 60 ◦C for 8 h, which was vacuum degassed 12 h at room temperature. The resulting solution was coated on a smooth and clean glass plate with a 150 μm thick casting knife, which was placed in a DC electric field membrane forming device with the DC electric field intensity of 2 kV/cm for 1 min, and then it was immediately immersed in deionized water bath to obtain a membrane. The control group was prepared without the DC electric field and all membranes were soaked in deionized water for 24 h before use. Apart from, different content of CQDs were added to DMAc and ultrasonically dispersed for 30 min to obtain homogeneous solution used for CQDs hybrid membranes. The schematic illustration of preparing the PES UF membrane is presented in Fig. 2. The membranes were named according to the amount of CQDs (as shown in Table 1) and the presence or absence of the DC electric field during the membrane formation process.
2.3. Characterization
The scanning electron microscope (SEM, Regulus 8100, Japan) was
Fig. 1.Electric field assisted membrane forming device.
operated at an accelerating voltage of 5 kV to observe the surface pores and the cross-section morphology of the membranes. All samples were freeze-dried for 12 h before spraying gold. The dynamic water contact angle analyzer (DSA100, KRUSS, Germany) was used to test the wetta- bility of the membrane surface in a specific volume mode. Atomic force microscope (AFM, Dimension Icon, Germany) was used to characterize the average roughness (Ra) and root mean square roughness (RMS) and reflect the surface morphology of the membranes. A precision electronic universal material testing machine (AGS-X 50 N, China) was applied to measure the mechanical properties of prepared PES UF membranes at a speed of 10 mm/min. A three-purpose ultraviolet analyzer (WFH–203B, China) was used to observe the prepared PES membranes under the irradiation of 365 nm UV lamp, which proved that CQDs were suc- cessfully immobilized on the surface of the membranes. X-ray photo- electron spectroscopy (XPS, K-Aepna, USA) was used to analyze the changes of chemical functional groups on the surface of the membranes.
Streaming potential technology (Sur Pass 3, Austria) was applied to examine the change of zeta potential of the membrane surface in a PH range of 4–9. Ultraviolet spectrophotometer was used to measure the absorbance of BSA and CR at 280 nm and 497 nm, respectively.
The membrane porosity[ ε (%)] measured by dry and wet weight method [28] and calculated by formula (1). The mean pore radius [rm
(nm)] was calculated by Guerout-Elford-Ferry ferry [29] formula (2) ε=
ω1− ω2
ρw×A×l×100% (1)
rm=
̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(2.9− 1.75ε) ×8μlQ ε×A×P
√
(2) where ω1 and ω2 are respectively the dry and wet weight of the mem- branes (g), ρw is the water density (0.998 g cm−3). l is the membrane thickness (m) and A is the effective area of the membrane (m2). μ is the water viscosity (8.9 ×10−4 Pa s), Q is the volume of permeate pure water per unit time (m3⋅s−1) and P is the operation pressure (Pa).
2.4. Separation, permeation, antifouling and self-cleaning tests
The filtration as well antifouling of resulting membranes were tested by a cross-flow device, of which the active surface area is 7.07 cm2. Before all filtration experiments were performed, the membranes were pre-pressed at 2 bar for 30 min to obtain a stable water permeability.
Then the pressure was adjusted to 1 bar and the pure water permeability [30] was recorded every 10 min, which is calculated by formula (3)
J= V
A×T×P (3)
where J is the pure water permeation permeability of the membrane (L m−2 h−1), V is the volume of permeate pure water at the transmembrane pressure of P (bar), A is the effective area of the membrane (m2), and T is the operation time (h).
In order to evaluate the separation performance of the prepared membranes, BSA (1000 ppm) was dissolved in PBS buffer as the feed solution. Under the same pressure, the concentration of permeate was measured to obtain the BSA rejection (R) of the membranes [30], which calculated by formula (4)
R= (
1− Cp
Cf
)
×100% (4)
R is the rejection of BSA (%), Cp and Cf are the concentrations of BSA in the permeate and feed solution, respectively (wt%).
BSA solution, as a good antifouling model, was usually used to evaluate the antifouling ability of membranes. The stable pure water permeability of the membranes was recorded as J0 and the permeability of BSA solution was recorded as J1. After the alternate test, the mem- brane was backwashed by 10% ethanol aqueous solution for 10 min and then tested for the water permeability JR. The total fouling rate (Rt), irreversible fouling rate (Rir) and reversible fouling rate (Rr) of the fabricated membranes were calculated by formula (5), (6), and (7), respectively [31].
Rt= (
1− J1
J0
)
×100% (5)
Rr= (JR− J1
J0
)
×100% (6)
Rir= (J0− JR
J0
)
×100% (7)
The pure water and BSA solution were alternately used to obtain the flux recovery rate (FRR) of membranes through a 280 min Fig. 2.Schematic illustration of preparing the PES UF membrane.
Table 1
Composition of the prepared PES membranes.
Samples CQDs Amounts (wt%) PES (wt%) PVP (wt%)
M-0 0 17 1
M-0-DC 0 17 1
M-0.5 0.5 17 1
M-0.3-DC 0.3 17 1
M-0.5-DC 0.5 17 1
M-0.7-DC 0.7 17 1
contamination test and the flux was recorded every 10 min [31]. The stable water flux and the water flux of the backwashed membrane were recorded as J0’ and JR’ respectively. The FRR was calculated as follows (8):
FRR= (JR＇
J0＇ )
×100% (8)
In order to test the dye pollution resistance of the membrane, M- 0 and M-0.5-DC were exposed to ultraviolet light for 2 h after 8 h of CR filtration experiment. The self-cleaning property of the membranes was characterized by comparing the residual amount of dirt on the mem- brane surface before and after illumination.
3. Results and discussions
3.1. Surface and cross-section morphology of the membranes
The surface and cross-section morphology of all PES membranes were observed by scanning electron microscope as shown in Fig. 3a and
b, respectively. Fig. 3a showed that there were only a few visible holes on the surface of M-0. However, some large visible holes appeared on the surface of M-0-DC, which was attributed to the effect of the DC electric field. On the other hand, it can be observed in Fig. 3b that the cross- section morphology of M-0-DC was significantly different from that of M-0. Compared with the dense cross-section structure of M-0, M-0-DC was porous and there are plenty of continuous channels near the surface.
These changes were very important to improve the pure water perme- ability of UF membrane. It is clear that the number of pores on the surface of PES membranes filled with hydrophilic CQDs was signifi- cantly increased and this can be attributed to the accelerating effect of the migration of CQDs to the top surface of the membrane during the membrane-formation process [32].
The mean pore size and porosity values of all prepared membranes were shown in Fig. 4a and those of all membranes doped with CQDs were higher than those of M-0. With the increase of the content of CQDs, the porosity increased to 85% and then decreased, which was similar to other reports [33,34]. The embedded hydrophilic CQDs could accelerate the phase transformation process and improve the porosity of the
Fig. 3.SEM images of PES UF membrane (a) surface; (b) Cross section.
membrane, but too much CQDs may agglomerate and increase the vis- cosity of the casting solution, which hinders the formation of pores. The changes in average pore size and porosity illustrated the changes in the internal structure of the membranes. The separation layer cross-section morphology of M-0 and M-0-DC were shown in Fig. 4b and c, respec- tively. It is obvious that the application of DC electric field will change the M-0-DC from a dense top layer to a thin top layer with through holes.
This can be explained by the polarization of PES molecular chain with the action of the DC electric field. According to the entanglement theory,
“topological entanglement” is bound to occur in polymer concentrated solution [35]. However, due to the polarization of the DC electric field, the molecular chain can adjust its conformation by single bond internal rotation and this kind of ordered behavior may lead to “cohesive entanglement” states between some molecular chains. The mechanism diagram of conformation transformation of PES molecular chains is shown in Fig. 4d. The ordered array of polarized PES molecular chains tended to reduce the viscosity of the casting solution. Moreover, the cohesive entanglement of PES molecular chain leaded to the increase of distance between the entangled node and its adjacent molecular chains.
These changes promoted the exchange rate of the solvent and water in the process of phase transformation, and leaded to the formation of thinner top layer, higher porosity and larger pores. The separation layer cross-section morphology of M-0.5 and M-0.5-DC were shown in Fig. S1.
After the DC process treatment, the channels near the upper surface were more continuous and perpendicular to the membrane surface, which can greatly promote the permeation efficiency of water after passing through the epidermis.
3.2. The membrane hydrophilicity
The test results of water contact angle of all prepared UF membranes
were shown in Fig. 5, which can directly reflect the change of membrane surface tension. It is well known that the surface tension of polymers is closely related to the chemical composition, molecular structure and condensed state structure of the surface [36]. As can be seen from Fig. 5, M-0 had the largest water contact angle of 71.0◦and the value of M-0-DC was reduced to 62.1◦, which was due to the thin separation layer of M-0-DC. Another possible reason for the decrease of the water contact angle was that the rearrangement of PES molecular chain segments changed the surface properties of M-0-DC [36]. CQDs with rich hydro- philic hydroxyl groups on the surface were used in this experiment and the FTIR spectra of CQDs was shown in Fig. S2. With the increase of CQDs content, the contact angle of the composite membranes decreased and then increased. M-0.5-DC had a minimum contact angle value of 52◦. This is mainly because the hydroxyl modified CQDs increased the hydrophilicity of the membranes, which was consistent with the results reported in many literatures that the modified carbon nano materials can effectively improve the hydrophilicity of the membranes [37,38].
However, excessive nano additives will significantly increase the vis- cosity of the casting solution, affect the phase transformation process of the membranes, and even block the membrane pores [39]. Therefore, the increase in contact angle when the content of CQDs was increased to 0.7% was attributed to the agglomeration of CQDs.
3.3. Surface roughness of membranes
Fig. 6 showed the surface morphology of the membranes charac- terized by AFM, in which the brightest and darkest areas represented the peaks (bulge) and valleys (pores), respectively. M-0.5-DC (Fig. 6e) showed a more loose and uniform morphology compared with M- 0 (Fig. 6c), which can be attributed to the electrophoresis phenomenon of CQDs [40]. Besides, the roughness parameters of fabricated Fig. 4. (a) The porosity and mean pore radius size of all membranes; The cross-section structure of the separation layer of M − 0 (b) and M-0-DC (c); (d) The conformational transition of PES molecular chains caused by electric field.
membranes were shown in Table 2. M-0 had the minimum RMS value of 2.95 nm and the addition of CQDs leaded to the increase of the surface roughness of the membranes. This phenomenon can be explained by the exposed and embedded CQDs on the membrane surface. However, the obvious large bright area on the surface and the RMS value of M-0.7-DC increased significantly, indicating the agglomeration of CQDs [32].
3.4. Chemical element characterization of membrane surface
In order to prove that CQDs were successfully immobilized in CQDs Fig. 5. Water contact angle value of all prepared membranes.
Fig. 6. AFM images of all prepared membranes: (a) M-0; (b) M-0-DC; (c) M-0.5; (d) M-0.3-DC; (e) M-0.5-DC; (f) M-0.7-DC.
Table 2
Surface roughness parameters of membranes.
Samples Ra (nm) RMS (nm)
M-0 2.4 ±0.1 2.95 ±0.3
M-0-DC 2.56 ±0.3 3.23 ±0.2
M-0.5 2.82 ±0.4 3.55 ±0.3
M-0.3-DC 2.65 ±0.1 3.35 ±0.5
M-0.5-DC 2.78 ±0.3 3.58 ±0.3
M-0.7-DC 3.53 ±0.8 4.29 ±0.6
doped membranes, it is a good choice to observe CQDs in PES membrane by using the green light of CQDs under UV lamp. Fig. 7a showed a photograph of the prepared PES membranes under UV light. It can be observed that the M-0 and M-0-DC without CQDs did not emit light, but the membranes doped with CQDs emitted obvious green light. XPS, as an important tool, was used to characterize the elements of the surface of the membrane and determine the valence states of the elements [41].
Fig. 7b represented the X-ray photoelectron spectra of the four kinds of prepared PES UF membranes with and without CQDs. The O 1s split peak spectra were shown in Fig. 7c–f, respectively. In the O 1s spectra of M-0.5 (Fig.7e) and M-0.5-DC (Fig. 7f), two new peaks appeared at 530.5 eV and 534.5 eV respectively, and they were believed to originate from O–H bond and C=O bond (the characteristic functional group of CQDs).
In addition, the element content of the prepared PES UF membranes was tested by XPS and the results were shown in Table 3. It can be noted that the total content of the characteristic functional group of CQDs in the surface of M-0.5-DC was 4.18%, which was higher than that in M-0.5 of 3.66%. These results further proved the fact that DC electric field pro- moted the enrichment of CQDs on the membrane surface.
3.5. Surface charge and mechanical properties of the membranes The measurement of zeta potential on the membrane surface is helpful to analyze the interaction between the membrane and the charged material in the feed solution [42]. The results of zeta potential of the prepared membranes in a specific pH range of 4–9 were shown in Fig. 8a, in which the red line and the black line represented the PES membrane with and without CQDs respectively. Due to the negative charge of CQDs, the electronegativity of M-0.5 and M-0.5-DC were significantly stronger than that of M-0 and M-0-DC. With the same CQDs concentration of 0.5%, M-0.5-DC had stronger electronegativity, which indicated that the applied DC electric field promoted the enrichment of CQDs on the membrane surface. This conclusion was consistent with the XPS results. In addition, compared with M-0, the application of the DC electric field weakened the electronegativity of M-0-DC, which may be related to the change of molecular arrangement state caused by the polarization of PES.
The entanglement between polymer chains directly affects the me- chanical strength of polymers [43]. In order to characterize the change
of mechanical strength of the prepared membrane, the tensile strength of the membranes was tested, and the results were presented in Fig. 8b.
The tensile strength of the membrane ranged from 2.8 MPa to 4.3 MPa.
The phenomenon that the tensile strength of M-0-DC was better than that of M-0 was attributed to the local “cohesive entanglement” of PES chains. In addition, increasing the content of CQDs could improve the tensile strength of the composite membranes, which could explained by the role of CQDs as a “nano wedge” between PES molecular chains. The tensile strength of M-0.5-DC was better than M-0.5, which benefited from the combined influence of DC electric field and CQDs on the properties of the membranes. However, when the content of CQDs continued to increase, the tensile strength of M-0.7-DC tended to decrease compared with M-0.5-DC. The reason for this behavior was that the aggregation and inhomogeneity of CQDs made the surface of the membrane tend to form microcracks [44–46]. After all, the appropriate CQDs content had a positive effect on the properties of PES composite membranes.
3.6. Permeability and selectivity of the membranes
The pure water permeability and the rejection of BSA of the fabri- cated PES UF membranes were shown in Fig. 9. As depicted in Fig. 9, all the modified membranes showed enhanced pure water permeability and M-0.5-DC had the most satisfying pure water permeability of 478.2 L m−2 h−1⋅bar−1. Compared with the control membrane M-0, the water
Fig. 7. Photograph of the prepared PES membranes under UV light (a). Survey XPS spectra of prepared UF membranes (b). O1s core energy levels of M-0 (c), M-0-DC (d), M-0.5 (e), and M-0.5-DC (f).
Table 3
Element content of prepared membranes tested by XPS atomic survey.
Samples Atomic composition (%) Surfaces chemical bonds ratio (%)
C S O N C=O
Content (%)
O–H Content (%)
The total content of O–H and C=O bonds (%)
M-0 75.94 5.04 15.44 3.61 0 0 0
M-0-DC 75.16 4.34 16.68 3.83 0 0 0
M-0.5 76.26 5.83 15.30 2.61 1.67 1.99 3.66 M-0.5-
DC 76.57 5.45 15.28 2.70 1.88 2.30 4.18
permeability of M-0-DC, M-0.5, and M-0.5-DC increased by 54.6%, 100.7%, and 164.1%, respectively. Another interesting phenomenon was that M-0.3-DC showed higher pure water permeability than M-0.5, which indicated that the structural modification by the DC electric field played an important role in the separation and permeability perfor- mance of PES UF membranes. The UV–vis spectra of BSA permeate of all membranes prepared was shown in Fig. S3 and it can be seen that the absorption peaks of BSA did not occur at 280 nm. Although there is a trade-off between permeability and selectivity [47], the rejection rate of M-0.5-DC to BSA remained above 98%. The significantly increased pure water permeability of M-0.5-DC was mainly due to the capture of water by hydrophilic CQDs fixed on the membrane surface and the structure of thin separation layer of electric field modified membrane, which greatly reduced the resistance of water molecules to pass through the membrane.
3.7. Antifouling and self cleaning properties of membranes
Low protein adsorption is one of the indicators to measure the antifouling performance of ultrafiltration membranes [48]. The Rt, Rr,
and Rir of all membranes obtained from the dynamic filtration experi- ment of “water - BSA - water” were shown in Fig. 10. Even though the Rt of M-0-DC was slightly higher than that of M-0, the Rir decreased from 31% (M-0) to 24% (M-0-DC), which was attributed to the fact that the membrane with high porosity can be cleaned more easily. The fouling rate of the membranes containing CQDs was significantly reduced. This was because the presence of CQDs reduced the hydrophobicity of the membrane and promoted the formation of a hydrated layer on the membrane surface, which reduced the possibility of BSA sticking to the membrane surface. Surprisingly, the Rir of M-0.5-DC was only 3.8%, which showed the great potential of the synergistic effect of the DC electric field and CQDs in improving the antifouling performance of the membranes.
The time dependent permeability of anti pollution experiment and FRR were shown in Fig. 11. The FRR of M-0 and M-0-DC was 71.8% and 76.7%, respectively. Although M-0.5-DC also showed a slight flux loss, its excellent recovery ratio (95.3%) proved that it can be cleaned easily.
Recently reported literature on novel membranes for studying the anti fouling properties of membranes were summarized in Table 4. In contrast, the M-0.5-DC prepared in this study showed excellent Fig. 8.(a) Zeta potential and (b) tensile strength of the prepared PES UF membranes.
Fig. 9. Pure water permeability and BSA rejection of the prepared PES membranes.
performance, which had a superior permeability (478.2 L m−2 h−1⋅bar−1, as shown in Fig. 9), effective BSA rejection (98.4%, as shown in Fig. 9), and outstanding permeability recovery (95.3%, as shown in Fig. 11b).
The absorption spectra of CR permeate of all membranes prepared was shown in Fig. S4 and it can be seen that the absorption peaks of CR
did not occur at 497 nm. The separation ability of the prepared mem- brane for CR was shown in Fig. 12a and the rejection of CR by all membranes remained above 97.9%. Compared with M-0, the larger pore size of the modified membranes was the main reasons for the decrease of CR rejection. In addition to good water solubility and dispersion, CQDs also has excellent optical stability, and its maximum absorption peak is Fig. 10.Fouling resistances for BSA fouling.
Fig. 11.Time dependent permeability of anti pollution experiment (a) and permeability recovery ratio (b).
Table 4
Performance comparison of different membranes in other literatures and the membranes prepared in this work.
Membrane samples Water permeability (L⋅m−2⋅h−1) BSA rejection (%) FRR (%) Pressure (bar) References
PES/GO-DC ~289 ~76 ~78 4 [20]
PES/PES-zwitterionic ~243 ~98 ~93 1 [49]
PSF/GO/PEG ~352 ~88 ~71 1 [50]
PSf/TiO2-PDA ~428 ~89 ~80 2 [51]
PES-Thr ~148 ~55 ~86 1 [52]
PES-Lys ~170 ~53 ~83 1 [52]
PES/ITF-0.3 wt% ~319 ~96 ~88 1 [53]
PES/CQDs-DC ~478 ~98 ~95 1 This work
in the ultraviolet band. Many studies have shown that CQDs can inhibit the recombination of electron pairs, which can effectively improve the efficiency of light conversion [54,55]. Therefore, it was expected that CQDs composite membrane can be used for dye separation and photo- catalytic degradation at the same time, which will effectively reduce the fouling of the membrane and prolong the life of the membrane. Fig. 12 (b1) and Fig. 12(b2) are the photos before and after illumination of M-0 after the CR filtration experiment. It can be observed that obvious cake layer appeared on the membrane surface, and then obvious dirt still existed on the membrane surface after illumination. Fig. 12(c1) and Fig. 12(c2) are the photos of M-0.5-DC after CR filtration before and after illumination, respectively. The surface of the M-0.5-DC became very clean after illumination, which indicated that a small amount of CQDs still endowed the membrane with self-cleaning ability and these novel membranes could achieve the perfect combination of dye sepa- ration and photocatalytic degradation.
4. Conclusion
In this work, PES ultrafiltration membranes named M-0-DC with a large number of finger like pores were obtained by using an electric field assisted membrane forming device, and its pure water permeability was increased by 54.6%. The change of cross-section structure of M-0-DC was characterized by SEM. Moreover, the pure water permeability of the membrane named M-0.5-DC was increased by 164.1%, which was due to the synergistic effect of the DC electric field and CQDs. On the one hand, the DC electric field promoted the conformation adjustment and polar- ization of PES molecular chain, which leaded to thinner epidermis, larger porosity and higher mechanical strength. On the other hand, the DC electric field promoted the uniform distribution and enrichment of CQDs on the membrane surface, which can be proved by water contact angle test and XPS analysis, respectively. Surprisingly, M-0.5-DC had excellent antifouling performance with the flux recovery rate up to 95.3%, and its BSA rejection rate was still above 98%. All in all, these results laid the foundation for further research on the complex proper- ties of polymers, indicating that DC electric field regulation has great application prospects in separation membranes, and provided a new idea and reference for the combined application of polymers and the electric field in the future. On this basis, more novel separation mem- branes will be successfully prepared.
Credit author statement
Bingjie Zhang: Conceptualization, Methodology, Investigation, Data curation, Writing - Original Draft, Writing - Review & Editing. Wenyi Wang: Conceptualization, Supervision, Writing - Reviewing and Editing,
Project administration, Resources. Liuyong Zhu: Validation, Investiga- tion. Ning Li: Formal analysis, Supervision. Xingyu Chen: Supervision.
Jiawei Tian: Investigation. Xuewen Zhang: Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The research was supported by the Science & Technology Develop- ment Fund of Tianjin Education Commission for Higher Education (2018ZD14). We would like to thank the Analytical & Testing Center of Tiangong University for scanning electron microscope, atomic force microscope, x-ray photoelectron spectrometer and other testing instruments.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.memsci.2021.119931.
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