Pharmaceuticals have been continuously detected from surface water and groundwater. In order to improve the rejection performance of pharmaceuticals by a nanofiltration membrane (NF), a positively charged membrane was prepared by co-deposition of natural gallic acid and polyethyleneimine on the polyacrylonitrile hydrolysis membrane. Effects of gallic acid concentration, polyethylene imine concentration, reaction time, and the molecular weight of polyethylene imine were documented. The physical and chemical properties of the membrane were also investigated by surface morphology, hydrophilicity, surface charge, and molecular weight cut-off. The optimized membrane had a molecular weight cut-off of about 958 Da and possessed a pure water permeability of 74.21 L·m−2·h−1·MPa−1. The results exhibited salt rejection in the following order: MgCl2 > CaCl2 > MgSO4 > Na2CO3 > NaCl > Na2SO4, while the rejection ability of pharmaceuticals is as follows: amlodipine > atenolol > carbamazepine > ibuprofen, suggesting that the positively charged membrane has enhanced retention to both divalent cations and charged pharmaceuticals. In addition, the antibacterial membrane was obtained by loading silver nanoparticles onto the positively charged membrane, which greatly improved the antibacterial ability of the membrane.

  • The NF membrane was prepared by using natural gallic acid that can be extracted from plants.

  • Improved water permeation flux due to loose pore structure and excellent hydrophilicity of the modified NF membrane.

  • Enhanced rejection ability of the membrane to both divalent cations and positively charged pharmaceuticals.

  • Improved antibiological fouling performance by loading AgNPs on the modified NF membrane.

Graphical Abstract

Graphical Abstract
Graphical Abstract

The continued presence of pharmaceuticals and personal care products (PPCPs) in water environments, as well as their potential negative effects on ecology and human health, has recently become one of the environment's research hotspots (Fatta-Kassinos et al. 2011; Fabbri et al. 2017; Mille et al. 2020). With the increase in urbanization, the content of PPCPs in the environment is getting higher and higher, such as in surface water, drinking water, and groundwater (Murthy & Gupta 1997). Researchers have found that the concentration of PPCPs has reached the level of ng/L in the water environment (McClellan & Halden 2010; Jelic et al. 2011). One way in which the aquatic environment is contaminated by PPCPs is through the direct discharge of drug-containing wastewater, and another source is excretion by patients treated with drugs (Bowen & Mohammad 1998a; Afonso et al. 2001). PPCPs in trace quantities have been proven to have possible harmful effects on human and animal lives, including disruption of several biological processes as well as development and reproduction inhibition (Ma et al. 2018; Ren et al. 2021). Herein, it is imperative to seek a highly efficient way to remove PPCPs.

Membrane separation technology plays an important role in solving the problems of water scarcity and pollution (Wang et al. 2018; Shokrollahi et al. 2020). Currently, membrane filtration has been used for the effective removal of individual PPCPs (Yang et al. 2017). Yoon et al. (2007) found that the nanofiltration (NF) membrane with 43–90% rejection for PPCPs was higher than the ultrafiltration membrane whose retention was about 40%. NF membrane technology has been widely used in water softening, brackish water desalination, wastewater treatment, dye purification, food processing, and organics removal (Lau et al. 2015; Mohammad et al. 2015). Also, surface coating is one of the methods to prepare the NF membrane. Han et al. (2014) utilized 2-hydroxypropyltrimethyl ammonium chloride chitosan with a positively charged character and good membrane-forming ability to fabricate the functional layer of the composite NF membrane. The composite membrane showed a classical positively charged membrane character which had higher rejection to multivalent cations.

Although membrane treatment technology has considerable advantages, there are many problems that need to be solved in practical applications, such as membrane biofouling (Guo et al. 2012). The key to preventing membrane biofouling is to prevent initial cell adhesion and its growth on the membrane surface (Rahaman et al. 2014). Therefore, the design of functional membranes with antibacterial properties is the most durable and effective method to prevent membrane biofouling (Jiang et al. 2013). Silver nanoparticles (AgNPs) have high thermal stability, chemical stability, and catalytic activity. It was often used in antibacterial preparation due to its ability to inhibit the growth of microorganisms (Basri et al. 2011). Researchers have successfully prepared various membranes containing AgNPs and proved their antibacterial properties (Yu 2003; Chou et al. 2005; Slistan-Grijalva et al. 2008; Taurozzi et al. 2008; Andrade et al. 2015). Also, research shows AgNPs can effectively inhibit the growth of bacteria because it can interfere with the synthesis of ribonucleic acid (Zhang et al. 2000).

Nevertheless, membranes are still facing great challenges in achieving the highest filtration efficiency due to the trade-off effect between permeability and rejection, as well as the need to introduce antibacterial properties in the membrane (Ayaz et al. 2019). Gallic acid (GA) is a natural polyphenolic compound containing carboxyl that is widely found in grapes, tea, and other plants in nature (Sileika et al. 2013). It has been widely used in food, biology, medical chemistry, and other fields due to its antioxidant (Rajalakshmi et al. 2001), water-soluble, antibacterial, and other characteristics. Besides, its carboxyl and hydroxyl groups provide good hydrophilicity, which could improve the permeation flux of the membrane. Also, in order to improve the separation performance, it is an effective method to improve the charge. Polyethyleneimine (PEI) is a water-soluble polymer with high cation density and strong adsorbability. Active primary and secondary ammonia on the PEI molecular chain can react with the epoxy group, carboxylic acid, and isocyanate (Dick & Ham 1970). Thus, it can be used to prepare a charged NF membrane (Chen et al. 2020).

Herein, this study aims to construct a positively charged composite NF membrane with natural GA and charged PEI via surface coating. This novel membrane contained a loose pore structure and excellent hydrophilicity, which could improve the permeate flux. The positively charged membrane had a stronger electrostatic repellent to both divalent cation and positively charged pharmaceuticals, which was mainly caused by the amine groups of PEI in the surface active layer. The characteristics of the membrane were analyzed by scanning electron microscope (SEM), atomic force microscope (AFM), water contact angle, and zeta potential, respectively. Meanwhile, the optimum conditions for membrane preparation were investigated. In addition, a GA/PEI-Ag antibacterial membrane was prepared by in situ reduction of AgNO3 on the membrane surface to improve the antibacterial property. The stability and antibacterial properties of the antibacterial membrane were studied.

Materials

The polyacrylonitrile ultrafiltration membrane (PAN, MWCO = 50 kDa) was purchased from Ande Membrane Separation Technology Engineering Co., Ltd (China). PEI was provided by Nanjing Lattice Chemical Technology Co., Ltd (China). GA was supplied by Shanghai Balinwei Chemical Technology Co., Ltd (China). Silver nitrate (AgNO3) was obtained from Nanjing Chemical Reagent Co., Ltd (China). Atenolol (ATE), Carbamazepine (CBZ), Amlodipine (AML) and Ibuprofen (IBU) provided by Shanghai Yuanye Biotechnology Co., Ltd (China) were used in this study.

Preparation of GA/PEI composite membrane

In this study, a GA/PEI composite NF membrane was prepared by surface coating modification technology. PEI had a positive charge due to many amine functional groups. After coming into contact with oxygen, GA lost electrons to produce quinones under alkaline conditions. The conjugated C = O double bond in quinones then underwent the Schiff base reaction with primary ammonia on the PEI molecular chain, followed by the Michael addition reaction, which produces macromolecular polymers (Cheng et al. 2018). The experimental process is shown in Figure 1. The PAN membrane was immersed in deionized water for 12 h to remove the glycerin on the PAN surface. Then, the substrate membrane was hydrolyzed in 2 mol/L NaOH solution at 45 °C for 1 h, afterward acidified in 0.2 mol/L HCl at room temperature for 3 h, followed by the formation of the polyacrylonitrile hydrolysis membrane (PAN-H). Meanwhile, a series of PEI with different concentrations and different molecular weights, a certain mass of GA fine powder, and 5 mL of Tris–HCl buffer solution (pH = 8.5) were weighed and fully dissolved and uniformly reacted for a certain time at room temperature. In the end, the PAN-H membrane was immersed in the uniform GA/PEI composite solution at 55 °C for 1 h, and then the membrane was immersed in deionized water for 30 min to remove the unattached substances on the surface for further characterization and evaluation.
Figure 1

Preparation principle of the GA/PEI composite membrane.

Figure 1

Preparation principle of the GA/PEI composite membrane.

Close modal

Preparation of the GA/PEI-Ag antibacterial membrane

The GA/PEI-Ag composite antibacterial membrane with a high antibacterial rate was prepared by using AgNO3 solution as the Ag+ source, using the binding force of phenolic hydroxyl group in GA, the in situ reduction of Ag+, the complexation of silver with –NH2 and –NH on PEI molecular branch chain, and uniformly loading silver nanoparticles on the membrane surface with the assistance of ultraviolet reduction. Firstly, 8 mM of AgNO3 was added to 0.3% PEI and stirred sufficiently at room temperature to form a composite solution of PEI-Ag +. Then, 0.2% GA was added and stirred sufficiently to form a uniform composite solution of GA–PEI-Ag +. Second, the PAN-H membrane was immersed in GA–PEI-Ag+ complex solution for 6 h with agitation to produce a GA–PEI-Ag antibacterial membrane. The surface of the PAN-H membrane was fully dried under the protection of water bath temperature and nitrogen blowing at 40 °C. Finally, the dried GA/PEI-Ag antibacterial membrane was irradiated with 200 W ultraviolet lamp for a period of time to assist in the reduction of silver ions to an elemental substance.

Characterization

The chemical structure of the membrane surface was characterized by Fourier transform infrared spectroscopy (Thermo Fisher). The surface and section morphology were observed by SEM (NEC Electronics Corporation, Japan), and the roughness was studied by AFM (Bruker, Germany). At the same time, the contact angle was measured by the contact angle analyzer (Bruker, Germany) to reflect the hydrophilicity of the membrane. Then, the zeta potential of membranes was characterized by an electrokinetic analyzer (Anton Paar Gmbh, Austria).

Determination of GA/PEI composite membrane parameters

The pure water permeability (PWP, L·m−2·h−1·MPa−1) was calculated by the following equation:
formula
(1)
where JV represents the penetration rate, LP is the water permeability, ΔP is the pressure, σ and Δπ are reflection coefficient and osmotic pressure, respectively.
When the retention rate of the retained substance was greater than 90%, the retention performance of the membrane was expressed by the molecular weight of the substance, which was defined as the retention molecular weight cut-off (MWCO) (Bowen et al. 1997). The relationship between the MWCO and the pore diameter was shown in the following equation (Afonso et al. 2001).
formula
(2)
The relationship between the membrane thickness and membrane aperture was shown in the following equation Hagen–Poiseuille empirical equation (Lee et al. 2015), which assumed that the membrane pore was cylindrical and of a single size:
formula
(3)
where JV represents the penetration rate (L·m−2·h−1), ΔP is the pressure (MPa), rP is the effective pore size of the membrane (nm), μ is the solution viscosity (Pa·s), ΔXe indicates the membrane thickness (μm).

Performance of the GA/PEI composite membrane

The separation experiment of fabricated membranes was tested in the cross-flow filtration membrane module. Membranes were pre-operated using pure water at 7 bar for 30 min to obtain a stable flux prior to the test. In the separation experiment, salts (MgCl2, MgSO4, Na2SO4, NaCl, Na2CO3, and CaCl2) with different charge properties and charge numbers of 0.5–10 g/L were selected. The rejection of membranes was determined under a pressure of 5 bar at 25 °C.

The GA/PEI composite membrane was used to intercept 50 μg/L of ATE, 500 μg/L of IBU, 5 μg/L of AML, and 5 μg/L of CBZ raw material solution, respectively. 100 mL of leachate was collected and the concentration of PPCPs in leachate was measured by the solid-phase extraction high-performance liquid chromatography (SPE-HPLC).

The rejection (R, %) was determined by the equation as follows:
formula
(4)
where Cf and Cp are the solute concentrations in feed and permeate, respectively.

Stability test of antibacterial membranes

The PAN-H membrane was cut into three identical small diaphragms with a diameter of 20 mm. Three AgNO3 solutions with different concentrations of 6, 8, and 10 mM were prepared, and three antibacterial membranes with different concentrations of silver were prepared by the in situ reduction method, respectively. Thereafter, Ag+ loading, Ag+ release rate, flux, and Ag+ release were tested to evaluate the stability of the antibacterial membrane.

Antibacterial performance test

The bacteriostatic circle experiment was based on our previous work (Nghiem et al. 2005). The bacteriostasis rate experiment was as follows: Firstly, the membrane sample was placed in a centrifuge tube containing diluted Escherichia coli solution and incubated in a biochemical incubator at 37 °C for 24 h. Then, 15 mL of LB liquid medium was treated with ultrasonic wave for 10 min to obtain colonies attached to the membrane. After that, the original solution was diluted to 0.1% with a sterile PBS buffer solution. Also, 20 μL of diluted suspension was cultivated on the solid medium. Finally, the number of colonies on the culture dish was counted with a biological microscope after 24 h incubation at 37 °C. Determination formula of antibacterial rate (Eb):
formula
(5)
Nb and Nm correspond to the colony number of the original PAN membrane (as control) and composite membrane on the culture dish, respectively.

Optimization of GA/PEI composite membrane conditions

Figure 2(a) shows the effect of GA concentration on the separation performance of the GA/PEI composite membrane. With the increase of GA concentration from 0.05 to 0.3%, the water flux decreased from 49.36 to 15.92 L·m−2·h−1, while the MgCl2 retention rate increased from 45.79 to 70.22%. The reason was more GA and PEI molecules react to form polymers, and functional groups that can react between them are cross-linked (Lv et al. 2015), resulting in smaller pore size and higher salt rejection. Therefore, the GA concentration of 0.2% was selected for this experiment to balance the water flux and salt retention.
Figure 2

Influence of GA concentration (a), PEI concentration (b), reaction time (c), and molecular weight of PEI (d) on the salt rejection and flux by the GA/PEI composite membrane.

Figure 2

Influence of GA concentration (a), PEI concentration (b), reaction time (c), and molecular weight of PEI (d) on the salt rejection and flux by the GA/PEI composite membrane.

Close modal

As illustrated in Figure 2(b), with the increase of PEI concentration from 0.05 to 0.3%, the water flux decreased from 31.05 to 14.33 L·m−2·h−1, while the MgCl2 retention rate increased from 74.40 to 83.93% at the beginning and then decreased to 78.84%. This was due to the moderate cross-linking of GA and PEI with a moderate concentration, forming a moderate pore size with high selectivity. However, when PEI was excessive, some positions may not form cross-linked structures, thus forming larger pores (Zhang et al. 2014). For this reason, 0.3% PEI concentration was selected as the best concentration.

As exhibited in Figure 2(c), when the reaction time was 3 h, the pure water flux was relatively high, reaching 51.91 L·m−2·h−1 at 0.5 MPa, but the rejection rate of salt was lower than 40%. With the increase of reaction time, the flux began to decrease and the salt rejection rate rose rapidly. That was because the membrane's mean effective pore size decreased and its membrane thickness grew as the reaction time increased (Zhang et al. 2016). Because the reaction between PEI and GA had likely reached a state of dynamic equilibrium by 9 h, the pure water flux was still maintained at 36.94 L·m−2·h−1, and the rejection rate had also improved to some extent. As a result, when the reaction time was greater than 9 h, the flux and the rejection rate almost did not change. Therefore, considering flux and rejection rate, the optimal reaction time was determined to be 9 h.

As shown in Figure 2(d), experiments demonstrated that using a PEI molecular weight of 600 Da of the NF membrane, the pure water flux was very high, but the interception of Na2SO4 and MgCl2 solution rate was relatively low. It was because the amount of amino groups with high relative molecular weight of PEI was relatively scarce, leading to higher reactivity, which increased the hydrophilicity of the basement membrane by forming a layer of smooth surface reaction layer (Weng et al. 2016). When the molecular weight of PEI was 1,800 Da, the reaction activity increased, the thickness of the reaction layer increased, and the pure water flux decreased slightly. However, the rejection rate of the composite membrane for Na2SO4 and MgCl2 solutions increased significantly because of the increase in the number of amino groups leading to the increase of effective charge. When the molecular weight of PEI was 10,000 Da, the number of reactive amino groups decreased due to the steric effect, leading to reduced reaction activity. Moreover, the macromolecular reticular structure was more prone to entanglement, which may lead to uneven local coating, resulting in greater roughness and reduced hydrophilicity. Thus, the flux of pure water decreased obviously, but the increasing trend of the rejection rate of salt solution slowed down (Ariza et al. 2002). Therefore, after comprehensive consideration, PEI with a molecular weight of 1,800 Da was adopted.

Characterization of the GA/PEI composite membrane

Figure 3(a) illustrates the Fourier infrared spectra (FTIR) of the PAN-H membrane and the GA/PEI composite NF membrane. The surface molecules of PAN membranes were prepared by polyacrylonitrile contain nitrile bonds (Feng et al. 2014). The spectrum of the PAN-H membrane showed that, in addition to C = O stretching vibration peaks at 1,539 cm−1, peaks at 3,355 cm−1 were due to the N–H stretching vibrations (Lv et al. 2015), indicating that C ≡ N on the surface of the PAN membrane hydrolyzed into –COOH bond successfully, and the amide bond formed after NaOH hydrolyzed. The peak at 2,955 cm−1 was the stretching vibration absorption peak of –CH2 (Niazi et al. 2020), and the peak at 2,359 cm−1 was caused by the stretching vibration of the –CN bond on the surface of the hydrolyzed membrane (Cheng et al. 2018). In comparison to the PAN-H membrane, the GA/PEI membrane had smaller peaks at 1,539 and 3,355 cm−1. This was caused by the fact that the GA/PEI coating was present on the PAN-H membrane surface, showing that the coating was successful (Zhang et al. 2019). Besides, the absorption peaks of the GA/PEI NF membrane at 783 and 1,716 cm−1 were enhanced, indicating that more phenolic groups and aromatic ketones were introduced on the surface of the PAN-H membrane (Zhang et al. 2019). In addition, the types of functional groups in the selective layer of GA/PEI were similar to those in the base membrane, so no new absorption peaks were observed.
Figure 3

The infrared spectra (a), hydrophilic characterization (b), and zeta potential with pH (c).

Figure 3

The infrared spectra (a), hydrophilic characterization (b), and zeta potential with pH (c).

Close modal

Figure 3(b) indicates the variation trend of the static contact angle of each composite membrane under different reaction times and different molecular weights of PEI. It can be seen from the figure that the contact angle of all composite membranes was less than 50°, indicating that both PAN-H membranes and composite membranes at different times and different molecular weights of PEI were hydrophilic. The contact angle of the PAN-H membrane as the base membrane was 48.31°. After coating on the surface of GA/PEI, the contact angle decreased to some extent. This was as a result of the membrane's porosity shape and the hydrophilic groups, including carboxyl, hydroxyl, amino, and methylamino, in the GA/PEI selection layer (Dye & Izake 2016). In addition, it was obvious that the contact angle of the composite membrane M3 was 33.19°, which greatly improved the hydrophilicity compared with the PAN-H membrane.

From Figure 3(c) it can be inferred that the variation of zeta potential on the surface of the PAN-H membrane and the GA/PEI composite membrane with pH 3–10. The zeta potential of the PAN-H membrane was about −49.67 mV under neutral conditions, and the surface of the PAN-H membrane was negatively charged in the pH range of 3–10. After GA and PEI surface coating, composite membrane zeta potential greatly improved because the positive charge carried by the amino group in PEI made the membrane surface positive (Wu et al. 2014). A more positive charge on the surface made the GA/PEI composite membrane improve the removal rate of divalent cation and positively charged PPCPs. However, the negative charge on the membrane surface was usually caused by protonated sulfonic acid or carboxylic acid groups at neutral pH. Due to the dissociation of functional groups, pH had an effect on the charge of the membrane, and the zeta potential of the membrane would also become more and more negative with the increase of pH and the deprotonation of functional groups (Tanninen & Nystrom 2002).

Determination of GA/PEI composite membrane parameters

As shown in Figure 4(a) that the pure water flux of the GA/PEI composite membrane was proportional to the operating pressure, and the linear equation obtained after fitting was y = 74.21 x −0.134. According to the formula (1), the PWP coefficient of the GA/PEI composite NF membrane was 74.21 L·m−2·h−1·MPa−1. Compared with previous studies in Table 1, due to the loose pore diameter and superior hydrophilicity, the GA/PEI composite NF membrane had excellent PWP.
Table 1

Comparison of the water permeability and separation performance of PPCPs between the current result and other previous NF membranes

MembranePWPPressure (bar)PPCPs rejection (%)
Ref.
ATEAML
GA/PEI/PAN-H 72.4a 88.04 91.35 This work 
(HTCC/PDA)3 NF 15.67a 76.22 – Ouyang et al. (2019)  
(PIP@rGO/TMC)/PES 49.86a 76 80 Li et al. (2022)  
HTCC-Ag/PES 16.27a 83.6 – Huang et al. (2016)  
ZIF-8-PEI NF 18.55b 78 – Guo et al. (2022)  
NF33 4.43b 11 70.9 – Taheri et al. (2020)  
MembranePWPPressure (bar)PPCPs rejection (%)
Ref.
ATEAML
GA/PEI/PAN-H 72.4a 88.04 91.35 This work 
(HTCC/PDA)3 NF 15.67a 76.22 – Ouyang et al. (2019)  
(PIP@rGO/TMC)/PES 49.86a 76 80 Li et al. (2022)  
HTCC-Ag/PES 16.27a 83.6 – Huang et al. (2016)  
ZIF-8-PEI NF 18.55b 78 – Guo et al. (2022)  
NF33 4.43b 11 70.9 – Taheri et al. (2020)  

aL·m–2·h−1·MPa−1.

bL·m–2·h−1·bar−1.

Figure 4

Permeability coefficient of pure water (a) and molecular weight cut-off (b) of the GA/PEI composite membrane.

Figure 4

Permeability coefficient of pure water (a) and molecular weight cut-off (b) of the GA/PEI composite membrane.

Close modal

Figure 4(b) reveals that the MWCO of the composite membrane was 958 Da, which belonged to the category of the NF membrane (interception molecular weight is 200–1,000 Da). According to Equation (3), the effective pore size of the composite NF membrane can be calculated as 0.69 nm.

Morphology analysis of the GA/PEI composite membrane

SEM surface and section morphology scanning results are exhibited in Figure 5. The original macroporous structure of the PAN-H membrane became a fine linear structure after coating the surface of the GA/PEI composite, the porosity decreased, and the surface became dense. It can be seen from the sectional images (g) and (h) that the surface layer thickness increased from 0.623 to 1.032 μm, proving that the surface of the composite membrane had been successfully loaded with GA/PEI selective layer (Wang et al. 2022). When PEI molecular weight reached 10,000 Da, the membrane surface became rough. It can be explained that when the molecular weight of PEI was too large, it was easy to have an uneven local coating (Kong et al. 2016). The rationale for using the molecular weight of 1,800 Da was displayed on the side. Figure 5(e) shows the pore size of the GA/PEI-Ag antibacterial membrane was further reduced and the surface became denser. Compared with the surface of the GA/PEI composite membrane, more dense metal ions appeared on the surface (Xiong et al. 2020), indicating that silver ions had been successfully complexed on the molecular chain of GA/PEI complex. The diameter of silver ions was about 265 nm from image (f), and the particle size was at the nanometer level, proving that the GA and PEI on the membrane surface successfully reduced silver ions to silver elementarity, but the agglomeration of AgNO3 was more serious.
Figure 5

Surface SEM images of (a) PAN-H (50 k), (b) GA/PEI1800 (50 k), (c) GA/PEI1800 (10 k), (d) GA/PEI10000 (10 k), (e) GA/PEI-Ag (10 k), (f) GA/PEI-Ag (25 k) membranes and cross-sectional SEM images of (g) PAN-H (5 k) and (h) GA/PEI1800 (5 k) membranes.

Figure 5

Surface SEM images of (a) PAN-H (50 k), (b) GA/PEI1800 (50 k), (c) GA/PEI1800 (10 k), (d) GA/PEI10000 (10 k), (e) GA/PEI-Ag (10 k), (f) GA/PEI-Ag (25 k) membranes and cross-sectional SEM images of (g) PAN-H (5 k) and (h) GA/PEI1800 (5 k) membranes.

Close modal
Figure 6 shows the AFM diagram of the GA/PEI1800 composite membrane, the GA/PEI10000 composite membrane and the GA/PEI-Ag antibacterial membrane in the scanning range of 2 μm × 2 μm. After coating on the surface of GA/PEI, the average roughness (RMS) value of the PAN-H membrane decreased from 23.5 to 12.1 nm, and the roughness of the PAN-H membrane decreased. That was because the deposition of GA/PEI composite filled the uneven defects on the surface of the base membrane and made the composite membrane smooth (Lv et al. 2015). The GA/PEI10000 composite membrane displayed nodular surface morphology, increased fold, decreased smoothness, and increased RMS to 18.6 nm when the molecular weight of PEI was changed from 1,800 to 10,000 Da. It attributed to the fact that macromolecular network structure was more likely to form when the PEI molecular quantity changed dramatically, leading to local coating unevenness, which produced the roughness (Xu et al. 2019). The GA/PEI-Ag antibacterial membrane had visible bulges, as shown in Figure 5(c), and agglomerated AgNPs were adsorbed on the membrane's surface, increasing the complexity of the meshwork structure and giving the surface more concave and convexity (Xiong et al. 2020). RMS increased to 27.2 nm, which proved the successful loading of AgNPs on the surface of the membrane.
Figure 6

AFM images of modified membranes: (a) GA/PEI1800; (b) GA/PEI10000; and (c) GA/PEI-Ag.

Figure 6

AFM images of modified membranes: (a) GA/PEI1800; (b) GA/PEI10000; and (c) GA/PEI-Ag.

Close modal

Desalination effect of the GA/PEI composite membrane

In this experiment, the rejection rate of the GA/PEI composite membrane for 0.5 g/L MgCl2, MgSO4, Na2SO4, NaCl, Na2CO3, and CaCl2 were determined at room temperature (25 °C). The rejection test results of the composite membrane for six kinds of salt solutions are revealed in Figure 7(a). It can be seen from the figure that the sequence of retention of inorganic salts by the composite membrane was MgCl2 > CaCl2 > MgSO4 > Na2CO3 > NaCl > Na2SO4. It was obvious that the removal rate of divalent cations (Mg2+, Ca2+, etc.) by the GA/PEI composite membrane was higher than that of negative ion, suggesting that the electrostatic effect of the composite membrane on cations was stronger. It also reflected the positive charge of the membrane surface, which was consistent with the above zeta potential test results. For the inorganic salt solution with the same cation, the removal rates of MgCl2 and MgSO4 were very high due to the Donnan effect, while the rejection rate of MgCl2 was higher than that of MgSO4. This was due to the negative charge of being greater than Cl. Since the composite membrane was positively charged, was easier to pass through the membrane hole due to electrostatic attraction.
Figure 7

Rejection of different salts (a) and effect of high concentration on rejection performance (b) by the GA/PEI composite membrane.

Figure 7

Rejection of different salts (a) and effect of high concentration on rejection performance (b) by the GA/PEI composite membrane.

Close modal

Figure 7(b) reflects the change in the rejection rate of the GA/PEI composite membrane for MgCl2 and NaCl at different concentrations. As can be seen from the figure, with the increase in salt solution concentration, the rejection rate of the GA/PEI composite membrane for salt solution showed a decreasing trend. However, it can still maintain a relatively high retention rate for high concentration salts. On the one hand, an increase in Cl concentration reduced electrostatic repulsion by balancing the positive charge on the membrane surface, thus reducing the rejection rate of cations (Song et al. 2017). On the other hand, the Debye length of salt will decrease with the increase in salt concentration. However, the electrostatic repulsion is obvious only at a distance shorter than the Debye length, which leads to a reduction in the rejection rate.

Separation performance of PPCPs by the GA/PEI composite membrane

Figure 8, the rejection of the modified membrane to Atenolol (ATE), CBZ, Ibuprofen (IBU), and Amlodipine (AML) were 88.04, 86.59, 78.83, 91.35, respectively. The removal rate of positively charged AML and ATE was the highest, followed by neutral CBZ, and negatively charged IBU was the lowest, which proved the positive charge of the composite membrane surface from the side. Due to electrostatic repulsion, the composite membrane had the highest repulsion rate to AML and ATE. Because pore size screening was involved in the interception of PPCPs in addition to the charge effect, the removal rate of AML by the composite membrane was higher than that of ATE. In the process of membrane separation, adsorption was also one of the removal mechanisms (Nghiem et al. 2005), which in turn was related to the hydrophilicity and hydrophobicity of the composite membrane. The higher the octyl-water partition coefficient (LogKOW) of organic pollutants, the stronger the hydrophobicity was, and the easier it was to be adsorbed on the surface of the charged membrane (Koyuncu et al. 2008; Verliefde et al. 2009). Although negatively charged IBU was more likely to pass through a positively charged membrane due to the action of charge, the hydrophobic polymer in the membrane matrix interacted with IBU and caused it to be adsorbed on the membrane surface, improving the rejection rate that reached 78.83%. Therefore, the removal mechanism of these four drugs by the NF membrane was mainly charge repulsion effect, as well as pore size screening and adsorption (Bowen & Mohammad 1998b; Grysakowski et al. 2008).
Figure 8

Rejection to four different PPCPs on the GA/PEI composite membrane.

Figure 8

Rejection to four different PPCPs on the GA/PEI composite membrane.

Close modal

Table 1 summarizes the comparison of water permeability and separation performance to PPCPs by different NF membranes in the previous literature. Compared with the membranes previously reported, the GA/PEI composite membrane had good water flux, good removal effect on ATE and AML, which had potential application value.

Stability analysis of antibacterial membranes

In this experiment, the variation of the silver fixation capacity of three antibacterial membranes with different concentrations over time was studied, as exhibited in Figure 9(a). It can be seen from Figure 9 that the density of fixed silver particles increased with the extension of the reaction time of the membrane in silver nitrate solution. After more than 6 h, the growth rate of fixed silver ion density decreased and gradually stabilized. Therefore, 6 h was determined to be the optimal reaction time.
Figure 9

Ag+ loading (a) and Ag+ release (b) on the GA/PEI-Ag antibacterial membrane at different concentrations of silver nitrate.

Figure 9

Ag+ loading (a) and Ag+ release (b) on the GA/PEI-Ag antibacterial membrane at different concentrations of silver nitrate.

Close modal

Table 2 describes the fixed Ag+ density and adsorption rate of the antibacterial membrane after the optimal reaction time. The results proved that with the increase in AgNO3 concentration, the Ag+ density attached to the surface of the antibacterial membrane gradually increased as well as the adsorption rate. When the concentration of AgNO3 increased from 6 to 8 mM, the Ag+ loading on the membrane increased significantly, and the latter was almost twice as much as the former. When the concentration of AgNO3 increased to 10 mM, the density of Ag+ fixed on the membrane only slightly increased compared with that of the GA/PEI-Ag (8 mM) antibacterial membrane. This phenomenon can be explained as the silver ions on the membrane surface tended to saturate when the concentration of AgNO3 was 8 mM. So when the concentration increased again, the adsorption rate will not be greatly improved. Therefore, 8 mM was selected as the optimal concentration of antibacterial membranes.

Table 2

Experimental data sheet of antibacterial membranes

The antibacterial membraneGA/PEI-Ag (6 mM)GA/PEI-Ag (8 mM)GA/PEI-Ag (10 mM)
Ag+ ion adsorption Initial Ag+ concentration (g/L) 0.65 0.86 1.08 
Ag+ concentration after immersion (g/L) 0.55 ± 0.03 0.61 ± 0.03 0.73 ± 0.02 
Fixed the density of Ag+ (mg/cm20.81 ± 0.02 2.00 ± 0.06 2.83 ± 0.07 
Adsorption rate (%) 15.67 ± 0.14 29.03 ± 0.26 32.87 ± 0.76 
Static release behaviors of Ag+ Ag+ concentration in deionized water (g/L) 0.55 ± 0.008 0.61 ± 0.011 0.73 ± 0.017 
Ag+ density after static release (mg/cm20.69 ± 0.005 1.85 ± 0.02 2.25 ± 0.05 
Static release rate (%) 14.76 ± 1.94 7.32 ± 1.72 20.56 ± 1.43 
Dynamic release behaviors of Ag+ Pure water flux (L·m−2·h−131.82 ± 0.74 27.03 ± 0.83 19.11 ± 0.56 
Release rate (%) 2.73 ± 0.05 2.03 ± 0.06 2.22 ± 0.03 
The antibacterial membraneGA/PEI-Ag (6 mM)GA/PEI-Ag (8 mM)GA/PEI-Ag (10 mM)
Ag+ ion adsorption Initial Ag+ concentration (g/L) 0.65 0.86 1.08 
Ag+ concentration after immersion (g/L) 0.55 ± 0.03 0.61 ± 0.03 0.73 ± 0.02 
Fixed the density of Ag+ (mg/cm20.81 ± 0.02 2.00 ± 0.06 2.83 ± 0.07 
Adsorption rate (%) 15.67 ± 0.14 29.03 ± 0.26 32.87 ± 0.76 
Static release behaviors of Ag+ Ag+ concentration in deionized water (g/L) 0.55 ± 0.008 0.61 ± 0.011 0.73 ± 0.017 
Ag+ density after static release (mg/cm20.69 ± 0.005 1.85 ± 0.02 2.25 ± 0.05 
Static release rate (%) 14.76 ± 1.94 7.32 ± 1.72 20.56 ± 1.43 
Dynamic release behaviors of Ag+ Pure water flux (L·m−2·h−131.82 ± 0.74 27.03 ± 0.83 19.11 ± 0.56 
Release rate (%) 2.73 ± 0.05 2.03 ± 0.06 2.22 ± 0.03 

The variation in the amount of Ag+ released from three antibacterial membranes with different concentrations is examined in Figure 9(b). The concentration of silver ion in water was almost unchanged after 24 h of deionized water immersion, and the order of silver ion release was as follows: GA/PEI-Ag (10 mM) > GA/PEI-Ag (8 mM) > PEI-Ag (6 mM). It can be seen that the amount of silver ion release increased with the increase in concentration. Table 2 showed the silver ion density and static release rate after the release of three different concentrations of antimicrobial membranes. According to the data in the table, the order of the static release rate was as follows: GA/PEI-Ag (10 mM) > GA/PEI-Ag (6 mM) > PEI-Ag (8 mM). This was because 8 mM AgNO3 fixed more silver ions, and the release values of GA/PEI-Ag (6 mM) and GA/PEI-Ag (8 mM) silver ions were consistent, so GA/PEI-Ag (8 mM) had a lower release rate. Therefore, 8 mM was determined to be the optimal concentration of the antimicrobial membrane.

After the filtration of pure water, the pure water flux, AgNO3 concentration, and dynamic release rate of the three antibacterial membranes are shown in Table 2. It can be seen from the table that in the antibacterial membrane, the flux increased as the concentration of AgNO3 decreased, showing a slow downward trend. The pure water flux in GA/PEI-Ag (8 mM) antibacterial membrane was slightly lower than that in the GA/PEI-Ag (6 mM) antibacterial membrane. However, the pure water flux of the GA/PEI-Ag (10 mM) antibacterial membrane decreased significantly, indicating that the AgNPs loaded on the surface of the GA/PEI-Ag (8 mM) antibacterial membrane were the most stable among the three kinds of membrane. Both pure water flux and release rate verified the rationality of choosing 8 mM AgNO3 concentration to prepare the antibacterial membrane from the side. While this study found that the silver ion concentration in the static release experiment was almost unchanged after 24 h of immersion in deionized water. However, the membrane in dynamic experiment did not undergo such a long immersion.

Study on antibacterial properties of membranes

The study results for antibacterial performance are revealed in Figure 10. Antimicrobial tests proved that the PAN-H membrane had almost no inhibition rate. The GA/PEI composite membrane and GA/PEI-Ag composite membrane had obvious bacteriostatic circles. The diameter of the antibacterial zone of the GA/PEI composite membrane was 22.76 ± 0.03 mm, while the GA/PEI-Ag antibacterial membrane increased by 11.3%. Furthermore, the bacteriostatic rate jumped from 27.4% of the GA/PEI composite membrane to 98.2% of the GA/PEI-Ag antibacterial membrane. This proved that the antibacterial effect of the GA/PEI-Ag antibacterial membrane was much better than that of the GA/PEI composite membrane. This phenomenon can be explained as Ag+ will interfere with the formation of cell wall and inactivate proteins, so as to inhibit the life activities of bacteria.
Figure 10

The bacteriostasis of the PAN-H membrane (a), GA/PEI composite membrane (b) and the GA/PEI-Ag membrane (c) on Escherichia coli, Antibacterial efficiency (Eb) of the PAN-H membrane, the GA/PEI composite membrane and the GA/PEI-Ag membrane (d).

Figure 10

The bacteriostasis of the PAN-H membrane (a), GA/PEI composite membrane (b) and the GA/PEI-Ag membrane (c) on Escherichia coli, Antibacterial efficiency (Eb) of the PAN-H membrane, the GA/PEI composite membrane and the GA/PEI-Ag membrane (d).

Close modal

In summary, this study successfully prepared a positively charged composite membrane for removal of PPCPs through surface coating technique. The success of the modified membrane was confirmed using FTIR. Also, the optimal membrane was determined to be the one with a GA and PEI concentration of a 0.2 and 0.3%, respectively. The optimal reaction time was 9 h and the optimal PEI molecular weight was 1,800 Da. SEM and AFM were used to study the surface microstructure of the optimal membrane. The optimal membrane showed a high PWP coefficient of 74.21 L·m−2·h−1·MPa−1 and MWCO of 958 Da. The optimal membrane also evaluated good repellent performance to various inorganic salts and PPCPs. After AgNPs were loaded on the modified membrane, the obtained GA/PEI-Ag antibacterial membrane possessed a great antibacterial ability. At the same time, the bacteriostatic rate of the GA/PEI-Ag antibacterial membrane was much higher than that of the GA/PEI composite membrane. Therefore, this study provides a new strategy for the preparation of NF membranes with effective PPCPs removal and antibacterial effect.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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