- Email: [email protected]
Grafting d -amino acid onto MF polyamide nylon membrane for biofouling control using biopolymer alginate dialdehyde as a versatile platform
Separation and Purification Technology 231 (2020) 115891
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Grafting D-amino acid onto MF polyamide nylon membrane for biofouling control using biopolymer alginate dialdehyde as a versatile platform ⁎
Rashid Khana, M. Kamran Khanb, Han Wanga, Kang Xiaoc,d, , Xia Huanga,d,
T
⁎
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Tsinghua University, Beijing 100084, China c College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China d Research and Application Center for Membrane Technology, School of Environment, Tsinghua University, Beijing 100084, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane fouling Membrane modification D-amino acid grafting Surface hydrophilicity Anti-microbial property
Biofouling is the Achilles heel to membrane-based separations. Development of novel fouling control approaches is very demanding for world-wide application of membrane technologies. Herein, a synergistic effect due to surface hydrophilicity and anti-microbial properties was endowed by D-tyrosine grafting onto a commercial microfiltration polyamide nylon membrane using alginate dialdehyde (ADA) as a green facile platform. The Dtyrosine grafted membrane surfaces were characterized using ATR-FTIR, XPS, SEM-EDX, and AFM. D-tyrosine and ADA grafting significantly increased membrane hydrophilicity and vastly decreased surface roughness thereby mitigating biofouling with improved permeability performance. During pure water filterability test about 74% higher permeability was observed compared to the pristine membrane. Further, the flux recovery ratio for the D-tyrosine-ADA-nylon membrane (91%) was significantly higher than the pristine membrane (42%). Also, the D-tyrosine-ADA-nylon membrane demonstrated excellent antibiofouling performance when incubated with E. Coli culture. Henceforth, this novel membrane functionalization establishes a new green facile platform to alleviate biofouling with the synergistic effects of anti-microbial and surface hydrophilicity properties. Moreover, this functionalization is quite simple and direct, having the potential for scaled-up industrial application.
1. Introduction Polymer membranes have been extensively used in various important separation technologies including but not limited to: removal of organic pollutants, macromolecules, virus, bacteria, salts, and gas molecules from various kinds of water treatment and wastewater reclamation systems [1–3]. However, significant membrane fouling has been seen, resulting from organic pollutants and microorganisms in the feed solutions. Organic pollutants present in the feed solutions easily deposit onto the membrane surface and thereby providing suitable environment for microorganisms to attach to the organic moieties. Consequently, microbial colonization and adhesion to the surfaces resulting in the biofilm formation deteriorate the performances of the membranes during operation. Hence, current trends in membrane processes are to devise effective fouling control strategies. At present researches have mainly been focused on modifying the surfaces of membrane by endowing their hydrophilicity. It was found that hydrophilic surfaces could reduce the interaction between membrane and
⁎
foulants. Various hydrophilic polymers, such as polydopamine (PDA), poly (N-vinyl pyrrolidone (PVP), poly(ethylene imine) (PEI), and poly (ethylene glycol) (PEG), have been used to increase the hydrophilicity of the membrane thereby decreasing the membrane fouling [4–7]. As a matter of fact, most of these anti-adhesive membrane coating strategies are not adequate to prevent membrane biofouling, since only a few initial microorganism colonies attachment are sufficient to form a mature biofilm. Therefore, a multipronged approach for the functionalization of the membrane having parallel anti-adhesion, anti-fouling and anti-microbial abilities would be promising for biofouling control. Recently, D-amino acids have been shown to inhibit bacterial adhesion onto the membrane surfaces, causing self-dispersal of biofilm at very low concentrations [8–10]. Kolodkin-Gal et al. demonstrated that four D-amino acids: D-tyrosine, D-methionine, D-tryptophan, and D-leucine not only inhibited biofilm formation but also effectively dispersed mature biofilm of Bacillus subtilis [11]. Kolodkin-Gal et al. further reported that D-tyrosine had the highest efficacy among all the D-amino acids used. D-amino acids were also demonstrated to reduce submerged
Corresponding authors at: Research and Application Center for Membrane Technology, School of Environment, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected] (K. Xiao), [email protected] (X. Huang).
https://doi.org/10.1016/j.seppur.2019.115891 Received 11 May 2019; Received in revised form 22 July 2019; Accepted 2 August 2019 Available online 03 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
antifouling MF membrane performance. The ADA-nylon membrane was washed with DI water many times, and then taken into an aqueous solution of D-tyrosine at different concentrations (50, 100, and 500 µM) for different time intervals (6–18 h, 25 °C and 120 rpm) [14] in order to optimize the conditions for membrane modification. After the modification process, the membranes were rinsed several times and kept in DI water at 4 °C for further tests.
solid surface biofilms formed by mixed culture from activated sludges [12,13]. Furthermore, Xing et al. have shown that D-amino acids contributed to the repulsive nature of the cell and eventually resulted in inhibition of the bacterial cell adhesion [8]. Jiang et al. grafted D-tyrosine on a polyether sulfone membrane using polydopamine as a precoating platform and showed excellent membrane surface hydrophilicity and anti-microbial properties [14]. However, polydopamine is expensive to use at industrial scale. Therefore, it is still of great interest and challenge to explore new less expensive, and up scalable precoating platform, which is the main purpose of this study. In the present study, we demonstrate that D-tyrosine could be grafted onto microfiltration (MF) polyamide membrane using biopolymer alginate dialdehyde (ADA) as a versatile platform. ADA is a promising bio-adhesive polymer, possessing carboxyl and aldehyde moieties for functionalization, having biocompatible, and biodegradable nature. ADA has been widely used in tissue engineering, pharmaceutical, cosmetics, and food industries [15]. However, to the best of our knowledge, there is no relevant report regarding the application of ADA as antifouling as well as a promising platform for D-tyrosine grafting. To modify the membrane, a simple, facile, and robust, twostep method was introduced. The fouling resistance, surface properties, and antimicrobial properties of the MF nylon membranes containing ADA and D-tyrosine were investigated in detail and compared to those of the pristine MF nylon membranes.
2.4. Characterization of membranes The membrane surface morphology was investigated by scanning electron microscope (SEM, S-4800, Hitachi Limited Inc., Japan). Prior to observation the membrane samples were sprayed with gold particles. Surface chemical structures of membranes were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy (ATRFTIR, NEXUS 670, Thermo Nicolet, USA). The spectra were taken in the region between 800 and 2000 cm−1. X-ray photoelectron spectroscopy (XPS, PHI 5300 spectrometer, USA) was used to analyze the membrane surface elemental composition. A monochromatic Al Kα radiation source was used as the X-ray source. The samples were washed with DI water and thoroughly vacuum freeze dried before ATR-FTIR and XPS characterization. Atomic force microscopy (AFM, Dimension Icon, Veeco Instruments, USA) was used to investigate the surface roughness in the tapping mode in air. The height profile of a 10.0 μm × 10.0 μm three-dimensional AFM image was used to estimate the root mean-square-roughness and mean roughness. To measure the membrane surface hydrophilicity, the static water contact angle was measured by a contact angle goniometer (OCA15EC, Dataphysics, Germany) equipped with a video camera using the sessile drop method. A 2 µL water droplet was used to minimize the gravity effect. At least 6 various locations were chosen on one membrane surface, to get a reliable contact angle value. Membrane surface streaming potential was investigated to find the surface charge by using a solid surface zeta potential analyzer (SurPASS, Anton Paar, Austria). The measurement was carried out with a background electrolyte solution containing 1 mM KCl at 25 °C and pH range of 3 to 10. The zeta potential of the membrane was calculated from the HelmholtzSmoluchowski equation. The membrane porosity (ɛ) was calculated by the gravimetric method, finding the weight of liquid (here pure water) contained in the membrane pores [18].
2. Materials and methods 2.1. Materials Nylon membranes with a pore size of 0.45 µm were bought from Millipore (USA). Sodium alginate, D-tyrosine, and bovine serum albumin (BSA, 67 kDa) were obtained from Sigma Aldrich (USA). Sodium periodate (NaIO4) was bought from Chengdu Kelong Chemical Reagent Company (China). Escherichia coli (E. Coli) LCT-EC106 was used as the model bacteria for the antifouling experiment. All other chemical agents were of analytical grade and used without any further purification. 2.2. Preparation of alginate dialdehyde (ADA) Na-alginate solution (12 g in 400 mL of deionized (DI) water and 100 mL of ethanol) was mildly stirred on a magnetic stirrer followed by adding 10 g of sodium periodate in the dark at room temperature to obtain ADA. The reaction was stopped after 24 h by adding ethylene glycol (40 mL) to reduce the excess periodate for 2 h with constant stirring (200 rpm). The resulting product was purified by precipitation with the addition of 10 g sodium chloride and 1600 mL pure ethanol. Then, the solution was dialyzed using dialysis tube (Molecular weight cutoff, 3500 Da) against DI water with several changes of water until the dialyzate was periodate free. The dialyzate was then lyophilized to obtain the product [16].
ε=
(w1 − w2)/ Dw (w1 − w2)/ Dw + w2/ Dp
(1)
where w1 is the wet membrane weight, w2 is the dry membrane weight, Dw is the pure water density (0.998 g cm−3), and Dp is the polymer density (Dp was here approximately 1.35 g cm−3 of polyamide membrane). The average pore size (rm) was calculated from the SEM images using the ImageJ software. 2.5. Filtration performance of membranes
2.3. Modification of membranes
The filtration performance of the membranes was evaluated employing a dead-end filtration vessel with an effective membrane area of 13.4 cm2. The membranes were pre-compacted for 30 min using DI water under a trans-membrane pressure of 70 kPa. After pre-compaction, an electronic balance was used to measure the pure water permeability at 40 kPa with 300 rpm stirring during the filtration. The water permeability was derived by Eq. (2):
Commercially available nylon MF membranes were utilized as the base membranes to be modified since they have the amide moieties for linkage. Fig. 1 is a schematic illustration of the modification procedure of the MF membranes. The nylon MF membranes were kept in DI water over-night and washed with 20% ethanol for 30 min to remove glycerol. The ADA treatment procedure of the MF membrane was carried out according to Khan et al. with slight modification [17]. The cleaned MF membrane was immersed in an aqueous solution of 10 mL ADA (1% w/ v) and 20 µL (85% v/v) phosphoric acid in a conical flask (shaken at 60 °C and 120 rpm) to react ADA to the nylon by one of its aldehyde moiety via a Schiff’s base reaction. A time range of 6–20 h was given for different sets of membrane to study the effect of ADA coating time on
Jw =
ΔV A × Δt × ΔP −2
(2) −1
−1
where Jw (L⋅m ⋅h ⋅bar ) is the pure water permeability, ΔV (L) is the permeate volume, A (m2) is the membrane surface area, Δt (h) is the permeation time, and ΔP (kPa) is the permeate pressure. For the filtration resistance test, a model protein solution containing 200 mg L−1 2
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 1. Scheme of two-step membrane modification process.
2.6.3. Antibiofouling property The antibiofouling properties of the membrane were assessed with model biofouling bacterium E. Coli culture. First, the pure water permeability of all the membranes were measured and then incubated for 48 h with E. Coli culture (30 mL LB medium having E. Coli culture). After 48 h incubation the membrane permeability was determined to check the biofouling effect due to E. Coli.
BSA with 10 mM ionic strength (9.9 mM NaCl and 0.1 mM NaHCO3) was forced to permeate through the membrane [19]. The concentration in the feed and permeate solutions was found by using BCA Protein Assay Kit (Sigma Aldrich) and the rejection was calculated by Eq. (3):
Cp R = 1 − ⎛ ⎞ × 100% ⎝ Cf ⎠ ⎜
⎟
(3)
where Cp and Cf (mg/L) are the permeate and feed BSA concentrations, respectively.
3. Results and discussion 3.1. Characterization of functionalized membranes
2.6. Antimicrobial and antifouling property of the membranes The schematic depiction of surface functionalization of microporous nylon membranes with ADA and D-tyrosine is shown in Fig. 1. ADA molecules were grafted on the nylon membranes by the reaction between aldehyde moieties of ADA and amide or amine moieties present on the membrane. FTIR and XPS scan measurements were employed for the confirmation of surface functionalization. The FTIR spectrum of pristine nylon, ADA-nylon and D-tyrosine-ADA-nylon were shown in Fig. 2. The peak at 1537 cm−1 corresponds to amides while the peak at 1632 cm−1 indicates carboxyl and carbonyl groups. The peak for amine group was observed around 3300 cm−1 for all the nylon membranes. A new peak around 1731 cm−1 as well as an increase in the carboxyl peak was observed for ADA-nylon membrane, which ascertained successful ADA coating. As far as D-tyrosine is concerned, the disappearance of aldehyde peak in the FTIR scan of D-tyrosine-ADA-nylon membrane confirms the successful grafting of D-tyrosine via a simple Schiff’s base reaction. XPS analysis was conducted to investigate the elemental composition of pristine nylon, ADA-nylon and D-tyrosine-ADA-nylon membranes. As shown in Fig. 3, the C1s core-level spectrum for different membranes can be distinguished into CeC/CeH, CeN/CeOH and NeC]O/O]CeOH species. For the ADA-nylon membrane the peak area for CeOH increased due to the presence of hydroxyl moiety on the ADA layer. The increase in peak area of O]CeOH group for ADA-nylon membranes was due to the occurrence of carboxyl moiety on ADA layer. Owing to the incorporation of amine and carboxyl functionalities by Dtyrosine for the D-tyrosine-ADA-nylon membranes, the peak area for CeN and O]C-OH further enhanced as compared to ADA-nylon membranes. Moreover, the XPS relative atomic concentrations of the major elements in the membrane surface are described in Table S1. The results reveal that the membranes grafting with ADA and D-tyrosine had higher O contents as compared to the pristine nylon membrane. This may be due to the introduction of ADA and D-tyrosine, which have high numbers of hydroxyl and carboxylic functionalities. While an increase in N contents was also seen for the D-tyrosine-ADA-nylon membranes, due to the presence of amine group in D-tyrosine. It is a well-established fact that surface morphology and hydrophilicity of the membrane surfaces defines the antifouling property [21–23]. Morphologies of all the membranes were studied by SEM,
2.6.1. Antimicrobial property The antimicrobial properties of the ADA and D-tyrosine modified MF nylon membranes against E. coli were performed using the plate count method. To prepare the bacteria suspension, E. coli was cultured in the Luria-Bertani (LB) broth solutions over-night at 37 °C. A single colony was transferred in 30 mL of LB broth, and then incubated over-night (120 rpm, 37 °C). Then, 10 mL of E. coli suspension with an initial concentration of 106 CFU/mL was taken into a conical flask having a sterile membrane sample. The E. coli suspension having the membrane sample was incubated for 2 h at 37 °C. The E. Coli suspension was diluted to different concentrations (20–100 fold), and then 0.1 mL of each diluent was spread onto the corresponding LB agar plates. Viable bacterial colonies were counted after 18 h culture at 37 °C. Each experiment was conducted in triplicate and values averaged. The bacterial inhibition rate I was calculated by Eq. (4):
I=
N0 − Ni × 100% N0
(4)
where I is the bacterial inhibition rate, N0 is the CFU of the sample without membrane and Ni is the bacterial CFU of the sample contacting with membrane. 2.6.2. Antifouling property The antifouling properties of the membrane were explored with BSA as model fouling agent. The process was mainly conducted in four steps as reported by other researchers [20]. Pure water filtration (Jw1) was carried at 40 kPa, then BSA solution filtration was conducted for 60 min. The BSA fouled membrane was washed for 40 min with DI water at room temperature, and the water permeability (Jw2) values were measured again with the washed membrane. The flux recovery ratio (FRR) was evaluated using Eq. (5):
FRR (%) =
Jw2 × 100 Jw1
(5) −2
−1
−1
where Jw1 and Jw2 (L⋅m ⋅h ⋅bar ) are the pure water permeability and the recovery pure water permeability after washing, respectively. 3
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 2. FTIR spectra of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes.
considered important to the membrane performance and antifouling property. Hydrophilic surface tends to reduce the attachment of hydrophobic foulants [26]. Hydrophilicity of all the modified membrane surfaces was improved via the successful introduction of hydroxyl and carboxyl groups by ADA coating and other hydrophilic ligands, as shown in Fig. 6. The highest hydrophilicity for the D-tyrosine-ADAnylon would probably be due to the presence of hydroxyl and carboxyl groups introduced by D-tyrosine. Nonetheless, no noticeable changes in contact angle were observed with further increasing of D-tyrosine concentration (500 µm). Pore size (rm) and porosity (ɛ) are shown in Table 1, which shows that very small amount of pore narrowing occurred due to ADA coating and subsequent functionalization. In contrast a very high membrane porosity has been observed for the modified membranes compared to the pristine membrane (Table 1).
showing likely higher smoothness (Fig. 4b, c) due to ADA coating and subsequent functionalization by D-tyrosine. The phenomena of surface smoothness with ADA coating and subsequent functionalization by Dtyrosine were further witnessed by AFM analysis. As noted in Fig. 4, an assembly of sharp crests were observed for pristine nylon membrane, while relatively smooth crests can be seen for the ADA modified membrane. A dramatic smoothness in crests was noticed for D-tyrosine grafted membrane, which might be the reason of D-tyrosine deposition in crest surfaces. This crest surface smoothness was also observed from values in terms of root-mean-square roughness (Rq) and mean roughness (Ra) for the ADA-nylon and D-tyrosine-ADA-nylon membranes compared to the pristine nylon membrane (Table S2). To further clarify the variation of functional groups on the nylon membranes, surface zeta potential with different pH was measured for all the membranes. As noted in Fig. 5, the pristine nylon membrane is negatively charged from pH 5 to 10 due to the dissociation of carboxylic functional groups in polyamide [24,25]. Because of the abundant carboxylic functional groups on the ADA layer, the ADA-nylon membrane showed negative charge from pH 3 to 10. The D-tyrosine-ADA-nylon membrane showed the most negative charge which was probably attributed to the dual functionalities due to ADA and D-tyrosine. EDS mapping showed a high increase in O percentage after ADA coating and further D-tyrosine grafting (Table S3). On the other hand, the hydrophilicity of the membrane is
3.2. Pure water permeability of the functionalized membranes To investigate the functionalized membrane filtration performance, microfiltration experiments were conducted. The pure water permeability for the ADA-nylon membrane (115.3 L⋅m−2⋅h−1⋅bar−1) was found to be a little higher than the pristine nylon membrane (107.3 L⋅m−2⋅h−1⋅bar−1) (Fig. 7). This higher permeability was probably due to the introduction of hydroxyl and carboxyl groups by ADA coating. The highest permeability was obtained for D-tyrosine-ADA-nylon
Fig. 3. C1s XPS spectra of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes. 4
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 4. SEM surface images of (a) pristine nylon, (b) ADA-nylon, and (c) D-tyrosine-ADA-nylon membranes. AFM surface images of (d) pristine nylon, (e) ADA-nylon, and (f) D-tyrosine-ADA-nylon membranes.
80 60
Pristine nylon ADA-nylon D-tyrosine-ADA-nylon
Zeta Potential (mV)
40 20 0 -20 -40 -60 -80 -100 3
4
5
6
7
8
9
10
pH
Fig. 6. Contact angles of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes.
Fig. 5. Surface zeta potential of pristine nylon, ADA-nylon, and D-tyrosine-ADAnylon membranes. −2
−1
Table 1 The average pore size and porosity of membranes.
−1
membrane (144.8 L⋅m ⋅h ⋅bar ), about 74.2% higher than the pristine membrane. The higher porosity (Table 1) and hydrophilicity (Fig. 6) due to the grafting were believed to weaken the resistance of water to pass through the membranes [27], thereby resulting in higher flux of the modified membrane than that of the pristine membrane. Furthermore, the very small amount of pore narrowing had no negative impact on the permeate permeability. However, when further increasing the grafting amount of D-tyrosine (500 µM), the pure water permeability started to decline. This might be due to the high concentrations of D-tyrosine blocking the membrane pores. Due to the nonspecific protein adsorption there has been no noticeable changes in BSA 5
Membrane
Average pore size rm (nm)
Porosity ε (%)
Pristine nylon ADA-nylon 50 µM D-tyrosine-ADA-nylon 100 µM D-tyrosine-ADA-nylon 500 µM D-tyrosine-ADA-nylon
448 442 440 440 438
58 69 72 74 73
± ± ± ± ±
1 2 2 3 3
± ± ± ± ±
1 2 3 3 3
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
evaluated with BSA filtration. The permeability of pristine nylon membrane decreased about 55% from initial permeability in 60 min filtration experiment as shown in Fig. 8a; while about 36% decrease was observed for ADA-nylon membranes. The least amount of decrease (30%) was seen for D-tyrosine-ADA-nylon membranes in 60 min BSA filtration experiment. Commonly, adsorption and deposition of BSA protein on the membrane decrease membrane permeability temporarily or permanently, termed as reversible or irreversible fouling. Consequently, water cleaning is used to remove reversible fouling. FRR has been widely regarded as the membrane antifouling ability as it reveals the water cleaning efficiency. The FRR values for all the membranes after four cycles were shown in Fig. 8b. All the ADA and D-tyrosine functionalized membranes showed higher FRR values than that of the pristine nylon membrane, revealing their excellent reusability and antifouling characteristics. The highest FRR of about 91% was seen for the 100 µM Dtyrosine grafted membrane; while the lowest FRR was observed for the pristine nylon membrane (42%). This outstanding antifouling property could be contributed by the increased hydrophilicity and enhanced smoothness. As matter of fact, the better hydrophilicity and smoother surfaces could result in a weaker interaction between membrane surface and hydrophobic BSA, preventing the adsorption or desorption of foulants on membrane surface [25,26]. Moreover, a smoother surface would contribute to lower total surface area thereby decreasing the adhesion efficiency of the foulants [28–31]. Furthermore, due to hydrophilicity of membrane surfaces, a tightly bound water layer is
Fig. 7. The pure water permeability and BSA rejection of pristine nylon, ADAnylon, and D-tyrosine-ADA-nylon membranes.
rejection for the ADA coating and subsequent functionalization compared to pristine membrane as shown in Fig. 7. 3.3. BSA-fouling and cleaning properties of D-tyrosine functionalized membranes In this study the membranes anti-adhesion properties were
Fig. 8. (a) The normalized flux of pristine and modified nylon membranes during 60 min filtration of BSA. (b) FRR of pristine nylon, ADA-nylon, and D-tyrosine-ADAnylon membranes. (c) Calculated bacterial inhibitions rate of pristine and modified nylon membranes. (d) The pure water permeability before and after bacterial growth of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes. 6
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Acknowledgement
formed, and foulants only interact with this shielding layer via shortranged forces, enabling the membrane to have antifouling properties.
This work was supported by the National Natural Science Foundation of China [grant number 51678336]. 3.4. Antibiofouling performance of D-tyrosine functionalized membranes Appendix A. Supplementary material The typical trend of biofilm cycle comprises of initial attachment of single cells, colonization of cells and then formation of a mature biofilm on the membrane surfaces [32]. In this study, E. coli was used as model bacteria to investigate the bacterial inhibition performance of the membrane surface. Fig. 8c displays the bacterial inhibition results of modified membranes calculated by Eq. (4), using the Fig. S1 images. The pristine nylon membrane did not exhibit any bacterial inhibition. The D-tyrosine functionalized membranes showed excellent bacterial inhibitions activity: the bacterial inhibition rates of 50 µM, 100 µM, and 500 µM membranes were 64.3, 73.6, and 83.7% respectively. Meanwhile, the ADA-nylon membrane showed about 40.7% bacterial inhibition. The D-tyrosine functionalization endowed the membrane with strong hydrophilicity. In contrast, most bacteria are hydrophobic in nature, which are usually prone to attach hydrophobic surfaces. Hence, it is important for membrane surfaces to be more hydrophilic in nature to avoid or mitigate bacterial contact for bacterial inhibition property. In the antibiofouling experiments, all the membranes were incubated with E. coli suspension having nutrients at 37 °C for 48 h to accelerate the biofouling process. During the culture growth, bacterial cells irreversibly attached to the membrane surface, resulting in the formation of matured biofilm on the membrane surface [33]. The pure water permeabilities of the membranes before and after this incubation were evaluated. The pure water permeability of all the incubated membranes decreased, due to biofilm formation. According to the filtration test results shown in Fig. 8d, the permeability of the pristine nylon membrane dropped by 52.5% after incubation with bacterial suspension, which is mostly caused by the biofilm developed. The permeability of the ADA-nylon membrane dropped by 43.6% indicating improved antibiofouling performance. The least amount of permeability decline was observed for the D-tyrosine-ADA-nylon membrane, which was in the range of 28–35%, showing excellent antibiofouling property. It is worth noting that the D-tyrosine-ADA-nylon membrane demonstrated lowest permeability reduction rates and outstanding antibiofouling performance. It could be easily inferred that the ADA and Dtyrosine functionalization endowed the membrane with strong antibiofouling characteristics. Besides, due to the strong hydrophilicity and smoothness, D-tyrosine could aggravate biofilm dispersal from the membrane surface [34].
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115891. References [1] K. Xiao, S. Liang, X. Wang, C. Chen, X. Huang, Current state and challenges of fullscale membrane bioreactor applications: a critical review, Bioresour. Technol. 271 (2019) 473–481. [2] K. Xiao, Y. Xu, S. Liang, T. Lei, J. Sun, X. Wen, H. Zhang, C. Chen, X. Huang, Engineering application of membrane bioreactor for wastewater treatment in China: current state and future prospect, Front. Environ. Sci. Eng. 8 (2014) 805–819. [3] H.-C. Yang, J. Luo, Y. Lv, P. Shen, Z.-K. Xu, Surface engineering of polymer membranes via mussel-inspired chemistry, J. Membr. Sci. 483 (2015) 42–59. [4] J. Mansouri, S. Harrisson, V. Chen, Strategies for controlling biofouling in membrane filtration systems: challenges and opportunities, J. Mater. Chem. 20 (2010) 4567–4586. [5] L. Duan, W. Huang, Y. Zhang, High-flux, antibacterial ultrafiltration membranes by facile blending with N-halamine grafted halloysite nanotubes, RSC Adv. 5 (2015) 6666–6674. [6] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.D. Freeman, A bioinspired fouling-resistant surface modification for water purification membranes, J. Membr. Sci. 413–414 (2012) 82–90. [7] M.S. Rahaman, H. Thérien-Aubin, M. Ben-Sasson, C.K. Ober, M. Nielsen, M. Elimelech, Control of biofouling on reverse osmosis polyamide membranes modified with biocidal nanoparticles and antifouling polymer brushes, J. Mater. Chem. B 2 (2014) 1724–1732. [8] S.F. Xing, X.F. Sun, A.A. Taylor, S.L. Walker, Y.F. Wang, S.G. Wang, D-Amino acids inhibit initial bacterial Adhesion: thermodynamic evidence, Biotechnol. Bioeng. 112 (2015) 696–704. [9] I.-S. Chang, P. Le Clech, B. Jefferson, S. Judd, Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng. 128 (2002) 1018–1029. [10] P. Le-Clech, V. Chen, T.A. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [11] I. Kolodkin-Gal, D. Romero, S. Cao, J. Clardy, R. Kolter, R. Losick, D-amino acids trigger biofilm disassembly, Science 328 (2010) 627–629. [12] A.I. Hochbaum, I. Kolodkin-Gal, L. Foulston, R. Kolter, J. Aizenberg, R. Losick, Inhibitory effects of D-amino acids on Staphylococcus aureus biofilm development, J. Bacteriol. (2011) JB. 05534-05511. [13] H. Xu, Y. Liu, Reduced microbial attachment by D-amino acid-inhibited AI-2 and EPS production, Water Res. 45 (2011) 5796–5804. [14] B.-B. Jiang, X.-F. Sun, L. Wang, S.-Y. Wang, R.-D. Liu, S.-G. Wang, Polyethersulfone membranes modified with D-tyrosine for biofouling mitigation: Synergistic effect of surface hydrophility and anti-microbial properties, Chem. Eng. J. 311 (2017) 135–142. [15] C. Hou, Z. Qi, H. Zhu, Preparation of core–shell magnetic polydopamine/alginate biocomposite for Candida rugosa lipase immobilization, Colloids Surf., B 128 (2015) 544–551. [16] F. Chen, M. Tian, D. Zhang, J. Wang, Q. Wang, X. Yu, X. Zhang, C. Wan, Preparation and characterization of oxidized alginate covalently cross-linked galactosylated chitosan scaffold for liver tissue engineering, Mater. Sci. Eng., C 32 (2012) 310–320. [17] M.K. Khan, J. Luo, R. Khan, J. Fan, Y. Wan, Facile and green fabrication of cation exchange membrane adsorber with unprecedented adsorption capacity for protein purification, J. Chromatogr. A 1521 (2017) 19–26. [18] J.F. Li, Z.L. Xu, H. Yang, Microporous polyethersulfone membranes prepared under the combined precipitation conditions with non-solvent additives, Polym. Adv. Technol. 19 (2008) 251–257. [19] S. Liang, Y. Kang, A. Tiraferri, E.P. Giannelis, X. Huang, M. Elimelech, Highly hydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membranes via postfabrication grafting of surface-tailored silica nanoparticles, ACS App. Mater. Interfaces 5 (2013) 6694–6703. [20] J.-J. Kim, K. Kim, Y.-S. Choi, H. Kang, D.M. Kim, J.-C. Lee, Polysulfone based ultrafiltration membranes with dopamine and nisin moieties showing antifouling and antimicrobial properties, Sep. Purif. Technol. 202 (2018) 9–20. [21] J.A. Brant, A.E. Childress, Colloidal adhesion to hydrophilic membrane surfaces, J. Membr. Sci. 241 (2004) 235–248. [22] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [23] A. Razmjou, J. Mansouri, V. Chen, The effects of mechanical and chemical modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes, J. Membr. Sci. 378 (2011) 73–84. [24] Y. Gao, S. Zhao, Z. Qiao, Y. Zhou, B. Song, Z. Wang, J. Wang, Reverse osmosis membranes with guanidine and amine enriched surface for biofouling and organic fouling control, Desalination 430 (2018) 74–85.
4. Conclusions In this study an MF nylon membrane was modified by D-tyrosine grafting using ADA as a green facile versatile platform, resulting in a novel D-tyrosine-ADA-nylon membrane. The results of ATR-FTIR and XPS confirmed that D-tyrosine had been successfully grafted on the MF polyamide commercial membrane surfaces. The SEM and AFM analysis showed that modified membranes had smoother surfaces compared to the pristine membrane. In addition, the membrane hydrophilicity was significantly enhanced. Excellent pure water permeability was witnessed for the modified membranes compared to the pristine membrane. Much higher membrane cleaning efficiency was achieved compared to the pristine membrane, with the best water permeability recovery reaching 91%. The anti-protein (BSA) adhesion performance of the modified membranes was also confirmed. Moreover, D-tyrosine not only inhibited microbial attachment but could also promote dispersal of the mature biofilm. Hence, this novel membrane functionalization may provide an easy route to alleviate membrane biofouling in various industrial applications.
7
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
[30] L. Zhang, J. Xu, Y. Tang, J. Hou, L. Yu, C. Gao, A novel long-lasting antifouling membrane modified with bifunctional capsaicin-mimic moieties via in situ polymerization for efficient water purification, J. Mater. Chem. A 4 (2016) 10352–10362. [31] A.R. Cooper, Ultrafiltration Membranes and Applications, Springer Science & Business Media, 2013. [32] A. Zaky, I. Escobar, A.M. Motlagh, C. Gruden, Determining the influence of active cells and conditioning layer on early stage biofilm formation using cellulose acetate ultrafiltration membranes, Desalination 286 (2012) 296–303. [33] C. Mattheis, H. Wang, C. Meister, S. Agarwal, Effect of Guanidinylation on the Properties of Poly (2-aminoethylmethacrylate)-Based Antibacterial Materials, Macromol. Biosci 13 (2013) 242–255. [34] H. Xu, Y. Liu, D-Amino acid mitigated membrane biofouling and promoted biofilm detachment, J. Membr. Sci. 376 (2011) 266–274.
[25] R. Zhang, Y. Li, Y. Su, X. Zhao, Y. Liu, X. Fan, T. Ma, Z. Jiang, Engineering amphiphilic nanofiltration membrane surfaces with a multi-defense mechanism for improved antifouling performances, J. Mater. Chem. A 4 (2016) 7892–7902. [26] J.R. Du, S. Peldszus, P.M. Huck, X. Feng, Modification of membrane surfaces via microswelling for fouling control in drinking water treatment, J. Membr. Sci. 475 (2015) 488–495. [27] J. Wang, Z. Wang, Y. Liu, J. Wang, S. Wang, Surface modification of NF membrane with zwitterionic polymer to improve anti-biofouling property, J. Membr. Sci. 514 (2016) 407–417. [28] D.E. Packham, Surface energy, surface topography and adhesion, Int. J. Adhes. Adhes. 23 (2003) 437–448. [29] V.M. Kochkodan, N. Hilal, V.V. Goncharuk, L. Al-Khatib, T.I. Levadna, Effect of the surface modification of polymer membranes on their microbiological fouling, Colloid J. 68 (2006) 267–273.
8
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Grafting D-amino acid onto MF polyamide nylon membrane for biofouling control using biopolymer alginate dialdehyde as a versatile platform ⁎
Rashid Khana, M. Kamran Khanb, Han Wanga, Kang Xiaoc,d, , Xia Huanga,d,
T
⁎
a
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China Department of Chemical Engineering, Tsinghua University, Beijing 100084, China c College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China d Research and Application Center for Membrane Technology, School of Environment, Tsinghua University, Beijing 100084, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Membrane fouling Membrane modification D-amino acid grafting Surface hydrophilicity Anti-microbial property
Biofouling is the Achilles heel to membrane-based separations. Development of novel fouling control approaches is very demanding for world-wide application of membrane technologies. Herein, a synergistic effect due to surface hydrophilicity and anti-microbial properties was endowed by D-tyrosine grafting onto a commercial microfiltration polyamide nylon membrane using alginate dialdehyde (ADA) as a green facile platform. The Dtyrosine grafted membrane surfaces were characterized using ATR-FTIR, XPS, SEM-EDX, and AFM. D-tyrosine and ADA grafting significantly increased membrane hydrophilicity and vastly decreased surface roughness thereby mitigating biofouling with improved permeability performance. During pure water filterability test about 74% higher permeability was observed compared to the pristine membrane. Further, the flux recovery ratio for the D-tyrosine-ADA-nylon membrane (91%) was significantly higher than the pristine membrane (42%). Also, the D-tyrosine-ADA-nylon membrane demonstrated excellent antibiofouling performance when incubated with E. Coli culture. Henceforth, this novel membrane functionalization establishes a new green facile platform to alleviate biofouling with the synergistic effects of anti-microbial and surface hydrophilicity properties. Moreover, this functionalization is quite simple and direct, having the potential for scaled-up industrial application.
1. Introduction Polymer membranes have been extensively used in various important separation technologies including but not limited to: removal of organic pollutants, macromolecules, virus, bacteria, salts, and gas molecules from various kinds of water treatment and wastewater reclamation systems [1–3]. However, significant membrane fouling has been seen, resulting from organic pollutants and microorganisms in the feed solutions. Organic pollutants present in the feed solutions easily deposit onto the membrane surface and thereby providing suitable environment for microorganisms to attach to the organic moieties. Consequently, microbial colonization and adhesion to the surfaces resulting in the biofilm formation deteriorate the performances of the membranes during operation. Hence, current trends in membrane processes are to devise effective fouling control strategies. At present researches have mainly been focused on modifying the surfaces of membrane by endowing their hydrophilicity. It was found that hydrophilic surfaces could reduce the interaction between membrane and
⁎
foulants. Various hydrophilic polymers, such as polydopamine (PDA), poly (N-vinyl pyrrolidone (PVP), poly(ethylene imine) (PEI), and poly (ethylene glycol) (PEG), have been used to increase the hydrophilicity of the membrane thereby decreasing the membrane fouling [4–7]. As a matter of fact, most of these anti-adhesive membrane coating strategies are not adequate to prevent membrane biofouling, since only a few initial microorganism colonies attachment are sufficient to form a mature biofilm. Therefore, a multipronged approach for the functionalization of the membrane having parallel anti-adhesion, anti-fouling and anti-microbial abilities would be promising for biofouling control. Recently, D-amino acids have been shown to inhibit bacterial adhesion onto the membrane surfaces, causing self-dispersal of biofilm at very low concentrations [8–10]. Kolodkin-Gal et al. demonstrated that four D-amino acids: D-tyrosine, D-methionine, D-tryptophan, and D-leucine not only inhibited biofilm formation but also effectively dispersed mature biofilm of Bacillus subtilis [11]. Kolodkin-Gal et al. further reported that D-tyrosine had the highest efficacy among all the D-amino acids used. D-amino acids were also demonstrated to reduce submerged
Corresponding authors at: Research and Application Center for Membrane Technology, School of Environment, Tsinghua University, Beijing 100084, China. E-mail addresses: [email protected] (K. Xiao), [email protected] (X. Huang).
https://doi.org/10.1016/j.seppur.2019.115891 Received 11 May 2019; Received in revised form 22 July 2019; Accepted 2 August 2019 Available online 03 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
antifouling MF membrane performance. The ADA-nylon membrane was washed with DI water many times, and then taken into an aqueous solution of D-tyrosine at different concentrations (50, 100, and 500 µM) for different time intervals (6–18 h, 25 °C and 120 rpm) [14] in order to optimize the conditions for membrane modification. After the modification process, the membranes were rinsed several times and kept in DI water at 4 °C for further tests.
solid surface biofilms formed by mixed culture from activated sludges [12,13]. Furthermore, Xing et al. have shown that D-amino acids contributed to the repulsive nature of the cell and eventually resulted in inhibition of the bacterial cell adhesion [8]. Jiang et al. grafted D-tyrosine on a polyether sulfone membrane using polydopamine as a precoating platform and showed excellent membrane surface hydrophilicity and anti-microbial properties [14]. However, polydopamine is expensive to use at industrial scale. Therefore, it is still of great interest and challenge to explore new less expensive, and up scalable precoating platform, which is the main purpose of this study. In the present study, we demonstrate that D-tyrosine could be grafted onto microfiltration (MF) polyamide membrane using biopolymer alginate dialdehyde (ADA) as a versatile platform. ADA is a promising bio-adhesive polymer, possessing carboxyl and aldehyde moieties for functionalization, having biocompatible, and biodegradable nature. ADA has been widely used in tissue engineering, pharmaceutical, cosmetics, and food industries [15]. However, to the best of our knowledge, there is no relevant report regarding the application of ADA as antifouling as well as a promising platform for D-tyrosine grafting. To modify the membrane, a simple, facile, and robust, twostep method was introduced. The fouling resistance, surface properties, and antimicrobial properties of the MF nylon membranes containing ADA and D-tyrosine were investigated in detail and compared to those of the pristine MF nylon membranes.
2.4. Characterization of membranes The membrane surface morphology was investigated by scanning electron microscope (SEM, S-4800, Hitachi Limited Inc., Japan). Prior to observation the membrane samples were sprayed with gold particles. Surface chemical structures of membranes were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy (ATRFTIR, NEXUS 670, Thermo Nicolet, USA). The spectra were taken in the region between 800 and 2000 cm−1. X-ray photoelectron spectroscopy (XPS, PHI 5300 spectrometer, USA) was used to analyze the membrane surface elemental composition. A monochromatic Al Kα radiation source was used as the X-ray source. The samples were washed with DI water and thoroughly vacuum freeze dried before ATR-FTIR and XPS characterization. Atomic force microscopy (AFM, Dimension Icon, Veeco Instruments, USA) was used to investigate the surface roughness in the tapping mode in air. The height profile of a 10.0 μm × 10.0 μm three-dimensional AFM image was used to estimate the root mean-square-roughness and mean roughness. To measure the membrane surface hydrophilicity, the static water contact angle was measured by a contact angle goniometer (OCA15EC, Dataphysics, Germany) equipped with a video camera using the sessile drop method. A 2 µL water droplet was used to minimize the gravity effect. At least 6 various locations were chosen on one membrane surface, to get a reliable contact angle value. Membrane surface streaming potential was investigated to find the surface charge by using a solid surface zeta potential analyzer (SurPASS, Anton Paar, Austria). The measurement was carried out with a background electrolyte solution containing 1 mM KCl at 25 °C and pH range of 3 to 10. The zeta potential of the membrane was calculated from the HelmholtzSmoluchowski equation. The membrane porosity (ɛ) was calculated by the gravimetric method, finding the weight of liquid (here pure water) contained in the membrane pores [18].
2. Materials and methods 2.1. Materials Nylon membranes with a pore size of 0.45 µm were bought from Millipore (USA). Sodium alginate, D-tyrosine, and bovine serum albumin (BSA, 67 kDa) were obtained from Sigma Aldrich (USA). Sodium periodate (NaIO4) was bought from Chengdu Kelong Chemical Reagent Company (China). Escherichia coli (E. Coli) LCT-EC106 was used as the model bacteria for the antifouling experiment. All other chemical agents were of analytical grade and used without any further purification. 2.2. Preparation of alginate dialdehyde (ADA) Na-alginate solution (12 g in 400 mL of deionized (DI) water and 100 mL of ethanol) was mildly stirred on a magnetic stirrer followed by adding 10 g of sodium periodate in the dark at room temperature to obtain ADA. The reaction was stopped after 24 h by adding ethylene glycol (40 mL) to reduce the excess periodate for 2 h with constant stirring (200 rpm). The resulting product was purified by precipitation with the addition of 10 g sodium chloride and 1600 mL pure ethanol. Then, the solution was dialyzed using dialysis tube (Molecular weight cutoff, 3500 Da) against DI water with several changes of water until the dialyzate was periodate free. The dialyzate was then lyophilized to obtain the product [16].
ε=
(w1 − w2)/ Dw (w1 − w2)/ Dw + w2/ Dp
(1)
where w1 is the wet membrane weight, w2 is the dry membrane weight, Dw is the pure water density (0.998 g cm−3), and Dp is the polymer density (Dp was here approximately 1.35 g cm−3 of polyamide membrane). The average pore size (rm) was calculated from the SEM images using the ImageJ software. 2.5. Filtration performance of membranes
2.3. Modification of membranes
The filtration performance of the membranes was evaluated employing a dead-end filtration vessel with an effective membrane area of 13.4 cm2. The membranes were pre-compacted for 30 min using DI water under a trans-membrane pressure of 70 kPa. After pre-compaction, an electronic balance was used to measure the pure water permeability at 40 kPa with 300 rpm stirring during the filtration. The water permeability was derived by Eq. (2):
Commercially available nylon MF membranes were utilized as the base membranes to be modified since they have the amide moieties for linkage. Fig. 1 is a schematic illustration of the modification procedure of the MF membranes. The nylon MF membranes were kept in DI water over-night and washed with 20% ethanol for 30 min to remove glycerol. The ADA treatment procedure of the MF membrane was carried out according to Khan et al. with slight modification [17]. The cleaned MF membrane was immersed in an aqueous solution of 10 mL ADA (1% w/ v) and 20 µL (85% v/v) phosphoric acid in a conical flask (shaken at 60 °C and 120 rpm) to react ADA to the nylon by one of its aldehyde moiety via a Schiff’s base reaction. A time range of 6–20 h was given for different sets of membrane to study the effect of ADA coating time on
Jw =
ΔV A × Δt × ΔP −2
(2) −1
−1
where Jw (L⋅m ⋅h ⋅bar ) is the pure water permeability, ΔV (L) is the permeate volume, A (m2) is the membrane surface area, Δt (h) is the permeation time, and ΔP (kPa) is the permeate pressure. For the filtration resistance test, a model protein solution containing 200 mg L−1 2
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 1. Scheme of two-step membrane modification process.
2.6.3. Antibiofouling property The antibiofouling properties of the membrane were assessed with model biofouling bacterium E. Coli culture. First, the pure water permeability of all the membranes were measured and then incubated for 48 h with E. Coli culture (30 mL LB medium having E. Coli culture). After 48 h incubation the membrane permeability was determined to check the biofouling effect due to E. Coli.
BSA with 10 mM ionic strength (9.9 mM NaCl and 0.1 mM NaHCO3) was forced to permeate through the membrane [19]. The concentration in the feed and permeate solutions was found by using BCA Protein Assay Kit (Sigma Aldrich) and the rejection was calculated by Eq. (3):
Cp R = 1 − ⎛ ⎞ × 100% ⎝ Cf ⎠ ⎜
⎟
(3)
where Cp and Cf (mg/L) are the permeate and feed BSA concentrations, respectively.
3. Results and discussion 3.1. Characterization of functionalized membranes
2.6. Antimicrobial and antifouling property of the membranes The schematic depiction of surface functionalization of microporous nylon membranes with ADA and D-tyrosine is shown in Fig. 1. ADA molecules were grafted on the nylon membranes by the reaction between aldehyde moieties of ADA and amide or amine moieties present on the membrane. FTIR and XPS scan measurements were employed for the confirmation of surface functionalization. The FTIR spectrum of pristine nylon, ADA-nylon and D-tyrosine-ADA-nylon were shown in Fig. 2. The peak at 1537 cm−1 corresponds to amides while the peak at 1632 cm−1 indicates carboxyl and carbonyl groups. The peak for amine group was observed around 3300 cm−1 for all the nylon membranes. A new peak around 1731 cm−1 as well as an increase in the carboxyl peak was observed for ADA-nylon membrane, which ascertained successful ADA coating. As far as D-tyrosine is concerned, the disappearance of aldehyde peak in the FTIR scan of D-tyrosine-ADA-nylon membrane confirms the successful grafting of D-tyrosine via a simple Schiff’s base reaction. XPS analysis was conducted to investigate the elemental composition of pristine nylon, ADA-nylon and D-tyrosine-ADA-nylon membranes. As shown in Fig. 3, the C1s core-level spectrum for different membranes can be distinguished into CeC/CeH, CeN/CeOH and NeC]O/O]CeOH species. For the ADA-nylon membrane the peak area for CeOH increased due to the presence of hydroxyl moiety on the ADA layer. The increase in peak area of O]CeOH group for ADA-nylon membranes was due to the occurrence of carboxyl moiety on ADA layer. Owing to the incorporation of amine and carboxyl functionalities by Dtyrosine for the D-tyrosine-ADA-nylon membranes, the peak area for CeN and O]C-OH further enhanced as compared to ADA-nylon membranes. Moreover, the XPS relative atomic concentrations of the major elements in the membrane surface are described in Table S1. The results reveal that the membranes grafting with ADA and D-tyrosine had higher O contents as compared to the pristine nylon membrane. This may be due to the introduction of ADA and D-tyrosine, which have high numbers of hydroxyl and carboxylic functionalities. While an increase in N contents was also seen for the D-tyrosine-ADA-nylon membranes, due to the presence of amine group in D-tyrosine. It is a well-established fact that surface morphology and hydrophilicity of the membrane surfaces defines the antifouling property [21–23]. Morphologies of all the membranes were studied by SEM,
2.6.1. Antimicrobial property The antimicrobial properties of the ADA and D-tyrosine modified MF nylon membranes against E. coli were performed using the plate count method. To prepare the bacteria suspension, E. coli was cultured in the Luria-Bertani (LB) broth solutions over-night at 37 °C. A single colony was transferred in 30 mL of LB broth, and then incubated over-night (120 rpm, 37 °C). Then, 10 mL of E. coli suspension with an initial concentration of 106 CFU/mL was taken into a conical flask having a sterile membrane sample. The E. coli suspension having the membrane sample was incubated for 2 h at 37 °C. The E. Coli suspension was diluted to different concentrations (20–100 fold), and then 0.1 mL of each diluent was spread onto the corresponding LB agar plates. Viable bacterial colonies were counted after 18 h culture at 37 °C. Each experiment was conducted in triplicate and values averaged. The bacterial inhibition rate I was calculated by Eq. (4):
I=
N0 − Ni × 100% N0
(4)
where I is the bacterial inhibition rate, N0 is the CFU of the sample without membrane and Ni is the bacterial CFU of the sample contacting with membrane. 2.6.2. Antifouling property The antifouling properties of the membrane were explored with BSA as model fouling agent. The process was mainly conducted in four steps as reported by other researchers [20]. Pure water filtration (Jw1) was carried at 40 kPa, then BSA solution filtration was conducted for 60 min. The BSA fouled membrane was washed for 40 min with DI water at room temperature, and the water permeability (Jw2) values were measured again with the washed membrane. The flux recovery ratio (FRR) was evaluated using Eq. (5):
FRR (%) =
Jw2 × 100 Jw1
(5) −2
−1
−1
where Jw1 and Jw2 (L⋅m ⋅h ⋅bar ) are the pure water permeability and the recovery pure water permeability after washing, respectively. 3
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 2. FTIR spectra of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes.
considered important to the membrane performance and antifouling property. Hydrophilic surface tends to reduce the attachment of hydrophobic foulants [26]. Hydrophilicity of all the modified membrane surfaces was improved via the successful introduction of hydroxyl and carboxyl groups by ADA coating and other hydrophilic ligands, as shown in Fig. 6. The highest hydrophilicity for the D-tyrosine-ADAnylon would probably be due to the presence of hydroxyl and carboxyl groups introduced by D-tyrosine. Nonetheless, no noticeable changes in contact angle were observed with further increasing of D-tyrosine concentration (500 µm). Pore size (rm) and porosity (ɛ) are shown in Table 1, which shows that very small amount of pore narrowing occurred due to ADA coating and subsequent functionalization. In contrast a very high membrane porosity has been observed for the modified membranes compared to the pristine membrane (Table 1).
showing likely higher smoothness (Fig. 4b, c) due to ADA coating and subsequent functionalization by D-tyrosine. The phenomena of surface smoothness with ADA coating and subsequent functionalization by Dtyrosine were further witnessed by AFM analysis. As noted in Fig. 4, an assembly of sharp crests were observed for pristine nylon membrane, while relatively smooth crests can be seen for the ADA modified membrane. A dramatic smoothness in crests was noticed for D-tyrosine grafted membrane, which might be the reason of D-tyrosine deposition in crest surfaces. This crest surface smoothness was also observed from values in terms of root-mean-square roughness (Rq) and mean roughness (Ra) for the ADA-nylon and D-tyrosine-ADA-nylon membranes compared to the pristine nylon membrane (Table S2). To further clarify the variation of functional groups on the nylon membranes, surface zeta potential with different pH was measured for all the membranes. As noted in Fig. 5, the pristine nylon membrane is negatively charged from pH 5 to 10 due to the dissociation of carboxylic functional groups in polyamide [24,25]. Because of the abundant carboxylic functional groups on the ADA layer, the ADA-nylon membrane showed negative charge from pH 3 to 10. The D-tyrosine-ADA-nylon membrane showed the most negative charge which was probably attributed to the dual functionalities due to ADA and D-tyrosine. EDS mapping showed a high increase in O percentage after ADA coating and further D-tyrosine grafting (Table S3). On the other hand, the hydrophilicity of the membrane is
3.2. Pure water permeability of the functionalized membranes To investigate the functionalized membrane filtration performance, microfiltration experiments were conducted. The pure water permeability for the ADA-nylon membrane (115.3 L⋅m−2⋅h−1⋅bar−1) was found to be a little higher than the pristine nylon membrane (107.3 L⋅m−2⋅h−1⋅bar−1) (Fig. 7). This higher permeability was probably due to the introduction of hydroxyl and carboxyl groups by ADA coating. The highest permeability was obtained for D-tyrosine-ADA-nylon
Fig. 3. C1s XPS spectra of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes. 4
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Fig. 4. SEM surface images of (a) pristine nylon, (b) ADA-nylon, and (c) D-tyrosine-ADA-nylon membranes. AFM surface images of (d) pristine nylon, (e) ADA-nylon, and (f) D-tyrosine-ADA-nylon membranes.
80 60
Pristine nylon ADA-nylon D-tyrosine-ADA-nylon
Zeta Potential (mV)
40 20 0 -20 -40 -60 -80 -100 3
4
5
6
7
8
9
10
pH
Fig. 6. Contact angles of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes.
Fig. 5. Surface zeta potential of pristine nylon, ADA-nylon, and D-tyrosine-ADAnylon membranes. −2
−1
Table 1 The average pore size and porosity of membranes.
−1
membrane (144.8 L⋅m ⋅h ⋅bar ), about 74.2% higher than the pristine membrane. The higher porosity (Table 1) and hydrophilicity (Fig. 6) due to the grafting were believed to weaken the resistance of water to pass through the membranes [27], thereby resulting in higher flux of the modified membrane than that of the pristine membrane. Furthermore, the very small amount of pore narrowing had no negative impact on the permeate permeability. However, when further increasing the grafting amount of D-tyrosine (500 µM), the pure water permeability started to decline. This might be due to the high concentrations of D-tyrosine blocking the membrane pores. Due to the nonspecific protein adsorption there has been no noticeable changes in BSA 5
Membrane
Average pore size rm (nm)
Porosity ε (%)
Pristine nylon ADA-nylon 50 µM D-tyrosine-ADA-nylon 100 µM D-tyrosine-ADA-nylon 500 µM D-tyrosine-ADA-nylon
448 442 440 440 438
58 69 72 74 73
± ± ± ± ±
1 2 2 3 3
± ± ± ± ±
1 2 3 3 3
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
evaluated with BSA filtration. The permeability of pristine nylon membrane decreased about 55% from initial permeability in 60 min filtration experiment as shown in Fig. 8a; while about 36% decrease was observed for ADA-nylon membranes. The least amount of decrease (30%) was seen for D-tyrosine-ADA-nylon membranes in 60 min BSA filtration experiment. Commonly, adsorption and deposition of BSA protein on the membrane decrease membrane permeability temporarily or permanently, termed as reversible or irreversible fouling. Consequently, water cleaning is used to remove reversible fouling. FRR has been widely regarded as the membrane antifouling ability as it reveals the water cleaning efficiency. The FRR values for all the membranes after four cycles were shown in Fig. 8b. All the ADA and D-tyrosine functionalized membranes showed higher FRR values than that of the pristine nylon membrane, revealing their excellent reusability and antifouling characteristics. The highest FRR of about 91% was seen for the 100 µM Dtyrosine grafted membrane; while the lowest FRR was observed for the pristine nylon membrane (42%). This outstanding antifouling property could be contributed by the increased hydrophilicity and enhanced smoothness. As matter of fact, the better hydrophilicity and smoother surfaces could result in a weaker interaction between membrane surface and hydrophobic BSA, preventing the adsorption or desorption of foulants on membrane surface [25,26]. Moreover, a smoother surface would contribute to lower total surface area thereby decreasing the adhesion efficiency of the foulants [28–31]. Furthermore, due to hydrophilicity of membrane surfaces, a tightly bound water layer is
Fig. 7. The pure water permeability and BSA rejection of pristine nylon, ADAnylon, and D-tyrosine-ADA-nylon membranes.
rejection for the ADA coating and subsequent functionalization compared to pristine membrane as shown in Fig. 7. 3.3. BSA-fouling and cleaning properties of D-tyrosine functionalized membranes In this study the membranes anti-adhesion properties were
Fig. 8. (a) The normalized flux of pristine and modified nylon membranes during 60 min filtration of BSA. (b) FRR of pristine nylon, ADA-nylon, and D-tyrosine-ADAnylon membranes. (c) Calculated bacterial inhibitions rate of pristine and modified nylon membranes. (d) The pure water permeability before and after bacterial growth of pristine nylon, ADA-nylon, and D-tyrosine-ADA-nylon membranes. 6
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
Acknowledgement
formed, and foulants only interact with this shielding layer via shortranged forces, enabling the membrane to have antifouling properties.
This work was supported by the National Natural Science Foundation of China [grant number 51678336]. 3.4. Antibiofouling performance of D-tyrosine functionalized membranes Appendix A. Supplementary material The typical trend of biofilm cycle comprises of initial attachment of single cells, colonization of cells and then formation of a mature biofilm on the membrane surfaces [32]. In this study, E. coli was used as model bacteria to investigate the bacterial inhibition performance of the membrane surface. Fig. 8c displays the bacterial inhibition results of modified membranes calculated by Eq. (4), using the Fig. S1 images. The pristine nylon membrane did not exhibit any bacterial inhibition. The D-tyrosine functionalized membranes showed excellent bacterial inhibitions activity: the bacterial inhibition rates of 50 µM, 100 µM, and 500 µM membranes were 64.3, 73.6, and 83.7% respectively. Meanwhile, the ADA-nylon membrane showed about 40.7% bacterial inhibition. The D-tyrosine functionalization endowed the membrane with strong hydrophilicity. In contrast, most bacteria are hydrophobic in nature, which are usually prone to attach hydrophobic surfaces. Hence, it is important for membrane surfaces to be more hydrophilic in nature to avoid or mitigate bacterial contact for bacterial inhibition property. In the antibiofouling experiments, all the membranes were incubated with E. coli suspension having nutrients at 37 °C for 48 h to accelerate the biofouling process. During the culture growth, bacterial cells irreversibly attached to the membrane surface, resulting in the formation of matured biofilm on the membrane surface [33]. The pure water permeabilities of the membranes before and after this incubation were evaluated. The pure water permeability of all the incubated membranes decreased, due to biofilm formation. According to the filtration test results shown in Fig. 8d, the permeability of the pristine nylon membrane dropped by 52.5% after incubation with bacterial suspension, which is mostly caused by the biofilm developed. The permeability of the ADA-nylon membrane dropped by 43.6% indicating improved antibiofouling performance. The least amount of permeability decline was observed for the D-tyrosine-ADA-nylon membrane, which was in the range of 28–35%, showing excellent antibiofouling property. It is worth noting that the D-tyrosine-ADA-nylon membrane demonstrated lowest permeability reduction rates and outstanding antibiofouling performance. It could be easily inferred that the ADA and Dtyrosine functionalization endowed the membrane with strong antibiofouling characteristics. Besides, due to the strong hydrophilicity and smoothness, D-tyrosine could aggravate biofilm dispersal from the membrane surface [34].
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115891. References [1] K. Xiao, S. Liang, X. Wang, C. Chen, X. Huang, Current state and challenges of fullscale membrane bioreactor applications: a critical review, Bioresour. Technol. 271 (2019) 473–481. [2] K. Xiao, Y. Xu, S. Liang, T. Lei, J. Sun, X. Wen, H. Zhang, C. Chen, X. Huang, Engineering application of membrane bioreactor for wastewater treatment in China: current state and future prospect, Front. Environ. Sci. Eng. 8 (2014) 805–819. [3] H.-C. Yang, J. Luo, Y. Lv, P. Shen, Z.-K. Xu, Surface engineering of polymer membranes via mussel-inspired chemistry, J. Membr. Sci. 483 (2015) 42–59. [4] J. Mansouri, S. Harrisson, V. Chen, Strategies for controlling biofouling in membrane filtration systems: challenges and opportunities, J. Mater. Chem. 20 (2010) 4567–4586. [5] L. Duan, W. Huang, Y. Zhang, High-flux, antibacterial ultrafiltration membranes by facile blending with N-halamine grafted halloysite nanotubes, RSC Adv. 5 (2015) 6666–6674. [6] B.D. McCloskey, H.B. Park, H. Ju, B.W. Rowe, D.J. Miller, B.D. Freeman, A bioinspired fouling-resistant surface modification for water purification membranes, J. Membr. Sci. 413–414 (2012) 82–90. [7] M.S. Rahaman, H. Thérien-Aubin, M. Ben-Sasson, C.K. Ober, M. Nielsen, M. Elimelech, Control of biofouling on reverse osmosis polyamide membranes modified with biocidal nanoparticles and antifouling polymer brushes, J. Mater. Chem. B 2 (2014) 1724–1732. [8] S.F. Xing, X.F. Sun, A.A. Taylor, S.L. Walker, Y.F. Wang, S.G. Wang, D-Amino acids inhibit initial bacterial Adhesion: thermodynamic evidence, Biotechnol. Bioeng. 112 (2015) 696–704. [9] I.-S. Chang, P. Le Clech, B. Jefferson, S. Judd, Membrane fouling in membrane bioreactors for wastewater treatment, J. Environ. Eng. 128 (2002) 1018–1029. [10] P. Le-Clech, V. Chen, T.A. Fane, Fouling in membrane bioreactors used in wastewater treatment, J. Membr. Sci. 284 (2006) 17–53. [11] I. Kolodkin-Gal, D. Romero, S. Cao, J. Clardy, R. Kolter, R. Losick, D-amino acids trigger biofilm disassembly, Science 328 (2010) 627–629. [12] A.I. Hochbaum, I. Kolodkin-Gal, L. Foulston, R. Kolter, J. Aizenberg, R. Losick, Inhibitory effects of D-amino acids on Staphylococcus aureus biofilm development, J. Bacteriol. (2011) JB. 05534-05511. [13] H. Xu, Y. Liu, Reduced microbial attachment by D-amino acid-inhibited AI-2 and EPS production, Water Res. 45 (2011) 5796–5804. [14] B.-B. Jiang, X.-F. Sun, L. Wang, S.-Y. Wang, R.-D. Liu, S.-G. Wang, Polyethersulfone membranes modified with D-tyrosine for biofouling mitigation: Synergistic effect of surface hydrophility and anti-microbial properties, Chem. Eng. J. 311 (2017) 135–142. [15] C. Hou, Z. Qi, H. Zhu, Preparation of core–shell magnetic polydopamine/alginate biocomposite for Candida rugosa lipase immobilization, Colloids Surf., B 128 (2015) 544–551. [16] F. Chen, M. Tian, D. Zhang, J. Wang, Q. Wang, X. Yu, X. Zhang, C. Wan, Preparation and characterization of oxidized alginate covalently cross-linked galactosylated chitosan scaffold for liver tissue engineering, Mater. Sci. Eng., C 32 (2012) 310–320. [17] M.K. Khan, J. Luo, R. Khan, J. Fan, Y. Wan, Facile and green fabrication of cation exchange membrane adsorber with unprecedented adsorption capacity for protein purification, J. Chromatogr. A 1521 (2017) 19–26. [18] J.F. Li, Z.L. Xu, H. Yang, Microporous polyethersulfone membranes prepared under the combined precipitation conditions with non-solvent additives, Polym. Adv. Technol. 19 (2008) 251–257. [19] S. Liang, Y. Kang, A. Tiraferri, E.P. Giannelis, X. Huang, M. Elimelech, Highly hydrophilic polyvinylidene fluoride (PVDF) ultrafiltration membranes via postfabrication grafting of surface-tailored silica nanoparticles, ACS App. Mater. Interfaces 5 (2013) 6694–6703. [20] J.-J. Kim, K. Kim, Y.-S. Choi, H. Kang, D.M. Kim, J.-C. Lee, Polysulfone based ultrafiltration membranes with dopamine and nisin moieties showing antifouling and antimicrobial properties, Sep. Purif. Technol. 202 (2018) 9–20. [21] J.A. Brant, A.E. Childress, Colloidal adhesion to hydrophilic membrane surfaces, J. Membr. Sci. 241 (2004) 235–248. [22] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712–717. [23] A. Razmjou, J. Mansouri, V. Chen, The effects of mechanical and chemical modification of TiO2 nanoparticles on the surface chemistry, structure and fouling performance of PES ultrafiltration membranes, J. Membr. Sci. 378 (2011) 73–84. [24] Y. Gao, S. Zhao, Z. Qiao, Y. Zhou, B. Song, Z. Wang, J. Wang, Reverse osmosis membranes with guanidine and amine enriched surface for biofouling and organic fouling control, Desalination 430 (2018) 74–85.
4. Conclusions In this study an MF nylon membrane was modified by D-tyrosine grafting using ADA as a green facile versatile platform, resulting in a novel D-tyrosine-ADA-nylon membrane. The results of ATR-FTIR and XPS confirmed that D-tyrosine had been successfully grafted on the MF polyamide commercial membrane surfaces. The SEM and AFM analysis showed that modified membranes had smoother surfaces compared to the pristine membrane. In addition, the membrane hydrophilicity was significantly enhanced. Excellent pure water permeability was witnessed for the modified membranes compared to the pristine membrane. Much higher membrane cleaning efficiency was achieved compared to the pristine membrane, with the best water permeability recovery reaching 91%. The anti-protein (BSA) adhesion performance of the modified membranes was also confirmed. Moreover, D-tyrosine not only inhibited microbial attachment but could also promote dispersal of the mature biofilm. Hence, this novel membrane functionalization may provide an easy route to alleviate membrane biofouling in various industrial applications.
7
Separation and Purification Technology 231 (2020) 115891
R. Khan, et al.
[30] L. Zhang, J. Xu, Y. Tang, J. Hou, L. Yu, C. Gao, A novel long-lasting antifouling membrane modified with bifunctional capsaicin-mimic moieties via in situ polymerization for efficient water purification, J. Mater. Chem. A 4 (2016) 10352–10362. [31] A.R. Cooper, Ultrafiltration Membranes and Applications, Springer Science & Business Media, 2013. [32] A. Zaky, I. Escobar, A.M. Motlagh, C. Gruden, Determining the influence of active cells and conditioning layer on early stage biofilm formation using cellulose acetate ultrafiltration membranes, Desalination 286 (2012) 296–303. [33] C. Mattheis, H. Wang, C. Meister, S. Agarwal, Effect of Guanidinylation on the Properties of Poly (2-aminoethylmethacrylate)-Based Antibacterial Materials, Macromol. Biosci 13 (2013) 242–255. [34] H. Xu, Y. Liu, D-Amino acid mitigated membrane biofouling and promoted biofilm detachment, J. Membr. Sci. 376 (2011) 266–274.
[25] R. Zhang, Y. Li, Y. Su, X. Zhao, Y. Liu, X. Fan, T. Ma, Z. Jiang, Engineering amphiphilic nanofiltration membrane surfaces with a multi-defense mechanism for improved antifouling performances, J. Mater. Chem. A 4 (2016) 7892–7902. [26] J.R. Du, S. Peldszus, P.M. Huck, X. Feng, Modification of membrane surfaces via microswelling for fouling control in drinking water treatment, J. Membr. Sci. 475 (2015) 488–495. [27] J. Wang, Z. Wang, Y. Liu, J. Wang, S. Wang, Surface modification of NF membrane with zwitterionic polymer to improve anti-biofouling property, J. Membr. Sci. 514 (2016) 407–417. [28] D.E. Packham, Surface energy, surface topography and adhesion, Int. J. Adhes. Adhes. 23 (2003) 437–448. [29] V.M. Kochkodan, N. Hilal, V.V. Goncharuk, L. Al-Khatib, T.I. Levadna, Effect of the surface modification of polymer membranes on their microbiological fouling, Colloid J. 68 (2006) 267–273.
8