Carbon Quantum Dots Grafted Antifouling Membranes for Osmotic

Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX-XXX

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Carbon Quantum Dots Grafted Antifouling Membranes for Osmotic Power Generation via Pressure-Retarded Osmosis Process Die Ling Zhao, Subhabrata Das, and Tai-Shung Chung* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 ABSTRACT: Osmotic power generated by pressure-retarded osmosis (PRO) has attracted global attention as a clean, abundant and renewable energy resource. However, the substrates of PRO membranes are particularly prone to fouling because of their direct contact with various foulants in raw water. This leads to a significant decline in power density and impedes the commercialization of PRO technology. In this work, a facile surface modification method has been developed to obtain a new type of nanoparticle functionalized antifouling PRO membranes. Carbon quantum dots (CQDs), with an average size around 3.2 nm, are fabricated from citric acid via a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on the substrate surface of poly(ether sulfone) (PES) membranes via covalent bonding. The bacteria diffusion tests show that the CQD modified PRO membranes possess much enhanced antibacterial activity and antibiofouling propensity. The continuous PRO operations at 15 bar also confirm that the CQD modified membranes exhibit a much higher power density (11.0 vs 8.8 W/m2) and water recovery after backwash (94 vs 89%) than the unmodified ones. This study may open up a new avenue in the fabrication of nanostructure functionalized polymeric membranes for wastewater treatment and osmotic power generation.

1. INTRODUCTION The growing awareness of energy scarcity has given rise to greater efforts for the search of alternative energy sources. In the past decade, osmotic energy released from the mixing of water streams with different salinities via pressure-retarded osmosis (PRO) has attracted global attention as an unexploited energy resource.1−4 It is estimated that the osmotic power harvested from the feed pair of seawater and river water can reach as high as 2000 TWh annually.5 It could be even higher when the concentrated reverse osmosis (RO) brine is employed as the draw solution because of its higher salinity.2,6−10 Moreover, this new draw solution can add extra benefits such as reducing the overall energy consumption for RO,11 diluting the seawater RO (SWRO) brine for environmentally friendly disposal,12 and saving cost on the seawater pretreatment.2 Rapid development of PRO membranes has advanced PRO technologies for osmotic power generation in both lab and industrial scale tests.13−20 A power density up to 27 W/m2 was achieved when using a 1 M NaCl solution and deionized water as the feed pair in lab scale tests,8 while a power density of 13.3 W/m2 was obtained from a Japanese prototype PRO plant using SWRO brine and freshwater as the feed pair.21,22 However, both lab and industrial results showed that the fouling on PRO membranes due to the depositions of inorganics, organics, and microorganisms causes a rapid decline of power density.23−26 For an example, replacing the feed solution from deionized water to municipal wastewater resulted in a significant drop in power density from 27 W/m2 to 4.6 W/ m2.8 Although the pretreatment of feed solutions offered some improvements, fouling on membranes was inevitable in long© XXXX American Chemical Society

term operations, especially when using natural water streams as the feed solutions.24,27−29 The fouling not only adversely affected the membrane performance remarkably, but also increased the operation cost on frequent cleaning and maintenance. Therefore, a new strategy to design PRO membranes with effective and long-lasting antifouling properties is essential for the PRO process to be economical and sustainable. Many attempts have been made to tailor membrane surface with desirable antifouling properties for conventional membrane processes, such as microfiltration, ultrafiltration, and reverse osmosis.30−32 However, due to the difference in fouling between PRO and conventional membranes, only limited antifouling approaches have been proposed to enhance the antifouling properties of PRO membranes. In conventional membranes, fouling usually takes place on the top surface of the selective layer, while in the PRO process, fouling not only occurs severely on and within the substrate, but also beneath the selective layer of PRO membranes.8,33 Recently, Cai et al., Zhao et al., and Le et al. designed zwitterions modified PRO membranes with superior fouling resistance to nonspecific foulants in municipal wastewater.34−36 Li et al. developed a dendritic hyperbranched polyglycerol anchored PRO membrane, which can reject proteins and bacteria, but not inorganic scaling.37 Therefore, in the later study, they further functionalized the hyperbranched polyglycerol polymer with negatively Received: August 15, 2017 Revised: November 7, 2017 Accepted: November 14, 2017

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DOI: 10.1021/acs.est.7b04190 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology charged groups.38 The resultant membranes provided a higher rejection to organic fouling and inorganic scaling from the feed solution. However, the synthetic procedures in previous works were complicated and the modifications included several procedures, which might hinder the scale-up of the antifouling PRO membranes. Carbon quantum dots (CQDs) comprising different surface functional groups with sizes less than 10 nm have attracted extensive attention.39,40 They not only inherit the merit of sizedependent optical property, but also have the advantages of high chemical inertness, tunable hydrophilicity, and facile fabrication approaches such as hydrothermal treatment, oxidation of graphite, and candle burning.41,42 To date, CQDs have found broad applications, such as drug delivery, sensors, energy storage and others.43 Recently, graphene oxide quantum dots (GOQDs) modified membranes were found to exhibit supreme antibacterial activity.44−46 Considering potential antifouling properties of CQDs similar as GOQDs, we aim to design and fabricate CQDs modified antifouling PRO membranes. In this work, the CQD material was first produced from a cheap material in a relatively large scale according to the Guo’s method.47 The carboxyl groups on the CQD surface can not only partially react with the amine groups (from PDA) on the substrate surface but also be negatively charged in solution for enhanced antifouling properties.46,48,49 Dopamine, comprising catechol and amine groups, readily oxidizes in alkaline environments and undergoes self-polymerization to form an adherent polydopamine (PDA) gutter layer onto a wide range of supports with high uniformity and stability.50−52 This facilitates the immobilization of CQDs in other membranes with different configurations and materials for various kinds of applications. In this work, a state-of-the-art thin-film composite poly(ether sulfone) (TFC-PES) hollow fiber membrane was employed for the surface modification. As illustrated in Figure 1, the CQDs

(3) Subsequently, CQDs anchored onto the substrate via covalent bonding between carboxyl groups on CQD surface and amine groups on the PDA layer. To the best of our knowledge, TFC-PES membranes grafted by CQDs have not been investigated for osmotic power generation via PRO processes.

2. MATERIALS AND METHODS 2.1. Materials and Methods. Citric acid (>99.5%), dopamine hydrochloride (99.0%), tris(hydroxymethyl) aminomethane (tris, ≥ 99.8%), alginic acid sodium salt, N-(3(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC, >99.0%) and N-hydroxysuccinimide (NHS, >99.0%) were all purchased from Sigma-Aldrich and used as-received without further purification. Gram-negative Escherichia coli (E. coli, ATCC DH5α) and Gram-positive Staphylococcus aureus (S. aureus, ATCC 25923) were bought from American Type Culture Collection, Manassas, VA, U.S.A.. Deionized water used in this work was produced by a Milli-Q unit (MilliPore) with a resistivity of 18 MΩ cm. 2.2. Synthesis of Carbon Quantum Dots (CQDs). The synthesis of CQDs was simple and straightforward as shown in Figure 1.47 In a typical reaction, citric acid solid powder was put into a glass beaker covered with a glass slide and was heated in air at 180 °C for 3 h. After the reaction, yellow powders containing CQDs passivated with carboxyl groups were produced. The CQDs were then dispersed in water and the resultant solution was dialyzed using Slide-A-Lyzer G2 dialysis cassettes (2K MWCO). 2.3. Preparation of TFC-PES-PDA-CQD Hollow Fiber Membranes. The preparation of TFC-PES-PDA-CQD hollow fiber membranes is schematically illustrated in Figure 1. The hollow fiber substrate was first prepared from poly(ether sulfone) (PES) via a previously described method.8,53 A thin polyamide selective layer was then synthesized on the inner surface of hollow fibers by interfacial polymerization.8,37,53 Subsequently, the outer surface was immersed in a tris buffer solution (10 mmol/L, pH 8.5) containing 2 mg/mL dopamineHCl for 1 h to obtain amine groups. Finally, the substrate was applied to a CQD aqueous solution to achieve TFC-PES-PDACQD via the covalent linkage between carboxyl groups on the CQD surface and amine groups on the PES-PDA, with the aid of EDC/NHS (10 and 30 mg, respectively). The CQD aqueous solutions with different concentrations were circulated in the shell side of hollow fibers for 24 h. Subsequently, the resulting hollow fibers were rinsed with deionized water to remove loosely attached CQDs and then immersed in deionized water for further characterizations. In the following characterizations (e.g., FESEM, XPS, and disk diffusion tests), the membrane samples were coated with a 25 mg/mL CQDs aqueous solution. 2.4. Characterizations of Hollow Fiber Membranes. Transmission electron microscopy (TEM) images of CQDs were scanned on a JEM-2100F electron microscope operating at an accelerating voltage of 200 kV. The corresponding histograms of particle size distribution were plotted by counting 200 nanoparticles. The substrate surface morphology of hollow fiber membranes was investigated by a field emission scanning electronic microscope (FESEM, JEOL JSM-6700F). Before FESEM observations, the membrane samples were freeze-dried, followed by platinum sputtering. X-ray photoelectron spectroscopy (XPS) characterizations were performed using a Kratos AXIS Ultra DLD spectrometer with a monochromatized Al

Figure 1. Synthesis route to produce CQDs from citric acid and the schematic procedures to modify the substrate surface by CQDs.

were immobilized on the substrate surface of TFC-PES membranes via three steps as follows. (1) CQDs passivated with carboxyl groups were produced via a one-step approach with an average size around 3.2 nm. (2) The substrate surface of TFC-PES membranes was treated with polydopamine (PDA) via Machael addition to create a PDA modified substrate surface (referred to as TFC-PES-PDA thereafter). B


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Figure 2. TEM images of CQDs and the size distribution based on 200 nanoparticles.

respective lumen and shell channels. The flow rates of both solutions were set as 0.2 L/min, and the temperature was kept at 25 °C. The membranes were first stabilized at 15.0 ± 0.1 bar for 10 min before any data were recorded. Water flux using deionized water as the feed solution was first measured under the same conditions, and this value was recorded as the initial water flux. After recording the weight decline of the feed solution for every 3 h, backwash was carried out by feeding deionized water into the lumen side at 15.0 ± 0.1 bar for 10 min while rinsing the shell side with deionized water without applying any hydraulic pressure. Water flux (Jv, L m−2 h−1, abbreviated as LMH) was determined by the weight decline of the feed solution during a certain duration using eq 1:

cathode as the X-ray source (1486.71 eV photons). Both wide scan and core-level spectra were recorded. The core-level signals were obtained at the photoelectron takeoff angle (α, with respect to the sample surface) of 90°. Binding energy peaks were all calibrated with the reference of neutral C 1s hydrocarbon peak at 284.6 eV. Fluorescence of membrane substrate surface under UV light illumination was investigated using a Leica DMLM fluorescence microscope (Leica Microsystems, Wetzlar, Germany). The dynamic water contact angle of the fiber was measured on a KSV Sigma 701 tensiometer (±0.01°, KSV instruments Ltd., Finland) via a force tensiometry method at about 25 °C. 2.5. Antibacterial Tests. The antibacterial properties of the membranes were investigated by the disk diffusion method.54 Briefly, E. coli and S. aureus strains were grown in Luria−Bertani (LB) solutions and incubated in a shaker incubator overnight at 37 °C with an agitation speed of 150 rpm. The bacterial culture was then centrifuged for 15 min at 5000 rpm, and the bacterial pellets were resuspended in LB solutions respectively for the two bacteria. The prepared cell suspension was used as the stock solution, which was diluted to 105 colony-forming units (CFU) per mL before experimental uses. The antibacterial properties of the membranes were measured as follows: A 100 μL aliquot from each E. coli and S. aureus working solutions was spread onto a nutrient agar plate separately. Three short hollow fiber membranes (i.e., pristine TFC-PES, TFC-PES-PDA, and TFC-PES-PDA-CQD) were then placed on the nutrient agar plates. After incubation at 37 °C for 24 h, the growth of cells around the membrane samples was inspected visually. 2.6. Antiorganic-Fouling Tests in PRO Processes. Membrane fouling by organics was examined in PRO tests on a laboratory-scale PRO setup using a synthetic seawater brine (0.81 mol/L NaCl) as the draw solution. A high alginate concentration (1 g/L) was employed in the feed solution to increase the rate of organic fouling. The selective layer of the membrane was facing the seawater brine in all tests. The draw and feed solutions were circulated counter-currently in their

Jv = Δm /(A × Δt × 1000)

(1)

where Δm (g) is the mass of water permeation across the effective membrane area A (m2) over a time period of Δt (h), assuming the density of water is 1000 g L−1. The power density (W, W/m2) was calculated by eq 2:

W = Jw × ΔP

(2)

where ΔP is the hydraulic pressure difference across the membrane and Jw is the water flux.

3. RESULTS AND DISCUSSION 3.1. Characterizations of Carbon Quantum Dots and Membranes. Figure 2 shows the as-prepared CQDs with an average diameter of ∼3.2 nm and a narrow size distribution. This is due to the fact that citric acid contains abundant −OH groups which would cause polymerization to form carbonaceous spheres at 180 °C. The yield of CQDs after the reaction is about 40%. Due to the existence of −COOH in citric acid molecules, CQDs are passivated with carboxyl groups on the surface which would partially react with the −NH2 groups of polydopamine using EDC/NHS as the coupling agents. After the modification, the outer surface morphology of PES-PDA and PES-PDAC


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Figure 3. A comparison of outer surface morphology among PES, PES-PDA (2 mg/mL PDA, 1 h), and PES-PDA-CQD (25 mg/mL CQD solution, 24 h) hollow fiber substrates.

Figure 4. XPS wide scan and C 1s core-level spectra of CQDs, and the substrate surfaces of PES, PES-PDA (2 mg/mL PDA, 1 h), and PES-PDACQD (25 mg/mL CQD solution, 24 h) hollow fibers.

surfaces of PES, PES-PDA, and PES-PDA-CQD. According to the C 1s core-level spectrum of CQDs, the high intensity of O− CO peak indicates that there are abundant carboxyl groups on the surface of CQDs. The pristine PES substrate possesses C 1s, O 1s, and small concentraions of S 2s and S 2p from sulfur. The high energy resolution spectrum of C 1s can be curve-fitted into three groups including C−H/C−C, C−O/C− SO2, and π−π* groups. After the PDA coating, the wide scan of PES-PDA reveals a strong signal of nitrogen, suggesting the successful coating of PDA. The C 1s core-level spectrum of PDA can slit into five peaks with binding energies at 284.6, 285.7, 286.2, 287.5, and 288.3 eV, corresponding to respective C−N, C−H, C−O, CO, and O−CO groups which are all

CQD substrates was compared with the pristine PES substrate by FESEM. As shown in Figure 3, the original PES substrate displays a porous outer structure. After the polydopamine deposition, the surface becomes less porous. More obvious changes are observed after the CQD modification. Contrary to the porous outer surface of PES hollow fibers, the surface of PES-PDA-CQD is uniformly covered by CQDs. Moreover, some CQDs fill in the surface pores because they have sizes much smaller than the surface pores. The existence of carboxyl groups on the CQD surface and successful covalent immobilization of CQDs on the substrate can be further evidenced by XPS. Figure 4 shows both the wide scan and the C 1s core-level spectra of CQDs, and the substrate D


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Figure 5. Fluorescence microscopy of substrate surfaces of PES-PDA and PES-PDA-CQD membranes under the excitation of UV light as a function of CQD concentration (0.1, 1, 5, 10, and 25 mg/mL) in the coating solutions.

hollow fiber membranes. Clearly, the presence of a CQD coating layer can effectively inactivate E. coli and S. aureus, and prevent the biofilm formation on the membrane surface. Thus, compared to PES and PES-PDA hollow fiber membranes, the CQD modified membranes have superior antimicrobial activity and antibiofouling capability. The superior antibacterial property of CQD coated PRO membranes may arise from three aspects. First, bacteria usually have an overall negative charge on their cell surface due to the presence of peptidoglycan in their cell wall, which is rich in carboxyl and amino groups. Since the CQD surface also has the negatively charged unreacted carboxyl groups, there is an electrostatic repulsion between the substrate surface and bacteria surface.38 Although the bacterial can still get close to the substrate surface, the highly dispersed CQDs with an ultrasmall size and large specific surface area may directly insert or cut the bacteria cells.44 Even for bacteria cells in motion, the immobilization of nanomaterials on the substrate surface could develop a disinfection system with high reliability.55,56 Oxidative stress is another cause that may lead to the death of bacterial cells. Since CQDs consist of abundant oxygencontaining functional groups on their surface, the homogeneous dispersion of CQDs could result in a high exposure of active edges to bacteria, providing a higher oxidative stress.44,57 Thus, vital cellular components could be oxidized or disrupted by CQDs. 3.3. Antiorganic Fouling Performance in PRO Processes. The PRO tests were performed at 15.0 ± 0.1 bar for both TFC-PES and TFC-PES-PDA-CQD hollow fiber membranes. The feed solution contains a high alginate concentration of 1 g/L because alginate is a typical organic foulant. Table 1 summarizes the results including the initial power density and the power density during three consecutive runs with backwash. When using deionized water as the feed, the initial water flux of TFC-PES-PDA-CQD drops slightly after the PDA and CQD modifications because its surface pores are covered by the CQD nanoparticles, as illustrated in Figure 3. However, the drop is not significant, possibly due to two reasons. First, according to the coating mechanism, CQDs would only form a single layer on the surface of PDA. As the size of the CQDs is only about 3 nm, it will not occupy a lot of pore space. However, in the previous step, namely the PDA coating, dopamine would deposit on the substrate surface with different thicknesses according to the coating conditions (such as concentration and time). So in this step, we minimized the effects of the PDA coating on water flux by optimizing the coating conditions (2 mg/mL, 1 h). Second, after the

components from PDA. In the case of the CQD modified substrate, the intensity of N 1s peak decreases due to the coverage of CQDs on the substrate surface. While in the C 1s core-level spectrum, the peak intensity corresponding to O− CO functional groups is significantly enhanced due to the carboxyl groups on the CQD surface. Figure 5 shows the images of outer surfaces under a fluorescence microscope with UV light illumination on membrane surface. The PES-PDA has mere fluorescence. However, the fluorescence appears after the CQD coating due to the unique fluorescence property of CQDs. The fluorescence intensity increases with an increase in CQD concentration in the coating solutions because of an increasing CQD loading on the substrate surfaces. 3.2. Antibacterial Tests. Antibiofouling ability is an important criterion for an antifouling membrane. Figure 6 shows the results of the disk diffusion tests where bacteria are in white color while the background is in black color). E. coli and S. aureus can grow around the PES and PES-PDA hollow fiber membranes, which means the two membranes do not have much resistance against the two model bacteria. In contrast, an obvious inhibition zone was developed around PES-PDA-CQD

Figure 6. Disk diffusion tests of PES, PES-PDA (1 mg/mL PDA, 2 h), and PES-PDA-CQD (25 mg/mL CQD solution, 24 h) hollow fibers membranes with (a) E. coli and (b) S. aureus. E


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Table 1. Summary of the Initial Water Flux (Using Deionized Water as the Feed), Salt Flux and Power Density of the TFC-PES and TFC-PES-PDA-CQD Hollow Fiber Membranes (Draw Solution: 0.81 M NaCl, Pressure: 15 bar) power density (W/m2) membrane

initial water flux (LMH)

salt flux (gMH)

initial

after 1 run

after 2nd run

after 3rd run

average

TFC-PES TFC-PES-PDA-CQD

33.4 32.9

29.0 32.7

13.9 13.7

8.5 11.3

9.3 11.3

8.5 10.5

8.8 11.0

st

Figure 7. PRO performance of TFC-PES and TFC-PES-PDA-CQD (25 mg/mL CQD solution, 24 h) membranes using an alginate solution (1 g/L) as the feed and 0.81 M NaCl solution as the draw solution at 15 bar.

unmodified TFC-PES membranes. Therefore, the CQD modified PRO membranes not only possesses higher antifouling propensity for osmotic power generation, but also have a higher recovery after backwash.

modification, the surface hydrophilicity of the substrate increased remarkably. For the uncoated PES substrate, the water contact angle is 85.5 ± 3.1°, while it is reduced to 45.5 ± 3.2° for PES-PDA-CQD. The enhanced surface hydrophilicity would help to improve the water flux. For a fair comparison of the antifouling performance, the initial water flux and power density of these two membranes were chosen at both about 33 LMH and 14 W/m2, respectively. Figure 7 shows the evolution of normalized water flux as a function of time. The normalized water flux is calculated as the ratio of water flux to the initial water flux using deionized water as the feed solution. For the pristine TFC-PES membranes, the water flux gradually decreases to about 60% of the initial water flux in the first 3 h as alginate molecules accumulate on the membrane surface along the time. The high propensity of the PES substrate to organic fouling arises from the hydrophobic− hydrophobic interaction between the conjugated benzene rings of PES and alginate. In contrast, the water flux of the TFC-PESPDA-CQD membranes declines much slowly and ends with 82% of the initial water flux after 3 h. The high resistance toward organic fouling can be attributed to the strong repulsive forces from CQDs because their carboxyl groups are negatively charged in a wide pH range from 3.8 to 10.2.44 After each 3 h’s operation and backwash, the water flux of TFC-PES recovers to about 89% of the initial value, while the recovery rate of the CQD modified membranes is higher than 94%. As CQDs are grafted on the substrate surface and its pores, the foulants are prevented from entering the substrate. This facilitates the membrane cleaning during the backwash. As shown Table 1, the average power density generated by TFC-PES-PDA-CQD in the 3 runs is 11.0 W/m2, while it is 8.8 W/m2 for the

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 65 6516 6645; fax: 65 6779 1936; e-mail: chencts@nus. edu.sg (T.S.C.). ORCID

Tai-Shung Chung: 0000-0002-6156-0170 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This research is supported by the Singapore National Research Foundation under its Environment and Water Research Programme and administered by Public Utility Board (PUB), under the following projects “Membrane Development for Osmotic Power Generation, Part 1. Materials Development and Membrane Fabrication” (1102-IRIS-11-01) and NUS grant no. R-279-000-381-279 and “Membrane Development for Osmotic Power Generation, Part 2. Module Fabrication and System Integration” (1102-IRIS-11-01) and NUS grant no. R-279-000382-279. Special thanks are due to Dr. Chun Xian Guo, Dr. Gang Han, Mr. Chunfeng Wan, and Mr. Tianshi Yang for their kind help and suggestions. F


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