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CDs@ZIF-8 modified thin film polyamide nanocomposite membrane for simultaneous enhancement of chlorineresistance and disinfection byproducts removal in drinking water Fei-Hong Wang, Tong Zheng, Ruohan Xiong, Panpan Wang, and Jun Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11006 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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ACS Applied Materials & Interfaces

CDs@ZIF-8 modified thin film polyamide nanocomposite membrane for simultaneous enhancement of chlorineresistance and disinfection byproducts removal in drinking water Feihong Wang, Tong Zheng, Ruohan Xiong, Panpan Wang*, Jun Ma* State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China * Corresponding author at: State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 73 Huanghe Road, Nangang District, Harbin 150090, China E-mail address: [email protected] (P.W.), [email protected] (J. Ma)

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Abstract: Reverse osmosis (RO) is an emerging membrane technology for disinfection byproducts (DBPs) removal. However, the chlorine-resistance and DBPs removal performance of thin film composite (TFC) polyamide membranes should be simultaneously improved when used in chlorinated drinking water. This study was dedicated to synthesize a novel nanoparticle of ZIF-8 with carbon dots (CDs@ZIF-8) and then modify thin film nanocomposite (TFN) membranes to enhance their performance in removing four trihalomethanes (THMs), four haloacetonitriles (HANs), and two haloketones (HKs) in chlorinated drinking water. The fabricated CDs@ZIF-8 nanoparticles and TFN membranes were characterized by FESEM, AFM, XPS, water contact angle, membrane surface potential, and three-dimensional excitation-emission matrix (EEM) to investigate the influences of CDs@ZIF-8 on TFN membranes. After chlorination, percentage reduction in salt rejection of the CDs@ZIF-8 TFN membranes was lower than that of the TFC membranes due to hydrogen bonding between CDs and polyamide, replacing amidic hydrogen with chlorine, rendering the membrane less susceptible to chlorine attack and enhancing chlorine-resistance. Results also showed that the rejection of DBPs in chlorinated drinking water by CDs@ZIF-8 TFN membranes was more than 95%. The large surface area and abundant oxygen-containing groups of CDs@ZIF8 made the nanoparticle act as a nanocarbon filler with high adsorption capacity of DBPs. The enhanced performances of chlorine-resistance and DBPs 2 / 38


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removal by CDs@ZIF-8 TFN membranes determined in this study provided valuable insights on the DBPs control in chlorinated drinking water by RO membranes.

Keywords: CDs@ZIF-8; Thin film nanocomposite membranes; Chlorine-resistance; Disinfection byproducts; Drinking water.

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Introduction Chlorine disinfection of municipal drinking water has effectively curbed waterborne diseases such as cholera, typhoid and diarrhoea.1 Chlorination of organic matter may form disinfection by-products (DBPs).2 Among the identified DBPs in water, trihalomethanes (THMs), haloacetonitriles (HANs), haloacetic acids (HAAs), N-nitrosodimethylamine (NDMA) and haloketones (HKs) are frequently investigated DBPs. Concern about formation and removal of DBPs has generated due to their potential carcinogenic effects to public health, including bladder cancer and miscarriages. Several methods have been demonstrated to control the concentration of DBPs in water including: (1) choice of alternative disinfectants; (2) removal of byproducts precursors; (3) removal of DBPs in chlorinated water.3-5 Reverse osmosis (RO) system is competitive in the removal of DBPs from chlorinated water with conventional treatments owing to the following advantages: simple operation, higher water flux with low fouling potentials, and lower operation cost.6 DBPs rejection varies with physiochemical properties of feed solution, membrane type, membrane fouling and operations in RO system.7 Studies have focused on the rejection of THMs, HAAs, HANs or NDMA by membrane. Rejection of HANs has been reported to be above 50% whereas THMs rejections have been above 60%.8,9 Xu et al. found that the rejection of NDMA and HANs in forward osmosis for wastewater recycling by Aquaporin was 31% and 48–76%, respectively.

10

On the contrary, the removal of HAAs from 4 / 38


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swimming pool water were >60% for NF270 and higher than 90% for NF90 according to the research of Yang et al.11 Obviously, it requires the fabrication process of RO membranes with high performance for the removal of diverse kinds of DBPs as much as possible in potable use. Currently, the most used RO membranes for desalination are thin-film composite (TFC) polyamide membranes, which are synthesized by the interfacial polymerization.12 To improve the TFC membranes performance of DBPs removal, Fujioka et al. immersed them into high-temperature ultrapure water to conduct heating modification of RO membrane.13 Results illustrated that the rejection of DBPs with low molecular weight was improved by tightening the membrane structure after heat treatment; however, water permeability could also decrease at the same time. Therefore, a proper modification method of RO membrane is urgently needed for balancing the membrane flux and DBP removal efficiency. Recently, addition of porous metal organic frameworks (MOFs) particle into polyamide membrane was demonstrated to be an effective method.14,15 Thereof, ZIF-8 with porous structure is relatively water-stable and effective for separation water and hydrated sodium ion in RO process.16 When using TFC membranes in chlorinated drinking water, these membranes are susceptible to free chlorine attack, causing membrane degradation and losing their performance after long time chlorine exposure.17 Carbon dots (CDs), also referred as carbon quantum dots (CQDs), possess considerable amounts of oxygen groups (hydroxy, epoxy, and carboxyl) in their basal composition.18 5 / 38



According to present researches, the CDs-modified thin film nanocomposite (TFN) membranes show higher chlorine resistance.19,20 Although the intrinsic carbonaceous quality of CDs endows high chlorine-torrent ability, their ultrasmall size makes an urgent need to encapsulate them to control CDs leaching and deterioration of membrane performance when using CDs in the modification of chlorine-torrent RO membrane.21 The objective of this study was to fabricate high chlorine-resistance RO membranes and enhance their removal efficiency of representative DBPs in chlorinated drinking water. Recent research showed that the carbonized ZIF-8 had a good adsorption capacity for dichloroacetonitriles (DCAN) with 5.08 mg/g, which was almost 9 times of ZIF-8.22 However, the carbonization of ZIF-8 need to be heated at 900 ℃ with N2 flow. Glucose (C6H12O6), a rich and economical carbon source, can be carbonized and produce CDs at low temperatures (200 ℃).23 We anticipated that the best strategy aiming to limit the release of CDs from the modified TFN polyamide membrane is in situ loading CDs into the intrinsic pore structures of zeolitic imidazolate framework-8 (ZIF-8) template with glucose to synthesize CDs@ZIF-8 nanoparticles. In this paper, we utilized the synthesized CDs@ZIF-8 as nanofillers to structurally modify the polyamide membranes. The effects of CDs@ZIF-8 on the membrane morphology, charge properties and chemical composition were investigated in details. We hypothesized that the incorporation of CDs@ZIF-8 could (1) create a channel for water to pass through fast and reject salt due to the intrinsic pore structure 6 / 38


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of ZIF-8; (2) enhance the chlorine resistance of TFN membranes to provides an important implication in chlorinated drinking water; and (3) improve the removal efficiency of DBPs in municipal drinking water by size exclusion, solute–membrane interaction and the adsorption of abundant functional groups on the CDs@ZIF-8.

Experimental Materials All of the analytical grade reagents in this study were used without further purification. Zinc nitrate hexahydrate (Zn(NO3)2.6H2O), trimesoyl-chloride (TMC), m-phenylenediamine (MPD), 2-methylimidazole, polyethylene glycol (PEG), EPA 501/601 trihalomethanes calibration mix, EPA 551B halogenated volatiles mix, and methyl tert-butyl ether (MtBE) were purchased from Sigma Aldrich Co. Isopar-G was purchased from Exxon Mobil Co (USA). Hydrochloric acid (HCl), sodium hypochlorite (NaOCl), glucose (C6H12O6), sodium chloride (NaCl), methyl alcohol (MeOH) and ethyl alcohol (EtOH) were got from Chemical Reagent Co., Ltd (China). Throughout the experiments, deionized (DI) water was used all the time. The polysulfone (PSF) ultrafiltration membrane (20,000 Da) was provided by Guochu Technology Co (China). Preparation of CDs@ZIF-8 0.714 g of Zn(NO3)2.6H2O in 50 mL MeOH was added into a solution of 0.816 g 2-methylimidazole in 50 mL of MeOH. The mixed solution was reacted for 2 h. Following by centrifugation and several washing steps, ZIF-8 nanoparticles 7 / 38



were obtained after drying in oven for 12 h. 0.5 g of the synthesized ZIF-8 was immersed into a solution of 2 g glucose in 100 mL of EtOH/H2O: 9:1 for 12 h to make glucose load into ZIF-8 pores from the liquid phase. The samples were collected by centrifugation and cleaned with EtOH/H2O (9:1) for several times. CDs formed by calcination at 250 ℃ in a nitrogen atmosphere for 2 h are confined into the MOF pores to fabricate the CDs@ZIF-8, as shown in Scheme 1.

Scheme 1. Representative scheme of preparation of carbon nanodots (CDs) on ZIF-8 template.

The stability of CDs@ZIF-8 in the aqueous solutions To confirm the water stability of CDs@ZIF-8 and ZIF-8, their pH-mediated dissolution property was examined by suspending 5 mg CDs@ZIF-8 or ZIF-8 in 20 mL water with different pH values regulated by nitric acid and sodium hydroxide. After 24 h, these samples were filtrated by ultrafiltrate membrane. The concentrations of Zn2+ released in various aqueous solutions with different pH values were analyzed using the inductively coupled plasma optical mass spectrometry (ICP-MS) technique. The release of 2-methylimidazole and CDs 8 / 38


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were analyzed by measuring total organic carbon (TOC) in different solutions indirectly. UV-visible absorption spectroscopy was used to confirm the release of CDs from CDs@ZIF-8. Preparation of TFC and TFN membranes Interfacial polymerization method was used to fabricate TFC membranes as mentioned. 12 Firstly, the PSF support membrane was immersed into aqueous solution MPD at 2% (w/v) for about 2 min, removing excess solution by blowing with N2. Then, the membrane was immersed into Isopar-G solution with the concentration of TMC at 0.1% (w/v) for 1 min. After removing the excess IsoparG solution, the synthesized membrane was washed several times with DI water. TFN membranes were fabricated similar to the process mentioned above except that CDs@ZIF-8 particles were dispersed into TMC/Isopar-G solution varying from 0.05 to 0.20 w/v %. Characterization of CDs@ZIF-8 and TFN membranes To investigate morphologies of CDs@ZIF-8 and membranes, field emission scanning electron microscopy (FESEM) was used to observe the different structure of materials with Hitachi SU-8000, Tokyo, Japan. The samples for observation were sputtered with platinum to void charging effects by using a platinum coater. The powder X-ray diffraction (XRD) was utilized to determine the composition of ZIF-8 and CDs@ZIF-8 with nickel-filtered Cu Kα radiation at 40 kV, 150 mA. Atomic force microscopy (AFM) was used to measure membrane surface roughness with instruments form Dimension Icon Bruker. 9 / 38



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The membrane surface element composition was measured by X-ray photoelectron spectroscopy (XPS) supplied by Thermo Scientific, USA. Hydrophilicity of membrane was inspected by the water contact angles using Data-physics OCA20 machine. The surface zeta potentials of TFN membranes were measured using a zeta-potential and particle-size analyzer (Zetasizer Nano ZS90, Malvern Panalytical, United Kingdom) with a zeta potential standard solution purchased from Malvern Panalytical. To confirm CDs@ZIF-8 particles were incorporated in the membranes, fluorescence spectorophotomet (F-4600, HITACHI) for three-dimensional excitationemission matrix (EEM) was used. Total organic carbon (TOC) was analyzed with Multi N/C 2100S, Analytic Jena, Germany. Membrane pore size and pore size distribution were calculated with solutions containing different molecular weights of PEG with the concentration of 200 ppm as the neutral solutes.24 The relationship between molecular weights (Mw, gmol-1) of PEG and Stokes radius (rs, nm) could be expressed as follows: r = 16.73 × 10

-12

×M

0.557

（1）

According to the traditional solute transport method, the effect of space and hydrodynamic interaction between solute and membrane pores is ignored, and the average effective pore size and pore size distribution of membrane are obtained. The average effective pore radius (μp) and the geometric standard deviation (σp) were calculated with μs (at R = 50%) and σg (at R = 84.1%).25 Then, based on μp and σp, the pore size distribution of TFC and TFN 10 / 38


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membranes can be deduced as the following probability density function:

[

]

𝑑𝑅(𝑑𝑝) 1 (𝑙𝑛 𝑑𝑝 ― 𝑙𝑛𝜇𝑝)2 = exp ― 𝑑𝑑𝑝 2(𝑙𝑛𝜎𝑝) 2𝜋𝑑𝑝𝑙𝑛 σp

（2）

Performance of CDs@ZIF-8 TFN membranes RO experiments were investigated in cross-flow membrane cells (Sterlitech Corp.) pressurized through a pump. The effective water transport area of cell was 34.5 cm2 in measurements. The feed solution was prepared with NaCl at the concentration of 2000 ppm for test. The water flux (Jw) and salt rejection (R) were evaluated to characterize the membrane RO performance by the following equations; 𝐽𝑤 = 𝛥𝑉𝑓𝑒𝑒𝑑/(𝐴𝑚𝛥𝑡) 𝑅 = (𝐶𝑓 ― 𝐶𝑝)/𝐶𝑓 × 100%

（3）（4）

Where ∆ Vfeed (L) is the volume of water passing through the effective membrane area Am (m2) after a certain time ∆t (h). The feed and permeate solutions are expressed as Cf and Cp respectively to calculate the NaCl concentration. The chlorine resistance of CDs@ZIF-8 TFN membranes were tested under the following process. Sodium hypochlorite solutions were prepared and adjusted to pH 7 with hydrochloric acid to maintain the chlorine stability. The synthesized membranes were immersed into the sodium hypochlorite solution and shaking for 2 h with PTFE-capped brown bottle to block light and evaporation of hypochlorous acid. Then, the membranes were washed with DI water to measure the water flux and salt rejection after chlorine treatment 11 / 38



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through the same method as mentioned in the above. The removal of DBPs by CDs@ZIF-8 TFN membranes tests were carried out with DBPs in municipal drinking water supplied by Harbin, China. Quantitative DBPs were spiked into the tap water to make up a feed solution. The feed and permeate samples were collected after 2 h for DBP analysis. All water samples were stored in a refrigerator to retard biological activity. Quality parameter of municipal water was summarized in Table 1. Table 1. Quality parameters of municipal water supplied in Harbin, China. Variables

Unit

Average values

TOC

mg/L

3.65

Ca2+

mg/L

12.15

Alkalinity

mg CaCO3/L

31.967

Residual chlorine

mg/L

0.05

pH

-

7

Temperature

℃

14-18

Analysis of DBPs Table 2 summarizes the investigated DBPs physiochemical properties in this study. DBPs samples were extracted with MtBE as provided elsewhere.11 Water samples (20 mL) were spiked with 4 mL MtBE and 8 g sodium sulfate for 2 min. DBPs were analyzed on a gas chromatography-mass spectrometry (Agilent 6890N) through GC-ECD method. 3 μL DBPs extract was injected. Column temperature was raised to 60 °C at 2 °C/ min from room temperature, 12 / 38


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and then raised to 100 °C at 5 °C/min and held for 5 min, and then raised to 250 °C at 10 °C/min for test. Table 2. Physicochemical properties of the selected DBPs. Disinfection byCompounds

MW

MV

MWidth

119

70

2.38

164

75

2.44

208

79

2.54

253

83

2.52

144

87

1.33

110

73

2.44

154

77

2.69

199

82

2.73

product Trichlormethane (TCM) Bromodichloromethane Trihalomethane

(BDCM)

s

Dibromochloromethan

(THMs)

e (DBCM) Bromoform (TBM) Trichloroacetonitrile (TCAN) Dichloroacetonitrile

Haloacetonitrile (DCAN) s Bromochloroacetonitril (HANs) e (BCAN) Dibromoacetonitrile 13 / 38



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(DBAN) 1,1-Dichloropropanone 127

92

1.14

(DCP) Haloketone 1,1,1(HKs)

10 Trichloropropanone

161

2.82 7

(TCP) Notes: MW=molecular weight (g/mol), MV=molecular volume (Å3), MWidth=molecular width (Å), and H-Acc=H-bond acceptor. DBPs were rejected by CDs@ZIF-8 TFN membranes.

Result and discussion Characterization of CDs@ZIF-8 The fabrication of CDs@ZIF-8 was accomplished by carbonization of glucose in the pores of ZIF-8. To inspect the different morphology between the nanocrystals, FESEM was carried out as shown in Fig. 1a. Typical rhombic dodecahedral shapes of ZIF-8 were observed which were similar to those of ZIF-8 reported in the literature.25 No new peaks were observed in the power XRD pattern (Fig.1b), illustrating that ZIF-8 lattice was not influenced by the CDs. But the increasing relative intensity of peaks at 6.8 º

and 13.6 º

demonstrated that the ZIF-8 pores were loaded with CDs.26 To further determine the influence of CDs in ZIF-8, the FTIR spectra and XPS were characterized to further confirm the chemical structure of nanocomposites. The peaks at 3134 cm-1 and 2931 cm-1 ascribing to the imidazole ring and methyl 14 / 38


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group were consistent with the infrared absorption spectra of the corresponding ZIF-8. Compared with pure ZIF-8, CDs@ZIF-8 samples showed a new characteristic peak at 3423 cm-1 attributed to the stretching vibrations of O-H, meaning the existence of hydroxyl groups. The peaks around 1587 cm-1 were assigned to the shift of the C=O bands, implying the existence of –COOH groups. The XPS data of C1s spectrum of the CDs@ZIF-8 indicated typical types of carbon bonds: C-N, C–O, and C=O, respectively. 27 Fig. 2a and 2c showed the TEM images of ZIF-8 and CDs@ZIF-8. Typical rhombic dodecahedral shapes became round after CDs were inserted into ZIF8 pore structures. To confirm the CDs inserted in ZIF-8, high resolution TEM (HRTEM) was carried out to distinguish between ZIF-8 and CDs@ZIF-8. HRTEM images in Fig. 2d exhibited lattice structures with a d-spacing value of 0.20 nm, in agreement with the basal spacing of graphite. Comparing with Fig. 2b, the results illustrated the existence and insertion of CDs in ZIF-8 porous structure. 28,29 Obviously, connecting layers of CDs between two ZIF-8 particles were also observed in red region of Fig. 2c. CDs existed not only in the ZIF-8, but also in the connecting layers between two nanoparticles. This may be caused by the carbonation of glucose which is located on the surface of ZIF-8. Through the above characterization of the synthesized particles, the CDs@ZIF8 samples had been successfully prepared.

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Fig. 1. (a) FESEM image of CDs@ZIF-8. (b) XRD patterns of powdered ZIF-8 and CDs@ZIF-8. (c) The IR absorbance spectrum of powder ZIF-8 and CDs@ZIF-8. (d) XPS spectra of C 1s in CDs@ZIF-8.

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Fig. 2. TEM (a, c) and HRTEM (b, d) images of ZIF-8 and CDs@ZIF-8.

Characterization of CDs@ZIF-8 TFN membranes CDs@ZIF-8 modified TFN membranes have many differences in structure morphology when comparing with TFC membranes. The influence of CDs@ZIF-8 on the surface and cross-section structure of the TFN membranes （loading of 0.15 wt%）was characterized by FESEM and shown in Fig.3. TFC membranes in Fig.3a show a typical “ridge and valley” morphology as reported.12 However, the surface structure of the membranes modified by CDs@ZIF-8 nanoparticles are visibly altered in Fig.3b. The surface of the TFN membranes are finely dispersed grainy structure. The different morphologies of the CDs@ZIF-8 TFN membranes are resulting from changes in the cross-linked polyamide structure incorporated with CDs@ZIF-8 into the skin layer. Due to the presence of hydrophilic groups on CDs@ZIF-8, the nanoparticles increase the interfacial area between aqueous solution and organic solution by repulsing organic components. The diffusivity of MPD molecule also increases with the addition of hydrophilic CDs @zif-8 through hydrogen bond, thus accelerating interfacial polymerization.30,31 On the other hand, Zn2+ in the CDs@ZIF-8 could activate the TMC preferentially during the interfacial polymerization reaction.32 As a result, a dense and dispersed grainy structure was spread out on the membrane surface. Cross-sectional view in Fig.3c-d shows a thinner skin layer in TFN samples than that of the TFC samples, shortening water pathway distance and reducing trans-membrane resistance.33 17 / 38



Fig. 3. FESEM images of surface and cross-section morphologies of (a, c) TFC and (b, d) CDs@ZIF-8 TFN membranes. (Particle loading of CDs@ZIF-8 was 0.15 wt%.)

To further characterize the influence of CDs@ZIF-8 on the membrane surface properties, we conducted AFM, water contact angle, and zeta potential measurements for evaluation. Fig. 4a-b show the three-dimensional AFM images of TFC and CDs@ZIF-8 TFN membranes, elucidating a few trends due to the addition of CDs@ZIF-8. The average roughness of TFC membranes is 52.6 nm, corresponding to the typical “ridge and valley” structure. When adding CDs@ZIF-8, the average roughness value for the TFN membranes sharply decreases to 45.5 nm. Fig. 4c exhibits the hydrophilic properties and zeta potential of TFC, ZIF-8 TFN, and CDs@ZIF-8 TFN membranes at pH 7. The water contact angle of TFC membranes is 68º. The improved hydrophilicity of CDs@ZIF-8 TNF membranes are confirmed by the obversion that the contact angle declines to 45 º . Furthermore, membrane pore size and pore size 18 / 38


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distribution were calculated with solutions containing different molecular weights of PEG. 24 The average pore size of CDs@ZIF-8 TNF membrane was 0.6 nm, less than the 0.68 nm of TFC membrane in Fig. 4d. Since the formation of HCl during the polymerization reaction, the local circumstance is expected to a strong acid condition. ZIF-8 is a pH sensitive material. As shown in Fig. 5a, it is found that a high concentration of Zn2+ release is observed, suggesting that ZIF-8 would decompose in such a strong acid solution. When in such a weak acid solution or a weak alkaline solution there is few Zn2+ release at pH 4–10. The results of the release of 2-methylimidazole and CDs measured by TOC in Fig. 5b. CDs@ZIF-8 would also degrade in a strong acid condition which could release CDs, free metal ions and linkers. UV-visible absorption spectroscopy was used to confirm the release of CDs from CDs@ZIF-8 in a strong acid solution. In the UV region of each curve, the peak at 280 nm in Fig. 6b is higher than that in Fig. 6a. This is attributed to the p–p* transition of CDs released by [email protected] The CDs and Zn2+ increase the interfacial area between aqueous solution and organic solution by repulsing organic components. The diffusivity of MPD molecule also increases with the addition of hydrophilic CDs @zif-8 through hydrogen bond, thus accelerating interfacial polymerization.32 Thereof, the average pore size reduced. It was more beneficial to reject the DBPs through size sieving. The presence of numerous negatively charged oxygen-containing groups on CDs@ZIF-8 was believed to facilitates a better water wettability and increase the surface charge density as observed in 19 / 38



literature.34-36

Fig. 4. Surface AFM images of (a) pristine TFC membranes, (b) CDs@ZIF-8 TFN membranes, (c) contact angle and Zeta potential at pH 7, (d) probability density function of TFC and CDs@ZIF-8 TFN membranes with the loading of nanoparticles 0.15 wt%.

The released CDs from CDs@ZIF-8 may be present on the interface during the interfacial polymerization process. Therefore, XPS was used to confirm CDs existed on the surface of the CDs@ZIF-8 TFN membranes. The characteristic peaks of C 1s and N 1s are shown in Fig. 7a-b. Three main peaks are observed at 285.4, 286.3 and 287.6 eV, representing C-O, C-N and C=O bands.37 Meanwhile, the peak at 399.8 eV is attributed to the bond (-NHCOO-), indicating the reaction of the hydroxy groups of CDs and MPD.38 The peak at 399.6 eV signifies the −NHCO− bond in polyamide layer.

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Fig. 5. Stability of ZIF-8 in aqueous solutions with pH values ranging from 2 to 10. (a) The concentrations of Zn2+ and (b) Total organic carbon (TOC) released by ZIF-8, CDs@ZIF8.

Fig. 6. The UV–Vis absorption spectra of the aqueous solutions filtrated by ultrafiltrate membrane with the pH values ranging from 2 to 10 (a) ZIF-8 and (b) CDs@ZIF-8.

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Fig. 7. High-resolution XPS spectrum of (a) C 1s and (b) N 1s of CDs@ZIF-8 TFN membranes. EEM fluorescence spectra of (c) TFC and (d) CDs@ZIF-8 membranes.

Yi Li et al provided a method by using the three-dimensional excitationemission matrix (EEM) fluorescence spectroscopy to study the influences of fluorescent nanomaterial on the TFN membranes.20 Studies on the photoluminescence (PL) intensity of TFC and TFN membranes were carried out to qualitatively state the effects of CDs@ZIF-8 on TFN membranes in this work. In Fig. 7c, the highest and strongest fluorescence peak (excitation (Ex)/ emission (Em) = λ 305 nm/ λ 366 nm) is observed, corresponding to the conjugated structure of benzene ring intrinsic in polyamide. A peak area the of TFN membranes at the same excitation/emission is shrunken as shown in Fig. 7d. That means the intensity of the PL decreased with the CDs@ZIF-8 embedded. This phenomenon may be explained that the released CDs could easily promote electron transfer and energy absorption from TFC membrane 22 / 38


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by quenching the fluorescence intensity by fluorescence resonance energy transfer (FRET).39 The amount of elemental zinc, carbon, nitrogen, and oxygen were quantified respectively to confirm their content on the pristine TFC and TFN membrane surface presented in Table 3. The atomic ratios of C/N and N/O could estimate the extent of polyamide thin film cross-linking.40,41 The abundant functional groups on the CDs@ZIF-8 could interact with MPD by hydrogen bonding interaction, impeding the IP process and loosening the chain-chain interactions. Table 3. XPS result of pristine TFC membranes and CDs@ZIF-8 TFN membranes prepared. Membranes

Zn(%)

C (%)

N (%)

O (%)

C/N

N/O

TFC

0

76.92

10.59

10.59

7.26

0.92

TFN

0.78

70.91

11.24

17.07

6.308

0.65

Chlorine resistance performance of CDs@ZIF-8 TFN membranes Fig. 8a displays the RO performance of CDs@ZIF-8 TFN membranes as the effects of CDs@ZIF-8 loading. The maximum rejection is observed at the loading of 0.15 %, when the water permeance is 1.9 LMH/bar and the rejection of salt is 98.2%. They are higher than the performance of TFC membrane, through which the water permeance is 1.12 LMH/bar and the rejection of salt is 95.3%. With addition of the CDs@ZIF-8, the hydrophilicity of the membrane surfaces was enhanced and thickness or roughness of polyamide layer were decreased. The intrinsic 3.4 Å pores of ZIF-8 exclude larger hydrated ions and 23 / 38



provided more channels for water to pass through the polyamide layer. Moreover, the hydrogen bonding interactions between the oxygen content groups on CDs@ZIF-8 and water molecules promote water to flow through smooth and frictionless surface structures of CDs rapidly.42 In summary, the CDs@ZIF-8 TFN membranes have an excellent water flux and separation performance, which was the result of the synergistic effect of CDs and ZIF-8. It is acknowledged that the sensitivity of chlorine to amidic nitrogen leads to poor chlorine resistance of polyamide membrane. The dominant chlorine uptake mechanism could be suggested that the amidic nitrogen shares its lone electron pair with the partial positively-charged Cl in oxidizing species via a reversible N-chlorination reaction, subsequently an irreversible ring-chlorination via Orton rearrangement happens.43 Therefore, when using polyamide membranes in chlorinated water, the effects of chlorine-resistance should be taken into consideration. The reduction in the salt rejection of the CDs@ZIF-8 TFN membranes is lower than that of the TFC membranes after chlorination as observed in Fig. 8b. The increased chlorine resistance of the CDs@ZIF-8 TFN membrane might be owing to hydrogen bonding between CDs and polyamide, because the hydrogen bonding for -N-C=O- and -N-C=O-OH groups could hinder the substitution of imine hydrogen with chlorine, rendering the membrane less susceptible to chlorine attack.44,45 On the other hand, the chlorine-resistance of membrane was improved by protecting the active site in MPD residual through the intrinsically electron-rich CDs released from 24 / 38


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CDs@ZIF-8, suggested to protect the amidic sites from being attacked by chlorine.46,47

Fig.8 (a) Salt rejections and water fluxes of the CDs@ZIF-8 TFN membranes with different CDs@ZIF-8 concentration; (b) Salt rejections of TFC and CDs@ZIF-8 TFN membranes (the loading concentration 0.15 w/v%) before and after chlorination with different NaOCl concentrations solution (pH 7) for 2 h.

Rejection of DBPs Fig. 9 shows DBPs rejection by TFC and CDs@ZIF-8 TFN membranes with the loading of nanoparticles at 0.15%. Four trihalomethanes (THMs), four haloacetonitriles (HANs), and two haloketones (HKs) DBPs were selected to investigate the DBPs removal performance of synthesized membranes. CDs@ZIF-8 TFN membranes exhibit higher rejections than those of TFC membrane. The lower DBP rejections by pristine TFC may be related with DBPs–membrane affinity as well as size exclusion. Compounds with a high affinity to the polyamide layer could be adsorbed onto the layer surface and partitioned into the support PSF membrane, then transporting through the membrane.48 The polyamide active layer is easily changed owing to the amine 25 / 38



and oxygen, resulting in the solutes partitioning via hydrophobic interaction or the formation of H-bonds. Therefore, TFC membranes may reject less DBPs which are hydrophobic and/or able to form H-bonds.49 In addition to adsorption by hydrophobic interaction or H-bonding, size exclusion could also have negatively influence on DBP rejection by RO membranes due to the probability density of TFC, and CDs@ZIF-8 TFN membranes shown in Fig. 4d. For these DBPs investigated, the rejection by CDs@ZIF-8 TFN membranes is more than 95%. Comparing with TFC membranes, CDs@ZIF-8 TFN membranes showed better performance on removal of DBPs. The removal process might be coupled by adsorption process as illustrated in Scheme 2. After DBPs were diffused into the membranes, CDs@ZIF-8 could adsorb DBPs similar to that happens in granular activated carbon (GAC) filter.50 The large surface area of ZIF-8 not only enhances its adsorption capacity, but also augments water transport channels akin to aquaporin in membranes.51 The abundant active sites of CDs increases the hydrophilicity of ZIF-8 nanoparticles in water to fully contact with DBPs molecules.52 CDs can also adsorb organic contaminants owing to abundant oxygen containing functional groups (−OH, − COOH, and −C=O) on its surface as mentioned in recent studies.53,54 Hence, the remainder concentrations of DBPs investigated in the study were lower than 10 μg/L, meeting the maximum contaminated level as 80 μg/L stipulated by US Environmental Protection Agency. The CDs@ZIF-8 TFN membranes with efficient DBPs rejection performance can be expected to further applications in 26 / 38


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treatments for drinking water when comparing with reported studies summarized in Table 4.

Fig. 9 Rejection of DBPs by TFC and CDs@ZIF-8 TFN membranes during filtration of DBPs-containing tap water supplied by Harbin, China.

Scheme 2. DBPs adsorption and chlorine-resistance of CDs@ZIF-8 modified thin film polyamide nanocomposite membrane Table 4. Performance comparison of TFN membrane with previously reported membranes. 27 / 38



Page 28 of 38

Rejection

Rejection

(%)

(%)

Disinfection by-

Referenc Compounds

product

e in this work

in reference

Trichlormethane 95.51

81

97.1

84

(TCM) Bromodichloromethane Trihalomethane (BDCM) 6

s Dibromochloromethane (THMs)

97.51

93

98.40

2.43

97

-

98.5

48

(DBCM) Bromoform (TBM) Trichloroacetonitrile (TCAN) Dichloroacetonitrile Haloacetonitrile (DCAN)

11

s Bromochloroacetonitrile (HANs)

8.95

65

98.98

76

98.2

90

(BCAN) Dibromoacetonitrile (DBAN) 1,1-Dichloropropanone Haloketone (DCP)

11

(HKs) 1,1,1-Trichloropropanone

98.49

28 / 38


95

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(TCP)

Conclusions A novel CDs@ZIF-8 was synthesized for incorporating into the polyamide active layer to fabricate CDs@ZIF-8 TFN membranes. The influences of CDs@ZIF-8 on membrane morphology and structure were characterized in details. CDs@ZIF-8 decreased membrane thickness, water contact angle, and surface roughness.

CDs@ZIF-8 TFN membrane has an increased water flux

and no obvious changes in salt rejection owing to the inherent pore structure of CDs@ZIF-8. CDs@ZIF-8 modified TFN membranes showed little deterioration in salt rejection after chlorine exposure, demonstrating good chlorineresistance. Then, this study evaluated the rejection of ten DBPs in chlorinated drinking water supplied by Harbin with TFC and CDs@ZIF-8 TFN membranes. Results illustrated that CDs@ZIF-8 TFN membrane exhibited better rejection performance for all DBPs investigated than those of TFC membranes. The rejection of ten DBPs by CDs@ZIF-8 TFN membrane was more than 95%. The large surface area of ZIF-8 not only enhances its adsorption capacity, but also CDs with abundant oxygen-containing groups infiltrated in ZIF-8 pores can adsorb DBPs to enhance the removal performance of DBPs. This work is anticipated to be a promising approach to modify RO membranes for removing DBPs in chlorinated drinking water in the near future. Acknowledgement 29 / 38



This work was financially supported by National Key R&D Program of China (Grant No.:2017YFA0207203), China Postdoctoral Science Foundation (Grant No.: 2017M611377), National Natural Science Foundation of China (Grant No.: 51508129), and National Science and Technology Major Projects for Water Pollution Control and Treatment (Grant No.2017ZX07201003). References 1. Fu, J.; Lee, W.-N.; Coleman, C.; Nowack, K.; Carter, J.; Huang, C.-H. Removal of disinfection byproduct (DBP) precursors in water by two-stage biofiltration treatment.

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Table Of Contents (TOC)

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