Positively Charged Nanofiltration Membrane with Dendritic Surface for

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Positively Charged Nanofiltration Membrane with Dendritic Surface for Toxic Element Removal Meng Li, Zhiwei Lv, Junfeng Zheng, Jiahui Hu, Chao Jiang, Mitsuru Ueda, Xuan Zhang, and Lianjun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b02119 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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ACS Sustainable Chemistry & Engineering

Positively Charged Nanofiltration Membrane with Dendritic Surface for Toxic Element Removal Meng Li

1‡

, Zhiwei Lv 1‡, Junfeng Zheng 1, Jiahui Hu 1, Chao Jiang

2*

, Mitsuru Ueda

1,3

, Xuan

Zhang 1*, and Lianjun Wang 1* 1) Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science & Technology, Nanjing 210094, China; 2) Department of Pharmaceutical Engineering, School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing, Jiangsu 210094, China; 3) Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-Ku, Tokyo 152-8552, Japan. ‡

Dr. Meng Li and Dr. Zhiwei Lv contribute equally to this work.

Corresponding Author * X. Zhang. E-mail: [email protected], Tel./fax: +86-25-84315916. * C. Jiang. E-mail: [email protected]. * L. Wang. E-mail: [email protected].

ACS Paragon Plus Environment


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ABSTRACT

A novel positively charged nanofiltration membrane has been prepared by the reaction of carboxylic acids on the surface of a polyamide thin film composite with poly(amidoamine) dendrimer (PAMAM, G2) in the presence of 2-chloro-1-methylpyridinium iodide (CMPI) as an activating agent. The membrane was prepared with excellent grafting efficiency and showed a high isoelectric point of pH 9.9. Due to the high density of free protonated amino groups, the membrane showed excellent rejections towards various toxic elements including Cu2+, Ni2+, and Pb2+. The rejection order also followed the size of the ions in terms of their hydrated radius. Furthermore, the membrane obtained by the surface grafting method exhibited outstanding alkaline stability compared to the membrane prepared by the conventional coating process. These results clearly indicate that grafting the poly(amidoamine) dendrimer onto the surface of the polyamide thin film composite membrane is a promising approach to improve the rejection of toxic containments.

KEYWORDS：：Positive charge, Nanofiltration membrane, Poly(amidoamine) dendrimer, Surface grafting, toxic element removal


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INTRODUCTION Potable water is a scarce resource in many parts of the world due to increasing water pollution from industrial effluents and other contaminants.1-5 Toxic elements are particularly problematic water contaminants as they are often present as soluble salts and are difficult to remove by conventional water purification techniques. Several containments such as lead, copper and chromium are highly toxic and can pose severe health hazards to humans and animals.6-9 One of the successful approaches used for removing them from water is the membrane technique as it offers several advantages over other common methods such as low energy consumption, simple operation, efficient separation and scalability.10-13 The commonly used membranes can be divided into three types by surface potential, i.e., negatively charged, neutral and positively charged membranes. Among them, the positively charged membranes are more attractive since the electrostatic interactions provide an effective way to reject positively charged ions, especially heavy metal ions.14-20 In this manner, those commonly-associated anions such as chloride or sulfate could also be simultaneously removed via the Donnan effect.14,18 Recently, many efforts have been made to develop new membranes with a positively charged surface, and the common approaches for fabricating membranes include the interfacial polymerization (IP) technique,21, 22 and the ion-pairs coating method.16, 23 For instance, several research groups have used polyethylene imine (PEI) as the aqueous reactant to fabricate thin film composite (TFC) nanofiltration membranes by a conventional IP method.24, 25 Due to the aminerich polyamide layer in the neutral condition, the resulting membranes were all positively charged, and thus showed excellent rejections to Mg2+. Recently, a different approach involving grafting has attracted attention as it can directly modify the surface structure to provide the


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desired properties.14, 15, 17 More importantly, the incorporated functional groups are chemically attached by covalent bonds, thus improving stability of the membranes. Wang et al. prepared a type of positively charged membrane by carbodiimide induced grafting with PEI.17 Due to the presence of a large amount of polar amine groups, the hydrophilicity of the membrane surface was dramatically improved, resulting in excellent anti-fouling properties of the membrane toward several basic proteins and cationic surfactants. Mariñas’s group modified a commercial nanofiltration (NF) membrane by three generations of aromatic amide dendrimers. The resulting membranes were found to have fairly high Ba2+ rejections while the water permeability was maintained over 70%.26,

27

This grafting approach provides the means to make a purposeful

modification to the membrane surface. However, the aromatic amines with low pKa value (e.g., 4.87 for aniline, 25 oC)28 would be in a partly protonated state under weakly acidic condition, leading to less charges on the membrane surface. Therefore, the amine precursors with higher basicity such as aliphatic amines would be of great interest. In this work, a surface grafting method was used to prepare a series of positively charged NF membranes. To enhance the surface charges, a commercially available aliphatic hyperbranched dendrimer, polyamidoamine (PAMAM) was used as the grafting agent. The separation performance of the membrane with regards to permeability, selectivity towards various toxic elements, as well as the surface physico-chemical properties was investigated in detail. Furthermore, for comparison purposes, the membrane was also prepared by a conventional coating process and used as a reference to verify the performance and chemical stabilities.


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EXPERIMENTAL Materials Poly (ether sulfone) (PES) ultrafiltration flat membranes (MWCO:20000 Da; Flux: 110 L m-2 h-1 bar-1；Average pore size: 12 nm) were supplied by RisingSun Membrane Technology Co., Ltd. (Beijing, China), and used as the substrates. De-ionized water used during the tests was purified using Millipore water purification with a minimum resistance of 18 MΩ. Poly(amidoamine) (PAMAM, G2, Mw=3256 g mol-1, 95%) dendrimer was purchased by Weihai CY Dendrimer Technology Co., Ltd (China). Trimesoyl chloride (TMC, >98.0%), piperazine (PIP, >98.0%) and 2-chloro-1-methylpyridinium iodide (CMPI, >98.0%) was purchased from TCI. Inorganic salts including Pb(NO3)2, CuCl2, NiCl2 , Na2HAsO4 and K2Cr2O7 of analytical grade and other reagents were employed without purification. Preparation of TFC Membranes The PAMAM grafted NF membranes were prepared on the PIP/TMC pristine TFC membrane by two steps, including IP and surface grafting method. In brief, the detailed procedure is described as follows. First, the aqueous solution (100 mL) containing 0.5% (w/v) of PIP was poured onto the surface of PES support membranes of ca. 78.5 cm2 (10 cm in diameter) and allowed to keep for at least 5 min. After mitigation of the redundant solution from the membranes surface, the membranes were rolled with rubber bar and tissue off under room temperature until no liquid remained. Then 50 mL n-hexane consisting of TMC 0.1% (w/v) was poured onto the amine-saturated membranes surface for 1 min. After pouring out the excessive solution, the membranes were rinsed with fresh n-hexane solution (50 mL) and then dried in an air for 3 min.12, 13 After that, a new aqueous solution (100 mL) containing different concentration


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of CMPI (0.01 - 0.04% (w/v)), PAMAM (0.05 - 0.2% (w/v)) and a trace amount of NaOH (equal to CMPI) was poured onto the membranes for 10 min, allowing for chemical grafting. Finally, the obtained membranes were washed by DI water and stored wetly until they were employed. Typically, the TFC membranes by surface coating method were also prepared. In brief, an aqueous solution (100 mL) containing different concentration of PAMAM (0.05 - 1.5% (w/v)) was poured onto the PIP/TMC polyamide membrane surface for 10 min, which was exactly the same condition as above. Finally, the obtained membranes were washed by DI water and stored wetly until they were employed. Characterizations All the membrane samples were cleaned with DI water and dried by supercritical drying apparatus (Leica EM CPD300, Germany) prior to all characterizations. Field emission scanning electron microscopy (FE-SEM, FEI Quant 250FEG, USA) was used to evaluate the surface and cross-section images of TFC membranes. Flourier Transformed Infrared Spectroscopy (FTIR) was employed to test the chemical structure of TFC membranes surface. The FTIR spectra were documented via a Nicolet IS-10 spectrometer equipped DTGS detector with a 4 cm-1 spectra resolution at room temperature. Surface hydrophilicity of TFC membrane was confirmed by static water contact angle measurement (DSA30, KRüSS, Germany) at ambient temperature. The DI water droplet was dropped onto the dry surface of the membranes. Five contact angles at different locations on one surface were calculated to obtain an averaged value. The surface images of the TFC membranes was recorded via the atomic force microscope (AFM, Bruker, Mutilmode8, Germany). AFM measurement (Intelligent mode) was carried out in air at room temperature with scanning area of 5 µm×5 µm. Silicon tips (NSG10, NT-MDT) with a resonance


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frequency of 330 kHz were applied. The surface wettability of membrane was evaluated by solid-liquid interfacial free energy (-∆GSL), as the following Young-Dupre Equation 1.

−∆GSL = γ L × (1 +

cos θ ) S

(1)

where θ is the water contact angle, γL is the pure water surface tension (72.8 mJ m-2 at 25 °C) and S ratio is obtained by the actual surface area to the projected area in AFM images, respectively. Separation Performance Tests The flux and rejection were investigated by laboratory-scale cross-flow membrane equipment with an effective membrane area of 12.56 cm2, and the feed flow was controlled to 3.75 LPM. Membranes were prepressed under a trans-membrane differential pressure of 0.8 MPa for at least 60 min to reach a steady state before measurement and then tested at 0.6 MPa.13 The flux was obtained by measuring the volume of permeate at 25 oC and calculated according to the following Equation 2:

J=

V A∆t

(2)

where V is the volume of permeated water, A is the effective area of membrane, ∆t is the time for collecting fixed volume water. The rejection was obtained by measuring the concentration of the ions in the feed and the permeate solution. In general, the concentrations for MgCl2 and various model wastewater solutions were determined by the electrical conductivity with a conductivity meter (UT30B, Shenzhen Uni-trend Electronics Company) and an inductively coupled plasma optical emission


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spectroscopy (ICP-OES, Optima 7300DV, PerkinElmer, USA), respectively.13 The rejection was determined according to the following Equation 3:

 Cp  R = 1 −  ×100%  C  f  

(3)

where Cp and Cf refer to the permeate and the feed concentration, respectively. To ensure the reproducibility, four membrane samples were prepared and tested for each testing condition. The obtained data were then averaged and presented with standard deviation in this study. Zeta Potential Measurement The membrane surface charge was characterized by streaming potential method using a set of AgCl electrodes analyzer (SurPASSTM 3, AntonPaar, Austria). Two membrane sheets with area of 1 cm × 2 cm were cut and fixed on the top of the cell so that the chemical composition and charge character of the barrier layer were able to be analyzed by the respective instruments. For streaming potential measurement, an electrolyte solution of 0.01 M KCl (aq.) was utilized to provide the back ground ionic strength, and automatic titration was performed using 0.05 M HCl (aq.) and 0.05 M NaOH (aq.) to investigate the effect of pH on the zeta potential as well as the isoelectric point. Alkaline Stability Test The alkaline stability test was measured by immersing the membrane sample in NaOH solution (pH was controlled at 10.0) at 25 oC for a certain period. After that, the film was taken


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out, washed thoroughly with DI water before evaluating the separation performance towards MgCl2 solution (1000 ppm) again.

RESULTS AND DISCUSSION Three types of NF membranes are used in this study, i.e., TFC, G-TFC and C-TFC. TFC refers to the membrane prepared by the conventional IP process using PIP and TMC, G-TFC stands for the membrane prepared by the grafting method in the presence of CMPI, and C-TFC refers to the membrane obtained by the direct coating method. Formation of G-TFC Polyamide Layer

Scheme 1. The surface grafting pathways of PAMAM dendrimer in the presence of CMPI.

PAMAM grafting on a TFC membrane was carried out with an activation agent, CMPI. Theoretically, the free carboxylic acid group generated after the IP process would initially react with pyridinium salt to give a carboxylic ester intermediate, which in turn would react with the amine to produce a carboxamide and N-methylpyridone.29 The formed hydrogen halide


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byproduct would be neutralized by the existing base. Scheme 1 shows the activation and grafting approaches. Compared to other widely reported activation methods such as using N-(3dimethylaminopropyl)-N’-ethylcarbodiimide (EDC) or N-hydroxysuccinimide (NHS) to form the active imide ester,17, 30, 31 the advantage of the present approach is the coupling of the activation and amide formation steps into a single “one-pot-like” step. In contrast, the barrier layer formed by the coating method is mainly derived from the ammonium salts formed between the carboxylic acids in the pristine polyamide and the amines in PAMAM, as shown in Figure 1.

Figure 1. Schematical illustration of the fabrication process of G-TFC and C-TFC membranes.

The chemical structures of the top surface of all membranes were characterized by FTIR, as shown in Figure 2. It can be seen that new absorption bands appear at 1656, 1575, 3100 and 3500 cm-1, which correspond to the stretching vibrations of amide (I), amide (II), and N-H


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groups (including Fermi resonance), respectively, indicating the formation of polyamide. However, no obvious differences could be seen between the three TFC membranes due to their similar chemical structures. Also, the distinct peak at 1484 cm-1 was assigned to C-N vibration.

Figure 2. FTIR spectra of (a) PES substrate, (b) TFC, (c) C-TFC, and (d) G-TFC membranes.

Optimization of TFC Membranes A model MgCl2 solution of 1.0 g L-1 was employed to optimize the separation performance for G-TFC as a function of CMPI and PAMAM concentrations, and the results are shown in Figure 3. Compared to the original TFC membrane which shows a medium rejection to MgCl2 (72%), the rejection rate significantly increases to ca. 89% even with a low CMPI concentration


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of 0.01 % (w/v). This result clearly illustrates the powerful effect of CMPI in the activation of free carboxylic acid groups. Further increase in the CMPI concentration leads to a slow variation in both the flux and rejection properties, and finally leads to a steady state, as shown in Figure 3(a). In a previous report, approximately 10 times excess of the activation agent was used for the free “-COOH” groups,26 while in this work, about 50 times excess of CMPI was used. Considering the trend seen in Figure 3(a), one possible explanation is that the actual residual amounts of carboxylic acid groups in our system might be greater than in the previously reported study. The membrane performance as a function of the PAMAM concentration was also investigated, as presented in Figure 3(b). With the aid of CMPI, even a low concentration of PAMAM was able to improve the salt rejection of the resulting membrane. When PAMAM concentration was set as low as 0.05% (w/v), the G-TFC membrane exhibits rejection of 84% to MgCl2, which is already much higher than the rejection rate of pristine TFC. The rejection and flux remain constant with further increase in the PAMAM concentration from 0.1 to 0.2% (w/v), indicating that a minimum of 0.1% (w/v) PAMAM is enough to entirely react with the activated intermediate. The optimum conditions for obtaining the best G-TFC membrane were 0.03% and 0.10% (w/v) of CMPI and PAMAM, respectively. Based on the stoichiometry, the molar ratio of CMPI to PAMAM was found to be 4:1. In the unique chemical structure of PAMAM G2, a total of 16 amino groups are located in the periphery and each set of four amino groups could be considered as one quarter branch. Therefore, one quarter branch could have only one reactive site, and possibly four amide bonds could be formed for one PAMAM molecule on average. In this case, the remaining 12 free “-NH2” groups are responsible for the positively charged sites, which contributed to the high rejection efficiency.


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For comparison purposes, C-TFC membranes prepared with the coating method were also optimized and the PAMAM concentration of 1.0% (w/v) gave the best membrane with 92% rejection to MgCl2. Although the rejection abilities of the two membranes are in the same range, their water permeability values are significantly different. The G-TFC membrane maintained about 70% (32.1±0.2 L m-2 h-1) of its initial permeability value, whereas only 48% (18.9±1.5 L m-2 h-1) of the initial permeability was observed for C-TFC, indicating a much denser surface for the latter.

Flux / L m-2 h-1

(a)45

100 95

40

90

TFC flux 35

Flux Rejection

30 25 20

85 80 75

TFC rejection

Rejection / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 0.00 0.01 0.02 0.03 0.04 Concentration of CMPI / % (w/v)


13


(b) 45

100 TFC flux

90

40 35

Flux Rejection

-2

Flux / L m h

-1

80

30

TFC rejection

70 60 50

25

Rejection / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 20 -0.05

0.00

0.05

0.10

0.15

0.20

Concentration of PAMAM / % (w/v)

Figure 3. Flux and rejection of MgCl2 solution for G-TFC membranes as a function of (a) CMPI concentration (PAMAM concentration was fixed at 0.1% (w/v); and (b) PAMAM concentration (CMPI concentration was fixed at 0.03% (w/v). The separation test was performed at 0.6 MPa, 25 °C, with the salt concentration of 1.0 g L-1.

Surface Properties The chemical compositions of the PES substrate, and TFC, C-TFC and G-TFC membranes were characterized by XPS, as shown in Table 1. Compared with the PES substrate and TFC membrane, the nitrogen content dramatically increases for both G-TFC and C-TFC membranes. Since the oxygen content remains the same in the PIP/TMC TFC barrier layer, the N/O ratio could reflect the relative amount of the PAMAM dendrimer grafted onto the surface. It is interesting to note that although only one tenth of the PAMAM dendrimer was used for the GTFC membrane compared to the C-TFC membrane, the N/O ratio of the former was still slightly higher, indicating the high loading efficiency for the grafting method. This is likely because most


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of the attached dendrimers were washed out during the coating fabrication, since the ionic bonds are much weaker than the covalent bonds. Therefore, the amount of the loadings is probably determined by the amount of the remaining carboxyl acid groups, suggesting no great discrepancy in XPS results for these two kinds of membranes. In this manner, the state of the PAMAM would have strong impact on the surface dense. In brief, the PAMAM might “grow” from the reactive site for the grafting method (G-TFC) in the presence of activation reagent, which could produce membrane surface with much more network pores or aggregate pores.27 On the contrary, the PAMAM would be only fixed through strong ionic interactions between “NH3+” and “-COO-” by the formation of ammonium salts for C-TFC membrane. In other words, those PAMAM might be dispersed over the C-TFC membrane surface area, which hindered some of the existing pores, as aforementioned in Figure 1. The water contact angle and −∆GSL results provide more support for this analysis (Figure 4). As previously reported by Wang, the surface wettability could be greatly enhanced by the presence of a large number of exposed polar groups (free amines).17 This theory also supports our observed results well, as a lower water contact angle of 46.9±3.5 degrees and higher −∆GSL of 118.1±2.9 mJ m-2 were obtained for the G-TFC than the C-TFC membrane, which further confirmed its higher N content and surface wettability.

Table 1. The XPS results for PES substrate, and TFC, C-TFC and G-TFC membranes.

Atomic concentraton (%) Membrane C

O

N


N/O

15


a

b

PES

81.1

18.9

0

-

TFC

74.3

16.9

8.8

0.52

C-TFC a

69.0

16.2

14.8

0.91

G-TFC b

68.4

15.3

16.3

1.07

prepared by the coating method in the presence of 1.0 % (w/v) PAMAM; prepared by the grafting method in the presence of 0.1% (w/v) PAMAM and 0.03% (w/v)

CMPI.

WCA

Water contact angle / deg

80

-∆GSL 120

60

100

80 40

Interfacial free energy / mJ m -2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

PES

TFC

C-TFC

G-TFC

Figure 4. Water contact angle and −∆GSL of PES substrate, TFC, C-TFC, and G-TFC membranes.


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Zeta potential curves were obtained for all three TFC membranes as a function of pH, as shown in Figure 5. Generally, G-TFC and C-TFC membranes showed similar curves which represent typical amphiprotic surfaces. It is worth noting that G-TFC exhibits slightly higher potential value than C-TFC over the entire pH range. Also, its isoelectric point (ca. 9.9) is higher than that of C-TFC (ca. 9.3). On one hand, the higher positive charges for G-TFC are mainly attributed to the higher amine concentration, as mentioned earlier. On the other hand, this specific property of G-TFC is responsible for the high rejection property toward cations, particularly for multivalent ions.

60

Zeta Potential / mV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


G-TFC C-TFC TFC

40 20 0 -20 -40

2

4

6

8

10

12

pH Figure 5. Zeta potential of TFC, C-TFC, and G-TFC membranes as a function of pH.

Surface Morphology of TFC Membranes


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Figure 6. FE-SEM surface images of (a) PES, (b) TFC; (c) C-TFC, (d) G-TFC and cross-section images of (a’) PES, (b’) TFC, (c’) C-TFC, (d’) G-TFC membranes.


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Figure 6 shows the surface and cross-section morphologies of all membranes, studied by FE-SEM. Compared to the PES substrate, there were no visible pores in all three TFC membrane samples, indicating the formation of barrier layer. Although the same PAMAM dendrimer was used for the formation of G-TFC and C-TFC membranes, their morphologies are extremely different. The C-TFC membrane based on ionic bonds shows a relatively smooth surface, which indicates that the PAMAM dendrimer is intertwined with the virgin polypiperazine-amide matrix. This is also responsible for the dense layer, as indicated by its much reduced water flux. In contrast, many protuberances are found on the surface of G-TFC due to the aggregation of PAMAM dendrimer. Due to the loose structure, the water flux is maintained at an acceptable range. Further studies by AFM also confirmed the FE-SEM results, as the highest average plane roughness (Ra) value of 11.2 nm was obtained for G-TFC, as shown in the AFM image (Figure 7).

Figure 7. AFM images of (a) PES substrate, (b) TFC, (c) C-TFC, and (d) G-TFC membranes.


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Separation Performance

(a)

30

-2

Flux / L m h

-1

25 20 15 10 5 0

(b) 100

Rejection / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90 80 70 60

CuCl2 50 ppm

100 ppm

NiCl2

Pb(NO3)2

200 ppm

500 ppm

Figure 8. Separation performance of (a) flux and (b) rejection of G-TFC membrane toward CuCl2, NiCl2 and Pb(NO3)2 as a function of concentration at pH of 4.8.

Separation ability of the G-TFC membrane towards typical toxic or potentially toxic elements (Cu2+, Ni2+ and Pb2+) at different concentrations was evaluated, and the results are shown in Figure 8. Four metal ion concentrations of 50, 100, 200 and 500 ppm were


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simultaneously tested to investigate the effect of ion concentration. To eliminate the influence of membrane surface potential, pH values of all solutions during the test were adjusted to the same value of 4.8. In general, the rejection ability of G-TFC membrane for the ions follows the sequence of Cu2+ > Ni2+ > Pb2+ at each of the tested concentrations. Moreover, for each solute, the rejection rate gradually decreases with the increase in concentration of ions. Considering the aforementioned rejection to MgCl2, it is obvious that the rejection rate decreases with size of the hydrated ions, i.e., Mg2+ > Cu2+ > Ni2+ > Pb2+ (Table 2). It is worth noting that although the feed concentrations are as high as 500 ppm, the rejection to all three ions still remains at a high level of over 90%, indicating potential applications for emergency water purification. The flux for different ions also displays a similar trend to that of rejection; it decreases slightly with increasing concentrations, likely due to the osmotic pressure.

Table 2. Hydrated radius of various ions.32, 33

Ions

Hydrated radius (Å)

H3O+

2.80-2.82

Mg2+

4.28

Cu2+

4.19

Ni2+

4.04

Pb2+

4.01

Rejection rates for two other specific pollutants, Cr and As, were also measured, as shown in Figure 9. The G-TFC membrane exhibits poor removal rates for both solutes under acidic


21


conditions. Both Cr and As are known to exist in their molecular forms under acidic conditions, thus the surface charges on the membrane serve no purpose and physical exclusion of the ions due to size becomes more relevant. The rejection remarkably increases to over 99% for both solutes at a higher pH of 12.0 due to the dissociation of the Cr and As salts into CrO42- (pKa2 = 6.49, 25 oC), and HAsO42-/AsO43- (pKa3 = 11.29, 25 oC),28 respectively, as well as the negatively charged membrane surface.

100 90 Rejection / %

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Page 22 of 33

K2CrO4 Na2HAsO4

80 70 60 50

pH = 4.7

pH = 12.0

Figure 9. Rejection of G-TFC membranes toward Cr and As solution of 200 ppm as a function of pH.

Stability of TFC Membrane in Alkaline Solution


22

Page 23 of 33

G-TFC C-TFC

100

Normalized rejection / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


95 90 85 80

PIP/TMC TFC

75 70

0

4

8

12 16 Time / h

20

24

Figure 10. Rejection variation to MgCl2 for TFC, G-TFC and C-TFC membranes during the alkaline stability test (stirred in NaOH solution, pH=10).

As stability of the membranes is an important criterion, the alkaline stability test was performed on the G-TFC and C-TFC membranes by immersing the membrane sheets into a NaOH solution of pH 10. Since the amide bond is chemically stable in the pH range of 4-10, the upper limit was selected as the test condition to eliminate the possible hydrolysis reactions and to accelerate the testing period. The pristine TFC membrane was also used as a reference. As shown in Figure 10, both G-TFC and C-TFC membranes showed a rapid decline in the rejection rate during the first 1 h and a slower decline subsequently. However, the former retained up to 95% of its initial rejection rate even after 24 h, whereas the latter retained only 80% of its initial rejection ability. This result clearly illustrates that the covalent bonds are of great importance for the membrane stability, particularly in applications such as treating alkaline wastewaters.


23


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Page 24 of 33

Nevertheless, it must be pointed out that although a huge excess of CMPI was added for the activation of carboxylic acid groups, the conversion still did not reach 100%, which resulted in the formation of some amount of ammonium salts.

Table 3 Comparison on the separation performance of G-TFC, commercially available and other lab-made membranes. Type of membrane

PWP/LMH/bar

Ions

Testing condition

Rejection/%

ref

Dow NF 90

13.2

Pb2+

20 ppm, 4.14 bar, pH 5-6

91~84

34

Dow NF 270

7.14

Pb2+[ Pb(NO3)2]

1000 ppm, 4 bar, pH 5

≈60 a

35

NF Hollow-Fiber

11.9 a

Ni2+

142 ppm, 4 bar, pH =2.31

95.0 a

Membranes

——

Cr6+

121 ppm, 4 bar, pH =2.31

95.8 a

(PEI) cross-linked P84

0.93±0.03

Pb2+[ Pb(NO3)2]

1000 ppm, 1 bar, pH 6.5

87.8±1.5

membranes

0.98±0.05

Pb2+[ Pb(NO3)2]

1000 ppm, 13 bar, pH 6.5

91.1±1.1

PBI/PES NF hollow

0.83

Pb2+[Pb(NO3)2]

200 ppm, 1 bar, pH 2.20

93.0

fiber membrane

——

Cr(VI)[Na2Cr2O7]

100 ppm, 1 bar, pH 12

98.9

2.4±0.3

Pb2+[ Pb(NO3)2]

1000 ppm, 10 bar, pH 5.34

99.8±0.2

Matrimid/PEI/

——

As(V)[NaH2AsO4]

1000 ppm, 10 bar, pH 8.55

99.9±0.1

Nexar flat sheet

——

Zn2+[ZnCl2]

1000 ppm, 10 bar, pH 5.88

99.3±0.3

membrane

——

Ni2+[NiCl2]

1000 ppm, 10 bar, pH 6.58

99.8±0.2

——

Cr(VI)[Na2Cr2O7]

1000 ppm, 10 bar, pH 4.81

92.3±0.5

4.15 (solute)

Pb2+[ Pb(NO3)2]

500 ppm, 6 bar, pH 4.8

89.8±0.07

4.15 (solute)

Ni2+[NiCl2]

500 ppm, 6 bar, pH 4.8

90.2±0.14

4.28 (solute)

Cu2+[CuCl2]

500 ppm, 6 bar, pH 4.8

95.1±0.14

——

Cr(VI)[K2CrO4]

200 ppm, 6 bar, pH 12.0

99.1±0.28

——

As(V)[Na2HAsO4]

200 ppm, 6 bar, pH 12.0

99.3±0.35

36

37

38

39

5.35±0.03

G-TFC

a

This study

Estimated from published figures;


24

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Table 3 sketches the separation performance of G-TFC, some lab-made, and commercially available membranes reported over the past few years. The two benchmark membranes, NF 90 and NF 270, exhibit higher PWP; however, the rejection ability seems insufficient due to their negatively-charged nature. Looking over the other listed membranes, G-TFC membrane shows comparable selectivity towards the positive ions with its water permeability also kept at an acceptable range, suggesting its promising potential for toxic elements removal applications.

CONCLUSION In this study, a positively charged NF membrane (G-TFC) was prepared by the surface grafting method from PAMAM in the presence of activation agent CMPI. Only a small feeding amount of PAMAM (0.1% (w/v)) and low concentration of CMPI (0.03% (w/v)) was sufficient to provide a significant improvement to the membrane’s rejection rate for ions, in particular, positive ions. The G-TFC membrane’s removal rates for typical toxic elements were maintained at the high level of over 90% at various ion concentrations. In brief, the rejection sequence followed the order of CuCl2 > NiCl2 > Pb(NO3)2, which is consistent with the hydrated radius of the ions. Additionally, the grafted membrane prepared via covalent bonds exhibited excellent alkaline stability while retaining most of its initial rejection ability (up to 95%).

AUTHOR INFORMATION Corresponding Author * X. Zhang. E-mail: [email protected], Tel./fax: +86-25-84315916.


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Page 26 of 33

* C. Jiang. E-mail: [email protected]. * L. Wang. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by NSFC (21406117, 21402092), Natural Science Foundation of Jiangsu Province (BK20140782), PAPD and the Fundamental Research Funds for the Central Universities (30915011306).

ABBREVIATIONS PAMAM, poly(amidoamine) dendrimer; CMPI, 2-chloro-1-methylpyridinium iodide; IP, interfacial polymerization; PEI, polyethylene imine; TMC, trimesoyl chloride;PIP, piperazine; PES, poly(ether sulfone); NF, nanofiltration; PA, polyamide; Ra, average plane roughness; J, water flux; R, salts rejection; -∆GSL, solid-liquid interfacial free energy.

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Page 32 of 33

For Table of Contents Use Only

Title: Positively Charged Nanofiltration Membrane with Dendritic Surface for Toxic Element Removal

List of Authors: Meng Li, Zhiwei Lv, Junfeng Zheng, Jiahui Hu, Chao Jiang, Mitsuru Ueda, Xuan Zhang, and Lianjun Wang

Synopsis Positively-charge nanofiltration membrane covalently bonded with poly(amidoamine) dendrimer is fabricated and shows potential application for toxic element removal.

32 ACS Paragon Plus Environment

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Title: Positively Charged Nanofiltration Membrane with Dendritic Surface for Heavy Metal Removal List of Authors: Meng Li, Zhiwei Lv, Junfeng Zheng, Jiahui Hu, Chao Jiang, Mitsuru Ueda, Xuan Zhang, and Lianjun Wang Synopsis Positively-charge nan 1230x600mm (96 x 96 DPI)


Positively Charged Nanofiltration Membrane with Dendritic Surface for

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