Supporting Information

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1 Supporting Information High-flux Positively Charged Nanocomposite Nanofiltration Membranes Filled with Poly (dopamine) Modified Multiwall Carbon Nanotubes Feng-Yang Zhao, a Yan-Li Ji, a Xiao-Dan Weng, b Yi-Fang Mi, a Chun-Chun Ye, a Quan-Fu An,* a and Cong-Jie Gao b,c a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou , China. b College of Chemical Engineering and Bioengineering, Zhejiang University, Hangzhou , China c The Development Center of Water Treatment Technology, Hangzhou , China * Corresponding author. Tel.: (+86) ; Fax: (+86) address: anqf@zju.edu.cn (Q.F. An). S-1

2 Figure S1. Reactions between PDA and PEI: a) Michael addition; b) Schiff-base reaction. Figure S2. Schematic representation of nanofiltration membrane test apparatus. S-2

3 Figure S3. SEM images of NF membranes by incorporating pristine MWCNTs. (C PEI = 0.7 wt%, C TMC = wt%, and the mass ratio of pristine MWCNTs to PEI in aqueous solution is 0.05) Figure S4. The optical images of NF membranes by incorporating pristine MWCNTs. (C PEI = 0.7 wt%, C TMC = wt%, and the mass ratio of pristine MWCNTs to PEI in aqueous solution is 0.05) Preparation of PEI-PDA-MWCNTs A typical synthesis process is described as follows: PDA-MWCNTs powder (X PDA-MWCNTs/PEI = 0.05) was first dispersed in DI water by 30 min sonication in an ice bath. Subsequently, PEI (0.7 wt%) was dissolved in the PDA-MWCNTs dispersion by continuous stirring at 25 o C for 30 min (ph 11.5). Afterwards, the product was washed with DI water and centrifuged (30 min, rpm) several times to remove un-reacted PEI. Finally, PEI-PDA-MWCNTs powder was dried at 40 o C under vacuum for 24 h. S-3

4 Figure S5. TEM images of a, a ) PDA-MWCNTs, b, b ) PEI-PDA-MWCNTs. Figure S6 Dispersion of PEI-PDA-MWCNTs, after standing for 1 min (left), 1h (middle), and 7 days (right). Effects of TMC and PEI concentration on PEI/TMC NF performance To obtain the optimized membrane preparation conditions, effects of TMC and PEI concentration were investigated thoroughly. As shown in Figure S7, by increasing TMC concentration from to 0.3 wt%, the water flux of PEI/TMC NF membranes dramatically decreases from 60.0 L m 2 h 1 to 19.8 L m 2 h 1. On the S-4

5 contrary, the MgCl 2 rejection increases from 83.5% to 93.2% and then keeps stable above 90% (within 3.5% variation). This phenomenon can be explained by promotion of the reaction between PEI and TMC with increasing TMC concentration, which is favorable to forming a denser polyamide selective layer. 1 In addition, a threshold PEI concentration at 0.7 wt% is observed beyond which the water flux begins to decrease (Figure S8). It is seen that the water flux increases from 20.7 to 31.2 L m 2 h 1 with increasing PEI concentration from 0.1 to 0.7 wt%. When PEI concentration is below 0.7 wt%, the unreacted amine groups of PEI are increased with increasing PEI concentration, which will increase the surface charged density (-NH + 3 ) and improve the surface hydrophilicity of the membrane, thus lead to enhance the water flux. 2 With further increasing X PDA-MWCNTs /PEI from 0.7 to 2.0 wt%, the water flux is decreased by 19.9%. This phenomenon could be explained by the increased thickness of the polyamide selective layer with the increase of PEI concentration. 3 Meanwhile, the MgCl 2 rejection significantly increases from 70.2 % to 90.9%, when PEI concentration increases from 0.1 to 0.2 wt%. By increasing PEI concentration from 0.2 to 2 wt%, the salt rejection keeps stable above 90% (within 3.5% variation) (Figure S8). Lee et al. 3 found that with increasing amine monomer concentration the water flux of membranes was firstly increased and then decreased with a slight variation of salt rejection. It is indicated that the polyamide active layers had the maximum water flux and salt rejection only at optimized monomer concentration ratio. Take both the salt rejection and water flux into consideration, the optimized conditions are: TMC concentration of wt% and PEI concentration of 0.7 wt%. The optimized membrane is defined as M-0 (Pure water permeability = 5.88 L m -2 h -1 bar -1, R MgCl2 = 93.2%), which is selected for the further study. The salt rejection for different inorganic salts of M-0 decreases in the following sequence ZnCl 2 > MgCl 2 > CuCl 2 CaCl 2 > MgSO 4 >Na 2 SO 4 > NaCl (Figure S9). This trend indicates that M-0 is a positively charged membrane. Moreover, the M-0 appears defect-free, dense, and smooth in stark contrast to the porous structure PSF-UF membrane, and the polyamide selective layer is well contact with the PSF-UF membrane (Figure S10). S-5

6 Water flux (L m -2 h -1 ) Salt rejection (%) TMC Concentration (wt%) 0 Figure S7. Effect of TMC contents on the separation performance of PEI/TMC NF membranes. (1 g L 1 aqueous MgCl 2 solution, 25 o C, and ) Water flux (L m -2 h -1 ) Salt rejection (%) PEI Concentration (wt%) 0 Figure S8. Effect of PEI contents on the separation performance of PEI/TMC NF membranes. (1 g L 1 aqueous MgCl 2 solution, 25 o C, and ) S-6

7 Water flux (L m -2 h -1 ) Salt rejection (%) 0 NaCl Na 2 SO 4 MgSO 4 CuCl 2 MgCl 2 CaCl2 ZnCl 2 0 Figure S9. Performance of M-0 for separating different inorganic salts. (1 g L -1 aqueous solution, 25 o C, and ). Figure S10. SEM surface ( 50.0k) and cross-section ( 50.0k) images: a, a ) PSF-UF; b, b ) M-0. S-7

8 Pure water permeability (Lm -2 h -1 bar -1 ) Salt rejection (%) Figure S11 Comparison of M-4 with various flat-sheet positively charged NF membranes reported in previous study. S-8

9 Table S1. Data collection for Figure S11. Membrane PEI/PDA-MWCNTs/T MC (M-4) PEI/ PAN PAN- OH-PEI(e)c (PSS 0.1M NaCl/PEI 0.5M NaCl) 1 /PES (PEI/CMCNa) 1 /PP (PEI/TMC) 1 /PES GO&EDA_HPEI PDA-PEI/PAN PEI-PDA/PES Fabrication method Interfacial Chemical Chemical Layer-by-layer self-assemble Layer-by-layer self-assemble Layer-by-layer self-assemble Surface modification Surface modification Surface modification PWP (LMH bar 1 ) R MgCl2 (%) Testing condition g L 1, g L 1, 10 bar g L 1, g L 1, 8 bar g L 1, 3 bar g L 1, 8 bar g L 1, 1 bar g L 1, 2bar g L 1, 8bar PDA/PEI/PAN Co-deposition 1.7 > g L 1, PEI/SA/MWCNT/PSF PEI/TMC/PVDF HPEI/IPC/PAI PEI-TMC/PIP/PSF In-situ ionic Interfacial Interfacial Surface modification g L 1, g L 1, 7 bar Mm, ~ g L 1, PDA/PEI/ZrO2/PAN Co-deposition ~10.0 ~ g L 1, PDADMAC/PSF UV grafting g L 1, PDMC/sPPSU UV grafting g L 1, PDMC-HEA/PSF PDHD/PSF Chemical Chemical g L 1, g L 1, Ref. This work S-9

10 PVAm/TMC/PSF Interfacial g L 1, PVAm/IPC/PSF Interfacial g L 1, PAH-PSS/PAA/PLLlip Layer-by-layer g L 1, osomes/pss/paa/pan self-assemble 4 bar PIL/PSF (KPF 6 ) Counter-ion g L 1, exchange 1 bar PHGH/TMC/PSF Interfacial Mm, PEG600-NH 2 /TMC/PE Interfacial g L 1, S PEG-4arm-NH 2 /TMC/ Interfacial g L 1, PES PVA/polyquaternium-1 Chemical g L 1, 0/PP 3 bar QAPPESK Phase inversion g L 1, 4 bar HACC/PAN Chemical g L 1, 10 bar HACC/PAN Chemical g L 1, 10 bar CS/TMC/PES Interfacial g L 1, Cross-Linking 10 bar CS/PDMC-HEA/PSF Chemical 3.45 ~ g L 1, HACC/Polyetherimide Chemical g L 1, 3 bar S-10

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