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Designing Multi-functional Coatings for Costeffectively Sustainable Water Remediation Xiquan Cheng, Zhen Xing Wang, Jing Guo, Jun Ma, and Lu Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03296 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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ACS Sustainable Chemistry & Engineering

Designing Multi-functional Coatings for Cost-effectively Sustainable Water Remediation Xi Quan Cheng,a,b Zhen Xing Wang,b,d Jing Guo, b Jun Ma,* a,c and Lu Shao*a,b

a.

School of Marine Science and Technology, State Key Laboratory of Urban Water Resource and

Environment (SKLUWRE), Harbin Institute of Technology, Weihai 264209, P.R. China b.

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State

Key Laboratory of Urban Water Resource and Environment (SKLUWRE), School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China c.

School of Environmental Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R.

China d

Department of Chemistry, Nanchang University, Nanchang 330031, P. R China

*Correspondence should be directed to Prof. L. Shao ([email protected] ) and Prof. J. Ma ([email protected])

ACS Paragon Plus Environment

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ABSTRACT

Multi-functional coatings derived from bio-inspired dopamine (DA) engage lots of attention for water environmental remediation recently. Nevertheless, high cost of DA monomers seriously hampers their broad applications. Herein, an elegant multi-functional coating which could be deployed as selective adsorbents or loose nanofiltration membranes for removing of dyes and antibiotics from water, were designed based on the Michael-addition and Schiff-base reactions between cost-effective plant-derived gallic acid (GA) and branched polyethyleneimine (PEI) under room temperature. Aside from the extremely low cost of GA (about 5% of DA), the resultant GA/PEI coatings possessed much better performance than DA/PEI coatings. When deployed as dye adsorbent, the bio-inspired GA/PEI coatings exhibited much higher selectivity, showing 40 % increment in anionic dye adsorptive capability and 30% decline in cationic dye adsorptive capability compared with DA/PEI coatings. When applied for nanofiltration membrane to separate antibiotics solution, the multi-functional coatings demonstrated solution flux as high as 41.3 L m-2 h-1 at 5 bar alongside 96.2 % azithromycin (AH) rejection, showing 20 % increment in flux and 10 % increment in rejection compared with DA/PEI nanofiltration membranes. Therefore, our cost-effective multi-functional GA/PEI coatings show promising applications for sustainable water remediation.

KEYWORDS: bio-inspired multi-functional coatings, adsorbent, nanofiltration, dye & antibiotics removal, water remediation


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Introduction For obtaining multi-functional properties towards diverse applications, surface modification of solid materials plays a crucial role in modern physical, chemical, biological, and materials science.1-5 Most recently, catecholamines (like dopamine) that inspired from mussels, provide the remarkable solution to effectively modify the solid surface by forming surface-adherent films virtually on any materials.6-11 After polymerization, as a majority of naturally occurring melamine materials, polydopamine (pDA) possesses many striking properties in optics, electricity, magnetics, and most importantly excellent biocompatibility.12-21 Different functional groups (catechol, amine and imine) and transition metals can also be introduced to the material surface during the coating process, thereby achieving desired properties.12-21 With these unique advantages, pDA is increasingly used as a multi-functional coating in the wide range of applications across the chemical, biological, medical, and materials sciences, as well as in applied science engineering up to now.12-21

Particularly, in response to the urgent need of safe water worldwide, the bio-inspired pDA based multifunctional coatings have been deployed as nanoparticles adsorbents,22-25 or high-efficient separation materials13-15,

26-35

. Notably, surface properties of the pDA coatings are always modified through

grafting/crosslinking with functional (macro)molecules to enhance the adsorptive capacity or separation performance. For instance, pristine pDA coated magnetic nanoparticles can be hardly utilized as dye adsorbent due to low separation capacity ( methanol (MeOH)> ethanol (EtOH) > isopropanol (IPA)> THF. The solute rejections of NF membranes were calculated using Eq.4 R=(1-CP/Cf )

(4)

where Cp and Cf are the solute concentrations in the permeate and the feed solution, respectively. Dye and antibiotics concentrations were measured with a UV–VIS CINTRA20-GBC apparatus at the maximum


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absorption wavelength. Each data point is an average of three repetitions of each test, with ± 5 % standard deviation.

Results and discussion Aforementioned, the GA/PEI coatings can be coated on different surfaces (Figure S2). During the coating process, we found that self-supporting layers can be formed on the surface of coating solution (Figure S2; introduced in SI in detail). Firstly, we characterized the chemical and physical structure of the self-supporting layer to clarify the reaction between the GA and PEI. Then, the GA/PEI coatings were coated on the Fe3O4 magnetic nanoparticles and hydrolyzed polyacrylonitrile (PAN) for removing organic contaminants from water, respectively (Figure 1B and Figure 1C). Compared with DA, there are more functional groups that can react with amino groups in PEI. Moreover, amino groups in DA may sacrifice the possibility of reaction between the DA and PEI. Thus, more PEI might be introduced to the solid surface through the coating process, leading to higher surface charge. The DA/PEI coatings were applied as control samples to demonstrate the advantages of our coating layers. Since the cost of plant-derived GA is low, the GA/PEI coatings might show intriguing promise in removing organic contaminants from water.

Characterization of GA/PEI self-supporting layers The possibile mechanism of the reaction between the GA and PEI was shown in Figure 1A. There are one carboxyl group and three phenolic hydroxyl groups in the GA molecules. On one hand, carboxyl group and phenolic hydroxyl groups in the GA molecules could react with PEI through acid-base reactions.41 On the other hand, the catechols (pyrogallol groups) were firstly oxidized to form quinone structures. Then, amino groups in PEI could react with the C=O or C=C-C=O through the Michael addition and Schiff-base reactions, inspired by the reaction mechanism of self-polymerization of dopamine.6, 12, 41 The inferences can be confirmed by the FTIR spectra and XPS results of the GA/PEI



freestanding membranes. From the FTIR spectra of GA (Figure 2), the strong peak at the wavenumber of 3496 cm-1, 3276 cm-1 and 1212 cm-1 belongs the stretching vibration peak of phenolic hydroxyl, the stretching vibration peak of -OH in the carboxyl group, and in-plane vibration peak of -OH in the carboxyl group, respectively. The absorption peak at 1608 cm-1 presents the in-plane bending vibration peak of C-C of the benzene ring.37, 41, 45-46 The vibration peak of C=O in the carboxyl group appears at 1691 cm-1. For the spectra of PEI, The peak at 3277 cm-1, 1584 cm-1 and 1347 cm-1 belongs to the stretching vibration peak of –NH, in-plane bending vibration peak of -NH, and the stretching vibration peak of –C-N, respectively. The peak around 2882-2887 cm-1 is the stretching peak and in-plane vibration peak of -CH2.47-48 For the FTIR spectra of GA/PEI self-supporting membranes, the peak at 1644 cm-1 may belong to C=N absorption peak or the C=O in CO-NH group. The peak at 1555 cm-1 belongs to the inplane vibration peak of NH in CO-NH group after the reaction between GA and PEI.37, 41, 45-46 The high resolution FTIR spectrum of the GA/PEI self-supporting layer was shown in Figure 2B. Peaks at 1349 cm-1 and 1434 cm-1 belong to the in-plane stretching vibration of phenolic hydroxyl group and in-plane stretching vibration of –OH bond in –COOH, respectively.41,

45-47

Peaks at 1159 cm-1 and 1060 cm-1

attribute to the in-plane stretching vibration of the C-N bond in the C-NH2 group.46-48 These peaks indicate that there are some unreacted amino, carboxyl and hydroxyl groups in the GA/PEI selfsupporting membranes. All of the aforementioned absorption peaks can be observed in the GA/PEI coated nanoparticles or GA/PEI coated nanofiltration membranes (Figure S3 & Figure S4). Atomic chemical bonding states of GA and GA/PEI self-supporting membranes were characterized by XPS (Figure 3). The wide spectra of XPS illustrate that the reaction between GA and PEI occurs since the occurrence of N1s peak is observed in the spectra of GA/PEI self-supporting membranes. From the high resolution XPS of the N1s of the GA/PEI self-supporting membranes, the peak at 398.3 eV belongs to the C-N-C bonds which are contributed from the Michael-addition and intrinsic C-N-C bond from PEI.41, 49-50 The wide peak around 399.4 eV is assigned to C=N-C and N-H bond which are contributed from Schiff-base reaction and the original bond (N-H) in PEI, respectively 41, 50-51. The peak at 401.7 eV is attributed to the


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-NH3+, which is contributed from the reaction between –COOH in GA and –NH2 in PEI.41, 52-53 The high resolution spectra of C1s elements of GA and GA/PEI membranes also confirm the reactions aforementioned. For the C1s spectra of GA, the peak at 284.1 eV is assigned to –C=C– bond and the peak at 291.3 eV is attributed to π-bond from the benzene ring.54-56 The peak at 286.1 eV is assigned to the aromatic C-OH bond 57 and the peak at 288.2 eV is attributed to the C=O in the carboxyl group 58. For the C1s spectra of GA/PEI membranes, the peak of C=C bond shifts to 283.9 eV due to the Schiff-base reaction and Michael-addition reaction, rearranging the conjunction of C=C bond of the benzene ring. The new peak at 284.9 eV belongs to the chemical shift of C-C bond,59 which is attributed to the rearrangement of the benzene ring and the intrinsic C-C bond in PEI. The peak at 285.6 eV is assigned to the C-OH and C-N, which is attributed to the rearrangement of benzene ring and Michael-additions, respectively.60-61 The peak at 286.7 eV belongs to the chemical shift of the C=N bond which is contributed from the Schiff-base reaction.41, 62 The results of XPS confirm that the Michael addition, the Schiff-base reaction and acid-base reaction occurred during the formation of the GA/PEI self-supporting membranes.

The morphologies of the GA/PEI self-supporting membranes characterized by SEM and AFM were shown in Figure S5 and Figure S6, respectively. Structure of cross-section of the GA/PEI membranes shows that the thickness of the GA/PEI self-supporting membranes increases with the increment of the molecular weight of PEI. The thickness of the self-supporting membranes fabricated by PEI-600 was about 1.4 µm; whereas that value was above 10 µm when the molecular weight of PEI is about 70,000 g mol-1. Moreover, more defects were observed when the free-standing membranes were fabricated with higher molecular weight of PEI from the cross-section and the surface morphology characterization of the GA/PEI self-supporting membranes. Especially for the GA/PEI-70,000 membranes, defects can be observed on the membrane surface even under low magnitude of SEM. The surface of GA/PEI-600 membrane is quite smooth, demonstrating smallest Ra and Rq (Figure S6 and Table S2). When the



molecular weight of PEI increases, the roughness of the membrane increases significantly. These results are consistent with the results of SEM characterization. The activity and steric effects of PEI may account for the phenomenon. Owning higher activity, the PEI with smaller molecular weight easily reacted with GA to form a dense layer on the surface of the solution, hampering the oxygen into the solution.7 Then, GA and PEI in the solution are hard to react with the functional groups on the self-supporting layer, resulting in the formation of thinner self-supporting layer.

In general, the surface of GA/PEI self-supporting layer is hydrophilic with water contact angle ranging from 35o to 40o (shown in Figure S7). The chemical composition of the self-supporting membranes may account for the hydrophilic surface of the GA/PEI self-supporting membranes. As shown in Figure 1, Figure 2 and Figure 3, amino, carboxyl and hydroxyl groups were detected on the self-supporting membranes, which will result in the high hydrophilic surface of the GA/PEI layer.

Deploying the GA/PEI coating layer as adsorbent Magnetic nanoparticles possess high specific surface and highly regenerative properties, attracting lots of attention. Deployed as adsorbent, one of the biggest advantages is that it can be easily recovered by magnetic field. The interaction between the adsorbent and dyes was the electrostatic attraction between the surface charge of the adsorbent and that of dyes.25 Higher surface charges of the dye adsorbent lead to higher adsorptive capacity for the dyes with the opposite surface charge.25 To clarify the advantages of the GA/PEI coatings, we compared their performance and basic properties with the pDA coated and DA/PEI coated magnetic nanoparticles, respectively. When applied as dye adsorbent, the catecholic hydroxyl groups in DA provide the adhesive properties, while the amino groups in DA and PEI provide positive surface charge which is the critical factor that plays important role in adsorbing dyes.25 Similarly, GA provides the pyrogallol groups and the amino group in PEI provides the adsorptive properties to dyes.


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From the morphology images characterized by TEM (Figure 4A), obvious coating structure is observed on Fe3O4 nanoparticles. As shown in Figure 4B, the zeta potential of the DA/PEI and GA/PEI coated magnetic nanoparticles are about 25 mV and 35 mV respectively in neutral aqueous solution, whereas, the value of DA coated membranes is about 15 mV below zero reported in our previous work,25 implying the formation of cationic PEI structure in the coating layer of nanoparticles. The great difference in the zeta potential for different coating can be explained by the formation mechanism of the coating layers. The formation of DA coatings mainly relies on the self-polymerization of DA. Besides self-polymerization of DA, the Michael addition and the Schiff’s base reaction between PEI and DA also occur during the formation of DA/PEI coating layers.2, 12, 17 However, for the GA/PEI coating layers, self-polymerization of GA can hardly occur. Besides the Michael addition and the Schiff’s base reaction between PEI and DA, neutral reaction between the –COOH in GA and –NH2 in PEI can easily occur as well. Compared with pDA layer, the DA/PEI layer shows more positively charged surface since more excessive amino groups can ionize.25, 63 Interestingly, since GA has more functional groups that can react with the amino group in PEI (three phenolic hydroxyl groups, one carboxyl group and the benzene ring), the GA/PEI coatings show higher positively charged surface than DA/PEI coatings. Moreover, self-polymerization of DA also sacrifices the possibility to react with PEI, which may be another reason accounting for the less positively charged of DA/PEI layer compared with GA/PEI coating layer.63-65 Due to higher electrostatic repulsion, the nanoparticles coated with GA/PEI coatings show less aggregation of nanoparticles.25 The BET surface area of the nanoparticles verified this point. The BET surface area of DA/PEI and GA/PEI coated nanoparticles are (29.6 m2 g-1 and 32.5 m2 g-1, respectively) higher than that of pDA coated nanoparticles (17.8 m2 g-1) .25 Because of the lowest positive charge and surface area, pDA coated magnetic nanoparticles exhibited lowest adsorptive capacity (below 30 mg g-1) for all kinds of dyes and poor selective adsorption.25 The adsorptive capacity of both DA/PEI coated nanoparticles and GA/PEI coated nanoparticles for all kinds of anionic dyes (over 80 mg g-1) are much higher than that of pDA coatings (Figure 5A and Figure 5B). The selective adsorption phenomenon can also be obviously



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observed from the visual color of the mixture solution of dyes after adsorption (Figure 5C). Interestingly, the GA/PEI coated ones not only show 40 % to 50 % higher adsorptive capacity to anionic dyes compared with DA/PEI coated adsorbent, but also show about 30 % lower adsorptive capacity to cationic dyes, indicating the much better selective adsorption of the GA/PEI coated nanoparticles. To further clarify the selective adsorption properties of the dye adsorbent towards the anionic dyes and the cationic dyes, the selectivity was calculated according to the adsorptive capacity (Eq. 1) and was shown in Figure S8. The calculated selectivities of GA/PEI coated dye adsorbents towards the anionic dyes/the cationic dyes were about twice that of DA/PEI coated one. For example, the calculated selectivity of DA/PEI coated dye adsorbent towards MB/MeB was about 24.8, whereas, the value of the GA/PEI coated one was about 56.3. The higher specific surface area and the higher positive surface charge of the GA/PEI coatings as discussed above account for their higher adsorptive capacity for anionic dyes and lower adsorptive capacity for cationic dyes.25 Most importantly, the GA/PEI coated magnetic nanoparticles also show outstanding recyclability, remaining over 90 % adsorptive capacity after 10 cycles (Figure S9). The sacrifice of the dye adsorptive capacity should be resulted from the loss of the dye adsorbent during adsorption-desorption operation process rather than the structural damage during the regeneration process, which can be confirmed by the FTIR spectra of the dye adsorbent (Figure S4). Compared with the FTIR of the fresh adsorbent, the new peaks at 1235 cm-1 and 945 cm-1 belong to the functional groups in RB, indicating that the dyes was absorbed on the surface of the dye adsorbent. Moreover, the FTIR spectrum of the regenerated adsorbent is same with that of the fresh adsorbent, which indicates that the structure of the dye adsorbent was not damaged and the dyes was removed from the adsorbent during the regeneration process. After treatment by NaOH, the volume of waste dye solutions is quite little, which could be applied for preparation of color cement. Taken together, the GA/PEI coatings are preferred to be an effective coatings on nanoparticles for organic electrolyte molecule removal in water compared with DA and DA/PEI coatings.


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Deploying the bio-inspired GA/PEI coating layer as selective layer of loose nanofiltration membranes Then GA/PEI coatings were also exploited as a selective layer coated on the PAN ultrafiltration membranes to obtain low-pressure loose NF membrane for removing organic contaminations from water and organic solvents in the current work. After coating with GA/PEI, the obvious pore structure in hydrolyzed PAN substrates is disappeared (Figure 6A). The selective layer is thin and the compatibility between hydrolyzed PAN and GA/PEI selective layer is well, resulting in no obvious interfacial layer that can be observed from the cross-section of the composite membranes (Figure 6A). As a positive charged coating, the zeta potential of composite membrane tends to the positive direction (Figure 6B). With the increment of molecular weight of PEI, the GA/PEI based NF membranes demonstrate lower permeance. As mentioned above, PEI with smaller molecular weight easily reacted with GA to form thinner layers, which could account for the higher permeance of the membranes made by PEI with lower molecular weight (Figure 7A). Moreover, membranes fabricated by PEI with lower molecular weight show better wettability which also contributes to their higher water permeance as well.66 The membrane fabricated by GA and PEI-1800 exhibits about 18.0 L m-2 h-1 bar-1 of water permeance together with over 99 % RB and 92 % CV rejection (Figure 7A). Since the zeta potential of the membrane is not high (-10 mV), steric effect plays more important role in rejecting organic molecules with dye rejection orders of RB>CV>MO and the rejection orders of antibiotics of AH>CP>TOB>CA (Figure 7B).53,

67

Specially applied in

separation of AH, the GA/PEI membrane exhibits initial solution flux as high as 41.3 L m-2 h-1 at 5 bar with 96.2 % AH rejection (Figure 7B), demonstrating intriguing promise in removing of AH from aqueous solutions. Compared with DA/PEI membranes, both the solution flux (20% increased) and the organic solute rejection (10% increased) of the GA/PEI membrane is slightly higher (Figure 7B). As mentioned above, the self-polymeriztion of DA generates agglomerated particles, blocking the membranes

25

and decreasing the water permeations33. Moreover, the more active moieties (functional

groups can react with PEI) in GA will introduce more PEI in molecules of the coating layer, which can



endow the layer with more positive charge contributing to less aggregation due to the electrostatic interaction65-66, 68. Besides, introducing more PEI in the selective layer also benefits for forming dense and crosslinked structures, thereby resulting in higher rejections to organic solutes.33 Most importantly, the solution flux of the membrane maintains about 86.7% of initial value after 48 hours alongside slightly changed AH rejection, and can recover to 98% of initial value once the membrane was washed by water (Figure 7C), which might be due to the high hydrophilic surface of the membranes (Figure S10).53 Besides, the nanofiltration membranes also demonstrated stable separation performance in the solutions with pH value ranging from 3 to 10 (Figure S11). Markedly, the membrane also shows higher comprehensive separation performance than most of other polydopamine-based nanofiltration membranes (Figure S12).64, 68-73 After treated by the nanofiltration membranes, the antibiotics and the dyes can be further evaporated for recovering the active molecules. Besides in water, the membrane also shows excellent stability and high flux in alcohols and THF thanks to the cross-linked structure of the selective layer with RB rejection nearly 100% (Figure S13), indicating that the GA/PEI nanofiltration membranes can be applied in recovery of solvents as well, thereby reducing the discharge of waste solvents. Moreover, GA is low cost and can be applied in large-scale. In a word, the newly developed GA/PEI coatings are much applicable to building loose NF membrane for organic separation in water or organic solvents owning to better separation performance and lower cost.

CONCLUSION In conclusion, the bio-inspired coatings that can be covered on various surfaces were developed based on the reaction between low-cost plant-derived GA and PEI in weak alkaline solution, and can be used for high-efficiency water remediation via acting as adsorbents and separating layer. Compared with traditional DA/PEI coatings, our multi-functioal coatings were proven to be much more effective as selective-adsorbent for dye removal and selective layers of nanofiltration membranes for removing organic contaminants from water. When deployed as dye adsorbent, the GA/PEI coatings exhibited much


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higher selectivity, showing 40% increment in anionic dye adsorptive capability and 30% decline in cationic dye adsorptive capability compared with DA/PEI coatings. When applied for nanofiltration membrane to separate antibiotics solution, the multi-functional coatings demonstrated solution flux as high as 41.3 L m-2 h-1 at 5 bar alongside 96.2% azithromycin (AH) rejection, showing 20% increment in flux and 10% increment in rejection compared with DA/PEI nanofiltration membranes. Taken excellent performance in water remediation and the low cost of GA/PEI coatings, our muti-functional bioinspired coatings show intriguing promise for large scale applications.

FIGURES



Figure 1. The formation of multi-functional GA/PEI coatings: (A) The structure of GA, PEI and the reaction mechanism between GA and PEI; (B) The coating process of GA/PEI on Fe3O4 nanoparticles and their selective-adsorption to dyes: anionic dyes were absorbed whereas cationic ones were not; (C) The coating of GA/PEI to hydrolysed PAN substrates and their application as nanofiltration membranes for organic solvent removal.

Figure 2. (A) FT-IR spectra of the monomers and the GA/PEI self-supporting membranes; (B) The FTIR spectrum of GA/PEI self-supporting membranes with high resolution

Figure 3. The XPS spectra of GA and GA/PEI self-supporting membranes: (A) wide-spectra; (B) N1s spectra of GA/PEI self-supporting membranes; (C) C1s spectra of GA; (D) C1s spectra of GA/PEI self-supporting membranes.


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Figure 4(A) The TEM images of the coated Fe3O4 nanoparticles; (B) The Zeta-potential of GA/PEI and DA/PEI coated Fe3O4 nanoparticles



Figure 5. (A) The adsorptive capability of the coated Fe3O4; (B) The relative adsorptive capability of GA/PEI coated Fe3O4 compared with DA/PEI coated ones, GA/PEI coated ones demonstrates higher adsorptive capacity to anionic dyes and lower adsorptive capacity to cationic ones (C) The selective adsorption to dyes of the GA/PEI coated Fe3O4 nanoparticles.


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Figure 6. (A) The morphologies and (B) the zeta-potential of loose nanofiltration membranes coated by GA/PEI layer



Figure 7. (A)The separation performance of the loose NF membranes to dyes: histogram represents the water permeance of nanofiltraton membranes and the scatter-line indicates the dye rejection performance of the nanofiltration membranes, Red=RB, Blue=CV and Orange=MO; (B) The separation performance of GA/PEI-1800 to antibiotics; (C) The long term separation performance of GA/PEI-1800 to AH, the operation time of each cycle is 50 hours. After the first 50 hours, the membrane was washed by fresh water for 2 hours about 3 times. Then, the separation performance of the nanofiltration membranes was re-measured to evaluate whether the fouling of the membranes is easy to wash.


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Conflict of Interest: The authors declare no conflict of interest. Supporting Information Available: The materials and characterization of the membranes, the detailed SEM images, FTIR, surface roughness of the membranes, water contact angle, solute and solvent properties are included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Correspondence should be addressed to [email protected], [email protected]

Acknowledgements X. Q. Cheng and Z. X. Wang contribute equally to this work. LS acknowledge the funds from National Natural Science Foundation of China (21676063, U1462103), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute Technology) (No. 2017DX07), and HIT Environment and Ecology Innovation Special Funds (No. HSCJ201619).



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Abstract Graphic

A kind of bio-inspired coating was designed and deployed as selective dye-adsorbents and loose nanofiltration membranes for sustainable water remediation.

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Designing Multifunctional Coatings for Cost ... - ACS Publications

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