Biomimetic Silicification on Membrane Surface for Highly-efficient

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Surfaces, Interfaces, and Applications

Biomimetic Silicification on Membrane Surface for Highly-efficient Treatments of both Oil-in-water Emulsion and Protein Wastewater Xiaobin Yang, Hongguang Sun, Avishek Pal, Yongping Bai, and Lu Shao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09218 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Materials & Interfaces

Biomimetic Silicification on Membrane Surface for Highly-efficient Treatments of both Oil-in-water Emulsion and Protein Wastewater Xiaobin Yang,† Hongguang Sun,† Avishek Pal,† Yongping Bai,† and Lu Shao*† †

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

Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, China

KEYWORDS: Membrane separation, biomimetic silicification, oil-in-water emulsion separation, protein interception, antifouling

1 Environment ACS Paragon Plus

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ABSTRACT: The worldwide water crisis and water pollution has put forward great challenges to the current membrane technology. Although polyvinylidene fluoride (PVDF) porous membranes can find diverse applications for water treatments, the inherent hydrophilicity must be tuned for energy/time-saving process. Herein, the surface wettability of PVDF membranes transforming from highly hydrophobicity to highly hydrophilicity was realized via one-step reaction of plant-derived phenol gallic acid (GA) and γ-aminopropyltriethoxysilane (APTES) under aqueous solutions. The surface hydrophilicization can be achieved on porous PVDF membranes by virtue of integration of mussel-inspired coating and in situ silicification via “pyrogallol-amino covalent bridge” towards excellent anti-fouling performance and highly efficent infiltration ability for oily emulsion and protein wastewater treatment. The water flux of surface manipulated microfiltration membrane can reach ca. 9246 L m-2 h-1 (54-fold increment compared to that of pristine membrane), oil rejection >99.5% in a 3-cycle emulsion separation; the modified ultrafiltration membrane demonstrated benign performance on bovine serum albumin (BSA) protein interception (rejection as high as ca. 96.6% with water flux of ca. 278.2 L m-2 h-1) and antifouling potential (increase of ca. 70.8%). Our in situ biomimetic silicification under “green” conditions exhibits the great potential of the developed strategy in fabrication of similar multifunctional membranes towards environmental remediation.


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1. INTRODUCTION The severe water pollution has stimulated the increasing concern on the advanced technology towards water-energy nexus.1-8 Therein, membrane technology has exhibited unique merits and rapidly spread for water remediation owing to no-phase transition, low-consumption and facile operations.9-17 As a presentative membrane material, polyvinylidene fluoride (PVDF) has come into the light for its extensive employment to fabricate microfiltration (MF) membrane or ultrafiltration (UF) membrane for oil/water separation and protein interception, respectively, owing to its excellent mechanical properties and chemical stability.18-24 However, there are several critical issues in porous PVDF membrane for water treatment. The inherent hydrophobicity of PVDF inevitably leads to a non-ideal water flux, and renders the membrane surface/interface vulnerable to protein adsorption and oil droplets accumulation during screening operation which blocks the pore channels resulting in deterioration of real-time flux.25-28 Meanwhile, the considerable risk of pollution adhered to membranes inevitably enlarge the operation cost and energy consumption. Thus, to break the limited performance of PVDF membrane for water environment remediation, the hydrophilic surface is the key point.29-36 Gallic acid (GA), typically extracted from plant Chinese Gall, has been considered as an emerging green and cost-effective platform for new surface manipulation based on in-depth understanding of mussel-inspired chemistry on the alkaline aqueous condition.37-47 In addition, Zhan et al reported the more favorable adhesive performance brought by pyrogallol group than catechol group in the synthesized copolymers, indicating the pyrogallol groups in the derived polymer exhibit the benign adhesion ability.48 Althought GA is a biocompatible and environmental friendly material, GA could not self-polymerize by itself for surface manipulation and the pyrogallol moiety in GA can generate similar mussel-inspired adhesive coating with the



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aid of amino-terminated substances via Michael addition/Schiff base reactions in alkaline conditions.49, 50 We noticed that the interesting alkaline condition for mussel-inspired reaction can also induce the siloxane to hydrolize/condensate into polysiloxane which is similar to the biomimetic silicification on earth.30, 36, 51 It is natural to conceive that the in situ biomimetic silicification and immobilization of superhydrophilic silicified polysiloxane on membrane surface via pyrogallol-amino covalent bridge can be realized by one-step deposition under the participation of GA and siloxane with amino functional groups via Michael addition/Schiff base reactions. Previosuly, some reported literatures are involved in the incorporation of polysiloxane with the aid of polydoapmine (PDA) feature; Yang et al fabricated a silica-decorated polypropylene MF membrane on the basis of pre-deposited PDA/ polyethyleneimine (PEI) coating layer,49 Wang et al introduced hydrolysis/condensation of ethoxylated and epoxyterminated siloxane precursors in PDA network with the aid of hydrogen bonding.30,

36

The

surface hydrophilic manipulation strategy developed in this study, different from conventional PDA-incorporated method,30,

36, 51

can integrate the deposition of cost-effective GA-based

mussel-inspired coating and introduction of silicification of hydrophilic siloxane together through covalent “amino bridge”, exhibiting the synergistic enhancement effect of excellent surface hydrophilicity. Herein, the hydrophilic hybrid network derived from polymerization of GA and APTESderived polysiloxane (PGA) was elegantly deposited on porous hydrophobic PVDF MF/UF membranes via co-incubating pristine membrane in an alkaline buffer solution containing GA and γ-aminopropyltriethoxysilane (APTES, amino-terminated siloxane) (Figure 1). The whole deposition procedure was conducted in an aqueous buffer solution at room temperature, in the absence of organic solvents and toxic chemicals. The surface chemistry, water contact angles,


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water uptake, and weight gain were evaluated to optimize the operating conditions. The leaching test was used to demonstrate the stability of as-deposited PGA coating. The water fluxes of porous MF membranes without modification and only with mono-component deposition of GA or APTES were measured in comparison revealing the synergistic enhancement brought by the PGA deposition. Afterwards, a 3-cycle separation of toluene-in-water emulsion was conducted using the membrane after the incubation of the optimized condition, evaluating the separation performance and its reusability. At last, the corresponding modified UF membrane was demonstrated to have a benign potential on BSA protein interception and antifouling performance.

Figure 1. The scheme of co-deposition of gallic acid (GA) and γ-aminopropyltriethoxysilane (APTES) on the PVDF MF/UF membranes for application in water remediation on oily and protein wastewater. PGA refers to the abbreviation of the hydrophilic hybrid network derived from polymerization of GA and APTES-derived polysiloxane.



2. EXPERIMENTAL SECTION 2.1. Materials. PVDF MF membranes with pore size of 0.22 µm were purchased from Beijing Shenghe, China. The porous PVDF UF membranes were fabricated by phase-inversion method using PVDF (FR904) from Inner Mongolia 3FWanHao Fluorine Chemical Co. Ltd. 3,4,5trihydroxybenzoic acid (GA), APTES, tris(hydroxymethyl)aminomethane (Tris), and phosphate buffer solution (PBS) mixture for preparation of PBS buffer solution were received from Aladdin (China). Polyvinylpyrrolidone (PVP, K30 Mw=40 000 g mol-1) was supplied by Sinopharm Chemical Reagent Co., Ltd, bovine serum albumin (BSA, Mn=68 000 g mol-1) by Beijing Aobo Star Biotechnology Co., Ltd. Oil red O, anhydrous ethanol and other chemicals were obtained from Tianjin Kermel Chemical Reagent Co., Ltd. (China). All the chemicals were used as recieved without further purifications. Ultrapure water was supplied by a Sartorius AG arium system. 2.2. Deposition of GA/APTES hybrid coating on PVDF MF membranes. First, PVDF MF membranes were immersed in acetone and ethanol for 6 h, respectively, then dried in air. A certain amount of APTES was dissolved in 20 mL ethanol, and 0.2 g GA was dissolved in 100 mL Tris-HCl solution (pH 8.5). Then, the two solutions were mixed together in the petri dishes and immediately had membranes palced in. The membranes were incubated at room temperature for 6 hours under shaking and followinglly rinsed by ethanol and water, respectively. Meanwhile, two membranes either without the addition of GA or APTES during surface modification were fabricated for contrast. The incubation conditions and abbreviation names of as-prepared membranes were listed in Table 1.


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Table 1. The abbreviation names of PVDF membranes and corresponding incubation conditions. For a sample of incubation solution, GA and APTES were dissolved in Tris-HCl buffer solution (100 mL) and ethanol (20 mL), respectively. These two solutions were mixed into the incubation solution. The incubation time for all samples was 6 h.

Samples

GA (g)

APTES (g)

pristine MF

-

-

MF-G/A-2/0

0.2

0

MF-G/A-0/6

0

0.6

MF-G/A-2/3

0.2

0.3

MF-G/A-2/4.5

0.2

0.45

MF-G/A-2/6

0.2

0.6

MF-G/A-2/7.5

0.2

0.75

MF-G/A-2/9

0.2

0.9

2.3. Deposition of GA/APTES hybrid coating on PVDF UF membranes. PVDF UF substrate membranes were home-made according to our previous work.26 The incubation solution was prepared by mixing 100 mL Tris-HCl solution (pH 8.5, containing 0.2 g GA) and 20 mL ethanol solution (containing 0.6 g APTES) together. Then, the substrate membranes were transferred into the incubation solution and incubated at room temperature for 2 h. Then, the as-prepared membranes were rinsed with ethanol and water for several times, respectively. The membrane was denoted as “UF-G/A-2/6”.



2.4. Characterization. Attenuated total reflectance Flourier transformed infrared spectroscopy (ATR-FTIR) was measured by a Spectrum One instrument (Perkin Elmer, USA). An X-ray photoelectron spectroscopy (XPS) analysis was recorded by a spectrometer (Shimadzu AXIS Ultra DLD) with an Al-Kα X-ray source at a photoelectron take-off angle of 90o towards the specimen surface. The scanning electron microscopy (SEM) and Energy-dispersive X-ray (EDX) mapping images were collected from samples with gold prior-spraying by Hitachi S-4500 to obtain the morphology. The water contact angles were obtained using a SL 200KB measuring system. The microscopic state of emulsion and filtrate were detected by optical microscopy (BM-60XCC). 2.5. Membrane Performance. As for PVDF MF membranes, the oil-in-water emulsion was used to simulate the oily wastewater. The corresponding water permeation measurement and oilin-water emulsion separation experiments were conducted using a glass sand core filter device. The oil-in-water emulsion was prepared by mixing toluene (20 mL) and water (980 mL) with surfactant (Tween-80, 20 mg) and stirred for 3h. The pure water permeation test and emulsion separation were measured under 0.09 MPa with the assistance of a pump. As for PVDF UF membranes, the BSA solution (1 g L-1) was used to simulate the protein wastewater. BSA purification assay was conducted on a homemade dead-end UF separator (the schematic was shown in Figure S1, Supporting Information) with the transmembrane pressure of 1 bar. The water flux was calculated using Equation 1: =

(1) ×

Where P corresponds to the permeance (L m-2 h-1). V, A, and t, refer to the volume of permeate (L), effective membrane area (m2), and operation time (h), respectively. Rejection was calculated using Equation 2:


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= 1 −

× 100% (2)

Where R refers to rejection of oil or BSA. Cp and Cf corresponds to oil or BSA concentrations in permeate and feed, which were determined by a UV-vis Cintra20-GBC measurer. The oil was dyed using oil red O. The relative ratio of weight gain, a parameter to reveal the relative deposited amount of hybrid coating onto resultant membranes, was obtained by Equation 3: =

− (3)

Where Wg refers to the relative weight gain percentage (%) during the modification, W0 and W1 refer to the weight of membranes without and with modification (g), respectively. The relative ratio of water uptake, a parameter to indicate the obtained wettability of resultant membranes, was calculated by Equation 4: % =

− (4)

Where Wup% refers to the relative weight uptake percentage as with respect to the dry ones, Wwet and Wdry refer to weight of wet membranes with immersion treatment in water for 24 h and dry membranes (g), respectively. The excessive water was wiped off the wet membranes. About the protein BSA antifouling measurement, a piece of membrane (2 cm × 5cm) was tailored and transferred into an enclosed container containing 20 mL BSA solution (2 g L-1, dissolved in PBS buffer solution, pH=7.4). The membranes were shaken under light-shading treatment (the containers were wrapped with black papers) at room temperature for one day. The BSA adsorption capacity of membrane was calculated using Equation 5: " =

( − ) × (5)



Where WA refers to the adsorbed BSA amount on membrane (µg cm-2), C0 and C1 (µg L-1) correspond to BSA concentrations before and after adsorption, respectively. V and A refer to the volume of solution (L) and membrane area (cm2), respectively. 3. RESULTS AND DISCUSSION 3.1. Possible underlying mechanism during co-deposition of GA and APTES. The dopamine self-polymerization composing of the procedure of oxidation, cyclization and branching (Supporting Information, Figure S2) can give clues to the co-deposition mechanism of GA and APTES. In fact, GA with catechol hydroxyl moiety also goes through the initial oxidation and results in quinone types (Figure 2). Then, it could occur Michael addition and Schiff base reactions with the amino moieties hanging outside APTES and the hydrolysed/condensate intermediates through the covalent bonds (it could be verified by following XPS analysis), realizing the adhesive deposition of hydrophilic polysiloxane through in situ biomimetic silicification with the aid of the adhesive deposition of mussel-inspired coating. The upper left part of the final product in Figure 2 exhibit the Schiff base reaction-derived imine; the amino group attack the quinone moiety generated by oxidation of pyrogallol group. The upper right part exhibit the Michael addition occurred between the amino group (Michael donor, electrondonating group) and benzoquinone moiety (Michael acceptor. electron-attracting group). In addition, the hydrogen bonds derived from polar groups and electrostatic interactions derived from the ionization states of amino and carboxyl groups also assist the entanglement and deposition of hybrid networks.


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HO

HO

O

HO

O

O

O

Oxidation

HO HO

HO

OH

HO

HN

N

O

OH

O

O

OH

O

HO OH

+

O Si O Si NH2

Si O OH

NH2

NH2

O Si O

O

OH

O

H3N

Hydrolyzation Condensation

OH O Si O Si OH

O

O

O

Si O OH HO O

H 2N

Figure 2. The possible mechanism of one-step deposition of GA and APTES, containing construction of mussel-inspired adhesive coating and in situ biomimetic silicification of hydrophilic polysiloxane. The undulating bonds represent the omission of the extended hybrid network with the similar structure.

3.2. Surface chemistry and morphology characterization of membranes. XPS analysis was utilized to evaluate the surface chemistry evolution of membranes. As shown in Figure 3a and Table 2, surface chemical components of the membrane after only incubation of APTES exhibited no obvious change compared to that of pristine membrane. It indicated that the generated polysiloxane could not deposit on hydrophobic PVDF membrane. The same scene could also be observed from MF-G/A-2/0 owing to failure in self-polymerization of GA. After the co-deposition of GA and APTES, compositions of N, O, and Si and atom ratios of N/C, O/C, and Si/C all increased obviously compared to that of the membranes without modification and only with independent deposition of GA or APTES. The overall data demonstrated that there exists synergistic effects in the co-deposition of GA and APTES. The terminal amino moiety of APTES bridges and crosslinks GA-based derivative substances together to realize the construction of GA-based mussel-inspired network. The rest siloxane portion of APTES went



through hydrolyzation/condensation into hydrophilic polysiloxane, boosting the resultant hydrophilicity of mussel-inspired biomimetic coating. XPS patterns increased and then slightly decreased along with the additive amount of APTES. Therein, MF-G/A-2/6 demonstrated the highest atomic composition of N, O, and Si (relatively lowest F composition, 1.45% compared to 45.84% of pristine membrane), also highest atom ratios of N/C, O/C, and Si/C among the membranes with GA and APTES co-deposition (The detailed XPS fitting analysis of MF-G/A2/6 see Figure S3, Supporting Information, indicating the successful construction of the PGA nanohybrid network). These features eliminate the negative influence of hydrophobic C-F bonds from PVDF substrates and render it with a positive effect of biomimetic silicified hydrophilic nanohybrids to enhance hydrophilicity of the membrane surface. When the amount of APTES in the incubation mixture beyond 0.6 g per standard solution, the atomic compositionof N, O, and Si, and the corresponding ratio compared to C decreased slightly; the phenomenon was attributed to the enhanced odds of the homogeneous nucleation in the solution to form bigger floccus rather than onto the membrane owing to the accelerated hydrolysis of siloxane. It also resulted in similar patterns of relative weight gain of as-fabricated membranes compared to pristine membranes (Figure 3b). In addition, FTIR spectra also indicated the successful co-deposition of GA and APTES on membrane surfaces (Supporting Information, Figure S4).


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Figure 3. (a) The surface chemical composition of pristine membranes and membranes after codeposition of GA and APTES with different mass ratios. (b) Weight gain relative ratios of asprepared membranes.

Table 2. Surface element characterization of membranes involved in this study. The XPS data of optimal membrane sample MF-G/A-2/6 was bolded.



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Composition (at. %)

Atom ratio

Samples C

F

N

O

Si

N/C

O/C

Si/C

Pristine MF

51.83 45.84 0.88

1.45

-

0.017 0.028

-

MF-G/A-2/0

54.36 41.92 1.34

2.38

-

0.025 0.044

-

MF-G/A-0/6

52.55 44.42 1.16

1.72

0.14 0.022 0.033 0.003

MF-G/A-2/3

53.01 40.19 1.79

4.44

0.57 0.034 0.084 0.011

MF-G/A-2/4.5 57.79 10.82 5.60 20.25 5.55 0.097 0.350 0.096 MF-G/A-2/6

57.73

1.45

6.93 26.00 7.89 0.120 0.450 0.137

MF-G/A-2/7.5 58.83

3.31

6.64 23.49 7.73 0.113 0.399 0.131

MF-G/A-2/9

3.57

6.29 23.42 7.87 0.107 0.398 0.134

58.86

In addition, surface morphology variation was demonstrated by SEM (Figure 4). As for the pristine MF membrane (Figure 4a), the micro-morphology was porous and the polymer substances were featured by sharp edges and corners. The independent deposition of GA or APTES demonstrated no significant change in micromorphology compared to that of pristine membrane (Figure 4a-c). Interestingly, after hybrid coating deposition, as for MF-G/A-2/3, some scattered nanoparticles arose on the membrane surface (Figure 4d). It was ascribed to the formation of mussel-inspired biomimetic coating and hydrolysis of siloxane. As for MF-G/A-2/6 and MF-G/A-2/9 (Figure 4e, f), the coating layer wrapped outside the sharp-shaped polymer substances is more obvious, evolving the micromorphology into one with obtuse shaped edges. Meanwhile, all modified membranes still maintained the porosity. In addition, the corresponding EDX mapping images also exhibited the distribution of typical element (element N, O, and Si; O derived from GA and APTES, N and Si derived from APTES) dots, indicating the uniformity of


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the coating (Figure 4g-i). Furthermore, the leaching and bending tests were also conducted on MF-G/A-2/6 to evaluate the stability of nanohybrid coating deposits (Supporting Information, Figure S5 and Figure S6). As a result, the deposited coating exhibited a benign stability even after vigorous rinsing or bending for 1000 times, which was benefited by the robust bio-adhesion derived from mussel-inspired coating.

Figure 4. SEM images of surface morphology of (a) pristine PVDF MF membrane and membranes only (b) after GA incubation or (c) APTES incubation, membranes (d-f) after codeposition of GA and APTES with various mass ratios. (g-i) The corresponding EDX mapping images (elements N, O, Si) of surface morphology of MF-G/A-2/6.

3.3. Wettability of membranes. Water contact angles (WCA) and water uptake ratios are two critical parameters to evaluate the wettability of membranes.30 As shown in Figure 5, the



membranes without modification and with independent treatment of GA or APTES all exhibited hydrophobic (WCAs>100o); therein, WCA of MF-G/A-2/0 demonstrated a slight decline owing to a small amount of attached hydroxyl moieties (it can refer to XPS data). After the codeposition, corresponding WCAs obviously declined, indicating the as-deposited nanohybrid coating significantly contributed to the enhanced wettability. In addition, the real-time record of WCAs indicated that PGA-modified membranes exhibited the greatly enhanced water infiltration ability across the membranes. It could be interpreted as the channel inside the membrane has been fully modified from highly hydrophobic to highly hydrophilic. As shown in the inset (visible state of water droplets on membranes) from Figure 5, water could simultaneously and instantaneously penetrate through MF-G/A-2/6 during the brief moment of spreading wetting on the membrane surface. Furthermore, water uptake ratios were evaluated for water affinity to entire membrane. As for MF-G/A-2/3, the improvement percentage of the water uptake ratio is higher than that of weight gain. It demonstrated the great potential of hydrophilicity derived from hybrid coating. The variation pattern is similar to that of weight gain ratios.


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Figure 5. (a) The real-time water contact angles on pristine and modified MF membranes. The insets are the digital images of water droplet status on pristine MF and MF-G/A-2/6 based on views from both sides. (b) Relative water uptake of as-prepared membranes.

3.4. Oil-in-water emulsion separation performance of PVDF MF membranes. The PGA hybrid coating deposited on membranes endowed it with benign hydrophilicity. This feature allowed for water penetration and oil repelling during membrane filtration, enabling oil-in-water



emulsion separation. The fluxes of pure water were measured to evaluate the water infiltration performance of the membranes (Figure 6). The pure water flux of MF-G/A-2/6 reached as high as ca. 9246 L m-2 h-1; it is nearly 54-fold compared to that of pristine membrane, 18-fold to that of MF-G/A-2/0, 37-fold to that of MF-G/A-0/6, respectively. It was attributed to the synergistic effect with integration of adhesive mussel-inspired biomimetic hybrid network and in situ biomimetic silicification-induced hydrophilic hydrolysed/condensed polysiloxane portion. As a demonstration, MF-G/A-2/6 was used to conduct the toluene-in-water emulsion separation. The real-time separation flux during the purification of oil-in-water emulsion was shown in Figure 7a (the shapes of water and underwater oil (toluene) droplets on membrane surface were demonstrated in the inset; MF-G/A-2/6 is super-hydrophilic (ca. 7.5o) and underwater superoleophobic (ca. 153o)). There exhibited a rapid decline in filtrate flux in the initial stage owing to the possible occurrence of a filter cake.30, 51 With the time going on, the decline rate of filtrate flux became slower until a nearly constant state. In addition, two more cycles of similar emulsion separation were conducted to investigate the reutilization of the measured membrane. One can observe, the simple water rinsing treatment to the membrane after each cycle of test validly recovered the membrane filtration performance. About the underlying reason, the water film formed and wrapped outside the membrane surface via the strong interactions between the polar moieties of the membrane surface and water molecules, and the typical feature of underwater super-oleophobicity all contributed to the results. It commendably repelled access of oil droplets to the membrane surface. Thus, it displayed the benign flux recovery to similar level compared to previous separation and guaranteed the reusability of as-fabricated membrane.


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Figure 6. The water flux of pristine and as-prepared membranes.

Figure 7. (a) The real-time flux during a 3-cycle toluene-in-water emulsion separation using MF-G/A-2/6 under the negative pressure of 0.09 MPa. (b) Separation device and optical images of (c) oil-in-water emulsion feed and (d) filtrate screened by MF-G/A-2/6.

In addition, the separation device was displayed in Figure 7b. In detail, the milky emulsion feed was purified into a transparent aqueous solution with the aid of the wetting-induced screening effect of MF-G/A-2/6. Furthermore, the optical microscopy images of the collected 19 Environment ACS Paragon Plus


filtrate exhibited the single phase compared to the biphasic one derived from feed (Figure 7c, d) (microscopy images of filtrate during every cycle see Figure S7, Supporting Information). Also the rejection rates of oil toluene after every cycle were displayed in Figure S8 (Supporting Information), all beyond 99.5%. It indicated that MF-G/A-2/6 successfully realized the oil-inwater emulsion separation, indicating the benign hydrophilicity brought by as-deposited PGA nanohybrids on PVDF membrane substrates. In addition, the aliphatic oil hexadecane was also used to simulate the oily emulsion wastewater. The 3-cycle separation toward hexadecane-inwater emulsion separation was also conducted, the results (The real-time fluxes, photos and macroscopic images of feed and filtrate see Figure S9, Supporting Information) indicated the benign feasibility of MF-G/A-2/6 toward oil hexadecane-in-water emulsion separation. 3.5. Protein interception performance of PVDF UF membranes. In addition, the homemade PVDF UF membrane was used to demonstrate the flexibility of co-deposition strategy towards the application of protein interception (FTIR results see Figure S10, Supporting Information). As shown in Figure 8a, a water droplet quickly spread on the membrane surface, instantly penetrated through the cross-section of PVDF UF membrane after GA and APTES co-deposition (UF-G/A-2/6), and wetted the back surface compared to the unchanged spherical state of water droplet on pristine PVDF UF membrane. It was attributed to the excellent wettability derived from the deposited hybrid coating of the membranes. The corresponding detailed wetting data is displayed in Figure 8b. The WCA (ca. 89o) of pristine PVDF UF membrane was almost unchanged within one minute, compared to that of UF-G/A-2/6 which dropped from ca. 12o to 0o within two seconds. The BSA solution was used to simulate the protein wastewater. The filtration performance of as-prepared UF membranes to BSA solution revealed the purification and antifouling ability toward the protein wastewater. Thus, the infiltration performance of UF-


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G/A-2/6 was investigated as a representative as compared to that of the pristine membrane. As shown in Figure 8c, the unmodified home-made UF membrane exhibited water flux of ca. 382.0 L m-2 h-1 and BSA rejection rate of ca. 54.6%. As for UF-G/A-2/6, the rejection rate to BSA reached as high as ca. 96.6% giving a water flux of ca. 278.2 L m-2 h-1. It indicated that the membrane after PGA deposition exhibited the benign protein BSA interception and water purification to the simulated protein wastewater.

Figure 8. Wettability and BSA resistance. (a) Water droplet status of top and corresponding back surfaces and (b) water contact angle evaluation on the surfaces of pristine PVDF UF membrane and UF-G/A-2/6 membrane, and the corresponding (c) filtration performance and (d) BSA adsorption evaluation.



Furthermore, the anti-fouling ability of membrane to protein BSA is an important evaluation parameter for membrane tolerance. The benign anti-fouling capacity demonstrates the benefits of maintaining separation performance and reduction of operation cost. Thus, dynamic protein adsorption measurement was conducted (Figure 8d). The BSA adsorption parameter of UF-G/A2/6 (ca. 19.9 µg cm-2) decreased by ca. 70.8% compared to that of pristine UF membrane (ca. 68.2 µg cm-2). The significantly enhanced antifouling property is benefited by the stabilized water film formed on as-deposited hydrophilic coating layer owing to the affinity of water molecules to the as-deposited polar moieties and hydrophilic silicified product.52 The water film validly defends the BSA molecules from approaching and depositing, thus greatly reducing the probability of pollutant BSA molecule irreversible adsorption and fouling to the membrane surface. 4. CONCLUSIONS This study integrates the adhesive deposition of cost-effective mussel-inspired biomimetic coating and introduction of silicification of hydrophilic siloxane together through covalent “amino bridge”. The developed in situ biomimetic silicification strategy brought the wettability transformation on widely-used PVDF MF/UF membranes from typical hydrophobicity to high hydrophilicity via co-incubating pristine membranes in an aqueous buffer solution containing plant-derived phenol GA and amino-terminated APTES. The surface chemistry, morphology, and weight gain ratios were measured to optimize the incubation conditions. The synergistic effect of hydrophilicity enhancement brought by co-deposition was verified according to the contrast deposition only with independent deposition of GA or APTES. The as-fabricated membranes after hybrid coating deposition exhibited the excellent hydrophilicity and water infiltration ability, thus were applied to conduct membrane-based water remediation from oil-in-


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water emulsion and protein wastewater. The optimal co-incubation condition endowed the membrane with significantly enhanced affinity for the water film attached on the membrane surface to defend the oil droplet and protein BSA molecule from approaching. The optimal MF membrane demonstrated the greatly enhanced water flux (ca. 9246 L m-2 h-1), the excellent oilin-water emulsion separation performance and reusability that membrane performance could be recovered just by water rinsing. The as-fabricated UF one demonstrated its benign performance on BSA protein interception (rejection of ca. 96.6% while giving a water flux of ca. 278.2 L m-2 h-1) and antifouling potential (ca. 70.8% increment in anti-adsorption of BSA).

ASSOCIATED CONTENT Supporting Information. The homemade UF separator, PDA deposition mechanism, XPS fitting analysis of MF-G/A-2/6, FTIR spectra results, leaching test, bending test, microscopic images of filtrate after every cycle of oil-in-water emulsion separation. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank for the financial support from National Natural Science Foundation of China (21676063), Open Project of State Key Laboratory of Urban Water Resource and Environment (Harbin Institute Technology) (No. HC201706), and HIT Environment and Ecology Innovation Special Funds (HSCJ201619). 23 Environment ACS Paragon Plus



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Biomimetic Silicification on Membrane Surface for Highly-efficient

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