Catecholic Coating and Silver Hybridization of Chitin Nanocrystals for

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Catecholic coating and silver hybridization of chitin nanocrystals for ultrafiltration membrane with continuous flow catalysis and gold recovery Yanwei Wang, Luting Zhu, Jun You, Fushan Chen, Lu Zong, Xiaofei Yan, and Chaoxu Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02633 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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ACS Sustainable Chemistry & Engineering

Catecholic coating and silver hybridization of chitin nanocrystals for ultrafiltration membrane with continuous flow catalysis and gold recovery Yanwei Wang, †,1,2 Luting Zhu, †,1 Jun You,1,* Fushan Chen,2 Lu Zong,1 Xiaofei Yan,1 and Chaoxu Li1,*

1

CAS Key Lab of Biobased Materials, Qingdao Institute of Bioenergy and Bioprocess Technology,

Chinese Academy of Sciences, Songling Road 189, Qingdao 266101, P. R. China. 2

Key Laboratory of Sensor Analysis of Tumor Marker Ministry of Education, College of Chemistry

and Molecular Engineering, Qingdao University of Science & Technology, Zhengzhou Road 53, Qingdao 266042, P. R. China.

E-mail: [email protected] (C. Li); [email protected] (J. You)

ACS Paragon Plus Environment


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ABSTRACT Despite great advantages of chitin for water purification, current researches have lacked the intensive study for the utilization of chitin in high-flux filters. To tackle this, we proposed, for the first time, a facile method for the construction of multifunctional nanoporous chitin ultrafiltration membranes derived from renewable marine resources (shells of shrimp and crab). Chitin nanocrystals with average diameter and length of 43.3 nm and 446.1 nm were prepared by H2SO4 hydrolysis, and a simple vacuum-filtration method was utilized to convert these uniform dispersed nanocrystals to nanoporous membranes. In order to greatly improve the permeation flux and functionality of the filtration membranes, a bio-inspired dopamine coating procedure was adopted to intercalate high content silver nanoparticles (57.2 wt%) into the chitin nanocrystal matrix, which might construct interfacial regions and foster low-resistance channels for enhancing solvent permeability. The resulting hybrid chitin membranes possessed numerous interconnected nanopores and its thickness could be easily tuned from 100 to 4000 nm by the volume of filtered nanocrystal suspensions. They allow fast permeation of water during vacuum assisted filtration. Typically, the flux of 100 nm thick hybrid filtration membranes with ~ 4 nm cut-off was up to 13400 L m-2 h-1 bar-1, which was nearly 3 orders of magnitude higher than that of commercial filtration membranes. More importantly, the hybridization of Ag nanoparticles also offered the membranes with continuous flow properties, e.g. super catalytic activity and substitution reactivity in decomposing toxic organic pollutants and recycling noble Au ions in water. Keywords: Chitin nanocrystal, catecholic chemistry, Ag nanoparticles, ultrafiltration membranes, water purification


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INTRODUCTION Ultrafiltration has attracted global attentions in diverse applications involving concentration, separation, purification and recovery of nanomaterials and macromolecules.1,2 In particular, in comparison to other methods (e.g. flocculation, adsorption, reverse osmosis and nanofiltration3-7) to remove pollutants (e.g. dyes, colloid etc.) for water purification, nanoporous ultrafiltration showed its advantages of high efficiency, flux and rejection. In order to further achieve cost-effectiveness, energy-conservation, environmental friendliness and time-saving, much endeavor has been made to produce sustainable ultrafiltration membranes with excellent separation performance, high permeation flux and solvent resistance.8 Biomass has particularly been engineered for nanoporous ultrafiltration membranes to simultaneously realize water purification and recovery of natural waste.9 In general, nanoporous filtration membranes are composed of a thin skin layer and a porous support. The thin skin layer was frequently constructed by various organic/inorganic nanomaterials with high specific surface area, e.g. Au nanoparticles,10 carbon nanotubes,11 protein nanofibers,8, 12-15

cellulose nanofibers,16 graphene,17 clay18 and transitional metal dichalcogenides.19 Among these

0~2 dimensional (0~2D) nanomateirals, 1D nanomaterials were able to form the membranes with interconnected open pore structures, which were beneficial to produce relatively high permeation flux.20 In sharp contrast, density packed nanoparticles and lamellar stacked nanosheets offered the membranes with low porosity and hereby low permeation flux. Nanofibers of synthetic polymers were firstly used to construct porous ultrafiltration membranes.21 Nanofibers of polysaccharides also attracted great interest on account of their unique combination of low-cost, widely-availability and benign environmental nature. For instance, sub-10 nm cellulose nanofibrils were employed to prepare ultrafiltration membranes with nano-pores from



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2.5 to 12 nm, which showed a high flux of 1.14×104 L h−1 m−2 bar−1 for pure water and excellent solvent resistance.9 Chitin nanofibrils were also used to produce ultrafiltration membranes, which, though having ~99.5% rejection ratio for ultrafiltration of oil/water emulsions, had their flux limited to 104.8 L h−1 m−2 bar−1.22 Moreover, lack of active groups for surface functionalization further hindered wide applications of these polysaccharides in binding and removing desired contaminants for water treatment.23 It remains challenged in sustainable purification technology to design polysaccharides-based membranes with the combination of high permeation flux, functionality, selectivity and solvent resistance. Chitin is one type of the most abundant marine polysaccharides in nature, widely distributing in crustacean shells, insect exoskeletons, and fungal cell walls.24 Due to the excellent advantages of non-toxic, biocompatibility, bio-degradability and high strength, chitin has attracted significant attention in the field of biomedicine, electronics, energy storage and agriculture.25-28 However, recent researches have lacked the intensive study for the utilization of chitin in high-flux filters. Herein we showed that chitin nanocrystals (ChNCs) were able to construct ultrafiltration membranes with the cutoff of 4 nm and the flux of 13 400 L m-2 h-1 bar-1. Chitin was selected not only due to its low-cost (e.g. from mariculture waste), environment-friendliness and natural abundance, but also due to its high structural stability under various organic/inorganic environments. Mussel-inspired catecholic chemistry was adopted to modify chitin nanocrystals, which offered the feasibility of hybridizing different nanomaterials to functionalize and improve performance of the ultrafiltration membranes. For example, Ag nanoparticles, when being in-situ reduced and bound homogeneously on chitin nanocrystals, might improve the flux of the ultrafiltration membranes by constructing their interfacial regions and fostering low-resistance channels for solvent permeability.17,29 More importantly, the hybridization of Ag nanoparticles also offered the


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membranes with continuous flow properties, e.g. super catalytic activity and substitution reactivity in removing organic pollutants and recycling Au ions in water. Thus by combining chitin nanocrystal

with

catecholic

chemistry

and

Ag

nanoparticles,

high

permeation

flux,

multi-functionality, stability and solvent resistance were successfully integrated into this novel type of ultrafiltration membranes.

EXPERIMENTAL SECTION Materials. Chitin powders were supplied by Golden-Shell Biochemical Co. Ltd (Zhejiang, China). Dopamine hydrochloride was purchased from Shanghai Jinsui Bio-Technology Co., Ltd. Tris (hydroxymethyl) aminomethane (Tris) was supplied by Sigma−Aldrich. All other reagents (HCl, NaOH, H2SO4, AgNO3, NaBH4, 4-nitrophenol etc.) were purchased from Sinopharm Chemical Reagent Co. Ltd. and used without further purification. All aqueous solutions were prepared with deionized water. Preparation of Chitin Nanocrystals (ChNCs). Chitin nanocrystals were prepared by sulfuric acid hydrolysis according to the literature.30 Typically, 3g of chitin powders were subjected to hydrolysis by H2SO4 aqueous solution (15~40 wt%) at 90 oC for 6 h with vigorous stirring. After a large amount of deionized water was added to stop the hydrolysis, the resultant suspension was centrifuged at 9000 rpm for >3 times to remove the acid and soluble hydrolyzates. Finally, the collected product was dialyzed against deionized water and stored at 4 °C. Catechol Activation of Chitin Nanocrystals (PDA(ChNCs)). 200 mL of ChNCs suspensions (2 mg/mL) was firstly adjusted to pH~8.0 by adding 0.24 g of Tris. After 0.4 g of dopamine hydrochloride was added, the catechol activation proceeded for >1 day at room temperature. Finally, the gray reaction products were purified by centrifuging at 6000 rpm for 5 min, and stored at 4 °C. Immobilization of Silver Nanoparticles (PDA(ChNCs)Ag). 20 mL of PDA-ChNC suspension ACS Paragon Plus Environment


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(0.26 mg/mL) was mixed with equivoluminal AgNO3 solution with designed concentrations under magnetic stirring. Thereafter, tris (hydroxymethyl) aminomethane was added dropwise into the mixture to provide weak alkali environment. After the reaction proceeded for 48 h under room temperature, the product was purified by centrifuging at 8000 rpm for 5 min, and washed for three times with deionized water. By changing the concentration of AgNO3 solution from 0.026 to 2.6 mg/mL, seven samples with different Ag content were obtained and coded as PDA(ChNCs)Ag. Membrane Fabrication Procedures. PDA(ChNCs)Ag was re-dispersed in Tris buffer solution to form a homogeneous suspension. A desired volume of the suspension was filtered with a cellulose nitrate filter membrane (pore size: 220 nm). During the filtration, PDA(ChNCs)Ag were assembled on the filtration membrane and form an ultrathin nanoporous membrane. The thickness of the nanoporous membrane was controlled by the filtration volume of the PDA(ChNCs)Ag suspension. Separation procedure. Separation performance of the membranes was evaluated using a vacuum filtration device. Deionized water (100 mL) was filtered across the filtration membrane to determine the pure water flux (J, L m−2 h−1 bar−1) according the equation J=0.1/(Atp), where A is the area of composite membrane (m2), t is the filtration time (h), and p is the applied suction pressure (bar). Gold nanoparticles (5 nm & 20 nm), metal ions and dyes were used to evaluate the membrane’s rejection and flow catalysis performance. The concentrations of gold nanoparticles and dyes were determined by UV−vis spectrophotometer. The concentrations of metal ions were characterized through a conductivity meter (FE30, Mettler-Toledo, Switzerland). The rejection (R, %) is calculated by R=(1-cp/cf)×100%, where cf and cp are the concentrations of compounds in the feed and permeate, respectively. Characterization. Scanning electron microscopy (SEM) was carried out with a Hitachi S-4800 instrument operated at 10 KV. The samples were sputtered with Au (thickness of ~3 nm) for


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observation. Transmission electron microscopy (TEM) measurement was performed on a Hitachi TEM (H-7650) instrument operating at a voltage of 100 kV. The samples were prepared by dropping the solutions on carbon-coated Cu grids followed by air-drying. X-ray diffraction (XRD) measurements were taken on a Bruker D8 ADVANCE X-ray diffractometer with a CuKα radiation (λ=1.5418A). Powder was leveled on sample holders and scanned with a 2θ angle from 10 to 85o with a step speed of 5o /min. FTIR spectra were recorded on a Nicolet 6700 Fourier transform infrared spectrometer. The specimens were prepared by the KBr-disk method. Thermal gravimetric analysis (TGA) was performed on a thermogravimetric analyzer (Ulvac TGD 9600). The samples were heated under dry nitrogen purge at a flow rate of 10 ml/ min from room temperature to 800℃ at the ramp rate of 10 oC/min. UV-Vis spectroscopy was performed on a DU800 UV-Vis spectrophotometer. All the solutions were diluted to appropriate concentrations and scanned in 1 cm-path-length quartz cuvettes.

RESULTS AND DISCUSSION Due to strong H-bonding and closely packing of chitin nanocrystals, the filtration membranes produced from ChNCs normally have low flux. Hybridization of nanoparticles (NPs) may be promising to solve this issue by creating extra porosity among ChNCs and fostering low-resistance channels for solvent permeability. Many unique properties of NPs, e.g. catalytic activity and selective reduction for Ag NPs, further offer additional dimensions to functionalize the filtration membranes for broad applications. However, as a typical type of marine polysaccharide, chitin lacks sufficient active groups for its strong affinity to NPs. In order to hybridize ChNCs with Ag NPs, a mussel activation method was utilized to modify chitin nanocrystals. Thus a combinational approach involving catecholic chemistry (dopamine coating) and hybridization of Ag NPs was employed to fabricate the ultrafiltration membranes from ChNCs (Scheme 1). ACS Paragon Plus Environment


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H2SO4 hydrolysis was adopted to produce ChNCs, which could introduce large amounts of negatively charged SO3− half-ester groups. The concentration of H2SO4 was optimized as 30 wt% to achieve the proper yield, aspect ratio and stability of ChNCs (Figure 1a, S1 & S2). Figure 1b shows TEM image and vision observation of the resultant ChNCs. Unambiguously they had the characteristic rod-like morphology with the average diameter of ~43.3 nm and the length of 446.1 nm, which produce a calculated aspect ratio of ~10. Due to the presence of SO3− groups, these ChNCs also had super dispersity in both water and buffer solution. In contrast, ChNCs obtained from HCl hydrolysis or NaOH deacetylation had the pKa value of ~6.3,30 which may show serious flocculation in the buffered solutions (pH=8.5) for subsequent dopamine coating (Figure S1a). The dopamine coating reaction was proceeded by optimizing the weight ratio of dopamine:ChNCs at 1:1. During the coating process, the suspension of ChNCs changed from light blue to pink due to oxidation of the catechol groups in dopamine molecules. The final black color indicated the successful polymerization of dopamine. UV spectroscopy, FT-IR spectra and TG were further used to confirm this reaction. As shown in Figure 2a, ChNCs had the characteristic peaks of FTIR absorption at 3445 cm-1 for –OH stretching, 3268 cm-1 for –NH– stretching, 1670 cm-1 and 1622 cm-1 for amide I, and 1552 cm-1 for amide II.31 After coating with PDA, the peaks at 3268 cm-1 and 1552 cm-1 intensified greatly due to the introduction of abundant primary amine groups within the polydopamine layers. In addition, a new peak at 1507 cm-1 might ascribe to the sp2 hybridized C−C bond within the aromatic ring of dopamine.32 Meanwhile, an small peak of UV absorption at 283.5 nm (Figure 2b), corresponding to the La-Lb transition of dopamine structures, emerged in the UV spectrum of polydopamine coated ChNCs (i.e. PDA(ChNCs)).33 All these results verified the presence of abundant catechol groups on ChNCs. The quantitative characterization of the PDA layer was carried out by thermogravimetric analysis (TGA) in nitrogen gas (Figure 2c). The final weights


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of ChNCs, PDA(ChNCs) and pure PDA at 800 °C were 31.4 wt%, 37.2 wt% and 45.6 wt%, respectively. These values calculated the content of PDA (ϕPDA) within PDA(ChNCs) as 40.8 wt% according the equation of 0.314(1- ϕPDA) +0.456 ϕPDA =0.372. Figure 2d shows XRD patterns of ChNCs and PDA(ChNCs). All the samples exhibited similar diffraction peaks at 2θ of 12.7 ﾟ, 19.3 ﾟ, 20.7 ﾟ, 23.3 ﾟ and 26.3 ﾟ, attributed to the (021), (110), (120), (130) and (013) crystal plane of α-chitin.31 These diffraction peaks suggested that the crystal structure of ChNCs remained unchanged during the dopamine coating process. Figure 2e & 2f showed that these rod-like morphologies of ChNCs had slight increase in their average diameter (64.5 nm) and length (485.6 nm). Thus the thickness of the PDA layer was calculated to be ~10 nm for PDA(ChNCs). The PDA layer of PDA(ChNCs) could not only reduce Ag+ into Ag NPs as the reducing agent, but also provide strong adhesive force for immobilization of Ag NPs. As shown in Figure S3, when mixing the ChNCs suspension with Ag+ solution at room temperature, Ag+ was hardly reduced to Ag NPs. When replacing ChNCs with PDA(ChNCs), Ag NPs distributed densely and uniformly on PDA(ChNCs) (Figure 3a). The XRD spectra in Figure 3b show five diffraction peaks at 2θ of 38.0º,44.1º,64.4º,77.4º and 81.5º, corresponding to the diffraction of the (111), (200), (220), (311) and (222) crystalline planes of the cubic phase of Ag NPs. These further confirmed the reduction of Ag+ to Ag NPs. The amount of Ag NPs immobilized on PDA(ChNCs) could be regulated by the Ag+ concentration. As shown in Figure 3c, the peak located near 415 nm in the spectrum is a typical peak for Ag NPs.34 With an increase of Ag+ concentration, this maximum absorbance dramatically intensified, suggesting the increasing amount of immobilized Ag NPs. The TEM images in Figure S4 also showed that more Ag NPs were produced on PDA(ChNCs) with the higher Ag+



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concentration. The amounts of Ag NPs on PDA(ChNCs) were quantitative determined by TGA in Figure 3d. If setting the weight percentage at 800 oC as x, the Ag content (ϕAg) was calculated according to the equation ϕAg+0.372 (1- ϕAg)= x, where 0.372 is the weight percentage of PDA(ChNCs) at 800 oC (Figure 2c). The calculated content of Ag NPs could be tuned within a broad range of 4.8~57.2 wt%. A simple vacuum-filtration apparatus was utilized to produce ultrafiltration membranes from PDA(ChNCs) hybridized by Ag NPs (i.e. PDA(ChNCs)Ag). The filter membrane of cellulose acetate was chosen as the porous support. The skin layer produced by this method usually shows good homogeneity, stability and strength.35 Indeed, during the vacuum-filtration process, the high-aspect-ratio PDA(ChNCs)Ag readily deposited on the support from their dispersion and formed smooth and porous nanostructures. As shown in Figure 4a, the as-fabricated membrane showed smooth surface. The SEM image in Figure 4b gives the surface-view of the membrane (thickness of 2.6 µm). The surface (Figure 4c) and cross-section (Figure 4b) SEM images indicate that the skin layer had numerous interconnected nanopores, in sharp contrast to compact membranes prepared from ChNCs and PDA(ChNCs) in Figure S5. This is because that Ag NPs immobilized on PDA(ChNCs) might construct interfacial regions and foster low-resistance channels. Such distinctive structures were expected to improve the permeation flux of these ultrafiltration membranes. Moreover, as shown in Figure 4d, the thickness of the resultant ultrafiltration membrane could be tuned from 100 nm to 2100 nm by the filtered volume of PDA(ChNCs)Ag suspensions. In order to investigate the effect of Ag NPs on the permeability of these ultrafiltration membranes, pure-water permeability was tested in Figure 5a. The ultrafiltration membrane (thickness of 4 µm for the skin layer) produced from ChNCs had a quite low water flux (100 L m-2


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h-1 bar-1). When replacing ChNCs with PDA(ChNCs)Ag, the flux increased ~4 times up to ~400 L m-2 h-1 bar-1. By increasing the content of Ag NPs within PDA(ChNCs)Ag, as shown in Figure 5b, the flux of the filtration membranes (thickness of 500 nm for the skin layer) could be further promoted. Thus it can be inferred that the introduction of Ag NPs would be a key factor to improve the permeation flux of this chitin-derived types of ultrafiltration membranes. This is also in agreement with the nanoporous nanostructures induced by hybridization of Ag NPs, as illustrated in Figure 4. It is well known that the flux of ultrafiltration membranes was greatly affected by their thickness. For the membranes produced by PDA(ChNCs)Ag, as shown in Figure 5c, the water flux dramatically decreased with the thickness, which could be explained by Hagen−Poiseuille theory. Impressively, the flux of 13 400 L m-2 h-1 bar-1 was achieved by the ultrafiltration membrane with the 100-nm-thick skin layer, which is nearly 3 orders of magnitude higher than that of commercial filtration membranes,36,37 and also much higher than that of other reported ultra-thin filtration membranes.8-10, 38 Even when increasing the thickness of the skin layer up to 4.5 µm, the filtration membrane still maintained a water flux up to 400 L m-2 h-1 bar-1, being comparable to most of commercial membranes. Beside these, due to strong mutual interaction between Ag NPs and ChNCs, the flux of these membranes has also super stability for cyclic uses (Figure 5d) as well as under harsh conditions (Figure S6). The separation performance of these chitin-derived filtration membranes were evaluated through pressure-driven filtration. Their pore sizes (i.e. cut-off) were determined by using Au NPs with different sizes (Figure 6a-c) as filtration objects. As shown in Figure 6d, the membrane rejection (with the thickness of the skin layer of 100 nm) reached up to >90 % for Au NPs with the size of 20 nm. The DLS analysis in Figure 6e indicates that these Au NPs hardly passed the



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membrane. For Au NPs with the size of 5 nm, the membrane showed much lower rejection

Catecholic Coating and Silver Hybridization of Chitin Nanocrystals for

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