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Wood-based Mesoporous Filter Decorated with Silver Nanoparticles for Water Purification Wenbo Che, Zefang Xiao, Zheng Wang, Jian Li, Haigang Wang, Yonggui Wang, and Yanjun Xie ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06001 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 15, 2019

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ACS Sustainable Chemistry & Engineering

Wood-based Mesoporous Filter Decorated with Silver Nanoparticles for Water Purification

Wenbo Che, Zefang Xiao, Zheng Wang, Jian Li, Haigang Wang, Yonggui Wang*, Yanjun Xie*

Key Laboratory of Bio-based Material Science and Technology (Ministry of Education), College of Material Science and Engineering, Northeast Forestry University, 150040, Hexing Road 26, Harbin, P. R. China

*Corresponding authors: [email protected] (Y. Wang); [email protected] (Y. Xie)

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ABSTRACT Wood is a versatile raw material valued highly for its abundance, low cost, biocompatibility, and natural composition. It has drawn increasing interest for its uses in green electronics, biological devices, and energy storage. Meanwhile, its potential application in water purification has not been adequately explored. This study reports the development of a wood-based filter decorated with silver nanoparticles (Ag NPs) and its application for water purification. The X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) confirmed the successful loading of Ag NPs on the surfaces of the wood mesoporous network. The effects of Ag NP content and filter thickness on the decomposition of methylene blue (MB) and bacterial removal were evaluated. The prepared Ag/wood filter can remove more than 98.5% of MB via physical adsorption and catalytic degradation. After the Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) suspensions passed through the Ag/wood filter, the inactivation and removal of E. coli and S. aureus reached up to 6 and 5.2 orders of magnitude, respectively. The findings demonstrate that the prepared Ag/wood filter, which is biomass based and easy to handle, has a potential for point-of-use water purification. Keywords: silver nanoparticles; porous wood; water purification; point-of-use

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INTRODUCTION The shortage of clean and safe drinking water is one of the major problems in developing countries.1 Biological contaminants in polluted water mainly cause the spread of waterborne diseases, which have been among the largest threats to human health.2 Point-of-use water treatment is usually the most effective method because of problems of microbial regeneration, by-products of disinfectants, pipeline corrosion, and pollution in the water supply system.3,4 Conventional point-of-use water disinfection techniques, including chlorination, ultraviolet disinfection, pasteurization, and boiling, have been intensively investigated, both theoretically and experimentally.3,5,6 However, several disadvantages limit the application of these methods, including low efficiency, time required to complete procedures, and the need for specialized equipment. New materials and approaches need to be identified or developed for low-cost, readily available, disposable, and effective point-of-use water sterilization in order to complement existing techniques under certain situations, such as during an emergency response or in remote areas.7 Nanotechnology is a promising approach to improving the effectiveness of water purification in many areas worldwide.8-10 Silver nanoparticles (Ag NPs) are of great interest because of their chemical catalytic ability, stability, antimicrobial properties, and low toxicity to humans.11 Owing to their larger surface area-to-volume ratio and higher reactivity compared with those of the bulk solid, Ag NPs exhibit excellent antimicrobial properties, hence their wide application as antimicrobial agents in diverse areas, particularly in water purification, by enhancing the performance of filtration materials. Generally, Ag NPs need to 3



be synthesized within porous carriers to prevent agglomeration and to achieve efficient loading. These porous carriers are generally artificial structures, such as polyurethane foams, polyamide membrane, polysulfone membrane, woven fabric, paper, and ceramic.12-19 After loading the Ag NPs, a series of point-of-use water filtration materials would be constructed for water purification. However, a natural porous structure (e.g., wood) as a carrier to construct functional composites is still rarely used. In recent years, considerable interest in the use of bioinspired wood nanotechnology for functional materials has been observed.20-22 Wood, a natural porous cellulosic material with a distinct hollow structure and membranes comprising nanoscale pores, has shown great potential for filtration and separation applications.23 Boutilier et al. prepared xylem filters by simply removing the bark of pine tree branches and inserting the xylem tissue into a tube.24 The xylem filter could effectively filter out bacteria from water with a rejection exceeding 99.9%. This is due to the presence of numerous pit membranes as the functional unit for the physical barrier of bacteria. This separation using sole wood is achieved by size exclusion of the bacterial, so it is incapable for purification of water containing small-size contaminants. In order to improve the purification capacity, various functional substances, such as palladium nanoparticles and gold nanoparticles, have been incorporated to modify the mesoporous structure of wood filter.25,26 However, to the best of our knowledge, wood with a porous structure loaded with Ag NPs has not been used in water purification. Accordingly, this study investigated the incorporation of Ag NPs into mesoporous wood to form a disposable wood-based filter (Ag/wood) for sustainable water puriﬁcation in emergency situations. The resulting Ag/wood filter could efficiently remove Escherichia coli 4


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(E. coli) and Staphylococcus aureus (S. aureus) from the water based a superimposed effect of the physical barrier feature of the pit membrane and the anti-bacterial properties of Ag NPs. The incorporated Ag NPs can also effectively catalyze the degradation of methylene blue (MB) in a fluid solution.

EXPERIMENTAL Materials and chemicals Radiata pine (Pinus radiata Don., softwood) sapwood was cut into samples measuring 3 mm × 30 mm × 30 mm, 5 mm × 30 mm × 30 mm, and 10 mm × 30 mm × 30 mm (L× T×R) for the preparation of wood filters. Silver nitrate (AgNO3), sodium citrate, MB, sodium borohydride (NaBH4), hydrochloric acid (HCl), sodium hydroxide (NaOH), and ethanol were supplied by Jiangtian Chemical Technology Co., Ltd. (Tianjin, China). Phosphate buffered saline, lysogeny broth (LB), agar, and the staining dyes fluorescein isothiocyanate (FITC) and propidium iodide were purchased from Solarbio (Beijing, China). E. coli (ATCC 25922, Gram-negative bacteria) and S. aureus (ATCC 22004, Gram-positive bacteria) were cultured at 37 ̊C in LB for subsequent use. Deionized water was used in all experiments. Preparation of Ag/wood filter The Ag/wood filter was prepared via in situ synthesis of Ag NPs in the wood (Scheme 1) in accordance with a previous report.27 A precursor aqueous solution was prepared by mixing AgNO3 (in the 0.01-0.09 mol L-1 range) solution with a sodium citrate solution (mole ratio 1:1). The wood chips were impregnated in the mixed solution with different AgNO3 concentrations under vacuum (0.01 MPa, 2 h) and pressure (0.6 MPa, 1 h). The mixed 5



solution with the wood chips was heated at 80 °C for 10 h, and the prepared wood samples were removed and ultrasonically rinsed in deionized (DI) water for 30 min. Ag/wood composite filters with different Ag NP contents were then obtained. Characterization of Ag/wood filter The Ag content of the prepared Ag/wood filters was determined by inductively-coupled plasma mass spectrometry (ICP-MS, NexION 300D, Perkin-Elmer; Waltham, MA, USA). The surface morphology of the natural wood and Ag/wood composite were analyzed by scanning electron microscopy (SEM, JSM-7500F, JEOL, Tokyo, Japan). The morphology of the Ag NPs was examined by transmission electron microscopy (TEM, JEM-2100, JEOL). The surface components of the samples were determined by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo-Fisher Scientific, Waltham, MA, USA). X-ray diffraction (XRD) analysis was performed on a D/MAX 2200 X-ray diffractometer (Rigaku Corp., Tokyo, Japan) operated with Cu Ka radiation (λ = 1.54 Å). The presence of Ag NPs in wood was established by measuring the absorbance spectra of the Ag/wood on an ultraviolet-visible (UV-vis) spectrophotometer (Cary100, Agilent Technologies, Santa Clara, CA, USA) at wavelengths of 200-800 nm. Removal of MB from effluent The flow rates of the Ag/wood filters with different thicknesses were obtained using a circulating water vacuum pump (SHZ-DIII, Jizhong Biotechnology, China) at 0.05 MPa. NaBH4 (100 ppm) and MB (~20 ppm) were mixed with DI water to prepare an aqueous solution. The Ag/wood filters with different wood thicknesses and Ag NP contents were fixed on the mini-filtration equipment for filtration (Fig. S1). The aqueous solution was filtered 6


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through the Ag/wood filters under 0.05 MPa vacuum using an SHZ-DIII circulating water vacuum pump. The pH of the mixed solution was adjusted using HCl or NaOH solution (1 mol L-1), and the pH value was measured by a PB-10 pH meter (Sartorius, Goettingen, Germany). UV-vis spectroscopy was conducted before and after filtration in the scanning range of 300-800 nm using a UV-vis spectrophotometer. The MB concentration was determined by measuring the absorption at 664 nm, and the MB removal efficiency was calculated using the following equation: Removal efficiency (%) = 100 × (C1 - C2)/C1 where C1 and C2 are the MB concentration before and after filtration, respectively. Bacterial removal during filtration Microbiological tests were performed with the nonpathogenic bacterial strain E. coli and S. aureus as model bacteria. The number of viable bacteria in water before and after filtration through the Ag/wood and pure wood samples was determined and expressed as colony-forming units (CFUs) per mL. The bacterial removal capacity of the filters was evaluated by filtering the prepared contaminated water with E. coli or S. aureus at a concentration of ∼106 CFU mL-1 under vacuum of 0.05 MPa and estimating the remaining bacteria in the filtered water by subsequent cultivation. The number of bacteria that remained in the treated water was calculated using the plate count method.28 The bacterial removal capacity is given as the log removal of the viability: log (bacterial removal) = log (A/B), where A and B are the number of bacteria before and after filtering, respectively. Dynamic monitoring of bacterial inactivation was also conducted by monitoring the optical density (OD) of the bacterial suspension.28 The treated effluent after filtering with natural wood or 7



Ag/wood filters was regrown in fresh LB. The OD values of the bacterial suspensions were then measured twice per hour of culturing to qualitatively monitor the growth kinetics by UV-vis spectrophotometry at a wavelength of 600 nm. In addition, confocal laser scanning microscopy (CLSM) was conducted to visualize the E. coli bacteria before and after the filtration treatment in accordance with a previously reported procedure.29 Anti-leaching effect of Ag/wood filter To evaluate the anti-leaching effect of the Ag/wood filter, the Ag content of the Ag/wood filter treated with an E. coli suspension (500 mL) was analyzed by ICP-MS. The ring diffusion method was also used to evaluate the anti-leaching effect of Ag/wood. This qualitative test was performed by evenly streaking 200 µL of bacterial culture (~107 CFU mL-1) on agar plates and then spreading the bacteria uniformly. The wood samples were gently placed over the solidified agar medium in Petri dishes. The Petri dishes were subsequently incubated at 37 °C overnight. The anti-leaching effect was evaluated by examining whether an inhibition zone appears around the samples.

RESULTS AND DISCUSSION Characterization of Ag/wood filter

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Scheme 1 The fabrication of Ag/wood composite filter for water purification.

Fig. 1 Photos of (a) natural wood and (b) Ag/wood; SEM graphs of (c) natural wood and (d) Ag/wood; (e) SEM graphs and (f) the corresponding Ag element mapping image of Ag/wood; (g) TEM graph of Ag NPs within the wood; (h) The selected area electron diffraction (SAED) pattern of the Ag/wood; and (i) The high-resolution TEM image exhibits the (111) lattice plane of Ag with an interplanar distance of 0.236 nm. The yellow dash lines in (c) highlighted the lengthwise profile of the tracheids.

The Ag/wood filter was prepared using natural pine wood decorated with Ag NPs by in situ synthesis (Scheme 1). Once heating up the precursor aqueous solution containing AgNO3 and sodium citrate in the wood porous structure, ion exchange interaction and/or 9



complexation between silver ions and chelating groups of wood (COOH and OH groups) are supposed to take place.27 Meanwhile, the sodium citrate acted as a reducer and stabilizing agent in the process of Ag NPs preparation. During the reaction, the citrate is oxidized producing acetone carboxylate and the Ag+ is in turn reduced to Ag NPs. The formed Ag NPs were in situ deposited on the surface of the wood cell walls. The obtained Ag/wood composite turned yellow (Fig. 1a and Fig.1b), exhibiting the plasmonic effect of the Ag NPs anchored to the surface of the wood channels.30 The Ag/wood filter interior also showed a uniform yellow color and relatively consistent Ag elemental content along the longitudinal direction (Fig. 1b and Fig. S2), which indicates the uniform distribution of Ag NPs throughout the entire wood structure. The uniform distribution was further confirmed by elemental mapping (Fig. 1e, Fig. 1f and Fig. S3). On the basis of the ICP-MS analysis, the Ag NP content of the Ag/wood composite synthesized with different concentrations of AgNO3, namely, 0.01, 0.05, and 0.09 M L-1, were determined to be 0.18 wt%, 1.25 wt%, and 2.09 wt%, respectively. This efficient deposition of Ag NPs is attributed to the efficient diffusion of AgNO3 and sodium citrate mix solution passing through tracheids and pits. Tracheids comprise more than 90% of the volume of the softwood and perform both mechanical and conductive functions in softwood.31 The parallel axial tracheids with a diameter of tens of micrometers within the softwood are closed at the ends and the connection between adjacent tracheids is mainly via pits (Fig. 1c and Fig. S4).24,32 The pit membranes have plenty of nanoscale pores that are the main passageway for fluid flow (Fig. S4). Because the tracheids have short length of around 0.5 mm. The water flow cannot directly pass through the micro-scaled tracheids vessels for the samples used in the present study with the thickness of 10


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≥3mm. Thus, the closed ends of tracheids would force water to follow a zigzag path from one cell to the next through circular bordered pit membranes during filtration (Scheme 1).33 After the synthesis of Ag NPs, the multi-channeled and three-dimensional structure of the tracheids and pits appears well-preserved (Fig. 1d). This promotes the efficient interaction between the Ag NPs and molecules of the compounds in the water, which facilitates water purification. Moreover, the particle size and crystal structure of the synthesized Ag NPs were determined by TEM (Fig. 1g and Fig. 1h). The Ag NPs synthesized in situ exhibited a spherical morphology with an average diameter of ~21 nm and a narrow size distribution in the 6-30 nm range (Fig. 1g and Fig. S5). The measured interplanar distance of the (111) lattice plane of Ag was 0.236 nm (Fig. 1i), which is consistent with the typical crystalline structure of Ag NPs.34

Fig. 2 (a) UV-visible absorbance spectra, and (b) XRD curves of natural wood and Ag/wood (c) Ag 3d XPS spectra of Ag/wood.

The UV-vis absorption spectra of the wood samples with and without Ag NP deposition are presented in Fig. 2a. The appearance of a new peak at 420 nm in the spectrum of the Ag/wood sample, which is attributed to surface plasmon resonance of conduction electrons on the surface of Ag NPs, confirmed the successful deposition of Ag NPs in Ag/wood.35 The formation of Ag NPs within the wood filter was further verified by XRD (Fig. 2b). The XRD 11



spectra of the starting natural wood exhibited a typical crystalline cellulose patterns with diffraction angles (2θ) at around 16°, 22°, and 35°.36 After in situ synthesis of Ag NPs, the XRD spectra of the Ag/wood composites revealed the appearance of new peaks at 38.1°, 44.4°, 64.6°, and 77.7°, which were assigned to the (111), (200), (220), and (311) lattice planes of Ag, respectively.37 In addition, the XPS analysis indicated the existence of the zero-valent state Ag (Fig. S6). It has two main components associated with a 3d doublet at 368.7 eV (Ag 3d5/2) and 374.7 eV (Ag 3d3/2), with a spin energy separation of 6.0 eV (Fig. 2c), which is a characteristic of metallic silver.38,39

Remediation of MB-polluted water

Fig. 3 (a) The rates of water flow through the wood under 0.05 MPa vacuum; (b) The MB removal efficiency using Ag/wood filter with different thickness; (c) UV-vis spectra of the MB solution before and after filtration treatment using natural wood and Ag/wood filter; (d) The MB removal efficiency using Ag/wood filter with different Ag NP content; (e) The MB removal efficiency using Ag/wood filter at different pH values adjusting by NaBH4; (f) The concentration-dependent MB removal efficiency using Ag/wood filter. The thickness of nature wood and Ag/wood, except (a) and (c), is 10mm. The Ag NPs content in Ag/wood, except (d), is 1.25 wt%. The pH value of MB solution, except (e), is 7. The concentration of MB solution, except (f), is 10 ppm. 12


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The widespread use of dyes, which present a health hazard for both humans and animals, has led to severe water pollution. The dye removal capacity of the Ag/wood filter was evaluated using MB as the model dye because of its wide use in the textile industry. First, the flow rate of samples with different thicknesses was evaluated under vacuum driven with 0.05 MPa. The results reveal that the introduction of Ag NPs caused no apparent change in flow rate in the Ag/wood, compared with natural wood (Fig. 3a), indicating that the mesoporous structure and nanochannels in wood was well maintained after in situ synthesis of Ag NPs. This result is consistent with the SEM analysis. Moreover, the water flow rate markedly decreased when the thickness reached 5 mm, and further decreased slowly with additional increase in thickness (Fig. 3a). This effect results from the increase in the closed ends of the axial tracheids, which interrupts the flow in the tracheids and forces the flow to pass through the nano-scaled pit membranes, thus promoting the efficient contact of Ag NPs with MB. The MB removal efficiency of both natural wood and Ag/wood increased with an increase in wood thickness (Fig. 3b). Owing to the physical adsorption of MB on the natural xylem,31 the natural wood treated with the MB/NaBH4 solution resulted in nearly 50% colorization (Fig. 3b and Fig. 3c). By contrast, the Ag/wood filter was more effective for the remediation of MB-polluted water, exhibiting an MB removal efficiency of up to 98.7% (Fig. 3b). This efficiency is related to the superposition between the physical adsorption by the wood structure and the catalytic degradation by Ag NPs.40,41 In comparison with Ag/wood filter, the filter paper decorated with Ag NPs using the same synthesis process exhibited inferior MB removal efficiency (

Wood-Based Mesoporous Filter Decorated with ... - ACS Publications

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