Mesoporous, Three-Dimensional Wood Membrane Decorated with

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Mesoporous, Three-Dimensional Wood Membrane Decorated with Nanoparticles for Highly Efficient Water Treatment Fengjuan Chen, Amy S. Gong, Mingwei Zhu, Guang Chen, Steven D. Lacey, Feng Jiang, Yongfeng Li, Yanbin Wang, Jiaqi Dai, Yonggang Yao, Jianwei Song, Boyang Liu, Kun Fu, Siddhartha Das, and Liangbing Hu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b01350 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Mesoporous, Three-Dimensional Wood Membrane Decorated with Nanoparticles for Highly Efficient Water Treatment Fengjuan Chen1(a), Amy S. Gong1(a), Mingwei Zhu1(a), Guang Chen2, Steven D. Lacey1, Feng Jiang1, Yongfeng Li1, Yanbin Wang2, Jiaqi Dai1, Yonggang Yao1, Jianwei Song1, Boyang Liu1, Kun Fu1, Siddhartha Das2, Liangbing Hu1,* 1

Department of Materials Science and Engineering, University of Maryland College Park, College Park, Maryland, 20742 2

Department of Mechanical Engineering, University of Maryland College Park, College Park, Maryland, 20742 (a) Equally contributed Email: [email protected] Keywords: Mesoporous structure, Nanoparticles, Three-dimensional membrane, Water treatment, High efficiency TOC Figure

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Abstract Wood, an earth-abundant material, is widely used in our everyday life. With its mesoporous structure, natural wood is comprised of numerous long, partially-aligned channels (lumens) as well as nanochannels that stretch along its growth direction. This wood mesostructure is suitable for a range of emerging applications, especially as a membrane/separation material. Here, we report a mesoporous, three-dimensional (3D) wood membrane decorated with palladium nanoparticles (Pd NPs/wood membrane) for efficient wastewater treatment. The 3D Pd NPs/wood membrane possesses the following advantages: (1) the uniformly distributed lignin within the wood mesostructure can effectively reduce Pd (II) ions to Pd NPs; (2) cellulose with its abundant hydroxyl groups can immobilize Pd NPs; (3) the partially-aligned mesoporous wood channels as well as their inner ingenious microstructures increase the likelihood of wastewater contacting Pd NPs decorating the wood surface; (4) the long, Pd NP-decorated channels facilitate bulk treatment as water flows through the entire mesoporous wood membrane. As a proof of concept, we demonstrated the use and efficiency of a Pd NPs/wood membrane to remove methylene blue (MB, C16H18N3ClS) from a flowing aqueous solution. The turnover frequency of the Pd NPs/wood membrane, ~2.02 molMB·molPd-1·min-1, is much higher than the values reported in literature. The water treatment rate of the 3D Pd NPs/wood membrane can reach 1×105 L·m2

·h-1 with a high MB removal efficiency (> 99.8%). The 3D mesoporous wood membrane with

partially-aligned channels exhibits promising results for wastewater treatment and is applicable for an even wider range of separation applications.


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Industrial development has led to the release of increasingly large amounts of contaminants, such as metal ions, organic dyes and cleaning agents.1 This extensive release of contaminants is a leading cause of water pollution, which is a severe environmental issue threatening our entire planet.2-6 Many emerging technologies using nanomaterials and nanostructures have been demonstrated toward clean water generations.7-10 Specifically, organic dyes, such as methylene blue (MB), Rhodamine 6G (Rh6G) or methylene orange (MO) have high toxicity, long-term chemical stability, slow degradation rate, and are potentially carcinogenic. However, they are still widely used in the printing and textile industries as well as in paper, paints, leather and pharmaceuticals, producing dye-contaminated wastewater that needs to be urgently treated.11, 12 A variety of technologies, such as physical adsorption, photocatalytic degradation, chemical oxidation, and membrane filtration, have been implemented to remove these organic compounds.13-15 However, these water treatment methods pose additional challenges. For example, adsorption and membrane filtration treatment methods can produce excessive amounts of solid waste.16 Photocatalytic reactions suffer from low solar energy conversion efficiencies and have difficulties separating catalysts for recycling.17 Therefore, there is an emerging demand for the methods that are rapid, efficient, cost-effective and recyclable for removing dyes from industrial wastewater. Wood is ubiquitously used as a structural material across the globe.18 Recently, nanomaterials such as cellulose nanocrystals and cellulose nanofibers extracted from wood have attracted much attention due to their optical and mechanical properties.19-22 For example, transparent paper composed of cellulose nanofibers has been demonstrated as a potential replacement for plastic substrates for flexible and biodegradable electronic devices.23-24 However, extracting cellulose nanofibers and nanocrystals is time-consuming and energy-intensive.25,


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In contrast to the

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aforementioned bottom-up approaches by extracting cellulose nanofibers, directly engineering natural wood can potentially open up a range of opportunities. In this way, the nanoscale nature of the hierarchical wood structures is preserved and tailored to yield efficient device performance without the use of intensive extraction and fabrication processes. Typically, hardwood, such as the basswood employed in this study, possesses a complex mesostructure with two types of long, partially-aligned channels along the growth direction. The vessel channel is several hundred micrometers in diameter while the fiber channel diameter is tens of micrometers.27 Commonly, vessel channels are connected together at the cell ends through perforation plates with micrometer sized pores, which enables continuous transportation of water along the wood channel. In contrast, the tapered ends of the fiber channels are interconnected through pits on the channel walls, which relies on capillary forces that limit water transportation rates. Since the hardwood is comprised of about 50% (pore volume) of vessel channels and 20% fiber channels,27 water filtration is a reasonable application for this material due to water transport through the complex vessel channels.18 Additionally, Pd NPs have shown excellent catalytic efficiency; these nanoparticles are often synthesized on nanoporous supports such as carbon, metal oxides, silica as well as in zeolite cages to avoid agglomeration.28, 29

Here, we developed a 3D wood membrane for wastewater treatment by in situ forming Pd NPs within the wood channels of basswood. The long, irregularly-shaped wood channels, which vary in diameter along the length direction, act as a 3D substrate to support the Pd NPs. Due to the mesostructure of wood, the pollutants in water will get much higher chances to contact with the anchored Pd NPs catalyst during flowing through the wood channels, which is essential for fast and high efficient wastewater treatment. Additionally, the wood structure remains stable in the


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presence of water, ensuring the membrane’s longevity for wastewater treatment. Hence, with this naturally developed hierarchical porous structure, wood membrane is more advantageous owning to the high flux rate of the large open vessel channels, as well as natural abundance and biodegradability, making it a better candidate compared to the fabricated polymeric and ceramic membranes.30, 31 As a proof of concept, we decorated the 3D wood membrane with Pd NPs to remove methylene blue (MB, C16H18N3ClS) from aqueous solutions. The MB removal rate and efficiency exceed previously reported values and methods.32-34 The proposed water treatment membrane composed of a 3D mesostructured wood provides a cost-effective opportunity for high efficient wastewater treatment. It can be envisioned that future catalyst selections can be applied to decorate wood-based supports for photocatalysis, electrocatalysis, organic reactors, and many other applications.

Results and discussion The microstructures in wood are unbelievably suitable for water treatment shown in Figure 1. Numerous spiral thickening and pits are decorated on the inner surface of the long, curved vessel channels (Figure 1a). These characteristics are crucial for the water treatment performance by determining the kinetics when water passing through the channels. After uniform and conformal Pd NPs decoration by an in situ way, all the structural characteristics in natural wood are well preserved in the Pd NPs/wood. The wood block becomes black due to strong light absorption by the plasmonic effect of the metal NPs anchored to the surfaces of the wood channels.35 The resulted 3D Pd NPs/wood membrane exhibits powerful ability for water treatment, which will be demonstrated by MB degradation tests in subsequent paragraphs (a treatment rate up to 1×105


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L·m-2·h-1 and the efficiency higher than 99.8%). Also, the demonstrated 3D Pd NPs/wood membrane can be readily scaled up for industry-scale applications.

Figure 1. Basswood decorated with Pd NPs for water treatment. (a) Schematic of a 3D wood membrane decorated with Pd NPs for water treatment. A piece of wood was cut perpendicular to the growth direction, where the channels are partially aligned through the wood block’s thickness. The magnified image shows the mesostructure of the wood which contains many channels. (b) In situ formed Pd NPs within the wood where lignin acts as the reducing agent. The plasmonic effect of the Pd NPs inside the wood channels causes the Pd NPs/wood to appear black. The magnified image shows the Pd NPs in the wood channels and the color change (blue to colorless) as the MB solution flows through the Pd NPs/wood membrane.


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Figure 2a shows digital images of natural wood (the yellow wood) and the Pd NPs/wood membrane (the black wood). Here should be noted that the lignin content in wood functions as an effective agent to reduce PdCl2 into Pd NPs.36, 37 To verify this process, the lignin within a wood block was completely removed with a previously reported method.38 This lignin-free wood block acted as a control sample. After employing the same fabrication process, no Pd NPs were formed, which further confirms that Pd (II) ions can be reduced by lignin (Figure S1), instead of the reducing ends hemiacetal groups in cellulose and hemicellulose. Although lignin extracted from pulping process has shown to serve as reducing agent in Pd nanoparticle synthesis, owning to the abundant aliphatic and phenolic hydroxyls formed during lignin depolymerization,

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to demonstrate the reducing ability of lignin within the lignocellulosic matrix, suggesting sufficient amounts of reducing groups are present in the natural lignin. Such in situ formation of Pd NPs within the wood channels can result in strong bonding between the NPs and the channel surface, presumably through binding of surface Pd atoms with the hydroxyl groups on cellulose and hemicellulose.40 Specifically, as shown by the red dashed lines in Figure 2b, the vessel channels within the wood are vertically-aligned with a range of diameters extend through the entire wood thickness (Figure 2b), which allows palladium (II) chloride (PdCl2) solution to penetrate throughout the entire wood block and realize in situ formation of Pd NPs within the 3D wood structure. This is proved by the fact that the Pd NPs/wood block turned black in color due to plasmonic effects after Pd NPs were formed within the wood structure. The high magnification SEM image of the channel in wood was shown in Figure S2. The 5mm thick Pd NPs/wood membrane was cut along the


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growth direction and exhibits a uniform black color (Figure 2c and Figure S3), which indicates the presence of Pd NPs throughout the entire wood structure. The formation of Pd NPs within the wood membrane was further confirmed by Fourier Transform Infrared spectroscopy (FTIR) (Figure S4). The additional absorption peaks around 682 cm−1, are assigned to the Pd-O stretching modes. This confirms the interaction between hydroxyl groups of the cellulose/hemicellulose and the palladium atoms.40 The particle size and crystal structure of the synthesized Pd NPs were determined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Figure 2d-e). Note that the Pd NPs have an average size of 5nm and exhibit good crystallinity. Figure 2f shows the measured interplanar distance of the (111) lattice plane of Pd was 0.23 nm, which confirms the identity of the nanoparticles.41 As shown in Figure S5, energy-dispersive X-ray (EDX) was employed to verify that elemental Pd was present on the 3D wood. The structure of the Pd NPs/wood membrane was further confirmed by XRD analysis. As shown in Figure S6, the strongest peak, which was centered at 35°, was assigned to wood (004),42 while the other peaks are indexed to the (111), (200) and (220) planes of the Pd NPs.16


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Figure 2. (a) The photo images of natural wood and the Pd NPs/wood membrane. The wood matrix changed from yellow to black due to the in situ synthesis of Pd NPs within the wood structure. (b) Magnified SEM image of natural wood. There are many long and irregular channels in the wood, which are highlighted with red dashed lines. (c) The Pd NPs/wood membrane was cut to show that both the surface and the inner side of the wood are black, thereby highlighting that Pd NPs are evenly distributed throughout the entire wood block. (d) TEM image of Pd NPs within the wood. Inset shows the size distribution of the Pd NPs. (e) SAED pattern of the Pd NPs/wood. (f) The HRTEM image exhibits the (111) lattice plane of Pd with an interplanar distance of 0.23 nm.


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The microstructure of the Pd NPs/wood membrane enables enhanced water treatment abilities. As shown in Figure 3a, besides the long intrinsic wood channels, perforation plates connect the channels together. These plates increase the amount of water-catalyst interaction as the water passes through the partially-aligned wood channels. To test the water treatment efficiency of the 3D Pd NPs/wood membrane, MB degradation tests were employed. Figure 3b depicts the experimental setup for MB degradation tests as well as the color change phenomenon as the solution flows through the Pd NPs/wood membrane. Specifically, the color change from blue to colorless is attributed to the destruction of the conjugated chromophore structure of MB, which is induced by the Pd NPs in the presence of NaBH4.32

The degradation ability of the Pd NPs/wood membrane towards MB is quantitatively confirmed by ultraviolet-visible spectroscopy (UV-Vis) measurements (Figure 3c). The characteristic absorbance band for MB (664 nm) completely disappeared after the MB solution was treated with Pd NPs/wood membrane. Note that the degradation efficiency was calculated using the following equation: 43 Degradation efficiency (%) = 100*(C0-C)/C0, where C0 and C are the MB concentrations before and after Pd NPs/wood membrane treatment, respectively. The Pd NPs/wood membrane shows great performance for MB degradation with an efficiency around 99.8%. In sharp contrast, when the aqueous MB/NaBH4 solution is flowed through the control sample, natural wood, no obvious color change is observed (Figure S7a) and the


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characteristic absorbance band of MB is nearly unchanged in the UV-Vis spectra (Figure S7b). This indicates that the natural wood sample shows no signs of MB adsorption. The Pd NPs/wood membrane maintains high water treatment efficiencies even at different pH values (Figure 3d). The degradation efficiency remains at 99.8% across all pH values (from 1 to 12), which is essential for industrial applications.32 Moreover, the Pd NPs/wood membrane shows excellent MB degradation performance over a wide range of MB concentrations (Figure 3e). Note that the degradation efficiency stayed constant at 99.8% for all MB concentration up to 40 mg·L-1. The degradation efficiency began to decrease slightly as the MB concentration was increased beyond 40 mg·L-1.

Figure 3. (a) SEM image of the long wood channels with vessels and perforation plates. (b) Demonstration of water treatment using a Pd NPs/wood membrane. The Pd NPs/wood membrane dimensions are 50mm×50mm×5mm; the blue solution is a mixture of MB (30 mg·L-1)


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and NaBH4 (100 mg·L-1) in water. The solution became colorless after flowing through the Pd NPs/wood membrane. (c) UV-Vis spectra of the MB solution before (in red) and after (in blue) flowing through Pd NPs/wood membrane. The degradation efficiency at different (d) pH values and (e) concentrations of the MB solution, respectively.

The Pd NPs/wood membrane also exhibits a high water treatment efficiency. The efficiency is evaluated by flowing the aqueous solution through the 3D wood membrane at various rates using a homemade filtration setup shown in Figure 4a-b. Note that the initial blue-colored solution became colorless after following through the 3D wood membrane (Figure 4c). A high degradation efficiency of more than 99.8% was maintained as the flow rate approached 1×105 L·m-2·h-1 (Figure 4d). To further evaluate the treatment efficiency of the Pd NPs/wood membrane, the turnover frequency (TOF; units of molMB·molPd-1·min-1) was calculated, which is defined as the number of degraded MB molecules (mol) per mole of Pd (mol-1) per minute (min-1). Figure 4e compares the TOF literature values for tetrahedral Pd nanocrystals (Pd-TNPs), Pd black and Pd nanocrystals/reduced graphene oxide nanohybrid (Pd-TNPs/RGO) catalyst materials to our proposed 3D wood membrane.32,44 The TOF value for our Pd NPs/wood membrane was calculated to be 2.02 molMB·molPd-1·min-1 at a rate of 1×105 L·m-2·h-1 (the blue bar), which is significantly higher than the Pd nanostructures reported in literature (Figure 4e). The ultra-fast and efficient degradation of MB by the proposed Pd NPs/wood membrane may be ascribed by two factors. Firstly, the presence of Pd NPs throughout the wood channels serves as an efficient electron relay between nucleophilic NaBH4 and electrophilic MB for the catalytic reductiondegradation process to occur. Secondly, the long and irregular channels, as well as the perforation plates within the wood membrane, result in intimate and efficient contact between the


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Pd NPs/wood membrane and the MB solution. This enables a highly efficient reaction towards the degradation of MB. The stability of the Pd NPs/wood membrane was also investigated by measuring the Pd weight loss after water treatment with inductively coupled plasma mass spectrometry (ICP-MS). The results showed that the loss of Pd is less than 1.0 wt% after 3L MB solution was treated with the wood membrane.

Figure 4. (a) The experimental setup for the water treatment tests, where vacuum is applied to control water flow rates. The blue solution in the funnel consisted of an aqueous mixture of MB (30 mg·L-1) and NaBH4 (100 mg·L-1). (b) Zoom-in of the black Pd NPs/wood membrane (thickness = 10 mm; effective diameter = 34 mm), which was used as a filter. (c) Zoom-in image of the MB solution flowing through the Pd NPs/wood membrane. The initial blue MB solution changed to a colorless liquid after filtration. (d) The degradation efficiency as a function of


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various treatment rates. (e) The turnover frequency (TOF) of our Pd NPs/wood membrane compared to other Pd-based materials reported in literature.

The promising water treatment performance of the Pd NPs/wood membrane originates from the fluid dynamics induced by the mesoporous structure of the 3D wood as water passes through the material. Figure 5a illustrates the 3D mesostructure where long, irregular vessel channels with nonuniform diameters travel throughout the basswood. The vessel channels are connected by perforation plates (highlighted in Figure 5a). Furthermore, many vessel pits and spiral thickenings were observed in the wood channels (highlighted in Figure 5b), which indicate that liquid flows through the pits in a more complex manner. In this case, for a fluid to flow to an adjacent cell, diffusion must occur instead of free liquid advection.45 Note that the water flow can be changed by tailoring the aforementioned features of the wood channels, e.g. irregular channel shapes with varying diameters as well as the number of vessel pits and spiral thickenings.

The effect of impurity concentration on the curved nature (partial alignment) of the wood channels of basswood was investigated. For our basswood (a type of hard wood), the water transport through the wood channels will be dictated by the vessel network.46 In contrast, the pits connecting the tapered ends of longitudinal cells becomes important for water transport in softwood.47The vessel network, relevant for hardwood, consists of bundles of vessels that are weaved to form a 3D network. This network dictates transport in all three directions (longitudinal, radial, and tangential). Note that individual vessels are interconnected through perforation plates as well as porous vessel walls to form this vessel network. A complete mechanism of the advective-diffusive transport in such a complicated vessel network is beyond


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the scope of this study. To provide an illustrative example of how the presence of the vessels (and the corresponding vessel network) leads to enhanced impurity removal, the transport within a single vessel was studied. Based on our SEM analysis as well as previously reported geometries for single vessels in hardwood,46 we can approximate a single representative vessel as a zigzag shape (see Figure 5c and Figure S8) we consider water-driven transport of MB (which we refer to as “dirt”) in such a zigzag channel. Therefore, in principle, our analysis is similar to how a spiralled microchannel increases the extent of reaction or mixing by forcing a solution to move across a longer flow path.48-50 The zigzag wood vessel ensures that water also follows in this curved trajectory and therefore covers a longer flow path. To model this system, we ran simulations on wood channels with two separate geometries – a straight channel and a zigzag channel. The zigzag channel has the same inlet-to-outlet vertical distance and wall-to-wall separation as the straight channel. The removal of dirt (or MB) is modeled based on an advection-diffusion phenomena. The liquid advection drives the dirt, which is assumed to be neutrally buoyant, downstream along the wood vessel. On the other hand, diffusion occurs between the dirt-carrying water and the dirt-free vessel wall, thereby removing the dirt from the bulk water. Detailed equations are found in the Supplementary Information. Our modeling confirms that there is a much larger removal of dirt in the zigzag vessel. This is evident from the concentration contour as well as the concentration profiles (at two selected locations across the vessel) shown in Figure 5c. The enhancement of dirt removal in the zigzag vessel can be attributed to the fact that the polluted water is exposed to a much larger extent of wall area. Consider A2 and A1 as the surface areas with which the incoming polluted water contacts zigzag and straight vessels, respectively. Since A2/A1 = 1/cosϕ > 1, the water is always exposed to a larger surface area in a zigzag-shaped vessel. This extended wall exposure ensures a


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greater amount of skin-friction drag, which leads to smaller fluid velocities at downstream locations within the zigzag vessel. We consider the flow to be driven by an identical pressure drop between the inlets and outlets for both the straight and zigzag vessels/channels. In Figure S8 (iii), we provide the velocity profiles across two different cross-sections of the zigzag and straight vessels. The profiles confirm the zigzag vessel exhibits much lower flow velocities. The order of magnitude for the flow velocity, which should be constant in both straight and zigzag vessels, can be predicted from the following condition:

u~

∆p h 2 2 × 10 4 ( N ⋅ m −2 ) ( 20 × 10 −6 m) 2 ~ ~ 10 m ⋅ s ∆x η 400 ×10 −6 m (10 −3 N ⋅ s ⋅ m − 2 )

(1)

Our simulations show similar orders of magnitude for the flow velocity in both straight and zigzag vessels (see Figure 5c). This confirms that are analytical prediction [Eq. (1)] and numerical approach are consistent in terms of disseminating the flow magnititude. This reduced velocity within the zigzag vessel ensures that the dirt remains within the channel and is exposed to the dirt-free wall for an extended period of time. In this case, the enhanced dirt-free wall exposure is increased, causing the dirt to remain in the channel and thus, effectively separate out contaminants from the flowing solution. The combination of these two factors ensures that the dirt concentration decreases dramatically at a given downstream location within the zigzag vessel. This trend is illustrated in Figure 5c (i-iii).


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Figure 5. (a) SEM image showing long and irregular channels in the radial section of the wood. One of the vessel channels and perforation plates are highlighted with red dashed lines. (b) A zoom-in SEM image of the vessel wood channel. Some of the vessel pits and the spiral thickening within the wood are highlighted with a yellow dashed circle and red dashed lines, respectively. (c) Contours of the dimensionless concentration profiles for (i) straight and (ii) zigzag channels. (iii) Concentration profiles for zigzag (z) and straight (s) channels at two different locations (middle, outlet) of the wood. Parameters used for the simulation are

D = 10−6 m2 ⋅ s−1 (see the Supplementary material for discussion on choice of the diffusivity value), (c)walls = 0 , and (c)inlet = 1.


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The zigzag nature of the wood vessels triggers a dual effect of reduced flow rate (or enhanced residence time for the dirt) and an augmented surface area for dirt diffusion (from bulk water to dirt-free walls). These two factors ultimately ensure that augmented dirt-removal (or equivalently higher MB degradation efficiency) occurs throughout the entire hardwood block due to the intrinsic mesostructure of the wood. It is useful to emphasize here that the curved channel does not alter the diffusivity, i.e. the water-to-surface diffusion coefficient of the dirt — however, a longer dirt residence time (or equivalently a weaker Peclet number, since the average velocity is lowered) as well as extended exposure to a larger amount of wall surface area implies an enhancement in diffusion-driven dirt removal.


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Conclusion In this work, we demonstrate that a mesoporous, 3D wood membrane decorated with Pd NPs can function as an excellent wastewater treatment material, which is attributed to the synergistic effect of evenly distributed nanosized catalyst particles and the channel structure of the hardwood. The mesoporous wood with partially-aligned yet irregular channels: (1) facilitates in situ NP synthesis, (2) allows for continuous water flow/transport, and (3) increases the exposure and interaction of impurities with Pd NPs that decorate the 3D wood channels. In this study, the wastewater used was an aqueous mixture of MB and NaBH4. The MB degradation efficiency of the Pd NPs/wood membrane reached 99.8% at a treatment flow rate up to 1×105 L·m-2·h-1. Experimental and modeling results show that the mesostructure of the wood channels with varying channel diameters, vessel pits, and spiral thickenings are integral to obtaining a high-flux, high MB degradation efficiency. The 3D mesoporous wood membrane with partially-aligned channels can be extended to a range of water treatment applications with different impurities. The proposed cost-effective and high-performance 3D wood membrane is scalable and enables potential industrial-scale applications.


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Experimental Section

Materials and Chemicals. The basswood used in this study was received from the Walnut Hollow Company. Palladium (II) chloride (PdCl2), methylene blue (MB, C16H18N3ClS), hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH) and sodium borohydride (NaBH4) were purchased from Sigma-Aldrich and used directly. Deionized (DI) water was used as the solvent.

Preparation of Pd NPs/wood membrane. A PdCl2 aqueous solution (1.5 mg·L-1) was prepared by mixing PdCl2 (150 mg) with HCl solution (20 mM, 100 mL), followed by a 1 h heat treatment at 60ºC until the PdCl2 powder dissolved completely. The wood slices were immersed in the PdCl2 solution and heated for 12 h at 80ºC to obtain the Pd NPs/wood membrane with a Pd NP content of 0.19wt%.

Water treatment measurements. The water treatment performance of the Pd NPs/wood membrane was evaluated using the degradation of MB in the presence of NaBH4 as the model reaction. The Pd NPs/wood membrane was used as a filter. Nominally, 30mL of NaBH4 (100 mg·L-1) and 300mL of MB (~40 mg·L-1) were mixed with DI water to form an aqueous solution. The aqueous solution was then filtrated through the Pd NPs/wood membrane. The pH of the mixed solution was adjusted by HCl or NaOH solution (1 mol·L-1) and tested with a METTLER TOLEDO Delta 320 digital pH-meter. Ultraviolet-visible spectroscopy (UV-Vis) measurements were conducted before and after filtration in the scanning range of 400-800 nm on a UV-1750


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spectrophotometer Lambda 35 (PerkInElmer, USA). The MB concentration was determined by the absorption peak at 664 nm, and the MB degradation efficiency was calculated as follows: degradation efficiency (%) = 100* (C0- C)/C0, where C0 is the initial MB concentration, and C is the MB concentration after filtration. A syringe pump (SP1-C1, Longer Precision Pump Co., China) was used to control the flow rates (0.5-1250 mL·min-1).

Characterization. The hardwood morphology was characterized by scanning electron microscopy (SEM, Hitachi SU-70). Transmission electron microscopy (TEM) images were taken using a JEM-2100F (JEOL, Tokyo, Japan) operated at 120 kV. High-resolution TEM images were taken using a JEOL 2100F microscope (JEOL, Tokyo, Japan). The TEM samples were prepared by mechanically grinding and pulverizing the composite wood membrane, followed by sonication in ethanol for 10 hours to obtain the dispersed NPs. Fourier Transform Infrared (FTIR) spectra were recorded on a Thermo Mattson FT-IR spectrometer using the KBr pellet technique. Powder X-ray Diffraction (XRD) analyses were performed on a Bruker AXS D8Advanced diffractometer with Cu Ka radiation (λ = 1.5418 Å) and the scanning angle (2θ) ranged from 10 to 110°. The amount of elemental Pd in the samples were determined using inductively coupled plasma mass spectrometry (ICP-MS, Perkin–Elmer Elan DRC II) measurements. The Pd NPs/wood membrane for ICP-MS was prepared by burning the wood and dissolving the Pd in aqua regia (HCl-HNO3). The resultant solution was further diluted to a ppb level of 100 for ICP-MS analysis.

Acknowledgement


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We acknowledge the support of the Maryland NanoCenter and its AIMLab. F. Chen would like to acknowledge the National Natural Science Foundation of China (21401091) and the China Scholarship Council (CSC) for financial support.

Supporting Information Available: Controlling experiments showing that Pd(II) ions were reduced by the presence of lignin in the wood; SEM images of the channels in wood; A photo of the black Pd NPs/wood membrane after fabrication; FTIR spectra of natural wood and Pd NPs/wood membrane; EDX and XRD spectra for the Pd NPs/wood membrane; the setup and results of the control experiment with natural wood for water treatment; the simulation and modeling of water flow in wood channels. This material is available free of charge via the Internet at http://pubs.acs.org.

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