Nanocrystalline celluloses-assisted preparation of hierarchical carbon monoliths for hexavalent chromium removal

Accepted Manuscript Nanocrystalline Celluloses-Assisted Preparation of Hierarchical Carbon Monoliths for Hexavalent Chromium Removal Haiping Su, Yanping Chong, Jitong Wang, Donghui Long, Wenming Qiao, Licheng Ling PII: DOI: Reference:

S0021-9797(17)30924-4 http://dx.doi.org/10.1016/j.jcis.2017.08.019 YJCIS 22666

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

3 May 2017 7 August 2017 7 August 2017

Please cite this article as: H. Su, Y. Chong, J. Wang, D. Long, W. Qiao, L. Ling, Nanocrystalline Celluloses-Assisted Preparation of Hierarchical Carbon Monoliths for Hexavalent Chromium Removal, Journal of Colloid and Interface Science (2017), doi: http://dx.doi.org/10.1016/j.jcis.2017.08.019

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Nanocrystalline Celluloses-Assisted Preparation of Hierarchical Carbon Monoliths for Hexavalent Chromium Removal Haiping Sua, Yanping Chonga, Jitong Wanga,b*, Donghui Longa,b, Wenming Qiaoa,b, Licheng Linga,b* a.

State Key Laboratory of Chemical Engineering, East China University of Science and Technology,

Shanghai 200237, P. R. China b.

Key Laboratory of Specially Functional Polymeric Materials and Related Technology, East China

University of Science and Technology, Shanghai 200237, P. R. China

 Corresponding author: Licheng Ling; Jitong Wang Tel.: +86 21 64252924; Fax: +86 021 64252914 E-mail: [email protected]; [email protected]

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Abstract Hierarchical porous carbon monoliths with a 3D framework were synthesized through a facile sol-gel process using resorcinol-melamine-formaldehyde (RMF) as carbon precursors, and nanocrystalline celluloses (NCCs) as the structural inducing agent, followed by ambient pressure drying and carbonization. Polymerization of the RMF resin occurs around the nanorod-like NCCs dispersed homogeneously in water, which is quite beneficial for the formation of an interconnected network and supports the rigid macroporous structure. A hierarchical porous carbon monolith with modest micropores and well-developed macropores was prepared after CO2 activation at 950 °C. The microporous structure was generated from the network of RMF polymer chains, while the macroporous structure was formed from the interconnection of polymer networks induced by NCCs. The obtained carbon monolith has a large specific surface area of 1808 m2 g-1 and shows a high adsorption capacity of 463 mg g-1 for toxic Cr(VI) ions. Moreover, the activated carbon monolith exhibits a high selectivity for Cr(VI) in the coexistence of several other metal ions. These outstanding advantages of carbon monoliths, including their micro/macroporous structures, rich functional groups, low cost and easy synthesis, endow them with potential for use in a wide range of applications.

Keywords: Nanocrystalline cellulose, hierarchically carbon monoliths, Cr(VI) adsorption, selectivity

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Introduction Because of their high surface area, large pore volume, rich surface chemistry, high conductivity or insulation, and excellent mechanical properties, carbon materials are outstanding candidates for many applications, ranging from water and air purification, adsorption, and catalysis to electrodes and energy storage[1-4]. As one of the most popular and widely used carbon materials, porous carbons, which can be produced from a variety of carbonaceous materials such as wood, coal, lignite and coconut shell, have been intensively studied due to the advantages mentioned above[5-7]. However, it is very difficult to control the pore structure of these materials, which may limit their wide application, because the porosity of natural materials usually varies depending on the source. Generally, the pore size distribution, specific surface area, channel connectivity, and architecture of the porous carbons strongly affect their performances[8-11]. Therefore, the design of porous carbons with tailored structures and porosity has become a promising research field, attracting intense attention in recent decades[12-14]. Hierarchical porous carbons are favorable in many applications where the macropores and mesopores could provide a large volume for ions storage with decreased ion diffusion distance[15, 16], while the presence of micropores can provide a high surface area for metal ion adsorption[17-19]. To date, the commonly used fabrication methods for hierarchical porous carbons include “nanocasting” (hard-templating) using silica or highly crystalline metal organic frameworks as templates[20, 21], the self-assembly (soft-templating) approach using supramolecular aggregates as templates[21, 22], chlorine extraction of the non-carbon components from carbides[23, 24], and post-activation of natural carbonaceous materials[25, 26]. Zhao et al.[27] prepared 3D hierarchical porous carbon using a combination of hard-templating (silica spheres nano-array) and soft-templating (block copolymer P123) methods. The results demonstrated that the tunable macro-mesopore size distribution, good pore connectivity and high specific surface area contributed greatly to the high capacitance of the supercapacitor. Xue et al.[28] presented the fabrication of macro-mesoporous carbonaceous materials through an organic-organic self-assembly method, resulting in

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macropores with a diameter of 100-450 m and adjustable uniform mesopores (3.8-7.5 nm). Presser et al.[29] synthesized hierarchical porous carbide-derived carbon by the chlorine extraction of silicon from carbides. The resulting porous carbon showed hierarchical porosity and a high specific surface area, offering an attractive high adsorption capacity and fast sorption velocity of cytokines from blood plasma. Cheng et al.[25] prepared hierarchically porous carbon by a facile two-step activation method using natural shiitake mushrooms as the raw material. They suggested that the resulting carbon was comprised of abundant micro-mesopores and naturally interconnected macropores, leading to a far superior performance as electrode material for supercapacitor, especially in the condition of a large current density. Although the pioneering works mentioned above are very interesting, the synthesis and post treatment procedure are somewhat time-consuming and arduous. Moreover, most of these carbon materials were obtained in the form of powders, which are undesirable in some practical uses. Therefore, it is highly essential to explore sustainable and easy methods for the synthesis of hierarchical porous carbons with monolithic morphology for widespread applications. Hexavalent chromium ion (Cr(VI)) is a highly toxic and carcinogenic metal one in waste water. Unlike most organic pollutants, Cr(VI) ions are refractory and can easily cause serious pollution to the environment. To date, Cr(VI) adsorption using porous carbons is considered to be one of the most economical methods because of its cost/performance[30-34]. Recently, hierarchical porous carbons have been widely used as adsorbents,

showing

excellent

performance

by

combining

the

advantages

of

micro

and

meso/macropores[35-37]. Despite several reports on this subject, further studies are still needed to improve the adsorption/desorption rate and the adsorption capacity. In this work, we developed a novel and facile method to prepare 3D hierarchical porous carbon monoliths through a nanocrystalline celluloses (NCCs)-assisted sol-gel process. NCCs with abundant hydroxyl groups on the surface serve as the structure-inducing agent and help to form the interconnected network[38-40]. This monolith could retain its 3D structure even after CO2 activation at 950 °C due to its

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rigid polymer chain network formed upon orientation of the NCCs. The as-prepared carbon monoliths simultaneously exhibit micro- and macro-porosity, which were formed from the network of RMF polymer chains and the interconnection of polymer networks induced by the NCCs, respectively. Furthermore, the effectiveness of the hierarchical porous carbon monoliths as adsorbents for Cr(VI) ion adsorption and selectivity in a coexisting system was evaluated. The present synthesis approach provides a facile and low-cost route for the preparation of hierarchical porous carbon monoliths with large specific areas and controllable morphologies. Moreover, it is reasonable to assume that these porous carbon monoliths with tunable hierarchical structure and surface chemistry can be used for water treatment.

2. Materials and methods 2.1. Preparation of nanocrystalline celluloses NCCs suspension was prepared via an acid-catalyzed hydrolysis of cellulose fiber[41]. Typically, 10 g of short cotton fiber was soaked in 400 mL of 1 M NaOH solution for 1 h to remove soluble impurities, washed with deionized water several times until the supernatant was neutral, and then dried at 60 °C overnight. The alkaline-cleaned cotton was hydrolyzed in 400 g of H2SO4 solution (65 wt.%) at 60 °C for 30 min, at which time the color of the solution changed to dark yellow. The reaction was quenched by adding excess distilled water and centrifuging several times to remove the acid residues. The obtained NCCs slurry was diluted with a defined quantity of water and subjected to ultrasonication for 30 min, thus dispersing the NCCs in water homogeneously, which show much higher stability than other man-made fibers or activated ones. Then, a colloid NCCs solution of 3 wt.% in water was obtained, which was stable for several weeks. The yield for this preparation was approximately 70 wt.%. 2.2. Preparation of hierarchical porous carbon monoliths The carbon monolith was synthesized via a sol-gel process by using resorcinol, melamine and formaldehyde solution as precursors and NCCs as the structural-inducing agent. In a typical synthesis, 2.0 g

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of resorcinol (R) and 2.9 g of formaldehyde (F) (37 wt. %) were dissolved in 20 mL of distilled water and then stirred at 40 °C for over 1 h (RF). Subsequently, 1.1 g of melamine (M) and 2.2 g of formaldehyde (F) (37 wt. %) were dissolved in 20 mL of distilled water at 80 °C with continuous agitation until the melamine was completely dissolved (MF). The above MF solution was cooled to 40 °C and then added to the RF solution with stirring for 10 min. Then, a specified amount of the aqueous NCCs suspension was added and stirred for 30 min at room temperature to form a homogeneous mixture (the weight ratio of NCCs/RMF=1/25). Blank experiments without NCCs were also carried out for comparison. The solution was then transferred into sealed plastic vials and heated at 80 °C for 24 h under static conditions. The obtained polymer monolith could be directly dried in air at 80 °C overnight to afford the dry monolith (denoted as NCCs/RMF-P), with a yield of approximately 76 wt.%. Hierarchical porous carbon monolith was obtained after carbonization at 800 °C for 3 h under N2 flow (denoted as NCCs/RMF-C, with a yield of approximately 35 wt.%) and further CO2 activation at 950 °C for 2 h (denoted as NCCs/RMF-A, with a yield of approximately 58 wt.%). 2.3. Characterizations The morphology and microstructure of the samples were observed using scanning electron microscopy (SEM, JEOL 7100F) and transmission electron microscopy (TEM, JEOL 2100F) operated at 200 kV. Nitrogen adsorption/desorption isotherms were measured at 77 K with a Quadrasorb SI analyzer. Prior to the measurements, samples were degassed under vacuum at 473 K for 12 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface area (SBET). The total pore volume was estimated from the adsorbed amount at P/P0 = 0.985. The pore size distributions were calculated from the adsorption isotherms by nonlocal density functional theory (NLDFT-cylindrical model). Elemental analysis was carried out using an Elementar Vario EL III analyzer. The surface composition of the porous carbon was obtained from an Axis Ultra DLD X-ray photoelectron spectrometer at 15 kV and 10 mA. The working pressure was lower than 2×10-8 Torr (1 Torr = 133.3 Pa). The C1s, N1s and O1s XPS spectra were measured at a 0.1 eV

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step size. The N1S XPS signals were fitted with mixed Lorentzian-Gaussian curves, and a Shirley function was used to subtract the background. The zeta potential of the polymeric precursor solution was determined by the British Malvem Zetasizer Nano-ZS type of nano particle sizer. 2.4. Adsorption test The Cr(VI) ion adsorption experiments were conducted using the batch equilibration technique at 30 °C. A stock solution of Cr(VI) (1000 mg L -1) was prepared by dissolving K2Cr2O7 into distilled water. The test solutions with different initial concentrations (20 - 400 mg L-1) were obtained by diluting the stock solution as required. The pH value for the test solution was adjusted using 0.1 M HCl and 0.1 M NaOH solution. The concentrations of Cr(VI) solutions before and after adsorption were measured using a UV-vis spectrophotometer (Shimadzu UV-2550) at the adsorption maximum of 540 nm. For the selective adsorption experiment, K2Cr2O7, Ni(NO3)2, Zn(NO3)2 and Cu(NO3)2 were used as the precursors to prepare the solutions with the initial metal ion concentration of 60 mg L -1 (pH=3). The final concentration of these metal ions were measured by the ICP-MS technique (NexlON 300). All materials were dried overnight at 100 °C prior to analysis. The dosage of the adsorbent is fixed at 0.2 g L-1. The equilibrium adsorption amount of metal ions, qe (mg g-1) was calculated as according to the below equation:

where V is the volume (L) of the metal ion solution, C0 and Ce are the initial and equilibrium concentrations (mg L-1) of metal ions in solution, and m is the weight (mg) of the adsorbent.

3. Results and discussion Fig. 1 shows the schematic illustration for the synthetic route of carbon monoliths, which suggests that the structure is induced well by the NCCs. Typically, cotton cellulose contains amorphous and crystalline regions[42]. The amorphous or paracrystalline regions are easier to hydrolyze due to the weak combination, while the crystalline regions show higher resistance to acid attack during the hydrolysis process, exhibiting different degrees of hydrolysis. Hence, the nanocrystal residuals (or NCCs) can be isolated by controlling

7

the reaction time, which show a rod-like morphology with a width of 5-10 nm and a length of 100-300 nm (shown in Fig. 2). The NCCs could disperse well in water and form a 3D transparent colloid due to the electrostatic repulsion of the surface hydroxyl groups[42-44]. Resorcinol, melamine, and formaldehyde could react along the axis of rod-like NCCs to crosslink each other into small clusters, and then continuously grow by further condensing into polymer networks. Due to the structural rigidity of the networks, the macropores among network units could be retained during drying at ambient pressure. After high-temperature pyrolysis and CO2 activation, the polymer networks convert into porous carbons, showing hierarchical micro-/macropores. The total synthesis process is facile, low cost and may have a great potential for use in industrial scale production.

Fig. 1. Schematic illustration of the NCCs/RMF monolith preparation.

Fig. 2. Optical image of NCCs dispersed in water (a) and SEM image of NCCs (b). 8

The typical SEM image of the as-prepared monolith NCCs/RMF-P is shown in Fig. 3a. Remarkably, a uniform 3D framework is formed when adding NCCs into the RMF system, and the macroporous structure generated from the condensation of RMF clusters could be observed. To better understand the role of NCCs in the reaction, experiments without NCCs were carried out for comparison. As seen in Fig. 3b, the pure RF system without any catalysts only forms nonuniform polymer microspheres under hydrothermal conditions, while the co-condensation of MF with RF successfully generates homogeneous RMF microspheres with much smaller diameters (Fig. 3c). Additionally, when NCCs were added into the pure RF system (Fig. 3d), spherical products alternated with fibrous aggregations were observed. The results demonstrate that only part of the RF reacts on the surface of NCCs, while some spheres also form simultaneously, indicating that the orientation effect of NCCs on this system was not sufficiently strong. Therefore, it is obvious that NCCs could prevent the formation of microspheres and guide the growth of the RMF polymer, leading to the formation of a uniform network. To further explore the role of the NCCs, the zeta-potential values of the RF, RMF and NCCs were measured to evaluate the surface charge properties. The obtained results show that the NCCs and RF are both negatively charged with zeta-potential values of -47.2 mV and -34.4 mV, respectively, while the RMF has a positively charged surface with a value of +0.86 mV. Hence, the electrostatic attraction between the NCCs and RMF could promote polymerization of RMF oligomers on the NCCs surface in order to form a 3D crosslinking network.

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Fig. 3. SEM images of NCCs/RMF-P macroporous structure (a), pure RF spheres (b), RMF spheres (c), and NCCs/RF products (d).

Fig. 4a (inset) shows the regular cylindroid shape of the obtained polymer monolith. Direct air drying conditions did not cause any cracks on the surface of the monolith, which means that the microstructure can withstand the capillary force related to the surface tension of the liquid during the drying. Moreover, the shape of the monolith could be tailored easily by changing the reaction container, and the large-scale preparation of such monoliths is expectable. After carbonization, the NCCs/RMF-C retains the original monolithic structure, with only a slight decrease in volume (Fig. 4b, inset), suggesting that the inner 3D network has high strength and can sustain the volume change during the heat treatment. Meanwhile, by changing the NCCs/RMF weight ratio, block morphology could also be formed with a slightly different inner structure. As seen in Fig. 4c and 4d, higher (1/10) or lower (1/50) NCCs/RMF weight ratios both result 10

in a 3D interconnected framework. Fig. 5 shows the high-resolution SEM and TEM images of the carbon monoliths with different NCCs/RMF weight ratios. These images clearly reveal that all samples with different NCCs/RMF weight ratios exhibit a typical crosslinking 3D network structure. The major difference is that the pore wall becomes thicker by adding less NCCs into RMF solution. Therefore, the addition of the NCCs could promote the formation of the monolith with rigid 3D macroporous structures, and prevent the generation of microspheres.

Fig. 4. SEM images of (a) NCCs/RMF-P (1/25) and (b) NCCs/RMF-C (1/25) with the photographs in the right corner, SEM images of (c) NCCs/RMF-P (1/10) and (d) NCCs/RMF-P (1/50).

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Fig. 5. SEM images of (a) NCCs/RMF-P (1/10), (b) NCCs/RMF-P (1/25), (c) NCCs/RMF-P (1/50) under high resolution, and TEM images of (d) NCCs/RMF-C (1/10), (e) NCCs/RMF-C (1/25), (f) NCCs/RMF-C (1/50).

Accordingly, carbon monoliths were obtained after carbonization and CO2 activation. The N2 adsorption/desorption isotherms of the carbonized monoliths and pure RMF carbon microspheres are presented in Fig. 6a. All of these are typical type-I isotherms with an initial rapid increase in nitrogen adsorption below P/P0=0.1, indicating the presence of abundant micropores. The pore size distributions obtained using the NLDFT model are shown in Fig. 6b, and demonstrate that in all samples, the majority of micropores are narrow with an average pore size approximately 0.7 nm, and a small fraction have a pore size of approximately 1.6 nm. The calculated porosity parameters are listed in Table 1. The BET surface area of NCCs/RMF-C with different weight ratios and pure RMF carbon spheres are all approximately 630 m2 g-1. Meanwhile, the total pore volume and average pore size values for different weight ratios and pure RMF

12

carbon spheres are fairly similar to each other. No obvious mesopores are observed in Fig. 6. These results suggest that the micro-porosity of NCCs/RMF-C monoliths mainly originates from the polymerization of the RMF polymer chains, while the existence of NCCs has little effect on the micro-structure and surface area of the monoliths.

Fig. 6. (a) N2 adsorption/desorption isotherms and (b) DFT pore size distributions of samples.

Table 1. Pore parameters of NCCs/RMF carbon monoliths and pure RMF spheres. SBETa

Smicb

Vt c

Vmicd

DDFTe

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

(nm)

NCCs/RMF-C (1/10)

619

547

0.29

0.22

0.61

NCCs/RMF-C (1/25)

629

556

0.30

0.22

0.72

NCCs/RMF-C (1/50)

630

558

0.29

0.22

0.72

RMF spheres-C

664

629

0.27

0.24

0.61

NCCs/RMF-A (1/25)

1808

1551

0.86

0.64

0.67

Samples

a

BET specific surface area from N2 adsorption. b DFT micropore surface area (< 2 nm). c Total pore volume

(P/P0 = 0.985). d DFT micropore volume (< 2 nm). e DFT pore diameter.

Since the porosities of carbon monoliths with different NCCs/RMF weight ratios are not significantly 13

different, NCCs/RMF-C (1/25) was chosen for CO2 activation to further enlarge the surface area. The SEM image shown in Fig. 7a illustrates that the activated carbon monolith maintains the 3D network with the amorphous carbon backbone (Fig. 7a, inset). The N2 adsorption/desorption isotherm (Fig. 7b) reveals that the BET surface area dramatically increases to 1808 m2 g-1, giving a hierarchical porous carbon with connected macropores and rich micropores. The chemical compositions of the NCCs/RMF-C (1/25) and NCCs/RMF-A (1/25) monoliths were characterized by elemental analysis and XPS survey (Fig. 7c and 7d). As summarized in Table 2, NCCs/RMF-C has a high nitrogen content of 4.0 wt.%, while it decreases to 2.0 wt.% after CO2 activation for NCCs/RMF-A. The high-resolution XPS analysis presents similar results, suggesting the uniform dispersion of nitrogen on the carbon backbone. The nitrogen peaks are represented by three peaks at the binding energies of 398.5  0.3 eV, 401.3  0.3 eV, and 403.6  0.3 eV, which are attributed to pyridinic N, graphitic N, and oxynitride, respectively[45, 46]. These nitrogen-containing sites are favorable for the adsorption. Moreover, a certain amount of oxygen also suggests the existence of oxygen functional groups. Hence, because the surface chemistry of porous carbons essentially depend on their surface functional groups, this may be beneficial for their applications in many fields such as adsorption and electrochemistry[3, 47-49].

Table 2. Chemical compositions of NCCs/RMF-C and NCCs/RMF-A measured by elemental analysis and XPS techniques. Elements analysis

XPS results

Samples N

C

O

N/C

N

C

O

N/C

wt. %

wt. %

wt. %

at./at.

wt. %

wt. %

wt. %

at./at.

NCCs/RMF-C

4.0

77.8

14.7

0.044

4.1

87.8

8.1

0.040

NCCs/RMF-A

2.0

86.0

9.5

0.020

2.3

92.6

5.1

0.021

(1/25)

14

Fig. 7. (a) SEM image of NCCs/RMF-A (1/25) with HRTEM in the right corner, (b) N2 adsorption/desorption isotherm and DFT pore size distribution of NCCs/RMF-A, (c) XPS survey and (d) high resolution N1s spectra of NCCs/RMF-C and NCCs/RMF-A.

The unique structural and chemical features of the prepared hierarchical porous carbons make them promising materials for use in heavy metal ions adsorption. Therefore, the Cr(VI) ion adsorption performance of NCCs/RMF-C and NCCs/RMF-A at 30 °C (at pH=1, selected based on the effect of pH shown in Fig. S1) were investigated, and the adsorption isotherms are presented in Fig. 8a. The results show that the activated carbon monolith has a much higher Cr(VI) adsorption capacity of 463 mg g-1 than the carbonized monolith (only 178 mg g-1), and that the adsorption capacity is even higher than those of most reported activated carbons (Table S1). Such a high capacity is mainly attributed to the high surface area. The heteroatom-doped carbon surface with nitrogen and oxygen functional groups should also show affinity to 15

the HCrO4- ion, which is the predominant Cr(VI) species in the low pH range[50, 51]. The Langmuir and Freundlich models are used to fit the adsorption behavior of NCCs/RMF-C and NCCs/RMF-A (Figs. S2 and S3). The calculated parameters are given in Table S2. The Langmuir model fits better with a higher value of correlation coefficient (R2), suggesting a homogenous surface and monolayer adsorption process[52] for both samples. The effect of adsorption time on the adsorption capacity for the two samples was also investigated and results are shown in Fig. S4. The results indicate that the Cr(VI) adsorption capacity increases with time until equilibrium after 24 h. The pseudo-first-order and the pseudo-second-order kinetic models were applied to understand the adsorption behaviors (Figs. S5 and S6). Examination of the calculated parameters summarized in Table S3 reveals that the pseudo-second-order kinetic model has higher R2, indicating that the rate-limiting step in the adsorption process is mainly chemisorption[52, 53]. Furthermore, the adsorption capacity increases with increasing temperature (Fig. S7), which may be attributed to the increasing mobility of the Cr(VI) ions induced by higher energy. Thus, the overall adsorption process of NCCs/RMF-A is an endothermic process. The positive H0 value also confirms that the adsorption process is endothermic (Table S4). In this regard, the adsorption mechanism can be ascribed to the advanced physical structure and chemistry interaction with the nitrogen-doped carbon surface. In addition, we further studied the selective adsorption ability of NCCs/RMF-A monoliths toward a mixed solution with several different heavy metal ions. Here, we used Ni(II), Zn(II) and Cu(II), which are very common in water contamination, as the coexisting metal ions. The initial concentration of each metal ion was 60 mg L-1 at pH=3.0. Fig. 8b shows the effect of coexisting metal ions for Cr(VI) adsorption, and the results indicate that the Cr(VI) adsorption capacity is much higher than that of the other metal ions. This high selectivity may be partly due to the surface charge properties of the NCCs/RMF-A. As shown in Fig. S9, the zeta potential value of the carbon surface was measured, which was assumed to be associated with the solution pH value. Under the test conditions of pH=3, the carbon surface is positively charged with a zeta potential value of +25 mV, which may be favorable for the adsorption of HCrO4- and improves the

16

adsorption capacity of Cr(VI). Meanwhile, the radius of HCrO4 - is much smaller than the hydrated radius of other metal ions (HCrO4- 0.230 nm < Ni(II) 0.404 nm < Cu(II) 0.419 nm < Zn(II) 0.43 nm)[54-56], allowing this ion to penetrate more easily through the boundary layer with a lower diffusion resistance in the micropores to reach the inner pores. Therefore, this makes the newly developed porous carbon monolith a competitive candidate for use as the adsorbent in future applications.

Fig. 8. (a) Adsorption isotherms of NCCs/RMF-C and NCCs/RMF-A on Cr(VI) removal at pH=1, (b) metal uptake performance for single/multi-components of the NCCs/RMF-A monolith at pH=3.

4. Conclusions In conclusion, a novel hierarchical porous carbon monolith with a 3D interconnected structure was prepared by a simple NCCs-assisted sol-gel process using resorcinol-melamine-formaldehyde as precursors. The NCCs was first used as the structural-incucing agent to form carbon monoliths with 3D frameworks, which has been verified to orient the growth of the polymer chain network. This rigid structure derived from the NCCs and RMF resin network could withstand collapse during the air dry process and CO2 activation at 950 °C, exposing no cracks on the surface of monoliths. The resulting porous carbon monolith has a hierarchical structure with micro-/macro pores, which are formed from the crosslinking of RMF polymer chains and the interconnection polymer networks, respectively. Such hierarchical porous carbon shows a high specific surface area of 1808 m2 g-1 and well-developed porous structure, which is beneficial for Cr(VI) 17

removal. Adsorption experiments demonstrate that the hierarchical porous carbons are excellent adsorbents for Cr(VI) with an adsorption capacity of 463 mg g-1 at pH=1, which is superior to that obtained in most of the recent studies.[53, 57, 58] In addition, the developed porous carbon monolith exhibits a high selectivity for Cr(VI) ions in a co-existing system of Ni(II), Zn(II) and Cu(II). Therefore, these new hierarchical porous carbons with controllable hierarchical porosity have good potential for application in water treatment. The improvement of the monolith strength, adjustment of the carbon surface chemistry, and their performance in a more realistic water treatment system will be done in our future work.

Acknowledgements This work was financially supported by MOST (2014CB239702), the National Natural Science Foundation of China (No. U1303291, 21506061, 21576090), the Petro China Innovation Foundation (No. 2015D50060405), the Fundamental Research Funds for the Central Universities and the Shanghai Rising-Star Program (17QB1401700).

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GRAPHICAL ABSTRACT Hierarchical porous carbon monolith was synthesized through nanocrystalline cellulose (NCCs) assisted sol-gel process. The obtained hierarchical porous carbons with rigid 3D structure show high adsorption capacity and selectivity for Cr(VI) ion.

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