Accepted Manuscript

Waste-Cellulose-Derived Porous Carbon Adsorbents for Methyl Orange Re-

Baofen Sun, Yanan Yuan, Hongliang Li, Xingyun Li, Chuanhui Zhang, Fuan
Guo, Xuehua Liu, Kean Wang, X.S. Zhao

PII: S1385-8947(19)30809-5
DOI: https://doi.org/10.1016/j.cej.2019.04.031
Reference: CEJ 21451

To appear in: Chemical Engineering Journal

Received Date: 12 January 2019

Please cite this article as: B. Sun, Y. Yuan, H. Li, X. Li, C. Zhang, F. Guo, X. Liu, K. Wang, X.S. Zhao, Waste-
Cellulose-Derived Porous Carbon Adsorbents for Methyl Orange Removal, Chemical Engineering Journal (2019),
doi: https://doi.org/10.1016/j.cej.2019.04.031

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Waste-Cellulose-Derived Porous Carbon Adsorbents for Methyl

Orange Removal

Baofen Sun a, Yanan Yuan a, Hongliang Li a, Xingyun Li a, Chuanhui Zhang a, Fuan Guo a, Xuehua

Liu a, Kean Wang b and X. S. Zhao a,c,*

a
Institute of Materials for Energy and Environment, College of Materials Science and Engineering,

Qingdao University, Qingdao 266071, China

b
Department of Chemical Engineering, Khalifa University of Science & Technology, Abu Dhabi,

United Arab Emirates

c
School of Chemical Engineering, The University of Queensland, St Lucia, Brisbane, QLD 4072,

Australia

Corresponding Authors:

TEL: +86-532-85951486; E-mail: chezxs@qdu.edu.cn; george.zhao@uq.edu.au (X. S. Zhao)

1
Graphical abstract:

2
Abstract

High-performance nitrogen-doped porous carbon adsorbents were prepared from waste cellulose

fibers using the spray-drying method and used to remove methyl orange (MO) from water. The

carbon adsorbents possessed a honeycomb-like turbostratic microstructure with hierarchical pores.

Such waste-cellulose-derived carbon adsorbents were found to exhibit a fast adsorption rate towards

MO. A sample thermally treated at 800 oC with the highest specific surface area (about 1259.4 m2/g)

and total pore volume (about 2.7 cm3/g) exhibited the best MO adsorption capacity (337.8 mg/g),

which is significantly higher than that of ZnCl2-activated carbon. The effect of MO initial

concentration, pH and temperature on the carbon adsorption was investigated systematically. It was

found that the adsorption system is heterogeneous while the isotherm data on the carbon sample can

be well described by both Langmuir and Freundlich isotherm models. The high-performance and

cost-effective carbon adsorbent described in this paper holds a great promise for dye removal from

aqueous solutions.

Keywords: Waste cellulose fibers; porous carbon; methyl orange; adsorption

3
1. Introduction

Many industrial processes discharge a large number of dye-containing wastewaters without

effective treatment. Such wastewaters pose threats to the environment because they are toxic and

carcinogenic [1, 2]. Amongst the various technologies available for the treatment of dye-pollutant

wastewaters [1-3], adsorption is the most competitive method because of a number of advantages,

such as low cost, high efficiency, and easy operation [4-6]. Currently, activated carbon is the most

popularly used adsorbent [7-9]. Activated carbon is generally produced from coal or petroleum

pitch [10, 11]. Due to concerns of cost and resource availability, researchers have been looking for

alternative carbon sources (e.g., biomasses and industrial wastes) to prepare porous carbon

materials with desired properties [12-19]. Jawad et al [20] have successfully transformed coconut

leaves into activated carbon for the removal of methylene blue. Guo et al [21] have reported the

preparation of nitrogen-rich carbon from phragmites australis with a superior adsorption capacity

towards Cd(II) ions. Industrial laundry sewage sludge has been converted into mesoporous

activated carbon by Silva and co-workers for removing reactive dyes from aqueous solutions [18].

Being the most abundant natural polysaccharide polymer on earth, cellulose has attracted

increasing attention for different applications [22-24]. As a result, a large amount of cellulose-rich

wastes is produced from many industries (e.g. textile industry, paper industry and agriculture). It is

important to convert such cellulose wastes into value-added products for a sustainable future [25,

26]. Fibrous adsorbents have been prepared from waste textile fibers for the removal of heavy

metals [27]. Natural cellulose fibers have also been used as carbon source to prepare activated

carbon adsorbents for wastewater treatments [28]. Waste cellulose-derived materials have been

employed for determination of Ag(I) coordination anions [29]. In utilization of waste cellulose,

however, pre-treatment or modification is needed for improving the properties of the resulting

functional materials [27, 30].

4
 Many practical applications require porous carbon materials with designated physical and

chemical properties [16, 31, 32]. Pore structure and surface properties are two important parameters

affecting the adsorption of porous carbon materials in aqueous solutions [33]. Therefore, control

over these parameters of a carbon plays a crucial role in developing high-performance carbon

adsorbents for the adsorptive removal of bulky dye molecules [34, 35]. Both physical and chemical

activations as well as the template strategy are often employed to control pore structure [30, 36-38].

Surface modification and element doping are also common means used for modifying surface

properties [31, 39].

In this paper, we demonstrate an approach to prepare nitrogen-doped porous carbons (NPCs)

with a hierarchical pore structure using waste cellulose fibers as the precursor without complicated

pretreatment. In this method, zincoxen solution plays multiple roles, including dissolving waste

cellulose fibers, providing nitrogen source, and templating the formation of pores. The NPCs

exhibited a high adsorption capacity towards methyl orange in aqueous solution.

2. Materials and Methods

2.1. Materials

Waste cellulose fibers were obtained from Qingdao ST. Meer Fiber Technology Co., Ltd.

Ammonium hydroxide, zinc chloride, ethylenediamine (EDA), nitric acid, hydrochloric acid (HCl),

sodium hydroxide (NaOH), and methyl orange (C14H14N3NaO3S) were obtained from Sinopharm

Chemical Reagent Co., Ltd, and used without further purification. Distilled water was used in all

experiments.

2.2. Preparation of samples

The NPCs were prepared via a spray drying process followed by thermal treatment. First, a

30% of zincoxen aqueous solution was prepared and used to solubilize cellulose fibers [40]. Then,

5
X g of dried waste cellulose fibers was added to 40 mL of the zincoxen aqueous solution at room

temperature. After being aged at -10 oC overnight, the mixture was thawed at room temperature and

spray dried to obtain solid Zn(EDA)32+/cellulose microspheres, which were subsequently calcined at

Y oC in a N2 atmosphere for 3 h. After washing with a 1.0 M nitric acid solution to remove ZnO

generated from Zn(EDA)32+, then with deionized water and ethanol, the solid product was dried at

80 oC for 12 h to obtain a carbon sample. By varying the mass of cellulose fibers and thermal

treatment temperature, different samples were prepared and are denoted as NPCs-X-Y (X = 0.5, 1.5,

2.5 in g was the mass of cellulose fibers used while Y = 500, 600, 700, 800 in oC was the thermal

treatment temperature). For example, NPCs-0.5-800 represents the carbon sample which was

prepared with 0.5 g of cellulose fibers and thermally treated at 800 C for 3 h.

For comparison purpose, another carbon sample was prepared according to the procedure

described above, but 0.5 g of cellulose power instead of waste cellulose fibers was used as carbon

source and ZnCl2 instead of zincoxen aqueous solution was employed as the activating agent. The

calcination temperature was 800 oC. This sample is denoted as PCs-0.5-800.

2.3. Characterization

Samples were characterized using X-ray diffraction (XRD, a RigakuUltima IV X-ray diffractometer,

Cu-Kα radiation, λ = 0.15418 nm), field emission scanning electron microscope (FESEM, JEOL

JSM-7800F), transmission electron microscope (TEM, JEOL JEM-2100) and X-ray photoelectron

spectroscopy (XPS, PHI Quantera II spectrometer) equipped with an anode of Monochromated Al

Kα radiation (1486.6 eV) techniques. Nitrogen adsorption/desorption isotherms were measured on a

Quantachrome Autosorb-IQ3. The specific surface areas were estimated by using the

Brunauer−Emmett−Teller (BET) method with the N2 adsorption data in the relative pressure range

of P/P0 = 0.005-0.2. The pore size distribution curves were derived using the density function

theory (DFT) model applied to the adsorption branch of N2 isotherms. The micropore volume (Vm)

and micropore surface area (Sm) were determined by using the t-plot method. The external pore

6
volume (Ve) and external surface area (Se) were determined by subtracting the Vm and Sm from the Vt

and St, respectively. UV-Vis spectra were collected on a UV-visible spectrophotometer (TU-1901).

2.4. Adsorption measurements

A methyl orange (MO) stock solution of concentration 1000 mg/L was prepared and then

diluted to different concentrations. The adsorption experiments were conducted using the batch

mode as described elsewhere [18, 41, 42]. The experiments in this work were carried out at room

temperature (293 K) without pH adjustment unless otherwise specified. To measure adsorption

isotherms, 0.02 g of solid sample was dispersed in 25 mL MO solutions of various initial

concentrations under stirring for 24 h, and then subject to sampling and analysis. To evaluate the

effect of temperature on the adsorption system, MO adsorption isotherms were measured at

different initial concentrations (in the range of 50 to 400 mg·L-1) at 278, 293 and 323 K,

respectively. The effect of pH on adsorption was investigated with the initial MO concentration of

200 mg/L, of which the pH was adjusted between 2.6 and 10.8 using HCl and NaOH solution,

respectively. For kinetic experiments, 0.24 g of NPCs was added into 300 mL of MO solution with

100 mg/L and 200 mg/L under stirring. At given time intervals, 1 mL of sample was withdrawn

from the suspension, then centrifuged to separate the liquid phase.

The concentrations of dye solutions were determined using a UV–Vis spectrophotometer at the

wavelength of 464 nm. A calibration curve of absorbance–concentration was established based on

Beer–Lambert’s law. The amount of MO adsorbed at equilibrium, qe (mg/g), was calculated using

Equation (1) below:

qe  (C0  Ce ) V / m (1)

where, V (L) is the volume of the MO solution; C0 and Ce (mg/L) are the initial and equilibrium

concentrations of MO, respectively; m (g) is the mass of the adsorbent.

7
3. Results and Discussion

3.1. Characterization of samples

Fig. 1a shows the XRD patterns of NPCs-0.5-800 before and after washing with 1.0 M nitric

acid solution. The diffraction peaks of ZnO (PDF No.36-1451) can be seen from the sample before

nitric acid washing, suggesting the formation of crystalline ZnO during the thermal treatment. After

washing with 1.0 M nitric acid, the sample exhibited only two broad peaks at 25.0 and 43 degrees

two theta, corresponding to the (002) and (10) planes of graphitic carbon [43]. The (002) diffraction

indicates a randomly oriented aromatic carbon sheets, while the diffraction of (10) bands can be

corresponded to the superposition of (100) and (101) planes, indicating a turbostratic structure [44,

45]. Such a turbostratic structural characteristic has been known to facilitate the adsorption of dyes

[46]. Similar XRD patterns can be observed on other NPCs (see Fig. S1), suggesting their similarity

of carbon structure and complete removal of ZnO by nitric acid washing.

Fig. 1. (a) XRD patterns, (b) N2 adsorption-desorption isotherm, and (c) pore size distribution of

NPCs-0.5-800.

Fig. 1b shows the nitrogen adsorption-desorption isotherms of NPCs-0.5-800. A composite of

Type I and Type IV isotherm with a H4 hysteresis loop is seen, according to the IUPAC

classification [47]. The hysteresis at the relative pressure range between 0.45 and 1.0 is the

characteristic of a mesoporous material, while the steep rise in adsorption in the relative pressure

range between 0 and 0.05 indicates the presence of micropores [46, 48, 49]. The hierarchical pore

8
structure was further verified by the pore size distribution derived from the DFT method (Fig. 1c). It

is seen that both micropores and mesopores were present in the sample. It is believed that the

formation of micropores was attributed to zinc activation and removal of oxygen-containing

moieties during the thermal treatment process [49], while the mesopores and macropores (see Fig.

2b) were evolved from the nitric acid etching of ZnO particles.

The nitrogen adsorption-desorption isotherms of other NPCs samples are depicted in Fig. S2,

which shows similar isotherms for all NPCs but different hysteresis loops for the NPCs-0.5

prepared at different temperatures. As thermal treatment temperature increased, the type H2(b)

loops changed to type H4 loops as shown in Fig. S2 (b). According to the IUPAC technical report,

the characteristic type of hysteresis loop is fairly closely related to particular features of the pore

structures [47]. The type H2(b) loop is attributed to pore-blocking with a large range of pore necks,

while type H4 loop is ascribed to the slits or interstices pores caused by non-rigid aggregates of

plate-like particles and the pore network consisting of macropores [47]. Therefore, it can be

concluded that, as thermal treatment temperatures increased, the pore necks can be destroyed and

the slits are gradually formed between the turbostratic carbon structures, which is supported by

XRD pattern above and the following TEM image (Fig. 2(c)). The slits or macropores in carbon

samples are favorable for the diffusion and adsorption of large dye molecules.

Table 1 compares the total BET surface area (St), t-plot micropore surface area (Sm), external

surface area (Se), total pore volume (Vt), micropore volume (Vm), meso/macropore volume (Ve) and

average pore sizes (APS) of the different carbon samples prepared under different experimental

conditions. It can be observed that, at the same thermal treatment temperature of 500 oC, NPCs-0.5

has the smallest St while the biggest external surface area proportion (Se/St), which indicates that the

increase in BET surface area of NPCs-1.5 and NPCs-2.5 is attributed to the increase of micropore

surface area. In addition, the increase in the mass of cellulose fibers decreased the Vt, Ve and APS.

These results indicate that the increase of dissolved cellulose fibers is favorable for the formation of

9
micropores but not for the formation of mesopores, which is further verified by the detailed pore

size distribution in Figs. S3a and S3b. As the thermal treatment temperature increased, St, Vt and Ve

increased (Table 1) and the larger pore size can be formed (Figs. S3c and S3d). It appears that 800
o
C is the optimal thermal treatment temperature to yield a sample with a BET specific surface area

of 1295.4 m2/g and a pore volume of 2.7 cm3/g.

Table 1. Specific Surface Area, Pore Volume and Average Pore Size (APS) of PCs samples

St Sm Se Vm Ve Vt APS
Samples Se/St Ve/Vt
2
(m /g) 2
(m /g) 2
(m /g) 3
(cm /g) 3
(cm /g) (cm3/g) (nm)

NPCs-0.5-500 516.3 277.5 238.8 0.46 0.11 0.75 0.86 0.87 6.7

NPCs-1.5-500 676.5 383.8 292.7 0.43 0.19 0.49 0.68 0.72 4.0

NPCs-2.5-500 565.5 382.5 183.0 0.32 0.15 0.35 0.49 0.70 3.5

NPCs-0.5-600 1049.6 727.3 322.3 0.31 0.35 1.12 1.47 0.76 2.6

NPCs-0.5-700 1139.9 871.4 268.5 0.24 0.31 1.68 2.00 0.84 3.8

NPCs-0.5-800 1295.4 891.4 404.0 0.31 0.40 2.29 2.69 0.85 4.2

PCs-0.5-800 1105.3 1025.1 80.2 0.07 0.43 0.06 0.49 0.13 0.9

In contrast, PCs-0.5-800 (the sample activated with ZnCl2) exhibited a typical Type-I isotherm

(Fig. S4a), the characteristic of a microporous carbon structure. The pores are mainly distributed at

0.6, 0.8 and 1.2 nm in the micropore region (Fig. S4b), while the size of a MO molecule is 1.31 

0.55  0.18 nm3 [37]. Therefore, although the specific surface area of 1105.3 m2/g, the smaller pore

size 0.9 nm) and volume (~0.5 cm3/g only) are unfavorable for the adsorption/diffusion of the bulky

dye molecules [33, 34], due to the strong diffusion resistance to MO molecules. From the

perspective of adsorption technology, carbon NPCs-0.5-800 represents an excellent candidate for

dye removal.

The FESEM images of NPCs-0.5-800 in Figs. 2a and 2b revealed a porous, honeycomb like

structure, with macropores. The TEM image (Fig. 2c) further confirmed the pore size of the

10
macropores ranging from about 70 to 120 nm in diameter and the turbostratic structure. Besides,

mesopores of about 20 nm in diameter can also be seen (Fig. 2d), in accordance with the pore size

distribution showed in Fig. 1c. Such hierarchical porous carbon structure is favorable for the

molecule diffusion and mass transfer [34, 50]. From the FESEM images in Fig. S5, it can be seen

that the NPCs samples prepared with high cellulose fiber contents showed a low porosity (Figs.

S5a-S5c), which is consistent with the average pore sizes listed in Table 1. It is also seen that larger

pores were formed at higher thermal treatment temperature (Figs. S5d and S5e). These results

indicate that higher thermal treatment temperature and high zinc contents are beneficial for the form

of hierarchical porous structure, thereby exposing more adsorption sites for dye adsorption. As the

FESEM image in Fig. S5f, PCs-0.5-800 revealed a bulky shape without meso-/macropores, which

was consistent with the DFT analysis.

Fig. 2. FESEM (a and b) and TEM (c and d) images of NPCs-0.5-800

11
 Fig. 3a shows the XPS survey of NPCs-0.5-800, of which the peaks at 284.8 eV, 400.1 eV and

531.5 eV correspond to C 1s, N 1s and O 1s, respectively. The nitrogen content was calculated to be

as high as 9.01%, demonstrating the facile N doping with the synthesis procedure. The N 1s

spectrum can be deconvoluted into five peaks at 398.3 eV (pyridinic-N), 399.6 eV (pyrrolic-N

and/or pyridone-N), 400.5 eV (graphitic-N), 401.5 eV (quaternary N) and 405.7 eV (oxidized N),

respectively (Fig. 3b) [51, 52]. Fig. S6 shows the N 1s spectra of different NPCs prepared at other

conditions. No obvious shift can be detected for all the peaks of N 1s spectra and similar five peaks

were deconvoluted. Fig. 3c shows the deconvolution of C 1s spectrum of NPCs-0.5-800 with the

main peak at 284.8 eV corresponding to the graphitic carbon (sp2 carbon) [53], and the peaks at

289.6, 287.8 and 286.2 eV attributed to the different bonding structures of the O-C=O, O-C-O/C=O,

and C-N/C-O/C=N bonds, respectively [52-54]. These results suggested the existence of abundant

functional groups and N-, O- in the NPCs samples.

Fig. 3. (a) XPS survey spectrum, (b) N 1s and (c) C 1s peak deconvolution spectra of NPCs-0.5-800.

3.2. Formation of NPCs

Fig. 4 illustrates the proposed mechanism for the formation of NPCs. Cellulose is composed of

-linked D-glucose chains crosslinked by numerous hydrogen bonds [23]. Upon the soaking in

zincoxen solution, the swelling of cellulose occurred [55], as Zn(EDA)32+ molecules strongly

interact with the –OH groups to break the intramolecular hydrogen bonds and form the soluble

cellulose-zinc complexes. Spherical zincoxen/cellulose composite droplets were formed upon the

12
spray-drying process and the zinc species were uniformly dispersed in the cellulose matrix. During

thermal treatment, Zn(EDA)32+ was decomposed to ZnO while cellulose was decomposed to form

carbon matrix encapsulates embedded with ZnO particles. Simultaneously, NH3 was released from

the decomposition of Zn(EDA)32+, which played the role of in-situ doping of nitrogen into the

carbon matrix while Zn did the role of activation. The NPCs were obtained after ZnO particles were

etched away with dilute nitric acid.

Fig. 4. A scheme showing the formation mechanism of NPCs.

3.3. Adsorption of MO

Fig. 5a shows the adsorption isotherms of MO on different carbon samples measured at room

temperature. The maximum adsorption capacity of different samples based on Langmuir adsorption

isotherm is listed in Table 2. It can be seen that sample NPCs-0.5-800 exhibited the best

performance among all NPCs samples. The adsorption capacity increased with the increase of

thermal treatment temperature, indicating that high thermal treatment temperature is favorable for

enhancing the adsorption capacity of MO. This is in agreement with the data in Table 1 because

higher BET surface area and pore volume were obtained at higher thermal treatment temperatures.

However, the adsorption capacity of NPCs decreased as the mass of dissolved cellulose fibers

13
increased, which is inconsistent with the change in BET surface area. Therefore, the BET surface

area is not necessarily proportional to the adsorption capacity, as has been observed from a

methylene blue (MB) adsorption system [56].

Pore size distribution and pore volume also play an important role in the adsorption system. It

can be seen that at 500 oC, the best adsorption capacity of NPCs-0.5-500 can be attributed to the

largest pore size (Table 1) and more mesopores (Figs. S3a). Fig. S7 reveals that the change of the

adsorption capacity is consistent with the changes of pore size and pore volume. The results shown

in Fig. S8 clearly indicate the relationship between external pore volume Ve and adsorption capacity.

Apparently, Ve is a critical factor for affecting the adsorption capacity of MO on the adsorbent.

Therefore, the increase in pore size and surface area promotes the adsorption of MO on the

adsorbents.

The pH of the dye solution is an important parameter affecting the adsorption capacity, because

it changes the surface charge as well as the solubility of the dye. As shown in Fig. 5b, the

adsorption capacity of MO on NPCs-0.5-800 decreased (from 346 to 208 mg/g) as the pH increased

from 2.6 to 10.8. Under acidic conditions, the carbon surface is ionized to carry a positive charge,

thus significantly enhancing adsorption forces towards anionic MO dye via electrostatic interactions

[57]. In addition, at very low pH, MO molecule carries both positive and negative charges to form

an intermolecular salt, resulting in a decrease in hydration between MO and water molecules [58] as

schematically illustrated in Scheme 1. Therefore, MO molecules easily adsorb on the surface of the

carbon adsorbent. In contrast, increasing pH would cause increased electrostatic repulsions between

the negatively charged surface and the dye molecules, leading to the reduced adsorption capacity.

14
 Scheme 1. Molecular structure of dye MO under acidic and basic conditions

Fig. 5. (a) Adsorption isotherms of MO on samples (adsorbent: 0.02 g; MO: 25 mL; T: 293 K; pH:

7.1). (b) effect of pH on the adsorption capacity of MO on sample NPCs-0.5-800 (adsorbent: 0.02 g;

MO: 25 mL; T: 293 K). (c) effect of temperature on the adsorption of MO on sample NPCs-0.5-800

(adsorbent: 0.02 g; MO: 25 mL; pH: 7.1). (d) Langmuir isotherms for MO adsorption on NPCs-0.5-

800 and PCs-0.5-800 (adsorbent: 0.02 g; MO: 25 mL, T = 293 K; pH 7.1).

15
 The plots of qe versus Ce at different temperatures are shown in Fig. 5c. It can be seen that

the adsorption capacity increased with increasing temperature at the same MO concentrations,

indicating that the adsorption of MO on NPCs samples is endothermic [59]. Such a phenomenon

has also been observed on dye adsorption on other adsorbents [49]. Enamul et al reported the

endothermic nature of adsorption of MO and MB (methylene blue) on a metal-organic framework

material (MOF-235) adsorbent, and attributed this abnormality to the stronger adsorption affinity of

the pre-adsorbed water molecules on the adsorbent [60]. It is noted that, although high operating

temperature promotes the adsorption/diffusion of the dye, it also incurs higher operating costs.

Table 2. Parameters derived from Langmuir and Freundlich adsorption isotherms for samples.

Langmuir Freundlich

Samples qm KL KF
R2 1/n R2
(mg/g) (L/mg) (mg1-1/n/L1/n/g)

NPCs-0.5-500 235.8 0.2777 0.9983 129.5 0.1144 0.9689

NPCs-1.5-500 166.1 0.0787 0.9792 86.9 0.1047 0.7716

NPCs-2.5-500 125.5 0.0442 0.9788 38.4 0.1975 0.9056

NPCs-0.5-600 256.4 0.2910 0.9972 153.4 0.0938 0.9779

NPCs-0.5-700 297.6 0.3625 0.9957 147.3 0.1422 0.9666

NPCs-0.5-800 337.8 0.3834 0.9956 160.1 0.1572 0.9832

PCs-0.5-800 187.6 0.0224 0.9882 21.3 0.3739 0.9671

Both Langmuir and Freundlich isotherms were employed to describe the adsorption

equilibrium of MO on NPCs in the present work (the model equations were listed in Supplementary

Material). The fitted isotherm lines of different NPCs are shown in Fig. S9. The derived isotherm

parameters of the two models are listed in Table 2 for MO adsorption onto different NPCs. It is

found that both isotherms can adequately describe the adsorption data on NPC-0.5-800. But

Langmuir model showed some superiority for NPCs derived at lower temperature due to a better

16
correlation coefficient. A maximum adsorption capacity of 337.8 mg·g-1 was obtained for MO

adsorption on NPCs-0.5-800 according to Langmuir isotherm model.

Whether an adsorption system is favorable or unfavorable can be predicted by the analysis of

the isotherm constants. In Langmuir theory, a dimensionless constant called the separation factor,

RL, is commonly used to express the adsorption nature defined by Webber and Chakkravorti [61]. RL

can be determined by the following equation (2):

1
RL  (2)
1  C0 K L

where KL (L/mg) is the Langmuir constants and C0 (mg/L) is the initial concentration of dye. In

general, the value of RL determines the type of the isotherm, which is favorable (0 < RL < 1),

unfavorable (RL >1), linear (RL = 1) or irreversible (RL = 0). It can be found that the RL values were

lower than 1 in all adsorption systems (see Table S1), indicating that the adsorption of MO onto the

NPCs is favorable.

The value of “1/n” in the Freundlich isotherm reflects the feasibility of the adsorption process

as well as the surface heterogeneity, becoming more heterogeneous as its value approaches zero

[62]. The value of 1/n below 0.5 indicates an easy adsorption of adsorbates on adsorbents, contrarily

the adsorption hardly occurs with the value of 1/n above 1 [63]. The values of 1/n are obtained to be

less than 0.5 for all samples, which suggests MO dye is easy to be adsorbed onto the NPCs sample.

It is also seen in Table 2 that, compared with microporous PCs sample derived at the same

temperature of 800 oC, the NPCs-0.5-800 presented a more heterogeneous surface (1/n = 0.16 vs

0.37) towards the adsorption of MO molecules, possibly due to the change in surface chemistry by

the doped nitrogen molecules.

The MO adsorption isotherm on PCs-0.5-800 was also measured under the same conditions

(Fig. 5d). It can be seen that the adsorption capacity of PCs-0.5-800 was less than half of that on

PCs-0.5-800. It is well-known that the adsorption capacity depends on the specific surface area,

17
porosity, as well as the surface chemistry of the adsorbent. Herein, smaller pore size and lower pore

volume (0.5 cm3/g) made PCs-0.5-800 unfavorable for the adsorption/diffusion of the bulky dye

molecules although it has a high surface area (1105.3 m2/g). The NPCs-0.5-800 got another

advantage, i.e. its hierarchical pore textures provide more adsorption sites available for MO

adsorption at a time, resulting in high adsorption capacities as well as fast uptake rate. Moreover,

the nitrogen-groups doped in NPCs-0.5-800 altered the polarity of the surface, thus improved the

adsorption performance of carbon surface towards MO molecules [64-66]. It is noted that the

surface chemistry is a complicated issue and the role of nitrogen doping on the overall adsorption

properties deserves more in-depth studies in the future.

Table 3. Adsorption capacities (qm) of MO on various adsorbents

Adsorbents qm (mg/g) T (K) pH Ref.

N-doped mesoporous carbon (N-OMC) 135.8 298 WpA* [64]

Nitrogen-doped mesoporous carbon 284.1 298 5.6 [65]

N-doped mesoporous carbon (NMC-3- 202.4 RT** Not reported [66]

Fe3O4-N-doped mesoporous carbon 200.0 298 7 [67]

Multiwall carbon nanotubes (MWCNT) 49.9 293 Not reported [68]

Pumpkin seed powder 200.3 318 3.0 [69]

Protonated cross-linked chitosan 89.29 293 6.7 [57]

PCs-0.5-800 187.6 293 WpA* This work

NPCs-0.5-800 337.8 293 WpA* This work

*WpA: without pH adjustment; **RT: Room temperature.

18
 Various adsorbents have been reported for MO adsorption from aqueous solution [57, 67-69].

Table 3 summarized their adsorption capacities (qm) and compares with the two samples developed

in this research. It can be seen that NPCs-0.5-800 has the highest capacity among all adsorbents

under experimental conditions, emphasizing the potential of NPCs for such applications.

Two kinetics models, namely pseudo-first order (PFO) and pseudo-second order (PSO), were

applied to simulate the kinetics data of MO onto NPCs. Fig. S10a shows the plot of qt versus t at

100 mg/L and 200 mg/L of MO initial concentrations, from which a fast adsorption rate was

observed. The majority of uptake is completed in the beginning 30 min. The plots of ln(qe-qt) and

t/qt versus t for the PFO and PSO kinetic models are depicted in Fig. S10b and S10c, respectively.

The simulated parameters of two kinetic models are listed in Table S2. According to the correlation

coefficient values (R2) in Table S2, the experimental data was in good agreement with the PSO

model, best describing the whole adsorption process of removing MO from aqueous solution.

3.4. Reuse of Adsorbent

Reusability of a solid adsorbent is a crucial factor to evaluate the feasibility for practical

applications. In the present work, washing of used adsorbent with ethanol and deionized water was

employed to regenerate the adsorbent (NPCs-0.5-800). It can be seen from Fig. 6 that the adsorption

capacity decreased by ~ 20% up to the second cycle, possibly due to the strong affinity of MO

adsorbed to some special sites (functional groups or small pores). From the second cycle to the sixth

cycle, the capacity decreased slightly from 80% to 69%, possibly due to the inadequate desorption

of each cycle. Considering its excellent capacity, the regeneration results indicated that the NPCs

can be used as an effective adsorbent for MO removal with a reasonably good reusability.

19
 Fig. 6. The reusability of NPCs-0.5-800 (the initial concentration of MO was 200 mg/L).

4. Conclusions

We have demonstrated a method for preparing high-performance nitrogen-doped porous carbon

adsorbents for the removal of azo dye methyl orange in aqueous solution. In this method, zincoxen

solution plays multiple roles, including dissolving waste cellulose fibers, providing nitrogen source,

and inducing the formation of pores. The carbon materials possess a hierarchical pore structure with

micro/meso/macropores, which are favorable for adsorption of large organic molecules. Increasing

thermal treatment temperature leads to the increase in specific surface area, pore size and external

pore volume. A sample thermally treated at 800 oC was found to exhibit the best performance in the

adsorption of azo dye methyl orange from aqueous solution in terms of adsorption capacity,

adsorption rate and reusability. The maximum adsorption capacity at 293 K estimated from the

Langmuir isotherm model is about 337 mg/g. It was also revealed that the external pore volume is a

key factor influencing the adsorption properties of the porous carbon materials for the dye

molecules. The simplicity in preparation, the cost-effective and renewable carbon source, and the

superior performance in removal of dye methyl orange suggest that the porous carbon materials

described in this paper hold a great promise for practical applications.

20
Acknowledgments

This work was supported by the World-Class Discipline Program and the Taishan Scholar’s

Advantageous and Distinctive Discipline Program of Shandong Province. The Australian Research

Council (ARC) is acknowledged for partially supporting this work under Project FL170100101.

References

[1] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, Adsorption of methylene blue on low-cost

adsorbents: A review, J. Hazard. Mater., 177 (2010) 70-80.

[2] T.D. Dang, A.N. Banerjee, Q.T. Tran, S. Roy, Fast degradation of dyes in water using

manganese-oxide-coated diatomite for environmental remediation, J. Phys. Chem. Solids., 98 (2016)

50-58.

[3] H. Ali, Biodegradation of Synthetic Dyes—A Review, Water Air Soil Pollut., 213 (2010) 251-

273.

[4] I. Anastopoulos, A. Hosseinibandegharaei, J. Fu, A.C. Mitropoulos, G.Z. Kyzas, Use of

nanoparticles to dye adsorption: Review, J. Disper. Sci. Technol., 39(6) (2018) 836-847.

[5] A.A. Adeyemo, I.O. Adeoye, O.S. Bello, Adsorption of dyes using different types of clay: a

review, Appl. Water Sci., 7 (2017) 543-568.

[6] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by

adsorption: a review, Adv. Colloid Interfac., 209 (2014) 172-184.

[7] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by

adsorption on various carbons—a comparative study, Dyes Pigments., 51 (2001) 25-40.

21
[8] K.R. Ramakrishna, T. Viraraghavan, Dye removal using low cost adsorbents, Water Sci.

Technol., 36 (1997) 189-196.

[9] L. Largitte, R. Pasquier, A review of the kinetics adsorption models and their application to the

adsorption of lead by an activated carbon, Chem. Eng. Res. Des., 109 (2016) 495-504.

[10] L. Liu, S. Kumar, Z. Wang, Y. He, J. Liu, K. Cen, Catalytic Effect of Metal Chlorides on Coal

Pyrolysis and Gasification Part I. Combined TG-FTIR Study for Coal Pyrolysis, Thermochim Acta,

655 (2017) 331-336.

[11] N. Byamba-Ochir, G.S. Wang, M.S. Balathanigaimani, H. Moon, Highly porous activated

carbons prepared from carbon rich Mongolian anthracite by direct NaOH activation, Appl. Surf.

Sci., 379 (2016) 331-337.

[12] Y. Feng, H. Zhou, G. Liu, J. Qiao, J. Wang, H. Lu, L. Yang, Y. Wu, Methylene blue

adsorption onto swede rape straw ( Brassica napus L.) modified by tartaric acid: Equilibrium,

kinetic and adsorption mechanisms, Bioresour Technol, 125 (2012) 138-144.

[13] A.A. Ahmad, B.H. Hameed, N. Aziz, Adsorption of direct dye on palm ash: Kinetic and

equilibrium modeling, J. Hazard. Mater., 141 (2007) 70-76.

[14] W. Liu, C. Yao, M. Wang, J. Ji, L. Ying, C. Fu, Kinetics and thermodynamics characteristics

of cationic yellow X‐ GL adsorption on attapulgite/rice hull‐ based activated carbon

nanocomposites, Environ Prog Sust Energ, 32 (2013) 655-662.

[15] H.C. Tao, H.R. Zhang, J.B. Li, W.Y. Ding, Biomass based activated carbon obtained from

sludge and sugarcane bagasse for removing lead ion from wastewater, Bioresour Technol, 192

(2015) 611-617.

[16] A. Jain, R. Balasubramanian, M.P. Srinivasan, Hydrothermal conversion of biomass waste to

activated carbon with high porosity - A review, Chem. Eng. J., 283 (2016) 789-805.

22
[17] M.A. Yahya, Z. Al-Qodah, C.W.Z. Ngah, Agricultural bio-waste materials as potential

sustainable precursors used for activated carbon production - A review, Renewable Sustain Engery

Reviews, 46 (2015) 218-235.

[18] T.L. Silva, A. Ronix, O. Pezoti, L.S. Souza, P.K.T. Leandro, K.C. Bedin, K.K. Beltrame, A.L.

Cazetta, V.C. Almeida, Mesoporous activated carbon from industrial laundry sewage sludge -

Adsorption studies of reactive dye Remazol Brilliant Blue R, Chem. Eng. J., 303 (2016) 467-476.

[19] X.Y. Li, K. Tie, Z. Li, Y.J. Guo, Z.J. Liu, X.H. Liu, X.W. Liu, H.B. Feng, X.S. Zhao,

Nitrogen-doped hierarchically porous carbon derived from cherry stone as a catalyst support for

purification of terephthalic acid, Appl. Surf. Sci., 447 (2018) 57-62.

[20] A.H. Jawad, R.A. Rashid, M.A.M. Ishak, L.D. Wilson, Adsorption of methylene blue onto

activated carbon developed from biomass waste by H2SO4 activation-kinetic, equilibrium and

thermodynamic studies, Desalination and Water Treatment, 57 (2016) 25194-25206.

[21] Z.Z. Guo, X.D. Zhang, Y. Kang, J. Zhang, Biomass-Derived Carbon Sorbents for Cd(II)

Removal- Activation and Adsorption Mechanism, ACS Sustainable Chem. Eng., 5 (2017) 4103-

4109.

[22] D.N.S. HON, Cellulose: a random walk along its historical path, Cellulose, 1 (1994) 1-25.

[23] S. Wang, A. Lu, L.N. Zhang, Recent advances in regenerated cellulose materials, Prog Polym

Sci., 53 (2016) 169-206.

[24] C.C. Wan, Y. Jiao, S. Wei, L.Y. Zhang, Y.Q. Wu, J. Li, Functional nanocomposites from

sustainable regenerated cellulose aerogels: A review, Chem. Eng. J., 359 (2019) 459-475.

[25] J.F. Wang, W.Z. Qian, Y.F. He, Y.B. Xiong, P.F. Song, R.M. Wang, Reutilization of discarded

biomass for preparing functional polymer materials, Waste Manage, 65 (2017) 11-21.

23
[26] Suhas, V.K. Gupta, P.J. Carrott, R. Singh, M. Chaudhary, S. Kushwaha, Cellulose: A review as

natural, modified and activated carbon adsorbent, Bioresour Technol, 216 (2016) 1066-1076.

[27] J.K. Bediako, W. Wei, Y.-S. Yun, Conversion of waste textile cellulose fibers into heavy metal

adsorbents, J. Ind. Eng. Chem., 43 (2016) 61-68.

[28] N.H. Phan, S. Rio, C. Faur, L.L. Coq, P.L. Cloirec, T.H. Nguyen, Production of fibrous

activated carbons from natural cellulose (jute, coconut) fibers for water treatment applications,

Carbon, 44 (2006) 2569-2577.

[29] Z.N. Lou, S.Q. Wen, L. Wan, P. Zhang, Y.J. Wang, F. Zhang, Y. Xiong, Y. Fan, High-

Performance Waste Cellulose Based Adsorbent Used for Determination of Ag(I) Coordination

Anions, J. Polym Environ., 26 (2017) 2650-2659.

[30] A. Jain, R. Balasubramanian, M.P. Srinivasan, Production of high surface area mesoporous

activated carbons from waste biomass using hydrogen peroxide-mediated hydrothermal treatment

for adsorption applications, Chem. Eng. J., 273 (2015) 622-629.

[31] A. Bhatnagar, W. Hogland, M. Marques, M. Sillanpää, An overview of the modification

methods of activated carbon for its water treatment applications, Chem. Eng. J., 219 (2013) 499-511.

[32] Q.L. Ma, Y.F. Yu, M. Sindoro, A.G. Fane, R. Wang, H. Zhang, Carbon-Based Functional

Materials Derived from Waste for Water Remediation and Energy Storage, Adv Mater, 29 (2017)

1605361-1605380.

[33] D. Yan, H.M. Lin, F.Y. Qu, Synthesis of ferromagnetic ordered mesoporous carbons for bulky

dye molecules adsorption, Chem. Eng. J., 193–194 (2012) 169–177.

[34] X.P. Dong, J. Fu, X.X. Xiong, C. Chen, Preparation of hydrophilic mesoporous carbon and its

application in dye adsorption, Mater Lett., 65 (2011) 2486-2488.

24
[35] X.M. Peng, D.P. Huang, T. Odoom-Wubah, D.F. Fu, J.L. Huang, Q.D. Qin, Adsorption of

anionic and cationic dyes on ferromagnetic ordered mesoporous carbon from aqueous solution-

Equilibrium, thermodynamic and kinetics, J. Colloid Interface Sci., 430 (2014) 272-282.

[36] F.L. Liu, Z.B. Guo, H.G. Ling, Z.N. Huang, D.Y. Tang, Effect of pore structure on the

adsorption of aqueous dyes to ordered mesoporous carbons, Micropor Mesopor Mat, 227 (2016)

104-111.

[37] J. Goscianska, M. Marciniak, R. Pietrzak, Mesoporous carbons modified with lanthanum(III)

chloride for methyl orange adsorption, Chem. Eng. J., 247 (2014) 258-264.

[38] D.J. Kim, H.I. Lee, J.E. Yie, S.-J. Kim, J.M. Kim, Ordered mesoporous carbons: Implication of

surface chemistry, pore structure and adsorption of methyl mercaptan, Carbon, 43 (2005) 1868-

1873.

[39] S.Z. Xu, Y.L. Lv, X.F. Zeng, D.P. Cao, ZIF-derived nitrogen-doped porous carbons as highly

efficient adsorbents for removal of organic compounds from wastewater, Chem. Eng. J., 323 (2017)

502-511.

[40] Y.N. Yuan, A.P. Fu, Y.Q. Wang, P.Z. Guo, G.L. Wu, H.L. Li, X.S. Zhao, Spray drying

assisted assembly of ZnO nanocrystals using cellulose as sacrificial template and studies on their

photoluminescent and photocatalytic properties, Colloids and Surfaces A: Physicochem. Eng.

Aspects, 522 (2017) 173-182.

[41] M.H. Dehghani, M. Mohammadi, M.A. Mohammadi, A.H. Mahvi, K. Yetilmezsoy, A.

Bhatnagar, B. Heibati, G. McKay, Equilibrium and Kinetic Studies of Trihalomethanes Adsorption

onto Multi-walled Carbon Nanotubes, Water Air Soil Pollut, 227 (2016) 332-349.

[42] K.Y. Foo, B.H. Hameed, Textural porosity, surface chemistry and adsorptive properties of

durian shell derived activated carbon prepared by microwave assisted NaOH activation, Chem. Eng.

J., 187 (2012) 53-62.

25
[43] F.B. Su, C.K. Poh, J.S. Chen, G.W. Xu, D. Wang, Q. Li, J.Y. Lin, X.W. Lou, Nitrogen-

containing microporous carbon nanospheres with improved capacitive properties, Energy Environ.

Sci., 4 (2011) 717-724.

[44] M. KEILUWEIT, P.S. NICO, M.G. JOHNSON, M. KLEBER, Dynamic Molecular Structure

of Plant Biomass-Derived Black Carbon (Biochar), Environ. Sci. Technol. , 44 (2010) 1247-1253.

[45] C. Wang, X.F. Wang, H. Lu, H.L. Li, X.S. Zhao, Cellulose-derived hierarchical porous carbon

for high-performance flexible supercapacitors, Carbon, 140 ( 2018) 139-147.

[46] S.W. Zhang, M.Y. Zeng, J.X. Li, J. Li, J.Z. Xu, X.K. Wang, Porous magnetic carbon sheets

from biomass as an adsorbent for the fast removal of organic pollutants from aqueous solution, J.

Mater. Chem. A, 2 (2014) 4391-4397.

[47] M. Thommes, K. Kaneko, A.V. Neimark, J.P. Olivier, F. Rodriguez-Reinoso, J. Rouquerol,

K.S.W. Sing, Physisorption of gases, with special reference to the evaluation of surface area and

pore size distribution (IUPAC Technical Report), Pure Appl. Chem., 87 (2015) 1051–1069.

[48] A. Alabadi, H.A. Abbood, Q.Y. Li, J. Ni, B. Tan, Imine-Linked Polymer Based Nitrogen-

Doped Porous Activated Carbon for Efficient and Selective CO2 Capture, Sci. Rep., 6 (2016)

38614-38623.

[49] X.W. Liu, X.H. Liu, B.F. Sun, H.L. Zhou, A.P. Fu, Y.Q. Wang, Y.-G. Guo, P.Z. Guo, H.L. Li,

Carbon materials with hierarchical porosity: Effect of template removal strategy and study on their

electrochemical properties, Carbon, 130 (2018) 680-691.

[50] X. Yu, S. Wang, Y. Gao, Z. Bao, Orthogonal design-guided preparation of multi-level porous-

activated carbon by pyrolysis of waste polyester textiles, Environ Sci Pollut Res Int, 25 (2018)

30567-30574.

26
[51] Y. Mao, H. Duan, B. Xu, L. Zhang, Y.S. Hu, C.C. Zhao, Z.X. Wang, L.Q. Chen, Y.S. Yang,

Lithium storage in nitrogen-rich mesoporous carbon materials, Energy Environ. Sci., 5 (2012)

7950-7955.

[52] M. Naushad, T. Ahamad, B.M. Al-Maswari, A.A. Alqadami, S.M. Alshehri, Nickel ferrite

bearing nitrogen-doped mesoporous carbon as efficient adsorbent for the removal of highly toxic

metal ion from aqueous medium, Chem. Eng. J., 330 (2017) 1351-1360.

[53] G.Z. Shen, B.Q. Mei, H.Y. Wu, H.Y. Wei, X.M. Fang, Y.W. Xu, Microwave Electromagnetic

and Absorption Properties of N-Doped Ordered Mesoporous Carbon Decorated With Ferrite

Nanoparticles, J. Phys. Chem. C, 121 (2017) 3846-3853.

[54] F.C.D. Godoi, E. Rodriguez-Castellon, E. Guibal, M.M. Beppu, An XPS study of chromate

and vanadate sorption mechanism by chitosan membrane containing copper nanoparticles, Chem.

Eng. J., 234 (2013) 423-429.

[55] S.G. Shenouda, F. Happey, X-ray study of crystalline character and mechanical properties of

cotton fibers swollen in zincoxen solution, J. Appl. Polym. Sci., 20 (1976) 2069-2081.

[56] L. Chen, T. Ji, L. Brisbin, J. Zhu, Hierarchical Porous and High Surface Area Tubular Carbon

as Dye Adsorbent and Capacitor Electrode, ACS Appl. Mater. Interf., 7(22) (2015) 12230-12237.

[57] R.H. Huang, Q. Liu, J. Huo, B.C. Yang, Adsorption of methyl orange onto protonated

crosslinked chitosan, Arab. J. Chem., 10 (2013) 24-32.

[58] T.Y. Dai, Y. Lu, Water‐ Soluble Methyl Orange Fibrils as Versatile Templates for the

Fabrication of Conducting Polymer Microtubules, Macromol Rapid Comm, 28 (2010) 629-633.

[59] M. Tanzifi, S.H. Hosseini, A.D. Kiadehi, M. Olazar, K. Karimipour, R. Rezaiemehr, I. Ali,

Artificial neural network optimization for methyl orange adsorption ontopolyaniline nano-adsorbent

- Kinetic, isotherm andthermodynamic studies, J. Mol. Liq., 244 (2017) 189-200.

27
[60] E. Haque, J.W. Jun, S.H. Jhung, Adsorptive removal of methyl orange and methylene blue

from aqueous solution with a metal-organic framework material, iron terephthalate (MOF-235), J

Hazard. Mater., 185 (2011) 507-511.

[61] T.W. Webber, R.K. Chakkravorti, Pore and solid diffusion models for fixed-bed adsorbers,

Proc. Acad. Sci. USSR Phys. Chem. Sect., 0 (1974) 228-238.

[62] K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem.

Eng. J., 156 (2010) 2-10.

[63] S.S. Fan, Y. Wang, Z. Wang, J. Tang, J. Tang, X.D. Li, Removal of methylene blue from

aqueous solution by sewage sludge-derived biochar: Adsorption kinetics, equilibrium,

thermodynamics and mechanism, J. Environ. Chem. Eng, 5 (2017) 601-611.

[64] A. Sanchez-Sanchez, F. Suarez-Garcia, A. Martinez-Alonso, J.M.D. Tascon, Synthesis,

characterization and dye removal capacities of N-doped mesoporous carbons, J Colloid Interface

Sci., 450 (2015) 91-100.

[65] Y.C. Jiao, L. Xu, H.L. Sun, Y.J. Deng, T. Zhang, G.J. Liu, Synthesis of benzxazine-based

nitrogen-doped mesoporous carbon spheres for methyl orange dye adsorption, J. Porous. Mater., 24

(2017) 1565-1574.

[66] H. Li, N.H. An, G. Liu, J.L. Li, N. Liu, M.J. Jia, W.X. Zhang, X.L. Yuan, Adsorption

behaviors of methyl orange dye on nitrogen-doped mesoporous carbon materials, J Colloid

Interface Sci, 466 (2016) 343-351.

[67] T. Liang, F.X. Wang, L. Liang, M.S. Liu, J.M. Sun, Magnetically separable nitrogen-doped

mesoporous carbon with high adsorption capacity, J Mater. Sci., 51 (2016) 3868-3879.

[68] D.L. Zhao, W.M. Zhang, C.L. Chen, X.K. Wang, Adsorption of methyl orange dye onto

multiwalled carbon nanotubes, Procedia Environ. Sci., 18 (2013) 890-895.

28
[69] M.V. Subbaiah, D.S. Kim, Adsorption of methyl orange from aqueous solution by aminated

pumpkin seed powder: Kinetics, isotherms, and thermodynamic studies, Ecotoxicol Environ Saf,

128 (2016) 109-117.

29
Highlights:

 Waste cellulose-derived carbon displays high adsorption capacity for MO.

30