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Carbon Microbelt Aerogel Prepared by Waste Paper: An Effi cient and Recyclable Sorbent for Oils and Organic Solvents

Hengchang Bi , Xiao Huang , Xing Wu , Xiehong Cao , Chaoliang Tan , Zongyou Yin , Xuehong Lu , Litao Sun , * and Hua Zhang *

Waste paper as the main constituent of the municipal waste

has resulted in many environmental problems. Therefore,

the development of applications for waste paper is being

demanded urgently. Herein, a novel kind of carbon microbelt

(CMB) aerogel with good selective sorption ability has been

prepared using the waste paper as its precursor material. The

CMB aerogel shows highly effi cient sorption of not only fats

and petroleum products (up to 188 times its own weight), but

also organic solvents such as chloroform (up to 151 times its

own weight). Moreover, the CMB aerogel could be regen-

erated many times without decrease of sorption capacity

by distillation, or squeezing, which depends on the type of

pollutants. In addition, the production method for CMB

aerogel is very simple and its precursor material, i.e., waste

paper, is cheapest among all reported sorbents. Therefore, we

believe the CMB aerogel is a cost-effective and promising

sorbent for the removal of pollutants.

Municipal wastes containing various solid wastes from

food, commercial materials, packaging, etc. have become

a severe problem in developed and developing countries.

Among these solid wastes, waste paper is the main con-

stituent of municipal wastes [ 1 ] due to the human daily life

and industrial activities, resulting in many environmental

problems. [ 2,3 ] The conventional paper recycling is based on

the process of conversion of waste paper into new paper. [ 4 ]

However, not all paper and paper products can be recycled.

During the recycling of waste paper, the pulping process

deteriorates the strength of the virgin fi ber, so that the prop-

erty of recycled paper is not comparable to those made from

virgin fi bers. [ 5 ] As a result, the production of high quality

paper still mainly relies on virgin fi bers, and the use of recy-

cled water paper is limited. [ 6 ] Meanwhile, a further increase

in the collection of waste paper is expected because of the

high cost for disposal of industrial wastes and the increased

concerning on the recycling of natural resources. Therefore,

the development of applications for waste paper other than

paper manufacturing has been demanded. [ 5 ] Some of the pos-

sible applications include the bioconversion of waste paper

to ethanol, [ 7 ] enzymatic production of glucose, [ 1 ] fabrication

of biodegradable polyurethane foam [ 8 ] and production of

methane. [ 9 ] The major constituent of paper is cellulose, [ 10 ]

indicating that the carbon-based aerogel can be fabricated by

using waste paper as the raw material.

Three-dimensional (3D) carbon-based aerogel has been

extensively investigated due to its high porosity, low den-

sity, high electrical conductivity, etc. [ 11–14 ] Particularly, the

hydrophobicity of aerogel makes it an ideal candidate for

the removal of pollutants and the separation of oil and

water, [ 11,15–18 ] because its hydrophobic surface can selectively

and effectively adsorb/absorb oily target compounds mixed

with water. The sorbent based on the exfoliated graphite

is cheap, but possesses quite low sorption capacity. [ 19,20 ]

Although carbon nanotube sponges or their derivatives

prepared by chemical vapor deposition (CVD) have high

sorption capacity, the expensive precursors and complex

equipment hamper their massive production for practical

applications. [ 21,22 ] Recently, 3D graphene has attracted inten-

sive attention because of their unique properties, such as high

compressive strength, high porosity and high electrical con-

ductivity. [ 23–29 ] Graphene-based aerogels used for sorption of

various oils and organic solvents have gained more and more

attention due to their high sorption capacity and excellent

recyclability. [ 15,30–34 ] However, the generation of acidic waste

and the use of large amount of chemicals during the prepara-

tion of aerogels seriously limit their industrialization, which

drives us to develop a low cost, facile and environmentally

friendly method to fabricate carbon-based 3D aerogels.

Herein, lightweight, hydrophobic and porous aerogels

made of carbon microbelts (CMBs) are fi rst produced via a

facile route by using waste paper as the raw material. Impor-

tantly, the CMB aerogel can absorb a wide range of organic DOI: 10.1002/smll.201303413

Aerogels

H. C. Bi, Dr. X. Wu, Prof. L. T. Sun SEU-FEI Nano-Pico Center Key Laboratory of MEMS of Ministry of Education Southeast University Nanjing 210096 , P. R. China Tel: 86-025-83792632-8813, Fax: 86-025-83792939 E-mail: slt@seu.edu.cn

H. C. Bi, Dr. X. Huang, Dr. X. H. Cao, C. L. Tan, Dr. Z. Y. Yin, Prof. X. H. Lu, Prof. H. Zhang School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798 , Singapore Tel: 65-67905175, Fax: 65-67909081 E-mail: HZhang@ntu.edu.sg

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solvents and oils with a maximum sorption capacity up to

188 times the weight of the pristine CMB aerogel. Moreover,

the CMB aerogel exhibits the excellent recyclability (up to

5 times), and maintains a high sorption capacity even after

fi ve cycles through distillation or squeezing. We believe that

such waste paper-derived novel carbon aerogel with good

performance will have great potential for industrial applica-

tions and environmental protection.

The fabrication process of CMB aerogel is illustrated in

Figure 1 a. Typically, a few pieces of waste offi ce paper were

soaked in distilled water for 12 h. They were then broken

down into strands of cellulose through strong agitation under

vigorous stirring, and the resulting mixture is called pulp.

Then the pulp was subject to freeze-drying to give the pulp

fi ber aerogel, which was pyrolyzed at 850 °C for 2 h at a low

pressure (∼0.5 mbar) under argon atmosphere to generate

the black and lightweight carbon microbelt (CMB) aerogel

(see the Experimental Section for details). The shape and size

of aerogel can be controlled by using various shapes of con-

tainers (Figure 1 a and Figure S1 in the Supporting Informa-

tion). For example, a typical pulp fi ber aerogel had a height

of ∼4.2 cm, and shrank to ∼2.5 cm after pyrolysis (Figure 1 a).

The CMB aerogel is lightweight (Figure S2 in the Supporting

Information) and has a low density of ∼ 5.8 mg cm −3 meas-

ured based on Archimedes’ principle and a high BET surface

area of ∼178 m 2 g −1 (Figure S3 in the Supporting Informa-

tion). The pulp fi ber aerogel can absorb water effectively,

indicated by the fact that it adsorbed a methylene blue (MB)

aqueous droplet rapidly and left a blue stain on its surface

(Figure 1 b).

On the contrary, the CMB aerogel is superhydrophobic

and can support a spherical water droplet on its surface

(Figure 1 c). To further confi rm the hydrophobicity of the

CMB aerogel, it was held by a pair of tweezers and immersed

into water. A uniform mirror-refl ection was observed on the

surface of the CMB aerogel (Figure 1 d), due to the formation

of an interface between the entrapped air in the 3D aerogel

and the surrounding water. [ 16,30 ] The different wettability

between the pulp fi ber aerogel and CMB aerogel can be

justifi ed by Fourier transform infrared spectroscopy (FTIR)

analysis. The FTIR spectrum of pulp fi ber aerogel showed

several peaks of hydrophilic functional groups, such as C=O,

C–O, and –OH (Figure S4a). In contrast, after pyrolysis, the

resultant CMB aerogel showed no functional groups, sug-

gesting its superhydrophobicity (Figure S4b).

Scanning electron microscope (SEM) image shows that

the pulp fi ber aerogels are porous and interconnected 3D net-

works ( Figure 2 a). The fi bers are belt-like, and most of them

are up to a few centimeters or even longer (Figure 2 a and

Figure S5a). High-magnifi cation SEM images indicate that

the width of belt fi bers is 15–20 µm (Figure 2 b). In contrast,

the belts in CMB aerogels show a reduced size of 5–10 µm,

and are cross-linked with each other tightly (Figure 2 c, 2 d and

Figure S5b in the Supporting Information). In addition, the

Figure 1. The fabrication process and wettability of CMB aerogel. (a) The fabrication process of CMB aerogel. 1: immersion and agitation; 2: freeze-drying; 3: pyrolysis. (b) A drop of water stained with MB was adsorbed by the pulp fi ber aerogel and a blue stain left. (c) Photograph of a water droplet supported on a CMB aerogel. (d) Mirror-refl ection can be observed when a CMB aerogel was immersed into water, which is a convictive and direct evidence for proving the hydrophobicity of CMB aerogel.

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selected area electron diffraction (SAED) pattern of several

belts (inset in Figure 2 d) indicates that they are amorphous.

The 3D porous structure and surface hydrophobicity of

the CMB aerogel make it an ideal candidate for the removal

of pollutants such as oils and organic solvents. Figures 3 a

and b show the strong sorption capability of CMB aerogels

(also see Movie S1 and Movie S2). When the CMB aerogel

was brought into contact with a heptane layer (stained with

Sudan red 5B) on a water surface, it absorbed the heptane

completely within 29 s (Figure 3 a and Movie S1). The CMB

aerogel could fl oat on the water surface after sorption of the

heptane, indicating its potential use for the facile removal

of oil spillage and chemical leakage. In addition, the CMB

aerogel can also be used to quickly absorb organic solvents

denser than water, such as chloroform which was stained with

Sudan red 5B at the bottom of water (Figure 3 b, Movie S2).

To study the sorption capacity quantitatively, here we

defi ne the weight gain (wt%) as the weight of absorbed sub-

stance per unit weight of the dried CMB aerogel. Various

types of organic liquids were studied, such as the commer-

cial petroleum products (e.g. pump oil), fats (e.g. olive oil,

colza oil) and ketones with different carbon chain lengths,

which are dominant components of crude oil. These materials

are common pollutants in our daily life as well as from the

industry. The sorption of organic solvents, such as heptane,

alcohol, benzyalcohol and octadecylene, was also tested. The

CMB aerogel showed a very high sorption capacity for all of

the aforementioned organic liquids. In general, CMB aerogel

can uptake these liquids at 56 to 188 times its own weight

(Figure 3 c).

Importantly, our CMB aerogel shows much higher

sorption capacity than many previously reported sorb-

ents ( Table 1 ), [ 11,16,19–21,30–44 ] such as activated carbons

(<1 times), [ 39 ] wool-based nonwoven (9–15 times), [ 35 ] nanowire

membrane (4–20 times), [ 37 ] polymers (5–25 times), [ 38 ] mag-

netic exfoliated graphite (30–50 times), [ 20 ] spongy gra-

phene (20–86 times) [ 15 ] and CNT sponge doped with boron

(25–125 times). [ 41 ] In addition, the sorption capacity of

CMB aerogel is also comparable to that of materials with

high sorption capacity, for example, the twisted carbon

fi ber (TCF) aerogel (50–192 times), [ 16 ] graphene spongy

(60–160 times), [ 32 ] graphene-based sponges (60–160 times) [ 30 ]

and carbon nanotube sponges (80–180 times). [ 21 ] Although

the sorption capacity of CMB aerogel is still lower than that

of nitrogen-doped graphene foam, [ 33 ] ultra-fl yweight aero-

gels [ 34 ] and cellulose nano-fi bers aerogel, [ 11 ] the fabrication

method of CMB aerogel is simpler and its precursor mate-

rial, i.e. waste paper, is cheapest among all these sorbents.

Therefore, our CMB aerogel is a cost-effective and promising

sorbent for the removal of pollutants.

Until now, many methods have been used to recyle sor-

bents and recover pollutants. This is very important because

Figure 2. SEM images of pulp fi ber aerogels and CMB aerogels. (a) Low- and (b) high-magnifi cation SEM images of the cellulose belts in pulp fi ber aerogels. (c) Low- and high-magnifi cation SEM images of the belts in CMB aerogels. Inset in (d): diffraction pattern of cellulose microbelt obtained with transmission electron microscope (TEM).

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most pollutants are either useful and/or precious raw mate-

rials or toxic, e.g., crude oil and toluene. For removal of

valuable pollutants or those with low boiling points, dis-

tillation is a common method. For precious or nonfl am-

mable pollutants with high boiling points, squeezing is an

alternative method. The recycle tests were performed for

CMB aerogel through distillation ( Figure 4 a) and squeezing

(Figure 4 b). To demonstrate the cyclic distillation test,

heptane with a boiling point of 98.5 °C was sorbed by the

CMB aerogel. After that, the material was heated to 95 °C

to release the vapor of heptane (note that the temperature

chosen for evaporation should be around the boiling point

Figure 3. Sorption of organic liquids by CMB aerogels. (a) Photographs showing the sorption process of heptane by using a CMB aerogel taken at intervals of 10 s. Heptane stained with Sudan red 5B fl oating on water was completely absorbed within 29 s. (b) Photographs showing the sorption process of chloroform by using a CMB aerogel. Chloroform stained with Sudan red 5B at the bottom of water was completely absorbed within 10 s. (c) Sorption effi ciency of the CMB aerogel on various organic liquids. Weight gain here is defi ned as the weight ratio of the absorbate to the dried CMB aerogel.

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Table 1. Comparison of various sorbent materials.

Sorbent materials Absorbed substances Sorption capacity (g g −1 )

Cost Ref.

Vegetable fi ber crude oil 1–100 low [36]

Nanowire membrane oils and some organic solvents 4–20 low [37]

Wool-based nonwoven diesel, crude oil, SN 150 9–15 low [35]

Polymers oils and organic solvents 5–25 medium [38]

Magnetic exfoliated graphite oils 30–50 high [20]

Exfoliated graphite heavy oil 60–90 low [19]

Activated carbons benzene, toluene <1 low [39]

Carbon nanotube sponges oils and organic solvents 80–180 high [21]

Graphene/CNT foam compressor oil, organic solvents 80–140 high [40]

CNT sponge doped with boron oils and organic solvents 25–125 high [41]

Graphene/a-FeOOH composite cyclohexane, toluene, vegetable oil, etc. 10–30 high [31]

Graphene-based sponges oils and organic solvents 60–160 high [30]

Graphene sponge oils and organic solvents 60–160 high [32]

Reduced graphite oxide foam cyclohexane, chlorobenzene, toluene, petroleum, motor oil 5–40 high [42]

Nitrogen doped graphene foam oils and organic solvents 200–600 high [33]

UFAs oils and organic solvents 215–913 high [34]

Carbonaceous nanofi ber aerogel oils and organic solvents 40–115 high [43]

CNF aerogels oils and organic solvents 106–312 low [11]

TCF aerogel oils and organic solvents 50–192 quite low [16]

Marshmallow-like gels oils and organic solvents 6–15 high [44]

CMB aerogel oils and organic solvents 56–188 low present work

Figure 4. Recyclability study of CMB aerogels. (a) Distillation was applied to recycle the CMB aerogel for sorption of heptane. (b) Squeezing was used to recycle the CMB aerogel for sorption of octadecene. (c) Photographs showing the sorption process of heptane by using a recycled CMB aerogel. Heptane stained with Sudan red 5B fl oating on water was completely absorbed within 20 s.

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of the absorbate). This sorption-evaporation process was

repeated up to 5 times. As shown in Figure 4 a, less than

1 wt% of residual heptane remained in the CMB aerogel

after each cycle, and no obvious change of sorption capacity

was observed after 5 cycles, indicating a stable sorption and

recycling performance of the CMB aerogel. In addition, no

structural damage was observed to the carobn belts after

the test (Figure S6).

As for the cyclic sorption-squeezing test, particularly for

those pollutants with higher boiling point or nonfl ammble

properties, octadecene with a high boiling point of 314 °C

was used as an example. In the fi rst cycle, 170 mg of octa-

decene could be sorbed by the CMB aerogel, but the rem-

nant mass was up to 10.8 mg after squeezing because of

incomplete compression of the CMB aerogel by tweezers.

From the second cycle onwards, the performance of CMB

aerogel became stable, i.e. the weight gain remained con-

tant (Figure 4 b). This is because the porous structure of the

aerogel and the carbon belts remained unchanged during

the whole process (Figure S7). When the squeezing-recycled

CMB aerogel was brought into contact with a heptane layer

(stained with Sudan red 5B) on a water surface, it can still

absorb the heptane completely and rapidly (Figure 4 c and

Movie S3). In a word, any of the two common methods

mentioned above, i.e. distillation and squeezing, or a combi-

nation of them can be applied for recycling CMB aerogels

dependent on the type of pollutants.

In summary, the smart use of waste paper to produce

CMB aerogels for selective oil adsorption with high effi ciency

and oil loading have been demonstrated without generation

of any additional pollution. The waste paper-produced CMB

aerogel possesses the high sorption capacity of 56–188 times

its own weight. The CMB aerogel can be recycled and repeat-

ably used via a simple method of distillation or squeezing.

Most importantly, the abundant source and simple prepara-

tion method make the CMB aerogel cost-effective for pos-

sible industrial applications, such as barrier separation and

water purifi cation. Furthermore, it is also anticipated that

CMB aerogel can be used as 3D electrode material for

energy storage devices, such as supercapacitors and lithium-

ion batteries, as well as building block for functional com-

posite materials.

Experimentals Section

Preparation of Carbon Microbelt (CMB) Aerogels : 60 mg of waste paper scraps were mixed with 40 mL of distilled water and left still for 24 h. Then 10 mL of hydrochloric aicd (10%) was added to the aforementioned mixture, which was left undisturbed for another 12 h. The solid in the mixture was washed by centrifuga-tion for several times with distilled water to remove chloride ions, and then dried overnight in an oven at 60 °C. The dried sample was poured into 30 mL of distilled water followed by strong agitation under vigorous magnetic stirring to form a uniform mixture which is called pulp. The pulp was then subjected to freeze-drying to form pulp fi ber aerogel. After that, the pulp fi ber aerogel was trans-ferred into a tubular furnace for pyrolysis. In order to remove the air trapped in the pulp fi ber aerogel completely, the furnace was

evacuated before introduction of argon gas, followed by evacua-tion of the furnace again. After that, the furnace was heated up to 850 °C at a heating rate of 5 °C min −1 and kept at 850 °C for 2 h in argon atomasphere at pressure of ∼0.5 mbar. Finally, the furnace was cooled down to room temperature naturally to obtain the low-density CMB aerogels.

Characterization of Pulp Fiber Aerogels and CMB Aerogels : All samples were characterized by a fi eld emission scanning elec-tron microscope (FESEM, JEOL, JSM-7600F), and Fourier transform infrared spectroscopy (FTIR, Perkin Elmer Instruments Spectra, GX FTIR spectrometer).

Sorption of Oils and Organic Solvents : In a typical sorption test, a CMB aerogel was placed in contact with an organic liquid until the aerogel was fi lled with the organic liquid completely, which was then taken out for weight measurement. In order to avoid evaporation of the absorbed organic liquid, especially for those with low boiling points, the weight measurement should be done quickly. The weight of a piece of CMB aerogel before and after sorption was recorded for calculation of the weight gain.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work was supported by the National Basic Research Program of China (Grant Nos. 2011CB707601 and 2009CB623702), the National Natural Science Foundation of China (Nos. 61274114 and 51071044), China Scholarship Council, Natural Science Founda-tion of Jiangsu Province (BK2012024 and BK2012123), Chinese postdoctoral fundings (No. 2012M520053) and Scientifi c Research Foundation of Graduate School of Southeast University (No. YBJJ1208) in China. It was also supported by the MOE under AcRF Tier 1 (RG 61/12) and Start-Up Grant (M4080865.070.706022) in Singapore. This research is also funded by the Singapore National Research Foundation and the publication is supported under the Campus for Research Excellence and Technological Enter-prise (CREATE) programme (Nanomaterials for Energy and Water Management).

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Received: November 2, 2013 Revised: December 16, 2013 Published online: February 10, 2014

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